Polyolefin Microporous Membrane and Separator For Lithium Ion Secondary Battery

Abstract

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 - 10 . (canceled) 
     
     
         11 . A polyolefin microporous membrane, wherein the polyolefin microporous membrane has a content of aluminum of not more than 70 ppm. 
     
     
         12 . The polyolefin microporous membrane according to  claim 11 , wherein the polyolefin microporous membrane has a total content of calcium, magnesium, zinc, and barium of not more than 300 ppm. 
     
     
         13 . The polyolefin microporous membrane according to  claim 11 , comprising polyolefin having a weight average molecular eight of not more than 500,000. 
     
     
         14 . A separator for a lithium ion secondary battery, comprising the polyolefin microporous membrane according to any one of  claims 11  to  13 .