JP5235324B2 - Polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a polyolefin microporous membrane. More specifically, a polyolefin microporous membrane that is used for separation of separators and substances in batteries, capacitors, etc., and particularly suitable as a separator for non-aqueous electrolyte batteries with excellent safety and practicality at high temperatures, The present invention relates to a battery separator and a lithium ion secondary battery used.

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.
Lithium ion secondary batteries have high output density and capacity density, but because the electrolyte uses an organic solvent, the electrolyte decomposes due to heat generated by abnormal situations such as short circuit and overcharge. May cause fire. In order to prevent such a situation, some safety elements are incorporated in the lithium ion secondary battery, and one of them is a shutdown function of the separator. The shutdown function is a function that stops the progress of electrochemical reaction by suppressing the ionic conduction in the electrolytic solution by blocking the micropores of the separator due to thermal melting when the battery generates abnormal heat.

  The behavior of the polyolefin microporous membrane at the time of shut-down does not mean that the pores are closed at a time when a certain temperature is exceeded, but actually the pores are continuously closed from a temperature lower than the shutdown temperature. Once the hole has been closed, it remains closed even after returning to room temperature, so if the battery is exposed to high temperatures, the separator becomes smaller and the resistance increases even if it is below the shutdown temperature. As a result, the cycle characteristics and rate characteristics deteriorate. For this reason, batteries using conventional separators have limitations in use in the temperature conditions of hot press in the square battery manufacturing process and in sub-high temperature regions (80 to 120 ° C.) that are less than abnormal heat generation.

On the other hand, the shutdown function of the separator is indispensable as a means of protecting safety during thermal runaway due to battery failure. In the semi-high temperature range (80 to 120 ° C), the permeability is maintained with a large pore diameter, and the high temperature range ( There is a need for a separator having seemingly contradictory characteristics of being quickly invalidated at 130 ° C. or higher. In addition, physical strength is also required for separators to prevent foreign matter contamination and internal short-circuiting during dendrite growth, but until now, "permeability in quasi-high temperature range", "shutdown characteristics during abnormal heat generation" and "high There was no separator that had a well-balanced “strength” characteristic.
For example, in Patent Document 1, although the pore diameter at normal temperature is large, since polyethylene is a simple substance, the pore diameter is reduced in the quasi-high temperature region, the permeability is lowered, and the thermal film breaking temperature after shutdown is also low.

In Patent Document 2, since it is stretched at a high temperature, it has excellent heat shrinkage resistance and can maintain permeability even at a quasi-high temperature. However, since it does not contain a low melting point polyethylene component, it has a high shutdown temperature. The film temperature is also low.
Patent Document 3 proposes a non-aqueous electrolyte secondary battery that uses a separator with excellent permeability at high temperatures in order to increase safety during overcharge. There is a problem with safety at high temperatures because the shutdown function is not expressed.
In order to achieve both a low shutdown temperature and a high thermal film breaking temperature, a microporous membrane in which polyethylene having a low melting point and polypropylene having a high melting point are blended has been studied. For example, the microporous membrane of Patent Document 4 blends a low molecular weight polyethylene component and polypropylene, has a low shutdown temperature and a high thermal membrane breakage temperature. However, since the draw ratio is as low as 2 times, the strength is insufficient, and there is no description about permeability at a semi-high temperature.

Since the microporous membrane of Patent Document 5 also contains polyethylene and polypropylene, the shutdown temperature is low and the thermal membrane breaking temperature is also high. However, in this manufacturing method, the original pore diameter is small, so the permeability at the quasi-high temperature is low, and stretching is also performed. Since the magnification is as low as 3 times, the strength is insufficient.
Since the microporous membrane of Patent Document 6 contains polyethylene and polypropylene, and is made by a manufacturing method using raw materials consisting of a polymer, a plasticizer, and inorganic powder, it has a low shutdown temperature, a high thermal film breaking temperature, and a quasi-high temperature. The hole diameter can be maintained. However, since the draw ratio is as low as 3 to 6 and the porosity is as high as 59 to 75%, the strength is insufficient and there is a problem in safety.
Since the microporous membrane of Patent Document 7 also contains polyethylene and polypropylene, the shutdown temperature is low and the thermal membrane breaking temperature is high, but the original pore size is small, so the permeability at the quasi-high temperature is low, and the draw ratio is 6 The puncture strength is low because it is as low as ˜7.5 times.

Japanese Patent Laid-Open No. 2002-88188 WO2004-20511 JP 2002-231209 A Japanese Patent No. 2952017 Japanese Patent No. 3507092 Japanese Patent No. 3305006 JP-A-9-180699

  An object of the present invention is to provide a microporous polyolefin membrane having a well-balanced maintenance of pore diameter at sub-high temperature, shutdown characteristics at high temperature, and strength, and having high safety and practicality as a separator for a nonaqueous electrolyte battery. And

As a result of intensive research on the above problems, the inventors of the present invention have a specific range of puncture strength, shutdown temperature, and a polyolefin microporous membrane having a bubble point after being left at 120 ° C. has excellent permeability in a quasi-high temperature region. The present invention has been found to have excellent shutdown characteristics and high strength, and in particular, when used as a separator for a lithium ion secondary battery, is excellent in charge / discharge characteristics in a quasi-high temperature region and safety in a high temperature region. It came.
That is, the present invention is as follows.
(1) A polyolefin microporous membrane having a puncture strength of 3 to 8 N / 20 μm, a shutdown temperature of 130 to 140 ° C., and a bubble point after standing at 120 ° C. of 300 to 580 kPa ,
From a polyolefin resin composition in which the polyolefin resin contains only 30 to 70 parts by mass of polyethylene having a viscosity average molecular weight of 100,000 to 300,000, 10 to 55 parts by mass of polyethylene having a viscosity average molecular weight of 700,000 or more, and 5 to 50 parts by mass of polypropylene. A polyolefin microporous membrane.
(2) The polyolefin microporous film according to (1) , wherein the film thickness is 5 to 50 μm, the porosity is 30 to 70%, and the thermal film breaking temperature is 160 ° C. or higher.
( 3 ) The polyolefin microporous membrane according to (1) or (2), wherein the product of the longitudinal draw ratio and the transverse draw ratio is 8 or more.
( 4 ) Obtained by melting and kneading a composition containing 25 to 50 parts by mass of polyolefin, 30 to 60 parts by mass of plasticizer and 10 to 40 parts by mass of inorganic powder, and extracting the plasticizer and inorganic powder. The polyolefin microporous membrane according to any one of (1) to ( 3) above.
( 5 ) A battery separator using the polyolefin microporous membrane according to any one of (1) to ( 4 ) above.
( 6 ) A lithium ion secondary battery using the separator described in ( 5 ) above.

  The microporous membrane of the present invention has excellent permeability in a quasi-high temperature region, and has excellent shutdown characteristics and high strength. The battery separator using the microporous membrane of the present invention is excellent in charge / discharge characteristics in the semi-high temperature region and safety in the high temperature region.

The microporous membrane of the present invention will be described in detail below, particularly focusing on its preferred form.
The puncture strength of the microporous membrane of the present invention is 3 to 8 N / 20 μm, preferably 3.5 to 7 N / 20 μm. When used as a separator for lithium ion batteries, the higher the puncture strength, the more the foreign matter can be mixed in during the battery manufacturing process and the microporous membrane can be prevented from breaking when lithium dendrite grows during charging. Excellent. On the other hand, if it is too high, the separator is hard and the winding property of the battery may be deteriorated.
The shutdown temperature is 130 to 140 ° C, more preferably 135 to 140 ° C. The shutdown temperature is preferably low in terms of safety, but if it is too low, the hole diameter becomes small at the time of hot pressing in the rectangular battery manufacturing process or under a sub-high temperature that does not cause abnormal heat generation, and the battery characteristics such as cycle characteristics and rate characteristics are impaired. May be. On the other hand, if the shutdown temperature is too high, thermal runaway does not stop when the battery is abnormally heated, resulting in poor safety.

The bubble point after leaving at 120 ° C. is 300 to 580 kPa, more preferably 360 to 520 kPa. The bubble point after standing at 120 ° C. indicates the pore size of the microporous membrane at the quasi-high temperature. If this value is too small, the pore size is large and the battery characteristics are impaired due to self-discharge, etc. Battery characteristics such as cycle characteristics and rate characteristics are insufficient. The bubble point before being left at 120 ° C. is preferably 300 kPa or more from the viewpoint of suppressing self-discharge, and is preferably 550 kPa or less from the viewpoint of cycle characteristics and rate characteristics. More preferably, it is 360-500 kPa.
The film thickness 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 is preferably 30% or more from the viewpoint of permeability, and preferably 70% or less from the viewpoint of strength. A more preferable porosity is 40 to 60%.
The thermal film breaking temperature is preferably 160 ° C. or higher, more preferably 180 ° C. or higher from the viewpoint of safety.
The air permeability is preferably 10 sec or more from the viewpoint of safety and 500 sec or less from the viewpoint of ion permeability, and more preferably 50 to 150 sec.

The microporous membrane of the present invention can be suitably produced by a production method including the following (Step 1) to (Step 6).
(Step 1) A step of mixing a polyolefin resin, a plasticizer and, if necessary, an inorganic powder with a Henschel mixer or the like;
(Step 2) Step of melt kneading the mixture prepared in Step 1 in an extruder;
(Step 3) Step of extruding the kneaded product obtained in Step 2 from a T-die, rolling with a roll, cooling and forming into a sheet form;
(Step 4) Step of extracting and removing the plasticizer from the sheet-like molded product obtained in Step 3 and drying;
(Step 5) Step of extracting and removing inorganic powder from the sheet-like molded product obtained in Step 4 and drying;
(Step 6) Step of stretching and heat-treating the sheet-like molded product obtained in Step 5;
In the above (Step 1), it is preferable to add an inorganic powder from the viewpoint of uniformity of pore diameter and strength of the resulting microporous membrane. Even when no inorganic powder is added, it is known that a uniform pore diameter can be obtained by reversing the order of (Step 4) and (Step 6) and extracting the plasticizer after stretching. The microporous membrane obtained by the manufacturing method has a small pore size. For this reason, in order to obtain a microporous film having a uniform and large pore diameter, it is preferable to add inorganic powder to the raw material, extract the plasticizer and the inorganic powder, and then stretch the film. Examples of the inorganic powder used in the production method of the present invention include silica, calcium silicate, aluminum silicate, alumina, calcium carbonate, magnesium carbonate, kaolin clay, talc, titanium oxide, carbon black, and diatomaceous earth. From the viewpoint of dispersibility and ease of extraction, it is particularly preferable to use silica.

  The polyolefin resin used in the microporous membrane of the present invention is obtained by blending, for example, a low molecular weight polyethylene having a viscosity average molecular weight of 100,000 or more and 300,000 or less, a high molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more, and polypropylene at a specific ratio. be able to. Low molecular weight polyethylene has a low melting point, so it improves the shutdown characteristics, high molecular weight polyethylene improves the strength, and polypropylene has an effect on maintaining the pore size at high temperatures and improving the thermal film breaking properties. The low molecular weight polyethylene having a viscosity average molecular weight of 100,000 to 300,000 is preferably 30 parts by mass or more for obtaining a shutdown temperature of 130 to 140 ° C., and preferably 70 parts by mass or less for preventing a decrease in strength. More preferably, it is 50-64 mass parts. The high molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more is preferably 10 parts by mass or more from the viewpoint of strength improvement, and 55 parts by mass or less is preferable so that the shutdown temperature does not become too high. More preferably, it is 20-40 mass parts. Polypropylene is preferably 5 parts by mass or more for maintaining the pore diameter at high temperatures and improving the thermal film breaking temperature, and is preferably 50 parts by mass or less because the strength decreases as the content increases. More preferably, it is 5-30 mass parts.

The molecular weight of the polypropylene used in the present invention is not particularly limited, but if a low molecular weight polypropylene having a viscosity average molecular weight of 50,000 or less is used, the pore size tends to increase, and a high molecular weight polypropylene having a viscosity average molecular weight of 300,000 or more. When is used, it is possible to obtain a microporous film excellent in heat-resistant film breaking property. These may be used alone or as a mixture.
Plasticizers used in the present invention are 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; trioctyl phosphate, etc. 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.

The blend ratio of the polyolefin resin, the plasticizer, and the inorganic powder in (Step 1) is not particularly limited, but the polyolefin resin concentration in the raw material is preferably 25 to 50 parts by mass from the viewpoint of strength and film formability. Since the viscosity suitable for extrusion is obtained, the plasticizer density | concentration in a raw material is 30-60 mass parts. The concentration of the inorganic powder is preferably 10 parts by mass or more in order to obtain a uniform pore size, and preferably 40 parts by mass or less from the viewpoint of film forming properties.
In addition to polyolefins, inorganic fine powders, and plasticizers, various additives such as antioxidants, antistatic agents, ultraviolet absorbers, lubricants, antiblocking agents and the like are added as necessary within a range that does not significantly impair the present invention. be able to.

The mixing of the three components of polyolefin, inorganic fine powder, and plasticizer in (Step 1) is performed using a general mixer such as a Henschel mixer, a V-blender, a pro shear mixer, or a ribbon blender.
In (Step 2), the mixture is kneaded by a melt kneader such as an extruder or a kneader. The obtained kneaded material is formed into a sheet by 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.
In (Step 3), as a cooling method, a method of cooling with air, a method of cooling by contact with a roll adjusted to 20 to 120 ° C. lower than the temperature of the T-die discharge resin, and 20 to less than the temperature of the T-die discharge resin. A method of cooling while forming a sheet by rolling with a calender roll having a temperature of 120 ° C. can be employed. 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. 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 (Step 4), the plasticizer in the film is extracted. Solvents used for extraction of plasticizers include organic solvents such as methanol, ethanol, methyl ethyl ketone, and acetone; ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran; halogens such as methylene chloride and 1,1,1-trichloroethane. Hydrocarbons can be used. These can be used alone or in combination.
In (Step 5), inorganic powder is extracted. When silica is used for the inorganic powder, an aqueous alkali solution such as sodium hydroxide or potassium hydroxide is preferably used as the solvent.
In (Step 6), 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. The stretching ratio is preferably biaxial stretching considering high strength and thinning. The draw ratio is preferably 8 times or more, more preferably 8.5 times or more in terms of surface magnification for improving the strength. In the case of biaxial stretching, either sequential biaxial stretching or simultaneous biaxial stretching may be used, but sequential biaxial stretching is preferable in order to obtain a film having a large pore diameter. 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.

When the microporous membrane of the present invention is used as a battery separator, for example, a battery may be prepared by the following method.
First, the microporous membrane is formed into a vertically long shape having a width of 10 mm to 100 mm and a length of 200 mm to 2000 mm. The separator is stacked in the order of positive electrode-separator negative electrode-separator, or negative electrode-separator positive electrode-separator, and wound into a circular or flat spiral shape. Further, the wound body is housed in a battery can and an electrolyte is injected.
Although the kind of battery in this invention is not specifically limited, From a viewpoint of the affinity of a polyolefin microporous membrane and electrolyte solution, it is preferable that it is a non-aqueous electrolyte battery. Moreover, it is more preferable that it is a lithium ion battery from a viewpoint that the outstanding safety | security can be provided when the microporous film of this invention is used as a separator.

Next, the present invention will be described in more detail with reference to examples. The test methods in the examples are as follows.
(1) Film thickness It measured using dial gauge "PEACOCK No.25" (Ozaki Seisakusho make, 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 It measured using the Gurley type air permeability meter based on JIS P-8117.
(3) Porosity A sample was cut into a size of 100 mm × 100 mm to obtain a volume (cm ³ ) and a 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 The puncture strength was measured 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 value obtained by converting the maximum puncture load per 20 μm was defined as the puncture strength.
(5) Calculated in ethanol according to Bubble Point ASTM E-128-61. After standing at 120 ° C., the bubble point was measured by cutting the sample into 100 mm × 100 mm, sandwiching it between two sheets of paper so that the hot air did not directly hit the sample, leaving it in an oven at 120 ° C. for 1 hour, taking it out and cooling it.

(6) Shutdown temperature, thermal membrane breakage temperature A multilayer porous membrane sufficiently impregnated with a prescribed 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 was continuously raised after the shutdown, and the temperature when the film melted and fell below 1000Ω 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) Battery evaluation A cylindrical battery was prepared according to the following procedure.
<Preparation of positive electrode> 92.2 wt% of lithium cobalt composite oxide LiCoO ₂ as an active material, 2.3 wt% of flake graphite and acetylene black as a conductive agent, and 3.2 wt% of polyvinylidene fluoride (PVDF) as a binder Was dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. 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 40 mm to form a strip.

<Preparation of negative electrode> A slurry was prepared by dispersing 96.9 wt% of artificial graphite as an active material, 1.4 wt% of ammonium salt of carboxymethylcellulose and 1.7 wt% of styrene-butadiene copolymer latex as binder. 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 40 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.

<Battery assembly> An electrode plate laminate was prepared by winding a strip-shaped negative electrode, a separator, a strip-shaped positive electrode, and a separator in this order and winding them in a spiral shape. 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 85 ° C. for 12 hours under vacuum, and then the above-described non-aqueous electrolyte was poured into a container in an argon box and sealed.
<Pretreatment> The assembled battery was charged at a constant current of 1/3 C to a voltage of 4.2 V, then charged at a constant voltage of 4.2 V for 5 hours, and then a final voltage of 3.0 V at a current of 1/3 C. Discharge was performed. 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.

(8-1) The battery pretreated in the high temperature history test (8) was put into an oven, heated from room temperature at 5 ° C./min, and then left at 120 ° C. for 30 minutes. Thereafter, the battery was taken out of the oven and allowed to cool to room temperature, and then discharged to 3.0 V at a current of 1 C, and the capacity retention rate with respect to the amount of charge before charging the oven was calculated. The case where the capacity retention rate was 80% or more was rated as ○, and the case where the capacity retention rate was 80% or less was rated as x.
(8-2) Oven Test The battery pretreated in (8) was put into an oven, heated from room temperature at 5 ° C./min, and allowed to stand at 150 ° C. for 30 minutes. Those that ignited at this time were marked with ×, and those that did not ignite were marked with ○.

[Example 1]
36 wt% polyethylene with a viscosity average molecular weight of 150,000, 27 wt% polyethylene with a viscosity average molecular weight of 300,000, 18 wt% polyethylene with a viscosity average molecular weight of 1 million, 9 wt% polyethylene with a viscosity average molecular weight of 2 million, 10 wt% high molecular weight polypropylene with a viscosity average molecular weight of 400,000 % Of a polymer consisting of 34% by weight, 45% by weight of DOP and 21% by weight of fine silica were mixed by a Henschel mixer and granulated. Then, it knead | mixed and extruded with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 100 micrometers. From the molded product, DOP was extracted with methylene chloride and finely divided silica was extracted with sodium hydroxide to obtain a microporous membrane. Two microporous membranes were stacked and heated to 120 ° C. and stretched 5.0 times in the longitudinal direction, and then heated to 130 ° C. and stretched 2.0 times in the transverse direction. Table 1 shows the physical properties of the obtained film.

[Example 2]
The polymer composition is 32 wt% polyethylene having a viscosity average molecular weight of 150,000, 24 wt% polyethylene having a viscosity average molecular weight of 300,000, 16 wt% polyethylene having a viscosity average molecular weight of 1 million, 8 wt% polyethylene having a viscosity average molecular weight of 2 million, and a high viscosity average molecular weight of 400,000. It was prepared in the same manner as in Example 1 except that a polyolefin resin having a molecular weight of 20 wt% was used. Table 1 shows the physical properties of the obtained film.

[Example 3]
The polymer composition is 24 wt% polyethylene with a viscosity average molecular weight of 150,000, 18 wt% polyethylene with a viscosity average molecular weight of 300,000, 12 wt% polyethylene with a viscosity average molecular weight of 1 million, 6 wt% polyethylene with a viscosity average molecular weight of 2 million, and a high viscosity average molecular weight of 400,000. It was prepared in the same manner as in Example 1 except that a polyolefin resin having a molecular weight of polypropylene of 40 wt% was used. Table 1 shows the physical properties of the obtained film.

[Example 4]
It was prepared in the same manner as in Example 1 except that polypropylene having a viscosity average molecular weight of 6,000 was used instead of high molecular weight polypropylene having a viscosity average molecular weight of 400,000. Table 1 shows the physical properties of the obtained film.

[Example 5]
It was prepared in the same manner as in Example 1 except that a polyolefin resin having a polymer composition of 63 wt% of polyethylene having a viscosity average molecular weight of 150,000, 27 wt% of polyethylene having a viscosity average molecular weight of 2 million, and 10 wt% of high molecular weight polypropylene having a viscosity average molecular weight of 400,000 was used. . Table 1 shows the physical properties of the obtained film.

[Comparative Example 1]
Except for using a polyolefin resin having a polymer composition of 40 wt% polyethylene having a viscosity average molecular weight of 150,000, 30 wt% polyethylene having a viscosity average molecular weight of 300,000, 20 wt% polyethylene having a viscosity average molecular weight of 1 million, and 10 wt% polyethylene having a viscosity average molecular weight of 2 million. Prepared in the same manner as in Example 1. Table 1 shows the physical properties of the obtained film.

[Comparative Example 2]
The polymer composition is 16 wt% polyethylene having a viscosity average molecular weight of 150,000, 12 wt% polyethylene having a viscosity average molecular weight of 300,000, 8 wt% polyethylene having a viscosity average molecular weight of 1 million, 4 wt% polyethylene having a viscosity average molecular weight of 2 million, and a high viscosity average molecular weight of 400,000. It was prepared in the same manner as in Example 1 except that a polyolefin resin having a molecular weight of 60 wt% was used. Table 1 shows the physical properties of the obtained film. Since this membrane had a higher polypropylene content than the high molecular weight polyethylene, a sufficient strength as a battery separator could not be obtained, so the battery was not evaluated.

[Comparative Example 3]
It was prepared in the same manner as in Example 1 except that a polyolefin resin having a polymer composition of 25 wt% polyethylene having a viscosity average molecular weight of 250,000 and 75 wt% polyethylene having a viscosity average molecular weight of 1 million was used. Table 1 shows the physical properties of the obtained film.

[Comparative Example 4]
It was prepared in the same manner as in Example 1 except that the draw ratio was 3.5 times in the vertical direction and 2.0 times in the horizontal direction. Table 1 shows the physical properties of the obtained film. Since this film had a low draw ratio and could not provide sufficient strength as a battery separator, battery evaluation was not performed.

[Comparative Example 5]
45 wt% polyethylene with a viscosity average molecular weight of 250,000, 45 wt% polyethylene with a viscosity average molecular weight of 1 million, 40 wt% polymer composed of a mixture of 10 wt% high molecular weight polypropylene with a viscosity average molecular weight of 40 wt%, and 60 wt% liquid paraffin A gel sheet having a thickness of 1850 μm was obtained by kneading and extruding with a twin-screw extruder. Next, it was led to a simultaneous biaxial tenter stretching machine and subjected to simultaneous biaxial stretching at a magnification of 7 × 7 under heating at 115 ° C. Next, it was sufficiently immersed in methylene chloride to extract and remove the liquid paraffin to form a microporous membrane. Table 1 shows the physical properties of the obtained microporous membrane.

  As described above, as shown in the Examples, it was found that the microporous membrane of the present invention is excellent in charge / discharge characteristics at a semi-high temperature and safety at a high temperature when used as a separator for a non-aqueous electrolyte secondary battery. From the comparative example, a separator with a bubble point of greater than 580 kPa after standing at 120 ° C. has insufficient charge / discharge characteristics at a semi-high temperature, and a separator with a shutdown temperature greater than 140 ° C. has insufficient battery safety at a high temperature. I understood it.

  The microporous membrane of the present invention has excellent permeability in a quasi-high temperature region, and has excellent shutdown characteristics and high strength. The battery separator using the microporous membrane of the present invention is excellent in charge / discharge characteristics in the semi-high temperature region and safety in the high temperature region. Therefore, it can be particularly suitably used as a separator for a non-aqueous electrolyte secondary battery of a hybrid vehicle that may be exposed to a sub-high temperature.

Claims (6)

A polyolefin microporous membrane having a puncture strength of 3 to 8 N / 20 μm, a shutdown temperature of 130 to 140 ° C., and a bubble point after standing at 120 ° C. of 300 to 580 kPa ,
From a polyolefin resin composition in which the polyolefin resin contains only 30 to 70 parts by mass of polyethylene having a viscosity average molecular weight of 100,000 to 300,000, 10 to 55 parts by mass of polyethylene having a viscosity average molecular weight of 700,000 or more, and 5 to 50 parts by mass of polypropylene. A polyolefin microporous membrane.   The polyolefin microporous membrane according to claim 1, having a thickness of 5 to 50 µm, a porosity of 30 to 70%, and a thermal membrane breaking temperature of 160 ° C or higher. The polyolefin microporous membrane according to claim 1 or 2 , wherein the product of the longitudinal draw ratio and the transverse draw ratio is 8 or more. A composition obtained by melting and kneading a composition containing 25 to 50 parts by mass of polyolefin, 30 to 60 parts by mass of a plasticizer and 10 to 40 parts by mass of an inorganic powder, extracting the plasticizer and the inorganic powder, and then stretching the composition. The polyolefin microporous membrane according to any one of 3 to 4 . A battery separator using the microporous polyolefin membrane according to any one of claims 1 to 4 . A lithium ion secondary battery using the separator according to claim 5 .