JP5592745B2 - Polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a polyolefin microporous membrane.

  Polyolefin microporous membranes are widely known as separation membranes and separators used for the separation and selective permeation of various substances. Examples of applications include microfiltration membranes, fuel cell materials, capacitor separators, functional membrane base materials for filling functional materials into holes and causing new functions, and battery separators. . Among these uses, polyolefin microporous membranes are particularly preferably used as separators in lithium ion batteries provided in notebook personal computers, mobile phones, digital cameras, and the like. The reason is that the membrane has high mechanical strength and pore blocking properties.

Pore plugging is the safety of the battery by melting when the inside of the battery is overheated due to an overcharged condition, etc., blocking the hole (hereinafter referred to as “hole blocking”) and blocking the battery reaction. It is the performance to ensure. The lower the temperature at which hole clogging occurs, the higher the safety effect.
In addition, the puncture strength of the separator needs to have a certain level or more in order to prevent film breakage when winding the separator or to prevent a short circuit due to foreign matter in the battery.
In addition, the separator is also required to have a low electrical resistance, that is, a high porosity, from the viewpoint of achieving a high output of the electricity storage device.

  Under such circumstances, Patent Document 1 proposes a microporous membrane that has both good permeation performance, a dense membrane structure, and low heat shrinkability, and can achieve both initial battery characteristics and cycle characteristics.

JP 2004-323820 A

  However, the microporous membrane described in Patent Document 1 still has room for improvement when considering applications that require high output such as in-vehicle lithium ion secondary batteries.

  Accordingly, the present invention provides a polyolefin microporous membrane having a membrane resistance equal to or lower than that of a conventional polyolefin microporous membrane and also having a high porosity, that is, high output characteristics, and the polyolefin microporous membrane. It aims at providing a nonaqueous electrolyte system secondary battery provided with a porous membrane.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have solved the above problem by controlling the heat shrinkage behavior at 60 to 120 ° C., which is lower than the melting point temperature of the polyolefin microporous film. As a result, the present invention has been completed.

That is, the present invention is as follows.
[1] A polyolefin microporous membrane that satisfies the conditions represented by the following formulas (1A) and (2A).
−0.010 ≦ (SM ₁₀₀ −SM ₆₀ ) /40≦0.070 (1A)
0.005 ≦ (ST ₁₂₀ −ST ₆₀ ) /60≦0.050 (2A)
(In the formulas (1A) and (2A), SM ₁₀₀ , SM ₆₀ , ST ₁₂₀ and ST ₆₀ are the length direction at 100 ° C., the length direction at 60 ° C., the width direction at 120 ° C. and the width at 60 ° C., respectively. ( The shrinkage stress (g) of the polyolefin microporous film in the direction is shown.)
[2] A polyolefin microporous membrane that satisfies the conditions represented by the following formulas (1B) and (2B).
−0.010 ≦ (SM ₁₀₀ −SM ₆₀ ) /40≦0.070 (1B)
0.015 ≦ (ST ₁₂₀ −ST ₆₀ ) /60≦0.030 (2B)
(In the formulas (1B) and (2B), SM ₁₀₀ , SM ₆₀ , ST ₁₂₀ and ST ₆₀ are the length direction at 100 ° C., the length direction at 60 ° C., the width direction at 120 ° C. and the width at 60 ° C., respectively. ( The shrinkage stress (g) of the polyolefin microporous film in the direction is shown.)
[3] The polyolefin microporous membrane according to [1] or [2], wherein the puncture strength is 200 g or more.
[4] A non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane according to any one of [1] to [3] as a separator.

  According to the present invention, a polyolefin microporous membrane having a membrane resistance equal to or lower than that of a conventional polyolefin microporous membrane and also having a high porosity, that is, high output characteristics, and the polyolefin microporous membrane. A non-aqueous electrolyte secondary battery including a porous film can be provided.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail.

The polyolefin microporous membrane of this embodiment satisfies the conditions represented by the following formulas (1A) and (2A).
−0.010 ≦ (SM ₁₀₀ −SM ₆₀ ) /40≦0.070 (1A)
0.005 ≦ (ST ₁₂₀ −ST ₆₀ ) /60≦0.050 (2A)
Here, in the formulas (1A) and (2A), SM ₁₀₀ , SM ₆₀ , ST ₁₂₀ and ST ₆₀ are respectively the length direction at 100 ° C. (also the resin discharge direction during film formation, hereinafter referred to as “MD”. ), The length direction at 60 ° C., the width direction at 120 ° C. (also a direction perpendicular to MD, and may be abbreviated as “TD” hereinafter) and the width direction at 60 ° C. The shrinkage stress of a polyolefin microporous film is shown.

  The polyolefin microporous membrane of the present embodiment (hereinafter, also simply referred to as “microporous membrane”) has the above-described structural characteristics, so that when it is provided as a separator in a lithium ion secondary battery, A lithium ion secondary battery having high output characteristics from a low membrane resistance can be realized. Although details are not clear, the present inventors consider the cause as follows. That is, by satisfying both of the conditions represented by the above formulas (1A) and (2A), the degree of deformation in the thickness direction of the film is improved by appropriate shrinkage behavior, the pore structure is maintained, and the liquid is retained. As a result, it is considered that a high output is possible when used as a separator of a lithium ion secondary battery. However, the factor is not limited to this.

  The shrinkage stress is measured according to the method described in the following examples.

  Since the microporous membrane of the present embodiment has the characteristics as described above, it is particularly suitable as a separator for a lithium ion secondary battery that requires high output characteristics, for example, a separator for an in-vehicle lithium ion secondary battery.

The microporous membrane of this embodiment preferably satisfies the conditions represented by the following formulas (1B) and (2B) from the viewpoint of the balance between membrane strength and permeability.
−0.010 ≦ (SM ₁₀₀ −SM ₆₀ ) /40≦0.070 (1B)
0.015 ≦ (ST ₁₂₀ −ST ₆₀ ) /60≦0.030 (2B)
Here, in the formulas (1B) and (2B), SM ₁₀₀ , SM ₆₀ , ST ₁₂₀ and ST ₆₀ are respectively microporous of MD at 100 ° C., MD at 60 ° C., TD at 120 ° C. and TD at 60 ° C. The shrinkage stress of the film is shown.

The microporous membrane of this embodiment preferably satisfies the condition represented by the following formula (1C) from the viewpoint of the balance between membrane strength and permeability.
0.030 ≦ (SM ₁₀₀ −SM ₆₀ ) /40≦0.070 (1C)

  The puncture strength (absolute strength) of the microporous membrane of this embodiment is preferably 200 g or more, and more preferably 300 g or more. When the puncture strength is 200 g or more, when a microporous membrane is used as a battery separator, pinholes or cracks may occur even when sharp portions such as electrode materials provided in the battery pierce the microporous membrane. It is preferable from the viewpoint that generation can be reduced. Although there is no restriction | limiting in particular as an upper limit of puncture strength, It is preferable that it is 1000 g or less. The puncture strength is measured according to the method described in the examples below.

  The porosity of the microporous membrane of this embodiment is preferably 40 to 90%, more preferably 55 to 80%. Setting the porosity to 40% or more is preferable from the viewpoint of following the rapid movement of lithium ions at the high rate when the microporous membrane is used as a separator of a lithium ion secondary battery. On the other hand, setting the porosity to 90% or less is preferable from the viewpoint of improving the film strength, and from the viewpoint of suppressing self-discharge when the microporous film is used as a separator of an electrochemical reaction device such as a lithium ion secondary battery. Is also preferable. The porosity is measured according to the method described in the following examples.

Further, the AC resistance of the microporous membrane of this embodiment, the viewpoint preferably 0.9Ω · cm ² or less from the output when used as a battery separator, more preferably not more than 0.6 ohm · cm ^2, 0.3 [Omega -More preferably cm ² or less. The AC resistance of the microporous membrane is measured according to the method described in the examples below.

  In the microporous membrane of the present embodiment, the capacity maintenance rate in the battery rate characteristic evaluation is preferably 10 to 20%, more preferably 16 to 20% from the viewpoint of output. The capacity retention rate is measured according to the method described in the following examples.

Examples of means for forming a microporous membrane having various properties as described above include, for example, the concentration of polyolefin during extrusion, the blending ratio of various polyolefins such as polyethylene and polypropylene in the polyolefin, the molecular weight of polyolefin, the draw ratio, and after extraction. And a method of optimizing the stretching and relaxation operations.
The values of the parameters of the formulas (1A) and (2A) can be adjusted by optimizing the stretching temperature and magnification during heat setting.

  Moreover, the aspect of the microporous membrane may be an aspect of a single layer or an aspect of a laminate.

  Next, the manufacturing method of the microporous film of this embodiment will be exemplarily described. However, if the obtained microporous membrane is the above-mentioned microporous membrane, the production method of the present embodiment includes a polymer type, a solvent type, an extrusion method, a stretching method, an extraction method, an opening method, a heat setting / heat treatment. The method is not limited at all.

  The manufacturing method of the microporous membrane of this embodiment includes a step of melt-kneading and molding a polymer and a plasticizer, or a polymer, a plasticizer and a filler, a stretching step, a plasticizer (and a filler if necessary) ) It is preferable from the viewpoint of appropriately controlling the balance between physical properties of permeability and membrane strength to include an extraction step and a heat setting step.

More specifically, for example, a method for producing a microporous membrane including the following steps (1) to (4) can be used.
(1) A kneading step in which a polyolefin, a plasticizer, and a filler as necessary are kneaded to form a kneaded product.
(2) A sheet forming step of extruding the kneaded material after the kneading step, forming it into a single-layered or laminated sheet and cooling and solidifying it.
(3) A stretching step of extracting a plasticizer and / or filler as necessary after the sheet molding step and further stretching the sheet (sheet-like molded body) in a direction of one axis or more.
(4) A post-processing step in which a plasticizer and / or a filler is extracted as necessary after the stretching step and further subjected to heat treatment.

  The polyolefin used in the kneading step (1) may be composed of one kind of polyolefin or a polyolefin composition containing plural kinds of polyolefin.

  Examples of the polyolefin include polyethylene, polypropylene, and poly-4-methyl-1-pentene, and two or more of these may be blended and used. Hereinafter, polyethylene may be abbreviated as “PE” and polypropylene may be abbreviated as “PP”.

  The viscosity average molecular weight (Mv) of polyolefin is preferably 50,000 to 3,000,000, more preferably 150,000 to 2,000,000. When the viscosity average molecular weight is 50,000 or more, an effect of developing the film-breaking resistance at the time of melting tends to be obtained, and when it is 3 million or less, the effect of facilitating the extrusion process tends to be obtained. preferable. A viscosity average molecular weight is measured based on the method as described in the following Example.

  The melting point of the polyolefin is preferably 100 to 165 ° C, more preferably 110 to 140 ° C. When the melting point is 100 ° C. or higher, the battery function tends to be stable, and when the melting point is 165 ° C. or lower, the fuse effect tends to be obtained. In addition, melting | fusing point means the temperature of the melting peak in DSC measurement. Further, the melting point of polyolefin when a plurality of types of polyolefin is used means the temperature of the peak having the largest melting peak area in the DSC measurement of the mixture.

  As the polyolefin, it is preferable to use high-density polyethylene from the viewpoint that heat fixation can be performed at a higher temperature while suppressing clogging of the pores.

  The proportion of such high density polyethylene in the polyolefin is preferably 5% by mass or more, and more preferably 10% by mass or more. When the ratio is 5% by mass or more, heat fixation can be performed at a higher temperature while further suppressing the blockage of the holes. On the other hand, the proportion of the high density polyethylene in the polyolefin is preferably 50% by mass or less, and more preferably 40% by mass or less. When the ratio is 50% by mass or less, the microporous membrane can have not only the effect of high density polyethylene but also the effect of other polyolefins in a well-balanced manner.

  Moreover, as a polyolefin, a viscosity average molecular weight (Mv) is 100,000-300,000 from a viewpoint of improving the shutdown characteristic at the time of using a microporous film as a battery separator, or improving the safety | security of a nail penetration test. It is preferable to use polyethylene.

  The ratio of such 100,000 to 300,000 polyethylene in the polyolefin is preferably 30% by mass or more, and more preferably 45% by mass or more. When the ratio is 30% by mass or more, the shutdown characteristics when the microporous membrane is used as a battery separator can be improved, or the safety of the nail penetration test can be improved. On the other hand, the ratio of 100,000 to 300,000 polyethylene in the polyolefin is preferably 100% by mass or less, and more preferably 95% by mass or less.

  Polypropylene may be used as the polyolefin from the viewpoint of improving safety.

  The proportion of such polypropylene in the polyolefin is preferably 5% by mass or more, more preferably 8% by mass or more. It is preferable that the ratio is 5% by mass or more from the viewpoint of improving the film resistance at high temperatures. On the other hand, the proportion of polypropylene in the polyolefin is preferably 20% by mass or less, and more preferably 18% by mass or less. It is preferable that the ratio is 20% by mass or less from the viewpoint of realizing a microporous membrane that has not only the effect of polypropylene but also the effect of other polyolefins in a well-balanced manner.

  The plasticizer used in the kneading step (1) may be those conventionally used for polyolefin microporous membranes, and may be abbreviated as, for example, dioctyl phthalate (hereinafter referred to as “DOP”). ), Phthalic acid esters such as diheptyl phthalate and dibutyl phthalate; organic acid esters other than phthalic acid esters such as adipic acid ester and glyceric acid ester; phosphoric acid esters such as trioctyl phosphate; liquid paraffin; solid wax; Mineral oil is mentioned. These are used alone or in combination of two or more. Among these, phthalate ester is particularly preferable in consideration of compatibility with polyethylene.

  In the kneading step (1), a polyolefin and a plasticizer may be kneaded to form a kneaded product, or a polyolefin, a plasticizer, and a filler may be kneaded to form a kneaded product. As the filler used in the latter case, at least one of organic fine particles and inorganic fine particles can be used.

  Examples of the organic fine particles include modified polystyrene fine particles and modified acrylic resin particles.

  Examples of inorganic fine particles include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide and iron oxide; nitrides such as silicon nitride, titanium nitride and boron nitride Ceramics: silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, zeolite, Ceramics such as calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; glass fiber.

  The blend ratio of the polyolefin, the plasticizer, and the filler used as necessary in the kneading step (1) is not particularly limited. The proportion of the polyolefin kneaded product is preferably 25 to 50% by mass from the viewpoint of the strength and film-forming property of the resulting microporous membrane. The proportion of the plasticizer in the kneaded product is preferably 30 to 60% by mass from the viewpoint of obtaining a viscosity suitable for extrusion. The proportion of the filler in the kneaded product is preferably 10% by mass or more from the viewpoint of improving the uniformity of the pore diameter of the obtained microporous membrane, and preferably 40% by mass or less from the viewpoint of film forming properties.

  In addition, the kneaded material may be further phenolic, phosphorus-based, sulfur-based, such as pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], if necessary. Antioxidants such as calcium soaps, metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; and various additives such as color pigments may be mixed.

  There is no restriction | limiting in particular in the kneading | mixing method in the kneading | mixing process of (1), The method used conventionally may be used. For example, the kneading order is a general mixing such as a Henschel mixer, a V-blender, a pro-shear mixer, and a ribbon blender in which polyolefin, a plasticizer, and a part of a filler used as necessary are mixed in advance. After mixing in advance using a machine, the remaining raw materials may be further kneaded, or all of the raw materials may be kneaded simultaneously.

  Moreover, there is no restriction | limiting in particular also in the apparatus used for kneading | mixing, For example, it can knead | mix using melt-kneading apparatuses, such as an extruder and a kneader.

  The sheet forming step (2) is, for example, a step of extruding the kneaded material into a sheet shape via a T die or the like, and bringing the extrudate into contact with a heat conductor to cool and solidify. As the heat conductor, metal, water, air, and the plasticizer itself can be used. Moreover, it is preferable to cool and solidify by sandwiching the extrudate between a pair of rolls from the viewpoint of increasing the film strength of the obtained sheet-like molded article and improving the surface smoothness of the sheet-like molded article.

  The stretching step (3) is a step of stretching a sheet (sheet-like molded body) obtained through the sheet forming step to obtain a stretched sheet. Sheet stretching methods in the stretching process include MD uniaxial stretching with a roll stretching machine, TD uniaxial stretching with a tenter, a combination of roll stretching machine and tenter, or sequential biaxial stretching with a combination of tenter and tenter, simultaneous biaxial tenter Or simultaneous biaxial stretching by inflation molding is mentioned. From the viewpoint of obtaining a more uniform film, the sheet stretching method is preferably simultaneous biaxial stretching. The total surface magnification at the time of stretching is preferably 8 times or more, more preferably 15 times or more, and further preferably 30 times or more, from the viewpoint of film thickness uniformity and the balance of tensile elongation, porosity and average pore diameter. preferable. When the total surface magnification is 30 times or more, a high-strength microporous film is easily obtained. The stretching temperature is preferably 121 ° C. or higher from the viewpoint of imparting high permeability and high temperature and low shrinkage, and is preferably 135 ° C. or lower from the viewpoint of film strength.

  The extraction prior to the stretching in the stretching step of (3) or the heat treatment in the post-processing step of (4) is a method of immersing the sheet or the stretched sheet in the extraction solvent or showering the extraction solvent on the sheet or the stretched sheet. It is done by. The extraction solvent is preferably a poor solvent for polyolefin and a good solvent for plasticizer and filler, and preferably has a boiling point lower than the melting point of polyolefin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane and fluorocarbon; alcohols such as ethanol and isopropanol; acetone and 2 -Ketones such as butanone; and alkaline water. An extraction solvent is used individually by 1 type or in combination of 2 or more types.

  Note that the filler may be extracted in whole or in part in any of the steps, or may remain in the finally obtained microporous membrane. Moreover, there is no restriction | limiting in particular about the order, method, and frequency | count of extraction.

  (4) As a heat treatment method in the post-processing step, the stretched sheet obtained through the stretching step is stretched and / or relaxed at a predetermined temperature using a tenter and / or a roll stretching machine. A heat setting method may be mentioned. The relaxation operation is a reduction operation performed at a predetermined relaxation rate on the MD and / or TD of the film. The relaxation rate is a value obtained by dividing the MD dimension of the film after the relaxation operation by the MD dimension of the film before the operation, or a value obtained by dividing the TD dimension after the relaxation operation by the TD dimension of the film before the operation, or MD In the case of relaxation by both TD and TD, it is a value obtained by multiplying the relaxation rate of MD and the relaxation rate of TD. The predetermined temperature is preferably 125 ° C. or lower and more preferably 123 ° C. or lower from the viewpoint of high porosity. On the other hand, from the viewpoint of suppressing heat shrinkage, the predetermined temperature is preferably 115 ° C. or higher. Further, from the viewpoint of heat shrinkage and permeability, in the post-processing step, the stretched sheet is preferably stretched 1.5 times or more to TD, and more preferably 1.8 times or more to TD. On the other hand, from the viewpoint of safety, it is preferable to stretch the stretched sheet to TD to 6.0 times or less, and from the viewpoint of maintaining a balance between film strength and permeability, 4.0 times or less is more preferable. The predetermined relaxation rate is preferably 0.9 times or less from the viewpoint of suppression of heat shrinkage, and is preferably 0.6 times or more from the viewpoint of preventing wrinkle generation, porosity, and permeability. The relaxation operation may be performed in both directions of MD and TD, but may be a relaxation operation of only one of MD and TD. Even if the relaxation operation is performed on only one of MD and TD, it is possible to reduce the thermal contraction rate not only in the operation direction but also in the other direction.

  The viscosity average molecular weight of the obtained microporous membrane is preferably 200,000 to 1,000,000. If the viscosity average molecular weight is 200,000 or more, the strength of the film is easily maintained, and if it is 1,000,000 or less, the moldability is excellent.

  The film thickness of the microporous membrane is preferably 5 μm or more from the viewpoint of safety, preferably 50 μm or less from the viewpoint of high output and high capacity density, and more preferably 25 μm or less. This film thickness is measured according to the method described in the following Examples.

  In addition to the above steps (1) to (4), the method for producing a microporous membrane may include a step of superimposing a plurality of single layer bodies as a step for obtaining a laminate. In addition, the production method may include a step of subjecting the microporous film to surface treatment such as electron beam irradiation, plasma irradiation, surfactant coating, and chemical modification.

  The polyolefin microporous membrane of this embodiment is used as a separation membrane for separation of substances, selective permeation, etc., and as a separator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, capacitors, etc. Can do.

  The microporous membrane of the above-described embodiment can be used as a separator for a non-aqueous electrolyte secondary battery, and is preferably used as a separator for a lithium ion secondary battery as described above. The separator is suitable for use in applications requiring high output characteristics, such as motorcycles, scooters, and automobiles, among lithium ion secondary batteries, and it is possible to impart battery characteristics higher than conventional ones. Become. The non-aqueous electrolyte secondary battery provided with the microporous membrane of the present embodiment is not particularly limited except that the microporous membrane is provided, and may have a known configuration other than the microporous membrane. Thus, according to the present embodiment, a polyolefin microporous membrane that can realize a lithium ion secondary battery that exhibits high output characteristics (film resistance, rate characteristics) can be obtained.

  As mentioned above, although the form for implementing this invention was demonstrated, this invention is not limited to the said this embodiment. The present invention can be variously modified without departing from the gist thereof.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. Various physical properties in the examples were measured by the following methods.

(1) Viscosity average molecular weight (Mv)
In order to prevent deterioration of the sample, 2,6-di-t-butyl-4-methylphenol was dissolved in decahydronaphthalene so as to have a concentration of 0.1% by mass (hereinafter abbreviated as “DHN”). Was used as a solvent for the sample. The sample was dissolved in DHN at 150 ° C. to a concentration of 0.1% by mass to obtain a sample solution. 10 mL of the sample solution was sampled, and the number of seconds (t) required to pass between the marked lines at 135 ° C. was measured with a Canon Fenske viscometer (SO100). Further, after heating the DHN to 0.99 ° C., and 10mL taken, the number of seconds required to pass between marked lines viscometer (t _B) was measured in the same manner. The intrinsic viscosity [η] was calculated by the following conversion formula using the obtained passing seconds t and t _B.
[Η] = ((1.651 t / t _B −0.651) ^0.5 −1) /0.0834

The viscosity average molecular weight (Mv) was calculated from the determined [η]. Mv of the raw material polyethylene, the raw material polyolefin composition, and the microporous membrane was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
For the raw material polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(2) Film thickness (μm)
The film thickness was measured at an ambient temperature of 23 ± 2 ° C. using KBM (trademark), which is a micro thickness gauge manufactured by Toyo Seiki.

(3) Porosity (%)
A sample of 10 cm × 10 cm square is cut from the microporous membrane, its volume (cm ³ ) and mass (g) are obtained, and from these and the membrane density (g / cm ³ ), the porosity is calculated using the following formula. Calculated.
Porosity = (volume−mass / film density) / volume × 100
The film density was calculated from the fraction of the composition, with 0.95 for polyethylene and 0.91 for polypropylene. In addition, the density calculated | required by the density gradient tube method of JISK-7112 can also be used as various film densities.

(5) Puncture strength ( g )
Using a KES-G5 (trademark) handy compression tester manufactured by Kato Tech, a piercing test was performed at an ambient temperature of 23 ± 2 ° C. under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec. . The maximum puncture load ( g ) was measured and the value was defined as the puncture strength.

(6) Shrinkage stress (g)
The sample was cut to 3 × 13.5 mm in accordance with the measurement direction of MD and TD. The sample length of the sample piece is 10.0 mm, set at a predetermined position of a thermal analyzer manufactured by Shimadzu Corporation (trade name “TMA-50”), and contracted at each temperature while raising the temperature at 10 ° C./min. Stress (g) was measured.

(7) Inclination of shrinkage stress (g / ° C)
From the measurement result of the contraction stress in the above “(6) contraction stress (g)”, (SM ₁₀₀ -SM ₆₀ ) / 40 and (ST ₁₂₀ -ST ₆₀ ) / 60 (both units are g / ° C.), respectively. Calculated.

(8) AC resistance of microporous membrane (Ω · cm ² )
In the cell, a platinum electrode, a spacer, a spacer, a spacer and a platinum electrode are laminated in this order, and 50/50% by volume of propylene carbonate (PC) and dimethyl carbonate (DMC) which are electrolytes are laminated between them. A blank cell was obtained by injecting 1M LiClO ₄ solution. Next, a clip was sandwiched between terminals at both ends of the cell, and the resistance (R0) of the blank cell was measured. Next, a platinum electrode, a spacer, a spacer, a microporous membrane, a spacer, and a platinum electrode are stacked in this order, and a 50/50 volume% 1M LiClO ₄ solution of PC and DMC as electrolyte is injected between them. A sample cell was obtained. Next, clips were sandwiched between terminals at both ends of the cell, and the resistance (R1) of the sample cell was measured. (R1-R0) was calculated as the AC resistance (Ω · cm ² ) of the microporous membrane.

(9) Rate characteristic evaluation a. Production of Positive Electrode LiCoO ₂ which is a lithium cobalt composite oxide as a positive electrode active material is 92.2% by mass, flake graphite and acetylene black are 2.3% by mass as a conductive material, and polyvinylidene fluoride (PVDF) is used as a binder. ) 3.2% by mass was dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied on one side to an aluminum foil serving as a positive electrode current collector with a die coater, dried, and further compression molded with a roll press to produce a positive electrode.

b. Production of negative electrode 96.9% by mass of artificial graphite as a negative electrode active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose as a binder, and 1.7% by mass of styrene-butadiene copolymer latex as a binder were dispersed in purified water. A slurry was prepared. This slurry was applied to a copper foil serving as a negative electrode current collector with a die coater, dried, and further compression molded with a roll press to produce a negative electrode.

c. Preparation of Nonaqueous Electrolyte Solution A nonaqueous electrolyte solution was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / liter. .

d. Rate characteristic evaluation The polyolefin microporous membrane was cut into a circular shape (disk) of 18 mmφ, the positive electrode and the negative electrode, and the positive electrode, the microporous membrane made of polyolefin, and the negative electrode were so arranged that the active material surfaces of the positive electrode and the negative electrode faced each other. Lamination was performed in order, and the laminate was stored in a stainless metal container with a lid. The container and the lid were insulated, and the container was placed in contact with the negative electrode copper foil and the lid in contact with the positive electrode aluminum foil. The non-aqueous electrolyte was poured into the container and sealed, and left in that state at room temperature for 1 day. Thereafter, the battery is charged at a current value of 2 mA (0.3 C) in a 25 ° C. atmosphere until the battery voltage reaches 4.2 V. After reaching the voltage, 4.2 V is maintained as it is, and the current value is 3 mA. The first charge after battery preparation was performed for a total of 8 hours by the method of starting to squeeze from. Subsequently, the battery was discharged at a current value of 6 mA (1 C) until the battery voltage reached 3.0 V (constant current discharge), and the discharge capacity was measured. Then, after charging in the same manner as described above, the battery was discharged (constant current discharge) at a current value of 30 mA (5 C) until the battery voltage reached 3.0 V, and the discharge capacity was measured.

  The ratio of the discharge capacity of the latter (5C) to the discharge capacity of the former (1C) is defined as the capacity maintenance ratio (%), and the case where the capacity maintenance ratio is 16% or more is “A”, and is 10% or more and less than 16% The rate characteristics (output) were evaluated with “B” being less than 10% and “C” being less than 10%.

[Example 1]
A polyethylene homopolymer having a Mv of 250,000 was prepared as a pure polymer. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99% by mass of the pure polymer, and a tumbler blender is used. A mixture such as a polymer was obtained by dry blending. The obtained mixture of polymers and the like was fed by a feeder under a nitrogen atmosphere to a twin-screw extruder in which the system was purged with nitrogen. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / sec) was injected as a plasticizer into the cylinder of the extruder by a plunger pump. @ Feeder and so that the amount ratio of liquid paraffin in the total mixture extruded and melt-kneaded by a twin-screw extruder is 65% by mass (that is, the amount ratio of the polymer mixture (PC) is 35% by mass). The pump was adjusted. The melt kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 100 rpm, and a discharge rate of 12 kg / hour.

Subsequently, the obtained melt-kneaded product was extruded through a T-die onto a cooling roll controlled at a surface temperature of 25 ° C. and cast to obtain a gel sheet having a thickness of 1550 μm.
Next, the obtained gel sheet was guided to a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched sheet. The set stretching conditions were an MD draw ratio of 7.0 times, a TD draw ratio of 6.1 times, and a set temperature of 121 ° C.
Next, the stretched sheet was introduced into a methyl ethyl ketone bath and sufficiently immersed in methyl ethyl ketone to extract and remove liquid paraffin from the stretched sheet, and then the methyl ethyl ketone was removed by drying.

  Next, the stretched sheet from which methyl ethyl ketone was removed by drying was guided to a TD tenter and heat-set. The heat setting temperature was 120 ° C., the TD maximum magnification was 2.0 times, and the relaxation rate was 0.90 times. The viscosity average molecular weight of the microporous membrane thus obtained was 250,000. Various characteristics of the microporous membrane are shown in Table 1 together with the composition, stretching conditions and the like.

[Examples 2 to 13, Comparative Examples 1 to 3]
A microporous membrane was obtained in the same manner as in Example 1 except for the conditions shown in Tables 1 and 2 below. In the table, polyethylene having an Mv of 700,000 is high density polyethylene. Further, the Mv of the microporous membrane obtained in Example 6 was 550,000.
Various characteristics of the microporous membrane are shown in Tables 1 and 2 together with the composition, stretching conditions and the like.

[Examples 14 and 15]
The conditions of the biaxial stretching ratio were set to a MD stretching ratio of 7.0 times and a TD stretching ratio of 5.0 times, and other conditions were the same as in Example 1 except for the conditions shown in Table 2 below. A membrane was obtained.
Various characteristics of the microporous membrane are shown in Table 2 together with the composition, stretching conditions and the like.

  The polyolefin microporous membrane of the present invention is suitably used as a lithium ion battery separator having high output characteristics.

Claims (4)

A polyolefin microporous membrane that satisfies the conditions represented by the following formulas (1A) and (2A).
−0.010 ≦ (SM ₁₀₀ −SM ₆₀ ) /40≦0.070 (1A)
0.005 ≦ (ST ₁₂₀ −ST ₆₀ ) /60≦0.050 (2A)
(In the formulas (1A) and (2A), SM ₁₀₀ , SM ₆₀ , ST ₁₂₀ and ST ₆₀ are the length direction at 100 ° C., the length direction at 60 ° C., the width direction at 120 ° C. and the width at 60 ° C., respectively. ( The shrinkage stress (g) of the polyolefin microporous film in the direction is shown.) A polyolefin microporous membrane that satisfies the conditions represented by the following formulas (1B) and (2B).
−0.010 ≦ (SM ₁₀₀ −SM ₆₀ ) /40≦0.070 (1B)
0.015 ≦ (ST ₁₂₀ −ST ₆₀ ) /60≦0.030 (2B)
(In the formulas (1B) and (2B), SM ₁₀₀ , SM ₆₀ , ST ₁₂₀ and ST ₆₀ are the length direction at 100 ° C., the length direction at 60 ° C., the width direction at 120 ° C. and the width at 60 ° C., respectively. ( The shrinkage stress (g) of the polyolefin microporous film in the direction is shown.)   The polyolefin microporous film according to claim 1, wherein the puncture strength is 200 g or more.   A non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane according to claim 1 as a separator.