In recent years, lithium-ion secondary batteries and lithium-ion capacitors have been drawing attention as electric storage devices for use in mobile electronic devices such as mobile phones or laptop personal computers, electric vehicles, and hybrid vehicles. As anode current collectors for such electric storage devices, porous metal foils are used or are being considered for use. This is because making the foil porous provides benefits such that the weight of the foil can be reduced (to improve fuel consumption in automobiles), that adhesive power of an active material can be improved by anchoring effect making use of the pores, and that pre-doping of lithium ions (e.g., vertical pre-doping) can be efficiently conducted by making use of the pores.
Known methods for producing such porous metal foils include (1) a method of masking the surface of a substrate in a desired pattern with an insulating film, onto which electrolytic plating is conducted to form pores in accordance with the pattern; (2) a method of providing the surface of a substrate with a specific surface roughness or a specific surface condition, onto which electrolytic plating is conducted to control nucleation; (3) a method of perforating a non-porous metal foil by etching or machining; and (4) a method of forming a three-dimensional network structure by techniques for producing metal foams or plating nonwoven fabrics.
In particular, various techniques have been proposed for the above method of (2) since its steps are relatively simple and suitable for mass production. For example, Patent Literature 1 discloses a method for producing a fine-porous metal foil by subjecting an anode having a surface roughness Rz of 0.8 μm or less to electrolytic plating. Patent Literature 2 discloses a method comprising forming an oxidized film on the surface of a cathode body made of titanium or a titanium alloy by anode oxidization method; electro-depositing copper on the surface of the cathode body to form a porous copper foil; and peeling the foil from the cathode body. Patent Literature 3 discloses a method for producing a porous metal foil provided with an aluminum alloy carrier, comprising forming even projections by etching aluminum; and gradually growing metal particles from the projections as cores for electro-deposition so as to connect the metal particles.
However, the actual situation is that the porous metal foils produced in accordance with these techniques fail to achieve a sufficiently high aperture ratio due to a concern that the foil strength would be lowered. In addition, a long foil is difficult to produce, and anode oxidation process had problems with the peelability of the porous metal foil and the stability of the aperture ratio, in that continuous peeling of the foil destroys the oxidized film. In particular, a higher aperture ratio is required for anode current collectors of electric storage devices, such as lithium-ion secondary batteries and lithium-ion capacitors, with improvement in performance.