Secondary battery positive electrode active material and secondary battery

A positive electrode active material for a secondary battery having an electrolytic solution prepared by dissolving a lithium salt in water, wherein

• the positive electrode active material includes a lithium transition metal oxide represented by the general formula LixM1-yLyO2, wherein 0.9≤x≤1.1, 0≤y<0.6, an element M is at least one selected from the group consisting of Ni and Co, and an element L is at least one selected from the group consisting of alkaline-earth elements, transition metal elements other than Ni and Co, rare earth elements, Group IIIb elements, and Group IVb elements, and
• the positive electrode active material is a complex oxide having an oxide of an element Me, which is at least one selected from the group consisting of B, Si, P, Ti, V, Mn, Al, Mg, Ca, Zr, W, Nb, Ta, In, Mo, and Sn, in an outer layer portion of the lithium transition metal oxide.