Avoiding CO₂ Improves Thermal Stability at the Interface of Li₇La₃Zr₂O₁₂ Electrolyte with Layered Oxide Cathodes

Abstract Solid‐state batteries promise higher energy densities and better safety than Li‐ion batteries with liquid electrolytes. However, the interface between solid electrolyte and cathode is unstable at the elevated temperatures that are needed while sintering to achieve good bonding between ceramic components. Here, the hypothesis is that, the gas environment, especially the presence of CO_2, is critical in determining the stability of the solid electrolyte–cathode interface. The effect of gas species on the interface is systematically probed, by a using Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte with a thin film LiNi_0.6Mn_0.2Co_0.2O₂ cathode as a model system to enable interface sensitivity. Detrimental phases formed at the interface and their onset conditions are identified by X‐ray absorption spectroscopy, X‐ray diffraction, and Gibbs free energy analysis. As a result, removing CO₂ and minimizing H₂O(g) during sintering is necessary to obtain good contact at the LLZO|cathode interface without forming secondary phases. Sintering in O₂ is ideal, yielding excellent chemical stability and low interfacial resistance. Secondary phases also do not form in N_2, but oxygen loss occurs at elevated temperatures. The interfacial resistance obtained upon sintering in pure O₂ is comparable to the lowest values at LLZO interfaces with protective coatings, but here without the need for interface coatings.