Active material, nonaqueous electrolyte battery, battery pack, and vehicle

1. An active material comprising:

an active material particle including

a core particle containing a monoclinic or orthorhombic niobium-titanium composite oxide, and

a shell layer, which covers at least a part of a surface of the core particle,

wherein

the shell layer contains a compound which is at least one compound selected from the group consisting of a lithium-titanium composite oxide, an Nb-containing lithium-titanium composite oxide, a lithium-niobium composite oxide, a lithium phosphate, and an Nb-containing lithium phosphate, and the compound has a composition different from that of the monoclinic or orthorhombic niobium-titanium composite oxide.

2. The active material according to claim 1 , wherein the lithium-titanium composite oxide has a tetragonal type crystal structure, the lithium-niobium composite oxide has a trigonal type crystal structure, the Nb-containing lithium-titanium composite oxide has a tetragonal type crystal structure, and the lithium phosphate has a monoclinic crystal structure; and

the shell layer contains at least one compound selected from the group consisting of the lithium-titanium composite oxide, the lithium-niobium composite oxide, the Nb-containing lithium-titanium composite oxide, and the lithium phosphate.

3. The active material according to claim 1 , wherein the shell layer contains an amorphous structure.

4. The active material according to claim 1 , wherein an element ratio of Nb/Ti in the core particle is 1.5≦Nb/Ti<2.5.

5. The active material according to claim 1 , wherein the shell layer contains the Nb-containing lithium-titanium composite oxide, and the Nb-containing lithium-titanium composite oxide has an Nb content within a range of 1 atm % to 20 atm %.

6. The active material according to claim 1 , wherein an atomic concentration of an Nb-element in the shell layer is gradually decreased from a point of on a surface of the core particle toward a surface of the active material particle.

7. The active material according to claim 1 , wherein the shell layer has a thickness within a range of 0.1% to 30% of an average particle size of the core particle.

8. The active material according to claim 1 , wherein the active material particle contains the shell layer in a content of 0.1% to 15% by weight relative to the mass of the active material particle.

9. The active material according to claim 1 , wherein the shell layer covers a whole surface of the core particle.

10. A nonaqueous electrolyte battery comprising:

a negative electrode containing the active material according to claim 1 ;

a positive electrode; and

a nonaqueous electrolyte.

11. A battery pack comprising the nonaqueous electrolyte battery according to claim 10 .

12. The battery pack according to claim 11 , further comprising:

a power distribution external terminal; and

a protective circuit.

13. A battery pack comprising nonaqueous electrolyte batteries, each of the nonaqueous electrolyte batteries comprising:

a negative electrode comprising the active material according to claim 1 ;

a positive electrode; and

a nonaqueous electrolyte;

wherein the nonaqueous electrolyte batteries are electrically connected in series, in parallel, or with a combination of series connection and parallel connection.

14. A vehicle comprising the battery pack according to claim 11 .

15. The vehicle according to claim 14 , wherein the battery pack is configured to recover a regenerative energy caused by a power of the vehicle.