A novel method for forming a positive electrode active material is provided. In the method for forming a positive electrode active material, a cobalt source and an additive element source are mixed to form an acidic solution; the acidic solution and an alkaline solution are made to react to form a cobalt compound; the cobalt compound and a lithium source are mixed to form a mixture; and the mixture is heated. The additive element source is a compound containing one or more selected from gallium, aluminum, boron, nickel, and indium.