Battery separator for lithium polymer battery

1. A method of making a battery separator for a lithium polymer battery comprises the steps of:

providing a microporous membrane,

providing a mixture of a gel forming polymer, a first solvent and a second solvent, the first solvent being at least 80% by weight of the mixture and more volatile than the second solvent, and said second solvent adapted to form pores in the gel-forming polymer, wherein said first solvent is selected from the group consisting of: tetrahydrofuran, methylethylketone, acetone, low molecular weight glymes, and mixtures thereof, wherein said second solvent is not a plasticizer, wherein said second solvent is selected from the group consisting of butanol, propanol, isopropanol, and mixtures thereof, and wherein water is added into said second solvent in a range of 0.25:1 to 2:1;

coating the mixture onto at least one side of the microporous membrane,

driving off the first solvent, thereby forming a microporous membrane coated with a non-porous coating of the gel forming polymer and the pore forming second solvent, and

driving off the second solvent, thereby forming a microporous membrane coated with a porous coating of the gel forming polymer.

2. The method of claim 1 wherein coating comprises dip coating.

3. The method of claim 1 wherein driving off the second solvent comprises heating.

4. The method of claim 1 wherein a weight ratio of gel forming polymer to said second solvent is 1:0.5 to 1:4.

5. The method of claim 1 wherein said gel forming polymer is selected from the group consisting of: polymers, polymers and comonomers and combinations thereof, where said polymer is selected from the group consisting of polyvinylidene fluoride, polyurethane, polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyacrylamide, polymethylacrylate, polymethylmethacrylate, polyvinylacetate, polyvinylpyrrolidnone, polytetraethylene glycol diacrylate, copolymers of the foregoing, and mixtures thereof;

where said comonomers are selected from the group consisting of:

hexafluoropropylene, octofluoro-l-butane, octofluoroisobutane, tetrafluoroethylene and combinations thereof.

6. The method of claim 5 wherein said gel forming polymer being a polyvinylidene fluoride:hexafluoropropylene copolymer.

7. The method of claim 1 wherein said first solvent is acetone.

8. The method of claim 1 wherein said second solvent is isopropanol.

9. The method of claim 5 wherein said comonomer content ranges from 3 to 20% by weight.

10. The method of claim 5 wherein said comonomer content ranges from 7 to 15% by weight.

11. The method of claim 5 wherein said gel forming polymer has a weight ratio of 91:9 where 91 is polyvinylidene fluoride and 9 is hexafluoropropylene.

12. The method of claim 1 wherein the coating step is performed to both external surfaces of said microporous membrane.

13. The method of claim 1 wherein said porous coating has a surface density ranging from 0.12 mg/cm 2 to 0.7 mg/cm 2 .

14. The method of claim 13 wherein said porous coating has a surface density ranging from 0.15 mg/cm 2 to 0.3 mg/cm 2 .

15. The method of claim 1 wherein driving off the first solvent comprises evaporating.

16. The method of claim 1 wherein the gel forming polymer is selected from the group consisting of polyvinylidene fluoride, polyurethane, polyethylene oxide, polypropylene oxide, polymethylacrylate, polymethylmethacrylate, polyvinylacetate, polyvinylpyrrolidnone, polytetraethylene glycol diacrylate, copolymers of the foregoing, and mixtures thereof.

17. The method of claim 1 wherein the microporous membrane coated with a porous coating of the gel-forming polymer has a uniformity of resistance McMullin Number in the range of 5 to 12.

18. The method of claim 17 wherein the microporous membrane coated with a porous coating of the gel-forming polymer has a uniformity of resistance McMullin Number in the range of 5 to 6.

19. The method of claim 1 wherein the relative humidity (%RH) during the coating step is below 45%.

20. The method of claim 19 wherein the relative humidity (%RH) during the coating step is in the range of 38 to 44%.