JP2024039479A - multilayer porous membrane - Google Patents

Abstract

To provide a multilayer porous membrane which can achieve heat resistance (in other words, thermal shrinkage suppression capability) and strength while suppressing increase in air permeability when a separator for a non-aqueous electrolyte battery is thinned, and can also improve energy density of a non-aqueous electrolyte battery, and a separator for a non-aqueous electrolyte battery and a non-aqueous electrolyte battery containing the same.SOLUTION: A multilayer porous membrane has a microporous membrane containing a polyolefin resin as a main component, and a porous layer which is laminated on at least one surface of the microporous membrane and contains inorganic particles and a binder polymer, wherein the inorganic particles are barium sulfate, the binder polymer contains poly(meth)acrylamide, an average particle diameter D50 of the inorganic particles is 0.01 μm or more and 0.60 μm or less, and a ratio D90/D50 of an average particle diameter D90 of the inorganic particles to an average particle diameter D50 of the inorganic particles is 1.6 or more and 2.5 or less.SELECTED DRAWING: None

Description

The present invention relates to a multilayer porous membrane, and more particularly to a multilayer porous membrane suitably used as a separator placed between a positive electrode and a negative electrode in a nonaqueous electrolyte battery.

Conventionally, in non-aqueous electrolyte batteries, an electrolytic solution is impregnated into a power generation element in which a separator is interposed between a positive electrode plate and a negative electrode plate. In general, separators are required to have ion permeability and safety such as a shutdown function, so separators that include a microporous membrane containing a polyolefin resin are used. Furthermore, from the viewpoint of electrical insulation, heat resistance, strength, safety and cycle characteristics of non-aqueous electrolyte batteries during thermal runaway, a polyolefin microporous membrane and a porous layer containing inorganic particles and a binder polymer are laminated. Multilayer porous membranes prepared by the above-mentioned methods have also been studied as separators (Patent Documents 1 to 5).

Patent Document 1 discloses a porous membrane composition comprising barium sulfate particles having a BET specific surface area of 3.0 m ² /g or more and 6.3 m ² /g or less, a water-soluble polymer such as poly(meth)acrylamide, and water. Studies have been conducted to reduce the amount of residual moisture in a non-aqueous secondary battery equipped with a separator, improve high-temperature cycle characteristics, and reduce wear on coating equipment by coating a separator base material with a substance. Since the particle size of the barium sulfate particles becomes relatively large from the upper limit of the BET specific surface area of Patent Document 1, it is difficult to make the coating layer thinner, and in order to maintain the strength of the coating layer, it is necessary to use a water-soluble polymer. Since the amount of is relatively large, it is thought that the increase in air permeability after coating on the base material is also large.

Patent Document 2 discloses that in a heat-resistant porous layer provided on one or both sides of a porous base material, the volume proportion of barium sulfate particles having an average primary particle size of 0.01 μm or more and less than 0.30 μm is 50% by volume to 90%. By adjusting the volume%, the electrolytic solution or electrolyte is difficult to decompose, reducing gas generation inside the non-aqueous secondary battery, and increasing the number of contact points between the barium sulfate particles and the binder resin such as polyacrylamide. Studies are underway to improve the heat resistance of separators for aqueous secondary batteries. In the separator for non-aqueous secondary batteries described in Patent Document 2, the binder resin is not suitable for barium sulfate particles having an average primary particle size of less than 0.30 μm, and in order to improve heat resistance, a relatively large amount of binder resin is required. Therefore, there remains room for improvement in achieving both improvement in heat resistance and suppression of increase in air permeability to the base material.

Patent Document 3 discloses a secondary battery separator including a heat-resistant porous layer containing barium sulfate particles and a synthetic resin binder provided on at least one side of a polyolefin porous membrane, the average particle diameter and volume ratio of barium sulfate particles, Barium sulfate particles can be Since it blocks X-rays in the same way as the metal foil of the electrode, it is possible to observe misalignment with the electrode during the X-ray inspection process in the manufacture of secondary batteries. Since it can be filled into a porous layer, it is being considered to reduce the water content while suppressing thermal shrinkage. In the secondary battery separator described in Patent Document 3, the synthetic resin binder is not suitable for barium sulfate particles having a small average particle size, and a relatively large amount of the synthetic resin binder is required to improve heat resistance. There remains room for improvement in achieving both improvement in heat resistance and suppression of increase in air permeability to the base material.

Patent Document 4 describes a battery separator having a heat-resistant porous layer on at least one side of a polyolefin microporous membrane, which has excellent heat shrinkage resistance even when the heat-resistant porous layer is thin, and which reduces shedding of the heat-resistant porous layer. From this point of view, a battery separator has been proposed which has a heat shrinkage rate of 5% or less at 150° C./1 hour and has a heat-resistant porous layer shedding amount of 0.6 mg or less. Patent Document 4 describes not only the polyacrylamide contained in the heat-resistant porous layer but also the BET specific surface area of the barium sulfate particles, but since the particle size distribution of the barium sulfate particles is unknown, it may be due to insufficient heat resistance or There is concern about deterioration of ion permeability.

Patent Document 5 proposes the median diameter (D50) of an ionic inorganic filler, the solid mass ratio of a binder polymer to an ionic inorganic filler, and the like in a porous layer disposed on at least one side of a polyolefin _microporous membrane, from the viewpoint of ensuring the ion permeability of the multilayer porous membrane while reducing the thermal shrinkage rate.

Japanese Patent Application Publication No. 2015-162313 International Publication No. 2019/146155 International Publication No. 2021/029397 JP2022-089292A JP2020-116925A

In recent years, in order to increase the energy density of non-aqueous electrolyte batteries, there has been a demand for thinner separators for non-aqueous electrolyte batteries. Furthermore, in order to ensure the safety of the non-aqueous electrolyte battery, an inorganic porous layer is provided on the surface of the base material with a layer containing inorganic particles or inorganic filler. From the viewpoint of improving the energy density of non-aqueous electrolyte batteries and making the separator thinner, it is also essential to make the inorganic porous layer thinner.

However, in recent years, the strength of separator base materials has increased, and such base materials are susceptible to heat shrinkage. Therefore, simply making the inorganic porous layer thinner will reduce the heat shrinkage resistance and strength of the inorganic porous layer, causing the entire separator to shrink. In other words, it becomes difficult to suppress thermal shrinkage of the product, that is, to ensure safety. Therefore, in order to reduce the thickness of the inorganic porous layer while suppressing thermal shrinkage, it is conceivable to increase the amount of binder resin in the inorganic porous layer, but this also increases the air permeability of the entire separator. As described above, comprehensive thinning of the separator by thinning the inorganic porous layer and the inorganic coating layer in accordance with the increase in the strength of the separator base material has not yet been achieved.

In view of the above circumstances, the present invention is capable of suppressing an increase in air permeability while improving heat resistance (i.e., thermal shrinkage suppressing ability) and strength in thinning a separator for non-aqueous electrolyte batteries, and It is an object of the present invention to provide a multilayer porous membrane that can also improve the energy density of a nonaqueous electrolyte battery, as well as a separator for a nonaqueous electrolyte battery and a nonaqueous electrolyte battery containing the same.

The present inventors have developed a multilayer porous film having a microporous polyolefin film and a porous layer containing inorganic particles and a binder polymer, which is laminated on at least one side of the microporous film, and the type and average particle size of the inorganic particles. The inventors have discovered that the above problems can be solved by specifying the relationship between the particle size distribution and the type of binder polymer, and have completed the present invention. Examples of embodiments of the present invention are listed below.
(1) A multilayer porous membrane comprising a microporous membrane containing a polyolefin resin as a main component, and a porous layer containing inorganic particles and a binder polymer laminated on at least one side of the microporous membrane,
the inorganic particles are barium sulfate,
the binder polymer includes poly(meth)acrylamide,
The average particle diameter D ₅₀ of the inorganic particles is 0.01 μm or more and 0.60 μm or less, and the ratio D ₉₀ /D ₅₀ of the average particle diameter D ₉₀ of the inorganic particles to the average particle diameter D ₅₀ of the inorganic particles is , 1.6 or more and 2.5 or less,
Multilayer porous membrane.
(2) The multilayer porous membrane according to item 1, wherein the ratio D _{10 /D 50 of the average particle diameter D 10 of the} inorganic particles to the average particle diameter D ₅₀ of the inorganic particles is 0.2 or more and 0.8 or less. .
(3) The ratio T/TB of the thickness TB of the microporous membrane and the thickness T of the porous layer is 0.05 or more and 0.25 or less, and the multilayer porous membrane is heated in an atmosphere at a temperature of 150° C. for 1 hour. The multilayer porous membrane according to item 1 or 2, wherein the average thermal shrinkage rate of the multilayer porous membrane in the MD direction and the TD direction when left is 0% or more and 10% or less.
(4) The multilayer porous membrane according to any one of items 1 to 3, wherein the inorganic particles have a BET specific surface area of 3.0 m ² /g or more and 50.0 m ² /g or less.
(5) The multilayer porous according to any one of items 1 to 4, wherein the proportion of the inorganic particles is greater than 90 volume % and 99 volume % or less, assuming the volume excluding voids in the porous layer as 100 volume %. film.
(6) Any one of items 1 to 5, wherein the ratio Wb/Wa of the volume proportion Wa of poly(meth)acrylamide in the porous layer to the volume proportion Wb of the water-insoluble binder in the porous layer is 1 or less. The multilayer porous membrane according to item 1.
(7) The multilayer porous membrane according to any one of items 1 to 6, wherein the porous layer has a thickness T of 2 μm or less.
(8) The multilayer porous membrane according to any one of items 1 to 7, wherein a thermoplastic polymer-containing layer containing a thermoplastic polymer is formed on at least one side of the multilayer porous membrane.
(9) The multilayer porous membrane according to item 8, wherein the thermoplastic polymer includes a particulate polymer, and the thermoplastic polymer-containing layer has a dot-like pattern.

According to the present invention, when making a separator for a non-aqueous electrolyte battery thinner, it is possible to suppress an increase in air permeability while improving heat resistance (i.e., heat shrinkage suppressing ability) and strength, and It is possible to provide a multilayer porous film that can also improve the energy density of , and using it, it is also possible to provide a separator for a non-aqueous electrolyte battery and a non-aqueous electrolyte battery.

EMBODIMENT OF THE INVENTION Hereinafter, the present invention will be described in detail for the purpose of illustrating the form (hereinafter abbreviated as "embodiment") for implementing the present invention, but the present invention is not limited to the following embodiment. In this specification, the upper and lower limits of each numerical range can be arbitrarily combined. Moreover, that a certain member contains a specific component as a main component means that the content of the specific component is 50% by mass or more based on the mass of the member. Unless otherwise specified, the physical properties or numerical values described in this specification are measured or calculated by the methods described in Examples.

<Multilayer porous membrane>
The multilayer porous membrane according to the present embodiment includes a porous membrane containing a polyolefin resin as a main component (PO microporous membrane), and a porous membrane containing inorganic particles and a binder polymer laminated on at least one side of the PO microporous membrane. It is a multilayer porous membrane comprising a layer.

The multilayer porous membrane according to the present embodiment can be used as a separator for non-aqueous electrolyte batteries (hereinafter sometimes abbreviated as separator), and in the porous layer, the type of inorganic particles and the average particle size as described below are By specifying the relationship between and particle size distribution and the type of binder polymer, it is possible to improve heat resistance (i.e. thermal shrinkage suppression ability) and strength when making the separator thinner while maintaining ion permeability. In addition, the energy density of the non-aqueous electrolyte battery can also be improved.

Regarding the structure of the multilayer porous membrane, the PO microporous membrane may have a porous layer on one or both sides, for example, a first porous layer containing inorganic particles and a binder polymer, and a polyolefin microporous membrane (PO microporous membrane). ), or a three-layer structure including, in this order, a first porous layer, a PO microporous membrane, and a second porous layer containing inorganic particles and a binder polymer.

The multilayer structure is not limited to the two-layer structure of the first porous layer-PO microporous membrane, and the three-layer structure of the first porous layer-PO microporous membrane-second porous layer, but if desired, for example, One or more further layers are formed between the first porous layer and the PO microporous membrane, between the second porous layer and the PO microporous membrane, or on at least one side or outside of the multilayer porous membrane. good. Further layers include, for example, an additional PO microporous membrane, an additional porous layer containing inorganic particles and a binder polymer, a resin layer containing 50% by mass or more of a resin other than polyolefin (PO), a thermoplastic polymer, etc. Examples include a thermoplastic polymer-containing layer containing a binder component having an adhesive function.

The constituent elements of the multilayer porous membrane according to this embodiment will be described below.

<Porous layer>
The porous layer is a layer laminated on at least one side of a microporous membrane containing polyolefin resin as a main component, and containing inorganic particles and a binder polymer.

(Interaction of constituent components of porous layer and average particle size and particle size distribution of inorganic particles)
In the porous layer according to this embodiment, the inorganic particles are barium sulfate, the binder polymer contains poly(meth)acrylamide, and the average particle diameter _D50 of the inorganic particles is 0.01 μm or more and 0.60 μm or less. , and the ratio D ₉₀ /D ₅₀ of the average particle diameter D ₉₀ of the inorganic particles to the average particle diameter D ₅₀ of the inorganic particles is 1.6 or more and 2.5 or less.

In this embodiment, the binder polymer may be either a homopolymer or a copolymer, so the fact that the binder polymer contains poly(meth)acrylamide means whether the binder polymer consists only of poly(meth)acrylamide or not. The binder polymer may be a mixture of poly(meth)acrylamide and other polymers, or may be a copolymer of (meth)acrylamide and other monomers.

In this embodiment, by combining barium sulfate and polyacrylamide in the porous layer, a strong porous layer is formed even if the binder polymer is small, and 0.01 μm≦average particle size D ₅₀ ≦0. By including barium sulfate as an inorganic particle that satisfies the ratio D ₉₀ /D ₅₀ ≦2.5 of .60 μm and 1.6≦average particle size, the number of bonding points between the barium sulfate particles and the binder polymer increases, and the resulting porous It is possible to further strengthen the layer and increase its heat resistance (ie, ability to suppress thermal shrinkage).

Although not wishing to be bound by theory, in conventional multilayer porous membranes, there is a limit to reducing the particle size of the granular binder polymer when the inorganic particles are made smaller. On the other hand, according to the present invention, the inorganic particles are small in size and sharp, satisfying the relationships of 0.01 μm≦D ₅₀ ≦0.60 μm and 1.6≦D ₉₀ /D ₅₀ ≦2.5. Barium sulfate particles with a specific particle size distribution contain poly(meth)acrylamide as a binder polymer, and since poly(meth)acrylamide is generally a non-particulate water-soluble polymer, barium sulfate particles with a specific particle size are By appropriately covering the granular poly(meth)acrylamide, it not only improves the thermal shrinkage suppression ability and strength when making the separator thinner, but also suppresses the increase in air permeability of the microporous membrane due to the arrangement of the porous layer. It has also been found that the energy density of non-aqueous electrolyte batteries can be improved. Furthermore, it has been found that remarkable effects can be obtained by using poly(meth)acrylamide among water-soluble polymers. This is presumed to be due to strong electrostatic interaction between barium sulfate, which is an ionic crystal, and poly(meth)acrylamide, which is highly polar. Such an interaction between barium sulfate and poly(meth)acrylamide occurs when the thickness of the porous layer (T) is 2 μm or less, when the binder polymer in the porous layer is significantly small, and when the thickness of the microporous membrane (TB ) and the thickness T of the porous layer (T/TB) is 0.05 or more and 0.25 or less.

The _D50 of barium sulfate is 0.05 μm from the viewpoint of increasing the number of bonding points between barium sulfate and poly(meth)acrylamide to further improve the thermal shrinkage suppressing ability and strength of the multilayer porous membrane and the energy density of the nonaqueous electrolyte battery. It is preferably in the range of 0.5 μm or more, and more preferably in the range of 0.1 μm or more and 0.4 μm or less.

From the same viewpoint as above, the D ₉₀ /D ₅₀ of barium sulfate is preferably in the range of 1.7 or more and 2.3 or less, and more preferably in the range of 1.8 or more and 2.1 or less.

The ratio D ₁₀ /D ₅₀ of the average particle diameter D ₁₀ of the inorganic particles to the average particle diameter D ₅₀ of barium sulfate as inorganic particles is such that the number of small-sized particles in the porous layer is relatively reduced, and the micropores created by the porous layer are increased. From the viewpoint of suppressing an increase in the air permeability of the membrane, it is preferably 0.2 or more and 0.8 or less, more preferably 0.3 or more and 0.7 or less, and 0.4 or more and 0.6 or less. It is more preferable that

The average particle diameter D ₉₀ of barium sulfate as an inorganic particle is preferably 0.016 μm to 1.0 μm from the viewpoint of improving the heat shrinkage suppressing ability and strength in thinning the separator and improving the energy density of the non-aqueous electrolyte battery. It is in the range of 5 μm, more preferably 0.085 μm to 1.15 μm, even more preferably 0.18 μm to 0.84 μm.

The average particle diameter _D10 of barium sulfate as an inorganic particle is determined from the viewpoints of thermal shrinkage suppressing ability in thinning the separator, suppressing the air permeability of the upper layer, improving strength, and improving the energy density of the non-aqueous electrolyte battery. It is preferably in the range of 0.01 μm to 0.48 μm, more preferably 0.015 μm to 0.35 μm, even more preferably 0.04 μm to 0.24 μm.

The interaction of barium sulfate and poly(meth)acrylamide in the porous layer described above as well as the particle size and particle size distribution of barium sulfate may be affected, for example, in the process of placing the porous layer on a microporous membrane or substrate or in the manufacture of the separator. In the process, the selection of raw materials can be used to combine barium sulfate with a water-soluble polymer such as poly(meth)acrylamide, or to significantly increase the ratio of barium sulfate to binder polymer (i.e., to form a porous layer). In the case of multiple binder polymers, use a larger amount of poly(meth)acrylamide than a general particulate binder; This can be achieved by dispersing, stirring, controlling particle size, etc. of inorganic particles.

Methods for adjusting the particle size distribution of barium sulfate as inorganic particles as described above include, for example, pulverizing barium sulfate using a ball mill, bead mill, jet mill, etc. to obtain a desired particle size distribution; Examples include a method in which barium sulfate having a particle size distribution is prepared and then blended.

The shape of the barium sulfate according to the present embodiment may be generally granular, for example, and more specifically, plate-like, scale-like, polyhedral, needle-like, columnar, granular, spherical, spindle-like, block-like, etc. A combination of two or more barium sulfates having the above shapes may be used. In addition, there are generally two types of barium sulfate: elutriated barium sulfate, which is a crushed product of natural minerals, and precipitated barium sulfate, which is produced by a chemical reaction. However, it is preferable to use precipitated barium sulfate, whose particle size distribution can be easily controlled. is preferred.

The BET specific surface area of barium sulfate as an inorganic particle is preferably 3.0 m ² /g or more and 50.0 m ² /g or less, and preferably 5.0 m ² /g or more and 40.0 m ² /g or less. More preferably, it is 6.4 m ² /g or more and 30.0 m ² /g or less. When the BET specific surface area of barium sulfate is within the above range, it is easy to achieve a particle size and particle size distribution suitable for this embodiment, which in turn improves the thermal shrinkage suppressing ability and strength in thinning the separator and the strength of the non-aqueous electrolyte battery. Energy density can be improved.

The barium sulfate according to the present embodiment may have a specific gravity of about 4.50, for example, although it is not limited, and may or may not be surface-treated.

The proportion of inorganic particles is preferably greater than 90 volume% and 99 volume% or less, more preferably 91 volume% or more and 99 volume% or less, assuming that the volume excluding voids in the porous layer is 100 volume%. , more preferably 93% by volume or more and 98% by volume or less. When the volume ratio of inorganic particles in the porous layer is within the above range, the ratio of barium sulfate to the binder polymer increases, suppressing the increase in air permeability of the microporous membrane due to the porous layer, and improving the multilayer porous membrane. The electrical resistance of can be reduced.

Further, from the same viewpoint as the volume ratio described above, the mass proportion of the inorganic particles in the porous layer is preferably 97 mass% or more and 100 mass% or less, with the mass of the porous layer being 100 mass%.

The binder polymer is a material that binds a plurality of inorganic particles together in a porous layer or binds a porous layer and a microporous membrane.

In the porous layer according to this embodiment, the binder polymer contains poly(meth)acrylamide. It is generally known that poly(meth)acrylamide is classified as a water-soluble polymer. In this technical field, even if the particle size of inorganic particles is reduced, there is a limit to reducing the particle size of conventionally used latex containing water-insoluble binder polymers. By incorporating a water-soluble polymer such as poly(meth)acrylamide into the porous layer, it is compatible with barium sulfate having the particle size distribution described above.

The ratio Wb/Wa of the volume proportion Wa of poly(meth)acrylamide contained in the porous layer to the volume proportion Wb of the water-insoluble binder is preferably 1 or less, more preferably smaller than 1, and 0.6. It is more preferably smaller than 0.4, particularly preferably smaller than 0.4, and most preferably around 0 or 0. When the porous layer contains a water-soluble polymer and the volume ratio of poly(meth)acrylamide is larger than the volume ratio of the water-insoluble binder, the binder polymer covers the barium sulfate as inorganic particles in a film, so that both The number of contact points of the multilayer porous membrane or the separator can be further improved by increasing the number of contact points.

The binder polymer according to this embodiment may also include another binder polymer as long as it includes poly(meth)acrylamide. Examples of binder polymers other than poly(meth)acrylamide include water-insoluble binder polymers and those that are insoluble in electrolyte and electrochemically stable within the range of use of non-aqueous electrolyte batteries.

Specific examples of binder polymers other than poly(meth)acrylamide include the following 1) to 7).
1) Polyolefin: For example, polyethylene, polypropylene, ethylene propylene rubber, and modified products thereof;
2) Conjugated diene polymer: For example, styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride;
3) Acrylic polymers (excluding poly(meth)acrylamide): For example, poly(meth)acrylic acid and its salts, poly(meth)acrylic esters, methacrylic ester-acrylic ester copolymers, styrene - Acrylic ester copolymer, acrylonitrile-acrylic ester copolymer;
4) Polyvinyl alcohol resin: For example, polyvinyl alcohol, polyvinyl acetate;
5) Fluorine-containing resin: For example, polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer;
6) Cellulose derivatives: for example, ethylcellulose, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose;
7) Resins with a melting point and/or glass transition temperature of 180°C or higher or polymers without a melting point but with a decomposition temperature of 200°C or higher (excluding poly(meth)acrylamide): For example, polyphenylene ether, polysulfone, polyether Sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, polyester.

The binder polymers 1) to 7) above can be used in combination with poly(meth)acrylamide. Furthermore, if desired, the monomer units constituting 1) to 7) above can be copolymerized with (meth)acrylamide.

From the viewpoint of safety in the event of a short circuit of the nonaqueous electrolyte battery, 3) an acrylic polymer, 5) a fluororesin, and 7) a polyamide as a polymer are preferable. From the viewpoint of durability, the polyamide is preferably a wholly aromatic polyamide, especially polymetaphenylene isophthalamide.

From the viewpoint of compatibility between the binder polymer and the electrode, the above 2) conjugated diene polymer is preferable, and from the viewpoint of voltage resistance, the above 3) acrylic polymer and 5) fluororesin are preferable.

The above 2) conjugated diene 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-chloro-1,3-butadiene, substituted linear Examples include conjugated pentadienes, substituted and side-chain conjugated hexadienes, and these may be used alone or in combination of two or more. Among these, 1,3-butadiene is particularly preferred.

The above 3) acrylic polymer is a polymer containing a (meth)acrylic compound as a monomer unit. The above (meth)acrylic compound refers to at least one selected from the group consisting of (meth)acrylic acid and (meth)acrylic acid ester.

Examples of the (meth)acrylic acid used in the above 3) acrylic polymer include acrylic acid and methacrylic acid.

Examples of the (meth)acrylic acid ester used in the above 3) acrylic polymer include (meth)acrylic acid alkyl esters, such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, -Ethylhexyl acrylate, 2-ethylhexyl amethacrylate; epoxy group-containing (meth)acrylic esters, such as glycidyl acrylate, glycidyl methacrylate; these may be used alone or in combination of two or more. Good too. Among the above, 2-ethylhexyl acrylate (EHA) and butyl acrylate (BA) are particularly preferred.

The acrylic polymer preferably contains EHA or BA as a main structural unit from the viewpoint of safety of the separator or the non-aqueous electrolyte battery. The main structural unit refers to a polymer portion corresponding to a monomer that accounts for 40 mol% or more of the total raw materials for forming the polymer.

The above 2) conjugated diene polymer and 3) acrylic polymer may be obtained by also copolymerizing other monomers copolymerizable with them. Other copolymerizable monomers that can be used include, for example, unsaturated carboxylic acid alkyl esters, aromatic vinyl monomers, vinyl cyanide monomers, and unsaturated monomers containing hydroxyalkyl groups. , unsaturated carboxylic acid amide monomer, crotonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, etc., and these may be used alone or in combination of two or more. good. Among the above, unsaturated carboxylic acid alkyl ester monomers are particularly preferred. Examples of the unsaturated carboxylic acid alkyl ester monomer include dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl itaconate, monomethyl fumarate, and monoethyl fumarate. They may be used alone or in combination of two or more.

Note that the above 2) conjugated diene polymer may be obtained by copolymerizing the above (meth)acrylic compound as another monomer.

As a binder polymer other than poly(meth)acrylamide, it is preferably in the form of latex from the viewpoint of strong binding force between multiple inorganic particles even at high temperatures exceeding room temperature and suppressing thermal shrinkage. , it is more preferable to include latex of an acrylic polymer.

The proportion of the binder polymer is preferably 1 volume% or more and less than 10 volume%, more preferably 1 volume% or more and 9 volume% or less, assuming that the volume excluding voids in the porous layer is 100 volume%. , more preferably 2% by volume or more and 7% by volume or less. When the volume ratio of the binder polymer in the porous layer is within the above range, the ratio of barium sulfate to the binder polymer increases, suppressing the increase in air permeability of the microporous membrane, and increasing the electrical conductivity of the multilayer porous membrane. Resistance can be reduced.

In addition, from the same viewpoint as the above-mentioned volume ratio, the mass proportion that the binder polymer occupies in the porous layer is 0.1 mass%, assuming that the total mass of the porous layer of the inorganic particles and the binder polymer is 100 mass%. It is preferably large and smaller than 3% by mass.

The thickness T of the porous layer is preferably 2 μm or less, more preferably 1.5 μm or less, per at least one side of the microporous membrane containing polyolefin resin as a main component. When the thickness T of the porous layer containing barium sulfate having the particle size and particle size distribution explained above is 2 μm or less, the porous layer formed by coating etc. is strong, so the resulting multilayer porous film or Even if the separator is thin, it has high heat resistance, that is, heat shrinkage is suppressed to a low level. The lower limit of the thickness T is not limited, but may be, for example, more than 0 μm or 0.5 μm or more per at least one side of the microporous membrane containing polyolefin resin as a main component.

From the viewpoint of the mechanism of action of the present invention, the basis weight of the porous layer is preferably within the range of 1.0 g/m ² to 5.0 g/m ² , and 1.5 g/m ² to 4.0 g/m 2 It is more preferable to fall within the range of ² .

<Polyolefin microporous membrane>
A porous membrane containing polyolefin as a main component (PO microporous membrane) contains polyolefin, and is preferably composed of polyolefin. The form of the polyolefin may be a microporous body of polyolefin, such as a polyolefin membrane, a woven fabric (woven fabric) of polyolefin fibers, or a nonwoven fabric of polyolefin fibers. Examples of polyolefins include homopolymers, copolymers, and multistage polymers obtained using ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene as monomers. These polymers may be used alone or in combination of two or more. The polyolefin is preferably at least one selected from the group consisting of polyethylene, polypropylene, and copolymers thereof, from the viewpoint of melt viscosity, shutdown and meltdown characteristics of a PO microporous membrane that can be used as a separator. , polypropylene, and even more preferably an ethylene-propylene copolymer or a mixture of polyethylene and polypropylene.

Specific examples of polyethylene include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), high molecular weight polyethylene (HMWPE), and ultra high molecular weight polyethylene ( UHMWPE), etc.

In the present specification, high molecular weight polyethylene (HMWPE) means polyethylene having a viscosity average molecular weight (Mv) of 100,000 or more. Generally, Mv of ultra-high molecular weight polyethylene (UHMWPE) is 1 million or more, and therefore, high molecular weight polyethylene (HMWPE) in the present specification includes UHMWPE by definition.

As used herein, high-density polyethylene refers to polyethylene with a density of 0.942 to 0.970 g/cm ³ . In the present invention, the density of polyethylene refers to a value measured according to D) density gradient tube method described in JIS K7112 (1999).

Specific examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene.

Specific examples of copolymers of ethylene and propylene include ethylene-propylene random copolymers, ethylene-propylene rubber, and the like.

When the polyolefin (PO) contained in the PO microporous membrane contains polyethylene (PE), the PE content is 50% by mass or more and 100% by mass or less, based on the total mass of the resin components constituting the PO microporous membrane. From the viewpoint of fuse characteristics or meltdown characteristics, the content is preferably 85% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 95% by mass or less.

When the PO contained in the PO microporous membrane contains polypropylene (PP), the PP content is 0% by mass or more and less than 50% by mass, based on the total mass of the resin components constituting the PO microporous membrane, and the PP content is 0% by mass or more and less than 50% by mass, and From the viewpoint of viscosity and fuse properties, the content is preferably 0% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 10% by mass or less.

In addition to the polyolefins listed above, PO microporous membranes also contain resins such as polyethylene terephthalate, polycycloolefin, polyether sulfone, polyamide, polyimide, polyimide amide, polyaramid, polyvinylidene fluoride, nylon, and polytetrafluoroethylene. It may further contain.

The melt index (MI) of the PO microporous membrane at 190°C is 0.02 g/10 min to 0.5 g from the viewpoint of suppressing the high viscosity of the PO resin composition during film formation and suppressing the occurrence of defective products. /10min is preferable, and 0.05g/10min to 0.3g/10min is more preferable.

The puncture strength when converted to the basis weight (g/m ² ) of the PO microporous membrane (hereinafter referred to as the basis weight-converted puncture strength) is 50 gf/(g/m ² ) or more, or 60 gf/(g/m ^{2 )} . ) or more is preferable. PO microporous membranes having a puncture strength converted to basis weight of 50 gf/(g/m ² ) or more or 60 gf/(g/m ² ) or more tend to be difficult to break. From the viewpoint of improving the safety of non-aqueous electrolyte batteries while maintaining the strength of the PO microporous membrane, the puncture strength in terms of basis weight is more preferably 70 gf/(g/m ² ) or more, and even more preferably 80 gf/( g/m ² ) or more. The puncture strength in terms of area weight is not limited, but may be, for example, 200 gf/(g/m ² ) or less, 150 gf/(g/m ² ) or less, or 140 gf/(g/m ² ) or less. can.

The lower limit of the puncture strength that is not converted into the basis weight of the PO microporous membrane (hereinafter simply referred to as puncture strength) is preferably 100 gf or more, more preferably 200 gf or more, and even more preferably 300 or more. A puncture strength of 100 gf or more is preferable from the viewpoint of suppressing breakage of the PO microporous membrane in an impact test. Further, the upper limit of the puncture strength of the PO microporous membrane is preferably 1000 gf or less, more preferably 800 gf or less, and even more preferably 700 gf or less, from the viewpoint of stability during film formation. Any lower limit value can be used as long as it allows stable production in film formation and battery production. The upper limit value is set in balance with other characteristics. The puncture strength can be increased by increasing the shear force applied to the molded product during extrusion or by increasing the orientation of molecular chains by stretching, but as the strength increases, thermal stability deteriorates due to the increase in residual stress. controlled.

The thickness TB of the PO microporous membrane is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 5 μm or more from the viewpoint of ensuring voltage resistance, and also ensuring the capacity of the non-aqueous electrolyte battery. From this point of view, the thickness is preferably 25 μm or less, more preferably 20 μm or less, even more preferably 14 μm or less, particularly preferably 10 μm or less. The thickness TB of the PO microporous membrane can be adjusted, for example, by controlling the die lip interval, the stretching ratio in the stretching process, and the like.

The porosity of the PO microporous membrane is preferably 20% or more, more preferably 30% or more, even more preferably 35% or more from the viewpoint of permeability, and preferably 70% or less from the viewpoint of membrane strength. It is more preferably 60% or less, and even more preferably 50% or less. The porosity of the PO microporous membrane is controlled, for example, by controlling the mixing ratio of the polyolefin resin composition and plasticizer, stretching temperature, stretching ratio, heat setting temperature, stretching ratio during heat setting, relaxation rate during heat setting, etc. It can be adjusted by combining these as well as by combining these.

The air permeability of the PO microporous membrane is preferably 10 sec/100 cm ³ or more, more preferably 50 sec/100 cm ³ from the viewpoint of preventing excessive current from flowing between the plurality of electrodes through the PO microporous membrane. Above, more preferably 80 sec/100 cm ³ or more, and from the viewpoint of transparency, preferably 1000 sec/100 cm ³ or less, more preferably 300 sec/100 cm ³ or less, still more preferably 200 sec/100 cm ^{3 or} less, particularly preferably 160 sec / ^100cm3 .

The viscosity average molecular weight (Mv) of the PO microporous membrane is preferably 400,000 or more and 1,300,000 or less, more preferably 450,000 or more and 1,200,000 or less, and even more preferably 500,000 or more and 1,150 or less. ,000 or less. When the Mv of the PO microporous membrane is 400,000 or more, the melt tension during melt molding becomes large, the moldability becomes good, and high film strength tends to be obtained due to the entanglement of the polymers. When Mv is 1,300,000 or less, it becomes easy to uniformly melt and knead the raw materials, and not only does it tend to have excellent sheet formability, especially thickness stability, but also when used as a separator for non-aqueous electrolyte batteries. In addition, the holes tend to become clogged when the temperature rises, and a good fuse function tends to be obtained.

The average pore diameter of the PO microporous membrane is preferably 0.03 μm or more and 0.70 μm or less, more preferably 0.04 μm or more and 0.20 μm or less, even more preferably 0.05 μm or more and 0.10 μm or less, and even more preferably 0.05 μm or more and 0.10 μm or less. 0.055 μm or more and 0.09 μm or less. The average pore diameter of the PO microporous membrane is preferably 0.03 μm or more and 0.70 μm or less from the viewpoints of ionic conductivity and voltage resistance. The average pore diameter depends on, for example, the composition ratio of the polyolefin, the type of polyolefin or plasticizer, the cooling rate of the extruded sheet, the stretching temperature, the stretching ratio, the heat-setting temperature, the stretching ratio during heat-setting, the relaxation rate during heat-setting, etc. Adjustments can be made by controlling and by combining these.

The PO microporous membrane preferably has low electronic conductivity, ionic conductivity, high resistance to organic solvents, and fine pore diameter. Further, the PO microporous membrane can be used alone as a separator for lithium ion secondary batteries, and can be particularly suitably used as a separator for laminate type lithium ion secondary batteries.

<Thermoplastic polymer containing layer>
At least one side or the outside of the multilayer porous membrane according to one embodiment may be provided with a thermoplastic polymer-containing layer, if desired. The thermoplastic polymer-containing layer includes a thermoplastic polymer. The thermoplastic polymer layer may optionally include particulate polymer and/or have a dot-like pattern.

(thermoplastic polymer)
Thermoplastic polymers are not particularly limited, but include, for example, polyolefin resins such as polyethylene, polypropylene, and α-polyolefin; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, and copolymers containing these; and conjugated dienes such as butadiene and isoprene. diene polymers containing as monomer units, copolymers containing these and their hydrides; acrylic polymers containing acrylic acid esters, methacrylic esters, etc. as monomer units, copolymers containing these and their hydrides; ethylene propylene rubber, polyvinyl Alcohol, rubbers such as polyvinyl acetate; cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose; melting points and /or resins with a glass transition temperature of 180° C. or higher, mixtures thereof, and the like. In addition, monomers having a hydroxyl group, a sulfonic acid group, a carboxyl group, an amide group, or a cyano group can also be used as monomers used when synthesizing the thermoplastic polymer.

Among these thermoplastic polymers, diene polymers, acrylic polymers, and fluorine polymers are preferred because they have excellent binding properties with electrode active materials, strength, and flexibility.

(Diene polymer)
The diene polymer is, for example, a polymer containing a monomer unit formed by polymerizing a conjugated diene having two conjugated double bonds, such as butadiene and isoprene, although it is not particularly limited. Conjugated diene monomers are not particularly limited, but include, for example, 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2-phenyl-1,3-butadiene, 1,3-pentadiene, 2 -Methyl-1,3-pentadiene, 1,3-hexadiene, 4,5-diethyl-1,3-octadiene, 3-butyl-1,3-octadiene and the like. These may be polymerized alone or may be copolymerized.

The proportion of monomer units formed by polymerizing a conjugated diene in the diene polymer is not particularly limited, but for example, it is 40% by mass or more, preferably 50% by mass or more, more preferably 60% by mass or more in the total diene polymer. be.

Examples of the diene-based polymer include, but are not particularly limited to, homopolymers of conjugated dienes such as polybutadiene and polyisoprene, and copolymers of conjugated dienes and monomers copolymerizable with them. Copolymerizable monomers are not particularly limited, and examples thereof include (meth)acrylate monomers described below and the following monomers (hereinafter also referred to as "other monomers").

Examples of "other monomers" include, but are not limited to, α,β-unsaturated nitrile compounds such as acrylonitrile and methacrylonitrile; unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, and fumaric acid; styrene , chlorostyrene, vinyltoluene, t-butylstyrene, vinylbenzoic acid, methyl vinylbenzoate, vinylnaphthalene, chloromethylstyrene, hydroxymethylstyrene, α-methylstyrene, divinylbenzene; styrenic monomers such as ethylene, propylene, etc. Olefins; halogen atom-containing monomers such as vinyl chloride and vinylidene chloride; vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; methyl vinyl Vinyl ketones such as ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, and isopropenyl vinyl ketone; Heterocycle-containing vinyl compounds such as N-vinylpyrrolidone, vinylpyridine, and vinyl imidazole; Acrylic acids such as methyl acrylate and methyl methacrylate Ester and/or methacrylic acid ester compounds; hydroxyalkyl group-containing compounds such as β-hydroxyethyl acrylate and β-hydroxyethyl methacrylate; amide monomers such as acrylamide, N-methylolacrylamide, and acrylamide-2-methylpropanesulfonic acid; These may be used alone or in combination of two or more.

(acrylic polymer)
The acrylic polymer is not particularly limited, but is preferably a polymer containing monomer units obtained by polymerizing (meth)acrylate monomers. When the thermoplastic polymer-containing layer contains an acrylic polymer as the thermoplastic polymer, it preferably contains a copolymer containing a monomer unit of a (meth)acrylate monomer. When the thermoplastic polymer of the thermoplastic polymer-containing layer contains a copolymer containing a monomer unit of a (meth)acrylic acid ester monomer, the adhesive strength is improved when the multilayer porous membrane or separator has a low basis weight. Therefore, it is preferable.

In addition, in this specification, "(meth)acrylic acid" indicates "acrylic acid or methacrylic acid", and "(meth)acrylate" indicates "acrylate or methacrylate".

Examples of (meth)acrylate monomers include, but are not limited to, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, t -Butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate , alkyl (meth)acrylates such as lauryl (meth)acrylate, n-tetradecyl (meth)acrylate, stearyl (meth)acrylate; hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, etc. Hydroxy group-containing (meth)acrylates; amino group-containing (meth)acrylates such as aminoethyl (meth)acrylate; and epoxy group-containing (meth)acrylates such as glycidyl (meth)acrylate (GMA).

The proportion of monomer units obtained by polymerizing (meth)acrylate monomers is not particularly limited, but is, for example, 40% by mass or more, preferably 50% by mass or more, and more preferably 60% by mass or more of the total acrylic polymer. Examples of the acrylic polymer include homopolymers of (meth)acrylate monomers and copolymers of monomers copolymerizable with the homopolymers.
Examples of copolymerizable monomers include the "other monomers" listed in the section of diene polymers, and these may be used alone or in combination of two or more.

(fluoropolymer)
Examples of the fluorine-based polymer include, but are not particularly limited to, homopolymers of vinylidene fluoride and copolymers of vinylidene fluoride and copolymerizable monomers. Fluoropolymers are preferred from the viewpoint of electrochemical stability.

The proportion of monomer units formed by polymerizing vinylidene fluoride is not particularly limited, but is, for example, 40% by mass or more, preferably 50% by mass or more, and more preferably 60% by mass or more.
Monomers copolymerizable with vinylidene fluoride are not particularly limited, but include, for example, vinyl fluoride, tetrafluoroethylene, trifluorochloroethylene, hexafluoropropylene, hexafluoroisobutylene, perfluoroacrylic acid, perfluoromethacrylic acid, Fluorine-containing ethylenically unsaturated compounds such as fluoroalkyl esters of acrylic acid or methacrylic acid; fluorine-free ethylenically unsaturated compounds such as cyclohexyl vinyl ether and hydroxyethyl vinyl ether; fluorine-free diene compounds such as butadiene, isoprene, chloroprene, etc. can be mentioned.

Among the fluorine-based polymers, vinylidene fluoride homopolymers, vinylidene fluoride/tetrafluoroethylene copolymers, vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene copolymers, and the like are preferred. A particularly preferred fluoropolymer is vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene copolymer, and its monomer composition is usually 30 to 90% by mass of vinylidene fluoride, 50 to 9% by mass of tetrafluoroethylene, and hexafluoropropylene. It is 20 to 1% by mass. These fluororesin particles may be used alone or in combination of two or more.

Moreover, as the monomer used when synthesizing the above-mentioned thermoplastic polymer, a monomer having a hydroxyl group, a carboxyl group, an amino group, a sulfonic acid group, an amide group, or a cyano group can also be used.

The monomer having a hydroxy group is not particularly limited, but examples include vinyl monomers such as pentenol.

The monomer having a carboxyl group is not particularly limited, and examples thereof include unsaturated carboxylic acids having an ethylenic double bond such as (meth)acrylic acid and itaconic acid, and vinyl monomers such as pentenoic acid.

The monomer having an amino group is not particularly limited, and examples include 2-aminoethyl methacrylate.

Monomers having a sulfonic acid group are not particularly limited, but examples include vinylsulfonic acid, methylvinylsulfonic acid, (meth)alisulfonic acid, styrenesulfonic acid, ethyl (meth)acrylate-2-sulfonate, and 2-acrylamide. -2-methylpropanesulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, and the like.

The monomer having an amide group is not particularly limited, and examples thereof include acrylamide (AM), methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, and the like.

The monomer having a cyano group is not particularly limited, and examples thereof include acrylonitrile (AN), methacrylonitrile, α-chloroacrylonitrile, α-cyanoethyl acrylate, and the like.

The thermoplastic polymer may be used alone or in combination of two or more types, but preferably contains two or more types of polymers. The thermoplastic polymer may be used together with a solvent, and the solvent may be one that can uniformly and stably disperse the thermoplastic polymer, such as N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide. , water, ethanol, toluene, hot xylene, methylene chloride, hexane, etc., among which aqueous solvents are preferred. Thermoplastic polymers can also be used in latex form.

(Glass transition temperature of thermoplastic polymer)
The thermoplastic polymer constituting the thermoplastic polymer-containing layer exhibits binding properties to the base material, suppresses blocking, and adhesive strength with the electrodes of the separator, while ensuring the distance between the electrodes and the separator in non-aqueous electrolyte batteries. In order to reduce the electrolyte injection time, it has at least two glass transition temperatures, at least one of the glass transition temperatures is in the region of 20°C or less, and one of the glass transition temperatures is It is preferable that at least one of them has thermal characteristics such that it exists in a range of 30° C. or higher and 120° C. or lower.

Here, the glass transition temperature is determined from a DSC curve obtained by differential scanning calorimetry (DSC). Note that in this specification, the glass transition temperature may be expressed as Tg.

Specifically, it is determined by the intersection of a straight line extending the low-temperature side baseline of the DSC curve toward the high-temperature side and a tangent at the inflection point of the step-like change portion of the glass transition. For more details, reference may be made to the methods described in the Examples.

Here, the "glass transition" refers to a change in heat amount occurring on the endothermic side due to a change in the state of the polymer, which is a test piece, in DSC. Such a change in heat amount is observed in the DSC curve as a step-like change shape or a shape that is a combination of a step-like change and a peak.

A "staircase change" refers to a portion of a DSC curve where the curve leaves the previous baseline and transitions to a new baseline. Note that shapes that are a combination of peaks and step-like changes are also included.

The term "inflection point" refers to a point where the gradient of the DSC curve of the step-like change portion is maximum. It can also be expressed as a point at which an upwardly convex curve changes to a downwardly convex curve in a step-like transition portion.

A "peak" refers to a portion of a DSC curve from when the curve departs from the baseline until it returns to the baseline again.

"Baseline" refers to a DSC curve in a temperature range in which no transition or reaction occurs in the test piece.

In one embodiment, at least one of the glass transition temperatures of the thermoplastic polymer used is in a region of 20°C or lower, so that it has excellent adhesion with the microporous membrane and blocking is suppressed. The effect is that the adhesion between the separator and the electrode is excellent. The glass transition temperature is preferably -100°C or higher, more preferably -50°C or higher, still more preferably -40°C or higher, or particularly preferably -6°C or higher from the viewpoint of handling properties and blocking resistance, and the microporous From the viewpoint of adhesion to the membrane, the temperature is preferably 20°C or lower, more preferably 10°C or lower, or particularly preferably 0°C or lower.

In one embodiment, at least one of the glass transition temperatures of the thermoplastic polymer used is in the range of 30°C or higher and 120°C or lower, thereby providing excellent adhesion and handling properties between the separator and the electrode, and further improving non-aqueous electrolysis. In a liquid battery, the distance between the electrode surface and the separator base material surface can be maintained, and the electrolyte injection time can be shortened. The glass transition temperature is preferably 30°C or higher from the viewpoint of handling properties and blocking resistance, or more preferably 40°C or higher, or even more preferably 70°C or higher, or particularly preferably 95°C or higher, and 150°C or higher from the viewpoint of adhesive strength. The temperature is preferably 130°C or lower, more preferably 130°C or lower, or particularly preferably 120°C or lower.

The thermoplastic polymer having two glass transition temperatures can be achieved, for example, by a method of blending two or more types of thermoplastic polymers, but is not limited to this method.

In particular, polymer blends can control the glass transition temperature of the entire thermoplastic polymer by combining polymers with high and low glass transition temperatures. Additionally, multiple functions can be imparted to the entire thermoplastic polymer. For example, in the case of blends, by blending two or more types of polymers, particularly one with a glass transition temperature in the region of 30°C or higher and one with a glass transition temperature in the region of 20°C or lower, it is possible to improve stickiness resistance and polyolefin microporosity. It is possible to achieve both good adhesion to the film. When blending, the mixing ratio of a polymer having a glass transition temperature of 30°C or higher and a polymer having a glass transition temperature of 20°C or lower is 0.1:99.9 to 99.9: The range is preferably 0.1, more preferably 5:95 to 95:5, even more preferably 50:50 to 95:5, even more preferably 60:40 to 90:10. be. Further, viscoelasticity can also be controlled by combining a polymer with high viscosity and a polymer with high elasticity.

In one embodiment, the glass transition temperature, ie, Tg, of the thermoplastic polymer can be adjusted as appropriate by, for example, changing the monomer components used to produce the thermoplastic polymer and the input ratio of each monomer. That is, for each monomer used in the production of thermoplastic polymers, the Tg of the homopolymer generally indicated (for example, described in "Polymer Handbook" (A WILEY-INTERSCIENCE PUBLICATION)) and the blending ratio of the monomers can be roughly estimated. I can do it. For example, a copolymer containing a high proportion of monomers such as styrene, methyl methacrylate, and acrylonitrile can give a polymer with a Tg of about 100°C, and a polymer with a Tg of about -80°C can be obtained. A copolymer with a low Tg can be obtained by blending a high proportion of monomers such as butadiene to give a polymer and n-butyl acrylate and 2-ethylhexyl acrylate to give a polymer with a Tg of about -50°C.

Further, the Tg of the polymer can be roughly estimated from the FOX formula (formula (1) below). Note that the glass transition point of the thermoplastic polymer of the present application is determined by the above method using DSC.
1/Tg=W ₁ /Tg ₁ +W ₂ /Tg ₂ +...+W _i /Tg _i +...W _n /Tg _n (1)
{In formula (1), Tg (K) represents the Tg of the copolymer, Tg _i (K) represents the Tg of the homopolymer of each monomer i, and W _i represents the mass fraction of each monomer. }

(Structure of thermoplastic polymer containing layer)
In the thermoplastic polymer-containing layer, a thermoplastic resin having a glass transition temperature of 30°C or more and 120°C or less is present on the outermost surface side of the multilayer porous membrane, and on the interface side between the polyolefin microporous membrane and the thermoplastic polymer-containing layer. Preferably, a thermoplastic resin having a glass transition temperature of 20° C. or lower is present. Note that the "outermost surface" refers to the surface of the thermoplastic polymer-containing layer that comes into contact with the electrode when the multilayer porous membrane or separator and electrode are laminated. Furthermore, the term "interface" refers to the surface of the thermoplastic polymer-containing layer that is in contact with the microporous polyolefin membrane or porous layer.

In the thermoplastic polymer-containing layer, the presence of a thermoplastic polymer having a glass transition temperature of 30°C or more and 120°C or less on the outermost surface side of the multilayer porous membrane provides excellent adhesion with the microporous membrane, resulting in a separator. tends to have excellent adhesion with electrodes. In addition, the presence of a thermoplastic polymer with a glass transition temperature of 20°C or less at the interface between the polyolefin microporous membrane and the thermoplastic polymer-containing layer tends to improve the adhesion and handling properties between the separator and the electrode. . When the separator includes such a thermoplastic polymer-containing layer, the adhesion between the separator and the electrode and the handling properties tend to be further improved.

The above structure consists of (a) a thermoplastic polymer comprising a particle thermoplastic polymer and a binder resin that adheres the particle thermoplastic polymer to a microporous polyolefin membrane with the particle thermoplastic polymer exposed on the surface; , and the glass transition temperature of the granular thermoplastic polymer is in the range of 30°C or more and 120°C or less, and the interface between the polyolefin microporous membrane and the thermoplastic polymer-containing layer has a glass transition temperature of 20°C or less. (b) the thermoplastic polymer has a laminated structure, and the glass transition temperature of the thermoplastic polymer in the outermost layer when used as a separator is in the range of 30°C or higher and 120°C or lower; This can be achieved by the presence of a thermoplastic polymer having a glass transition temperature of 20° C. or lower at the interface between the polyolefin microporous membrane and the thermoplastic polymer-containing layer. Note that (b) the thermoplastic polymer may have a laminated structure of polymers having different Tg.

(Average particle size of thermoplastic polymer)
The structure of the thermoplastic polymer is not particularly limited, but may be configured, for example, in a granular shape. Having such a structure tends to improve the adhesion between the separator and the electrode and the handling of the separator. Here, granular refers to the state in which individual thermoplastic polymers have a contour as measured by a scanning electron microscope (SEM), and whether it is an elongated shape, a spherical shape, or a polygonal shape. etc. may be used.

The particle size distribution and median diameter of the granular thermoplastic polymer can be measured using a laser particle size distribution analyzer (Microtrac MT3300EX manufactured by Nikkiso Co., Ltd.). If desired, the particle size distribution of the particulate thermoplastic polymer can be adjusted using the particle size distribution of the water or binder polymer as a baseline. Let _D50 be the particle size at which the cumulative frequency is 50%, and let _D50 of the particulate thermoplastic polymer be D _P.

The average particle diameter D _P of the granular thermoplastic polymer is such that it can maintain the distance between multiple electrodes via the separator while exhibiting adhesive strength with the electrodes of the separator, and is suitable for electrolysis in a non-aqueous electrolyte battery equipped with a separator. From the viewpoint of shortening the injection time of the liquid, it is preferably 100 nm or more and 1000 nm or less, more preferably 130 nm or more and 700 nm or less, even more preferably 320 nm or more and 590 nm or less, and 400 nm or more and 550 nm or less. is most preferable.

(Weight per side of thermoplastic polymer-containing layer)
In the separator according to one embodiment, the basis weight per one side of the thermoplastic polymer-containing layer is preferably 0.03 g/m ² or more and 0.3 g/m 2 or less, and 0.04 g/m ² or less, from the viewpoint of adhesive strength. It is more preferably 0.15 g/m ² or ^more , and most preferably 0.06 g/m ² or more and 0.10 g/m ² or less. The basis weight of the thermoplastic polymer-containing layer can be adjusted by changing the polymer concentration of the coating liquid and the coating amount of the polymer solution. A range exceeding 0.08 g/m ² is preferable from the viewpoint of suppressing deformation of the cell shape due to expansion and contraction of the electrode and improving the cycle characteristics of the battery within a range that does not impede the effects of the present embodiment.

(Form of thermoplastic polymer-containing layer, coverage ratio of base material surface by thermoplastic polymer-containing layer)
The existing form (pattern) of the thermoplastic polymer-containing layer may be, for example, a state in which the thermoplastic polymers are present in a mutually dispersed manner over the entire surface of the multilayer porous membrane, or a state in which the thermoplastic polymers are present in a sea-island shape. When the thermoplastic polymer is present in a striped pattern, examples of the arrangement pattern include dots, stripes, lattice, stripes, hexagonal, random, etc., and combinations thereof. Among these, it is preferable that the thermoplastic polymer-containing layer has a dot-like pattern.

The dot shape indicates that on the polyolefin microporous membrane, there are parts containing a thermoplastic polymer and parts not containing a thermoplastic polymer, and the parts containing a thermoplastic polymer are present in an island shape. Note that in the thermoplastic polymer-containing layer, the portion containing the thermoplastic polymer may be independent.

The diameter of the dots in the thermoplastic polymer-containing layer is preferably 20 μm or more from the viewpoint of adhesion with the PO microporous membrane or electrode, improving the strength of the multilayer porous membrane, or suppressing an increase in air permeability with respect to the PO microporous membrane. It is in the range of 1,200 μm, more preferably 30 μm to 1,100 μm, even more preferably 40 μm to 1,000 μm.

The distance between the dots of the thermoplastic polymer-containing layer is preferably 100 μm or more from the viewpoint of adhesion with the PO microporous membrane or electrode, improving the strength of the multilayer porous membrane, or suppressing an increase in air permeability with respect to the PO microporous membrane. It is in the range of 3,500 μm, more preferably 120 μm to 3,300 μm, even more preferably 140 μm to 3,000 μm.

In one embodiment, the total coverage area ratio of the thermoplastic polymer-containing layer to the base material surface is set such that the resistance of the battery is lowered while maintaining the adhesive force of the multilayer porous membrane or the separator with the electrode, and the separator is provided with the separator. 3% or more, or 4% or more, or 5% or more, or 10% or more, or 20% or more, or 30% or more, or It is preferably 40% or more, and preferably 90% or less, or 80% or less, or 75% or less, or 70% or less. If the coverage area of the thermoplastic polymer-containing layer is small, the distance between the separator and the electrode interface becomes uneven, resulting in non-uniform current distribution, which makes it easier for the temperature to rise in the (heating) safety test. Furthermore, if the area covered by the thermoplastic polymer-containing layer is large, the resistance of the battery will increase, leading to deterioration of the rate test. The total coverage area ratio S of the thermoplastic polymer-containing layer present on the surface of the base material is calculated from the following formula.
S (%) = Total coverage area of thermoplastic polymer-containing layer ÷ Surface area of base material x 100

The total coverage area ratio (%) of the coating pattern of the thermoplastic polymer-containing layer to the substrate surface is measured using a microscope (model: VHX-7000, manufactured by Keyence Corporation). After photographing the separator as a sample at 30 times magnification (coaxial epi-illumination), select automatic area measurement as the measurement mode and measure the total coverage area ratio of the thermoplastic polymer. The coverage area ratio in each sample is determined by performing the above measurement three times and using the arithmetic average value thereof.

The form or total coverage area ratio of the thermoplastic polymer-containing layer can be adjusted by changing the polymer concentration of the coating liquid, the coating amount of the polymer solution, the coating method, and coating conditions.

<Characteristics of multilayer porous membrane>
The multilayer porous membrane according to the present embodiment preferably satisfies at least one of the following relationships (i) and (ii), and satisfies both relationships (i) and (ii) from the viewpoint of the mechanism of action of the present invention. It is more preferable to meet:
(i) The ratio T/TB of the thickness TB of the PO microporous membrane and the thickness T of the porous layer is 0.05 or more and 0.25 or less;
(ii) The average thermal shrinkage rate of the multilayer porous film in the MD direction and the TD direction when the multilayer porous film is left in an atmosphere at a temperature of 150° C. for 1 hour is 0% or more and 10% or less.

A multilayer porous film that satisfies the following relationships (i) and/or (ii) not only enables the separator to be made thinner and contributes to improved productivity, but also can be incorporated into nonaqueous electrolyte batteries as a thin film separator. Sometimes it also contributes to improved energy density and safety.

Regarding the above relationship (i), the thickness ratio T/TB is preferably 0.05 or more and 0.25 or less, more preferably 0.07 or more and 0.23 or less, and 0.10 or more. It is more preferably within the range of 0.20, and particularly preferably within the range of 0.10 to 0.14. The thickness ratio T/TB may be measured and calculated on one side or both sides of the PO microporous membrane, and from the viewpoint of the mechanism of action and thickness measurement principle of the invention, It is preferable to measure and calculate a hit, and specifically, measure and calculate by setting the total thickness of the porous layers arranged in the multilayer porous membrane as T.

Regarding the above relationship (ii), when the multilayer porous membrane is left in an atmosphere at a temperature of 150°C for 1 hour, the average thermal shrinkage rate of the multilayer porous membrane in the MD direction and the TD direction is 0% or more and 10% or less. It is preferably at least 0% and less than 8%, even more preferably at least 0% and less than 5%.

From the same viewpoint as the above relationship (ii), the 150°C heat shrinkage rate of the multilayer porous membrane is preferably 0.0% or more and 10.0% or less in both the MD direction and the TD direction, and more preferably 0.0% or more and 10.0% or less. It is .0% or more and less than 8.0%, more preferably 0.0% or more and less than 5.0%.

The heat shrinkage rate at 130°C of the multilayer porous membrane is not particularly limited, but for example, it is preferably 0.0% or more and 5.0% or less, more preferably 0.0% in both the MD direction and the TD direction. The content is preferably 0.0% or more and 2.0% or less, more preferably 0.0% or more and 2.0% or less. It is preferable that the 130° C. heat shrinkage rate is 5.0% or less in both the MD direction and the TD direction from the viewpoint of suppressing rupture of the multilayer porous membrane when abnormalities occur in the battery and suppressing short circuits.

In order to ensure voltage resistance, the total thickness of the multilayer porous membrane is preferably over 1.5 μm, more preferably 2.5 μm or more, and even more preferably 4.0 μm or more. Further, the total thickness of the multilayer porous membrane is preferably 30 μm or less, since the capacity of the non-aqueous electrolyte battery in which the multilayer porous membrane is mounted is unlikely to deteriorate, more preferably 26 μm or less, and 24 μm or less. It is more preferable that it is, even more preferably that it is 20 μm or less, and particularly preferably that it is 15 μm or less.

The air permeability of the multilayer porous membrane is preferably 50 sec/100 cm ³ from the viewpoint of ensuring the safety of the non-aqueous electrolyte battery by preventing excessive current from flowing between the plurality of electrodes through the multilayer porous membrane. More preferably, it is 80 sec/100 cm ³ or more. Further, the air permeability of the multilayer porous membrane is preferably 250 sec/100 cm ³ or less, more preferably 200 sec/100 cm ³ or less from the viewpoint of ion permeability.

<Method for manufacturing multilayer porous membrane>
The multilayer porous membrane according to the present embodiment can be manufactured by a known method, for example, by forming a PO microporous membrane and then arranging a porous layer on at least one side of the PO microporous membrane. be able to.

The multilayer porous membrane may be prepared by, for example, forming a PO microporous membrane, and then disposing a first porous layer on one surface of the PO microporous membrane and disposing a second porous layer on the other surface of the PO microporous membrane. It can be manufactured by Alternatively, the PO microporous membrane and the porous layer can be manufactured by coextrusion, the first porous layer and the second porous layer can be extruded on both sides of the PO microporous membrane, or the PO microporous membrane can be manufactured separately. and a porous layer can be bonded together.

Furthermore, the method for producing a multilayer porous membrane includes, if desired, obtaining a multilayer porous membrane by disposing a porous layer on at least one side of a PO microporous membrane, and forming a thermoplastic polymer-containing layer on at least one side of the obtained multilayer porous membrane. The method may include a step of forming.

(Method for manufacturing microporous polyolefin membrane)
There are no particular restrictions on the method for producing the polyolefin microporous membrane (PO microporous membrane), and known production methods can be employed. 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 and extruding it at a high draw ratio, and then making it porous by exfoliating the polyolefin crystal interface by heat treatment and stretching;
(3) A method in which a polyolefin resin composition and an inorganic filler are melt-kneaded, formed into a sheet, and then made porous by peeling off the interface between the polyolefin and the inorganic filler by stretching;
(4) A method of making the polyolefin porous by dissolving the polyolefin resin composition and then immersing it in a poor solvent for polyolefin to coagulate the polyolefin and simultaneously removing the solvent;
etc.

Hereinafter, as an example of a method for producing a PO microporous membrane, a method will be described in which a polyolefin resin composition and a pore-forming material are melt-kneaded, molded into a sheet shape, and then the pore-forming material is extracted.

First, a polyolefin resin composition and the above-mentioned pore-forming material are melt-kneaded. As a melt-kneading method, for example, the polyolefin resin and other additives as necessary are put into a resin kneading device such as an extruder, a feeder, a laboplast mill, a kneading roll, a Banbury mixer, etc., while the resin components are heated and melted. Examples include a method of introducing and kneading a pore-forming material in an arbitrary ratio.

Pore-forming materials can include plasticizers, inorganic materials, or combinations thereof. Plasticizers include, but are not particularly limited to, non-volatile solvents that can form a homogeneous solution above the melting point of the polyolefin, such as hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; Examples include higher alcohols such as oleyl alcohol and stearyl alcohol. Among plasticizers, liquid paraffin has high compatibility with polyolefin resins such as polyethylene and/or polypropylene, and even if the melt-kneaded product is stretched, interfacial peeling between the resin and plasticizer is unlikely to occur, resulting in a uniform This is preferable because it tends to be easier to carry out stretching. Inorganic materials are not particularly limited, and include, for example, oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; silicon nitride, titanium nitride, and nitride. Nitride 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 , ceramics such as asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber. These may be used alone or in combination of two or more. Among these inorganic materials, silica, alumina, and titania are preferred from the viewpoint of electrochemical stability, and silica is particularly preferred from the viewpoint of easy extraction.

Next, the melt-kneaded product is formed into a sheet shape. As a method for producing a sheet-like molded product, for example, a melt-kneaded product is extruded into a sheet-like form through a T-die, etc., and cooled to a temperature sufficiently lower than the crystallization temperature of the resin component by contacting with a heat conductor. An example of this method is to solidify the material. Thermal conductors used for cooling and solidification include metals, water, air, plasticizers, and the like. Among these, it is preferable to use metal rolls because of their high heat conduction efficiency. In addition, when the extruded kneaded material is brought into contact with metal rolls, sandwiching it between the rolls further increases the efficiency of heat conduction, and the sheet is oriented, increasing film strength and improving the surface smoothness of the sheet. This is more preferable because it tends to The die lip interval when extruding the melt-kneaded material into a sheet form from a T-die is preferably 200 μm or more and 3,000 μm or less, more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, smearing and the like are reduced, and the influence on film quality such as streaks and defects is small, and the risk of film breakage etc. in the subsequent stretching process can be reduced. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is fast and uneven cooling can be prevented, and the thickness stability of the sheet can be maintained.

Alternatively, the sheet-like molded body may be rolled. Rolling can be carried out, for example, by a pressing method using a double belt press machine or the like. By rolling, it is possible to increase the orientation, especially in the surface layer portion. The rolling surface magnification is preferably more than 1 time and no more than 3 times, more preferably more than 1 time and no more than 2 times. When the rolling ratio exceeds 1, the planar orientation tends to increase and the strength of the porous membrane finally obtained tends to increase. On the other hand, when the rolling ratio is 3 times or less, the difference in orientation between the surface layer portion and the center interior 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-like molded body to form a porous membrane. As a method for removing the pore-forming material, for example, a method of immersing the sheet-like molded body in an extraction solvent to extract the pore-forming material and thoroughly drying it can be mentioned. The method for extracting the pore-forming material may be either a batch method or a continuous method. In order to suppress shrinkage of the porous membrane, it is preferable to restrain the ends of the sheet-like molded body during the series of steps of dipping and drying. Further, the amount of pore forming material remaining in the porous membrane is preferably less than 1% by mass based on the mass of the entire porous membrane.

As the extraction solvent used when extracting the pore-forming material, it is preferable to use one that is a poor solvent for the polyolefin resin, a good solvent for the pore-forming material, and has a boiling point lower than the melting point of the polyolefin resin. . 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-chlorinated solvents such as hydrofluoroether and hydrofluorocarbon. Examples 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. Note that these extraction solvents may be recovered and reused by operations such as distillation. Furthermore, when an inorganic material is used as the pore-forming material, an aqueous solution of sodium hydroxide, potassium hydroxide, etc. can be used as the extraction solvent.

Moreover, it is preferable to stretch the sheet-like molded body or porous membrane. The stretching may be performed before extracting the pore-forming material from the sheet-like molded body. Alternatively, the treatment may be performed on a porous membrane obtained by extracting the pore-forming material from the sheet-like molded body. Furthermore, the step may be performed before and after extracting the pore-forming material from the sheet-like molded body.

As the stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used, but biaxial stretching is preferable from the viewpoint of improving the strength etc. of the obtained PO microporous membrane. When a sheet-like molded body is stretched biaxially at a high magnification, the molecules are oriented in the plane direction, and the microporous membrane finally obtained becomes difficult 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 preferred from the viewpoint of improving puncture strength, uniformity of stretching, and shutdown properties. Further, from the viewpoint of ease of controlling plane orientation, sequential biaxial stretching is preferable.

Here, simultaneous biaxial stretching refers to stretching in which MD (machine direction of continuous molding of PO microporous membrane) and TD (direction crossing MD of PO microporous membrane at an angle of 90°) are performed simultaneously. The stretching ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD stretching is performed independently, and when MD or TD is stretched, the other direction is unrestricted or fixed to a constant length. state.

The stretching 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. The stretching ratio in each axial direction is preferably in the range of 4 times or more and 10 times or less in MD, 4 times or more and 10 times or less in TD, 5 times or more and 8 times or less in MD, and 5 times or more and 8 times or less in TD. It is more preferable that it is in the range of . When the total area magnification is 20 times or more, sufficient strength tends to be imparted to the resulting PO microporous membrane, while when the total area magnification is 100 times or less, membrane breakage during the stretching process is prevented, and a high They tend to be more productive.

In order to suppress shrinkage of the PO microporous membrane, heat treatment can be performed for the purpose of heat fixation after the stretching process or after the formation of the PO microporous membrane. Further, the PO microporous membrane may be subjected to post-treatments such as hydrophilization treatment using a surfactant or the like, or crosslinking treatment using ionizing radiation or the like.

The PO microporous membrane is preferably subjected to heat treatment for the purpose of heat fixation from the viewpoint of suppressing shrinkage. Heat treatment methods include stretching operations performed at a predetermined temperature atmosphere and a predetermined stretching rate for the purpose of adjusting physical properties, and/or relaxation performed at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing stretching stress. One example is manipulation. A relaxing 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, the MD and/or TD of the membrane is stretched by 1.1 times or more, more preferably 1.2 times or more, from the viewpoint of obtaining a PO microporous membrane with higher strength and higher porosity. preferable.

A relaxation operation refers to a reduction operation of the membrane to MD and/or TD. The relaxation rate is the value obtained by dividing the dimension of the membrane after the relaxation operation by the dimension of the membrane before the relaxation operation. Note that when both MD and TD are relaxed, the value is the product of the MD relaxation rate and the TD relaxation rate. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and even more preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. The relaxation operation may be performed in both the MD and TD directions, or may be performed only in the MD or TD.

The stretching and relaxing operations after the plasticizer extraction are preferably performed in TD from the viewpoint of process control and control of hole area in the 400° C. solder test. The temperature in the stretching and relaxing operations is preferably lower than the melting point (hereinafter also referred to as "Tm") of the polyolefin resin, and more preferably in a range of 1 to 25 degrees Celsius lower than Tm. It is preferable that the temperature in the stretching and relaxing operations is within the above range from the viewpoint of a balance between reduction in thermal shrinkage rate and porosity.

(Porous layer arrangement method)
As a method for arranging the porous layer on at least one side of the PO microporous membrane, known arrangement methods, coating methods, lamination methods, extrusion methods, etc. can be employed. For example, a method of forming a porous layer by applying a coating liquid or slurry containing barium sulfate and poly(meth)acrylamide as a binder polymer as described above to a PO microporous membrane can be mentioned.

Methods for arranging the first porous layer on one side of the polyolefin microporous membrane (PO microporous membrane) and arranging the second porous layer on the other side of the PO microporous membrane include known arrangement methods, coating methods, Lamination methods, extrusion methods, etc. can be employed. For example, a method may be used in which a porous layer is formed by coating both sides of a PO microporous membrane with a coating liquid or slurry containing barium sulfate and poly(meth)acrylamide as a binder polymer as described above.

When the total of barium sulfate as inorganic particles and binder polymer contained in the coating liquid or slurry is 100 volume %, the proportion of barium sulfate is preferably greater than 90 volume % and 99 volume % or less, and 91 volume %. % or more and 99 volume % or less, and still more preferably 93 volume % or more and 98 volume % or less. When the volume ratio of the inorganic particles in the coating solution is within the above range, the ratio of barium sulfate to the binder polymer increases, suppressing the increase in air permeability of the microporous membrane due to the porous layer, and improving the multilayer porous membrane. The electrical resistance of can be reduced.

In addition, from the same viewpoint as the volume ratio above, the mass proportion of barium sulfate in the coating liquid or slurry is 97 mass% or more and 100 mass% or less, assuming that the total of barium sulfate and binder polymer is 100 mass%. It is preferable that there be.

The particle size distribution of the barium sulfate-containing slurry was determined from the viewpoint of strengthening the interaction between barium sulfate and poly(meth)acrylamide to further improve the thermal shrinkage suppressing ability and strength of the multilayer porous membrane and the energy density of the nonaqueous electrolyte battery. , preferably satisfy at least one of the following relationships:
- Regarding the average particle diameter D ₅₀ , 0.01 μm≦D ₅₀ ≦0.60 μm, 0.05 μm≦D ₅₀ ≦0.5 μm, or 0.1 μm≦D ₅₀ ≦0.4 μm;
- The ratio D ₉₀ /D ₅₀ of the average particle diameter D ₉₀ to the average particle diameter D ₅₀ is 1.6≦D ₉₀ /D ₅₀ ≦2.5, 1.7≦D ₉₀ /D ₅₀ ≦2.3, or 1.8≦ _D90 / _D50 ≦2.1;
- Regarding the ratio D ₁₀ /D ₅₀ of the average particle diameter D ₁₀ to the average particle diameter D ₅₀ , 0.2≦D ₁₀ /D ₅₀ ≦0.8, 0.3≦D ₁₀ /D ₅₀ ≦0.7, or 0.4≦D ₁₀ /D ₅₀ ≦0.6 or less;
- Average particle size D ₉₀ : 0.016 μm to 1.5 μm, 0.085 μm to 1.15 μm, or 0.18 μm to 0.84 μm;
- Average particle size D ₁₀ is 0.01 μm to 0.48 μm, 0.015 μm to 0.35 μm, or 0.04 μm to 0.24 μm.

Methods for adjusting the particle size distribution of the barium sulfate-containing slurry as described above include, for example, pulverizing barium sulfate using a ball mill, bead mill, jet mill, etc. to obtain a desired particle size distribution; Examples include a method in which a distributed barium sulfate is prepared as an inorganic filler and then blended.

In forming a barium sulfate-containing slurry or coating solution, from the viewpoints of interaction between water-soluble polymers such as poly(meth)acrylamide and barium sulfate, increase in the number of contact points between the two, and heat resistance of the multilayer porous membrane or separator, The ratio Wb'/Wa' of the volume proportion Wa' of poly(meth)acrylamide contained in the slurry or coating liquid to the volume proportion Wb' of the water-insoluble binder is preferably 1 or less, more preferably less than 1. Preferably, it is smaller than 0.6, more preferably smaller than 0.4, and most preferably around 0 or 0.

A dispersant such as a surfactant may be added to the barium sulfate-containing coating liquid or barium sulfate-containing slurry in order to stabilize dispersion or improve coatability. The dispersant adsorbs onto the surface of the inorganic particles in the slurry and stabilizes the inorganic particles through electrostatic repulsion, and includes, for example, polycarboxylate, sulfonate, polyoxyether, and the like. The amount of the dispersant added is preferably 0.2 parts by weight or more and 5.0 parts by weight or less, more preferably 0.3 parts by weight or more and 1.0 parts by weight or less in terms of solid content.

The coating liquid contains a thickener; a wetting agent; an antifoaming agent; a PH adjuster containing an acid or alkali, etc. in order to stabilize dispersion or improve coating properties, and to adjust the contact angle on the surface of the porous layer. Various additives may be added. The total amount of these additives added is preferably 20 parts by mass or less of their active ingredients (if the additive is dissolved in a solvent, the mass of the dissolved additive component) based on 100 parts by mass of the inorganic particles. , more preferably 10 parts by mass or less, still more preferably 5 parts by mass or less.

Regarding additives, anionic surfactants such as higher fatty acid salts, alkyl sulfonates, alpha olefin sulfonates, alkanesulfonates, alkylbenzene sulfonates, sulfosuccinate salts, alkyl sulfate salts, alkyl Examples include ether sulfate ester salts, alkyl phosphate ester salts, alkyl ether phosphate ester salts, alkyl ether carboxylates, alpha sulfofatty acid methyl ester salts, and methyl taurate. Examples of nonionic surfactants include glycerin fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene These include fatty acid esters, fatty acid alkanolamides, and alkyl glucosides. Examples of amphoteric surfactants include alkyl betaines, fatty acid amidopropyl betaines, and alkyl amine oxides. Examples of the cationic surfactant include alkyltrimethylammonium salts, dialkyldimethylammonium salts, alkyldimethylbenzylammonium salts, and alkylpyridinium salts. In addition, there are polymeric surfactants such as fluorine-based surfactants, cellulose derivatives, polycarboxylate salts, and polystyrene sulfonate salts.

The viscosity of the barium sulfate-containing coating liquid or the barium sulfate-containing slurry is preferably 10 mPa·sec or more and 200 mPa·sec when measured with a B-type viscosity device (at 60 rpm). More preferably 40 mPa·sec or more and 150 mPa·sec, still more preferably 50 mPa·sec or more and 130 mPa·sec. A viscosity of 10 mPa·sec or more is preferable from the viewpoint of suppressing the precipitation of barium sulfate in the coating liquid, and a viscosity of 200 mPa·sec or less is preferable to stabilize the dispersion of the coating liquid and to transfer the coating liquid to the PO microporous membrane. This is preferable from the viewpoint of suppressing the surface pattern of the porous layer after coating.

The medium for the coating solution is preferably one that can uniformly and stably disperse or dissolve barium sulfate and poly(meth)acrylamide as a binder polymer, such as N-methylpyrrolidone, N,N-dimethylformamide, N,N - Dimethylacetamide, water, ethanol, toluene, hot xylene, methylene chloride, hexane, etc.

There is no particular limitation on the method of dispersing or dissolving the inorganic particles and the binder polymer in the medium of the coating liquid, as long as the method can achieve the dispersion characteristics of the coating liquid necessary for the coating process. Examples include ball mills, bead mills, planetary ball mills, vibrating ball mills, sand mills, colloid mills, attritors, roll mills, high-speed impeller dispersion, dispersers, homogenizers, high-speed impact mills, ultrasonic dispersion, and mechanical stirring using stirring blades.

There is no particular limitation on the method of applying the barium sulfate-containing coating solution or the barium sulfate-containing slurry to the PO microporous membrane as long as it can achieve the required layer thickness or application area.For example, gravure coater method, small diameter gravure Coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen Examples include printing methods and spray coating methods.

In one embodiment, the coating of the barium sulfate-containing coating solution or the barium sulfate-containing slurry to the PO microporous membrane suppresses an increase in the air permeability of the PO microporous membrane, forms a strong porous layer, and the resulting multilayer porous membrane. Alternatively, from the viewpoint of thinning the separator and heat resistance (namely, thermal shrinkage suppressing ability), the coating thickness of the porous layer per at least one side of the PO microporous membrane is preferably more than 0 μm and 2 μm or less, more preferably 1.0 μm or less. This is done so that the thickness is 5 μm or less.

In one embodiment, when applying the barium sulfate-containing coating solution or the barium sulfate-containing slurry to the PO microporous membrane, from the viewpoint of the mechanism of action of the present invention, the coating area of the porous layer is preferably 1.0 g/ m ² to 5.0 g/m ² , more preferably 1.5 g/m ² to 4.0 g/m ² .

If desired, the PO microporous membrane may be subjected to surface treatment prior to application of the barium sulfate-containing coating solution or barium sulfate-containing slurry. When a PO microporous membrane is surface-treated, it becomes easier to apply a coating liquid or slurry, and the adhesion between the barium sulfate-containing porous layer and the surface of the PO microporous membrane after coating may be improved. The surface treatment method is not particularly limited as long as it does not significantly damage the porous structure of the PO microporous membrane, and examples include corona discharge treatment, plasma discharge treatment, mechanical roughening, solvent treatment, Examples include acid treatment method and ultraviolet oxidation method.

The method for removing the medium from the coated film after coating is not particularly limited as long as it does not adversely affect the PO microporous film; for example, drying at a temperature below its melting point while fixing the PO microporous film. method, method of drying under reduced pressure at low temperature, extraction drying, etc. Further, a portion of the solvent may remain as long as it does not significantly affect the characteristics of the non-aqueous electrolyte battery. For a multilayer porous membrane in which a PO microporous membrane and a porous layer are laminated, it is preferable to adjust the drying temperature, winding tension, etc. as appropriate from the viewpoint of controlling shrinkage stress in the MD direction.

(Method for forming thermoplastic polymer-containing layer)
The method of forming the thermoplastic polymer-containing layer on the PO microporous membrane or porous layer as a base material is not particularly limited. Can be mentioned.

There is no particular limitation on the method of applying the coating solution containing the thermoplastic polymer to the PO microporous membrane or porous layer, as long as the required layer thickness and application area can be achieved. 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 coater method, die coater method, screen printing method, spray coating method, spray coater coating method, inkjet coating, and the like. Among these, the gravure coater method or the spray coating method is preferred from the viewpoint of having a high degree of freedom in the coating shape of the thermoplastic polymer and easily obtaining a preferred area ratio. Regarding the formation of a dot-like pattern in the thermoplastic polymer-containing layer, a gravure coater method, inkjet coating, and a coating method that allows easy adjustment of the printing plate are preferred.

When coating a thermoplastic polymer on a PO microporous membrane, if the coating solution penetrates into the microporous membrane or porous layer, the adhesive resin will fill the surface and inside of the pores, reducing permeability. It ends up. Therefore, as a medium for the coating liquid, a poor solvent for a thermoplastic polymer is preferable.

When a poor solvent for a thermoplastic polymer is used as the coating liquid medium, the coating liquid does not penetrate into the microporous membrane or porous layer, and the adhesive polymer exists mainly on the surface of the microporous membrane. , is preferable from the viewpoint of suppressing a decrease in permeability. Water is preferred as such a medium. Moreover, the medium that can be used in combination with water is not particularly limited, but examples include ethanol, methanol, and the like. If desired, an antifoaming agent may be added to the thermoplastic polymer-containing coating solution.

Thermoplastic polymer-containing coating liquids (hereinafter simply referred to as paints) are subject to high-temperature storage tests while ensuring adhesion to the separator electrodes, the difficulty of increasing the temperature of the separator, and the difficulty of cycle deterioration. From the viewpoint of compatibility, the paint viscosity is preferably within the range of 30 or more and 100 cP or less, and more preferably within the range of 50 or more and 80 cP or less. From the same viewpoint, the pH of the paint is preferably within the range of 5 to 7.9, more preferably within the range of 5.5 to 7.7.

Furthermore, it is preferable to subject the microporous membrane as a separator base material to a surface treatment prior to coating, since this makes it easier to apply the coating liquid and improves the adhesion between the microporous membrane or porous layer and the adhesive polymer. The surface treatment method is not particularly limited as long as it does not significantly damage the porous structure of the microporous membrane, and examples include corona discharge treatment, plasma treatment, mechanical roughening, solvent treatment, and acid treatment. method, ultraviolet oxidation method, etc.

In the case of the corona discharge treatment method, the corona treatment strength on the surface of the substrate is preferably in the range of 1 W/(m ² /min) to 40 W/(m ² /min), and 3 W/(m ² /min). /min) to 32 W/(m ² /min), and more preferably 5 W/(m ² /min) to 25 W/(m ² /min).

The method for removing the solvent from the coating film after coating is not particularly limited as long as it does not adversely affect the microporous membrane or porous layer. For example, a method in which the microporous membrane and/or porous layer is fixed while drying at a temperature below its melting point, a method in which drying is carried out under reduced pressure at a low temperature, and a method in which the adhesive polymer is simultaneously solidified by immersion in a poor solvent for the adhesive polymer. Examples include a method of extracting a solvent.

In drying the coating film, the drying rate is preferably within the range of 0.03 g/(m ² s) or more and 4.0 g/(m ² s) or less, and 0.05 g/(m ² s ) is more preferably within the range of 3.5 g/(m ²・s) or less, and more preferably within the range of 0.08 g/(m ²・s) or more and 3.0 g/(m ²・s) or less. More preferably. In drying the coating film, it is also preferable to raise the temperature by warming or heating to an extent that does not impair the particle shape of the thermoplastic polymer-containing layer.

<Separator for non-aqueous electrolyte batteries and non-aqueous electrolyte batteries>
The multilayer porous membrane according to this embodiment can be used as a separator for non-aqueous electrolyte batteries. A nonaqueous electrolyte battery includes a positive electrode, a separator, a negative electrode, and a nonaqueous electrolyte, and specifically includes a lithium battery, a lithium secondary battery, a lithium ion secondary battery, a sodium secondary battery, and a sodium ion battery. Secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel metal hydride batteries, nickel cadmium batteries, electric double layer capacitors, Examples include lithium ion capacitors, redox flow batteries, and lithium sulfur batteries. Among these, from the viewpoint of practicality, lithium batteries, lithium secondary batteries, lithium ion secondary batteries, nickel hydride batteries, or lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred.

A non-aqueous electrolyte battery is manufactured by, for example, stacking a positive electrode and a negative electrode via a separator made of the above-described multilayer porous membrane, and winding or folding them as necessary to form a laminated electrode. After forming a body or a wound electrode body or a ninety-nine folded body, it is loaded into an exterior body, the positive and negative electrodes and the positive and negative terminals of the exterior body are connected via a lead body, etc. It can be produced by injecting a non-aqueous electrolyte containing a non-aqueous solvent such as a cyclic carbonate and an electrolyte such as a lithium salt into the exterior body, and then sealing the exterior body.

A nonaqueous electrolyte battery includes the laminate described above, a wound body in which the laminate is wound, or a zigzag body in which the laminate is zigzag folded, together with a nonaqueous electrolyte, inside an exterior body such as a cylindrical can, a pouch-type case, or a laminate case. A nonaqueous electrolyte battery using the multilayer porous membrane according to this embodiment as a separator may not only be excellent in safety, but also in energy density and cycle characteristics.

When the non-aqueous electrolyte battery is a secondary battery, a positive electrode terminal is welded to the end of a positive electrode stack consisting of a positive electrode current collector and a positive electrode active material layer, and By welding a negative terminal to the end of the negative electrode laminate, it is possible to charge and discharge a secondary battery including the terminal-equipped positive electrode laminate and the terminal-equipped negative electrode laminate.

Furthermore, the positive electrode laminate with a terminal and the negative electrode laminate with a terminal are laminated with a separator interposed therebetween, and the resulting laminate, wound body, or folded body is rolled or folded as desired. A secondary battery can be obtained by storing it in an exterior body, injecting a non-aqueous electrolyte into the exterior body, and sealing the exterior body.

When manufacturing a nonaqueous electrolyte secondary battery using the multilayer porous membrane according to this embodiment as a separator, known positive electrodes, negative electrodes, and nonaqueous electrolytes may be used.

Examples of the positive electrode material include, but are not limited to, lithium-containing composite oxides such as LiCoO ₂ , LiNiO ₂ , spinel-type LiMnO ₄ , and olivine-type LiFePO ₄ .

Examples of the negative electrode material include, but are not limited to, carbon materials such as graphite, non-graphitizable carbon, easily graphitizable carbon, and composite carbon materials; silicon, tin, metallic lithium, and various alloy materials. .

The nonaqueous electrolyte is not particularly limited, but an electrolyte in which an electrolyte is dissolved in an organic solvent can be used. Examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate. Examples of the electrolyte include lithium salts such as LiClO ₄ , LiBF ₄ , and LiPF ₆ .

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples and Comparative Examples.

Testing and evaluation methods

<Viscosity average molecular weight (Mv)>
The intrinsic viscosity [η] (dl/g) of decalin solvent at 135°C was determined based on ASTM-D4020.
The Mv of polyethylene and polyolefin microporous membranes was calculated using the following formula.
[η]=6.77× ^10-4 Mv ^0.67
Mv of polypropylene was calculated using the following formula.
[η]=1.10×10 ⁻⁴ Mv ^0.80

<Thickness (μm) of PO microporous membrane, multilayer porous membrane, and multilayer membrane>
The thickness of the polyolefin microporous membrane and the multilayer porous membrane was measured at room temperature (23±2°C) using a micro thickness meter "KBM (trademark)" manufactured by Toyo Seiki Co., Ltd., and the thickness of the porous layer was determined from the respective thicknesses. The coating thickness was calculated. Furthermore, from the viewpoint of detection from a multilayer porous membrane, it is also possible to measure the thickness of each layer using a cross-sectional SEM image.

<Melt index (MI) of polyolefin microporous membrane (g/10 minutes)>
The melt index (MI) of a polyolefin microporous membrane (PO microporous membrane) was measured in accordance with JIS K7210:1999 (plastic-thermoplastic melt mass flow rate (MFR) and melt volume flow rate (MVR)). A load of 21.6 kgf was applied to the membrane at 190°C, and the amount of resin (g) flowing out from an orifice with a diameter of 1 mm and a length of 10 mm in 10 minutes was measured, and the value rounded to the first decimal place was defined as MI. .

<Average particle size and particle size distribution of inorganic particles>
The particle size distribution and median diameter (μm) of the inorganic particle dispersion or slurry coating liquid were measured using a laser particle size distribution measuring device (Microtrac MT3300EX manufactured by Nikkiso Co., Ltd.). The particle size distribution was measured. If necessary, the particle size distribution of the inorganic particle dispersion liquid or slurry coating liquid was adjusted using the particle size distribution of water or the binder polymer as a baseline. The particle size at which the cumulative frequency was 50% was defined as _D50 , the particle size at which the cumulative frequency was 10% was defined as D10 _{, and} the particle size at which the cumulative frequency was 90% was defined as _D90 .

<Air permeability (sec/100cm ³ ) and air permeability ratio of multilayer porous membrane to polyolefin microporous membrane>
The air permeability of a multilayer porous membrane and the air permeability of a PO microporous membrane are measured by Toyo Seiki Co., Ltd. in accordance with JIS P-8117. ) to measure the air permeability resistance of the multilayer porous membrane and the PO microporous membrane in an atmosphere of 23°C and 40% humidity using the Gurley air permeability meter "G-B2 (trademark)" manufactured by It was carried out by
Further, the value obtained by subtracting the air permeability of the PO microporous membrane from the air permeability of the multilayer porous membrane was defined as the increase in air permeability of the multilayer porous membrane relative to the PO microporous membrane.

<Content of inorganic particles in porous layer (mass%)>
It can be calculated from the mixing ratio of the constituent materials when preparing the coating liquid.
Furthermore, from the viewpoint of detection from a multilayer porous membrane, it is also possible to measure weight changes of organic substances and inorganic particles using TG-DTA. Specifically, the porous layer portion of the multilayer porous membrane is scraped with a glass plate to collect 8 mg to 10 mg. The collected sample of the porous layer is set in a device and raised from room temperature to 600° C. at a rate of 10° C./min in an air atmosphere, and the change in weight is measured and calculated.

<Content of inorganic particles in porous layer (volume %)>
The content (volume %) of inorganic particles in the porous layer can be calculated from the composition ratio of the constituent materials and the specific gravity of each constituent material when preparing the coating liquid.

<Porosity (%)>
A 10 cm x 10 cm square sample was cut from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and the porosity was calculated from these and the membrane density (g/cm ³ ) using the following formula.
Porosity (%) = (volume - mass / density) / volume x 100

<Piercing strength (gf) and area weight conversion piercing strength (gf/(g/m ² ))>
Using a handy compression tester "KES-G5 (trademark)" manufactured by Kato Tech, the microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm. Next, by performing a puncture test on the central part of the fixed microporous membrane at a temperature of 23°C and a humidity of 40% with a needle tip radius of curvature of 0.5 mm and a puncture speed of 2 mm/sec, Raw puncture strength (gf) was obtained as the maximum puncture load. The value obtained by converting the obtained puncture strength (gf) into basis weight (gf/(g/m ² )) was also calculated.

<Heat shrinkage rate (%) at 130°C and 150°C>
As a sample, a multilayer porous membrane was cut into pieces of 100 mm in the MD direction and 100 mm in the TD direction, and left in an oven at 130°C or 150°C for 1 hour. At this time, the sample was sandwiched between 10 sheets of paper so that the hot air did not directly hit the sample. After taking out the sample from the oven and cooling it, the length (mm) was measured, and the heat shrinkage rate was calculated using the following formula. The measurement was performed in the MD direction and the TD direction, and the average of both values was expressed as the heat shrinkage rate.
Heat shrinkage rate (%) = {(100-length after heating)/100} x 100

<Glass transition temperature of thermoplastic polymer (℃)>
An appropriate amount of a thermoplastic polymer coating solution (non-volatile content = 30%) was placed on an aluminum plate and dried for 30 minutes in a hot air dryer at 130°C. Approximately 17 mg of the dried film was packed into an aluminum container for measurement, and a DSC curve and a DDSC curve were obtained in a nitrogen atmosphere using a DSC measuring device (manufactured by Shimadzu Corporation, DSC6220). The measurement conditions were as follows.
(1st stage temperature increase program)
Start at 70°C and increase temperature at a rate of 15°C per minute. After reaching 110°C, maintain for 5 minutes.
(Second stage temperature reduction program)
The temperature drops from 110℃ at a rate of 40℃ per minute. After reaching -50°C, maintain for 5 minutes.
(3rd stage temperature increase program)
The temperature increases from -50℃ to 130℃ at a rate of 15℃ per minute. DSC and DDSC data are acquired during this third stage of temperature rise.
The intersection of the baseline (the straight line extending the baseline in the obtained DSC curve toward the high temperature side) and the tangent at the inflection point (the point where the upwardly convex curve changes to a downwardly convex curve) is the glass transition temperature ( Tg).

<Dot diameter and distance between dots>
For the thermoplastic polymer-containing coating solution, the dot diameter of the coating pattern was measured using a microscope (model: VHX-7000, manufactured by Keyence Corporation). A sample separator was photographed at 100 times magnification (coaxial epi-illumination), the diameters of multiple (5 points) dots were measured in measurement mode, and the average value thereof was calculated as the dot diameter. In addition, the distance from the outer edge of a certain dot to the outer edge of the nearest other dot is defined as the "inter-dot distance", and measurements are taken at five observation points in measurement mode, and the average value of these is taken as the inter-dot distance. calculate.

<Adhesion to electrode>
The multilayer porous membrane or separator obtained in each example and comparative example and the negative electrode as an adherend (manufactured by Enertech, negative electrode material: graphite, conductive agent: acetylene black, L/W: 20 mg/cm ² on both sides) , Cu current collector thickness: 10 μm, negative electrode thickness after pressing: 140 μm) were cut into a rectangular shape with a width of 15 mm and a length of 60 mm, and the thermoplastic polymer-containing layer of the multilayer porous membrane or separator and the negative electrode active material were cut. After obtaining a laminate by overlapping them so that they faced each other, the laminate was pressed under the following conditions.
Press pressure: 1MPa
Temperature: 90℃
Pressing time: 5 seconds After pressing, the laminate was pressed using force gauges ZP5N and MX2-500N (product names) manufactured by Imada Co., Ltd., by fixing the electrodes and gripping and pulling the multilayer porous membrane or separator. A 90° peel test was conducted at a peel rate of 50 mm/min to measure the peel strength. At this time, the average value of peel strength in a peel test over a length of 40 mm conducted under the above conditions was adopted as the adhesive strength with the electrode. When a separator that achieves an adhesive force of 2.1 N/m or less obtained by this method is used in a non-aqueous electrolyte battery, the adhesive force with the opposing positive electrode and negative electrode will be good.

<Battery evaluation>
(a. Preparation of positive electrode)
91.2 parts by mass of lithium nickel manganese cobalt composite oxide (Li[Ni _1/3 Mn _1/3 Co _1/3 ]O ₂ ) as a positive electrode active material, and 2 parts each of flaky graphite and acetylene black as conductive materials. .3 parts by mass and 4.2 parts by mass of polyvinylidene fluoride (PVdF) as a resin binder were prepared, and these were dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was coated on one side of a 20 μm thick aluminum foil serving as a positive electrode using a die coater so that the coating amount of the positive electrode active material was 120 g/m ² . After drying at 130° C. for 3 minutes, the positive electrode active material was compression-molded using a roll press so that the bulk density of the positive electrode active material was 2.90 g/cm ³ to obtain a positive electrode. This positive electrode was punched out into a circular shape with an area of 2.00 cm ² .

(b. Preparation of negative electrode)
Prepare 96.6 parts by mass of artificial graphite as a negative electrode active material, 1.4 parts by mass of ammonium salt of carboxymethyl cellulose and 1.7 parts by mass of styrene-butadiene copolymer latex as a resin binder, and disperse them in purified water. A slurry was prepared. This slurry was coated on one side of a 16 μm thick copper foil serving as a negative electrode current collector using a die coater so that the negative electrode active material was 53 g/m ² . After drying at 120° C. for 3 minutes, the negative electrode active material was compression molded using a roll press so that the bulk density of the negative electrode active material was 1.35 g/cm ³ to obtain a negative electrode. This was punched out into a circle with an area of 2.05 cm ² .

(c. Preparation of non-aqueous electrolyte)
A nonaqueous electrolyte was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl carbonate = 1:2 (volume ratio) to a concentration of 1.0 ml/L.

(d. Battery assembly)
The negative electrode, multilayer porous membrane, and positive electrode were stacked in this order from the bottom so that the active material surfaces of the positive and negative electrodes faced each other. A cell is obtained by storing this laminate in a stainless metal container with a lid, in which the container body and lid are insulated, so that the copper foil for the negative electrode and the aluminum foil for the positive electrode are in contact with the container body and the lid, respectively. Ta. This cell was dried at 70° C. for 10 hours under reduced pressure. Thereafter, a non-aqueous electrolyte was injected into the container in an argon box and the container was sealed to obtain an evaluation battery.

(e. Cycle test)
The battery assembled in the above (d. battery assembly) was charged to a battery voltage of 4.2V at a temperature of 25°C and a current value of 3mA (approximately 0.5C), and then the current value was increased to 3mA while maintaining 4.2V. The first charge after fabrication of the battery was performed for a total of about 6 hours by starting to squeeze the battery, and then the battery was discharged to a battery voltage of 3.0 V at a current value of 3 mA.
Next, at 25°C, charge the battery to a voltage of 4.2V with a current value of 6mA (approximately 1.0C), then hold the current at 4.2V and start reducing the current value from 6mA for a total of about 3 hours. The battery was charged, and then discharged to a battery voltage of 3.0 V at a current value of 6 mA, and the discharge capacity at that time was defined as a 1C discharge capacity (mAh).
Next, at 25°C, charge the battery to a voltage of 4.2V with a current value of 6mA (approximately 1.0C), then maintain the current value of 4.2V and start reducing the current value from 6mA. The battery was charged for a period of time, and then discharged to a battery voltage of 3.0 V at a current value of 60 mA (approximately 10 C), and the discharge capacity at that time was defined as a 10 C discharge capacity (mAh).
Thereafter, the battery was discharged to a final discharge voltage of 3 V at a discharge current of 1 C at a temperature of 25° C., and then charged to a final discharge voltage of 4.2 V at a charging current of 1 C. This was regarded as one cycle, and charging and discharging were repeated. Then, the cycle characteristics were evaluated based on the following criteria using the capacity retention rate after 500 cycles with respect to the initial capacity (capacity in the first cycle).
Good: Capacity retention rate of 80% or more Bad: Capacity retention rate of less than 80%

[Example 1]
Using a tumble blender, 46.5% by mass of polyethylene (PE), a homopolymer with a viscosity average molecular weight (Mv) of 700,000, 46.5% by mass of PE, a homopolymer with Mv 250,000, and polypropylene (PP), a homopolymer with Mv 400,000. A 7% by weight polymer mixture was formed. To 99 parts by mass of the polymer mixture, 1 part by mass of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant, and the mixture was heated again in a tumbler blender. A mixture of polymers and the like was obtained by dry blending. The resulting mixture of polymers, etc. was purged with nitrogen and then fed to a twin-screw extruder by a feeder under a nitrogen atmosphere. Liquid paraffin (kinematic viscosity 7.59×10 ⁻⁵ m ² /s at 37.78° C.) was also injected into the extruder cylinder using a plunger pump.

The mixture was melt-kneaded and the feeder and pump were adjusted so that the liquid paraffin amount ratio in the entire extruded mixture was 68% by mass (resin composition concentration was 32% by mass). The melt-kneading conditions were a set temperature of 200° C., a screw rotation speed of 70 rpm, and a discharge rate of 145 kg/h.

Subsequently, the melt-kneaded product was extruded and cast through a T-die onto a cooling roll whose surface temperature was controlled to 25°C, to obtain a gel sheet with a thickness of 1350 μm.

Next, the gel sheet was introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched. The stretching conditions were as follows: MD magnification 7.0 times, TD magnification 6.38 times, and temperature set at 122°C. Next, the sample was introduced into a methylene chloride bath, and thoroughly immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, the methylene chloride was removed by drying to obtain a porous body.

Next, the porous body was introduced into a TD tenter and heat-set. The heat setting temperature was 132° C., the maximum TD magnification was 1.85 times, the relaxation rate was 0.784, and a polyolefin microporous membrane with a thickness of 12 μm was obtained as a base material.

Next, to barium sulfate having the average particle size, particle size distribution, and BET specific surface area shown in Table 1, an appropriate amount of water and an aqueous sodium polycarboxylate solution as a dispersant were added to the solids of each component except for water. 0.7% by mass (solid content equivalent) was mixed and stirred and dispersed.
At that time, bead milling was performed, if necessary, under the conditions of a bead diameter of 0.1 mm and a rotation speed in the mill of 3000 rpm. A coating solution was prepared by mixing poly(meth)acrylamide as a water-soluble polymer and acrylic latex as a water-insoluble binder with the volume or mass content shown in Table 1 in the treated mixed solution. .
In Table 1, "vol%" represents the volume ratio of each component when the total of barium sulfate, sodium polycarboxylate, poly(meth)acrylamide, and acrylic latex is 100 volume%, and ""wt%" represents the mass ratio of each component when the total of barium sulfate, sodium polycarboxylate, poly(meth)acrylamide, and acrylic latex is 100% by mass.

After corona discharge treatment was performed on the surface of the polyolefin microporous membrane, a coating liquid was applied to the treated surface using a gravure coater. Thereafter, the coating solution on the microporous polyolefin membrane was dried at 60°C to remove water, and a coating layer containing 90.5% by volume of barium sulfate was coated on one side of the microporous polyolefin membrane to a thickness of 1.5 μm. A porous layer was formed to obtain a multilayer porous membrane. The physical properties of the obtained multilayer porous membrane and the evaluation results of a battery equipped with the multilayer porous membrane as a separator are also listed in Table 1.

[Examples 2 to 16 and Comparative Examples 1 to 11]
The base film thickness of the polyolefin microporous membrane, the raw material type of the porous layer, the coating liquid composition and coating conditions, and the presence or absence of dot pattern printing of the thermoplastic polymer-containing layer were set as shown in Table 1. A multilayer porous membrane was formed in the same manner as in Example 1 except for this. Various properties of the obtained multilayer porous membrane and a battery equipped with the same as a separator were evaluated by the above methods. The evaluation results are shown in Table 1.

In Examples 14 to 16, 80 parts by mass of adhesive resin (acrylic polymer, glass transition temperature 90°C, average particle size 380 nm, electrolyte swelling degree 2.8) and adhesive resins having different glass transition temperatures were used. (acrylic polymer, glass transition temperature -6°C, average particle size 132 nm, electrolyte swelling degree 2.5) is mixed with 20 parts by mass, and ion-exchanged water is added to the adhesive resin-containing coating solution (adhesive resin A concentration of 3% by mass) was prepared. An adhesive resin-containing coating liquid was applied in dots using a gravure coater onto the porous layer of a microporous polyolefin membrane having a porous layer to form a dot pattern having the dot diameter and inter-dot distance shown in Table 1. Thereafter, water was removed by drying at 60°C.

Claims (9)

A multilayer porous membrane comprising a microporous membrane containing a polyolefin resin as a main component, and a porous layer laminated on at least one side of the microporous membrane and containing inorganic particles and a binder polymer,
the inorganic particles are barium sulfate;
the binder polymer includes poly(meth)acrylamide,
The average particle diameter D ₅₀ of the inorganic particles is 0.01 μm or more and 0.60 μm or less, and the ratio D ₉₀ /D ₅₀ of the average particle diameter D ₉₀ of the inorganic particles to the average particle diameter D ₅₀ of the inorganic particles is , 1.6 or more and 2.5 or less,
Multilayer porous membrane. The multilayer porous membrane according to claim 1, wherein the ratio D ₁₀ /D ₅₀ of the average particle diameter _{D 10} of the inorganic particles to the average particle diameter D 50 of the inorganic particles is 0.2 or more and 0.8 or less. The ratio T/TB of the thickness TB of the microporous membrane and the thickness T of the porous layer is 0.05 or more and 0.25 or less, and when the multilayer porous membrane is left in an atmosphere at a temperature of 150° C. for 1 hour. The multilayer porous film according to claim 1 or 2, wherein the average thermal shrinkage rate of the multilayer porous film in the MD direction and the TD direction is 0% or more and 10% or less. The multilayer porous membrane according to claim 3, wherein the inorganic particles have a BET specific surface area of 3.0 m ² /g or more and 50.0 m ² /g or less. 5. The multilayer porous membrane according to claim 4, wherein the proportion of the inorganic particles is greater than 90% by volume and equal to or less than 99% by volume, with the volume of the porous layer excluding voids being 100% by volume. The multilayer porous membrane according to claim 5, wherein the ratio Wb/Wa of the volume proportion Wa of poly(meth)acrylamide in the porous layer to the volume proportion Wb of the water-insoluble binder in the porous layer is 1 or less. The multilayer porous membrane according to claim 6, wherein the porous layer has a thickness T of 2 μm or less. The multilayer porous membrane according to claim 7, wherein a thermoplastic polymer-containing layer containing a thermoplastic polymer is formed on at least one side of the multilayer porous membrane. The multilayer porous membrane according to claim 8, wherein the thermoplastic polymer includes a particulate polymer, and the thermoplastic polymer-containing layer has a dot-like pattern.