Nonaqueous electrolyte secondary battery and battery pack

1. A nonaqueous electrolyte secondary battery comprising:

a positive electrode containing olivine-structured Fe or a Mn-containing phosphorus compound as a positive electrode active material;

a negative electrode containing a titanium-containing metal oxide capable of inserting and extracting lithium ions as a negative electrode active material;

a nonwoven fabric separator, which contains an electrically insulating fiber and is bonded to a surface of at least one of the positive electrode and the negative electrode; and

a nonaqueous electrolyte, wherein

in a thickness direction of the nonwoven fabric separator, a density of the fiber on a side having contact with the positive electrode is high, and a density of the fiber on a side having contact with the negative electrode is low, and

wherein in the nonwoven fabric separator, an overall thickness is 20 μm or less, and a thickness of part, where the density of the fiber is high, is ⅓ or less of the overall thickness of the nonwoven fabric separator.

2. The nonaqueous electrolyte secondary battery according to claim 1 , wherein in the nonwoven fabric separator, a porosity of part, where the density of the fiber is high, is 25% to 40%, and a porosity of part, where the density of the fiber is low, is 40% to 75%.

3. The nonaqueous electrolyte secondary battery according to claim 1 , wherein an average fiber diameter of the fibers is within a range of 10 nm to 1000 nm.

4. The nonaqueous electrolyte secondary battery according to claim 1 , wherein the fiber is made from an organic material.

5. The nonaqueous electrolyte secondary battery according to claim 4 , wherein the organic material contains polyamide-imide.

6. The nonaqueous electrolyte secondary battery according to claim 1 , wherein the nonwoven fabric separator is bonded to the surface of the negative electrode.

7. The nonaqueous electrolyte secondary batter) according to claim 1 , wherein the nonwoven fabric separator contains an inorganic filler.

8. The nonaqueous electrolyte secondary battery according to claim 7 , wherein a content of the inorganic filler at part, where the density of the fiber is high, is higher than a content of the inorganic filler at part where the density of the fiber is low.

9. The nonaqueous electrolyte secondary battery according to claim 7 , wherein the inorganic filler is at least one selected from the group consisting of alumina, titan oxide and zinc oxide.

10. The battery pack comprising the nonaqueous electrolyte secondary battery according to claim 1 .

11. The nonaqueous electrolyte secondary battery according to claim 1 , wherein the titanium-containing metal oxide is at least one selected from the group consisting of titanium oxide, lithium titanium oxide and niobium titanium oxide.

12. The nonaqueous electrolyte secondary battery according to claim 1 , wherein the negative electrode includes a negative electrode current collector and a negative electrode layer which is formed on one surface or both surfaces of the negative electrode current collector and contains the negative electrode active material, an electroconductive agent and a binder.

13. The nonaqueous electrolyte secondary battery according to claim 12 , wherein the electroconductive agent is at least one selected from the group consisting of acetylene black, carbon black, coke, a carbon fiber, graphite, a metal compound powder, and a metal powder.