MICROPOROUS POLYOLEFIN FILM, SEPARATOR FOR BATTERY, SECONDARY BATTERY, AND METHOD FOR PRODUCING MICROPOROUS POLYOLEFIN FILM

The present invention addresses the problem of providing a microporous polyolefin film which is superior in output characteristics and strength to conventional ones. A first aspect of the present invention relates to a microporous polyolefin film which satisfies the relationship [Y (N)] ≥ [-6.7×10-3×(X(/µm3))+4.5] where Y (N) is the piercing strength in terms of 10-µm film thickness and X (/µm3) is the number of pores per unit volume, the number of pores X being 40 /µm3 or larger.