Deciphering Emergent Oxyhalide Solid-State Electrolytes for Next-Generation All-Solid-State Lithium Metal Batteries.

The conventional lithium-ion batteries face safety risks from flammable electrolytes and stagnating energy density. All-solid-state lithium metal batteries (ASSLMBs) represent a paradigm shift, leveraging non-flammable solid-state electrolytes (SSEs) and high-capacity lithium metal anodes to overcome these hurdles. Among SSEs, halides are promising candidates due to their high ionic conductivity, exceptional oxidation stability, and deformability. However, their commercial viability is hindered by hygroscopicity, inadequate ionic conductivity compared to sulfide benchmarks, and interfacial instability with lithium metal. Recently, oxyhalide SSEs have been designed to bridge this gap by integrating oxygen into halide structures for superior electrochemical properties. This review systematically summarizes recent advances in emerging oxyhalide SSEs, with a focus on Li─M─ -O─Cl systems. We discuss their development, synthesis, and structural classification, and analyze ion-transport behavior in both crystalline and amorphous states. Design strategies for enhancing humidity stability, electrochemical window, and mechanical robustness are critically evaluated. We further examine battery-level applications, emphasizing the role of interfacial chemistry and microstructural control in determining electrochemical performance. Finally, we outline key challenges and future directions to accelerate the practical implementation of oxyhalide-based ASSLMBs.