Origin of the Outstanding Performance of Dual Halide Doped Li₇P₂S₈X (X = I, Br) Solid Electrolytes for All-Solid-State Lithium Batteries

Halide-doped sulfide solid electrolytes have attracted great attention due to their high lithium-ion conductivity, Li-metal compatibility, and deformability for use in all-solid-state Li batteries. Although the effects of halide mixing in Li6PS5X and Li4PS4X (X = halogen) solid electrolytes are fully investigated, studies on the origin of the remarkable increase in the ionic conductivity by dual halide doped Li7P2S8X (X = I, Br) solid electrolytes are scarce. Here, we systematically investigate the crystal phase evolution in glassy matrix and the corresponding ionic conductivity variation, revealing that a metastable Li10GeP2S12-like Li3PS4 phase with halide-doped glassy matrix is responsible for the outstanding performance.