Halide solid-state electrolytes: structures, properties, anodic interfacial challenges and modification strategies

All-solid-state lithium batteries (ASSLBs) employing solid-state electrolytes (SSEs) and lithium metal anodes (LMAs) offer a promising solution toward high energy density and improved safety. Among SSE candidates, halide SSEs have attracted increasing attention due to their relatively balanced advantages in terms of high-voltage stability, ionic conductivity, mechanical deformability, and air stability. Nevertheless, practical application of halide SSEs still faces significant challenges, particularly at the LMA/SSE interface, where complex interfacial degradation affects long-term electrochemical performance. This review presents a comprehensive overview of halide SSEs and provides an in-depth analysis of the dynamic evolution mechanisms at the LMA/SSE interface. First, the crystal structure, ion conduction mechanisms, and key physicochemical properties of representative halide SSEs are summarized. Next, the origins of interfacial instability between lithium metal and halide SSEs are thoroughly examined, with emphasis on the intrinsic and interconnected chemical, electrochemical, and mechanical degradation processes. Additionally, recent advancements in anode interface engineering strategies are reviewed, including lithium metal modification, dual-electrolyte architectures, and crystal structure engineering of the electrolyte. Finally, perspectives on future research directions and remaining challenges associated with the integration of halide SSEs and LMAs in ASSLBs are proposed.