JP2010202828A - Polyolefin microporous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyolefin microporous membrane which is suitable as a separator having high safety to the internal short-circuit and satisfying both of good cycle characteristic and windability. <P>SOLUTION: The polyolefin microporous membrane has communication holes in the thickness direction. The polyolefin microporous membrane is characterized in that the tensile elongation in MD (machine direction) is 30-55%, the ratio of the tensile elongation in TD (transverse direction) to the tensile elongation in MD; (TD tensile elongation/MD tensile elongation) is 5-15, and the bubble point is 400-550 kPa. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polyolefin microporous membrane, a battery separator, a lithium ion secondary battery, and a method for producing a polyolefin microporous membrane.

Polyolefin microporous membranes are widely used as separators in batteries, capacitors and the like because they exhibit excellent electrical insulation and ion permeability. In particular, in recent years, with the increase in functionality and weight of portable devices, lithium ion secondary batteries with high output density and high capacity density have been used as power sources. Polyolefin microporous membranes are mainly used for such battery separators.
The basic role of the separator of the lithium ion secondary battery is to be disposed between the positive electrode and the negative electrode to prevent short-circuit between both electrodes and to allow ions to permeate through the microporous structure. In order to improve battery safety, separators are required to have a certain degree of strength and elongation so that even if an external force is applied and the battery is deformed, the separator does not break and both electrodes are short-circuited.
However, depending on the usage environment, a large external force that cannot prevent the separator from breaking may be applied. To obtain higher safety, the lithium ion secondary battery should be kept safe even if it is short-circuited. Design is required, and various studies are being conducted.

For example, Patent Document 1 focuses on the product of tensile rupture strength (hereinafter sometimes abbreviated as “tensile strength”) and tensile rupture elongation (hereinafter sometimes abbreviated as “tensile elongation”). The winding direction of the value (the same as the length direction of the separator or the raw material resin discharge direction at the time of film formation; hereinafter, sometimes abbreviated as “MD”) and the vertical direction (the same as the width direction of the separator) When a separator having a ratio of 0.67 to 1.5 is applied to the battery, the short-circuit area at the time of an internal short circuit is increased, so that the temperature due to local concentration of current is increased. It has been proposed that the abnormal increase in the amount is suppressed and the safety is improved.
In Patent Document 2, the use of a separator having a tensile elongation in the width direction of 1000 to 3000% and a decrease in permeability at high temperature suppresses internal short circuit due to the breakage of the separator during the crushing test and oxygen diffusion. It has been reported that the safety of the battery is improved.

Japanese Patent Publication No. 05-051143 JP-A-10-125299

However, when the microporous membrane described in Patent Documents 1 and 2 is used as a battery separator, there is still room for improvement from the viewpoint of battery safety. The battery is also required to have good cycle characteristics and good winding properties during production.
It is an object of the present invention to provide a polyolefin microporous membrane suitable as a separator capable of achieving both good safety against internal short circuit, good cycle characteristics, and good winding properties during production. .

The present inventors have intensively studied to solve the above problems. As a result, the inventors have found that a specific polyolefin microporous membrane (hereinafter, sometimes simply abbreviated as “microporous membrane”) can achieve the above-described problems, and have made the present invention.
That is, the present invention is as follows.
[1]
It has communication holes in the film thickness direction, the tensile breaking elongation in the length direction (MD) is 30% or more and 55% or less, and the ratio of the tensile breaking elongation in the width direction (TD) to the length direction (TD tensile) A polyolefin microporous membrane having a breaking elongation / MD tensile breaking elongation) of 5 to 15 and a bubble point of 400 kPa to 550 kPa.
[2]
The microporous membrane according to [1], wherein the TD tensile elongation at break is 250% or more and 450% or less.
[3]
The microporous membrane according to [1] or [2], wherein the dynamic friction coefficient is 0.4 to 0.6.
[4]
20 to 40% by mass of ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 1.5 to 2.5 million, and 30 to 50 mass of linear low density polyethylene having an Mv of 100,000 to 300,000 and a melting point of 130 ° C. or less. The microporous membrane according to any one of [1] to [3].
[5]
A battery separator using the microporous membrane according to any one of [1] to [4].
[6]
[5] A lithium ion secondary battery using the battery separator according to [5], a positive electrode, a negative electrode, and an electrolytic solution.

[7]
[1] A method for producing a polyolefin microporous membrane according to [1], wherein the following steps (1) to (5):
(1) a mixing step of mixing a polyolefin resin, a plasticizer, and an inorganic powder;
(2) a kneading step of melt-kneading the mixture obtained in the mixing step in an extruder;
(3) A sheet forming step in which the kneaded product obtained in the kneading step is extruded from a T die, cooled and formed into a sheet shape,
(4) An extraction process for extracting the plasticizer and the inorganic powder from the sheet-like molded product obtained in the sheet molding process,
(5) Stretching step for stretching the sheet-like porous body obtained in the extraction step,
The drawing method (winding speed / feeding speed) in the extraction step is 1.001 or more and 1.080 or less.
[8]
The production method according to [7], wherein a ratio of MD and TD draw ratio (MD draw ratio / TD draw ratio) in the drawing step is 1.0 or more and 4.0 or less.

  The microporous membrane of the present invention is suitable as a separator that can achieve both good safety against internal short circuits, good cycle characteristics, and good winding properties during production.

  The best mode for carrying out the present invention (hereinafter abbreviated as “embodiment”) will be described in detail below. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

The polyolefin microporous membrane of the present embodiment has communication holes in the film thickness direction, and has, for example, a three-dimensional network skeleton structure. Moreover, MD tensile elongation is 30 to 55%, TD / MD ratio of tensile elongation is 5 to 15, and bubble point is 400 to 550 kPa.
And when such a microporous film is used as a battery separator, safety against internal short circuit, cycle characteristics, and winding performance are improved.

One of the safety evaluation methods against an internal short circuit is a nail penetration test in which a nail is artificially penetrated from the side of a battery to force a short circuit. In the nail penetration test, the positive electrode and the negative electrode react directly when the separator breaks at the nail penetration location, and the heat generation at this time causes the separator to melt, further expanding the hole and expanding the short-circuit area. It is considered that thermal runaway occurs due to this reaction, and when the positive electrode active material is decomposed and oxygen is released, the battery with low safety leads to ignition.
Conventionally, when a battery is configured using a separator having a single-layer structure, in order to improve battery safety against an internal short circuit, as described in Patent Document 1, the short circuit area is increased to localize current locally. It has been said that it is preferable to prevent the temperature from rising abnormally or prevent it from breaking as described in Patent Document 2.
On the other hand, the inventors thought that when a separator that does not easily crack is used, diffusion of the oxygen is suppressed, and the safety of the battery as a whole is improved.

  Lithium ion secondary batteries generally have a structure in which strip-shaped electrodes and separators are wound in order to obtain high output density and capacity density, and in the MD direction to suppress sagging in the winding process. It is wound with tension. As a characteristic of the biaxially oriented film, if a hole is made in a state where tension is applied in any one of the uniaxial directions, cracks easily run in a direction perpendicular to the direction of the tension. Therefore, the lithium ion secondary battery separator wound with tension in the MD direction is likely to crack along the TD direction during the nail penetration test.

  However, as a result of intensive studies against the above technical common sense, the present inventors have communication holes in the film thickness direction, MD tensile elongation, TD / MD ratio of tensile elongation, and bubble points within a certain range. The polyolefin microporous film set to 1 has been found to realize a separator in which cracks are difficult to run and the short-circuit area is difficult to expand. Moreover, such a separator is excellent in battery cycle characteristics and winding properties during production.

The ratio of TD tensile elongation / MD tensile elongation is 5 or more and 15 or less, preferably 6-13. If a microporous membrane with TD tensile elongation / MD tensile elongation in this range is used for the separator, cracks will not easily run in the TD or MD direction during the nail penetration test, and safety for the nail penetration test will be improved. I found.
The MD tensile elongation is 30 to 50% or more, preferably 35 to 50%. 30% or more is preferable from the viewpoint of suppressing tearing in the MD direction at the time of an internal short circuit, and 55% or less is preferable from the viewpoint of winding property.
The bubble point is 400 to 550 kPa, preferably 420 to 500 kPa. A battery using a separator with a microporous membrane of 400 kPa or more has a moderately small pore size, so that diffusion of oxygen derived from the positive electrode decomposed due to abnormal heat generation at the time of internal short circuit is suppressed, so that it is difficult to cause ignition and internal short circuit Excellent safety. Moreover, 400 kPa or more is preferable from the viewpoint of self-discharge and withstand voltage. On the other hand, a battery using a separator having a bubble point of 550 kPa or less has a moderately large pore size, so that clogging due to a decomposition product of an electrolyte during a cycle test is suppressed, and cycle characteristics are improved.

  A battery using the microporous membrane as a separator may be a cylindrical type or a rectangular type as long as it is a wound type battery, but the details are not clear, but it is particularly high when used for a cylindrical type battery. Cycle characteristics and safety are easily obtained.

The TD tensile elongation of the microporous membrane is preferably 250% or more from the viewpoint of suppressing tearing in the TD direction at the time of an internal short circuit, and 450% or less from the viewpoint of obtaining an appropriate TD tensile elongation / MD tensile elongation ratio. Is preferred. More preferably, it is 300 to 400%.
The thickness of the microporous membrane is preferably 5 μm or more from the viewpoint of strength, and is preferably 50 μm or less from the viewpoint of increasing the battery capacity. A more preferable film thickness is 10 to 30 μm.
The porosity of the microporous membrane is preferably 35% or more from the viewpoint of permeability, and preferably 60% or less from the viewpoint of strength and winding properties. A more preferable porosity is 40 to 55%.
The air permeability of the microporous membrane is preferably 10 sec / 100 cc or more from the viewpoint of safety and 500 sec / 100 cc or less from the viewpoint of ion permeability, and more preferably 50 to 150 sec / 100 cc.

The puncture strength of the microporous membrane is preferably 3.0 N or more from the viewpoint of suppressing foreign matter contamination in the battery and rupture by lithium dendrite, and 8.0 N or less from the ease of winding in the battery manufacturing process. preferable. More preferably, it is 3.5-7.0N.
In order to obtain an appropriate TD / MD tensile elongation ratio, the microporous membrane preferably has an MD tensile strength of 150 to 300 MPa and a TD tensile strength of 20 to 35 MPa in order to obtain an appropriate TD / MD tensile elongation ratio. More preferably, the MD tensile strength is 180 to 250 MPa, and the TD tensile strength is 20 to 35 MPa.
The dynamic friction coefficient of the microporous membrane is preferably 0.4 or more from the viewpoint of safety against internal short circuit, and preferably 0.6 or less from the viewpoint of winding property. When the dynamic friction coefficient is moderately high, the adhesion between the separator and the electrode is improved, and it is easy to suppress the expansion of the hole due to the heat shrinkage after fracture. More preferably, it is 0.45-0.55.

The shutdown temperature of the microporous membrane is preferably 140 ° C. or lower from the viewpoint of safety, and preferably 130 ° C. or higher from the viewpoint of cycle characteristics. The lower the shutdown temperature, the lower the diffusion of oxygen derived from the positive electrode decomposed due to abnormal heat generation. Moreover, since the blockage | closure of the hole under high temperature can be suppressed, so that shutdown temperature is high, it is excellent in the cycling characteristics of a high temperature state. A more preferable shutdown temperature is 134 to 137 ° C.
In addition, as the adjustment method of each parameter regarding the microporous membrane, there are a method for adjusting the molecular weight of the following polyolefin resin, a ratio of the polyolefin resin, a stretching temperature in the following production process, a stretching ratio, a method for adjusting the heat treatment conditions, and the like. Can be mentioned.

The microporous membrane comprises the following steps (1) to (5):
(1) A mixing step of mixing a polyolefin resin, a plasticizer, and, if necessary, an inorganic powder with a Henschel mixer,
(2) a kneading step of melt kneading the mixture obtained in the mixing step in an extruder or the like;
(3) A sheet forming step in which the kneaded product obtained in the kneading step is extruded from a T die, cooled and formed into a sheet shape,
(4) An extraction step of extracting a plasticizer and, if necessary, an inorganic powder from a sheet-like molded product obtained in the sheet molding step,
(5) Stretching step for stretching the sheet-like porous body obtained in the extraction step,
It can manufacture with the manufacturing method containing.

In addition, you may include the process of drying after an extraction process, and the process of heat-processing after an extending process.
In the production of the microporous membrane, the plasticizer may be extracted after stretching the sheet (stretching before extraction), but it is easy to obtain a microporous membrane having a specific elongation and a uniform and moderately large pore size. In view of the above, it is preferable that the plasticizer or inorganic powder is extracted and then stretched (stretching after extraction).

  The polyolefin resin used in the step (1) may be a kind of polyolefin or a polyolefin composition containing several kinds of polyolefin. Examples of the polyolefin include polyethylene, polypropylene, poly-4-methyl-1-pentene, and two or more of these may be blended and used. From the viewpoint of moldability and strength, a composition based on polyethylene is preferable. In particular, in order to obtain an appropriate strength and elongation, and an appropriate shutdown temperature, an extremely high viscosity average molecular weight (Mv) of 1.5 to 2.5 million. It is preferable to use a polyolefin resin containing 20 to 40% by mass of molecular weight polyethylene, 30 to 50% by mass of linear low density polyethylene having an Mv of 100,000 to 300,000 and a melting point of 130 ° C. or less.

Examples of the plasticizer include dioctyl phthalate (hereinafter referred to as DOP), phthalic acid esters such as diheptyl phthalate and dibutyl phthalate; organic acid esters such as adipic acid ester and glyceric acid ester; and trioctyl phosphate. Examples thereof include liquid paraffins; liquid paraffin; solid wax; mineral oil and the like, and phthalic acid esters are particularly preferable in consideration of compatibility with polyethylene. These may be used alone or as a mixture.
Examples of the inorganic powder include silica, calcium silicate, aluminum silicate, alumina, calcium carbonate, magnesium carbonate, kaolin clay, talc, titanium oxide, carbon black, and diatomaceous earth. These may be used alone or as a mixture. From the viewpoint of dispersibility and ease of extraction, it is particularly preferable to use silica.

(1) The blend ratio of the polyolefin resin, the plasticizer and the inorganic powder in the step is not particularly limited, but the polyolefin resin concentration in 100% by mass of the blend raw material is 25 to 50 mass in terms of strength and film forming property. % Is preferred.
Further, the plasticizer concentration in 100% by mass of the blend raw material is preferably 30 to 60% by mass because a viscosity suitable for extrusion can be obtained.
Further, the concentration of the inorganic powder in 100% by mass of the blend raw material is preferably 0% by mass or more in order to obtain a uniform pore diameter, and preferably 10 to 40% by mass from the viewpoint of film forming property.
In addition to the polyolefin resin, inorganic powder, and plasticizer, various additives such as an antioxidant, an antistatic agent, an ultraviolet absorber, a lubricant, and an antiblocking agent can be added as necessary.

(1) The mixing in a process can be performed using common mixers, such as a Henschel mixer, a V-blender, a pro shear mixer, and a ribbon blender.
In the step (2), the mixture is kneaded by a melt kneader such as an extruder or a kneader.
In the step (3), the obtained kneaded material is formed into a sheet by, for example, melt molding using a T die. In this case, it is preferable from the viewpoint of dimensional stability to form through a gear pump, and it is particularly preferable from the viewpoint of dimensional stability that the pressure before the gear pump is controlled to be constant.

  (3) In the step, as a method of cooling the melt-extruded mixture, for example, a method of cooling with air, a method of cooling by contacting with a roll temperature-controlled 20 to 120 ° C. lower than the T-die discharge resin temperature, It is possible to take a method of cooling while rolling and forming into a sheet shape with a calender roll that is 20 to 120 ° C. lower than the T-die discharge resin temperature. It is preferable in terms of film thickness uniformity to adopt a method in which it is cooled while being formed into a sheet by rolling with a calender roll that is 20 to 120 ° C. lower than the T-die discharge resin temperature. A more preferable difference between the T-die discharge resin temperature and the calender roll temperature is 40 to 80 ° C. In this case, when the roll is used, it is preferable that the distance between the contact points of the T dice and the roll sheet is 5 to 500 mm. The die discharge temperature can be measured by making the terminal not touch the die with a normal thermocouple thermometer and bringing it into contact with the discharge resin.

  In the step (4), the plasticizer in the film and, if necessary, the inorganic powder are extracted. Examples of the solvent used for extraction of the plasticizer include organic solvents such as methanol, ethanol, methyl ethyl ketone, and acetone; ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran; methylene chloride, 1,1,1-trichloroethane, and the like. Of halogenated hydrocarbons can be used. These can be used alone or in combination. On the other hand, an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide is preferably used as the solvent used for extraction of the inorganic powder.

  Here, the draw ratio (winding speed / feeding speed) in the extraction step is preferably 1.001 or more and 1.080 or less in order to obtain an appropriate MD tensile elongation and an appropriate bubble point of the microporous membrane. . Usually, in the extraction process, the film shrinks by several percent with extraction, so that winding is performed at a draw ratio of 1 or less. However, by setting this to 1.001 or more, winding can be performed in a high stress state. Therefore, it is easy to obtain a moderately high bubble point, and if it is 1.080 or less, a decrease in MD tensile elongation can be suppressed. A more preferable extraction draw ratio is 1.010 to 1.070. In addition, when performing the extraction process of a plasticizer and the extraction process of inorganic powder separately, the product of each draw ratio is made into the draw ratio of an extraction process.

  In the step (5), the sheet-like molded product is stretched in at least a uniaxial direction. The method of stretching in the uniaxial direction may be roll stretching or stretching using a tenter, but biaxial stretching is preferred in order to obtain an appropriate TD tensile elongation / MD tensile elongation ratio and an appropriate bubble point. In the case of biaxial stretching, either sequential biaxial stretching or simultaneous biaxial stretching may be used, but in order to obtain an appropriate TD tensile elongation / MD tensile elongation ratio and an appropriate bubble point, sequential biaxial stretching may preferable. The stretching may be performed by one sheet or a plurality of sheets, but it is preferable to stretch two or more sheets in order to improve the strength.

The MD draw ratio is preferably 3.5 to 6.5 times, more preferably 4.0 to 5.5 times. The TD stretch ratio is preferably 1.5 to 2.5 times, more preferably 1.7 to 2.0 times. When the MD stretch ratio and the TD stretch ratio are in this range, there are few breaks, wrinkles, etc., and film formation stability is improved.
The MD draw ratio / TD draw ratio ratio is preferably 1.0 to 4.0, more preferably 2.0 to 3.5 from the viewpoint of obtaining an appropriate TD tensile elongation / MD tensile elongation ratio. More preferably, it is 2.4 to 2.8.

  The maximum heating temperature of TD stretching is preferably in the range of −2 ° C. to + 4 ° C., more preferably in the range of −1 ° C. to + 2 ° C., based on the film melting point of the sheet-like molded product before stretching. Such heat setting conditions are preferable from the viewpoint of expressing an appropriate dynamic friction coefficient. More preferably, it is 0 degreeC-+1 degreeC. In addition, it is preferable to perform heat treatment such as heat fixation or heat relaxation after stretching to improve heat shrinkage resistance.

  Various parameters described in the present embodiment are measured according to the measurement methods in the following examples unless otherwise specified.

  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. In addition, the physical property in an Example was measured with the following method.

(1) Film thickness (μm)
Measurement was performed using a dial gauge “PEACOCK No. 25” (trademark, manufactured by Ozaki Seisakusho Co., Ltd.). A sample was cut into a size of 100 mm × 100 mm, the thickness of the center part of each grid divided into 9 grids was measured, and the average value of 9 points was taken as the film thickness.

(2) Air permeability (sec / 100cc)
It measured using the Gurley type air permeability meter based on JIS P-8117.

(3) Porosity (%)
The sample was cut into a size of 100 mm × 100 mm to determine the volume (cm ³ ) and mass (g), and calculated from these and the resin density (g / cm ³ ) using the following formula.
Porosity (%) = (1− (mass / volume) / (resin density)) × 100

(4) Puncture strength (N)
The measurement was performed using a handy compression tester “KES-G5” (trade name, manufactured by Kato Tech). The puncture test was performed with a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / s, and the maximum puncture load was defined as the puncture strength.

(5) Bubble point (kPa)
Calculation was performed using ethanol in accordance with ASTM E-128-61.

(6) MD, TD tensile strength (MPa), tensile elongation (%)
Based on JIS K7127, it measured about MD and TD sample (shape; width 10mm x length 100mm) using the tensile tester by Shimadzu Corporation, and autograph AG-A type (trademark). Moreover, the sample used the thing which stuck the cellophane tape (Nitto Denko Packaging System Co., Ltd. make, brand name: N.29) on the single side | surface of the both ends (25 mm each) of the sample for the distance between chuck | zippers to 50 mm. Furthermore, in order to prevent sample slippage during the test, a fluororubber having a thickness of 1 mm was attached to the inside of the chuck of the tensile tester.
The tensile elongation (%) was determined by dividing the amount of elongation (mm) up to fracture by the distance between chucks (50 mm) and multiplying by 100. The tensile strength at break (MPa) was determined by dividing the strength at break by the cross-sectional area of the sample before the test. The measurement was performed at a temperature of 23 ± 2 ° C., a chuck pressure of 0.30 MPa, and a tensile speed of 200 mm / min.

(7) Shutdown temperature (℃)
A multilayer porous membrane sufficiently impregnated with a specified electrolyte is sandwiched between 10 μm thick nickel foils fixed to a glass plate, and the glass plate is fixed with a commercially available clip. A cell was fabricated by fixing a thermocouple to the glass plate with heat-resistant tape.
More specifically, heat-resistant tape is bonded to one nickel foil, and a 15 mm × 10 mm window portion is left in the central portion of the foil for masking. The windows are overlapped so as to be covered with the multilayer porous film, and the multilayer porous film is sandwiched between the other nickel foils. The prescribed electrolyte is a 1 mol / l lithium borofluoride solution, and the solvent is propylene carbonate / ethylene carbonate / γ-butyllactone = 1/1/2 (volume ratio).
The cell was placed in an oven and the temperature and the electrical resistance between the nickel foils were measured. The oven was heated from 30 ° C. to 200 ° C. at a rate of 2 ° C./min, and the electrical resistance value was measured at an alternating current of 1 kHz. The temperature at which the electric resistance value reached 1000Ω was taken as the shutdown temperature.

(8) Coefficient of dynamic friction Using a KES-SE friction tester manufactured by Kato Tech Co., Ltd., load 50 g, contact area 10 × 10 = 100 mm ² (winding 20 piano wires made of 0.5 mmφ hard stainless steel wire SUS304), A sample cut into a width of 50 mm and a measurement direction of 200 mm under the conditions of a contact feed speed of 1 mm / sec, a tension of 6 kPa, a temperature of 25 ° C., and a humidity of 50%, was measured three times each on the front and back in the MD and TD directions. Asked.

(9) Viscosity average molecular weight The viscosity average molecular weight of polyethylene and polypropylene was measured at a measurement temperature of 135 ° C. using decalin as a solvent, and was calculated from the viscosity [η] by the Chain equation.
In the case of polyethylene [η] = 6.77 × 10 ⁻⁴ × Mv ^0.67
In the case of polypropylene [η] = 1.10 × 10 ⁻⁴ × Mv ^0.80

(10) Melting point (° C)
Measurement was performed using DSC60 manufactured by Shimadzu Corporation. In the case of a film, it was punched into a circle having a diameter of 5 mm, and several sheets were stacked to make 3 mg. In the case of a polymer, 3 mg of powder was used as a measurement sample. This was placed on an aluminum open sample pan having a diameter of 5 mm, and a clamping cover was placed thereon and fixed in the aluminum pan with a sample sealer. In a nitrogen atmosphere, the temperature was raised from 30 ° C. to 200 ° C. at a rate of 10 ° C./min, held at 200 ° C. for 5 minutes, then lowered to 30 ° C. at 10 ° C./min and held at 30 ° C. for 5 minutes. Finally, the temperature was increased again to 200 ° C. at a rate of temperature increase of 10 ° C./min, and a melting endothermic curve was measured. Here, the melting point of the film was the peak temperature of the melting endothermic curve at the first temperature increase, and the melting point of the polymer was the peak temperature of the melting endothermic curve at the second temperature rising. Further, when a plurality of melting endothermic curves were present, the peak was obtained from the peak having the largest peak area.

(11) Density (g / cm ³ )
Measured according to ASTM D1238.

(12) Evaluation as a battery A cylindrical battery was prepared according to the following procedure.
<Preparation of positive electrode>
92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as the active material, 2.3% by mass of flake graphite and acetylene black as the conductive agent, and 3.2% by mass of polyvinylidene fluoride (PVDF) as the binder are N -A slurry was prepared by dispersing in methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the positive electrode is 250 g / m ² and the active material bulk density is 3.00 g / cm ³ . This was cut into a width of about 57 mm to form a strip.

<Production of negative electrode>
A slurry was prepared by dispersing 96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as binders in purified water. This slurry was applied to one side of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. In this case, the active material coating with the amount of the negative electrode 106 g / ^{m 2,} bulk density of the active material was 1.55 g / cm ³ and high packing density. This was cut into a width of about 58 mm to form a strip.

<Preparation of Nonaqueous Electrolyte> LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 1/2 (volume ratio) so as to have a concentration of 1.0 mol / l.
<Separator>
The microporous membranes described in the examples and comparative examples were slit into 60 mm to form strips.

<Battery assembly>
(12-1) Winding property A negative electrode plate, a separator, a belt-like positive electrode, and a separator were stacked in this order, and wound several times in a spiral shape with a winding tension of 250 gf to prepare an electrode plate laminate. At this time, the separators were visually observed for the presence or absence of wrinkles or wrinkles, and among the 10 batteries that were produced, no kinks or wrinkles occurred. The case where one occurred was “Δ”, and the case where two or more appearance defects such as twists and wrinkles occurred was “×”.
The electrode plate laminate is housed in a stainless steel container having an outer diameter of 18 mm and a height of 65 mm, and an aluminum tab derived from the positive electrode current collector is used as a container lid terminal portion, and a nickel tab derived from the negative electrode current collector Was welded to the container wall. Thereafter, drying was performed at 80 ° C. for 12 hours under vacuum, and then the above-described nonaqueous electrolytic solution was injected into the container in an argon box and sealed.

<Pretreatment>
The assembled battery was charged at a constant current of 1 / 3C to a voltage of 4.2V, then charged at a constant voltage of 4.2V for 5 hours, and then discharged at a current of 1 / 3C to a final voltage of 3.0V. It was. Next, after constant current charging to a voltage of 4.2 V with a current value of 1 C, a constant voltage charge of 4.2 V was performed for 2 hours, and then discharging was performed to a final voltage of 3.0 V with a current of 1 C. Finally, after constant current charging to 4.2 V with a current value of 1 C, 4.2 V constant voltage charging was performed for 2 hours as a pretreatment.

(12-2) Cycle characteristics (%)
The battery pretreated in (12) was discharged at a discharge current of 1 A to a discharge end voltage of 3 V under a temperature of 25 ° C., and then charged to a charge end voltage of 4.2 V with a charge current of 1 A. Charging / discharging was repeated with this as one cycle, and the capacity retention rate (%) after 500 cycles with respect to the initial capacity was expressed as cycle characteristics.

(12-3) Nail penetration test For the battery pretreated in (12), the temperature when the nail having a diameter of 2.5 mm was penetrated from the side surface at a speed of 5 mm / sec was measured, and the battery surface maximum Evaluations were made for those having an ultimate temperature of less than 100 ° C., evaluations for evaluations of 100 ° C. or more and less than 120 ° C., evaluations for evaluations of 120 ° C. or more and less than 140 ° C.

[Example 1]
Mv 2 million, melting point 134 ° C., density 0.936 g / cm ³ ultrahigh molecular weight polyethylene (“Mv 2 million PE” and listed in Table 1) 30% by mass, Mv 150,000, melting point 127 ° C. and density 0.926 g / cm ³ linear low density polyethylene ( "Mv15 ten thousand PE" and described in Table 1) 40 mass% of, mv12 ten thousand melting point 132 ° C. at a density of 0.954 g / cm ³ and a propylene unit content 1 mol% of co 45 parts by mass of DOP, 21 parts by mass of fine silica (trade name Nipsil LP, manufactured by Tosoh Silica Co.), 34 parts by mass of polymer comprising 30% by mass of polymerized polyethylene (described in Table 1 as “Mv120,000 PE”), antioxidant As 0.3 parts by weight of BHT (dibutylhydroxytoluene) and 0.3 parts by weight of PLTP (dilaurylthiodipropionate) Mix with granule and granulate. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T dice | dies, and it shape | molded in the sheet form of thickness 100 micrometers with the calender roll cooled at 150 degreeC. From the molded product, DOP was extracted with methylene chloride and finely divided silica was extracted with sodium hydroxide, and the resultant was taken up at a draw ratio of 1.030 in the entire extraction process to obtain a microporous membrane. The melting point of this microporous membrane was 127.5 ° C. In addition, DOP and fine silica did not substantially remain in the extracted film.
Two microporous membranes are stacked and stretched 4.90 times to MD with a stretching roll heated to 120 ° C. (stretching after extraction), then 1 in the TD direction in a tenter with a maximum heating temperature of 128.0 ° C. The film was stretched 85 times. Various characteristics of the obtained microporous membrane were evaluated. The evaluation results are shown in Table 1.

[Examples 2-11, Comparative Examples 1-8]
A microporous membrane was obtained in the same manner as in Example 1 except for the conditions shown in Table 1. Various characteristics of the obtained microporous membrane were evaluated. The evaluation results are shown in Table 1.

[Comparative Example 9]
Mv 120,000, melting point 132 ° C., density 0.954 g / cm ³ and propylene unit content 1 mol%, 30% by mass of copolymer polyethylene, Mv 250,000, melting point 136 ° C., density 0.957 g / cm ³ polyethylene 30 wt%, MV100 ten thousand melting point of 135 ° C., the ultra high molecular weight polyethylene 15 wt% of the density of 0.955 g / cm ^3, MV 200 ten thousand melting point 134 ° C., the ultra-high molecular weight density of 0.936 g / cm ³ 35 parts by mass of a polymer mixture composed of 25% by mass of polyethylene and 65 parts by mass of liquid paraffin were kneaded and extruded at 200 ° C. with a twin screw extruder equipped with a T die, and formed into a sheet having a thickness of 1000 μm. The sheet was guided to a simultaneous biaxial tenter and stretched 7.0 times in the MD direction and 6.5 times in the TD direction at a maximum heating temperature of 120 ° C. The melting point of the pre-stretch film was 122.0 ° C. Finally, liquid paraffin was extracted with methylene chloride at a draw ratio of 1.000 to obtain a microporous membrane. Various characteristics of the obtained microporous membrane were evaluated. The evaluation results are shown in Table 1.

  As described above, when the microporous membrane of the present embodiment is used as a battery separator, as shown in the examples, it has an excellent balance of safety against internal short circuit, cycle characteristics, and winding performance.

  ADVANTAGE OF THE INVENTION According to this invention, the polyolefin microporous film | membrane suitable as a separator which can make compatible the favorable safety | security with respect to an internal short circuit, a favorable cycle characteristic, and favorable winding property is provided.

Claims (8)

There is a communication hole in the film thickness direction,
The tensile elongation at break in the length direction (MD) is 30% or more and 55% or less,
The ratio of the tensile elongation at break in the width direction (TD) and the length direction (TD tensile elongation at break / MD tensile elongation at break) is 5 or more and 15 or less,
Bubble point is 400 kPa or more and 550 kPa or less,
A polyolefin microporous membrane.   2. The polyolefin microporous membrane according to claim 1, which has a TD tensile breaking elongation of 250% or more and 450% or less.   The polyolefin microporous membrane according to claim 1 or 2, wherein the dynamic friction coefficient is 0.4 to 0.6.   20 to 40% by mass of ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 1.5 to 2.5 million, and 30 to 50 mass of linear low density polyethylene having an Mv of 100,000 to 300,000 and a melting point of 130 ° C. or less. The polyolefin microporous membrane according to any one of claims 1 to 3, comprising:   The battery separator using the polyolefin microporous film in any one of Claims 1-4.   A lithium ion secondary battery using the battery separator according to claim 5, a positive electrode, a negative electrode, and an electrolytic solution. It is a manufacturing method of the polyolefin microporous film of Claim 1, Comprising: Each process of following (1)-(5),
(1) a mixing step of mixing a polyolefin resin, a plasticizer, and an inorganic powder;
(2) a kneading step of melt kneading the mixture obtained by the mixing step,
(3) a sheet forming step of cooling and forming the kneaded product obtained in the kneading step into a sheet,
(4) An extraction process for extracting a plasticizer from the sheet-like molded product obtained in the sheet molding process,
(5) Stretching step for stretching the sheet-like porous body obtained in the extraction step,
The drawing method (winding speed / feeding speed) in the extraction step is 1.001 or more and 1.080 or less.   The method according to claim 7, wherein a ratio of MD to TD draw ratio (MD draw ratio / TD draw ratio) in the drawing step is 1.0 or more and 4.0 or less.