Patent ID: 8906539

Claim:
A multi-layer, microporous polyolefin membrane comprising three or more layers, including (a) a first microporous layer constituting each outer layer and comprising polyethylene and polypropylene, wherein the polyethylene contains 7% or less by mass of ultra-high-molecular-weight polyethylene based on the total mass of the polyethylene in the first microporous layer, wherein the polypropylene content is 25% or less by mass based on the total mass of the polyethylene and the polypropylene in the first microporous layer, and wherein the ultra-high-molecular-weight polyethylene has a weight-average molecular weight of 1×10 6 or more, the first microporous layer having a structure in which a pore size distribution curve obtained by mercury intrusion porosimetry has at least two peaks, and (b) a second microporous layer constituting at least one intermediate layer comprising polyethylene containing 8% or more by mass of ultra-high-molecular-weight polyethylene based on the total mass of the polyethylene in the second microporous layer, wherein the first microporous layer comprises dense domains having a main peak in a range of 0.01 to 0.07 μm, and coarse domains having at least one sub-peak in a range of more than 0.08 μm and 1.5 μm or less in the pore size distribution curve.