Halide solid-state electrolytes for all-solid-state batteries: structural design, synthesis, environmental stability, interface optimization and challenges

Since the huge breakthrough in 2018, research on halide solid-state electrolytes (SSEs) has set off a new craze. In comparison with oxide and sulfide SSEs, halide SSEs have more balanced properties in various aspects, including ionic conductivity, electrochemical stability window, and moisture resistance. Herein, the overall knowledge and deep understanding of halide SSEs and their practical applications in all-solid-state batteries (ASSBs) are introduced. Firstly, the principle of screening halide SSE components is proposed. Among F, Cl, Br and I anions, the Cl anion is excellent owing to its suitable ionic conductivity and electrochemical stability window. The Sc, Y, and lanthanide elements are also more compatible with Cl anions in terms of electronegativity. Secondly, the structural design theory of halide SSEs with high ionic conductivity and the mechanism of Li ion migration are described. A monoclinic structure is more conducive to Li ion migration, compared with trigonal and orthorhombic structures. Additionally, substitution strategies for halide SSEs are discussed, mainly including dual-halogen, isovalent cation substitution, and aliovalent cation substitution. Furthermore, the mechanism of moisture resistance and synthesis method of halide SSEs are analyzed. Compared with the solid-state reaction and mechanochemistry method, wet chemical synthesis is more likely to achieve scale-up production of halide SSEs. Finally, the application prospects and challenges of halide SSEs in ASSBs are outlined.