Positive electrode active material, method for producing positive electrode active material, positive electrode, and secondary battery

A method for producing a positive electrode active material for a magnesium secondary battery or a lithium ion secondary battery, comprising:

• a step (1) of mixing a nucleation material solution in which the nucleation material is dissolved or dispersed and a solution containing a vanadium compound to form a mixed solution;
• a step (2) of adjusting the mixed solution to be acidic;
• and
• a precipitation step (3) of precipitating a crystal of vanadium oxide,
• wherein the precipitation step (3) is
• a step (3a) of precipitating vanadium oxide starting from a nucleus by precipitating the nucleus in the mixed solution, or
• a step (3b) of precipitating vanadium oxide starting from the nucleation material dispersed in the mixed solution.