COMPOSITE CATHODE ACTIVE MATERIAL FOR ALL-SOLID-STATE BATTERY, PREPARATION METHOD THEREOF, CATHODE LAYER FOR ALL-SOLID-STATE BATTERY, AND ALL-SOLID-STATE BATTERY INCLUDING THE CATHODE LAYER

A composite cathode active material for an all-solid-state battery including a sulfide solid electrolyte, a preparation method thereof, a cathode layer for an all-solid-state battery, and an all-solid-state battery including the cathode layer, the composite cathode active material including a secondary particle including a plurality of primary particles; and a buffer layer on a surface of the secondary particle, wherein the secondary particle includes a nickel lithium transition metal oxide represented by Formula 1 (LiNiMO), the buffer layer includes a first buffer layer adjacent to a surface of the secondary particle and including an oxide represented by Formula 2 (LiAO); and a second buffer layer including an oxide represented by Formula 3 (LiEO).