Negative electrode active material containing a titanium oxide compound having a crystal structure of monoclinic system titanium dioxide

1. A negative electrode active material comprising a titanium oxide compound having a crystal structure of monoclinic system titanium dioxide,

wherein the titanium oxide compound has solid acid points on a surface thereof;

at least one kind of ion selected from the group consisting of Na + , K + , Mg 2+ , Ca 2+ , Cu 2+ , S 2− , SO 4 2− and Cl − exists on the surface of the titanium oxide compound;

the ion is bound to the solid acid points to make the solid acid points electrochemically inactive.

2. The negative electrode active material according to claim 1 , wherein said at least one kind of ion is selected from the group consisting of Na + , K + , and S 2− .

3. The negative electrode active material according to claim 1 , wherein an aspect ratio of the titanium oxide compound is in a range from 1 to 50, a length of the titanium oxide compound in the direction of its short axis is in a range from 0.1 to 50 μm and a length of the titanium oxide compound in the direction of its long axis is in a range from 0.1 to 200 μm.

4. The negative electrode active material according to claim 1 , which satisfies the following equation (1):

0.01≦X/O≦1  (1)

wherein X is an atomic percent of an element constituting said at least one kind of ion and O is an atomic percent of oxygen.

5. A nonaqueous electrolyte battery comprising:

a positive electrode which lithium can be inserted to and released from;

a negative electrode comprising the negative electrode active material as claimed in claim 1 ; and

a nonaqueous electrolyte.

6. A battery pack comprising the nonaqueous electrolyte battery as claimed in claim 5 .

7. The negative electrode active material according to claim 1 , wherein at least a part of the solid acid points on the surface of the titanium oxide compound are deactivated by the at least one kind of ion.

8. The negative electrode active material according to claim 1 , wherein said ion does not contribute to charge or discharge of the titanium oxide compound.

9. The negative electrode active material according to claim 1 , wherein each of the solid acid points comprises a hydroxyl group, and the ion is bond to the hydroxyl group.

10. The negative electrode active material according to claim 1 , wherein the crystal structure belongs to a space group C2/m.

11. The negative electrode active material according to claim 1 , wherein a BET specific surface area of the titanium oxide compound is 5 to 100 m 2 /g.

12. The nonaqueous electrolyte battery according to claim 5 , wherein the positive electrode comprises a positive electrode active material, and the positive electrode active material comprises at least one selected from the group consisting of manganese dioxide, iron oxide, copper oxide, nickel oxide, lithium-manganese composite oxide, lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-nickel-cobalt composite oxide, lithium-manganese-cobalt composite oxide, lithium-manganese-nickel composite oxide having a spinel structure, lithium-phosphorous oxide having an olivine structure, iron sulfate, vanadium oxide and lithium-nickel-cobalt-manganese composite oxide.

13. The nonaqueous electrolyte battery according to claim 5 , wherein the nonaqueous electrolyte comprises at least one selected from the group consisting of a liquid nonaqueous electrolyte, a gel-like nonaqueous electrolyte, an ionic liquid, a polymer solid electrolyte, and an inorganic solid electrolyte.