Solid secondary battery, and method of preparing the same

A solid secondary battery comprising:

• a cathode layer;
• an anode layer;
• and
• a solid electrolyte layer between the cathode layer and the anode layer,
• wherein the cathode layer comprises:
• a cathode current collector;
• and
• a cathode active material layer on at least one side of the cathode current collector,
• wherein the cathode active material layer comprises a composite cathode active material,
• wherein the composite cathode active material comprises a composite of i) M2S, M being Li or Na, ii) an alkali metal salt, iii) an inorganic electronically-conductive structure, and iv) a two-dimensional carbonaceous structure and/or a fibrous carbonaceous material, the fibrous carbonaceous material having an aspect ratio of 2 or more,
• wherein the inorganic electronically-conductive structure has an electronic conductivity of 10−3 S/cm or more,
• wherein the composite comprises a solid solution of the M2S and the alkali metal salt, and
• wherein the two-dimensional carbonaceous structure is graphene, graphene oxide, or a combination thereof.