The adhesion between metal foil serving as a current collector and a negative electrode active material is increased to enable long-term reliability. An electrode active material layer (including a negative electrode active material or a positive electrode active material) is formed over a base, a metal film is formed over the electrode active material layer by sputtering, and then the base and the electrode active material layer are separated at the interface therebetween; thus, an electrode is formed. The electrode active material particles in contact with the metal film are bonded by being covered with the metal film formed by the sputtering. The electrode active material is used for at least one of a pair of electrodes (a negative electrode or a positive electrode) in a lithium-ion secondary battery.