Sulfide-Based Solid Electrolyte and Preparation Method Thereof

A sulfide-based solid electrolyte contains a nickel (Ni) element and a halogen element. For example, a sulfide-based solid electrolyte can include, with respect to 100 parts by mole of a mixture of lithium sulfide (LiS) and diphosphorus pentasulfide (PS), 5 parts by mole to 20 parts by mole of nickel sulfide (NiS), and 5 parts by mole to 40 parts by mole of lithium halide.