Multilayer battery separators

Abstract

A microporous battery separator is provided having a first co-extruded multilayered portion and a second co-extruded multilayered portion. The two portions are bonded together. In a preferred embodiment, the battery separator has two substantially identical multilayered portions bonded together face-to-face. Each of the two multilayered portions has at least one strength layer and at least one shutdown layer. Methods for making the battery separators are also provided. Preferably, a tubular multilayered film is extruded, and collapsed onto itself to form a multilayered battery separator precursor. The precursor is then bonded and annealed before it is stretched to form a microporous multilayer battery separator.

What is claimed is: 
     
         1 . A method for making a multilayer battery separator comprising the steps of:
 co-extruding a tubular film with multiple layers;   collapsing and face-to-face bonding the tubular film onto itself to form a battery separator precursor with two plies of the tubular film bonded face-to-face;   annealing the precursor; and   stretching the bonded and annealed precursor to form a microporous multilayer battery separator.   
     
     
         2 . The method of  claim 1  wherein said tubular film has two layers including a shutdown layer and a strength layer. 
     
     
         3 . The method of  claim 2  wherein said strength layer consists essentially of polypropylene, and said shutdown layer consists essentially of polyethylene. 
     
     
         4 . The method of  claim 1  wherein said tubular film has three layers including two strength layers sandwiching a shutdown layer. 
     
     
         5 . The method of  claim 4  wherein said strength layer consists essentially of polypropylene, and said shutdown layer consists essentially of polyethylene. 
     
     
         6 . The method of  claim 1  wherein said tubular film has four layers, the first and fourth layers consisting essentially of polypropylene, the second and third layers being bonded together and consisting essentially of polyethylene. 
     
     
         7 . The method of  claim 1  wherein said tubular film has six layers, the second and fifth layers consisting essentially of polyethylene, the first, third, fourth and sixth layers consisting essentially of polypropylene, said third and fourth layers being bonded together. 
     
     
         8 . The method of  claim 1  wherein said tubular film has a thickness of from about 0.5 mil to about 1.5 mils. 
     
     
         9 . The method of  claim 1  wherein the microporous multilayer battery separator has a puncture strength of at least about 500 grams.