Multi-layer polyolefin porous membrane, battery separator obtained using the same, and method for producing the same

A battery separator comprising a multi-layer polyolefin porous membrane, comprising:

• first and second layers;
• and
• a plurality of protrusions comprising polyolefin, wherein the protrusions have a protrusion width (W) satisfying 5 μm≦W≦50 μm and have a protrusion height (H) satisfying 0.5 μm≦H,
• wherein the protrusions are randomly disposed on a first side of the membrane, and wherein the protrusions are disposed with a density of not less than 3 protrusions/cm2 and not greater than 200 protrusions/cm2,
• wherein a meltdown temperature of the membrane is not lower than 165° C.,
• wherein an air permeation resistance of the membrane is not greater than 300 sec/100 cc Air, and
• wherein a thickness of the membrane is not greater than 20 μm.

A method of manufacturing a multi-layer polyolefin porous membrane, the method comprising:

• preparing a first polyolefin resin solution by:
• mixing a first forming solvent with a second polyolefin resin for a first layer to form a first mixture, and
• melting and kneading the first mixture;
• preparing a second polyolefin resin solution by:
• mixing a second forming solvent with a second polyolefin resin for a second layer to form a second mixture, and
• melting and kneading the second mixture;
• extruding the first polyolefin resin solution through a first die;
• extruding the second polyolefin resin solution through a second die;
• forming a multi-layer gel-like product by chilling at least one of the first and second polyolefin resin solutions with one or more cooling rolls having a surface while removing forming solvent from the chilled polyolefin resin solution;
• stretching the gel-like product in a machine direction (MD) and in a transverse direction (TD) to obtain a stretched and molded material;
• forming a multi-layer porous molded material by:
• extracting and removing forming solvent from the stretched and molded material, and
• drying the stretched and molded material;
• and
• heating the multi-layer porous molded material.