1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery, a battery pack using the nonaqueous electrolyte battery, and a vehicle having the battery pack mounted thereto.
2. Description of the Related Art
A nonaqueous electrolyte battery is charged and discharged by the migration of lithium ions between the negative electrode and the positive electrode. Research is being conducted in an attempt to develop a nonaqueous electrolyte battery as a battery having high energy density.
Nowadays, a nonaqueous electrolyte battery comprising a lithium-transition metal composite oxide used as a positive electrode active material and a carbonaceous material used as a negative electrode active material has been commercialized. In general, Co, Mn, Ni, etc., are used as the transition metal included in the lithium-transition metal composite oxide.
In recent years, a nonaqueous electrolyte battery comprising a lithium-titanium composite oxide as the negative electrode active material has been put to practical use. The lithium-titanium composite oxide has a Li absorption-release potential higher than that of the carbonaceous material. Also, the lithium-titanium composite oxide is small in change of the volume accompanying the charge-discharge operation and, thus, is excellent in the charge-discharge cycle characteristics. Among the lithium-titanium composite oxides, the lithium-titanium oxide having the spinel structure offers particular hope for further improving the nonaqueous electrolyte battery.
For example, Japanese Patent Application KOKAI Publication No. 07-302587 discloses a nonaqueous electrolyte lithium secondary battery in which a mixture of the lithium-titanium oxide having the spinel structure and Nb2O5 is used as the negative electrode. The secondary battery disclosed in this prior art is intended to obtain excellent charge-discharge cycle characteristics by utilizing the feature of the lithium-titanium oxide having the spinel structure. At the same time, the secondary battery is intended to moderate the rapid drop of the battery voltage that is generated in the end period of the discharge of the secondary battery in the case where the lithium-titanium oxide having the spinel structure is used in the negative electrode. Therefore, use is made of Nb2O5 that is relatively close to the lithium-titanium oxide in the potential in the stage of releasing lithium ions and that is small in the change of the potential. For achieving these objects, the lithium-titanium oxide and Nb2O5 are mixed at a ratio of 60:30 by weight in the Example described in the patent document quoted above. To be more specific, the weight ratio X/Y of the weight X of the lithium-titanium oxide to the weight Y of Nb2O5 in the negative electrode is set at 2.
However, it was impossible to obtain excellent charge-discharge cycle characteristics in the secondary battery disclosed in Japanese Patent Application KOKAI Publication No. 07-302587 quoted above because the negative electrode was constructed to contain a large amount of Nb2O5 relative to the amount of the lithium-titanium oxide.