Halide‐based solid electrolytes: The history, progress, and challenges

Abstract Lithium metal solid‐state batteries (LMSBs) have attracted extensive attention over the past decades, due to their fascinating advantages of safety and potential for high energy density. Solid‐state electrolytes (SEs) with fast ionic transport and excellent stability are indispensable components in LMSBs. Heretofore, a series of inorganic SEs have been extensively explored, such as sulfide‐ and oxide‐based electrolytes. Unfortunately, they both have difficulty in achieving a satisfactory balance of conductivity and stability, and oxides suffer from a high impedance of grain boundaries, while sulfides encounter poor stability. Halide‐based solid electrolytes are gradually emerging as one of the most promising candidates for LMSBs due to their advantages of decent room temperature ionic conductivity (>10 −3 S cm −1 ), good compatibility with oxide cathode materials, good chemical stability, and scalability. Herein, research and development of the widely studied metal halide SEs including fluorides, chlorides, bromides, and iodides are reviewed, mainly focusing on the structures and ionic conductivities as well as preparation methods and electrochemical/chemical stabilities. And then, based on typical metal halide solid electrolytes, we emphasize the interface issues (grain boundaries, cathode−electrolyte and electrolyte–anode interfaces) that exist in the corresponding LMSBs and summarize the related work on understanding and engineering these interfaces. Furthermore, the typical (or in situ) characterization tools widely used for solid‐state interfaces are reviewed. Finally, a perspective on the future direction for developing high‐performance LMSBs based on the halide electrolyte family is put out.