Interplay of Cathode–Halide Solid Electrolyte in Enhancing Thermal Stability of Charged Cathode Material in All-Solid-State Batteries

All-solid-state batteries (ASSBs) are expected to address the thermal instability of conventional rechargeable batteries, given nonflammable inorganic solid electrolytes (SEs). However, the interaction between sulfide SEs and electrode materials can cause an exothermic reaction accompanied by the formation of explosive decomposition products. Herein, we demonstrate the enhanced thermal stability of a charged cathode material (Li1–xNi0.6Co0.2Mn0.2O2, x ≈ 0.5) with a Li3InCl6 halide SE compared to sulfide SEs. Li3InCl6 and the cathode composite not only delay the decomposition of NCM622 but also mitigate oxygen evolution from the cathode via oxidation decomposition of the halide SE. Furthermore, the halide SE suppresses combustible oxygen-gas evolution by capturing oxygen species through a mitigated exothermic reaction accompanying an endothermic phase transition from oxychloride to oxide. Oxygen capture was also observed in other halide SEs (Li3YCl6 and Li2ZrCl6). These findings emphasize the pivotal role of the cathode–SE interfacial interplay in governing the thermal stability of ASSBs and suggest SE design criteria for thermally safe battery systems.