Polyolefin microporous membrane and separator for lithium ion secondary battery
A polyolefin microporous membrane that can realize a lithium ion secondary battery having favorable resistance against foreign matters or the like, and high cycle characteristics at a high temperature is provided. The present invention provides a polyolefin microporous membrane having a ratio of tensile strength in a length direction to that in a width direction of 0.75 to 1.25, and a thermal shrinkage rate in the width direction at 120° C. of less than 10%.
1. A polyolefin microporous membrane having a ratio of tensile strength in a length direction to tensile strength in a width direction of 0.75 to 1.25, and a thermal shrinkage rate in the width direction at 105° C. of less than 1%,
wherein the polyolefin comprises not less than 30% by mass of polyethylene, and wherein the tensile strength in the width direction is not less than 160 MPa.
2. The polyolefin microporous membrane according to claim 1 , wherein the polyolefin microporous membrane has the ratio of tensile strength of 0.85 to 1.25.
3. The polyolefin microporous membrane according to claim 1 , wherein the polyolefin microporous membrane has the tensile strength of not less than 160 MPa and not more than 250 MPa.
4. The polyolefin microporous membrane according to claim 1 , wherein the polyolefin microporous membrane has a content of aluminum of not more than 10 ppm.
5. A separator for a lithium on secondary battery, comprising the polyolefin microporous membrane according to any one of claims 1 , 2 , 3 and 4 .
6. A lithium on secondary battery comprising the separator for a lithium on secondary battery according to claim 5 , a positive electrode, a negative electrode, and an electrolytic solution.