Battery separator for lithium polymer battery

Abstract

A battery separator for a lithium battery is disclosed. The separator has a microporous membrane and a coating thereon. The coating is made from a mixture of a gel forming polymer, a first solvent, and a second solvent. The first solvent is more volatile than the second solvent. The second solvent acts as a pore former for the gel-forming polymer.

That which is claimed:  
     
         1 . A battery separator for a lithium battery comprises 
 a microporous membrane, and    a coating thereon, said coating being made from a mixture comprising a gel forming polymer, a first solvent and a second solvent, the first solvent being more volatile than the second solvent, and said second solvent adapted to form pores in the gel-forming polymer.    
     
     
         2 . The battery separator according to  claim 1  wherein said second solvent includes water.  
     
     
         3 . The battery separator according to  claim 1  wherein said gel forming polymer being 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.  
     
     
         4 . The battery separator according to  claim 3  wherein said gel polymer being a polyvinylidene fluoride:hexafluoropropylene copolymer.  
     
     
         5 . The battery separator according to  claim 1  wherein said first solvent being selected from the group consisting of tetrahydrofuran, methylethylketone, acetone, low molecular weight glymes, and mixtures thereof.  
     
     
         6 . The battery separator according to  claim 1  wherein said second solvent being selected from the group consisting of butanol, propanol, isopropanol, and mixtures thereof.  
     
     
         7 . A lithium polymer battery comprising 
 an anode,    a cathode, and    a separator sandwiched between said anode and said cathode, said separator comprising a microporous membrane and a coating thereon, said coating being made from a mixture comprising a gel forming polymer, a first solvent, and a second solvent, the first solvent being more volatile than the second solvent, and said second solvent adapted to form pores in the gel forming polymer.    
     
     
         8 . The lithium polymer battery of  claim 7  further comprising an electrolyte being impregnated within said separator and in electrochemical communication with said anode and said cathode.  
     
     
         9 . The lithium polymer battery of  claim 7  wherein said second solvent includes water.  
     
     
         10 . The lithium polymer battery of  claim 7  wherein said gel forming polymer being 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.  
     
     
         11 . The lithium polymer battery of  claim 10  wherein said gel forming polymer being a polyvinylidene fluoride:hexafluoropropylene copolymer.  
     
     
         12 . The lithium polymer battery of  claim 7  wherein said first solvent being selected from the group consisting of tetrahydrofuran, methylethylketone, acetone, low molecular weight glymes, and mixtures thereof.  
     
     
         13 . The lithium polymer battery of  claim 7  wherein said second solvent being selected from the group consisting of butanol, propanol, isopropanol, and mixtures thereof.  
     
     
         14 . 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 more volatile than the second solvent, and said second solvent adapted to form pores in the gel-forming polymer,    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.    
     
     
         15 . The method of  claim 14  wherein coating comprises dip coating.  
     
     
         16 . The method of  claim 14  wherein driving off the second solvent comprises heating.  
     
     
         17 . The method of  claim 14  wherein said second solvent includes water.  
     
     
         18 . The method of  claim 14  wherein said gel forming polymer being 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.  
     
     
         19 . The method of  claim 14  wherein said gel forming polymer being a polyvinylidene fluoride:hexafluoropropylene copolymer.  
     
     
         20 . The method of  claim 14  wherein said first solvent being selected from the group consisting of tetrahydrofuran, methylethylketone, acetone, low molecular weight glymes, and mixtures thereof.  
     
     
         21 . The method of  claim 14  wherein said second solvent being selected from the group consisting of butanol, propanol, isopropanol, and mixtures thereof.  
     
     
         22 . A method of forming a lithium polymer battery comprises the steps of: 
 providing an anode,    providing a cathode,    providing a separator, said separator comprising a microporous membrane and a coating thereon, said coating being made from a mixture comprising a gel forming polymer, a first solvent, and a second solvent, the first solvent being more volatile than the second solvent, and said second solvent adapted to form pores in the gel forming polymer,    treating the separator with a solution of a plasticizer for the gel forming polymer in a solvent for the plasticizer,    removing the solvent for the plasticizer,    sandwiching the treated separator between the anode and the cathode, and    bonding the anode and the cathode to the separator.    
     
     
         23 . The method of  claim 22  wherein treating comprises immersing the separator in said solution.  
     
     
         24 . The method of  claim 22  wherein removing the solvent for the plasticizer comprises evaporating the solvent.  
     
     
         25 . The method of  claim 22  wherein bonding comprises laminating with heat and pressure.  
     
     
         26 . The method of  claim 22  wherein the gel forming polymer being 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.  
     
     
         27 . The method of  claim 26  wherein said gel forming polymer being a polyvinylidene fluoride:hexafluoropropylene copolymer.  
     
     
         28 . The method of  claim 22  wherein the first solvent being selected from the group consisting of tetrahydrofuran, methylethylketone, acetone, low molecular weight glymes, and mixtures thereof.  
     
     
         29 . The method of  claim 22  wherein the second solvent being selected from the group consisting of butanol, propanol, isopropanol, and mixtures thereof.