In addition, a wide variety of batteries can be designed without severe limitations on the selection of a negative electrode active material. This can increase the potential difference between the positive electrode and the negative electrode, thereby allowing the design of a battery with a high energy density. Moreover, the sodium ion battery of the present invention, compared to the lithium ion secondary battery, is a battery obtainable at a lower cost because it can be composed of lower-cost materials due to the more abundant resources. Thus the present invention is extremely practical. For example, the claims in European patent application EP 2 413 416 A1 filed this year are:
1. A sodium ion battery comprising a positive electrode, a negative electrode and a sodium ion nonaqueous electrolyte, wherein the negative electrode comprises a negative electrode active material and a negative electrode current collector made of aluminum or aluminum alloy.
2. The sodium ion battery according to claim 1, wherein the negative electrode active material is a negative electrode active material capable of being doped and dedoped with sodium ions.
3. The sodium ion battery according to claim 1, wherein the negative electrode active material is a carbonaceous material capable of being doped and dedoped with sodium ions.
4. The sodium ion battery according to claim 1, wherein the positive electrode comprises a positive electrode active material capable of being doped and dedoped with sodium ions.
5. The sodium ion battery according to claim 4, wherein the positive electrode active material is a sodium inorganic compound.
6. The sodium ion battery according to claim 1, wherein the sodium ion nonaqueous electrolyte is liquid.
7. The sodium ion battery according to claim 6, comprising a separator.
8. The sodium ion battery according to claim 7, wherein the separator is a laminated film which has a heat resistant porous layer and a porous film laminated to each other.
9. Use of the negative electrode current collector made of aluminum or aluminum alloy as a negative electrode current collector of a sodium ion secondary battery.