All-solid-state batteries (ASSBs) are a rising alternative for boosting the volumetric energy density and are considered safer than conventional Li-ion batteries. However, Li-ion transport across the solid electrolyte (SE)/active materials (AMs) interfaces is limited by the parasitic (electro-)chemical side reactions responsible for the impedance rise and lower performance. The fundamental understanding of such interfaces has not been fully achieved yet, mainly due to the limitations in current surface-sensitive analytical techniques, especially in operando mode.

In this contribution, we will report the recent developments in operando X-ray photoelectron spectroscopy (XPS)[1], X-ray absorption spectroscopy (XAS)[2], and X-ray photoemission electron microscopy (XPEEM)[3] to monitor in real-time the interface evolution of operational ASSB working electrodes (WE) at multiple measurement length scales. Operando measurements are made possible thanks to the unique and versatile electrochemical custom-made cells designed to operate in ultra-high vacuum, providing reliable electrochemistry.[4] We will highlight how the combination of in-house operando XPS and synchrotron-based operando XAS/XPEEM using soft and tender X-rays can provide a reliable platform to investigate (i) the (electro‑)chemical evolution of the SE/AMs interfaces, (ii) the surface modifications of the AMs, and (iii) the electronic properties across electrified solid-solid interfaces. Different examples will be presented describing in a novel way the interface degradation mechanism observed in real time upon cycling.

Specifically, we will show results from operando XPS performed on WE made of ball milled Li₃PS₄ (LPS) mixed with VGCF conductive carbon. We identified accurately, by combining cyclic voltammetry and operando XPS, the onset potentials of the LPS oxidation and reduction, and the various formed byproducts species, when cycled in high voltage [2.4 V - 5 V vs. Li⁺/Li] and low voltage [2.4 V – 5 mV] ranges. We demonstrate the non-reversibility of the polysulfide with bridging sulfur species formed in the high voltage range during the 1^st charge, and the reversibility of the Li₂S and Li_xP species formed in the low voltage range over multiple cycles.

In operando XAS, we performed measurements at LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111) and LPS WE by acquiring the Mn, Ni, and Co L-edge spectra, as well as at the S and P K-edge spectra in both total electron yield (TEY) and total fluorescence yield (TFY) modes.[5] From the TEY surface sensitive mode (~10 of nm), we detected the formation of electrochemically inactive NCM surface rich in reduced Ni²⁺, Co²⁺, and Mn³⁺ transition metals co-responsible for the impedance rise, together with Li₃PS₄ oxidized byproducts. Additionally, from the TFY bulk sensitive mode (100s of nm) the charge compensation mechanism and the onset potentials during the NCM111 delithiation is elucidated by following the Ni and Co L-edges oxidation evolution to +4 state up to 4.5 V vs. Li⁺/Li. Interestingly, by monitoring the O K-edge we evidence the oxygen oxidation above 4.5 V and its reversible involvement in the charge compensation process. [6]  

Finally, with the support of operando XPEEM performed on both NCM622 and LiNbO₃ coated NCM622 mixed with Li₆PS₅Cl (LPSCl) as WEs, we elucidate the missing piece of the exact electrolyte-electrode degradation mechanism and the role played by the LiNbO₃ coating to reduce the impedance. Thanks to the excellent nanoscale lateral resolution better than 70 nm and the local spectroscopic capability on single NCM particles, with a depth of analysis of 3-4 nm, we revealed that the reduced transition metals formed at the cathode surface are mainly initiated by the layered oxygen loss which is further enhanced in contact with the LPSCl. Interestingly, we confirm that the presence of LiNbO₃ as coating mitigates the formation of the reduced transition metal and improve the Li kinetics at the interface with the SE.