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 is a complex oxide represented by the general formula LiaNixCoyMnzMbO2, wherein
• 0.9<a<1.1,
• 0.4≤x<1.0,
• 0≤y<0.4,
• 0≤z<0.4,
• 0≤b<0.2, and
• 0.9<(x+y+z+b)<1.1
• are satisfied, and
• an element M includes at least one selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al, Ga, and In;
• and
• a ratio of a BET specific surface area after a particle compression test (B) to a BET specific surface area before the particle compression test (A) (B/A) of particles of the complex oxide is 1.0 or more and 3.0 or less.