Microporous membranes, methods for making such membranes, and the use of such membranes as battery separator film

1. A microporous membrane comprising a mixture of polyolefin, the mixture of polyolefin consisting essentially of a first polyethylene, a second polyethylene having a Tm ≧131.0° C. and an Mw of 5.6×10 5 to 7.5×10 5 , a third polyethylene having an Mw >1.0×10 6 , and polypropylene, wherein 1) the first polyethylene is a copolymer of ethylene and a comonomer which is selected from the group consisting of butene-1, pentene-1, hexane-1, 4-methyl-pentene-1, octane-1, vinyl acetate, methyl methacrylate and styrene, 2) the first polyethylene has a Mw of 3.8×10 4 to 2.6×10 5 , a molecular weight distribution of from 1.8 to 3.5 and a Tm of 123.0° C. to 126.0° C., 3) the second polyethylene and the third polyethylene are ethylene homopolymer, 4) an amount of the polypropylene is <5.0 wt. %, based on the weight of the polymer used to produce the microporous membrane and 5) the microporous membrane has a normalized air permeability of 1.0×10 2 seconds/100 cm 3 /20 μm to 3.58×10 2 seconds/100 cm 3 /20 μm.

2. The microporous membrane according to claim 1 , wherein the membrane has a shutdown temperature ≦130.5° C.

3. The microporous membrane according to claim 1 , wherein the first polyethylene is present in an amount of 4.0 wt. % to 20.0 wt. %, based on the total weight of polymer in the polymer-diluent mixture.

4. The microporous membrane according to claim 1 , wherein the second polyethylene is present in an amount of 50.0 wt. % to 95.0 wt. %, based on the total weight of polymer in the polymer-diluent mixture, and the third polyethylene is present in an amount of 1.0 wt. % to 46.0 wt. %, based on the total weight of polymer in the polymer-diluent mixture.

5. A battery separator film comprising the microporous membrane of claim 1 .

6. A battery comprising an electrolyte, an anode, a cathode, and a separator situated between the anode and the cathode, wherein the separator comprises the microporous membrane of claim 1 .

7. A method of producing a microporous membrane, comprising:

(1) extruding a mixture of diluent and polymer, the polymer consisting essentially of a first polyethylene, a second polyethylene having a Tm ≧131.0° C. and an Mw of 5.6×10 5 to 7.5×10 5 , a third polyethylene having an Mw >1.0×10 6 , and polypropylene, wherein 1) the first polyethylene is a copolymer of ethylene and a commoner which is selected from the group consisting of butene-1, pentene-1, hexane-1, 4-methyl-pentene-1, octane-1, vinyl acetate, methyl methacrylate and styrene, 2) the first polyethylene has a Mw of 3.8×10 4 to 2.6×10 5 , a molecular weight distribution of from 1.8 to 3.5 and a Tm of 123.0° C. to 126.0° C., 3) the second polyethylene and the third polyethylene are ethylene homopolymer, and 4) an amount of the polypropylene is <5.0 wt. %, based on the weight of the polymer used to produce the microporous membrane;

(2) stretching the extrudate in at least one planar direction; and

(3) removing at least a portion of the diluent from the stretched extrudate to form the microporous membrane which has a normalized air permeability of 1.0×10 2 seconds/100 cm 3 /20 μm to 3.58×10 2 seconds/100 cm 3 /20 μm and a shutdown temperature ≦130.5° C.

8. The method of claim 7 , wherein the first polyethylene is present in an amount of 4.0 wt. % to 17.0 wt. %, based on the weight of the polymer in the polymer-diluent mixture, the second polyethylene is present in an amount of 50.0 wt. % to 95.0 wt. % , based on the weight of the polymer in the polymer-diluent mixture, and the third polyethylene is present in an amount of 1.0 wt. % to 46.0 wt. %, based on the weight of the polymer in the polymer-diluent mixture.

9. A membrane product of the method of claim 7 .