Nonaqueous electrolyte secondary battery and separator

1. A battery comprising:

a positive electrode;

a negative electrode; and

a separator disposed between the positive electrode and the negative electrode,

wherein the separator comprises

a substrate layer, and

a surface layer formed on at least one principal plane of the substrate layer,

wherein the surface layer comprises fibrils made of polyvinylidene fluoride having an average diameter of not more than 1.0 μm and inorganic particles composing at least 70 wt % of the surface layer.

2. The battery according to claim 1 , wherein the negative electrode has a negative electrode active material comprising at least one member selected from silicon and tin.

3. The battery according to claim 1 , wherein the negative electrode active material is a material comprising:

a first constituent element,

a second constituent element, and

a third constituent element;

the first constituent element is silicon or tin;

the second constituent element is at least one member selected from the group consisting of cobalt, iron, magnesium, titanium, vanadium, chromium, manganese, nickel, copper, zinc, gallium, zirconium, niobium, molybdenum, silver, indium, cerium, hafnium, tantalum, tungsten, bismuth and silicon; and

the third constituent element is at least one member selected from the group consisting of boron, carbon, aluminum and phosphorus.

4. The battery according to claim 1 , wherein a porosity of the surface layer is 20% or more and not more than 90%.

5. The battery according to claim 1 , wherein an average particle size of the inorganic particles is not more than 50% relative to a thickness of the surface layer.

6. The battery according to claim 1 , wherein the inorganic particles comprise alumina or silica.

7. The battery according to claim 1 , wherein the substrate layer comprises a polyolefin resin.

8. A separator comprising:

a substrate layer, and

a surface layer formed on at least one principal plane of the substrate layer, wherein the surface layer comprises fibrils made of polyvinylidene fluoride having an average diameter of not more than 1.0 μm and inorganic particles composing at least 70 wt % of the surface layer.

9. The separator according to claim 8 , wherein a porosity of the surface layer is 20% or more and not more than 90%.

10. The separator according to claim 8 , wherein the inorganic particles comprise alumina or silica.

11. The separator according to claim 8 , wherein the substrate layer comprises a polyolefin resin.

12. The battery according to claim 1 , wherein the surface layer comprises a three-dimensional network structure in which fibrils made of the polyvinylidene fluoride as a component are continuously connected with each other.

13. The battery according to claim 1 , wherein an amount of deformation against pressure of the surface layer is larger than that of the substrate layer.

14. The battery according to claim 13 , wherein the deformation is measured by vertically applying a pressure of 1, 5, 10 or 20 kgf/cm 2 and measurement of a displacement of the surface layer or the substrate layer.

15. The battery according to claim 14 , wherein the deformation of the surface layer measured by vertically applying the pressure of 20 kgf/cm 2 ranges from 23% to 60%.

16. The battery according to claim 1 , wherein an air permeability of the separator as measured by a Gurley densometer ranges from 250 to 400 sec/100 cc.

17. The battery according to claim 1 , wherein a thickness of the substrate layer ranges from 12 μm to 23 μm.

18. The battery according to claim 1 , wherein the negative electrode active material is a material comprising:

a first constituent element, and

a second constituent element,

the first constituent element is silicon or tin; and

the second constituent element is oxygen (O) or carbon (C).

19. An electronic appliance comprising the battery according to claim 1 .