Humidity Stability of Halide Solid-State Electrolytes

Halide solid-state electrolytes (HSSEs) are regarded as highly promising electrolyte materials for solid-state batteries due to their high ionic conductivity, wide electrochemical window, and excellent mechanical processability. However, their extreme sensitivity to humidity leads to chemical decomposition in humid environments, causing crystalline structure collapse and electrochemical performance decay, which severely restricts practical applications. This review is designed to serve as an all-inclusive handbook for studying this critical issue. First, this paper systematically reviews the research progress in the humidity stability of HSSEs, and delves into the origins of humidity instability through hard and soft acids and bases (HSAB) theory, thermodynamic, and kinetic analysis. Next, the characterization of humidity stability is discussed across three dimensions: macroscopic reaction phenomena; microstructure and chemical composition; electrochemical performance.Finally, strategies for enhancing humidity stability are summarized from three aspects: elemental substitution, novel material design, and surface engineering. Furthermore, promising future research directions for developing highly humidity-stable HSSEs are proposed.