Abstract: A polyolefin microporous film includes: a polyethylene-based resin; and a polyolefin (B) other than polyethylene. The polyolefin microporous film has peaks at temperatures of lower than 150° C. and 150° C. or higher respectively in a differential scanning calorimeter (DSC). A half width of the peak at lower than 150° C. is 10° C. or lower. A puncture strength in terms of 10 ?m is 2.0 N or more.

Abstract: A porous polyolefin film has a shutdown temperature of 133° C. or lower, a porosity of 41% or more, and a value of 12,500 or more, which is calculated by (tensile elongation (%) in the machine direction (MD)×tensile strength (MPa) in the machine direction (MD)+tensile elongation (%) in the transverse direction (TD)×tensile strength (MPa) in the transverse direction (TD))/2, the TSD (° C.) and Tm satisfying formula (1): Tm?TSD?0 ??(1), where TSD represents the shutdown temperature (° C.), and Tm represents the lowest among the melting point (° C.) of all layers, wherein excellent safety such as protection from internal short circuit and/or thermal runaway is achieved in the porous polyolefin film, without reducing the permeability as shown in conventional microporous films.

Abstract: To achieve an enhanced safety of a lithium ion battery while increasing the energy density and output of the battery, it is important to allow a separator to have a smaller maximum pore size, a higher pin puncture strength and a lower melt-heat shrinkage in the transverse direction. A polyolefin microporous membrane has a pin puncture strength per 20 ?m of membrane thickness of 4.5 N or more, and contains a polyolefin, wherein the polyolefin has such a molecular weight distribution that the ratio of the polyolefin having a molecular weight of 5.0×106 or more is 0.6 to 1.5%.