Solid Halide Electrolytes with High Lithium‐Ion Conductivity for Application in 4 V Class Bulk‐Type All‐Solid‐State Batteries

New lithium halide solid-electrolyte materials, Li₃ YCl₆ and Li₃ YBr₆ , are found to exhibit high lithium-ion conductivity, high deformability, and high chemical and electrochemical stability, which are required properties for all-solid-state battery (ASSB) applications, particularly for large-scale deployment. The lithium-ion conductivities of cold-pressed powders surpass 1 mS cm^-1 at room temperature without additional intergrain or grain boundary resistances. Bulk-type ASSB cells employing these new halide solid electrolyte materials exhibit coulombic efficiencies as high as 94% with an active cathode material of LiCoO₂ without any extra coating. These superior electrochemical characteristics, as well as their material stability, indicate that lithium halide salts are another promising candidate for ASSB solid electrolytes in addition to sulfides or oxides.