JP5511329B2 - Polyolefin microporous membrane - Google Patents

Description

  The present invention relates to 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. With the recent increase in energy of batteries, the load on battery members is increasing, and higher safety performance is also required for separators.
Polyolefin microporous membranes are mainly composed of polyethylene as a main component, but various studies of blending polypropylene for the purpose of improving high-temperature safety as a battery have been conducted.

For example, Patent Document 1 proposes a membrane in which the molecular weight of polyethylene and polypropylene is defined by the ratio Mw / Mv of the weight average molecular weight (Mw) and the viscosity average molecular weight (Mv), and the molecular weight is defined as 10,000 or less. ing.
In Patent Document 2, a polyethylene copolymer having a viscosity average molecular weight (Mv) of 500,000 to 1,000,000 and a polypropylene copolymer having a melt index of 1 g / 10 min or less and containing an ethylene component is copolymerized with an α-olefin. Membranes mixed at a specific ratio have been proposed.
In Patent Document 3, a separator is proposed in which polypropylene is added in order to maintain the hole diameter at high temperature and improve the thermal film breaking property, and has a specific range of puncture strength, a shutdown temperature, and a bubble point after being left at 120 ° C.

International Publication No. 2005/113657 Pamphlet Japanese Patent No. 4136008 JP 2008-266457 A

However, when the microporous membrane described in Patent Documents 1 to 3 is used as a battery separator, there is still room for improvement from the viewpoint of achieving both oxidation resistance and cycle characteristics.
An object of the present invention is to provide a polyolefin microporous membrane suitable as a separator capable of achieving both oxidation resistance and cycle characteristics.

  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]
Including 5 to 50% by mass of a polypropylene component and 50 to 95% by mass of a polyethylene component, and the polyethylene component includes 10% by mass or more of ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 1 million or more,
A polyolefin microporous membrane having a temperature difference between the melting point Tme of the polyethylene component and the melting point Tmp of the polypropylene component of −20 ° C. <Tmp−Tme ≦ 20 ° C. and a bubble point of 400 to 600 kPa.
[2]
The polyolefin microporous film according to [1], wherein a fraction having a molecular weight of 1 million or more on a GPC curve is 5% or more, and an intensity ratio of polypropylene to polyethylene infrared absorption spectrum is 0.01 to 0.45. (Absorption wavelength of polypropylene: 998 cm ⁻¹ , absorption wavelength of polyethylene: 719 cm ⁻¹ )
[3]
The polyolefin microporous membrane according to [1] or [2], wherein the polypropylene component is a random copolymer of ethylene and propylene.
[4]
The polyolefin microporous membrane according to any one of [1] to [3], wherein the air permeability / porosity ratio is 2.2 to 4.0.
[5]
A battery separator using the polyolefin microporous membrane according to any one of [1] to [4].
[6]
A lithium ion secondary battery comprising the 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 by the mixing step,
(3) A sheet forming step in which the kneaded product obtained in the kneading step is extruded from a slit, 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) a stretching step of stretching the sheet-like porous body obtained in the extraction step,
The step (1) is a step of mixing 25 to 50 parts by mass of a polyolefin resin, 30 to 60 parts by mass of a plasticizer, and 10 to 40 parts by mass of an inorganic powder so that the total becomes 100 parts by mass, The polyolefin resin contains 5 to 50% by mass of a polypropylene component and 50 to 95% by mass of a polyethylene component, and the polyethylene component contains 10% by mass or more of ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 1 million or more. The manufacturing method whose temperature difference of melting | fusing point Tme of the said polyethylene component and melting | fusing point Tmp of the said polypropylene component is -20 degreeC <Tmp-Tme <= 20 degreeC.

  According to the present invention, there is provided a polyolefin microporous membrane suitable as a separator capable of achieving both oxidation resistance and cycle characteristics.

  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, while containing 5-50 mass% of polypropylene components and 50-95 mass% of polyethylene components, while the said polyethylene component contains ultra high molecular weight polyethylene, melting | fusing point Tme of the said polyethylene component, and melting | fusing point Tmp of the said polypropylene component The temperature difference is −20 ° C. <Tmp−Tme <23 ° C., and the bubble point is 400 to 600 kPa.
And when such a microporous film is used as a battery separator, it is possible to provide a high-performance secondary battery having both oxidation resistance and cycle characteristics.

The oxidation resistance can be judged by the degree of blackening of the separator. Blackening is attributed to polyeneization due to the radical chain oxidation reaction of the polymer simultaneously with the reduction of cobalt at the positive electrode. When blackening occurs, the film strength is deteriorated as a result. Polyethylene undergoes an oxidation reaction in a chain from its molecular structure, whereas polypropylene has the property of stopping the chain reaction and can be expected to prevent blackening.
And in this Embodiment, the said Tmp-Tme and bubble point are prescribed | regulated in the microporous film | membrane with which the polypropylene component and the polyethylene component were mix | blended in a fixed ratio. Although the details of the mechanism that achieves both good oxidation resistance and good cycle characteristics by specifying in this way are not detailed, a microporous film having a constant pore size is formed of a material exhibiting a constant melting behavior. Thus, it is considered that clogging due to the decomposition product of the electrolytic solution during the cycle test is suppressed and the cycle characteristics are improved, and the oxidation resistance of the film surface can be expressed.

  Conventionally, in order to efficiently draw out the characteristics of polypropylene in a mixed system of polyethylene and polypropylene, homopolypropylene is often selected instead of a copolymer copolymer with a comonomer such as ethylene or α-olefin. From the standpoint, a homopolymer having a higher melting point has been considered preferable. The inventors of the present invention, in a manner contrary to these conventional attempts, use a copolymerized polypropylene having a difference in melting point from polyethylene within a certain range and having ethylene as a comonomer component, while maintaining cycleability. It has been found that high oxidation resistance can be achieved despite the low propylene content compared to homo, and in addition, a lower shutdown temperature can be achieved.

  The reason for this is considered to be that compatibility with polyethylene is improved by having an ethylene component in the polymer main chain and having a difference in melting point within an appropriate range. In the first place, it has been considered that polyethylene and polypropylene are incompatible with each other due to their properties and difficult to disperse uniformly. In this embodiment, copolymer polypropylene (especially, a random copolymer of ethylene and propylene) is preferably used as a preferred form, enabling uniform fine dispersion of polypropylene in a polyethylene matrix, and exhibiting oxidation resistance. It is thought that.

In the present embodiment, the temperature difference between the melting point Tme of the polyethylene component and the melting point Tmp of the polypropylene component is preferably in the range of −20 ° C. <Tmp−Tme <23 ° C., and further −15 ° C. <Tmp−. Tme <22 ° C. is preferred. Within this range, it is preferable from the viewpoint of maintaining good compatibility between the polyethylene component and the polypropylene component and improving uniformity.
When a plurality of types of raw materials are used, “melting point of polyethylene component” and “melting point of polypropylene component” mean “melting point of mixture of polyethylene component” and “melting point of mixture of polypropylene component”, respectively. Examples of the method for adjusting Tmp-Tme include a method of changing the mixing ratio of raw materials having different melting points.

The bubble point of the microporous membrane is preferably 400 kPa or more from the viewpoints of suppression of self-discharge and good withstand voltage characteristics, etc., and prevents clogging due to electrolyte decomposition products and realizes good cycle characteristics and rate characteristics. From the viewpoint of achieving this, 600 kPa or less is preferable. More preferably, it is 440-530 kPa.
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 air permeability / porosity ratio of the microporous membrane is preferably 2.2 or more from the viewpoint of trickleability and withstand voltage characteristics, and the upper limit is preferably 6.0 or less, more preferably 4.0 from the viewpoint of cycle characteristics. It is as follows.
The puncture strength of the microporous membrane is preferably 3N or more from the viewpoint of suppressing foreign matter contamination in the battery and breakage by lithium dendrite, and preferably 8N or less from the viewpoint of ease of winding in the battery manufacturing process.
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 more effective for early suppression of thermal runaway during abnormal heat generation, and the higher the shutdown temperature, the better the high-temperature cycle characteristics because the blockage of holes at high temperatures can be suppressed. A more preferable shutdown temperature is 135 to 140 ° C.
Further, the thermal membrane breaking temperature of the microporous membrane is preferably 150 ° C. or higher, more preferably 170 ° C. or higher, and even more preferably 200 ° C. from the viewpoint of safety.

  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 is, for example, the following steps (1) to (5):
(1) A mixing step of mixing a polyolefin resin, a plasticizer, and, if necessary, an inorganic powder with, for example, 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 slit such as a T die, and cooled to be 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 can be obtained, for example, by blending various polyethylene components and polypropylene components.
Examples of the polyethylene component include ultra high molecular weight polyethylene, high density polyethylene, low density polyethylene, and linear low density polyethylene. These may be used alone or in combination of two or more.
The viscosity average molecular weight (Mv) of the polyethylene component is preferably 100,000 or more and 2.5 million or less. Examples of the polyethylene component include ultra high molecular weight polyethylene (particularly homopolyethylene) having a viscosity average molecular weight (Mv) of 1,000,000 to 2,500,000, linear having an Mv of 100,000 to 300,000 and a melting point of 130 ° C. or less. Preferably it contains low density polyethylene.

Since ultra high molecular weight polyethylene has a high melt viscosity, it has the effect of improving kneadability with polypropylene during film formation, and is also effective in maintaining strength. A low molecular weight polyethylene having an Mv of less than 1 million has an effect of improving shutdown characteristics because of its low melting point.
The proportion of ultrahigh molecular weight polyethylene in the polyolefin resin is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 30% by mass or less in terms of moldability. On the other hand, when the linear low density polyethylene is used, the proportion of the linear low density polyethylene in the polyolefin resin is preferably 20% by mass or more, and preferably 40% by mass or less in terms of strength.

  Examples of the polypropylene component include homopolymers and copolymers copolymerized with an ethylene component. These may be used alone or in combination of two or more. In consideration of compatibility with the polyethylene component, a copolymer copolymerized with the ethylene component is preferably used rather than a homopolymer.

Examples of the copolymer include an ethylene propylene random copolymer and a block copolymer. The content of the ethylene component in the copolymer is preferably 2% by mass or more from the viewpoint of compatibility with the polyethylene component, and preferably 30% by mass or less from the viewpoint of maintaining the properties of polypropylene.
The polypropylene content of the film can be calculated using infrared absorption (IR) measurements. The ratio of the peak intensity (A ^PP ) at the absorption wavelength (988 cm ⁻¹ ) peculiar to polypropylene and the peak intensity (A ^PE ) at the absorption wavelength (719 cm ⁻¹ ) peculiar to polyethylene is taken as the vertical axis, and the mixing ratio of polypropylene is taken as the horizontal axis. By doing so, a quadratic curve is drawn. By obtaining an approximate expression from the curve, the polypropylene content can be calculated from the peak intensity of IR.
The polymerization catalyst for the polypropylene component used is not particularly limited, and examples of such a catalyst include Ziegler-Natta catalysts and metallocene catalysts.

The viscosity average molecular weight (Mv) of the polypropylene component is preferably 150,000 or more from the viewpoint of high temperature characteristics, and preferably 700,000 or less from the viewpoint of film quality.
The proportion of polypropylene in the polyolefin resin is preferably 5% by mass or more from the viewpoint of oxidation resistance, maintaining the pore diameter at high temperature, and improving the thermal film breaking temperature, and from the viewpoint of film forming property, 50% by mass or less. Is preferable, and 40% by mass or less is more preferable from the viewpoint of physical property balance between puncture strength and air permeability and film quality.

  Examples of the plasticizer include phthalic acid esters such as dioctyl phthalate (hereinafter referred to as DOP), diheptyl phthalate, and dibutyl phthalate; organic acid esters such as adipic acid ester and glyceric acid ester; trioctyl phosphate, etc. Phosphoric acid esters; liquid paraffin; solid wax; mineral oil and the like. In view of compatibility with polyethylene, a phthalate ester is particularly preferable. 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 from the viewpoint of strength and film formability. % 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 1% 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 properties.
As such a blending example, for example, 25 to 50 parts by mass of a polyolefin resin, 30 to 60 parts by mass of a plasticizer, and 10 to 40 parts by mass of an inorganic powder are blended so that the total is 100 parts by mass. Can be mentioned.
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 product is formed into a sheet by melt molding using, for example, a T die or a ring 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 process, as a cooling method of the melt-extruded mixture, for example, a method of cooling with air, a method of cooling by contacting with a roll adjusted to 20 to 120 ° C. lower than the die discharge resin temperature, a die It is possible to adopt a method of cooling while forming a sheet by rolling with a calender roll 20 to 120 ° C. lower than the discharged 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 20 to 120 ° C. lower than the die discharge resin temperature. A more preferable difference between the 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 die 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.

  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 view of high strength and thin film formation. In the case of biaxial stretching, either sequential biaxial stretching or simultaneous biaxial stretching may be used, but sequential biaxial stretching is preferred in order to obtain a medium to large pore film. 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. After stretching, it is preferable to perform heat treatment such as heat fixation or heat relaxation in order to improve heat shrinkage resistance.

The polyolefin microporous membrane of this embodiment can be used for separation of separators and substances in batteries, capacitors and the like. In particular, it can be suitably used as a separator for a non-aqueous electrolyte battery excellent in safety and practicality.
A battery (for example, a lithium ion secondary battery) formed using such a microporous membrane may be a cylindrical type or a rectangular type as long as it is a wound type battery, but is particularly suitable for a cylindrical type battery. High charge / discharge characteristics, cycle characteristics, and high safety are easily obtained.
The 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)
The measurement was performed at a room temperature of 23 ° C. using a micro thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark). 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
The film density of “Example / Comparative Example” was calculated as 0.95 (g / cm 3).

(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 / second, 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) Shutdown temperature (° C), thermal film breaking temperature (° C)
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. Further, the temperature at which the electric resistance value again fell below 1000Ω after the shutdown was defined as the thermal film breaking temperature.

(7) 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

(8) Melting point (° C)
Measurement was performed using DSC60 manufactured by Shimadzu Corporation. 3 mg of polymer 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. The melting peak obtained by the method described in JIS-K7121 in the DSC curve with the exotherm obtained by 1st heat as the positive direction was taken as the melting point.

(9) Ethylene content (% by mass)
It calculated | required based on the method as described in Macromolecules 1982,15,1150-1152 from the 13 C-NMR spectrum. A sample was prepared by uniformly dissolving about 200 mg of PP resin in 3 ml of orthodichlorobenzene in a 10 mmφ test tube, and the 13 C-NMR spectrum of the sample was measured under the following conditions.
Measurement temperature: 135 ° C
Pulse repetition time: 10 seconds Pulse width: 45 °
Integration count: 2500 times

(10) GPC measurement <Pretreatment>
After weighing the sample, an eluent was added to adjust the concentration to 0.5 mg / ml. Next, it melt | dissolved by standing (160 degreeC * 1hr) + rocking | fluctuation (160 degreeC * 3hr) with a high temperature dissolver. Then, it filtered with a 0.45 micron filter with the heating state (160 degreeC), and used the filtrate as the GPC measurement sample.

<Measurement condition>
Using Tosoh HLC-8121GPC / HT, measurement was performed under the following conditions, and a calibration curve was prepared using standard polystyrene. A molecular weight distribution curve in terms of polyethylene was obtained by multiplying each molecular weight component by 0.43 (Q factor of polyethylene / Q factor of polystyrene = 17.7 / 41.3).
Column: Tosoh TSKgel GMH _HR -H (20) HT (7.8 mm ID × 30 cm) 2 mobile phase: o-dichlorobenzene (containing 0.05% BHT)
Detector: differential refractometer Flow rate: 1.0 ml / min
Column temperature: All paths 155 ° C
Sample volume: 500 μl (0.5 mg / ml)

(11) IR measurement It measured on condition of the following using FTS-60A / 896 UMA300 by BIO RAD. The film sample had a thickness of 20 μm, and the background was measured before the measurement.
Measurement mode: Microscopic transmission method Detector: MCT
Beam splitter: Potassium bromide Scan speed: 20 kHz
Integration count: 64 times Resolution: 4 cm ^-1
Measurement wavelength: 4000 to 700 cm ⁻¹

(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. At this time, the active material application amount of the negative electrode was 106 g / m ² , and the active material bulk density was 1.55 g / cm ^3, which was a 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>
A strip-shaped negative electrode, a separator, a strip-shaped positive electrode, and a separator were stacked in this order, and the electrode plate laminate was fabricated by winding a plurality of times in a spiral shape with a winding tension of 250 gf. 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 8 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 3 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 3 hours as a pretreatment.

(12-1) Cycle characteristics (%)
The pretreated battery 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 after 500 cycles with respect to the initial capacity was evaluated as cycle characteristics.
Capacity retention: 95% or more and 100% or less: ○
Capacity retention 90% or more and less than 95%:
Capacity retention less than 90%: ×

(12-2) Blackening The pretreated battery was charged at a constant current up to a voltage of 4.2 V with a current of 1 C, and then charged at a constant voltage of 4.2 V at a temperature of 70 ° C. for 100 hours. The separator is removed from the battery, and ultrasonic cleaning is performed in dimethoxyethane, ethanol, and 1N hydrochloric acid for 15 minutes each to remove the deposits. Then, it dried in the air, the black discoloration condition of the positive electrode contact surface side of a separator was observed visually, and oxidation resistance evaluation was performed. The determination was made based on the ratio of black discoloration per area.
Less than 5%: ◎
5% or more and less than 10%: ○
10% or more and less than 20%: △
20% or more: ×

The raw materials used in the following examples and comparative examples are as follows.
PE (Mv 120,000): Mv 120,000, melting point 132 ° C., density 0.954 g / cm ³ and propylene unit content 1 mol% PE (Mv = 150,000): Mv 150,000, melting point 127 ° C. and density Is a low-density polyethylene PE having a viscosity of 0.926 g / cm ³ (Mv = 250,000): a high-density polyethylene PE having an Mv of 250,000, a melting point of 136 ° C., and a density of 0.957 g / cm ³ (Mv = 1 million): Mv100 Ultra high molecular weight polyethylene PE having a melting point of 135 ° C. and a density of 0.955 g / cm ³ (Mv = 2 million): Ultra high molecular weight polyethylene PPa having an Mv of 2 million, a melting point of 134 ° C. and a density of 0.936 g / cm ³ : Mv40 ten thousand melting point 148 ° C. at a density of 0.91 g / cm ³ and an ethylene content of 3% by weight of the random copolymer polypropylene PPb: Random polypropylene containing ethylene component as comonomer at 125 ° C PPc: block copolymerized polypropylene with Mv 350,000, melting point 160 ° C and ethylene content 6 mass% PPd: homopolypropylene with Mv 400,000 and melting point 163 ° C Silica: finely divided silica . Product name Nippon Sil LP, manufactured by Tosoh Silica

[Example 1]
45 parts by mass of DOP, 21 parts by mass of finely divided silica (trade name Nipsil LP, manufactured by Tosoh Silica Co., Ltd.), and BHT (dibutylhydroxytoluene) 0 as an antioxidant with respect to 34 parts by mass of the polyolefin resin obtained by blending the raw materials shown in Table 1 3 parts by mass and 0.3 part by mass of DLTP (dilauryl thiodipropionate) were mixed with a Henschel mixer and granulated. 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 fine silica was extracted with sodium hydroxide. Two films after the extraction were stacked and stretched 5.8 times in MD with a stretching roll heated to 120 ° C. (stretching after extraction), and then doubled in the TD direction in a tenter having a maximum temperature of 132.0 ° C. Stretched. Various characteristics were evaluated about the obtained polyolefin microporous film. The evaluation results are shown in the table below.

[Example 2]
Two membranes after extraction obtained by blending the raw materials shown in the following table are stacked and stretched 5.3 times to MD with a stretching roll heated to 126 ° C (stretching after extraction), and a tenter with a maximum temperature of 142 ° C. A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that the film was stretched twice in the TD direction. The evaluation results are shown in the table below.

[Examples 3-5, Comparative Examples 1-4]
A microporous membrane was obtained in the same manner as in Example 1 except for the conditions shown in the table below. Various characteristics of the obtained microporous membrane were evaluated. The evaluation results are shown in the table below.

[Example 6]
Two membranes after extraction obtained by blending the raw materials shown in the table below are stacked and stretched 5.3 times to MD (stretching after extraction) with a stretching roll heated to 126 ° C, and a tenter having a maximum temperature of 140 ° C. A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that the film was stretched twice in the TD direction. The evaluation results are shown in the table below.

[Comparative Example 5]
35 parts by mass of polyolefin resin obtained by blending the raw materials shown in the table below and 65 parts by mass of liquid paraffin were kneaded and extruded at 200 ° C. in a twin screw extruder equipped with a T die, and formed into a sheet having a thickness of 1000 μm. did. 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. Finally, liquid paraffin was extracted with methylene chloride to obtain a microporous membrane. Various characteristics of the obtained microporous membrane were evaluated. The evaluation results are shown in the table below.

[Comparative Example 6]
Two membranes after extraction obtained by blending the raw materials shown in the following table were stacked and stretched 4.6 times to MD (stretching after extraction) with a stretching roll heated to 126 ° C, and a tenter having a maximum temperature of 140 ° C. A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that the film was stretched 2.5 times in the TD direction. The evaluation results are shown in the table below.

[Comparative Example 7]
32 parts by mass of copolymerized polyethylene having an Mv of 800,000 density of 0.93 g / cm ³ and a propylene unit content of 1.4 mol%, 8 parts by mass of polypropylene copolymerized with an ethylene component and having a density of 0.9 g / cm ³ , DOP42. After 4 parts by mass and 17.6 parts by mass of finely divided silica were mixed and granulated in a super mixer, they were kneaded and extruded by a twin-screw extruder equipped with a T-die to form a sheet having a thickness of 100 μm. The molded product was immersed in methylene chloride to extract and remove DOP, and then immersed in an aqueous sodium hydroxide solution to extract and remove silica to form a microporous membrane. The microporous membrane was stretched 4 times in the longitudinal direction under heating to 120 ° C., and then stretched 1.5 times in the transverse direction. When various characteristics were evaluated about the obtained microporous film, compared with Examples 1-6, it was inferior to oxidation resistance and cycling characteristics.

  As described above, as shown in the examples, the microporous membrane of the present embodiment is excellent in oxidation resistance and cycle characteristics when used as a battery separator.

  According to the present invention, there is provided a polyolefin microporous membrane suitable as a separator capable of achieving both good oxidation resistance and good cycle characteristics.

Claims (7)

Including 5 to 50% by mass of a polypropylene component and 50 to 95% by mass of a polyethylene component, and the polyethylene component includes 10% by mass or more of ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 1 million or more,
A polyolefin microporous membrane having a temperature difference between the melting point Tme of the polyethylene component and the melting point Tmp of the polypropylene component of −20 ° C. <Tmp−Tme ≦ 20 ° C. and a bubble point of 400 to 600 kPa. 2. The polyolefin microporous membrane according to claim 1, wherein a fraction having a molecular weight of 1 million or more on a GPC curve is 5% or more, and an intensity ratio of polypropylene to polyethylene infrared absorption spectrum is 0.01 to 0.45. (Absorption wavelength of polypropylene: 998 cm ⁻¹ , absorption wavelength of polyethylene: 719 cm ⁻¹ )   The polyolefin microporous membrane according to claim 1 or 2, wherein the polypropylene component is a random copolymer of ethylene and propylene.   The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the air permeability / porosity ratio is 2.2 to 4.0.   The battery separator using the polyolefin microporous film in any one of Claims 1-4.   The lithium ion secondary battery containing the separator of Claim 5, a positive electrode, a negative electrode, and electrolyte 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 in which the kneaded product obtained in the kneading step is extruded from a slit, 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) a stretching step of stretching the sheet-like porous body obtained in the extraction step,
The step (1) is a step of mixing 25 to 50 parts by mass of a polyolefin resin, 30 to 60 parts by mass of a plasticizer, and 10 to 40 parts by mass of an inorganic powder so that the total becomes 100 parts by mass, The polyolefin resin contains 5 to 50% by mass of a polypropylene component and 50 to 95% by mass of a polyethylene component, and the polyethylene component contains 10% by mass or more of ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 1 million or more. The manufacturing method whose temperature difference of melting | fusing point Tme of the said polyethylene component and melting | fusing point Tmp of the said polypropylene component is -20 degreeC <Tmp-Tme <= 20 degreeC.