Battery pack and vehicle

1. A battery pack comprising nonaqueous electrolyte batteries, each comprising:

a positive electrode containing a positive electrode current collector, and a positive electrode layer which is formed on both surfaces of the positive electrode current collector and contains a lithium-transition metal oxide having a layered crystal structure;

a negative electrode containing a negative electrode current collector, and a negative electrode layer which is formed on both surfaces of the negative electrode current collector and contains a lithium-titanium composite oxide having a spinel structure; and

a nonaqueous electrolyte,

wherein the positive electrodes and the negative electrodes satisfy the formula (1) given below:

1.02≦X≦2  (1)

where X is a ratio of an available electric capacity of each of the negative electrodes at 25° C. to an available electric capacity of each of the positive electrodes at 25° C.,

the available electric capacity of each of the positive electrodes is an electric capacity obtained by charging under a constant current of 0.2 C to reach 4.25 V, followed by discharging under a constant current of 0.2 C to reach 3.5 V, and

the available electric capacity of each of the negative electrodes is an electric capacity obtained by charging under a constant current of 0.2 C to reach 1.0 V, followed by discharging under a constant current of 0.2 C to reach 2.0 V.

2. The battery pack according to claim 1 , wherein an amount of increase in an available electric capacity of each of the positive electrodes accompanying a temperature elevation is small, compared with an amount of increase in an available electric capacity of each of the negative electrodes accompanying a temperature elevation.

3. The battery pack according to claim 1 , wherein the positive electrodes and the negative electrodes satisfy the formula (2) given below:

1.02≦X≦1.44  (2)

where X is the ratio of the available electric capacity of each of the negative electrodes at 25° C. to the available electric capacity of each of the positive electrodes at 25° C.

4. The battery pack according to claim 1 , wherein the positive electrodes and the negative electrodes satisfy the formula (3) given below:

1.03≦X≦1.44  (3)

where X is the ratio of the available electric capacity of each of the negative electrodes at 25° C. to the available electric capacity of each of the positive electrodes at 25° C.

5. The battery pack according to claim 1 , wherein the lithium-transition metal oxide includes at least one composite oxide selected from the group consisting of lithium-cobalt composite oxide, lithium-nickel composite oxide and lithium-manganese composite oxide.

6. The battery pack according to claim 1 , wherein the layered crystal structure of the lithium-transition metal oxide is a layered halite type structure.

7. The battery pack according to claim 1 , wherein the lithium-transition metal oxide is represented by a composition formula of Li y M1 z1 M2 z2 O 2 , where M1 is at least one element selected from the group consisting of Co, Ni and Mn, M2 is at least one element selected from the group consisting of Fe, Al, B, Ga and Nb, and 0<y≦1.2, 0.98≦z1+Z2≦1.2 and 0≦z2<0.2, and where a ratio of a Ni amount to a sum of M1 and M2 is not larger than 0.85 (including 0).

8. The battery pack according to claim 1 , wherein the lithium-transition metal oxide is represented by a composition formula of Li y M1 z1 M2 z2 O 2 , where M1 is at least one element selected from the group consisting of Co, Ni and Mn, M2 is at least one element selected from the group consisting of Fe, Al, B, Ga and Nb, and 0<y≦1.2, 0.98≦z1+Z2≦1.2 and 0≦z2<0.2, and where a ratio of a Ni amount to a sum of M1 and M2 is not smaller than 0.3 and not larger than 0.85.

9. The battery pack according to claim 1 , wherein the nonaqueous electrolyte contains at least two kinds of solvents selected from the group consisting of propylene carbonate, ethylene carbonate and γ-butyrolactone.

10. The battery pack according to claim 1 , wherein each of the nonaqueous electrolyte batteries has a rated capacity not smaller than 2 Ah and not larger than 100 Ah.

11. The battery pack according to claim 1 , which includes at least 5 nonaqueous electrolyte batteries and not larger than 500 nonaqueous electrolyte batteries.

12. The battery pack according to claim 1 , which further comprising a protective circuit which detects a voltage of the nonaqueous electrolyte batteries.

13. The battery pack according to claim 1 , wherein each of the nonaqueous electrolyte batteries has a rated capacity not smaller than 3 Ah and not larger than 15 Ah.

14. The battery pack according to claim 1 , which includes at least 5 nonaqueous electrolyte batteries and not larger than 200 nonaqueous electrolyte batteries.

15. The battery pack according to claim 1 , wherein the negative electrode comprises a negative electrode current collector formed of an aluminum foil or an aluminum alloy foil.

16. The battery pack according to claim 1 , wherein each of the aluminum foil and the aluminum alloy foil has an average crystal grain size not larger than 50 μm.

17. The battery pack according to claim 1 , wherein at least some of the nonaqueous electrolyte batteries further comprises:

a case including a first sealing section formed at one edge portion and a second sealing section formed at another edge portion opposite to said one edge portion;

a positive electrode terminal including a tip portion withdrawn to an outside via the first sealing section of the case; and

a negative electrode terminal including a tip portion withdrawn to the outside via the second sealing section of the case.

18. The battery pack according to claim 17 , wherein each of the negative electrode terminal and the positive electrode terminal is formed of aluminum or aluminum alloy.

19. The battery pack according to claim 1 , wherein the case is formed of a laminate film.

20. A vehicle comprising a battery pack defined in claim 1 .