Interfacial Instability of Halide Solid Electrolytes and Oxide Cathodes Under Low‐Humidity Dry‐Room Conditions

ABSTRACT Halide solid electrolytes are widely regarded as promising catholytes for all‐solid‐state batteries because of their high ionic conductivity, good deformability, and relative compatibility with oxide cathodes. However, their interfacial stability has been largely evaluated under inert laboratory conditions, whereas practical battery manufacturing is typically conducted in low‐humidity dry rooms rather than inert‐atmosphere gloveboxes. Here, we demonstrate that halide solid electrolyte/oxide cathode composite electrodes can undergo severe interfacial degradation even under a low‐humidity dry‐room environment with a dew point of −40°C. Using Li 3 InCl 6 and LiNi 0.83 Co 0.12 Mn 0.05 O 2 as a representative model system, we reveal that dry‐room exposure induces interfacial reactions that generate a passivation layer associated with hindered Li + transport, leading to markedly deteriorated electrochemical performance. Similar degradation behavior was also observed in several representative halide electrolyte/layered oxide combinations, suggesting that this issue is not limited to a single material pair. These findings identify a previously underappreciated processing‐induced interfacial challenge in halide‐based all‐solid‐state batteries and underscore the need to place greater emphasis on composite‐cathode stability under realistic dry‐room manufacturing conditions.