Patent ID: 8252450

Claim:
A multi-layer, microporous polyolefin membrane comprising first microporous layers constituting at least both surface layers, and at least one second microporous layer disposed between both surface layers, said first microporous layer comprising (a) a first polyethylene resin containing 8% or more by mass of ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1×10 6 or more, based on the total mass of the polyethylene in the first microporous layer, and (b) 25% or less by mass of polypropylene based on the total mass of the polyethylene and the polypropylene in the first microporous layer, said second microporous layer comprising a second polyethylene resin containing 7% or less by mass of said ultra-high-molecular-weight polyethylene, and having a structure in which a pore diameter distribution curve obtained by mercury intrusion porosimetry has at least two peaks, and the total thickness of said first microporous layers being 15-60% per 100% of the total thickness of said first and second microporous layers.