Research Progress on Halide Solid‐State Electrolytes: Synthesis, Mechanism, and Modification

ABSTRACT Halide solid‐state electrolytes (HSSEs) have gained significant attention as key components for all‐solid‐state lithium ion batteries due to their notable advantages, including high ionic conductivity (> 1 mS cm −1 ), wide electrochemical window (> 4 V vs. Li/Li⁺), and good compatibility with high‐voltage cathodes. Despite progress, major challenges such as ionic conductivity, air stability, and interface compatibility still remain. This review systematically summarizes their representative classifications (e.g., Li a ‐M‐X 8 , Li a ‐M‐X 6 , Li a ‐M‐X 4 , Li a M b O c X d , M = In, Y, Al…; X = Cl, F, Br…), synthesis methods (e.g., solid phase, liquid phase, gas phase), and ion conduction mechanisms (e.g., vacancy‐driven transport). The merits and demerits of different synthesis methods are analyzed, and the factors affecting ion conductivity are also discussed. Moreover, various modification strategies (e.g., structure optimization, doping, and surface coating) are analyzed to address the above issues. Meanwhile, research guidelines for developing advanced HSSEs are also proposed. Additionally, we provide a systematic outlook on HSSEs in terms of novel synthesis methods and interface modification technologies (such as plasma and supercritical fluid technologies), high‐precision characterization methods for interface components (such as solid‐state nuclear magnetic resonance), artificial intelligence (AI)‐assisted mechanism analysis, and material synthesis. This review offers new research insights into the design and development of advanced solid‐state electrolytes for energy storage.