Microporous membrane, battery separator and battery

1. A multi-layer, microporous membrane comprising a first porous layer comprising primarily polyethylene, and a second porous layer comprising 0.1 to 99.9 wt % polyethylene and 0.1 to 99.9 wt % polypropylene, the polypropylene having (1) a weight-average molecular weight of 6×10 5 or more, (2) a heat of fusion of 90 J/g or more, and (3) a fraction having a molecular weight of 5×10 4 or less being 5% or less by mass of the polypropylene.

2. The multi-layer, microporous membrane of claim 1 wherein the fraction of polypropylene having a molecular weight of 5×10 4 or less is 3% or less by mass of the polypropylene.

3. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a heat of fusion of 100 J/g or more.

4. The multi-layer, microporous membrane of claim 1 wherein the polyethylene of the first and/or second layer has a weight-average molecular weight of 2×10 5 or more.

5. The multi-layer, microporous membrane of claim 2 wherein the polyethylene of the first and/or second layer has a weight-average molecular weight of 5×10 5 or more.

6. The multi-layer, microporous membrane of claim 3 wherein the polyethylene of the first and/or second layer has a weight-average molecular weight of from 1×10 6 to 15×10 6 .

7. The multi-layer, microporous membrane of claim 1 wherein the polyethylene of the first and/or second layer has a weight-average molecular weight of 1×10 4 or more and less than 5×10 5 .

8. The multi-layer, microporous membrane of claim 1 wherein the polyethylene of the first and/or second layer has an Mw/Mn of 5 to 300.

9. The multi-layer, microporous membrane of claim 1 wherein the polypropylene of the second layer has a weight-average molecular weight of 6.5×10 5 or more and a heat of fusion of 95 J/g or more, the fraction of the polypropylene having a molecular weight of 5×10 4 or less being 4.5% or less by mass of the polypropylene.

10. The multi-layer, microporous membrane of claim 1 wherein the polypropylene content of the second layer is from 5 to 95% by mass.

11. The multi-layer, microporous membrane of claim 1 wherein the polypropylene content of the second layer is from 20 to 80% by mass.

12. The multi-layer, microporous membrane of claim 1 wherein the polypropylene content of the second layer is from 30 to 70% by mass.

13. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a weight-average molecular weight of 6×10 5 to 1×10 6 .

14. The multi-layer, microporous membrane of claim 11 wherein the polypropylene has a Mw/Mn of 2.5 or less.

15. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a weight-average molecular weight of 1×10 6 to 1.5×10 6 .

16. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a Mw/Mn of 3 or less.

17. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a weight-average molecular weight of 1.5×10 6 to 2×10 6 .

18. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a Mw/Mn of 4 or less.

19. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a weight-average molecular weight of greater than 2×10 6 .

20. The multi-layer, microporous membrane of claim 1 wherein the polypropylene has a Mw/Mn of 5 or less.

21. The multi-layer, microporous membrane of claim 1 having a porosity of 25 to 80%, an air permeability of 20 to 2,000 seconds/100 cc (converted to the value at 20-gm thickness), a pin puncture strength of 2,000 mN/20 μm or more, a shut down temperature of 120 to 140° C., and a meltdown temperature of 170° C. or higher.

22. The multi-layer, microporous membrane of claim 1 having a thickness of 3-200 μm.

23. The multi-layer, microporous membrane of claim 1 comprising a two-layer structure comprising the first porous layer and the second porous layer; a three-layer structure comprising the first porous layer, the second porous layer and the first porous layer; or a three-layer structure comprising the second porous layer, the first porous layer and the second porous layer.

24. A method for producing a multi-layer, microporous membrane comprising the steps of

(1) preparing a first melt-blend polyolefin solution comprising polyethylene and a first diluent;

(2) preparing a second melt-blend polyolefin solution comprising a polyolefin composition comprising 0.1 to 99.9 wt % polyethylene, 0.1 to 99.9 wt % polypropylene having a weight-average molecular weight of 6×10 5 or more, and a second diluent; wherein a fraction of polypropylene having a molecular weight of 5×10 4 or less is 5% or less by mass of the polypropylene; and wherein a heat of fusion of the polypropylene is 90 J/g or more;

(3) extruding the first and second polyolefin solutions through separate dies to form a first extrudate and a second extrudate;

(4) cooling each of the resultant first and second extrudates to form first and second gel-like sheets;

(5) stretching the first and second gel-like sheets in at least one direction;

(6) removing the first and second diluents from the first and second gel-like sheets;

(7) drying the first and second gel-like sheets;

(8) heat-treating the first and second gel-like sheets; and

(9) laminating the resultant first and second gel-like sheets to form the multi-layer, microporous polyolefin membrane.

25. The method of claim 24 wherein the polypropylene has a weight-average molecular weight of 6×10 5 to 1×10 6 .

26. The method of claim 24 wherein the polypropylene has a Mw/Mn of 2.5 or less.

27. The method of claim 24 wherein the polypropylene has a weight-average molecular weight of 1×10 6 to 1.5×10 6 .

28. The method of claim 24 wherein the polypropylene has a Mw/Mn of 3 or less.

29. The method of claim 24 wherein the polypropylene has a weight-average molecular weight of 1.5×10 6 to 2×10 6 .

30. The method of claim 24 wherein the polypropylene has a Mw/Mn of 4 or less.

31. The method of claim 24 wherein the polypropylene has a weight-average molecular weight of greater than 2×10 6 .

32. The method of claim 24 wherein the polypropylene has a Mw/Mn of 5 or less.

33. The method of claim 24 wherein the fraction of polypropylene having a molecular weight of 5×10 4 or less is 3% or less by mass of the polypropylene.

34. The method of claim 24 wherein the polypropylene has a heat of fusion of 100 J/g or more.

35. The method of claim 24 wherein the polyethylene of the first and/or second polyolefin solution has a weight-average molecular weight of 2×10 5 or more.

36. The method of claim 24 wherein the polyethylene of the first and/or second polyolefin solution has a weight-average molecular weight of 5×10 5 or more.

37. The method of claim 36 wherein the polyethylene of the first and/or second polyolefin solution has a weight-average molecular weight of from 1×10 6 to 15×10 6 .

38. The method of claim 24 wherein the polyethylene of the first and/or second polyolefin solution has a weight-average molecular weight of 1×10 4 or more and less than 5×10 5 .

39. The method of claim 24 wherein the polypropylene of the second polyolefin solution has a weight-average molecular weight of 6.5×10 5 or more and a heat of fusion of 95 J/g or more, the fraction having a molecular weight of 5×10 4 or less being 4.5% or less by mass of the polypropylene.

40. The method of claim 24 wherein the polypropylene content of the second polyolefin solution is from 5 to 95% by mass.

41. The method of claim 24 wherein the polypropylene content of the second polyolefin solution is from 20 to 80% by mass.

42. The method of claim 24 wherein the polypropylene content of the second polyolefin solution is from 30 to 70% by mass.

43. The method of claim 24 wherein step (9) comprises forming an at least three layer microporous membrane by laminating the resultant first gel-like sheet to each opposite side of the second gel-like sheet.

44. The method of claim 24 wherein step (9) comprises forming an at least three layer microporous membrane by laminating the resultant second gel-like sheet to each opposite side of the first gel-like sheet.

45. The method of claim 24 wherein the polyethylene resin in the first melt-blend polyolefin solution and the polyolefin composition in the second melt-blend polyolefin solution are 1 to 50% by mass of the respective melt-blend polyolefin solution.

46. A method for producing a multi-layer, microporous membrane comprising the steps of

(1) preparing a first melt-blend polyolefin solution comprising a polyethylene resin and a first diluent;

(2) preparing a second melt-blend polyolefin solution comprising a second diluent and a polyolefin composition comprising a polyethylene and polypropylene having a weight-average molecular weight of 6×10 5 or more and a heat of fusion (measured by a differential scanning calorimeter) of 90 J/g or more, a fraction of the polypropylene having a molecular weight of 5×10 4 or less being 5% or less by mass of the polypropylene;

(3) simultaneously coextruding the first and second polyolefin solutions through dies to form a laminar coextrudate;

(4) cooling the laminar coextrudate to form a multi-layer gel-like sheet;

(5) stretching the multi-layer gel-like sheet in at least one direction;

(6) removing the first and second diluents from the stretched multi-layer gel-like sheet;

(7) drying the multi-layer gel-like sheet; and

(8) heat-treating the multi-layer gel-like sheet to form the multi-layer, microporous polyolefin membrane.

47. The method of claim 46 wherein the laminar coextrudate product of step (3) comprises layers of the first gel-like sheet on each opposite side of the second gel-like sheet to form the multi-layer, microporous polyolefin membrane.

48. The method of claim 46 wherein the laminar coextrudate product of step (3) comprises layers of the second gel-like sheet on each opposite side of the first gel-like sheet to form the multi-layer, microporous polyolefin membrane.

49. A battery separator composed of a multi-layer, microporous membrane comprising a first porous layer comprising primarily polyethylene, and a second porous layer comprising polyethylene and polypropylene, the polypropylene having a weight-average molecular weight of 6×10 5 or more and a heat of fusion (measured by a differential scanning calorimeter) of 90 J/g or more, and a fraction of the polypropylene having a molecular weight of 5×10 4 or less being 5% or less by mass.

50. The battery separator of claim 49 wherein said microporous membrane comprises a two-layer structure comprising the first porous layer and the second porous layer; a three-layer structure comprising the first porous layer, the second porous layer and the first porous layer; or a three-layer structure comprising the second porous layer, the first porous layer and the second porous layer.

51. The battery separator of claim 49 having a porosity of 25 to 80%, an air permeability of 20 to 2,000 seconds/100 cc (converted to the value at 20-μm thickness), a pin puncture strength of 2,000 mN/20 μm or more, a shut down temperature of 120 to 140° C., and a meltdown temperature of 170° C. or higher.

52. The battery separator of claim 51 having a thickness of 3-200 μm.

53. A battery comprising a battery separator comprising of a multi-layer, microporous polyolefin membrane comprising a first porous layer comprising primarily polyethylene, and a second porous layer comprising a polyethylene and polypropylene, the polypropylene having a weight-average molecular weight of 6×10 5 or more and a heat of fusion (measured by a differential scanning calorimeter) of 90 J/g or more, a fraction of the polypropylene having a molecular weight of 5×10 4 or less being 5% or less by mass.

54. The battery of claim 53 wherein the battery separator comprises a two-layer structure comprising the first porous layer and the second porous layer; a three-layer structure comprising the first porous layer, the second porous layer and the first porous layer; or a three-layer structure comprising the second porous layer, the first porous layer and the second porous layer.

55. The battery of claim 53 wherein the battery separator has a porosity of 25 to 80%, an air permeability of 20 to 2,000 seconds/100 cc (converted to the value at 20-μm thickness), a pin puncture strength of 2,000 mN/20 μm or more, a shut down temperature of 120 to 140° C., and a meltdown temperature of 170° C. or higher.

56. The battery of claim 53 wherein the battery separator has a thickness of 3-200 μm.