WO2009038233A1

WO2009038233A1 - Microporous membranes and methods for making and using such membranes

Info

Publication number: WO2009038233A1
Application number: PCT/JP2008/067581
Authority: WO
Inventors: Kotaro Takita; Shintaro Kikuchi
Original assignee: Tonen Chemical Corporation
Priority date: 2007-09-20
Filing date: 2008-09-19
Publication date: 2009-03-26
Also published as: KR101135077B1; KR101138860B1; KR20100066516A; KR20100066518A; JP2010540690A; JP2010540692A; KR101160462B1; KR20100066517A; WO2009038231A1; JP2010540691A; WO2009038229A1; JP5453272B2

Abstract

The invention relates to a microporous membrane having suitable permeability, mechanical strength, electrolytic solution absorption characteristics, compression resistance, and heat shrinkage resistance; and methods for producing and using such a microporous polyolefin membrane. It also relates to battery separators comprising such a microporous polyolefin membrane, and to batteries utilizing such battery separators.

Description

MICROPOROUS MEMBRANES AND METHODS FOR MAKINGAND USING

SUCH MEMBRANES

FIELD OF THE INVENTION

[0001] The invention relates to a microporous membrane having suitable permeability, mechanical strength, heat shrinkage resistance, and excellent electrolytic solution absorption characteristics, compression resistance; and methods for producing and using such a microporous membrane. It also relates to battery separators comprising such a microporous membrane, and to batteries utilizing such battery separators.

BACKGROUND OF THE INVENTION

[0002] Microporous membranes are useful as separators for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc. When the microporous membrane is used as a battery separator, particularly for lithium ion batteries, the membrane's performance significantly affects the battery's properties, productivity, and safety. Accordingly, the microporous membrane should have appropriate permeability, mechanical properties, heat resistance, dimensional stability, shutdown properties, meltdown properties, etc. It is desirable for such batteries to have a relatively low shutdown temperature and a relatively high meltdown temperature for improved battery safety properties, particularly for batteries exposed to high temperatures under operating conditions. High separator permeability is desirable for high capacity batteries. A separator with high mechanical strength is desirable for improved battery assembly and fabrication.

[0003] The optimization of material compositions, stretching conditions, heat treatment conditions, etc., has been proposed to improve the properties of microporous membranes used as battery separators. For example, JP6-240036A discloses a microporous polyolefin membrane having improved pore diameter and a sharp pore diameter distribution. The membrane is made from a polyethylene resin containing 1 % or more by mass of ultra-high molecular weight polyethylene having a weight average molecular weight ("Mw") of 7 x 10⁵ or more, the polyethylene resin having a molecular weight distribution (weight average molecular weight/number-average molecular weight) of 10 to 300, and the microporous polyolefin membrane having a porosity of 35 to 95 %, an average penetrating pore diameter of 0.05 to 0.2 μm, a rupture strength (15 mm width) of 0.2 kg or more, and a pore diameter distribution (maximum pore diameter/average penetrating pore diameter) of 1.5 or less. This microporous membrane is produced by extruding a melt-blend of the above polyethylene resin and a membrane-forming solvent through a die, stretching the gel-like sheet obtained by cooling at a temperature from the crystal dispersion temperature ("Ted") of the above polyethylene resin to the melting point +10 °C, removing the membrane-forming solvent from the gel-like sheet, re-stretching the resultant membrane to 1.5 to 3 fold as an area magnification at a temperature of the melting point of the above polyethylene resin -10 °C or less, and heat-setting it at a temperature from the crystal dispersion temperature of the above polyethylene resin to the melting point. [0004] WO 1999/48959 discloses a microporous polyolefin membrane having suitable strength and permeability, as well as a uniformly porous surface without local permeability variations. The membrane is made of a polyolefin resin, for instance, high density polyethylene, having an Mw of 50,000 or more and less than 5,000,000, and a molecular weight distribution of 1 or more to less than 30, which has a network structure with fine gaps formed by uniformly dispersed micro-fibrils, having an average micro-fibril size of 20 to 100 run and an average micro-fibril distance of 40 to 400 nm. This microporous membrane is produced by extruding a melt-blend of the above polyolefin resin and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling at a temperature of the melting point of the above polyolefin resin -50 °C or higher and lower than the melting point, removing the membrane-forming solvent from the gel-like sheet, re-stretching it to 1.1 to 5 fold at a temperature of the melting point of the above polyolefin resin -50 ⁰C or higher and lower than the melting point, and heat-setting it at a temperature from the crystal dispersion temperature of the above polyolefin resin to the melting point. [0005] WO 2000/20492 discloses a microporous polyolefin membrane of improved permeability which is characterized by fine polyethylene fibrils having an Mw of 5 x 10⁵ or more, the composition comprising polyethylene. The microporous polyolefin membrane has an average pore diameter of 0.05 to 5 μm, and the percentage of lamellas at angles θ of 80 to 100° relative to the membrane surface is 40 % or more in longitudinal and transverse cross sections. This polyethylene composition comprises 1 to 69 % by weight of ultra-high molecular weight polyethylene having a weight average molecular weight of 7 x 10⁵ or more, 98 to 1% by weight of high density polyethylene and 1 to 30 % by weight of low density polyethylene. This microporous membrane is produced by extruding a melt-blend of the above polyethylene composition and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling, heat-setting it at a temperature from the crystal dispersion temperature of the above polyethylene or its composition to the melting point +30 °C, and removing the membrane-forming solvent. [0006] WO 2002/072248 discloses a microporous membrane having improved permeability, particle-blocking properties and strength. The membrane is made using a polyethylene resin having an Mw of less than 380,000. The membrane has a porosity of 50 to 95 % and an average pore diameter of 0.01 to 1 μm. This microporous membrane has a three-dimensional network skeleton formed by micro-fibrils having an average diameter of 0.2 to 1 μm connected to each other throughout the overall microporous membrane, and openings defined by the skeleton to have an average diameter of 0.1 μm or more and less than 3 μm. This microporous membrane is produced by extruding a melt-blend of the above polyethylene resin and a membrane-forming solvent through a die, removing the membrane-forming solvent from the gel-like sheet obtained by cooling, stretching it to 2 to 4 fold at a temperature of 20 to 140 ⁰C, and heat-treating the stretched membrane at a temperature of 80 to 140 °C.

[0007] WO 2005/113657 discloses a microporous polyolefin membrane having suitable shutdown properties, meltdown properties, dimensional stability, and high-temperature strength. The membrane is made using a polyolefin composition comprising (a) polyethylene resin containing 8 to 60 % by mass of a component having a molecular weight of 10,000 or less, and an Mw/Mn ratio of 11 to 100, wherein Mn is the number-average molecular weight of the polyethylene resin, and a viscosity-average molecular weight ("Mv") of 100,000 to 1,000,000, and (b) polypropylene. The membrane has a porosity of 20 to 95 %, and a heat shrinkage ratio of 10 % or less at 100 °C. This microporous polyolefin membrane is produced by extruding a melt-blend of the above polyolefin and a membrane-forming solvent through a die, stretching the gel-like sheet obtained by cooling, removing the membrane-forming solvent, and annealing the sheet.

[0008] With respect to the properties of separators, not only permeability, mechanical strength, dimensional stability, shutdown properties and meltdown properties, but also properties related to battery productivity such as electrolytic solution absorption, and battery cyclability, such as electrolytic solution retention properties, have recently been given importance. In particular, electrodes for lithium ion batteries expand and shrink according to the intrusion and departure of lithium, and an increase in battery capacity leads to larger expansion ratios. Because separators are compressed when the electrodes expand, it is desired that the separators when compressed suffer as little a decrease as possible in electrolytic solution retention.

[0009] Moreover, even though improved microporous membranes are disclosed in JP6-240036A, WO 1999/48959, WO 2000/20492, WO 2002/072248, and WO 2005/113657, further improvements are still needed, particularly in membrane permeability, mechanical strength, heat shrinkage resistance, compression resistance, and electrolytic solution absorption properties. It is thus desired to form battery separators from microporous membranes having improved permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolytic solution absorption.

SUMMARY OF THE INVENTION

[0010] The invention relates to the discovery of a microporous membrane having good permeability, mechanical strength, heat shrinkage resistance, and improved electrolytic solution absorption and compression resistance properties. An embodiment of the invention relates to a microporous membrane comprising pores characterized by a pore size (or pore diameter when the pores are approximately cylindrical) distribution curve obtained by mercury intrusion porosimetry having at least two peaks. It has been discovered that such a membrane has good permeability, mechanical strength, heat shrinkage resistance, and improved compression resistance and electrolytic solution absorption characteristics. The microporous membrane can be produced by a method comprising: (1) combining a polyolefin composition and at least one diluent, for example a membrane-forming solvent, the polyolefin composition comprising (a) from about 74 wt. % to about 99 wt. % of a first polyethylene having a weight average molecular weight < 5 10⁵, e.g., in the range of from about 2.5 x 10⁵ to about 5 x 10⁵ and a molecular weight distribution of from about 5 to about 100, (b) from about 1 wt. % to about 5 wt. % of a second polyethylene having a weight average molecular weight >5 x 10⁵, e.g., in the range of from about 5.1 x 10⁵ to about 1 x 10⁶ and a molecular weight distribution of from about 5 to about 100, and (c) from about 0 wt. % to about 25 wt. % of a polypropylene having a weight average molecular weight of from about 3 x 10⁵ to about 1.5 x 10⁶, a molecular weight distribution of from about 1 to about 100, and a heat of fusion of 80 J/g or higher, the percentages based on the weight of the polyolefin composition, (2) extruding the combined polyolefin composition and diluent through a die to form an extrudate, (3) cooling the extrudate to form a cooled extrudate (4) stretching the cooled extrudate to a magnification of from, e.g., about 9 to about 400 fold in at least one direction at a high stretching temperature of from about Ted of the combined polyethylene of the cooled extrudate to about Ted + 30 °C to form a stretched sheet, (5) removing at least a portion of the diluent, e.g. the membrane-forming solvent, from the stretched sheet to form a membrane, (6) stretching the membrane, e.g., to a magnification in the range of from about 1.1 to about 1.8 fold in at least one direction to form a stretched membrane, and (7) heat-setting the stretched membrane to form the final microporous membrane. [0011] In an embodiment, the microporous membrane has dense domains corresponding to a main peak in a range of 0.01 to 0.08 μm in the pore size (or pore diameter when the pores are approximately spherical) distribution curve, and coarse domains corresponding to at least one sub-peak in a range of more than 0.08 μm to 1.5 μm in the pore size (or pore diameter when the pores are approximately cylindrical) distribution curve. In an embodiment, the pore volume ratio of the dense domains to the coarse domains is 0.5 to 49. In an embodiment, the microporous membrane has surface roughness of 3 x 10² nm or more as the maximum height difference between any two points on the surface of the membrane. In an embodiment, the upper limit on the surface roughness of the microporous membrane is 3 x 10³ nm. With surface roughness within this range, the microporous membrane has a large contact area with an electrolytic solution when used as a battery separator, exhibiting suitable electrolytic solution absorption characteristics. [0012] The resins used in forming the polyolefin composition comprise (a) a first polyethylene resin having a weight average molecular weight < 5 x 10⁵, e.g., in the range of from about 2.5 x 10⁵ to about 5 x 10⁵, for example from about 2.5 x 10⁵ to about 4 x 10⁵, and a molecular weight distribution of from about 5 to about 100, for example from about 7 to about 50, (b) a second polyethylene resin having a weight average molecular weight > 5 x 10⁵, e.g., of from about 5.1 x 10⁵ to about 1 x 10⁶ and a molecular weight distribution of from about 5 to about 100, for example from about 5 to about 50, and (c) optionally a polypropylene resin having a weight average molecular weight of from about 3 x 10⁵ to about 1.5 x 10⁶, for example from about 6 x 10⁵to about 1.5 x 10⁶, and a molecular weight distribution of from about 1 to about 100, for example from about 1.1 to about 50, and a heat of fusion of 80 J/g or higher, for example from 80 to about 200 J/g. The microporous membrane may suitably comprise 25 wt. % of polypropylene obtained from polypropylene resin and 75 wt. % or more of polyethylene obtained from polyethylene resins, based on the weight of the microporous polyolefin membrane. [0013] In an embodiment, the microporous membrane is manufactured by a method comprising the steps of (1) combining a polyolefin composition and at least one membrane-forming solvent to form a polyolefin solution, the polyolefin composition comprising (a) from about 74 wt. % to about 99 wt. % of a first polyethylene resin having a weight average molecular weight of from about 2.5 x 10⁵ to about 5 x 10⁵ and a molecular weight distribution of from about 5 to about 100, (b) from about 1 wt. % to about 5 wt. % of a second polyethylene resin having a weight average molecular weight of from 5.1 x 10³ to 1.25 x 10⁶ and a molecular weight distribution of from about 5 to about 100, and (c) from about 0 wt. % to about 25 wt. % of a polypropylene resin having a weight average molecular weight of from about 3 x 10⁵ to about 1.5 x 10⁶, a molecular weight distribution of from about 1 to about 100, and a heat of fusion of 80 J/g or higher, percentages based on the weight of the polyolefin composition, with the solution preferably having a solvent concentration of from about 25 wt. % to about 50 wt. %, for example from about 30 wt. % to about 50 wt. %, based on the weight of the polyolefin solution, (2) extruding the polyolefin solution through a die to form an extrudate, (3) cooling the extrudate to form a cooled extrudate having a high polyolefin content, (4) stretching the cooled extrudate to a magnification of, e.g., from about 9 to about 400 fold in at least one direction at a high stretching temperature of from about Ted of the combined polyethylene of the cooled extrudate to about Ted + 30 °C to form a stretched sheet, (5) removing at least a portion of the membrane-forming solvent from the stretched sheet to form a membrane, (6) stretching the membrane to a high magnification of from about 1.1 to about 1.8 fold, for example from about 1.2 to about 1.6 fold, in at least one direction to form a stretched membrane, and (7) heat-setting the stretched membrane to form the microporous membrane. [0014] In the above method, the stretching of the microporous membrane in step (6) may be called "re-stretching", because it is conducted after the stretching of the cooled extrudate in step (4).

[0015] In yet another embodiment, the invention related to a microporous membrane comprising a microporous polyolefin membrane comprising polyethylene and having an electrolytic solution absorption speed > 4.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The inventions relates to a microporous membrane (or film) having enhanced properties, especially electrolyte injection and compression properties. As an initial step, in the process for making such a membrane certain specific polyethylene resins and, optionally, a certain specific polypropylene resin can be combined, e.g. by melt-blending, to form a polyolefin composition.

[1] Materials used to produce the microporous membrane

(1) Polyolefin Composition

[0017] The polyolefin composition comprises (a) from about 74 wt. % to about 99 wt. %, for example from about 75 wt. % to about 98 wt. % of a first polyethylene resin having a weight average molecular weight < 5 xlO⁵, e.g., in the range of from about 2.5 x 10⁵ to 5 x 10⁵ and a molecular weight distribution of from about 5 to about 100, (b) from about 1 wt. % to about 5 wt. %, for example from about 2 wt. % to about 5 wt. %, of a second polyethylene resin having a weight average molecular weight > 5 x 10⁵, e.g., in the range of from 5.1 x 10⁵ to about 1 x 10⁶, and (c) from 0 wt. % to about 25 wt. % polypropylene resin having a weight average molecular weight of from about 3 x 10⁵ to about 1.5 x 10⁶, a molecular weight distribution of from about 1 to about 100, and a heat of fusion of 80 J/g or higher, the weight percents being based on the weight of the polyolefin composition. [0018] In another embodiment, the polyolefin composition comprises 90 wt% to 97 wt% of the first polyethylene, 2 wt% to 5 wt% of the second polyethylene, and 5 wt% to 10 wt% of the polypropylene, based on the weight of the polyolefin composition.

(a) Polyethylene resins (V) Composition

[0019] The first polyethylene resin, for example a high density polyethylene (HDPE) resin, has a weight average molecular weight (Mw") of from about 2.5 x 10³ to 7 x 10⁵, or 3 xlO⁵ to 6 x 10⁵, and a molecular weight distribution ("MWD") of from about 5 to about 100. A non-limiting example of the first polyethylene resin for use herein is one that has an Mw of from about 2.5 x 10⁵ to about 4 x 10⁵ and an MWD of from about 7 to about 50. The first polyethylene resin can be an ethylene homopolymer, or an ethylene/α-olefin copolymer, such as, for example, one containing a small amount, e.g. about 5 mole %, of a third α-olefin. The third α-olefin, which is not ethylene, is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, or styrene or combinations thereof. Such copolymer is preferably produced using a single-site catalyst.

[0020] The second polyethylene resin, for example an ultra-high molecular weight polyethylene (UHMWPE) resin, has an Mw greater than that of the first polyethylene resin, e.g., in the range of from 5.1 x 10⁵ to 1 x 10⁶, and an MWD of from about 5 to about 100. A non-limiting example of the second polyethylene resin for use herein is one that has an MWD of from about 5 to about 50. The second polyethylene resin can be an ethylene homopolymer, or an ethylene/α-olefin copolymer, such as, for example, one containing a small amount, e.g. about 5 mole %, of a third α-olefin. The third α-olefin, which is not ethylene, can be, for example, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, or styrene or combinations thereof. Such copolymer is preferably produced using a single-site catalyst. (H) Mw and MWD Determination [0021] MWD is equal to the ratio of Mw to the number-average molecular weight ("Mn). The MWD of the polymers used to produce the microporous membrane can be controlled e.g., by a multi-stage polymerization. The MWD of the polyethylene composition can be controlled by the molecular weights and mixing ratios of the polyethylene components. [0022] Mw and Mn of the polyethylenes are determined using a High Temperature Size Exclusion Chromatograph, or "SEC", (GPC PL 220, Polymer Laboratories), equipped with a differential refractive index detector (DRI). Three PLgel Mixed-B columns (available from Polymer Laboratories) are used. The nominal flow rate is 0.5 cmVmin, and the nominal injection volume was 300 DL. Transfer lines, columns, and the DRI detector were contained in an oven maintained at 145°C. The measurement is made in accordance with the procedure disclosed in "Macromolecules, Vol. 34, No. 19, pp. 6812-6820 (2001)". [0023] The GPC solvent used is filtered Aldrich reagent grade 1 ,2,4-Trichlorobenzene (TCB) containing approximately 1000 ppm of butylated hydroxy toluene (BHT). The TCB was degassed with an online degasser prior to introduction into the SEC. Polymer solutions were prepared by placing dry polymer in a glass container, adding the desired amount of above TCB solvent, then heating the mixture at 160⁰C with continuous agitation for about 2 hours. The concentration of UHMWPE solution was 0.25 to 0.75mg/ml. Sample solution will be filtered off-line before injecting to GPC with 2Dm filter using a model SP260 Sample Prep Station (available from Polymer Laboratories). [0024] The separation efficiency of the column set is calibrated with a calibration curve generated using a seventeen individual polystyrene standards ranging in Mp from about 580 to about 10,000,000, which is used to generate the calibration curve. The polystyrene standards are obtained from Polymer Laboratories (Amherst, MA). A calibration curve (logMp vs. retention volume) is generated by recording the retention volume at the peak in the DRI signal for each PS standard, and fitting this data set to a 2nd-order polynomial. Samples are analyzed using IGOR Pro, available from Wave Metrics, Inc. 06) Polypropylene resin [0025] The polypropylene for use herein has an Mw of from about 3 x 10⁵ to about 1.5 x 10⁶, for example from about 6 x 10⁵ to about 1.5 x 10⁶, a heat of fusion ("ΔHm") of 80 J/g or higher, for non-limiting example from about 80 to about 200 J/g, and an MWD of from about 1.0 to about 100, for example from about 1.1 to about 50, and can be a propylene homopolymer or a copolymer of propylene and another, i.e. a fourth, olefin, though the homopolymer is preferable. The copolymer may be a random or block copolymer. The fourth olefin, which is an olefin other than propylene, includes α-olefins such as ethylene, ctane-1, pentene-1, ctane-1, 4-methylpentene-l, ctane-1, vinyl acetate, methyl methacrylate, styrene, etc., and diolefins such as butadiene, 1,5-hexadiene, 1 ,7-octadiene, 1 ,9-decadiene, etc. The percentage of the fourth olefin in the propylene copolymer is preferably in a range not deteriorating the properties of the microporous polyolefin membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc., and is preferably less than about 10 mole %, e.g. from about 0 to less than about 10 mole %.

[0026] In an embodiment, the MWD of polypropylene is from about 1.0 to about 100, for example from about 2 to about 50, or about 2.5 to 6.

[0027] The Mw, MWD and the ΔHm of the polypropylene are measured in accordance with the procedures described in patent publication US 2008/0057389, which is incorporated by reference herein in its entirety.

[0028] The amount of polypropylene resin in the polyolefϊn composition is 25 wt. % or less based on the weight of the polyolefϊn composition. When the percentage of polypropylene is more than 25 wt. %, the resultant microporous membrane has relatively lower strength and is difficult to form into a bimodal structure. The percentage of polypropylene resin may be, for example, from about 5 wt. % to about 20 wt. %, and for further example from about 7 wt. % to about 15 wt. %, based on the weight of the polyolefϊn composition. In an embodiment, the propylene has a ΔHm in the range of about 110 J/g to 120 J/g. (2) Other components

[0029] In addition to the above components, the polyolefϊn composition can contain (a) additional polyolefϊn and/or (b) heat-resistant polymer resins having melting points or glass transition temperatures ("Tg") of about 170°C or higher, in amounts not deteriorating the properties of the microporous membrane, for example 10% or less by mass based on the polyolefϊn composition.

(a) Additional Polyolefϊn's

[0030] The additional polyolefin can be at least one of (a) polybutene-1 , polypentene-1 , poly-4-methylpentene-l, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and an ethylene/α-olefϊn copolymer, each of which may have an Mw of from 1 x 10⁴ to 4 x 10⁶, and (b) a polyethylene wax having an Mw of from 1 x 10³ to 1 x 10⁴. Polybutene-1, polypentene-1, poly-4-methylpentene-l, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene are not restricted to homopolymers, but may be copolymers containing still other α-olefϊns.

(b) Heat-resistant resins [0031] The heat-resistant resins are preferably (i) amorphous resins having melting points of about 170°C or higher, which may be partially crystalline, and (ii) completely amorphous resins having a Tg of about 170°C or higher and mixtures thereof. The melting point and Tg are determined by differential scanning calorimetry (DSC) according to method JIS K7121. Specific examples of the heat-resistant resins include polyesters such as polybutylene terephthalate (melting point: about 160-230°C), polyethylene terephthalate (melting point: about 250-270⁰C), etc., fluororesins, polyamides (melting point: 215-265°C), polyarylene sulfide, polyimides (Tg: 280°C or higher), polyamideimides (Tg: 280°C), polyether sulfone (Tg: 223°C), polyetheretherketone (melting point: 334°C), polycarbonates (melting point: 220-240°C), cellulose acetate (melting point: 22O⁰C), cellulose triacetate (melting point: 300⁰C), polysulfone (Tg: 190⁰C), polyetherimide (melting point: 216⁰C), etc. (c) Content

[0032] The total amount of the additional polyolefin and the heat-resistant resin is preferably 20 % or less, for example from 0 wt. % to about 20 wt. %, based on the weight of the combined diluent and polyolefin composition.

[2] Method for producing the microporous membrane [0033] The invention relates to a method for producing the microporous membrane comprising the steps of (1) combining certain specific polyolefins (generally in the form of polyolefin resins) and at least one solvent or diluent to form a polyolefin solution, (2) extruding the polyolefin solution through a die to form an extrudate, (3) cooling the extrudate to form a cooled extrudate, (4) stretching the cooled extrudate at a certain specific temperature to form a stretched sheet, (5) removing the solvent or diluent from the stretched sheet to form a solvent/diluent-removed membrane, (6) stretching the solvent/diluent-removed membrane at a certain specific temperature and to a certain specific magnification to form a stretched membrane, and (7) heat-setting the stretched membrane to form the microporous membrane. A heat-setting treatment step (4i), a heat roll treatment step (4ii), and/or a hot solvent treatment step (4iii) may be conducted between the steps (4) and (5), if desired. A heat-setting treatment step (5i) may be conducted between the steps (5) and (6). A step (5Ii) of cross-linking with ionizing radiations following step (5i) prior to step (6), and a hydrophilizing treatment step (7i) and a surface-coating treatment step (7ii) may be conducted after the step (7), if desired. (1) Combining the polymer and diluent

[0034] The polyolefin composition comprising polyethylene and polypropylene is blended with at least one diluent to form a mixture. The mixture may contain various additives such as anti-oxidants, fine silicate powder (pore-forming material), etc., in ranges not deteriorating the effects of the present invention, if desired.

[0035] To better enable stretching at relatively higher magnifications, the diluent can be preferably liquid at room temperature. When the diluent is a solvent for one or more of the polymers in the mixture, it can be called a "solvent" or "membrane-forming solvent". When the diluent is a solvent (or membrane-forming solvent), the mixture can be called a "polyolefin solution". The diluent can be, for example, aliphatic, alicyclic or aromatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, mineral oil distillates having boiling points comparable to those of the above hydrocarbons, and phthalates liquid at room temperature such as dibutyl phthalate, dioctyl phthalate, etc. To most effectively obtain an extrudate in the form of a gel-like sheet having a stable solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In an embodiment, one or more solid solvents which are miscible with the polyolefin composition during melt-blending but solid at room temperature may be added to the liquid solvent. Such solid solvents are preferably stearyl alcohol, ceryl alcohol, paraffin waxes, etc. In another embodiment, solid solvent can be used without liquid solvent. However, when only the solid solvent is used, uneven stretching, etc., can occur. [0036] The viscosity of the liquid solvent is preferably from about 30 to about 500 cSt, more preferably from about 30 to about 200 cSt, when measured at a temperature of 25°C. When the viscosity at 25°C is less than 30 cSt, the polyolefin solution may foam, resulting in difficulty in blending. On the other hand, when the viscosity is more than 500 cSt, the removal of the liquid solvent can be difficult. [0037] Though not particularly critical, the polyolefin solution is preferably melt-blended in a double-screw extruder to prepare a high resin concentration polyolefin solution. The membrane-forming solvent may be added before starting melt-blending, or supplied to the double-screw extruder in an intermediate portion during blending, though the latter is preferable. [0038] The melt-blending temperature of the polyolefin solution is preferably in a range of the melting point ("Tm") of the polyethylene resin +10°C to Tm +12O⁰C. The melting point can be measured by differential scanning calorimetry (DSC) according to JIS K7121. In an embodiment, the melt-blending temperature is from about 140 to about 250°C, preferably from about 170 to about 240°C, particularly where the polyethylene resin has a melting point of about 130 to about 140⁰C.

[0039] To obtain a hybrid structure, the concentration of the polyolefin composition in the polyolefin solution is preferably from about 25 wt. % to about 50 wt. %, for example from about 25 wt. % to about 45 wt. %, based on the weight of the polyolefin solution.

[0040] The ratio L/D of the screw length L to the screw diameter D in the double-screw extruder is preferably in a range of from about 20 to about 100, more preferably in a range of from about 35 to about 70. When L/D is less than 20, melt-blending can be inefficient. When L/D is more than 100, the residence time of the polyolefin solution in the double-screw extruder can be too long. In this latter case, the membrane's Mw deteriorates as a result of excessive shearing and heating, which is undesirable. The cylinder of the double-screw extruder preferably has an inner diameter of from about 40 to about 100 mm. [0041] In the double-screw extruder, the ratio Q/Ns of the amount Q (kg/h) of the polyolefin solution charged to the number of revolution Ns (rpm) of a screw is preferably from about 0.1 to about 0.55 kg/h/rpm. When Q/Ns is less than 0.1 kg/h/rpm, the polyolefin can be damaged by shearing, resulting in decrease in strength and meltdown temperature. When Q/Ns is more than 0.55 kg/h/rpm, uniform blending cannot be achieved. Q/Ns is more preferably from about 0.2 to about 0.5 kg/h/rpm. The number of revolutions Ns of the screw is preferably 180 rpm or more. Though not particularly critical, the upper limit of the number of revolutions Ns of the screw is preferably about 500 rpm. (2) Extrusion [0042] The polyolefin solution can be melt-blended in the extruder and extruded from a die. In another embodiment, the polyolefin can be extruded and then pelletized. In this latter embodiment, the pellets can be melt-blended and extruded in a second extrusion to make the gel-like molding or sheet. In either embodiment, the die may be a sheet-forming die having a rectangular orifice, a double-cylindrical, hollow die, an inflation die, etc. In the case of the sheet-forming die, the die gap is preferably from about 0.1 to about 5 mm. The extrusion temperature is preferably from about 140 to about 250°C, and the extruding speed is preferably from about 0.2 to about 15 m/minute. The direction of extrusion (i.e., the direction of extrudate travel during extrusion and steps downstream of extrusion) is called the machine direction or "MD". The direction perpendicular to both the thickness of the membrane and MD is called the transverse direction or "TD".

(3) Cooling the extrudate [0043] The extrudate produced from the die is cooled to form a cooled extrudate, e.g., a high resin content gel-like molding or sheet. Cooling can be conducted by exposing the extrudate to a temperature < the extrudate's gelation temperature at a cooling rate of about 50°C/minute or more. Cooling can be preferably conducted to about 25°C or lower. Such cooling sets the micro-phase of the polyolefϊn separated by the membrane-forming solvent. Generally, the slower cooling rate provides the gel-like sheet with larger pseudo-cell units, resulting in a coarser higher-order structure. On the other hand, a higher cooling rate results in denser cell units. A cooling rate of less than 50°C/minute can lead to increased crystallinity, making it more difficult to provide the gel-like sheet with suitable stretchability. Usable cooling methods include bringing the extrudate into contact with a cooling medium such as cooling air, cooling water, etc.; bringing the extrudate into contact with cooling rollers; etc.

[0044] In an embodiment, the relative amounts of polyolefϊn composition and diluent combined for extrusion are selected so that the cooled extrudate has high polyolefϊn content. [0045] By high polyolefϊn content, it is meant that the cooled extrudate comprises at least about 25 wt. %, for example from about 25 wt. % to about 50 wt. %, polyolefin derived from the polyolefϊn used to produce the polyolefin composition, based on the weight of the cooled extrudate. It is believed that a polyolefin content of less than about 25 wt. % of the cooled extrudate makes it more difficult to form a hybrid microporous membrane structure having both small and large pores. A polyolefin content of more than about 50 wt. % can lead to higher viscosity which makes it more difficult to form the desired hybrid structure. The cooled extrudate preferably has a polyolefin content at least as high as that of the polyolefin solution.

(4) Stretching the cooled extrudate [0046] The cooled extrudate can be stretched in at least one direction. While not wishing to be bound by any theory or model, it is believed that the cooled extrudate (e.g., a gel-liked sheet) can be uniformly stretched because the sheet contains the _ _

membrane-forming solvent. The gel-like sheet is preferably stretched to a predetermined magnification after heating by, for example, a tenter method, a roll method, an inflation method or a combination thereof. The stretching may be conducted monoaxially or biaxially, though biaxial stretching is preferable. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching or multi-stage stretching (for instance, a combination of the simultaneous biaxial stretching and the sequential stretching) may be used, though simultaneous biaxial stretching is preferable. When biaxial stretching is used, the amount of stretching in each direction need not be the same. [0047] The stretching magnification of this first stretching step is preferably 2 fold or more, more preferably 3 to 30 fold in the case of monoaxial stretching. In the case of biaxial stretching, the stretching magnification is preferably 3 fold or more in any direction, namely 9 fold or more, more preferably 16 fold or more, most preferably 25 fold or more, in area magnification. In an embodiment, the cooled extrudate is simultaneously stretched 3 to 7 fold (e.g., 5 fold) in MD and 3 to 7 fold (e.g., 5 fold) in TD. An example for this first stretching step would include stretching from about 9 fold to about 400 fold. A further example would be stretching from about 16 to about 49 fold. With the area magnification of 9 fold or more, the pin puncture strength of the microporous membrane is improved. When the area magnification is more than 400 fold, stretching apparatuses, stretching operations, etc., involve large-sized stretching apparatuses, which can be difficult to operate.

[0048] During stretching, the extrudate can be exposed to a temperature in the range of from about the crystal dispersion temperature ("Ted") of the polyethylene used to produce the extrudate to about Ted + 30 °C, e.g. in a range of Ted of the combined polyethylene content of the cooled extrudate to Ted + 25°C, more preferably in a range of Ted + 10°C to Ted + 25°C, most preferably in a range of Ted + 15°C to Ted + 25°C. When the stretching temperature of the cooled extrudate is lower than Ted, it is believed that the polyethylene resin is so insufficiently softened that the gel-like sheet is easily broken by stretching, failing to achieve high-magnification stretching. [0049] The crystal dispersion temperature is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D 4065. Because the polyethylene resin has a crystal dispersion temperature of about 90 to 100°C, the stretching temperature is from about 90 to 125°C; preferably form about 100 to 125°C, more preferably from 105 to 125°C.

[0050] The above stretching causes cleavage between polyethylene lamellas, making the polyethylene phases finer and forming large numbers of fibrils. The fibrils form a three-dimensional network structure. The stretching is believed to improve the mechanical strength of the final microporous membrane and expands its pores, making the microporous membrane suitable for use as a battery separator.

[0051] Depending on the desired properties, stretching may be conducted with a temperature distribution in a thickness direction, to provide the final microporous membrane with further improved mechanical strength. The detailed description of this method is given by Japanese Patent 3347854. (5) Diluent Removal

[0052] Any convenient method can be used for removing at least a portion of the diluent from the cooled extrudate. For example, the diluent can be removed (e.g., washed, displaced, or dissolved) using a washing solvent. Because the polyolefin composition phase is phase-separated from a membrane-forming solvent phase, the removal of the diluent provides a microporous membrane. The removal of the liquid solvent can be conducted by using one or more suitable washing solvents, i.e., one capable of displacing the liquid solvent from the membrane. Examples of the washing solvents include volatile solvents, e.g., saturated hydrocarbons such as pentane, hexane, heptane, etc., chlorinated hydrocarbons such as methylene chloride, carbon tetrachloride, etc., ethers such as diethyl ether, dioxane, etc., ketones such as methyl ethyl ketone, etc., linear fluorocarbons such as trifluoroethane, C₆Fi₄, etc., cyclic hydrofluorocarbons such as C₅H₃F₇, etc., hydrofluoroethers such as C₄F₉OCH₃, C₄F₉OC₂Hs, etc., perfluoroethers such as C₄F₉OCF₃, C₄F₉OC₂F₅, etc., and mixtures thereof. [0053] The washing of the stretched membrane can be conducted by immersion in the washing solvent and/or showering with the washing solvent. The washing solvent used is preferably from about 300 to about 30,000 parts by mass per 100 parts by mass of the stretched membrane. The washing temperature is usually from about 15 to about 30°C, and if desired, heating may be conducted during washing. The heating temperature during washing is preferably about 80°C or lower. Washing is preferably conducted until the amount of the remaining liquid solvent becomes less than 1 wt. % of the amount of liquid solvent that was present in polyolefin solution prior to extrusion. [0054] The microporous membrane deprived of the membrane-forming solvent can be dried by a heat-drying method, a wind-drying (e.g., air drying using moving air) method, etc. Any drying method capable of removing a significant amount of the washing solvent can be used. Preferably, substantially all of the washing solvent is removed during drying. During drying, the membrane can be exposed to a temperature that is preferably equal to or lower than Ted, more preferably 5°C or more lower than Ted. Drying is conducted until the remaining washing solvent becomes preferably 5 wt. % or less, more preferably 3 wt. %, based on the weight (on a dry basis) of the microporous membrane. Insufficient drying can be recognized because it results in an undesirable decrease in the porosity of the microporous membrane.

(6) Stretching the dried microporous membrane

[0055] The dried microporous membrane is stretched in a second stretching step (re-stretched) at least monoaxially at high magnification. The re-stretching of the microporous membrane can be conducted, for example, while heating, by a tenter method, etc., as in the first stretching step. The re-stretching may be monoaxial or biaxial. In the case of biaxial stretching, any one of simultaneous biaxial stretching or sequential stretching may be used, though simultaneous biaxial stretching is preferable. Because re-stretching is usually conducted on the microporous membrane in a long sheet form, which is obtained from the stretched gel-like sheet, the directions of MD and TD during the re-stretching is usually the same as those in the stretching of the cooled extrudate. The high stretching magnification in this step is from about 1.1 to about 1.8 fold in at least one direction, for example from about 1.2 to about 1.6 fold. The amount of stretching need not be the same in each stretching direction. [0056] The second stretching is conducted while exposing the membrane at a second temperature preferably equal to Tm or lower, more preferably in a range of Ted to Tm, (the "second stretching temperature"). When the second stretching temperature is higher than Tm, it is believed that the melt viscosity is generally too low to conduct good stretching, resulting in low permeability. When the second stretching temperature is lower than Ted, it is believed that the polyolefϊn is insufficiently softened so that the membrane might be broken by stretching, i.e., a failure to achieve uniform stretching. In an embodiment, the second stretching temperature is usually from about 90 to about 135°C, for example from about 95 to about 130°C. _ _

[0057] The monoaxial second stretching magnification of the microporous membrane in this step, as mentioned above, is preferably from about 1.1 to about 1.8 fold. A magnification of 1.1 to 1.8 fold generally provides the microporous membrane of the present invention with a hybrid structure having a large average pore size. In the case of monoaxial second stretching, the magnification can be form 1.1 to 1.8 fold in a longitudinal or transverse direction. In the case of biaxial second stretching, the microporous membrane may be stretched at the same or different magnifications, though preferably the same, as long as the stretching magnifications in both directions are form 1.1 to 1.8 fold. [0058] When the second stretching magnification of the microporous membrane is less than 1.1 fold, it can be more difficult to produce a membrane having a hybrid structure, and having good permeability, electrolytic solution absorption capability, and compression resistance in the finished membrane. When the second stretching magnification is more than 1.8 fold, it can be more difficult to maintain a fibrous structure, and it is believed that the heat shrinkage resistance and the electrolytic solution absorption characteristics of the membrane are reduced. This second stretching magnification can be, e.g., from 1.2 to 1.6 fold.

[0059] The stretching rate is preferably 3%/second or more in the stretching direction. In the case of monoaxial stretching, stretching rate is 3%/second or more in a MD or TD. In the case of biaxial stretching, stretching rate is 3%/second or more in both MD and TD. A stretching rate of less than 3%/second can make it more difficult to produce a membrane having good permeability without undue permeability variation across TD. The stretching rate is preferably 5%/second or more, more preferably 10%/second or more. Though not particularly critical, the upper limit of the stretching rate is e.g., 50%/second to avoid membrane rupture during stretching. (7) Heat treatment

[0060] The dried microporous membrane can be thermally treated (heat-set) to stabilize crystals and make uniform lamellas in the membrane. The heat-setting can be conducted by a tenter method or a roll method. The heat-setting temperature can be conducted while exposing the membrane to a temperature in a range of the second stretching temperature ±5°C, more preferably in a range of the second stretching temperature ±3°C. It is believed that too low a heat-setting temperature makes it more difficult to produce a membrane having the desired pin puncture strength, tensile rupture strength, tensile rupture elongation and heat shrinkage resistance, while too high a heat-setting temperature can adversely affect membrane permeability. [0061] An annealing treatment can be conducted after the heat-setting step. The annealing is a heat treatment with no load applied to the microporous membrane, and may be conducted by using, for example, a heating chamber with a belt conveyer or an air-floating-type heating chamber. The annealing may also be conducted continuously after the heat-setting with the tenter slackened. The annealing can be conducted while exposing the membrane to a temperature of Tm or lower, more preferably in a range from about 60°C to about Tm -5°C. Annealing is believed to provide the microporous membrane with improved permeability and strength. Optionally, the membrane can be annealed without prior heat-setting.

(8) Heat-setting treatment of stretched gel-like sheet

[0062] The stretched extrudate between the steps (4) and (5) may be heat-set, if desired. The heat-setting method may be conducted the same way as described above for step (7).

(9) Heat roller treatment

[0063] Following step (4), at least one surface of the stretched extrudate step (4) may be brought into contact with one or more heat rollers following any of steps (4) to (7). The roller temperature is preferably in a range of from Ted +10°C to Tm. The contact time of the heat roll with the stretched extrudate is preferably from about 0.5 second to about 1 minute. The heat roll may have a flat or rough surface. The heat roll may have a suction functionality to remove the solvent. Though not particularly critical, one example of a roller-heating system may comprise holding heated oil in contact with a roller surface. (10) Hot solvent treatment

[0064] The stretched extrudate may be contacted with a hot solvent between steps (4) and (5). A hot solvent treatment turns fibrils formed by stretching to a leaf vein form with relatively thick fiber trunks, providing the microporous membrane with large pore size and suitable strength and permeability. The term "leaf vein form" means that the fibrils have thick fiber trunks, and thin fibers extending in a complicated network structure from the trunks. The details of the hot solvent treatment method are described in WO 2000/20493. (11) Heat-setting of microporous membrane containing washing solvent [0065] The microporous membrane containing at least a portion of the washing solvent can be heat set between the steps (5) and (6) if desired. The heat-setting procedure can be the same as described above in step (7). (12) Cross-linking [0066] The heat-set microporous membrane can be cross-linked by ionizing radiation rays such as α-rays, β-rays, γ-rays, electron beams, etc. In the case of irradiating electron beams, the amount of electron beams is preferably from about 0.1 to about 100 Mrad, and the accelerating voltage is preferably form about 100 to about 300 kV. The cross-linking treatment is believed to elevate the meltdown temperature of the microporous membrane. (13) Hvdrophilizing treatment

[0067] The heat-set microporous membrane may be subjected to a hydrophilizing treatment (a treatment that makes the membrane more hydrophilic). The hydrophilizing treatment may be a monomer-grafting treatment, a surfactant treatment, a corona-discharging treatment, etc. Optionally, the monomer-grafting treatment is conducted after the cross-linking treatment.

[0068] In the case of surfactant treatment hydrophilizing the heat-set microporous membrane, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants may be used, and the nonionic surfactants are preferred. The microporous membrane can be dipped in a solution of the surfactant in water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, etc., or coated with the solution by a doctor blade method. (14) Surface-coating treatment

[0069] Optionally, the heat-set microporous membrane resulting from step (7) can be coated with porous polypropylene, porous fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, porous polyimides, porous polyphenylene sulfide, etc., to improve meltdown properties when the membrane is used as a battery separator. The polypropylene used for the coating preferably has Mw of form about 5,000 to about 500,000, and a solubility of about 0.5 grams or more in 100 grams of toluene at 25°C. Such polypropylene more preferably has a racemic diade fraction of from about 0.12 to about 0.88, the racemic diade being a structural unit in which two adjacent monomer units are mirror-image isomers to each other. The surface-coating layer may be applied, for instance, by applying a solution of the above coating resin in a good solvent to the niicroporous membrane, removing part of the solvent to increase a resin concentration, thereby forming a structure in which a resin phase and a solvent phase are separated, and removing the remainder of the solvent. Examples of the good solvents for this purpose include aromatic compounds, such as toluene, xylene. [31 Structure, properties, and composition of microporous membrane

(1) Structure

[0070] The microporous membrane of this invention has a hybrid structure derived from the polyethylene resins, in which its pore size distribution (represented as a differential per volume curve) obtained by mercury intrusion porosimetry has at least two peaks (main peak and at least one sub-peak). The main peak and sub-peak respectively correspond to the dense domains and coarse domains of the polyethylene phase. [0071] The microporous membrane produced by the above-described method has a relatively wide pore size distribution when plotted as a differential pore volume curve. Pore size distribution can be measured, e.g., by conventional methods such as mercury porosimetry.

[0072] When mercury porosimetry is used to measure the distribution of pore sizes and pore volume in the membrane, it is conventional to measure pore diameter, pore volume, and the specific surface area of the membrane. The measurements can be used to

determine a differential pore volume expressed as ^-τ-τ where Vp is the pore volume, dLog[r) and r is the pore radius, assuming cylindrical pores. The differential pore volume when plotted on the y axis with pore diameter on the x axis is conventionally referred to as the "pore size distribution." For membranes exhibiting a hybrid structure, at least about 25%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60%, of the differential pore volume is associated with pores that are about 100 nanometers in size

(diameter) or larger. In other words, for the curve of ₇-^ vs. pore diameter, the dLog(r) fraction of the area under the curve from a pore diameter of about 100 nanometers to about 1 ,000 nanometers is at least about 25%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60% of the total area under that curve for pore sizes (or diameters assuming cylindrical pores) of from about 10 nanometers to about 1,000 nanometers. In an embodiment, the area under the curve for pore diameters of from about 100 nanometers to about 1,000 nanometers is in the range of about 25% to about 60%, or about 30% to about 55%, or about 35% to about 50% of the total area under the curve for pore diameters of from about 10 nanometers to about 1,000 nanometers. [0073] Though not critical, dense domains (relatively small pores) and coarse domains (relatively large pores) are irregularly entangled to form a hybrid structure in any cross sections of the microporous polyolefϊn membrane viewed in longitudinal and transverse directions. The hybrid structure can be observed by a transmission electron microscope (TEM), etc. [0074] Because the microporous membrane of the present invention has relatively large internal space and openings due to coarse domains, it has suitable permeability and electrolytic solution absorption, with little air permeability variation when compressed. This microporous membrane also has relatively small internal space and openings which influence safety properties of the membrane when used as a battery separator, such as shutdown temperature and shutdown speed. Accordingly, lithium ion batteries such as lithium ion secondary batteries comprising separators formed by such microporous membrane have suitable productivity and cyclability while retaining their high safety performance.

[0075] The mercury intrusion porosimetry method used to determine the microporous membrane structure involves use of a Pore Sizer 9320 (Micromeritics Company, Ltd.), a pressure range of from 3.6 kPa to 207 MPa, and a cell volume of 15 cm³. For the measurements, a contact angle of mercury of 141.3 and a surface tension of mercury of 484 dynes/cm was employed. The parameters obtained by this included pore volume, surface area ratio, peak top of pore size, average pore size and porosity. References teaching this method include Raymond P. Mayer and Robert A. Stowe, J. Phys. Chem.70,12(1966); L. C. Drake, Ind.Eng.Chem., 41,780(1949); H. L. Ritter and L. C. Drake, Ind.Eng.Chem.Anal.,17,782(1945) and E. W. Washburn, Proc.Nat.Acad.Sci.,7,115(1921).

[0076] The mercury intrusion porosimetry method can be briefly summarized as follows. Pressurized mercury is applied to a planar surface of the microporous membrane at a pressure P. This pressure does work Wl on the mercury, which causes the mercury to intrude into the pores of the microporous membrane until equilibrium is reached. In equilibrium, the force Fl on the surface of the mercury applied by pressure P is balanced by a Force F2 equal in magnitude to Fl but acting in the opposite direction. According to the conventional Washbum model, the amount of work needed to move the surface of the mercury in to the pore W\ = 2wLγcosθ is balanced by the work done by the opposing force, which can be expressed as WI = Pw²L . In these equations, L is the depth and r is the radius of the pores, assuming the pores are cylindrical. The contact angle of the mercury is expressed as θ. The surface tension of mercury is expressed as γ, which is generally recognized as 0.48Nm^"1. Consequently, according to the Washbum model, pore radius r can be expressed as a function of the pressure P according to the equation

r = . The measurement of differential pore volume can then proceed as

follows. First, the volume V of mercury intruded into the pores is measure as a function of pressure P. The measured value of P is used to calculate pore radius r, as described above. P is increased incrementally, and the volume of mercury is determined at each value of P. In this way, a table can be constructed showing the pore volume associated with pores of a particular r, tabulated over the range of r as determined by the range of P selected for the measurement. The tabulated values of r can be conveniently converted to Log (r). Pore volume Vp is generally expressed as cm³ per gram of the microporous

membrane. Differential pore volume expressed as ÷-rτ can be calculated from the dLog{r) tabulated values of Vp and r, where dVp is approximated by the difference between adjacent tabulated values of Vp, and where dLog(r) is approximated by the difference between adjacent tabulated values of Log( r). Because the microporous polyolefin membrane has relatively large internal space and openings due to coarse domains, it has suitable permeability and electrolytic solution absorption, with little air permeability variation when compressed. Accordingly, lithium ion batteries such as lithium ion secondary batteries comprising separators formed by such microporous polyolefin membrane have suitable productivity and cyclability.

[0077] In an embodiment, the microporous polyolefin membrane is a single-layer membrane, where the membrane has a hybrid structure, e.g., the microporous layer material is characterized by a differential pore volume curve having an area under the curve over the range of pore diameters of from about 100 nm to about 1,000 nm that is about 25% or more of a total area under the curve over the range of pore diameters of from about 10 nm to about 1,000 nm.

[0078] A hybrid structure is more difficult to produce when the percentage of the first polyethylene in the membrane, is more than 99 wt% based on the total weight of polyolefin in the membrane. It is also more difficult to produce a hybrid structure when the percentage of polypropylene in the membrane is more than 25 wt% based on the total weight of polyolefin in the membrane. It is also more difficult to produce a hybrid structure when the percentage of the second polyethylene in the membrane is more than 5 wt%. Membranes having a hybrid structure generally have better electrolytic solution absorption characteristics then membranes that do not have a hybrid structure.

[0079] In a preferred embodiment of the microporous polyolefin membrane, the main peak is in a pore size range of 0.01 to 0.08 μm, and at least one sub-peak is in a pore size ranging from more than 0.08 μm to 1.5 μm. In other words, at least one sub-peak has a pore size of greater than 0.08 μm to 1.5 μm or less. More preferably, the main peak is a first peak in a pore size range of about 0.04 to 0.07 μm, and the sub-peaks comprise a second peak in a pore size range of about 0.1 to 0.11 μm, a third peak at a pore size of about 0.7 μm, and a fourth peak in a pore size range of about 1 to 1.1 μm. However, the sub-peaks need not have the third and fourth peaks. For example, the pore size distribution curve may have first to fourth peaks at about 0.06 μm, about 0.1 μm, about 0.7 μm, and about 1.1 μm, respectively.

[0080] The pore volume ratio of the dense domains to the coarse domains of the microporous membrane of the present invention is determined by standard methods using transmission electron microscopy, mercury intrusion porosimetry, etc. as described in PCT publication WO2008/016174, which is incorporated by reference herein in its entirety. As shown in the PCT reference, a hatched area Si on the smaller diameter side and a vertical line Li passing the first peak corresponds to the pore volume of the dense domains, and a hatched area S₂ on the larger diameter side and a vertical line L₂ passing the second peak corresponds to the pore volume of the coarse domains. The pore volume ratio Si /S₂ of the dense domains to the coarse domains is preferably from about 0.5 to about 49, more preferably from about 0.6 to about 10, most preferably from 0.7 to 2. When distinct peaks are not observed in the raw porosimetry data, the approximate positions can be obtained using, e.g., conventional peak deconvolution analysis. For example, the first derivative of the curve representing the number of pores on the y-axis and the pore size on the x-axis can be used to more clearly identify a change in slope (e.g., an inflection) on the curve.

[0081] Though not critical, dense domains and coarse domains are irregularly entangled to form a hybrid structure in any cross sections of the microporous polyolefin membrane viewed in longitudinal and transverse directions. The hybrid structure can be observed by a transmission electron microscope (TEM), etc.

(2) Properties

[0082] The thickness of the final membrane is generally in the range of 3 μm to 200 μm. For example, the membrane can have a thickness in the range of from about 5 μm to about 50 μm, e.g., from about 15 μm to about 30 μm. The thickness of the microporous membrane can be measured, e.g., by a contact thickness meter at 1 cm longitudinal intervals over the width of 10 cm, and then averaged to yield the membrane thickness.

Thickness meters such as the Litematic available from Mitsutoyo Corporation are suitable. Non-contact thickness measurement methods are also suitable, e.g. optical thickness measurement methods.

(a) Air permeability of 500 seconds/ 100cm² or less (converted to the value at 20 μm thickness)

[0083] The membrane's air permeability is measured according to JIS P8117. In an embodiment, the membrane air permeability is in the range of form 20 to 400 seconds/100 cm³. If desired, air permeability Pi measured on a microporous membrane having a thickness Ti according to JIS P8117 can be converted to air permeability P₂ at a thickness of 20 μm by the equation of P₂ = (Pi x 2O)AT₁.

(b) Porosity of from about 25 to about 80% [0084] The membrane's porosity is measured conventionally by comparing the membrane's actual weight to the weight of an equivalent non-porous membrane of 100% polyethylene (equivalent in the sense of having the same length, width, and thickness). Porosity is then determined using the formula: Porosity % = 100 x (w2-wl)/w2, wherein "wl" is the actual weight of the microporous membrane and "w2" is the weight of an equivalent non-porous membrane of 100% polyethylene having the same size and thickness. (c) Pin puncture strength of 2,000 mN or more (converted to the equivalent value for a membrane having a 20 μm thickness)

[0085] The membrane's pin puncture strength (converted to the value at membrane thickness of 20 μm) is represented by the maximum load measured when the microporous membrane is pricked with a needle of 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second. When the pin puncture strength is less than 2,000 mN/20 μm, short-circuiting might occur in batteries with separators formed by the microporous membrane. In an embodiment, the membrane's pin puncture strength (converted to 20 μm) is in the range of 4,000 to 5,000 mN. [0086] The maximum load is measured when each microporous membrane having a thickness of Ti is pricked with a needle of 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second. The measured maximum load Li is converted to the maximum load L₂ at a thickness of 20 μm by the equation of L₂ = (Li x 20)/Ti, and defined as pin puncture strength.

(d) Tensile strength of 49,000 kPa or more [0087] A tensile strength of 49,000 kPa or more in both longitudinal and transverse directions (measured using a 10mm wide test piece according to ASTM D-882), is characteristic of suitable durable microporous membranes, particularly when used as battery separators. The tensile rupture strength is preferably about 80,000 kPa or more.

(e) Tensile elongation of 100% or more [0088] A tensile elongation of 100% or more in both longitudinal and transverse directions (measured according to ASTM D-882), is characteristic of suitably durable microporous membranes, particularly when used as battery separators. (T) Heat shrinkage ratio of 12% or less in MD and TD [0089] The heat shrinkage ratio of the microporous membrane orthogonal planar directions (e.g., machine direction or transverse direction) at 105⁰C is measured as follows: (i) Measure the size of a test piece of microporous membrane at ambient temperature in both the machine direction and transverse direction, (ii) equilibrate the test piece of the microporous membrane at a temperature of 105°C for 8 hours with no applied load, and then (iii) measure the size of the membrane in both the machine and transverse directions. The heat (or "thermal") shrinkage ratio in either the machine or transverse directions can be obtained by dividing the result of measurement (i) by the result of measurement (ii) and expressing the resulting quotient as a percent. [0090] In an embodiment, the microporous membrane has a TD heat shrinkage ratio at 105°C in the range of 3% to 10%, e.g., 3.5% to 5%; and an MD heat shrinkage ratio at 105°C in the range of 1% to 8%, e.g., 1.5% to 3%.

(g) Thickness variation ratio of 21 % or less after heat compression (expressed as an absolute value)

[0091] The thickness variation ratio after heat compression at 90°C under pressure of 2.2 MPa for 5 minutes is generally 20% or less per 100% of the thickness before compression. Batteries comprising microporous membrane separators with a thickness variation ratio of 20% or less have suitably large capacity and good cyclability. In an embodiment, the membrane's thickness variation ratio is in the range of 10% to 20%.

[0092] To measure the thickness variation ratio after heat compression, a microporous membrane sample is situated between a pair of highly flat plates, and heat-compressed by a press machine under a pressure of 2.2 MPa (22 kgf/cm²) at 90°C for 5 minutes, to determine an average thickness in the same manner as above. A thickness variation ratio is calculated by the formula of (average thickness after compression - average thickness before compression) / (average thickness before compression) x 100. (h) Air permeability after heat compression of 700 sec/100 cm³ or less [0093] The microporous polyolefin membrane when heat-compressed under the above conditions generally has air permeability (Gurley value) of 700 sec/100 cm³ or less. Batteries using such membranes have suitably large capacity and cyclability. The air permeability is preferably 650 sec/100 cm³ or less, e.g., in the range of 400 sec/100cm³ to 500 sec/cm³.

[0094] Air permeability after heat compression is measured according to JIS P8117. (Q Surface roughness of 3 x IQ² nm or more [0095] The surface roughness of the membrane measured by an atomic force microscope (AFM) in a dynamic force mode is generally 3 x 10² nm or more (measured as the average maximum height difference across the membrane). The membrane's surface roughness is preferably 3.5 x 10² nm or more, e.g., in the range of 400 nm to 700 nm. (j) Electrolytic solution absorption speed of 4 or greater [0001] Using a dynamic surface tension measuring apparatus (DCAT21 with high-precision electronic balance, available from Eko Instruments Co., ltd.), a microporous membrane sample is immersed in an electrolytic solution for 600 seconds (electrolyte: 1 mol/L of LiPF₆, solvent: ethylene carbonate/dimethyl carbonate at a volume ratio of 3/7) kept at 18°C, to determine an electrolytic solution absorption speed by the formula of [weight (in grams) of microporous membrane after immersion / weight (in grams) of microporous membrane before immersion]. The electrolytic solution absorption speed is expressed by a relative value, assuming that the electrolytic solution absorption rate in the microporous membrane of Comparative Example 1 is 1. In an embodiment, the membrane's electrolytic solution absorption speed is in the range of 4 to 10, e.g., 4.1 to 6. (k) Pore size distribution [0096] The pore size distribution of the microporous membrane is determined by mercury intrusion porosimetry. At least about 25% of the differential pore volume is associated with pores larger than 100 nanometers (nm). (1) Surface roughness

[0097] The maximum height difference of a surface measured by AFM in a dynamic force mode (DFM) is used as surface roughness. [31 Microporous membrane composition

[0098] The microporous membrane generally comprises the same polymers used to produce the polymeric composition, in generally the same relative amounts. Washing solvent and/or process solvent (diluent) can also be present, generally in amounts less than 1 wt% based on the weight of the microporous membrane. A small amount of polymer molecular weight degradation might occur during processing, but this is acceptable. In an embodiment where the polymer is polyolefin and the membrane is produced in a wet process, molecular weight degradation during processing, if any, causes the MWD value of the polymers in the membrane to differ from the Mw/Mn of the polymers used to produce the membrane by no more than about 5%, or no more than about 1%, or no more than about 0.1%.

(1) Polyolefin

[0099] An embodiment of the microporous membrane of the present invention comprises (a) from about 74 wt. % to about 99 wt. % of a first polyethylene having an Mw of from 2.5 x 10⁵ to 5 x 10⁵, for example from about 2.5 x 10⁵ to about 4 x 10⁵, and an MWD of from about 5 to about 100, for example from about 7 to about 50, (b) from about 1 wt. % to about 5 wt. % of a second polyethylene having an Mw of from 5.1 x 10⁵ to about 1 x 10⁶, for example from about 5.2 x 10⁵ to about 8 x 10⁵, and an MWD of from about 5 to about 100, for example from about 5 to about 50, and (c) optionally from about 0 wt. % to about 25 wt. % of a polypropylene having an Mw of from about 3 x 10⁵ to about 1.5 x 10⁶, for example from about 6 x 10⁵ to about 1.5 x 10⁶, and an MWD of from about 1 to about 100, for example from about 1.1 to about 50, and a heat of fusion of 80 J/g or higher, for example from 80 to about 200 J/g, percentages based on the mass of the membrane. In an embodiment, the polypropylene's heat of fusion is in the range of 110 J/g to 120 J/g.

[41 Battery separator

[00100] In an embodiment, the battery separator formed from any of the above microporous polyolefin membranes of the present invention has a thickness of form about 3 to about 200 μm, or from about 5 to about 50 μm, or from about 7 to about 35 μm, though the most suitable thickness is properly selected depending on the type of battery to be manufactured. f5I Battery [00101] Though not particularly critical, the microporous polyolefin membranes of the present invention may be used as separators for primary and secondary batteries, particularly such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, particularly for lithium ion secondary batteries.

[00102] The lithium ion secondary battery comprises a cathode and an anode laminated via a separator, and the separator contains an electrolyte, usually in the form of an electrolytic solution ("electrolyte"). The electrode structure is not critical. Conventional structures are suitable. The electrode structure may be, for instance, a coin type in which a disc-shaped positive and anodes are opposing, a laminate type in which planar positive and anodes are alternately laminated, a toroidal type in which ribbon-shaped positive and anodes are wound, etc.

[00103] The cathode usually comprises a current collector, and a cathodic active material layer capable of absorbing and discharging lithium ions which is formed on the current collector. The cathodic active materials may be inorganic compounds such as transition metal oxides, composite oxides of lithium and transition metals (lithium composite oxides), transition metal sulfides, etc. The transition metals may be V, Mn, Fe, Co, Ni, etc. Preferred examples of the lithium composite oxides are lithium nickelate, lithium cobaltate, lithium manganate, laminar lithium composite oxides based on Ct-NaFeO₂, etc. The anode comprises a current collector, and a negative-electrode active material layer formed on the current collector. The negative-electrode active materials may be carbonaceous materials such as natural graphite, artificial graphite, coke, carbon black, etc.

[00104] The electrolytic solution can be a solution obtained by dissolving a lithium salt in an organic solvent. The lithium salt may be LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, LiN(C₂F₅ SO₂)₂, LiPF₄(CF₃)₂, LiPF₃(C₂Fs)₃, lower aliphatic carboxylates of lithium, LiAlCl₄, etc. These lithium salts may be used alone or in combination. The organic solvent may be an organic solvent having a high boiling point and high dielectric constant such as ethylene carbonate, propylene carbonate, ethylmethyl carbonate, γ-butyrolactone, etc.; and/or organic solvents having low boiling points and low viscosity such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, dimethyl carbonate, etc. These organic solvents may be used alone or in combination. Because the organic solvents having high dielectric constants generally have high viscosity, while those having low viscosity generally have low dielectric constants, their mixtures are preferably used. [00105] When the battery is assembled, the separator is impregnated with the electrolytic solution, so that the separator (microporous polyolefin membrane) is provided with ion permeability. The impregnation treatment is usually conducted by immersing the microporous membrane in the electrolytic solution at room temperature. When a cylindrical battery is assembled, for instance, a cathode sheet, a microporous membrane separator and an anode sheet are laminated in this order, and the resultant laminate is wound to a toroidal-type electrode assembly. The resultant electrode assembly is charged/formed into a battery can and then impregnated with the above electrolytic solution, and the battery lid acting as a cathode terminal provided with a safety valve is caulked to the battery can via a gasket to produce a battery. [00106] The present invention will be explained in more detail referring to Examples below without intention of restricting the scope of the present invention.

Example 1 [00107] Dry-blended to form a mixture are 100 parts by polyolefin composition mass of (i) 2 % of ultra-high molecular weight polyethylene (UHMWPE) resin having a weight-average molecular weight (Mw) of 0.7 x 10⁶ and a molecular weight distribution (Mw/Mn) of 8, and (ii) 97.8 % of high density polyethylene (HDPE) resin having Mw of 3.0 x 10⁵ and Mw/Mn of 8.6, and 0.2 parts by mass of the final polyolefin solution of tetrakis[methylene-3-(3,5-ditertiary- butyl-4-hydroxyphenyl)-propionate] methane as an antioxidant. The polyethylene in the mixture has a melting point of 135 ⁰C, and a crystal dispersion temperature of 100 °C. [00108] Forty parts by mass of the resultant mixture, i.e. polyolefin solution, is charged into a strong-blending double-screw extruder having an inner diameter of 58 mm and L/D of 42, and 60 parts by mass of liquid paraffin (50 cst at 40 °C) is supplied to the double-screw extruder via a side feeder. Melt-blending is conducted at 210°C and 200 rpm to prepare a polyethylene solution. This polyethylene solution is extruded from a T-die mounted to the double-screw extruder. The extrudate is cooled while passing through cooling rolls controlled at 40 °C, to form a cooled extrudate, i.e. gel-like sheet.

[00109] Using a tenter- stretching machine, the gel-like sheet is simultaneously biaxially stretched at 118.5 °C to 5 fold in both longitudinal and transverse directions. The stretched gel-like sheet is fixed to an aluminum frame of 20 cm x 20 cm, immersed in a bath of methylene chloride controlled at 25 °C to remove the liquid paraffin with vibration of 100 rpm for 3 minutes, and dried by an air flow at room temperature. The dried membrane is re-stretched by a batch-stretching machine to a magnification of 1.5 fold in a transverse direction at 129 °C. The re-stretched membrane, which remains fixed to the batch-stretching machine, is heat-set at 129 °C for 30 seconds to produce a microporous polyethylene membrane. Example 2

[00110] Example 1 is repeated except for the polyolefin composition comprising 3 % by mass of UHMWPE resin having a weight average molecular weight of 0.54 x 10⁶ and an Mw/Mn of 8, and 97 % by mass of the HDPE resin having a weight average molecular weight of 3 x 10⁵ and an Mw/Mn of 8.6. Example 3

[00111] Example 1 is repeated except for the polyolefin composition comprising 90 % by mass of the HDPE resin having a weight average molecular weight of 3 x 10⁵ and an Mw/Mn of 8.6, and 8 % by mass of a PP resin having a weight average molecular weight of 6.6 x 10⁵ and a ΔHm of 83.3 J/g, the stretching temperature is 118 °C, the re-stretching temperature is 129.5 °C, and the heat setting is at 129.5 °C.

Comparative Example 1 [00112] Example 1 is repeated except for the polyolefin composition comprising 20 % by mass of UHMWPE resin having a weight average molecular weight of 2 x 10⁶ and an Mw/Mn of 8; and 80 % by mass of HDPE resin having a weight average molecular weight of 3.5 x 10⁵ and an Mw/Mn of 8.6. Other differences from Example 1 for this Comparative Example 1 are that 30 parts by mass of the resultant polyolefin composition and 70 parts by mass of the liquid paraffin (50 cst at 40 °C) is charged into the double-screw extruder, the stretching temperature is 115 ⁰C, and the heat setting temperature is 126.8 °C.

[00113] The properties of the microporous membranes obtained in the Examples and Comparative Example are shown in Table 1. Comparative Example 2

[00114] Example 1 is repeated except for the polyolefin composition comprising 100 % by mass of the HDPE resin having a weight average molecular weight of 3 x 10⁵ and a molecular weight distribution of 8.6; the stretching is done at 118 °C; and the re-stretching is to a magnification of 1.4 fold at 129 °C. No PP or UHMWPE are included in this Comparative Example. Comparative Example 3

[00115] Example 1 is repeated except for the polyolefin composition comprising 3% by mass of the UHMWPE resin having a weight average molecular weight of 2 x 10⁶ and a molecular weight distribution of 8; 92 % by mass of the HDPE resin having a weight average molecular weight of 3 x 10⁵ and a molecular weight distribution of 8.6; and 5 % by mass of a PP resin having a weight average molecular weight of 5.3 x 10⁵ and a ΔHm of only 77.2 J/g; the stretching is at 116 °C; the re-stretching is to a magnification of 1.4 fold at 127 °C; and the heat setting is at 127 °C. Another exception from Example 1 for this Comparative Example 3 is that 35 parts by mass of the resultant polyolefin composition and 65 parts by mass of the liquid paraffin (50 cst at 40°C) is charged into the double-screw extruder. [00116] The properties of the microporous membranes obtained in the Examples and Comparative Examples are shown in Table 1.

TABLE 1

[00117] It is noted from Table 1 that the microporous membrane of the present invention exhibits a pore size distribution curve obtained by mercury intrusion porosimetry having a first peak at a pore size of from 0.01 to 0.08 μm, and second to fourth peaks at pore sizes of more than 0.08 μm and 1.5 μm or less, and the surface roughness as a maximum height difference was 3 x 10² nm or more. The microporous membrane of the present invention has suitable air permeability, pin puncture strength, tensile rupture strength, tensile rupture elongation and heat shrinkage resistance, as well as improved electrolytic solution absorption, with little variation of thickness and air permeability after heat compression. On the other hand, the microporous membrane product of the Comparative Example exhibits relatively lower electrolytic solution absorption, lower compression properties, higher heat shrinkage, and poorer processability. [00118] Battery separators formed by the microporous polyolefin membrane of the present invention provide batteries with suitable safety, heat resistance, storage properties and productivity.

Claims

1. A microporous polyolefin membrane comprising polyethylene and having an electrolytic solution absorption speed > 4.

2. The microporous membrane of claim 1, wherein the membrane's electrolytic solution absorption speed is in the range of 4.1 to 10.

3. The microporous membrane of claims 1 or 2, wherein the membrane comprises from 74 wt. % to 99 wt. % of a first polyethylene having an Mw < 5 x 10⁵, based on the weight of the membrane.

4. The microporous membrane of any of the preceding claims wherein the membrane comprises from 1 wt. % to 5 wt. % of a second polyethylene, the second polyethylene having an Mw > 5 x 10⁵, based on the weight of the membrane.

5. The microporous membrane of any preceding claim further comprising from 0 wt. % to 25 wt. % of a polypropylene having a heat of fusion > 80 J/g and an Mw in the range of 3 x 10⁵ to 1.5 x 10⁶, based on the weight of the membrane.

6. The microporous membrane of any preceding claim, wherein the membrane has a pore size distribution curve having at least two peaks.

7. The microporous membrane of any preceding claim, wherein 25% to 60% of the membrane's differential pore volume is associated with pores having diameters > 1 OOnm.

8. The microporous membrane of any preceding claim, wherein the membrane's differential pore volume curve exhibits a first area associated with pores having diameters in the range of 10 run to 1000 nm and a second area associated with pores having diameters of 200 nm to 1000 nm, and wherein the second area is in the range of 25% to 60% of the first area.

9. The microporous membrane of any preceding claim, wherein the membrane has a melt down temperature >170°C.

10. The microporous membrane of claims 3 through 9, wherein the membrane comprises 90 wt. % to 97 wt. % of the first polyethylene, 5 wt. % to 10 wt. % of the polypropylene, and 2 wt. % to 5 wt. % of the second polyethylene, based on the weight of the microporous membrane.

11. A method for manufacturing a microporous membrane, comprising: (1) combining a polyolefin composition and a diluent, the polyolefin composition comprising (a) from 74 wt. % to 99 wt. % of a first polyethylene having an Mw < 5 x 10⁵ and (b) from 1 wt. % to 5 wt. % of a second polyethylene, the second polyethylene having an Mw > 5 x 10⁵, the weight percentages based on the weight of the polyolefin composition,

(2) extruding the combined polyolefin composition and diluent through a die to form an extrudate,

(3) cooling the extrudate to form a cooled extrudate,

(4) stretching the cooled extrudate to a magnification of from 9 to 400 fold in at least one direction at a stretching temperature of from Ted to Ted + 30 °C to form a stretched cooled extrudate,

(5) removing at least a portion of the membrane-forming solvent from the stretched gel-like sheet to form a membrane,

(6) stretching the membrane to a magnification of from 1.1 to 1.8 fold in at least one direction to form a stretched membrane, and

(7) heat-setting the stretched microporous membrane to form the microporous membrane.

12. The method of claim 11 further comprising a heat-setting treatment step (4i) between steps (4) and (5) wherein the stretched extrudate is heat-set at a temperature of the stretching temperature ±5°C.

13. The method of any of claims 11 or 12 further comprising a heat roll treatment step (4ii) following step (4i) and before step (5) wherein the stretched extrudate contacts a heated roller at a temperature from the crystal dispersion temperature of the polyolefin composition to the melting point +10°C of the polyolefin composition.

14. The method of any of claims 11-18, further comprising a hot solvent treatment step (4iii) following step (4ii) and before step (5) wherein the stretched extrudate is contacted with a hot solvent.

15. The method of any of claims 11-14, further comprising a heat-setting treatment step (5i) conducted between the steps (5) and (6) wherein the membrane is heat-set at a temperature of the stretching temperature ±5⁰C.

16. The method of any of claims 11-15, further comprising a cross-linking step (5ii) following step (5i) and before step (6) wherein the heat-set membrane is cross-linked by ionizing radiation rays selected from one or more of α-rays, β-rays, γ-rays, and electron beams.

17. The method of any of claims 11-16, further comprises a hydrophilizing treatment step (7i) following step (7) wherein the heat-set microporous membrane is made more hydrophilic by one or more of a monomer-grafting treatment, a surfactant treatment, and a corona-discharging treatment.

18. The method of any of claims 11-17, further comprising a surface-coating treatment step (8) following step (7) wherein the heat-set microporous membrane is coated with one or more of a porous polypropylene, a porous fluororesin, a porous polyimide, and a porous polyphenylene sulfide.

19. The method of any of claims 11-18, wherein the polyolefin composition further comprises (c) from 0 to 25 % of a polypropylene having a heat of fusion > 80 J/g and an Mw in the range of 3 x 10³ to 1.5 x 10⁶, based on the weight of the membrane.

20. The method of any of claims 11-19, wherein the membrane-forming solvent is a liquid at room temperature.

21. A membrane made by the process of claim 11.

22. A battery comprising an anode, a cathode, an electrolyte, and a least one battery separator located between the anode and the cathode, the separator comprising the membrane of any of claims 1-11 and 21.

23. The battery of claim 22, wherein the battery is a lithium ion battery.

24. The battery of claims 21 or 22 used as a power source for an electric vehicle or hybrid electric vehicle.