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.