Electrochemical Behavior and Interfacial Failure of Li _2.22 Zr _1.11 Cl _5.33 O _0.67 in High-Performance All-Solid-State Batteries

All-solid-state lithium-ion batteries (ASSBs) are promising energy storage systems owing to their high energy density and intrinsic safety. Halide solid electrolytes, especially Li 2 ZrCl 6 -based materials, have been extensively reported for their good compatibility with high-voltage cathodes; however, their interfacial instability and the associated failure mechanisms in practical cell configurations remain insufficiently understood. In this work, without introducing new electrolyte compositions or cell architectures, we systematically investigate the electrochemical behavior and failure mechanisms of a previously reported anion-regulated oxychloride electrolyte, Li 2.22 Zr 1.11 Cl 5.33 O 0.67 (LZCO), in all-solid-state lithium-ion batteries. The LZCO electrolyte exhibits a room-temperature ionic conductivity of 1.2 × 10 –3 S cm –1, yet pronounced interfacial degradation is observed under cycling. To enable mechanistic analysis under realistic operating conditions, a multilayer configuration consisting of a Li–In alloy anode, a sulfide interlayer, an LZCO electrolyte, and a high-nickel NCM955 cathode is employed as a model system. The assembled cells deliver stable cycling with 78.3% capacity retention after 300 cycles at 1C and an average Coulombic efficiency exceeding 99.7%. Combined electrochemical analysis and XPS characterizations reveal that capacity fading of the cells is primarily associated with the partial reduction of NCM955 during repeated charge–discharge cycles, accompanied by interfacial degradation. This study provides mechanistic insights into interfacial failure in halide-based ASSBs and offers guidance for their reliable practical implementation.