Battery separator with Z-direction stability

Abstract

A battery separator is a microporous membrane. The membrane has a major volume of a thermoplastic polymer and a minor volume of an inert particulate filler. The filler is dispersed throughout the polymer. The membrane exhibits a maximum Z-direction compression of 95% of the original membrane thickness. Alternatively, the battery separator is a microporous membrane having a TMA compression curve with a first substantially horizontal slope between ambient temperature and 125° C., a second substantially horizontal slope at greater than 225° C. The curve of the first slope has a lower % compression than the curve of the second slope. The curve of the second slope is not less than 5% compression. The TMA compression curve is graphed so that the Y-axis represents % compression from original thickness and the X-axis represents temperature.

1 . A battery separator comprising: 
 a microporous membrane comprising a major volume of a thermoplastic polymer and a minor volume of an inert particulate filler, said filler being dispersed throughout said polymer,    wherein said membrane exhibits a maximum Z-direction compression of 95% of the original membrane thickness.    
     
     
         2 . The battery separator of  claim 1  wherein said thermoplastic polymer being selected from the group consisting of: polyethylene, polypropylene, polybutene, polymethylpentene, ultrahigh molecular weight polyethylene, copolymers thereof, and blends of the foregoing.  
     
     
         3 . The battery separator of  claim 1  wherein said inert particulate filler being selected from the group consisting of: carbon based materials, metal oxides and hydroxides, metal carbonates, minerals, synthetic and natural zeolites, cements, silicates, glass particles, sulfur-containing salts, synthetic polymers, and mixtures thereof.  
     
     
         4 . The battery separator of  claim 1  wherein said major volume of said thermoplastic polymer being 50-90% by volume of said membrane.  
     
     
         5 . The battery separator of  claim 1  wherein said minor volume of said inert particulate filler being 1-50% by volume of said membrane.  
     
     
         6 . The battery separator of  claim 5  wherein said minor volume of said inert particulate filler being 5-45% by volume of said membrane.  
     
     
         7 . The battery separator of  claim 1  wherein said membrane exhibits a maximum Z-direction compression of 85% of the original membrane thickness.  
     
     
         8 . A battery separator comprising: 
 a microporous membrane having a TMA compression curve with a first substantially horizontal slope between ambient temperature and 125° C., a second substantially horizontal slope at greater than 225° C., wherein a Y-axis represents % compression from original thickness and a X-axis represents temperature, said curve of said first slope having a lower % compression than said curve of said second slope, and said curve of said second slope not being less than 5% compression.