Separator media for electrochemical cells

Abstract

A separator medium for electrochemical cells that contains at least one nonwoven sheet of polymeric fibers. The nonwoven sheet has a surface area of about 0.5 to about 1.5 m 2 /g and has a maximum pore size that is equal to or more than 2.5 times the mean flow pore size and more than 11 times the minimum pore size. The sheet may be sulfonated to a level of 0.67% and demonstrates superior tensile properties after sulfonation and relative to previously known separators.

We claim: 
     
         1 . A separator medium for electrochemical cells comprising at least one nonwoven sheet comprising polymeric fibers, wherein the nonwoven sheet has a surface area of 0.5 to 1.5 m 2 /g and the separator retains at least 70% of its machine direction (MD) tensile strength relative to the medium when it is not subjected to sulfonation. 
     
     
         2 . The separator medium of  claim 1 , wherein the polymeric fibers are sulfonated and contain at least 0.67% by weight of sulfur. 
     
     
         3 . The separator medium of  claim 1 , wherein the polymeric fibers comprise polymers selected from the group consisting of polyolefins, polyesters, polyamides, polyaramids, polysulfones, polyimides, fluorinated polymers and combinations thereof. 
     
     
         4 . The separator medium of  claim 3 , wherein the polymeric fibers are made from polyolefin polymers selected from the group consisting of polyethylene, polypropylene, polybutylene and polymethylpentene. 
     
     
         5 . The separator medium of  claim 1 , wherein the polymeric fibers have non-circular cross sections. 
     
     
         6 . The separator medium of  claim 1 , wherein the polymeric fibers are plexifilamentary fiber strands. 
     
     
         7 . The separator medium of  claim 1 , wherein the nonwoven sheet is a uniaxially stretched nonwoven sheet in the machine direction. 
     
     
         8 . The separator medium of  claim 1 , wherein the nonwoven sheet has a surface area of 0.5 to 1.0 m 2 /g. 
     
     
         9 . The separator medium of  claim 1 , wherein the nonwoven sheet consists of fibers that have a number average fiber diameter greater than 1 micrometer for 100% of the fibers. 
     
     
         10 . The separator medium of  claim 1 , wherein the nonwoven sheet has an ammonia trapping capacity of 0.20 mmole/g and a machine direction tensile strength retention of at least 16 newtons/centimeter (N/cm). 
     
     
         11 . The separator medium of  claim 1 , wherein the nonwoven sheet has a maximum pore size that is equal to or more than 2.5 times the mean flow pore size and more than 11 times the minimum pore size. 
     
     
         12 . A process for producing a separator medium for electrochemical cells comprising:
 flash spinning a solution of 12% to 24% by weight polyethylene in a spin agent consisting of a mixture of normal pentane and cyclopentane at a spinning temperature from 205° C. to 220° C. to form plexifilamentary fiber strands and collecting the plexifilamentary fiber strands into an unbonded web;   uniaxially stretching the unbonded web in the machine direction between heated draw rolls at a temperature between 124° C. and 154° C., positioned between 5 cm and 30 cm apart and stretched between 3% and 25% to form the stretched web; and   bonding the stretched web between heated bonding rolls at a temperature between 124° C. and 154° C. to form a nonwoven sheet wherein the nonwoven sheet has a surface area of 0.5 to about 1.5 m 2 /g and   a maximum pore size that is more than 2.5 times the mean flow pore size and more than 11 times the minimum pore size.   
     
     
         13 . The process for producing the separator medium of  claim 12 , further comprising sulfonating the nonwoven sheet after bonding the stretched web. 
     
     
         14 . An electrochemical cell of  claim 1 , wherein the cell is either a battery or a capacitor. 
     
     
         15 . An electrochemical cell, wherein the cell is an alkaline battery comprising separator medium that further comprises at least one nonwoven sheet comprising polymeric fibers wherein the nonwoven sheet has a surface area of 0.5 to 1.5 m 2 /g and wherein the nonwoven sheet has a maximum pore size that is equal to or more than 2.5 times the mean flow pore size and more than 11 times the minimum pore size, the polymeric fibers are sulfonated and contain at least 0.67% by weight of sulfur and wherein the separator retains at least 70% of its machine direction (MD) tensile strength relative to the medium when it is not subjected to sulfonation.