Single-layer lithium ion battery separators exhibiting high low-shrinkage rates at high temperatures

Abstract

An insulating (nonconductive) microporous polymeric battery separator comprised of a single layer of enmeshed microfibers and nanofibers is provided. Such a separator accords the ability to attune the porosity and pore size to any desired level through a single nonwoven fabric. Through a proper selection of materials as well as production processes, the resultant battery separator exhibits isotropic strengths, low shrinkage, high wettability levels, and pore sizes related directly to layer thickness. The overall production method is highly efficient and yields a combination of polymeric nanofibers within a polymeric microfiber matrix and/or onto such a substrate through high shear processing that is cost effective as well. The separator, a battery including such a separator, the method of manufacturing such a separator, and the method of utilizing such a separator within a battery device, are all encompassed within this invention.

1 . A battery separator comprising fibers of length less than 2.54 cm (1 inch), such fibers comprising at least 5% of thermally stable fibers that have no melting point, glass transition temperature or thermal degradation below about 300° C., wherein said fibers are polymeric materials selected from the group consisting essentially of polyacrylonitriles, polyolefins, polyolefin copolymers, polyamides, polyvinyl alcohol, polyethylene terephthalate, polybutylene terephthalate, polysulfone, polyvinyl fluoride, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polymethyl pentene, polyphenylene sulfide, polyacetyl, polyurethane, aromatic polyamide, semi-aromatic polyamide, polypropylene terephthalate, polymethyl methacrylate, polystyrene, synthetic cellulosic polymers, polyaramids, and blends, mixtures and copolymers including said polymeric materials, wherein such battery separator exhibits no observable spectral reflectance of after application of a drop of an electrolyte on the surface thereof, wherein such battery separator has a mean flow pore size less than 2000 nm, wherein said separator allows for lithium ion transport with an electrolyte within a lithium ion battery, and wherein said separator exhibits an apparent density of at least 0.564 g/cm 3 . 
     
     
         2 . A battery separator of  claim 1 , said separator further comprised of a single layer of said fibers, and wherein said fiber layer includes enmeshed microfibers and nanofibers, and wherein said separator exhibits a porosity of no greater than 51%. 
     
     
         3 . The battery separator of  claim 2 , wherein said microfibers are entangled with one another; wherein said separator exhibits interstices between each entangled microfiber; and wherein said interstices include nanofibers present therein. 
     
     
         4 . The battery separator of  claim 2  in which said microfibers comprise fibrillated microfibers. 
     
     
         5 . The battery separator of  claim 2 , wherein said microfibers have an average fiber diameter greater than 1000 nm. 
     
     
         6 . The battery separator of  claim 2 , wherein said microfibers have an average fiber diameter greater than 3000 nm. 
     
     
         7 . The battery separator of  claim 2 , wherein said nanofibers have an average fiber diameter less than 1000 nm. 
     
     
         8 . The battery separator of  claim 2 , wherein said nanofibers have an average fiber diameter less than 700 nm. 
     
     
         9 . The battery separator of  claim 2  comprising nanofibers with a transverse aspect ratio greater than 1.5:1. 
     
     
         10 . The battery separator of  claim 1  wherein said separator exhibits a tensile strength greater than 59 kg/cm 2 .