Laminated polyolefin microporous membrane, battery separator, and production method thereof

A battery separator comprising:

• a laminated polyolefin microporous membrane, comprising:
• a first polyolefin microporous membrane, wherein the first polyolefin microporous membrane comprises not less than 80% mass polyethylene, and
• a second polyolefin microporous membrane, wherein the second polyolefin microporous membrane comprises not less than 50% mass high density polyethylene and between 20% and 35% mass low melting point resin, wherein the resin has a melting point of at least 120° C. and less than 130° C.,
• wherein:
• a shutdown temperature of the laminated polyolefin microporous membrane is from 128° C. to 135° C., wherein the shutdown temperature is the temperature at which air permeation resistance becomes greater than 1×105 sec/100 cc Air,
• the laminated polyolefin microporous membrane has a thickness between 5 and 25 μm,
• an air permeation resistance increase rate from 30° C. to 105° C. per 20 μm of thickness of the laminated polyolefin microporous membrane is less than 1.5 sec/100 cc Air/° C., and
• a variation range in an F25 value of the laminated polyolefin microporous membrane in a longitudinal direction is not greater than 1 MPa, wherein the F25 value represents a value determined by dividing the load at 25% elongation of a sample of the laminated polyolefin microporous membrane as measured with a tensile tester by the cross-sectional area of the sample polyolefin microporous membrane;
• and
• a porous layer on one or more sides of the laminated polyolefin microporous membrane, wherein the porous layer comprises a water-soluble resin or a water-dispersible resin and heat-resistant particles and has an average thickness T(ave) of from 1 μm to 5 μm.

A method of manufacturing a laminated polyolefin microporous membrane, the method comprising:

• (a) preparing a first polyolefin solution by melt-kneading a membrane-forming solvent into a first polyolefin composition;
• (b) preparing a second polyolefin solution by melt-kneading a membrane-forming solvent into a second polyolefin composition;
• (c) forming an unstretched gel-like sheet by simultaneously extruding the first and second polyolefin solutions from a single die and cooling the first and second polyolefin solutions;
• (d) obtaining a longitudinally stretched gel-like sheet by passing the unstretched gel-like sheet between three or more pairs of longitudinal stretching rollers and increasing a peripheral speed between each of the rollers in a stepwise manner so as to stretch the sheet in a longitudinal direction,
• wherein each pair of longitudinal stretching rollers comprises a longitudinal stretching roller and a nip roller contacting the longitudinal stretching roller in parallel, and a contact pressure of the nip roller against the longitudinal stretching roller is not less than 0.05 MPa and not greater than 0.5 MPa;
• (e) obtaining a biaxially stretched gel-like sheet by holding the longitudinally stretched gel-like sheet with clips so that a distance between the clips is not more than 50 mm and stretching the sheet in the transverse direction;
• (f) extracting the membrane-forming solvent from the biaxially stretched gel-like sheet and drying the biaxially stretched gel-like sheet;
• (g) heat-treating the dried sheet to obtain the laminated polyolefin microporous membrane.

A method of manufacturing a battery separator, the method comprising:

• providing a laminated polyolefin microporous membrane, comprising:
• a first polyolefin microporous membrane, and
• a second polyolefin microporous membrane,
• wherein:
• a shutdown temperature of the laminated polyolefin microporous membrane is from 128° C. to 135° C.,
• an air permeation resistance increase rate from 30° C. to 105° C. per 20 μm of thickness of the laminated polyolefin microporous membrane is less than 1.5 sec/100 cc Air/° C., and
• a variation range in an F25 value of the laminated polyolefin microporous membrane in a longitudinal direction is not greater than 1 MPa, wherein the F25 value represents a value determined by dividing the load at 25% elongation of a sample of the laminated polyolefin microporous membrane as measured with a tensile tester by the cross-sectional area of the sample polyolefin microporous membrane;
• and
• applying a coating solution containing a water-soluble resin or a water-dispersible resin and heat-resistant particles to one or more sides of the laminated polyolefin microporous membrane with a roller coating method using a coating roller having an eccentricity tolerance not greater than 10 μm/Φ 100 mm.