ASSYMETRIC POROUS MEMBRANE

Disclosed herein is an asymmetric porous membrane having two outer layers and at least one inner layers. The outer layers may be asymmetric in thickness, pore size, porosity, tortuosity, or combinations thereof. In some embodiments, the asymmetric porous membrane has two outer layers and no inner layers. The thickness of one of the outer layers to the other of the outer layers is from 1.1 to 25:1, preferably from 1.1:1 to 10:1, 1.1:1 to 5:1 or 4:1 to 10:1. In some embodiments, both outer layers are PP-containing layers and in some embodiments, one outermost layer may be a PE-containing layer and the other may be a PP-containing layer. The asymmetric porous membrane may be used as or to make a battery separator. In some embodiments, the battery separator may include an asymmetric porous membrane having the thinner of the two outer layers or the layer facing the anode coated with a coating such as a ceramic coating. The battery separator may be used in a liquid electrolyte secondary battery. In the battery, the thicker of the outer layers and/or the PP-containing layer faces or is closest to the cathode and the thinner of the two outer layers and/or the PE-containing layer faces or is closest to the anode. The outer layer facing the anode may be coated with a coating such as a ceramic coating. The battery separators described herein may have electrochemical stability voltage equal to or above 4.2 v.s. Li/Li+.