Method of making a co-extruded, multi-layered battery separator

1. A method of making a polyolefinic microporous multi-layered co-extruded membrane according to a dry process comprising the steps of:

co-extruding a nonporous polyolefinic multi-layered precursor through an extrusion die with a minimum shear rate of 4 sec −1 and a throughput of 18-100 lbs/hr (8.2-45.4 Kg/hr) per layer, wherein at least two layers of said polyolefinic multi-layered precursor are polymerically dissimilar from one another; and

processing the nonporous polyolefinic multi-layered co-extruded precursor to obtain the microporous polyolefinic multi-layered co-extruded membrane, wherein processing further includes: annealing the nonporous polyolefinic multi-layered microporous polyolefinic multi-layered co-extruded precursor; and stretching the annealed nonporous polyolefinic multi-layered microporous polyolefinic multi-layered co-extruded precursor,

whereby the microporous polyolefinic multi-layered co-extruded membrane has a thickness in the range of 4-50 μm and a uniform thickness defined by a standard deviation of <0.8 μm.

2. The method of claim 1 wherein the shear rate is 8 sec −1 .

3. The method of claim 1 wherein the co-extruding further comprises:

adding a material to at least one layer of said multi-layered precursor.

4. The method of claim 3 wherein said material is selected from the group of materials adapted to: lower the melting temperature of one or more layers; improve the melt integrity of the membrane; improve the strength or toughness of the membrane; improve the antistatic properties of the membrane; improve the surface wettability of the membrane; improve the surface tribology performance of the membrane; improve the processing of one or more layers; improve the flame retardance of the membrane; facilitate nucleation of one or more layers; color one or more layers of the membrane; and combinations thereof.