Multi-layer, microporous polyolefin membrane, its production method, battery separator, and battery

1. 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.

2. The multi-layer, microporous polyolefin membrane according to claim 1 , wherein said first microporous layer has an average pore diameter of 0.005-0.1 μm, and said second microporous layer has an average pore diameter of 0.02-0.5 μm.

3. The multi-layer, microporous polyolefin membrane according to claim 1 , wherein the average pore diameter ratio of said second microporous layer to said first microporous layer is more than 1/1 and 10/1 or less.

4. The multi-layer, microporous polyolefin membrane according to claim 1 , wherein said second microporous layer comprises dense domains having a main peak in a range of 0.01-0.08 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 said pore diameter distribution curve.

5. The multi-layer, microporous polyolefin membrane according to claim 4 , wherein the pore volume ratio of said dense domains to said coarse domains is 0.5-49.

6. The multi-layer, microporous polyolefin membrane according to claim 1 , wherein it has a three-layer structure having a pair of said first microporous layers on both surfaces of said second microporous layer.

7. The multi-layer, microporous polyolefin membrane according to claim 1 , wherein said first microporous layer is made of (i) a mixture of said ultra-high-molecular-weight polyethylene and polypropylene, the percentage of polypropylene being 25% or less by mass, or (ii) a mixture of (a) a first polyethylene composition comprising said ultra-high-molecular-weight polyethylene and high-density polyethylene having a weight-average molecular weight of 1×10 4 to 5×10 5 , the percentage of said ultra-high-molecular-weight polyethylene being 8% or more by mass, based on the total mass of said first polyethylene composition and (b) polypropylene, the percentage of polypropylene being 25% or less by mass based on the total mass of said first polyethylene composition and said polypropylene; and wherein said second microporous layer is made of (i) said high-density polyethylene, (ii) a second polyethylene composition comprising said ultra-high-molecular-weight polyethylene and said high-density polyethylene, the percentage of said ultra-high-molecular-weight polyethylene being 7% or less by mass, (iii) a mixture of said high-density polyethylene and polypropylene, the percentage of polypropylene being 25% or less by mass, or (iv) a mixture of said second polyethylene composition and polypropylene, the percentage of polypropylene being 25% or less by mass.

8. A battery separator formed by the multi-layer, microporous polyolefin membrane recited in claim 1 .

9. A battery comprising a separator formed by the multi-layer, microporous polyolefin membrane recited in claim 1 .

10. The multi-layer, microporous polyolefin membrane according to claim 2 , wherein the average pore diameter ratio of said second microporous layer to said first microporous layer is more than 1/1 and 10/1 or less.