Abstract: Provided is a positive electrode for an all-solid-state lithium ion secondary battery which can reduce the internal resistance of the all-solid-state lithium ion secondary battery. The positive electrode includes an active material layer containing a positive electrode active material and a solid electrolyte material. The positive electrode active material contains secondary particles comprising an aggregate of a plurality of primary particles containing a lithium transition metal composite oxide. A smoothness of the secondary particles is more than 0.73, and a degree of circularity of the secondary particles is more than 0.83.

Abstract: The positive electrode active material includes secondary particles formed by aggregation of a plurality of primary particles that contain a lithium transition metal composite oxide having a layered structure and containing lithium and nickel. The secondary particles have a smoothness greater than 0.73, and a circularity greater than 0.83. The secondary particles contain cobalt and have a first region at a depth of 150 nm from a surface of the respective secondary particle and a second region at a depth of 10 nm or less from the surface of the respective secondary particle, and a ratio of a number of moles of cobalt to a total number of moles of metal elements other than lithium in the second region is larger than a ratio of a number of moles of cobalt to a total number of moles of metal elements other than lithium in the first region.