JP7015159B2 - Multi-layer separator, its winding body and manufacturing method - Google Patents

Description

The present invention relates to a multilayer separator (hereinafter, also simply referred to as "separator") used for a separator for a power storage device, a wound body thereof, and a manufacturing method thereof.

In recent years, among energy storage devices, non-aqueous electrolyte batteries centering on lithium ion batteries have been actively developed. Normally, a non-aqueous electrolyte battery is provided with a microporous membrane (separator) between the positive and negative electrodes. Such a separator has a function of preventing electron conduction due to direct contact between positive and negative electrodes or a short circuit, and also has a function of allowing ions to permeate through an electrolytic solution held in the microporous.

Improvement of the separator is being studied in order to improve the cycle characteristics, safety, etc. of the non-aqueous electrolyte battery. In recent years, with the miniaturization and thinning of portable devices, there is a demand for miniaturization and thinning of power storage devices such as lithium ion secondary batteries. On the other hand, in order to enable the portable device to be carried for a long time, the capacity of the power storage device is also increased by improving the volumetric energy density.
In addition, a large, high-capacity non-aqueous electrolyte battery is used as a power source for powering electric vehicles, etc., and in addition to further improving safety, it is highly suitable for quick charging and regenerative braking. It is necessary to have input / output characteristics, and performance requirements are diversifying and becoming more sophisticated.

The separator has the characteristic that the battery reaction is stopped immediately when abnormally heated (fuse characteristic), and the shape is maintained even at high temperatures to prevent the dangerous situation where the positive electrode material and the negative electrode material directly react. Performance related to safety such as performance (short characteristic) is required. In addition to these, recently, from the viewpoint of making the charge / discharge current uniform and suppressing lithium dendrite, the separator is also required to improve the adhesion to the electrode. By improving the closeness between the separator and the battery electrode, non-uniformity of the charge / discharge current is less likely to occur, and lithium dendrite is less likely to precipitate. As a result, the charge / discharge cycle life can be extended. Therefore, it is being enthusiastically studied to impart various functions by using multiple layers of separators.

For example, Patent Document 1 discloses that the surface of a porous film is coated with organic fine particles having a melting point of 80 to 130 ° C. to exhibit fuse behavior at a relatively low temperature. Patent Document 2 proposes coating a porous membrane substrate with thermoplastic polymer particles to bond and fix the separator and the electrode. In Patent Document 3, a coating liquid containing a polyvinylidene fluoride-based resin is applied on the surface of the microporous membrane, immersed in a coagulating liquid, solidified, washed with water, and dried to form a polyfoam on the surface of the microporous membrane. A technique for forming an adhesive porous layer having good permeability made of a vinylidene fluoride resin is disclosed. Further, Patent Document 4 proposes a composite film having good binding property by forming a polymer thin film on the surface of a support layer and stretching it.

Japanese Unexamined Patent Publication No. 2006-164761 International Publication No. 2016/047165 International Publication No. 2014/136838 Japanese Unexamined Patent Publication No. 2008-302359

In Patent Documents 1 to 4, it is proposed to impart functions such as fuse characteristics and adhesiveness to electrodes by increasing the number of layers, but there is a problem that the same functions are exhibited after long-term storage. That is, the separator is usually commercialized and distributed in a state of being wound around a core, and is unwound from the wound body at the time of use. The wound state is a state in which the separators are in close contact with each other, and may be exposed to a high temperature of 40 to 60 ° C. in some cases. In such a case, there may be a problem that the originally desired function is not normally expressed. INDUSTRIAL APPLICABILITY The present invention is a method for manufacturing a multilayer separator, a multilayer separator wound body, and a multilayer separator capable of maintaining adhesiveness with an electrode even when the separator is stored for a long period of time in a separator having a functional layer on the surface. It is one of the purposes to provide.

The present inventors have diligently studied to solve the above problems. As a result, the multilayer separator including the microporous membrane (A) containing the thermoplastic resin (a) as the main component and the porous layer (B) containing the thermoplastic resin (b) as the main component is the above-mentioned porous. The layer (B) is formed so as to form a continuum on at least one surface of the microporous film (A), and the repetitive peel strength retention rate of the porous layer (B) is adjusted to 60% or more. We have found that the problem can be solved and arrived at the present invention.
That is, the present invention is as follows.
[1]
A multilayer separator comprising a microporous membrane (A) containing a thermoplastic resin (a) as a main component and a porous layer (B) containing a thermoplastic resin (b).
The porous layer (B) is formed so as to form a continuum on at least one surface of the microporous membrane (A).
A multilayer separator having a repetitive peel strength retention rate of 60% or more for the porous layer (B).
[2]
The multilayer separator according to item 1, wherein the melting point of the thermoplastic resin (b) or the glass transition temperature is 5 ° C. higher than the melting point of the thermoplastic resin (a).
[3]
The multilayer separator according to item 1 or 2, wherein the air permeability is 500 seconds / 100 ml or less.
[4]
The multilayer separator according to any one of items 1 to 3, wherein the porous layer (B) has an average thickness of 10 nm or more and less than 500 nm.
[5]
The multilayer separator according to any one of items 1 to 4, wherein the loss elastic modulus of the thermoplastic resin (b) at the melting point temperature of the thermoplastic resin (a) is 0.01 MPa or more and 3.00 MPa or less.
[6]
A multilayer separator winding body in which the multilayer separator according to any one of items 1 to 5 is wound 1000 m or more on a core having an outer diameter of 3 inches (7.62 cm) to 20 inches (50.8 cm).
[7]
A method for producing a multilayer separator including a microporous membrane (A) containing a thermoplastic resin (a) as a main component and a porous layer (B) containing a thermoplastic resin (b).
A slurry in which the thermoplastic resin (b) is dissolved or dispersed in a solvent is applied to at least one surface of the microporous film (A) containing the thermoplastic resin (a) as a main component so that the slurry forms a continuum. Step (1) to obtain a coated body,
After the step (1), the step (2) of stretching the coated body 1.01 times or more at a temperature 5 ° C. lower than the melting point of the thermoplastic resin (b) or the glass transition temperature or higher.
A method for manufacturing a multilayer separator, including.
[8]
The method for producing a multilayer separator according to item 7, wherein in the step (1), the coating basis weight of the slurry after drying is 0.05 g / m ² or more and 3.00 g / m ² or less in terms of solid content.
[9]
The stretching temperature in the step (2) is equal to or higher than the melting point or the glass transition temperature of the thermoplastic resin (b), and the above method is 1% or more in the stretching direction with respect to the post-stretching after the step (2). The method for producing a multilayer separator according to item 7 or 8, further comprising a step (3) of relaxation.

According to the present invention, it is possible to provide a multilayer separator capable of maintaining the adhesiveness between the separator and the electrode for a long period of time, and a wound body and a manufacturing method thereof.

FIG. 1 is a schematic view showing a cross section in the length direction of a measurement sample for explaining a method for measuring a repetitive peel strength retention rate in the present embodiment.

Hereinafter, embodiments for carrying out the present invention (hereinafter, abbreviated as “the present embodiment”) will be described in detail. The present invention is not limited to the present embodiment, and can be variously modified and implemented within the scope of the gist thereof.

[Poor layer (B)]
Hereinafter, the porous layer (B) containing the thermoplastic resin (b) (hereinafter, also referred to as “functional porous layer”) will be described.
The functional porous layer contains the thermoplastic resin (b). The functional porous layer is formed so as to form a continuum on at least one surface of the microporous membrane (A) (hereinafter, also referred to as “base material”) containing the thermoplastic resin (a) as a main component. The thermoplastic resin (b) is preferably a resin different from that of the thermoplastic resin (a). As used herein, the term "different" resin refers to a resin that differs in any one or more of all the properties of the resin, such as the type of monomer constituting the resin, the molecular weight of the resin, the melting point, and the glass transition temperature. The details of the microporous membrane (A) will be described later. The functional porous layer may be formed on at least one side of the base material so that the functional porous layer forms a continuum, and may be formed on both sides of the base material. In the present embodiment, "to form a continuum" means a thermoplastic resin constituting the skeleton of the lower microporous membrane (A) when observing the surface of the multilayer separator on which the porous layer (B) is formed. (A) is not exposed. In other words, when the surface of the multilayer separator on which the porous layer (B) is formed is observed with a microscope, SEM, or the like, the thermoplastic resin (a) constituting the skeleton of the microporous membrane (A) is thermoplastic. It is covered with resin (b). Therefore, the functional porous layer can efficiently express its function. If it is dispersed in the form of particles, or if it is partially coated or patterned, it is not preferable because the functionality of the functional porous layer may be impaired.

The functional porous layer can be adhered between the electrode and the separator by undergoing a heat pressing process in the battery manufacturing process. That is, the functional porous layer can function as an adhesive layer.

The thermoplastic resin (b) preferably has a melting point of 5 ° C. or higher than the melting point of the thermoplastic resin (a). In the case of an amorphous resin having no melting point, it is preferable to have a glass transition temperature of 5 ° C. or higher than the melting point of the thermoplastic resin (a). Examples of such resins include polyolefins such as polyethylene and polypropylene, fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers and ethylene-tetrafluoroethylene. Fluoropolymer-containing rubber such as copolymer, styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, methacrylic acid ester-acrylic acid Ester copolymers, styrene-acrylic acid ester copolymers, acrylonitrile-acrylic acid ester copolymers, rubbers such as ethylene propylene rubber, polyvinyl alcohol, vinyl acetate, polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, Examples thereof include resins having a melting point and / or a glass transition temperature of 180 ° C. or higher, such as polyetherimide, polyamideimide, polyamide, and polyester. These can be used alone or in combination of two or more. When two or more types are used in combination, the melting point of the thermoplastic resin having the highest ratio (weight ratio) is defined as the melting point of the thermoplastic resin (b). The melting point of the thermoplastic resin (b) is measured using a multilayer separator, but if measurement is difficult, the melting point of the raw material thermoplastic resin (b) may be used for measurement.

The loss elastic modulus of the thermoplastic resin (b) at the melting point temperature of the thermoplastic resin (a) is preferably 3.00 MPa or less, more preferably 2.00 MPa or less. When the loss elastic modulus is 3.00 MPa or less, it is easy to obtain a multilayer layer (B) forming a continuum when the slurry containing the thermoplastic resin (b) is applied to produce a multilayer separator, and the peel strength is repeated. The maintenance rate is likely to improve. It also has the advantage that the fuse temperature tends to drop. The lower limit of the loss elastic modulus of the thermoplastic resin (b) is not particularly limited, but 0.01 MPa or more is preferable because the air permeability tends to be low, and 0.10 MPa or more is more preferable.

Specific examples of the thermoplastic resin (b) include the following 1) to 4).
1) Conjugated diene polymer,
2) Acrylic polymer,
3) Polyvinyl alcohol-based resin and 4) Fluororesin.
Among them, the above 1) conjugated diene polymer is preferable from the viewpoint of easy compatibility with the electrode, and the above 2) acrylic polymer and the above 4) fluororesin are preferable from the viewpoint of withstand voltage. Further, the polymer layer may contain other components other than the thermoplastic polymer to the extent that the problem solving of the present invention is not impaired.

The above 1) conjugated diene-based polymer is a polymer containing a conjugated diene compound as a monomer unit. Examples of the conjugated diene compound include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chlor-1,3-butadiene, and substituted linear chain. Conjugated pentadiene, substitution and side chain conjugated hexadiene and the like can be mentioned, and these may be used alone or in combination of two or more. Of these, 1,3-butadiene is particularly preferable.
Further, the conjugated diene-based polymer may contain a (meth) acrylic compound or another monomer described later as a monomer unit. Specifically, for example, a styrene-butadiene copolymer and its hydride, an acrylonitrile-butadiene copolymer and its hydride, an acrylonitrile-butadiene-styrene copolymer and its hydride, and the like.

The above 2) acrylic polymer is a polymer containing a (meth) acrylic compound as a monomer unit. The (meth) acrylic compound refers to at least one selected from the group consisting of (meth) acrylic acid and (meth) acrylic acid ester. Examples of such a compound include a compound represented by the following formula (P1).
CH ₂ = CR ^Y1 -COO- ^RY2 (P1)
In the formula (P1), ^RY1 represents a hydrogen atom or a methyl group, and ^RY2 represents a hydrogen atom or a monovalent hydrocarbon group.
When ^RY2 is a monovalent hydrocarbon group, it may have a substituent and may have a heteroatom in the chain. Examples of the monovalent hydrocarbon group include a chain alkyl group which may be linear or branched, a cycloalkyl group, and an aryl group. Examples of the substituent include a hydroxyl group and a phenyl group, and examples of the hetero atom include a halogen atom and an oxygen atom. The (meth) acrylic compound may be used alone or in combination of two or more. Examples of such (meth) acrylic compounds include (meth) acrylic acid, chain alkyl (meth) acrylate, cycloalkyl (meth) acrylate, (meth) acrylate having a hydroxyl group, and phenyl group-containing (meth) acrylate. Can be mentioned.
As a chain alkyl group which is a kind of ^RY2 , more specifically, a methyl group, an ethyl group, an n-propyl group, and a chain alkyl group having 1 to 3 carbon atoms, which is an isopropyl group; n- Examples thereof include chain alkyl groups having 4 or more carbon atoms such as a butyl group, an isobutyl group, a t-butyl group, an n-hexyl group, a 2-ethylhexyl group, and a lauryl group. Moreover, as an aryl group which is one kind of ^RY2 , for example, a phenyl group can be mentioned.
Among these, from the viewpoint of improving the adhesion to the electrode (electrode active material), ^RY2 is preferably a monomer having a chain alkyl group having 4 or more carbon atoms, and more specifically. ^RY2 is preferably a (meth) acrylic acid ester monomer which is a chain alkyl group having 4 or more carbon atoms. More specifically, ^RY2 is preferably at least one selected from the group consisting of butyl acrylate, butyl methacrylate, and 2-ethylhexyl acrylate. The upper limit of the number of carbon atoms in the chain alkyl group having 4 or more carbon atoms is not particularly limited, and may be, for example, 14, but 7 is preferable. These (meth) acrylic acid ester monomers may be used alone or in combination of two or more.

Examples of the above 3) polyvinyl alcohol-based resin include polyvinyl alcohol and polyvinyl acetate, and examples of the above 4) fluororesin include polyvinylidene fluoride, polytetrafluoroethylene, and vinylidene fluoride-hexafluoropropylene. Examples thereof include copolymers, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, and ethylene-tetrafluoroethylene copolymers. When the polyvinylidene fluoride-based copolymer contains 70 mol% or more of the resin constituting the porous layer (B) as a constituent unit of vinylidene fluoride, it tends to be able to maintain sufficient strength in addition to adhesiveness to the electrode. It is preferable because it is located in.

The basis weight of the functional porous layer to the substrate is preferably 0.05 g / m ² or more and 3.00 g / m ² or less, more preferably 0.10 g / m ² or more and 2.00 g / m ² or less in terms of solid content. Yes, more preferably 0.20 g / m ² or more and 1.5 g / m ² or less. By controlling the amount of the functional porous layer supported on the substrate in the range of 3.00 g / m ² , the transparency of the multilayer separator is suppressed while suppressing the deterioration of the cycle characteristics due to the blockage of the pores of the substrate in the obtained separator. It is preferable because it is easy to control the temper to 500 seconds / 100 ml or less. When the amount of the functional porous layer supported on the substrate is 0.05 g / cm ² or more, it is preferable from the viewpoint of exhibiting the effect of further improving the adhesive force between the functional porous layer and the substrate.
The amount of the functional porous layer supported on the substrate can be adjusted, for example, by changing the content of the thermoplastic resin in the coating liquid, the coating amount of the thermoplastic resin solution, the draw ratio after coating, and the like. However, the method for adjusting the loading amount is not limited to the above.

In the present embodiment, the functional porous layer has a repeated peel strength retention rate of 60% or more. The functional porous layer may not exhibit sufficient functions when it is poorly bonded to the base material or in the functional layer. Conventionally, the binding property has been discussed from the viewpoint of the peel strength method, but even if the peel strength by one measurement is high, if the functional porous layer is peeled off or partially slipped off, the functional porous layer is formed. Function may not be fully expressed. Therefore, the conventional peel strength is not always an appropriate configuration in the subject of the present invention. As a result of diligent studies, the inventors have found that a separator adjusted to show a retention rate in a specific range in the repeated peel strength can sufficiently exert the function of the functional porous layer, and have reached the present invention. For example, when a multilayer separator is wound and stored for a long period of time, it is between overlapping separators (that is, between the functional porous layer of a certain separator and the functional porous layer of the separators above and below it, or the function of a certain separator. It has been found that even if the adhesive force is developed between the porous layer and the base materials of the separators above and below the porous layer, the function of the functional porous layer is fully exhibited after the separator is unwound from the core.

The repetitive peel strength maintenance rate of the functional porous layer is 60% or more, preferably 65% or more. When the repetitive peel strength maintenance rate is 60% or more, it is easy to be able to exhibit the conventional function even when the separator wound body is exposed to a high temperature.

The average thickness of the functional porous layer is not particularly limited, but is preferably less than 1.0 μm, more preferably less than 500 nm, still more preferably less than 400 nm. Further, it is preferably 10 nm or more, more preferably 20 nm, still more preferably 50 nm or more. It is preferable that the average thickness of the functional porous layer is less than 1.0 μm from the viewpoint of enhancing the binding property in the functional porous layer and suppressing the decrease in permeability. Further, it is preferable that the lower limit of the average thickness of the functional porous layer is 10 nm or more because the function can be sufficiently exhibited.

[Microporous Membrane (A)]
In the present embodiment, the microporous membrane (A) contains the thermoplastic resin (a) as a main component. The thermoplastic resin (a) is preferably a resin different from the thermoplastic resin (b). In the present embodiment, "main component" means that the target material is contained in an amount of 50 vol% or more of the constituent materials. In the present embodiment, the microporous membrane (A) has a porous layer containing an inorganic filler and a resin binder, which is different from the porous layer (B) described above, on at least one surface or inside of the microporous membrane. You may have.

The thermoplastic resin (a) is, for example, a polyolefin such as polyethylene or polypropylene, a fluororesin such as polyvinylidene fluoride or polytetrafluoroethylene, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer or 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, methacrylic acid ester-acrylic Acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, rubbers such as ethylene propylene rubber, polyvinyl alcohol, vinyl acetate, polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide , Polymerimide, polyamideimide, polyamide, polyester and the like, and examples thereof include resins having a melting point and / or a glass transition temperature of 180 ° C. or higher. These can be used alone or in combination of two or more. When two or more types are used in combination, the melting point of the thermoplastic resin having the highest ratio (weight ratio) is defined as the melting point of the thermoplastic resin (a). The melting point of the thermoplastic resin is measured using a microporous membrane, but if measurement is difficult, the melting point may be measured using the raw material thermoplastic resin (a).

When the multilayer separator of the present invention is used as a separator for a power storage device, the thermoplastic resin (a) is preferably a polyolefin resin. Polyolefin resins are suitable because they are excellent in electrochemical stability, stability to an electrolytic solution, mechanical strength, and the like.
The polyolefin resin is a polyolefin resin used for ordinary extrusion, injection, inflation, blow molding, etc., and is a polymer containing olefin hydrocarbon as a monomer component. For example, ethylene, propylene, 1-butene, 2 -Homopolymers such as butene, 4-methyl-1-pentene, 1-hexene, and 1-octene, copolymers, and multistage polymers. In addition, polyolefins selected from these homopolymers, copolymers, and multistage polymers can be used alone or in combination.

Specific examples of the polyolefin resin include low-density polyethylene (density 0.910 g / cm ³ or more and less than 0.930 g / cm ³ ) and linear low-density polyethylene (density 0.910 g / cm ³ or more and 0.940 g / cm ³ ). Medium density polyethylene (density 0.930 g / cm ³ or more and less than 0.942 g / cm ³ ), high density polyethylene (density 0.942 g / cm ³ or more), ultra high molecular weight polyethylene (density 0.910 g / cm ³ ) (More than 0.970 g / cm ³ or less), isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene, polybutene, polymethylpentene, and ethylene propylene rubber.
From the viewpoint of improving the heat resistance of the microporous polyolefin membrane, the polyolefin resin preferably contains polypropylene. The proportion of polypropylene in the polyolefin resin is preferably 1% by mass or more, more preferably 5% by mass or more, and further preferably 15% by mass or more. The proportion of polypropylene in the polyolefin resin is preferably 30% by mass or less, more preferably 20% by mass or less, still more preferably 10% by mass or less. It is preferable that the proportion of polypropylene is 1% by mass or more from the viewpoint of improving the heat resistance of the microporous polyolefin membrane. On the other hand, from the viewpoint of enhancing the uniformity of the microporous membrane, the proportion of polypropylene is preferably 30% by mass or less.

The viscosity average molecular weight (Mv) of the polyolefin resin is preferably 300,000 or more and 5 million or less, more preferably 400,000 or more and 4 million or less, and further preferably 500,000 or more and 3 million or less. The viscosity average molecular weight can be obtained by measuring the ultimate viscosity of the decalin solvent at 135 ° C. based on ASTM-D4020 and calculating from the calculation formula according to the polyolefin resin.
For polyethylene, Mv can be calculated by the following formula.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
For polypropylene, Mv can be calculated by the following equation.
[Η] = 1.10 × 10 ^-4 Mv ^0.80

Setting the viscosity average molecular weight of the polyolefin resin to 300,000 or more is from the viewpoint of maintaining high melt tension and ensuring good moldability when the polyolefin resin composition is melt-kneaded, and for the polyolefin resin molecule. It is preferable from the viewpoint of imparting sufficient entanglement and increasing the strength of the polyolefin microporous film. On the other hand, it is preferable that the viscosity average molecular weight is 5 million or less from the viewpoint of improving the stability of extrusion molding of the polyolefin resin.

The polyolefin resin used in the embodiment may contain a filler of an inorganic oxide such as alumina, silica, titania or zirconia, lithium sulfate, magnesium sulfate, barium sulfate, as necessary, as long as the advantages of the present invention are not impaired. Fillers of ionic compounds such as barium titanate, lithium phosphate, lithium carbonate, antioxidants such as phenol-based, phosphorus-based and sulfur-based, metal soaps such as calcium stearate and lithium stearate, ultraviolet absorbers, light Additives such as stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed and used.
The microporous polyolefin membrane has a large number of very small pores of 1 μm or less, and forms dense communication pores. Therefore, when impregnated with the electrolytic solution, it has high ionic conductivity and strength, and is often used as a separator for a power storage device.

[Multilayer separator]
The average thickness of the multilayer separator is preferably 0.1 μm or more and 100 μm or less, more preferably 1 μm or more and 50 μm or less, and further preferably 3 μm or more and 25 μm or less. From the viewpoint of mechanical strength, 0.1 μm or more is preferable, and from the viewpoint of increasing the capacity of the battery, 100 μm or less is preferable.

From the viewpoint of ion conductivity and withstand voltage, the average pore size of the multilayer separator is preferably 10 nm or more and 500 nm or less, more preferably 20 nm or more and 200 nm or less, and further preferably 30 nm or more and 100 nm or less. In the multilayer separator, it is preferable that at least one or more layers have a pore diameter in the above range.

The average porosity of the multilayer separator is preferably 25% or more and 95% or less, more preferably 30% or more and 85% or less, still more preferably 35% or more and 75% or less, and particularly preferably 40% or more and 65% or less. From the viewpoint of improving ionic conductivity, 25% or more is preferable, and from the viewpoint of withstand voltage resistance, 95% or less is preferable. The average porosity in the multilayer separator is an average porosity calculated from the thickness of each layer, the weight of each layer, and the density of the constituent materials of each layer.

The air permeability of the multilayer separator is preferably 10 seconds / 100 ml or more and 500 seconds / 100 ml or less, more preferably 20 seconds / 100 ml or more and 400 seconds / 100 ml or less, and further preferably 30 seconds / 100 ml or more and 300 seconds / 100 ml. It is less than or equal to, and particularly preferably 40 seconds / 100 ml or more and 200 seconds / 100 ml or less. When the air permeability is 10 seconds / 100 ml or more, self-discharge tends to decrease when used as a separator, and when it is 500 seconds / 100 ml or less, good charge / discharge characteristics tend to be obtained, which is preferable. ..

The puncture strength of the multilayer separator is preferably 100 gf or more, more preferably 200 gf or more, still more preferably 300 gf or more, preferably 600 gf or less, more preferably 500 gf or less, still more preferably 400 gf or less. It is preferable that the puncture strength is 100 gf or more from the viewpoint of suppressing the film rupture due to the active material or the like that has fallen off when the battery is wound. It is also preferable from the viewpoint of reducing the concern of short circuit due to expansion and contraction of the electrode due to charging and discharging. On the other hand, setting the puncture strength to 600 gf or less is preferable from the viewpoint of reducing shrinkage due to relaxation of orientation during heating.

The upper limit of the fuse temperature of the multilayer separator is preferably 145 ° C. or lower, more preferably 140 ° C. or lower, and further preferably 135 ° C. or lower. If the fuse temperature is 145 ° C. or lower, the thermal runaway of the battery tends to be reduced. The lower limit is preferably 90 ° C. or higher, more preferably 110 ° C. or higher. If the fuse temperature is 90 ° C. or higher, it tends to be difficult to affect the input / output characteristics at high temperatures. The fuse temperature can be adjusted by the melting point of the thermoplastic resin (a) or the thermoplastic resin (b), but in the multilayer separator, it is adjusted by the melting point or the glass transition temperature of the thermoplastic resin (b), the loss elastic modulus, and the amount of grain. can.
The fuse temperature can be measured according to the method described in Examples described later.

The multilayer separator is preferably a multilayer separator wound body in which a core (also referred to as a "winding core") having an outer diameter of 3 inches (7.62 cm) or more and 20 inches (50.8 cm) or less is wound by 1000 m or more. .. The term "rolled body" as used herein means that a multilayer separator having a uniform width is wound on a core by a predetermined length. The “core” refers to a core having a cylindrical outer shape used for winding a multilayer separator, and examples thereof include a cylindrical winding core made of a paper tube, ABS resin, phenol resin, or the like. One inch can be converted to 25.4 mm. When the outer diameter is 3 inches or more, the number of turns can be reduced, so that the internal pressure is low even when exposed to high temperatures and the like, and it is easy to maintain the performance of the functional layer, which is preferable. Further, it is preferable because the curl (warp) tends to be reduced after the multilayer separator is unwound from the core. It is preferable that the outer diameter is 20 inches or less from the viewpoint of handling. The winding length and width are not particularly limited, but the winding length is 1000 m or more and 10000 m or less, and the width is 5 mm or more and 2000 mm or less. When the winding length is 1000 m or more, the number of times of laminating is large and the effect of the present invention is likely to be exhibited.

[Manufacturing method of multilayer separator]
(Manufacturing method of microporous membrane (A))
The method for producing the microporous membrane (A) containing the thermoplastic resin (a) as a main component is not particularly limited, and a known production method can be adopted. for example,
(1) A method in which a polyolefin resin composition and a pore-forming material are melt-kneaded, formed into a sheet, stretched as necessary, and then made porous by extracting the pore-forming material.
(2) A method of melt-kneading a polyolefin resin composition, extruding it at a high draw ratio, and then exfoliating the polyolefin crystal interface by heat treatment and stretching to make it porous.
(3) A method in which the polyolefin resin composition and the inorganic filler are melt-kneaded and molded on a sheet, and then the interface between the polyolefin and the inorganic filler is peeled off by stretching to make the polyolefin porous, and (4) the polyolefin resin. A method of dissolving a composition and then immersing it in a poor solvent for polyolefin to solidify the polyolefin and at the same time removing the solvent to make it porous.
And so on.

Hereinafter, as an example of the method for producing the microporous film (A), a method of melt-kneading the polyolefin resin composition and the pore-forming material to form a sheet and then extracting the pore-forming material will be described.

First, the polyolefin resin composition and the pore-forming material are melt-kneaded. As a melt-kneading method, for example, a polyolefin resin and, if necessary, other additives are heated and melted by using a resin kneading device such as an extruder, a kneader, a lab plast mill, a kneading roll, or a Banbury mixer. Examples thereof include a method of introducing a pore-forming material at an arbitrary ratio and kneading.

Examples of the pore-forming material include a plasticizer, an inorganic material, or a combination thereof.

The plasticizer is not particularly limited, but it is preferable to use a non-volatile solvent capable of forming a uniform solution above the melting point of the polyolefin. Specific examples of such a non-volatile solvent 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. .. In particular, liquid paraffin is preferable from the viewpoint of compatibility with multiple components and uniformity during stretching.
As a method for mixing the plasticizer, preferably, the polyolefin resin, other additives, and the plasticizer are pre-kneaded at a predetermined ratio using a Henshell mixer or the like before being put into the resin kneading apparatus. be. More preferably, in the pre-kneading, only a part of the plasticizer is added, and the remaining plasticizer is side-fed to the resin kneading device while being appropriately heated for kneading. By using such a kneading method, the dispersibility of the plasticizer is enhanced, and when the sheet-shaped molded product of the resin composition and the melt-kneaded plasticizer is stretched in a later step, the film is not broken and the magnification is high. It will be possible to stretch with.

The ratio of the polyolefin resin composition and the plasticizer is not particularly limited as long as they can be uniformly melt-kneaded and molded into a sheet. For example, the mass fraction of the plasticizer in the composition composed of the polyolefin resin composition and the plasticizer is preferably 20 to 90% by mass, more preferably 30 to 80% by mass. When the mass fraction of the plasticizer is 90% by mass or less, the melt tension at the time of melt molding tends to be sufficient for improving the moldability. On the other hand, when the mass fraction of the plasticizer is 20% by mass or more, the polyolefin molecular chain is not cut even when the mixture of the polyolefin resin composition and the plasticizer is stretched at a high magnification, and a uniform and fine pore structure is formed. It is preferable because it is easy to form and the strength is easily increased.

The non-equipment is not particularly limited, and for example, oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, itria, zinc oxide, iron oxide; silicon nitride, titanium nitride, nitride. Nitride-based ceramics such as boron; silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite. , Asbestos, zeolite, calcium silicate, magnesium silicate, kaolinite, ceramics such as silica sand; glass fiber and the like. These may be used alone or in combination of two or more. Among these, silica, alumina, or titania is preferable from the viewpoint of electrochemical stability, and silica is particularly preferable from the viewpoint of easy extraction.

The ratio of the polyolefin resin composition to the inorganic material is preferably 5% by mass or more, more preferably 10% by mass or more, based on the total mass of these, from the viewpoint of obtaining good isolation. From the viewpoint of ensuring high strength, it is preferably 99% by mass or less, more preferably 95% by mass or less, based on the total mass of the polyolefin resin composition and the inorganic material. Further, when it is 10% by mass or more, it is preferable that the permeability does not easily decrease even if the heat is stretched above the melting point. It is presumed that the polyolefin and the inorganic material are highly entangled and the melt viscosity is increased.

Next, the melt-kneaded product is formed into a sheet. As a method for producing a sheet-shaped molded product, for example, the melt-kneaded product is extruded into a sheet shape via a T-die or the like, brought into contact with a heat conductor, and cooled to a temperature sufficiently lower than the crystallization temperature of the resin component. There is a method of solidifying. Examples of the heat conductor used for cooling solidification include metal, water, air, and a plasticizer. Among these, it is preferable to use a metal roll because of the high efficiency of heat conduction. Further, when the extruded kneaded product is brought into contact with the metal roll, sandwiching it between the rolls further enhances the efficiency of heat conduction, and the sheet is oriented to increase the film strength and the surface smoothness of the sheet. It is more preferable because it tends to be. The die lip interval when extruding the melt-kneaded product from the T-die into a sheet is preferably 200 μm or more and 3,000 μm or less, and 500 μm or more and 2,
It is more preferably 500 μm or less. When the die lip interval is 200 μm or more, the shavings and the like are reduced, the influence on the film quality such as streaks and defects is small, and the risk of film breakage and the like can be reduced in the subsequent stretching step. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is high, cooling unevenness can be prevented, and the thickness stability of the sheet can be maintained.

Further, the sheet-shaped molded product may be rolled. Rolling can be carried out by, for example, a pressing method using a double belt press machine or the like. By rolling, the orientation of the surface layer portion can be increased in particular. The rolled surface magnification is preferably more than 1 times and 3 times or less, and more preferably more than 1 times and 2 times or less. When the rolling ratio exceeds 1 times, the surface orientation tends to increase and the film strength of the finally obtained microporous film (A) tends to increase. On the other hand, when the rolling ratio is 3 times or less, the orientation difference between the surface layer portion and the inside of the center is small, and a uniform porous structure tends to be formed in the thickness direction of the film.

Next, the pore-forming material is removed from the sheet-shaped molded product to form a microporous film. Examples of the method for removing the pore-forming material include a method in which a sheet-shaped molded product is immersed in an extraction solvent to extract the pore-forming material and then sufficiently dried. The method for extracting the pore-forming material may be either a batch method or a continuous method. In order to suppress the shrinkage of the microporous membrane, it is preferable to restrain the end portion of the sheet-shaped molded product during a series of steps including dipping and drying. When removing the pore-forming material from the sheet-shaped molded product, it is preferable that the residual amount of the pore-forming material in the microporous membrane is less than 1% by mass with respect to the total mass of the microporous membrane.

As the extraction solvent used when extracting the pore-forming material, a solvent that is poor with respect to the pore-forming material, is a good solvent with respect to the pore-forming material, and has a boiling point lower than the melting point of the pore-forming material should be used. Is preferable. When a polyolefin resin is used as the pore-forming material as such an extraction solvent, for example, hydrocarbons such as n-hexane and cyclohexane; and halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane. Non-chlorine halogenated solvents such as hydrofluoroether and hydrofluorocarbon; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone can be used. When an inorganic material is used as the pore-forming material, for example, an aqueous solution of sodium hydroxide, potassium hydroxide or the like can be used as the extraction solvent, respectively.

It is preferable to stretch the sheet-shaped molded product or the microporous film. The stretching may be performed before extracting the pore-forming material from the sheet-shaped molded product, or may be performed on the microporous film from which the pore-forming material is extracted from the sheet-shaped molded product. Further, it may be performed before and after extracting the hole forming material from the sheet-shaped molded body.
As the stretching treatment, either uniaxial stretching or biaxial stretching can be preferably used. Biaxial stretching is preferable from the viewpoint of improving the strength of the obtained microporous membrane (A). In particular, when the sheet-shaped molded product is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, the finally obtained microporous membrane (A) is less likely to tear, and has high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Simultaneous biaxial stretching is preferable from the viewpoint of improving puncture strength, uniformity of stretching, and shutdown property. Further, from the viewpoint of ease of controlling the plane orientation, successive biaxial stretching is preferable.

The draw ratio is preferably in the range of 20 times or more and 100 times or less, and more preferably in the range of 25 times or more and 70 times or less as the surface magnification. The draw ratio in each axial direction is preferably in the range of 4 times or more and 10 times or less for MD and 4 times or more and 10 times or less for TD; 5 times or more and 8 times or less for MD and 5 times or more and 8 times or less for TD. It is more preferable that the range is. When the total area magnification is 20 times or more, sufficient strength tends to be imparted to the obtained microporous membrane (A), while when the total area magnification is 100 times or less, film breakage in the stretching step is prevented. , High productivity tends to be obtained.

The microporous membrane (A) is preferably heat-treated for the purpose of heat fixation from the viewpoint of suppressing shrinkage. Examples of the heat treatment method include one or more of a stretching operation for the purpose of adjusting physical properties and a relaxation operation for the purpose of reducing the stretching stress. The relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretching machine.

In the stretching operation, it is possible to stretch 1.1 times or more, more preferably 1.2 times or more in one or more directions of the MD and TD of the film, thereby further increasing the strength and porosity. It is preferable from the viewpoint that the porous membrane (A) can be obtained.
The relaxation rate in the relaxation operation (value obtained by dividing the size of the membrane after the relaxation operation by the size of the membrane before the relaxation operation) shall be 1.0 or less in the direction of one or more of the MD and TD of the film. Is more preferable, 0.97 or less is more preferable, and 0.95 or less is further preferable. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. The relaxation operation may be performed in both directions of MD and TD, or may be performed only for one of MD or TD.
The stretching and relaxation operations after the plasticizer extraction is preferably performed in TD. The temperature in the stretching and relaxation operations is preferably lower than the melting point of the polyolefin resin (hereinafter, also referred to as “Tm”), and more preferably in the range of 1 ° C. to 25 ° C. lower than Tm. When the temperature in the stretching and relaxation operations is in the above range, it is preferable from the viewpoint of the balance between the heat shrinkage rate reduction and the porosity.
As the microporous membrane (A) to which the slurry containing the thermoplastic resin (b) is applied, the microporous membrane that has not been subjected to the above-mentioned final stretching and heat relaxation may be used, and after stretching and heat relaxation, the microporous membrane may be used. A microporous membrane may be used.

(Method for forming the porous layer (B))
The multilayer separator of the present embodiment is not particularly limited, but can be manufactured by, for example, the following method. (1) A method in which a slurry in which a thermoplastic resin (b) is dissolved or dispersed in a solvent is applied to at least one surface of a microporous film (A) containing the thermoplastic resin (a) as a main component and stretched; (2). ) The thermoplastic resin (b) and, if necessary, a plasticizer or the like are heated and mixed with an extruder or the like, co-extruded with the constituent material of the microporous film (A) containing the thermoplastic resin (a) as a main component, and laminated. A method of forming a sheet and performing stretching, plastic extraction, drying and the like; (3) a method of separately producing a microporous film (A) and a porous layer (B) and then laminating them.

As an example, a method (1) for applying a slurry containing a thermoplastic resin (b) will be described.
The form of the thermoplastic resin (b) in the slurry may be an aqueous solution or a dispersion liquid dissolved or dispersed in water, or an organic medium-based solution or a dispersion liquid dissolved or dispersed in a general organic medium. You may. As the medium of the slurry, a medium capable of uniformly and stably dispersing or dissolving the thermoplastic resin (b) is preferable. Specific examples thereof include N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, water, ethanol, toluene, hot xylene, methylene chloride, hexane and the like. From the viewpoint of reducing the air permeability of the multilayer separator to 500 seconds / 100 ml or less, the slurry is preferably a dispersed liquid. Further, it is preferable that the thermoplastic resin (b) has a uniform particle size because it exhibits uniform functionality. For example, a latex-like dispersion having a sharp particle size distribution by using a known emulsion polymerization method or the like is used. Can be obtained.

A water-soluble polymer or the like may be added to the slurry in order to improve the binding property, and a dispersant such as a surfactant; a thickener; a wetting agent is used to stabilize the dispersion and improve the coatability. Defoaming agents; various additives such as pH regulators containing acids or alkalis may be added. Further, inorganic particles or the like may be added in order to exhibit other functions as long as the effects of the present invention are not impaired.

The method of applying the slurry to the microporous membrane (A) is not particularly limited as long as it can realize the required layer thickness and coating area. For example, gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast. Examples include a coater method, a die coater method, a screen printing method, and a spray coating method.

The amount of slurry applied can be adjusted by adjusting the solid content concentration of the slurry, the clearance between the die and the microporous membrane (A), the take-up speed, and the selection of the transfer roll. The coating texture after drying is preferably 0.05 g / m ² or more and 3.00 g / m ² or less in terms of solid content, more preferably 0.10 g / m ² or more and 2.00 g / m ² or less, and further preferably. Is 0.20 g / m ² or more and 1.5 g / m ² or less. Controlling the basis weight of the functional porous layer to the substrate in the range of 3.00 g / m ² or less suppresses the deterioration of the cycle characteristics due to the blockage of the pores of the substrate in the obtained separator, and the transparency of the multilayer separator. It is preferable because it is easy to control the temper to 500 seconds / 100 ml or less. When the coating basis weight is 0.05 g / cm ² or more, it is preferable from the viewpoint of exhibiting the effect as a functional layer that further improves the adhesive force between the functional porous layer and the base material.

Further, if the surface of the microporous membrane (A) is surface-treated prior to the application of the slurry, the slurry can be easily applied, and the surface of the porous layer (B) and the microporous membrane (A) after coating can be easily applied. It is preferable because the adhesiveness is improved. The surface treatment method is not particularly limited as long as it does not significantly impair the porous structure of the microporous membrane (A). For example, a corona discharge treatment method, a plasma discharge treatment method, a mechanical roughening method, a solvent treatment method, an acid treatment method, an ultraviolet oxidation method and the like can be mentioned.

The method for removing the medium from the coating film after coating is not particularly limited as long as it does not adversely affect the microporous film (A) and the porous layer (B). For example, a method of fixing the microporous membrane (A) and the porous layer (B) and drying at a temperature below the melting point, a method of drying under reduced pressure at a low temperature, extraction drying and the like can be mentioned.

After the coating step, the stretching process is performed. The stretching temperature is 5 ° C. lower than the melting point or glass transition temperature of the thermoplastic resin (b), preferably equal to or higher than the melting point or glass transition temperature of the thermoplastic resin (b), and the melting point or glass of the thermoplastic resin (b). It is more preferable that the temperature is 5 ° C. higher than the transition temperature. The stretching temperature is preferably not more than the melting point or the glass transition temperature of the thermoplastic resin (a), preferably 3 ° C. or less lower than the melting point or the glass transition temperature of the thermoplastic resin (a), and is preferably not more than the temperature of the thermoplastic resin (a). More preferably, the temperature is 5 ° C. or lower than the melting point or the glass transition temperature. When the thermoplastic resin (b) is stretched at a temperature 5 ° C. lower than the melting point or the glass transition temperature, a part of the thermoplastic resin (b) is melted to develop fluidity. Then, the melted resin (b) is continuously coated on a large number of fine fibrils of the microporous membrane (A) containing the thermoplastic resin (a) as a main component. Then, when the resin (a) and the resin (b) are fused in a state where the contact area is remarkably large when cooled, a very strong binding property is exhibited and the repeated peel strength maintenance rate of 60% or more is achieved. It is speculated that it will be possible.

The draw ratio is preferably 1.01 times or more, more preferably 1.5 times or more, still more preferably 1.8 times or more. When it is 1.01 times or more, it is preferable because the repeated peeling strength is improved by increasing the contact area and the permeability is excellent.

It is more preferable to carry out the relaxation treatment after the stretching. It is preferable to relax by 1% or more in the stretching direction, more preferably 2% or more, and further preferably 3% or more based on the dimensions after stretching. If it is relaxed by 1% or more, the peel strength is improved repeatedly and the heat shrinkage rate is improved, so that it is suitable as a separator for a power storage device.

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 as long as the gist is not exceeded. The physical properties in the examples or comparative examples were measured by the following methods, respectively. When the measurement atmosphere was not specified, the measurement was performed in the atmosphere at 23 ° C. ± 3 ° C. and relative humidity 40% ± 5%.

[Measurement and evaluation method]
<Average thickness (microporous membrane (A) and multilayer separator)>
The average thickness (μm) of the microporous membrane (A) and the average thickness (μm) of the multilayer separator were measured by cutting out a sample having an MD of 10 mm and a TD of 10 mm. Nine points (3 points x 3 points) are selected in a grid pattern on the sample surface, and the thickness of each point is adjusted to a room temperature of 23 ± 2 ° C. using a dial gauge (PEACOCK No. 25 (registered trademark) manufactured by Ozaki Seisakusho). It was measured. The average value of the measured values at each of the nine points was taken as the average thickness (μm) of the separator.

<Average thickness (porous layer (B))>
The average thickness of the porous layer (B) was measured by observing the cross section of the separator using a scanning electron microscope (SEM) "Model S-5500, manufactured by Hitachi, Ltd.". The sample was stained with ruthenium, embedded in acrylic resin, and ultrathin sections were prepared by the cryomicrotome method, and used as a sample for microscopy.
Observation was performed at an acceleration voltage of 30 kV, the thickness of the porous layer (B) in the visual field was measured at arbitrary 5 points, and the average value was calculated. For the average value, the value rounded off to the first digit in nm is used as the average thickness (nm) of the porous layer (B).

<Air permeability>
The air permeability of the microporous membrane (A) (seconds / 100 ml) and the air permeability of the multilayer separator (seconds / 100 ml) are determined by a Garley type air permeability meter (GB2 manufactured by Toyo Seiki Co., Ltd.) conforming to JIS P-8117. It refers to the air permeability resistance measured using (Trademark)).

<Puncture strength (gf)>
The puncture strength of the microporous membrane (A) and the multilayer separator was measured by the following method. A sample was fixed with a sample holder having an opening diameter of 11.3 mm using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech Co., Ltd. Next, a puncture test is performed on the central part of the fixed sample using a needle with a diameter of 1.0 mm and a radius of curvature of 0.5 mm at the tip under an environment of 23 ° C ± 3 ° C and a puncture speed of 2 mm / sec. Therefore, the piercing strength (gf) was measured as the maximum piercing load.

<Melting point>
The melting points of the thermoplastic resins (a) and (b) were measured using DSC-60 manufactured by Shimadzu Corporation. When the thermoplastic resin was a slurry (in a state of being dispersed or dissolved in a solution), an appropriate amount of the slurry was collected in an aluminum dish and dried in a hot air dryer at 130 ° C. for 30 minutes. About 4 mg of the dried film after drying was packed in an aluminum container for measurement and measured under a nitrogen atmosphere to obtain a DSC curve. If the thermoplastic resin is a solid such as powder, pellets, or film, measure it directly.
(1st stage temperature rise / fall program)
Start from 50 ° C or lower, raise the temperature at a rate of 10 ° C per minute, and maintain for 5 minutes after reaching 200 ° C.
(2nd stage temperature rise / fall program)
The temperature is lowered from 200 ° C to 10 ° C per minute, and maintained for 5 minutes after reaching 30 ° C.
(3rd stage temperature rise / fall program)
The temperature rises from 30 ° C to 200 ° C at a rate of 10 ° C per minute.
The DSC curve of the temperature raising / lowering program of the third stage was used, and the temperature at the top of the melting peak was taken as the melting point.

<Glass-transition temperature>
The glass transition temperature of the thermoplastic resin (b) was measured using DSC6220 manufactured by Shimadzu Corporation. When the thermoplastic resin was a slurry (in a state of being dispersed or dissolved in a solution), an appropriate amount of the slurry was collected in an aluminum dish and dried in a hot air dryer at 130 ° C. for 30 minutes. About 17 mg of the dried film after drying was packed in an aluminum container for measurement and measured under a nitrogen atmosphere to obtain a DSC curve. If the thermoplastic resin is a solid such as powder, pellets, or film, measure it directly.
(1st stage temperature rise / fall program)
Start at 70 ° C, raise the temperature at a rate of 15 ° C per minute, and maintain for 5 minutes after reaching 110 ° C.
(2nd stage temperature reduction program)
The temperature was lowered from 110 ° C to 40 ° C per minute, and maintained for 5 minutes after reaching -50 ° C.
(3rd stage temperature rise / fall program)
The temperature rises from -50 ° C to 130 ° C at a rate of 15 ° C per minute.
Using the DSC curve of the 3rd stage temperature rise / fall program, the baseline (the straight line that extends the baseline in the obtained DSC curve to the high temperature side) and the inflection point (the curve that is convex upward is the curve that is convex downward). The intersection with the tangent line at (the point where it changes to) was determined as the glass transition temperature (Tg).

<Viscosity average molecular weight of polyolefin>
The ultimate viscosity [η] (dl / g) at 135 ° C. in a decalin solvent was determined according to ASTM-D4020.
The Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
The Mv of polypropylene was calculated by the following formula.
[Η] = 1.10 × 10 ^-4 Mv ^0.80

<Repeat peel strength maintenance rate>
It was calculated by measuring the peel strength at 180 ° C.
(Preparation of measurement sample)
FIG. 1 is a schematic view showing a cross section in the length direction of the measurement sample (10) for explaining the method for measuring the repetitive peel strength retention rate in the present embodiment. Double-sided tape (Nichiban Nystack NWBB-15) (length 76 mm x width 15 mm) (2) was attached to the entire surface of one side of the slide glass (length 76 mm x width 26 mm) (1). At this time, attach the first double-sided tape from the widthwise end of the slide glass, attach the second double-sided tape next to it so that it does not overlap with the first double-sided tape, turn the slide glass over, and 2 The protruding part of the double-sided tape was cut off with a cutter. As a result, two double-sided tapes having a width of 15 mm and a width of 11 mm were attached to the entire surface of one side of the slide glass. On the double-sided tape, a multilayer separator piece (3) cut out in an MD direction of 76 mm and a TD direction of 26 mm was attached so that the porous layer (B) was on the upper surface. A peeling tape (3M mending tape MP-12) (length 180 mm × width 12 mm) (4) was attached onto the porous layer (B). At this time, the peeling tape was attached in the length direction from one end of the porous layer (B) of the multilayer separator piece, and the other end of the porous layer (B) was left for grasping by the chuck of the tensile elongation tester. (Hereinafter referred to as a "grip portion"; indicated by a alternate long and short dash line (5)). The remaining portion of the peeling tape was folded 180 ° in the length direction (hereinafter, the tip of the folding is referred to as a "free end" and is indicated by a broken line (6)). As described above, a measurement sample was prepared. It should be noted that FIG. 1 is a schematic diagram, and the scale and the like are different from the actual measurement sample. For example, in FIG. 1, for the sake of explanation, a gap is drawn between the porous layer (B) and the peeling tape, and between the folded peeling tapes, but they actually come into contact with each other.
(Measurement of peel strength)
It was measured using a tensile elongation tester (RTC-1210A manufactured by ORIENTEC). First, with the peeling tape folded 180 ° along the length direction of the slide glass, the free end of the folded peeling tape was grasped by the chuck of the testing machine. Further, the grip portion (including the slide glass, double-sided tape, and the multilayer separator piece) was grasped by the chuck of the testing machine. Subsequently, the grip portion gripped by the chuck was fixed, and the free end of the peeling tape was pulled at a speed of 100 mm / min in the direction indicated by the arrow (7). The peel strength (first time) was measured from the stress at that time. When the displacement of the peeling tape at the start of measurement was 0 mm, the average value of the stresses in which the displacement of the peeling tape was between 5 mm and 40 mm was defined as the peel strength [gf] (first time). After the measurement, the peel strength (second time) was measured again using the peeling tape. At this time, new slide glass, double-sided tape, and multilayer separator were used, and the peeling tape used in the first measurement was used to prepare a measurement sample. The measurement conditions are the same as the first measurement. After the measurement, the peel strength (third time) was measured again using the peeling tape. At this time, new slide glass, double-sided tape, and multilayer separator were used, and the peeling tape used in the second measurement was used to prepare a measurement sample. The measurement conditions are the same as the first measurement.
In the second and third sample preparations, if the peeling tape used repeatedly did not adhere to the porous layer (B), it was judged that measurement was not possible.
(Repeat peel strength maintenance rate)
The repeated peel strength maintenance rate was determined from the values of the first and third peel strengths.
Repeated peel strength maintenance rate (%) = 3rd peel strength / 1st peel strength x 100

<Electrode adhesive strength>
The electrode adhesive strength was calculated by 90 ° C. peeling measurement.
(Preparation of measurement sample)
Multilayer separator and positive electrode as an adherend (manufactured by enertech, positive electrode material: LiCoO ₂ , conductive aid: acetylene black, binder: PVDF, LiCoO ₂ / acetylene black / PVDF (weight ratio) = 95/2/3, L / W: 36 mg / cm ² on both sides, density: 3.9 g / ml, thickness of Al current collector: 15 μm, thickness of positive electrode after pressing: 107 μm) are rectangular with TD direction 15 mm and MD direction 60 mm, respectively. It was cut into a shape. The porous layer (B) of the separator and the positive electrode active material were superposed so as to face each other to obtain a laminated body. The obtained laminate was pressed under the following conditions.
Press pressure: 1MPa
Temperature: 100 ° C
Press time: 5 seconds (measurement of peel strength)
For the laminated body after pressing, the electrodes are fixed using the force gauges ZP5N and MX2-500N (product name) manufactured by Imada Co., Ltd., and the separator is gripped and pulled by a method, and the peeling speed is 90 mm / min. ° A peeling test was performed to check the peeling strength. At this time, the process was performed under the above conditions, and the average value of the peel strength in the peel test in which the displacement was 20 mm to 50 mm was taken as the electrode adhesive strength.
(evaluation)
The multi-layer separator has a peel strength (before storage) using the wound body of the multi-layer separator that has been stored and managed at 35 ° C or lower for 7 days or less after the wound body is prepared, and the wound body is stored in an oven at 50 ° C for 7 days. The peel strength (after storage) using the rolled body was measured and compared. The measurement was performed by sampling at 20 points from the wound body, and the average value and standard deviation of the 20-point measurement were obtained and compared. In addition, the rate of change of the average value before and after storage was compared and evaluated.
Adhesive strength maintenance rate (%) = Electrode adhesive strength after storage / Electrode adhesive strength before storage x 100

<Fuse temperature (° C)>
The fuse temperatures of the microporous membrane (A) and the multilayer separator were measured as follows. Two 10 μm thick nickel foils (A and B) are prepared, and one nickel foil A is masked and fixed on a slide glass with Teflon (registered trademark) tape, leaving a square portion of 10 mm in length and 10 mm in width. did.
Another nickel foil B is placed on a ceramic plate to which a thermocouple is connected, and a microporous film (A) or a multilayer separator of a measurement sample immersed in a specified electrolytic solution for 3 hours is placed on the nickel foil B, and nickel is placed on the microporous film (A) or a multilayer separator. A slide glass with a foil attached was placed, and then silicon rubber was placed.
After setting this on a hot plate, the temperature was raised at a rate of 15 ° C./min with a pressure of 1.5 MPa applied by a hydraulic press.
The impedance change at this time was measured under the condition of AC 1V and 1 kHz. In this measurement, the temperature at the time when the impedance reached 1000Ω was defined as the fuse temperature, and the temperature at the time when the impedance fell below 1000Ω again after reaching the hole closed state was defined as the short temperature.
The composition ratio of the specified electrolytic solution is as follows.
Solvent composition ratio (volume ratio): propylene carbonate / ethylene carbonate / γ-butyl lactone = 1/1/2
Composition ratio of electrolytic solution: LiBF ₄ was dissolved in the above solvent to a concentration of 1 mol / liter, and trioctylphosphate was added to a concentration of 0.5% by weight.

<Loss modulus (E'')>
The loss elastic modulus (E ″) of the thermoplastic resin (b) was calculated from the dynamic viscoelasticity (DMA) measurement.
-Preparation of sample pieces Weigh 7 g of thermoplastic resin (b), sandwich it between two SUS plates with a mirror-finished surface of 250 mm in length x 250 mm in width x 3 mm in thickness, and MINI TEST PRESS, a desktop lab press machine manufactured by Toyo Seiki Co., Ltd. A resin sheet was prepared by heating and pressurizing the entire SUS plate using -10. After preheating the thermoplastic resin (b) at the melting point (glass transition temperature in the case of an amorphous resin) + 30 ° C. and 0.5 MPa for 3 minutes, the thickness of the resin sheet is continuously adjusted to 0.1 to 1 mm. A resin sheet was obtained by pressurizing and allowing it to stand for 2 minutes, and then immediately cooling the SUS plate together with the SUS plate at 25 ° C. for 3 minutes using a separately prepared press machine of the same type. When it is difficult to obtain the desired sheet thickness, the thickness of the resin sheet may be adjusted by sandwiching a metal spacer having a desired thickness between the SUS plates, if necessary. The above temperature and pressure refer to the displayed values of the press machine.
From the obtained resin sheet, a homogeneous portion without any air bubbles mixed in was visually selected, and a sample piece was cut into a size of 30 mm in length × 5 mm in width and used as a sample piece.
-Dynamic Viscoelasticity (DMA) Measurement For the sample piece prepared above, use Q800 (trademark) manufactured by TA Instruments Japan, and perform dynamic viscoelasticity measurement in accordance with JIS K 7244: 1999. went. The sample pieces were set so that the distance between the chucks was 10 mm, and the test pieces protruding from the chucks were excised as necessary. The measurements were taken in tensile mode, frequency 1.0 Hz, strain 0.1%, preload load 0.01 N, heating rate 2 ° C./min. The temperature-dependent curve of the loss elastic modulus E'' of the thermoplastic resin (b) was obtained. From the obtained curve of E'', the loss elastic modulus of the thermoplastic resin (b) at the melting point of the thermoplastic resin (a) was determined.

[Manufacturing of microporous membrane (A)]
<Manufacturing Example A-1>
21.0 parts by mass of high-density polyethylene with a viscosity average molecular weight (Mv) of 270,000 and a melting point of 136 ° C, 14.0 parts by mass of ultra-high molecular weight polyethylene with an Mv of 700,000 and a melting point of 135 ° C. Methylene- (3', 5'-di-t-butyl-4'-hydroxyphenyl) propionate] 0.1 part by mass of methane was placed in a tumbler blender and dry blended. The obtained mixture was supplied to the feed port of a twin-screw co-directional screw extruder by a feeder and melt-kneaded. In addition, a twin-screw extruder of liquid paraffin "Smoyl P-350P" (manufactured by Matsumura Petroleum Research Institute Co., Ltd.) is used so that the ratio of the amount of liquid paraffin to the total mixture (100 parts by mass) extruded is 67 parts by mass. Side-fed to the cylinder. The set temperature was 160 ° C. for the kneaded portion and 200 ° C. for the T-die. Subsequently, the melt-kneaded product was extruded into a sheet from a T-die and cooled with a cooling roll controlled to a surface temperature of 70 ° C. to obtain a sheet-shaped polyolefin resin composition having a thickness of 2000 μm. Next, they were continuously guided to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the vertical direction and 6.4 times in the horizontal direction. The stretching set temperature at this time was 122 ° C. Next, it was guided to a methylene chloride tank and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. After that, methylene chloride was dried and the microporous polyolefin membrane was wound up.

<Manufacturing example A-2>
The microporous polyolefin membrane produced in Production Example A-1 is not wound after drying, is further guided to a lateral tenter, stretched 1.9 times in the lateral direction, and then the final outlet is 1.7 times. The relaxation rate was 10.5% (relaxation rate = (1.9-1.7) /1.9 × 100 = 10.5%), and the microporous polyolefin membrane was wound. The set temperature of the transversely stretched portion was 127 ° C., and the set temperature of the relaxation portion was 132 ° C.

<Manufacturing Example A-3>
High-density polyethylene with a viscosity average molecular weight (Mv) of 270,000 and a melting point of 136 ° C. is 13.4 parts by mass, and ultra-high molecular weight polyethylene having an Mv of 2 million and a melting point of 134 ° C. is 5.8 parts by mass, with an average primary particle size of 12 nm. A certain silica "RX200" (manufactured by Nippon Aerodil Co., Ltd.) is 12.8 parts by mass, and a liquid paraffin "Smoyl P-350P" (manufactured by Matsumura Petroleum Research Institute Co., Ltd.) is 15.4 parts by mass as a plasticizing agent. As an antioxidant, 0.1 part by mass of tetrakis- [methylene- (3', 5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane was premixed with a super mixer. The obtained mixture was supplied to the feed port of a twin-screw co-directional screw extruder by a feeder and melt-kneaded. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the ratio of the amount of liquid paraffin to the total mixture (100 parts by mass) extruded was 68 parts by mass. The set temperature was 160 ° C for the kneaded portion and 230 ° C for the T die. Subsequently, the melt-kneaded product was extruded into a sheet from a T-die and cooled with a cooling roll controlled to a surface temperature of 70 ° C. to obtain a sheet-shaped polyolefin resin composition having a thickness of 1800 μm. Next, they were continuously guided to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the vertical direction and 6.4 times in the horizontal direction. The stretching set temperature at this time was 122 ° C. Next, it was guided to a methylene chloride tank and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. After that, methylene chloride was dried. After being further guided to the lateral tenter and stretched 1.6 times in the lateral direction, the relaxation rate was set to 12.5% so that the final outlet was 1.4 times (relaxation rate = (1.6-1.4) / 1). .6 × 100 = 12.5%), the inorganic-containing polyolefin microporous film was wound. The set temperature of the transversely stretched portion was 135 ° C., and the set temperature of the relaxation portion was 140 ° C.
Table 1 shows the physical characteristics of the obtained microporous membranes (A-1) to (A-3). For the microporous membrane (A-2), the repeated peeling strength maintenance rate between the surface of the microporous membrane (A-2) and the peeling tape (3M mending tape MP-12) was determined according to the above measurement method. It was measured. This measurement result can be considered as a measure of the performance of the peeling tape used.

[Manufacturing of slurry of thermoplastic resin (b)]
Slurry b-1
PVDF-HFP copolymer suspension (melting point 125) having 98.5 mol% of repeating unit derived from vinylidene fluoride (VDF) and 1.5 mol of repeating unit derived from hexafluoropropylene (HFP) produced by the aqueous emulsion polymerization method. Aqueous slurry b-1 in which 0.5 part by mass of sodium carboxymethyl cellulose (CMC1220 manufactured by Daicel FineChem) was dissolved in 100 parts by mass (solid content) of the PVDF-HFP copolymer at ° C. and a solid content concentration of 28%). Made.

Slurry b-2
PVDF-HFP copolymer suspension (melting point 160) having 98.5 mol% of repeating unit derived from vinylidene fluoride (VDF) and 0.2 mol of repeating unit derived from hexafluoropropylene (HFP) produced by the aqueous emulsion polymerization method. An aqueous slurry b-2 in which 0.5 part by mass of sodium carboxymethyl cellulose (CMC1220 manufactured by Daicel FineChem) was dissolved in 100 parts by mass (solid content) of the PVDF-HFP copolymer at ℃, solid content concentration 28%). Made.

Slurry b-3
An aqueous polyethylene dispersion (manufactured by Mitsui Chemicals, Chemipearl W410) having a melting point of 110 ° C. and a solid content concentration of 40 wt% was used as the slurry b-3.

Slurry b-4
MMA-BA copolymer suspension (glass transition temperature) containing 83 mol% of repeating units derived from methyl methacrylate (MMA) and 17 mol% of repeating units derived from n-butyl acrylate (BA) produced by the aqueous emulsion polymerization method. Aqueous slurry b-3 in which 0.5 parts by mass of sodium carboxymethyl cellulose (CMC1220 manufactured by Daicel FineChem) is dissolved in 100 parts by mass (solid content) of the MMA-BA copolymer at 55 ° C. and a solid content concentration of 40 wt%). Was produced.

Slurry b-5
A PVDF-HFP copolymer (melting point 160 ° C.) having 98.5 mol% of a repeating unit derived from vinylidene fluoride (VDF) and 0.2 mol of a repeating unit derived from hexafluoropropylene (HFP) was dissolved in dimethylacetamide in an amount of 5 wt%. An organic slurry b-5 was prepared by adding 30% by mass of tripropylene glycol to 100% by mass of the solution.

[Example 1]
A multilayer separator wound body was prepared by using slurry b-1 on the microporous polyolefin membrane produced in Production Example A-1. The porous polyolefin resin film was unwound from a feeding machine, the surface was subjected to a corona discharge treatment, and then the slurry b-1 was applied using a gravure reverse coater. After drying at 60 ° C. to remove water, the mixture was guided to a transverse stretching machine, stretched twice in the transverse direction, and subsequently subjected to a 10% relaxation treatment in the transverse direction. The set temperature of the transverse stretching machine was 130 ° C. A multi-layer separator was prepared by winding with a winder of 2100 m or more. The obtained multilayer separator was slit to a width of 60.0 mm and wound 2000 m on an ABS core having an inner diameter of 3 inches and an outer diameter of 8 inches. Table 2 shows the physical characteristics and manufacturing conditions of the obtained wound body.

[Examples 2 to 7]
In Example 1, each wound body was prepared by changing the conditions shown in Table 1. The basis weight of the coating was adjusted to a predetermined value by appropriately changing the cell volume of the gravure roll. The physical characteristics of the obtained wound body are shown in Table 2.

[Comparative Examples 1 to 4]
In Example 1, each wound body was prepared by changing the conditions shown in Table 1. The basis weight of the coating was adjusted to a predetermined value by appropriately changing the cell volume of the gravure roll. The physical characteristics of the obtained wound body are shown in Table 2.

[Comparative Example 5]
A multilayer separator wound body was prepared by using slurry b-4 on the microporous polyolefin membrane produced in Production Example A-2. The porous polyolefin resin film was unwound from a feeding machine, the surface was subjected to a corona discharge treatment, and then the slurry b-4 was formed into a dot-like pattern using a direct gravure coater. A gravure roll engraved so that the dot diameter after drying was 500 um, the dot height was 1 um, and the coverage was 40% was used. After drying at 60 ° C. to remove water, the mixture was guided to a transverse stretching machine, stretched twice in the transverse direction, and subsequently subjected to a 10% relaxation treatment in the transverse direction. The set temperature of the transverse stretching machine was 130 ° C. A multi-layer separator was prepared by winding with a winder of 2100 m or more. The obtained multilayer separator was slit to a width of 60.0 mm and wound 2000 m on an ABS core having an inner diameter of 3 inches and an outer diameter of 8 inches. Table 2 shows the physical characteristics and manufacturing conditions of the obtained wound body.

[Comparative Example 6]
A multilayer separator wound body was prepared by using slurry b-5 on the microporous polyolefin membrane produced in Production Example A-2. The porous polyolefin resin film was unwound from a feeding machine, and slurry b-5 was applied using a gravure reverse coater. Subsequently, it was solidified by immersing it in a coagulation liquid consisting of water / dimethylacetamide = 70 wt% / 30 wt%. After washing with water and drying (60 ° C.) to remove water, the mixture was guided to a transverse stretching machine, fixed once in the transverse direction, and heat-treated. The set temperature of the transverse stretching machine was 80 ° C. A multi-layer separator was prepared by winding with a winder of 2100 m or more. The obtained multilayer separator was slit to a width of 60.0 mm and wound 2000 m on an ABS core having an inner diameter of 3 inches and an outer diameter of 8 inches. Table 2 shows the physical characteristics and manufacturing conditions of the obtained wound body.

From Examples and Comparative Examples, it can be seen that the wound body having a repetitive peel strength retention rate of 60% or more has a small change in adhesiveness to the electrode after high temperature storage and a small variation variation. Further, in Comparative Example 1 and Comparative Example 5, although the first peel strength (conventional peel strength method) is large, the adhesiveness varies greatly after high temperature storage.

1 Slide glass 2 Double-sided tape 3 Multi-layer separator piece 4 Peeling tape 5 Grip part of measurement sample 6 Free end of peeling tape 7 Tension direction 10 Measurement sample

Claims (11)

A multilayer separator comprising a microporous membrane (A) containing a thermoplastic resin (a) as a main component and a porous layer (B) containing a thermoplastic resin (b).
The porous layer (B) is formed so as to form a continuum on at least one surface of the microporous membrane (A).
The following formula of the porous layer (B):
Repeated peel strength maintenance rate (%) = 3rd peel strength / 1st peel strength x 100
The repetitive peel strength maintenance rate required by the above is 60% or more, and
A multilayer separator in which the loss elastic modulus of the thermoplastic resin (b) at the melting point temperature of the thermoplastic resin (a) is 0.01 MPa or more and 3.00 MPa or less . A multilayer separator comprising a microporous membrane (A) containing a thermoplastic resin (a) as a main component and a porous layer (B) containing a thermoplastic resin (b).
The porous layer (B) is formed so as to form a continuum on at least one surface of the microporous membrane (A).
The following formula of the porous layer (B):
Repeated peel strength maintenance rate (%) = 3rd peel strength / 1st peel strength x 100
The repetitive peel strength maintenance rate required by the above is 60% or more, and
A multilayer separator in which the thermoplastic resin (a) is polyethylene . A multilayer separator comprising a microporous membrane (A) containing a thermoplastic resin (a) as a main component and a porous layer (B) containing a thermoplastic resin (b).
The porous layer (B) is formed so as to form a continuum on at least one surface of the microporous membrane (A).
The following formula of the porous layer (B):
Repeated peel strength maintenance rate (%) = 3rd peel strength / 1st peel strength x 100
The repetitive peel strength maintenance rate required by the above is 60% or more, and
A multilayer separator having a fuse temperature of 90 ° C or higher and 140 ° C or lower . The multilayer separator according to claim 1 or 3, wherein the thermoplastic resin (a) is a polyolefin resin, and the amount of polypropylene in the polyolefin resin is 30% by mass or less based on the total mass of the polyolefin resin. The multilayer separator according to any one of claims 1 to 4, wherein the melting point of the thermoplastic resin (b) or the glass transition temperature is 5 ° C. higher than the melting point of the thermoplastic resin (a). The multilayer separator according to any one of claims 1 to 5 , wherein the air permeability is 500 seconds / 100 ml or less. The multilayer separator according to any one of claims 1 to 6 , wherein the porous layer (B) has an average thickness of 10 nm or more and less than 500 nm. A multilayer separator winding body in which the multilayer separator according to any one of claims 1 to 7 is wound 1000 m or more on a core having an outer diameter of 3 inches (7.62 cm) to 20 inches (50.8 cm). A method for producing a multilayer separator including a microporous membrane (A) containing a thermoplastic resin (a) as a main component and a porous layer (B) containing a thermoplastic resin (b).
A slurry in which the thermoplastic resin (b) is dissolved or dispersed in a solvent is applied to at least one surface of the microporous film (A) containing the thermoplastic resin (a) as a main component so that the slurry forms a continuum. Step (1) to obtain a coated body,
After the step (1), the step (2) of stretching the coated body 1.01 times or more at a temperature 5 ° C. lower than the melting point of the thermoplastic resin (b) or the glass transition temperature or higher.
Including
A method for producing a multilayer separator, wherein the thermoplastic resin (a) is polyethylene . The method for producing a multilayer separator according to claim 9 , wherein in the step (1), the coating weight of the slurry after drying is 0.05 g / m ² or more and 3.00 g / m ² or less in terms of solid content. The stretching temperature in the step (2) is equal to or higher than the melting point or the glass transition temperature of the thermoplastic resin (b), and the method is performed after the step (2) by 1% or more in the stretching direction with reference to the post-stretching. The method for producing a multilayer separator according to claim 9 or 10 , further comprising a step (3) of relaxation.

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