JP2008186722A - Porous membrane having high thermal resistance and high permeability, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-layer porous membrane having a high safety and practicality, especially as a separator for a non-aqueous electrolyte battery since it has thermal resistance and high permeability at the same time. <P>SOLUTION: This is a multi-layer porous membrane in which at least one face of the polyolefin resin porous membrane has a wetting index of 40 mN/m or more and which has a porous layer consisting of an inorganic filler and a resin binder and its manufacturing method. Also a separator for non-aqueous electrolyte battery using it and the non-aqueous electrolyte battery are provided. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a porous membrane suitably used for separation of separators and substances in batteries, capacitors and the like, and a method for producing the same. Furthermore, the present invention relates to a separator for a non-aqueous electrolyte battery and a non-aqueous electrolyte battery using the same.

Polyolefin porous membranes are widely used as separators in batteries, capacitors and the like because they exhibit excellent electrical insulation and ion permeability. In recent years, lithium ion secondary batteries with a high output density and a high capacity density have been used as power sources in connection with the increasing functionality and weight of portable devices in recent years. Polyolefin porous membranes are mainly used for such battery separators. It has been.
Lithium ion secondary batteries have high output density and capacity density, but because the electrolyte uses an organic solvent, the electrolyte decomposes due to heat generated by abnormal situations such as short circuit and overcharge. May cause fire. In order to prevent such a situation, some safety functions are incorporated in the lithium ion secondary battery, and one of them is a shutdown function of the separator. The shutdown function is a function that stops the progress of the electrochemical reaction by suppressing the ionic conduction in the electrolytic solution by blocking the micropores of the separator by heat melting or the like when the battery generates abnormal heat. Generally, the lower the shutdown temperature, the higher the safety. One of the reasons why polyethylene is used as a component of the separator is that it has an appropriate shutdown temperature. However, in a battery with high energy, the amount of heat generated during thermal runaway is large, and if the temperature continues to rise even after exceeding the shutdown temperature, both electrodes are short-circuited due to the film breakage due to the thermal contraction of the separator, causing further heat generation There is a risk.

In order to solve such a problem, a method of forming a layer mainly composed of an insulating inorganic filler between a separator and an electrode has been proposed. (Patent Documents 1, 2, 3, 4, 5, 6, 7) In this method, even if the temperature continues to rise beyond the shutdown temperature and the separator breaks down, the inorganic filler layer exists as an insulating layer. It is described that it is extremely safe because it can prevent a short circuit between the two electrodes.
Polyolefin surface treatment by a corona discharge treatment method or the like is a generally used technique for improving the wettability and adhesion of the polyolefin surface. In the case of forming a thin film consisting only of an inorganic substance on the pore wall surface of a polyolefin porous film, a method for increasing the adhesion of the pore wall surface by surface treatment and preventing the peeling of the thin film consisting only of an inorganic substance has been proposed (Patent Document 8). ing.

Japanese Patent No. 3756815 Japanese Patent No. 3752913 JP 2005-276503 A JP 2004-227972 A JP 2000-040499 A Japanese Patent Laid-Open No. 11-080395 JP 09-237622 A Japanese Patent No. 3797729

When the inorganic filler layer is formed on the separator surface by applying a dispersion containing the inorganic filler and the resin binder to the separator surface, which is a porous film, a resin binder for binding the inorganic filler and the inorganic filler is obtained. Since it inevitably enters the pores of the separator, many pores are blocked, and the permeability of the separator is lowered, resulting in inferior charge / discharge characteristics. This problem was more noticeable as the permeability of the separator itself was higher and the ratio of the resin binder to the inorganic filler was larger.
An object of the present invention is to provide a porous membrane excellent in heat resistance and permeability, particularly a porous membrane useful as a separator for a nonaqueous electrolyte battery. Moreover, it aims at providing the manufacturing method which can provide such a porous membrane with high productivity, the separator for nonaqueous electrolyte batteries and the nonaqueous electrolyte battery provided with high safety | security and practicality.

The inventor of the present invention has arrived at the present invention as a result of intensive studies to solve the above-mentioned problems.
That is, the present invention is as follows.
[1] A multilayer porous membrane having a polyolefin resin porous membrane having a wetting index of at least one surface of 40 mN / m or more and a porous layer comprising an inorganic filler and a resin binder on the surface.
[2] A separator for a non-aqueous electrolyte battery using the multilayer porous membrane according to [1].
[3] A nonaqueous electrolyte battery using the battery separator according to [2].
[4] At least one surface of the polyolefin resin porous membrane is subjected to a surface treatment so that the wetting index is 40 mN / m or more, and then a dispersion containing an inorganic filler and a resin binder is applied to the surface, thereby the polyolefin resin. A method for producing a multilayer porous membrane, comprising forming a porous layer on the surface of the porous membrane.

  According to the present invention, a multilayer porous membrane that exhibits excellent heat resistance and permeability and is useful as a separator for storage batteries such as non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries and electric double layer capacitors, and the production thereof A method, a separator for a nonaqueous electrolyte battery using the method, and a nonaqueous electrolyte battery can be provided.

The present invention is described in detail below.
The multilayer porous membrane of the present invention has a polyolefin resin porous membrane having a wetting index of at least one surface of 40 mN / m or more and a porous layer made of an inorganic filler and a resin binder on the surface.
The polyolefin resin porous membrane, which is a base material for forming a porous layer composed of an inorganic filler and a resin binder, has a wetting index of 40 mN / m or more and 476 mN / m or less, preferably 45 mN / m or more on the surface on the porous layer forming side. By setting it to 476 mN / m or less, more preferably 55 mN / m or more and 476 mN / m or less, and most preferably 70 mN / m or more and 476 mN / m or less, excellent heat resistance and permeability can be achieved at the same time.

The polyolefin resin porous membrane is a porous membrane in which the polyolefin resin occupies 50% or more and 100% or less of the mass fraction of the resin component constituting the porous membrane from the viewpoint of shutdown performance when used as a battery separator. Preferably, it is more preferably 55% or more and 100% or less, and most preferably 60% or more and 100% or less.
The polyolefin resin refers to a polyolefin resin used for normal extrusion, injection, inflation, blow molding and the like, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Homopolymers and copolymers, multistage polymers, and the like can be used. In addition, polyolefins selected from the group of these homopolymers, copolymers, and multistage polymers can be used alone or in combination. 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, ethylene-propylene random copolymer, polybutene. And ethylene propylene rubber. When the microporous membrane of the present invention is used as a battery separator, it is preferable to use a resin mainly composed of high-density polyethylene because of its low melting point and high strength required performance.

The viscosity average molecular weight of the polyolefin resin is preferably from 30,000 to 12 million, more preferably from 50,000 to less than 2 million, and most preferably from 100,000 to less than 1 million. A viscosity average molecular weight of 30,000 or more is preferable because the melt tension at the time of melt molding becomes large and the moldability is easily improved and sufficient entanglement is easily imparted and the strength is easily increased. If the viscosity average molecular weight is 12 million or less, it tends to be easy to obtain uniform melt-kneading and is preferable because it tends to be excellent in sheet formability, particularly thickness stability. Furthermore, the viscosity average molecular weight is 1
If it is less than 00000, when it is used as a battery separator, it is preferable because it is easy to close the hole when the temperature rises and a good shutdown function is easily obtained. For example, instead of using a polyolefin having a viscosity average molecular weight of less than 1 million alone, the polyolefin resin to be used is a mixture of polyethylene having a viscosity average molecular weight of 2 million and 270,000, and the viscosity average molecular weight of the mixture is less than 1 million. Good.

The polyolefin resin can also contain an inorganic filler described later, and, as necessary, within the range not impairing the advantages of the present invention, an antioxidant such as phenol, phosphorus or sulfur, calcium stearate or stearin. Metal soaps such as zinc acid, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed and used.
The multilayer porous membrane of the present invention comprises a porous layer made of an inorganic filler and a resin binder on at least one surface of the polyolefin resin porous membrane, thereby exhibiting excellent heat resistance.
The layer thickness of the porous layer is preferably 0.5 μm or more from the viewpoint of improving heat resistance, preferably 100 μm or less, more preferably 2 μm or more and 50 μm or less, and more preferably 3 μm or more and 30 μm or less from the viewpoint of permeability or high capacity of the battery. Most preferably, it is 4 μm or more and 20 μm or less.
The mass fraction occupied by the inorganic filler in the porous layer is preferably from 50% to less than 100%, more preferably from 55% to 99.99%, more preferably from 60% to 99, from the viewpoint of heat resistance. Is more preferably 9% or less, and particularly preferably 65% or more and 99% or less.

  As the inorganic filler used for the porous layer, a filler having a melting point of 200 ° C. or higher, high electrical insulation, and electrochemically stable within the use range of the lithium ion secondary battery is preferable. For example, oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide, nitride ceramics such as silicon nitride, titanium nitride, boron nitride, silicon carbide, calcium carbonate, sulfuric acid Aluminum, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatom Examples thereof include ceramics such as soil and silica sand, glass fibers, and the like, and these may be used alone or in combination. From the viewpoint of electrochemical stability, alumina and titania are more preferable. The average particle size of the inorganic filler is preferably 0.1 μm or more and 2.0 μm or less, and more preferably 0.2 μm or more and 1.0 μm or less. If it is less than 0.1 μm, the short circuit temperature tends to be low. On the other hand, if it exceeds 2.0 μm, it becomes difficult to form a thin porous layer.

  As the resin binder used for the porous layer, an inorganic filler can be bound, it is insoluble in the electrolyte solution of the lithium ion secondary battery, and it is electrochemically stable in the usage range of the lithium ion secondary battery. It is preferable. For example, polyolefins such as polyethylene and polypropylene, fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, fluorine-containing resins such as vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer Rubber, styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, methacrylic acid ester-acrylic acid ester copolymer, styrene Acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, rubber such as ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate, polyphenylene ether, Risuruhon, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, melting point and / or glass transition temperature of such polyesters are 180 ° C. or more resins. The viscosity average molecular weight of the polyolefin used for the resin binder is preferably 1000 or more and less than 12 million, more preferably 2000 or more and less than 2 million, and still more preferably 5000 or more and less than 1 million.

According to the present invention, it is possible to obtain a multilayer porous membrane that simultaneously achieves excellent heat resistance and permeability, but after forming a porous layer with respect to the air permeability of the polyolefin resin porous membrane as a substrate. The rate of increase in air permeability of the multilayer porous membrane is preferably 0% or more and 100% or less, more preferably 0% or more and 70% or less, and particularly preferably 0% or more and 50% or less. However, if the air permeability of the polyolefin resin porous membrane as the substrate is less than 100 seconds / 100 cc, the rate of increase in air permeability of the multilayer porous membrane after forming the porous layer is from 0% to 500% It can be preferably used.
The air permeability of the multilayer porous membrane is 10 seconds / 100 cc or more and 650 seconds / 100 cc or less, preferably 20 seconds / 100 cc or more and 500 seconds / 100 cc or less, more preferably 30 seconds / 100 cc or more and 450 seconds / 100 cc or less, particularly preferably 50 The range is from second / 100 cc to 400 second / 100 cc. When the air permeability is 10 seconds / 100 cc or more, self-discharge is small when used as a battery separator, and when 650 seconds / 100 cc or less, good charge / discharge characteristics are obtained.

The final film thickness of the multilayer porous membrane is preferably in the range of 2 μm to 200 μm, more preferably in the range of 5 μm to 100 μm, and still more preferably in the range of 7 μm to 50 μm. When the film thickness is 2 μm or more, the mechanical strength is sufficient, and when the film thickness is 200 μm or less, the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the capacity of the battery.
The thermal shrinkage rate at 150 ° C. of the multilayer porous membrane is preferably 0% or more and 15% or less in both the MD direction and the TD direction, more preferably 0% or more and 10% or less, and more preferably 0% or more and 5% or less. It is particularly preferred that If the MD direction and the TD direction are 15% or less, it is possible to prevent the separator from breaking even when the battery is abnormally heated. Therefore, it is possible to suppress contact between the positive and negative electrodes, and thus better safety performance tends to be obtained. This is preferable.

The shutdown temperature of the multilayer porous membrane is preferably 120 ° C. or higher and 160 ° C. or lower, more preferably 120 ° C. or higher and 150 ° C. or lower. When the temperature is 160 ° C. or lower, even when the battery generates heat, current interruption is promptly promoted, and better safety performance tends to be obtained, which is preferable. On the other hand, it is preferable that the temperature is 120 ° C. or higher because, for example, use of a high temperature around 100 ° C., heat treatment, etc. can be performed.
The short-circuit temperature of the multilayer porous membrane is preferably 180 ° C. or higher and 1000 ° C. or lower, and more preferably 200 ° C. or higher and 1000 ° C. or lower. When the temperature is 180 ° C. or higher, contact between the positive and negative electrodes can be suppressed until heat is dissipated even in abnormal battery heat generation, which is preferable because better safety performance tends to be obtained.
In the multilayer porous membrane of the present invention, at least one surface of the polyolefin resin porous membrane is subjected to a surface treatment to increase the wetting index to 40 mN / m or more, and then a dispersion containing an inorganic filler and a resin binder is applied to the surface. By this, it can manufacture suitably by the method of forming a porous layer in the polyolefin resin porous membrane surface.

  The method for surface-treating the polyolefin resin porous membrane is particularly limited as long as the wetness index of the treated polyolefin resin porous membrane surface is 40 mN / m or more and the porous structure of the polyolefin resin porous membrane is not significantly impaired. It can be adopted without. Examples thereof include a corona discharge treatment method, a plasma treatment method, a mechanical surface roughening method, a solvent treatment method, an acid treatment method, and an ultraviolet oxidation method. By applying surface treatment to the polyolefin resin porous membrane, not only can excellent heat resistance and permeability be achieved at the same time, but also it becomes easier to apply the inorganic filler-containing resin solution more uniformly due to improved wettability. Since the effect generally known as the effect of the surface treatment with respect to polyolefin, such as improving the adhesion between the inorganic filler-containing resin layer after coating and the surface of the polyolefin resin porous membrane, can be obtained, it is preferable.

As the polyolefin resin, those described above can be preferably used. Polyolefin resins include phenolic, phosphorus and sulfur-based antioxidants, metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light as necessary, as long as the advantages of the present invention are not impaired. Additives such as stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed and used.
As a manufacturing method of a polyolefin resin porous membrane, a general manufacturing method can be adopted without particular limitation. For example, as a general manufacturing method, a polyolefin resin and a plasticizer are melt-kneaded and formed into a sheet shape, and then made porous by extracting the plasticizer. A polyolefin resin is melt-kneaded and extruded at a high draw ratio. After that, the method is made porous by exfoliating the polyolefin crystal interface by heat treatment and stretching, melt-kneading the polyolefin resin and the inorganic filler and forming on the sheet, and then stretching the interface between the polyolefin resin and the inorganic filler. Examples thereof include a method of making it porous by peeling, a method of dissolving a polyolefin resin and then immersing it in a poor solvent for the polyolefin resin to solidify the polyolefin resin and simultaneously making it porous by removing the solvent.

Hereinafter, a method of making a porous structure by extracting a plasticizer after melt-kneading a polyolefin resin and a plasticizer to form a sheet and then forming the sheet will be described.
The plasticizer 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. In particular, when the polyolefin resin is polyethylene, liquid paraffin is preferable because it is highly compatible with polyethylene and is difficult to cause interface peeling between the resin and the plasticizer during stretching.

  In the present invention, the inorganic filler can be melt-kneaded together with the polyolefin resin and the plasticizer. The inorganic filler used in this case is not particularly limited as long as it has a melting point of 200 ° C. or higher, has high electrical insulation, and is electrochemically stable within the usage range of the lithium ion secondary battery. Can be used. As long as these conditions are satisfied, they may be used alone or in combination. Examples of the inorganic filler include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide and iron oxide, and nitride ceramics such as silicon nitride, titanium nitride and boron nitride, silicon Carbide, calcium carbonate, aluminum sulfate, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, Examples thereof include ceramics such as diatomaceous earth and silica sand, ceramics such as glass fiber, etc., and these may be used alone or in combination.

  The content of the inorganic filler relative to the polyolefin resin is such that uniform melt film formation is possible with the addition of a plasticizer, a sheet-like porous film precursor can be formed, and productivity is not impaired. The mass fraction of the inorganic filler in the composition comprising the polyolefin resin and the inorganic filler is preferably 0% or more and 90% or less, more preferably 1% or more and 80% or less, and more preferably 3% or more. It is particularly preferably 70% or less, and most preferably 5% or more and 60% or less. The addition of an inorganic filler is preferable because the affinity with the electrolytic solution is improved and the impregnation property of the electrolytic solution can be improved. A mass fraction of the inorganic filler of 90% or less is preferable because a uniform and sheet-like porous film precursor can be formed by melt film formation without impairing productivity.

  The ratio between the polyolefin resin and the plasticizer or between the polyolefin resin, the inorganic filler, and the plasticizer is a ratio that enables uniform melt-kneading, and a ratio sufficient to form a sheet-like microporous membrane precursor. The mass fraction of the plasticizer in the composition comprising the polyolefin resin and the plasticizer or the composition comprising the polyolefin resin, the inorganic filler and the plasticizer is preferable. Is preferably 30 to 80% by mass, more preferably 40 to 70% by mass. When the plasticizer has a mass fraction of 80% by mass or less, the melt tension at the time of melt molding is hardly insufficient and the moldability tends to be improved, which is preferable. On the other hand, when the mass fraction is 30% by mass or more, it is preferable because the film becomes thinner in the thickness direction as the draw ratio increases and a thin film can be obtained. In addition, since the plasticizing effect is sufficient, the crystalline folded lamellar crystal can be efficiently stretched, and when stretched at a high magnification, the polyolefin chain is not broken and a uniform and fine pore structure is obtained, and the strength is likely to increase. Furthermore, the extrusion load is reduced and productivity is improved.

  As a method of melt-kneading polyolefin resin and plasticizer, or polyolefin resin, inorganic filler and plasticizer, polyolefin resin alone, or polyolefin resin and inorganic filler, extruder, kneader, lab plast mill, kneading roll, Banbury mixer Into a resin kneading apparatus such as a plasticizer, introducing a plasticizer at an arbitrary ratio while heating and melting the resin, and kneading a composition comprising a polyolefin resin and a plasticizer or a polyolefin resin, an inorganic filler and a plasticizer. Thus, a method of obtaining a uniform solution is preferable. As a more preferable method, a polyolefin resin and a plasticizer, or a polyolefin resin, an inorganic filler, and a plasticizer are previously kneaded at a predetermined ratio using a Henschel mixer or the like, and the kneaded product is put into an extruder. Examples of the method include introducing a plasticizer at an arbitrary ratio while heating and melting and further kneading. Specifically, polyolefin resin and plasticizer, or polyolefin resin, inorganic filler and plasticizer pre-kneaded with a Henschel mixer etc. are put into a twin screw extruder and the remaining amount of the specified plasticizer added is side By feeding, a sheet with better dispersibility can be obtained, and stretching at a high magnification can be performed without breaking the membrane.

  The melt-kneaded product is formed into a sheet shape. The method of producing a sheet-like molded body by extruding the melt and cooling and solidifying is to extrude a uniform melt of polyolefin resin and plasticizer or polyolefin resin, inorganic filler 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 used for cooling and solidification, metal, water, air, plasticizer itself, or the like can be used. In particular, a method of cooling by contacting with a metal roll has the highest heat conduction efficiency and is preferable. Further, it is more preferable that the metal roll is sandwiched between the rolls because the heat conduction efficiency is further increased, the sheet is oriented and the film strength is increased, and the surface smoothness of the sheet is also improved. The die lip interval when extruding into a sheet form from a T die is preferably 400 μm or more and 3000 μm or less, and more preferably 500 μm or more and 2500 μm or less. When the die lip interval is 400 μm or more, the mess and the like are reduced, and there is little influence on the film quality such as streaks and defects, and film breakage and the like can be prevented in the subsequent stretching process. When the thickness is 3000 μm or less, the cooling rate is high and uneven cooling can be prevented, and the thickness stability can be maintained.

  As the stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used, but biaxial stretching is more preferable from the viewpoint of the obtained film strength and the like. When the film is stretched at a high magnification in the biaxial direction, it has a stable structure that is difficult to tear because of molecular orientation in the plane direction, and a high puncture strength is obtained. The stretching method may be simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, multi-stretching, etc., either alone or in combination, but if the stretching method is simultaneous biaxial stretching, the piercing strength is increased. And is most preferable from the viewpoints of uniform stretching and shutdown properties. Here, the simultaneous biaxial stretching is a method in which stretching in the MD direction and stretching in the TD direction are performed simultaneously, and the deformation rate in each direction may be different. Sequential biaxial stretching is a technique in which stretching in the MD direction or TD direction is performed independently. When stretching is performed in the MD direction or TD direction, the other direction is in an unconstrained state or a constant state. It is in a state of being fixed to the length. The draw ratio is preferably in the range of 20 times to 100 times, more preferably in the range of 25 times to 50 times in terms of surface magnification. The stretching ratio in each axial direction is preferably 4 to 10 times in the MD direction, preferably 4 to 10 times in the TD direction, 5 to 8 times in the MD direction, and 5 to 8 times in the TD direction. The range of is more preferable. When the total area magnification is 20 times or more, sufficient strength can be imparted to the film, and when it is 100 times or less, film breakage is prevented and high productivity is obtained.

The rolling process may be used in combination with the biaxial stretching process. Rolling can be performed by a press method using a double belt press or the like. Rolling can particularly increase the orientation of the surface layer portion. The rolling surface magnification is greater than 1 and preferably 3 or less, more preferably greater than 1 and 2 or less. If it is larger than 1 time, the plane orientation increases and the film strength increases. If it is 3 times or less, the difference in orientation between the surface layer portion and the center is small, and it is preferable for expressing a uniform porous structure in the surface layer portion and inside in the stretching step, and also preferable for industrial production.
The method of extracting the plasticizer may be either a batch type or a continuous type. However, the plasticizer is extracted by immersing the porous membrane precursor in an extraction solvent and sufficiently dried, so that the plasticizer is substantially removed from the porous membrane. It is preferable to remove. In order to suppress the shrinkage of the porous film, it is preferable to constrain the end of the porous film during a series of steps of immersion and drying. Moreover, it is preferable that the plasticizer residual amount in the porous membrane after extraction is less than 1% by mass.

It is desirable that the extraction solvent is a poor solvent for the polyolefin resin and a good solvent for the plasticizer, and has a boiling point lower than the melting point of the polyolefin porous 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, and non-chlorine-based solvents such as hydrofluoroether and hydrofluorocarbon. Examples thereof include halogenated solvents, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, and ketones such as acetone and methyl ethyl ketone. Needless to say, the recovered extraction solvent may be used by operations such as distillation.
When the inorganic filler is melt-kneaded with the plasticizer, the inorganic filler may be extracted as necessary. The extraction solvent in this case is preferably a poor solvent for the polyolefin resin and a good solvent for the inorganic filler, and has a boiling point lower than the melting point of the polyolefin porous membrane.

It is preferable to add a heat treatment step such as heat setting and heat relaxation subsequent to each stretching step within a range not impairing the advantages of the present invention, since it has an effect of further suppressing shrinkage of the polyolefin resin porous membrane.
Moreover, 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 multilayer porous membrane of the present invention is a porous layer on the surface of a polyolefin resin porous membrane by applying an inorganic filler-containing resin solution in which an inorganic filler and a resin binder are dissolved or dispersed in a solvent to at least one surface of the polyolefin resin porous membrane. It is preferable to manufacture by forming.

  As the inorganic filler and the resin binder, the above-mentioned ones can be preferably used. As the solvent, those in which the inorganic filler and the resin binder can be dissolved or dispersed uniformly and stably are preferable. For example, N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, water, ethanol, toluene, Examples include hot xylene and hexane. Further, in order to stabilize the inorganic filler-containing resin solution or to improve the coating property to the polyolefin resin porous membrane, a dispersant such as a surfactant, a thickener, a wetting agent, an antifoaming agent, an acid, Various additives such as a pH adjusting agent including an alkali may be added. These additives are preferably those that can be removed upon solvent removal or plasticizer extraction, but are electrochemically stable in the range of use of the lithium ion secondary battery, do not inhibit the battery reaction, and are up to about 200 ° C. If stable, it may remain in the battery.

The method for dissolving or dispersing the inorganic filler and the resin binder in the solvent is not particularly limited as long as it can realize the solution or dispersion characteristics necessary for the coating process described later. Examples thereof include a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, an attritor, a roll mill, a high-speed impeller dispersion, a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and mechanical stirring using a stirring blade.
The method for applying the inorganic filler-containing resin solution to the polyolefin resin porous membrane is not particularly limited as long as it can realize the required layer thickness and application area. For example, gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast Examples include a coater method, a die coater method, a screen printing method, and a spray coating method. Moreover, according to a use, you may apply | coat an inorganic filler containing resin solution only to the single side | surface of a polyolefin resin porous film, and may apply | coat to both surfaces.

By removing the solvent from the inorganic filler-containing resin solution applied to the polyolefin resin porous membrane, a porous layer consisting of an inorganic filler and a resin binder is formed. As a method for removing the solvent, the polyolefin resin porous membrane is adversely affected. If it is a method which does not affect, it can employ | adopt without specifically limiting. For example, a method of drying a polyolefin resin porous membrane while fixing it at a temperature below its melting point, a method of drying under reduced pressure at a low temperature, and dipping in a poor solvent for a resin binder to solidify the resin binder and simultaneously extract the solvent The method etc. are mentioned.
Since the multilayer porous membrane of the present invention is excellent in heat resistance and permeability, it is particularly useful when used as a separator for non-aqueous electrolyte batteries, and it is preferable that the multilayer porous membrane of the present invention is used as a separator. A nonaqueous electrolyte battery having safety performance can be obtained.

EXAMPLES Next, although an Example demonstrates this invention further in detail, these do not restrict | limit the scope of the present invention. The test methods in the examples are as follows.
<Evaluation of microporous membrane>
(1) Viscosity average molecular weight Mv
Based on ASTM-D4020, the intrinsic viscosity [η] (dl / g) at 135 ° C. in a decalin solvent is determined. Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80
(2) Wetting index (mN / m)
It measured by the method based on JISK-6768.
(3) Film thickness Measured with a dial gauge (PEACOCK No. 25 (trademark) manufactured by Ozaki Seisakusho). A sample of MD 10 mm × TD 10 mm was cut out from the porous film, and the film thickness was measured at nine locations (3 points × 3 points) in a lattice shape. The average value obtained was defined as the film thickness (μm).

(4) Air permeability (sec / 100cc)
A Gurley type air permeability meter (G-B2 (trademark) manufactured by Toyo Seiki Co.) conforming to JIS P-8117 was used. The inner cylinder weight was 567 g, and the time required for 100 ml of air to pass through an area of 28.6 mm in diameter and 645 mm ² was measured. The air permeability increase rate due to the formation of the porous layer is calculated by the following formula.
Air permeability increase rate (%) = (Air permeability of porous multilayer film−Air permeability of polyolefin resin porous film)
/ Air permeability of polyolefin resin porous membrane x 100

(5) Shutdown temperature, short-circuit temperature a. Production of positive electrode 92.2% by mass of lithium cobalt composite oxide (LiCoO ₂ ) as a positive electrode active material, 2.3% by mass of flake graphite and acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) 3 as a binder A slurry is prepared by dispersing 2% by mass 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 bulk density of the active material is 3.00 g / cm ³ .
b. Preparation of Negative Electrode 96.6% by mass of artificial graphite as a negative electrode active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. To do. 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 .

c. Nonaqueous electrolyte solution Prepared by dissolving LiBF ₄ as a solute in a mixed solvent of propylene carbonate: ethylene carbonate: γ-butyllactone = 1: 1: 2 (volume ratio) to a concentration of 1.0 mol / L.
d. Evaluation On a ceramic plate connected with a thermocouple, a negative electrode cut into 65 mm × 20 mm and immersed in a non-aqueous electrolyte for 1 minute or more is placed, and a thickness cut into 50 mm × 50 mm with a hole having a diameter of 16 mm is formed on the negative electrode. Place a 9 μm thick aramid film, cut it into 40 mm × 40 mm, place a porous film of the sample soaked in non-aqueous electrolyte for 1 hour or more so as to cover the hole of the aramid film, and cut it into 65 mm × 20 mm A positive electrode immersed in a non-aqueous electrolyte for 1 minute or longer is placed so as not to contact the negative electrode, and a Kapton film and a silicon rubber having a thickness of about 4 mm are placed thereon.
After this was set on a hot plate, the temperature was raised at a rate of 15 ° C./min with a pressure of 4.1 MPa applied by a hydraulic press machine, and the impedance change between the positive and negative electrodes at this time was AC 1V, Measurement was performed up to 200 ° C. under the condition of 1 kHz. In this measurement, the temperature at which the impedance reached 1000Ω was taken as the shutdown temperature, and the temperature at which the impedance again fell below 1000Ω after reaching the hole closed state was taken as the short-circuit temperature.

(6) Battery evaluation a. Production of Positive Electrode The positive electrode produced in (5) a was punched into a circle having an area of 2.00 cm ² .
b. Production of Negative Electrode The negative electrode produced in (5) b was punched into a circle having an area of 2.05 cm ² .
c. Nonaqueous electrolyte solution Prepared 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 ml / L.
d. Battery assembly and evaluation The negative electrode, the separator, and the positive electrode are stacked in this order from the bottom so that the active material surfaces of the positive electrode and the negative electrode face each other, and stored in a stainless steel container with a lid. The container and the lid are insulated, the container is in contact with the negative electrode copper foil, and the lid is in contact with the positive electrode aluminum foil. The non-aqueous electrolyte described above is injected into this container and sealed.

The simple battery assembled as described above is charged to a battery voltage of 4.2 V at a current value of 3 mA (about 0.5 C) in a 25 ° C. atmosphere, and the current value is reduced from 3 mA so as to maintain 4.2 V. In the method of starting, the first charge after battery preparation was performed for a total of about 6 hours, and then discharged to a battery voltage of 3.0 V at a current value of 3 mA.
Next, in a 25 ° C. atmosphere, the battery is charged to a battery voltage of 4.2 V at a current value of 6 mA (about 1.0 C), and the current value starts to be reduced from 6 mA so as to hold 4.2 V. The battery was charged for 3 hours and discharged at a current value of 6 mA to a battery voltage of 3.0 V. The discharge capacity at that time was set to 1 C discharge capacity (mAh).

Next, in a 25 ° C. atmosphere, the battery is charged to a battery voltage of 4.2 V at a current value of 6 mA (about 1.0 C), and the current value starts to be reduced from 6 mA so as to hold 4.2 V. The battery was charged for 3 hours, and discharged at a current value of 12 mA (about 2.0 C) to a battery voltage of 3.0 V. The discharge capacity at that time was 2 C discharge capacity (mAh).
The ratio of the 2C discharge capacity to the 1C discharge capacity was calculated, and this value was used as the rate characteristic.
Rate characteristics (%) = 2C discharge capacity / 1C discharge capacity × 100
Furthermore, in a 60 ° C. atmosphere, the battery is charged to a battery voltage of 4.2 V at a current value of 6 mA (about 1.0 C), and further, the current value starts to be reduced from 6 mA so as to maintain 4.2 V. The cycle of charging for a time and discharging to a battery voltage of 3.0 V at a current value of 6 mA was repeated.
The ratio of the discharge capacity after a predetermined cycle to the discharge capacity at the first cycle in this cycle was determined as the capacity retention rate (%), and the cycle characteristics were judged.

[Example 1]
16.6 parts by weight of polyethylene with a viscosity average molecular weight (Mv) of 700,000, 16.6 parts by weight of polyethylene with an Mv of 250,000 and 1.8 parts by weight of polypropylene with an Mv of 400,000, 40 parts by weight of liquid paraffin (LP) as a plasticizer, oxidized What added 0.3 weight part of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an inhibitor was premixed with a Henschel mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the liquid paraffin content ratio in the total mixture (100 parts by weight) melt-kneaded and extruded was 65 parts by weight. The melt-kneading conditions were set at a preset temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1300 μm. Next, it was continuously led to a simultaneous biaxial tenter stretching machine, and simultaneous biaxial stretching was performed 7 times in the MD direction and 6.4 times in the TD direction. At this time, the set temperature of the simultaneous biaxial tenter was 120 ° C. Next, the plasticizer was removed by being led to a methyl ethyl ketone bath, and then methyl ethyl ketone was removed by drying. Furthermore, it was led to a TD tenter heat fixing machine and heat fixed. The heat setting temperature was 125 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having a film thickness of 16 μm, a porosity of 48%, and an air permeability of 195 seconds / 100 cc was obtained.

When the surface of the polyolefin resin porous membrane was subjected to corona discharge treatment at a discharge amount of 50 W, the surface wetting index was 73 mN / m or more. On the treated surface side, titania particles (average particle size 0.4 μm) 90 parts by weight, polyphenylene ether (reduced viscosity 0.51, obtained by oxidative polymerization of 2,6-xylenol, glass transition temperature 209 ° C.) 10 weights After each part was uniformly dispersed in 200 parts by weight of toluene using a bar coater, the solution was dried at 60 ° C. to remove toluene, and a porous layer having a thickness of 6 μm was formed on the porous film. A multilayer porous film having a total film thickness of 22 μm was obtained.
The obtained multilayer porous membrane had an air permeability of 260 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 33%, maintaining excellent permeability. Further, the shutdown temperature was observed at 149 ° C., and the short circuit was not observed even when the temperature exceeded 200 ° C., indicating very high heat resistance.
When the battery was evaluated using this multilayer porous membrane as a separator, the rate characteristics were as high as 90% or more, the capacity retention after 100 cycles was 90% or more, and the cycle characteristics were also good.

[Example 2]
In Example 1, the total film thickness was obtained in the same manner as in Example 1 except that the discharge amount of corona discharge treatment on the surface of the polyolefin resin porous membrane was 20 W and the surface wetting index was 45 mN / m. A multilayer porous membrane of 22 μm was obtained.
The obtained multilayer porous membrane had an air permeability of 270 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 38%, maintaining excellent permeability. Further, the shutdown temperature was observed at 148 ° C., and the short circuit was not observed even when the temperature exceeded 200 ° C., indicating very high heat resistance.
When the battery was evaluated using this multilayer porous membrane as a separator, the rate characteristics were as high as 90% or more, the capacity retention after 100 cycles was 90% or more, and the cycle characteristics were also good.

[Example 3]
When the surface of the polyolefin resin porous membrane used as the substrate in Example 1 was subjected to corona discharge treatment at a discharge amount of 50 W, the surface wetting index was 73 mN / m or more. 90 parts by weight of alumina particles (average particle size 0.7 μm) and 10 parts by weight of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly dispersed in 150 parts by weight of water on the treated surface side. The aqueous solution was applied using a gravure coater and then dried at 60 ° C. to remove water to obtain a multilayer porous film having a total thickness of 19 μm in which a porous layer having a thickness of 3 μm was formed on the porous film. .
The obtained multilayer porous membrane had an air permeability of 245 sec / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 26%, maintaining excellent permeability. Further, the shutdown temperature was observed at 146 ° C., and the short circuit was not observed even when the temperature exceeded 200 ° C., indicating very high heat resistance.
When the battery was evaluated using this multilayer porous membrane as a separator, the rate characteristics were as high as 90% or more, the capacity retention after 100 cycles was 90% or more, and the cycle characteristics were also good.

[Comparative Example 1]
The wetting index of the surface of the polyolefin resin porous membrane used as the substrate in Example 1 was 38 mN / m. On the surface, 90 parts by weight of titania particles (average particle size 0.4 μm) and 10 parts by weight of polyphenylene ether (reduced viscosity 0.51, glass transition temperature 209 ° C. obtained by oxidative polymerization of 2,6-xylenol) Each of the solutions uniformly dispersed in 200 parts by weight of toluene was applied using a bar coater, then dried at 60 ° C. to remove toluene, and a porous layer having a thickness of 6 μm was formed on the porous film. A multilayer porous membrane having a thickness of 26 μm was obtained.
The resulting multilayer porous membrane had an air permeability of 410 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as high as 110%, resulting in poor permeability.
When this multilayer porous membrane was used as a separator for battery evaluation, the rate characteristics were as low as about 80%, and the capacity retention after 100 cycles was as low as about 70%.
This multilayer porous membrane showed a very high heat resistance, with a shutdown temperature of 150 ° C. observed and no short circuit observed even at 200 ° C. or higher.

[Comparative Example 2]
The wetting index of the surface of the polyolefin resin porous membrane used as the substrate in Example 1 was 38 mN / m. An aqueous solution in which 90 parts by weight of alumina particles (average particle size 0.7 μm) and 10 parts by weight of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly dispersed in 150 parts by weight of water on the surface. Was applied using a gravure coater and then dried at 60 ° C. to remove water to obtain a multilayer porous film having a total film thickness of 19 μm in which a porous layer having a thickness of 3 μm was formed on the porous film.
The obtained multilayer porous membrane had an air permeability of 360 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as high as 85%, so that the permeability deteriorated.
When this multilayer porous membrane was used as a separator for battery evaluation, the rate characteristics were as low as about 80%, and the capacity retention after 100 cycles was as low as about 70%.
This multilayer porous film showed a very high heat resistance, with a shutdown temperature observed at 146 ° C. and no short circuit observed even at 200 ° C. or higher.

[Comparative Example 3]
The wetting index of the surface of the polyolefin resin porous membrane used as the substrate in Example 1 was 38 mN / m. When the same evaluation was performed without forming a porous layer on the surface of the porous film, the shutdown temperature was observed at 147 ° C., but the short-circuit temperature was as low as 154 ° C.
The air permeability of this porous film showed excellent permeability of 195 sec / 100 cc. When the battery was evaluated using this porous film as a separator, the rate characteristics were as high as 90% or more, and after 100 cycles. The capacity retention rate was 90% or more and the cycle characteristics were good.

[Comparative Example 4]
When the surface of the polyolefin resin porous membrane used as the substrate in Example 1 was subjected to corona discharge treatment at a discharge amount of 50 W, the surface wetting index was 73 mN / m or more. When the same evaluation was performed without forming a porous layer on the surface of the porous film, the shutdown temperature was observed at 148 ° C., but the short-circuit temperature was as low as 156 ° C.
The air permeability of this porous film showed excellent permeability of 195 sec / 100 cc. When the battery was evaluated using this porous film as a separator, the rate characteristics were as high as 90% or more, and after 100 cycles. The capacity retention rate was 90% or more and the cycle characteristics were good.
The physical properties in the above Examples and Comparative Examples are summarized in Table 1.

Since the multilayer porous membrane of the present invention exhibits excellent heat resistance and permeability, it is particularly useful as a separator for storage batteries such as non-aqueous electrolyte secondary batteries and electric double layer capacitors that are required to have excellent safety and reliability. Useful.

Claims (4)

  A multilayer porous membrane having a wetting index of at least one surface of a polyolefin resin porous membrane of 40 mN / m or more and a porous layer comprising an inorganic filler and a resin binder on the surface.   A separator for a non-aqueous electrolyte battery using the multilayer porous membrane according to claim 1.   A non-aqueous electrolyte battery using the battery separator according to claim 2. The surface of the polyolefin resin porous membrane is subjected to a surface treatment on at least one side of the polyolefin resin porous membrane so that the wetting index is 40 mN / m or more, and then the surface of the polyolefin resin porous membrane is coated with a dispersion containing an inorganic filler and a resin binder. A method for producing a multilayer porous membrane, comprising forming a porous layer on the substrate.