Emerging Halide Superionic Conductors for All-Solid-State Batteries: Design, Synthesis, and Practical Applications

Recently, halide superionic conductors have emerged as promising solidelectrolyte (SE) materials for all-solid-state batteries (ASSBs), owing to their inherentproperties combining high Li(+)conductivity, good chemical and electrochemical oxidationstabilities, and mechanical deformability, compared to sulfide or oxide SEs. In thisReview, recent advances in halide Li+- and Na+-conducting SEs are comprehensivelysummarized. After introducing the ionic diffusion mechanism and related governingfactors of the crystal structures, we discuss the design strategies, such as the substitutionand synthesis protocols, of the halide materials for further improving their properties. Wereview theoretical and experimental results on electrochemical stabilities andcompatibilities with electrode materials. Moreover, we offer a critical assessment of thechallenges and issues associated with the development of practical ASSB applications,such as cost considerations, stabilities in atmospheric air, aqueous solutions, and slurry-processing, and the wet-slurry or dry fabrication of sheet-type electrodes (or SEmembranes) for large-format ASSBs. Based on these discussions, we provide a perspective on the future research directions ofhalide SEs, emphasizing the need for expanding the materials space.