Alkali ion conducting plastic crystals

A solid electrolyte represented by general formula Li y SiR x (MO 4 ), where x is an integer from 1 to 3 inclusive, y=4−x, each R present is independently C1-C3 alkyl or C1-C3 alkoxy, and M is sulfur, selenium, or tellurium. Methods of making the solid electrolyte include combining a phenylsilane and a first acid to yield mixture including benzene and a second acid, and combining at least one of an alkali halide, and alkali amide, and an alkali alkoxide with the second acid to yield a product d represented by general formula Li y SiR x (MO 4 ) y . The second acid may be in the form of a liquid or a solid. The phenylsilane includes at least one C1-C3 alkyl substituent or at least one C1-C3 alkoxy substituent, and the first acid includes at least one of sulfuric acid, selenic acid, and telluric acid.