JP2004323820A - Polyolefin microporous membrane and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyolefin microporous membrane having a dense membrane structure and good permeation characteristics together with low shrinkage at a high temperature, making initial battery performance compatible with cycle performance, and suitable as a separator for a secondary battery such as lithium ion secondary battery excellent also in safety, and to provide a method for producing the membrane. <P>SOLUTION: This polyolefin microporous membrane has water permeability/air permeability balance of 0.8×10<SP>-3</SP>-1.7×10<SP>-3</SP>and a bubble point of >0.5 MPa and <1.0 MPa. This method for producing the membrane comprises not less than three times stretching in transverse direction (TD) before extraction as indispensable process and after the extraction, stretching at least in machin direction (MD) is performed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a microporous polyolefin membrane, a method for producing the same, and its application to a battery separator.

Polyolefin microporous membranes are used for microfiltration membranes, battery separators, condenser separators, and the like. In recent years, in particular, demand for lithium ion secondary battery applications has been growing, and as the energy density of batteries has increased, high performance and high safety have also been required for separators.
Lithium-ion secondary battery separators have high mechanical strength that is closely related to battery assembly performance expressed by puncture strength, high ion transmission performance to improve battery output characteristics, and ion transmission during abnormal heat generation. It is also required to have a safety performance that suppresses a short circuit and prevents a fire or the like.

  The output characteristics of a battery are current characteristics such as discharge performance at a large current and discharge performance at a low temperature, and life-related characteristics such as cycle characteristics and high-temperature storage characteristics. The higher the separator's ion permeability, the better the ion permeability. Physical properties closely related to the above include gas permeability represented by a bubble point and air permeability, and liquid permeability represented by a water permeability. In addition, with the recent demand for higher energy density of batteries, it has become important to keep the initial capacity high. On the other hand, the amount of heat generated during abnormal times and the rate of heat generation tend to increase due to the increase in energy density, and since the current is maintained at a high temperature for a while even after current interruption by a fuse, it is desired that film shrinkage at high temperatures be low. .

  In the microporous membrane manufacturing technology, a microporous membrane precursor is formed from a composition consisting of a polymer and a plasticizer by a phase separation process, and a plasticizer extraction and removal process and a stretched thinning process are applied to the precursor. Techniques for forming a porous membrane are known. In such known techniques, depending on whether the step of performing stretching is performed before or after the extraction process, it is referred to as pre-extraction stretching or post-extraction stretching, respectively. The orientation can be imparted to the matrix resin to increase the strength, and a microporous membrane having a fine pore structure with a small pore size can be manufactured. On the other hand, in the case of the process by stretching after extraction, the interface breakdown predominantly proceeds by stretching, so that a microporous membrane having a large pore size and high permeability can be obtained.

  When the microporous membrane obtained by these two methods is used as a separator for a lithium ion secondary battery, it exhibits contradictory characteristics in view of the above mechanical characteristics and battery characteristics. In other words, a small-pore membrane formed by stretching before extraction has high mechanical strength and is excellent in maintaining the initial capacity at the time of battery preparation, but is inferior in cycle characteristics. On the other hand, a large-pore membrane formed by stretching after extraction is excellent in cycle properties but has an initial capacity. And the mechanical strength is weak.

  It can be easily inferred that the strength of the mechanical strength is caused by each manufacturing method. On the other hand, the superiority of battery characteristics can be interpreted as follows. That is, the factor that the small pore membrane is inferior in the cycle characteristics is due to the material generated by the separation of the active material due to the expansion and contraction of the positive and negative electrode active materials accompanying the charge and discharge and the side reaction of the electrolytic solution in the secondary battery which repeats the charge and discharge. This is because microporous clogging occurs gradually, and in a small-pore membrane, the clogging becomes severe during repeated charge / discharge cycles, and the initial battery performance cannot be maintained. On the other hand, large pore membranes are hardly clogged and maintain their performance.

  On the other hand, it is considered that the reason why the large pore membrane is inferior to the initial capacity of the battery is that a large amount of metallic lithium is deposited and is not used due to current concentration at the time of initial charge / discharge due to uneven surface pore structure. Regarding the initial capacity of batteries, despite their importance, little attention has been paid to them.However, in today's strict demand for higher energy density of batteries, separators that can maintain high initial capacity and maintain cycle characteristics are extremely very popular. It can be useful.

  As for the safety performance, there is a fuse function as a function of the separator. This is a function of blocking the permeation of ions by closing the pores of the microporous membrane when the battery generates abnormal heat due to a short circuit or exposure to a high temperature, thereby suppressing the further heat generation. However, at present, as the capacity and density are increasing, the amount of heat generated when abnormal heat is generated is large and the temperature rises rapidly, so that even after the fuse, the inside of the battery may reach an extremely high temperature at which the separator melts. . In the molten state, the highly stretched separator shrinks greatly, and when the electrodes come into contact with each other, a dangerous state of re-short circuiting may occur. For this reason, there is a strong demand for a separator having excellent shape retention at a high temperature of the melting point or higher. In most batteries, since the separator is wound in the MD direction, it is particularly excellent in shape retention in the TD direction. The separator is very useful. Here, the MD direction is a machine direction, that is, a winding direction during continuous film formation of the separator, and the TD direction is a width direction perpendicular to the MD direction.

In the following known methods for the purpose of maintaining the permeability and the membrane strength, the production methods such as using both pre-extraction stretching and post-extraction stretching are disclosed, but together with the problems described below, both of the initial capacity of the battery are reduced. No attention has been paid to maintenance, and a microporous membrane that can maintain the cycle capacity while maintaining a high initial capacity has not yet been obtained.
In Patent Document 1, although having high permeability, the porosity is high and the film strength is low for use as a battery separator.
Patent Document 2 discloses that a microporous polyolefin membrane which can be freely adjusted in permeation performance without impairing strength by simultaneously using stretching before extraction and stretching after extraction and further performing heat treatment, and at the same time, has excellent dimensional stability at high temperatures. Is disclosed, but a microporous membrane which can maintain a high initial capacity and also maintain a cycle property has not been obtained.
Patent Document 3 discloses a method for producing a microporous membrane having a moderately large through-hole and a sharp pore size distribution by regulating the stretching temperature after extraction to a low temperature. Has a large pore size and a high porosity, and therefore has poor electrolyte retention properties in battery separator applications. It is not clear why the pore size increases and the porosity increases, but stretching at low temperatures does not cause local stretching, but the stretching stress is not used for stretching the resin itself, but only for expanding pores. It is thought that it is done.

  Regarding safety, Patent Document 2 discloses a polyolefin microporous membrane having excellent dimensional stability at high temperatures, but does not describe in detail how high the temperature is. Is an index for evaluating the dimensional stability of the microporous membrane at high temperatures. "However, since the measurement condition of the heat shrinkage is 100 ° C. for 2 hours, the separator which is a problem in the present invention, It is not a description of a high temperature at which the material becomes a molten state.

Patent Document 4 discloses a microporous membrane and a manufacturing method in which the heat shrinkage in the vertical direction is 20% or less and the heat shrinkage in the horizontal direction is 15% or less by heat treatment at a specific temperature. This is a shrinkage rate at 105 ° C., and focuses only on shrinkage at a low temperature equal to or lower than the fuse temperature, but does not describe shrinkage at a fuse temperature or higher.
Patent Document 5 discloses a microporous film excellent in heat resistance and low shrinkage at a high temperature equal to or higher than the fuse temperature and a manufacturing method. The film obtained by this method has a draw ratio of 3 times or more in the MD direction. After stretching, a step of reducing by 20 to 80% in the TD direction is indispensable, so that the transmission performance is inferior. Further, from the viewpoint of high strength, it is described that the film is stretched 3 times or more, preferably 5 to 20 times in the MD direction. However, since the film is significantly stretched only in the MD direction, the anisotropy in the MD direction and the TD direction is large. In this case, the structure is apt to be vertically torn, and the film is easily broken by dendrite precipitation crystals or the like. Therefore, the initial battery characteristics, cycle characteristics, and safety cannot be satisfied.

Patent Documents 1 and 3 do not pay attention to shrinkage.
A separator for a lithium ion secondary battery that combines the cycle characteristics of a large pore membrane with the initial battery properties of a small pore membrane, and high safety, that is, a microporous membrane that has both good permeability and a dense membrane structure, and low shrinkage at high temperatures. The emergence of porous membranes has been eagerly desired.
Japanese Patent Publication No. 6-21177 JP-A-11-60789 Japanese Patent No. 2657430 JP-A-9-12756 JP 2003-119306 A

  The present invention has both good permeability performance, a dense membrane structure, and low shrinkage at high temperatures, and when used as a separator for a secondary battery such as a lithium ion, can achieve both cycle characteristics and initial battery characteristics, and can be used safely. And to provide a method for producing the same.

  The present inventors have conducted intensive studies on the above problems, and as a result, in the method for producing a polyolefin microporous membrane, by stretching in a specific direction and magnification in each of the stretching steps before and after extraction, a specific range of A separator that has a balance of water permeability / air permeability and a specific range of bubble points, is capable of reducing shrinkage at high temperatures, and has excellent safety, in which the membrane maintains a high initial capacity of the battery and maintains cycleability. As a result, the present invention has been found to be useful.

That is, the present invention is as follows.
(1) Polyolefin fine particles characterized in that the ratio of water permeability / air permeability is 0.8 × 10 ^{−3 to} 1.7 × 10 ⁻³ and the bubble point is more than 0.5 MPa and less than 1.0 MPa. Porous membrane.
(2) The microporous polyolefin membrane according to (1), wherein the shrinkage in the TD direction in a state where the MD direction is fixed is less than 20% at 150 ° C.
(3) (a) a step of melt-kneading a composition comprising a polyolefin resin and a plasticizer, extruding, cooling and solidifying to form a sheet, and (b) stretching three or more times in the TD direction after the step (a). (C) extracting the plasticizer after the step (b), and (d) performing at least one stretching in the MD direction at least once after the step (c). The method for producing a microporous membrane according to the above (1) or (2), characterized in that:
(4) A battery separator comprising the polyolefin microporous membrane according to (1) or (2).

  The microporous polyolefin membrane of the present invention has both good permeability and a dense membrane structure, and low shrinkage at high temperatures, and is capable of satisfying both initial battery characteristics and cycle characteristics and excellent in safety, such as lithium ion. It is suitable as a separator for a secondary battery.

Hereinafter, the present invention will be described in detail with particular emphasis on preferred embodiments. The water permeation amount and the air permeation amount in the present invention are the permeation amount of water and air per unit time, unit pressure, and unit area of the microporous membrane, and the water permeation rate constant Rliq (m ³ / (m ² ··· sec · Pa)) is the amount of water permeation, and the air permeation rate constant Rgas (m ³ / (m ² · sec · Pa)) is the amount of air permeation.
Here, the air permeability is a value obtained by converting the value of the air permeability measured by a Gurley air permeability meter based on JIS P-8117.
The ratio of water permeability / air permeability of the membrane of the present invention needs to be in the range of 0.8 × 10 ^{−3 to} 1.7 × 10 ⁻³ . A preferred range is from 1.0 × 10 ^{−3 to} 1.5 × 10 ⁻³ . When the ratio is less than 0.8 × 10 ⁻³ , the discharge characteristics tend to decrease, and when the ratio exceeds 1.7 × 10 ⁻³ , the initial battery capacity tends to decrease, which is not preferable.

The water permeability is preferably in the range of 3 × 10 ⁻¹⁰ to 4 × 10 ⁻⁹ m ³ / (m ² · sec · Pa), and more preferably 4 × 10 ⁻¹⁰ to 2 × 10 ⁻⁹ m ³ / ( m ² · sec · Pa), more preferably 4 × 10 ^{−10 to} 1.5 × 10 ⁻⁹ m ³ / (m ² · sec · Pa). When applied to a battery separator, if the water permeability is smaller than 3 × 10 ⁻¹⁰ m ³ / (m ² · sec · Pa), the impregnation rate of the electrolyte tends to be slow. If the water permeability is larger than 4 × 10 ⁻⁹ m ³ / (m ² · sec · Pa), the liquid retention of the electrolytic solution tends to decrease.
The air permeability of the membrane of the present invention is preferably 3 × 10 ^{−7 to} 3 × 10 ⁻⁶ m ³ / (m ² · sec · Pa), and more preferably 4 × 10 ^{−7 to} 1.5 ×. 10 ⁻⁶ m ³ / (m ² · sec · Pa), and more preferably 4 × 10 ⁻⁷ to 1 × 10 ⁻⁶ m ³ / (m ² · sec · Pa). If the air permeability is less than 3 × 10 ⁻⁷ m ³ / (m ² · sec · Pa), the discharge characteristics of the battery at low temperatures tend to be reduced, and 3 × 10 ⁻⁶ m ³ / (m ² (Sec · Pa), a short circuit between the positive and negative electrodes is likely to occur when used as a battery separator.

  Further, the bubble point of the film of the present invention needs to be in a range of more than 0.5 MPa and less than 1 MPa. A preferred range is 0.6 to 0.9 MPa. The bubble point method is known as a simple method of expressing the maximum pore diameter. However, when the bubble point is 0.5 MPa or less, the pores may be coarsened and the film strength may be reduced, which may cause poor winding during battery production. On the other hand, if the pressure is 1.0 MPa or more, the pores are too dense and sufficient cycle characteristics cannot be obtained.

  Further, a major feature of the film of the present invention is that the heat shrinkage is less than 20%. The term “thermal shrinkage” as used herein refers to a shrinkage ratio in the TD direction at 150 ° C. in a state where both ends of the MD are fixed so as not to shrink in the MD direction. In the battery, the separator is wound in the MD direction, and particularly in a rectangular battery, the wound material is crushed. Therefore, the MD direction of the separator is fixed and cannot be shrunk. On the other hand, the TD direction is not fixed and contraction is easy. If the shrinkage at 150 ° C. is 20% or more, the separator shrinks greatly, so the electrodes are exposed after the current is cut off by the fuse function, and short-circuit occurs again when the electrodes come into contact with each other, increasing the risk of heat generation and ignition. I do. The separator shrinkage at 150 ° C. is less than 20%, preferably less than 15%, more preferably less than 10%.

  Next, a preferred example of the method for producing the microporous polyolefin membrane of the present invention will be described. In the step (a) of the production method of the present invention, the method of melt-kneading the polyolefin resin and the plasticizer is as follows. The polyolefin resin is charged into a resin kneading device such as an extruder or a kneader, and the resin is heated and melted at an arbitrary ratio. Is preferable to obtain a uniform solution by introducing a plasticizer and kneading a composition comprising a resin and a plasticizer. Moreover, what kneaded the resin and the plasticizer in advance may be charged. The form of the polyolefin resin to be charged may be any of powder, granule and pellet. When kneading by such a method, the form of the plasticizer is preferably a liquid at room temperature. As the extruder, a single screw type extruder, a twin screw different direction screw type extruder, a twin screw same direction screw type extruder, or the like can be used.

  In the step (a) of the production method of the present invention, the method for producing a sheet-like microporous membrane precursor by extruding and cooling and solidifying is a method of extruding a uniform solution of a resin and a plasticizer into a sheet via a T-die or the like. It is preferable that the heat treatment is performed by contacting with a heat conductor and cooling to a temperature sufficiently lower than the crystallization temperature of the resin. As the heat conductor to be used, metal, water, air, or the plasticizer itself can be used, but a method of cooling by contacting with a metal roll is particularly preferred because it has the highest heat conduction efficiency. Also, when contacting a metal roll, calendering or hot rolling by sandwiching between the rolls, etc., further increases the efficiency of heat conduction, and the sheet is oriented to increase the film strength, and the sheet strength is increased. It is preferable because the surface smoothness is also improved.

In the case of rolling, the ratio between the sheet thickness before rolling and the sheet thickness after rolling is referred to as a rolling ratio. A preferable range of the rolling ratio is 1.5 to 8 times, and more preferably a range of 2 to 5 times. . Here, the sheet thickness before rolling refers to the lip interval of a T-die or the like, and the sheet thickness after rolling refers to the thickness of the sheet obtained after rolling. If the rolling ratio is less than 1.5 times, the orientation of the sheet may be insufficient. On the other hand, if it exceeds 8 times, it tends to be difficult to obtain a sheet having a uniform thickness. The air gap from the exit of the T die to the contact point of the rolling roll is usually preferably in the range of 10 to 200 mm.
The temperature of the rolling roll is preferably in the range of 20 ° C to less than 110 ° C, more preferably in the range of 70 ° C to 100 ° C. If the temperature of the rolling roll is lower than 20 ° C., the rollability tends to decrease. When the temperature of the rolling roll is 110 ° C. or higher, the melting point of the resin tends to be close to the melting point of the resin, and it is difficult to sufficiently solidify a uniform solution of the resin and the plasticizer.

In the step (c) of the production method of the present invention, the method of extracting the plasticizer may be any of a batch type and a continuous type. However, the plasticizer is extracted by immersing the microporous membrane in an extraction solvent and sufficiently extracted. It is important to dry and substantially remove the plasticizer from the microporous membrane. In order to suppress shrinkage of the microporous membrane, it is preferable to restrain the end of the microporous membrane during a series of steps of dipping and drying. Further, it is preferable that the residual amount of the plasticizer in the microporous membrane after extraction is less than 1% by weight.
In the production method of the present invention, the stretching performed before the extraction step is referred to as “pre-extraction stretching (step (b)”), and a step of stretching three times or more in the TD direction is essential. Further, simultaneous biaxial stretching and sequential biaxial stretching may be included. Further, the number of times of stretching may be any of one-stage stretching, multi-stage stretching, many-time stretching, and the like, and a known stretching device such as a tenter or a roll stretching machine can be used for stretching.

  In the stretching before extraction in the present invention, stretching in the TD direction by 3 times or more is indispensable for obtaining a dense membrane structure referred to in the present invention, and the stretching ratio in the TD direction is preferably 4 to 10 times. It is. In the pre-extraction stretching, the plasticizer is stretched in a state in which the plasticizer is highly dispersed in the micropores, crystal gaps, and amorphous portions of the microporous membrane. Has an effect of suppressing an increase in the porosity of the steel. When the stretching ratio is less than 3 times, when a subsequent stretching step after extraction is performed, a dense membrane structure is not maintained, and the pore diameter becomes coarse, so that a dense membrane structure cannot be obtained. On the other hand, if the stretching ratio exceeds 10 times, the amount of shrinkage in the TD direction at high temperatures tends to increase.

  The stretching temperature is preferably in the range of from 50 ° C. lower than the melting point Tm ° C. of the polyolefin microporous membrane to less than Tm ° C., and more preferably from 40 ° C. lower than the melting point Tm ° C. of the polyolefin microporous membrane to 5 ° C. below Tm ° C. Perform at temperatures below the lower temperature. If the stretching temperature is lower than a temperature 50 ° C. lower than Tm ° C., the stretchability tends to decrease. If the stretching temperature is higher than Tm ° C., the microporous membrane tends to melt and impair the permeability.

In the production method of the present invention, the stretching performed after the extraction step is referred to as post-extraction stretching (step (d)), and at least one stretching step is required at least in the MD direction.
At least the MD direction means that simultaneous biaxial stretching and / or sequential biaxial stretching may be included. The term "at least once" means one-stage stretching, multi-stage stretching, or many-time stretching. In the production method of the present invention, at least one stretching step is required at least in the MD direction after the extraction step for two reasons. One is that if the film is not stretched in the MD direction after extraction, the bubble point becomes 1.0 MPa or more, so that good cycle characteristics cannot be exhibited when incorporated into a battery. Although the reason is not clear by stretching, shrinkage at high temperature can be suppressed.

The magnification of stretching after extraction can be set to an arbitrary magnification, but in the MD direction only, the magnification is preferably 5 times or less, more preferably 1.1 to 3 times, and the area magnification in the biaxial direction, preferably 10 times or less. , More preferably 1.4 to 6 times. Stretching after extraction by more than 5 times in the MD direction tends to excessively increase the pore size despite being used in combination with stretching before extraction. On the other hand, if it is less than 1.1 times, the bubble point may be 1.0 MPa or more.
The temperature for stretching after extraction is preferably a temperature in the range of 50 ° C. lower than the melting point Tm ° C. of the polyolefin microporous membrane to less than Tm ° C., and more preferably 40 ° C. lower than the melting point Tm ° C. of the polyolefin microporous membrane to Tm ° C. It is performed at a temperature in the range of less than 5 ° C. below the temperature. If the stretching temperature is lower than 50 ° C. lower than Tm ° C., the stretchability tends to decrease, and the strain component after stretching tends to remain, and the shape stability at high temperatures tends to be insufficient. If the stretching temperature is higher than Tm ° C., the microporous membrane tends to melt and impair the permeability.

Since the stretching after the extraction is performed in a state where the plasticizer is substantially removed from the microporous membrane, the polymer interface is predominantly destroyed with the stretching, and the microporous membrane has an effect of increasing the pore diameter. Therefore, when only stretching after extraction is performed without performing stretching before extraction, which is essential in the present invention, an excessive increase in the pore diameter is unnecessarily caused, and stretching orientation cannot be imparted to the microporous membrane. turn into.
In contrast, in the case of the production method of the present invention in which the stretching before extraction and the stretching after extraction are used in combination, the pore size can be appropriately increased without impairing the strength of the microporous membrane, which is useful. As a result, the bubble point can be more than 0.5 MPa and less than 1.0 MPa, and good cycle characteristics can be exhibited when incorporated in a battery.

In the production method of the present invention, adding a heat treatment step such as heat setting and thermal relaxation subsequent to or after each stretching step within a range that does not impair the advantages of the present invention enhances the shape stability of the microporous membrane at high temperatures. effective.
In the manufacturing method of the present invention, post-processing may be performed within a range not to impair the advantages of the present invention. Examples of the post-treatment include a hydrophilization treatment with a surfactant or the like and a cross-linking treatment with ionizing radiation or the like.

  The polyolefin resin used in the present invention refers to a resin used for ordinary extrusion, injection, inflation, and blow molding, and includes ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-hexene. -Octene homopolymers and copolymers can be used. In addition, a polyolefin selected from the group of these homopolymers and copolymers can be used as a mixture. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber, and the like. . When using the microporous membrane obtained by the production method of the present invention as a battery separator, it is a low melting point resin, and from the performance required for high strength, it is particularly preferable to use a resin mainly composed of high-density polyethylene. .

The viscosity average molecular weight of the polyolefin resin raw material used in the present invention and the microporous membrane of the present invention is preferably 50,000 or more and less than 12 million, more preferably 100,000 or more and less than 4,000,000, and most preferably 200,000 or more and less than 2,000,000. is there. If the viscosity average molecular weight is less than 50,000, the melt tension during melt molding is small, and moldability is likely to be reduced. If the viscosity average molecular weight exceeds 12,000,000, it tends to be difficult to obtain a uniform resin composition.
The plasticizer used in the present invention may be a non-volatile solvent that can form a uniform solution at a temperature equal to or higher than the melting point of the polyolefin resin when mixed with the polyolefin resin. Examples thereof include hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol.

  The ratio of the polyolefin resin and the plasticizer used in the present invention is a ratio sufficient to cause microphase separation and to form a sheet-like microporous membrane precursor, and to such an extent that productivity is not impaired. I just want it. Specifically, the weight fraction of the polyolefin resin in the composition comprising the polyolefin resin and the plasticizer is preferably 10 to 70%, more preferably 20 to 60%. If the weight fraction of the polyolefin resin is less than 10%, the melt tension during melt molding tends to be insufficient, and the moldability tends to decrease. On the other hand, when the weight fraction exceeds 70%, productivity is hardly improved due to an increase in extrusion load.

  The extraction solvent used in the present invention is a poor solvent for the polyolefin and a good solvent for the plasticizer, and preferably has a boiling point lower than the melting point of the microporous polyolefin membrane. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, alcohols such as ethanol and isopropanol, diethyl ether and the like. Examples include ethers such as tetrahydrofuran and ketones such as acetone and 2-butanone.

In the composition used in the present invention, an additive such as an antioxidant, a crystal nucleating agent, an antistatic agent, a flame retardant, a lubricant, an ultraviolet absorber, etc., within a range not impairing the advantages of the present invention, according to the purpose. Can be mixed.
The microporous membrane of the present invention refers to a porous sheet or film substantially composed of polyolefin, and is used, for example, as a microfiltration membrane or a battery separator, and is particularly used for a battery separator. It is suitable.
The thickness of the microporous membrane of the present invention is preferably 1 to 100 µm, more preferably 5 to 50 µm. If the film thickness is smaller than 1 μm, the mechanical strength may be insufficient, and if it is larger than 100 μm, the volume occupied by the separator increases, which tends to be disadvantageous in terms of increasing the capacity of the battery.

  The porosity of the microporous membrane of the present invention is preferably in the range of 25% to 60%, more preferably 30% to 55%. If the porosity is less than 25%, the permeability tends to decrease, while if it exceeds 60%, the mechanical strength tends to decrease. The piercing strength of the microporous membrane of the present invention is preferably at least 3.0N, more preferably at least 4.0N. If it is less than 3.0 N, when used as a battery separator, the separator is easily broken by the dropped active material or the like.

Next, the present invention will be described in more detail by way of examples, which do not limit the scope of the present invention. The test method shown in the examples is as follows.
<Evaluation of microporous membrane>
(1) Film thickness It was measured with a dial gauge (Ozaki Seisakusho: trademark, PEACK No. 25).
(2) Porosity A sample having a size of 10 cm square was cut out from the microporous membrane, and its volume and weight were obtained. From the obtained results, calculation was made using the following equation.
Porosity (%) = (volume (cm ³ ) -weight (g) / density) / volume (cm ³ ) × 100
(3) Piercing strength Using a KAT-G5 handy compression tester manufactured by Kato Tech, a piercing test was performed under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec. (N).

(4) Air permeability Measured with a Gurley air permeability meter according to JIS P-8117. At this time, the pressure was 0.01276 atm, the membrane area was 6.424 cm ² , and the amount of permeated air was 100 cc.
(5) Air Permeability The air permeation rate constant Rgas is obtained from the air permeability (sec) using the following equation. The measurement was performed in a room at room temperature of 23 ° C.
Rgas (m ³ / (m ² · sec · Pa)) = 0.0001 / air permeability
/0.0006424/(0.01276×101325)
(6) Water Permeability A microporous membrane previously immersed in alcohol is set in a stainless steel liquid permeable cell having a diameter of 41 mm, and the alcohol of the membrane is washed with water, and then water is permeated at a differential pressure of about 50,000 Pa. , The amount of water per unit time, unit pressure, and unit area was calculated from the amount of water permeation (cm ³ ) after elapse of 120 seconds, and was calculated as the water permeability (cm ³ / (cm ² · sec · Pa)). . The measurement was performed in a room at room temperature of 23 ° C.
The water transmission rate constant Rliq is determined from the water permeability (cm ³ / (cm ² · sec · Pa)) using the following equation.
Rliq (m ³ / (m ² · sec · Pa)) = water permeability / 100

(7) Bubble point Measured with an ethanol solvent according to ASTM F316-86.
(8) Viscosity average molecular weight Mv
To prevent deterioration of the sample, 2,6-di-t-butyl-4-methylphenol was dissolved in decahydronaphthalene to a concentration of 0.1% by weight, and this (hereinafter abbreviated as DHN) was used as a sample solvent. Used. The sample is dissolved in DHN at 150 ° C. to a concentration of 0.1% by weight to prepare a sample solution. 10 ml of the prepared sample solution is sampled, and the number of seconds passed between the marked lines at 135 ° C. (t) is measured by a Cannon-Fenske viscometer (SO100). After heating DHN to 150 ° C., 10 ml was sampled, and the number of seconds (tB) passing between the marked lines of the viscometer was measured by the same method.
Using the obtained passing seconds t and tB, the intrinsic viscosity [η] was calculated by the following conversion formula.
[Η] = ((1.651 t / tB-0.651) 0.5-1) /0.0834
From the obtained [η], in the case of polyethylene, Mv was calculated by the following equation.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
In the case of polypropylene, Mv was calculated by the following equation.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(9) Shrinkage ratio A sample of MD50 mm × TD40 mm is cut out from the microporous membrane and sandwiched between two slide glasses having a width of 26 mm so that the MD direction of the microporous membrane protrudes. Turn over the protruding part and fix with heat resistant tape. This is sandwiched between glass plates of 80 mm × 50 mm × 5 mm, placed horizontally in an oven at 150 ° C., and left for 1 hour. Then, it cools by air and measures the shortest length of TD width (mm).
Shrinkage (%) = (1−shortest TD width (mm) / 40) × 100

<Production and evaluation of battery>
(1) Preparation of positive electrode 92.2% by weight of lithium-cobalt composite oxide LiCoO ₂ as an active material, 2.3% by weight of flaky graphite and acetylene black as conductive agents, and polyvinylidene fluoride (PVDF) 3 as a binder A slurry is prepared by dispersing 0.2% by weight in N-methylpyrrolidone (NMP). This slurry is applied to one surface 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 by a roll press. At this time, the active material coating amount of the positive electrode is set to 250 g / m ² , and the active material bulk density is set to 3.00 g / cm ³ . This is cut to a width of about 40 mm to form a band.

(2) Preparation of negative electrode 96.9% by weight of artificial graphite as an active material, 1.4% by weight of an ammonium salt of carboxymethyl cellulose and 1.7% by weight of a styrene-butadiene copolymer latex as a binder were dispersed in purified water to form a slurry. Is prepared. This slurry is applied to one surface of a copper foil having a thickness of 12 μm as a negative electrode current collector by a die coater, dried at 120 ° C. for 3 minutes, and then compression-molded by a roll press. At this time, the active material coating amount of the negative electrode is set to 106 g / m ² , and the active material bulk density is set to 1.35 g / cm ³ . This is cut to a width of about 40 mm to form a band.
(3) Adjustment of Nonaqueous Electrolyte LiPF ₆ is dissolved as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) so as to have a concentration of 1.0 mol / liter.

(4) Battery assembly The above-described microporous membrane separator, strip-shaped positive electrode, and strip-shaped negative electrode are stacked in the order of strip-shaped negative electrode, separator, strip-shaped positive electrode, and separator, and spirally wound to form an electrode plate laminate.
After pressing this electrode plate laminate into a flat plate shape, it is housed in an aluminum container, an aluminum lead is led out from the positive electrode current collector, and the nickel lead is led out from the negative electrode current collector to the bottom of the container. Weld. Further, the above-mentioned non-aqueous electrolyte is injected into the container and sealed. The lithium-ion battery manufactured in this manner is 6.3 mm in length (thickness), 30 mm in width, and 48 mm in height, and is designed to have a nominal discharge capacity of 620 mAh.

(5) Battery evaluation (under 25 ° C atmosphere)
The lithium ion battery assembled as described above is charged at a constant current and constant voltage (CCCV) for 6 hours under the conditions of a current value of 310 mA (0.5 C) and a final battery voltage of 4.2 V. At this time, the current value immediately before the end of the charging is almost zero. Then, it is left (aged) in a 25 ° C. atmosphere for one week.
Next, the battery was charged at a constant current and constant voltage (CCCV) for 3 hours under the conditions of a current value of 620 mA (1.0 C) and a final battery voltage of 4.2 V, and discharged to a battery voltage of 3.0 V at a constant current value (CC) of 620 mA. Cycle. The discharge capacity at this time is defined as an initial discharge capacity. The initial discharge capacity is a value close to the nominal discharge capacity, or that the initial discharge capacity is higher than the initial discharge capacity is maintained high, which is an index that the initial battery characteristics in the present invention are good. .
Thereafter, the above-described cycle is further repeated 300 times. In this cycle, the ratio (%) of the capacity at the 300th cycle to the initial discharge capacity is defined as the capacity retention rate. The higher the capacity retention ratio, the better the cycle characteristics.

[Example 1]
To 60 parts by weight of HDPE having an Mv of 270,000 and 40 parts by weight of HDPE having an Mv of 950,000, 0.3 of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added. Parts by weight were added and dry blended using a tumbler blender to obtain a mixture of polymers and the like. The obtained mixture of polymers and the like was supplied to a feed port of a twin screw co-rotating screw type extruder having a diameter of 37 mm by a feeder. Liquid paraffin (kinematic viscosity at 37.78 ° C. 7.59 × 10 ⁻⁵ m ² / s) was injected into a twin-screw extruder cylinder by a plunger pump.

The feeder and pump were adjusted such that the liquid paraffin content ratio in the total mixture (100 parts by weight) melt-kneaded and extruded was 62 parts by weight. Melt kneading was performed at a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded material was extruded through a T-die onto a cooling roll controlled at a surface temperature of 85 ° C., and was rolled at a rolling ratio of 4 to obtain a gel sheet having a thickness of 200 μm.
Next, the obtained sheet was guided to a horizontal tenter stretching machine, subjected to horizontal stretching before extraction at a stretching temperature of 115 ° C. and a stretching magnification of 5 times, and then subjected to 10% thermal relaxation. Next, it was immersed in methylene chloride to extract and remove liquid paraffin and dried.

Next, the membrane after the extraction was guided to a multi-roll longitudinal stretching machine, and the membrane was extracted and longitudinally stretched so as to have a stretching temperature of 110 ° C. and a total stretching ratio of 2 times in the MD direction to obtain a microporous membrane.
Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator. Further, in order to clarify the differences between the following examples and comparative examples, Table 2 shows a stretching process comparison table.

[Example 2]
At a rolling ratio of 3 times, the thickness of the gel sheet is 250 μm, the magnification of the longitudinal stretching after extraction is 1.5 times, and then the lateral stretching after the extraction is stretched at a stretching temperature of 125 ° C. and a stretching ratio of 1.4 times to 10% thermal relaxation. A microporous membrane was obtained in the same manner as in Example 1 except for performing the above. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Example 3]
A microporous membrane was obtained in the same manner as in Example 2 except that the thickness of the gel sheet was 150 μm, the stretching ratio after extraction was 1.5 times, and then the stretching after extraction was 1.7 times. . Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Example 4]
As raw materials, 30 parts by weight of Mv270,000 HDPE, 40 parts by weight of Mv950, HDPE, and 30 parts by weight of linear low-density polyethylene, Mv120,000 were used. The thickness of the gel sheet was 150 μm. A microporous film was obtained in the same manner as in Example 2 except that the stretching temperature was 110 ° C. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Example 5]
The melt-kneaded product extruded from the T-die was taken out by a cooling roll controlled at a surface temperature of 40 ° C. to obtain a gel sheet having a thickness of 2 mm. The obtained sheet was stretched by a simultaneous biaxial tenter at a stretching temperature of 122 ° C. to 7 × 7 before extraction, and then immersed in methylene chloride to extract and remove liquid paraffin and dried. Next, the film is extracted at a stretching temperature of 110 ° C. and a stretching ratio of 1.5 times, and then longitudinally stretched. Subsequently, the film is extracted at a stretching temperature of 125 ° C. and a stretching ratio of 1.4 times, and then transversely stretched to relax the heat by 10%. Got. The steps up to extrusion other than those described above were the same as in Example 1. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Comparative Example 1]
A microporous membrane was obtained in the same manner as in Example 5, except that longitudinal stretching was not performed after extraction. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Comparative Example 2]
Mv 280,000 HDPE 40 parts by weight, dioctyl phthalate 42 parts by weight, dibutyl phthalate 18 parts by weight and 0.3 parts by weight of tetrakis- [methylene-3- (3 ′, 5′-di- [t-Butyl-4′-hydroxyphenyl) propionate] methane was kneaded at 250 ° C. using a twin-screw extruder, extruded from a T-die and taken up by a cooling roll to obtain a gel sheet having a thickness of 1600 μm. The obtained sheet was stretched 7 × 7 times at a stretching temperature of 130 ° C. using a simultaneous biaxial tenter, then dipped in methylene chloride to remove the plasticizer, and then dried. Next, the membrane was extracted at a stretching temperature of 125 ° C. and a stretching magnification of 1.8 times and then horizontally stretched, and then thermally relaxed by 17% to obtain a microporous membrane. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Comparative Example 3]
A microporous membrane was obtained according to the method of Example 2 of JP-A-11-60789 (no longitudinal stretching after extraction) except that the final film thickness was adjusted to 20 μm. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Comparative Example 4]
A microporous membrane was obtained according to the method of Example 1 of Japanese Patent No. 2657430 (no longitudinal stretching after extraction), except that a twin screw co-axial screw extruder having a diameter of 37 mm was used. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

[Comparative Example 5]
Example 1 except that the thickness of the gel sheet was 150 μm, the stretching before extraction was not performed, the stretching ratio after stretching was 4 times, and then the stretching after extraction was stretched at a stretching temperature of 125 ° C. and a stretching ratio of 2 times. Similarly, a microporous membrane was obtained. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of a battery using the same as a separator.

  As is clear from Table 1, when the microporous membrane of the present invention was used as a separator for a secondary battery, the cycle characteristics were higher than in Comparative Examples 1 and 3, and the initial discharge capacity was higher than in Comparative Examples 2, 4, and 5. Is maintained high, and it can be seen that both initial battery characteristics and cycle characteristics can be achieved. In addition, it can be seen that, in the examples, since the longitudinal stretching was performed after the extraction, the shrinkage of the obtained microporous membrane at a high temperature was suppressed.

  The polyolefin microporous membrane of the present invention can be suitably used particularly as a battery separator.

Claims (4)

A microporous polyolefin membrane, wherein the ratio of water permeability / air permeability is 0.8 × 10 ^{−3 to} 1.7 × 10 ⁻³ and the bubble point is more than 0.5 MPa and less than 1.0 MPa.   2. The microporous polyolefin membrane according to claim 1, wherein the shrinkage in the TD direction in a state where the MD direction is fixed is less than 20% at 150 ° C. 3.   (A) a step of melt-kneading a composition comprising a polyolefin resin and a plasticizer, extruding, cooling and solidifying to form a sheet, and (b) after the step (a), stretching at least three times in the TD direction. And (c) extracting the plasticizer after the step (b); and (d) performing at least one stretching in at least the MD direction after the step (c). The method for producing a microporous membrane according to claim 1 or 2, wherein   A battery separator comprising the polyolefin microporous membrane according to claim 1.