JP4931083B2 - Multilayer porous membrane and method for producing the same - Google Patents

Description

  The present invention relates to a multilayer porous membrane, a separator for a nonaqueous electrolyte battery, a nonaqueous electrolyte battery, and a method for producing a multilayer porous membrane.

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, with the increase in functionality and weight of portable devices, lithium ion secondary batteries with high output density and high capacity density have been used as power sources. Polyolefin porous membranes are often used as separators in such high power density and high capacity density lithium ion secondary batteries.
Here, in the lithium ion secondary battery, an organic solvent is usually used as an electrolytic solution. Therefore, when an abnormal situation such as a short circuit or overcharge occurs in the lithium ion secondary battery, there is a possibility that the electrolytic solution is decomposed and, in the worst case, ignition occurs. In order to prevent such a situation, some safety functions are incorporated in the lithium ion secondary battery. An example is the separator shutdown function.

  The shutdown function means a function of stopping the progress of the electrochemical reaction by suppressing the ionic conduction in the electrolytic solution by blocking the micropores of the separator due to heat melting or the like when the battery is abnormally heated. Generally, the lower the shutdown temperature, the higher the safety. Since polyethylene has an appropriate shutdown temperature, polyethylene is preferably used as a component of the separator.

However, a battery having high energy may generate a large amount of heat during thermal runaway. If the temperature continues to rise even after the shutdown temperature is exceeded, there is a danger that both electrodes will be short-circuited due to the membrane breakage of the separator (hereinafter sometimes referred to as “short-circuit”), causing further heat generation.
Under such circumstances, a method of forming a layer mainly composed of an insulating inorganic filler between the separator and the electrode has been proposed (Patent Documents 1, 2, 3, 4, 5, 6, 7). Further, these patent documents describe a method of forming an inorganic filler layer on the separator surface by applying a dispersion containing an inorganic filler and a resin binder to the separator surface which is a porous film.

  Here, Patent Documents 1, 3 and 4 describe a method in which polyvinyl alcohol is used as a binder in a layer mainly composed of the insulating inorganic filler. Patent Documents 8, 9, and 10 describe a method in which polyvinyl alcohol is used as an adhesive for bonding between an electrode layer and a separator. Further, Patent Document 11 discloses a method of preventing adhesion of a thin film made of only an inorganic material by increasing the adhesion of the porous wall surface by surface treatment when forming a thin film made of only an inorganic material on the porous wall surface of a polyolefin porous film. Is described.

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. 3426253 Japanese Patent No. 3393145 WO99 / 31750 publication Japanese Patent No. 3797729

However, 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, the resin binder for binding the inorganic filler and the inorganic filler. May enter the pores of the separator and close many of the pores, thereby reducing the permeability of the separator. When the permeability of the separator is lowered, the charge / discharge characteristics tend to be inferior. Such blockage of pores is more likely to occur as the thickness of the inorganic filler layer increases and as the ratio of the resin binder to the inorganic filler increases.
On the other hand, if the layer thickness of the inorganic filler layer is excessively reduced, the melted separator and the inorganic filler layer may break together when the temperature continues to rise beyond the shutdown temperature. Such a rupture may cause a short circuit between the two electrodes. Further, if the ratio of the resin binder to the inorganic filler is excessively small, the inorganic filler may not be sufficiently bound. When the binding of the inorganic filler is not sufficient, the inorganic filler tends to be easily peeled off from the separator surface and missing.

  Also, if the heat shrinkage stress of the polyolefin porous film on which the inorganic filler layer is laminated is excessively large, the temperature may continue to rise beyond the shutdown temperature, and the molten separator and the inorganic filler layer may break together. is there. Such a rupture may cause a short circuit between the two electrodes. Such film breakage tends to occur more prominently as the temperature increase rate of the separator increases. On the other hand, excessively increasing the thickness of the inorganic filler layer on the separator surface in order to prevent such film breakage may cause a decrease in the permeability of the separator.

  Furthermore, if the resin binder used to form the inorganic filler layer or the surface condition of the separator as the substrate on which the inorganic filler layer is laminated are selected incorrectly, the temperature will continue to rise beyond the shutdown temperature. There is a case where a large variation occurs in a temperature (short circuit temperature) at which both electrodes are short-circuited due to a film or the like, or a case where it is difficult to ensure a higher short circuit temperature.

  An object of this invention is to provide the multilayer porous membrane excellent in heat resistance and permeability | transmittance. Moreover, it aims at providing the manufacturing method which can provide such a porous membrane with high productivity, the separator for nonaqueous electrolyte batteries provided with high safety | security and practicality, and a nonaqueous electrolyte battery.

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]
Provided with a porous layer containing an inorganic filler and a resin binder on at least one side of the polyolefin resin porous membrane,
The multilayer porous membrane, wherein the polyolefin resin porous membrane has a maximum heat shrinkage stress of 10 g or less.
[2]
A separator for a non-aqueous electrolyte battery using the multilayer porous membrane according to claim 1.
[3]
A nonaqueous electrolyte battery using the separator for a nonaqueous electrolyte battery according to claim 2.
[4]
By applying a dispersion liquid containing an inorganic filler and a resin binder to at least one surface of a polyolefin resin porous membrane having a maximum heat shrinkage stress of 10 g or less, the inorganic filler and the resin binder are applied to at least one surface of the polyolefin resin porous membrane. A method for producing a multilayer porous membrane comprising forming a porous layer comprising:

  According to the present invention, a multilayer porous membrane excellent in heat resistance and permeability is provided. In addition, a manufacturing method capable of providing such a porous membrane with high productivity, a separator for nonaqueous electrolyte batteries and a nonaqueous electrolyte battery having high safety and practicality are provided.

The best mode for carrying out the present invention (hereinafter abbreviated as “embodiment”) will be described in detail below. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.
The multilayer porous membrane of the present embodiment includes a porous layer containing an inorganic filler and a resin binder on at least one surface of the polyolefin resin porous membrane.

[Polyolefin resin porous membrane]
The polyolefin resin porous membrane of the present embodiment is formed of a polyolefin resin composition containing a polyolefin resin as a main component. The inclusion of a polyolefin resin as a main component is suitable from the viewpoint of satisfactorily realizing shutdown performance when used as a battery separator.
In the present embodiment, the “main component” means that the proportion of a specific component in all the components is preferably 50% by mass or more, more preferably 55% by mass or more, and still more preferably 60% by mass or more. It means preferably 80% by mass or more, most preferably 90% by mass or more, and may mean 100% by mass.

As said polyolefin resin, the polyolefin resin used for normal extrusion, injection, inflation, blow molding, etc. can be used. More specifically, as the polyolefin resin, for example, a homopolymer or copolymer obtained by using ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like as monomers. Examples thereof include a coalescence or a multistage polymer. These can be used alone or in combination of two or more.
Examples of the polyolefin resin 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, Examples include polybutene and ethylene propylene rubber.
In addition, as said polyolefin resin, it is preferable to contain a high density polyethylene as a main component from a viewpoint of making low melting | fusing point and high intensity | strength as a battery separator of a multilayer porous membrane compatible.

The polyolefin resin has a viscosity average molecular weight of preferably 30,000 to 12 million, more preferably 50,000 to less than 2 million, and still more preferably 100,000 to less than 1 million. A viscosity average molecular weight of 30,000 or more is preferable from the viewpoint of realizing a good moldability by setting a large melt tension during melt molding and from the viewpoint of realizing a high strength by imparting sufficient entanglement. . On the other hand, setting the viscosity average molecular weight to 12 million or less is preferable from the viewpoint of realizing uniform melt-kneading and realizing good moldability of the sheet, particularly good thickness stability. When the viscosity average molecular weight is less than 1 million, when used as a battery separator, it is preferable from the viewpoint of realizing a good shutdown function because the holes are likely to close when the temperature rises.
Here, as a method for adjusting the viscosity average molecular weight of the polyolefin resin, there is a method using a plurality of polymers having different viscosity average molecular weights in addition to a method using a polymer having a specific viscosity average molecular weight alone. Can be mentioned. For example, when adjusting the viscosity average molecular weight to less than 1 million, instead of using a polyolefin having a viscosity average molecular weight of less than 1 million, for example, polyethylene having a viscosity average molecular weight of 2 million, for example, polyethylene having a viscosity average molecular weight of 270,000 Can be mixed and used.
In addition, the “viscosity average molecular weight” in the present embodiment is a value measured according to the measurement method of Examples described later.

The polyolefin resin composition may contain an inorganic filler. As such an inorganic filler, an inorganic filler having a melting point of 200 ° C. or higher, high electrical insulation, and electrochemically stable in the usage range of the lithium ion secondary battery is preferably used.
More specifically, examples of such inorganic fillers include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide;
Nitride ceramics such as silicon nitride, titanium nitride, 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 , Ceramics such as diatomaceous earth and quartz sand, ceramics such as glass fiber;
Etc.
These can be used alone or in combination of two or more.

When the inorganic filler described above is blended with the polyolefin resin, the blending ratio thereof can be uniformly melted in a state where a plasticizer described later is added, and can form a sheet-like porous membrane precursor, And it is preferable that it is a grade which does not impair productivity.
The proportion of the inorganic filler in the total amount of the polyolefin resin and the inorganic filler (mass fraction) is preferably 0% or more, more preferably 1% or more, still more preferably 3% or more, particularly The upper limit is preferably 5% or more, and the upper limit is preferably 90% or less, more preferably 80% or less, still more preferably 70% or less, and particularly preferably 60% or less. The addition of the inorganic filler is preferable from the viewpoint of improving the affinity with the electrolytic solution and improving the impregnation property of the electrolytic solution. On the other hand, it is preferable that the mass fraction of the inorganic filler is 90% or less from the viewpoint that a uniform and sheet-like porous film precursor (described later) can be formed by melt film formation without impairing productivity.

  If necessary, the polyolefin resin composition may be an antioxidant such as phenol, phosphorus or sulfur, a metal soap such as calcium stearate or zinc stearate, an ultraviolet absorber, a light stabilizer, or an antistatic agent. Further, additives such as an antifogging agent and a coloring pigment can be mixed and used.

As a manufacturing method of the said polyolefin resin porous membrane, a general manufacturing method is employable, without restrict | limiting in particular. More specifically, as a manufacturing method, for example,
(I) A method in which a polyolefin resin and a plasticizer are melt-kneaded and formed into a sheet shape, and then made porous by extracting the plasticizer,
(II) A method for making a polyolefin resin porous by exfoliating the polyolefin crystal interface by heat treatment and stretching after melt kneading and extruding the polyolefin resin at a high draw ratio,
(III) A method in which a polyolefin resin and an inorganic filler are melt-kneaded and formed into a sheet shape, and then made porous by peeling the interface between the polyolefin resin and the inorganic filler by stretching,
(IV) After dissolving the polyolefin resin, it is immersed in a poor solvent for the polyolefin resin to solidify the polyolefin resin and at the same time remove the solvent to make it porous.
Etc.

Hereinafter, the method (I) will be further described.
The plasticizer used in the method (I) is preferably a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than the melting point of the polyolefin resin when mixed with the polyolefin resin. Examples of such plasticizers include hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, higher alcohols such as oleyl alcohol and stearyl alcohol, and the like. In particular, the use of liquid paraffin when the polyolefin resin contains polyethylene as a main component is that liquid paraffin has good compatibility with polyethylene, so that it is difficult to cause interfacial delamination with the polyolefin resin during stretching. From the viewpoint of performing proper stretching.

The blending ratio of the plasticizer to the polyolefin resin is preferably such that uniform melt kneading is possible, a sheet-like microporous membrane precursor can be formed, and productivity is not impaired.
The proportion (mass fraction) of the plasticizer in the total amount of the polyolefin resin, the plasticizer, and the inorganic filler blended as necessary is preferably 30% or more, more preferably 40%. The upper limit is preferably 80% or less, more preferably 70% or less. Setting the mass fraction of the plasticizer to 80% or less is preferable from the viewpoint of maintaining melt tension during melt molding and ensuring moldability. On the other hand, setting it to 30% or more is preferable from the viewpoint of obtaining a homogeneous thin film. In other words, by setting it to 30% or more, the plasticizing effect is sufficient, the lamellar crystals folded in a crystalline form are efficiently stretched, and a uniform and fine pore structure is realized without breaking the polyolefin chain even at high magnification. As a result, high film strength can be realized. Furthermore, setting it to 30% or more tends to reduce the extrusion load at the time of extrusion molding, and is preferable from the viewpoint of realizing high productivity.

As a method of obtaining a melt-kneaded product containing the polyolefin resin and the plasticizer, or a melt-kneaded product containing the polyolefin resin, the inorganic filler, and the plasticizer, the polyolefin resin alone or the polyolefin resin and other blends The product is put into a resin kneading apparatus (extruder, kneader, lab plast mill, kneading roll, Banbury mixer, etc.), and a plasticizer is introduced and kneaded at an arbitrary ratio while the resin is heated and melted to obtain a uniform solution. The method is preferred.
Among them, a polyolefin resin and a plasticizer or a polyolefin resin, an inorganic filler, and a plasticizer are preliminarily kneaded at a predetermined ratio using a Henschel mixer or the like, and the kneaded product is extruded into an extruder (for example, a twin screw extruder). The remaining amount of the predetermined plasticizer addition amount is introduced at an arbitrary ratio (for example, by side-feeding or the like) while being heated and melted, and further kneaded. By adopting such a method, a sheet with better dispersibility can be obtained, and stretching at a high magnification can be carried out without film breakage.

In the method (I), the melt-kneaded product is formed into a sheet. As a method for producing a sheet-like molded product by extruding the melt-kneaded material and cooling and solidifying, a polyolefin resin and a plasticizer, or a uniform melt of a polyolefin resin, an inorganic filler, and a plasticizer is formed into a sheet shape via a T-die or the like. It is possible to employ a method of extruding to a thermal 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 from a T-die into a sheet is preferably 400 μm or more, more preferably 500 μm or more, and the upper limit is preferably 3000 μm or less, preferably 2500 μm or less. It is preferable to set the die lip interval to 400 μm or more from the viewpoint of reducing spears and the like, reducing the influence on the film quality such as streaks and defects, and preventing film breakage in the subsequent stretching step. On the other hand, the thickness of 3000 μm or less is preferable from the viewpoint of fast cooling rate and prevention of uneven cooling and maintaining the thickness stability.

The sheet-like molded body (porous membrane precursor) formed by the method (I) may be subjected to stretching treatment as necessary. As such a stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used. Among these, biaxial stretching is preferable from the viewpoint of the obtained film strength and the like. When stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, so that the structure is difficult to tear and a stable structure tends to be obtained. Further, the stretching method may be any one of simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, multi-stretching and the like, but the stretching method is simultaneous biaxial stretching. Most preferable from the viewpoints of an increase in thickness, 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 simultaneously performed, and the deformation rate (stretching ratio) 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 unconstrained or fixed. 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. Setting the total surface magnification to 20 times or more is preferable from the viewpoint of imparting sufficient strength to the film. On the other hand, setting it to 100 times or less is preferable from the viewpoint of preventing film breakage and ensuring high productivity.
In the present embodiment, the MD direction means the resin extrusion direction (machine direction, flow direction). On the other hand, the TD direction means a width direction (a direction perpendicular to the machine direction) of the sheet extruded into a sheet shape.

  In the said extending | stretching process, you may use a rolling process together. A rolling process is implemented using the press method which uses a double belt press etc., for example. Employing such a rolling step is particularly preferable because the orientation of the surface layer portion can be increased. The rolling surface magnification is preferably greater than 1 and 3 or less, more preferably greater than 1 and 2 or less. A ratio larger than 1 is preferable from the viewpoint of increasing the plane orientation and increasing the film strength. On the other hand, it is preferable to make it 3 times or less from the viewpoint of maintaining a small orientation difference between the surface layer portion and the center inside, expressing a uniform porous structure in the surface layer portion and inside in the stretching step, and from the viewpoint of industrial production.

In the method (I), a plasticizer is extracted from the formed sheet-like molded body (porous membrane precursor) to form a polyolefin resin porous membrane.
The method for extracting the plasticizer may be either a batch type or a continuous type, but 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 it. 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.

The extraction solvent is preferably a poor solvent for the polyolefin resin and a good solvent for the plasticizer. The extraction solvent preferably has a boiling point lower than the melting point of the polyolefin resin 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.
Here, as these extraction solvents, it is also possible to use extraction solvents recovered by distillation or the like.
When the inorganic filler is melt-kneaded together 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.

The manufacturing methods (I) to (IV) described above may have a heat treatment step such as heat fixation or heat relaxation, if necessary. Such a heat treatment step is preferably performed subsequent to or after the stretching step from the viewpoint of suppressing thermal shrinkage of the polyolefin resin porous membrane.
More specific examples of such a heat treatment step include a method of heat setting with a tenter heat fixing machine.

The maximum value of heat shrinkage stress of the polyolefin resin porous membrane is preferably 10 g or less, more preferably 9 g or less, still more preferably 8 g or less, still more preferably 7 g or less, particularly preferably 6 g or less, and most preferably 5 g. The lower limit is preferably 0 g or more. Setting the maximum value of the heat shrinkage stress within the above range is preferable from the viewpoint of achieving both heat resistance and permeability of the obtained multilayer porous membrane.
In the present embodiment, the “maximum value of heat shrinkage stress” means the MD heat shrinkage stress value measured by a measurement method “maximum heat shrinkage stress (g)” of an example described later under a constant high temperature condition. And the TD heat shrinkage stress value, the larger numerical value is meant. In the case of a polyolefin porous film produced by a general manufacturing method, the MD heat shrinkage stress is larger than the TD heat shrinkage stress. By adopting a polyolefin resin porous film having a maximum value of MD heat shrinkage stress of 10 g or less, the MD direction A multilayer porous membrane having a low thermal shrinkage at high temperatures in both the TD direction can be obtained. Such a multilayer porous membrane can be particularly suitably used for applications requiring dimensional stability in both the MD direction and the TD direction at high temperatures. In addition, as such a use, the separator use for stack type nonaqueous electrolyte batteries etc. are mentioned, for example.

Examples of the method for reducing the maximum value of the heat shrinkage stress of the polyolefin resin porous membrane to 10 g or less include a method of lowering the viscosity average molecular weight of the polyolefin resin used, and an increase in the amount of plasticizer when using a plasticizer. To reduce the polyolefin resin ratio, to add an inorganic filler to the polyolefin resin, to increase the resin temperature during melt extrusion, to decrease the discharge rate during melt extrusion, to widen the die lip interval such as T-die, Method of lowering the stretching ratio in the process, Method of raising the stretching temperature in the stretching process, Method of raising the processing temperature in the heat treatment process, Method of raising the relaxation ratio in the heat setting process, Method of raising the relaxation temperature in the heat setting process Etc. These can be used alone or in combination of two or more.
Among such methods, in particular, a polyolefin resin containing 50% or more of high-density polyethylene having a viscosity average molecular weight of 100,000 or more by mass fraction is secured (to ensure melt tension at high temperature) and oriented by heat fixing at 120 ° C. or more. The relaxing method is preferable from the viewpoint of adjusting the maximum value of the heat shrinkage stress to 10 g or less while maintaining the film strength of the polyolefin resin porous film.

Moreover, the manufacturing method of (I)-(IV) mentioned above may have a surface treatment process as needed. Implementing such a surface treatment step is a viewpoint of simultaneously achieving excellent heat resistance and permeability of the multilayer porous membrane, and a viewpoint of more uniformly applying an inorganic filler-containing resin solution that forms a porous layer described later. Furthermore, it is preferable from the viewpoint of improving the adhesiveness between the porous layer and the polyolefin resin porous membrane.
Here, as such a surface treatment process, for example, a corona discharge treatment method, a plasma treatment method, a mechanical surface roughening method, a solvent treatment method, an acid treatment method, an ultraviolet oxidation method, a hydrophilic treatment by a surfactant or the like. And a crosslinking treatment method using ionizing radiation and the like.

The polyolefin resin porous membrane has a surface wetting index (measurement method: JIS K-6768), preferably 40 mN / m or more, more preferably 45 mN / m or more, from the viewpoint of achieving both heat resistance and permeability. More preferably, it is 55 mN / m or more, particularly preferably 70 mN / m or more, and the upper limit is preferably 476 mN / m or less.
In addition, what is necessary is just to select suitably the conditions of the said surface treatment process as a method of adjusting the surface wetting index of the polyolefin resin porous membrane to the said range.

The porosity of the polyolefin resin porous membrane is preferably 30% or more, more preferably 40% or more, and the upper limit is preferably 85% or less, more preferably 70% or less, and still more preferably 55% or less. It is to be noted that the porosity in the present embodiment, cut a sample of 100 mm × 100 mm square of a polyolefin resin porous membrane, determined and the volume (mm ³⁾ mass (mg), than those with the membrane density (g / cm ³⁾ The value is calculated using the following formula.
Porosity = (volume−mass / film density) / volume × 100
The air permeability of the polyolefin resin porous membrane is preferably 10 seconds / 100 cc or more, more preferably 150 seconds / 100 cc or more, and the upper limit is preferably 650 seconds / 100 cc or less, preferably 400 seconds / 100 cc or less. It is.
In addition, the said porosity and air permeability can be adjusted by selecting suitably the manufacturing conditions of a polyolefin resin porous membrane.

[Porous layer, multilayer porous film]
The porous layer of the present embodiment is formed using, for example, an inorganic filler-containing resin solution (dispersion) containing an inorganic filler and a resin binder.
The inorganic filler is preferably one having a melting point of 200 ° C. or higher, high electrical insulation, and electrochemically stable in the usage range of the lithium ion secondary battery. 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, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, zeolite, silicic acid Examples thereof include ceramics such as calcium, magnesium silicate, diatomaceous earth, and silica sand, and glass fibers. These can be used alone or in combination of two or more. Among these, alumina and titania are more preferable from the viewpoint of electrochemical stability.

The average particle size of the inorganic filler is preferably 0.1 μm or more, more preferably 0.2 μm or more, still more preferably 0.3 μm or more, and the upper limit is preferably 3.0 μm or less, more preferably 1.0 μm. It is as follows. Setting the average particle size to 0.1 μm or more is preferable from the viewpoint of reducing the thermal shrinkage rate of the multilayer porous film and making it difficult to break the film, and from the viewpoint of realizing a high short-circuit temperature. On the other hand, setting the average particle size to 3.0 μm or less is preferable from the viewpoint of reducing the thermal contraction rate of the multilayer porous membrane and making it difficult to break the membrane. Moreover, it is preferable that an average particle diameter shall be 1.5 micrometers or less from a viewpoint of forming a porous layer with small layer thickness favorably, and the viewpoint of the dispersibility in the porous layer of an inorganic filler.
In the present embodiment, the “average particle diameter of the inorganic filler” is a value measured according to a method using SEM in the measurement methods of the examples described later.

  The proportion (mass fraction) of the inorganic filler in the porous layer is preferably 50% or more, more preferably 55% or more, still more preferably 60% or more, and particularly preferably 65 from the viewpoint of heat resistance. The upper limit is preferably less than 100%, preferably 99.99% or less, more preferably 99.9% or less, and particularly preferably 99% or less.

On the other hand, as the resin binder, it is preferable that 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. .
Examples of such resin binders include polyolefins such as polyethylene and polypropylene, fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, and ethylene-tetrafluoro. Fluorine-containing rubber such as ethylene copolymer, styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, methacrylate ester-acrylic Acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate, etc. Arm, poly (phenylene ether), polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, melting point and / or glass transition temperature of such polyesters are 180 ° C. or more resins. These can be used alone or in combination of two or more.
The viscosity average molecular weight of the polyolefin used for the resin binder is preferably 1000 or more, more preferably 2000 or more, and further preferably 5000 or more from the viewpoint of moldability, and the upper limit is preferably less than 12 million, preferably Is less than 2 million, more preferably less than 1 million.

When polyvinyl alcohol is used as the resin binder, the saponification degree is preferably 85% or more and 100% or less. Setting the saponification degree to 85% or more is preferable from the viewpoint of improving the short-circuit temperature and suppressing variations in the short-circuit temperature and realizing good safety performance. The saponification degree is more preferably 90% or more, still more preferably 95% or more, and particularly preferably 99% or more.
In the present embodiment, the “degree of saponification” is a value measured according to the measurement method of Examples described later.

The average degree of polymerization of the polyvinyl alcohol (measurement method: JIS K-6726) is preferably 200 or more, more preferably 300 or more, still more preferably 500 or more, and the upper limit is preferably 5000 or less, more preferably 4000 or less. More preferably, it is 3500 or less. Setting the average degree of polymerization to 200 or more tends to enable the inorganic filler to be firmly bound in a small amount, and is preferable from the viewpoint of suppressing an increase in air permeability of the multilayer porous membrane while maintaining the mechanical strength of the porous layer. On the other hand, an average polymerization degree of 5000 or less is preferable from the viewpoint of preventing gelation and the like when preparing a dispersion with an inorganic filler.
In addition, a commercial item can be used as said polyvinyl alcohol, and the catalog value of a commercial item can be used also as the average degree of polymerization.

  The proportion of the resin binder in the total amount of the inorganic filler and the resin binder is preferably 0.5% or more, more preferably 0.7% or more in terms of volume fraction from the viewpoint of the binding property of the two. More preferably, it is 1.0% or more, particularly preferably 2% or more, most preferably 2.5% or more, and preferably 8% or less as the upper limit. Setting the ratio to 0.5% or more is preferable from the viewpoint of sufficiently binding the inorganic filler and making it difficult to cause peeling, missing, and the like (to ensure sufficient handling properties). On the other hand, setting the ratio to 8% or less is preferable from the viewpoint of realizing good ion permeability of the separator.

  The layer thickness of the porous layer is preferably 0.5 μm or more, more preferably 2 μm or more, still more preferably 3 μm or more, and particularly preferably 4 μm or more from the viewpoint of improving heat resistance. The upper limit is preferably 100 μm or less, more preferably 50 μm or less, still more preferably 30 μm or less, particularly preferably 20 μm or less, and most preferably 10 μm or less, from the viewpoint of permeability and increase in battery capacity.

The multilayer porous membrane of the present embodiment is obtained by applying an inorganic filler-containing resin solution (dispersion) in which the inorganic filler and the resin binder are dissolved or dispersed in a solvent to at least one surface of the polyolefin resin porous membrane. It can be produced by forming a porous layer on the surface of the polyolefin resin porous membrane.
Here, when an inorganic filler layer is formed on the separator surface by applying a dispersion containing an inorganic filler and a resin binder to the separator surface, which is a porous film, such a method is excellent in productivity but inorganic. In some cases, the resin binder for binding the filler and the inorganic filler enters into the pores of the separator and closes many of the pores, thereby reducing the permeability of the separator.
However, in this embodiment, clogging is unexpectedly reduced by setting the dispersion composition, the maximum value of the heat shrinkage stress of the separator as the base material, or the wetting index of the separator surface within a specific range. It has been found that good separator permeability can be realized.

  As the solvent, it is preferable to use a solvent in which the inorganic filler and the resin binder can be dissolved or dispersed uniformly and stably. Examples of such a solvent include N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, water, ethanol, toluene, hot xylene, and hexane. In addition, in order to stabilize the inorganic filler-containing resin solution or to improve the coating property to the polyolefin resin porous membrane, the dispersion liquid contains a dispersant such as a surfactant, a thickener, a wetting agent, a disinfectant. Various additives such as foaming agents, pH adjusting agents including acids and alkalis, and the like 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 (in the multilayer porous membrane).

  Examples of the method for dissolving or dispersing the inorganic filler and the resin binder in the solvent include, for example, 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, and a high-speed impact. Examples thereof include a mechanical stirring method using a mill, ultrasonic dispersion, a stirring blade, and the like.

  The method for applying the dispersion to the surface of the polyolefin resin porous membrane is not particularly limited as long as it can realize the required layer thickness and application area. Examples of such coating methods include 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 Examples include a coater method, a squeeze coater method, a cast coater method, a die coater method, a screen printing method, and a spray coating method. Moreover, the said dispersion liquid may be apply | coated only to the single side | surface of a polyolefin resin porous film in light of the use, and may be applied to both surfaces.

  The solvent is preferably a solvent that can be removed from the dispersion applied to the polyolefin resin porous membrane. The method for removing the solvent is not particularly limited as long as it does not adversely affect the polyolefin resin porous membrane. As a method for removing the solvent, for example, a method in which the polyolefin resin porous membrane is fixed and dried at a temperature below its melting point, a method in which the polyolefin resin is dried at a low temperature under reduced pressure, and a resin binder is solidified by being immersed in a poor solvent for the resin binder. At the same time, a method of extracting the solvent can be mentioned.

  In addition, the multilayer porous membrane of this Embodiment can also be manufactured using the method different from the manufacturing method mentioned above. For example, a raw material for a polyolefin resin porous membrane (for example, a polyolefin resin and a plasticizer) is introduced into one extruder, and a raw material for a porous layer (for example, an inorganic filler and a resin binder, and plastic as necessary) is supplied to the other extruder. It is also possible to adopt a method in which a plasticizer is extracted after a plasticizer is added, integrated with one die (coextrusion), and formed into a sheet shape.

  In the multilayer porous membrane, the ratio of the layer thickness of the porous layer to the thickness (total layer thickness) of the multilayer porous membrane is preferably 15% or more, more preferably 16% or more, and preferably 50% as the upper limit. % Or less, more preferably 47% or less. Setting the ratio to 15% or more is preferable from the viewpoint of increasing the short-circuit temperature and realizing good heat resistance, while setting it to 50% or less is preferable from the viewpoint of suppressing a decrease in separator permeability. It is.

The rate of increase in air permeability when the air permeability of the polyolefin resin porous membrane is compared with the air permeability of the multilayer porous membrane (after laminating the porous layer) is preferably 0% or more, preferably as the upper limit. Is 100% or less, more preferably 70% or less, and still more preferably 50% or less. In the present embodiment, the air permeability increase rate is used as one of indexes for evaluating the ion permeability (battery charge / discharge characteristics) of the multilayer porous membrane.
When the air permeability of the polyolefin resin porous membrane as the substrate is less than 100 seconds / 100 cc, the multilayer porous membrane is preferably used as a separator even if the air permeability increase rate is 0% or more and 500% or less. It is possible.

  The air permeability of the multilayer porous membrane is preferably 10 seconds / 100 cc or more, more preferably 20 seconds / 100 cc or more, still more preferably 30 seconds / 100 cc or more, and particularly preferably 50 seconds / 100 cc or more. On the other hand, the upper limit is preferably 650 seconds / 100 cc or less, more preferably 500 seconds / 100 cc or less, further preferably 450 seconds / 100 cc or less, and particularly preferably 400 seconds / 100 cc or less. Setting the air permeability to 10 seconds / 100 cc or more is preferable from the viewpoint of suppressing self-discharge when used as a battery separator. On the other hand, setting to 650 seconds / 100 cc or less is suitable from the viewpoint of obtaining good charge / discharge characteristics.

  The thickness (total layer thickness) of the multilayer porous membrane is preferably 2 μm or more, more preferably 5 μm or more, still more preferably 7 μm or more, and the upper limit is preferably 200 μm or less, more preferably 100 μm or less, and even more preferably 50 μm or less. is there. Setting the film thickness to 2 μm or more is preferable from the viewpoint of ensuring sufficient mechanical strength. On the other hand, setting it to 200 μm or less is preferable from the viewpoint of increasing the capacity of the battery by reducing the occupied volume of the separator.

  The heat shrinkage rate at 150 ° C. of the multilayer porous membrane is preferably 0% or more and 15% or less, more preferably 0% or more and 10% or less, and particularly preferably 0% or more and 5% or less. preferable. Setting the heat shrinkage rate to 15% or less from the viewpoint of preventing the separator from breaking even during abnormal battery heat generation and suppressing contact between the positive and negative electrodes (from the viewpoint of realizing better safety performance). preferable. The heat shrinkage rate is preferably set in the above range in both the MD direction and the TD direction.

  The shutdown temperature of the multilayer porous membrane (the temperature at which the micropores of the separator are blocked by heat melting or the like when the battery has abnormal heat generation) is preferably 120 ° C. or higher, and preferably 160 ° C. or lower as the upper limit, preferably It is 150 degrees C or less. Setting the shutdown temperature to 160 ° C. or lower is preferable from the viewpoint of promptly promoting current interruption even when the battery generates heat and obtaining better safety performance. On the other hand, it is preferable to set it as 120 degreeC or more from the viewpoint of the usability in the high temperature of about 100 degreeC, for example, and the viewpoint which can perform various heat processing.

The short-circuit temperature of the multilayer porous membrane is preferably 180 ° C. or higher, more preferably 200 ° C. or higher, and the upper limit is preferably 1000 ° C. or lower. Setting the short-circuit temperature to 180 ° C. or higher is preferable from the viewpoint of suppressing contact between the positive and negative electrodes until the heat is dissipated even in abnormal battery heat generation, and realizing better safety performance.
Note that the air permeability, film thickness, thermal contraction rate, shutdown temperature, and short circuit temperature of these multilayer porous membranes can all be measured according to the measurement methods of the examples described later.

  The multilayer porous membrane of the present embodiment is particularly useful as a separator used in a storage battery such as a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery or an electric double layer capacitor because it has excellent heat resistance and ion permeability. is there. And the nonaqueous electrolyte battery provided with high safety | security and practicality can be obtained by using the multilayer porous film of this Embodiment as a separator.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(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) Film thickness (μm)
It measured with the dial gauge (PEACOCK No.25 (trademark) 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).

(3) 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} × 100

(4) Average particle size of inorganic filler (μm)
A 10 μm × 10 μm field of view magnified by a scanning electron microscope (SEM) directly or after printing on a photo from a negative, it is read into an image analyzer and the diameter of each particle calculated from this (the same area) The number average value of the diameters of the circles was defined as the average particle size (μm) of the inorganic filler. However, if the staining boundary is unclear when inputting from the photograph to the image analysis apparatus, the photograph was traced and input to the image analysis apparatus using this figure. If there is no notice in this Embodiment, "the average particle diameter of an inorganic filler" is measured using a scanning electron microscope (SEM).
The average particle size of the inorganic filler can also be measured using a laser type particle size distribution measuring device. In this case, an inorganic filler is added to distilled water, a small amount of a sodium hexametaphosphate aqueous solution is added, and the mixture is dispersed with an ultrasonic homogenizer for 1 minute, and then a laser particle size distribution analyzer (Microtrack MT3300EX manufactured by Nikkiso Co., Ltd.) is used. By measuring the particle size distribution, the particle size at which the cumulative frequency is 50% can be set as the average particle size of the inorganic filler. In the present embodiment, when the average particle size of the inorganic filler is measured using a laser particle size distribution measuring device, this is clearly stated.

(5) Bulk density of inorganic filler (g / cm ³ )
The heavy bulk density was measured by a method based on JIS R-9301-2-3.

(6) Volume fraction of resin binder (%)
The volume fraction (%) of the resin binder was calculated by the following formula.
Vb = {(Wb / Db) / (Wb / Db + Wf / Df)} × 100
Vb: Volume fraction of resin binder (%)
Wb: Weight of resin binder (g)
Wf: Weight of inorganic filler (g)
Db: Resin binder density (g / cm ³ )
Df: Bulk density of inorganic filler (g / cm ³ )

(7) Saponification degree of PVA (%)
The measurement was performed according to JIS K-0070.

(8) MD maximum heat shrinkage stress (g), TD maximum heat shrinkage stress (g)
It measured using Shimadzu Corporation TMA50 (trademark). When measuring the value in the MD (TD) direction, a sample cut to a width of 3 mm in the TD (MD) direction is fixed to the chuck so that the distance between the chucks is 10 mm, and set on a dedicated probe. The initial load is 1.0 g, the sample is heated from 30 ° C. to 200 ° C. at a heating rate of 10 ° C./min, the load (g) generated at that time is measured, and the maximum value is the MD (TD) maximum heat shrinkage stress ( g).

(9) 150 ° C. heat shrinkage The separator is cut to 100 mm in the MD direction and 100 mm in the TD direction, and left in an oven at a predetermined temperature (150 ° C.) for 1 hour. At this time, the sample is sandwiched between two sheets of paper so that the hot air does not directly hit the sample. After taking out the sample from the oven and cooling, the length (mm) was measured, and the thermal shrinkage rate of MD and TD was calculated by the following formula.
MD thermal shrinkage rate (%) = {(100−length of MD after heating) / 100} × 100
TD heat shrinkage rate (%) = {(100−length of TD after heating) / 100} × 100

(10) Wetting index (mN / m)
It measured by the method based on JISK-6768.

(11) 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 A slurry was prepared by dispersing 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 in purified water. 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.

(12) Battery evaluation a. Production of Positive Electrode The positive electrode produced in (11) a was punched into a circle having an area of 2.00 cm ² .
b. Production of Negative Electrode The negative electrode produced in (11) 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]
47.5 parts by mass of polyethylene having a viscosity average molecular weight (Mv) of 200,000, 2.5 parts by mass of polypropylene having an Mv of 400,000, 30 parts by mass of liquid paraffin (LP) as a plasticizer, and pentaerythrityl-tetrakis- [ What added 0.5 part by mass of 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] 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 amount ratio in the total mixture (100 parts by mass) melt-kneaded and extruded was 50 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 200 rpm, and a discharge rate of 15 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 1050 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 118 ° 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 conditions were a maximum draw ratio of 1.5 times, a final draw ratio of 1.3 times, a maximum drawing temperature of 123 ° C., and a final drawing temperature of 128 ° C. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 3.8 g, a TD maximum heat shrinkage stress of 2.9 g, a film thickness of 16 μm, a porosity of 45%, and an air permeability of 235 seconds / 100 cc was obtained.

An aqueous solution in which 95 parts by mass of alumina particles (average particle size 0.7 μm) and 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly dispersed in 150 parts by mass of water, After coating with a gravure coater on the surface of the polyolefin resin porous membrane, it was dried at 60 ° C. to remove water, and a porous layer (binder volume fraction 3.4%) having a thickness of 4 μm was formed on the porous membrane. The formed multilayer porous film having a total film thickness of 20 μm was obtained.
The resulting multilayer porous membrane had an air permeability of 255 sec / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 9%, maintaining excellent permeability. In addition, MD heat shrinkage rate at 150 ° C is 3%, TD heat shrinkage rate is 3%, shutdown temperature is observed at 146 ° C, short circuit is not observed even at 200 ° C or higher, and has very high heat resistance. Indicated.
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]
After the corona discharge treatment (discharge amount 50 W) was performed on the surface of the polyolefin resin porous film used as the base material in Example 1, alumina particles (average) were formed on the treated surface side (surface wetting index of 73 mN / m or more). Particle size 0.7 μm) 95 parts by mass, SB latex (solid content concentration 50%, minimum film formation temperature 0 ° C. or less) 10 parts by mass, ammonium polycarboxylate aqueous solution (San Disco SN Dispersant 5468) 1 part by mass, polyoxy An aqueous solution in which 1 part by weight of an alkylene surfactant (San Nopco SN wet 980) was uniformly dispersed in 150 parts by weight of water was applied using a gravure coater and then dried at 60 ° C. to remove the water. Thus, a multilayer porous film having a total film thickness of 23 μm, in which a porous layer having a thickness of 7 μm (a binder volume fraction of 7.8%) was formed on the porous film, was obtained.
The obtained multilayer porous membrane had an air permeability of 280 sec / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 19%, maintaining excellent permeability. In addition, MD thermal shrinkage rate at 150 ° C is 2%, TD heat shrinkage rate is 2%, shutdown temperature is observed at 145 ° C, short circuit is not observed even at 200 ° C or higher, and very high heat resistance. Indicated.
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]
16.5 parts by weight of polyethylene with a viscosity average molecular weight (Mv) of 700,000, 16.1 parts by weight of polyethylene with a Mv of 300,000 and 2.5 parts by weight of polypropylene having a Mv of 400,000, 40 parts by weight of liquid paraffin (LP) as a plasticizer, What added 0.3 mass part of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant 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 mass) melt-kneaded and extruded was 65 parts by mass. 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 2400 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 7.0 times in the TD direction. At this time, the set temperature of the simultaneous biaxial tenter was 125 ° 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 133 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 4.8 g, a TD maximum heat shrinkage stress of 3.7 g, a film thickness of 20 μm, a porosity of 40%, and an air permeability of 280 seconds / 100 cc was obtained.

After the surface of the polyolefin resin porous membrane is subjected to corona discharge treatment (discharge amount 50 W), titania particles (average particle size 0.4 μm) 95 mass on the treated surface side (surface wetting index of 73 mN / m or more) Part, SB latex (solid content concentration 50%, minimum film formation temperature 0 ° C. or less) 10 parts by mass, ammonium polycarboxylate aqueous solution (San Nopco SN Dispersant 5468), 1 part by mass, polyoxyalkylene surfactant (San Nopco) SN wet 980) After applying an aqueous solution in which 1 part by mass of water was uniformly dispersed in 150 parts by mass of water using a bar coater, the water was removed by drying at 60 ° C. A multilayer porous film having a total film thickness of 26 μm formed by a 6 μm porous layer (binder volume fraction 6.2%) was obtained.
The resulting multilayer porous membrane had an air permeability of 315 sec / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 13%, maintaining excellent permeability. Also, MD thermal shrinkage at 150 ° C is 8%, TD heat shrinkage is 5%, shutdown temperature is observed at 147 ° C, short circuit is not observed even at 200 ° C or higher, and very high heat resistance. Indicated.
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 4]
16.6 parts by mass of polyethylene having a viscosity average molecular weight (Mv) of 700,000, 16.6 parts by mass of polyethylene having an Mv of 250,000, 1.8 parts by mass of polypropylene having an Mv of 400,000, 40 parts by mass of liquid paraffin (LP) as a plasticizer, What added 0.3 mass part of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant 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 mass) melt-kneaded and extruded was 65 parts by mass. 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 118 ° 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 122 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 6.7 g, a TD maximum heat shrinkage stress of 2.8 g, a film thickness of 17 μm, a porosity of 49%, and an air permeability of 165 seconds / 100 cc was obtained.

After the surface of the polyolefin resin porous membrane is subjected to corona discharge treatment (discharge amount 50 W), titania particles (average particle size 0.4 μm) 95 mass on the treated surface side (surface wetting index of 73 mN / m or more) After applying an aqueous solution in which 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) is uniformly dispersed in 150 parts by mass of water using a gravure coater, drying is performed at 60 ° C. Then, water was removed to obtain a multilayer porous film having a total film thickness of 24 μm in which a porous layer having a thickness of 7 μm (a binder volume fraction of 4.6%) was formed on the porous film.
The obtained multilayer porous membrane had an air permeability of 200 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 21%, maintaining excellent permeability. In addition, MD heat shrinkage at 150 ° C is 5%, TD heat shrinkage is 4%, shutdown temperature is observed at 144 ° C, short circuit is not observed even at 200 ° C or higher, and very high heat resistance. Indicated.
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 5]
12 parts by mass of ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 2 million, 12 parts by mass of high-density polyethylene of Mv 280,000, 16 parts by mass of linear low-density polyethylene of Mv 150,000 and silica (average particle size 8.3 μm) 17 After mixing and granulating 6 parts by mass and 42.4 parts by mass of dioctyl phthalate (DOP) as a plasticizer, the mixture is kneaded and extruded by a twin-screw extruder equipped with a T-die to form a sheet having a thickness of 90 μm. Molded. From the molded product, DOP was extracted with methylene chloride and silica was extracted with sodium hydroxide to obtain a microporous membrane. The microporous membrane was heated to 118 ° C., stretched 5.3 times in the longitudinal direction, and then stretched 1.8 times in the transverse direction. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 8.7 g, a TD maximum heat shrinkage stress of 0.9 g, a film thickness of 11 μm, a porosity of 48%, and an air permeability of 55 seconds / 100 cc was obtained.

On the surface of the polyolefin resin porous membrane, 95 parts by mass of titania particles (average particle size 0.4 μm) and 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly distributed in 150 parts by mass of water. After the aqueous solution dispersed in is coated using a bar coater, it is dried at 60 ° C. to remove water, and a porous layer (binder volume fraction 4.6%) having a thickness of 8 μm is formed on the porous film. The formed multilayer porous film having a total film thickness of 19 μm was obtained.
The obtained multilayer porous membrane had an air permeability of 240 seconds / 100 cc and increased to a preferable air permeability. In addition, MD thermal shrinkage at 150 ° C is 4% and TD thermal shrinkage is as small as 3%, shutdown temperature is observed at 150 ° C, short circuit is not observed even at 200 ° C or higher, and has very high heat resistance. Indicated.
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 6]
16.6 parts by mass of polyethylene having a viscosity average molecular weight (Mv) of 700,000, 16.6 parts by mass of polyethylene having an Mv of 250,000, 1.8 parts by mass of polypropylene having an Mv of 400,000, 40 parts by mass of liquid paraffin (LP) as a plasticizer, What added 0.3 mass part of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant 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 mass) melt-kneaded and extruded was 65 parts by mass. 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 1000 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 118 ° 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 130 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 3.6 g, a TD maximum heat shrinkage stress of 3.3 g, a film thickness of 12 μm, a porosity of 36%, and an air permeability of 230 seconds / 100 cc was obtained.
After the surface of the polyolefin resin porous membrane is subjected to corona discharge treatment (discharge amount 50 W), alumina particles (average particle size 2.0 μm) 95 mass on the treated surface side (surface wetting index of 73 mN / m or more) After applying an aqueous solution in which 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) is uniformly dispersed in 150 parts by mass of water using a gravure coater, drying is performed at 60 ° C. Then, water was removed to obtain a multilayer porous film having a total film thickness of 20 μm in which a porous layer having a thickness of 8 μm (a binder volume fraction of 4.6%) was formed on the porous film.
The resulting multilayer porous membrane had an air permeability of 330 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 43%, maintaining excellent permeability. In addition, MD thermal shrinkage rate at 150 ° C is 2%, TD heat shrinkage rate is 2%, shutdown temperature is observed at 147 ° C, short circuit is not observed even at 200 ° C or higher, and very high heat resistance. Indicated.
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 7]
47 parts by mass of polyethylene having an Mv of 700,000, 46 parts by mass of polyethylene having an Mv of 250,000, and 7 parts by mass of polypropylene having an Mv of 400,000 were dry blended using a tumbler blender. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99% by mass of the obtained pure polymer mixture, and again A mixture such as a polymer was obtained by dry blending using a tumbler blender. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected as a plasticizer into the extruder cylinder by a plunger pump. The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 65% by mass. 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 material was extruded and cast on a cooling roll controlled at a surface temperature of 25 ° C. through a T-die to obtain a sheet-like polyolefin composition having a thickness of 2000 μm.
Next, it led to the simultaneous biaxial tenter stretching machine, and simultaneous biaxial stretching was performed 7 times in the MD direction and 7 times in the TD direction. At this time, the setting temperature of the simultaneous biaxial tenter was 125 ° C. Next, the solution was introduced into a methyl ethyl ketone tank, and liquid paraffin was extracted and removed, 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 133 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 3.2 g, a TD maximum heat shrinkage stress of 3.1 g, a film thickness of 16 μm, a porosity of 40%, and an air permeability of 165 seconds / 100 cc was obtained.
After the surface of the polyolefin resin porous membrane is subjected to corona discharge treatment (discharge amount: 50 W), alumina particles (average particle size of 0.51 μm (laser type) are formed on the treated surface side (surface wetting index of 73 mN / m or more). Average particle diameter measured by a particle size distribution measuring apparatus 0.61 μm)) 98.2 parts by mass, polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) 1.8 parts by mass uniformly in 150 parts by mass of water After the aqueous solution dispersed in is applied using a gravure coater, it is dried at 60 ° C. to remove water, and a porous layer having a thickness of 5 μm (a binder volume fraction of 1.7%) is formed on the porous film. The formed multilayer porous film having a total film thickness of 21 μm was obtained.
The resulting multilayer porous membrane had an air permeability of 180 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 9%, maintaining excellent permeability. In addition, MD heat shrinkage rate at 150 ° C is 3%, TD heat shrinkage rate is 2%, shutdown temperature is observed at 145 ° C, short circuit is not observed even at 200 ° C or higher, and has very high heat resistance. Indicated.
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 8]
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 a Mv of 250,000, 1.8 parts by weight of polypropylene with a Mv of 400,000, 40 parts by weight of liquid paraffin (LP) as a plasticizer, oxidized What added 0.3 mass 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 mass) melt-kneaded and extruded was 65 parts by mass. 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 an MD maximum heat shrinkage stress of 6.0 g, a TD maximum heat shrinkage stress of 2.1 g, a film thickness of 16 μm, a porosity of 46%, and an air permeability of 190 seconds / 100 cc was obtained.
After the surface of the polyolefin resin porous membrane is subjected to corona discharge treatment (discharge amount: 50 W), alumina particles (average particle size of 0.51 μm (laser type) are formed on the treated surface side (surface wetting index of 73 mN / m or more). Average particle diameter measured by a particle size distribution measuring apparatus 0.61 μm)) 98.2 parts by mass, polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) 1.8 parts by mass uniformly in 150 parts by mass of water After the aqueous solution dispersed in is coated using a gravure coater, it is dried at 60 ° C. to remove water, and a 6 μm thick porous layer (a binder volume fraction of 1.7%) is formed on the porous film. The formed multilayer porous film having a total film thickness of 22 μm was obtained.
The resulting multilayer porous membrane had an air permeability of 210 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 11%, maintaining excellent permeability. In addition, MD heat shrinkage at 150 ° C is 5%, TD heat shrinkage is 3%, shutdown temperature is observed at 145 ° C, short circuit is not observed even at 200 ° C or higher, and very high heat resistance. Indicated.
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 9]
Example 3 was the same as Example 3 except that the TD relaxation rate during heat setting was 0.90. As a result, the maximum MD thermal contraction stress was 4.9 g, the maximum TD thermal contraction stress was 3.7 g, and the film thickness was 25 μm. A polyolefin resin porous membrane having a porosity of 43% and an air permeability of 290 sec / 100 cc was obtained.
After the surface of the polyolefin resin porous membrane is subjected to corona discharge treatment (discharge amount: 50 W), alumina particles (average particle size of 0.85 μm (laser type) are formed on the treated surface side (surface wetting index of 73 mN / m or more). Average particle diameter measured by particle size distribution measuring apparatus 1.2 μm)) 98.4 parts by mass, polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) 1.6 parts by mass uniformly in 150 parts by mass of water After the aqueous solution dispersed in is applied using a gravure coater, it is dried at 60 ° C. to remove water, and a 6 μm thick porous layer (binder volume fraction 1.6%) is formed on the porous film. The formed multilayer porous film having a total film thickness of 31 μm was obtained.
The resulting multilayer porous membrane had an air permeability of 310 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 7%, maintaining excellent permeability. In addition, MD thermal shrinkage rate at 150 ° C is 2%, TD heat shrinkage rate is 2%, shutdown temperature is observed at 147 ° C, short circuit is not observed even at 200 ° C or higher, and very high heat resistance. Indicated.
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]
19.2 parts by weight of polyethylene with a viscosity average molecular weight (Mv) of 1 million, 12.8 parts by weight of polyethylene with a viscosity average molecular weight (Mv) of 250,000, 20 parts by weight of silica (average particle size 8.3 μm), and dioctyl phthalate (DOP) ) 48 parts by mass were mixed and granulated, and then kneaded and extruded by a twin-screw extruder equipped with a T-die to form a sheet having a thickness of 90 μm. From the molded product, DOP was extracted with methylene chloride and finely divided silica was extracted with sodium hydroxide to obtain a microporous membrane. The microporous membrane was stretched 4.5 times in the longitudinal direction under heating to 110 ° C., and then stretched 2.0 times in the lateral direction under heating to 130 ° C. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 12.9 g, a TD maximum heat shrinkage stress of 1.2 g, a film thickness of 18 μm, a porosity of 48%, and an air permeability of 125 seconds / 100 cc was obtained.

On the surface of the polyolefin resin porous membrane, 95 parts by mass of titania particles (average particle size 0.4 μm) and 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly distributed in 150 parts by mass of water. After coating the aqueous solution dispersed in the film using a bar coater, the film was dried at 60 ° C. to remove water, and a porous layer having a thickness of 13 μm was formed on the porous film. Got.
The obtained multilayer porous membrane had an air permeability of 180 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as low as 44%, maintaining excellent permeability. Further, the TD heat shrinkage at 150 ° C. was as small as 3%, but the MD heat shrinkage was as large as 32%, and the short-circuit temperature was as low as 156 ° C.

[Comparative Example 2]
19.2 parts by weight of polyethylene with a viscosity average molecular weight (Mv) of 1 million, 12.8 parts by weight of polyethylene with a viscosity average molecular weight (Mv) of 250,000, 20 parts by weight of silica (average particle size 8.3 μm), and dioctyl phthalate (DOP) ) 48 parts by mass were mixed and granulated, and then kneaded and extruded by a twin-screw extruder equipped with a T-die to form a sheet having a thickness of 90 μm. From the molded product, DOP was extracted with methylene chloride and finely divided silica was extracted with sodium hydroxide to obtain a microporous membrane. The microporous membrane was stretched 6.0 times in the longitudinal direction under heating to 110 ° C., and then stretched 2.0 times in the transverse direction under heating to 130 ° C. As a result, a polyolefin resin porous film having an MD maximum heat shrinkage stress of 23.5 g, a TD maximum heat shrinkage stress of 1.8 g, a film thickness of 16 μm, a porosity of 48%, and an air permeability of 110 seconds / 100 cc was obtained.

On the surface of the polyolefin resin porous membrane, 95 parts by mass of titania particles (average particle size 0.4 μm) and 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly distributed in 150 parts by mass of water. After applying the aqueous solution dispersed in the film using a gravure coater, the film was dried at 60 ° C. to remove water, and a porous layer having a thickness of 16 μm was formed on the porous film. Got.
The obtained multilayer porous membrane had an air permeability of 780 seconds / 100 cc, and the rate of increase in air permeability due to the formation of the porous layer was as extremely high as about 600%, and the permeability deteriorated. Regarding heat resistance, MD thermal shrinkage at 150 ° C is 5%, TD heat shrinkage is 3%, shutdown temperature is observed at 145 ° C, and short circuit is not observed even at 200 ° C or higher. It was very good.

[Comparative Example 3]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the substrate in Example 1, the MD heat shrinkage rate at 150 ° C. was 69%, and the TD heat shrinkage rate was 67%. It was very big. The shutdown temperature was observed at 147 ° C, but the short-circuit temperature was as low as 149 ° C. 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 4]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the base material in Example 3, the MD heat shrinkage rate at 150 ° C. was 68% and the TD heat shrinkage rate was 64%. It was very big. The shutdown temperature was observed at 146 ° C, but the short-circuit temperature was as low as 153 ° C. 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 5]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the base material in Example 4, the MD heat shrinkage rate at 150 ° C. was 74% and the TD heat shrinkage rate was 54%. It was very big. The shutdown temperature was observed at 148 ° C, but the short-circuit temperature was as low as 152 ° C. 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 6]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the base material in Example 5, the MD heat shrinkage rate at 150 ° C. was 80% or more, and the TD heat shrinkage rate was 20%. And it was very big. The shutdown temperature was observed at 144 ° C., but the short-circuit temperature was as low as 153 ° C. When this multilayer porous membrane was used as a separator for battery evaluation, the rate characteristics were as high as 90% or higher, the capacity retention rate after 100 cycles was 90% or higher, and the cycle characteristics were also good.

[Comparative Example 7]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the base material in Example 6, the MD heat shrinkage rate at 150 ° C. was 57% and the TD heat shrinkage rate was 59%. It was very big. The shutdown temperature was observed at 147 ° C, but the short-circuit temperature was as low as 153 ° C. 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 8]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the base material in Example 7, the MD heat shrinkage rate at 150 ° C. was 60%, and the TD heat shrinkage rate was 50%. It was very big. The shutdown temperature was observed at 145 ° C, but the short-circuit temperature was as low as 155 ° C. 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 9]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the base material in Example 8, the MD heat shrinkage rate at 150 ° C. was 68% and the TD heat shrinkage rate was 51%. It was very big. The shutdown temperature was observed at 152 ° C, but the short-circuit temperature was as low as 155 ° C. 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 10]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the base material in Example 9, the MD heat shrinkage rate at 150 ° C. was 69% and the TD heat shrinkage rate was 62%. It was very big. The shutdown temperature was observed at 152 ° C, but the short-circuit temperature was as low as 154 ° C. 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 11]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the substrate in Comparative Example 1, the MD heat shrinkage rate at 150 ° C. was 80% or more, and the TD heat shrinkage rate was 23%. And it was very big. The shutdown temperature was observed at 152 ° C, but the short-circuit temperature was as low as 153 ° C. When this multilayer porous membrane was used as a separator for battery evaluation, the rate characteristics were as high as 90% or higher, the capacity retention rate after 100 cycles was 90% or higher, and the cycle characteristics were also good.

[Comparative Example 12]
When the same evaluation was performed without forming a porous layer on the surface of the polyolefin resin porous film used as the substrate in Comparative Example 2, the MD heat shrinkage rate at 150 ° C. was 80% or more, and the TD heat shrinkage rate was 24%. And it was very big. The shutdown temperature was observed at 145 ° C, but the short-circuit temperature was as low as 147 ° C. In addition, 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 rate after 100 cycles was 90% or more, and the cycle characteristics were also good. Table 1 summarizes the physical properties in the comparative examples.

Claims (12)

Provided with a porous layer containing an inorganic filler and a resin binder on at least one side of the polyolefin resin porous membrane,
The multilayer porous membrane, wherein the polyolefin resin porous membrane has a larger value of MD maximum heat shrinkage stress and TD maximum heat shrinkage stress under a temperature condition of 30 ° C to 200 ° C of 7 g or less.   2. The multilayer porous membrane according to claim 1, wherein the inorganic filler has an average particle size of 0.1 μm to 3.0 μm.   The multilayer porous membrane according to claim 1 or 2, wherein a ratio of the resin binder to a total amount of the inorganic filler and the resin binder is 0.5% or more and 8% or less in terms of volume fraction.   The multilayer porous membrane according to any one of claims 1 to 3, wherein a ratio of the total thickness of the multilayer porous membrane to the total layer thickness is 15% or more and 50% or less.   The multilayer porous membrane according to any one of claims 1 to 4, wherein an air permeability of the multilayer porous membrane is 10 seconds / cc or more and 650 seconds / cc or less.   The multilayer porous membrane according to any one of claims 1 to 5, wherein a total thickness of the multilayer porous membrane is 2 µm or more and 200 µm or less.   The multilayer porous membrane according to any one of claims 1 to 6, wherein the resin binder is polyvinyl alcohol having a saponification degree of 85% or more.   The multilayer porous membrane according to any one of claims 1 to 7, wherein the polyolefin resin porous membrane surface facing the porous layer has a wetting index of 40 mN / m or more.   The multilayer porous membrane according to any one of claims 1 to 8, wherein a shutdown temperature of the multilayer porous membrane is 150 ° C or lower.   The separator for non-aqueous electrolyte batteries using the multilayer porous membrane of any one of Claims 1-9.   A nonaqueous electrolyte battery using the separator for a nonaqueous electrolyte battery according to claim 10.   The method for producing a multilayer porous membrane according to any one of claims 1 to 9, wherein a dispersion containing an inorganic filler and a resin binder is applied to at least one surface of the polyolefin resin porous membrane. A method for producing a multilayer porous membrane.