An object is to provide a power storage device with high discharge capacity and high energy density. The power storage device includes a positive electrode in which a positive electrode active material is formed over a positive electrode current collector; and a negative electrode which faces the positive electrode with an electrolyte interposed therebetween. The positive electrode active material includes a film-form first region which includes a compound containing lithium and nickel; and a film-form second region which includes a compound containing lithium and one or more of iron, manganese, and cobalt, but not containing nickel. The first region is covered with the second region. Since a superficial portion of the positive electrode active material does not contain nickel, nickel is not in contact with an electrolyte solution; thus, generation of a catalyst effect of nickel can be suppressed, and a high discharge potential of nickel can be utilized.