The present invention provides a polyolefin multilayer microporous membrane that has a low air permeability value, maintains high porosity and mechanical strength even when formed into a thin film, has excellent impedance characteristics, and has excellent battery characteristics when used as a battery separator.
(Solution)
A polyolefin multilayer microporous membrane including a first microporous layer and a second microporous layer; the first microporous layer including a first polyolefin resin containing a polypropylene; the second microporous layer including a second polyolefin resin containing an ultrahigh molecular weight polyethylene; and the polyolefin multilayer microporous membrane satisfying the following requirements (I) and (II).
(I) A membrane thickness is not greater than 25 µm, and the membrane thickness (µm) and a porosity (%) satisfy the relationship in the formula below. Porosity % / thickness µm ≥ 3.0
(II) An air permeability in terms of a membrane thickness of 16 µm is not less than 100 sec/100 cm 3 and not greater than 300 sec/100 cm 3 .