JP4220329B2 - Polyolefin microporous membrane and method for producing the same - Google Patents

Description

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

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

  The output characteristics of a battery are current characteristics such as discharge performance at large current and discharge performance at low temperature, and characteristics related to life such as cycle characteristics and high temperature storage characteristics. The higher the ion permeation performance of the separator, the better the ion permeation performance. As physical properties closely related to the above, gas permeability typified by bubble point and air permeability, liquid permeability typified by water permeability, and the like are used. 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 and the rate of heat generation at the time of abnormality tend to increase due to the increase in energy density, and the film shrinkage at high temperature is desired to be low because it is maintained at a high temperature for a while after the current interruption by the fuse. .

  In 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 removal process and a stretched thinning process are applied to this. A technique for forming a porous film is known. In such a known technique, depending on whether the stage of stretching is performed before or after the extraction process, it will be referred to as pre-extraction stretching or post-extraction stretching, respectively. In addition, the matrix resin can be oriented to increase the strength, and a microporous membrane having a dense pore structure with a small pore diameter can be produced. On the other hand, in the case of a process by stretching after extraction, interfacial fracture proceeds predominantly by stretching, so that a microporous film having a large pore diameter and high permeability can be obtained.

  When the microporous membrane obtained by these two manufacturing methods is used as a separator for a lithium ion secondary battery, the above-mentioned mechanical characteristics and battery characteristics are contradictory. That is, the small pore diameter membrane by stretching before extraction is excellent in maintaining the initial capacity at the time of battery preparation with high mechanical strength, but inferior in cycle characteristics, while the large pore diameter membrane by stretching after extraction has excellent cycle characteristics, but the initial capacity. The characteristic is that the decrease in mechanical strength and mechanical strength are weak.

  It can be easily inferred that the mechanical strength is caused by each manufacturing method. On the other hand, the superiority or inferiority of battery characteristics can be interpreted as follows. In other words, the reason why the small-diameter 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 or the side reaction of the electrolyte in the secondary battery that repeats charge and discharge This is because microporous clogging occurs gradually, and clogging becomes severe during repeated charge / discharge cycles in a small-pore film, and the initial battery performance cannot be maintained. On the other hand, the large pore diameter membrane is not easily clogged and the performance is maintained.

  On the other hand, the reason why the large pore diameter film is inferior to the initial capacity of the battery is thought to be that there is a large amount of metallic lithium that is deposited due to current concentration during the initial charge / discharge due to the nonuniform surface pore structure. Despite the importance of the initial capacity of the battery, not much attention has been paid to it, but today, when the demand for higher energy density of the battery is severe, separators that can maintain high initial capacity and maintain cycle characteristics are very It can be said that it is useful.

  Regarding the safety performance, there is a fuse function as a separator function. This is a function of blocking the permeation of ions by blocking the pores of the microporous membrane and suppressing the progress of further heat generation when the battery is abnormally heated due to short circuit or high temperature exposure. However, with the progress of higher capacity and higher density, the amount of heat generated when abnormal heat is generated is large and the temperature rises rapidly. Therefore, even after the fuse, the inside of the battery may reach an extremely high temperature where the separator melts. . In a molten state, a highly stretched separator may be greatly shrunk and may be in a dangerous state where the electrodes are re-short-circuited due to contact between the electrodes. For this reason, separators that are excellent in shape retention at high temperatures above the melting point are eagerly desired. Among them, in most batteries, the separator is wound in the MD direction, and thus particularly excellent in shape retention in the TD direction. The separator is very useful. The MD direction referred to here is the machine direction, that is, the winding direction during continuous film formation of the separator, and the TD direction is the width direction perpendicular to the MD direction.

For the purpose of maintaining permeability and membrane strength, the following known methods disclose production methods such as using both pre-extraction stretching and post-extraction stretching. However, both of the following problems are associated with the initial capacity of the battery. No attention has been paid to the maintenance, and a microporous membrane that can maintain the initial capacity at a high level and maintain the cycleability has not been obtained.
In patent document 1, although it is highly permeable, its porosity is high and its membrane strength is weak for battery separator applications.
In Patent Document 2, a polyolefin microporous membrane that can freely adjust the permeation performance without damaging the strength by simultaneously using the pre-extraction stretching and the post-extraction stretching, and at the same time without losing the strength, and at the same time having excellent dimensional stability at high temperatures. Has been disclosed, but a microporous membrane capable of maintaining the initial capacity high and maintaining the cycleability has not been obtained.
Further, Patent Document 3 discloses a method for producing a microporous membrane having through holes of an appropriate size and a sharp pore size distribution by regulating the stretching temperature after extraction to a low temperature. Since the pore diameter is large and the porosity is also high, the electrolyte retainability is poor for battery separator applications. The reason why the pore diameter becomes larger and the porosity becomes higher is not clear, but local stretching does not occur by stretching at a low temperature, but stretching stress is not used for stretching orientation of the resin itself, but only for expanding pores. It is thought that it is to be.

  Regarding safety, Patent Document 2 discloses a polyolefin microporous membrane having excellent dimensional stability at high temperatures, but it does not describe in detail how high temperature is, and “thermal shrinkage” Is an index for evaluating the dimensional stability of a microporous membrane at a high temperature ", but since the measurement condition of the heat shrinkage rate is 100 ° C for 2 hours, the separator in question in the present invention It is not a description of the high temperature at which the metal enters 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 the shrinkage rate at 105 ° C., focusing only on the shrinkage at a low temperature below the fuse temperature, and does not describe the shrinkage above the fuse temperature.
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 thereby has a stretch ratio of 3 times or more in the MD direction. After stretching, the transmission performance is inferior because a step of shrinking 20 to 80% in the TD direction is essential. Furthermore, 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, but the MD and TD directions have a large anisotropy in order to stretch significantly only in the MD direction. The structure is easy to split longitudinally, and film breakage due to dendrite-precipitated crystals tends to occur. Therefore, the initial battery characteristics, cycle characteristics, and safety cannot be satisfied.

Further, Patent Documents 1 and 3 do not focus on shrinkage.
A separator for lithium ion secondary batteries that combines the cycle characteristics of large pore membranes with the initial battery properties of small pore membranes, and high safety, that is, the fine permeability that combines good permeation performance with a dense membrane structure and low shrinkage at high temperatures. The appearance of a porous membrane was eagerly desired.
Japanese Examined Patent Publication No. 6-21177 Japanese Patent Laid-Open No. 11-60789 Japanese Patent No. 2657430 Japanese Patent Laid-Open No. 9-12756 JP 2003-119306 A

  The present invention has both good permeation performance, a dense membrane structure, and low shrinkage at high temperature. When used as a secondary battery separator such as lithium ion, the cycle characteristics and the initial battery characteristics can be achieved at the same time. The present invention also provides an excellent polyolefin microporous membrane and a method for producing the same.

  As a result of intensive studies on the above problems, the inventors of the present invention have made a specific range and magnification by stretching in a specific direction and magnification in each of the stretching steps before and after the extraction in the polyolefin microporous membrane production method. A separator that has a water permeability / air balance and a specific range of bubble points, can reduce shrinkage at high temperatures, and can maintain cycle capacity while maintaining high initial capacity of the battery. As a result, the present invention has been found to be useful.

That is, the present invention is as follows.
(1) 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 polyolefin microporous membrane according to (1), wherein the shrinkage rate in the TD direction with the MD direction 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, solidifying by cooling, and forming into a sheet shape, (b) stretching at least 3 times in the TD direction after the step (a) (C) a step of extracting the plasticizer after the step (b), (d) a step of performing at least one stretching in the MD direction after the step (c). The method for producing a microporous membrane according to the above (1) or (2), wherein
(4) A battery separator comprising the polyolefin microporous membrane as described in (1) or (2) above.

  The polyolefin microporous membrane of the present invention has both good permeation performance, a dense membrane structure, and low shrinkage at high temperatures, and is compatible with both initial battery characteristics and cycle characteristics, and is excellent in safety. It is suitable as a separator for a secondary battery.

Hereinafter, the present invention will be described in detail with a focus on preferred embodiments. Water permeability in the present invention, the unit time of the microporous membrane and the air permeability amount, unit pressure is that of the permeation amount of water and air per unit area, water permeation rate constants ^{Rliq (m 3 / (m 2} · sec · Pa)) is the water permeability, and the air permeation rate constant Rgas (m ³ / (m ² · sec · Pa)) is the air permeability.
Here, the air permeability is a value obtained by converting an air permeability value measured by a Gurley type air permeability meter according to 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 1.0 × 10 ^{−3 to} 1.5 × 10 ⁻³ . If this ratio is less than 0.8 × 10 ⁻³ , the discharge characteristics tend to decrease, and if it exceeds 1.7 × 10 ⁻³ , the initial battery capacity tends to decrease.

The water permeability is preferably in the range of 3 × 10 ⁻¹⁰ to 4 × 10 ⁻⁹ m ³ / (m ² · sec · Pa), 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 electrolytic solution tends to be slow. Moreover, when the water permeation amount is larger than 4 × 10 ⁻⁹ m ³ / (m ² · sec · Pa), the liquid retaining property of the electrolytic solution tends to be lowered.
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), more preferably 4 × 10 ^{−7 to} 1.5 ×. 10 ⁻⁶ m ³ / (m ² · sec · Pa), more preferably 4 × 10 ⁻⁷ to 1 × 10 ⁻⁶ m ³ / (m ² · sec · Pa). If the air permeability is smaller than 3 × 10 ⁻⁷ m ³ / (m ² · sec · Pa), the discharge characteristics at low temperatures of the battery tend to be lowered, and 3 × 10 ⁻⁶ m ³ / (m ² If it is greater than 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 the 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 for representing the maximum pore diameter. However, when the bubble point is 0.5 MPa or less, the pores become coarse, resulting in a decrease in film strength, which may cause a winding failure during battery production. On the other hand, at 1.0 MPa or more, the pores are too dense and sufficient cycle characteristics cannot be obtained.

  Furthermore, a great feature of the film of the present invention is that the thermal shrinkage rate is less than 20%. The term “thermal shrinkage” as used herein refers to the shrinkage rate 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. In particular, in the case of a square battery, the wound product is crushed. Therefore, the MD direction of the separator is fixed and cannot be contracted. On the other hand, the TD direction is not fixed and the contraction is easy. When the shrinkage rate at 150 ° C is 20% or more, the separator shrinks so much that the electrodes are exposed after the current is interrupted by the fuse function, and the electrodes come into contact with each other to cause a short circuit, increasing the risk of heat generation and ignition. To do. The shrinkage of the separator at 150 ° C. is less than 20%, preferably less than 15%, more preferably less than 10%.

  Next, a preferred method for producing the polyolefin microporous 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 performed by adding the polyolefin resin to a resin kneading apparatus such as an extruder or a kneader and heating and melting the resin at an arbitrary ratio. A method of obtaining a uniform solution by introducing a plasticizer and kneading a composition comprising a resin and a plasticizer is preferred. Moreover, what knead | mixed resin and a plasticizer previously may be thrown in. The polyolefin resin to be introduced may be in any form of powder, granules, and pellets. Moreover, when knead | mixing by such a method, it is preferable that the form of a plasticizer is a normal temperature liquid. As an extruder, a single screw type extruder, a biaxial different direction screw type extruder, a biaxial same direction screw type extruder, etc. can be used.

  In the step (a) of the production method of the present invention, the method of producing a sheet-like microporous membrane precursor by extruding and solidifying by cooling is to extrude a uniform solution of resin and plasticizer into a sheet form via a T die or the like. It is preferably carried out 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. In particular, a method of cooling by contacting with a metal roll has the highest heat conduction efficiency and is preferable. Also, when contacted with a metal roll, calendering or hot rolling such as sandwiching between rolls further increases the efficiency of heat conduction, and the sheet is oriented to increase the film strength. Since surface smoothness also improves, it is preferable.

When rolling, the ratio of the sheet thickness before rolling and the sheet thickness after rolling is called the rolling ratio, but the preferred range of the rolling ratio is 1.5 to 8 times, more preferably 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 film thickness. The air gap from the T die exit to the rolling roll contact point 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. When 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, there is a tendency that the uniform solution of the resin and the plasticizer is not sufficiently solidified, close to the resin melting point.

In the step (c) of the production method of the present invention, the method of extracting the plasticizer may be either a batch type or a continuous type, but the plasticizer is extracted by immersing the microporous membrane in the extraction solvent, It is essential that the plasticizer be substantially removed from the microporous membrane by drying. In order to suppress the shrinkage of the microporous membrane, it is preferable to constrain the end of the microporous membrane during a series of steps of immersion and drying. Further, the residual amount of plasticizer in the microporous membrane after extraction is preferably less than 1% by weight.
In the production method of the present invention, stretching performed before the extraction step is referred to as pre-extraction stretching [(b) step], and a step of stretching three times or more in the TD direction is essential. Moreover, simultaneous biaxial stretching and sequential biaxial stretching may be included. Furthermore, the number of stretching operations may be any of one-stage stretching, multi-stage stretching, and multi-stage stretching, and a known stretching apparatus such as a tenter or a roll stretching machine can be used for stretching.

  In the pre-extraction stretching in the present invention, stretching in the TD direction by 3 times or more is essential for obtaining a dense film structure as 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 highly dispersed state in the micropores, crystal gaps, and amorphous parts of the microporous membrane, so that the stretchability is improved by the plasticizing effect and the microporous membrane is improved. This has the effect of suppressing the increase in porosity. If the draw ratio is less than 3, the dense film structure is not maintained and the pore diameter becomes coarse when the drawing process after the subsequent extraction is performed, and a dense film structure cannot be obtained. On the other hand, when the draw ratio exceeds 10, the amount of shrinkage in the TD direction at high temperatures tends to increase.

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

In the production method of the present invention, the stretching performed after the extraction step is called post-extraction stretching [(d) step], and at least one stretching step is essential in at least the MD direction.
At least the MD direction means that simultaneous biaxial stretching and / or sequential biaxial stretching may be included. In addition, at least once means one-stage stretching, multi-stage stretching, or multi-stage stretching. In the production method of the present invention, there are two reasons why at least one stretching step is required at least in the MD direction after the extraction step. One is that if it is not stretched in the MD direction after extraction, the bubble point becomes 1.0 MPa or more, and good cycle characteristics cannot be expressed when incorporated in a battery, and the other is in the MD direction. Although the reason is not clear by extending | stretching, it is a point which can suppress shrinkage | contraction at the time of high temperature.

The ratio of the stretching after extraction can be set to an arbitrary ratio, but the ratio in the MD direction alone is preferably within 5 times, more preferably 1.1 to 3 times, and the area magnification in the biaxial direction, preferably within 10 times. More preferably, it is 1.4 to 6 times. If the film is stretched after extraction exceeding 5 times in the MD direction, the pore diameter tends to be excessively increased in spite of the combined use 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 stretching temperature 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., more preferably 40 ° C. lower than the melting point Tm ° C. of the polyolefin microporous membrane. At a temperature in the range of less than 5 ° C below the temperature. If the stretching temperature is less than 50 ° C. lower than Tm ° C., the stretchability tends to decrease, and a strain component after stretching tends to remain, which is insufficient in terms of shape stability at high temperatures. When the stretching temperature is Tm ° C. or higher, the microporous membrane tends to melt and impair the permeation performance.

Stretching after extraction is performed in a state in which the plasticizer is substantially removed from the microporous membrane, and therefore, the polymer interface breaks predominantly with stretching, and the microporous membrane has an effect of increasing the pore size. Therefore, if only the post-extraction stretching is performed without performing the pre-extraction stretching, which is essential in the present invention, the pore diameter is excessively increased, and the stretch orientation cannot be imparted to the microporous film, resulting in low strength. turn into.
Compared to this, the production method of the present invention in which the pre-extraction stretching and the post-extraction stretching are used in combination is useful because the pore diameter can be increased appropriately without impairing the strength of the microporous membrane. Thereby, a bubble point can be made to exceed 0.5 Mpa and less than 1.0 Mpa, and favorable cycling characteristics can be expressed when it is incorporated in a battery.

In the production method of the present invention, it is possible to increase the shape stability of the microporous membrane at a high temperature by adding a heat treatment step such as heat fixation and thermal relaxation after each stretching step within a range that does not impair the advantages of the present invention. effective.
Moreover, in the manufacturing method of this invention, you may post-process in the range which does not impair the advantage of this invention. Examples of the post-treatment include a hydrophilic treatment with a surfactant and the like, and a crosslinking treatment with ionizing radiation.

  The polyolefin resin used in the present invention refers to a resin used for ordinary extrusion, injection, inflation, and blow molding. Ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1 -Octene homopolymers and copolymers can be used. In addition, polyolefins selected from the group of these homopolymers and copolymers can be mixed and used. 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, and ethylene propylene rubber. . When the microporous membrane obtained by the production method of the present invention is used as a battery separator, it is preferable to use a resin having a high melting point polyethylene as a main component because it is a low-melting-point resin and requires high strength. .

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 from 50,000 to less than 12 million, more preferably from 100,000 to less than 4 million, and most preferably from 200,000 to less than 2 million. is there. When the viscosity average molecular weight is less than 50,000, the melt tension at the time of melt molding becomes small and the moldability tends to be lowered. When the viscosity average molecular weight exceeds 12 million, it tends to be difficult to obtain a uniform resin composition.
The plasticizer used in the present invention may be any non-volatile solvent that can form a uniform solution above 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 between the polyolefin resin and the plasticizer used in the present invention is a ratio sufficient to cause microphase separation and form a sheet-like microporous membrane precursor and does not impair productivity. I just need 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%. When the weight fraction of the polyolefin resin is smaller than 10%, the melt tension at the time of melt molding tends to be insufficient, and the moldability tends to be lowered. On the other hand, when the weight fraction exceeds 70%, it is difficult to improve productivity due to an increase in extrusion load.

  The extraction solvent used in the present invention is preferably a poor solvent for polyolefin and a good solvent for plasticizer, and its boiling point is preferably lower than the melting point of the polyolefin microporous membrane. Examples of such extraction solvents 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, 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., as long as the advantages of the present invention are not impaired. Can be mixed.
The microporous membrane of the present invention refers to a porous sheet or film substantially composed of polyolefin. For example, it is used as a microfiltration membrane or a battery separator, particularly for use in battery separators. It is suitable.
The film 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 occupied volume of the separator increases, which tends to be disadvantageous in terms of increasing the battery capacity.

  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 puncture strength of the microporous membrane of the present invention is preferably 3.0N or more, and more preferably 4.0N or more. If it is less than 3.0N, when used as a battery separator, the separator is easily broken by the dropped active material or the like.

The invention will now be described in more detail by way of examples, which do not limit the scope of the invention. The test methods shown in the examples are as follows.
<Evaluation of microporous membrane>
(1) Film thickness Measured with a dial gauge (Ozaki Seisakusho: trademark, PEACOCK No. 25).
(2) Porosity A 10 cm square sample was cut from the microporous membrane, its volume and weight were determined, and the obtained results were used for calculation.
Porosity (%) = (volume (cm ³ ) −weight (g) / density) / volume (cm ³ ) × 100
(3) Puncture strength A piercing test was conducted using a Kato Tech, trademark, KES-G5 handy compression tester under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. (N).

(4) Air permeability It measured with the Gurley type air permeability meter based on JISP-8117. The pressure at this time is 0.01276 atm, the membrane area is 6.424 cm ² , and the amount of permeated air is 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 carried out indoors at room temperature 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 permeation cell having a diameter of 41 mm, and after the alcohol in the membrane is washed with water, water is allowed to permeate at a differential pressure of about 50000 Pa. The water permeation amount per unit time, unit pressure, and unit area was calculated from the water permeation amount (cm ³ ) when 120 seconds passed, and this was defined as the water permeability (cm ³ / (cm ² · sec · Pa)). . The measurement was carried out indoors at room temperature 23 ° C.
The water permeation rate constant Rliq is obtained 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 It measured with the ethanol solvent based on ASTMF316-86.
(8) Viscosity average molecular weight Mv
To prevent deterioration of the sample in decahydronaphthalene, 2,6-di-t-butyl-4-methylphenol is dissolved to a concentration of 0.1% by weight, and this (hereinafter abbreviated as DHN) is used as the sample solvent. Use. A sample solution is prepared by dissolving the sample in DHN at 150 ° C. to a concentration of 0.1% by weight. 10 ml of the prepared sample solution is sampled, and the number of seconds passing through the marked line (t) at 135 ° C. is measured with a Canon Fenceke viscometer (SO100). Moreover, after heating DHN to 150 degreeC, 10 ml is extract | collected and the number of seconds (tB) which passes between the marked lines of a viscometer is measured by the same method.
The intrinsic viscosity [η] was calculated by the following conversion formula using the obtained passing seconds t and tB.
[Η] = ((1.651t / tB−0.651) 0.5−1) /0.0834
In the case of polyethylene, Mv was calculated from the obtained [η] by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
In the case of polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(9) Shrinkage A sample of MD 50 mm × TD 40 mm is cut 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 it with heat-resistant tape. This is sandwiched between 80 mm × 50 mm × 5 mm glass plates, 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 rate (%) = (1-shortest TD width (mm) / 40) × 100

<Production and evaluation of battery>
(1) Production of positive electrode 92.2% by weight of lithium cobalt composite oxide LiCoO ₂ as an active material, 2.3% by weight of flake graphite and acetylene black as a conductive agent, and polyvinylidene fluoride (PVDF) 3 as a binder A slurry is prepared by dispersing 2% by weight in N-methylpyrrolidone (NMP). This slurry is applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the positive electrode is 250 g / m ² and the active material bulk density is 3.00 g / cm ³ . This is cut to a width of about 40 mm to form a strip.

(2) Production of Negative Electrode 96.9% by weight of artificial graphite as an active material, 1.4% by weight of ammonium salt of carboxymethyl cellulose and 1.7% by weight of styrene-butadiene copolymer latex as a binder were dispersed in purified water to form a slurry. To prepare. This slurry is applied to one side of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material 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 strip.
(3) Adjustment of Nonaqueous Electrolyte Solution It is adjusted by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / liter.

(4) Battery assembly The above-mentioned microporous membrane separator, strip-like positive electrode and strip-like negative electrode are stacked in the order of the strip-like negative electrode, separator, strip-like positive electrode, and separator, and wound in a spiral shape to produce an electrode plate laminate.
This electrode plate laminate is pressed into a flat plate shape, and then stored in an aluminum container. The aluminum lead is led out from the positive electrode current collector to the battery lid, and the nickel lead is led out from the negative electrode current collector to the bottom of the container. Weld. Further, the non-aqueous electrolyte described above is injected into the container and sealed. The lithium ion battery thus produced is designed to have a vertical (thickness) size of 6.3 mm, a horizontal size of 30 mm, and a height of 48 mm, and a nominal discharge capacity of 620 mAh.

(5) Battery evaluation (at 25 ° C atmosphere)
The lithium ion battery assembled as described above is charged with 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 charging is almost zero. Then, it is left (aged) for 1 week in an atmosphere of 25 ° C.
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 termination 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. The cycle that is done. The discharge capacity at this time is defined as the initial discharge capacity. If the initial discharge capacity is a value close to or higher than the nominal discharge capacity, the initial capacity is maintained high, which is an indicator of good initial battery characteristics in the present invention. .
Thereafter, the above-described cycle is further repeated 300 times. In this cycle, the capacity ratio is the ratio (%) of the capacity at the 300th cycle to the initial discharge capacity. A high capacity retention rate means good cycle characteristics.

[Example 1]
As an antioxidant, 0.3 parts of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was added 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. Part by weight was added and dry blended using a tumbler blender to obtain a polymer mixture. The obtained mixture of polymers and the like was fed to a feed port of a twin screw co-directional screw type extruder having a diameter of 37 mm by a feeder. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the twin-screw extruder cylinder by a plunger pump.

The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture (100 parts by weight) melted and kneaded and extruded was 62 parts by weight. The melt-kneading conditions were 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 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 transverse tenter stretching machine, subjected to transverse stretching before extraction at a stretching temperature of 115 ° C. and a stretching ratio 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 extracted membrane was guided to a multi-stage roll type longitudinal stretching machine, and extracted and longitudinally stretched so that the stretching temperature was 110 ° C. and the total stretching ratio in the MD direction was doubled to obtain a microporous membrane.
Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of the battery using this as a separator. Moreover, in order to clarify the difference between the Example shown below and a comparative example, the extending | stretching process comparison table was shown as Table 2.

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

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

[Example 4]
As raw materials, 30 parts by weight of HDPE with Mv 270,000, 40 parts by weight of HDPE with Mv 950,000, and 30 parts by weight of linear low density polyethylene with Mv 120,000, the thickness of the gel sheet is 150 μm, the stretching temperature for longitudinal stretching after extraction is 105 ° C., A microporous membrane was obtained in the same manner as in Example 2 except that the stretching temperature for stretching was 110 ° C. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of the battery using this as a separator.

[Example 5]
The melt-kneaded product extruded from the T-die was taken up with 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 before extraction by 7 × 7 times at a stretching temperature of 122 ° C. using a simultaneous biaxial tenter, and subsequently immersed in methylene chloride to extract and remove liquid paraffin, followed by drying. Next, the film was extracted at a stretching temperature of 110 ° C. and a draw ratio of 1.5 times, and then longitudinally stretched. Got. The other steps up to extrusion were the same as in Example 1. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of the battery using this 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 the battery using this 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 with 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, and subsequently immersed in methylene chloride to remove the plasticizer and then dried. Next, this membrane was extracted at a stretching temperature of 125 ° C. and stretched at a draw ratio of 1.8, and then stretched laterally, followed by 17% thermal relaxation to obtain a microporous membrane. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of the battery using this as a separator.

[Comparative Example 3]
A microporous membrane was obtained in accordance with 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 the battery using this as a separator.

[Comparative Example 4]
A microporous membrane was obtained in accordance with the method of Example 2 of Japanese Patent No. 2657430 (no longitudinal stretching after extraction) except that a biaxial co-directional screw type extruder having a diameter of 37 mm was used. Table 1 shows the physical properties of the obtained microporous membrane and the characteristics of the battery using this as a separator.

[Comparative Example 5]
Example 1 with the exception that the gel sheet thickness is 150 μm, stretching before extraction is not performed, the ratio of longitudinal stretching after extraction is 4 times, and then lateral stretching after extraction is 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 the battery using this as a separator.

  As is apparent from Table 1, the microporous membrane of the present invention, when used as a secondary battery separator, has higher cycle characteristics than Comparative Examples 1 and 3, and the initial discharge capacity compared to Comparative Examples 2, 4 and 5. It can be seen that the initial battery characteristics and the cycle characteristics can be compatible. Moreover, in the Example, since the longitudinal stretch after extraction is performed, it turns out that the shrinkage | contraction at the time of the high temperature of the obtained microporous film is suppressed.

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

Claims (4)

A polyolefin microporous membrane having a water permeability / air permeability ratio of 0.8 × 10 ^{−3 to} 1.7 × 10 ⁻³ and a bubble point of more than 0.5 MPa and less than 1.0 MPa.   2. The polyolefin microporous membrane according to claim 1, wherein a shrinkage ratio 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, solidifying by cooling, and forming into a sheet; (b) after the step (a), stretching 3 times or more in the TD direction; And (c) a step of extracting the plasticizer after the step (b), and (d) a step of performing at least one stretching in the MD direction after the step (c). The method for producing a microporous membrane according to claim 1 or 2.   A battery separator comprising the microporous polyolefin membrane according to claim 1.