Aging Property of Halide Solid Electrolyte at the Cathode Interface

Abstract Halide solid electrolytes have recently emerged as a promising option for cathode‐compatible catholytes in solid‐state batteries (SSBs), 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, the stability of halide solid electrolytes (e.g., Li₃InCl₆) when used with conventional layered oxide cathodes during extended calendar aging is investigated. It is found that, contrary to their well‐known oxidation stability, halide solid electrolytes can be vulnerable to reductive side reactions with oxide cathodes (e.g., LiNi_0.8Co_0.1Mn_0.1O₂) 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 SSBs is related to the reduction propensity of metal ions in halide solid electrolytes and 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.