Multilayer separator exhibiting improved strength and stability

1. A multi-layer microporous battery separator comprising:

a high molecular weight polypropylene layer having a melt flow index (MFI) of <1.0 g/10 minutes;

a polyethylene layer;

a high molecular weight polypropylene layer having a melt flow index of <1.0 g/10 minutes;

which forms a microporous battery separator by a dry stretch process, where said microporous battery separator has a porosity of ≦37% while maintaining a Gurley from 13 to 25 seconds, a thickness of ≦25 microns, said microporous battery separator exhibits an increase of 5% or more in mixed penetration strength compared to a tri-layer dry-stretched microporous battery separator of the same thickness and does not use said high molecular weight polypropylene, a net shrinkage of said microporous battery separator is less than 5% measured for 6 hours at 105° C., and an ionic resistance of said microporous battery separator is less than 2.5 ohms-cm 2

made by a process for the preparation of a multi-layer microporous battery separator comprising the steps of:

providing a polypropylene having a MFI≦1.0 measured at pellet before processing and a polyethylene;

extruding said high molecular weight polypropylene to form a precursor polypropylene film;

extruding said polyethylene to form a precursor polyethylene film;

laminating said precursor polypropylene films to each side of said precursor polyethylene film to form a non-porous tri-layer precursor;

annealing said non-porous tri-layer precursor;

stretching said non-porous tri-layer precursor to form a stretched microporous tri-layer film;

allowing a relaxation of said stretched microporous tri-layer film to form a microporous tri-layer film; and

where net stretch of said microporous tri-layer film is less than 90%.

2. The multi-layer microporous battery separator according to claim 1 , where the polyethylene layer is a high density polyethylene.