LAUSR

Halide solid electrolytes have recently emerged as a promising option for cathode-compatible catholytes in solid-state batteries, owing to their superior oxidation stability at high voltage and their interfacial stability. However, their day-to-month-scale aging at the cathode interface has remained unexplored until now, while its elucidation is indispensable for practical deployment. Herein, we investigate the stability of halide solid electrolytes (e.g., Li3 InCl6 ) when used with conventional layered oxide cathodes during extended calendar aging. We find that, contrary to their well-known oxidation stability, halide solid electrolytes can be vulnerable to reductive side reactions with oxide cathodes (e.g., LiNi0.8 Co0.1 Mn0.1 O2 ) in the long term. More importantly, the calendar aging at a low state of charge or as-fabricated state causes more significant degradation than at a high state of charge, in contrast to typical lithium-ion batteries, which are more susceptible to high-state-of-charge calendar aging. This unique characteristic of halide-based solid-state batteries is related to the reduction propensity of metal ions in halide solid electrolytes and is correlated to the formation of an interphase due to the reductive decomposition triggered by the oxide cathode in a lithiated state. This understanding of the long-term aging properties provides new guidelines for the development of cathode-compatible halide solid electrolytes. This article is protected by copyright. All rights reserved.