A method of forming a highly purified positive electrode active material is provided. A method of forming a positive electrode active material whose crystal structure is not easily broken even when charge and discharge are repeated is provided. The method of forming a positive electrode active material including lithium and a transition metal includes a first step of preparing a lithium source and a transition metal source and a second step of crushing and mixing the lithium source and the transition metal source to form a composite material. In the first step, a material with a purity of greater than or equal to 99.99% is prepared as the lithium source and a material with a purity of greater than or equal to 99.9% is prepared as the transition metal source. In the second step, crushing and mixing are performed using dehydrated acetone.