JPWO2020137336A1 - Polyolefin microporous membrane and method for producing polyolefin microporous membrane - Google Patents

Abstract

A microporous polyolefin membrane having an air permeation resistance of 300 seconds / 100 cm3 or more and 500 seconds / 100 cm3 or less, a porosity of 18% or more and 30% or less, a tensile elongation in TD of 105% or more, and a film thickness of less than 6 μm.

Description

The present invention relates to a polyolefin microporous membrane and a method for producing a polyolefin microporous membrane.

Microporous membranes are used in various fields such as filters for filtration membranes and dialysis membranes, separators for batteries and separators for electrolytic capacitors. Among these, microporous membranes made of polyolefin are widely used as separators for secondary batteries because they are excellent in chemical resistance, insulation, mechanical strength, etc., and have shutdown characteristics.

When an external short circuit occurs in a battery, a large current flows momentarily, which may generate heat or gas. Therefore, the separator is required to have a shutdown characteristic in which the holes are closed when abnormal heat is generated to shut off the battery reaction. In addition, it is assumed that the secondary battery will be subject to impact such as dropping or compression during use, and there is a risk that the battery will short-circuit due to contact between the electrodes due to deformation of the electrodes. The tensile elongation is required to be able to follow the above. Furthermore, with the extension of life of mobile phones and electric vehicles in recent years, secondary batteries are required to have cycle characteristics that maintain the same level of capacity as the initial stage of battery use even if charging and discharging are repeated many times. Therefore, the separator is required to have a high porosity.

In Reference Experiment 2 of Patent Document 1, a polyethylene separator having a film thickness of 4 μm and an air permeation resistance of 380 seconds or 420 seconds is disclosed.

In Example 2 of Patent Document 2, two-stage simultaneous biaxial stretching was performed using the raw sheet from which the plasticizer was extracted and removed, and the film thickness was 2.7 μm, the air permeation resistance was 315 seconds / 100 ml, and the porosity was 16. % And 36% elongation separators are disclosed.

In Example 6 of Patent Document 3, Mw by using a 50 wt% 1.6 × 10 ⁶ polypropylene of high density Porichiren and Mw of 5.6 × 10 ⁵ as a raw material for an intermediate layer of the three-layered separator, By performing wet stretching at 113 ° C. and then re-stretching the TD 1.6 times at a temperature of 124 ° C., a multilayer separator having a film thickness of 6 μm and an air permeation resistance of 470 seconds / sec can be obtained. It is disclosed.

Also in Patent Document 4, in Example 1, 7.5 × 10 ⁵ of the first polyethylene resin 60 wt%, 1.9 × 10 ⁶ Mw of the second three-layer separator of polyethylene resin 40 wt% of the Mw Wet-stretched at 112.5 ° C. and then re-stretched 1.2 times over TD at a temperature of 122 ° C. to achieve a film thickness of 20 μm and a porosity of 280. It is disclosed that a multilayer separator having a porosity of 49% and an elongation of 150% can be obtained at seconds / sec.

Japanese Unexamined Patent Publication No. 2006-32246 Japanese Unexamined Patent Publication No. 11-60790 International release 2015/182689 Special Table 2013-517152

A secondary battery, for example, a lithium ion secondary battery, has a high energy density and is therefore widely used as a battery used in personal computers, mobile phones, and the like. Secondary batteries are also expected as a power source for driving motors of electric vehicles and hybrid vehicles.

In recent years, as the volume of electrodes has increased due to the increase in energy density of secondary batteries, there has been a demand for thinner separators. However, when the film thickness of the separator is thin, the amount of resin constituting the separator is small, so that the shutdown function is less likely to occur as compared with the separator having a thick film thickness. In addition, the porosity is a physical property that requires design changes according to the ease of passage of ions and the required characteristics related to dendrites, but the thinner the film, the weaker the strength of the film itself, making it difficult to use as an independent film. Therefore, the range of adjustment for high porosity is limited. Therefore, the thinner the separator, the more difficult it becomes to achieve both the shutdown function and the porosity. Further, in general, the thinning and the high porosity are adjusted by increasing the draw ratio, but the crystallization of the polyolefin resin proceeds due to the high stretching, so that the obtained separator has a small tensile elongation.

As described above, it becomes more difficult to balance the shutdown function, high tensile elongation and high porosity with the thin film separator. The above-mentioned Patent Documents 1 to 4 do not mention a thin film separator having an excellent shutdown function and a balance between tensile elongation and porosity.

An object of the present invention is a microporous polyolefin membrane having a specific air permeability resistance, porosity and tensile elongation, which is a thin film of less than 6 μm, and has an excellent balance of shutdown characteristics, cycle characteristics and impact resistance. And its manufacturing method.

The conventional shutdown function evaluates the shutdown temperature, and it is preferable to stop the current flow at the earliest possible stage (low temperature). In the case of the thin film separator, the present inventors cannot sufficiently exert the function only by lowering the shutdown temperature, and in the thin film separator, the temperature rise rate of the battery is moderated by setting the air permeation resistance within a certain range. It was found that a sufficient shutdown function can be expressed by using. (1) The air permeation resistance is 300 seconds / 100 cm ³ or more and 500 seconds / 100 cm ³ or less, the porosity is 18% or more and 30% or less, the tensile elongation in the width direction is 105% or more, and the film thickness is less than 6 μm. Polyolefin microporous membrane.
(2) The polyolefin microporous membrane according to (1), wherein the polyolefin microporous membrane contains 50% by mass or more of long-chain branched polyethylene having a molecular weight distribution of 10 or more and 20 or less.
(3) The polyolefin microporous membrane according to (1) or (2), which has a tensile strength in the mechanical direction of 230 MPa or more.
(4) The polyolefin microporous membrane according to (1) to (3), wherein the tensile elongation in the width direction is 150% or more.
(5) The polyolefin microporous membrane according to any one of (1) to (4), wherein the heat absorption amount for melting measured by a differential scanning calorimeter is 200 J / g or less.
(6) A battery using the separator containing the polyolefin microporous membrane according to any one of (1) to (5).
(7) A step of melt-kneading a polyolefin resin containing long-chain branched polyethylene with a plasticizer to obtain a resin solution.
The step of discharging the resin solution from the die and cooling it to form a gel-like sheet, and
A step of simultaneously biaxially wet-stretching the gel-like sheet at 120 ° C. or lower and a surface magnification of 25 times or lower.
A step of extracting a plasticizer from the gel-like sheet after wet stretching and drying it.
A method for producing a microporous polyolefin membrane, which comprises.
(8) The method for producing a microporous polyolefin membrane according to (7), which comprises a step of second heat fixing at a surface magnification of 1.0 times at 130 ° C. or lower after the drying step.

According to the present invention, it is possible to provide a polyolefin microporous membrane having a specific air permeation resistance, porosity and tensile elongation, which is a thin film of less than 6 μm, and which has a rapid shutdown function. The polyolefin microporous membrane of the present invention can impart excellent balance characteristics such as quick shutdown performance, cycle characteristics and impact resistance to a battery using the polyolefin microporous membrane, and is a thin film suitable for increasing the capacity of the battery. It is a film that can guarantee high safety.

The curve of the air permeation resistance with respect to the temperature and the linear approximation straight line in the microporous membrane obtained in Example 1 and Comparative Example 5 are shown. The curve of the air permeation resistance with respect to the temperature and the linear approximation straight line in the microporous membranes obtained in Example 2 and Comparative Examples 1 and 6 are shown. The curve of the air permeation resistance with respect to the temperature and the linear approximation straight line in the microporous membrane obtained in Example 3 and Comparative Example 3 are shown. The curve of the air permeation resistance with respect to the temperature and the linear approximation straight line in the microporous membranes obtained in Example 4 and Comparative Examples 2 and 4 are shown. It is a characteristic figure which shows the DSC curve obtained in Example 1. FIG. It is a characteristic figure which shows the DSC curve obtained in the comparative example 1. FIG.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the embodiments described below.

1. 1. Polyolefin microporous membrane The microporous membrane of the present invention has an air permeation resistance of 300 seconds / 100 cm ³ or more and 500 seconds / 100 cm ³ or less, a porosity of 18% or more and 30% or less, and a tensile elongation in the width direction of 105%. As described above, the polyolefin microporous membrane having a film thickness of less than 6 μm. The present invention can be obtained by using a raw material described later and adjusting the stretching to a range in which the amount of heat absorption for melting of the microporous membrane obtained by DSC measurement is easily melted and the membrane strength is not significantly reduced.

[Air permeation resistance]
The lower limit of the air permeation resistance of the microporous membrane less than 6 μm is 300 seconds / 100 cm ³ or more, and the upper limit is 500 seconds / 100 cm ³ or less. If the air permeation resistance of the microporous membrane is lower than the lower limit, the shutdown function is deteriorated. Further, when the air permeation resistance of the microporous membrane exceeds the upper limit, it becomes difficult for ions to move between the electrodes, which leads to an increase in impedance and a decrease in battery cycle characteristics and rate characteristics.

The lower limit of the air permeation resistance of the microporous membrane ^{is preferably 350 seconds / 100 cm 3} or more. The upper limit is preferably 450 seconds / 100 cm ³ or less, and more preferably 420 seconds / 100 cm ³ or less. When the air permeation resistance is within the above range, when the microporous membrane is used as a separator for a secondary battery, the ion permeability is excellent, so that the impedance of the secondary battery changes at a low level and the battery output becomes good. .. The air permeation resistance can be set in the above range by adjusting the amount of heat absorption for melting of the microporous membrane obtained by the DSC measurement described later to a range in which the amount of heat absorbed by melting is easily melted and the membrane strength is not significantly reduced.

The air permeability of a thick-film secondary battery separator obtained by converting the air permeability of a microporous membrane of less than 6 μm to 10 μm or more is in the range that is said to be preferable from the viewpoint of maintaining output characteristics and cycle characteristics. It is out of the upper limit. It is not preferable to use the present invention in the thick film separator because it becomes difficult for ions to move in the battery. Since the microporous membrane is a thin film, the battery safety can be improved without impairing the output characteristics of the battery, and therefore the above range is effective.

In the present invention, the reason why the air permeation resistance is intentionally set in such a range is that it is difficult for air to pass through the microporous membrane in the film thickness direction, in other words, ions pass through the microporous membrane. This means that the flow path is long (residence time is long) and the pore diameter is small. As a result, when the microporous membrane is incorporated into the battery, the time required for ions to pass through the microporous membrane is set as long as possible, so that the temperature rise rate of the battery can be suppressed when an external short circuit occurs. Then, even if it is a thin film, the shutdown function can be exhibited before the melting film of the separator occurs due to the temperature rise of the battery.

The air permeability resistance of the microporous membrane can be obtained by converting the value measured by a method conforming to JIS P-8117 using an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) to 6 μm. ..

[Porosity]
The lower limit of the porosity of the microporous membrane is 18% or more, and the upper limit is 30% or less. If the porosity of the microporous membrane is less than 18%, the cycle characteristics will deteriorate. Further, if the porosity of the microporous membrane exceeds 30%, the shutdown function is deteriorated. In the present invention, as will be described later, long-chain branched polyethylene having a long branched chain is used as a raw material for the microporous membrane, and even if the low molecular weight component in the long-chain branched polyethylene is inside the microporous membrane after production. I make it easy to move. The amount of heat absorption for melting of the microporous membrane obtained by the DSC measurement described later is adjusted to a range that facilitates melting and does not significantly reduce the membrane strength so that the long-chain branched polyethylene contributes to pore formation and melting behavior. Therefore, a large number of small pores are formed by the long-chain branched polyethylene, and as a result, a high porosity is achieved. Therefore, it is possible to obtain a thin film in which ions can sufficiently pass through the microporous membrane and have excellent cycle characteristics, and the shutdown function works quickly.

The porosity can be calculated from the film thickness, area, mass, and density of the microporous membrane (for example, in the case of polyethylene alone, it is 0.99 g / cm ^3). The film thickness, area, and mass of the microporous membrane were measured, and the porosity was calculated by the following formula.
Porosity (%) = 1-mass / (film thickness x area x density)
The density of the polyolefin microporous membrane can be determined by a method based on K7112: 1999.

[Tensile elongation]
The lower limit of the tensile elongation in the width direction (TD) of the microporous membrane is 105% or more. If the tensile elongation of the TD of the microporous membrane is less than the lower limit, safety such as impact resistance will be reduced. The lower limit of the tensile elongation of the TD of the microporous membrane is preferably 150% or more, and more preferably 180% or more. The upper limit of the tensile elongation of the TD of the microporous membrane is preferably 300% or less, more preferably 200% or less.

When an impact is applied to the battery due to dropping or compression, the microporous membrane must stretch well in TD in order for the microporous membrane to follow the deformation of the electrodes and suppress the short circuit of the battery. Is as above.

Inside the battery, the MD of the microporous membrane is the same as the battery winding direction, and the dimension of the TD of the microporous membrane corresponds to the dimension in the width direction of the battery. Therefore, when an impact is applied to the battery, if the balance between strength and elongation (toughness) is adjusted so that the TD of the microporous membrane has sufficient tensile elongation, the impact will be generated. Even if the electrodes are deformed, the microporous film can follow the deformation, so that the electrodes are not exposed at both ends in the width direction of the battery, and a short circuit is prevented. Further, by setting the elongation in such a range, when the microporous film is used as the separator, the separator can follow the unevenness of the electrode, the deformation of the battery, the generation of internal stress due to the heat generation of the battery, and the like.

The lower limit of the tensile elongation of the MD of the microporous membrane is preferably 50% or more, more preferably 100% or more, and further preferably 110% or more. The upper limit of the tensile elongation of the MD of the microporous membrane is preferably 200%, more preferably 150% or less, and even more preferably 120% or less. When the breaking elongation of the MD of the microporous film is within the above range, it is difficult to deform even when a high tension is applied when coating the coat layer, and wrinkles are unlikely to occur, so coating defects are suppressed and the coating surface is coated. It is preferable because the flatness of the is good.

The tensile elongation can be measured by a method according to ASTM D-882A using a strip-shaped microporous membrane having a chuck-to-chuck distance of 20 mm and a width of 10 mm at room temperature of 25 ° C.

[Film thickness]
The upper limit of the film thickness of the microporous membrane is less than 6 μm. If it exceeds the upper limit, the energy density per built-in battery volume will decrease. The upper limit of the film thickness of the microporous membrane is preferably 5 μm or less, and the lower limit is preferably 1 μm or more, more preferably 3 μm or more. When the film thickness of the microporous membrane is within the above range, when the microporous membrane is used as a battery separator, the number of turns of the laminate composed of the positive electrode, the negative electrode and the separator can be increased inside the battery package. Battery capacity is improved. That is, by using the thin film separator, the energy density per unit volume of the battery can be increased. The film thickness of the microporous membrane can be measured with a contact thickness gauge.

[Tensile strength]
The lower limit of the mechanical direction (MD) tensile strength of the microporous membrane is preferably 230 MPa or more, more preferably 250 MPa or more. The upper limit of the tensile strength of the MD of the microporous membrane is preferably 400 MPa or less, more preferably 300 MPa or less. When the tensile strength of the MD of the microporous membrane is within the above range, it is difficult to break even when a high tension is applied, and it can be used in applications requiring high durability. For example, when a microporous membrane having excellent strength as described above is used as a separator, short circuits during battery fabrication and battery use can be suppressed, and the separator can be wound while applying high tension, resulting in high battery capacity. Can be achieved. Further, when a coating layer is formed on at least one surface of the thinned microporous film, higher MD tensile strength is required. Therefore, from the viewpoint of improving the coatability of the coating layer, when the tensile strength of the MD is within the above range, it can be suitably used as a base material for coating.

The lower limit of the tensile strength of the TD of the microporous membrane is preferably 100 MPa or more, more preferably 150 MPa or more. The upper limit of the tensile strength of the TD of the microporous membrane is preferably 300 MPa or less.

[Melting heat absorption of microporous membrane (ΔH)]
The upper limit of the heat absorption amount (ΔH) for melting of the microporous membrane is preferably 200 J / g or less, more preferably 190 J / g or less, and further preferably 180 J / g or less. Here, the heat absorption of fusion (ΔH) of the microporous film is the heat of fusion per weight of the microporous film when the area of the heat of fusion curve obtained by measurement with a differential scanning calorimeter is taken as the heat of fusion.

When the amount of heat absorbed by the microporous film is within the above range, softening or melting is likely to occur even if the resin constituting the microporous film absorbs a very small amount of heat. Therefore, the shutdown function is promptly performed, and the microporous membrane is easily stretched. In order to obtain a microporous film exhibiting such characteristics, long-chain branched polyethylene described later is used as a raw material, and manufacturing conditions such as stretching temperature and stretching ratio are set so that this resin does not crystallize as much as possible. It is necessary. Therefore, since the long-chain branched polyethylene keeps such a state, the polyethylene contributes to the formation of fibrils, so that the tensile elongation in TD is secured so as to have resistance to the battery when an impact is applied. However, the porosity can be ensured until good cycle characteristics are obtained. On the other hand, if the melt heat absorption amount is too small, the resin may not easily flow to a level that closes the pores even if the resin softens and melts. Therefore, the lower limit of the melt heat absorption amount is 100 J / g or more.

[Maximum pore diameter and average flow pore diameter]
The upper limit of the maximum pore diameter is preferably 50 nm or less, more preferably 40 nm or less, and further preferably 35 nm or less. The lower limit of the maximum pore diameter is preferably 15 nm or more, and more preferably 20 nm or more.

The upper limit of the average flow hole diameter is preferably 40 nm or less, more preferably 30 nm or less, and further preferably 25 nm or less. The lower limit of the average flow hole diameter is preferably 10 nm or more, more preferably 15 nm or more, and further preferably 20 nm or more.

The average flow hole diameter in such a range can generally be said to be a small level value, but the reason for setting it in this way is related to the above-mentioned shutdown function and air permeation resistance. That is, as described above, by setting the air permeation resistance to a high level, the shutdown function can be easily expressed and a sufficient time until the shutdown is secured. However, in the present invention, the pore size of the entire membrane is further increased. By making the hole diameter smaller, the size of the hole to be closed at shutdown is reduced. Therefore, the high air permeation resistance not only narrows the flow path through which the ions pass through the microporous membrane, but also reduces the maximum pore diameter and average pore diameter of the entire microporous membrane. Shutdown occurs when the resin constituting the microporous membrane is slightly softened or melted.

By reducing the pore size of the microporous membrane, when a battery incorporating the microporous membrane is used, it becomes difficult for the dendrite (solid foreign matter) to pass through the microporous membrane even if it is generated. The pore size can also be set within the above range by adjusting the amount of heat absorption for melting of the microporous membrane obtained by the DSC measurement described later to a range in which the membrane strength is easily melted and the membrane strength is not significantly reduced. That is, the fact that the resin constituting the microporous film is easy to move even below the melting point means that the resin is not stretched so much, and therefore small pores are formed.

The average flow rate pore diameter and the maximum pore diameter can be determined by the following method of measuring in the order of Dry-up and Wet-up using a Palm Porometer (trade name, model: CFP-1500A) manufactured by PMI. In Dry-up, pressure is applied to the microporous polyolefin membrane, and the air flow rate through the membrane is measured. In Wet-up, pressure is applied to a polyolefin microporous membrane sufficiently immersed in Galwick (trade name), and the maximum pore diameter converted from the pressure at which air begins to penetrate is set. The average flow diameter can be converted from the pressure obtained by the Dry-up measurement and the pressure at the intersection of the curve showing the slope of 1/2 of the flow rate curve and the curve obtained by the Wet-up measurement. The following formula is used to convert the pressure and pore diameter.
d = C · γ / P (in the formula, d (μm) is the pore size of the microporous membrane, γ (dynes / cm) is the surface tension of the liquid, P (Pa) is the pressure, and C is the pressure constant (2860). be.)
[Puncture strength]
The lower limit of the puncture strength of the microporous membrane is preferably 1.4 N or more, more preferably 1.7 N or more, and further preferably 1.9 N or more. The upper limit of the puncture strength is preferably 3.8 N or less, more preferably 2.8 N or less, and further preferably 2.4 N or less. When the piercing strength is within the above range, the membrane strength of the microporous membrane is excellent. Further, in the secondary battery using this microporous membrane as a separator, the occurrence of short-circuiting of electrodes and self-discharge are suppressed. The puncture strength is determined by, for example, adding ultra-high molecular weight polyethylene or adjusting the weight average molecular weight (Mw) and draw ratio of the polyolefin resin constituting the microporous film when producing the microporous film. Can be a range.

[Heat shrinkage rate]
The heat shrinkage of the MD after heating the microporous membrane at 105 ° C. for 8 hours is preferably 10% or less, more preferably 7% or less, still more preferably 5% or less. The heat shrinkage of the TD of the microporous membrane at 105 ° C. for 8 hours is preferably 10% or less, more preferably 8% or less, and further preferably 4% or less. The lower limit of the heat shrinkage rate of MD and the lower limit of the heat shrinkage rate of TD are preferably 0.5% or more. When the heat shrinkage rate of MD and the heat shrinkage rate of TD are within the above ranges, the heat shrinkage property is excellent, and when a microporous membrane is used as a separator, expansion and contraction due to heat can be suppressed. In adjusting the heat shrinkage of the microporous membrane in this way, the characteristics related to DSC, which will be described later, contribute. That is, the fact that the amount of heat absorbed by the microporous membrane is small means that the microporous membrane is not subjected to such a large stretching stress, and when heat is applied, the heat softens the resin rather than shrinking the microporous membrane. Since it is easily consumed for melting, the heat shrinkage rate of the microporous membrane can be suppressed to a small value.

[Ratio of increase in air permeation resistance up to 120 ° C. (inclination) ((Sec / 100 cm ³ ) / ° C.)]
When the rate of increase in air permeation resistance up to 120 ° C is large, ions form the microporous membrane as the temperature of the microporous membrane rises toward the shutdown completion temperature in the temperature range before the shutdown is completed. It becomes difficult to pass. Therefore, even at the time of a short circuit, the temperature of the battery gradually rises, so that the temperature of the battery does not rise so much when the shutdown of the microporous membrane is completed. In other words, by increasing the rate of increase in the air permeation resistance up to 120 ° C., the battery can be safely detoxified (shutdown completed) in a low temperature state when a short circuit occurs. The rate of increase in air permeation resistance up to 120 ° C. is preferably 1.2 or more, more preferably 1.5 or more, still more preferably 1.8 or more. Such an increase rate of the air permeation resistance can be obtained by using the raw materials described later and adjusting the DSC characteristics. Note that FIG. 2 is a graph obtained by extracting a characteristic example in FIG. In FIGS. 1 and 2, the linear equations of the approximate curves obtained in each example and their slopes are also shown on the right side of the legend.

2. Method for Producing Microporous Membrane The method for producing microporous membrane will be described below. The following description is an example of a manufacturing method, and is not limited to this method.

[composition]
In the present specification, the polyolefin microporous membrane (hereinafter, also simply referred to as “microporous membrane”) refers to a microporous membrane containing polyolefin as a main component, and the “main component” refers to the total amount of the microporous membrane. It contains 90% by mass or more of polyolefin. Hereinafter, the microporous membrane of the present embodiment will be described.

The microporous membrane contains a polyolefin resin as a main component. As the polyolefin resin, polyethylene can be used. For example, polyethylene can be contained in an amount of 50% by mass or more based on the total amount of the microporous membrane. As the polyethylene, high density polyethylene (HDPE), medium density polyethylene, branched polyethylene, linear low density polyethylene and the like are used. The polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and another α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like.

Here, the high-density polyethylene (density: 0.920 g / m ³ or more and 0.970 g / m ³ or less) is a long-chain branched polyethylene. A long-chain branched polyethylene is a polyethylene in which another carbon chain is branched from a carbon chain which is a main chain and another carbon chain is extended as a branched chain, and another carbon chain is extended from the branched chain, and the weight average thereof. The molecular weight (Mw) is, for example, about 1 × 10 ⁴ or more and less than 1 × 10 ^6. The ultimate viscosity (dl / g) of long-chain branched polyethylene is, for example, 16.

The lower limit of the molecular weight distribution (MwD) of the long-chain branched polyethylene is 7 or more, preferably 10 or more. The upper limit of the molecular weight distribution (MwD) of the long-chain branched polyethylene is 20 or less, preferably 15 or less. By using a resin having a wide molecular weight distribution in this way, relatively low molecular weight components are increased. The molecular weight distribution of polyethylene can be determined by the gel permeation chromatography method.

Comprises at least 1 × 10 ⁴ 1 × 10 ⁵ less components, and long chain branched polyethylene of molecular weight distribution (MWD) is the above range, when the battery is exposed to high temperature environment enough to cause a shutdown, quickly softened, While having the role of melting and closing the hole, it plays the role of forming the hole. That is, as will be described later, in the present invention, the microporous membrane is stretched at a low temperature at which the low molecular weight component does not melt or is difficult to melt. Therefore, the low molecular weight component is formed into a fibril formed by another resin. It is not merely melted and attached, but forms part of such a fibril, which contributes to the formation of numerous pores, in other words, the microporous membrane having a high porosity. There is. Further, even if a refractory resin that is difficult to stretch at a low temperature such as ultra-high molecular weight polyethylene, which will be described later, is contained due to the inclusion of such a low molecular weight component, the refractory resin is sandwiched between the refractory resins. By playing an auxiliary role in stretching, that is, by stretching even the amount of the high melting point resin in which the resin having a low melting point component is difficult to stretch, even a thin film can be prevented from breaking during stretching. Such long-chain branched polyethylene is difficult to crystallize even when stretched, and even if pores are formed (under stretching), it is still easily softened and melted, so that it is easy to stretch and the pores of the microporous membrane are quickly formed. It plays a role of closing in. Mw is a value measured by gel permeation chromatography (GPC). The content of high-density polyethylene, which is a long-chain branched polyethylene, is preferably 50% by mass or more, more preferably 60% by mass or more, and 70% by mass or more, based on 100% by mass of the entire polyolefin resin. It is more preferable to have. The upper limit of the content of the high-density polyethylene is preferably 100% by mass or less, and other components may be contained.

Further, the microporous membrane can contain ultra high molecular weight polyethylene (UHMwPE). Ultra high molecular weight polyethylene used as the raw material is the weight average molecular weight (Mw) of 1 × 10 ⁶ or more, preferably 1 × 10 ⁶ or more 8 × 10 ⁶ or less. When Mw is in the above range, moldability is good. As the ultra-high molecular weight polyethylene, one type may be used alone or two or more types may be used in combination, and two or more types of ultra-high molecular weight polyethylene having different Mw may be mixed and used.

The ultra-high molecular weight polyethylene can be contained in an amount of 0% by mass or more and 70% by mass or less with respect to 100% by mass of the entire polyolefin resin. When the content of the ultra-high molecular weight polyethylene is 10% by mass or more and 60% by mass or less, the Mw of the obtained microporous membrane can be easily controlled within a specific range, and the productivity such as extrusion kneading tends to be excellent. be. Further, when the ultra-high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the microporous membrane is thinned.

First, a polyolefin resin and a film-forming solvent are melt-kneaded to prepare a resin solution. As the melt-kneading method, for example, a method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is known, the description thereof will be omitted. The resin solution may contain components other than the above-mentioned polyolefin resin and solvent for film formation, and may contain, for example, an antioxidant.

Next, the molten resin is extruded and cooled to form a gel-like sheet. For example, the resin solution prepared above is fed from an extruder to one die and extruded into a sheet to obtain a formed product. A gel-like sheet is formed by cooling the obtained molded product.

As a method for forming the gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed at a rate of 50 ° C./min or higher, at least up to the gelation temperature. Cooling is preferably performed up to 25 ° C. or lower. By cooling, the microphase of the polyolefin separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, crystallization of the resin can be suppressed, and extremely small pores can be formed.

Then, the gel-like sheet is stretched. Stretching of a gel-like sheet is also referred to as wet stretching. After heating, the gel-like sheet is stretched by, for example, the tenter method, using the simultaneous biaxial stretching method. Since the gel-like sheet contains a film-forming solvent, it is uniformly stretched. The final area stretching ratio (plane magnification) in wet stretching is 25 times or less. The draw ratio is 5 times or less in both the mechanical direction (MD) and the width direction (TD).

That is, the draw ratio is an index of how large the gel-like sheet is formed into a film, in other words, it is the area of the obtained microporous film, which leads to productivity. It can be said that the larger the stretched surface magnification, the larger the area of the obtained microporous film and the higher the productivity. On the other hand, in the present invention, as described above, the stretched surface magnification is reduced to 25 times or less, the area of the obtained microporous film is reduced, and the productivity is sacrificed, but the physical properties of the obtained microporous film, that is, The shutdown function and hole diameter are set to the desired range. When stretched at a surface magnification of more than 25 times, the strength of the microporous film is improved, while the pore size is increased and the heat shrinkage is increased.

The upper limit of the wet stretching temperature is 120 ° C. or lower. If the wet stretching temperature exceeds the upper limit, the pore size of the film becomes large and the air permeation resistance decreases. The upper limit of the wet stretching temperature is preferably 115 ° C. or lower, more preferably 110 ° C. or lower. The lower limit of the stretching temperature is preferably 100 ° C. or higher. When the stretching temperature is within the above range, the high melting point component (ultra-high polyethylene) of the resin used as a raw material is set to the minimum temperature range of a stretchable level, but the low melting point component of the raw material resin (ultra-high polyethylene) is set. The low melting point component in long-chain branched polyethylene) is stretched without melting. Therefore, the low melting point component can contribute to the formation of the fibril together with the high melting point component, not in a state of being attached to the fibril formed by the high melting point component. Therefore, a large number of holes are formed and the porosity can be set within the above-mentioned range.

That is, in general, when stretching using resins having different melting points as raw materials, the stretching temperature is set within a range in which the high melting point component can be stretched (softened). The reason is that if the entire raw material resin is in a stretchable temperature range, stretching is uniformly performed and film rupture is prevented. In that case, the component having a melting point lower than the stretching temperature is melted into a lump, and is in a state of being attached to the fibril formed by the high melting point component. In such a state, if the melt of the low melting point resin adheres to the gaps between the fibrils formed by the high melting point component, it leads to a decrease in the porosity and an increase in the air permeation resistance beyond the design range.

On the other hand, compared to the conventional stretching at a temperature higher than the melting start temperature, stretching at a low temperature has a high risk of film rupture because the load on the film is large, and the risk increases as the film thickness becomes thinner. Originally, stability is required from the viewpoint of production efficiency, but in the present invention, the characteristics of the present invention are prioritized over productivity, and the stretching temperature is not adjusted to the resin having a melting point on the high temperature side, but the resin having a melting point on the low temperature side. I dare to match it. Then, as described above, the draw ratio is set low. Therefore, the resin having a melting point on the low temperature side is stretched at a level that does not cause film rupture, while the resin having a melting point on the low temperature side contributes to the formation of fibrils. Therefore, even a thin film can be stretched without breaking the film, and a microporous film having a complicated porosity and internal structure (high air permeation resistance) can be obtained.

Due to the above stretching, cleavage occurs between the polyethylene lamellae, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. The mechanical strength is improved and the pores are expanded by stretching, but since the stretching conditions are set as described above, it is possible to control the through-hole diameter and have a high porosity even with a thinner film thickness. Become. Therefore, the microporous membrane of the present invention is suitable for a safer and higher performance battery separator.

Next, the gel-like sheet after stretching is heat-treated. The heat treatment stabilizes the crystals. As a heat treatment method, a heat fixing treatment (first heat fixing) is performed. The first heat fixing treatment is a heat treatment in which the microporous membrane is heated while being kept so that the dimensions of MD and TD do not change (surface magnification 1.0 times). By this heat fixing treatment, the resin crystals are fixed, and the microporous film is less likely to be heat-shrinked. The temperature of the heat fixing treatment is preferably 120 ° C. or lower, more preferably 110 ° C. or lower. The lower limit is preferably 100 ° C. or higher.

Next, the film-forming solvent is removed from the stretched gel-like sheet to form a microporous film. The film-forming solvent is removed by cleaning with a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, when the film-forming solvent is removed, it is composed of fibrils that form a fine three-dimensional network structure, and is porous with pores that communicate irregularly in three dimensions. A quality film is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the cleaning solvent are known, the description thereof will be omitted. For example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

Next, the microporous film from which the film-forming solvent has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably not less than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C. or more lower than Tcd. Drying is preferably carried out with the microporous membrane film as 100% by mass (dry weight) until the residual cleaning solvent is 5% by mass or less, and more preferably 3% by mass or less. When the residual cleaning solvent is within the above range, the porosity of the microporous membrane is maintained when the subsequent stretching step and heat treatment step of the microporous membrane film are performed, and deterioration of permeability is suppressed. Here, the crystal dispersion temperature (Tcd) means a value obtained by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065.

Next, the microporous membrane after drying is heat-treated. The heat treatment stabilizes the crystals and homogenizes the lamellae. As a heat treatment method, a heat fixing treatment (second heat fixing treatment) is performed following the heat relaxation treatment. The heat relaxation treatment is a heat treatment in which the microporous membrane is heated while narrowing the dimensions between the ends in the width direction so as to forcibly reduce the dimensions of the TD. The temperature of the heat relaxation treatment is preferably 140 ° C. or lower, more preferably 130 ° C. or lower. The heat relaxation rate (ratio of the TD size of the microporous membrane before the heat relaxation treatment to the shrinkage size of the TD of the microporous membrane after the heat relaxation treatment) is preferably 10% or less, more preferably 5% or less. .. Since the stress remaining in the microporous membrane is relaxed by the heat relaxation treatment, the heat shrinkage rate of the treated microporous membrane becomes small. Further, since the size of the TD of the microporous membrane is reduced by the heat shrinkage treatment, the pore diameter is reduced and the tensile elongation in the TD is increased (easily stretched). Then, the smaller the pore diameter means that the air permeation resistance becomes larger, which contributes to the prompt manifestation of the shutdown function. The second heat-fixing treatment is a heat treatment in which the microporous membrane is heated while being maintained so that the dimensions of MD and TD do not change (surface magnification 1.0 times). By this heat fixing treatment, the resin crystals are fixed, and the microporous film is less likely to be heat-shrinked. The temperature of the heat fixing treatment is preferably 130 ° C. or lower, more preferably 110 ° C. or lower.

By the way, in the present invention, so-called dry stretching is not performed after removing the film-forming solvent. When the dry stretching is performed, the strength of the microporous membrane is improved and the area of the microporous membrane is also increased, which leads to an improvement in productivity. However, in the present invention, such dry stretching is intentionally avoided. There is. Therefore, the fibrils formed by wet stretching are maintained as they are, and the pore diameter remains small, and since the pore diameter remains small, the flow of through holes from one side surface to the other side surface in the thickness direction of the microporous membrane. The path is complex and therefore remains highly resistant to air permeation. When the pore size is increased by dry stretching, the fibrils constituting the microporous film become thinner and the strength of the microporous film decreases as the pore size increases. In order to secure as much as possible, a microporous film with excellent porosity and shutdown function is obtained without intentionally performing dry stretching.

The microporous membrane in the present invention is a so-called monolayer membrane composed of one layer.

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to these examples.

1. 1. Measurement method and evaluation method [Weight average molecular weight (Mw)]
The weight average molecular weight (Mw) of the polyolefin resin was determined by the gel permeation chromatography (GPC) method under the following conditions.
-Measuring device: GPC-150C manufactured by Waters Corporation
-Column: Showa Denko Corporation Shodex UT806M
-Column temperature: 135 ° C
-Solvent (mobile phase): o-dichlorobenzene-Solvent flow rate: 1.0 ml / min-Sample concentration: 0.1 wt% (dissolution condition: 135 ° C./1 h)
・ Injection amount: 500 μl
-Detector: Waters Corporation differential refractometer (RI detector)
-Calibration curve: A calibration curve obtained using a monodisperse polystyrene standard sample was prepared using a predetermined conversion constant.

[Film thickness (μm)]
Using a contact thickness meter (Lightmatic manufactured by Mitutoyo Co., Ltd.), the film thickness of the sample (95 mm × 95 mm) cut out from the microporous membrane was measured at 5 points, and the average value was taken as the film thickness.

[Air permeation resistance (sec / 100 cm ³ )]
^{The air permeability resistance (sec / 100 cm 3} ) of the microporous membrane was measured using an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117.

[Porosity (%)]
This is a method of calculating the volume fraction of pores from the film thickness, area, mass, and density of the microporous membrane (for example, in the case of polyethylene alone, it is 0.99 g / cm ^3). The film thickness and mass of the sample (95 mm × 95 mm) cut out from the microporous membrane were measured, and the porosity was calculated by the following formula.
Porosity (%) = 1-mass / (film thickness x area x density)
The density of the polyolefin microporous membrane was determined by a method based on K7112: 1999.

[Puncture strength (N)]
The maximum load when the microporous membrane was pierced at a speed of 2 mm / sec with a needle having a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm was defined as the puncture strength.
[Tensile strength (Mpa)]
The MD tensile strength and the TD tensile strength were measured at room temperature of 25 ° C. using a strip-shaped test piece having a chuck-to-chuck distance of 20 mm and a width of 10 mm by a method according to ASTM D882.

[Tensile elongation (%)]
The MD tensile elongation and the TD tensile elongation were measured at room temperature of 25 ° C. using a strip-shaped test piece having a chuck-to-chuck distance of 20 mm and a width of 10 mm by a method according to ASTM D-882A.

[Heat shrinkage rate (%)]
The MD heat shrinkage rate and the TD heat shrinkage rate at 105 ° C. for 8 hours were measured as follows.
(1) The length of the test piece (95 mm × 95 mm) of the microporous membrane at room temperature (25 ° C.) is measured for both MD and TD.
(2) The test piece of the microporous membrane is equilibrated at a temperature of 105 ° C. for 8 hours without applying a load.
(3) The length of the microporous membrane is measured for both MD and TD.
(4) The heat shrinkage to MD and TD was calculated by dividing the measurement result (3) by the measurement result (1), subtracting the obtained value from 1, and expressing the value as a percentage (%).

[Pore diameter (average flow rate pore diameter (nm) and maximum pore diameter (nm))]
The average flow pore diameter and the maximum pore diameter (nm) of the polyolefin microporous membrane were measured as follows. The measurement was performed in the order of Dry-up and Wet-up using a PMI palm porometer (trade name, model: CFP-1500A). In Dry-up, pressure is applied to the microporous polyolefin membrane, and the air flow rate through the membrane is measured. In Wet-up, pressure was applied to a polyolefin microporous membrane sufficiently immersed in Galwick (trade name) having a known surface tension, and the pore diameter converted from the pressure at which air began to penetrate was set as the maximum pore diameter. The average flow diameter was converted from the pressure obtained by the Dry-up measurement and the pressure at the intersection of the curve showing the slope of 1/2 of the flow rate curve and the curve obtained by the Wet-up measurement. The following formula was used to convert the pressure and pore diameter.
d = C · γ / P (in the formula, d (μm) is the pore size of the microporous membrane, γ (dynes / cm) is the surface tension of the liquid, P (Pa) is the pressure, and C is the pressure constant (2860). be.).

[DSC measurement]
The melting point and melting heat absorption ΔH of the microporous membrane were determined by differential scanning calorimetry (DSC measurement) using a Diamond DSC manufactured by PerkinElmer. The microporous membrane was punched into a circle with a diameter of 5 mm, and several 5 to 10 mg measurement samples were placed on an aluminum open sample pan with a diameter of 5 mm, and a clamping cover was placed on the sample and fixed in the aluminum pan with a sample sealer. .. Then, the aluminum pan was allowed to stand at 30 ° C. for 1 minute in a nitrogen atmosphere, and then the temperature was raised from 30 ° C. to 230 ° C. at a heating rate of 10 ° C./min. In the melting endothermic curve at this time, the maximum temperature was taken as the melting point of the microporous membrane. From the area below the curve, the amount of heat absorbed by melting per weight of the measurement sample was defined as the amount of heat absorbed by melting ΔH of the microporous membrane.

[Shutdown temperature (° C)]
After measuring the air permeation resistance, the temperature at which the air permeation resistance of the microporous membrane first ^{exceeds 100,000 seconds / 100 cm 3} while continuing to raise the temperature was defined as the shutdown temperature of the microporous membrane.

[Ratio of increase in air permeation resistance up to 120 ° C (Sec / 100 cm ³ ) / ° C]
Using an air permeability meter (EGO-1T manufactured by Asahi Seiko Co., Ltd.), the microporous membrane is exposed to a temperature atmosphere of 30 ° C. and then heated from 30 ° C. to 120 ° C. at a heating rate of 5 ° C./min. While measuring the air permeability resistance. From the obtained data of the air permeability resistance to the temperature, a graph of the curve in which the temperature and the air permeability resistance as shown in FIG. 1 are set on the horizontal axis and the vertical axis, respectively, is created, and the curve up to the temperature of 120 ° C. is created. A first-order approximate straight line was obtained, and the slope was taken as the rate of increase in air permeation resistance up to 120 ° C.

[Impedance]
An impedance measuring device (manufactured by Solartron, SI1250, SI1287) was used for impedance measurement. On a glass plate (50 mm (W) x 80 mm (L) x 3 mm (T)), Ni foil (30 mm x 20 mm), microporous film (30 mm (W) x 20 mm (L)), Ni foil (30 mm x 20 mm) are stacked in this order, and an electrolyte typical of a microporous film (composed of 1 mol / L LiPF ₆ , lithium salt, ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 40: 60 vol%) is used as a separator at about 0. It was impregnated with 0.02 ml, 1.0 kV was applied (constant voltage), and the value after 10 seconds was taken as the impedance (Ω / cm ² ).
Impedance was evaluated as follows.
Extremely good (A): ^{less than 1.00 Ω / cm 2} Good (B): 1.00 Ω / cm ² or more, less than 1.50 Ω / cm ² Defective (C): 1.50 Ω / cm ² or more.

[Manufacturing method of evaluation battery]
The battery used for the evaluation (evaluation battery) was _{a 1 mol / L LiPF 6 prepared in a mixed solvent of lithium cobalt composite oxide LiCoO 2} as the positive electrode active material, graphite as the negative electrode active material, and EC / EMC / DMC as the electrolytic solution. After laminating a positive electrode, a separator made of a microporous film, and a negative electrode, a wound electrode body was prepared by a conventional method, inserted into a battery can, impregnated with an electrolytic solution, and sealed. .. The details of the method for manufacturing the evaluation battery will be described below.

[Preparation of positive electrode]
Lithium cobalt composite oxide LiCoO ₂ as the positive electrode active material, acetylene black as the conductive material, and polyvinylidene fluoride (PVDF) as the binder are mixed at a mass ratio of 93.5: 4.0: 2.5, and the solvent N is used. -A slurry was prepared by mixing and dispersing in methylpyrrolidone (NMP). This slurry was applied to both sides of a 12 μm-thick aluminum foil serving as a positive electrode current collector, dried, and then rolled by a roll press. The rolled product was slit to a width of 30 mm to obtain a positive electrode.

[Preparation of negative electrode]
Artificial graphite as the negative electrode active material, carboxymethyl cellulose as the binder, and styrene-butadiene copolymer latex were mixed and dispersed in purified water so as to have a mass ratio of 98: 1: 1 to prepare a slurry. This slurry was applied to both sides of a copper foil having a thickness of 10 μm as a negative electrode current collector, dried, and then rolled by a roll press. The rolled product was slit to a width of 33 mm to form a negative electrode.

[Non-aqueous electrolyte]
_{LiPF 6} as a solute was dissolved in a mixed solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate = 3: 5: 2 (volume ratio) so as to have a concentration of 1.15 mol / liter. Further, 0.5% by mass of vinylene carbonate was added to 100% by mass of the non-aqueous electrolytic solution to prepare a non-aqueous electrolytic solution.

[Battery production]
After laminating the above positive electrode, the microporous film and the above negative electrode, a flat wound electrode body (height 2.2 mm × width 36 mm × depth 29 mm) was produced. A tab with a sealant was welded to each electrode of this flat wound electrode body to form a positive electrode lead and a negative electrode lead. The flat wound electrode body is sandwiched between aluminum laminate films, sealed with some openings left, and dried in a vacuum oven at 80 ° C. for 6 hours. After drying, 0.7 mL of electrolyte is poured immediately. It was liquid, sealed with a vacuum sealer, and press-molded at 80 ° C. and 1 MPa for 1 hour. Subsequently, charging and discharging were carried out. The charging / discharging condition was a current value of 300 mA, and after charging with a constant current to a battery voltage of 4.2 V, constant voltage charging was performed with a battery voltage of 4.2 V until reaching 15 mA. After a 10-minute pause, a constant current discharge was performed at a current value of 300 mA to a battery voltage of 3.0 V, followed by a 10-minute pause. The above charging and discharging were carried out for 3 cycles to prepare a test secondary battery having a battery capacity of 300 mAh.

[Volume energy density]
The volumetric energy density was measured by the following formula.
Volumetric energy density (Wh / L) = average operating voltage (V) x battery capacity (Ah) / battery volume (L)
The volumetric energy density was evaluated as follows.
Very good (A): 490 Wh / L or more Good (B): 480 Wh / L or more and less than 490 Wh / L Defective (C): 480 Wh / L or less.

[Rate characteristics]
The rate characteristics were evaluated by the following method. For the measurement of the rate characteristics, the test secondary battery prepared in the above (method for manufacturing the evaluation battery) was used. This battery is constantly charged to a battery voltage of 4.2 V with a current value of 1.0 C, then charged with a constant current of a battery voltage of 4.2 V to a current value of 0.05 C, and then charged with a current value of 0.2 C. The discharge capacity was measured by discharging (constant current discharge) until the battery voltage became 3.0 V. Subsequently, after charging to 4.2 V again by the above procedure, the battery was discharged (constant current discharge) at a current value of 5 C until the battery voltage became 3.0 V, and the discharge capacity was measured. The discharge capacity ratio (%) was calculated by the following formula.
Formula: Discharge capacity ratio = (Discharge capacity at 5C / Discharge capacity at 0.2C) x 100
The rate characteristics were evaluated as follows.
Extremely good (A): 90% or more Good (B): 85% or more, less than 90% Defective (C): Less than 85%.

[Cycle characteristics]
The cycle characteristics were evaluated by the following method. For the measurement of the cycle characteristics, the test secondary battery prepared in the above (preparation of the evaluation battery) was used. The battery was constantly charged with a current value of 1.0 C up to a battery voltage of 4.2 V, and then charged with a constant voltage until the current value reached 0.05 C with a battery voltage of 4.2 V. After a 10-minute pause, a constant current discharge was performed at a current value of 1.0 C until the battery voltage reached 3.0 V, followed by a 10-minute pause. This charging / discharging was regarded as one cycle, and charging / discharging was repeated 500 times.
The remaining capacity ratio was calculated by the following formula.
Formula: Remaining capacity ratio = 500th cycle discharge capacity × 100/1st cycle discharge capacity The cycle characteristics were evaluated as follows.
Extremely good (A): 90% or more Good (B): 85% or more, less than 90% Defective (C): Less than 85%.

[Crushing test]
The crush test was evaluated by the following method. For the measurement of the crush test, the test secondary battery prepared in the above (preparation of the evaluation battery) was used. The battery was pressurized with two flat plates, and the temperature rise of the battery was confirmed in 5 cells.
The crush test was evaluated as follows.
Very good (A): In all 5 cells, the surface temperature rise is 80 ° C. or less Good (B): In 1 to 2 cells, the surface temperature rise is more than 80 ° C. and 120 ° C. or less, and the surface temperature rise of the remaining cells is 80 ° C. or lower Normal (C): In 3 to 5 cells, the surface temperature rise exceeds 80 ° C. and 120 ° C. or lower, and the surface temperature rise of the remaining cells is 80 ° C. or lower. The temperature rise exceeds 120 ° C.

[External short circuit test]
The external short circuit test was evaluated by the following method. For the measurement of the external short-circuit test, the test secondary battery prepared in the above (preparation of evaluation battery) was used. The positive electrode terminal and the negative electrode terminal were connected to an external resistor, and the temperature rise of the battery was confirmed in 5 cells.
The external short circuit test was evaluated as follows.
Very good (A): In all 5 cells, the surface temperature rise is 80 ° C. or less Good (B): In 1 to 2 cells, the surface temperature rise is more than 80 ° C. and 120 ° C. or less, and the surface temperature rise of the remaining cells is 80 ° C. or lower Normal (C): In 3 to 5 cells, the surface temperature rise exceeds 80 ° C. and 120 ° C. or lower, and the surface temperature rise of the remaining cells is 80 ° C. or lower. The temperature rise exceeds 120 ° C.

(Example 1)
Table 1 shows that the weight average molecular weight is 2.2 × 10 ⁶ g / mol, the molecular weight distribution is 6, and the ultra-high molecular weight polyethylene having an extreme viscosity of 16 dl / g is 40% by mass, and the weight average molecular weight is 4.1 × 10 ⁵ g / mol, the molecular weight. Tetrakiss (methylene-3- (3,5)) as an antioxidant with respect to 100% by mass of a polyolefin composition consisting of 60% by mass of high-density polyethylene (long-chain branched polyethylene) having a distribution of 13.5 and an ultimate viscosity of 4.0 dl / g. -Different butyl-4-hydroxyphenyl) -propionate) The raw material obtained by dry blending 0.2% by mass of methane is put into a twin-screw extruder, and liquid paraffin is further supplied from the side feeder of the twin-screw extruder. Then, a polyethylene solution was prepared by melt-kneading the mixture in a twin-screw extruder so that the polyethylene composition was 25% by mass and the liquid paraffin was 75% by mass.

This polyolefin solution was extruded from the T-die of the twin-screw extruder and cooled while being taken up by a cooling roller to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially wet-stretched at 112 ° C. at 112 ° C. with a biaxial stretching machine, and fixed to the biaxial stretching machine as it was so that there was no dimensional change in both the MD and TD directions. The first heat was fixed at a temperature of 110 ° C. Next, the stretched gel-like sheet was immersed in a washing tank of a methylene chloride bath to remove liquid paraffin and washed, and the obtained microporous membrane was dried by a dryer. The obtained microporous membrane was heat-relaxed at 128 ° C. to the TD of the microporous membrane at a relaxation rate of 2% without dry stretching, and fixed so that there was no dimensional change in both the MD and TD directions. The second heat was fixed in. Then, the microporous membrane was cooled to room temperature to obtain a polyolefin microporous membrane.

Table 1 shows the production conditions for the microporous membrane, and Table 2 shows the evaluation results. The curve of the air permeation resistance to the temperature and the linear approximation straight line thereof in the polyolefin microporous membrane of Example 1 are shown in FIG. Moreover, the DSC curve of the polyolefin microporous membrane obtained in Example 1 is shown in FIG.

(Examples 2 to 4)
A polyolefin microporous film was obtained in the same manner as in Example 1 except that the resin composition and production conditions shown in Table 1 were used. Curves of air permeation resistance to temperature in the polyolefin microporous membranes of Examples 2, 3 and 4 are shown in FIGS. 2, 3 and 4, respectively. A first-order approximate straight line is also drawn with a thin solid line on the curves of Examples 1 to 4. In addition, the formula of the linear approximation straight line is added to the legend.

(Comparative Example 1)
Table 1 shows weight average molecular weight 2.2 × 10 ⁶ g / mol, molecular weight distribution 6, ultra-high molecular weight polyethylene 40% by mass with extreme viscosity 16 dl / g, weight average molecular weight 3.0 × 10 ⁵ g / mol, molecular weight. Tetraquis (methylene-3- (3,5-ditershally butyl-4) as an antioxidant is used for 100% by mass of a polyolefin composition consisting of 60% by mass of high-density polyethylene having a distribution of 6.0 and an ultimate viscosity of 3.7 dl / g. -Hydroxyphenyl) -propionate) The raw material obtained by dry blending 0.2% by mass of methane is put into a twin-screw extruder, and liquid paraffin is further supplied from the side feeder of the twin-screw extruder to be inside the twin-screw extruder. A polyethylene solution was prepared by melt-kneading the mixture so that the polyethylene composition was 25% by mass and the liquid paraffin was 75% by mass. This polyolefin solution was extruded from the T-die of the twin-screw extruder and cooled while being taken up by a cooling roller to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially wet-stretched at 112 ° C. at 112 ° C. with a biaxial stretching machine, and fixed to the biaxial stretching machine as it was so that there was no dimensional change in both the MD and TD directions. The first heat was fixed at a temperature of 110 ° C.

Next, the stretched gel-like sheet was immersed in a washing tank of a methylene chloride bath to remove liquid paraffin and washed, and the obtained microporous membrane was dried by a dryer. The obtained microporous membrane was heat-relaxed at 128 ° C. to the TD of the microporous membrane at a relaxation rate of 2% without dry stretching, and fixed so that there was no dimensional change in both the MD and TD directions. The second heat was fixed in. Then, the microporous membrane was cooled to room temperature to obtain a polyolefin microporous membrane.

(Comparative Example 2)
A polyolefin microporous film was obtained in the same manner as in Example 1 except that the resin composition and production conditions shown in Table 1 were used.

(Comparative Example 3)
Table 1 shows weight average molecular weight 2.2 × 10 ⁶ g / mol, molecular weight distribution 6, ultra-high molecular weight polyethylene 18% by mass with extreme viscosity 16 dl / g, weight average molecular weight 4.1 × 10 ⁵ g / mol, molecular weight. Tetraquis (methylene-3- (3,5-ditershally butyl-4)) as an antioxidant is used for 100% by mass of a polyolefin composition consisting of 82% by mass of high-density polyethylene having a distribution of 13.5 and an ultimate viscosity of 4.0 dl / g. -Hydroxyphenyl) -propionate) The raw material obtained by dry blending 0.2% by mass of methane is put into a twin-screw extruder, and liquid paraffin is further supplied from the side feeder of the twin-screw extruder to be inside the twin-screw extruder. A polyethylene solution was prepared by melt-kneading the mixture so that the polyethylene composition was 30% by mass and the liquid paraffin was 70% by mass.

This polyolefin solution was extruded from the T-die of the twin-screw extruder and cooled while being taken up by a cooling roller to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially wet-stretched at 112 ° C. at 112 ° C. with a biaxial stretching machine, and fixed to the biaxial stretching machine as it was so that there was no dimensional change in both the MD and TD directions. The first heat was fixed at a temperature of 110 ° C.

Next, the stretched gel-like sheet was immersed in a washing tank of a methylene chloride bath to remove liquid paraffin and washed, and the obtained microporous membrane was dried by a dryer. The obtained microporous membrane was dry-stretched 1.40 times to TD at 132 ° C. and heat-relaxed to TD at 132 ° C. with a relaxation rate of 2%, and the dimensions were measured in both the MD and TD directions. It was fixed so that there was no change, and a second heat fixing treatment was performed at 128 ° C. Then, the microporous membrane was cooled to room temperature to obtain a polyolefin microporous membrane.

(Comparative Example 4)
A microporous film was obtained in the same manner as in Example 1 except that the resin composition and production conditions shown in Table 1 were used.

(Comparative Example 5)
A microporous film was obtained in the same manner as in Comparative Example 3 except that the resin composition and production conditions shown in Table 1 were used and heat relaxation and second heat fixation were not performed.

(Comparative Example 6)
A microporous film was obtained in the same manner as in Example 1 except that the resin composition and production conditions shown in Table 1 were used.

(Comparative Example 7)
A microporous film was obtained in the same manner as in Example 1 except that the resin composition and production conditions shown in Table 1 were used and heat relaxation and second heat fixation were not performed.

Table 2 shows the evaluation results and the like of the obtained microporous membrane. The curves obtained in each Comparative Example, which are the basis of the increase rate of the air permeation resistance up to 120 ° C., are shown in each figure. The DSC curve obtained in Comparative Example 1 is shown in FIG.

[evaluation]
Although the microporous membranes of Examples 1 to 4 were extremely thin films, they had high porosity and air permeability resistance, and were shown to be excellent in battery characteristics. In FIGS. 1 and 2, it can be seen that in each embodiment, the air permeation resistance tends to increase as the temperature rises in the temperature range of 120 ° C. or lower. Therefore, it is easy to prevent the passage of ions at a low temperature until the shutdown is completed, and it can be seen that the temperature of the battery does not rise so much when the shutdown is completed. On the other hand, in the comparative example, the rate of increase in the air permeation resistance up to 120 ° C. is small, and a large number of ions flow between the electrodes after the short circuit occurs and before the shutdown is completed. It was found that the temperature was likely to rise.

Comparing FIG. 5 showing the results of the examples relating to DSC and FIG. 6 showing the results of the comparative example, as described above, in the examples, the heat absorption amount of the resin constituting the microporous film is smaller than that in the comparative example. You can see that. Therefore, it can be seen that the microporous membrane is easily softened and melted in the examples.

Claims (8)

Polyolefin microporous with air permeation resistance of 300 seconds / 100 cm ³ or more and 500 seconds / 100 cm ³ or less, porosity of 18% or more and 30% or less, tensile elongation in the width direction of 105% or more, and film thickness of less than 6 μm. film. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane contains 50% by mass or more of long-chain branched polyethylene having a molecular weight distribution of 10 or more and 20 or less. The polyolefin microporous membrane according to claim 1 or 2, wherein the tensile strength in the mechanical direction is 230 MPa or more. The microporous polyolefin membrane according to claims 1 to 3, wherein the tensile elongation in the width direction is 150% or more. The polyolefin microporous membrane according to claims 1 to 4, wherein the amount of heat absorbed by melting measured by a differential scanning calorimeter is 200 J / g or less. A battery using the separator containing the microporous polyolefin membrane according to any one of claims 1 to 5. A process of melt-kneading a polyolefin resin containing long-chain branched polyethylene with a plasticizer to obtain a resin solution.
The step of discharging the resin solution from the die and cooling it to form a gel-like sheet, and
A step of simultaneously biaxially wet-stretching the gel-like sheet at 120 ° C. or lower and a surface magnification of 25 times or lower.
A step of extracting a plasticizer from the gel-like sheet after wet stretching and drying it.
A method for producing a microporous polyolefin membrane, which comprises. The method for producing a microporous polyolefin membrane according to claim 7, further comprising a step of second heat fixing at a surface magnification of 1.0 times at 130 ° C. or lower after the drying step.

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