Solid state secondary battery and positive electrode active material

A secondary battery comprising a power generating element, the power generating element comprising:

• a positive electrode which includes a positive electrode active material layer containing a positive electrode active material composed of a particle of a lithium-containing composite oxide, the particle of the lithium-containing composite oxide including a central portion having a composition represented by chemical formula (1):
• Li1+qNixCoyMnzMpO2 (1)
• wherein −0.02≤q≤0.20, x+y+z+p=1, 0.5≤x≤1.0, 0≤y≤0.5, 0≤z≤0.5, 0≤p≤0.1, and M is one or more elements selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr and Cr;
• a surface region of the particle of the lithium-containing composite oxide having a depth within 100 nm from a surface of the particle of the lithium-containing composite oxide;
• a solid electrolyte layer which contains a sulfide solid electrolyte containing sulfur and phosphorus;
• and
• a negative electrode which includes a negative electrode active material layer containing a negative electrode active material, wherein
• the positive electrode, the solid electrolyte layer, and the negative electrode are laminated in this order, and
• one or more added element selected from the group consisting of B, S and Si is doped into the surface region of the particle such that the one or more added element is present in a molar concentration larger than that of Ni, and the Ni is present in the surface region.