A positive electrode active material in which the number of defects that cause deterioration is small or progress of the defect is suppressed is provided. The positive electrode active material is used for a secondary battery. The positive electrode active material contains lithium cobalt oxide containing an additive element. After a cycle test is performed on a cell that uses the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, the positive electrode active material includes a defect and contains at least the same element as the additive element in a region in the vicinity of the defect. The additive element is contained also in a surface portion of the positive electrode active material.