The fundamentals of ion migration in crystalline ionic solids are well established. Ion migration is essentially a classical process, owing to the high masses of the ions. It is mediated by point defects, either vacancies or interstitials. It is thermally activated, exhibiting Arrhenius behaviour, with an activation enthalpy of defect migration that varies, according to the ion that is moving and the crystal structure in which the ion is moving, from a fraction of an eV to several eV.

Determining experimentally for a specific material the point defect that is responsible for ion migration and its associated migration enthalpy are challenging tasks. Computer simulations have proved extremely helpful in this regard, by providing data for closely controlled systems, by aiding the interpretation of experimental data and by providing atomic-scale insights. Caution needs to be exercised, however, in performing simulations and in using them to interpret experimental data. In this presentation, focussing on ion migration in crystalline solids, I will examine various themes from this intersection of experiment and simulation for which caution is required.

Conductivity degradation is one of the key limitations in the commercial implementation of doped zirconia electrolytes.  This closely aligns with odering phenomena in both the anion and cation lattices.  We briefly review these phenomena and illustrate this with a special example, a  magnesia and india co-doped zirconia composition that exhibits an unusual conductivity degradation-recovery profile where the conductivity degrades for a short period of time then recovers and exceeds its initial value. The conductivity at 850 °C in air was found recovered to the initial value of 0.096 S cm-1 in less than 25 hours, and then it continues to improve up to 0. 11 S cm-1 after 250 hours. This special behaviour in a stable single-phased composition allows us to unambiguously correlate conductivity changes over time to changes in short-range structure. Through a detailed structural characterisation by electron diffraction, TOF neutron diffraction and other techniques, the long-standing recognition that the conductivity degradation is related to short-range ordering is confirmed experimentally. We conclude that the conductivity degradation is caused by pyrochlore and rare-earth C-type ordering within the fluorite, whereas the conductivity increase and stabilisation observed in this composition is a result of eliminating such defect short-range ordering and moving towards a stable disordered fluorite structures.

Ceria and its solid solutions exhibit excellent redox properties and stability in a wide range of temperatures and oxygen partial pressures (pO₂). These materials therefore find applications in solid oxide cells (SOCs), oxygen separation membranes, catalytic surface coatings, and oxygen sensors. In the present study, we have investigated the electrical conductivity of Gd_0.10Ce_0.90O_2-δ (GCO) as a function of both pO₂ (10^-0.36-10^-25 bar) and temperature (650-850  ͦC). Experimental data are modeled in terms of the underlying defect chemistry. Our work clearly highlights departures from ideal solution behavior by concurrently modeling data of oxygen non-stoichiometry and electrical conductivity. Our analysis starts with analyzing literature data on the oxygen non-stoichiometry of GCO.¹ Following Mizusaki et al.^2,3, and in agreement with earlier reports for doped ceria,^4,5 we demonstrate that the observed decrease in reduction enthalpy for GCO with decreasing pO₂ can be accounted for by the interactions between the involved ionic and electronic defects. The non-ideality of the experimental data of electrical conductivity can also be traced back to the interactions of defects. The results indicate that upon reduction the small polarons formed in GCO retain their mobile character, rather than being trapped in complex defect associates as has been suggested previously.¹

Nuclear magnetic resonance offers plenty of advanced techniques, such as 1D and 2D EXSY [1,2], to study dynamic features of solid-state lithium ion conductors. Provided ultrafast ion dynamics is present at temperatures around ambient, i.e., dealing with jump rates in the order of some GHz, extremely low, if not cryogenic, temperatures are needed to completely freeze any Li⁺ ion dynamics on the NMR time scale. Taking advantage of two substances, viz. Ge-bearing Li₆PS₅I [3] and LiTi₂(PS₄)₃ [4], we measured ⁷Li NMR line shapes and spin-lattice relaxation rates over a wide temperature range down to 9 K. The NMR experiments resolve the various Li⁺ exchange processes that ultimately give rise to overall, fast ionic transport at ambient conditions. As an example, for Ge-containing Li₆PS₅I we identified the spin-lock NMR rate peak appearing at 163 K as the one reflecting the rate-limiting intercage Li⁺ diffusion process that enables the ions to be transported over long distances. The corresponding Einstein-Smoluchowski diffusion coefficient excellently agrees with that indirectly probed by macroscopic conductivity spectroscopy.

All Solid-State Batteries (ASSBs) are looked as the most promising technology to go beyond the limitation of LiBs. The presence of a solid electrolyte may reduce the propagation of dendrites, and absence of a flammable liquid eases the explosion risk in case of thermal runaway. This kind of batteries, denominated Gen4b, couples the solid electrolyte with Li metal (theoretical specific capacity: 3860 mA h/g) and high voltage cathodes, which use, allowed by the low flammable risk, will in addition increase the gravimetric and volumetric energy density beyond incumbent LIBs^[1].

Sulfide-based materials emerge from the historical proposed ceramic materials, such oxides, as a favorable candidate to be used as solid electrolyte in ASSBs in view of their promising performance and ease of manufacturing. Sulfides are characterized by high ionic conductivity (above 1mS/cm at RT), they show mechanical strength together with easy processability, which allows their manufacturing from densification under pressure at room temperature (an advantage towards oxides, which require high sintering temperature).  However, their difficulty to withstand high current density and long-term cycling, as well as their sensitivity to moisture are hampering their entrance into the battery market.

This work aims to provide a better understanding of the transport properties of sulfide-based ASSBs, for which, as a case of study, argyrodite Li₆PS₅Cl separators are considered. First the conduction properties were analyzed and correlated with Li-ion mobility in the electrolyte itself. Secondly the interfacial processes taking place at the battery level were investigated, for this purpose, distribution of relaxation times has been applied^[2,3]. This technique allows to correlate the factors determining the cell performance to the various impedance sources happening at the Li|electrolyte and cathode|electrolyte interfaces.  As result, the degradation process at the electrolyte interfaces have been followed during cycling, giving precious insight on the parasitic reaction happening during real operating condition.

Over the past four decades, protonic ceramics have captivated increasing attention, with a notable surge in interest observed in recent years. This trend can be explained by a better understanding of their properties and the development of scalable and reproducible manufacturing processes.

This presentation will begin with a comprehensive overview of protonic ceramics, specifically emphasizing their conductivity and hydration properties. Subsequently, it will delve into the diverse applications of protonic ceramic cells, encompassing fuel cells, electrolyzers, and membrane reactors. The core of the presentation will revolve around electrolysis cells (protonic ceramic electrolysis cells, PCECs), with an initial focus on the latest advancements in the field. Following this, the major challenges and corresponding solutions will be addressed:

(1) The faradaic efficiency, which is proportional to the ratio of the net hydrogen molar flux over the imposed current density, is an important performance metric: the discrepancy in the results in the literature and potential solutions to achieve high faradaic efficiencies will be discussed [1];

(2) The steam electrode (positrode) mechanisms are more complex than in solid oxide cells, and the community is working on developing highly performant materials at intermediate temperatures [2]. In addition, the positrode materials must also be stable under high steam contents.

Proton-conducting oxides are extremely promising for playing a central role in future energy supplies based on renewables – for example as electrolyte and electrode materials in solid oxide fuel or electrolysis cells, or as mixed proton- and electron-conducting ceramic membranes for hydrogen purification. Materials with perovskite-type structures have emerged as possible candidates for both applications. However, their composition, and in particular, their basicity influence the achievable concentration of protons and thus the ionic conductivity of the material. What makes the situation even more difficult is that the proton concentration in an oxide cannot be easily measured, especially not in-situ, which would be important if one wants to obtain this information on a running device.

This study is dedicated to examining the achievable proton concentration in the perovskite-type model material BaFe_0.8Y_0.2O_3-δ (BFY) in various oxidation and protonation states, as the protonation of BFY is intricately linked to the oxidation state of iron. Since this material is an auspicious candidate as a potential electrode material in proton-conducting fuel/electrolysis cells, its proton uptake has already been extensively studied, revealing favorable behavior[1]. The determination of the proton concentration in BFY, however, remains a challenge and primarily relies on techniques that involve indirect analysis of protons, such as thermogravimetry and conductivity measurements, which are considered state-of-the-art methods.

Here we present two alternative analytical methods that can be used for direct detection of protons in perovskites and are also very well suited for in-situ quantification: Infrared (IR) spectroscopy and laser induced breakdown spectroscopy (LIBS). IR spectroscopy provides a potential avenue for directly analyzing protons, since they can be expected to exhibit a characteristic OH vibration band. Indeed such an absorption band is observable on BFY, and its position in the IR spectrum is in good accordance with simulations in literature[2]. LIBS allows direct measurement of the proton concentration via analyzing the characteristic emission from the plasma generated by irradiating the material with an UV-laser. Here we present a customized sample chamber that enables LIBS measurements in different and even changing atmospheres, allowing in-situ analysis of protons in oxides.

The results obtained were validated by gravimetric measurements and supplemented by X-ray diffraction analyses, and both techniques demonstrated their ability to directly detect protons in perovskite-type oxides. This is particularly noteworthy as it makes them immune to artifacts arising from competing reactions, such as oxygen uptake, a challenge often encountered in thermogravimetry. The findings underscore the reliability of our analytical approach, and validate it as a valuable alternative to conventional methods, thus contributing to advancements in renewable energy applications.

The fundamental relationships between performance metrics, impedance spectra, and electrochemical processes in electrolysis devices and other electrochemical systems are highly complex and often difficult to disentangle. This lack of knowledge hinders identification and evaluation of physicochemical processes, limits understanding of how materials and device architecture influence performance, and ultimately inhibits principled design choices. There is a need for practical, robust procedures to gain physical insight into electrochemical device performance. This requires effective design of experiments to sufficiently characterize varying conditions within reasonable time constraints. In addition, new methods for coherent analysis of large electrochemical datasets are necessary to extract meaning from vast amounts of raw data.

Here, we introduce a novel approach to accelerate electrochemical characterization with standard instrumentation by utilizing rapid measurements in both the time and frequency domains. This “hybrid electrochemical impedance spectroscopy” (HEIS) method provides excellent resolution across a broad range of timescales while decreasing measurement time by more than an order of magnitude compared to conventional electrochemical impedance spectroscopy. The technique can be applied to quickly construct detailed electrochemical maps of energy conversion devices across multiple measurement condition dimensions, revealing physicochemical relationships that are hidden in sparse conventional datasets. We use this new approach to comprehensively characterize a commercial Li-ion battery as a function of C-rate and state-of-charge during charging/discharging and a reversible protonic ceramic electrolyzer/fuel cell (PCEC) under different temperature, atmosphere, and bias conditions. Altogether, more than 20,000 distinct HEIS experiments are performed, resulting in a rich dataset that can be assembled to form relaxation hypersurfaces - multi-dimensional analogs of the one-dimensional distribution of relaxation times (DRT) - which are then processed with new analysis techniques to reveal the underlying processes that govern device performance. This approach describes the electrochemical behavior of both batteries and PCECs with an unprecedented level of detail made possible by accelerated measurement and scalable data processing strategies. However, this study represents just one possible form of the mapping concept. The underlying principles and techniques can be adapted to develop various new ways to examine a wide variety of energy-conversion devices. The conceptual approach demonstrated here has the potential to reduce the time required to understand electrochemical materials and device architectures, enabling a faster design cycle.

Electrochemical impedance spectroscopy is a powerful tool to monitor systems in operation. The analysis of impedance data mostly involves pattern recognition and fitting with a simple electric equivalent circuit model. This has become the standard for studying liquid systems, but the analysis of inhomogeneous solid-state systems is more complex. This is due to the microstructure of the sample and the morphology of interfaces, which introduce additional degrees of freedom. As a result, the transport through real structures cannot be adequately described by 1D models. However, this is typically neglected when analyzing the impedance in terms of the transport processes taking place in the system, which in turn can lead to serious misinterpretations.

For example, diffusion processes on the metal anode interface strongly influence the cycling performance of a solid-state battery. This includes charge transfer driven morphological (in)stabilities, e.g., at the interface between lithium and Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO). For a long time, the interface impedance response has been mistakenly attributed to charge transfer reactions, leading to the misconception that an inherently high transfer resistance prevents the realization of the reversible metal anode concept. However, experiments by Krauskopf et al. have shown that the actual cause is the non-ideal physical contact between lithium and LLZO, i.e., the presence of pores at the interface.[1] The charge transfer resistance, on the other hand, is negligible.

Although the current constriction effect is well-known as a DC phenomenon, its AC behavior is not yet fully understood. Thus, we have built on earlier work by Fleig and Maier[2],[3] to understand the various dependencies of the constriction phenomenon, e.g., on the microstructure of LLZO, or the interface morphology. We show that the signature of the current constriction signal in the impedance spectrum is identical to that of ion transport. However, it is not a migration process in the strict sense. It is related to the frequency-dependent change of the electrode area that actively contributes to the transport.[4],[5] It is a geometric effect that is expected at different length scales.

As a result, various processes (e.g., charge transfer reactions, morphological and chemical instabilities, etc.) affect the interface properties of metal anodes. Identifying the dominant interface effect(s) is crucial, as the strategy for improving a solid-state battery depends on the rate-limiting process that needs to be overcome. For this purpose, we have developed a guideline for the interpretation of experimental impedance data of parent metal anodes to increase the reliability of future impedance analyses.[6]

The advent of solid-state batteries has spawned a recent increase in interest in lithium conducting solid electrolytes. However, many open questions remain when trying to optimize electrolytes and understand solid state battery chemistries.

In this presentation, we will show that it is not only important to find fast ionic conductors, but that fast ionic conduction is paramount within solid state battery composites. Measuring the effective ionic transport in electrode composites provides an avenue to explore transport and stability limitations that in turn provide better criteria for solid state battery performance. These transport limitations will be explored as a function of materials composition, particle sizes as well as temperatures.

By understanding these limitations, we will then explore two approaches to overcome these transport limitations.

Finally, we will show how pressure induces dislocation densities in solid ionic conductors. By increasing the strain and dislocations, the overall ionic transport properties improve extending to solid state battery performance.

Li(FSA)(SN)₂ molecular crystal solid electrolytes, consisting of lithium bis-(fluorosulfonyl)imide (LiFSA) and succinonitrile (SN), exhibit comparable ionic conductivity (1.1×10^-4 S·cm^-1) and improved transference number (0.95) with traditional solid polymer electrolytes [1]. FSA anions in an ionic liquid electrolyte contribute to the passivation layer formed on the positive electrode surface [2], suggesting that Li(FSA)(SN)₂ may enable battery operation under high voltage by passivating the interface. However, the adaptability of Li(FSA)(SN)₂ solid electrolytes in high-voltage operation is still unclear. In this study, we fabricated a battery using a thin-film 5 V-class LiNi0_.5Mn_1.5O₄ (LNMO) positive electrode and investigated the operation of the battery under high voltages.

In the early cycles, the battery composed of Li(FSA)(SN)₂ and LNMO shows successful charge and discharge cycles up to 4.8 V vs. Li/Li⁺. However, the capacity shows severe degradation (54% after 100 cycles with the current density of 10 μA·cm^-2). Continuous interfacial side reactions and thick interphase layer formation at the Li(FSA)(SN)₂ | LNMO interface may have led to capacity degradation. An insufficient passivation of the interface is suggested. Therefore, a highly stable amorphous Li₃PO₄ buffer layer was inserted on the Li(FSA)(SN)₂ | LNMO interface to passivate the interface. With the insertion of Li₃PO₄, the capacity degradation was suppressed to 4% after 100 charge-discharge cycles, indicating that the insertion of Li₃PO₄ stabilized the interface even at a high voltage of 4.8 V vs. Li/Li⁺. This study reveals the potential of molecular crystal solid electrolytes for 5 V-class all-solid-state batteries and highlights the importance of interfacial engineering on battery performance.

SOEC has a huge potential as an enabling technology to facilitate the transition of the global energy system to a sustainable one. The technology offers unrivalled efficiency in hydrogen generation and has a potential to reach low cost. Generation of hydrogen is the first step in realizing a range of P2X-schemes for providing green fuels (e.g. methanol, ammonia or jet fuel) for the transport sector [1,2].

The largest SOEC units manufactured/operated today are in the range of ~1 MW. However, numerous scale up projects are under way and industrial stakeholders are these years investing massively to bring up production capacity.  

In this paper we shall discuss some of the challenges in upscaling SOECs and address some materials issues/opportunities for improving SOEC performance with view to large scale deployment.

The brittle nature of the ceramics used in the cells limits the size of the cells to edge lengths in the range of 10-15 cm. Ideally this should be larger, however, this requires care in the manufacturing, and a tough and strong backbone in the cell. Many stakeholders rely on zirconia to provide the structural backbone of the cell. Transformable metastable tetragonal with  2 – 3.5 mol.% Y₂O₃ substitution is advantageous over fully stabilized cubic version with ~8 mol. % Y₂O₃, as the former has higher strength and toughness. We shall present recent results on optimizing composition and processing routes to maximize toughness and strength, whilst preventing unintended spontaneous transformation to the monoclinic phase during use.

In SOEC one does not need any PGM-materials. However, typically the state of the art cells relies on use of several critical raw materials. Several LREE/HREE materials are used (Ce, Gd, Y, La) and also some critical transition metals like Co/Mn and Ni. For large-scale deployment, it is obviously an advantage if the use of these Critical Raw Materials can be minimized. Introducing performance improvements in terms of production capacity per m² of cell or kg of material is an important strategy to reduce the CRM-use and is likely a part of the strategy among most stakeholders.

We shall here present recent results on activities to increase the competitiveness of the SOEC technology by introducing a metal-support as the structural backbone and replace the Ni/YSZ-fuel electrode with one based on a mixed conducting Ti-based oxide (Ni and Fe-doped (La,Sr)TiO₃). Recently, we have found that such a material combination can sustain SOEC operation for extended periods (tested over 5000 hr) at current densities exceeding 0.5 A/cm². These durability results shall be presented as shall be findings on the fundamental charge transport properties of the titanates.  

Finally, results of endeavors to improve electrode performance of both fuel and oxygen electrodes will be discussed. On the fuel electrode side, opportunities and challenges of using Ni/CGO in thin electrolyte cells will be addressed and on the oxygen electrode side, results on boosting of performance via various infiltrates will be summarized.

Nowadays, climate changes attributed to the continuous and excessive consumption of carbon-based fossil fuels are causing escalating concerns, leading to a growing demand for sustainable energy technologies. However, the intermittent nature of many renewable energy sources, such as wind and solar, necessitates the development of reliable energy storage technologies. Among the most promising technologies, solid oxide electrolysis cells (SOECs) can convert electrical energy into chemical energy at high operating temperatures (typically 500-1000 ℃), offering the advantage of achieving higher efficiency and lower capital expenditures [1]. Despite their potential, SOECs are not without challenges. The performance of state-of-the-art cells tends to degrade over time, with a significant portion of this degradation originating from the Ni/ytttria-stabilized zirconia (YSZ) electrode [2]. This study aims to investigate the influence on electrochemical performance and microstructure of Ni/YSZ fuel electrode based SOEC, by adding two common impurities, namely Si and Al. As refractory compounds, silica and alumina are frequently used in SOEC systems at both laboratory and industry scales, serving various roles including glass sealing, insulation, and as parts in vacuum pumps and heating elements to mention a few [3,4]. Alumina is also used as a sintering aid in SOEC cells to reduce the sintering temperature [5]. In this study, a Ni contact layer containing alumina and silica was applied on the top of fuel electrode, to mimic accelerated degradation caused by Si/Al impurities. The electrochemical performance and microstructure information were investigated in detail and compared with a cell without such contaminated layer. Based on the results, possible explanation on the mechanism of Si/Al poisoning the Ni/YSZ SOEC cathode was provided.

Nuclear Magnetic Resonance (NMR) plays a critical role in understanding structural and dynamical disorder, and ionic transport diffusion pathways in solids, complementing diffraction or impedance spectroscopy methods. Here, we present a recently published approach[1] combining experimental and computational Magic Angle Spinning (MAS) NMR aimed at elucidating the local configurational disorder and oxide ion diffusion mechanism in promising structural families of fast oxide ion conductors having the melilite,[1] [2] langasite[3] and perovskite[4] structures.

Layered tetrahedral network melilite possessing the La_1.54Sr_0.46Ga₃O_7.27 composition exhibits the flexibility required to accommodate interstitial oxide anions leading to excellent ionic transport properties at moderate temperatures[2]. ¹⁷O and ⁷¹Ga MAS NMR spectra display complex spectral line shapes that could be accurately predicted using a computational ensemble-based approach to model site disorder across multiple cationic and anionic sites, thereby enabling the assignment of bridging/non-bridging oxygens (Figure 1) and the identification of distinct gallium coordination environments. The ¹⁷O and ⁷¹Ga MAS NMR spectra of La_1.54Sr_0.46Ga₃O_7.27 display additional features that are not observed for the parent LaSrGa₃O₇ phase[submitted] and that can only be detected at the most powerful NMR magnet in the world operating at 35.2 T.[5] These features are attributed to interstitial oxide ions incorporated upon cation doping and stabilised by the formation of five-coordinate Ga centres conferring framework flexibility.[2]

¹⁷O high temperature (HT) MAS NMR experiments capture exchange within the bridging oxygens at 130 °C and reveal coalescence of all oxygen signals in La_1.54Sr_0.46Ga₃O_7.27 at approximately 300 °C (Figure 1), indicative of the participation of both interstitial and framework oxide ions in the transport process. These results, further supported by the coalescence of the ⁷¹Ga resonances in the ⁷¹Ga HT MAS NMR spectra of La_1.54Sr_0.46Ga₃O_7.27, unequivocally provide evidence for the conduction mechanism in this melilite phase.

Other related examples on langasite[3] and perovskite[4] highlight the potential of MAS NMR spectroscopy to enhance the understanding of ionic motion in solids.

Solid oxide fuel cells (SOFC) have gained particular interest due to the demands for sustainable energy sources and reached a mature state of development. In contrast, there are still challenges in materials engineering for solid state electrolysis cells (SOEC). If an external electric current is driven through the cell in electrolysis mode, degradation effects due to electrochemical redox processes can occur, which may limit the lifetime. Electric current induced redox phenomena also gain importance for processing of ceramic materials using Field-Assisted Flash Sintering (FAST) and Spark Plasma Sintering (SPS). Intermediate reduction during the sintering process was re­cently reported. [1] Generally, when considering a contact between two mixed electronic and ionic conductors, electrochemical redox phenomena will occur, if the transference number of the electronic charge carriers is changing at the interface. This includes reduction phenomena as well as oxidative pore formation at electrode interfaces or in the interface between solid electrolytes and mixed conductors. [2, 3]

In this contribution we want to focus on the formation of blackened zirconia, known since the first reports by Casselton in 1968 and subject of various studies in recent decades. The cathodic electroreduction of yttria-stabilized zirconia (YSZ) will lead to a dendrite-shaped reaction front of a reduced phase moving to the anode side. In contrast to the solid electrolyte YSZ, reduced YSZ is a good electronic conductor, leading to an interface between reduced and unreduced phase with a large change of the electronic transference number. However, the composition and structure of the oxygen deficient phases is still not cleared in detail. In many studies it is reported that the mechanical properties of reduced single crystal or ceramic material degrades significantly (i.e. getting brittle).

Our new investigations on YSZ single crystals reveal strong strain fields leading to the for­mation of checkerboard-like structures on the surface above a highly oxygen deficient region below the surface. [4] Transmission electron microscopy (TEM), energy dispersive x-ray spectro­scopy (EDX) and x-ray photoelectron spectroscopy (XPS) have been employed to understand its physical nature. The lattice constant of one cubic axes has shrunk significantly, Zr⁴⁺ is reduced to lower valences, resulting in an approximate stoichiometry of ZrO ∙ Zr₂O. Moreover, there are many stacking faults in the reduced phase and misfit dislocations in the interface to the unreduced phase. The strong change of the molar volume may be the origin of the strain field resulting in the checkerboard-like surface structure and of the deterioration of the mechanical properties.

Improving the ionic conductivity of outstanding, composition-optimised crystalline electrolytes is a major challenge. Achieving increases of orders of magnitude requires, conceivably, highly non-linear effects. One known possibility is the use of high electric fields to increase the point-defect mobility. In this study, we investigate quantitatively a second possibility, that high electric fields can increase substantially point-defect concentrations. As a model system we take a pyrochlore oxide (La₂Zr₂O₇) for its combination of structural vacancies and dominant anti-Frenkel disorder; and we perform molecular-dynamics simulations with many-body potentials as a function of temperature and applied electric field. Results within the linear regime yield the activation enthalpies and entropies of oxygen-vacancy and oxygen-interstitial migration, and from three independent methods, the enthalpy and entropy of anti-Frenkel disorder. Transport data for the non-linear regime are consistent with field-enhanced defect concentrations and defect mobilities. A route for separating the two effects is shown, and an analytical expression for quantitative prediction of the field-dependent anti-Frenkel equilibrium constant is derived. In summary, we demonstrate that the one stone of a non-linear driving force can be used to hit two birds of defect behaviour.

Solid state electrolytes for use in all solid-state batteries face demanding performance criteria.  They must conduct Li⁺ ions between electrodes during battery cycling and withstand the electrical and chemical potentials in the cell, all whilst maintaining stable interfaces as electrode materials (de-)swell during lithium (de-)intercalation and redox. 

We have developed fast-ion conducting phases for this purpose in the garnet family by manipulation of the lithium content, charge carrier concentration and framework charge [1, 2].  These materials are characterised using combinations of crystallographic and local structural probes, and lattice dynamics are measured by complementing impedance analysis with muon spin relaxation measurements.  Using in situ and operando measurements we can determine the influences of reaction pathway and dopant chemistry on the electrolyte performance.  These reveal some subtle differences in the products arising from sol-gel synthesis compared to more conventional solid state reaction routes.  The choice of synthesis route can have an important impact in determining the Li transport properties of the resultant material.

Deployment of these electrolytes in all solid-state batteries requires matching of electrolyte properties with cathode and anode materials.  We will present examples where the chemical stability and redox (in-)activity of the garnet phase is key in enabling solid state electrolytes to function with metallic lithium anode [3].

All-solid-state lithium-ion batteries is considered a front runner among next-generation battery technologies. Solid-electrolytes which allows for Li-ion only mobility and practically inflammable nature, it solves safety and cross migration issues. Additionally, all solid-sate batteries promise significantly high energy as well as power densities. [1], [2] Among the many challenges pertaining to all-solid-state batteries, the electrochemical stability of solid-electrolytes as well as the interfaces remains a key challenge. Recent computational as well as experimental studies have shown that many of these lithium-ion solid electrolytes has poor electrochemical stability at high voltage. [3]–[5] Lithium-stuffed garnet-type solid-electrolyte is one the most widely studied material for all-solid-state batteries for its’ high ionic conductivity and kinetic stability against lithium metal anode. [6], [7] The computational works for Li₇La₃Zr₂O₁₂ (LLZO) garnets have predicted poor electrochemical stability, with oxidation onsetting as low as 2.9 V vs. Li/Li+. [8], [9] Indeed, poor cycling performance has been noted in many experimental studies employing LLZO-type garnets solid-electrolytes, [10]–[12]  which among other factors, possibly, implicate as predicted low oxidation potential. Earlier studies based on utilizing electrochemical techniques have suggested oxidative decomposition at potential as low as 3.7 V vs. Li/Li⁺. [9], [13], [14]. Nonetheless, there remains a gap in the literature for local structural changes in these solid-electrolytes in response to relevant high electrochemical potential.

In this study, we studied local structural and impedance change of representative Al-doped LLZO garnet solid electrolyte as a result of electrochemical reactivity. The experimental techniques utilized the Zr K-edge extended x-ray absorption fine structure (EXAFS) to characterize the local structure and O K-edge soft x-ray absorption spectroscopy (XAS) to deconvolute degradation products. The EXAFS spectra were collected at Advanced Photon Source (APS) beamline 10-ID-B, Argonne National Laboratory. The O K-edge XAS was collected at beamline 23-ID-2 at NSLS-II, Brookhaven National Laboratory.The results shows that Zr coordination shells in LLZO underwent major shrinkage with added, seemingly, loss of coordination number. These results are in agreement with electrochemical induced loss of oxygen from the lattice, a mechanism reported earlier. [9], [15] The O K-edge XAS which further confirms the degradation of LLZO by showing suppression of its’ signature feature as a result of electrochemical oxidation. The growth of impedance after polarization suggest Li-mobility is significantly reduced as a result of electrochemical reactivity at higher potential. In summary, our results show significant local structural change in garnet-type solid-electrolytes under electrochemical potentials at 4.3 V vs. Li/Li⁺. These local changes in structure directly impedes Li-ion mobility. Cell degradation at these potentials could result from oxidation instability of the garnet electrolyte.

With the rapidly growing EV industry, the development of safer batteries with solid-state electrolytes is highly important. The design of composite polymer electrolytes (CPE) incorporating Li⁺ conductor ceramic particles inside a polymeric matrix is an interesting approach to combine the flexibility and processability of the polymers with the high ionic conductivity and electrochemical stability of ceramics. Despite the improved stability of these electrolytes, their ionic conductivity is insufficient for high-rate applications. To address this issue, a heat treatment process was proposed to remove the secondary phases from the surface of the ceramic particles and form a better interphase with the polymer matrix.

In this work, solid-state NMR spectroscopy was applied to investigate the chemical and phase evolution of LLZO garnets during heat treatment and its impact on the local Li dynamics. Using relaxometry, ¹H-⁷Li heteronuclear correlations, and variable temperature NMR experiments, it was shown that heat treatment can effectively remove the LiOH and Li₂CO₃ from the surface of LLZO, and eliminate the protons from its bulk, leading to a significant improvement in the Li dynamics in LLZO powders.^[1] Careful analysis of the ⁷Li spectra showed that removing the secondary phases on the surface of LLZO also improves the local interactions between the PEO and LLZO, ultimately leading to higher ionic conductivity and mechanical properties.^[2] In the next step, 2D ⁷Li-⁷Li and ⁶Li-⁶Li EXSY NMR experiments were employed to detect the Li-ion exchange and quantify the exchange rates. Using pseudo-3D experiments, LiOH was identified as an intermediate phase for the interfacial Li exchange. Finally, ⁷Li → ⁶Li trace exchange experiments were employed to determine the Li-ion pathway through the electrolyte. It was concluded that the heat treatment of LLZO is essential for achieving CPEs with improved micro and macro-scale properties.

Protonic ceramic electrochemical cells (PCECs) offer significant promise as efficient and sustainable energy devices, capable of converting chemical fuels into electric power and vice versa, especially when operating at temperatures below 600°C. However, challenges arise due to the formation of secondary phases within perovskite electrolyte materials, stemming from harsh processing conditions involving high temperatures and prolonged annealing periods. These challenges contribute to a notable decline in the electrochemical performance of PCECs. Furthermore, PCECs encounter issues with poor catalytic activity at lower operating temperatures, resulting from sluggish kinetics at the air electrode. To address these obstacles, our research adopts an innovative approach that employs an ultra-fast sintering process, drastically reducing sintering times to just 5 min. Additionally, we introduce air electrodes based on newly doped BaCoO3-𝛿, strategically designed to enhance oxygen permeability and phase stability. These developments aim to improve the activity and durability of PCECs. In this presentation, I will discuss our recent progress in advancing highly efficient PCECs, highlighting our efforts to overcome these critical challenges.

Bulk protonic conduction may enhance the positrode performance for proton ceramic electrochemical cells by increasing the electrochemically active area beyond the triple-phase boundaries. Significant hydration and proton conduction is observed in positrode perovskites like lanthanum doped barium ferrite (BLF) where the B-site is acceptor doped with Ni and Zn [1], [2]. To further explore the properties as a positrode, electrochemical impedance spectroscopy (EIS) is performed on Ba_0.95La_0.05(Fe_1‑x‑yNi_xZn_y)O_3-δ (BLFNZ) deposited on proton conducting electrolyte Ba(Zr_0.7Ce_0.2Y_0.1)O_3-δ.

Various BLFNZ compositions are studied in a symmetrical cell with reference electrode. Key variables such as activation energies, pre-exponentials and predominant charge carriers are investigated by considering polarization resistance (R_p) as a function of pO₂ and pH₂O across a wide temperature range. Extracting individual area specific polarization resistances (R_p,i) at open circuit voltage reflects equilibrium conditions, while applying cathodic and anodic DC voltage to EIS sweeps delineate oxygen reduction- and oxygen ervolution reaction conditions, respectively. The voltage application results in a net current (I) through the cell and R_p,i is later converted into net overpotentials by integrating R_p,i over I.

This work combines EIS studies with thermogravimetric analysis (TGA) of Ba_0.95La_0.05(Fe_0.7Ni_0.2Zn_0.1)O_3-δ (BLFNZ10), amongst other BLFNZ compositions between 250-700 °C in varying pO₂ and pH₂O. The activation energy of 1.2 eV extracted at 350-450 °C in humid air (~3% H₂O) is relatively high, but a small pre-exponential value (10^‑7 Ωcm²) renders the R_p comparable to state-of-the-art positrode materials [3]. Assuming hydration dominates the mass change in humid air, TGA experiments reveal the proton concentration ranges from 1.8-0.03 mol% for 250-500 °C, i.e., sufficient for appreciable bulk protonic conduction. The relatively high activation energy may suggest a sluggish surface oxygen exchange reaction. Continued experiments include EIS with and without DC bias, and TGA experiments that may yield further insight to the reaction mechanisms of BLFNZ10. Thus, this study aims to establish relationships between hydration and the electrochemical properties of BLF based positrodes.

Proton ceramic electrochemical cells (PCECs) show promise for cost-efficient steam electrolysis, power generation, and membrane reactors. However, the air/steam electrode is a restricting factor in their performance, with the oxygen evolution and reduction reactions being rate limiting. A mechanistic understanding of the reaction steps at the electrodes is therefore required to further develop high-performance electrodes and cell architectures.

Pulsed Isotope Exchange (PIE) is a gas phase analysis method that allows for fast characterisation of the oxygen surface exchange rate of powder samples [1]. The surface exchange of oxygen is measured as a function of oxygen pressure and temperature, as well as the effect of water on the exchange rate. The reaction mechanisms and rate determining steps are investigated further by electrochemical impedance spectroscopy on model electrodes at temperatures from 500 °C down to 200°C in different pO₂s and pH₂Os.

Various compositions of Ba_1-xGd_0.8-yLa_0.2+x+yCo₂O_6-δ, which are double perovskite type materials, have been shown to be efficient triple conducting air/steam electrodes for PCECs at intermediate temperatures [2, 3]. BaGd_0.3La_0.7Co₂O_6-δ shows an oxygen exchange limited by dissociative adsorption in all measured pO₂s with activation energies ranging from 0.8 eV to 1.1 eV in dry atmospheres, which is comparable to other double perovskite cobaltites [4, 5]. While the presence of water does not change the rate limiting step, it lowers the exchange rate and increases the activation energy in high pO₂ and increases the exchange rate and lowers the activation energy in low pO₂s. Electrochemical impedance measurements show similar activation energies to PIE in the same atmospheres and temperature range.

Raman spectroscopy is a non-destructive vibrational spectroscopy technique which offers valuable insights into the structural characteristics of materials by using scattered light to measure the vibrational energy modes of a sample. As the vibrational frequency of a specific mode is inversely proportional to the effective reduced mass, its wavenumber is sensitive to the mass of the atoms constituting the molecule or the crystal, and varies for different isotopes. It is thus possible to determine the concentration of a specific isotope in a material and to measure its spatial or temporal variations by carrying out in situ Raman measurements.

The Isotope Exchange Raman Spectroscopy (IERS) technique is based in the continuous measurement of a specific Raman mode of a given material (such as an oxide) while it is exposed to an isotope enriched atmosphere (e.g. ¹⁸O) within a temperature-controlled environment. It allows for the estimation of the isotopic concentration as a function of time (and/or position), facilitating the measurement of ion transport kinetics, including oxygen surface exchange and diffusion coefficients, in functional materials. This innovative approach was first developed for gadolinium-doped ceria (CGO), a well know ionic conducting oxide typically used as an electrolyte in solid oxide cells.^[1] For thin films we demonstrated that employing various sample configurations allows for the in situ measurement of both in-plane and out-of-plane kinetic coefficients. In addition, the IERS technique has been further developed and expanded to the study of several mixed ionic electronic perovskites thin films, but also to bulk materials and composites. During my presentation, I will discuss the latest findings, as well as the advantages and limitations of the technique.

Solid oxide electrolysis and fuel cells (SOEC/SOFC) have drawn much attention due to their outstanding efficiency and superior fuel flexibility [1-3]. However, a significant challenge in scaling SOEC/SOFC technologies is the degradation of cells during operation. To address this, advanced operando characterization needs to be developed, which may offer insights into the mechanisms behind cell degradation.

Electrochemical impedance spectroscopy (EIS) emerges as a powerful technique for the nondestructive investigation of degradation in the solid oxide cell, or its individual components [4]. Integrating EIS with operando transmission electron microscopy (TEM) allow us to perform detailed structural and compositional analyses during EIS testing. In previous research, we successfully achieved analysis of pure gadolinia-doped ceria (CGO) material in relevant gas environments, at elevated temperatures, and under electrical polarization inside a TEM, and introduced the EIS-TEM methodology. This was achieved by combining an environmental TEM (ETEM) with a heating-biasing holder and a special sample preparation procedure [5, 6].

Based on previous works, this study employs the EIS-TEM methodology to investigate a model solid oxide cell composed of CGO electrodes and an yttria-stabilized zirconia (YSZ) electrolyte. The cell was fabricated using pulsed laser deposition (PLD) followed by focused ion beam scanning electron microscopy (FIB-SEM) processing. We determined the activation energies for YSZ ion transport and CGO surface reactions to be 0.9 eV and 0.5 eV, respectively, which are in close agreement with the literature. This study demonstrates the viability of conducting direct SOEC/SOFC cell tests within a TEM, including EIS analysis during operation, thereby providing profound insights into the reaction mechanisms of the cell.

Finding the optimal mixed conducting materials for use in Solid Oxide Cells (SOCs) or catalytic applications requires detailed knowledge of the electrochemical charge transport properties and oxygen exchange kinetics. These properties are governed by the defect chemistry of the material and cation segregation phenomena, which both are very closely linked to the oxygen stoichiometry under various temperature, atmosphere and ageing conditions.

Here, we present two new, fast and precise cell types for coulometric titration and quasi in-situ XPS measurements to investigate the bulk defect chemistry and link it to the surface chemistry of the mixed conducting perovskite Sr_1-xTi_0.6Fe_0.4O_3-δ with x ranging from 0 to 0.07. For powder samples, we use a miniature titration cell with dedicated pumping and sensing electrodes. Thin film samples deposited by Pulsed Laser Deposition (PLD) on YSZ were used in sealed oxide ion battery [1] type cells, on which oxide ions are directly exchanged by pumping through the electrolyte. The big advantage of coulometry over thermogravimetry is the much broader pO₂ range that is accessible electrochemically, compared to gas mixtures. By running multiple oxidation and reduction cycles within and beyond the thermodynamic stability limit, we can differentiate between reversible oxygen stoichiometry changes and irreversible metal exsolution reactions, with corresponding phase transitions. This enables us to investigate differences in oxygen stoichiometry, thermochemical stability and kinetics of metal exsolution between materials on a fundamental level. For the thin film samples, these coulometric techniques were combined with simultaneous Electrochemical Impedance Spectroscopy (EIS) and in-situ X-ray Spectroscopy (XPS), thus quantifying surface stoichiometry, and linking transition metal oxidation states on the surface with bulk oxygen vacancy concentration determined from coulometry.

In summary, we could show that coulometric techniques and in-situ XPS are valuable techniques to characterise the bulk defect chemistry and surface reducibility of various oxide materials.

Solid oxide electrolysis cells (SOECs) represent a promising avenue for sustainable energy storage, conversion, and the synthesis of fuels and chemical components. While state-of-the-art nickel and yttria-stabilised zirconia (Ni|YSZ) cermet cathodes exhibit high efficiency for CO₂ and H₂O electrolysis, they are plagued by issues such as coking and morphological degradation. In contrast, ceria-based cathodes, such as nickel and gadolinium-doped ceria (Ni|GDC) cermets, and particularly single-phase GDC cathodes, emerge as compelling alternatives as the mixed conductivity of GDC under reducing conditions leads to a high electrochemical activity. A major benefit of especially single phase GDC cathodes is their high coking resistance during CO₂ electrolysis. However, the notable drawback of ceria-based electrodes lies in their pronounced chemical expansion, causing mechanical stress, fractures, and potential cell failure. Furthermore, similar to Ni|YSZ, morphological transformations under intense electrochemical polarisation are anticipated for Ni|GDC due to solid-state (de)wetting phenomena, yet the intricate details of these transformations need further investigations.

In this study, our objectives are two-fold: (i) Investigate the (de)wetting behaviour of nickel on YSZ and GDC (ii) Unravel the intricacies of the chemical expansion observed in GDC under electrochemical polarisation.

For our first topic, dense Ni thin film electrodes are sputter deposited onto bare YSZ single crystals or YSZ substrates that have previously had a GDC film grown using pulsed laser deposition (PLD). Employing techniques such as electron microscopy, focused ion beam cutting, and electron backscatter diffraction, we examine the wetting behaviour under varying atmospheres and polarisations. The data obtained were matched with a computational phase field model and thus form the basis for understanding and predicting the morphological changes in SOEC cermet electrodes.

To fulfil our second goal, single phase, PLD grown GDC thin film electrodes are investigated by the means of in-situ X-ray diffraction (XRD) to study their chemical expansion behaviour under electrochemical polarisation. Electrochemical impedance spectroscopy is employed to simultaneously obtain the chemical capacitance of GDC electrodes, which provides us with the minority point defect concentration related to the expansion. Correlating in-situ XRD results with the material’s defect chemistry, establishes the groundwork for tailoring the electro-chemo-mechanics of SOEC cathodes, that will be done in forthcoming doping-variation experiments.

Improving battery safety is a top priority for consumers and battery manufactures. For that reason, a variety of approaches have been investigated to prevent thermal runaway, which is the uncontrollable accelerating heating of batteries that causes fires/explosions.  

We have recently pioneered the use of ceramic materials, introduced inside the batteries, to achieve a fast and early shutdown of the electrochemical reactions triggering the thermal runaway [1]. The ceramic material was a La and Nb co-doped BaTiO₃ whose resistivity drastically increased with temperature. As a result, when the battery heated under abuse conditions (overcharge and overheating testing), the ceramic material became insulating, thus effectively stopping the battery electrochemical reactions that release heat and induce further, escalating battery heating. Therefore, the introduction of this ceramic material can effectively complement other safety measures to produce batteries that remain safe even when they are operated outside their specifications.

Another approach to enhance battery safety is the implementation of solid electrolytes, which can be achieved with the help of a small amount of liquid electrolytes to facilitate materials’ contacts. Unfortunately, the interface between solid and liquid electrolytes associates a large resistance, which then compromises battery performance, but we have shown that the introduction of water in low concentrations as an additive effectively suppresses such interfacial resistance [2].  

The demand for extended-range electric vehicles has created a renaissance of interest in replacing the common metal-ion with higher energy-density metal-anode batteries, i.e. lithium, sodium and zinc. However, metal cells suffer from capacity fading and potential safety issues due to uneven metal electrodeposition.

"Anode-free" (AF) batteries comprise a metal-ion cathode and a current collector, the cathode consisting of the metal source. The metal is then plated on the current collector during charging. These batteries present a significant advantage due to their higher energy density, superior safety, and ease of production. However, realising metal and AF batteries requires a better understanding of alkali and multivalent metal plating, short-circuiting mechanisms, and metal battery degradation.

A considerable performance gap between lithium symmetric cells and practical lithium batteries motivated us to explore the correlation between the shape of voltage traces and degradation. The coupling of operando nuclear magnetic resonance (NMR) and galvanostatic electrochemical impedance spectroscopy (GEIS) allowed us to observe metal batteries' electrochemical and chemical dynamics and degradation in real-time without affecting the electrochemical reactions in the cell. [1-2]

"Soft shorts" are small localised electrical connections between two electrodes that allow the co-existence of direct electron transfer and interfacial reaction. Although soft shorts were identified as a potential safety issue for lithium-ion batteries in the early nineties, their detection and prevention were not widely studied. Using coupled EIS and operando NMR, we showed that transient (i.e., soft) shorts form in realistic conditions for battery cycling.

The typical rectangular-shaped voltage trace, widely considered ideal, was proven, under the conditions studied here, to be a result of soft shorts. Recoverable soft-shorted cells were demonstrated during a symmetric cell polarisation experiment, defining a new type of critical current density: the current density at which the soft shorts are not reversible. We showed that soft shorts are predictive towards the formation of hard shorts, demonstrating the potential use of EIS as a relatively low-cost and non-destructive method for early detection of catastrophic shorts and battery failure while demonstrating the strength of operando NMR as a research tool for metal plating in metal batteries. [1-2]

While today's lithium-ion battery chemistries are ubiquitous, powering mobile devices and increasingly electric vehicles, they contain critical minerals such as cobalt and minerals with supply concerns, mainly if these batteries are used at the scale needed to meet the 2050 climate goals. To address this challenge, "beyond-lithium" technologies are investigated. These battery chemistries allow earth-abundant, safer, and low-cost energy storage.

Sodium metal anodes have been studied broadly, assuming sodium and lithium metal anodes respond similarly to cycling. We established that lithium and sodium short circuit formation mechanisms fundamentally differ and strongly depend on electrolyte composition and SEI stability.

Zinc metal anodes have gained increasing interest due to the sustainability of the aqueous electrolytes; however, a greater insight into their unique plating, hydrogen evolution, and corrosion mechanisms is critical.

We demonstrated how SEI composition heterogeneity determines lithium plating morphology. [3] These findings inspired us to explore scanning electrochemical microscopy (SECM) methods for electrochemical mapping of the SEI. In our SECM study of zinc plating, we compared the effects of various electrolyte compositions and plating conditions on SEI heterogeneity and zinc metal morphology.

This new understanding of the metal plating mechanism is crucial to developing the next generation of rechargeable batteries with high energy density, prolonged cycling life and improved sustainability. A fundamental understanding and reliable testing methods for soft short circuits are critical for commercialising metal (e.g., metal-air, metal-sulphur) and AF batteries.

The use of inorganic ceramic solid electrolytes (SEs) to produce all-solid-state lithium batteries (ASSLBs) can overcome the safety issues related to conventional liquid electrolytes and enable Li batteries to operate at higher voltage. To make this happen, one of the most crucial requirements for the SE is to possess a large electrochemical stability window (ESW). Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte has attracted a lot of attention given its high ionic conductivity and because it was first reported with a very wide ESW (0–9 V vs. Li⁺/Li). It was not until recently that this value of the ESW was challenged, mainly proving that the oxidation of LLZO happened at a much earlier potential than 9 V. In the same line of thought, our work describes a methodology to accurately determine the ESW of doped M:LLZO (M = Al, Ta and Nb). The ESW of M:LLZO was found to be identical for all three dopants and much narrower than previously documented, on both reduction and oxidation sides. The reduction reaction was evidenced through ex situ ⁷Li nuclear magnetic resonance and operando powder X-ray diffraction analyses.

Batteries undergo both active cycling and prolonged idle storage throughout their lifespan. While degradation mechanisms induced by cycling and their mitigation techniques have been deeply studied, the specific effects of storage without cycling remain largely underexplored. Notably, battery performance also sees a unique decline over time, contingent on the state-of-charge (SoC) when the batteries are at rest. Capacity decline during SoC70 storage primarily arises from electrode slippage and Li inventory loss in a full cell. This is accompanied by a minor structural breakdown of Ni-rich layered oxide cathodes. Conversely, SoC100 storage leads to a more pronounced structural impairment of Ni-rich cathodes and pronounced side reactions. Intriguingly, these severe side reactions curb the Li inventory loss, electrode slippage, and the reduction of full-cell capacity during SoC100 storage. In addition to standard degradation processes, such as Li/Ni cation mixing, the formation of surface reconstruction layers, and the emergence of exhausted phases, cathodes stored at SoC100 displayed an unexpected contraction of interlayer spacing during post-storage cycling, highlighting the atypical effects of storage. Based on the mechanisms of capacity reduction highlighted in this study, we propose strategies to counteract the aging caused by storage. This research offers valuable perspectives for refining battery production and management to enhance their calendar lifespan.

High temperature fuel cells (SOFC) are efficient and clean energy generation systems. An important characteristic of these devices is that they can operate in reversible mode, acting as electrolyzers (SOEC) for the generation of hydrogen. The use of surplus electricity from renewable or continuous sources of electricity allows the production of so-called green hydrogen (and oxygen), making them a key element in reducing the carbon footprint of the global energy system. In this sense, these cells also allow, through the coelectrolysis of CO2 and water, the production of other fuels widely used industrially for the production, among others, of synthetic natural gas.
Achieving efficient and durable operation at low fabrication and operation costs is the main challenge for the implementation of large scale high temperature electrolysis. We explore here different strategies to reach this goal. On one hand, the optimization of air electrodes is crucial in order to allow high density currents and we deal with the addition of nanocatalysts on conventional LSM electrodes. On the other hand, we explore the potential of the operation under pressurized conditions in order to increase the efficiency through the decrease of the overvoltage contributions at cell level and ease the integration on the whole system. Microtubular geometries overcome the poor mechanical behaviour of flat membranes while facilitating easier sealing.

Key Words: Solid oxide fuel cells; Hydrogen production.

A solid oxide fuel cell (SOFC), an eco-friendly energy conversion device, has gained significant attention as a power generator due to its high fuel flexibility and efficiency and low emissions of pollutants. However, SOFC has inherently poor thermomechanical stability, ascribed to ceramic properties, such as high elastic modulus and low thermal conductivity. Therefore, it is prone to crack formation and failure of the cell in the thermal shock caused by rapid temperature changes, leading to slow start-up/shutdown and unstable operation under thermal cycling. To achieve exceptional thermomechanical strength, an innovative design of the solid oxide cell is indispensable, thereby overcoming the inherent vulnerability of ceramics to thermal shock.

This study examines the temperature and stress distribution in a solid oxide cell exposed to rapid heating conditions via computational simulation. Thermal stress analyses were employed to assess the thermomechanical robustness of anode-supported and electrolyte-supported SOFCs. Critical vulnerabilities within each SOFC configuration were identified, providing essential insights into their respective thermomechanical stabilities.

For the next generation energy conversion and storage systems, the development of fuel cells and electrolysis cells is an important endeavor to achieve a sustainable and clean alternative energy source in support of the global sustainable development goals. Thus, R&D activities in developing green and reliable energy storage and conversion technologies are at the heart of successful incorporation in the power mix and securing global future energy needs. One of the research efforts at our laboratory focuses on development of a reversible solid oxide electrochemical cells (SOEC), from materials considerations to device fabrication. In this presentation, new material electrode composite compositions and synthesis methods such as for LSM-ScYSZ, Ni-ScSZ, and highly conducting solid electrolyte material of Sc and Y co-doped ZrO2 have been investigated and its electrochemical performance in a solid oxide fuel and electrolysis cell applications will be reported. For Sc and Y co-doped ZrO₂, the XRD, Raman and PES analyses revealed the complementary structural characteristics of the material while SEM/EDS results showed the morphological and compositional analyses. The Sc and Y co-doped ZrO₂ solid electrolyte thin film also revealed a high total conductivity of about 2.8x10^-1 at 700 ^oC. In addition, I-V curves revealed the reversible performance of an electrolyte-supported fabricated single cell (5x5cm²) in both the fuel cell and electrolysis cell modes. 

Solid oxide electrolysis technology is gaining attention for its potential in clean synthetic fuel production. However, electrode degradation remains a significant issue, particularly in the microstructure change of the Ni-YSZ fuel electrode. This study involved the fabrication of Ni-YSZ cross-sectional model electrodes, with the morphological changes of the fuel electrodes being quantitatively evaluated by laser microscopic observations under operating condition. To evaluate the microstructure evolution of Ni/YSZ fuel electrodes, comb-shaped symmetric Ni pattern electrodes were designed on the top layer of the YSZ thin film. When operated in fuel cell mode, one side of the Ni pattern electrode spreaded, resulting in an increase in triple phase boundary (TPB) length and a decrease in electrolyte width. On the other hand, when operated in electrolysis mode, the Ni pattern electrode slightly migrated away from the electrolyte.  To prevent electrochemical oxidation, platinum electrode was used as a counter electrode. Then, quantitative analysis of the Ni microstructure change in electrolysis operation became possible.

Proton and oxide-ion conductors are attractive materials having a wide range of potential applications such as PCFC, PCEC, SOFC, and SOEC. The conventional strategy to improve the proton and oxide-ion conductivity is acceptor doping into oxides without oxygen vacancies. However, the acceptor doping results in proton and vacancy/oxide-ion trapping near dopants, leading to the high apparent activation energy and low proton conductivity at intermediate and low temperatures. Intrinsic oxygen vacancies are the oxygen vacancies in a parent material. In this keynote, I present the high proton and oxide-ion conduction via the intrinsic oxygen vacancies.

The hypothetical cubic perovskite BaScO_2.5 may have intrinsic oxygen vacancies without the acceptor doping. Herein, I report that the cubic perovskite-type BaSc_0.8Mo_0.2O_2.8 stabilized by Mo donor-doing into BaScO_2.5 exhibits high proton conductivity within the ‘Norby gap’ (e.g., 0.01 S cm⁻¹ at 320 °C) and extremely high chemical stability under oxidizing, reducing and CO₂ atmospheres [1]. The high proton conductivity of BaSc_0.8Mo_0.2O_2.8 at intermediate and low temperatures is attributable to high proton concentration, high proton mobility due to reduced proton trapping, and three-dimensional proton diffusion in the cubic perovskite stabilized by the Mo-doping into BaScO_2.5. The donor doping into the perovskite with disordered intrinsic oxygen vacancies would be a viable strategy towards high proton conductivity at intermediate and low temperatures. I also report high proton and oxide-ion conduction in hexagonal perovskite-related oxides with intrinsically deficient oxygen layers [2-7].

Many Bi-containing compounds exhibit high oxide-ion conductivity via conventional vacancy mechanism. However, interstitial oxide-ion conduction is rare in Bi-containing materials. Herein, I also report high oxide-ion conductivity through interstitial oxygen (intrinsic oxygen vacancy) sites in Sillén oxychlorides, LaBi_2-xTe_xO_4+x/2Cl [8]. Oxide-ion conductivity of LaBi_1.9Te_0.1O_4.05Cl is 20 mS cm⁻¹ at 702 °C, and higher than best oxide-ion conductors as Bi₂V_0.9Cu_0.1O_5.35 below 201 °C. Despite of the presence of Bi and Te species, LaBi_1.9Te_0.1O_4.05Cl shows extremely high chemical and electrical stability at 400 °C from oxygen partial pressure 10⁻²⁵ to 0.2 atm and high chemical stability under CO₂ flow, wet 5% H₂ in N₂ flow, and air with natural humidity. Neutron scattering length density analysis, DFT calculations, and ab initio molecular dynamics simulations indicate that the extremely high oxide-ion conduction is attributed to cooperative diffusion through interstitial oxygen sites (interstitialcy diffusion mechanism) in triple fluorite layers. The present findings demonstrate the ability of LaBi_2-xTe_xO_4+x/2Cl as superior oxide-ion conductors, which could open new horizons for oxide ion conductors.

Complex oxides are renowned for their diverse functionalities that are essential to the advancement of electronics, energy, and information technologies. These functionalities include for example, ferroelectricity, piezoelectricity, and pyroelectricity. The extraordinary physical properties of these materials originate from the complex interactions among lattice, orbital, charge, and spin dynamics. In this talk, I will present our most recent research, focusing on how we can manipulate the properties of oxide heterostructures by intentionally altering their symmetry. A significant part of my presentation will be dedicated to exploring the advancements in freestanding oxide membranes. These have emerged as a groundbreaking platform for experimentalists to design materials with novel properties. Mainly made up of transition metal oxides, these membranes are produced as ultrathin, quasi-2D layers and can be reassembled into multilayered structures, allowing for precision in controlling the twist angles between layers. I will also introduce a pioneering method for creating high-quality complex heterostructures integrated with Si, using SrTiO3 membranes as a universal platform. This aspect of our study paves a new way for embedding electronic devices with multifunctional physical properties into silicon-based technology, promising significant advancements in the field.

Electrochemical control has emerged over the last decade as a powerful approach for developing reprogrammable microelectronic devices. Such oxide based devices are characterized by remarkably large changes in chemical composition at low energetic cost. Both from a fundamental science and applications-oriented perspective, the ability to control the functional properties of metal oxides dynamically through their chemical composition opens a broad perspective in developing new classes of microelectronic devices. While such concepts have been demonstrated, important questions remain about how to optimize their device performance best. Information regarding ionic transport within the oxides remains elusive given the challenge of isolating it from the dominant electronic conduction. Moreover, many current device concepts not only rely on the transport of ions within a single oxide layer but across interfaces with additional layers as well. Adequate methods for studying the ionic properties in these ultra-thin films and isolating their transport kinetics are lacking. Their development will be vital for mastering overall device performance, such as speed, retention, and predictability.

To address these challenges, we have been investigating a model system capable of reversible ion transport and exchange under an applied field between two adjacent thin film solid oxide layers. Our investigations have focused on studying a Pr_xCe_1-xO₂/La_2-xCe_xCuO₄ bilayer stack that utilizes model materials for which the defect chemical and transport properties have previously been carefully characterized, that offer high ionic mobilities and can accommodate large levels of non-stoichiometry [1,2]. Building on our previous observations that we could reversibly modulate the resistance of the stack by over an order of magnitude through the transfer of ionic species, we discuss how our conductivity results can be interpreted via defect chemical models and show how such transfer can be tracked in situ inside a TEM by monitoring local valence and lattice dimension changes. Furthermore, we demonstrate that we can isolate the ionic transport kinetics, both within and across the solid oxide interfaces, by a combination of DC bias impedance measurement and dynamic current-voltage studies. This allows us, for example, to systematically study how the ionic mobility varies in Pr_xCe_1-xO₂ as a function of stoichiometry, as controlled by the degree of ionic titration. The findings in this work can be expected to aid in developing material selection and design criteria for similar bilayer systems and be used to achieve faster resistance switching speeds, larger resistance switching ranges, and longer device retentions.

[1] Tuller, H. L., Bishop, S. R., Chen, D., Kuru, Y., Kim, J.J. & Stefanik, T. S. Praseodymium Doped Ceria: Model Mixed Ionic Electronic Conductor with Coupled Electrical, Optical, Mechanical and Chemical Properties. Solid State Ion 225, 194–197 (2012).

[2] Kim, C. S. & Tuller, H. L. Fine-Tuning of Oxygen Vacancy and Interstitial Concentrations in La_1.85Ce_0.15CuO_4+Δ by Electrical Bias. Solid State Ion 320, 233–238 (2018).

Mixed ionic and electronic conductors (MIECs) are key components for a vast number of electrochemical devices, e.g. as intercalation electrodes, sensors or permeation membranes.  The functionality of the components rely on the ability of the material either to store or to transport the neutral species, i.e. the electrons as well as the ions. However, most materials with a high ionic conductivity exhibits a poor electronic one and vice versa. A common approach of dealing with this challenge is the preparation of artificial mixed conductors by mechanical mixing of a good electronic conductor with an ion conductor to prepare transport pathways for both electrons and ions. Consequently, these artificial MIECs are characterized by a high number of interfaces, which may significantly affect charge storage and charge transport in the composites. A detailed understanding of the impact of the nanostructure and, in particular, electron-conducting/ion-conductor interface on charge transport and charge storage properties of the composites is essential for the design and the improvement of electrochemical devices with improved functionality. However, the number of systematic studies to address this question is still scarce. Motivated by recent results regarding the mixed-conducting properties of mesoporous CeO2/YSZ nanocomposites [1], we present the preparation and characterization of multilayer heterostructures formed by ceria (CeO2) and yttria-stabilized zirconia (YSZ). The multilayers were prepared by pulsed laser deposition to achieve thin film structures with well-defined interfaces. To identify the impact of the interfaces on charge transport, samples with constant thickness but varying number of interfaces were deposited. Structural characterization was performed using Raman microscopy, X-Ray diffraction analysis, scanning electron microscopy and time-of-flight secondary ion mass spectrometry. The electrochemical properties were characterized using electrochemical impedance spectroscopy at different temperatures and under varying atmospheric conditions.

In the wake of global efforts to meet climate change goals, solid-state ionic conductors are receiving increasing interest as promising materials for electrochemical applications. One crucial feature of solid-state ionic conductors is high ionic conductivity. Enhancing ionic conductivity would allow a lower operative temperature while guaranteeing higher currents and lower overpotentials, making the material a good candidate for superior fuel cells or battery applications. Ionic conductivity follows an Arrhenius-type law, suggesting an increase in conductivity with decreasing activation energy. However, decreasing activation energy is also typically correlated with orders of magnitude decrease in the pre-exponential factor, which is known as compensation law [1]. Previous designs of ion conductors have struggled to overcome such compensation law due to a lack of fundamental understanding and descriptors (tunable design variables). Therefore, a deeper understanding of the processes governing ionic conductivity and fundamental descriptors to overcome compensation law could enable superior designs of solid-state ionic conductors.

In this presentation, we discuss designs of solid-state ion conductors with low activation energies and high pre-exponential factors. Using density functional theory simulations on perovskite oxygen ion conductors, we highlight that the entropy of migration regulates changes in the pre-exponential factor across many orders of magnitude. We uncover that such entropy of migration is the result of changes in the vibrational structure of the local environment of oxygen atoms occurring from equilibrium to the transition state during ion hops. Based on this understanding, we highlight strategies and descriptors to enable high pre-exponential factors by enabling the softening of the local environment of mobile ions during migration. On the other hand, we recently showed that the local electronic structure of lattice point defects regulates the migration barrier, which can be decreased by increasing the charge screening capability of the local host lattice [2]. Therefore, independently tuning the vibrational structure of the local environment of mobile ions and the electronic structure of lattice point defects can open a new design space to overcome compensation law. This framework is tested on perovskites and other crystal structures and mobile species. Our new findings and proposed descriptors hold great potential for accelerating the discovery of superior solid-state ion conductors leveraging emerging characterization techniques or growing approaches of computation-aided designs.

The success of rechargeable lithium-ion batteries (LIBs) has brought evident convenience to human society, but state-of-the-art LIBs with a graphite anode are approaching their energy density limits. Li metal is considered the ultimate anode material due to its ultra-high theoretical specific capacity of 3860 mAh g^-1, which is more than 10 times higher than lithiated graphite. Nonetheless, Li metal anode suffers from poor safety and low cycling efficiency due to its high reactivity. Electrolytes that work with Li anode should possess excellent stability against Li metal or form a highly passivating interface. It is also critical to control the amount of Li in the cell, preferably having no excess Li at the anode side.  In this presentation, next-generation electrolytes that enable such types of high-performance Li metal batteries will be discussed, including advanced garnet-type ceramic electrolytes and hybrid electrolytes. Also, recent progress in the development of solid state sodium ion electrolytes based on silicate-structure and solid state polymer electrolytes for solid state Na batteries will be presented.   

In order to realize the promise of superior energy density, the solid electrolytes in solid-state batteries have to synergize fast ionic conductivity with a low contribution to the overall cell mass. While the dead mass of the “separator” solid electrolyte layer can be minimized by reducing its thickness to the extent limited by operational safety and processability, the catholyte within the inevitably thick composite cathodes will constitute 80-90% of the total electrolyte volume in high energy density cells. Thus, in the absence of fast-ion conducting high energy density cathode materials, low density catholytes are of key importance to maximize the energy density of all-solid-state batteries. Additional constraints for fast-ion conductors include electrochemical and electromechanical compatibility as well as interface wettability with the electrode materials. Soft, compressible electrolytes not only facilitate densification, they also increase both the effective interfacial contact area and prolong cycle life by retaining close interfacial contact with breathing electrodes.

Here we discuss two design strategies to realize such compressible low-density fast-ion conductors. The first strategy involves ultrathin ceramic-in-polymer Composite Solid Electrolytes (CSEs). We developed CSEs combining Li-doped polyacrylonitrile with various ceramic electrolytes (Li_1.5Al_0.5Ge_1.5(PO₄)₃, LiTa₂PO₈ etc.) with and without plasticizers. Optimized composites reach Li⁺ conductivities of 0.1 – 1 mS·cm^-1 at densities < 2 g cm^-3 and retain wide electrochemical windows. Temperature-dependent analysis of Li⁺ paths shows that the ceramic filler induces a percolating network of fast ion-conductive interphases below the glass transition of the bulk polymer. Interfacial resistance is drastically reduced by spin coating and in situ UV-curing the precursor slurry on Li anode and composite cathode, respectively. Symmetric cells cycled with up to 2 mA·cm^-2 over 500h retain a stable overpotential and no dendrite growth is observed. LFP/CSE/Li cells achieve a maximum specific discharge capacity of 148 mAh·g^-1 and retain 89% of that after 200 cycles. Stable room temperature cycling is demonstrated for cells with a cathode mass loading of 6 mg·cm^-2. Strong binding and three-dimensional architecture of the in situ-formed interfaces is crucial for the favourable cycling stability.[1]

An alternative all-ceramic design strategy is to exploit the correlation between fast ionic conductivity, density and glass forming ability in glass-ceramic oxides[2] and oxyhalides[3]. The higher number of monovalent light halides required to balance the charges of typically heavier high valent glass-former cations reduces the overall density and turns the material less brittle. Compared to close-packed halides, oxyhalides typically have lower coordination numbers, creating free volume for ionic motion. In view of their favorable oxidation stability yet limited reduction stability, oxyhalides are in particular suitable to function as catholytes. The recent report[3] of fast room temperature ionic conductivity exceeding 10 mS cm^-3 in LiMOCl₄ (M = Nb, Ta) exemplifies the strong potential of the so far hardly explored class of crystalline oxyhalides. Our redetermination of LiNbOCl₄ (density 2.7 g cm^-3) finds a simpler and more plausible, Li-disordered, tetragonal crystal structure with smaller unit cell than the originally reported orthorhombic phase. The revised structure model [4] is consistent with DFT simulations. Molecular dynamics simulations utilizing our bond-valence related embedded-atom method type forcefield softBV-EAM reveal the ion transport mechanism, the origin of the temperature dependent activation energy and its relation to the rotational mobility of (NbOCl₄^–)_n polyanions therein. The simulations thus indicate that LiNbOCl₄ is in a plastic-crystalline state at room temperature, which explains its favorably high compressibility and formability of LiNbOCl₄.

Recently, halide materials have emerged as potential inorganic electrolytes for all-solid-state lithium batteries (ASSLB) thanks to their moderate-high ionic conductivity at room temperature and their high-voltage wide electrochemical stability compared to other solid-state electrolytes such as polymers or sulfides, allowing halides to be electrochemically compatible with 4V cathode materials [1]. On another hand, their high ductility and the low-moderate temperatures needed for their synthesis make halides ideal candidates for the upscaling of material preparation, stacking and densification steps on cell manufacturing. Nevertheless, halides must overcome some challenges that might prevent their use such as the stability with the lithium (Li) metallic anode. In contact with Li, the halide undergoes a reduction reaction, leading to the formation of a mixed ionic-electronic SEI [2] until either the lithium electrode or the halide is consumed. As such, understanding the ionic transport and the Li/solid electrolyte interface dynamics is essential to optimize halide-based ASSLB.

In the present work, we report the advantages and the potential of Li₃YCl₄Br₂ (LYCB) halide as solid-state electrolyte. First, LYCB presents a high ionic conductivity at room temperature, greater than 1 mS cm^-1, which can be further improved after a densification step at moderate temperature. The cycling of full cells with a high-voltage NMC622 cathode, LYCB and metallic lithium anode leads to a discharge capacity up to 150 mAh g^-1. Using computational methods, impedance spectroscopy measurements and physico-chemical characterizations, the dynamics at the Li/LYCB interface are evaluated. An increase of the interfacial resistance with time witnesses the formation of a resistive interface between the halide and the Li electrodes. With a good agreement, both simulations and experimentations demonstrate the reduction of LYCB by the Li. However, the tests performed suggest that the evolution of the Li/LYCB interface due to the reactivity between the two components leads to the formation of a SEI which is beneficial to the cycling performances with Li electrodes, enabling the stripping and plating of a symmetric cell during 1000 h applying a current density of 0.1 mA cm^-2 with a capacity of 0.5 mAh cm^-2. The cells show an overpotential as low as 40 mV, one order of magnitude lower compared to other halides in contact with Li electrodes [3,4]. Post-mortem analysis evidence the formation, during cycling, of a multi-layer interface, made of the resulting products issued from the reduction of LYCB, that might protect LYCB from further degradation by the Li electrodes. Overall, this study highlights LYCB as a promising candidate for use as electrolyte Li-metal solid-state batteries.

All solid-state batteries are a promising technology due to their employment of inorganic solid electrolytes (SEs), which are safer compared to their flammable, liquid counterparts. Recently, halide SEs have gained significant attention due to their relatively high ionic conductivity (> 1 mS/cm), wide electrochemical stability window, and compatibility with various electrode materials. However, prior to 2023, no known halide materials could reach the conductivities of the state-of-the-art sulfide SEs (> 10 mS/cm). This can be attributed to the close-packed anion arrangement of existing halide conductors, which confines lithium ions to narrow and high-energy conduction pathways. Recently, a new Li oxyhalide superionic conductor LiMOCl₄ (M=Nb/Ta) was reported with extremely high ionic conductivities greater than 10 mS/cm [1]. Unlike previously found halide conductors, LiMOCl₄ has a non-close-packed anion framework that provides a large conduction pathway. We present a first-principles investigation of the phase stability, electrochemical stability, and Li-ion conductivity in the LiMXCl₄ (M = Ta, Nb, Sb, Ti, Zr, Hf, Sn, and X = O, F, Cl) family of superionic conductors. We identify a mixed-anion framework as key to stabilizing the oxyhalide structure and demonstrate the impact of aliovalent anion and cation substitutions on material properties. Long ab-initio molecular dynamics simulations spanning 10’s of ns confirm room temperature superionic conductivity in all systems. Probabilistic analysis of the rotational dynamics at the picosecond timescale contributes direct, quantitative evidence correlating the high Li-ion conductivity in oxyhalide systems with the soft tilting motion of MO₂Cl₄ octahedral groups present in the material. Our findings supply the theoretical underpinnings for the effectiveness of LiMXCl₄ family of superionic conductors as catholytes compatible with 4-V-class layered cathodes and provide essential design criteria for constructing new Li halide superionic conductors with conductivity greater than 10 mS/cm.

Among the different solid electrolytes, lithium halides show promising ionic conductivity and cathode compatibility. The complex composition-properties relationships makes rational design very challenging. In the first part of this work use the ionic potential, the ratio of charge number and ion radius, can effectively capture the key interactions within halide materials, making it possible to guide the design of especially complex halide compositions. This is demonstrated by the preparation of a family of complex layered halides that combine an enhanced conductivity with a favorable isometric morphology. In the second part we evaluate the the role of the Cl vs Br anion in the Li3YBrxCl6-x halide and investigate the trade-off between ionic conductivity and electrochemical stability. One of the main limitations of this material, and in general halide solid electrolytes, is the electrochemical instability at low working potentials, limiting its application. In this context we study the electrochemical and structural transformation and their (ir)reversibility, providing more insights into the stability limits of this material.

Despite their relatively simple stoichiometries, compounds in the barium–group(V) oxide series Ba₄M₂O₉ (M = V, Nb, Ta) adopt three distinct structure types depending on composition and synthetic conditions: a completely unique and complex γ phase; [1,2] a uniquely distorted 6H-perovskite; [3] and a partially disordered α phase related to Sr₄Ru₂O₉-type. [4] They all hydrate to very high degrees of up to 1/3 (γ) or 1/2 (6H and α) H₂O per formula unit and show appreciable mixed oxide/proton conductivity. Intriguingly, none of these structure types contain oxide vacancies that would explain their hydration and conductivity in terms of a conventional hydroxylation mechanism, nor layers for intercalation. We have used diffraction, electron microscopy, physical property measurements, NMR, inelastic neutron scattering and DFT calculations to show how water is incorporated into the crystal lattices in ways that depend upon and guide their temperature-dependent structural distortions. The potential significance of these results extends beyond their ionic conductivity to the Earth sciences, because even at ppm levels, the pressure- and temperature-dependencies of lattice hydration processes mean that they affect the total amount of water available on Earth and the rheological properties of the lithosphere (plate tectonics, earthquakes etc.). 

Ionic conducting materials are the most important parts in electrochemical devices such as batteries, fuel cells and electrolysers. Different types of ionic conducting materials, which conduct O^2-, H⁺ or OH^- ions, have been widely used in solid oxide fuel cells (SOFCs), solid oxide electrolytic cells (SOECs), proton exchange membrane fuel cells (PEMFCs) and electrolysers (PEM electrolysers), anion exchange membrane fuel cells (AEMFCs) and electrolysers (AEM electrolysers). The typical materials are solid oxides such as doped ZrO₂ or CeO₂ with fluorite structures or doped BaCeO₃ (BCO)/BaZrO₃ (BZO)/LaGaO₃ (LGO) with perovskite structures^1-10. The representative high temperature proton conductors based on doped perovskite oxides BCO/BZO were discovered in 1980 and 1990s respectively^{2, 6}, while the high temperature O^2- ion conductor based on doped LGO was discovered in 1994^{3, 11}. Those materials have been applied in SOFCs and SOECs. The typical operating temperature is above 550 °C while the durability of the solid oxide cells (SOCs) are not ideal due to the high operating temperature. When temperature drops, the O^2- or H⁺ ionic conductivity in oxides is not high enough to provide high power density for the SOCs. Therefore it is desired to develop low temperature O^2-, H⁺ or OH^- ionic conducting materials.

Polymer membranes such as acidic proton-conducting Nafion membrane have been used in PEMFCs and PEM electrolysers but the acidic membrane are very expensive. Expensive noble electrode materials such as Pt, IrO₂ have to be used in PEMFCs or PEM electrolyers leading to high cost, which is the key barrier for large scale applications. Also European proposes to ban all fluorine-based membrane such as Nafion in 2026-27. It is desired to find a replacement membrane for Nafion. The alkaline OH^- ionic conducting membranes based quaternary ammonium groups have been investigated for over a decade but their stability is not ideal, also reacting with CO₂ in air leading to reduced OH^- ionic conductivity. It is urgent to discover new stable OH^- ionic conducting materials for AEMFCs and AEM electrolysers.

Fortunately, in 2020, Tao's group at University of Warwick found that some ceramic materials exhibit high mixed OH^-/H⁺ ionic conduction in water, with ionic conductivity exceeds 0.01 S cm^-1 and ion transfer number above 98% at a temperature below 100 °C. Under certain conditions, the ionic conductivity exceeds 0.1 S cm^-1. Some of ceramic OH^- and H⁺ ionic conductors are very stable, ideal electrolyte materials for low temperature electrochemical devices such as fuel cells, electrolysers and batteries.

For the first time, ceramic low temperature OH^- and H⁺ ionic conducting materials have been discovered. The application in direct ammonia fuel cells has been demonstrated as well.

Structural features of materials, such as lattice distortions, water uptake, and oxide vacancies, are key factors that define protonic conduction. It is also worth noting that fast oxygen diffusion is advantageous for hydration kinetics since proton incorporation requires the migration of oxygen vacancies.

Here, a combination of ¹⁸O tracer isotopic exchange and Secondary Ion Mass Spectrometry (SIMS) measurements was applied to obtain oxygen surface exchange and diffusion coefficients of BaCe_0.8Y_0.1Yb_0.1O_3-δ (BCYYb), BaSn_0.8Y_0.1Yb_0.1O_3-δ (BSYYb), and BaZr_0.8Y_0.1Yb_0.1O_3-δ (BZYYb). The isotopic exchange was conducted in dry (¹⁸O₂) and wet (H₂¹⁸O) atmospheres at various temperatures. Lower oxygen diffusion coefficients were observed for the wet exchange, indicating that the humidified and dry atmospheres affect oxygen transport differently. In some cases, variations in the surface exchange and diffusion coefficients were observed at the phase transition temperature, suggesting a different rate-limiting step in the oxygen exchange and diffusion processes. Oxygen diffusivity was correlated to the conductivity measured by impedance spectroscopy (EIS), and the equilibrium constants associated with water incorporation were derived from thermogravimetric analysis (TGA) in the temperature range 100-900 °C under a water partial pressure of 0.023 atm. The constants, calculated under the assumption of a negligible hole concentration, were linear in the Arrhenius plot. In contrast, high-temperature X-ray diffraction (XRD) revealed complex structural transitions. The structural changes linked to the water uptake were confirmed by Raman spectroscopy carried out at variable temperatures under dry and wet atmospheres. Results were in good agreement with data reported by Grimaud et al [1]. for BaCe_0.9Y_0.1O_3-d, although the detection of OH bands using Raman spectroscopy remains an open discussion.

Mixed anion oxides, such as oxyhydrides and oxyhalides, showing exotic transport properties, are of interest in the research field of solid state ionics^[1,2]. The anion-doping effects have also attracted much attention for perovskite-type proton conductors^[3,4]. Meanwhile, unlike conventional cation-doping techniques, it is challenging for anion doping to precisely control and quantify the doping concentration, which is essential for interpreting their defect equilibrium. In this study, anion-doped Ba-Zr-based proton conductors were prepared by a topochemical reaction at high temperatures, and their defect equilibrium was clarified based on exact anion content analyzed by solid-state NMR spectroscopy and DFT calculations. The impacts of anion doping on the transport properties were also investigated.
    Sc-doped, Co-doped, and (Sc, Co)-co-doped BaZrO₃ were prepared as a single-phase host oxide by a solid-state reaction. BaF₂, an F source, was mixed with the barium zirconate powders to have a molar ratio of 1:1. The topochemical reaction was carried out at 1200 °C for 10 h. The excess BaF₂ that is unreacted can be removed by hot water after the reaction. ¹⁹F MAS NMR was conducted to identify the occupation site and amount of F ions introduced into the perovskite-type phase combined with shielding coefficients derived from DFT calculations.
    ¹⁹F MAS NMR spectra of undoped, Sc-, Co- and (Sc, Co)-doped BaZrO₃ were taken after the topochemical reaction at 1200 °C. In addition to a BaF₂ peak (-14.7 ppm), a new peak was observed at around -90 ppm for Sc-containing BaZrO₃. The peak intensity increases with increasing Sc content. The DFT-calculated chemical shift of 2-coordinated F (Zr-F-Zr) in BaZrO₃ was also found to be around -90 ppm, depending on local structures. Meanwhile, undoped BaZrO₃ and Co-doped BaZrO₃ show no extra peak other than BaF₂. This NMR analysis implies that F can fully occupy O-sites of BaZrO₃ to compensate for the effective charge of Sc, i.e., [Sc_Zr’] = [F_o^•], if enough F source is available at high temperatures. The F content in 5, 10, and 20%Sc-doped BaZrO₃ was determined as 4.9, 10.2, and 17.1 mol%, respectively. Even though proton concentration in the F-doped samples was reduced due to the presence of F_o^•, its impact on proton and hole conductivities strongly depended on the F content and the presence of Co. In addition to these F-doped samples, the defect equilibrium in hydride-ion(H^-)-doped Ba-Zr-based oxides prepared by the topochemical reaction will also be discussed.

Interfacial reactions at the solid electrolyte interphase control the performance of batteries and electrochemical N2 and CO2 reduction. In batteries, degradation is typically accompanied by parasitic gas evolution reaction. In this contribution I will explore the commonalities between battery science and electrocatalysis and demonstrate the insight to be gained by studying different electrochemical reactions in parallel.

We recently demonstrated that on chip electrochemistry mass spectrometry - previously developed for aqueous electrocatalysis1 - can detect and quantify gases such as O2, CO2, C2H4 and H2 with ultra high sensitivity and time resolution during lithium ion battery operation.2 It allows us to probe degradation processes, which were previously unobservable. Augmented by 18O2 isotopic labelling, we reveal that lattice oxygen, not singlet oxygen is the more probable reactive species during the degradation of lithium nickel manganese cobalt oxide electrodes. We also show that CO2 reduction to ethylene takes place at graphite anodes.

Electrochemical nitrogen reduction to ammonia is a potentially sustainable and scalable alternative to the Haber Bosch process. Thus far, amongst solid electrodes, only lithium and calcium based electrodes in organic electrolytes can unequivocally reduce nitrogen to ammonia.3-5 Even so, at present, the lithium based system is far too inefficient and unstable for many practical uses. We hypothesise that the reactivity of lithium and calcium is due to (a) their ability to bind and dissociate N2 and (b) their ability to form solid electrolyte interphases that can moderate access to Li+ and H+ but facilitate access to N2.6 Herein, I will peruse upon these hypotheses, using a combination of electrochemical experiments, cryo-microscopy, infrared spectroscopy, electrochemistry mass spectrometry, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy and density functional theory On the basis of our insight, we propose new avenues towards going beyond lithium and calclium in electrochemical nitrogen fixation.

In Solid Oxide Fuel Cells, oxygen electrode polarization related to electrochemical reactions at the gas/solid interface is often the dominant flux limiting mechanism. The most common materials used as oxygen electrodes are metal oxides with mixed ionic and electronic conductivity. Their performance as an oxygen electrode is a function of the kinetics of exchange between lattice oxygen defects and molecular oxygen in the gas phase. As such, there has been tremendous efforts to characterize the rate of this oxygen exchange on mixed conducting oxides, with a wide variety of techniques.

One technique uses electrical conductivity as tool to track the stoichiometry changes within the oxide. With stepwise changes of oxygen pressure in the atmosphere, the stoichiometry goes through a transient state and stabilize to a value that equilibrate the oxygen chemical potential with that of the gas phase. This technique, called electrical conductivity relaxation, is usually performed on highly dense ceramics, and carries information of oxygen surface exchange kinetics as well as bulk ionic diffusion of oxygen defects. The latter can often be predominant in the total conductivity transient, which make the determination of surface exchange kinetics rates less accurate.

In a recent work¹, we have introduced a variation of the electrical conductivity relaxation technique, which uses porous ceramics to improve the sensitivity to surface exchange processes. In this presentation, we will present this technique in-depth, and discuss the relevant parameters to consider for accurate measurements of surface exchange coefficients. The interpretation of relaxation profiles measured on porous ceramics requires an accurate description of the microstructure. The procedure proposed in this work features a simple determination of microstructural parameters from 2D images recorded by scanning electron microscopy. Then, those parameters are directly accounted for in the fitting procedure of the relaxation profiles with an adapted analytical model. The accuracy of the procedure is demonstrated on a wide range of mixed conducting oxides and microstructures.

Achieving facile, non-destructive chemical analysis techniques that can provide microscopic insights into the phase evolution of materials is essential for creating high-performance devices for energy harvesting and storage, among other applications. However, many of the most powerful techniques, such as isotopic ion exchange methods, in situ TEM, and synchrotron radiation-based techniques, are complex and limit easy access to crucial data.

Raman spectroscopy is a fast, non-destructive optical technique that provides quantitative chemical and structural information about the material under investigation. However, conventional Raman microscopy is diffraction-limited and unable to provide spatial resolution below ~ 1 µm. To overcome this limitation, we employ Tip-Enhanced Raman Spectroscopy (TERS), which combines the chemical sensitivity of Raman spectroscopy with high spatial resolution of scanning probe microscopy (SPM) and enables chemical imaging of surfaces at the nanometer length scale.

In this talk, we will discuss the fundamentals of TERS and its recent advancements in the implementation of the technique for electroceramic materials. We will showcase our latest results on Li-ion battery research using TERS, both ex-situ and operando configurations. The technique's spatial resolution capabilities will be highlighted, showing, for instance, the phase evolution at grain boundaries in battery cathodes with a resolution < 20 nm when cycled in an aqueous electrolyte.[1] We will also show our recent advancements in implementing operando TERS in battery cathodes with outstanding spatial resolution supported by experimental modeling emphasizing again the differences between grain and grain boundaries. Our efforts aim to demonstrate the versatility of TERS as a new approach for nanometric chemical insights of electroceramics in the energy field.

Since 2014, we have suggested a superior description of the impedance of the polycrystalline solid electrolytes [1-4] compared to the brick-layer model heavily stained by constant-phase-elements (CPEs) in most applications. Time- and painstaking parameter determination from the individual fitting analysis hindered further progress. The recent dissemination of Python-based data science tools and AI assistance like ChatGPT allowed the layman to code the algorithms that fit up to hundreds of spectra or thousands of impedance data labeled with temperature values. We applied the techniques to various Li7La3Zr2O12 (LLZ) samples. In essence, the AC response of the polycrystalline electrolytes can be described by the additive electric polarization relaxations, which can be represented by Havriliak-Negami (HN) capacitance functions or generalized Debye models, originating from the mobile charge carriers by the current constriction effects at the grain boundaries and also in bulk. The capacitance magnitude is inversely proportional to the temperature, and the time constants are activated similarly to the ionic conductivity. All the data are thus collapsed to the single trace of the complex capacitance. The overall sample resistance of the grain and grain boundaries is connected in parallel. Temperature-independent geometric capacitance with nearly constant loss (NCL) can be identified only at cryostatic temperature. Low-frequency blocking effects by the inert metal electrodes can be best described by the transmission line model (TLM) with the longitudinal resistance and the Warburg-interfacial impedance parameter activated by higher activation energy alike than the bulk one. While the interfacial and transverse shunt capacitance is inversely proportional to the temperature, as for the polarizations in the sample, the presence of the temperature-independent double-layer capacitance is also noted, which can be put at the other terminal of the TLM for the electrode response. The exponents of the Havriliak-Negami capacitance functions, similar to the power exponents of CPEs, can be and need to be fixed, unlike the conventional CPE modeling, the physics of which is related to the universal dielectric relaxation phenomena or Kohlraush-Williams-Watt stretched exponential functions. It has been found the low-frequency and high-frequency limiting exponents of one or half can be widely applied [5], which corresponds to the ideal relaxation and the diffusion-related relaxations, respectively. Havriliak-Negami capacitance functions (DE31) and the transmission line models with the generalized Havariliak-Negami capacitance elements (DX26) have been implemented in ZView®  for the convenient preliminary analysis of the individual spectra. The models are directly indicated in the raw data presented in AC conductivity Arrhenius plots and capacitance and admittance Bode plots, and hence, are physics-based. The models can be successfully applied to the extremely nontrivial individual impedance of the composite solid electrolytes with the percolating conducting components for the battery electrode applications. Surface or near-surface conduction often leads to flattened activation at lower temperatures. The short-circuit paths as parallel resistance hardly affect the capacitance relaxations in the present modeling, so the original AC response and the surface conduction can be determined and separated.  

Ordered mesoporous oxides are characterized by a regular pore structure surrounded by single interconnected nanocrystallites. This unique architecture makes this class of material of high interest for a variety of electrochemical applications, e.g., for gas sensing, energy storage or catalysis. The open pore-solid framework provides good accessibility of the internal surfaces to the surrounding medium, as gases or liquids can completely penetrate into the mesopores, resulting in superior device performance compared to nanoparticles or disordered mesoporous counterparts. In addition, the electrochemical properties of ordered mesoporous oxides can be tailored, at least to some extent, by adjusting the pore size or by surface functionalization. However, a detailed knowledge about the impact of the mesoporous architecture on the electrical properties is necessary for further device optimization making a reliable electrochemical characterization of the materials indispensable. Electrochemical impedance spectroscopy is the method of choice to determine the electrical properties. However, the interconnected pore network hinders a simple estimation of the material-specific conductivity from the measured impedance.

Here, we use a 3D impedance network [1] to elucidate the impact of porosity on the impedance response of mesoporous thin films. The results demonstrate that the regular pore structure gives rise to a geometric current constriction effect, which dominates the impedance spectra in the whole frequency range.  As a consequence, the effective conductivity values estimated from the total resistance and the electrode geometry underestimate the material-specific conductivity of the material by more than one order of magnitude. However, by a detailed analysis of computed impedance spectra for varying pore size, we were able to derive an empirical expression, which allows the estimation of the material-specific conductivity from the measured impedance with an error of less than 8% [2].

Rate capability is a limiting factor in solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), and oxide-based solid-state lithium (SSLiBs) and sodium (SSNaBs) batteries due to high area specific resistance (ASR). Lower ASR SOFC, SOEC, SSLiB, and SSNaB structures would enable higher power density and lower cost, dramatically improving market adoption of these energy conversion and storage technologies. In this presentation we will explain the roles of solid ion-conductor composition, structure, and interfaces in reducing ASR to achieve extremely high current densities. Results will be presented for GDC based SOFCs/SOECs, Li-garnet based SSLiBs, and NASICON based SSNaBs, and similarities between these dissimilar materials and electrochemical devices elucidated. Moreover, we will demonstrate that SOFC and SOEC current densities over 5 A/cm2 can be achieved at 650°C, and SSNaB and SSLiB current densities of 30 mA/cm2and 100 mA/cm2, respectively, can be achieved at room temperature, based on these similarities.

Some active materials used in lithium-ion battery (LIB) electrodes undergo phase separation into Li-rich and Li-poor phases upon lithium intercalation. Typical examples include LiFePO₄ (LFP) at the cathode and graphite at the anode. While phase separation enables useful features, such as a constant equilibrium potential as a function of state-of-charge, such behaviour poses challenges during material characterisation and for high-rate operation.

This study provides a perspective of phase separation in LIB active materials by combining non-equilibrium thermodynamics principles [1] with in-operando techniques [2]. The results show that classical techniques used to estimate solid-state diffusion coefficients, such as the galvanostatic intermittent titration technique (GITT) [3], must be revisited for this class of materials. In fact, although the rapid equilibration of the interface between Li-rich and Li-poor phases within a particle leads to a quick voltage relaxation, the solid-state diffusivity can be significantly lower than what this fast dynamics may suggest. This has significant implications on the distribution of lithium within secondary particles because, upon fast lithiation, the Li-rich phase grows at the particle surface and prevents further lithiation. This is especially critical for graphite anodes, since the Li-rich phase (also known as stage I) at the particle surface is the primary cause for the plating of metallic lithium outside the particle, as quantified in in-operando experiments [2]. Nevertheless, experiments also show that Li plating is not irreversible and part of the plated lithium can be stripped and back-intercalated in graphite when the current is interrupted. This discovery opens opportunities for alternative fast-charging protocols as long as the particle size distribution is well controlled to prevent excessive plating on small particles. On the other hand, experimental characterisation and simulation of a disordered carbon, which does not undergo phase separation, reveal an effectively faster solid-state diffusion and a more significant resistance to lithium plating even at high C-rate [4], thus enabling for a comprehensive comparison between solid-solution and phase-separating active materials when it comes to characterisation and fast-charging capabilities.

Solid-state batteries using lithium metal anode has a huge potential as post lithium-ion batteries due to high energy density, safety, and fast charging capability. Lithium Lanthanum Zirconate oxide (Li₇La₃Zr₂O₁₂, LLZO) electrolyte is a promising candidate as it demonstrates an excellent compatibility with Li metal anode and notable performance in terms of their critical current density and cycling stability. Despite the promise, establishing a full “solid" cell architecture still faces challenges, mainly in achieving robust contact and low impedance for composite cathodes due to the formation of secondary phases. Thus, integrating with a choice of cathode material into practical devices has lagged, leading to the dearth of lab-scale prototype cell data. Here we report the electrical resistance of composite cathode, LLZO-LiCoO₂, is reduced 3-4 orders of magnitude by manipulating sintering atmosphere. Raman mapping reveals the second phase of Li_0.5La₂Co_0.5O₄ are avoided in the composite cathode sintered in Li-rich atmosphere as compared to ambient condition. Practically thin electrolyte is fabricated either by co-sintering with the cathode composite or by pulsed laser deposition on the cathode substrate. Discussion is aimed toward further opportunities for all-ceramic cathode-supported cell development to meet important target parameters including interfacial resistance, cathode active loading, specific capacity, and cycling stability.

One of the most widely used electroanalytical methods for understanding the kinetics of composite battery electrodes is GITT – the Galvanostatic Intermittent Titration Technique [1,2]. At the same time, it is common for battery researchers to question the reliability of GITT measurements. The level of skepticism associated with GITT varies; some question its quantitative accuracy (but not much on otherwise trends), whereas others raise more serious questions on its relevancy for kinetics. The root causes for the unreliability of GITT are also perceived quite differently among researchers. Some believe that the unreliability is a result of poor implementation of the technique, implying that careful measurements could produce reliable results. Others believe that there are more fundamental limitations, dismissing any kinetic information obtained from GITT measurements. In this talk, we will isolate the implementation issues and focus on answering the question: "if the measurement protocol is optimized, can we extract relevant kinetic parameters from GITT measurements on composite battery electrodes?" We will take lessons from recent microscopic kinetics studies [3-7] and understand their implications from the perspective of using electroanalytical techniques such as GITT.

Inhomogeneous metal plating in batteries not only results in irreversible capacity losses, but also poses a serious safety concern for their operation. Dendritic growth of plated metal can cause internal short circuits and catastrophic failure of the cell [1].  Understanding the transport conditions that lead to heterogeneous metal deposition is essential for the development of a safer lithium-ion battery. Scanning Electrochemical Microscopy (SECM) is a scanning probe technique which is increasingly being used in battery research to characterise the heterogeneity of the solid electrolyte interphase (SEI), the transport properties of which directly affect the morphology of the plated metal [2]. Here we present a study linking the heterogeneity of an SEI to metal electroplating and battery performance using zinc as a model compound. Zinc is an ideal candidate for this study, as it has been shown to form an SEI and can be used in an aqueous electrolyte [3]. We present a comprehensive study demonstrating how different additives to the electrolyte affect the heterogeneity of the SEI and therefore impact metal electroplating within the cell. This research aims to provide a methodology to predict and prevent where inhomogeneous metal plating will occur and aid a strategy for mitigating dendrite induced failures.

Solid Oxide Electrolysis Cells (SOEC) are solid-state devices that play a pivotal role in the efficient conversion of renewable electrical power into fuels, making them a key technology for the green transition. The SOEC technology allows for the production of green hydrogen through steam electrolysis, as well as the operation of SOEC in co-electrolysis mode (i.e., electrolysis of H₂O+CO₂) to generate synthesis gas (CO+H₂). This gas can then be processed further into a variety of carbon-containing fuels.

To achieve high electrochemical performance and mechanical robustness of SOEC, it is essential to manufacture these ceramic solid-state devices to obtain a mechanically strong support layer, thin and gas tight oxide ion conduction electrolyte and porous electrodes having structures optimized to ensure the highest possible density of active sites for the electrochemical reactions e.g. large percolating triple-phase-boundary density in cermet-based electrodes like the often applied nickel/yttria-stabilized-zirconia based fuel electrode.

Advanced and complementary characterization techniques are needed for the further development and improvements of SOEC towards marked entry to ensure a high performing, reliable and long-term durable SOEC technology for production of green fuels. The characterization techniques include combinations of electrochemical characterization (e.g. impedance spectroscopy), element mapping (e.g. via EDS, Raman spectroscopy) and micro- and nano-structural characterization (e.g. via TEM, STEM, SEM, FIB-SEM/3D reconstructions).

At Topsoe, we’re constructing an SOEC manufacturing facility in Denmark, set to be operational in 2025. This facility will produce electrolysis cells, stacks and modules with a capacity of 500 MW per year. We’ve already announced the first agreement, a reservation of 500 MW capacity. However, in parallel with the up-scaling and commercialization of our TOPSOE™ SOEC electrolyzer technology, it is remains crucial to continue the R&D work on SOEC – cells, components, and stacks. In this perspective, this presentation will provide an overview of Topsoe’s expertise, results, and initiatives in the field of SOEC. This spans from studies of materials’ properties, electrode nanostructures as illustrated in Figure 1, to the electrochemical performance of cells and stacks, and the design of MW-sized systems. We’ll present examples of performance and durability testing  and will concluded with a status on Topsoe’s up-scaling of SOEC production capacity.

Figure 1: Example of electrode nano-structure characterization via TEM of Ni/YSZ based fuel electrode produced and tested at Topsoe [1].

Over the past 10 years, significant development on materials, cells, stacks and sytem integrating proton-conducting oxides have been conducted in a series of European projects (WINNER, GAMER, PROTOSTACK). Proton conducting oxide materials have several unique characteristics that distinguish them from oxygen ion conducting oxides and proton-conducting polymers. By enabling proton-mediated electrochemistry under both dry and wet environments at moderate temperatures (e.g. 400–600 °C), these materials provide unique opportunities to enhance a diverse range of complementary electrochemical and thermochemical processes while providing storage solutions. Leveraging these characteristics, the projects have focused on designing, manufacturing, assembling and testing tubular cells and stacks to alleviate thermo-mechanical constraints when operating in high pressure (10 bar).  As starting point, the cells contain BZCY-based electrolyte, BZCY-Ni tubular hydrogen electrode and BGLC-BZCY composite steam + O₂ electrode. Further improvement of the cells is achieved by designing a dual layered electrode with addition of a BGLC top layer, as will be shown by comparing the electrochemical performance of these architectures. The cells are integrated in tubular steel shells forming the so called "single engineering units" (SEU). The SEUs have ca. 50-60 cm² active surface area and are tested in pressurized steam electrolysis operation at intermediate temperature (600°C) and up to 10 bar pressure demonstrating high faradaic efficiency (> 80%) and reasonable area specific resistance. A dedicated design assembly of the SEUs to produce racks of 16 SEUs mounted in series has been developed in the project, as well as an integrated system design with necessary balance of plant components. Results of this system operated at ambient pressure and up to 7 bar will be presented. Knowledge gained from operations of the SEUs and system have highlighted the dramatic impact of Cr evaporation on cells performance, microstructure and composition, as will be presented. Mitigations strategies will also be discussed.  

Solid oxide fuel cell (SOFC) and Solid oxide electrolysis cell (SOEC) stacks are assembly of metal interconnector, ceramic cells, and have multiple layers of various ceramics such as an electrolyte and electrodes. It is necessary to clarify the deterioration mode of each material and between materials to suppress the deterioration rate. Cr poisoning of the cathode was one of the most significant deteriorations in SOFC. As a candidate for suppressing Cr poisoning deterioration of cathodes, we have developed a ceramic coating for metal-interconnectors using an electrodeposition coating method and confirmed that it can suppress Cr poisoning under SOFC operating environments. SOFC cells, which consist of the electrolyte and electrodes, undergo large amount of deterioration, and many reports have been published on the reaction mechanism at the solid-solid interface and the deterioration due to impurity poisoning. However, in SOFC cell stacking, it is necessary to discuss the effects of ceramic interfaces on durability other than electrode reactions. For example, thermal cycles associated with startup and shutdown may cause peeling of multilayers interfaces and effects on other layers of ceramic bonding material such as bonding materials that electrically connect metal-interconnectors and SOFC cathodes. In this study, we have evaluated the effect on the long-term durability of the metal-interconnector / ceramic coating / bonding material multilayer interface to clarify the effect on the durability of SOFC and SOEC cell stacks. For the electrode contact materials, we selected the similar composition of Co-Mn spinel oxides as the electrode contact material so that it can be sintered even at lower temperatures than the general stacking temperatures. In designing the SOFC/SOEC interface, we focused on two methods: diffusion bonding and metal addition. The compositions of Co and Mn were graded at the ceramic coating / the electrode contact material interface. As a result, during stacking and during SOFC/SOEC operation, mutual diffusion occurred by virtue of the difference in activity between Co and Mn, and the ASR was reduced by improved adhesion between the ceramic coating and the electrode contact material. Mutual diffusion between the ceramic coating and the electrode contact material occurred over time during the long-term durability test, and the ASR continued to decrease. As a second method, by adding metallic Co to the electrode contact material (Co-Mn oxide) and sintered it, the adhesion at the ceramic coating/the electrode contact material interface was significantly improved. By utilizing the heat of oxidation when the metallic Co oxidized for sintering, we have achieved sintering at low temperatures by locally increasing the temperatures at the interface, even at low stacking temperatures. In the presentation, we will explain the design of the electrode contact material for SOFC cathodes and SOEC anodes and metal interconnectors based on the results of long-term high temperature accelerated tests more than 10,000 hours and the thermal cycle tests.

Proton Ceramic Electrolysis (PCEs) presents an intriguing advantage compared to other electrolysis technologies due to its potential for direct production of pressurized dry hydrogen, in addition to an intermediate operating temperature that allows for high electrical efficiency, thermal integration with industrial processes and renewable energy sources and slower diffusion-related degradation. Recent years have shown an impressive development in reported cell performances for PCEs, confirming the potential of the technology. Pressurized operation of PCEs is particularly advantageous due to a combination of favourable thermodynamics for proton transport in the electrolyte and the possibility of direct production of dry pressurized hydrogen without any downstream separation or compression.

This contribution will present the development and upscaling of tubular PCEs (10-60 cm²) based on a robust and chemically stable BZCY81 electrolyte for operation at high steam pressures. Scalable and reproducible production of tubular cells with a demonstrated tolerance to long-term operation at 10 bar operating pressure is achieved for more than 10 tubular cells with different cell configurations - highlighting the robustness of the proposed technology. Significant improvements in electrochemical performance and faradaic efficiency is achieved by optimizing the architecture and composition of the steam electrode. The impact of operating pressure on electrochemical activity, faradaic efficiency and overall energy efficiency is investigated at a single-cell level and discussed in terms of the defect chemistry of the electrolyte and the electrochemical processes at the electrode interfaces. Moreover, a combined electrochemical and fluid-dynamic model is used to corroborate experimental measurements over a range of process conditions to elucidate the impact of pressurized operation on the overall efficiency of PCEs, and discuss the trade-offs in different operating modes in terms of overall energy efficiency vs hydrogen production rate at a given delivery pressure at system level. 

Electrochemical membrane reactors offer the opportunity to increase the yield of relevant chemical reactions such as the production of synthetic liquid fuels as well as to improve their energy efficiency by coupling exothermic and endothermic processes.

This work, developed in the frame of the EU Horizon 2020 “eCOCO2” project, presents the CO₂ electro-catalytic reduction into methane by using tubular protonic membrane reactors. Two different modes of operation were employed to provide the needed H₂ for the methanation reaction: H₂ pumping and electrolysis. In both operation modes, the protonic membrane was composed of BaZr_0.8Ce_0.1Y_0.1O₃ as electrolyte and the cermet Ni+BaZr_0.7Ce_0.2Y_0.1O₃ as inner metallic electrode acting as support and as methanation catalyst. In the case of the H₂ pumping, the outer electrode was made of Ni+BaZr_0.7Ce_0.2Y_0.1O₃ whereas for the electrolysis, an electrode made of PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) was used.

Transport properties of the electrochemical cells were evaluated by impedance spectroscopy, i-V curves and H₂ production as a function of the applied current density and the operational conditions, from 600 ºC to 450 ºC and pressures up to 30 bar. Then, methanation reaction performance was also evaluated as a function of the operational parameters, analyzing the CO₂ conversion and CO and CH₄ yields.

In both operational modes, it was observed that the total system pressure plays a key role in both the electrochemical and catalytic performances of the protonic cells. The improvement in the performance at high pressure is due to two effects occurring simultaneously as it was inferred from DRT analysis: (i) an increased hydration of the electrolyte which enhances the proton conductivity and (ii) the improvement of the surface kinetics and mass transfer of the electrodes. Regarding catalytic performance, CO₂ conversion and CH₄ selectivity increase significantly with pressure, reaching values of 86% and 94% respectively, for a stoichiometric H₂/CO₂ ratio of 4 at 450 °C and 30 bar.

In addition, computational fluid dynamics (CFD) simulations were conducted to gain critical insights into the reaction kinetics, transport phenomena, and electrochemical performance of the membrane reactor.

Fast oxide-ion conducting solids are used as electrolytes in solid oxide fuel cells (SOFCs), solid oxide electrolyser cells (SOECs), gas sensors, and oxygen pumps. Commercial devices are typically based on electrolytes showing a defect fluorite structure. The fluorite structure is well-known as a host for fast oxide-ion conduction, with archetypal examples including yttrium doped zirconia, delta-Bi₂O₃ and lanthanide doped ceria. High oxide-ion conductivity in these systems is facilitated by large concentrations of oxide-ion vacancies within the cubic fluorite structure. While the long-range structures of these materials are similar and essentially based on a cubic close packed array of cations with anions in the tetrahedral sites, local structure can vary significantly, particularly with respect to dopant cations and oxide-ion vacancies.

Until fairly recently, analysis of local structure in such systems relied on careful examination of average structural models, derived from diffraction data, to speculate on local coordination environments, in some cases supported by local structural probes such as solid-state NMR and EXAFS as well as computational modelling. However, recent developments in reverse Monte Carlo (RMC) analysis of total neutron scattering data have allowed for a more detailed analysis of local structure based on physical data using a combination of Bragg and diffuse scattering. The former is associated with the long-range order while the latter arises from short-range order. The large box approach of RMC modelling uniquely allows for examination of the resulting model for local vacancy ordering as well as preferred vacancy association and dopant clustering. 

Using this approach we have examined local structure in a number of fluorite structured systems. In the case of delta-Bi₂O₃, the highly conducting fluorite phase is only stable above ca. 730 °C but can be preserved to room temperature through incorporation of dopant cations. However, even for isovalent dopants, where the nominal vacancy concentration is unaltered through doping, conductivity is lower than in the parent delta-Bi₂O₃, and has been associated with oxide-ion vacancy trapping by the dopant cation. In early work, using RMC modelling of total neutron scattering data we were able to show that local ordering of oxide-ion vacancy pairs occurs in these systems with a statistically non-random preference for ordering in the <100> direction.^1,2,3  

In seeking to lower the operating temperature of SOFCs, electrolytes based on lanthanide doped cerias, such as gadolinium doped ceria (GDC), have been utilised. Substitution of tetravalent Ce⁴⁺ by trivalent lanthanide (Ln³⁺) ions (Ce_1-xLn_xO_2-x/2) introduces oxide ion vacancies into the fluorite lattice. We have carried out RMC analysis of neutron total scattering data from Nd doped ceria and GDC. In the latter case, samples prepared with isotopically enriched ¹⁶⁰Gd were used to overcome the high neutron absorption coefficient of naturally abundant Gd. 

The study reveals three distinct features in the local structure of lanthanide doped cerias. Firstly, clustering of the dopant cations, secondly preferred association between oxide-ion vacancies and the dopant cations and finally oxide-ion vacancy clustering. Details of these findings will be presented.

For a sustainable energy future, solid-oxide fuel cells (SOFC) play an essential role: they generate electricity from industrial waste gases, with water as their only byproduct. The cornerstone of this technology are materials capable of splitting gaseous oxygen (O₂) to conduct it as oxygen anions (O^2-) in the device.¹ Commercial SOFC currently rely on strontium-doped lanthanum perovskites (La_1-xSr_xMO_3-d with M=Co, Mn, Fe) as a cathode. The high-performance of this perovskite comes from three key factors: the atomic arrangement is cubic, it is inherently oxygen deficient, and contains multivalent metals that allow for mixed ionic-electronic conductivity. However, the activation of the oxygen-ion conductivity requires high temperature (T > 800°C), and lanthanum is a rare-earth element with extreme supply issues.^2,3

The quest for new materials with equivalent (or improved) performances and a more attractive elements composition starts with the need to stabilise a cubic perovskite arrangement, optimal for ionic and electronic conductivity. However, with the same ideal composition (ABO₃, where A = non-metal, B = metal) the cubic structure might be unachievable or unstable.
The established approach in materials chemistry is to mix the components in B with one element at a time (A(B’B’’)O₃).⁴

A well-known, rare-earth free material for fuel cells is BaInO_2.5 ⁵, whose ionic conductivity (normally active at temperatures above 1040°C) can be lowered with a variety of doping of the B-site (V, Mo, W, Cu, Ga, Y, Zr, Sn, Ti, Al, Fe, Co, Mn, etc) to yield a cubic perovskite ^6-13. This compositional flexibility makes it a great candidate for a systematic study on the synthesis of high-entropy perovskites (HEPs): a cubic perovskite in which the B position is equally shared by five or more atoms, where the disorder of a multi-element system creates an entropic advantage. Once synthesised, HEPs are often reported to be better performing their they low-doping equivalent, with lower activation temperature and higer conductivity. ¹⁴

In this study we perform a systematic study on sequential on multi-elemental doping of BaInO_2.5, attempting a rational incremental approach of 2->3->4->5 elements sharing the B position, taking from a pool of 10 elements (In, Sn, Ti, V, Cr, Mn, Fe, Co, Ni, Zr) that exclude rare-earth materials. All the syntheses were performed with solid state ceramic methods, with maximum reaction temperature fixed at 1100°C.

We will highlight 8 new stable compositions in the BaIn_xM_1-xO_3-d (M = mixed metal) compositional space, of which three medium entropy and one high entropy. The structural results in this system highlight a complex interplay between charge, size and orbital configuration and the capability (or not) to form a multi-doped system. The thermogravimetric characterisation provides insight on the protonic and electronic conductivity performance, in terms of amount of mobile vacancies and the temperature of activation for their mobility.

This systematic study opens up to answer the question: is high-entropy stabilisation always achievable, and always beneficial, for a functional material?

The drastic Effect of A-site non-stoichiometry on the Cation Diffusion and Core-Shell Formation in NBT-based Ceramics

Sophie Bauer ^a, Till Frömling ^a,b

^a Technical University of Darmstadt, Department of Materials and Earth Science, Peter‑Grünberg‑Straße 2, 64287 Darmstadt, Germany, sophie.bauer@mr.tu-darmstadt.de

^b Fraunhofer Research Institution for Materials Recycling and Resource Strategies (IWKS), Aschaffenburger Straße 121, 63457 Hanau, Germany

Since Bi₂O₃ tends to evaporate easily during the solid-state synthesis of Na_0.5Bi_0.5TiO₃‑based (NBT-based) ceramics, it is extremely challenging to control the oxygen and bismuth vacancy formation, and consequently, the stoichiometry of the composition. [1] As the diffusion processes in NBT‑based ceramics strongly depend on the vacancy concentration, the addition of 0.1 % bismuth can supress the grain boundary diffusion coefficient by six orders of magnitude. [1] Therefore, different microstructures, each with distinct properties, emerge depending on the degree of bismuth deficiency and, consequently, the concentration of oxygen and bismuth vacancies. [1, 2]  In the case of Na_0.5Bi_0.5TiO₃‑SrTiO₃ (NBT-ST), a homogeneous material can be synthesized by introducing a slight bismuth deficiency in the starting material, while bismuth enrichment results in a core-shell microstructure. [2, 3] As expected, both materials exhibit fundamentally different properties. Bi-enrichment causes an exceptionally high achievable strain in this ferroelectric material and a high energy density that can be stored, offering potential applications in actuators and high-energy capacitors. In contrast, Bi deficiency results in a seven times larger grain size and significantly higher conductivity.

This work sheds light on the possibilities of tailoring the microstructure, transport properties, and electromechanical properties of NBT-based materials. Understanding the diffusion mechanisms of cations in NBT-based ceramics is inevitably crucial in this context.

Photovoltaic (PV) conversion, as an eco-friendly energy solution to contemporary challenges, has garnered extensive attention, particularly in studying PV cells at ambient temperatures. Notably, the renowned photocatalyst SrTiO₃ has exhibited photo-responsiveness even at high temperatures [1], achieving a remarkable photovoltage exceeding 1.0 V [2] and a high short-circuit current density of approximately 1.5 mA•cm^-2 [3] under optimized conditions.

Detailed analysis reveals that the induction of photo-response at high temperatures is influenced by photo-oxidation in SrTiO₃ and the heterojunction between the photoelectrode and SrTiO₃. Inspired by these insights, our research group explored the photo-response of ionic conducting materials at high temperatures. While experimenting with SrTiO₃ deposited on Gd-doped CeO₂ (GDC) electrolytes, it was found that the bare GDC electrolytes also showed a photo response under heating. This study focuses on the photo response of GDC at high temperatures and aims to unveil the photovoltage mechanism.

In our experiments, sintered GDC was mirror-polished, and Pt and Au electrodes were sputtered onto it. The samples were placed in a quartz chamber and heated within the range of 200 ºC to 400 ºC under O₂-Ar gas flow. Open circuit voltage (OCV), electrochemical impedance spectroscopy (EIS), and DC polarization measurements were conducted under both UV (365 nm) irradiation and non-irradiation conditions. This presentation will discuss the impact of gas composition and temperature on the photo-response of GDC. Additionally, the contribution of oxygen transport on photovoltage will be validated by measuring the changes in oxygen partial pressure during UV irradiation using a yttria-doped zirconia oxygen sensor.

Oxyfluoride type materials are very interesting for their potential as mixed conductors as charge carriers for various applications. Some Oxyfluorides can act as O^2-, F^-, e^- and h⁺ conductors. Materials based on TiO_2­-x and can have a very wide range on electronic conductivity based on the oxygen-titanium ratio. Materials based on highly doped SrF₂ tend to show some ionic conductivity with little to no electronic conductivity. The interaction between these two compounds is highly interesting.

SrF2-TiO2 composite materials were produced by hot-pressing SrF₂ and TiO₂ powders under a reducing atmosphere at 1100°C. Ratios of 1:1-1:5 were produced including TiO₂ and SrF₂ only. Up-to three phases were found in all samples. The very SrF₂ rich samples (1:3-1:5) showed SrF₂ and a minority phase which was not recognized by XRD or SEM. The samples with more TiO₂ such as (1:1 and 1:2) showed also a TiO_2-x phase in the middle of the unrecognized phase.

The materials were measured in argon atmosphere at 50-300°C using electrochemical impedance spectroscopy. The response was fitted with a modified Debye circuit to separate the electronic and ionic conductivity of the samples showing very high ionic conductivity and comparative electronic conductivity. Proof of the ionic conductivity was received with electron blocking layers of La_0.2Sr_0.8F_2.2 showing that the ionic conductivity is very high for the SrF₂ rich samples but less so for the 1:1 ratio compound. XPS analysis showed several Ti oxidation states allowing for electronic conductivity. Activation energies were calculated for electronic and ionic conduction showing ionic conduction with a very low activation energy of under 0.1eV for fluorine conduction and an activation energy of 0.2eV for electronic conduction.

The development of all-solid-state batteries (ASSBs) has the potential to be transformational as a safer more efficient storage of energy for stationary and transport applications.

However, progress in developing well-performing and long-lasting ASSB cells has been delayed due to the difficulty to develop solid electrolytes that possess high cation conductivity combined with the adequate electro-chemo-mechanic stability required at cell level. This is associated to the complex and dynamic solid/solid interfaces that are generated during processing and that evolve upon cycling, which are difficult to study and therefore, optimize.

Most studies are trying to find a solution to this in the use of composite and hybrid solid electrolytes which adds even more complexity to understand which are the key factors governing the performance of the devices.

In this work, we analyze a range of inorganic and polymer-inorganic solid electrolytes with a combination of experimental and computational techniques, we discuss structural and chemical factors affecting the ion mobility in these systems and how it can be altered by the processing conditions at a bulk and interface level.  High frequency (up to 3GHz) electrochemical testing allows for the quantification of ionic motion and separate bulk processes from processes taking places at the different interfaces. Synchrotron and neutron diffraction analysis is employed to understand how structural features and degrees of order-disorder might impact the cations conduction paths and conductivity. Surface-sensitive analysis ex-situ and operando are used to understand how the interfaces evolved and how this can be correlated to the electrochemical performance (1,2). Finally, good adhesion and controlled reactivity among materials is analyzed and routes for optimization are proposed.  

Thiophosphate solid electrolytes (SE) are promising candidates for solid state battery systems as they exhibit both reasonable ionic conductivity and ease of processability. Unfortunately, several challenges still hinder their commercialisation, one of them is linked to the control of the interfaces, and another one is linked to their electro-chemo-mechanical degradation. For the former, coating techniques can play a role to better handle the interfaces, whereas the second one is still unclear as it is declared that those degradation are electrochemical driven, when they could be there during the cell assembly, during the densification for example. Indeed, densification is a key parameter to control since it impacts the ionic conductivity, and ensures the contact between the active materials and the electrolyte¹. Finally, proper densification of the SE could limit the propagation of dendrites. Investigating the densification is far from trivial, and here we rely on novel advanced in situ synchrotron techniques to provide new insight into the sintering of the electrolyte.

The densification of two thiophosphate electrolytes (one amorphous and one crystalline) was investigated using the UToPEc set-up at Psiché beamline of Soleil synchrotron. This unique set-up² allowed for the simultaneous acquisition of absorption X-ray tomography, X-ray diffraction, density measurement and impedance measurements whilst applying a compressive force of up to 4 GPa meaning we can follow both structural and morphological changes during densification. As can be seen in Figure 1, the amorphous solid electrolyte can lose up to 53% of its own volume during the densification whereas the crystalline one could only be reduced by 20%.

Following the densification, we cycle the solid electrolyte to track the electro-chemo-degradation using X-ray diffraction computed tomography (XRD-CT) carried out at ID15a at the European Synchrotron Radiation Facility. It allows localizing the extend of the electrochemical and chemical degradation after long-term cycling and reveal that the thiophosphate solid electrolyte (LPSCL) decomposes electrochemically in known products such as S, Li2S, LiCl³ and that the chemical decomposition at the Li interface is more prominent than the electrochemical one.

Both advanced techniques reveal the difficulties in disentangling the extent of processing in solid state batteries.

Li₂S-P₂S₅ (LPS) glass-ceramic solid-state electrolytes (SSEs) have garnered substantial interest in solid-state battery research due to remarkable electrochemical properties and enhanced safety over conventional organic liquid electrolytes.[1, 2]   While conventional mechanical synthesis techniques for processing LPS SSEs are generally energy-intensive and time-consuming, recent advances in solution processing techniques have produced LPS SSEs with exceptional ionic conductivities and mechanical properties at low reaction temperatures and in a fraction of the amount of time required by mechanical processes.[3]  A major challenge lies in the limited-understanding of the underlying chemistries involved in solution processing, despite considerable efforts to elucidate full reaction mechanisms.[4] Here we contribute to the growing body of research investigating solvent and precursor interactions by exploring how the thiol functional group of the solvent ethyl mercaptan (EthSH) interacts with Li₂S and P₂S₅ under common reaction conditions to facilitate the formation of LPS SSEs.

Three common molar stoichiometries of LPS systems (50:50, 70:30, 75:25) and the individual precursors Li₂S and P₂S₅ were reacted in EthSH separately at room temperature for 24 hr with magnetic stirring, dried, and then annealed at 225 °C for 1 hr following common solution processing methodology.[5]  During each process step, subsamples of each reaction were collected and characterized electrochemically and physiochemically by AC electrical impedance (EIS), powder X-ray diffraction (PXRD), Raman shift, and nuclear magnetic resonance (NMR) spectroscopies, and scanning electron microscopy.

The low boiling point of ethyl mercaptan allows for rapid drying of the processed SSEs. However, the solvent does not complex with Li₂S and P₂S₅ when the Li₂S content of the LPS system is at or above 70 mol%, as other polar organic solvents do, nor does it complex with the individual precursors.   Instead, the 70:30 and 75:25 samples exhibited a moderate amount amorphous characteristic, commonly associated with solid-state reactions, and expressed XRD peaks assigned to Li₂S and Li₄P₂S₆.  At 50 mol% Li₂S, the ethyl mercaptan stabilized a Li₂S-P₂S₅/EthSH complex similar to Li₂P₂S₆ with a high order of crystallinity at room temperature, which was verified with PXRD, Raman, and SEM.  All three systems exhibited low or immeasurable ionic conductivity with EIS. To the best of our knowledge room temperature crystallization of any LPS system has not been reported before.

The all-solid-state battery (ASSB) is composed of a solid electrolyte, cathode and anode, unlike LIBs (lithium ion batteries) containing a combustible liquid electrolyte, and has recently been in the spotlight for its advantages of high energy density as well as high safety.¹ A sulfide-based all-solid-state battery with a high ion conductivity (10^-3 to 10^-2 S cm^-1) of solid electrolytes is attracting particular attention as a high-capacity and high-power battery.² The sulfide-based solid electrolyte, the core material of this battery, has the advantage of being stable against heat, so when designing battery modules and packages, it is expected to increase the energy density at the package level by simplifying the cooling parts.

The sulfide-based solid electrolyte has high reactivity with atmospheric moisture and generates hydrogen sulfide (H₂S), which leads to an increase in process cost and safety issues when mass-producing the battery. In order to overcome this, it is necessary to improve the moisture stability of the solid electrolyte. Through many studies, it has been confirmed that moisture stability is able to be improved through substitution or doping of oxides³, however the conductivity is largely reduced, and further improvement is required. In this study, as a way to improve the problems, we combined Argyrodite solid electrolyte and ion conductive oxide (Li₃PO₄) to minimize the decrease in ionic conductivity and improve atmospheric stability. In addition, the electrochemical stability of the material was investigated by evaluating the electrochemical properties to which the high-capacity cathode material (NCM811) was applied as pouch-type all solid state battery cell.

All-solid-state batteries with non-flammable inorganic solid electrolytes are a key technology to address the safety issues of lithium-ion batteries with flammable organic liquid electrolytes. However, conventional electrode materials suffer from substantial volume change during lithium-ion (de)intercalation, leading to the failure of the interface between the electrode materials and solid electrolytes and then severe performance degradation. In this work, we report strain-free charge storage via an interface between a transition-metal carbide nanosheet (MXene) and solid electrolytes, where MXene shows negligible structural change during lithium-ion (de)intercalation. Combined assessment including operando STEM-EELS elemental mapping clarified the strain-free nature of the MXene electrodes in the all solid-state batteries. In addition, the irreversible reactions at the MXene-electrolyte interface is visualized, explaining the inital irreversible capacities and relatively low rate capability of the MXene electrodes. A strain-free all-solid-state battery, which consists of Ti₃C₂T_x anode and disordered rocksalt Li_8/7Ti_2/7V_4/7O₂ cathode, demonstrates a long-term operation owing to the strain-free nature of both electrode materials.

Ba(Zr,Ce,Y)O3-δ proton-conducting materials: manufacture, performance and stability

W. A. Meulenberg1,2,*, J. Malzbender1, J. L. Wolter1, W. Deibert1, O. Guillon1, J. M. Serra3,

C. Segarra3, , A. Dos Santos3, L. Almar3, D. Catalán-Martínez3, M. Fabuel3, S. Escolástico3,

W. Zhou1

1 Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research (IEK), 52425 Jülich, Germany

2 University of Twente, Faculty of Science and Technology, Inorganic Membranes, P.O. Box 217, 7500 AE, Enschede, the Netherlands

3 Instituto de Tecnología Química, Universitat Politècnica de València – Consejo Superior de Investigaciones Científicas, Av. Los Naranjos, s/n, 46022 Valencia, Spain

Abstract

Proton-conducting membranes have great potential for applications in proton conducting membrane reactors for the production of commodity chemicals or synthetic fuels as well as for use in solid oxide fuel cells. Ba(Zr,Ce, Y)O3-δ perovskite oxides are of particular interest due to its proven high proton conductivity and good chemical stability. In this work, fabrication of Ba(Zr,Ce, Y)O3-δ samples, including thick pellet, thin tape-cast and multi-layered half-cell, were studied to prevent the typical challenges like warping, cracking and delaminating. Materials with different stoichiometries were investigated, on their conductivity and chemical stability against NH3 and H2O, to select the most promised candidate for future compact membrane reactor. Mechanical behaviors including elastic modulus, hardness, fracture toughness and creep were investigated to warrant long-term structural stability. Thus, a systematic investigation on Ba(Zr,Ce, Y)O3-δ materials was carried out to pave the road towards widespread application of proton-conducting membranes.

Donor-substituted (W, Mo) LaNbO₄ materials have been reported to be good oxide ion conductors exhibiting excellent chemical stability with acceptable ionic conductivity, while their protonic conductivity is negligible [1]-[6]. However, most of these works were conducted at ambient conditions, and the conductivity behaviours at reducing conditions can be significantly different from non-reducing conditions.

In this work, LaNb_0.84W_0.16O_4+δ and LaNb_0.84Mo_0.16O_4+δ have been successfully synthesized via a solid-state reaction route. An abnormal conductivity behaviour at reducing conditions was found in electrochemical impedance spectroscopy measurements. At first, this abnormality is attributed to the emergence of the electronic conductivity. However, more evidence from different characterisations such as DC measurement, XPS and XAS suggest the simultaneous emergence of protonic conductivity later on. Protons originating from the protonic incorporation are indeed participated in the conductivity behaviour and worth further investigating.

In conclusion, the conductivity behaviour can be significantly different depending on the dopants. For W-substituted LaNbO₄, the materials are stable at reducing conditions without the change of oxidation states of cations as well as the oxygen stoichiometry. Therefore, the electronic conductivity in W-substituted LaNbO₄ is surprisingly low, and the conductivity enhancement is mainly, if not all, from the emergence of the protonic conductivity. In contrast, Mo-substituted LaNbO4 materials suffer from severe reductions and their conductivity behaviour is more complex than W-substituted LaNbO₄ as there is a trade-off between the emergence of the electronic and protonic conductivity and the degradation of oxide ionic conductivity.

Discovering efficient and innovative materials for protonic ceramic cells requires the characterization of countless compositions. The knowledge of structural properties, hydration parameters, and electrochemical performances is essential for selecting the most promising electrolyte materials. However, traditional measurements are performed on individual samples and require extremely long stabilization times. A high-throughput approach was chosen to evaluate the electrochemical performance of hundreds of compositions inside the Ba(Ce,Sn,Zr)_0.8Y_0.1Yb_0.1O_3-δ (BCSZYY) ternary system. Thin film combinatorial libraries were produced by pulsed laser deposition on 100 mm diameter wafers. The composition gradient is obtained through alternate depositions of BaCe_0.8Y_0.1Yb_0.1O_3-δ (BCYY), BaSn_0.8Y_0.1Yb_0.1O_3-δ (BSYY), and BaZr_0.8Y_0.1Yb_0.1O_3-δ (BZYY) centered on three opposite edges of the substrate to recreate the ternary diagram. XY-resolved characterization techniques were performed to map the elemental, structural, and hydration properties of each composition inside the BCSZYY ternary system.

In particular, X-ray diffraction (XRD) was carried out to study the thin film structure. More than 300 diffractograms were collected on different positions of a single sample and analyzed through a custom-made code to calculate the pseudocubic cell parameter of the thin film. The resulting values of the "a" cell parameters range from 4.176 Å for the BSYY‑rich part, to 4.251 Å for the BZYY area, up to 4.418 Å for the BCYY one, in good accordance with the values obtained by Rietveld refinement for reference powders of single materials.

In situ measurements were performed at the DiffAbs beamline of the SOLEIL synchrotron by simultaneously collecting X-ray diffraction and fluorescence signals. XRF allowed the evaluation of the elemental distribution of the BCSZYY samples at room temperature and gave insight into thickness variation, complementary to spectroscopic ellipsometry results. A custom-made furnace was specifically developed to perform in situ XRD measurements on wide-surface thin-film samples at high temperatures in dry and wet conditions. The furnace consists of a hotplate and an X-ray transparent PEEK dome, allowing the control of temperature and atmosphere. Due to the large surface of the hotplate, the temperature distribution was calibrated using a platinum internal reference. The BCSZYY sample was subsequently measured in dry and wet N₂ to calculate the unit cell expansion due to water incorporation and to extract some information about hydration thermodynamics for the entire BCSZYY ternary system.

A custom-made setup for electrochemical impedance spectroscopy measurements is under construction to measure the conductivity of the entire ternary system. Electrochemical results will be related to the diffusion coefficients obtained using ¹⁸O₂, H₂¹⁸O, and D₂O isotopic exchange followed by Time of Flight Secondary Ions Mass Spectrometry (ToF-SIMS). Recently, we successfully applied this technique to combinatorial samples, and it will be systematically performed inside high-throughput procedures.

The synergy between ionic liquids (ILs) and metal-organic frameworks (MOFs) has attracted considerable scholarly attention as an innovative hybrid ionic conductor [1-2]. The incorporation of ionic liquids into the nanomaterial structure of MOFs has led to the successful synthesis of a novel solid electrolyte characterized by exceptional thermal stability and high ionic conductivity [3]. Here, we use an efficient capillary action method to introduce two proton conductive ionic liquids, namely 1-Ethyl-3-methylimidazolium triflate ([Emim][TfO]) and 1-Ethyl-3-methylimidazolium bis(trifluoromethyls​ulfonyl)​imide ([Emim][TFSI]), into the pores of ZIF-8, separately. Our investigation focuses on the influence of varying ILs quantities on pore volume, thermal stability, microstructure, and proton conductivity. The cages of ZIF-8 expand, and the decomposition temperature decreases following the encapsulation of ILs molecules. Notably, when the pores of the ZIF-8 material are entirely saturated with [Emim][TFSI], the composite material exhibits an impressive ionic conductivity of 1.140 mS·cm⁻¹ (at 0 % RH and 180 °C). The activation energy for this particular composite is 0.25(4) eV, which is notably low when compared to other IL@MOF conductive composites. These findings underscore the significant promise of IL@MOF hybrid composites as potential ionic conductors, attributable to their elevated electrical conductivity and relatively low activation energy. Additional, this composite can be used as electrolytes of High-Temperature Proton Exchange Membrane Fuel Cells, which are a type of fuel cell that operates at higher temperatures compared to traditional proton exchange membrane fuel cells (PEMFCs).

Literature:

[1] J. Jia et al, ACS Sustain. Chem. Eng. 2023, 11, 13502-13507. [2] A. B. Kanj et al, Nano Lett. 2019, 19, 2114-2120. [3] W. L. Xue et al, Angew. Chem. Int. Ed. Engl. 2021, 60, 1290-1297.

Electrochemical devices using a solid electrolyte are expected as next-generation energy conversion and storage devices. Typical examples are solid oxide cells using a solid-state oxide-ion (SOC) or proton conductor (PCC) and all-solid-state batteries using a solid-state lithium-ion conductor (ASSB). In such devices, the difference in chemical potential of reaction component, which is maintained across the electrolyte between the cathode and the anode, is the driving force for ionic transport and reaction. The chemical potential in the electrolyte also has a great impact on thermodynamic stability of the electrolyte, thus the device performance and durability [1, 2]. Therefore, in order to optimize designs and operating conditions of the devices or to ensure stability and durability of the devices, it is important to understand the chemical potential distribution in solid electrolyte under operation.

In this study, we aimed to experimentally evaluate the chemical potential distribution in solid state ionics devices. So far, a lot of numerical studies have been carried to estimate the chemical potential distribution in solid state ionics devices [1-3]. However, only a few studies have been reported to directly observe the distribution of chemical potential in an experimental manner [4-6]. And as far as the authors know, there exist no studies for its operando observation under device operation.

Challenges for the direct observation of chemical potential in solid state ionics devices are (i) how to detect the chemical potential change in the solid electrolyte and (ii) how to achieve operando measurement with high positional and temporal resolutions at desired temperature in controlled atmosphere under polarization. For the detection of chemical potential change, we embedded a potential probe into or on the electrolyte. In addition, as an operando analytical technique with high positional and temporal resolutions, micro-beam X-ray absorption fine structure (XAFS) was employed. In this presentation, our recent results on the direct observation of chemical potential distribution in SOC and ASSB will be given. The distribution of oxygen chemical potential in the former and that of lithium chemical potential in the latter, respectively, were evaluated under device operation. The obtained results were compared with the simulated results based on the ambipolar diffusion of ion and electron in the electrolyte.

High-temperature electrolysis is a compelling choice for efficient renewable energy storage, with electrochemical CO₂ and H₂O splitting serving as key reactions. Solid Oxide Electrolysis Cells (SOECs), known for their stability at elevated temperatures, stand out for such applications. Perovskite-type and fluorite-type oxides functioning as Mixed Ionic and Electronic Conductors (MIECs) prove to be well-suited materials for SOEC cathodes. Crucial properties for these materials include catalytic activity, electronic and ionic conductivity, redox-cycling stability, and resilience against graphitic carbon deposition during CO₂ splitting. However, many available materials combine only some of these properties, or degrade in the course of operation.

In order to enhance the efficiency and stability of SOEC cathode materials, it is imperative to attain a fundamental understanding regarding the mechanisms of degradation processes and surface reactions. This objective can be achieved through the utilization of in-situ surface-sensitive measurements, such as in-situ X-ray Photoelectron Spectroscopy (XPS) or in-situ Scanning Electron Microscopy (SEM). By combining these techniques with precisely crafted model electrodes, fabricated using Pulsed Laser Deposition (PLD) and magnetron sputtering, it becomes possible to correlate the surface reaction rate to surface compositions and morphologies.

Particularly intriguing materials for CO₂ and H₂O splitting are metal/metal-oxide composites. The interactions of the metal with both the supporting oxide and the gas phase significantly influences the surface activity of cathode materials in high-temperature electrolysis. Examples include B-site exsolved nanoparticles on perovskite-type oxides as well as the dewetting behavior of metallic nickel on Gd-doped Ceria (GDC). In both cases, differences in atmosphere, temperature and applied voltage lead to surface chemical and morphological changes, which furthermore influence the surface activity as well as the formation kinetics of undesired byproducts like graphitic carbon in the case of electrochemical CO₂ splitting.

In conclusion, our study on metal-support interactions on SOEC cathode materials using in-situ surface-sensitive measurements provides valuable insights for enhancing the efficiency and stability of high-temperature electrolysis for renewable energy storage.

Mixed ionic electronic conductors (MIECs) are an important class of materials characterized by simultaneous conduction of ions and electrons. One key aspect of this family of materials is the ability to vary their concentration of charged point defects (e.g. vacancies, interstitials, substitutional ions) through electrochemical ionic insertion. Indeed, the application of an electrochemical potential across an electrolyte can modify the chemical equilibrium of point defects in MIEC materials, forcing a variation of the defect’s concentration. This fundamental characteristic has allowed the development of non-stoichiometric materials as efficient insertion electrodes for Lithium-ion batteries or solid oxide cells’ electrodes for hydrogen technology. Additionally, the strong correlation between ionic point defects and structural/electronic properties offers a useful approach to modulate their functional properties, such as magnetism for novel magnetoionic devices. For all these systems, it is of high importance to understand equilibrium and kinetics of ionic insertion, possibly in-situ and/or operando.

In this work, we present optoionic impedance spectroscopy (OIS) as a novel technique to study ionic insertion in MIEC thin films. OIS is a direct modification of Electrochemical Impedance Spectroscopy (EIS), one of the most common techniques employed in the study of electrochemical systems. Indeed, both techniques involves the application of an external sinusoidal electrical stimulus able to promote ionic insertion over a wide range of frequencies. However, while EIS measures the sinusoidal electrical response of the system, OIS focuses on the dynamic optical properties. Thanks to the great sensitivity of optical properties of many MIECs to the concentration of point defects, ionic insertion can be studied dynamically.[1] In contrast with previous works,[2] we employed spectroscopy ellipsometry for increasing the sensitivity of OIS, a technique able to optically detect various structural and optical attributes of a thin film, including optical constants and thickness, regardless of the substrate's optical properties.

Different case studies were devised for the demonstration of OIS technique capabilities. First, we investigated the OIS response to oxygen insertion in La_0.5Sr_0.5FeO_3−δ (LSF50) thin films deposited on Yttria-stabilized Zirconia electrolyte. By comparing EIS and OIS we demonstrated that while the former measures the variation of charge in time (i.e. current) of the system, the latter probes the variation of chemical charge in the layer (i.e. variation of charged ions). Moreover, we showed that, as long as stimuli-response linearity is maintained, OIS can be used to retrieve the chemical capacitance without employing an ellipsometry optical fitting model. Then, we applied OIS for the characterization of the oxygen insertion in SrFeO_3−δ (SFO) thin films, a material characterized by a complex topotactic phase transition under reducing conditions. Studying the OIS response as a function of different equilibrium potential, we demonstrated that OIS technique can be employed to obtain the concentration of oxygen in a material with a complex defect chemistry such as SFO. Finally, we showed that OIS can be also employed for study the kinetics of ion insertion in LSF50 thin films in liquid alkaline electrolyte, where a significant contribution from ion diffusivity is observed. Overall, OIS is presented as a powerful technique for studying equilibrium and kinetics of ionic insertion in MIEC thin films.

The composite of nickel and yttria stabilised zirconia (Ni-YSZ) is a frequent choice as the fuel electrode in solid oxide cells (SOC). Despite progress in electrode research, many degradation mechanisms are still up for debate and degradation is usually correlated with operating conditions. However, the determination of degradation mechanisms during device-scale testing remains complicated due to the intertwined effects of temperature, gas phase, overpotential and material properties. The microstructural variations in the porous electrodes introduce further uncertainties as local conditions vary and destructive sample preparation is required to assess the microstructure. This makes it difficult to directly relate microstructural changes to the electrochemical measurements.

Hence, complementary tools and experiments are warranted. Here, we present an approach based on a thin nickel film/YSZ electrode featuring a well-defined and controlled triple phase boundary for the conversion of H₂O to H₂ or vice versa without distributions in temperature and gas phase. Sample fabrication in clean room facilities using photolithography and magnetron sputtering provides micrometre-controlled electrode dimensions. Single atmosphere testing with a non-polarised control probe electrode grants the opportunity to separate potential induced degradation from temperature and gas phase driven effects.

During durability tests, the impedance response is measured while the phase stability and defect concentration is monitored by operando micro-Raman spectroscopy. The restructuring of nickel is tracked by ex-situ atomic force microscopy and electron microscopy, whereas x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy are used to analyse material compositions. In addition, alloying of nickel and co-doping of YSZ are investigated for a possible suppression of degradation and to improve the mechanistic understanding. Results on nickel mobility, impurity effects, and interfacial contact loss are presented for Ni/YSZ, alloyed Ni, and co-doped YSZ.

Transition metal doped SrTiO₃ is of significance for devices in energy^1,2, information^3,4, and catalysis⁵. With the growth characterization of Ni-doped SrTiO₃ (Ni:STO), spontaneous formation of separated nano-phases is observed. In view of controlling the formation of such nano-phases and in a wider context for vertically aligned nanocolumn engineering⁶, our focus is on the growth mechanism of Ni:STO with Ni dopant enrichment in designated areas of the perovskite lattice, associated to the occasional formation of nano-phases embedded in a Ni-doped STO matrix.

Scanning Tunneling Microscopy and Spectroscopy (STM/S) are unique surface techniques for mapping both precise topography and electronic properties down to the atom scale, making them a promising instrument for studying nanostructures, even within device structures⁷.

In this study, we investigate the reflection high-energy electron diffraction-controlled pulsed laser deposition (PLD) grown 0.25 unit cell (u.c.), 0.5 u.c., and 10 u.c. Ni:STO on Nb-doped SrTiO₃ (Nb:STO) using low-temperature (LT) STM/S at 4.6 K in ultra-high vacuum (UHV). Upon in situ transfer to the LT-STM, the as-deposited samples with less than 1 u.c. show surface morphology with Ni:STO islands and bare Nb:STO spaces in between the islands, called trenches. Surprisingly, the trenches of the 0.5 u.c. sample already show specific patterns that can be corresponding to the nanocolumn patterns on a thick film, similar to vertically aligned nanostructures (VAN) morphologies observed in the literature.⁸ Film thickness of about 4.1 Å (lattice constant of STO~3.9 Å) is observed on both the 0.25 and 0.5 u.c. samples. Above the monolayer Ni:STO, homogeneously dispersed nanoparticles with diameters ranging between 2 and 3 nm exhibit low-density distribution. Local electronic properties of the nanostructures are investigated by performing STS and dI/dV, which yield spectra proportional to the local density of states. A bandgap of about 3.4 eV is seen on the Ni:STO film (tip-induced band bending takes part in the STS measurement), while the nanoparticles show a similar bandgap with extra sharp states 1.1 eV above the Fermi level, indicating increased p-doping concentration in the oxide nanoparticle. The STS on the Nb:STO substrate reveals a broader bandgap, exceeding 4 eV, attributed to the formation of Sr vacancies creating a space charge at the surface region⁹.

The study presents the very early stage formation process and the dopant distribution of the embedded nano-phase Ni:STO, allowing for improved control of such nanostructures, likely also relevant to understand the nucleation of VANs.

To mitigate the climate change and reach a carbon neutral society before it is too late, a mix of sustainable energy technologies are needed.

Batteries will continue to play a vital role in decarbonising transportation as well as in storing the intermittent renewable energy. Li ion batteries have revolutionised the electrification of transportation and contributed significantly to grid storage. However, there are increasing concerns with the availability of the minerals currently used in Li-ion batteries today, especially looking at the predicted growth of batteries demand. Diversification of battery technologies with more sustainable options in mind, not only for the raw minerals used in future batteries but also for more sustainable manufacturing practices for cells and packs are needed.

In my talk I will touch on some of these sustainable practices needed to be implemented today while showing the 12 principles of “green batteries” inspired from “green chemistry” my research group introduced. I will than focus on Na-ion batteries, the next battery technology in line for commercialisation in 2024, with emphasis on our research on hard carbon anodes on understanding the fundamentals on Na ion storage using a mix of characterisation techniques coupled with electrochemistry. I will also discuss the importance and complexity of solid electrolyte interfaces and some perspectives on commercialisation from our group.

I will also resent some new insights onto a very old research topic: intercalation of alkali metals into graphite. We have performed in depth study on K intercalation in graphite using a combination of Raman (including looking at the low frequency band), XRD, dilatometry, optical microscopy as well as DFT calculation which when coupled with electrochemistry and post-mortem TOF-SIMS reveal the formation of a solid electrolyte interface responsible for capacity loss in the first cycles.

I will present also on Al-ionic liquid-graphite dual ion batteriey configurations while focusing on the Al anode corrosion and degradation using XPS, XAFS and post-mortem microscopy after various cycles as well as understanding the intercalation of AlCl4- in graphite, soft and hard carbon and the difference in the storage mechanism among such classes of carbons.

Continuous solid electrolyte interphase (SEI) and dendrite growth, as well as formation of ion blocking interfaces are some of the crucial issues preventing the commercialization of high energy density batteries depending on the implementation of alkali and alkaline earth metal anodes.[1,2] Electrochemical impedance spectroscopy is a powerful tool to study processes in full battery cells, but also ion transport in naturally formed and artificial SEIs, and its evolution in time from symmetric cells. The technique can be successfully used in batteries containing liquid, solid-state or hybrid electrolytes. In this talk, an overview on the recent studies on ion transport and evolution of SEIs on Li, Na, K, Mg, and most recently silicon anodes will be given.[3-6] In natural SEIs formed in contact with liquid carbonate and glyme-based electrolytes at open circuit voltage, main findings include changes from surface to diffusion controlled growth mechanism (Li), high porosity and predominant liquid transport pathways (Na), as well as inherent mechanical instabilities (Mg). Artificial sulfide and Al₂O₃-based SEIs on Li and Na metal are porous and reactive in contact with liquid and thiosulphate solid-state electrolytes.[7,8]

The replacement of conventional lithium-ion batteries with solid-state batteries is currently being investigated by a large number of players in both the academic and industrial sectors [1]. Sulfide-based electrolytes are among the materials considered most promising, especially for applications in the transport field [2]. For this type of solid electrolyte, manually assembled cells such as Swagelok cells, EL-CELLs and self-built pressure vessels are typically used to evaluate the performance of the anode, cathode and solid electrolyte materials [3]. However, coin cells are often out of consideration. Although coin cells cannot accurately predict how a material will perform in a battery cell format in an end-use application, they are easy to assemble and can provide reproducible data in comparison to the other cell types, making them an interesting option for testing materials under conditions that are more relevant to the intended application. This round-robin test for the preparation of solid-state coin cell batteries with a solid sulfide electrolyte, a lithium nickel manganese cobalt oxide cathode and a lithium metal anode is intended for academic researchers and provides guidelines for research in this field. The cell preparation method presented in this work has been evaluated for its reproducibility and can be modified depending on the parameters used to optimize the solid electrolyte, the cathode material, the bilayer consisting of cathode and solid electrolyte, the lithium metal anode and the cell in general. Defined electrochemical tests of a sulfide solid-state battery in coin cell configuration are measured in a round-robin test between four laboratories (see Figure 1).

All solid-state fluoride ion batteries can be based on intercalation (e.g. La₂NiO4) as well as conversion-based electrode materials (e.g., BiF₃, CuF₂, ZnF2, etc.) with solid electrolytes being typically tysonite-type of fluorite-type alkaline eart / rare earth fluorides (e.g. La_0.9Ba_0.1F_2.9 or La_0.6Ba_0.4F_2.6). If they want to compete with existing battery technologies, significant improvements in terms of cyclic stability are necessary to fully access the high specific capacities this battery concept can provide in theory.

In this contribution [1], we report the development of a high-pressure, high-temperature battery operation stand for battery cycling under inert conditions inside a glovebox. This stand was then used to investigate the effect of stack pressure on the cell performance of conversion-based as well as intercalation-based electrode materials for fluoride ion batteries. It was found that cyclic stability as well as energy efficiency is strongly increased compared to non-pressure conditions, which is assigned to sustained inter-particle contact. Thus, the cell-design must be considered carefully to be able to distinguish intrinsic material properties from percolation- and interphase-related impacts on the cell behavior. Further, the effect of pressure on the ionic conductivity of common solid fluoride ion conductors is investigated.
 

Today, state of the art lithium batteries are based on liquid electrolytes, due to their high ionic conductivity at room temperature. However, these batteries still exhibit shortcomings related to safety, durability and complexity of manufacturing or maintenance. For these reasons, all-solid-state lithium-metal batteries, employing solid electrolytes, have gained attention in recent decades due to their enhanced safety, chemical stability, and thermal stability [1,2].  Among solid electrolytes, ceramic ones offer superior chemical and thermal stability, resistance against dendrite growth, superior mechanical properties and wide electrochemical window. Despite their potential, their full deployment is still hindered by their performances and interfacial detrimental interactions with the electrodes [3,4]. 
Apart of the efforts of the scientific community at materials level, attention needs to be placed also at processing level. In this regard our group is exploring disruptive manufacturing processes for ceramic electrolytes and cathodes: 3D printing and ultra-fast high temperature sintering (UHS), in order to approach next generation of full ceramic devices. 
In the present study NASICON-type Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolytes were printed by stereolithography (SLA) in complex geometries difficult or sometimes impossible to produce with conventional ceramic forming techniques. Despite its benefits, only a limited number of studies have utilized 3D printing in the production of full ceramic electrolytes [5,6].  SLA is a promising method that allows the fabrication of intricate shapes with high resolution [7]. Here we present the development of SLA processes to produce LAGP full-ceramic electrolytes exploring two different geometrical approaches: the conventional flat electrolyte and a corrugated one which offer an increased interfacial area with electrodes thus leading to a reduced area specific resistance with the same projected area respect to its flat counterpart. Self standing LAGP structures were printed, debinded and sintered. Crack-free LAGP electrolytes with flat and corrugated geometries were succesfully produced  with high densification (>80%), and uniform shrinkage in all directions. Those electrolytes demonstrated to have conductivities in good agreement with LAGP manufactured with conventional techniques (10⁻⁵-10⁻⁴ S/cm) and no detrimental reactions or degradation due to the debinding step were detected. Furthermore simmetrical cells Li/LAGP/Li flat and corrugated were tested in plating/stripping galvanostatic tests up to 200h. 
The printed electrolytes were also coupled with Co-free LiFePO₄ (LFP) cathodes printed by robocasting and attached by conventional thermal treatments (600ºC, 4h, N₂ atmosphere). Even if 3D printing of LFP cathodes by robocasting has been reported in literature before it was limited to Li-ion batteries (employing liquid electrolytes) [8]. Our approach will potentially open doors to a full-printed, full-phosphate ASSB.
Simultaneously, we are developing UHS for the same electrolyte and cathode materials. UHS was introduced in 2020 as a sintering technique able to densify ceramics in seconds, with the promise of significantly decreasing both the production times and energy usage respect to conventional time-consuming and energy hungry thermal processes [9]. This method also aims to mitigate degradation phenomena and minimize lithium losses [9-11]. Results on UHS of the raw materials and printed pieces will be presented. LAGP was sintered by UHS in less than 2 minutes reaching very high densification (88%) and ionic conductivities comparable with the ones obtained by conventional sintering (10⁻⁴ S/cm). Furthermore, LFP was successfully attached on LAGP electrolytes by this technique avoiding detrimental interfacial interactions. Finally, UHS was successfully applied to 3D printed LAGP electrolytes lowering the times needed for debinding and sintering from days to minutes.
In conclusion, we believe that the combination of 3D printing (performances enhanced by design) and UHS (limitation of Li losses and degradation phenomena) will pave the way to the next generation of Co-free full-phosphate batteries with complex geometries.

Ni-YSZ is the state of the art fuel electrode in solid oxide cells, but microstructural changes often occur near the electrode/electrolyte interface during operation that can cause performance degradation. This talk will review current experimental results and present phase-field simulations of microstructural evolution, with the initial structures obtained from measured three-dimensional Ni-YSZ microstructures. Ni migration and particle disconnection, as reported by some groups, is investigated by imposing a spatial gradient in Ni/YSZ interfacial tension.  An analysis of the electro-wetting effect at Ni/YSZ interfaces is presented to explain the interfacial tension gradient. Both surface diffusion and capillary driven evaporation/condensation are explored as possible transport mechanisms; the latter is found to be too slow to explain observed Ni migration at normal operating temperatures. Effects of different Ni-YSZ starting microstructures, e.g., different Ni/YSZ ratios, are also explored. Results from analytical calculations of Ni migration velocity, assuming that the interfacial tension gradient arises from electro-wetting and that transport is via surface diffusion, are also presented.

Seeking effective approaches for the efficient utilization of CO₂ emission is desperately interested in the development of a sustainable community. Converting CO₂ into highly valuable synthetic chemicals would be a promising way for energy storage. Using solid oxide electrolysis cells (SOECs) driven by renewable energy is expected to be one of the most efficient ways to convert electricity and industrial waste heat into chemical energy. Considering efficient energy conversion, recent development has focused on the simultaneous electrolysis of both CO₂ and H₂O in SOECs to directly produce industrially useful hydrocarbon feedstocks (e.g. H₂/CO “syngas” mixtures) and fuels by means of well-established Fischer–Tropsch (F–T)-based processes in the chemical industry so that a “Power-to-Syngas” concept can be fulfilled. For practical application, developing a highly active fuel electrode with sufficient electrolysis current density and low degradation rate is an essential task for CO₂/H₂O co-electrolysis. In addition, to achieve the direct synthesis of useful fuels, controlling CO/H₂ syngas product ratios from the feed gas of CO₂/H₂O is essential. However, the key challenge is particularly the significant effect of water gas shift reaction (WGSR) at relatively low temperatures (< 800^oC).

In previous studies, we have discovered several potential fuel-electrode candidates, such as La(Sr)Fe(Mn)O₃^[1] perovskite and CuFe₂O₄^[2] spinel for high-temperature CO₂/H₂O co-electrolysis. To facilitate their accessibility in intermediate temperatures; that is, particularly to suppress the influence of WGSR, their electrolysis performance with sufficient electrolysis current and appropriate syngas control is required to be further improved. Through an intuitive technique, infiltration of binary metal oxide,^[3] fabricating active nanocatalysts over electrode matrix has shown intensively growing interest in designing highly efficient fuel electrodes for the application of CO₂ and H₂O electrolysis.

In this study, we investigated the influence of electrolysis performance by introducing various selected functional binary oxide catalysts such as lanthanide and transition metal oxides into the scaffold of the potential La(Sr)Mn(Fe)O₃ and CuFe₂O₄ fuel electrodes by the infiltration technique for intermediate-temperature CO₂/H₂O co-electrolysis. We found that by the single- and multiple-infiltration of the selective oxides on these potential electrodes, significantly enhanced electrolysis current density and syngas product control can be achieved. For example, one of the particularly promising results shows a superior electrolysis current density (~2 A/cm² at 800^oC) can be achieved by Ce-infiltration, accompanying with markedly decreased internal resistances of the cells and exceptional Faradaic efficiency (nearly 100 %). The cell performance can also be tuned by the adjustments of infiltration concentrations and selecting multiple infiltrations of different binary metal oxides. In addition, syngas control can be significantly improved through varying oxide infiltrations and feed flow rates to increase CO₂ conversion, resulting in a considerable suppression of water gas-shift reaction, and facilitating the operation in the intermediate-temperature range.

Composite materials based on molten salts and solid oxide phases have been suggested in the 90's as promising electrolytes for intermediate temperature fuel cells. The conductivity as a function of temperature presents a peculiar variation, with a huge gain of 2-3 orders of magnitude around the melting temperature of the salts. Conductivity close to 0.1 S.cm^-1 can be reached at 500°C as in the case of Sm-doped ceria (SDC) associated to (Li,Na,K)₂CO₃ ternary eutectic composition. Our work aimed to go deeper in the determination of species and transport mechanisms in this electrolyte. We performed systematic studies as a function of temperature and gas compositions (oxidizing and reducing atmospheres, wet or dry) by impedance spectroscopy. DSC/TGA additional analyses, as well as post mortem X-Ray Diffraction were carried out.  All results yielded to the important role of interfaces between molten carbonates and solid oxide phase, thus a partial transformation of carbonates into hydroxides in H₂-rich atmospheres, accelerated in presence of water and avoided with CO₂ addition [1]. This allowed us to define the atmospheric conditions for fuel cells / electrolysis operations, as well as the design of the electrodes. The reversibility has been highlighted with "symmetrical cells", as well as power density of 0.4 W.cm-2 at 0.7V in these conditions. 

In this research, we address the critical challenge of fabricating a thin and dense Gd-doped ceria (GDC) buffer layer to impede interfacial elemental diffusion between the yttrium-doped zirconia electrolyte and the perovskite functional cathode within metal supported solid oxide fuel cell (MS-SOFC). To overcome the limitations of conventional sintering processes (>1400 ºC), we successfully synthesized GDC particles with a precise size of 3 nm using a novel precipitation method. The subsequent application of a spin-coating process enabled the formation of a thin (<1 μm) buffer layer at temperatures below 1000 ºC. Microstructure analysis at varying temperatures confirmed the efficacy of the synthesized buffer layers. These advancements were applied to a cutting-edge MS-SOFC, manufactured through co-firing in a reducing atmosphere. Electrochemical investigations, employing 2-probe AC impedance and DC measurements, highlighted the superior performance achieved through the optimization of microstructure in each layer and the incorporation of a thin ceria-blocking layer. This study presents a groundbreaking approach to enhance the efficiency and feasibility of MS-SOFC, emphasizing the significance of dense GDC buffer layer at low temperature and its application in next-generation metal-supported fuel cell technologies.

The insufficient long-term stability of Solid Oxide Cells (SOCs) is one of the major drawbacks, limiting commercial usage of these electrochemical devices. The main reason for possible damage or degradation comes from the thermal expansion coefficient (TEC) mismatch between the electrode and electrolyte, which causes unwanted strain, cracks, and layer delamination. The problem may be mitigated by using negative thermal expansion coefficient materials (NTEs) that have been known for decades for their ability to decrease their spatial dimensions with increasing temperature. This interesting property along with the high electronic conductivity was recently reported for perovskite-based compounds like NdMnO₃ and Sm_1-xA_xMnO_3-δ (A = Zn, Cu; x ≤ 0.15) [1,2,3]. The addition of such materials to form composite-type oxygen electrodes for SOCs significantly lowers the TEC, adjusting it to the chosen electrolyte material, and diminishing possible degradation mechanisms. Therefore, the appropriately designed and produced composite electrode should possess suitably low polarization resistance, as well as is expected to deliver enhanced long-term stability, which in effect would greatly increase not only the performance but also the practical applicability of the SOC-based devices.

Double perovskite oxides from REBa_0.5Sr_0.5CoCuO_5+δ (RE: Pr, Nd, Sm, Gd) group are known for their high mixed ionic-electronic conductivity, as well as suitable electrocatalytic activity, and are reported as attractive oxygen electrode material for SOCs. In this work, those materials were prepared by the typical sol-gel method or the electrospinning method. Among initially studied compositions, SmBa_0.5Sr_0.5CoCuO_5+δ (SBSCCO) was found to be the best candidate, exhibiting demanded physicochemical and electrocatalytic properties. Also, this single-phase material is chemically stable and compatible with LSGM electrolyte. The total conductivity of SBSCCO is around 100 S·cm^-1 between 300°C and 900°C, while if used as the oxygen electrode, the measured polarization resistance (R_p) in SBSCCO|LSGM|SBSCCO symmetrical cell was only 0.17 Ω·cm² at 800°C. The long-term stability tests showed that the relative increase of R_p is around 20% after 5 days at 700°C.

To further enhance the characteristics of the SBSCCO electrode, selected NTEs: Sm_1-xZn_xMnO_3-δ (SZMx) and Sm_1-xCu_xMnO_3-δ (SCMx) were synthesized through the solid state reaction, with final annealing performed at 1300°C for 10 h in air. The conducted dilatometry measurements confirmed the negative TEC for those materials, ranging from -0.1·10^-6 1/K to -11·10^-6 1/K between in the 200-900°C range. Also, the total conductivity of SZM15 was found to be around 20 S·cm^-1 at 900°C. Then, the previously obtained SBSCCO and chosen NTEs were used to prepare several composite oxygen electrodes, by adding different amounts of the respective powders. The preliminary results show that the R_p of the composite electrode with 10 wt.% addition of SZM15, as measured in the SBSCCO:SZM15|LSGM|SBSCCO:SZM15 symmetrical cell in the 700-900°C range, was significantly lower when compared with the unmodified SBSCCO electrode. For instance, R_p at 800°C was 0.12 Ω·cm² giving a 30% relative decrease. Additionally, the I-V characteristics were measured for commercial anode-supported fuel cells comprising Ni-YSZ|YSZ support, GDC10 buffer layer, and the SBSCCO:SZM15 composite electrode. The maximum peak power density recorded at 800°C was 850 mW·cm^-2, which is around 60% higher when compared to the same cell prepared with the SBSCCO electrode without NTE addition.

All the obtained results clearly show that the state-of-the-art innovative application of NTEs in SOCs indeed allows for decreasing the TEC of the composite and adjusting it to the solid electrolyte, but also enables a significant decrease of the polarization resistance. Further experiments are ongoing, regarding the long-term performance of the composite electrodes, SEM and TEM measurements involving the electrolyte/electrode interface.

Recent years have seen monumental and exciting developments in the field of all-solid-state batteries (ASSBs). I will talk about several exciting scientific breakthroughs that have led to a renaissance of solid state chemistry. Despite its promises, solid state batteries still face a multitude of technical hurdles before commercialization can be achieved. Amongst these challenges, none are more daunting than the ability for scale-up prototyping, specifically, enabling technology transition from the laboratory to the pilot scale. A vast majority of ASSB reports to date are still limited to form factors impractical for actual device operation. In this plenary talk, I will provide a perspective on a wide range of scalability challenges and considerations for ASSBs, including solid electrolyte design and synthesis, dry electrode processing, cell assembly for optimum interfacial control, and stack pressure considerations at the cell level. More importantly, I will discuss ways to bridge the development gap between fundamental research and applications, through partnerships and collaborations between the industry, univeristies and national laboratories. 

The ability to design structures that can regulate and control the ionic functionalities of electroceramics for applications in a wide range of devices requires a fundamental understanding of transport pathways. Ionic transport typically takes place in the presence of a wide range of defects including dopant species, dislocations, precipitates, grain boundaries, and surfaces. The ability to elucidate the relationship between local structure and transport remains an ongoing challenge for both experimental and computational techniques. Modern transmission electron microscopy (TEM) allows the structure and composition of electroceramics to be characterized down to the atomic level. However, the dream of in situ electron microscopy is to characterize the transport and reactivity functionalities with high spatial resolution. Can transport/diffusion processes be directly observed at the atomic level? What does such a statement even mean?

Molecular timescales are typically in the picosecond range and atomic level information is available with advanced computational methods like ab initio molecular dynamics(Kang, Vincent et al. 2022). But large-scale complex systems with processes involving correlated atomic motions remain computationally inaccessible. Experimental ultrafast approaches to high resolution TEM can yield time resolutions of picoseconds, at least for cyclic processes, but the associated spatial resolution is limited to 2 nm (Zhang and Flannigan 2019). Bridging this spatiotemporal gap is an ongoing challenge in the field. An emerging new generation of electron detectors have greatly improved sensitivity and readout rates allowing atomic resolution information to be captured on the millisecond time scale. While this remains far from molecular timescales, it does make it possible to consider looking for repetitive atomic dynamic processes, such as diffusion pathways, that may be matched to millisecond temporal resolution(Lawrence, Levin et al. 2020, Lawrence, Levin et al. 2021). The rate of structural dynamics can be manipulated and matched to the system readout speed through temperature control. For example, a process with an activation energy of 0.6 eV will manifest about 1000 jumps per second at room temperature. This lies within the range of activation energies for oxygen migration pathways in many oxygen ion conductors.

Here we use aberration corrected in situ transmission electron microscopy equipped with cutting edge detector technology to explore ionic transport and diffusion in oxides with a focus on cerium dioxide and strontium titanate. Oxygen transport is easier to detect, at least in comparison to say, lithium transport, because of the larger anion size and the significant perturbation of the cation sublattice during transport. However, even at millisecond time resolutions, the signal per frame gets very small for atomic resolution analysis with moderate electron dose rate. To compensate for this, we have developed convolutional neural networks to denoise data sets (Marcos-Morales, Leibovich et al. 2023). With oxygen transport, local short-lived structural changes can be rather subtle to detect. Consequently, novel sophisticated image processing techniques must be employed to characterize the nature of the structural dynamics (Thomas, Crozier et al. 2023). This presentation will review what is currently possible in measuring atomic resolution structural dynamics in electroceramics and how the field is likely to evolve over the near future.

In a search for new, improved mixed ionic-electronic conductors (MIEC) exhibiting a desired set of structural, thermomechanical, transport, and electrocatalytic properties, various complex perovskites and related oxides have been proposed i.a. as excellent oxygen electrode materials for Solid Oxide Cells (SOCs). Apart from their relatively complex and often highly anisotropic structural features, usually manifested through formation of various superstructures, such oxides may also show compositional complexity, with the emerging group of the high entropy materials being developed as well. In this work several systems are described in more details, including the A-site layered RE(Ba,Sr)Co_2-yMn_yO_5+δ (RE: selected rare-earth cations; 0 ≤ y ≤ 2), for which, depending on the Mn content, the physicochemical properties can be optimized for the SOC, or alternatively, for oxygen storage-related applications [1, 2]. In another group, RE(Ba,Sr)Co_2-yCu_yO_5+δ, doping with copper is found to occur in a limited range, however, materials at the limit of formation of solid solutions may exhibit enhanced electrocatalytic properties [3]. Together with the appropriate other characteristics, as well as significantly decreased cobalt content, they appear as very good candidates for SOC oxygen electrodes. Importantly, depending on the chemical composition and preparation method (e.g. by electrospinning), some of those oxides can be obtained in a single perovskite structure, showing altered properties in comparison to their respective double perovskite counterparts. Also, pure Cu-based perovskite-related oxides with a general formula of La_1-x(Ba,Sr)_xCuO_3-δ are found as very promising candidates for SOC oxygen electrodes, however, possibility of obtaining single phase compounds is relatively limited in the considered system, and often, the oxygen sublattice shows strong ordering, which hinders the ionic conductivity [4]. In this aspect, finding systems with the (partially) disordered oxygen vacancies is reported as beneficial, especially regarding electrocatalytic activity at moderate temperatures [5]. In another approach, multicomponent at either the A- and/or B-site perovskites have recently become of interest, however, the actual role of the high entropy in terms of influencing the properties (in terms of variation from the expected solid solution-based rules) is not yet understood [6]. Finally, the (small RE cation-based) REMnO₃-type hexagonal oxides, with compositions close to the perovskite-hexagonal phase transition border, can be optimized for the temperature swing absorption processes, which enable oxygen separation from air at relatively low temperatures of 200-300 °C [7]. The oxides show unique set of the physicochemical properties, including presence of interstitial oxygen defects. This work is summarized with several guidelines, which should be helpful for designing and obtaining such complex oxides exhibiting the desired useful properties.

The technology of solid oxide fuel cells (SOFCs) is widely considered to be one of the essential components of the next-generation power grid, offering unmatched efficiency, low pollution level, and the possibility of both producing (fuel cell mode) and storing energy (electrolysis mode). Its main deficiencies are high operating temperatures and still limited longevity, both factors being affected primarily by insufficient performance and degradation of the cathode materials. Unfortunately, in most state-of-the-art solutions, the excellent performance of materials walks hand in hand with their susceptibility to deterioration under cell’s operating conditions. A common reason for this behavior is the presence of alkali ions, mainly Sr and Ba, which on the one hand are considered indispensable for achieving sufficient catalytic activity, but on the other tend to form secondary phases, such as carbonates, which over time limit the amount of active cathode sites. One of the possible solutions proposed to counter these effects is the application of the high-entropy approach, which enables obtaining properties beyond the rule-of-mixture, and therefore overcoming some of the limitations of more conventional analogs. However, strict adherence to the high-entropy guidelines might become a limitation by itself, as the more elements we combine, the more difficult it becomes to preserve the single-phase structure and avoid the unwanted interactions between them.

In this study, we present the possibility of obtaining highly functional, alkali-free perovskites of superior performance, obtained through the progressing simplification of the starting La-Co-Cu-Fe-Mn-Ni and La-Cu-Fe-Mn-Ni-Ti high entropy systems. The properties of all materials are systematically studied, including their structure, oxygen nonstoichiometry, thermal expansion behavior, electrical properties, and stability against both typical electrolytes and under the operating conditions of the fuel cell. For the selected, most promising materials, the level of electrochemical performance is assessed using both symmetrical cells (cathodic polarization value R_p) and full cells (power output). The results show that most of the beneficial properties characterizing the base, high entropy systems, can be preserved in simpler, 3- and 4-component compositions while at the same bringing several new features, thanks to eliminating some of the suboptimal elements from the mixture. The electrochemical performance of the best materials, such as La(Co,Cu,Fe)O₃ and La(Cu,Fe,Ni)O₃, despite the lack of alkali ions is superior to the state-of-the-art LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ) by a considerable margin, reaching the cathodic polarization value of 0.15 Ω·cm² in the vicinity of 675 °C, making these materials potential candidates even for the low-temperature SOFCs (LT-SOFCs). This is especially impressive for the La(Cu,Fe,Ni)O₃ composition, which not only does not contain alkali ions, but cobalt as well. Overall, it can be stated that the proposed approach can lead to materials characterized by a superior combination of functionality and electrochemical performance with respect to the currently utilized alternatives, while simultaneously addressing some of the most pressing issues of the SOFC cathode materials.

The anxiety for long driving range and for reducing the cost of lithium-ion cells is pushing to increase the nickel content in layered oxide cathodes. This is because Ni is less expensive and more abundant than Co. As the Ni^3+/4+: 3d band barely touches the top of the O^2-: 2p band, Ni³⁺ can be oxidized all the way close to Ni⁴⁺ without much oxygen loss from the layered lattice, resulting in capacities of as high as 240 mAh/g. In contrast, the overlap of Co^3+/4+: 3d band with the top of the O^2-: 2p band leads to an oxidation of O^2- ions to oxygen beyond about 50% charge, resulting in a limited capacity of < 150 mAh/g. However, increasing the Ni contents above about 70% is met with a few hurdles: (i) The tendency of Ni³⁺ to get reduced to Ni²⁺ at the synthesis temperatures of about 700 ^oC necessitates a high stream of oxygen gas flow, introducing scale-up challenges for obtaining high quality materials with consistency. (ii) The instability of Ni⁴⁺ in contact with the liquid electrolyte in the charged state introduces aggressive surface reactivity, which results in rapid capacity fade and thermal runway. The surface reactivity is accompanied by loss of oxygen from the surface, reduction of Ni⁴⁺ to Ni²⁺ on the surface and formation of resistive rock-salt phases, formation of cracks, loss of active lithium in cathode-electrolyte interphase formation, Ni dissolution and migration to the anode due to the electronically driven lattice instability associated with the Jahn-Teller active Ni³⁺, and consequent capacity fade during extended cycling [1]. The reactivity also leads to thermal instability, oxygen loss, gas evolution, and safety concerns [2]. These challenges become exponentially aggravated with Ni contents > 80% and even much more for > 90%.

To overcome the challenges, generally doping of high-Ni cathodes with various dopants is pursued. However, no clear understanding is available in the literature on which dopant does what. Doping is generally carried out randomly in the literature by a trial and error process. This presentation will provide a systematic investigation of doping LiNiO₂ with 5% or 10% of various common dopants like Co, Mn, Al, and Mg. The effect of dopants on capacity, cycle life, thermal stability, and gas evolution will be presented. The surface reactivity depends both on the surface characteristics of the high-Ni cathodes and the electrolyte. This presentation will then focus on the development of cathodes with a robust surface as well as more stable electrolytes to overcome the challenges. The ability to achieve long cycle life, while reducing the amount of gas evolution, by tuning the cathode and electrolyte compositions will be presented. Also, the fundamental understanding developed with the use of advanced analytical techniques, such as in-situ XRD, SEM, XPS, and TOF-SIMS will be discussed.  

The spinel material LiNi_0.5Mn_1.5O₄ is a good candidate as a positive electrode material for Li-ion batteries, thanks in particular to its high energy density and the absence of cobalt. But the performance of this material is highly dependent on the stoichiometry and order of the two transition metals. These parameters are closely linked to the conditions under which the material is synthesized.

Molten salt synthesis has enabled us to obtain a multi-faceted platelet material for which, by varying the temperature and atmosphere of the heat treatment, it is possible to control the order of the transition metals in the absence of impurities detectable by diffraction [1]. The electrochemical properties of this platelet material are fully comparable to those of a material with a classical octahedral morphology.

Raman and NMR spectroscopies were used to explore the local order in these platelet materials. 4D-STEM was used to observe transition metal order at the particle level with a spatial resolution of 10 nm. By comparing two globally ordered materials with a difference in order homogeneity at the particle level, we were able to show that heterogeneity in transition metal order was beneficial to lithium mobility, enabling a globally ordered material to achieve electrochemical performance comparable to a disordered material.

A large proportion of the batteries marketed worldwide are Li-ion batteries, which generally use a mixture of lithium salts and organic liquid as an electrolyte (LE). But this technology poses several issues, the main one being safety due to the high flammability of the organic solvent, and the energy density also reaches limits with LE since Li metal cannot be used as negative electrode. All solid-state batteries could provide a suitable alternative by replacing LE with a solid electrolyte that is less flammable and allows lithium metal to be used as a negative electrode to increase energy density [1]. The thiophosphate materials family is one of the most promising ones owing to their good ionic and suitable mechanical properties. However, the numerous solid-solid interfaces especially the electrode/electrolyte ones are complex to handle as it is one of the principal causes of electrochemical fading [2]. Especially on the positive electrode, the optimal contact between the two solids promotes the chemical decomposition due to the contact between an oxide (electronic conductor) and a thiophosphate (ionic conductor), leading to the creation of several insulating products that generate an increase in impedance. Composite electrode engineering is not an easy task due to the intrinsic engineering of the solid-state batteries where there must be no (or almost no) porosity after sintering to improve wetting between the active material and the electrolyte. Indeed, the contact is the sole option to guarantee an optimized electrochemical activities and transport properties [3]. Here, we proposed to investigate an Li₆PS₅Cl/NCM523 composite electrode prior cycling to understand the impact of the electrode engineering such as the ratio of the solid electrolyte/electroactive materials, the nature of the electronic/ionic conductor. The investigation will be carried out by means of electrochemical impedance spectroscopy performed at different temperature coupled with several advanced characterizations performed at ESRF synchrotron (XRD, XAS). The goal being to find the optimal ratio to ensure a proper ionic and electronic conductivities while giving satisfactory electrochemical performance.

Chemo-mechanical durability challenges in electrochemical applications, arising from chemically-induced strains, drive the search for near-zero-strain materials. We focus our studies on ABO₃ perovskite oxides as prototype structures, evaluating the impact of B-O bond features on redox and hydration coefficients of chemical expansion (CCEs). CCEs normalize the chemical strain to the stoichiometry change, and our goal is to enable efficient design of near-zero-CCE materials through uncovering crystal chemical descriptors.  We combine in-situ neutron diffraction, X-ray diffraction, dilatometry, and thermogravimetric analysis, with ex-situ X-ray absorption spectroscopy and DFT simulations to quantify CCEs and correlate structure and behavior across multiple length scales. Contrary to expectations, when considering a wide enough range of zirconate and cerate proton-conducting compositions, hydration CCEs do not scale monotonically with lattice parameter nor with chemical strain anisotropy.  Instead, we have found that there appears to be a minimum in hydration CCE for perovskites with intermediate octahedral tilt angles, which can accommodate hydration through the largest decreases in B-O-B angles. This mechanism does not significantly enlarge the lattice parameters. For redox CCEs, the charge distribution in the B-O bonds of mixed ionic/electronic conductors plays an important role. While we have observed minimal redox CCEs for systems where redox takes place fully on the anion rather than the cation, we also sought to quantify the link between octahedral tilt angles, hybridization, and CCEs.  When comparing a limited set of ferrites where tolerance factor is varied by changing the A-site cation, lower CCEs correlate to both lower B-O-B angles and higher degrees of hybridization. Ongoing work expands these studies to a wider compositional range with other B-site cations to verify these relationships.

 Electrochemical conversion from fuel to electricity and vice versa, with high efficiency, is a key technology for achieving carbon neutrality. Electrochemical devices that operate at intermediate temperatures of around 300°C are an undeveloped technological area and thus offer promising possibilities for new applications. Particularly, in terms of catalytic reactions, the intermediate temperature range is expected to provide better control over product selectivity and reaction activity compared to low-temperature or high-temperature devices. Currently, only a few electrochemical devices for CO₂ conversion in this temperature range have been reported, utilizing CsH₂PO₄ proton conductors as electrolytes [1,2].

 In this study, instead of CsH₂PO₄, which lacks long-term stability, a new type of electrochemical cell was fabricated using a phosphate glass proton conductor developed by alkali-proton substitution (APS) method [3,4] as the electrolyte in combination with a cermet-type porous substrate.

 Previously, single cells for fuel cells were fabricated by hot pressing glass electrolyte blocks onto Pd plates [5]. However, for practical reasons, such as cell scale-up and cost considerations, it became necessary to develop a cell with an electrolyte fabricated on a conventional support. Therefore, we developed a process to form a glass electrolyte on a porous cermet, similar to those used in SOFCs and SOECs. The glass electrolyte, with a composition of 36HO_1/2 - 4NbO_5/2 - 2BaO - 4LaO_3/2 - 4GeO₂ - 1BO_3/2 - 49PO_5/2 [3], was prepared by APS method and then powdered for cell fabrication. This powder was coated on a porous Ni-Gd_0.1Ce_0.9O_1.95 cermet support (φ18mm), and a dense glass electrolyte layer was formed by hot-pressing at 330^oC. The electrolyte layer's thickness, around 40μm, was confirmed by cross-sectional SEM observations. The proton conductivity of the glass electrolyte remained consistent post hot-pressing, measuring 2x10^-3 Scm^-1 at 300°C under H₂ atmosphere using a Pd sputtered film electrode (φ10mm). This demonstrates that the powdering and hot-pressing procedure did not affect the proton conductivity. We successfully fabricated a cermet-supported type phosphate glass electrolyte electrochemical cell. To confirm its potential in CO₂ reduction reactions, experiments were carried out using cermet-supported type cell employing Ru fine particles as the electrocatalyst. 100% H₂ gas was supplied to the anode and 100% CO₂ gas to the cathode at 300^oC and the exhaust gas was analyzed by gas-chromatography. When a voltage of 5V was applied, corresponding to the direction of proton transport from the anode to the cathode, hydrogen production was observed, and CO and CH₄ gases were also detected. Faraday efficiencies for each gas production were 19%, 12% and 68% for H₂, CO and CH₄, respectively. The high Faraday efficiency for CH₄ production indicates that the supplied H₂ gas is being efficiently utilized in the reaction with CO₂, and this result proves that our cell can be effectively used for CO₂ reduction. The ability to electrochemically control the potential of reactants opens up the possibility of developing a new field of research in reaction chemistry using ionic conductors. Currently, we are carefully examining whether an electrochemical reaction acceleration effect is being observed.

In the vast majority of technologically relevant electrochemical devices, the reactions of interest occur at interfaces, where defects often serve to mediate the reaction pathway. In oxides, point defects, and in particular surface oxygen vacancies are generally the relevant defects in mediating electrocatalysis. Furthermore, surface defect formation energies are often lower than those of the bulk, resulting in defect enrichment on surfaces. In the case of cerium dioxide (ceria), an exemplary (electro)catalyst material, computational prediction of enhanced oxygen vacancy concentrations on {011} surfaces has motivated the development of synthetic routes for preparing nanoparticles terminated with these surfaces. In many instances, such nanoparticles indeed display higher activity for reactions of interest than those with predominant {100} or {111} termination. Yet, studies quantifying the concentration and crystallographic environment of such defects, particularly under relevant reaction conditions, are scarce. Here we demonstrate glancing angle X-ray absorption spectroscopy about Ce L edges as a tool for studying the surface features of epitaxial ceria thin films. Analysis of the near-edge (XANES) portion of the data yields the Ce³⁺ concentration, whereas the thin-film geometry avoids ambiguities that otherwise arise from the corners and edges of nanoparticles and provides access to (001), (110) and (111) termination via selection of the substrate (yttria-stabilized zirconia) termination. The use of X-rays enables easy integration with environmental chambers for gas and temperature control, a stark contrast to electron-based methods. At selected conditions we find the surface Ce³⁺ concentration to be surprisingly insensitive to termination. Using a series of angle-resolved measurements, we extract the depth profile for the (111) termination at high temperature (800 °C) and reducing conditions (oxygen partial pressure of 10^-19 atm). We find the surface to be essentially fully reduced, followed by a sharp drop in Ce³⁺ concentration to the bulk value within approximately 5 nm of the surface. The implications of these findings on (electro)catalytical reaction pathways and rates is discussed.

The evolution of materials under irradiation is driven by the enhanced transport of atoms during irradiation. To understand, predict, and ultimately design new materials destined for nuclear energy applications, it is imperative to understand the motion of these atoms. Currently, there are no probes to directly measure this transport during irradiation. Electrochemical impedance spectroscopy (EIS) has been extensively applied to these types of materials from the point of view of their conductivity and functionality. Los Alamos National Laboratory (LANL) recently established the capability of combining electrochemical measurements with ion beam irradiations to develop a first-of-its-kind capability to directly measure the transport of atoms during irradiation. Different oxides have propensity for being radiation tolerant either through rapid self-annealing of radiation induced defects or through the formation of meta-stable disordered structures or phase changes. The current work explores the changes in conductivity under light ion irradiation of oxide materials with varying levels of radiation tolerance.

Solid Oxide Electrolyser Cells (SOEC) can achieve unrivalled efficiency in converting renewable electrical energy to hydrogen and therefore are an indispensable part of our transition to a sustainable energy economy, but the technology is not yet fully developed. One approach to improve the performance of SOEC is using gadolinia-doped ceria (GDC) as electrolyte, since its higher ionic conductivity compared to the current standard, yttria-stabilised zirconia (YSZ), allows significantly higher electrolysis currents at a given voltage. However, GDC suffers from electrochemical expansion upon reduction; with pO₂ as low as 10^–19 Pa at the fuel electrode-electrolyte interface, this expansion can lead to cell cracking, limiting its lifetime. In order to design a cell that can withstand such effects, it is necessary to precisely characterize the electrochemical expansion under cathodic bias. To achieve this, we have developed an in situ cell for investigating the electrochemical expansion of GDC at relevant temperatures, in reducing gas atmosphere and with applied cathodic bias simultaneously. In our setup, the SOEC is mounted between the air compartment, which houses a heater, and the fuel compartment, which features an X-ray window for diffraction and spectroscopy. The cell is electrically contacted from both sides to monitor the cell voltage and determine the cathode overpotential. In conjunction with the X-ray techniques, operational temperature of up to 800 °C and separate air and water/hydrogen atmospheres for the air and fuel electrodes, allow conducting operando experiments on real working SOEC.

The need for new materials to tackle societal challenges in energy and sustainability is widely acknowledged. As demands for performance increase while resource constraints narrow available options, the vastness of composition, structure and process parameter space make the apparently simple questions of where to look for and how to then find the materials we need a grand challenge to contemporary physical science. This talk will emphasise that discovery synthesis of new inorganic materials is at the extreme forefront of this endeavour.

Since 2017, it has been clear that computational prediction can usefully direct the search for new structure types [1]. In the context of discovery, it is helpful to consider both appropriate definition of the word “new” and also how the claimed numbers of materials and the speeds with which they are accessed fit with the size of the space investigated, and the differences in how composition can vary between solids and molecules.

In this presentation, I will address the role of digital and robotic tools in discovery from the perspective of the experimental realisation of new materials with structures that differ from those in the databases in a manner that has consequence for their functional performance. This will include the demonstration that it is now possible under clear assumptions to guarantee to predict the crystal structure of a material based solely on its composition [2], the role of machine learning from data in supporting decisions by experimental researchers [3], and the acceleration of inorganic materials discovery with robots [4].

The role of these digital tools in a modern integrated materials discovery workflow will be presented with an example of the discovery (i.e., the experimental realisation in the laboratory) of a quaternary inorganic solid that displays high lithium ion conductivity that arises from its new structure. This leads to a different perspective on how lithium ions can attain high mobility in solids (accepted for publication, 2024). Such perspectives may prove generally helpful in the design of the fast ion transporting materials that we will need across future energy technologies.

Thin layers have traditionally represented exceptional model systems for fundamental prospects in materials science. However, the extent of such an approach to be utilized in real devices e.g., for energy conversion and storage, is still limited. To date, one of the few successful examples of a commercial thin film energy device is the lithium solid-state battery (SSB) based on a lithium phosphorus oxynitride (LiPON) electrolyte. LiPON remains as the only option in this arena thanks to its stability towards Li-metal and low electronic conductivity. However, LiPON’s low ionic conductivity (c.a. 1 μS·cm^-1) limits cells’ performance and its difficult processability hinders large-scale production. Among the alternative ceramic electrolytes, NASICON superionic solid electrolyte Li_1+xAl_xTi_2-x(PO₄)₃ (LATP) is very promising due to its good ionic conductivity (c.a. 1 mS·cm^-1) and stability in ambient air at high temperatures. Despite these advantages, all-ceramic devices based on LATP have not been reported, mainly due to interfacial reactivity between components.

In the present talk, we will present our latest advances in the implementation of thin films in ceramic SSBs and the development of novel investigation tools for materials research and device integration. With this aim, LATP and different electrode materials were deposited by Large-Area Pulsed Laser Deposition. Spinel LiMn₂O₄ and Li₄Ti₅O₁₂, and olivine LiFePO₄ were chosen, aiming to exploit their outstanding stability, low cost, and environmental friendliness. Bi-layers of LATP with these electrodes were fabricated in order to study the structure of the different interfaces. Various strategies such as the introduction of protective coatings or Rapid Thermal Processing (RTP) procedures are implemented with the aim of improving the quality of these electrode-electrolyte interfaces. The nature of such interfaces and their evolution after being subjected to electrochemical cycles has been examined from different perspectives applying a series of complementary techniques such as Focused Ion Beam-Secondary Ion Mass Spectroscopy (FIB-SIMS), Atom Probe Tomography (APT), Grazing Incidence Wide Angle X-Ray Scattering (GIWAXS), among others. Our results show the interface chemical stability and electrochemical performance of these bi-layers demonstrating their suitability for future thin-film solid state lithium batteries (SSLBs).

Solid-state batteries (SSBs) are attracting great interest leading to potentially higher energy and power densities compared conventional Li-ion batteries based on liquid electrolytes. However, they are plagued by the development of advanced solid electrolytes (SEs), mainly lacking in ionic conductivity and electrochemical stability; thus, the ongoing quest for exploration of new materials and compositions.

Inducing a large structural disorder, i.e. high configurational entropy, has recently emerged as a new strategy to overcome limitations of conventional SE materials. In this regard, few multicomponent SE materials have been reported up to now, showing favorable charge-transport properties [1,2]. However, a thorough understanding on how configurational entropy (ΔS_conf) affects ionic conductivity is lacking. Here, we successfully synthesized a solid solution series of halogen-rich lithium argyrodites with the general formula Li_5.5PS_4.5Cl_xBr_1.5−x (0 ≤ x ≤ 1.5) [3]. Using neutron powder diffraction and ³¹P magic-angle spinning nuclear magnetic resonance spectroscopy, the shared S²⁻/Cl⁻/Br⁻ occupancy on the anion sublattice was quantitatively analyzed. We show that anion disorder positively affects Li-ion dynamics, leading to a room-temperature ionic conductivity of 22.7 mS cm⁻¹ (9.6 mS cm⁻¹ in cold-pressed state) for Li_5.5PS_4.5Cl_0.8Br_0.7 (ΔS_conf = 1.98R). To the best of our knowledge, this is the first experimental evidence that configurational entropy of the anion sublattice correlates with ion mobility. Our results indicate the possibility of improving ionic conductivity in ceramic ion conductors by tailoring the degree of compositional complexity. Moreover, the Li_5.5PS_4.5Cl_0.8Br_0.7 SE allowed for stable cycling of single-crystal LiNi_0.9Co_0.06Mn_0.04O₂ (s-NCM90) composite cathodes in SSB cells, emphasizing that dual-substituted lithium argyrodites hold great promise in enabling high-performance electrochemical energy storage.

The targeted variation of oxide material properties by changing their composition is an integral part of everyday scientific work in the field of solid state ionics. This approach can be used, for example, to change the conductivity of an oxide for a specific charge carrier species, to influence the space charge that forms at an interface, or to manipulate the surface reaction kinetics of an electrode. The latter in particular has attracted a great deal of attention in recent years in several respects. On the one hand, the enormous variability of the reaction kinetics of mixed-conducting oxide electrodes has been linked to small changes in their surface chemistry, which has also opened up the possibility of compensation strategies for degradation phenomena. On the other hand, the concept of exsolution (which involves the intentional partial reductive decomposition of perovskite-type materials forming catalytically active surface particles) has emerged as an exceptionally compelling tool for preparing novel heterogeneous catalysts with highly attractive properties. What can be learnt from the investigation of such highly surface-sensitive systems is that materials with completely new electro-catalytic properties can be generated by applying only small changes that have their effect primarily on the surface.

In this contribution a number of various possibilities of manipulating the surface chemistry of mixed conducting electrodes will be explored:

(i) The perovskite-type electrode material (La,Sr)FeO_3-δ (LSF) is modified by application of tiny amounts of platinum. First, as a dopant by introducing Pt during the synthesis procedure of the oxide. This yields the remarkable result that Pt in LSF has no extra catalytic activity, but nevertheless enhances the electrode’s oxygen exchange kinetics. This at first glance peculiar behavior, however, can be explained by an influence of the noble metal dopant on the available number of redox-active sites. Second, Pt is applied as nanosized, metallic surface particles. In this form, its effect is completely different with the p(O₂) dependence of the oxygen reaction being even inversed compared to bare LSF.

(ii) The complex behavior of exsolution particles under electrochemical polarization of the corresponding parent oxide is highlighted. More specifically, the oxidation state and the associated catalytic activity of particles exsolved from ferrite-based thin-film electrodes are selectively switched by applying a bias voltage. Interestingly, in case of iron particles, the oxidation state of the particles is almost exclusively governed by the parent oxide, while nickel particles are significantly more influenced by the atmosphere. We explain the behavior with a 'kinetic competition' between the gas atmosphere and the applied electrochemical driving force with different oxygen transport kinetics in Fe- and Ni-based particles as a decisive factor. Additionally, we demonstrate voltage-controlled sequential exsolution of Ni and Fe nanoparticles, and extend our exploration to bimetallic exsolution systems with three distinct activity states.

Dynamic structural and chemical changes of catalyst materials under operation conditions play a crucial role for the activity and stability of electrocatalysts. It is essential to understand the mechanistic processes that govern the material response, oftentimes particularly affecting the catalyst surface and therefore the electrochemical interface, to improve the lifetime of energy conversion devices such as water splitting catalysts or solid oxide cells. Exploring the underlying effects may further allow to develop design principles for tailoring the functionality of energy materials.

Metal exsolution reactions can be driven in fuel electrode materials under the reducing operation conditions of solid oxide cells. The process enables the synthesis of nanostructured catalysts based on the release of a fraction of reducible dopants from a perovskite host to its’ surface and the subsequent nucleation of finely dispersed oxide-supported metal nanoparticles. The performance of such catalysts strongly depends on the nanoparticle characteristics, such as the nanoparticle size and nanoparticle density, correlated to the properties of the active triple-phase-boundaries.

We employ epitaxial thin films with atomically defined surface morphologies to study the exsolution kinetics with respect to the mass transport of Ni dopants from the bulk to the perovskite surface. In addition, we investigate the subsequent growth and coalescence behavior of the exsolved nanoparticles during the reducing thermal annealing. We demonstrate that the electrostatic interactions of exsolution-active dopants with the surface potential that is correlated to the inherent surface space charge region of perovskite oxides determines the kinetics of metal exsolution in our material system [1]. Moreover, we reveal that defects that form under the reducing reaction conditions at the surface of the perovskite support can accelerate nanoparticle clustering and hence degradation of metal exsolution catalysts [2]. Based on our findings, we derive strategies for the control of the metal exsolution dynamics and for the stabilization of exsolved nanoparticles.

In situ exsolution has emerged as an outstanding route for producing oxide-supported metal nanoparticles. A variety of transition-metal cations can be incorporated into the host lattice – typically a perovskite-oxide – under oxidising conditions and exsolved as metallic nanoclusters after reduction. Consistent and comprehensive descriptions of the thermodynamics and kinetics of this phenomenon are lacking, however.

Herein, supported by hybrid Density-Functional-Theory calculations, we propose a single model that explains diverse experimental observations; why transition-metal cations (but not host cations) exsolve from perovskite lattices upon reduction; why different transition-metal cations exsolve under different conditions; why the metal nanoparticles are embedded at the surface; why the oxide’s surface orientation affect behaviour; why exsolution occurs surprisingly rapidly at relatively low temperatures; and why the re-incorporation of exsolved species involves far longer times and much higher temperatures. Our model’s foundation is that the transition-metal cations are completely reduced within the perovskite lattice as the Fermi level is shifted upwards within the bandgap.[1] The calculations also emphasise the importance of oxygen vacancies and A-site vacancies. Our model provides a fundamental basis for improving existing, and creating new, exsolution catalysts.

Oxide-ion conductors and proton conductors have attracted considerable attention because of their potential applications such as fuel cells, and gas sensors. Exploring novel ionic conductors with high ionic conductivity is an important but challenging task. Our group tackled this task by several strategies, and successfully discovered novel ionic conductors. We also investigate ion migration path and mechanism through the precise structure analysis.

Although Dion−Jacobson phases have been investigated in various fields, there are no reports on oxide-ion conduction. Through the screening by the bond valence method, we have found that Dion−Jacobson phase CsBi₂Ti₂NbO_10−δ exhibits high oxide-ion conductivity.[1] From the high temperature neutron diffraction study, we revealed that the high conductivity is attributable to the large anisotropic thermal motions of oxygen atoms, the presence of oxygen vacancies, and the formation of oxide-ion conducting layers in the crystal structure. Ba₃Y₂O₅Cl₂ is discovered as the first examples of oxide-ion conducting oxychlorides.[2] Ruddlesden–Popper (RP) oxychlorides were expected to show high oxide-ion conductivity, because they have larger free and lattice volumes than RP oxides, leading to low activation energies. Two dimensional oxide-ion migration in Ba₃Y₂O₅Cl₂ is indicated from the DFT calculations. By investigating Ba₇Nb_4–xMo_1+xO_20+x/2 materials, we found that Ba₇Nb_3.8Mo_1.2O_20.1 exhibits high oxide-ion and proton conductivity.[3] Total direct current conductivity at 400 °C in wet air of Ba₇Nb_3.8Mo_1.2O_20.1 was 13 times higher than that of Ba₇Nb₄MoO₂₀. Ab initio molecular dynamics (AIMD) simulations, neutron-diffraction experiments at 800 °C, and neutron scattering length density analyses of Ba₇Nb_3.8Mo_1.2O_20.1 indicated that the excess oxygen atoms are incorporated by the formation of both 5-fold coordinated (Nb/Mo)O₅ monomer and its (Nb/Mo)₂O₉ dimer with a corner-sharing oxygen atom and that the breaking and reforming of the dimers lead to the high oxide-ion conduction. Ba₂LuAlO₅ exhibits high proton conductivities, high diffusivity and high chemical stability without chemical doping.[4] Ba₂LuAlO₅ is a hexagonal perovskite-related oxide with highly oxygen-deficient hexagonal close-packed h′ layers, which enables a large amount of water uptake x = 0.50 in Ba₂LuAlO₅·x H₂O. Ba₇Nb₄MoO₂₀ is one of the promising ionic conductors. However, the chemical order/disorder of Mo and Nb atoms was not revealed because Mo and Nb have similar scattering power for both X-ray and neutron. Therefore, we have combined resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations to reveal the chemical order in Ba₇Nb₄MoO₂₀.[5] NMR provided direct evidence that Mo atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. Resonant X-ray diffraction determined the occupancy factors of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively.

A relevant point in circular economy is the possibility to recycle wastes in a so-called “waste to treasure” attitude. For this approach to be economically sustainable, the purification and separation steps should be minimized, thus requiring robust catalysts. Double perovskite samples of composition Sr₂Mg_1-xMn_xMoO_6-δ (SMMO) have been considered because of their versatility towards different feeding gasses and resistance towards sulfur impurities, opening to the possible use of these catalysts with direct waste-derived fuels. [1] Catalysts characterized by different amount of Mg/Mn have been prepared (with x = 0, 0.2, 0.5 and 1) and studied as both catalysts for the Reverse Water Gas Shift (RWGS) reaction – relevant for optimization of H₂ content in syngas for E-fuels synthesis – and anode materials in Solid Oxide Fuel Cells (SOFC) – with direct utilization of the fuel thanks to the high operating temperature. The synthesis was carried out via the Pechini route, using a previously optimized quantity of ethylene glycole (EG/CA = 3). X-Ray Diffraction (XRD) has shown the formation of the desired phase, with only minor impurities of MgO and SrMoO₄, that could lower the overall performance of the cell. [2,3] Scanning Electron Microscopy (SEM) images have shown polyhedric-like shape, principally due to the high calcination temperature. The thermo-catalytic tests, performed using a gas blend of 80% H₂ and 20% CO₂, have shown a “twins”-like behaviour of the samples, also supported by the H₂-TPR analysis (x = 0 is similar to x = 0.5, while x = 0.2 is similar to x = 1), hypothesising the absence of Mn (II) in the x = 0.5 sample; this results in the presence of Mn (III), known in previous work to possess a good activity toward oxygen exchange that, together with the Lewis basicity of the Mg (II) cation favours the coordination of CO₂ and, consequently, its reduction. [4,5] The same gas blend was used to evaluate the electrochemical behaviour of the SMMO materials as SOFC anode by means of the Electrochemical Impedance Spectroscopy (EIS) technique, comparing the results to a benchmark test in H₂ only as fuel. Interesting outcomes have been achieved in complete cells’ tests, using different feeding mixtures. SMMO materials demonstrate to be sustainable, versatile and robust catalyst – and electrocatalysts. The detailed characterization of the electrode powder, and the comparison between catalytic and electro-catalytic behaviour allows to go deeply into the fuel oxidation mechanism and to relate composition, structure and properties.

Dense membranes made of mixed ionic and electronic conducting oxides have been widely applied for high-purity O2 production[1] and partial oxidation of CH4[2], as well as water splitting[3]. As one of the most promising membrane materials, doped alkaline earth titanates, e.g. SrTiO3 doped by Mg or Fe (≤ 25 mol%) at Ti site, showcase great advantages in chemical stability under reducing conditions over the barium/lanthanum strontium cobalt ferrite perovskite-type oxides[4, 5]. However, a conductivity minimum, caused by a depletion of charge carriers at intermediate oxygen partial pressures (~10-8-10-6 bar), may limit the performance of the doped alkaline earth titanates in application scenarios such as water splitting. One solution proposed here is to introduce additional dopants that sustain a mixed valence state for charge transfer over a broad range of oxygen partial pressure (Po2). In addition, the thickness reduction of membrane components also represents a state-of-the-art strategy for performance promotion due to the physically shortened transport passes[6]. The current work has explored and validated both of the two strategies based on characterizations of crystal structure, microstructure, and conductivity as well as oxygen permeation.

The doping strategy has been applied for CaTiO3 as an example. The Ti site is doped by Sc with a stable valence of 3+ together with Cr or Ru with variable valence states. The Sc dopant can introduce mobile charge carriers (oxygen vacancies) supporting ionic conduction[7], while the Cr or Ru dopant are expected to provide charge carriers that are mobile and sustainable over a wide Po2 range guaranteeing a high electronic conductivity[8, 9]. However, the synthesized materials with dual dopants show lower total conductivities as compared with the materials with a single dopant, suggesting a cancel-out effect likely caused by the charge balancing between two dopants. Further efforts to alleviate such an effect can rely on the concentration optimisation of the two dopants.

The strategy towards the membrane component has been conducted for the Sr0.98Ti0.75Fe0.25O3-δ materials using commercially available powders. As the free-standing membrane becomes highly fragile during production and handling when the thickness is continuously decreased to ~100μm, it is necessary to apply a thicker but porous layer as the mechanical support for the dense thin membrane layer, forming a so-called asymmetric membrane. By employing a well-developed tape casting technique, a two-layer tape has been cast in sequence and co-sintered, obtaining the asymmetric membrane. The thickness of the membrane layer has been successfully decreased to ~20 μm, which effectively enhances the oxygen permeation suggesting a promising prospect for application in water splitting. A further development of catalyst is required to overcome the increased limitation from the surface exchange.

Most of Solid Oxide Fuel/Electrolyser Cells (SOFC/SOEC) air electrode materials, such as
La_0.8Sr_0.2MnO₃ (LSM) or La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF), display the perovskite structure or related
structures. They are made of Rare Earth Elements (REE) on the A site whose partial substitution allows
to tune their transport properties.^[1] However, REE have been listed as critical resource by the European
Union, mostly due to the concentration of the mining industries in China. In this context, the aim of the
NOUVEAU project, funded by the EU, is the development of novel electrode coatings and interconnect
for sustainable and reusable SOEC with the objective to reduce by at least 30 % the amount of REE
used in a SOEC stack. This implies to find new REE free materials, mainly for the air electrode.

Interestingly, recent papers showed good mixed ionic-electronic conductivity, promising oxygen
permeability, thermal expansion coefficients (TECs) compatible with typical solid electrolytes and low
area specific resistance (ASR) values for Ca2Fe2O5 derivatives. ^[2-7] ASR of 0.22 Ω.cm² at 750 °C with
a cerium gadolinium oxide (CGO) electrolyte and 0.23 Ω.cm² at 700 °C with Sm0.2Ce0.8O1.9 (SDC)
electrolyte were reported for Ca₂Fe_1.3Mn_0.7O_5+δ and Ca₂Fe_1.8Co_0.2O_5+δ, respectively, the target for the
application being an ASR lower than 0.15 Ω cm² at 700°C.^[6-7]

These materials display the brownmillerite structural type which can be viewed as an oxygen deficient
perovskite with the nominal formula A₂B₂O_5+δ or ABO_2.5+δ. The oxygen vacancies ordering in the
brownmillerite structure results in alternating layers of corner-sharing BO₆ octahedra and BO₄
tetrahedra. The latter being oxygen deficient, consequently, oxygen diffusion along these chains is
expected while a good electronic conductivity can be obtained using transition metal ions with flexible
red-ox properties such as manganese, iron and cobalt on the B-site.

Based on the promising results reported in literature, di calcium iron oxides-based materials with
manganese or cobalt in the B site were synthesized via a citrate-nitrate route. First ASR measurements
were carried out by electrochemical impedance spectroscopy. An ASR of only 1.99 Ω cm² at 800 °C
was measured for the most promising Ca₂Fe_0.5Mn_1.5O_6-δ material mainly because of a lack of ionic and
electrical conductivity. The ASR was considerably improved when the material was studied in
composite with CGO, reaching 0.60 Ω.cm² at 800 °C. Moreover, the electronic conductivity was
improved by doping with Bi, Y or La on the Ca site. This led to an ASR of only 0.74 Ω.cm² at 800 °C
for the pure Ca_1.9Bi_0.1Fe_0.5Mn_1.5O_6-δ. Study of composite cathode is in progress.

A careful study of the electrochemical response of these systems will be presented.

Li-ion batteries contain an ensemble of micron-sized redox-active particles. Understanding lithium reaction and transport rates in individual battery particles are essential for resolving the fundamental lithium insertion process and for accurate modelling and design of battery devices. Unfortunately, it is very difficult to probe the behavior in individual battery particles using electrochemical measurements conducted on porous electrodes. In this presentation, we use the microelectrode array to enable the electrochemical charge and discharge of individual NMC battery particles. By characterizing a statistically significant number of particles, we show that the smaller particles are no faster than the larger particles [1]. This result is in stark contradiction to conventional battery transport models, and is a result of electrolyte penetration into the polycrystalline secondary particles which were previously not considered in these models. We further show that single-crystal particles have slow reaction and diffusion times even in single particles, a result of the absence of cracks. These results highlight promises for microelectrodes to uncover previously hidden transport mechanisms in energy storage materials.

Solid state batteries (SSBs) are one of the emerging options for next-generation energy storage systems due to their improved safety and high power/energy densities. In composite SSB electrodes, many active material (AM) particles are randomly and three-dimensionally distributed together with solid electrolyte (SE) particles, forming complex ion and electron conduction pathways. Such complicated mass transport pathways lead to a local depletion of ion and/or electron suppy in the electrode, especially at high current densities, resulting in a three-dimensional (3D) heterogeneous electrochemical reaction within the AM particle ensemble and within each individual AM particle. Such heterogeneous reactions can degrade the capacity/power output and lifetime of SSBs. Therefore, an understanding of the influence of heterogeneous reactions on battery performance is important for the development of high-performance SSBs. However, there are few techniques that allow tracking of 3D heterogeneous reactions within an AM particle ensemble in a composite SSB electrode with sufficiently high spatial and temporal resolution.

          Here, we developed an imaging technique that allows 3D operando tracking of the evolution of the heterogeneous reaction within the AM particle ensemble in the electrode and each individual particle in the ensemble with sufficiently high spatial (~200 nm) and temporal (40 min.) resolution, based on our previously reported technique using computed-tomography with X-ray absorption fine structure spectroscopy (CT-XAFS) [1], and investigated the heterogeneous reaction in composite SSB electrodes. Furthermore, we quantitatively analyzed the optimal particle parameters (e.g., particle size, shape) based on the data set of both chemical and geometric information of over 10⁷ voxels and 10²~10³ of AM particles obtained by the 3D imaging.

          Composite SSB cathodes were prepared by mixing octahedral single-crystal Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC) particles with an average size of ~ 3 or 10 µm and Li_2.2C_0.8B_0.2O₃ (LCBO) solid electrolyte in a weight ratio of 5:5. A model SSB was then fabricated using this composite cathode, Li foil as the anode, and LCBO as the solid electrolyte. We performed the imaging nano CT-XAFS measurements near the Ni-K edge, employing an imaging optics consisting of a condenser zone plate and an objective Fresnel zone plate. The charge state distribution in the electrode was evaluated from the peak top energy of the Ni-K edge at each voxel in the obtained 3D image.

          We three-dimensionally observed the evolution of the electrochemical reaction in 10²~10³ NMC particles in the region of 50×50×50 µm³ in the composite SSB electrode during constant-current electrochemical of 0.05~0.3C. The electrochemical reaction proceeded heterogeneously both in the whole AM particle ensemble and in each single particle. We statistically analyzed the relationship between the features of the electrochemical reaction in each particle such as average state-of-charge (SOC), intra-particle reaction inhomogeneity, and inter-particle reaction inhomogeneity, and particle parameters (e.g. spatial coordinates, volume, and shape). In the investigated electrode, larger particles tended to react heterogeneously within each particle while smaller particles tended to react heterogeneously between each particle, thus there was an optimal AM particle size that allowed for a uniform reaction. Our technique allows for the quantitative determination of the optimal active material parameters through the data-driven analysis of the experimentally obtained information of the heterogeneous reaction in the AM particle ensemble in a composite SSB electrode. Therefore, our technique provides useful information for SSB electrode design that cannot be obtained by other existing techniques.

The continuous demand for improved performance in electrical devices, ranging from small-scale applications like mobile phones to larger ones such as electric vehicles, necessitates an ongoing pursuit of enhancing battery energy density. A critical component defining energy density in batteries is the cathode material. Among the candidates for improving battery performance, high-nickel layered materials stand out as one of the best, offering higher capacity and voltage than currently employed alternatives [1].

Traditionally, these materials are used in the form of polycrystals, facing challenges associated with surface reactivity leading to microcrack formation during lithiation and delithiation [2]. In recent years, materials with single-crystal morphology have garnered significant attention, particularly in the context of advancing all-solid-state battery technologies. Single crystals, in contrast to polycrystals, exhibit substantially lower specific surface area, which can be a tremendous advantage for materials with limited structural and electrochemical stability towards the electrolyte.

The most commonly used method for producing single crystals is based on the molten salts technique, which, however, is time-consuming and energy-intensive. This study introduces a synthesis method for high-Ni NMC955 materials using the Pechini process followed by annealing. Diverse morphologies were achieved by adjusting synthesis parameters at various stages. This developed synthesis method provides a tool for tailoring the morphology of single-crystal NMC for future applications in cells with both liquid and solid electrolytes.

Testing was conducted across different voltage ranges, examining the effect of the electrolyte on cell performance, life cycle, and the formation, stability, and morphology of the cathode/electrolyte interface (CEI). Results indicate that single crystals exhibit better cyclability than polycrystals in full cells. In-depth characterization of the cathode/electrolyte interface in cells with different electrolytes was performed. Charge transfer and interface stability were further assessed through Electrochemical Impedance Spectroscopy (EIS) during the lithiation/delithiation process for both all-solid-state and traditional cells.

Single crystals demonstrated superior performance within a wider voltage window compared to polycrystals. Interestingly, the increase in the voltage window did not lead to capacity fade, contrasting with the polycrystal case. These findings underscore the importance of investigating single-crystal materials for the development of high-energy-density lithium-ion battery technologies, particularly in cells with solid-state electrolytes. The results also contribute significantly to the understanding and characterization of interfaces in single-crystal NMC, with potential implications for future advancements in battery technology.

Intensive research is being conducted to further improve the performance of lithium-ion secondary batteries. Conventional positive electrode materials exhibit excellent charge-discharge cycle characteristics because only Li-ions are extracted and inserted while maintaining the crystalline structure of the material. In contrast, Li-rich layered oxide (LLO) materials with a low-crystalline structure achieve high energy density and extraction/insertion of a large amount of Li-ions. In this study, X-ray total scattering measurements were performed at the high-energy X-ray diffraction beamline BL04B2 at the SPring-8 synchrotron radiation facility (Sayo, Japan). The crystal structure variations during the charging and discharging of LLO were evaluated. The structural parameters in the various states of charge/discharge of LLO material 0.4Li₂MnO₃−0.6LiNi_0.5Mn_0.5O₂ were determined through a crystal pair distribution function-based structural refinements using experimental G(r). Crystal pair distribution function analysis reveals that the constituent transition metals migrate substantially due to delithiation in the initial irreversible charging. We clarified the existence of a low-crystal structure in LLO formed by the crystal structure change accompanying the migration of pillar metal ions that support the Li-deficient layer during the extraction of Li-ions [1].

Protonic ceramic electrochemical cells (PCECs) have been developed as a promising technology for efficient power generation and hydrogen production via steam electrolysis. However, the current operating temperature range of 450-600°C still limits their widespread application and adoption in ceramic electrochemical cell technology. To address this issue, lowering the operating temperature range to 250-450°C has been proposed, which would expand the range of materials for building PCEC systems and reduce their cost. In this presentation, I will discuss our group's series of approaches to redesign PCECs. These approaches include using readily fabricated single-grain thick, chemically homogeneous, and robust electrolytes, as well as newly developed positive electrode materials. Our innovations have simultaneously reduced the electrolyte ohmic resistance, electrolyte-positive electrode contact resistance, and electrode polarization resistance, while also improving durability. We have demonstrated that through the PCEC materials and architecture we have designed, PCECs can be employed for power generation and hydrogen production at 250-450°C, thereby transforming the architectures and removing previous operational constraints of ceramic electrochemical cells for scaled operation. PCECs can therefore be considered as one part of the growing hydrogen economy, while leveraging the existing fossil fuel infrastructure and achieving higher energy efficiency and lower emissions.

Protonic ceramic electrochemical cells (PCECs) provide the potential to boost the electrochemical characteristics in diverse energy conversion devices by efficiently converting chemical fuels into electricity and vice versa at low temperatures below 600 °C on demand. However, the poor sinterability nature of proton conducting oxide electrolyte of PCECs inevitably fabricate the device at high temperature (>1500 ℃). This extreme heat treatment process gives rise to significant challenges for devices, including low chemical and physical stability, ultimately resulting in performance degradation. Herein, we provide a new approach to lower the co-sintering temperature of electrolyte/fuel electrode support layers by accelerating the diffusion of vapor-phase sintering activators through a ultra-fast microwave sintering process. PCECs were successfully manufactured at temperatures below 1000 ℃ using a microwave-driven diffusion method, demonstrating superior performance compared to conventionally sintered PCECs processed at temperatures exceeding 1500 ℃. Our findings offer insight into addressing that current challenges encountered by proton conducting oxide electrolyte currently face and demonstrate their feasibility for low-temperature manufacturing process of PCECs.

Triple-conducting oxides, which conduct oxygen ions and electrons or holes as well as protons, are a promising option for electrodes of protonic ceramic fuel and electrolyser cells (PCFCs, PCECs), as the reaction zone extends beyond the three-phase boundary of the air electrode, increasing the rate of oxygen reduction. However, for single-phase triple-conducting perovskites, a conflict between electronic conductivity and proton uptake has been observed [1].

The present study focuses on self-generated Ba(Ce,Fe,In)O_3-δ composites of Ce-rich and Fe-rich perovskites that collectively satisfy the desired properties for proton uptake and transport properties of air electrodes in PCFCs and PCECs. In the system BaCe_0.5Fe_0.5O_3-δ [2], In³⁺ is substituted on the B-site to induce oxygen vacancies and increase the hydration capacity. Crystal structures, lattice parameters and the relative amounts of the formed phases were determined by X-ray diffraction. The local chemical composition (cation distribution) within the individual phases, which is decisive for the transport properties of the composite, was determined by scanning transmission electron microscopy including energy-dispersive X-ray spectroscopy. Proton uptake was analysed by thermogravimetry.

Annealing experiments showed that the miscibility gap of the BaCe_0.8-xFe_xIn_0.2O_3-δ system ranges from [Ce]/([Ce]+[Fe]) ratios of approximately 0.2 to 0.9 and narrows with increasing dopant concentration. The acceptor In³⁺ tends to accumulate in the Fe-rich phase, similar to Y³⁺ in the Ba(Ce,Fe,Y)O_3-δ system [3]. The ratio of In(Ce-rich phase) / In(Fe-rich phase) ranges from 0.3 to 0.7.

The proton concentrations of the Ba(Ce,Fe,In)O_3-δ composites are within the range of 1-4 mol% at 400°C. The proton uptake increases with higher amounts of In and lower amounts of Fe in the precursor in the composite samples as well as in Ce- or Fe-rich Ba(Ce,Fe,In)O_3-δ single phases. Measurements on BaCe_0.4Fe_0.4Acc_0.2O_3‑δ (Acc = Y, Yb, Gd, Sm, Sc) composites show a general increase in proton uptake compared to the undoped system BaCe_0.5Fe_0.5O_3-δ [4]. The variation of the acceptor ion has only a minor effect on the proton uptake. The driving forces for the dopant accumulation in the Fe-rich phase such as size mismatch and acid/base properties will be discussed.

Oxygen electrode/electrolyte interface is critical to the performance and stability of solid oxide fuel cells. Typically, the formation of a tight oxygen electrode/electrolyte interface is achieved by minimizing the surface energy between solid phases during high temperature sintering. For conventional LSM and LSCF electrodes, a large number of studies have shown that the formation of LSM/YSZ and LSCF/GDC interfaces is accompanied by cationic interdiffusion, which may subsequently lead to phase segregation, interfacial reactions, and performance degradation under operating conditions. However, researchers have paid little attention to the oxygen electrode/electrolyte interface of protonic ceramic fuel cells (PCFCs), and the formation and evolution mechanism of the interface under practical working conditions are still unclear. Herein, LSM, LSF, PBSCF, and BCFZY oxygen electrodes were prepared on BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ electrolyte by high-temperature sintering and treated under actual working conditions. The effects of electrode composition, treatment conditions on ionic interdiffusion and elemental segregation were systematically investigated by exploring the morphology, phase composition, elemental distribution and valence states on the electrolyte surface, and the effects on the oxygen electrode/electrolyte interface were further investigated.

The performance of solid oxide cells (SOCs) is highly influenced by the electrocatalytic activity of the working electrode material at the solid/gas interface. New generations of working electrode materials are usually highly functionalized oxides and cermets of complex stoichiometries, designed to perform as mixed ionic/electronic conductors for efficient operation as anode or cathode at intermediate temperatures (500 – 700 °C)^[1]. Examples of these materials include doped Sr(Ti,Fe)O₃ perovskites and co-doped CeO₂ with alloyed with 3d metals. Both undergo complex transformations, specifically surface modifications such as nanoparticle ex-solution and alloying upon reduction to boost electrocatalytic activity ^[2].

Here we present how to gain insights into the role of each element while by using highly brilliant synchrotron light for element specific techniques such as time-temperature resolved near ambient pressure X-ray photoelectron and absorption spectroscopies (NAP-XPS/XAS) to probe the (near) surface chemistry, oxidation states and elemental segregation at or near the solid/gas interface. Grazing incidence X-ray diffraction provides a complementary structural perspective. In a next step we present device-driven optimization using model cells to apply polarization and other electrochemical techniques such as voltammetry, chronoamperometry and electrochemical impedance spectroscopy along with the synchrotron-based techniques. Polarization proved to be an important variable capable of completely modifying the surface chemistry of the working electrode, offering opportunities for tuning the surface chemistry or regenerating the working electrode material by reversing exsolution, remove carbon deposits or investigate SOCs in fuel or electrolysis (SOFC/SOEC) operation modes.  

We include three study cases on yttria-stabilized zirconia electrolyte supported model cells: The first uses a Ni-doped Sr(Ti,Fe)O₃ perovskites as working electrode for bimetallic Ni-Fe exsolution and reversible SOFC/SOEC operation in H₂/O₂ gases ^[3]. The second uses a Ni-Co co-doped Sr(Ti,Fe)O₃ electrode for trimetallic exsolution of Ni-Co-Fe nanoparticles and SOFC operation with the C1 fuel CH₄ ^[4]. The third case is a cermet, co-doped CeO₂ with a Ni-Fe alloy, for CO₂/CO/H₂ co-electrolysis in SOEC mode ^[5,6].  

The change of the oxygen content in a material using an electrochemical cell is an important asset to understand the variation of the Fermi energy in materials, paving the way for Fermi level engineering as a versatile toolkit for designing a broad range of materials according to specific applications. The electrochemical cell is constructed by top and bottom electrodes deposited on an oxygen ion conducting electrolyte. The reduction or increase of the oxygen content in the electrodes by cathodic or anodic polarization, is accompanied by an increase or lowering of the Fermi energy, respectively. The latter can be monitored together with chemical changes of the sample if the electrochemical cell is operated in an X-ray photoelectron spectrometer. We will present experiments using Y-stabilized zirconia, Fe-doped SrTiO₃, or (anti-)ferroelectric perovskites as electrolytes, and (Sn-doped) In₂O₃, SrFeO_3-δ and Sr-doped LaCoO_3-δ as electrode materials. The use of the setup to quantify the Fermi energies of charge transition levels in either the electrolyte or the electrode will be demonstrated.

Although solid oxide fuel and electrolysis cells are already commercially available, the exact surface chemistry and reaction mechanisms for oxygen exchange at the air and fuel side are still only partly understood. It was realised that slight alterations of the surface chemistry during operation can have substantial impact on the electrochemical activity, which explains the tremendous scatter in the measured electrochemical activity of a given material¹. In-situ ambient pressure XPS measurements on electrochemical model cells reveal the surface chemistry during electrochemical operation and its changes with time², atmosphere and electrochemical polarisation^3–5 and provide a direct link between surface chemical state and electrochemical activity. This talk presents a survey of own experiments that show the complex interplay of operation conditions, surface chemistry and electro-catalytic activity in oxygen and fuel atmospheres.

Specifically, this talk will present advanced designs for two and three electrode model cells⁶ that are optimised for in-situ characterisation in ambient pressure XPS as well as UHV⁷ and corresponding results. In oxidising conditions, primarily degrading factors were observed. Interestingly, the primary source of the fast initial degradation is not A-site cation segregation per se, but the formation of a SO₄^2- adsorbate layer from sulphur trace impurities on various different materials¹. In reducing conditions, mixed oxidation states of Fe²⁺/Fe³⁺ in Fe containing perovskites and Ce³⁺/Ce⁴⁺ in doped ceria⁴ are observable, and most likely the mediators for charge transfer to hydroxide⁴ or carbonate^3,8 reaction intermediates in the rate determining step. Interestingly, only ceria exhibits a strongly enhanced Ce³⁺ fraction at the surface, whereas the Fe²⁺ on perovskite-type SrTi_0.6Fe_0.4O_3-δ and La_0.6Sr_0.4FeO_3-δ is not enriched at the surface. Lastly, cathodic polarisation in a reducing atmosphere can trigger the exsolution of catalytically active Fe⁰ metal particles^9–11, on which the H₂ release rate can be greatly enhanced, compared to the bare perovskite.

The growing scale of renewable electricity sources (i.e., solar/wind energies) underscores the necessity for electricity storage to balance grid supply/demand across various time scales, which is critically required for the widespread integration of renewables into our energy infrastructure. One of the promising next-generation energy devices, solid oxide cells (SOCs) have been highlighted as a means of storing excess electricity as chemical energy. SOCs offer several advantages, including:(1) High efficiency combined with utilizing waste heat sources (e.g., geothermal energy/nuclear energy), (2) Flexible operations in electrolysis/fuel cell modes, and (3) Selective chemical product generation (e.g., CO/NH₃/C₂H₆). Despite ongoing efforts in the commercialization of SOC technology, it continues to face hindrances primarily associated with degradation-related issues.^[1] Notably, the UK government has set an ambitious long-term stability goal of over 87,500 hours (equivalent to 10 years) for SOCs by 2050.^[2] Therefore, the “long-term durability” of the SOCs under operating conditions will be crucial for achieving the ambitious goals set for SOC.

The primary objective of this research is to comprehend the intricate structure and chemistry of interfaces in SOCs by systematically characterising the evolution from the pristine state to the postmortem state of aged SOCs. The investigation has specifically focused on internal interfaces, such as grain boundaries, establishing connections between the findings and the microscopic outcomes of the interfaces. A range of advanced characterization tools, including scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), low energy ion scattering (LEIS), secondary ion mass spectrometry (SIMS), and atom probe tomography (APT), has been employed to establish correlations among interfacial changes. The investigation spans a range of length scales, from scrutinizing interactions at individual grain boundaries (APT) to assessing areas exceeding 400 mm² (SIMS). The advanced characterisation techniques play a critical role in better understanding the degradation mechanism of SOC and achieving in-depth knowledge, providing insight to overcome durability issues.

Conventional cathode materials of sodium-ion batteries (SIBs) are pre-sodiated and store Na-ions electrochemically. Interestingly, pre-potassiated materials can be appealing cathodes for SIBs. Potassium Prussian blue analogues (K-PBAs) is an excellent example. K-PBAs are cost effective and sustainable materials that are based on earth-abundant elements, and they have the structural remits to allow fast migration of large-sized ions. I will show in this talk that in a hybrid SIB cell, where Na+ is in the electrolyte and K+ is in the K-PBA cathode, not only is the electrochemical mechanism of cation intercalation in K-PBA dominated by K-ion rather than Na-ion, but the mechanism can be affected by the [Fe(CN)6]4- anion vacancies present in K-PBA. K-PBA with 25% anion vacancies exhibits two cation intercalation steps in a hybrid SIB cell, both being dominated by K-ion and displaying a ~0.2 V voltage increase compared with Na-PBA. In contrast, K-PBA with 7% anion vacancies exhibits both K-ion intercalation (>2.8 V vs. Na+/Na) and Na-ion intercalation (<2.8 V). This is because a higher vacancy level enhances K-ion diffusion in the PBA framework, which facilitates K-ion intercalation and suppresses Na-ion intercalation. A lower vacancy level deteriorates K-ion diffusion and thus enhances Na-ion intercalation. In a full-cell that is consisted of the K-PBA cathode and the hard carbon anode, the electrochemical process proceeds in a similar manner as the half-cells. K-ion dominates the intercalation in the K-PBA cathode while Na-ion dominates the intercalation in the hard carbon anode; therefore, a hybrid SIB is formed.

Heightened attention has been directed towards high entropy materials, owing to their distinct properties as potential battery cell components.^[1] Lithium-doped high-entropy oxides (LiHEOs) exemplified by Li_0.33(MgCoNiCuZn)_0.67O exhibit notable lithium ion and electron conductivity.^[2,3] This renders them promising as coating materials for NCM (Lithium-Nickel-Cobalt-Manganese Oxide) in conventional Li-ion battery (LIBs) cells, due to their high ionic and electronic conductivity. However, the stringent conditions and high temperatures required for their synthesis impose limitations on their practical application.

To overcome these challenges, a photonic curing strategy is proposed for the synthesis of LiHEO/HEO (high entropy oxide) materials. This approach successfully yields, for the first time, a nanoscale, homogeneous coating layer on NCM particles.[WO2023/280534 A1] The modified surface of NCM with LiHEO demonstrates exceptional electrochemical cycling stability, exhibiting a noteworthy 91% capacity retention at 1C after 350 cycles. This enhanced electrochemical performance can be attributed to the uniform coating, which effectively mitigates structural changes resulting from the interaction between the electrode active material and electrolyte.

In comparison to the uncoated NCM, the presence of a coating substantially reduces the formation of cracks in secondary particles. This reduction in crack formation prevents the electrolyte from further reacting with primary particles along the cracks. The findings suggest the potential of LiHEO-modified materials as a viable solution for improving the electrochemical performance and cycling stability of high nickel cathode active materials and thereby increasing the cycle life of LIBs. Further, photonic curing presents a novel synthesis and coating procedure for cathode active material particles that paves the way for surface modification of any heat sensitive material for a number of applications using a cost and energy efficient approach.

Solid-state batteries (SSBs) are a potentially safe, next-generation energy-storage technology [1-2]. For SSBs to be commercially viable, the development of solid electrolytes (SEs) with high ionic conductivity, high (electro)chemical stability, and good processability is imperative [2]. A novel approach to modifying materials, potentially leading to improved properties, is the high-entropy concept, characterized by a ΔS_conf > 1.5R (where ΔS_conf and R represent configurational entropy and ideal gas constant, respectively) [3]. However, the effect of configurational entropy on lithium transport remains largely elusive.

Recently, our group has extended the choice of high-entropy SEs to include lithium argyrodites by multianionic and/or -cationic substitution [4-6]. For multicationic substituted litihum argyrodites with the composition Li_6.5[Ge_0.25Si_0.25Sb_0.25P_0.25]S₅I, high ionic conductivity (> 10 mS/cm at 25 °C) and a low activation energy for conduction (0.20 eV) were observed, as shown by ⁷Li pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and electrochemical impedance spectroscopy (EIS) [5-6]. These results were rationalized from a structural perspective via neutron powder diffraction combined with magic angle spinning (MAS) NMR spectroscopy. In this case, high S^2‒/I^‒ site disorder (up to ~11%) and lithium re-distribution led to shortened Li-Li jump distances, facilitating long-range ion diffusion [5-6]. Subsequently, we have explored a series of multicationic substituted lithium argyrodites by tuning the cationic stoichiometry. Through complementary EIS and ⁷Li PFG NMR measurements, we found that compositionally complex substitution helps to significantly improve Li-ion mobility. Notably, our study provides clear evidence for the correlation between configurational entropy and ion mobility and indicates the potential for enhancing conductivity in ceramic ion conductors by tailoring the compositional complexity, opening up new avenues for research and development in this field.

Due to their solid nature, solid-state batteries (SSBs) pose distinctive challenges in realizing their potential benefits on a practical scale. There is a significant lack of fundamental understanding of the complex processes governing the behavior of solid electrolyte-electrode interfaces and how engineering modifications affect their kinetics and thermodynamic stability.¹ A novel strategy for material optimization includes element substitutions in the crystalline network, effectively altering material properties. When multiple different elements are employed, one enters the high‑entropy regime imposing local distortions that lead to a distribution of overlapping site energies, activating favorable percolation paths for the Li-ions and improving material kinetics². However, one might wonder about the effect of the high-entropy approach on electrochemical stability at the interfaces, a largely unexplored concept.

In this work, we put into perspective the pristine Li₃InCl₆ and its high‑entropy counterpart Li_2.8In_0.2Zr_0.2Yb_0.2Sc_0.2Lu_0.2Cl₆ halide solid electrolyte materials. We experimentally showcase superior electrochemical stability, higher diffusivity, and lower electronic conductivity for the high-entropy material, leading to more stable interfaces with longer cycle life.

A variety of questions arise as to why this improved behavior occurs. Is it an indirect effect of higher ionic conductivity that helps with the current distribution, avoiding hot spots? Is it the intrinsic properties of the selected elements offering localized electron states resistant to reduction? Or is it a property evoked by the number of different elements, increasing complexity and thus increasing the energy barrier for reduction and eventually decomposition?

To answer these questions and initiate a discussion, we dive into the fundamentals, employing density functional theory calculations to investigate and deconvolute the mechanism behind the improved behavior. Employing advanced theoretical principles developed for accurately determining the electrochemical window^3,4, we showcase the importance of electronic configuration and connect these insights with the effective electron mass. Overall, we link the concepts of high entropy, local electronic structure modifications, and electrochemical stability, delivering advanced optimization strategies of solid electrolytes for enabling SSBs.

A-site layered double perovskites, derived from the parent material PrBaMn2O_5+δ, have demonstrated exceptional suitability as electrode materials in solid oxide fuel cells (SOFCs). By introducing catalytically active metal cations onto the B-site of a single perovskite oxide, it becomes feasible to induce the exsolution of metal nanoparticles on the surface through a phase transition to the double perovskite structure under reducing conditions. Palladium, renowned for its outstanding catalytic activity, has been utilized in various applications, leveraging exsolution as a means to employ noble metals in minimal quantities. In this study, we scrutinize the phase segregation behavior and ionic environment of palladium incorporated on the B-site of Pr_0.5Ba_0.5MnO_3-δ. The substitution of palladium in the manganese sites induces the creation of oxygen vacancies to accommodate square planar Pd²⁺. Upon undergoing reduction treatment, a phase transformation occurs, resulting in the formation of the A-site layered double perovskite, PrBaMn_1.9Pd_0.1O_5+δ (L-PBMPd), accompanied by the exsolution of monometallic and bimetallic nanoparticles on its surface. Notably, this study reports, for the first time, the exsolution of metallic manganese in the form of an alloy nanoparticle with a composition of Pd₃Mn. The achieved maximum power densities of 0.59 W cm^-2 at 800 °C and 0.80 W cm^-2 at 850 °C underscore the high performance of L-PBMPd as an anode material in SOFCs. This outstanding performance suggests potential synergistic effects of the Pd₃Mn nanoparticle in fuel oxidation, positioning it as a promising alloy for catalytic applications.

Metal nanoparticles on oxide surfaces hold considerable potential for application in heterogeneous catalysis and in electrocatalysis, particularly in the context of solid-state electrochemical devices such as fuel flexible fuel cells, electrolysers, and electrochemical membrane reactors. Exsolution is a process to achieve this modification of the solid support. It offers important advantages compared to conventional deposition of nanoparticles, the most prominent being the reduced tendency of the nanoparticles to agglomerate. Some insights into the structure, stoichiometry and functionality of individual material systems suitable for exsolution have already been gained, however, there is still a lack of knowledge in defect chemistry and the kinetics of exsolution.

From its size Ni²⁺ is expected to reside on the B-site positions of the perovskite structure of SrTiO₃. However, B-site diffusion is known to be relatively slow because of the stable Ti-O-skeleton of the material which seems contradictory to the much faster exsolution process. Here, an interstitial diffusion mechanism for nickel and its dependency on different defects is investigated by density-functional theory (DFT) calculations as well as transient thermogravimetry.

Exsolution involves reduction of Ni²⁺ and the complementary oxidation of O^2- which leaves the sample, resulting in a mass loss which can be tracked over time. With nickel diffusion being expected to be the dominant process diffusion information can be extracted from fitting this data. The studies are performed with systematic variation of several parameters, including the temperature, amount of nickel, amount of A-site and oxygen vacancies, and the particle sizes of the sample.

This approach aims to develop a more generalized understanding of the kinetics of exsolution, with the overarching goal of facilitating the tailored design of materials according to their applications.

Since its inception a decade ago, exsolution has proven to be a powerful tool for the design of advanced energy materials. Therefore, the ability to tailor the size, morphology, and composition of the exsolved nanoparticles is critical to implement this technique to further improve the performance of such nanostructured catalysts [1]. Previously, ex-situ techniques had been used to probe this rather complex system, but the growing interest to understand the exsolution processes and the material changes during catalytic operation has paved way for the development of in-situ methods such as XRD, AP-XPS, and TEM [2]. So far however, poor information could be gathered with respect the nanoparticle size evolution during the exsolution process.

In this work, the exsolution of nano alloy with Ru-Ni composition from an A-site deficient double perovskite of the type (La_2−xNiRuO_6−δ, x = 0.15) [3], was investigated via in-situ X-ray scattering methods. In particular, the exsolution process was studied at 450 and 550°C for 5 h both by in situ XRD and small-angle X-ray scattering (SAXS). This latter method represents a novel approach and offers a direct and highly statistically relevant investigation of the size evolution of exsolved nanoparticles. The analysis of the SAXS) curves gave an insight into the formation of both surface and sub-surface nanoparticles.At 450 °C the particles formed on the surface increased from 0.5 to 2 nm, whereas the buried component grew from 3 to 10 nm in diameter. which agreed with the particle size distribution seen in the TEM images [3]. During the exsolution process over 5 hours we could monitor different nucleation events which corresponded well to the structural changes of the perovskite matrix and the reduction of the metal species monitored by in situ XRD and XANES spectroscopy, respectively.

The development of nanostructured composite materials is critical for improving the efficiency of a variety of heterogeneous catalytic processes. Metal exsolution is a promising synthesis route for the formation of oxide-supported metal catalysts, allowing for the formation of well dispersed and highly active metal nanoparticles. While exsolution is often proposed as a route for enhancing particle stability (thus leading to catalytic systems with increased efficiency and extended lifetimes), recent work has shown that there are still limitations to the thermal stability of exsolved nanoparticles. Provided this, a fundamental understanding of the factors which impact the stability of exsolved nanoparticles remains essential to the development of optimized catalytic systems. In this work, the dynamics of exsolved Ni nanoparticles are investigated using an environmental STEM equipped with a secondary electron detector, providing a route for atomic-resolution characterization of the sample surface during and after the exsolution process. An epitaxially grown Ni-doped strontium titanate thin film is utilized for in situ experiments, which has a unique nanostructure that consists of a Ni-doped strontium titanate matrix with embedded, heteroepitaxial NiO nanocolumns. Exsolution from this material is characterized, providing evidence that two distinct populations of Ni nanoparticles form – those that precipitate above second-phase nanocolumns, and those which nucleate above the matrix. The dynamics of the two nanoparticle populations post-exsolution are highlighted and characterized by in situ STEM, with particular focus on the different behaviors of the two types in regards to Ostwald Ripening and particle migration.

Li-ion batteries are ubiquitous; powering our cell phones, tablets, and laptop computers. Yet, fundamentally, we’ve exclusively relied on materials that adopt the same ordered rock salt structure type for over 20 years. During the time, we have continually optimized the performance of these materials to the point that we are rapidly approaching the theoretical limits on energy density. So where do we go from here? This talk will present an overview of the work our group has done to develop a deeper understanding of the fundamental structural and compositional requirements to effectively move charge through the solid state and discuss nascent work to develop reversible F-ion insertion hosts that function at room temperature. We will begin with an overview of the new design principles that must be applied in designing F-ion batteries and go on to highlight two major structural families that have been developed. We will conclude with a discussion of some of the challenges surrounding electrolyte development.

Introducing structural or compositional disorder within solid electrolytes is a common practical strategy for increasing their ionic conductivity. Dual cation (M, Sn)F₂ fluorites have been proposed to exhibit two forms of disorder within their cationic host frameworks: occupational disorder between cations and orientational disorder of Sn(II) stereoactive lone pairs. To understand these two forms of disorder and their effect on fluoride-ion conductivity, we have performed a joint experimental and computational study of the exemplar cubic-BaSnF₄, using X-ray diffraction, ¹¹⁹Sn Mössbauer spectroscopy, ¹⁹F NMR spectroscopy, and ab initio molecular dynamics simulations. We find that cubic-BaSnF₄ shows extreme intrinsic fluoride-ion disorder, where 1/3 of fluoride ions occupy interstitial sites, with this disorder a consequence of repulsion between Sn lone pairs and fluoride ions. We also identify highly inhomogeneous fluoride-ion dynamics, where fluoride ions in Sn-rich environments are significantly more mobile than those in Ba-rich environments. Our simulations reveal that the Sn lone pairs dynamically reorient on a ps timescale, and that this lone-pair dynamics is coupled to the dynamics of local fluoride ions. These results illustrate how different forms of host-framework disorder can interact in an exemplar solid electrolyte to give complex mobile-ion dynamics, and highlight the important role of Sn(II) lone pair dynamics in this solid electrolyte.

The vast majority of rechargeable batteries currently used for mobile electronics and vehicle applications are lithium-ion, exploiting their impressive energy densities and relatively high voltage. However, concerns around factors including safety, high cost and geopolitical distribution of the raw materials has motivated studies of other battery chemistries. Na⁺-ion cells are at the most advanced stage of development, driven by the high earth abundance of sodium, whilst multivalent mobile cations such as Mg²⁺ and Al³⁺ potentially offer increased energy densities, but are hampered by large structural changes associated with the insertion/removal of these highly charged species into/from the electrodes.

Until recently, batteries based on mobile anions (rather than cations) have been largely ignored, despite reports of high mobility of F^- ions within solids dating back to Michael Faraday’s studies of the ‘fluoride of lead’, β-PbF₂, in 1838. A major challenge for cells based on fluorine chemistry is the lack of a suitable liquid electrolyte, as most candidates contain HF2- ions with a small voltage stability window and/or react with atmospheric moisture to form HF. However, a significant advance occurred in 2011, with reports of viable cells using solid electrolytes such as BaSnF₄ [1], which has recently attracted the interest of car companies including Honda and Toyota. However, these batteries have a major drawback, needing to be operated at elevated temperatures (typically 80-150^oC) to achieve sufficiently high F^--ion conductivity within the solid electrolyte [2].

This presentation will focus on the structure-property relationships within a number of F^--ion conductors, including a discussion of our current understanding of the archetypal compound β-PbF₂. This will be followed by a more detailed presentation of a number of ternary derivatives of PbF₂, such as KPbF₃ [3], PbSnF₄ [4], CsSn₂F₅ [5] and β-KSbF₄ [6]. Information on the crystal structure, including the nature of the dynamic F^- disorder, has been provided by neutron powder diffraction studies, exploiting the sensitivity of the technique to the locations of the anions in the presence of heavier cations. Finally, the factors that promote extensive F^- disorder within such solid phases will be considered, including the influence of crystal structure on the preferred anion diffusion mechanisms and the role of electron lone-pairs associated with cations such as Pb²⁺ and Sn²⁺ in promoting high F^- mobility.

All solid-state fluoride ion batteries are promising and suitable for electric vehicles and grid storage due to their high energy density. The use of very high fluoride ion conducting solid electrolyte is necessary to achieve the potential of fluoride ion batteries. Various fluoride ion conducting solid electrolytes in the tysonite, and fluorite structured systems are under development. Among these fluoride ion conducting solid electrolytes the fluorite structured materials especially PbSnF₄ system shows high ionic conductivity at ambient temperatures (~10^-3 Scm^-1). Similar to PbSnF₄, the barium variant (BaSnF₄) which exists in the same structure as PbSnF₄, also shows conductivity in the order of 10^-4 Scm^-1 at room temperatures. In recent years, further increase in conductivity has been achieved by varying the barium to tin ratio. Especially, the solid solutions Ba_1-xSn_xF₄ prepared by mechanical milling technique (x=0.5-0.57) with Sn rich ratio shows ionic conductivity nearly in order of 10^-2 Scm^-1 at room temperature^. Therefore, we have studied the fluoride ion diffusion coefficient in this system since the diffusion coefficient plays a vital role in the mass transfer during battery operation. In the present work, the ¹⁹F diffusion coefficient in Ba_1-xSn_xF₄ solid solution with high ionic conductivity has been studied using ¹⁹F pulsed field gradient nuclear magnetic resonance Spectroscopy (PFG-NMR). The fluoride ion diffusion coefficient is found to be in the order of 10^-12 m²s^-1 at room temperature. Further, we have investigated the fluoride ion conduction mechanism in Ba_1-xSn_xF₄ by comparing the diffusion coefficient measured by ¹⁹F PFG-NMR and calculated from ac impedance measurements and the results will be discussed in detail.

The development of high-performance fluoride-ion (F⁻) conductors is essential for the practical application of all-solid-state F⁻ batteries. To date, the crystal structures of the F⁻ conductors have been mainly limited to perovskite-, fluorite- and tyssonite-type fluorides. However, the room temperature conductivity of these F⁻ conductors is several orders of magnitude lower than that of cationic conductors. Furthermore, strategies to improve F⁻ conductivity are mainly limited to introducing defects in the crystal structure, which lags considerably behind the abundant means of conductivity enhancement in cationic conductors, such as inductive effects¹ and paddlewheel effects².

In mixed-anion compounds that containing more than one anion in the structure, the presence of anions with different polarization rates and electronegativity from F⁻ allows the construction of new crystal structures that cannot be obtained in single-anion compounds³. In this study, F⁻ conduction in fluorosulfide La₂SrF₄S₂ was investigated^[4]. In La₂SrF₄S_2, the triple-fluorite layer La₂SrF₄ allows for a two-dimensional F⁻ conduction, which is not found in single anion fluorides. The presence of a sulphur layer widens the conduction space of the F⁻ layer and weakens the interaction between cations and F⁻. Furthermore, we attempted to improve the F⁻ conductivity by cation-anion engineering. Specifically, (i) La₂SrF_4+xS_2-xCl_x, in which S²⁻ was changed to Cl⁻ to introduce excess F⁻, and (ii) La₂Sr_1-xPb_xF₄S₂, in which Sr²⁺ was changed to Pb²⁺ with 6s² lone pair electrons, were synthesized and the change in F⁻ conductivity were explored and explained from a crystallographic point of view.

(i) The ionic conductivity of La₂SrF_4.1S_1.9Cl_0.1 at 160 °C was 6.26 × 10⁻⁶ S cm⁻¹, which is five times higher than that of La₂SrF₄S₂. On the other hand, the activation energy of La₂SrF_4.1S_1.9Cl_0.1 is 0.51 eV, which is comparable to 0.53 eV of La₂SrF₄S₂. The increase in the pre-exponential factor can be attributed to the introduction of excess F⁻ in the lattice by replacing divalent S²⁻ with monovalent Cl⁻. This suggests that the use of mixed-anion compounds allows carrier engineering by anions, which is different from the carrier control of by cation substitution in previous F⁻ conductors.

(ii) The ionic conductivity of La₂Sr_0.6Pb_0.4F₄S₂ at 160 °C was 2.56 × 10⁻⁴ S cm⁻¹, which are both higher than those of La₂SrF₄S₂ and La₂SrF_4.1S_1.9Cl_0.1. The amount of F⁻ carriers and the F⁻-F⁻ distance did not change much before and after the introduction of Pb²⁺, suggesting that there are factors other than those previously considered responsible for the increased pre-exponential factor. ¹⁹F MAS NMR showed that the number of F⁻ in the hetero-cationic environment increased after the introduction of Pb²⁺, suggesting that the distorted cationic sublattice is partly responsible for the conductivity enhancement. Also, 6s² lone electron pairs in Pb²⁺ may also contribute to the increase in F⁻ conductivity.

In conclusion, using the mixed-anion compound La₂SrF₄S₂ as a model, we proposed new strategies to improve F⁻ conductivity by cation-anion engineering.

LiM_0.5Mn_1.5O₄ (where M = Ni, Fe) compositions have emerged as a focal point of extensive research in the context of positive electrode materials for lithium-ion batteries. These materials, crystallizing in the spinel structure, have captivated significant attention owing to their remarkable ability to substantially enhance the average voltage compared to layered oxides or Mn-rich compositions, thereby holding great promise for advancing the performance of Li-ion battery systems. Moreover, these compositions are of particular interest due to their sustainable nature, being cobalt-free. This attribute aligns with the growing emphasis on eco-friendly battery technologies, contributing to a more sustainable and environmentally conscious approach to energy storage.

Exploring the intricate details of the crystal structures and compositional variations within this family of compounds becomes imperative for unlocking their full potential and contributing to the ongoing advancements in energy storage technologies. This study delves into a comprehensive analysis of the crystal chemistry of LiM_0.5Mn_1.5O₄ (M = Ni, Fe) compositions, shedding light on their structural and microstructural details (including disorder and defects) and the pivotal role these play in enhancing the capacity and rate performance of these materials. [1-3]

One of the biggest challenges facing lithium-ion batteries is how to increase their energy density. The cathode, typically a layered lithium transition metal oxide, represents a major limitation. One route to increase the energy density is to store charge at high voltage on the oxide ions in the cathode material. However, removing electrons from lattice oxide ions typically results in structural instability leading to voltage hysteresis and voltage fade over cycling. Understanding the mechanism behind oxygen redox is critical to overcoming these issues.

Our recent investigations into Li-rich cathodes have revealed that oxidized oxygen takes the form of O₂ molecules which are trapped in nanovoids in the structure. We have also shown that these trapped O₂ molecules can be reduced back to O^2- on discharge providing a viable charge storage mechanism to explain oxygen redox. In this talk, I will discuss the evidence for the formation and reduction of trapped O₂ [1-3] and explore the impacts this has on the performance of oxygen redox cathodes[4]. I will show how the formation of O₂ extends to 4d and 5d transition metal oxides[5], disordered rocksalt cathodes[6] and even to non-Li-rich cathodes[7]. Finally, I will show that it is possible to suppress this structural change and undergo reversible, high voltage O-redox without voltage hysteresis[8]. Altogether, this understanding helps to explain the unusual properties of oxygen redox cathodes and informs how they might be harnessed to boost the energy density of batteries.

LiNi_0.5Mn_1.5O₄ (LNMO) stands out as a highly promising spinel-type positive electrode material for Li-ion batteries (LIBs). Its high operating voltage (4.8 V vs Li⁺/Li, associated with the Ni⁴⁺/Ni²⁺ redox couple) provides high energy density, a crucial factor for the development of next-generation LIBs [1].

LNMO crystallizes with different extents of Ni/Mn ordering. In the fully ordered phase (P4332 space group), Ni and Mn cations occupy distinct crystallographic octahedral sites, whereas, in the fully disordered phase (Fd3m space group), Ni and Mn share the same crystallographic octahedral site. High temperatures (T > 750°C), required to obtain the disordered phase, lead at T ≥ 700°C to oxygen loss in LNMO and, as a result, to the formation of rock salt-based impurity and the reduction of Mn(IV) cations to Mn(III) [1,2].

The extent of Ni/Mn ordering significantly influences the performance of this positive electrode material. It is shown that the disordered LNMO exhibits superior performance at high C-rates and better cycling stability than the ordered LNMO [3,4]. Moreover, the disordered Mn-rich LNMO (LiNi_0.44Mn­_1.56O₄, Ni/Mn = 22/78) can be prepared without inducing oxygen deficiency, resulting in superior electrochemical performance [5]. This unexpected result encourages this systematic investigation of the impact of the Ni/Mn ratio on the phase equilibrium and electrochemical performance of LNMO. In this study, we performed ex situ and in situ Neutron powder diffraction under air and oxygen atmospheres to follow the Ni/Mn ordering process during the synthesis of LNMO with varying initial Ni/Mn of 25/75 (corresponding to stoichiometric LNMO), and Ni/Mn of 23/77 (Mn-rich LNMO).

Our findings reveal that for both Ni/Mn ratios, the Ni/Mn ordering process is disrupted by the oxygen release. In other words, the fully disordered LNMO phase cannot be prepared without rock salt-based impurities, caused by the oxygen release, for the selected Ni/Mn ratios. Our data also suggest that the extent of Ni/Mn ordering in LNMO is controlled solely by a concentration of Mn(III) in the crystal structure of LNMO, which can be adjusted by both the Ni/Mn ratio and partial oxygen pressure during the synthesis of LNMO.

The development of high-energy-density batteries with lithium (de-)intercalation reaction is one of the most promising solutions to realize electric vehicles and power storage system applications with long-term stability. In this study, anti-fluorite structured Li_5+xFe_1-xMn_xO₄ was synthesized by a solid-state reaction, and lithium intercalation properties were characterized by electrochemical and structural investigations.

A mixed phase of the Li₅FeO₄-type and Li₆MnO₄-type structure was obtained for x < 0.6 composition. A new phase with the Li₆MnO₄-type structure was formed at x = 0.4, and a single phase with Li₆MnO₄-type structure was obtained in the range between 0.6 and 1.0 composition. The particle size of the synthesized samples was about 50 µm, which is quite large compared with conventional cathode materials for lithium batteries. The as-prepared Li_5.6Fe_0.4Mn_0.6O₄ exhibited first charge and discharge capacities of 680 and 300 mAh/g, respectively, with a large irreversible capacity of 380 mAh/g (coulombic efficiency = 44%). This indicates that the new iron-manganese composition with antifluorite-type structure is lithium insertion/extraction-active although a large irreversible electrode reaction was observed.

To improve the charge-discharge performance of the prepared sample, the electrode fabrication process was changed as shown in the experimental section. The ball milling process was introduced to decrease the particle size and compensate for homogeneous electronic conductivity in the composite electrode. Charge-discharge measurements showed that the first discharge capacity of the Li_5.6Fe_0.4Mn_0.6O₄ (x = 0.6) electrode treated by ball-milling was 450 mAh/g. The coulombic efficiency was 60%. A reversible reaction continued to proceed with ca. 200 mAh/g after the following cycles. A new iron-manganese system with an anti-fluorite structure could become a potential for high-capacity cathode material.

One of the major problems with proton-conducting oxide fuel cells is the large chemical expansion upon hydration and the resulting mechanical stress within the electrolyte. The purpose of this study was to establish a method for numerical calculation of stress distribution under fuel cell (and electrolysis) operating conditions and a method for experimental verification of the estimated results. In this study, Ba(Zr,Y)O3 and Ba(Zr,Yb)O3 were selected as the target proton-conducting oxides. First, the lattice volume change was modeled using published thermodynamic parameters of defect equilibrium, along with experimental results from high-temperature X-ray diffraction. Although a significant change in lattice volume was observed upon hydration, the influence of redox reactions was found to be negligible. Then the elastic modulus of the material was measured using a resonance method at elevated temperatures in controlled atmospheres. The chemical potential distribution of oxygen and hydrogen (and water vapor) under fuel cell (or electrolysis) conditions were estimated using an equivalent circuit containing three transmission lines, for proton, oxide ion, and hole, terminated by gas-solid exchange reaction resistors for oxygen, hydrogen, and water.  The obtained proton concentration distribution was used to calculate the chemical strain distribution across the sample. The stress distribution and deformation of the electrolyte were calculated for the free-standing and constrained states. The information of the deformation of the free-standing sample was used for experimental validation of the calculated results. In the verification experiment, a sample pellet with Pt and Pd electrodes on the air and fuel sides of the surface was placed on a quartz tube with using mica as a gas sealant, and the surface profile was measured using a reflection-type laser profilometer. When the water vapor partial pressure on the air side was increased, the sample deformed concavely toward the fuel side. This change was confirmed reversible by increasing and decreasing the water vapor pressure. The amount of deformation agreed well with the calculated value, confirming that the calculated results were valid.

Protonic ceramic cells (PCCs) show great promises in the fields of electrochemical energy conversion and storage, while the fabrication of dense protonic electrolytes is a key step towards their practical applications. Most of the protonic ceramic electrolytes take a ABO3 perovskite-type lattice structure with cerium-rich content in B site and barium in A site, which usually show poor sinterability and was assigned to Ba evaporation. Here, through a systematic and comparative study of BaCeO3 and BaZrO3, we demonstrate that Ba is actually not lost but segregated inside samples, and thermal redox of Ce4+/Ce3+ plays an important role in the abnormal sintering behavior of cerium-rich protonic perovskites. During the heating process, partial Ce in the B-site is thermally reduced to Ce3+, which is too large to stay at B-site, thus exsolves or partially displaces to the A site, resulting in the formation of impurity CeO2 phase and a shrinkage of the perovskite lattice, both detrimentally affecting on the electrolyte densification. Contrary to previous beliefs that high-temperature evaporation-induced barium deficiency inhibits the electrolyte sintering, our findings indicate that Ba deficiency alone does not hinder the sintering of BaZrO3 perovskites, instead, excess Ba negatively affects the sintering as it tends to accumulate at the grain boundaries. As to BaCeO3, barium deficiency promotes cerium exsolution while barium excess suppresses such exsolution to improve electrolyte sintering. Co-doping Zr and Ce in the B-site of protonic perovskite can optimize the sintering characteristic. These findings offer new insights into sintering of protonic perovskites and provide guidance for the development of new protonic devices.

This research focuses on the interplay between the processing conditions, structure, transport, and proton surface exchange kinetics, of vertically aligned nanocomposites (VANs), targeting protonic ceramic electrochemical cells (PCECs). The VANs consist of a proton conductor (BaZr_0.9Y_0.1O_3-δ, BZY) as the first phase and a mixed p-type/ oxide-ion conductor (Ce_0.9Pr_0.1O_2-δ, PCO) as the second phase. By separating the proton-conducting and p-type mixed conducting phases for task sharing, the BZY-PCO VANs possess the potential for rapid proton surface exchange at the solid-gas interface and transport along the solid-solid heterointerfaces.

These VAN thin films were deposited on MgO (100) substrates with pulsed laser deposition (PLD) under different conditions. Owing to the small lattice mismatch between MgO and BZY, along with the different crystal structures of perovskite BZY and fluorite PCO, it is expected that this combination can lead to vertically aligned phase-separated structures. To optimize the PLD recipe for high-quality VANs, we varied the substrate temperature, laser repetition rate, laser power, and processing oxygen pressure. The crystallinity and phases were characterized by grazing-incidence X-ray diffraction (GI-XRD), and the structural analysis and elemental mapping were performed by scanning/transmission electron microscopy (S/TEM), energy-dispersive X-ray spectroscopy (EDS), and electron energy-loss spectroscopy (EELS). Additional characterization of strain and structural order as a function of depth was enabled by angle-dependent synchrotron X-ray pair distribution function (PDF) analysis.

Furthermore, proton surface exchange kinetics of VANs were evaluated by simultaneous electrical conductivity relaxation (ECR) and optical transmission relaxation (OTR) measurements. This research reveals how processing conditions influence mesostructural ordering in VANs, heterointerface chemistry, and the kinetics of proton surface exchange.

Acceptor-doped barium zirconates are of major interest as proton conducting electrolyte materials for electrochemical applications at intermediate operating temperatures. The conduction of protons through polycrystalline yttrium doped barium cerium zirconates (BZCY) is hindered by the formation of space charge regions at grain boundaries caused by a positive core charge. During high temperature sintering, the positive core charge additionally acts as a driving force for acceptor dopant segregation. Yttrium segregation to grain boundaries has been observed in sintered ceramics but the fundamental relationships between the degree of segregation and protonic conductivity are not explored. Here, we present a comprehensive study of the influence of yttrium segregation on the chemical composition and structure at grain boundaries in BZCY and its impact on electrochemical properties. We designed an out-of-equilibrium model material, that displays no observable segregation and used it as a starting point to observe the kinetics of segregation and the induced changes in  grain boundary conductivity after varied thermal histories. Furthermore, we coupled the electrochemical results derived from impedance spectroscopy to atomic resolution transmission electron microscopy. We discovered that atomically sharp acceptor segregation drastically increases the grain boundary conductivity both in the model system and reference samples processed by the industrially applied solid state reactive sintering (SSRS) route.

For decades, much effort has been devoted to developing proton conductors applicable to electrochemical devices at intermediate temperatures. However, promising materials that possess sufficient conductivity have not been realized yet. This study demonstrates the development of novel proton conductors that are operative at intermediate temperatures, especially 300–400 °C, through the simple ion-exchange method. The Li⁺/H⁺ ion-exchange process was conducted for Li_(14−2x)/4Zn_(1+x)/4GeO₄ (lithium super ionic conductors, LISICONs) in non-aqueous solutions. The chemical formula of the resultant sample was determined as Li_3.13H_0.37Zn_0.25GeO₄. The electrical conductivity of this material was evaluated to be 87.0 mS cm⁻¹, 39.0 mS cm⁻¹, and 5.5 mS cm⁻¹ at 400 °C, 300 °C, and 200 °C, respectively, in 10% H₂O–90% N₂. Furthermore, the main charge carrier in this electrolyte was identified as a proton from the H/D isotopic exchange study. These findings open up the possibility of realizing new electrochemical devices that are operative at 200–400 °C.

Solid-state batteries based on a ceramic electrolyte and lithium metal anode promise to transform battery safety and energy density, but both charging and discharging rates are limited to below what is required. On discharge, voids form in the lithium metal at the Li/solid electrolyte interface due to limited Li metal creep at practical stack pressures and under practical current densities. These voids accumulate on cycling, leading to detachment of the Li anode and consequently high local currents during charge, triggering the growth of dendrites (filaments of Li metal that penetrate the ceramic electrolyte) resulting in short-circuit and cell failure. Furthermore, even without any prior voiding, charging currents are limited to levels too low for practical applications by dendrite formation. Important efforts are being made to understand dendrites in ceramic electrolytes and to mitigate them.

We describe the formation and progression of lithium dendrites, informed by observing dendritic cracks operando using X-ray computed tomography (XCT). Dendrites are found to follow a two-stage process of initiation then propagation, with distinct mechanisms for each. Initiation occurs in sub-surface pores where pressure builds to exceed the local fracture strength at the grain boundaries, by the mechanism of slow lithium extrusion. Propagation involves dry cracks, with Li driving the crack forward from the rear by a wedge-opening mechanism, rather than lithium at the crack tip, as had often been assumed previously. Informed by the description of dendrite cracks, we control the microstructure of an Argyrodite (Li₆PS₅Cl) solid electrolyte and examine the impact of different microstructures on the dendrites. These studies consider how microstructure influences critical current densities for dendrite growth.

Solid-state batteries have the potential to be a disruptive technology because of their ability to improve safety and increase energy density by incorporating Li metal anodes.  However, all solid-state interfaces present unique challenges, including high interfacial impedances, accommodation of mechanical stresses due to solid-solid interfacial contact, and (electro)chemical instabilities that can evolve during dynamic cycling conditions. Furthermore, a significant challenge facing the scale-up of SSBs is to improve our understanding of processing science needed to enable manufacturing.

To address these challenges, our group focuses on gaining new fundamental insights into the coupled phenomena occurring at interfaces, and applied this knowledge to rationally design interfacial composition and structure to address the root cause of performance limitations. As an enabling technology, we have developed operando video microscopy methods that allow for a dynamic visualization of the morphological and electrochemical evolution of solid-state electrochemical interfaces.

In this talk, I will describe our journey to deepen our understanding of interfacial phenomena at both the Li metal anode side, as well as in composite solid-state cathode materials.  On the anode side, there is a critical need to understand and control the coupling between electrochemistry, morphology, and mechanics, to improve the uniformity and reversibility of plating and stripping during charge and discharge, respectively [1-4].  On the cathode side, we focus on overcoming energy/power tradeoffs, which exhibit unique phenomena owing to the single-ion conducting nature of ceramic electrolytes, as well as improving stability at high voltages [5].  Throughout the discussion, a common theme will be the identification of electrochemical signatures of the coupled thermodynamic, kinetic, and transport phenomena that occur at solid-state battery interfaces.  Finally, equipped with this fundamental knowledge, I will discuss strategies to rationally design and manufacture optimized electrode architectures to improve stability and reversibility during cycling.

In the conventional view, electrode kinetic properties are obtained ex-situ near equilibrium, where the lithium concentration is only slightly perturbed. However, during high cycling rates, lithium transport within the electrode creates substantial lithium concentration differences, leading to dynamic phase separation. Therefore, we discovered that conventional kinetic properties fail to assess the effective rate capability at fast C-rates. Instead, kinetic phase heterogeneity, characterized by dynamic separation observed in-situ with X-ray diffraction (XRD) and deviations in lattice parameters, exclusively correlates with the capacity at high C-rate cycling.

 To understand such phase evolution or separation, it is crucial to reveal the lithium transport pathway within the individual crystalline particles or grains. Using operando X-ray microscopy technology, our group has unveiled how lithium inserts and de-inserts within individual crystalline battery particles during cycling. We note that lithium transport and phase evolution behavior on the surface are critically different from those in the bulk and also play a crucial role in determining the rate capability.

 By performing electrode-level operando XRD and single-particle-level operando X-ray microscopy, lithium insertion and de-insertion kinetics could be elucidated much further, aiding the understanding of fast battery cycling protocols.

Interfaces have been a key region of interest in solid-state batteries for many years. The dynamic interplay between the solid-electrolyte and the respective anode defines crucial characteristics of the resulting Solid-Electrolyte Interphase (SEI), particularly in terms of chemical states and acts as a key indicator of the solid-electrolyte’s suitability for a specific battery system. Understanding the fundamentals of the SEI formation is paramount in developing next-generation batteries. Previous studies of interface characterisation have been confined to ex-situ analyses, which require numerous repetitions to yield reproducible and representative data for comparison with industrial systems. In this work, we present an in situ characterisation method for interfaces, that is simple and adaptable to various sample conditions such as, as-prepared material or electrochemically tested samples.

In this work, the solid-electrolyte used was based on the NASICON composition with a stochiometry of Na_3.4Zr₂Si_2.4P_0.6O₁₂ synthesised through solution-assisted solid state reaction. In situ metallic anode|NASICON interface was formed by inducing negative charge across the analysis area of the sample, to form a ‘negative’ electrode, through the applied combination of flood gun and primary-ion gun within a Time-of-Flight Secondary Ion Mass Spectrometer (ToF-SIMS). This created charge imbalance between the ‘negative’ electrode and metallic anode, promoting the diffusivity of Na⁺ ions. To facilitate the mobility of charge carriers, and provide optimal connection to both negative and positive electrode, samples were setup in an in-house metallic casing. Ion maps and profiles were obtained using the same ToF-SIMS instrument, and cross-validated to results obtained via ex situ X-Ray Photoelectron Spectroscopy.

Evolution in the chemical states of the sodium anode|NASICON interface was observed indicating an interphase formation, suggested to be Na₂ZrO₃. The condition of the sample, as-prepared or electrochemically tested, was also found to influence the homogeneity of the Na⁺ plating, thus resulting in direct observation of metal filament/dendrite propagation. Our findings demonstrate that Na⁺ mobility can be controlled to induce galvanostatic charge/discharge cycling, that effectively mimick a battery’s mode of operation and the SEI formation that would occur in a conventional symmetric cell. This work not only advances our understanding of solid-electrolyte interfaces using NASICON in sodium-ion batteries but also presents a practical and versatile approach to in situ characterisation, laying the foundation for the development of more efficient solid-electrolytes for next-generation batteries.