| abstract |
Polyolefin multilayer microporous membrane with low air permeability, high porosity and mechanical strength even when thinned, excellent impedance characteristics, and excellent battery characteristics when used as a battery separator Offer. A polyolefin multilayer microporous membrane comprising at least a first microporous layer and a second microporous layer, wherein the first microporous layer comprises a first polyolefin resin containing polypropylene. The second microporous layer is made of a second polyolefin resin containing ultrahigh molecular weight polyethylene, and satisfies the following requirements (I) to (II): by. (I) The film thickness is 25 μm or less, and the film thickness (μm) and the porosity (%) satisfy the relationship of the following formula. Porosity (%) / film thickness (μm) ≧ 3.0 (formula) (II) 16 [mu] m in terms of air permeability is less than 100 sec / 100 cm 3 or more 300 sec / 100 cm 3. |