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, 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 for the all-solid-state battery 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 and the buffer layer includes a metal oxide represented by Formula LiNiMO Formula LiAEO Formula