The present disclosure relates to solid-state electrolytes and methods of making the same. The method includes admixing a sulfate precursor including one or more of Li2SO4 and Li2SO4.H2O with one or more carbonaceous capacitor materials. The first admixture is calcined to form an electrolyte precursor that is admixed with one or more additional components to form the solid-state electrolyte. When a ratio of the sulfate precursor to the one or more carbonaceous capacitor materials in the first admixture is about 1:2, the electrolyte precursor consists essentially of Li2S. When a ratio of the sulfate precursor to the one or more carbonaceous capacitor materials in the first admixture is less than about 1:2, the electrolyte precursor is a composite precursor including a solid-state capacitor cluster including the one or more carbonaceous capacitor materials and a sulfide coating including Li2S disposed on one or more exposed surfaces of the solid-state capacitor cluster.