JP6406066B2 - Nonaqueous secondary battery porous membrane binder, nonaqueous secondary battery porous membrane composition, nonaqueous secondary battery porous membrane and nonaqueous secondary battery - Google Patents

Description

  The present invention relates to a binder for a nonaqueous secondary battery porous membrane, a composition for a nonaqueous secondary battery porous membrane, a porous membrane for a nonaqueous secondary battery, and a nonaqueous secondary battery.

  Non-aqueous secondary batteries such as lithium ion secondary batteries (hereinafter sometimes simply referred to as “secondary batteries”) have the characteristics of being small and light, having high energy density, and capable of repeated charge and discharge. Is used in a wide range of applications. The secondary battery generally includes an electrode (positive electrode, negative electrode) and a battery member such as a separator that separates the positive electrode and the negative electrode to prevent a short circuit between the positive electrode and the negative electrode.

Here, in recent years, in secondary batteries, battery members including a porous film as a protective layer are used for the purpose of improving heat resistance and strength.
Specific examples of the porous film include those formed by binding non-conductive particles such as organic particles and inorganic particles with a binder. Such a porous film is usually referred to as a slurry composition in which a porous film material such as non-conductive particles or a binder is dissolved or dispersed in a dispersion medium such as water (hereinafter referred to as “a composition for a porous film”). And the porous membrane composition is applied and dried on an electrode base material or a separator base material provided with an electrode mixture layer on a current collector.

In recent years, porous membranes have been actively improved for the purpose of further improving the performance of non-aqueous secondary batteries. Specifically, a technique has been conventionally proposed in which a porous film is firmly bonded to a substrate such as an electrode substrate or a separator substrate by using a particulate polymer and a water-soluble polymer in combination as a binder. ing. (For example, refer to Patent Documents 1 and 2).
For example, in patent document 1, 20-50 mass% of ethylenically unsaturated carboxylic acid monomer units, 40-70 mass% of (meth) acrylic acid ester monomer units, and a fluorine-containing (meth) acrylic acid ester monomer A porous film comprising a water-soluble polymer containing 1 to 20% by mass and having a glass transition temperature of 30 to 80 ° C. and a particulate polymer is excellent in adhesion strength with an electrode substrate or a separator substrate. It has been reported.
Moreover, in patent document 2, the porous film which used together the water-soluble polymer, such as a cellulose semisynthetic polymer, and the particulate polymer containing 0.5-40 mass% of monomer units containing a hydrophilic group. However, it has been reported that it has high binding properties and adheres closely to the electrode substrate.

International Publication No. 2013/125645 International Publication No. 2009/123168

Here, in recent years, in order to improve the performance of non-aqueous secondary batteries, for example, electrical characteristics (life characteristics, output characteristics, etc.), it is desired to further improve the adhesion of the porous membrane to the substrate. ing. Specifically, the porous film has a sufficient amount of powder fall (dropping of fine powder from the porous film) that occurs during winding after the porous film is manufactured or when it is cut and curved during the production of the secondary battery. In order to be able to suppress, not only the excellent adhesion with the base material before being incorporated in the secondary battery (that is, before immersion in the electrolyte solution) is required, but also when the substrate is incorporated in the secondary battery, In order to adhere firmly to the material, it is required to maintain excellent adhesiveness even after immersion in the electrolytic solution.
However, with the conventional technique described above, it has been difficult to produce a porous film having excellent adhesion to the substrate both before and after immersion in the electrolyte solution.

  Moreover, when forming the porous film on the substrate, the composition for the porous film used for forming the porous film is subjected to a high shearing force due to the rotation of the gravure roll, for example, when applied with a gravure coating apparatus. However, the composition for a porous film used for the formation of the above-mentioned conventional porous film has low dispersion stability when subjected to a high shearing force, and therefore the composition for a porous film is porous for a long time. When the film is formed or the rotational speed of the gravure roll is increased in order to perform high-speed molding, there is a problem that the contained components aggregate and it is difficult to obtain a porous film having a uniform thickness.

Therefore, the present invention is a non-aqueous system that can form a porous film that is excellent in adhesion to a substrate before and after immersion in an electrolytic solution, and that can enhance the stability of the composition for porous film under high shear. It aims at providing the binder for secondary battery porous membranes.
In addition, the present invention provides a composition for a porous membrane of a non-aqueous secondary battery that is excellent in stability under high shear and can form a porous membrane excellent in adhesion to a substrate before and after immersion in an electrolyte solution. The purpose is to provide.
And this invention provides the nonaqueous secondary battery provided with the porous film for nonaqueous secondary batteries which is excellent in adhesiveness with a base material before and behind electrolyte solution immersion, and the said porous film for nonaqueous secondary batteries Objective.

  The present inventor has intensively studied for the purpose of solving the above problems. The present inventor has identified a particulate polymer comprising a random copolymer containing a (meth) acrylic acid alkyl ester monomer unit and an aromatic monovinyl monomer unit in a ratio within a specific range, A polycarboxylic acid-based copolymer that is water-insoluble at a pH below and water-soluble at a specific pH or higher and contains a carboxylic acid group-containing monomer unit in a proportion within a specific range, and By using a porous membrane binder having a degree of swelling with respect to the non-aqueous electrolyte solution within a specific range, the porous membrane composition is secured before being immersed in the porous membrane while maintaining the stability under high shear. The inventors have found that excellent adhesion to the substrate can be exhibited both after immersion, and thus completed the present invention.

  That is, the present invention aims to advantageously solve the above problems, and the binder for a nonaqueous secondary battery porous membrane of the present invention comprises a particulate polymer and a polycarboxylic acid copolymer. The particulate polymer is a random copolymer containing 35% by mass or more of (meth) acrylic acid alkyl ester monomer units and 20% by mass to 65% by mass of aromatic monovinyl monomer units. The polycarboxylic acid-based copolymer contains 20% by mass or more and 50% by mass or less of carboxylic acid group-containing monomer units, has a pH of 6.5 or less and is water-insoluble, and has a pH of 8 or more and is water-soluble. And the swelling degree with respect to the non-aqueous electrolyte is more than 1 time and 2 times or less. Thus, a particulate polymer composed of a random copolymer containing a (meth) acrylic acid alkyl ester monomer unit and an aromatic monovinyl monomer unit within the above ranges, and a non-specific pH below a specific pH, respectively. A polycarboxylic acid-based copolymer that is water-soluble and water-soluble at a specific pH or higher and contains a carboxylic acid group-containing monomer unit in a specific range, and non-aqueous electrolysis If a porous membrane binder having a degree of swelling with respect to the liquid is within a specific range, the stability of the porous membrane composition using the binder under high shear can be improved, and the binder can be used. The adhesion of the porous film formed by using the base material before and after the electrolytic solution immersion can be made excellent.

  Here, the binder for a nonaqueous secondary battery porous membrane of the present invention preferably contains 0.5 to 50 parts by mass of the polycarboxylic acid copolymer per 100 parts by mass of the particulate polymer. . If the blending amount of the polycarboxylic acid copolymer with respect to the particulate polymer is within the above range, the stability of the composition for the porous membrane is further enhanced under high shear, and before and after immersion of the porous membrane in the electrolyte solution This is because it is possible to improve the electrical characteristics of the secondary battery by further reducing the moisture content of the porous film and reducing the amount of moisture brought into the secondary battery while further improving the adhesion of the secondary battery.

  Furthermore, the binder for a nonaqueous secondary battery porous membrane according to the present invention has a degree of swelling of the particulate polymer with respect to the nonaqueous electrolyte of more than 1 time and 2 times or less, and the non-reactivity of the polycarboxylic acid copolymer. The degree of swelling with respect to the aqueous electrolyte is preferably 2 to 5 times. If the degree of swelling of the particulate polymer and polycarboxylic acid copolymer with respect to the non-aqueous electrolyte solution is within the above-mentioned range, the adhesion of the porous membrane before and after immersion in the electrolyte solution is further improved, This is because electrical characteristics such as output characteristics can be improved.

  Moreover, this invention aims at solving the said subject advantageously, The composition for non-aqueous secondary battery porous films of this invention is for any of the above-mentioned non-aqueous secondary battery porous films. It contains a binder, non-conductive particles and water. By using any of the above-described compositions containing a binder for a nonaqueous secondary battery porous membrane for forming a porous membrane, a porous membrane having excellent adhesion to a substrate before and after immersion in an electrolytic solution can be obtained. Moreover, the composition containing any of the binders for a non-aqueous secondary battery porous membrane described above is excellent in stability under high shear and hardly aggregates during the formation of the porous membrane.

  Moreover, this invention aims at solving the said subject advantageously, The porous film for non-aqueous secondary batteries of this invention is formed from the above-mentioned composition for non-aqueous secondary battery porous films. It is characterized by that. The porous film is excellent in adhesiveness with the substrate before and after immersion in the electrolyte solution.

  Moreover, this invention aims at solving the said subject advantageously, The non-aqueous secondary battery of this invention is equipped with the porous film for non-aqueous secondary batteries mentioned above. The non-aqueous secondary battery has excellent durability and high performance.

According to the present invention, a non-aqueous system capable of forming a porous film excellent in adhesiveness with a substrate before and after immersion in an electrolytic solution and also improving the stability of the composition for porous film under high shear A binder for a secondary battery porous membrane can be provided.
In addition, according to the present invention, the composition for a porous membrane of a non-aqueous secondary battery is excellent in stability under high shear and can form a porous membrane excellent in adhesion to a substrate before and after immersion in an electrolyte. Things can be provided.
And according to this invention, the non-aqueous secondary battery provided with the porous film for non-aqueous secondary batteries which is excellent in adhesiveness with a base material before and after electrolyte solution immersion, and the said porous film for non-aqueous secondary batteries is provided. be able to.

Hereinafter, embodiments of the present invention will be described in detail.
Here, the binder for nonaqueous secondary battery porous membranes of the present invention is used as a material for preparing a composition for nonaqueous secondary battery porous membranes. And the composition for non-aqueous secondary battery porous films of this invention is prepared using the binder for non-aqueous secondary battery porous films of this invention. Moreover, the porous film for nonaqueous secondary batteries of this invention is formed using the composition for nonaqueous secondary battery porous films of this invention. In addition, the non-aqueous secondary battery of the present invention is such that at least one battery member includes the porous membrane for a non-aqueous secondary battery of the present invention.

(Binder for non-aqueous secondary battery porous membrane)
The binder for a porous film of the present invention is a composition containing a particulate polymer and a polycarboxylic acid copolymer having a binding force, other components, and a dispersion medium such as water.
And the composition for porous films containing the binder for porous films of this invention is excellent in stability under high shear. Moreover, the porous film formed using the binder for porous films of this invention is excellent in adhesiveness with a base material both before and after electrolyte solution immersion.

<Particulate polymer>
The particulate polymer is at least the following (1), (2):
(1) 35% by mass or more of (meth) acrylic acid alkyl ester monomer unit, 20% by mass to 65% by mass of aromatic monovinyl monomer unit,
(2) a random copolymer;
It has the characteristics of.
Such a particulate polymer has excellent binding force, in particular, it is difficult for components to elute into the electrolyte, and it can retain sufficient binding strength even after immersion in the electrolyte, contributing to ensuring the durability of the porous membrane. To do. In addition, due to the contribution including the aromatic monovinyl monomer unit at a predetermined ratio, the composition for porous membrane has the property of improving the stability under high shear. In general, the particulate polymer is not water-soluble and is present in particulate form in the aqueous medium.
In the present invention, “comprising a monomer unit” means “a monomer-derived structural unit is contained in a polymer obtained using the monomer”. In the present invention, “(meth) acryl” means acrylic and / or methacrylic.

[(Meth) acrylic acid alkyl ester monomer unit]
Examples of (meth) acrylic acid alkyl ester monomers that can form (meth) acrylic acid alkyl ester monomer units include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, and t-butyl. Alkyl acrylates such as acrylate, isobutyl acrylate, n-pentyl acrylate, isopentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate Esters: methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate , Methacrylic acid alkyl esters such as stearyl methacrylate and glycidyl methacrylate; These can be used alone or in combination of two or more. Among these, 2-ethylhexyl acrylate, ethyl acrylate, butyl acrylate, and methyl methacrylate are preferable from the viewpoint of improving the adhesion between the porous film and the base material before and after immersion in the electrolytic solution. From the viewpoint of further improving the adhesive property of the porous film with the base material before and after the above-mentioned electrolytic solution immersion liquid and reducing the water content of the porous film to improve the electrical characteristics of the secondary battery, 2 -Ethylhexyl acrylate is more preferred. From the viewpoint of improving the electrical characteristics (especially the life characteristics) of the secondary battery by reducing the amount of moisture brought into the secondary battery due to the particulate polymer and suppressing the decomposition of the electrolyte in the electrolyte. The (meth) acrylic acid alkyl ester monomer preferably does not have a hydrophilic group such as an acidic group (for example, a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, etc.).

  The content ratio of the (meth) acrylic acid alkyl ester monomer unit in the particulate polymer needs to be 35% by mass or more, preferably 45% by mass or more, more preferably 55% by mass or more. The content is preferably 80% by mass or less, more preferably 70% by mass or less, still more preferably 65% by mass or less, and particularly preferably 60% by mass or less. When the content ratio of the (meth) acrylic acid alkyl ester monomer unit is within the above range, the binding force of the particulate polymer is ensured, and the degree of swelling of the particulate polymer with respect to the non-aqueous electrolyte is moderate. It is possible to suppress elution of components due to excessive swelling. And the adhesiveness with the base material before and behind the electrolyte solution immersion of a porous film improves, and the lifetime characteristic and output characteristic of a secondary battery can be improved.

[Aromatic monovinyl monomer unit]
Examples of the aromatic monovinyl monomer that can form an aromatic monovinyl monomer unit include styrene, styrene sulfonic acid and its salt (for example, sodium styrene sulfonate), α-methylstyrene, p-tert-butoxystyrene, Vinyl toluene etc. are mentioned. These can be used alone or in combination of two or more. Among these, styrene, sodium styrenesulfonate, and p-tert-butoxystyrene are preferable, and styrene is more preferable. In addition, the amount of moisture brought into the secondary battery due to the particulate polymer is surely reduced to suppress the decomposition of the electrolyte in the electrolytic solution, thereby improving the electrical characteristics (especially the life characteristics) of the secondary battery. From the viewpoint, the aromatic monovinyl monomer preferably does not have a hydrophilic group such as an acidic group (for example, a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, etc.).

  The content of the aromatic monovinyl monomer unit in the particulate polymer needs to be 20% by mass or more and 65% by mass or less, preferably 25% by mass or more, more preferably 30% by mass or more, and further Preferably it is 35 mass% or more, Most preferably, it is 37 mass% or more, Preferably it is 55 mass% or less, More preferably, it is 45 mass% or less. When the content ratio of the aromatic monovinyl monomer unit is within the above-mentioned range, the degree of swelling of the particulate polymer with respect to the non-aqueous electrolyte can be set to an appropriate level, and elution of components due to excessive swelling can be suppressed. Therefore, the durability of the porous membrane can be improved by improving the adhesion with the base material after immersion in the electrolyte solution. Moreover, the stability under the high shear of the composition for porous films containing the binder for porous films can be improved. Furthermore, the moisture content of the porous membrane can be reduced to reduce the amount of moisture brought into the secondary battery. And the lifetime characteristic and output characteristic of a secondary battery can be improved.

[Other monomer units]
The particulate polymer may contain a monomer unit other than the above-described (meth) acrylic acid alkyl ester monomer unit and aromatic monovinyl monomer unit. Such other monomer units are not particularly limited, and examples thereof include acidic group-containing monomer units and crosslinkable monomer units.
As described above, the acidic group-containing monomer unit and the crosslinkable monomer unit are monomer units other than the (meth) acrylic acid alkyl ester monomer unit and the aromatic monovinyl monomer unit. Accordingly, the acidic group-containing monomer that can form an acidic group-containing monomer unit and the crosslinkable monomer that can form a crosslinkable monomer unit include the above-mentioned (meth) acrylic acid alkyl ester monomers. And aromatic monovinyl monomers (such as styrene sulfonic acid and its salts) are not included.

[[Acid group-containing monomer unit]]
Examples of the acidic group-containing monomer that can form an acidic group-containing monomer unit include a carboxylic acid group-containing monomer, a sulfonic acid group-containing monomer, and a phosphoric acid group-containing monomer.

Examples of the carboxylic acid group-containing monomer include ethylenically unsaturated monocarboxylic acid and derivatives thereof, ethylenically unsaturated dicarboxylic acid and acid anhydrides thereof, and derivatives thereof.
Examples of the ethylenically unsaturated monocarboxylic acid include acrylic acid, methacrylic acid, crotonic acid and the like. Examples of ethylenically unsaturated monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E monomethoxyacrylic. Examples include acid and β-diaminoacrylic acid.
Examples of the ethylenically unsaturated dicarboxylic acid include maleic acid, fumaric acid, itaconic acid, mesaconic acid and the like. Examples of acid anhydrides of ethylenically unsaturated dicarboxylic acids include maleic anhydride, acrylic anhydride, methyl maleic anhydride, dimethyl maleic anhydride, and the like. In addition, examples of ethylenically unsaturated dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, diphenyl maleate, nonyl maleate, decyl maleate , Dodecyl maleate, octadecyl maleate, fluoroalkyl maleate and the like.

Examples of the sulfonic acid group-containing monomer include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth) allyl sulfonic acid, (meth) acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methylpropane sulfonic acid, 3 -Allyloxy-2-hydroxypropane sulfonic acid etc. are mentioned.
In the present invention, “(meth) allyl” means allyl and / or methallyl.

Examples of phosphoric acid group-containing monomers include 2- (meth) acryloyloxyethyl phosphate, methyl-2- (meth) acryloyloxyethyl phosphate, ethyl- (meth) acryloyloxyethyl phosphate, and the like.
In the present invention, “(meth) acryloyl” means acryloyl and / or methacryloyl.

These can be used alone or in combination of two or more. Among these, the acidic group-containing monomer is preferably an acidic group-containing monomer having two or more acidic groups in one molecule. When the particulate polymer has an acidic group-containing monomer unit, the presence of the acidic group makes the porous membrane composition stable under high shear and the adhesion of the porous membrane to the substrate before and after immersion in the electrolyte solution. Can be further improved, and the degree of swelling of the particulate polymer with respect to the non-aqueous electrolyte can be increased to improve the output characteristics of the secondary battery. However, on the other hand, when the amount of the acidic group-containing monomer used in preparing the particulate polymer is increased, the amount of moisture brought into the secondary battery due to the particulate polymer is increased. Battery life characteristics are degraded. Therefore, while suppressing an increase in the amount of moisture brought into the secondary battery, the stability of the composition for the porous membrane under high shear, the adhesion of the porous membrane with the substrate before and after the electrolyte immersion, and the secondary From the viewpoint of enhancing the output characteristics of the battery, it is preferable to use an acidic group-containing monomer having two or more acidic groups in one molecule.
In addition, while improving the stability of the composition for porous membranes under high shear, it improves the durability by improving the adhesion with the substrate after immersion of the porous membrane in the electrolyte, and the output characteristics of the secondary battery From the viewpoint of enhancing the acid group-containing monomer, the acidic group-containing monomer is preferably a carboxylic acid group-containing monomer, a sulfonic acid group-containing monomer, or a phosphoric acid group-containing monomer, and more preferably a carboxylic acid group-containing monomer. preferable.
Therefore, from these viewpoints, a carboxylic acid group-containing monomer having two or more carboxylic acid groups as acidic groups in one molecule is preferable, and more specifically, the above-described ethylenically unsaturated dicarboxylic acid is preferable. Among these, maleic acid, fumaric acid, and itaconic acid are more preferable, and itaconic acid is more preferable.
That is, the particulate polymer preferably contains a monomer unit derived from an ethylenically unsaturated dicarboxylic acid, a monomer unit derived from maleic acid, a monomer unit derived from fumaric acid, and a single amount derived from itaconic acid. It is more preferable to include at least one selected from the group consisting of body units, and it is further preferable to include a monomer unit derived from itaconic acid.

The content ratio of the acidic group-containing monomer unit in the particulate polymer is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and further preferably 0.5% by mass or more, preferably Is 5% by mass or less, more preferably 2% by mass or less, and still more preferably 1.5% by mass or less. When the content ratio of the acidic group-containing monomer unit is 0.1% by mass or more, the stability of the composition for a porous membrane under high shear, the adhesiveness with the substrate before and after the electrolyte solution immersion of the porous membrane, And the output characteristics of the secondary battery are improved. On the other hand, when the content ratio of the acidic group-containing monomer unit is 5% by mass or less, the elution of the particulate polymer into the non-aqueous electrolyte solution is suppressed, and the porous film adheres to the substrate after being immersed in the electrolyte solution. In addition to improving the durability and durability, the amount of moisture brought into the secondary battery due to the particulate polymer is reduced, so that the electrical characteristics (especially the life characteristics) of the secondary battery are improved.
In addition, the composition for porous membranes using the binder for porous membranes containing the particulate polymer mentioned above is dispersible by the aromatic monovinyl monomer unit contained in the particulate polymer in a relatively high proportion. And improved stability under high shear. Therefore, in this particulate polymer, even when the content ratio of the acidic group-containing monomer unit is reduced to improve the durability of the porous membrane and reduce the amount of moisture brought into the secondary battery, The film composition can sufficiently exhibit stability under high shear.

[[Crosslinkable monomer unit]]
As the crosslinkable monomer capable of forming a crosslinkable monomer unit, a monomer capable of forming a crosslinked structure upon polymerization can be used. Specifically, a monofunctional monomer having a thermally crosslinkable crosslinkable group and one ethylenically unsaturated bond per molecule, and a polyfunctionality having two or more ethylenically unsaturated bonds per molecule Monomer. Examples of thermally crosslinkable groups contained in the monofunctional monomer include epoxy groups, N-methylolamide groups, oxetanyl groups, oxazoline groups, and combinations thereof. Examples of such a crosslinkable monomer include those described in International Publication No. 2012/029805.
And by containing a crosslinkable monomer unit, the adhesion to the substrate after immersion of the electrolyte solution in the porous membrane is improved and durability is improved, while the non-aqueous electrolyte solution of the particulate polymer described later is added. The degree of swelling can be made moderate.

The crosslinkable monomer may be hydrophobic or hydrophilic.
In the present invention, that the crosslinkable monomer is “hydrophobic” means that the crosslinkable monomer does not contain a hydrophilic group, and the crosslinkable monomer is “hydrophilic”. The term “crosslinkable monomer” includes a hydrophilic group. Here, the “hydrophilic group” in the crosslinkable monomer means a carboxylic acid group, a hydroxyl group, a sulfonic acid group, a phosphoric acid group, an epoxy group, a thiol group, an aldehyde group, an amide group, an oxetanyl group, or an oxazoline group.

Hydrophobic crosslinkable monomers include allyl (meth) acrylate, ethylene di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, trimethylol Polyfunctional (meth) acrylates such as propane-tri (meth) acrylate; polyfunctional allyl / vinyl ethers such as dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxyethane And divinylbenzene and the like.
Examples of the hydrophilic crosslinkable monomer include vinyl glycidyl ether, allyl glycidyl ether, methylol acrylamide, acrylamide, and allyl methacrylamide.
These can be used alone or in combination of two or more.
Among these, from the viewpoint of reducing the amount of moisture brought into the secondary battery and improving the electrical characteristics (especially life characteristics) of the secondary battery, a hydrophobic cross-linkable monomer is preferable, and allyl methacrylate is preferable. More preferred is ethylene dimethacrylate. And from a viewpoint of improving the stability under the high shear of the composition for porous films containing a porous film binder, ethylene dimethacrylate is especially preferable.
In the present invention, “(meth) acrylate” means acrylate and / or methacrylate.

  The content ratio of the crosslinkable monomer unit in the particulate polymer is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and further preferably 0.5% by mass or more, preferably It is 5 mass% or less, More preferably, it is 2 mass% or less, More preferably, it is 1.8 mass% or less, Most preferably, it is 1.5 mass% or less. When the content of the crosslinkable monomer unit is 0.1% by mass or more, the elution of the particulate polymer into the electrolytic solution is suppressed, and the adhesion of the porous membrane to the base material after immersion in the electrolytic solution is improved. To do. Further, the shape of the particulate polymer is hardly deformed even under high shear, and the stability of the porous membrane composition under high shear can be enhanced. On the other hand, when the content ratio of the crosslinkable monomer unit is 5% by mass or less, the adhesion of the particulate polymer can be sufficiently ensured, and the adhesion with the base material before and after immersion of the porous membrane in the electrolyte solution Will improve.

[Preparation of particulate polymer]
The particulate polymer is prepared by polymerizing a monomer composition containing the above-described monomer in an aqueous solvent described in, for example, JP-A-2014-160651. By starting the polymerization in the monomer state rather than the oligomer state obtained by polymerizing the monomers in the monomer composition to some extent, the formation of block copolymers and graft copolymers can be suppressed, and random copolymerization can be achieved. The particulate polymer of the present invention which is a polymer can be obtained.
Here, the content ratio of each monomer in the monomer composition is usually the same as the content ratio of the monomer units in the desired particulate polymer.
The polymerization mode of the particulate polymer is not particularly limited as long as the obtained particulate polymer becomes a random copolymer, and examples thereof include a solution polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method. Any method may be used. As the polymerization reaction, addition polymerization such as ionic polymerization, radical polymerization, and living radical polymerization can be used. And generally used emulsifiers, dispersants, polymerization initiators, polymerization aids and the like used for the polymerization can be used, and the amount used is also generally used.

[Properties of particulate polymer]
[[Random copolymer structure and glass transition temperature]]
The particulate polymer used in the present invention needs to be a random copolymer. In addition, even when a particulate polymer is provided with a core structure and a shell structure, when the shell structure is a random copolymer, it is included in the particulate polymer of the present invention.

When the particulate polymer is a random copolymer, the polymer can be homogenized, the durability of the polymer with respect to the electrolytic solution can be increased, and the dispersibility in the composition for a porous membrane can be improved. it can. Furthermore, since the viscosity of the composition for porous membrane can be suppressed, it becomes easy to remove moisture from the porous membrane during drying.
In the present invention, whether or not the particulate polymer is a random copolymer is determined by measuring the glass transition temperature.
Specifically, when the particulate polymer that is a copolymer has one glass transition temperature, the particulate polymer corresponds to a random copolymer. On the other hand, when the particulate polymer has two or more glass transition temperatures, the particulate polymer does not have a random copolymer structure and corresponds to a block copolymer or a graft copolymer.
In the present invention, the “glass transition temperature” of the particulate polymer can be measured using the measuring method described in the examples of the present specification.

  And the glass transition temperature of the particulate polymer which is a random copolymer becomes like this. Preferably it is 10 degrees C or less, More preferably, it is 5 degrees C or less, Most preferably, it is 0 degrees C or less. Moreover, although the minimum of the glass transition temperature of a particulate polymer is not specifically limited, Usually, it is -100 degreeC or more.

[[Swelling degree with respect to non-aqueous electrolyte]]
In the present invention, the “swelling degree with respect to the non-aqueous electrolyte” of the particulate polymer is the immersion when a film (binder film) formed from the particulate polymer is immersed in a specific non-aqueous electrolyte under predetermined conditions. It can be determined as a value (times) obtained by dividing the subsequent weight by the weight before immersion. Specifically, a binder film is formed using the method described in the examples of the present specification, and is described in the examples. Measure using the measurement method of
Here, the degree of swelling of the particulate polymer with respect to the non-aqueous electrolyte is preferably more than 1 time, more preferably 1.2 times or more, still more preferably 1.4 times or more, preferably 2 times or less, more Preferably it is 1.8 times or less, More preferably, it is 1.6 times or less. If the degree of swelling of the particulate polymer with respect to the non-aqueous electrolyte is more than 1 time, the adhesion of the porous membrane to the substrate after immersion in the electrolyte can be improved, and the output characteristics of the secondary battery can be improved. be able to. On the other hand, if the degree of swelling of the particulate polymer with respect to the non-aqueous electrolyte is 2 times or less, elution of the particulate polymer into the electrolyte can be suppressed, and the adhesion of the porous membrane after immersion in the electrolyte can be improved. it can.

[[Contact angle with water]]
In the present invention, the “contact angle with water” of the particulate polymer means the water droplet contact angle of a film (binder film) formed from the particulate polymer, and specifically, measured by the following method. can do.
The aqueous dispersion of particulate polymer is applied to an electrolytic copper foil (NC-WS (registered trademark) manufactured by Furukawa Electric) using a table coater, dried at 50 ° C. for 20 minutes, at 120 ° C. for 20 minutes, and dried with hot air Dry in a vessel to produce three 1 cm × 1 cm binder films (thickness 500 μm). A drop of distilled water was dropped on the film, and the contact angle of the formed water drop was measured with a contact angle meter (model CA-DT-A, mfd. Kyowa) under an RH condition of an ambient temperature of 23 degrees and 50%. Measured by Interface Science Co., Ltd. For each of the three binder films, the contact angle is measured at two points, and the average of the measured values for a total of six times is taken as the contact angle. In addition, the diameter of the water droplet of distilled water is 2 mm, and the value measured 1 minute after dropping the water droplet of distilled water is adopted as the numerical value of the contact angle appearing on the measuring meter.
The contact angle of the particulate polymer with water is preferably more than 80 °, more preferably 85 ° or more, particularly preferably 90 ° or more, preferably 120 ° or less, more preferably 115 ° or less, particularly Preferably it is 110 degrees or less.

[[Particle size]]
The volume average particle diameter D50 of the particulate polymer is preferably 100 nm or more, more preferably 110 nm or more, still more preferably 120 nm or more, preferably 500 nm or less, more preferably 400 nm or less, still more preferably 300 nm or less. . When the volume average particle diameter D50 of the particulate polymer is 100 nm or more, the aggregation of the particulate polymer is suppressed, and the stability of the porous membrane composition under high shear is improved. On the other hand, when the volume average particle diameter D50 of the particulate polymer is 500 nm or less, adhesion with the substrate before and after immersion of the porous membrane in the electrolytic solution is secured, and durability of the porous membrane can be improved.
The “volume average particle diameter D50” of the particulate polymer represents a particle diameter at which the cumulative volume calculated from the small diameter side is 50% in the particle size distribution (volume basis) measured by the laser diffraction method.

<Polycarboxylic acid copolymer>
The polycarboxylic acid-based copolymer has at least the following (1) and (2):
(1) 20% by mass or more and 50% by mass or less of a carboxylic acid group-containing monomer unit,
(2) pH is 6.5 or lower and water-insoluble, and pH is 8 or higher and water-soluble.
It has the characteristics of. In addition, the polycarboxylic acid copolymer is a monomer unit other than a carboxylic group-containing monomer unit, such as a (meth) acrylic acid alkyl ester monomer unit, a crosslinkable monomer unit, a reactive surfactant. Includes units.
Such a polycarboxylic acid-based copolymer has excellent binding force due to the contribution of the carboxylic acid group, and in addition, hardly aggregates, and has high storage stability of the binder for porous membranes and high composition of the porous membrane composition. Contributes to improved stability under shear.
Furthermore, since the polycarboxylic acid copolymer is water-insoluble at pH 6.5 or less, it exhibits acidity immediately after polymerization of the polymer under acidic conditions or after being combined with the particulate polymer. In the porous membrane binder, the solubility in water is extremely low. For this reason, the gelation is suppressed, and the polycarboxylic acid copolymer and the porous membrane binder containing the copolymer can be stably stored for a long period of time. On the other hand, since the polycarboxylic acid copolymer is water-soluble at a pH of 8 or more, in a porous membrane composition that is usually alkaline due to the influence of the isoelectric point of non-conductive particles, Thicken by dissolving well. Therefore, an appropriate viscosity can be imparted to the porous membrane composition, and stability under high shear can be ensured. In addition, the above-mentioned polycarboxylic acid-based copolymer having a pH of 6.5 or less and water-insoluble has a lower acid amount than that of a water-soluble polymer having a pH of 6.5 or less. The affinity of is low. Therefore, drainage of the porous membrane composition during drying is relatively good, and the moisture content of the resulting porous membrane can be reduced.

[Carboxylic acid group-containing monomer unit]
Examples of the carboxylic acid group-containing monomer that can form a carboxylic acid group-containing monomer unit include those described above in the section “Particulate Polymer”. These can be used alone or in combination of two or more. Among these, methacrylic acid, acrylic acid, and itaconic acid are preferable, and methacrylic acid is more preferable.

  The content ratio of the carboxylic acid group-containing monomer unit in the polycarboxylic acid-based copolymer needs to be 20% by mass or more and 50% by mass or less, preferably 23% by mass or more, more preferably 25% by mass. % Or more, more preferably 27% by mass or more, preferably 40% by mass or less, more preferably 35% by mass or less, and still more preferably 33% by mass or less. When the content ratio of the carboxylic acid group-containing monomer unit is 20% by mass or more, the adhesion with the base material before and after immersion of the porous membrane in the electrolyte solution can be improved. In addition, the stability of the porous membrane composition under high shear can be ensured. On the other hand, when the content ratio of the carboxylic acid group-containing monomer unit is 50% by mass or less, the moisture content of the porous film can be reduced to reduce the amount of moisture brought into the secondary battery. The electrical characteristics (especially life characteristics) of the secondary battery are improved. Moreover, the excessive thickening in the composition for porous films is suppressed, and the stability under the high shear of the composition for porous films can be ensured.

[(Meth) acrylic acid alkyl ester monomer unit]
Examples of the (meth) acrylic acid alkyl ester that can form a (meth) acrylic acid alkyl ester monomer unit include those described above in the section of “Particulate polymer”. These can be used alone or in combination of two or more. Here, in the (meth) acrylic acid alkyl ester, the number of carbon atoms of the alkyl group bonded to the non-carbonyl oxygen atom is preferably 1 or more and 20 or less, more preferably 1 or more and 10 or less, and still more preferably 1. It is 8 or less. When the number of carbon atoms of the alkyl group bonded to the non-carbonyl oxygen atom is within the above range, the polycarboxylic acid copolymer is easily polymerized, and the obtained polymer is excessively hydrophobic. Can be suppressed.

  The content ratio of the (meth) acrylic acid alkyl ester monomer unit in the polycarboxylic acid-based copolymer is preferably 50% by mass or more, more preferably 55% by mass or more, and further preferably 60% by mass or more. On the other hand, the upper limit is 70% by mass or less. When the content ratio of the (meth) acrylic acid alkyl ester monomer unit is 50% by mass or more, flexibility is imparted to the porous film, and the adhesion with the substrate before and after immersion of the porous film in the electrolytic solution is improved. Can do.

Here, in a particularly preferred embodiment, the polycarboxylic acid copolymer has, as a (meth) acrylic acid alkyl ester monomer unit, an alkyl group bonded to a non-carbonyl oxygen atom having 1 to 3 carbon atoms. It includes both (meth) acrylic acid alkyl ester monomer units (U1) and (meth) acrylic acid alkyl ester monomer units (U2) having 4 to 6 carbon atoms. Thus, the storage stability of the binder for porous membranes improves by combining a monomer unit (U1) and a monomer unit (U2).
Examples of the (meth) acrylic acid alkyl ester that can form the monomer unit (U1) include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, and isopropyl methacrylate. It is done. Among these, ethyl acrylate is preferable.
Examples of the (meth) acrylic acid alkyl ester that can form the monomer unit (U2) include n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, isopentyl acrylate, hexyl acrylate, and n-butyl methacrylate. , T-butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, and hexyl methacrylate. Of these, butyl acrylate is preferred.

The content ratio of the monomer unit (U1) in the polycarboxylic acid copolymer is preferably 30% by mass or more, more preferably 40% by mass or more, still more preferably 55% by mass or more, and preferably 70% by mass. % Or less, more preferably 65% by mass or less, and still more preferably 60% by mass or less. When the content ratio of the monomer unit (U1) is 30% by mass or more, the adhesion of the porous membrane with the base material before immersion in the electrolytic solution can be further improved, and when it is 70% by mass or less, Excessive swelling of the carboxylic acid copolymer in the electrolytic solution is suppressed, and the structure of the porous film can be sufficiently retained.
The content ratio of the monomer unit (U2) in the polycarboxylic acid copolymer is preferably 2% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, and preferably 20% by mass. % Or less, more preferably 15% by mass or less, and still more preferably 14% by mass or less. When the content ratio of the monomer unit (U1) is 2% by mass or more, the adhesion of the porous membrane with the base material before immersion in the electrolyte solution can be further improved. Excessive swelling of the carboxylic acid copolymer in the electrolytic solution is suppressed, and the structure of the porous film can be sufficiently retained.

  Moreover, the ratio (U1 / U2) of the content ratio of the monomer unit (U1) and the monomer unit (U2) in the polycarboxylic acid-based copolymer is preferably 1.0 or more on a mass basis. Preferably it is 2.0 or more, More preferably, it is 2.5 or more, Preferably it is 10.0 or less, More preferably, it is 7.0 or less, More preferably, it is 6.0 or less, Most preferably, it is 5.0 or less. When U1 / U2 is 1.0 or more, the polycarboxylic acid copolymer can be easily solubilized in water at a pH of 8 or more, and a sufficient viscosity can be expressed in the porous membrane composition. And generation | occurrence | production of coating defect can be reduced and electrical characteristics, such as a lifetime characteristic, can be improved. In addition, when U1 / U2 is 10.0 or less, flexibility is imparted to the porous film, and adhesion with the substrate before and after immersion of the porous film in the electrolytic solution can be improved.

[Crosslinkable monomer unit]
Examples of the crosslinkable monomer capable of forming a crosslinkable monomer unit include those described above in the section “Particulate polymer”. These can be used alone or in combination of two or more. Among these, ethylene dimethacrylate, allyl glycidyl ether, and glycidyl methacrylate are preferable.

  When the polycarboxylic acid copolymer includes a crosslinkable monomer unit, the content of the crosslinkable monomer unit is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and still more preferably. It is 0.5 mass% or more, Preferably it is 2.0 mass% or less, More preferably, it is 1.5 mass% or less, More preferably, it is 1.0 mass% or less. When the content ratio of the crosslinkable monomer unit is 0.1% by mass or more, the polycarboxylic acid copolymer is prevented from eluting into the electrolyte in the secondary battery, and the output characteristics of the secondary battery and Life characteristics can be enhanced. On the other hand, when the content ratio of the crosslinkable monomer unit is 2.0% by mass or less, flexibility is imparted to the porous film, and the adhesion with the base material before and after immersion of the porous film in the electrolyte solution can be improved. it can.

[Reactive surfactant unit]
The reactive surfactant unit refers to a structural unit having a structure obtained by polymerizing a reactive surfactant. Increasing the solubility of the polycarboxylic acid copolymer in water at pH 8 or higher and the dispersibility of the polycarboxylic acid copolymer in water at pH 6.5 or lower by including a reactive surfactant unit Can do. Examples of the reactive surfactant that can form a reactive surfactant unit include those described in JP-A-2014-160651. These can be used alone or in combination of two or more.

  Examples of suitable reactive surfactants include compounds represented by the following formula (I).

In the formula (I), R represents a divalent linking group. Examples of R include -Si-O- group, methylene group, phenylene group and the like. In formula (I), R ¹ represents a hydrophilic group. An example of R ¹ includes —SO ₃ NH ₄ . Furthermore, in the formula (I), n represents an integer of 1 to 100.

Another example of a suitable reactive surfactant has a structural unit having a structure formed by polymerizing ethylene oxide and a structural unit having a structure formed by polymerizing butylene oxide. Examples thereof include compounds having an alkenyl group having a terminal double bond and —SO ₃ NH ₄ (for example, trade names “Latemul PD-104” and “Latemul PD-105”, manufactured by Kao Corporation).

  When the polycarboxylic acid copolymer includes a reactive surfactant unit, the content of the reactive surfactant unit is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and still more preferably. Is 0.5% by mass or more, preferably 15% by mass or less, more preferably 10% by mass or less, and still more preferably 5% by mass or less. When the content ratio of the reactive surfactant unit is 0.1% by mass or more, aggregation of the polycarboxylic acid copolymer is suppressed, and a uniform porous film can be obtained. On the other hand, when the reactive surfactant unit content is 15% by mass or less, the moisture content of the porous battery can be reduced to reduce the amount of moisture brought into the secondary battery. Electrical characteristics (especially life characteristics) are improved.

[Other monomer units]
As long as the effects of the present invention are not significantly impaired, the polycarboxylic acid copolymer is a carboxylic acid group-containing monomer unit, a (meth) acrylic acid alkyl ester monomer unit, a crosslinkable monomer unit, a reaction Arbitrary structural units other than the functional surfactant unit may be included. Arbitrary structural units can be used individually by 1 type or in combination of 2 or more types. As an arbitrary structural unit, a fluorine-containing (meth) acrylic acid ester monomer unit is mentioned, for example. However, from the viewpoint of satisfactorily applying the porous membrane composition to the substrate, the content ratio of the fluorine-containing (meth) acrylate monomer unit in the polycarboxylic acid copolymer is less than 1% by mass. It is preferable that

[Preparation of polycarboxylic acid copolymer]
The polycarboxylic acid copolymer is prepared by polymerizing a monomer composition containing the above-described monomer in an aqueous solvent described in, for example, JP-A-2014-160651. Here, the content ratio of each monomer in the monomer composition is usually the same as the content ratio of the monomer units (and structural units) in the desired polycarboxylic acid copolymer.
The polymerization mode of the polycarboxylic acid copolymer is not particularly limited, and any method such as a solution polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method may be used. As the polymerization reaction, for example, addition polymerization such as ionic polymerization, radical polymerization, and living radical polymerization can be used. And generally used emulsifiers, dispersants, polymerization initiators, polymerization aids and the like used for the polymerization can be used, and the amount used is also generally used.

  By the method described above, an aqueous dispersion in which a polycarboxylic acid copolymer is dispersed in an aqueous solvent having a pH of usually 6.5 or less is obtained. Although the polycarboxylic acid copolymer may be taken out from the obtained aqueous dispersion, the present invention is usually combined with the aqueous dispersion of the polycarboxylic acid copolymer and the aqueous dispersion of the particulate polymer. A binder for a porous film is prepared.

The aqueous dispersion of the polycarboxylic acid copolymer obtained as a result of the polymerization reaction is usually acidic with a pH of 6.5 or less as described above, but it is adjusted to increase the pH if necessary. May be. Thus, when adjusting the pH of the aqueous dispersion of the polycarboxylic acid copolymer, the pH after the adjustment is preferably 3.0 or more, more preferably 3.2 or more, and even more preferably 3.5 or more. , Preferably 6.5 or less. If the aqueous dispersion of the polycarboxylic acid copolymer is too low in pH, it tends to agglomerate when the pH suddenly rises when mixed with other alkaline materials. On the other hand, if the pH is too high, the polycarboxylic acid copolymer is dissolved and easily aggregated when mixed with the aqueous dispersion of the particulate polymer. Therefore, by adjusting the pH of the aqueous dispersion of the polycarboxylic acid copolymer to an appropriate range, a favorable binder for a porous film and a composition for a porous film can be obtained.
Examples of the method for increasing the pH of the aqueous dispersion of the polycarboxylic acid copolymer include a method of adding an alkaline aqueous solution. Examples of alkaline components constituting the alkaline aqueous solution include ammonia (ammonium hydroxide); alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; alkalis such as calcium hydroxide and magnesium hydroxide. Examples include earth metal hydroxides. These can be used alone or in combination of two or more.

[Properties of polycarboxylic acid copolymer]
[[Water soluble and water insoluble]]
The polycarboxylic acid copolymer used in the present invention needs to have a pH of 6.5 or less and water insolubility, and a pH of 8 or more and water solubility.
Here, the polycarboxylic acid-based copolymer contains a carboxylic acid group-containing monomer unit at a high rate of at least 20% by mass, and the polymer containing a large amount of carboxylic acid groups in this way has a higher pH. There is a tendency for solubility in water to increase. Therefore, in the present invention, if the polycarboxylic acid copolymer is water-insoluble at pH 6.5, the polycarboxylic acid copolymer is water-insoluble at pH 6.5 or lower. If the polycarboxylic acid copolymer is water-soluble at pH 8, the polycarboxylic acid copolymer is water-soluble at pH 8 or higher.
In the present invention, the water-solubility and water-insolubility of the polycarboxylic acid copolymer at pH 6.5 and pH 8 can be determined by the following method.
First, an evaluation liquid (aqueous solution, aqueous dispersion) adjusted so that the polycarboxylic acid copolymer has a concentration of 10% by mass at each pH is prepared and stirred at 25 ° C. for 1 hour. In addition, this evaluation liquid is usually added to the aqueous dispersion of the acidic polycarboxylic acid copolymer obtained by using the above-described preparation method, and added with water or the above-described alkaline aqueous solution as necessary. The one with adjusted partial concentration and pH is used. Therefore, the evaluation liquid contains substantially no polymer other than the predetermined polycarboxylic acid copolymer.
The evaluation liquid after stirring described above is transferred to a cell having an optical path length of 30 mm, and the haze meter is used to measure the scattered light and the total transmitted light, and the haze of the evaluation liquid is determined by the following equation.
Haze (%) = scattered light / total light transmitted light × 100
When the haze of the evaluation solution is 60% or more, it is determined that the polycarboxylic acid copolymer is water-insoluble at that pH, and when it is less than 60%, the polycarboxylic acid copolymer at that pH. Is determined to be water-soluble.

In the above-described determination method, the haze of the polycarboxylic acid copolymer pH 6.5 evaluation solution is preferably 70% or more, more preferably 80% or more. By using a polycarboxylic acid copolymer having such a high haze at pH 6.5, that is, a high water-insoluble property, in the preparation of the porous membrane binder, aggregation in the porous membrane binder is suppressed. Therefore, preparation and storage of the porous membrane binder are facilitated.
In addition, the haze degree of the evaluation liquid (dispersion liquid) at pH 6.5 is less than 60%, that is, the polycarboxylic acid copolymer exhibiting water solubility at pH 6.5 or less has a high acid amount and has an affinity for moisture. Is expensive. Therefore, drainage of the porous membrane composition during drying is poor, the moisture content of the resulting porous membrane is increased, and the electrical characteristics of the secondary battery are lowered. In addition, since the thickening effect is very strong, the viscosity of the porous membrane composition tends to be excessively high. Therefore, it is necessary to reduce the solid content concentration of the porous membrane composition, and the production efficiency of the porous membrane cannot be ensured.
On the other hand, in the above-described determination method, the haze of the pH 8 evaluation solution of the polycarboxylic acid copolymer is preferably less than 50%, more preferably less than 40%. By using a polycarboxylic acid copolymer having such a low haze, that is, high water solubility at pH 8, for the preparation of a porous membrane binder, a sufficient viscosity can be expressed in the porous membrane composition containing the binder. The occurrence of coating defects can be reduced, and the output characteristics and life characteristics of the secondary battery can be improved.
In addition, the solubility (water-soluble, water-insoluble) of the polycarboxylic acid copolymer at a specific pH is, for example, the content ratio of the carboxylic acid group-containing monomer unit or the alkyl (meth) acrylate. It can be appropriately adjusted by changing the carbon number of the alkyl group bonded to the non-carbonyl oxygen atom of the ester monomer unit.

[[Swelling degree with respect to non-aqueous electrolyte]]
In the present invention, the “swelling degree with respect to the non-aqueous electrolyte” of the polycarboxylic acid-based copolymer is a predetermined condition in which a film (binder film) formed from the polycarboxylic acid-based copolymer is used as a specific non-aqueous electrolyte. Can be obtained as a value (times) obtained by dividing the weight after soaking by the weight before soaking. Specifically, a binder film is formed using the method described in the examples of this specification. The measurement is performed using the measurement method described in the same example.
Here, the degree of swelling of the polycarboxylic acid copolymer with respect to the non-aqueous electrolyte is preferably 2 times or more, more preferably 2.5 times or more, preferably 5 times or less, more preferably 4 times or less. is there. If the degree of swelling of the polycarboxylic acid copolymer with respect to the non-aqueous electrolyte is 2 times or more, the adhesion of the porous membrane to the substrate after immersion in the electrolyte can be improved, and the output characteristics of the secondary battery Can be improved. Further, when the degree of swelling of the polycarboxylic acid copolymer with respect to the non-aqueous electrolyte solution is 5 times or less, the dissolution of the polycarboxylic acid copolymer into the electrolyte solution is suppressed, and the base after the porous membrane is immersed in the electrolyte solution is suppressed. Adhesiveness with the material can be improved.

[[Weight average molecular weight]]
The weight average molecular weight of the polycarboxylic acid copolymer is preferably 20,000 or more, more preferably 50,000 or more, still more preferably 100,000 or more, preferably 500,000 or less, more preferably 250, 000 or less. When the weight average molecular weight of the polycarboxylic acid copolymer is 20,000 or more, the strength of the polycarboxylic acid copolymer can be increased and a stable porous film can be formed. The high temperature storage characteristics of the secondary battery can be improved. On the other hand, when the weight average molecular weight of the polycarboxylic acid copolymer is 500,000 or less, the polycarboxylic acid copolymer can be softened. Adhesion can be improved.
The “weight average molecular weight” of the polycarboxylic acid-based copolymer is polyethylene oxide using a solution obtained by dissolving 0.85 g / ml sodium nitrate in a 10 vol% aqueous solution of acetonitrile by GPC (gel permeation chromatography). It can be obtained as a conversion value.

[[Glass-transition temperature]]
The glass transition temperature of the polycarboxylic acid copolymer is preferably −50 ° C. or higher, more preferably −30 ° C. or higher, further preferably −10 ° C. or higher, preferably lower than 30 ° C., more preferably 25 ° C. or lower. More preferably, it is 20 degrees C or less. When the glass transition temperature of the polycarboxylic acid copolymer is −50 ° C. or higher, the solubility of the polycarboxylic acid copolymer in water at pH 8 or higher is increased, and a sufficient viscosity is expressed in the porous membrane composition. The occurrence of coating defects can be reduced and the life characteristics of the secondary battery can be improved. On the other hand, when the glass transition temperature of the polycarboxylic acid copolymer is less than 30 ° C., the porous film is given flexibility, and the adhesion with the base material before and after immersion of the porous film in the electrolyte solution can be improved. .
The “glass transition temperature” of the polycarboxylic acid copolymer can be measured according to the method for measuring the glass transition temperature of the particulate polymer.

<Blending ratio of particulate polymer and polycarboxylic acid copolymer>
The compounding ratio of the particulate polymer and the polycarboxylic acid copolymer in the binder for porous membrane is not particularly limited. For example, the binder for a porous film is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, further preferably 2 parts by mass or more, based on 100 parts by mass of the particulate carboxylic acid copolymer. In particular, it contains 3 parts by mass or more, preferably 50 parts by mass or less, more preferably 30 parts by mass or less, still more preferably 15 parts by mass or less, and particularly preferably 8 parts by mass or less. When the blending amount of the polycarboxylic acid copolymer is 0.5 parts by mass or more per 100 parts by mass of the particulate polymer, the adhesion with the base material before and after immersion of the porous membrane in the electrolytic solution is improved. On the other hand, when the blending amount of the polycarboxylic acid-based copolymer is 50 parts by mass or less per 100 parts by mass of the particulate polymer, adhesion with the substrate before and after immersion of the porous membrane in the electrolyte solution is improved. The water content in the porous film is reduced. And the stability under the high shear of the composition for porous films is securable.

<Preparation of binder for nonaqueous secondary battery porous membrane>
The method for preparing the porous membrane binder is not particularly limited. For example, when the preparation of the particulate polymer and the preparation of the polycarboxylic acid copolymer are both carried out in an aqueous medium and obtained as an aqueous dispersion, respectively. Is a porous membrane binder by combining an aqueous dispersion of a particulate polymer, a polycarboxylic acid copolymer and any other components. Here, as other components, other components described in the section “Composition for non-aqueous secondary battery porous membrane” described later can be given. Further, the ratio of the total amount of the particulate polymer and the polycarboxylic acid copolymer contained in the porous membrane binder is usually 10% by mass or more, preferably 20% by mass or more, more preferably 30% by mass or more. .

<Properties of binder for nonaqueous secondary battery porous membrane>
[Swelling degree with respect to non-aqueous electrolyte]
The “swelling degree with respect to the non-aqueous electrolyte” of the binder for porous membrane of the present invention is after immersion when a film (binder film) formed by molding the binder for porous membrane is immersed in a specific non-aqueous electrolyte under predetermined conditions. Can be obtained as a value (times) divided by the weight before dipping, specifically, a binder film is formed using the method described in the examples of the present specification, and is described in the examples. Measure using the measuring method.
Here, the degree of swelling of the porous membrane binder with respect to the non-aqueous electrolyte solution needs to be more than 1 time and 2 times or less, preferably 1.1 times or more, more preferably 1.2 times or more, and still more preferably. It is 1.3 times or more, preferably 1.9 times or less, more preferably 1.8 times or less, and still more preferably 1.7 times or less. When the swelling degree of the binder for the porous membrane with respect to the non-aqueous electrolyte is more than 1 time, the adhesion of the porous membrane to the base material after immersion in the electrolyte can be improved, and the output characteristics of the secondary battery are improved. be able to. On the other hand, when the degree of swelling of the binder for porous membrane with respect to the nonaqueous electrolytic solution is 2 times or less, elution of components in the porous membrane (particulate polymer, polycarboxylic acid copolymer, etc.) into the electrolytic solution is suppressed. The adhesion with the substrate after immersion of the porous membrane in the electrolytic solution can be improved.

[PH]
The binder for the porous membrane of the present invention has a pH of preferably 4.0 or higher, more preferably 4.5 or lower, and preferably 6.5 or lower. When the pH of the porous membrane binder is 4.0 or more, the stability of the polycarboxylic acid copolymer is improved, and aggregation during storage can be suppressed. And the adhesiveness with the base material before and behind electrolyte solution immersion of the porous film obtained can be improved. Moreover, pH adjustment of the composition for porous films at the time of preparation of the composition for porous films using the binder for porous films becomes easy. On the other hand, by setting the pH of the binder for porous membranes to 6.5 or less, it is possible to keep the polycarboxylic acid copolymer in a water-insoluble state and suppress aggregation and excessive thickening during storage.
In addition, pH of the binder for porous films can be adjusted suitably by adding acidic aqueous solution or alkaline aqueous solution.

(Composition for non-aqueous secondary battery porous membrane)
The composition for a non-aqueous secondary battery porous membrane of the present invention is an aqueous slurry composition in which the above-mentioned particulate polymer, polycarboxylic acid copolymer, and non-conductive particles are dispersed in water as a dispersion medium. is there. Incidentally, the particulate polymer and the polycarboxylic acid-based copolymer were contained in the binder for the nonaqueous secondary battery porous membrane of the present invention, and their preferred abundance ratio was determined in the porous membrane binder. Is the same as the preferred abundance ratio.
The composition for a porous film of the present invention is excellent in stability under high shear, and the porous film formed using the composition for a porous film of the present invention has a base material before and after immersion in an electrolyte solution. Excellent adhesion.

<Non-conductive particles>
Non-conductive particles are non-conductive and do not dissolve in water used as a dispersion medium in the composition for porous membranes and non-aqueous electrolytes of secondary batteries, and their shape is maintained in them. Particles. And since nonelectroconductive particle is electrochemically stable, it exists stably in a porous film under the use environment of a secondary battery. Since the porous membrane composition contains non-conductive particles, the network structure of the obtained porous membrane is moderately clogged, lithium dendrites and the like are prevented from penetrating the porous membrane, and the short circuit of the electrode is suppressed. This is because it can be made even more certain. As the non-conductive particles, for example, various inorganic particles and organic particles can be used.

Examples of the inorganic particles include aluminum oxide (alumina) and hydrates thereof, silicon oxide, magnesium oxide, titanium oxide, BaTiO ₂ , ZrO, alumina-sili force composite oxide and other oxide particles, aluminum nitride, boron nitride Nitride particles such as silicon, covalently-bonded crystal particles such as silicon and diamond, sparingly soluble ion crystal particles such as barium sulfate, calcium fluoride and barium fluoride, and clay fine particles such as talc and montmorillonite.
Examples of organic particles include polyethylene, polystyrene, polydivinylbenzene, styrene-divinylbenzene copolymer cross-linked products, and various types of cross-linked polymers such as polyimide, polyamide, polyamideimide, melamine resin, phenol resin, and benzoguanamine-formaldehyde condensate. Examples thereof include molecular particles and heat-resistant polymer particles such as polysulfone, polyacrylonitrile, polyaramid, polyacetal, and thermoplastic polyimide. Here, the organic particles differ from the above-mentioned particulate polymer in that the particulate polymer has binding ability, whereas the organic particles have no binding ability. The particulate polymer is not included.

Among these, from the viewpoint of improving the durability of the porous membrane and the electrical characteristics of the secondary battery including the porous membrane, the nonconductive particles are preferably inorganic particles, and aluminum oxide and hydrates thereof are more preferable. Boehmite is more preferable.
In addition, the particle size of nonelectroconductive particle is not specifically limited, It can be made to be the same as that of the nonelectroconductive particle conventionally used.

<Combination ratio of non-conductive particles and binder for porous film>
The compounding ratio of the non-conductive particles and the porous membrane binder in the porous membrane composition is not particularly limited. For example, in the composition for a porous membrane, the total amount of the particulate polymer and the polycarboxylic acid copolymer is preferably 1 part by mass or more, more preferably 2 parts by mass per 100 parts by mass of the nonconductive particles. As described above, the porous film binder is contained in an amount of 3 parts by mass or more, preferably 10 parts by mass or less, more preferably 8 parts by mass or less, and still more preferably 6 parts by mass or less. When the total amount of the particulate polymer and the polycarboxylic acid copolymer is 1 part by mass or more per 100 parts by mass of the non-conductive particles, adhesion to the substrate before and after immersion of the porous membrane in the electrolyte is ensured. The durability of the porous membrane can be improved, and if it is 10 parts by mass or less, the internal resistance of the secondary battery can be reduced and the output characteristics can be improved.

<Other ingredients>
The composition for porous films may contain other arbitrary components in addition to the components described above. The optional component is not particularly limited as long as it does not have an excessively unfavorable effect on the battery reaction in the secondary battery using the porous film. Further, the kind of the arbitrary component may be one kind or two or more kinds.
Examples of the optional component include a wetting agent, a leveling agent, an electrolytic solution decomposition inhibitor, a dispersant, and a water-soluble polymer.

<Preparation of composition for nonaqueous secondary battery porous membrane>
The method for preparing the composition for the porous film is not particularly limited. Usually, the porous film binder, the non-conductive particles, water, and any components used as necessary are mixed. can get. The mixing method is not particularly limited, but in order to disperse each component efficiently, mixing is performed using a disperser as a mixing device.
The disperser is preferably an apparatus capable of uniformly dispersing and mixing the above components. Examples include a ball mill, a sand mill, a pigment disperser, a crusher, an ultrasonic disperser, a homogenizer, and a planetary mixer. Among them, a high dispersion apparatus such as a bead mill, a roll mill, or a fill mix is particularly preferable because a high dispersion share can be added.

<Properties of composition for nonaqueous secondary battery porous membrane>
The pH of the composition for a porous membrane of the present invention is preferably more than 7.0, more preferably 7.5 or more, still more preferably 8.0 or more, and particularly preferably 8.5 or more. By setting the pH of the porous membrane composition to more than 7.0, the solubility of the polycarboxylic acid copolymer in water is increased, and the thickening effect imparts sufficient viscosity to the porous membrane composition. can do. And generation | occurrence | production of the defect of application | coating can be reduced and the lifetime characteristic and output characteristic of a secondary battery can be improved.
In addition, pH of the composition for porous films can be suitably adjusted by adjusting pH of the binder for porous films, or adding acidic aqueous solution or alkaline aqueous solution.

  And normally, what is necessary is just to arbitrarily set the solid content density | concentration of the composition for porous films in the range which the composition for porous films has the viscosity of the range which does not impair workability | operativity, when manufacturing a porous film. Specifically, the solid content concentration of the composition for a porous membrane can usually be 10 to 60% by mass.

(Porous membrane for non-aqueous secondary batteries)
For example, the above-described composition for a secondary battery porous membrane is applied to the surface of an appropriate base material to form a coating film, and the formed coating film is dried to provide a non-aqueous secondary battery on the base material. A porous film can be formed. This porous film is excellent in durability because it is excellent in adhesiveness with the substrate before and after immersion in the electrolyte.

Here, the base material which apply | coats the composition for porous films is a member used as the object which forms the coating film of the composition for porous films. There is no limitation on the substrate, for example, forming a coating film of the composition for porous film on the surface of the release substrate, drying the coating film to form a porous film, and peeling the release substrate from the porous film It may be. Thus, the porous film peeled off from the mold release substrate can be used for a secondary battery as a self-supporting film.
However, from the viewpoint of improving the production efficiency by omitting the step of peeling the porous film, it is preferable to use a separator substrate or an electrode substrate as the substrate. The porous film provided on the separator substrate and the electrode substrate can be suitably used as a protective layer for improving the heat resistance and strength of the separator and the electrode.

<Separator base material>
Although it does not specifically limit as a separator base material, Known separator base materials, such as an organic separator base material, are mentioned. Here, the organic separator substrate is a porous member made of an organic material, and examples of the organic separator substrate include a microporous film or a nonwoven fabric containing a polyolefin resin such as polyethylene and polypropylene, an aromatic polyamide resin, and the like. Among them, a polyethylene microporous film and a nonwoven fabric are preferable because of their excellent strength. In addition, the thickness of an organic separator base material can be made into arbitrary thickness, Usually, 0.5 micrometer or more, Preferably it is 5 micrometers or more, Usually, 40 micrometers or less, Preferably it is 30 micrometers or less, More preferably, it is 20 micrometers or less.

<Electrode base material>
Although it does not specifically limit as an electrode base material (a positive electrode base material and a negative electrode base material), The electrode base material with which the electrode compound-material layer was formed on the electrical power collector is mentioned.
Here, the current collector, the electrode active material in the electrode mixture layer (positive electrode active material, negative electrode active material) and the binder for electrode mixture layer (binder for positive electrode mixture layer, binder for negative electrode mixture layer) As the method for forming the electrode mixture layer on the current collector and the current collector, known methods can be used, and examples thereof include those described in JP2013-145663A.

<Method for forming porous film for non-aqueous secondary battery>
Examples of a method for forming a porous film on a substrate such as the above-described separator substrate or electrode substrate include the following methods.
1) A method of applying a composition for a porous membrane to the surface of a separator substrate or an electrode substrate (in the case of an electrode substrate, the surface on the electrode mixture layer side, the same applies hereinafter), and then drying;
2) A method of drying the separator substrate or electrode substrate after immersing the separator substrate or electrode substrate in the porous membrane composition;
3) A method for producing a porous film by applying a composition for a porous film on a release substrate and drying it, and transferring the resulting porous film to the surface of a separator substrate or an electrode substrate;
Among these, the method 1) is particularly preferable because the film thickness of the porous film can be easily controlled. Specifically, the method 1) includes a step of applying the porous membrane composition onto the substrate (application step), and drying the porous membrane composition applied onto the substrate to form a porous membrane. A process (porous film formation process) is provided.

In the coating process, the method for coating the porous membrane composition on the substrate is not particularly limited, and examples thereof include a doctor blade method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. A method is mentioned. Of these, the gravure method is preferable in that a uniform porous film can be obtained.
In the porous film forming step, the method for drying the composition for the porous film on the substrate is not particularly limited, and a known method can be used, for example, drying with hot air, hot air, low-humidity air, vacuum drying, A drying method by irradiation with infrared rays or electron beams can be mentioned. The drying conditions are not particularly limited, but the drying temperature is preferably 50 to 150 ° C., and the drying time is preferably 5 to 30 minutes.

  In addition, the separator and the electrode may be provided with components other than these separator base material or electrode base material, and the porous film of this invention mentioned above, unless the effect of this invention is impaired remarkably. For example, you may provide another layer between the separator base material or electrode base material, and the porous film of this invention as needed. In this case, the porous film of the present invention is indirectly provided on the surface of the separator substrate or the electrode substrate. Further, another layer may be provided on the surface of the porous membrane of the present invention.

  The thickness of the porous film formed on the substrate is preferably 0.01 μm or more, more preferably 0.1 μm or more, particularly preferably 1 μm or more, preferably 20 μm or less, more preferably 10 μm or less, particularly Preferably it is 5 micrometers or less. When the thickness of the porous film is 0.01 μm or more, the strength of the porous film can be sufficiently ensured, and when it is 20 μm or less, the diffusibility of the electrolytic solution is secured and the secondary film using the porous film is used. The output characteristics of the battery can be improved.

(Non-aqueous secondary battery)
The non-aqueous secondary battery of the present invention includes the above-described porous membrane for a non-aqueous secondary battery of the present invention. More specifically, the non-aqueous secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the above-described porous film for a non-aqueous secondary battery is a battery member of a positive electrode, a negative electrode, and a separator. Included in at least one.
Since the nonaqueous secondary battery of the present invention includes the porous film obtained from the composition for porous film of the present invention, it has excellent durability and high performance.

<Positive electrode, negative electrode and separator>
At least one of the positive electrode, the negative electrode, and the separator used in the secondary battery of the present invention includes a porous film. Specifically, as a positive electrode and a negative electrode having a porous film, an electrode in which a porous film is provided on an electrode substrate formed by forming an electrode mixture layer on a current collector can be used. Moreover, as a separator which has a porous film, the separator which provides a porous film on a separator base material, and the separator which consists of a porous film can be used. In addition, as an electrode base material and a separator base material, the thing similar to the thing quoted by the term of the "porous film for non-aqueous secondary batteries" can be used.
Moreover, as a positive electrode which does not have a porous film, a negative electrode, and a separator, it does not specifically limit, The electrode which consists of an electrode base material mentioned above, and the separator which consists of a separator base material mentioned above can be used.

<Electrolyte>
As the electrolytic solution, an organic electrolytic solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. As the supporting electrolyte, for example, a lithium salt is used in a lithium ion secondary battery. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and the like. Among these, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li are preferable because they are easily dissolved in a solvent and exhibit a high degree of dissociation. In addition, electrolyte may be used individually by 1 type and may be used in combination of 2 or more types. Usually, the lithium ion conductivity tends to increase as the supporting electrolyte having a higher degree of dissociation is used, so that the lithium ion conductivity can be adjusted depending on the type of the supporting electrolyte.

The organic solvent used in the electrolytic solution is not particularly limited as long as it can dissolve the supporting electrolyte. For example, in a lithium ion secondary battery, dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC). Carbonates such as propylene carbonate (PC), butylene carbonate (BC), and methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfolane, Sulfur-containing compounds such as dimethyl sulfoxide; are preferably used. Moreover, you may use the liquid mixture of these solvents. Among these, carbonates are preferable because they have a high dielectric constant and a wide stable potential region. Usually, the lower the viscosity of the solvent used, the higher the lithium ion conductivity tends to be, so the lithium ion conductivity can be adjusted depending on the type of solvent.
The concentration of the electrolyte in the electrolytic solution can be adjusted as appropriate. Moreover, you may add a known additive to electrolyte solution.

<Method for producing non-aqueous secondary battery>
A non-aqueous secondary battery, for example, stacks a positive electrode and a negative electrode through a separator, and rolls or folds this as necessary into a battery container, and injects an electrolyte into the battery container to seal it. Can be manufactured. Of the positive electrode, the negative electrode, and the separator, at least one member is a member with a porous film. Here, an expanded metal, an overcurrent prevention element such as a fuse or a PTC element, a lead plate, or the like may be placed in the battery container as necessary to prevent an increase in pressure inside the battery or overcharge / discharge. The shape of the battery may be any of a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, a flat shape, and the like.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited thereto. In addition, unless otherwise indicated, the part and% in a present Example are a mass reference | standard.
In Examples and Comparative Examples, the glass transition temperature of the particulate polymer, the degree of swelling of the particulate polymer and the polycarboxylic acid copolymer with respect to the non-aqueous electrolyte, and the pH 6.5 and pH 8 of the polycarboxylic acid copolymer Water-soluble and water-insoluble, the degree of swelling of the porous membrane binder with respect to the non-aqueous electrolyte, the stability of the porous membrane composition under high shear, the moisture content of the porous membrane, before immersion of the porous membrane in the electrolyte and The adhesion to the substrate after immersion, and the life characteristics and output characteristics of the lithium ion secondary battery were measured and evaluated by the following methods.

<Glass transition temperature>
The aqueous dispersion of the particulate polymer was dried for 3 days in an environment of 50% humidity and 23 to 25 ° C. to obtain a film having a thickness of 1 ± 0.3 mm. Using the dried film as a sample, a glass using a differential scanning calorimeter (DSC6220SII, manufactured by Nanotechnology) at a measurement temperature of −100 ° C. to 180 ° C. and a heating rate of 5 ° C./min according to JIS K7121. The transition temperature (° C.) was measured.
<Swelling degree of particulate polymer and polycarboxylic acid copolymer with respect to non-aqueous electrolyte>
Each aqueous dispersion of the particulate polymer and the polycarboxylic acid copolymer was dried for 3 days under an environment of 50% humidity and 23 to 25 ° C. to obtain a film having a thickness of 1 ± 0.3 mm. This film was cut into 1 cm × 1 cm to form a binder film, and the weight M0 was measured. Thereafter, the obtained film was placed in a nonaqueous electrolytic solution (solvent: EC / DEC / vinylene carbonate (VC) = 68.5 / 30 / 1.5 (volume ratio), electrolyte: LiPF _{6 having a} concentration of 1 M) at 60 ° C., Soaked for 72 hours. The non-aqueous electrolyte solution on the surface of the film after immersion was wiped off, and the weight M1 was measured. And the swelling degree with respect to non-aqueous electrolyte solution was computed according to the following formula.
Swelling degree with respect to non-aqueous electrolyte = M1 / M0
<Water-soluble and water-insoluble in polycarboxylic acid copolymer at pH 6.5 and pH 8>
An aqueous dispersion of a polycarboxylic acid-based copolymer was prepared as an evaluation liquid prepared to have a concentration of 10% by mass and pH 6.5, and an evaluation liquid prepared to have a concentration of 10% by mass and pH 8, respectively, at 25 ° C. Stir for 1 hour. The evaluation liquid after stirring was transferred to a cell having an optical path length of 30 mm, and the scattered light and total light transmitted light were measured using a haze meter, and the haze of the evaluation liquid was determined by the following formula.
Haze (%) = scattered light / total light transmitted light × 100
When the haze of the evaluation solution is 60% or more, it is determined that the polycarboxylic acid copolymer is water-insoluble at that pH, and when it is less than 60%, the polycarboxylic acid copolymer at that pH. Was determined to be water-soluble. When the pH is 6.0 and water-insoluble, the pH is determined to be water-insoluble at 6.0 or less, and when the pH is 8 and water-soluble, the pH is 8 or higher. It was determined that there was.
<Swelling degree of porous membrane binder with respect to non-aqueous electrolyte>
The binder for a nonaqueous secondary battery porous membrane was dried for 3 days in an environment of 50% humidity and 23 to 25 ° C. to obtain a film having a thickness of 1 ± 0.3 mm. This film was cut into 1 cm × 1 cm to form a binder film, and the weight M0 was measured. Thereafter, the obtained film was immersed in a nonaqueous electrolytic solution (solvent: EC / DEC / VC = 68.5 / 30 / 1.5 (volume ratio), electrolyte: LiPF _{6 having a} concentration of 1 M) at 60 ° C. for 72 hours. . The non-aqueous electrolyte solution on the surface of the film after immersion was wiped off, and the weight M1 was measured. And the swelling degree with respect to non-aqueous electrolyte solution was computed according to the following formula.
Swelling degree with respect to non-aqueous electrolyte = M1 / M0
<Stability of porous membrane composition under high shear>
The composition for porous membrane was applied on a separator substrate (made of polyethylene) using a gravure roll (number of lines: 95) under the conditions of a conveyance speed of 50 m / min and a gravure rotation ratio of 100%. The material was cut out and the coating amount M0 (mg / cm ² ) per unit area was calculated. Moreover, the coating amount M1 (mg / cm ² ) was similarly calculated after 1 hour from the start of coating. Then, a coating amount change rate ΔM (%) was calculated using an equation of ΔM = (| M0−M1 |) / M0 × 100 (%) and evaluated as follows. It shows that stability under the high shear of the composition for porous films is so high that this value is small.
A: Application rate change rate ΔM is less than 5% B: Application rate change rate ΔM is 5% or more and less than 10% C: Application rate change rate ΔM is 10% or more and less than 20% D: Application rate change rate ΔM is 20% or more <Moisture content of porous membrane>
A separator (with a porous film) was cut out to 10 cm × 10 cm to obtain a test piece. This test piece is left for 24 hours at a temperature of 25 ° C. and a humidity of 50%, and then a test piece is obtained by a Karl Fischer method (JIS K-0068 (2001) water vaporization method, vaporization temperature 150 ° C.) using a coulometric titration moisture meter. The water content W (ppm) was measured and evaluated as follows. The smaller this value, the smaller the water content in the porous film, and the smaller the amount of water brought into the secondary battery.
A: Moisture content W is 500 ppm or less B: Moisture content W is more than 500 ppm to 600 ppm or less C: Moisture content W is more than 600 ppm to 700 ppm or less D: Moisture content W is more than 700 ppm <base before immersion of electrolyte in porous membrane Adhesiveness with materials>
A separator (with a porous membrane) is cut into a rectangle with a length of 100 mm and a width of 10 mm to form a test piece, and a cellophane tape (as defined in JIS Z1522) is attached to the porous membrane surface with the porous membrane surface facing down. One end of the material was pulled in the vertical direction at a pulling speed of 100 mm / min to measure the stress (the cellophane tape was fixed to the test stand). The measurement was performed 3 times, the average value was calculated | required, this was made into peel strength, and it evaluated as follows. It shows that a porous film is excellent in adhesiveness with the base material (separator base material) before electrolyte solution immersion, so that peel strength is large.
A: Peel strength is 100 N / m or more B: Peel strength is 75 N / m or more and less than 100 N / m C: Peel strength is 50 N / m or more and less than 75 N / m D: Peel strength is 25 N / m or more and less than 50 N / m E: Peel strength is less than 25 N / m <Adhesiveness with porous substrate after immersion in electrolyte>
A separator (with a porous membrane) was cut into a rectangle having a length of 100 mm and a width of 10 mm to obtain a test piece, a non-aqueous electrolyte (solvent: EC / DEC / VC = 68.5 / 30 / 1.5 (volume ratio), electrolyte) : Impregnated with 1M LiPF ₆ ) for 1 minute. Then, it was immersed in diethyl carbonate, washed and dried at room temperature for 12 hours. When a cellophane tape (as defined in JIS Z1522) is applied to the surface of the porous membrane with the porous membrane surface down, and one end of the separator substrate is pulled in a vertical direction at a pulling speed of 100 mm / min and peeled off. The cellophane tape was fixed to a test stand. The measurement was performed 3 times, the average value was calculated | required, this was made into peel strength, and it evaluated as follows. It shows that a porous film is excellent in adhesiveness with the base material (separator base material) after electrolyte solution immersion, so that peel strength is large.
A: Peel strength is 100 N / m or more B: Peel strength is 75 N / m or more and less than 100 N / m C: Peel strength is 50 N / m or more and less than 75 N / m D: Peel strength is 25 N / m or more and less than 50 N / m E: Peel strength is less than 25 N / m <Life characteristics of lithium ion secondary battery>
About the produced lithium ion secondary battery, charging / discharging which is charged from 3V to 4.2V at 0.5C at 25 ° C and then discharged from 4.2V to 3V at 0.5C at 25 ° C was repeated 100 cycles. A value obtained by calculating the ratio of the 0.5C discharge capacity at the 100th cycle to the 0.5C discharge capacity at the 1st cycle as a percentage was regarded as a capacity maintenance ratio (%) and evaluated as follows. Larger values indicate less discharge capacity loss and better cycle characteristics.
A: Capacity maintenance ratio is 70% or more B: Capacity maintenance ratio is 60% or more and less than 70% C: Capacity maintenance ratio is 50% or more and less than 60% D: Capacity maintenance ratio is less than 50% <Output of lithium ion secondary battery Characteristics>
The manufactured lithium ion secondary battery was charged to 4.2 V by a constant current method with a charge rate of 0.2 C, and then discharged to 3.0 V at a discharge rate of 0.2 C, resulting in a value of 0.8. The battery capacity during 2C discharge was determined. Subsequently, after charging to 4.2 V by a constant current method with a charge rate of 0.2 C, the battery capacity at the time of 2 C discharge was determined by discharging to 3.0 V at a discharge rate of 2 C. The same measurement is performed on 10 cells, and the average value of the battery capacity at the time of 0.2C discharge (Cap0.2C) and the average value of the battery capacity at the time of 2C discharge (Cap2C) are calculated. did. And the value calculated | required by Cap2C / Cap0.2C * 100 was made into the capacity maintenance rate (%) at the time of 2C discharge, and it evaluated as follows. The larger this value, the higher the discharge capacity during high-rate (2C) discharge, and the better the output characteristics.
A: Capacity maintenance ratio during 2C discharge is 90% or more B: Capacity maintenance ratio during 2C discharge is 75% or more and less than 90% C: Capacity maintenance ratio during 2C discharge is 60% or more and less than 75% D: Capacity during 2C discharge Maintenance rate is less than 60%

Example 1
<Preparation of particulate polymer>
In a reactor equipped with a stirrer, 90 parts of ion-exchanged water and 0.5 part of potassium persulfate were supplied, the gas phase part was replaced with nitrogen gas, and the temperature was raised to 80 ° C. On the other hand, 40 parts of ion-exchanged water in another container, 0.8 part of sodium dodecylbenzenesulfonate as an emulsifier, and 58.2 parts of 2-ethylhexyl acrylate (2-EHA) as a (meth) acrylic acid alkyl ester monomer, Mix 40 parts of styrene (ST) as an aromatic monovinyl monomer, 0.8 part of itaconic acid (IA) as an acidic group-containing monomer, and 1.0 part of ethylene dimethacrylate (EDMA) as a crosslinkable monomer. As a result, a monomer composition was obtained. This monomer composition was continuously added to the reactor over 4 hours for polymerization. The reaction was carried out at 80 ° C. during the addition. After completion of the addition, 0.5 part of ammonium persulfate was added, and the reaction was terminated by further stirring at 80 ° C. for 3 hours to produce an aqueous dispersion of a particulate polymer. The pH of this aqueous dispersion was 2.5. Moreover, using this aqueous dispersion, the glass transition temperature and the degree of swelling with respect to the non-aqueous electrolyte were measured. The results are shown in Table 1.
<Preparation of polycarboxylic acid copolymer>
A reactor equipped with a stirrer is supplied with 100 parts of ion-exchanged water, 0.6 part of polyoxyalkylene alkenyl ether ammonium sulfate and 0.2 part of potassium persulfate, and the gas phase part is replaced with nitrogen gas. The temperature rose. On the other hand, 50 parts of ion-exchanged water in another container, 0.6 part of polyoxyalkylene alkenyl ether ammonium sulfate as an emulsifier, 30 parts of methacrylic acid (MAA) as a carboxylic acid group-containing monomer, and a single amount of (meth) acrylic acid alkyl ester The body was mixed with 57 parts of ethyl acrylate (EA) (corresponding to U1), 12.2 parts of butyl acrylate (BA) (corresponding to U2) and 0.8 part of ethylene dimethacrylate (EDMA) as the crosslinkable monomer. A monomer composition was obtained. This monomer composition was continuously added to the reactor over 3 hours for polymerization. The reaction was carried out at 70 ° C. during the addition. After completion of the addition, the temperature was raised to 80 ° C. and stirred for 3 hours to complete the reaction, and an aqueous dispersion of a polycarboxylic acid copolymer was produced. The pH of this aqueous dispersion was 2.0. Further, using this aqueous dispersion, water solubility and water insolubility at pH 6.5 and pH 8 were determined, and the degree of swelling with respect to the nonaqueous electrolyte solution was measured. The results are shown in Table 1.
<Preparation of binder for nonaqueous secondary battery porous membrane>
Mix the aqueous dispersion of the particulate polymer and the aqueous dispersion of the polycarboxylic acid copolymer so that the polycarboxylic acid copolymer is 5 parts per 100 parts of the particulate polymer, Further, an appropriate amount of 5% aqueous sodium hydroxide solution was added. Then, the porous membrane binder was prepared by stirring for 10 minutes at 300 rpm with a three-one motor. The pH of this porous membrane binder was 4.7. Moreover, the swelling degree with respect to non-aqueous electrolyte solution was measured using this binder for porous films. The results are shown in Table 1.
<Preparation of composition for nonaqueous secondary battery porous membrane>
With respect to 100 parts of boehmite filler ("BMM-W" manufactured by Kawai Lime Industry Co., Ltd.) as non-conductive particles, 0.5 part of polycarboxylic acid / ammonium sulfonate salt as a dispersant, the above-mentioned secondary battery porous membrane 4 parts of binder for solids, xanthan gum ("KELZAN" manufactured by Sanki Co., Ltd.) 0.2 part as a thickener, polyethylene glycol type surfactant ("San Nopco (registered trademark) SN wet 366" manufactured by San Nopco) ”) 0.2 part, and ion-exchanged water were added so as to have a solid concentration of 40% by mass and mixed to prepare a porous membrane composition. The pH of the composition for porous membrane was 9.0.
Using the obtained porous membrane composition, the stability under high shear was evaluated. The results are shown in Table 1.

<Manufacture of porous membrane and separator>
An organic separator substrate made of a polyethylene porous substrate (manufactured by Celgard, thickness 16 μm) was prepared. The porous membrane composition obtained as described above was applied to one side of the prepared organic separator substrate, and dried at 60 ° C. for 10 minutes. As a result, a separator having a thickness of 18 μm (including a porous film on the separator substrate) was obtained.
Using the obtained separator (with a porous film), the adhesion to the substrate and the water content before and after immersion of the porous film in the electrolytic solution were evaluated. The results are shown in Table 1.

<Manufacture of negative electrode>
In a 5 MPa pressure vessel with a stirrer, 33 parts of 1,3-butadiene, 3.5 parts of IA, 63.5 parts of ST, 0.4 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water and potassium persulfate as a polymerization initiator After adding 0.5 part and stirring sufficiently, it heated to 50 degreeC and superposition | polymerization was started. When the polymerization conversion rate reached 96%, the reaction was stopped by cooling to obtain a mixture containing the binder for negative electrode mixture layer (SBR). After adding a 5% aqueous sodium hydroxide solution to the mixture containing the binder for the negative electrode mixture layer and adjusting the pH to 8, the unreacted monomer is removed by heating under reduced pressure, and then up to 30 ° C. or lower. The mixture was cooled to obtain an aqueous dispersion containing the desired binder for the negative electrode mixture layer.
In a planetary mixer with a disper, artificial graphite (specific surface area 5.5 m ² / g: average particle size: 15.6 μm) 100 parts as a negative electrode active material, carboxymethylcellulose sodium salt as a thickener (Nippon Paper Chemical Co., Ltd.) 1 part of a 1% aqueous solution (manufactured “MAC-800LC”) corresponding to the solid content and ion-exchanged water were added to prepare a solid content concentration of 60%, followed by mixing at 25 ° C. for 60 minutes. Further, ion exchange water was added to this planetary mixer to adjust the solid content concentration to 52%, and further mixed at 25 ° C. for 15 minutes to obtain a mixed solution. To this mixed solution, 1.5 parts of the above binder for the negative electrode mixture layer and an amount of ion-exchanged water corresponding to the solid content are added to adjust the final solid content concentration to 45%, and mixed for 10 minutes. did. This was defoamed under reduced pressure to obtain a slurry composition for a secondary battery negative electrode having good fluidity.
The obtained negative electrode slurry composition was applied on a copper foil having a thickness of 20 μm, which was a current collector, with a comma coater so that the film thickness after drying was about 150 μm, and dried. This drying was performed by conveying the copper foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Thereafter, heat treatment was performed at 120 ° C. for 2 minutes to obtain a negative electrode raw material before pressing. The negative electrode raw material before pressing was rolled with a roll press to obtain a negative electrode after pressing with a negative electrode mixture layer thickness of 80 μm.

<Production of positive electrode>
100 parts of LiCoO ₂ having a volume average particle diameter of 12 μm as a positive electrode active material, ₂ parts of acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., “HS-100”) as a conductive material, and as a binder for a positive electrode mixture layer Polyvinylidene fluoride (manufactured by Kureha, “# 7208”) in an amount corresponding to the solid content and 2 parts of N-methylpyrrolidone were mixed to make the total solid content concentration 70%. These were mixed by a planetary mixer to prepare a positive electrode slurry composition.
The obtained positive electrode slurry composition was applied onto a 20 μm-thick aluminum foil as a current collector by a comma coater so that the film thickness after drying was about 150 μm and dried. This drying was performed by transporting the aluminum foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Thereafter, heat treatment was performed at 120 ° C. for 2 minutes to obtain a positive electrode raw material before pressing. The positive electrode raw material before pressing was rolled with a roll press to obtain a positive electrode after pressing with a positive electrode mixture layer thickness of 80 μm.

<Manufacture of lithium ion secondary batteries>
An aluminum packaging exterior was prepared as the battery exterior. The positive electrode obtained was cut into a square of 4.6 cm × 4.6 cm and arranged so that the surface on the current collector side was in contact with the aluminum packaging material exterior. The obtained separator was cut into a 6 cm × 6 cm square and arranged so that the porous film was in contact with the surface of the positive electrode mixture layer side of the positive electrode. Furthermore, the negative electrode obtained above was cut into a 5 cm × 5 cm square and arranged so that the surface of the negative electrode mixture layer side was in contact with the separator substrate surface of the separator. An electrolytic solution (solvent: EC / DEC / VC = 68.5 / 30 / 1.5 (volume ratio), electrolyte: LiPF ₆ having a concentration of 1 M) was injected so that no air remained. Further, in order to seal the opening of the aluminum packaging material, heat sealing at 150 ° C. was performed to close the exterior of the aluminum packaging material, thereby manufacturing a lithium ion secondary battery. Using this lithium ion secondary battery, output characteristics and life characteristics were evaluated. The results are shown in Table 1.

(Examples 2, 3, and 10)
In the same manner as in Example 1, except that the blending amount of the polycarboxylic acid copolymer per 100 parts of the particulate polymer in the porous membrane binder was changed as shown in Table 1, the particulate polymer and polycarboxylic acid were used. An acid copolymer, a porous membrane binder, a porous membrane composition, a separator (with a porous membrane), a negative electrode, a positive electrode, and a lithium ion secondary battery were produced, and the same items were evaluated. The results are shown in Table 1.

(Examples 4-6, 11)
In the same manner as in Example 1 except that the amount of 2-EHA and ST used was changed as shown in Table 1 during the preparation of the particulate polymer contained in the porous membrane binder, the particulate polymer and polycarboxylic acid type were used. A copolymer, a binder for a porous film, a composition for a porous film, a separator (with a porous film), a negative electrode, a positive electrode, and a lithium ion secondary battery were produced, and the same items were evaluated. The results are shown in Table 1.

(Example 7)
Except that butyl acrylate (BA) was used instead of 2-EHA as the (meth) acrylic acid alkyl ester monomer when preparing the particulate polymer contained in the porous membrane binder, the same procedure as in Example 1 was performed. Manufacturing particulate polymer, polycarboxylic acid copolymer, porous membrane binder, porous membrane composition, separator (with porous membrane), negative electrode, positive electrode, and lithium ion secondary battery. evaluated. The results are shown in Table 1.

(Examples 8 and 9)
In the same manner as in Example 1, except that the amounts of MAA, EA, and BA were changed as shown in Table 1 when preparing the polycarboxylic acid-based copolymer contained in the porous membrane binder, A polycarboxylic acid-based copolymer, a porous membrane binder, a porous membrane composition, a separator (with a porous membrane), a negative electrode, a positive electrode, and a lithium ion secondary battery were produced, and the same items were evaluated. The results are shown in Table 1.

(Comparative Example 1)
In the same manner as in Example 1, except that the blending amount of the polycarboxylic acid copolymer per 100 parts of the particulate polymer in the porous membrane binder was changed as shown in Table 1, the particulate polymer and polycarboxylic acid were used. An acid copolymer, a porous membrane binder, a porous membrane composition, a separator (with a porous membrane), a negative electrode, a positive electrode, and a lithium ion secondary battery were produced, and the same items were evaluated. The results are shown in Table 1.

(Comparative Example 2)
Instead of polycarboxylic acid copolymer, polyacrylic acid (“AQUALIC IH” manufactured by Nippon Shokubai Co., Ltd.) (100% monomer unit derived from acrylic acid (AA) as carboxylic acid group-containing monomer) ) Was used in the same manner as in Example 1 except that a particulate polymer, a porous membrane binder, a porous membrane composition, a separator (with a porous membrane), a negative electrode, a positive electrode, and a lithium ion secondary battery were produced. The same items were evaluated. The results are shown in Table 1.

(Comparative Example 3)
In the same manner as in Example 1 except that the amount of 2-EHA and ST used was changed as shown in Table 1 during the preparation of the particulate polymer contained in the porous membrane binder, the particulate polymer and polycarboxylic acid type were used. A copolymer, a binder for a porous film, a composition for a porous film, a separator (with a porous film), a negative electrode, a positive electrode, and a lithium ion secondary battery were produced, and the same items were evaluated. The results are shown in Table 1.

(Comparative Example 4)
A polycarboxylic acid copolymer, a porous membrane binder, a porous membrane composition, a porous membrane composition in the same manner as in Example 1 except that a particulate polymer that is a graft copolymer prepared as follows was used. A separator (with a porous film), a negative electrode, a positive electrode, and a lithium ion secondary battery were manufactured, and the same items were evaluated. The results are shown in Table 1.
<Preparation of particulate polymer (graft copolymer)>
In a reactor equipped with a stirrer, 230 parts of ion exchange water, 0.15 part of sodium lauryl sulfate (“Emal (registered trademark) 2F” manufactured by Kao Chemical Co., Ltd.) as an emulsifier, 50 parts of n-butyl acrylate (BA), Add 50 parts of styrene macromonomer (STMM, one-end methacryloylated polystyrene oligomer, “AS-6” manufactured by Toagosei Co., Ltd.), 1 part of t-butylperoxy-2-ethylhexanate as a polymerization initiator, After stirring, the mixture was polymerized by heating to 90 ° C. to obtain an aqueous dispersion of a particulate polymer. When the glass transition temperature of the obtained particulate polymer was measured, two glass transition temperatures (−40 ° C. and 90 ° C.) were confirmed, and it was confirmed that the particulate polymer was a graft copolymer.

From Examples 1 to 11 and Comparative Examples 1 to 4 in Table 1 above, a random copolymer containing (meth) acrylic acid alkyl ester monomer units and aromatic monovinyl monomer units in proportions within a specific range, respectively. A particulate polymer comprising a polymer and a polycarboxylic acid which is water-insoluble at a specific pH or lower and water-soluble at a specific pH or higher, and contains a carboxylic acid group-containing monomer unit in a specific range. And a porous membrane composition that is excellent in stability under high shear, and an electrolytic solution by using a porous membrane binder that includes an acid copolymer and has a degree of swelling with respect to a non-aqueous electrolytic solution within a specific range. It can be seen that a porous film excellent in adhesion to the substrate before and after immersion and having a low water content, and a secondary battery excellent in life characteristics and output characteristics can be obtained.
Here, from Examples 1 to 3 and 10 in Table 1 above, by adjusting the blending ratio of the carboxylic acid copolymer to the particulate polymer, adhesion to the substrate before and after immersion of the porous membrane in the electrolyte solution It can be seen that the secondary battery life characteristics and output characteristics can be further improved, and the water content of the porous film can be further reduced.
In addition, from Examples 1, 4 to 7 and 11 in Table 1 above, by changing the composition of the particulate polymer, stability of the porous membrane composition under high shear, immersion of the porous membrane in the electrolyte solution It can be seen that the adhesion to the substrate before and after, the life characteristics and output characteristics of the secondary battery can be further improved, and the moisture content of the porous film can be further reduced.
And from Examples 1, 8, and 9 in Table 1 above, by changing the composition of the carboxylic acid copolymer, the stability of the composition for porous membranes under high shear, the electrolyte immersion of the porous membranes It can be seen that the adhesion to the substrate before and after, the life characteristics and output characteristics of the secondary battery can be further improved, and the moisture content of the porous film can be further reduced.

According to the present invention, a non-aqueous system capable of forming a porous film excellent in adhesiveness with a substrate before and after immersion in an electrolytic solution and also improving the stability of the composition for porous film under high shear A binder for a secondary battery porous membrane can be provided.
In addition, according to the present invention, the composition for a porous membrane of a non-aqueous secondary battery is excellent in stability under high shear and can form a porous membrane excellent in adhesion to a substrate before and after immersion in an electrolyte. Things can be provided.
And according to this invention, the non-aqueous secondary battery provided with the porous film for non-aqueous secondary batteries which is excellent in adhesiveness with a base material before and after electrolyte solution immersion, and the said porous film for non-aqueous secondary batteries is provided. be able to.

Claims (6)

Including a particulate polymer and a polycarboxylic acid copolymer,
The particulate polymer is a random copolymer containing 35% by mass or more of (meth) acrylic acid alkyl ester monomer units and 20% by mass to 65% by mass of aromatic monovinyl monomer units,
The polycarboxylic acid-based copolymer contains 20% by mass or more and 50% by mass or less of a carboxylic acid group-containing monomer unit, is water-insoluble at pH 6.5 or less, and is water-soluble at pH 8 or more. ,
And the binder for non-aqueous secondary battery porous films whose swelling degree with respect to non-aqueous electrolyte solution is more than 1 time and 2 times or less.   The binder for a nonaqueous secondary battery porous film according to claim 1, comprising 0.5 to 50 parts by mass of the polycarboxylic acid copolymer per 100 parts by mass of the particulate polymer.   The degree of swelling of the particulate polymer with respect to the non-aqueous electrolyte is more than 1 to 2 times or less, and the degree of swelling of the polycarboxylic acid copolymer with respect to the non-aqueous electrolyte is 2 to 5 times, The binder for non-aqueous secondary battery porous membranes of Claim 1 or 2.   The composition for non-aqueous secondary battery porous films containing the binder for non-aqueous secondary battery porous films in any one of Claims 1-3, a nonelectroconductive particle, and water.   A porous film for a non-aqueous secondary battery, formed from the composition for a non-aqueous secondary battery porous film according to claim 4.   A non-aqueous secondary battery comprising the porous membrane for a non-aqueous secondary battery according to claim 5.