CROSS REFERENCE TO RELATED APPLICATIONS

The present invention application claims priority to Japanese Patent Application No. 2017-188282 filed in the Japan Patent Office on Sep. 28, 2017, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to a separator for nonaqueous electrolyte secondary batteries and a nonaqueous electrolyte secondary battery.

Description of Related Art

Separators which are porous resin sheets having ionic permeability and insulating properties are used in electrode assemblies for nonaqueous electrolyte secondary batteries such as lithium ion batteries. For example, Japanese Patent No. 3939778 (Patent Document 1) discloses a separator which is composed of a porous multilayer film including a porous polypropylene layer formed from polypropylene with a weight-average molecular weight of 660,000 to 980,000 and a porous layer containing a material with a melting point ranging from 100° C. to 140° C. and which has a needle penetration strength of 470 gf to 510 gf. Japanese Patent No. 5202866 (Patent Document 2) discloses a separator which is a polyolefin multilayer microporous membrane including first porous layers forming two surface layers and a second porous layer placed between the surface layers.

The first porous layers are made of ultra-high molecular weight polyethylene and the second porous layer is made of ultra-high molecular weight polyethylene and high-density polyethylene.

BRIEF SUMMARY OF THE INVENTION

Nonaqueous electrolyte secondary batteries are required to have further increased capacity and power. For a separator which is one of members of a secondary battery, a reduction in thickness for achieving high capacity and an increase in porosity for achieving high capacity are trends. It is an issue to ensure the strength of the separator. In the case where, for example, an electrode assembly is contaminated with a conductive contaminant, the conductive contaminant may possibly penetrate the separator to cause an internal short circuit when the separator has low strength.

A separator for nonaqueous electrolyte secondary batteries according to an aspect of the present disclosure includes a porous resin sheet having at least a three-layer structure consisting of an A-layer, a B-layer, and a C-layer stacked in that order. The average thermal expansion coefficient of the A-layer at a temperature of 0° C. to 50° C. is 100 ppm/K or more less than the average thermal expansion coefficient of the B-layer at a temperature of 0° C. to 50° C. The average thermal expansion coefficient of the C-layer at a temperature of 0° C. to 50° C. is 100 ppm/K or more less than the average thermal expansion coefficient of the B-layer at a temperature of 0° C. to 50° C.

A nonaqueous electrolyte secondary battery according to an aspect of the present disclosure includes an electrode assembly including a positive electrode, a negative electrode, and the separator and a nonaqueous electrolyte.

According to are aspect of the present disclosure, a separator, having high puncture strength, for nonaqueous electrolyte secondary batteries can be provided, the separator being capable of achieving a reduction in thickness and an increase in porosity. A nonaqueous electrolyte secondary battery according to an aspect of the present disclosure includes the separator. In the nonaqueous electrolyte secondary battery, even if an electrode assembly is contaminated with, for example, a conductive contaminant, the contaminant is sufficiently inhibited from penetrating the separator, thereby preventing the occurrence of an internal short circuit.

