State‐of‐Charge Dependent Aging Mechanism of Halide‐Based Composite Cathodes for All‐Solid‐State Batteries

ABSTRACT Halide‐based solid‐state electrolytes (SSEs), such as Li 3 InCl 6 (LIC), are promising catholytes for all‐solid‐state batteries (ASSBs) because of their high ionic conductivity and high‐voltage stability. However, the aging mechanism between halide SSEs and Ni‐rich cathodes (LiNi x Co y Mn 1 −x−y O 2 , NCM) remain poorly understood. Herein, we investigate the state‐of‐charge (SoC)‐dependent aging behavior of LIC/NCM composite cathodes and reveal a non‐monotonic relationship between SoC and capacity retention after aging. Severe capacity loss occurs under both low‐ and high‐SoC conditions, whereas high capacity retention is achieved after aging at a mid‐range SoC. Comprehensive structural and chemical analyses uncover a synergistic aging mechanism: reductive decomposition of LIC dominates aging at low SoCs, while structural degradation of NCM accounts for aging at high SoCs. Furthermore, an ultrathin coating layer introduced onto the NCM particle surface via atomic layer deposition effectively suppresses interfacial reactions and enhances electrochemical stability, particularly during low‐SoC storage. This work provides mechanistic insights into SoC‐dependent interfacial aging in halide‐based ASSBs and proposes both SoC management and interface engineering strategy to extend their calendar life.