Microlayer membranes, improved battery separators, and related methods

A battery separator comprising a multilayer microporous film comprising 9 or more layers, and at least 8 or more interfaces, wherein at least three consecutive layers of the 9 or more layers each have a thickness of from 0.1 to 2 microns, and further wherein said at least three consecutive layers are coextruded and then laminated with at least one other layer to form the multilayer microporous film;

• wherein the multilayer microporous film comprises a first region comprising two or more layers, wherein the first region comprises polypropylene and discontinuous amorphous regions;
• and a second region comprising at least one layer and the multilayer microporous film having one or more of the following properties:
• an average dielectric breakdown value (V) that is higher than that of a tri-layer microporous film having the same thickness, Gurley and/or porosity as the multilayer microporous film;
• an average dielectric breakdown value (V) that is 1 to 35% higher than, 5 to 35% higher than, 10 to 35% higher than, 15 to 35% higher than, or 20 to 35% higher than that of a tri-layer microporous film having the same thickness, Gurley and/or porosity as the multilayer microporous film;
• when measured using mercury intrusion porosimetry, exhibits a peak mercury intrusion value of log differential intrusion 5 mL/g or less, 4.5 mL/g or less, 4.0 mL/g or less, or 3.5 mL/g or less;
• a MacMillan number above 5, above 5.5, above 6, above 7, above 7.5, above 8, above 9, or above 10;
• a tortuosity value of 1.6 or higher, 1.7 or higher, or 2.0 or higher;
• a pin removal force of less than 50 N, less than 40N, less than 30N, less than 20N, less than 15N, or less than 10N;
• an electrical resistance less than or equal to 2.0, less than or equal to 1.7, less than or equal to 1.6, less than or equal to 1.5, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, less than or equal to 1.1, or less than or equal to 1.0 and;
• lower standard deviation value for the dielectric break down than a tri-layer microporous film having the same thickness, Gurley, and/or porosity as the multilayer microporous film.