Abstract: Provided are a thin-film separator for a power storage device, the separator having high strength and reduced clogging, and a power storage device separator that provides high strength, high level of safety, and high dimensional stability at high temperature, and that can be formed in a thin film. One aspect provides a power storage device separator comprising a fine-porous layer (X) consisting mainly of polyolefin (A), the fine-porous layer (X) having a melt flow rate of less than or equal to 0.9 g/10 min, and having an average long-hole diameter of more than or equal to 100 nm according to MD-TD surface observation or ND-MD cross-sectional observation with a scanning electron microscope.

Abstract: The purpose of the present invention is to provide a separator which is for a power storage device, and which has high strength and can be thinned. The separator for a power storage device has a microporous membrane containing polyolefin as a main component, wherein the melt tension of the microporous membrane as measured at a temperature of 230° C. is 30 mN or less, and the melt flow rate (MFR) of the microporous membrane as measured under a load of 2.16 kg and at a temperature of 230° C. is 0.9 g/10 min or less.