Patent ID: 7794511

Claim:
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.