Microporous polymeric membrane, battery separator, and battery

Abstract

The invention relates to microporous polymeric membrane having a good balance of rupture temperature and air permeability. The invention also relates to a battery separator formed by such a microporous membrane, and a battery comprising such a separator. Another aspect of the invention relates to a method for making the microporous polymeric membrane, a method for making a battery using such a membrane as a separator, and a method for using such a battery.

What is claimed is: 
     
         1 . A multi-layer microporous membrane comprising:
 a first layer material comprising polyethylene and a second layer material comprising polypropylene, the polypropylene having (1) a weight-average molecular weight ≧6×10 5 , (2) a heat of fusion ≧90 J/g, and (3) an MWD of 2 to 6, wherein   (a) the multi-layer microporous membrane has at least a first microporous layer containing the first layer material, a third microporous layer containing the first layer material, and a second microporous layer containing the second layer material, the second microporous layer being located between the first and third microporous layers, and   (b) the total amount of polypropylene in the multi-layer microporous membrane is at least 2.0 wt. % based on the total weight of the multi-layer microporous membrane,   the membrane being produced by a method comprising:   (1) combining a polyethylene resin and a first diluent,   (2) combining a polypropylene resin and a second diluent; the polypropylene resin having the polypropylene having (a) a weight-average molecular weight of 6×10 5  or more, (b) a heat of fusion of 90 J/g or more, and (c) an MWD of 2 to 6;   (3) extruding at least a portion of the combined first polyethylene resin and first diluent, and extruding at least a portion of the combined first polypropylene resin and second diluent to produce a multi-layer extrudate which comprises
 a first layer comprising the combined polyethylene and first diluent, a second layer comprising the polypropylene and the second diluent, and a third layer comprising the combined polyethylene and first diluent, wherein the second layer is located between the first and third layers and wherein the polypropylene is present in the extrudate in an amount ≧2.0 wt. % based on the weight of polymer in the extrudate; and then 
   (4) cooling the multi-layer extrudate to form a multi-layer sheet,   (5) removing at least a portion of the first and second diluents from the multi-layer sheet to form a diluent-removed sheet, and   (6) removing at least a portion of any volatile species from the sheet to form the microporous membrane.   
     
     
         2 . The membrane of  claim 1 , wherein the combined polyethylene and first diluent is a first polyolefin solution, wherein the combined polypropylene and second diluent is a second polyolefin solution, and wherein
 the polyethylene resin is present in the first polyolefin solution in an amount of about 0.5 wt. % to about 75 wt. % based on the total weight of polyolefin in the first polyolefin solution,   the first polyolefin solution comprises polypropylene resin, the polypropylene resin being present in the first polyolefin solution in an amount of about 0 wt. % to about 10 wt. % based on the total weight of polyolefin in the first polyolefin solution,   the second polyolefin solution comprises polyethylene resin, the polyethylene resin being present in the second polyolefin solution in an amount of about 40 wt. % to about 90 wt. % based on the total weight of polyolefin in the second polyolefin solution,   the polypropylene resin is present in the second polyolefin solution in an amount of about 10 wt. % to about 60 wt. % based on the total weight of polyolefin in the second polyolefin solution;   the first diluent is present in the first polyolefin solution in an amount of about 25 wt. % to about 99 wt. % based on the weight of the first polyolefin solution; and   the second diluent is present in the second polyolefin solution in an amount of about 25 wt. % to about 99 wt. % based on the weight of the second polyolefin solution.   
     
     
         3 . The membrane of  claim 2 , wherein
 (a) the polyethylene of the first, second, and third layer independently comprise a PE1, a PE2, or both PE1 and PE2, wherein   (1) the first polyethylene has an Mw of 1×10 5  to 5×10 6 ;   (2) the second polyethylene has an Mw of 1×10 5  to 5×10 6 ;   (3) PE1 is one or more of a high-density polyethylene, a medium-density polyethylene, a branched low-density polyethylene, or a linear low-density polyethylene;   (4) PE1 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer selected from propylene, butene-1, hexene-1;   (5) PE2 has an Mw of at least 1×10 6 ;   (6) PE2 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer selected from propylene, butene-1, hexene-1;   (7) the amount of the PE1 in the first microporous layer material is 50 wt. % to 100 wt. %, based on the weight of the first microporous layer material;   (8) the amount of the PE2 in the first microporous layer material is 0 wt. % to 50 wt. %, based on the weight of the first microporous layer material;   (9) the amount of the PE1 in the second microporous layer material is 40 wt. % to 60 wt. %, based on the weight of the second microporous layer material;   (10) the amount of the PE2 in the second microporous layer material is 0 wt. % to 50 wt. %, based on the weight of the second microporous layer material;   (11) the first and/or second polyethylene has Mw/Mn of 5 to 300; and   (b) the second polypropylene has at least one of:   (1) an Mw/Mn of 2 to 6;   (2) an MW of 9×10 5  to 2×10 6 ; and   (3) a heat of fusion is 100 J/g or more.   (b) the polypropylene has at least one characteristic selected from:   (1) a weight-average molecular weight of 9×10 5  to 2×10 6 ,   (2) a heat of fusion of 100 J/g or more, and   (3) a molecular weight distribution (Mw/Mn) of 2 to 6.   
     
     
         4 . A battery comprising an anode, a cathode, an electrolyte and the multi-layer microporous membrane of  claim 1 , wherein the multi-layer microporous membrane separates at least the anode from the cathode. 
     
     
         5 . The battery of  claim 4 , wherein the electrolyte contains lithium ions and the battery is a secondary battery. 
     
     
         6 . The battery of  claim 4  which is a source or sink of electric charge. 
     
     
         7 . The battery of  claim 5  which is a source or sink of electric charge. 
     
     
         8 . The membrane of  claim 1 , having a Rupture temperature 180° C. or higher and comprising polypropylene wherein the total amount polypropylene in the membrane is ≧2 wt. %, based on the total weight of the membrane. 
     
     
         9 . The membrane of  claim 8 , wherein the total amount polypropylene in the membrane having
 (a) an Mw >6×10 5 ,   (b) an MWD of 2 to 6, and   (c) a heat of fusion of 90 J/g or higher   is ≧2 wt. %, based on the total weight of the membrane.   
     
     
         10 . The membrane of  claim 8 , wherein the membrane has a Normalized Air Permeability satisfying A≦(M*P)−I, where A is Normalized Air Permeability thickness of the microporous membrane, P is Normalized Pin Puncture Strength of the microporous membrane, M is a slope of about 0.09 to about 0.1, or about 0.95 to about 0.99, and I is ≧100. 
     
     
         11 . The membrane of  claim 9  wherein the membrane has Normalized Air Permeability satisfying A≦(M*P)−I, where A is Normalized Air Permeability thickness of the microporous membrane, P is Normalized Pin Puncture Strength of the microporous membrane, M is a slope of about 0.09 to about 0.1, or about 0.95 to about 0.99, and I is ≧100. 
     
     
         12 . A battery comprising the separator of  claim 10 . 
     
     
         13 . A battery separator made by the process recited in  claim 1 .