Polyolefin microporous membrane and production method thereof

1. A polyolefin microporous membrane, comprising:

at least one microporous membrane layer,

wherein the microporous membrane layer has an air permeability between 100 sec/100 cc and 220 sec/100 cc, a pin puncture strength of at least 550 gf, and a crystallization half time t 1/2 of from 10 to 35 minutes when subjected to isothermal crystallization at 117° C., wherein the air permeability and the pin puncture strength are normalized to a thickness of 16 μm, and

wherein the microporous membrane layer comprises an ultra-high molecular weight polyethylene concentration of at least 30 wt. %, and wherein the ultra-high molecular weight polyethylene has an average molecular weight of at least 1.0×10 6 and less than 2.0×10 6 .

2. The polyolefin microporous membrane according to claim 1 , wherein a curl of a 18.5 mm×95 mm section of the microporous membrane layer is less than 2 mm.

3. The polyolefin microporous membrane according to claim 1 , wherein a thickness of the microporous membrane layer is between 5 μm and 30 μm.

4. The polyolefin microporous membrane according to claim 1 , wherein the microporous membrane layer has a shrinkage in a transverse direction at 105° C. which is less than 6%, and a shrinkage in a longitudinal direction at 105° C. which is between 1.1 and 2.0 times the shrinkage in the transverse direction.

5. A separator for a non-aqueous electrolyte secondary battery comprising a polyolefin microporous membrane comprising:

at least one microporous membrane layer,

wherein the microporous membrane layer has an air permeability between 100 sec/100 cc and 220 sec/100 cc, a pin puncture strength of at least 550 gf, and a crystallization half time t 1/2 of from 10 to 35 minutes when subjected to isothermal crystallization at 117° C., wherein the air permeability and the pin puncture strength are normalized to a thickness of 16 μm, and

wherein the microporous membrane layer comprises an ultra-high molecular weight polyethylene concentration of at least 30 wt. %, and wherein the ultra-high molecular weight polyethylene has an average molecular weight of at least 1.0×10 6 and less than 2.0×10 6 .

6. A non-aqueous electrolyte secondary battery comprising the separator according to claim 5 .

7. The separator according to claim 5 , wherein a curl of a 18.5 mm×95 mm section of the microporous membrane layer is less than 2 mm.

8. The separator according to claim 5 wherein a thickness of the microporous membrane layer is between 5 μm and 30 μm.

9. The separator according to claim 5 , wherein the microporous membrane layer has a shrinkage in a transverse direction at 105° C. which is less than 6%, and a shrinkage in a longitudinal direction at 105° C. which is between 1.1 and 2.0 times the shrinkage in the transverse direction.

10. A method of producing a polyolefin microporous membrane, the method comprising:

(a) preparing a polyolefin solution by melt-kneading:

(i) a polyolefin resin containing an ultra-high molecular weight polyethylene having a concentration of at least 30 wt. % and an average molecular weight of at least 1.0×10 6 and less than 2.0×10 6 , and

(ii) a membrane-forming solvent;

(b) molding a sheet by extruding the polyolefin solution from an extruder to form an extrudate and subsequently cooling the extrudate, wherein a crystallization half time t 1/2 of the gel-like sheet between 10 and 35 minutes when subjected to isothermal crystallization at 117° C.;

(c) stretching the sheet in a longitudinal direction;

(d) stretching the sheet in a transverse direction;

(e) extracting a membrane-forming solvent from the stretched sheet; and

(f) drying the stretched sheet.