METHOD OF PREPARING SULFIDE-BASED SOLID ELECTROLYTE, SULFIDE-BASED SOLID ELECTROLYTE PREPARED THEREFROM, AND SOLID SECONDARY BATTERY INCLUDING THE SULFIDE ELECTROLYTE

A method of preparing a sulfide solid electrolyte, the method including: first contacting a starting materials including LiS, PS, and LiI in a first solvent to provide a precursor; and second contacting the precursor with a second solvent to prepare the sulfide solid electrolyte, wherein the first solvent includes a C-Calkyl group or a cyclic ether compound which is unsubstituted or substituted with a C-Calkoxy group, and the second solvent includes a C-Chydrocarbon substituted with a Cto Calkoxy group.