JPWO2016181993A1 - Copolymer, secondary battery electrode binder, secondary battery electrode composition, secondary battery electrode - Google Patents

Abstract

At least one unit selected from the group consisting of unsaturated carboxylic acid monomers, salts of unsaturated carboxylic acid monomers, unsaturated carboxylic acid ester monomers, vinyl ester monomers, and unsaturated nitrile monomers. A copolymer of N-vinylacetamide, and the copolymer includes a number of moles of a structural unit derived from N-vinylacetamide and a number of moles of another structural unit derived from N-vinylacetamide. The ratio is 1.00: 0.010 to 1.00: 0.250.

Description

The present invention relates to a copolymer, a binder for an electrode of a secondary battery, an electrode composition for a secondary battery, and an electrode for a secondary battery.
This application claims priority based on Japanese Patent Application No. 2015-096330 filed in Japan on May 11, 2015, the contents of which are incorporated herein by reference.

  The secondary battery is used as a battery for consumer equipment such as a mobile phone, and an in-vehicle battery such as a hybrid electric vehicle, a plug-in hybrid electric vehicle, and an electric vehicle. Among these, lithium ion batteries are widely used because they have excellent energy density and charge / discharge cycle life.

  The positive electrode of the lithium ion battery has a current collector formed of aluminum foil or the like, and an active material layer formed on the current collector. The active material layer includes an active material containing a lithium transition metal oxide such as lithium cobalt oxide, a conductive aid such as carbon black, and a binder. The binder contained in the active material layer plays a role as a binder for fixing the active material layer on the current collector. The active material layer is generally formed by a method in which an active material layer material such as an active material is dispersed in a solvent in which a binder is dissolved to form an active material solution, which is applied onto a current collector.

Currently, in the positive electrode of a lithium ion battery, polyvinylidene fluoride (PVDF), which is a fluorine-containing resin, is mainly used as a binder. However, PVDF has insufficient binding properties with the current collector.
In addition, in recent years, it is desired to apply water as a solvent for a binder for an electrode of a secondary battery in order to cope with environmental problems in the future. However, PVDF is only soluble in special high polarity solvents such as N-methyl-2-pyrrolidone (NMP). For this reason, a binder that can be dissolved or dispersed in water is desired.

As a binder for an electrode of a secondary battery having excellent binding properties, for example, Patent Document 1 discloses fluororubber mainly composed of a copolymer of hexafluoropropylene and vinylidene fluoride, and trifluorochloroethylene and fluoride. It has been proposed to use a fluororubber whose main component is a copolymer with vinylidene.
In Patent Document 2, a fluorine-based polymer copolymer mainly composed of —CH ₂ —CF ₂ —, —CF (CF ₃ ) —CF ₂ —, and —CF ₂ —CF ₂ — is used as a binder. It has been proposed to use.
Patent Document 3 proposes the use of a polymer having an N-vinylformamide unit as a binder.

JP-A-4-95363 Japanese Patent Publication No. 8-4007 International Publication No. 2012/176895

  However, the binder for electrodes of the conventional secondary battery has insufficient binding force with the current collector. For this reason, in the electrode of the conventional secondary battery, the active material layer may peel from the current collector. In the secondary battery electrode, the separation of the active material layer greatly affects the performance life and safety of the secondary battery.

  Moreover, in the electrode of the conventional secondary battery, it has been a problem that the electrode binder contained in the active material layer swells when immersed in the electrolytic solution. When the binder for an electrode contained in the active material layer swells, peeling of the active material layer from the current collector is promoted, or the internal resistance of the active material layer increases and the performance of the secondary battery may deteriorate. is there.

  In addition, conventional secondary battery electrode binders have insufficient ability to disperse a conductive auxiliary such as carbon black. For this reason, when forming an active material layer, it was necessary to contain a dispersing agent in addition to a binder in the active material solution. The dispersant may be a factor that increases the internal resistance in the secondary battery. For this reason, it is desirable not to contain.

The present invention has been made in view of the above circumstances, and is soluble in water, soluble in NMP, which is usually used when forming an electrode of a secondary battery, binding with a current collector, electrical conductivity. An object of the present invention is to provide a copolymer excellent in dispersibility of an auxiliary agent and suppressed in swelling when immersed in an electrolyte solution, and suitable as a material for a binder for an electrode of a secondary battery. .
Moreover, this invention makes it a subject to provide the binder for electrodes of a secondary battery containing the said copolymer, the composition for electrodes of a secondary battery, and the electrode for secondary batteries.

In order to solve the above-mentioned problems, the present inventor has paid attention to N-vinylacetamide, which is a water-soluble monomer, and has made extensive studies.
As a result, at least one selected from the group consisting of unsaturated carboxylic acid monomers, salts of unsaturated carboxylic acid monomers, unsaturated carboxylic acid ester monomers, vinyl ester monomers, and unsaturated nitrile monomers. What is necessary is just to use the copolymer which is a copolymer of a seed monomer and N-vinylacetamide, and fully contains the structural unit derived from N-vinylacetamide as a binder for electrodes of a secondary battery. The inventor came up with the present invention.

That is, the present invention relates to the following matters.
(1) At least one selected from the group consisting of an unsaturated carboxylic acid monomer, a salt of an unsaturated carboxylic acid monomer, an unsaturated carboxylic acid ester monomer, a vinyl ester monomer, and an unsaturated nitrile monomer. It is a copolymer of a seed monomer and N-vinylacetamide, and the copolymer comprises the number of moles of the structural unit derived from N-vinylacetamide and other structural units derived from N-vinylacetamide. The copolymer whose ratio with the number of moles is 1.00: 0.010-1.00: 0.250.
(2) At least one selected from the group consisting of an unsaturated carboxylic acid monomer, a salt of an unsaturated carboxylic acid monomer, an unsaturated carboxylic acid ester monomer, a vinyl ester monomer, and an unsaturated nitrile monomer. The copolymer according to (1), wherein the seed monomer has a solubility parameter of 13 (cal / cm ³ ) ^1/2 or less.

(3) The copolymer according to (1), wherein the unsaturated carboxylic acid monomer is (meth) acrylic acid.
(4) The copolymer according to (1), wherein the unsaturated carboxylic acid ester monomer is a (meth) acrylic acid ester.
(5) The copolymer according to (1), wherein the vinyl ester monomer is vinyl acetate.
(6) The copolymer according to (1), wherein the unsaturated nitrile monomer is acrylonitrile.
(7) The copolymer according to any one of (1) to (6) and a fluorine-containing resin, wherein the fluorine-containing resin is in a total amount of the copolymer and the fluorine-containing resin. The binder for electrodes of the secondary battery which is 90 mass% or less.

(8) A composition for an electrode of a secondary battery, comprising the copolymer according to any one of (1) to (6), a solvent, an active material, and a conductive additive.
(9) It has an electrical power collector and the active material layer formed on the electrical power collector, and the said active material layer is an active material, a conductive support agent, and in any one of (1)-(6) The electrode for secondary batteries containing the copolymer of description.

  The copolymer of the present invention is excellent in solubility in water, solubility in NMP usually used when forming an electrode of a secondary battery, binding property with a current collector, and dispersibility of a conductive auxiliary agent. Swelling when immersed in an electrolytic solution is suppressed. Therefore, the copolymer of the present invention is suitable as a binder for electrodes of secondary batteries.

  Moreover, the composition for electrodes of the secondary battery of the present invention includes the copolymer of the present invention having excellent solubility in water and NMP and excellent dispersibility of the conductive auxiliary agent. For this reason, the composition for an electrode of the secondary battery of the present invention can be easily manufactured without using a dispersant by dissolving the copolymer in a solvent and dispersing the active material and the conductive additive therein. it can. In addition, the composition for an electrode of the secondary battery of the present invention is excellent in binding property with the current collector by applying the composition on a current collector and drying, and swelling when immersed in an electrolytic solution. A suppressed active material layer can be formed.

  Moreover, as for the electrode for secondary batteries of this invention, an active material layer contains the copolymer of this invention. For this reason, it has the active material layer which was excellent in the binding force of an active material layer and an electrical power collector, and the swelling at the time of being immersed in electrolyte solution was suppressed. Therefore, the secondary battery electrode of the present invention can suppress deterioration of the secondary battery.

It is a figure which shows the shape of the test body which performed the tension test, and the condition of a tension test.

Hereinafter, the copolymer of this embodiment, the binder for electrodes of a secondary battery, the composition for electrodes of a secondary battery, and the electrode for secondary batteries will be described in detail.
"Copolymer, binder for secondary battery electrodes"
The copolymer of this embodiment comprises an unsaturated carboxylic acid monomer, a salt of an unsaturated carboxylic acid monomer, an unsaturated carboxylic acid ester monomer, a vinyl ester monomer, and an unsaturated nitrile monomer. It is a copolymer of at least one monomer selected from the group and N-vinylacetamide (hereinafter sometimes referred to as “N-vinylacetamide copolymer”).

Other monomers of N-vinylacetamide used for the polymerization of N-vinylacetamide copolymer (hereinafter sometimes referred to as “other monomers”) are unsaturated carboxylic acid monomers, unsaturated carboxylic acids. It is at least one monomer selected from the group consisting of a salt of an acid monomer, an unsaturated carboxylic acid ester monomer, a vinyl ester monomer, and an unsaturated nitrile monomer.
As another monomer, the SP value δ (solubility parameter) is non-polar compared to N-vinylacetamide (SP value δ (fedors estimation method) 13 (cal / cm ³ ) ^1/2 ) ( It is preferable to use one having a small SP value.

In this embodiment, the N-vinylacetamide copolymer sufficiently contains a structural unit derived from N-vinylacetamide. Therefore, when the other monomer having an SP value δ of N-vinylacetamide or less (13 (cal / cm ³ ) ^1/2 or less) is used, the SP value δ of the N-vinylacetamide copolymer is It is close to NMP (SP value δ (fedors estimation method) 11.3 (cal / cm ³ ) ^1/2 ). An N-vinylacetamide copolymer having an SP value δ close to NMP is preferable because of its excellent solubility in NMP.

  Examples of monomers suitable for use as other monomers for the polymerization of the N-vinylacetamide copolymer having an SP value δ that is nonpolar compared to N-vinylacetamide are shown in Table 1, for example. Compounds. Table 1 shows the SP value δ calculated using the fedors estimation method for monomers that can be used as other monomers, N-vinylacetamide, and NMP.

  The unsaturated carboxylic acid monomer used for the polymerization of the N-vinylacetamide copolymer has a structure containing a polymerizable unsaturated group and a carboxyl group. The salt of the unsaturated carboxylic acid monomer used for the polymerization of the N-vinylacetamide copolymer is obtained by replacing the hydrogen atom of the carboxyl group contained in the unsaturated carboxylic acid monomer with a metal or the like.

  In the polymerization of the N-vinylacetamide copolymer, when an unsaturated carboxylic acid monomer and / or a salt thereof is included as another monomer of N-vinylacetamide, the polymerizable unsaturated group of the monomer Can be copolymerized with N-vinylacetamide. Moreover, the N-vinylacetamide copolymer obtained after superposition | polymerization has a structure containing the carboxyl group which is a polar group. For this reason, high hydrogen bond strength with the metal surface is obtained. Therefore, when an active material layer is provided on a current collector made of metal using an electrode binder containing this copolymer, an active material layer having excellent binding properties with the current collector can be obtained.

  Specific examples of unsaturated carboxylic acid monomers and / or salts thereof used for other monomers include acrylic acid, methacrylic acid, crotonic acid, and ammonium salts, organic amine salts, and monovalent metal salts thereof. Divalent metal salts are preferred. Among these, it is preferable to use (meth) acrylic acid and / or a salt thereof as the unsaturated carboxylic acid monomer and / or a salt thereof. Acrylic acid or a salt thereof is preferable as the unsaturated carboxylic acid monomer and / or a salt thereof from the viewpoint of enhancing the binding property with the current collector and suppressing the swelling when immersed in the electrolytic solution. The salt is preferably a sodium salt or an ammonium salt from the viewpoint of stability.

  In this specification, “(meth) acrylic acid” means acrylic acid or methacrylic acid.

The unsaturated carboxylic acid ester monomer used for the polymerization of the N-vinylacetamide copolymer is preferably one having an SP value δ of 13 (cal / cm ³ ) ^1/2 or less. Examples of such unsaturated carboxylic acid ester monomers include (meth) acrylic acid esters. Specifically, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isopropyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, (meth ) Methoxyethyl acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, and the like. Among these, it is preferable to use methyl methacrylate or methyl acrylate as the unsaturated carboxylic acid ester monomer because an improvement in cohesive force due to interaction with N-vinylacetamide can be expected.

  In the polymerization of an N-vinylacetamide copolymer, when an unsaturated carboxylic acid ester monomer is included as another monomer, the N-vinylacetamide copolymer obtained after the polymerization has an affinity for NMP rather than an amide group. Highly ester site is included. For this reason, it becomes the N-vinylacetamide copolymer excellent in the solubility with respect to NMP. In addition, when an unsaturated carboxylic acid ester monomer is included as another monomer, N obtained after polymerization is adjusted by adjusting the ratio of the amount of N-vinylacetamide and the unsaturated carboxylic acid ester monomer. -The affinity of the vinylacetamide copolymer to the electrolyte can be adjusted.

As the vinyl ester monomer used for the polymerization of the N-vinylacetamide copolymer, those having an SP value δ of 13 (cal / cm ³ ) ^1/2 or less are preferable. Examples of such vinyl ester monomers include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl laurate, vinyl decanoate, vinyl stearate, vinyl hexanoate, vinyl octoate, and vinyl palmitate. Etc. Among these vinyl ester monomers, it is preferable to use vinyl acetate because the reaction rate and molecular size are similar to those of N-vinylacetamide.

  In the polymerization of the N-vinylacetamide copolymer, when a vinyl ester monomer is included as another monomer, the copolymerizability with N-vinylacetamide is high, which is preferable. This is because the vinyl ester monomer is a non-conjugated monomer having a Q value (conjugation effect) of 0.2 or less, like N-vinylacetamide.

The unsaturated nitrile monomer used for the polymerization of the N-vinylacetamide copolymer is preferably one having an SP value δ of 13 (cal / cm ³ ) ^1/2 or less. Examples of such unsaturated nitrile monomers include acrylonitrile, methacrylonitrile, α-alkylacrylonitrile and the like. Among these unsaturated nitrile monomers, acrylonitrile is preferably used from the viewpoints of copolymerizability with N-vinylacetamide and electrochemical stability.

  In the polymerization of the N-vinylacetamide copolymer, when an unsaturated nitrile monomer is included as another monomer, structural alternation can be expected in the copolymerization, and electrochemical stability is improved. This is because the e-value (polar effect) of the unsaturated nitrile monomer is greater than 0 and the e-value of N-vinylacetamide is −1.57, and the product of both reaction rate ratios is less than 1. It is.

  The N-vinylacetamide copolymer is a ratio of the number of moles of structural units derived from N-vinylacetamide to the number of moles of other structural units derived from N-vinylacetamide (structural units derived from N-vinylacetamide: other Is 1.00: 0.010-1.00: 0.250.

  The N-vinylacetamide copolymer has a molar ratio of other structural units derived from N-vinylacetamide exceeding 0.250 when the structural unit derived from N-vinylacetamide is 1.00. And solubility in water is insufficient. The N-vinylacetamide copolymer having a molar ratio of the above structural units exceeding 0.250 is a secondary battery electrode in which an active material layer containing this copolymer is provided on a current collector. When formed, the active material layer is greatly swollen by the electrolytic solution. For this reason, an active material layer peels from a collector, or the internal resistance of an active material layer increases, and the performance of a secondary battery deteriorates. The N-vinylacetamide copolymer preferably has a molar ratio of the above structural units of 0.150 or less, more preferably 0.100 or less.

  In addition, the N-vinylacetamide copolymer having a molar ratio of the above structural units of less than 0.010 has insufficient solubility in NMP that is usually used when forming an electrode of a secondary battery. Further, the N-vinylacetamide copolymer having a molar ratio of the above structural units of less than 0.010 has insufficient compatibility with the fluorine-containing resin. For these reasons, the N-vinylacetamide copolymer has a molar ratio of the above structural units of 0.010 or more, and preferably 0.050 or more.

  The weight average molecular weight of the N-vinylacetamide copolymer is such that the viscosity can be easily adjusted when the electrode composition containing the N-vinylacetamide copolymer is produced, the dispersibility of the conductive assistant, and the active material layer containing the same are formed. From the standpoint of binding properties to the current collector, it is preferably 0.1 to 3 million, more preferably 10 to 1.5 million. When the weight average molecular weight of the N-vinylacetamide copolymer is 10,000 or more, it is easy to obtain a binder for an electrode of a secondary battery in which swelling when immersed in an electrolytic solution is suppressed. When the weight average molecular weight of the N-vinylacetamide copolymer is 3 million or less, when an electrode composition containing this copolymer is produced, an electrode composition having a viscosity that is easy to apply is easily obtained.

  The weight average molecular weight of the N-vinylacetamide copolymer in the present embodiment is a value calculated by the following method. GPC (gel permeation chromatography) of N-vinylacetamide copolymer using a calibration curve created from the measurement result of absolute molecular weight of N-vinylacetamide in each molecular weight band by GPC-MALS (multi-angle light scattering detector) It is a value calculated from the measurement result.

The N-vinylacetamide copolymer of this embodiment can be used alone as a binder for an electrode of a secondary battery.
The electrode binder may contain one or more fluorine-containing resins in addition to the N-vinylacetamide copolymer.
Examples of the fluorine-containing resin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), and polyvinyl fluoride (PVF).

The N-vinylacetamide copolymer is excellent in compatibility with the fluorine-containing resin. Therefore, the N-vinylacetamide copolymer and the fluorine-containing resin can be mixed and used at an arbitrary ratio.
In addition to the N-vinylacetamide copolymer, the electrode binder containing a fluorine-containing resin is superior in the dispersibility of the conductive auxiliary agent compared to the case where the fluorine-containing resin is used alone, and when immersed in an electrolytic solution. In addition to being a binder for an electrode in which swelling is suppressed, when an active material layer containing the binder is produced, the binder with the current collector is excellent.

  When the binder for electrodes of the secondary battery of this embodiment contains a fluorine-containing resin, the content of the fluorine-containing resin relative to the total amount of the N-vinylacetamide copolymer and the fluorine-containing resin is 90% by mass or less. More preferably, it is 70 mass% or less, More preferably, it is 50 mass% or less, and 1 mass% or more is preferable. The smaller the content of the fluorine-containing resin in the electrode binder, the more remarkable the effect due to the N-vinylacetamide copolymer being contained in the electrode binder. When the content of the fluorine-containing resin with respect to the total amount is 50% by mass or less, the peel strength described later is sufficiently obtained and the dispersibility of the conductive additive is excellent.

  The electrode binder of the secondary battery of the present embodiment preferably has a peel strength of 0.015 N / m or more measured by a method described later. When the peel strength is 0.015 N / m or more, when a secondary battery electrode having an active material layer containing the electrode binder is formed, sufficient binding force between the active material layer and the current collector is obtained. It is done.

  Moreover, it is preferable that the binder for electrodes of the secondary battery of this embodiment has an electrolytic solution immersion swelling ratio measured by a method described later of 10% or less. When the electrolytic solution immersion swelling rate is 10% or less, when a secondary battery including an electrode for a secondary battery having an active material layer containing the electrode binder is formed, the active material and the conductive material in the active material layer are electrically conductive. The distance from the auxiliary agent can be kept appropriate. Moreover, it can prevent that a binder melt | dissolves in electrolyte solution because an active material layer swells in the active material layer containing the binder for electrodes as electrolyte solution immersion swelling rate is 10% or less. By these things, the performance deterioration of a secondary battery can be prevented as electrolyte solution immersion swelling rate is 10% or less.

  The binder for an electrode of the secondary battery of this embodiment may be used for the positive electrode of the secondary battery or may be used for the negative electrode.

"Method for producing copolymer, method for producing binder for electrode of secondary battery"
Next, a method for producing the copolymer of the present embodiment and a method for producing a binder for an electrode of a secondary battery will be described.
In order to manufacture the binder for an electrode of the secondary battery of the present embodiment, first, an N-vinylacetamide copolymer is manufactured by the following method.

  N-vinylacetamide copolymer is used in the reaction apparatus to produce unsaturated carboxylic acid monomer, salt of unsaturated carboxylic acid monomer, unsaturated carboxylic acid ester monomer, vinyl ester monomer, unsaturated It can be produced by polymerizing at least one monomer (other monomer) selected from the group consisting of nitrile monomers and N-vinylacetamide in the presence of a polymerization initiator.

  The ratio of N-vinylacetamide in the total amount of monomers used for polymerization of the N-vinylacetamide copolymer (total amount of N-vinylacetamide and other monomers) is 85.0 to 99.99. It is preferably 9% by mass. When the ratio of N-vinylacetamide used for the polymerization of the N-vinylacetamide copolymer is 85.0 to 99.9% by mass, the number of moles of the structural unit derived from N-vinylacetamide and the above-mentioned N-vinylacetamide origin N-vinylacetamide copolymer in which the ratio of other structural units to the number of moles (structural units derived from N-vinylacetamide: other structural units) is 1.00: 0.010-1.00: 0.250 A polymer is easily obtained.

  The polymerization method for producing the N-vinylacetamide copolymer is not particularly limited. For example, polymerization methods such as solution polymerization, dropping polymerization, reverse phase suspension polymerization, emulsion polymerization, and precipitation polymerization can be used. Of these, precipitation polymerization is particularly suitable as a method for polymerizing the N-vinylacetamide copolymer.

  When producing the N-vinylacetamide copolymer, all of the monomers used for the polymerization may be supplied to the reaction vessel before the start of the polymerization, or a part of the monomers used for the polymerization may be supplied during the polymerization. You may supply to a reaction container. When supplying a part of monomer used for polymerization to the reaction vessel during polymerization, it is preferable to supply the monomer dissolved in the solvent to the reaction vessel.

  In particular, when the proportion of other monomers in the total amount of monomers used for polymerization exceeds 5% by mass, a mixture of a part of other monomers used for polymerization and a solvent is polymerized. It is preferable to use the method of supplying to the reaction vessel inside. This is to properly control the difference in reaction rate between N-vinylacetamide and other monomers. In the case of producing an N-vinylacetamide copolymer in which the monomer component is localized (block structure), all monomers used for polymerization are dissolved and mixed in a solvent in advance. It is preferable to use a method that keeps the data.

  When the N-vinylacetamide copolymer is produced, all of the solvent used for the polymerization may be supplied to the reaction vessel before the start of polymerization, or a part of the solvent used for the polymerization is supplied to the reaction vessel during the polymerization. May be.

  As the solvent used for the polymerization reaction of the N-vinylacetamide copolymer, it is preferable to use a solvent in which the monomer used for the polymerization of the N-vinylacetamide copolymer is dissolved and the resulting copolymer is precipitated. . In addition, when performing the polymerization reaction, a solvent in which the above monomer is easily dissolved is used. After synthesizing the copolymer, the copolymer is precipitated using another solvent in which the copolymer is easily precipitated. May be.

  As a solvent used for the polymerization reaction of the N-vinylacetamide copolymer, a solvent that can be generally used for the polymerization reaction of a vinyl compound can be used. Specific examples include water, ethyl acetate, butyl acetate, acetone, methyl ethyl ketone, methyl isobutyl ketone, benzene, toluene, methanol, ethanol, isopropanol, and the like. Of these solvents, it is particularly preferable to use ethyl acetate.

  In this embodiment, an N-vinylacetamide copolymer having a weight average molecular weight of 0.1 to 1,500,000 can be easily obtained by producing by a polymerization method using an organic solvent as a solvent. Moreover, the N-vinylacetamide copolymer whose weight average molecular weight is about 5 to 3 million is easily obtained by manufacturing by the polymerization method using water as a solvent.

  As a polymerization initiator used for the polymerization of the N-vinylacetamide copolymer, those generally used for radical polymerization of vinyl compounds can be used without limitation. Specifically, persulfates such as sodium, potassium and ammonium, benzoyl peroxide, hydrogen peroxide, caproyl peroxide, peroxygen compounds such as sodium peracetello, sodium percarbonate, azobisisobutyronitrile, 2 , 2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [N- (carboxyethyl) -2-methylpropionamide], 2,2′-azobis {2- [N- (2 Azo compounds such as -carboxylethyl) amidino] propane}, dimethyl 2,2'-azobis (2-methylpropionate), 2,2'-azobis (2-methylpropionic acid), and the like.

Among the above polymerization initiators, it is particularly preferable to use azobisisobutyronitrile or dimethyl-2,2′-azobis (2-methylpropionate) that can be dissolved in an organic solvent. Furthermore, it is most preferable to use dimethyl-2,2′-azobis (2-methylpropionate) containing no nitrile group and halogen as the polymerization initiator.
Moreover, the usage-amount of a polymerization initiator should just be able to start and advance a polymerization reaction, for example, the range of 0.005-5.0 mass part with respect to 100 mass parts of the sum total of the monomer used for superposition | polymerization. And is not particularly specified.

  The reaction temperature when producing the N-vinylacetamide copolymer is preferably 30 to 120 ° C. By setting the reaction temperature within the above range, polymerization can be performed at a reaction rate suitable for polymerization of the N-vinylacetamide copolymer. The reaction temperature may be constant from the start to the end of the polymerization, or may be changed during the polymerization reaction.

The polymerization of the N-vinylacetamide copolymer is radical polymerization and is preferably performed in a nitrogen gas atmosphere because of the large influence of oxygen.
In this embodiment, after obtaining a reaction product containing an N-vinylacetamide copolymer by a polymerization reaction, the reaction product may be washed using a solvent as necessary.

The N-vinylacetamide copolymer thus obtained can be used alone as a binder for an electrode of a secondary battery.
Moreover, when the binder for electrodes of the secondary battery of this embodiment contains an N-vinylacetamide copolymer and one or more fluorine-containing resins, the N obtained by the above production method. -What mixed the vinyl acetamide copolymer and 1 type, or 2 or more types of fluorine-containing resin in the above-mentioned ratio can be used as a binder for the electrodes of a secondary battery.

"Electrode composition for secondary battery"
The composition for electrodes of the secondary battery of this embodiment includes the N-vinylacetamide copolymer of this embodiment, a solvent, an active material, and a conductive additive.
As the solvent, for example, a solvent in which an N-vinylacetamide copolymer such as NMP, water, methanol, butanol, propylene glycol monomethyl ether, dimethyl sulfoxide, or ethylene glycol is dissolved can be used.

As the active material, for example, a conventionally known material used as an active material such as a lithium transition metal oxide such as lithium cobaltate can be used depending on the use of the secondary battery.
As a conductive support agent, conventionally well-known things used as a conductive support agent can be used according to the use etc. of a secondary battery, such as carbon black, acetylene black, and graphite, for example.

  The composition for an electrode of the secondary battery according to the present embodiment includes one or more fluorine-containing resins in addition to the N-vinylacetamide copolymer, the solvent, the active material, and the conductive auxiliary as necessary. It may be a thing and a conventionally well-known additive may be included.

The composition for electrodes of the secondary battery of the present embodiment can be produced, for example, by the method shown below.
First, the N-vinylacetamide copolymer of this embodiment and one or more fluorine-containing resins contained as necessary are dissolved in a solvent. Next, an active material, a conductive additive, and an additive contained as necessary are dispersed in a solvent in which the N-vinylacetamide copolymer (or N-vinylacetamide copolymer and fluorine-containing resin) is dissolved. It is obtained by the method.

"Secondary battery electrodes"
The electrode for secondary batteries of this embodiment has a current collector and an active material layer formed on the current collector.
As the current collector, a metal foil such as an aluminum foil, a stainless steel foil, or a copper foil can be used. As the current collector, it is particularly preferable to use an aluminum foil that has good binding properties with the binder for electrodes of the secondary battery of the present embodiment.
The active material layer includes an active material, a conductive additive, and the N-vinylacetamide copolymer of the present embodiment. The active material layer is manufactured by applying the electrode composition of the secondary battery of the present embodiment on a current collector and drying it.

  In the N-vinylacetamide copolymer of this embodiment, the ratio of the number of moles of the structural unit derived from N-vinylacetamide to the number of moles of the other structural unit derived from N-vinylacetamide is 1.00: 0. 0.010 to 1.00: 0.250. For this reason, the N-vinylacetamide copolymer of this embodiment is excellent in solubility in water and NMP, binding property with the current collector, and dispersibility of the conductive auxiliary agent, and swelling when immersed in the electrolytic solution. It has been suppressed.

  Moreover, the N-vinylacetamide copolymer of the present embodiment has a thickening property against NMP and water, so that it is easy to apply and is suitable as a binder for electrodes of secondary batteries. Moreover, the solution which melt | dissolved the N-vinylacetamide copolymer of this embodiment in NMP or water easily thickens, and high binding force is obtained. For this reason, when manufacturing the composition for electrodes containing the N-vinylacetamide copolymer of this embodiment, it is not necessary to perform the kneading | mixing process for improving a viscosity and binding force, and a kneading | mixing process is performed. Even if it is a case, it can simplify compared with the case where PVDF is used, for example.

  Further, when the electrode binder of the secondary battery of this embodiment does not contain a fluorine-containing resin, no corrosive acid component is generated even if the secondary battery produced using the binder becomes high temperature due to thermal runaway or the like. Therefore, it is preferable.

  The electrode composition of the present embodiment includes the N-vinylacetamide copolymer of the present embodiment, which is excellent in solubility in NMP and water and excellent in dispersibility of the conductive additive. For this reason, the electrode composition of the present embodiment can be easily dissolved without using a dispersant by dissolving an N-vinylacetamide copolymer in a solvent and dispersing the active material and the conductive additive therein. Can be manufactured. In addition, the electrode composition of the present embodiment was coated on a current collector and dried, thereby being excellent in binding property with the current collector and suppressing swelling when immersed in an electrolytic solution. An active material layer can be formed.

  In the secondary battery electrode of the present embodiment, the active material layer includes the N-vinylacetamide copolymer of the present embodiment. For this reason, it has the active material layer which was excellent in the binding force of an active material layer and an electrical power collector, and the swelling at the time of being immersed in electrolyte solution was suppressed. Therefore, according to the secondary battery electrode of the present embodiment, the deterioration of the secondary battery can be prevented.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

(Examples 1-8, Comparative Example 2)
N-vinylacetamide and other monomers shown in Table 2 were used at a mass ratio shown in Table 2 (N-vinylacetamide / other monomers), and dimethyl-2,2′- as a polymerization initiator. Content (mass) shown in Table 3 for azobis (2-methylpropionate) (trade name: V-601 (oil-soluble azo polymerization initiator) manufactured by Wako Pure Chemical Industries, Ltd.) with respect to 100 parts by mass in total of the monomers. The copolymer (binder) of Examples 1 to 8 and Comparative Example 2 was obtained by polymerization using the method described below. Table 2 shows polymer forms of the binders of Examples 1 to 8 and Comparative Example 2.

(Comparative Example 1)
Only N-vinylacetamide was used as a monomer used for polymerization, and the polymerization initiator shown in Table 3 was used at a content (parts by mass) shown in Table 3 with respect to 100 parts by mass of N-vinylacetamide. Polymerization was performed by the method shown to obtain a binder of Comparative Example 1. Table 2 shows the polymer form of the binder of Comparative Example 1.

(Comparative Example 3)
Polyvinylidene fluoride (trade name: Kynar PVDF 760: manufactured by ARKEMA), which is a fluorine-containing resin, was used.

"Method for producing copolymers (binders) of Examples 1, 2, 4 to 6, 8"
A three-neck separable flask was prepared as a reaction vessel, and a nitrogen gas insertion tube, a stirrer, a solvent dropping device, and a thermometer were attached. Next, in the reaction vessel, initially charged ethyl acetate and N-vinylacetamide at a mass ratio of 20% with respect to ethyl acetate were added, and the reaction vessel was stirred up to the polymerization start temperature shown in Table 3 under nitrogen flow. The inside was heated up. Table 3 shows the volume ratio of the initially charged ethyl acetate to the reaction vessel.
Subsequently, while maintaining the inside of the reaction vessel at the polymerization initiation temperature, the other monomers shown in Table 2 were charged into the reaction vessel so as to have the mass ratio shown in Table 2, and after a certain time had elapsed, the polymerization initiator was added. The proportions shown in Table 3 were added.

  Thereafter, the stirring speed was increased stepwise while dropping ethyl acetate, and the polymerization reaction was carried out with the polymerization time shown in Table 3. The reaction product precipitated as a powder. Thereafter, the reaction product was filtered with suction, washed with ethyl acetate, and air-dried with nitrogen gas at the drying temperature and drying time shown in Table 3 to obtain a binder.

“Method for Producing Copolymer (Binder) of Examples 3 and 7”
Except that the volume ratio (50%) of the initially charged ethyl acetate to the reaction vessel shown in Table 3 was set, that the polymerization initiation temperature was 62 ° C., and that other monomers and a polymerization initiator were added simultaneously. Was polymerized in the same manner as in Examples 1, 2, 4 to 6, and 8.

"Method for producing binder of Comparative Example 1"
Polymerization was carried out in the same manner as in Examples 1, 2, 4 to 6 and 8, except that no other monomer was charged into the reaction vessel.

“Method for Producing Copolymer (Binder) of Comparative Example 2”
In the same manner as in Examples 1, 2, 4 to 6, and 8, after performing the steps up to the addition of another monomer into the reaction vessel, the polymerization initiator was added at the ratio shown in Table 3. Thereafter, another monomer diluted to 20% by mass with ethyl acetate was dropped into the reaction vessel so as to have a mass ratio shown in Table 2, and a polymerization reaction was carried out with a polymerization time shown in Table 3. The reaction product is suction filtered in the same manner as in Examples 1, 2, 4 to 6, and 8, washed with ethyl acetate, and air-dried with nitrogen gas at the drying temperature and drying time shown in Table 3. Thus, a binder was obtained.

  Table 3 shows the polymerization reaction conditions of Examples 1 to 8 and Comparative Examples 1 and 2.

  Next, residual monomers were measured by HPLC (high performance liquid chromatography) for the binders of Examples 1 to 8 and Comparative Examples 1 and 2, and it was confirmed that the polymerization reaction was completed.

Moreover, about the binder of Examples 1-8 and Comparative Examples 1 and 2, the structural state was confirmed by NMR (nuclear magnetic resonance) measurement. Specifically, methylene / methine / methyl derived from N-vinylacetamide and other monomers in the binders (copolymers) of Examples 1 to 8 and Comparative Examples 1 and 2 in ¹ H-NMR. The content was determined from the ratio of proton accumulation and the ratio of C═O accumulation derived from each monomer by ¹³ C-NMR. Using this, the ratio (molar ratio) of N-vinylacetamide-derived and other structural units was determined. In addition, about Example 8 which contains two types of monomers as another monomer, the ratio (molar ratio) is calculated about other two types of structural units derived from N-vinylacetamide, respectively, and the total The value was the ratio (molar ratio) of other structural units derived from N-vinylacetamide. The results are shown in Table 4.

  Next, the following items were examined for the binders of Examples 1 to 8 and Comparative Examples 1 to 3. The results are shown in Table 5.

(Solubility of binder in N-methyl-2-pyrrolidone (NMP) and pure water)
NMP was placed in a glass bottle with a tight stopper, and the binder was added and sealed so that the binder concentration was 5% by mass. And the glass bottle with a sealing stopper was shaken up and down manually 20 times, and NMP and the binder were mixed. Thereafter, the glass bottle with a tight stopper was left in a thermostat at 20 ° C., and the time required for complete dissolution was measured by visually confirming the dissolution state of the binder.
In addition, it is necessary to completely dissolve in the same manner as the solubility of the binder in NMP except that pure water is put into a glass bottle with a stopper and the binder is added so that the concentration of the binder is 10% by mass. Time was measured.

(Solution viscosity)
A binder solution in which the binder was dissolved in NMP at a concentration of 5% by mass was put in a container. The container was allowed to stand in a circulation type constant temperature water bath adjusted to 20 ° C. Then, it confirmed that the temperature of the binder solution put into the container was adjusted to 20 degreeC, and measured the viscosity on the following conditions.
The viscosity of the binder solution in which the binder was dissolved in pure water at a concentration of 10% by mass was measured in the same manner as the binder solution in which the binder was dissolved in NMP.

Viscometer: DVE (Brookfield) viscometer Spindle: No. 4 spindle Rotation speed: 50 rpm
Temperature: 20 ° C
Measurement time: The viscometer spindle was placed in the binder solution, and the value 30 minutes after the spindle started rotating was taken as the measured value.

(Electrolytic solution immersion swelling rate)
The binder was dissolved in NMP so as to have a concentration of 5% by mass, placed in a petri dish, and dried in vacuum at 150 ° C. for 4 hours to form a 1 mm thick film. The obtained film was cut into a 1 cm horizontal square with a length of 1 cm, and the mass was precisely weighed.
Subsequently, the precisely weighed film was immersed in an electrolytic solution (ethyl carbonate / dimethyl carbonate / propylene carbonate = 1/1/1 (volume ratio)) in a test tube. And the test tube which put the film and electrolyte solution was put into the warm bath adjusted to 95-98 degreeC, and it took out from the warm bath 30 minutes later. Thereafter, the film was taken out from the electrolytic solution. The electrolyte solution adhered to the film was wiped off, and the mass of the film was precisely weighed.

Thereafter, from the mass of the film before and after being immersed in the electrolytic solution, the electrolytic solution immersion swelling rate was calculated using the following formula.
Electrolytic solution immersion swelling rate (%) = (film mass after immersion−film mass before immersion) / film mass before immersion) × 100

(Peel strength)
1.5 g of a solution obtained by dissolving the binder in NMP so as to have a concentration of 5% by mass was uniformly coated on an aluminum foil test piece (3 cm × 10 cm) using a bar coater. Next, another aluminum foil test piece was laminated and bonded on the aluminum foil test piece on the side to which the solution was applied, and adhered with a roller. Thereafter, it was vacuum-dried at 150 ° C. for 6 hours to obtain a test specimen.

Then, the peeling strength of the test body was measured at a speed of 500 mm / min using a tensile tester (ORIENTEC PTM-100). As shown in FIG. 1, the bonding area in the test body subjected to the tensile test is 21 cm ² (3 cm × 7 cm), and the distance between chucks is 5 cm. FIG. 1 shows the shape of the specimen subjected to the tensile test and the state of the tensile test.
Three test specimens were prepared for each binder. The maximum value of the tensile test in the three specimens was averaged to obtain the peel strength of each binder.

(Dispersibility of conductive aid)
A binder was added to pure water stirred at 1000 rpm with a stirrer at room temperature so that the concentration would be 1% by mass. It was visually confirmed that the added binder was uniformly dissolved, and an aqueous binder solution was obtained. Subsequently, carbon black (trade name: VGCF, manufactured by Showa Denko KK) was added to the binder aqueous solution as a conductive auxiliary agent so as to be 5% by mass. Stirring was stopped after 20 minutes to obtain a carbon black dispersion.

2 g of the obtained carbon black dispersion was collected, applied to a 0.2 cm × 5 cm × 10 cm glass plate using a bar coater, and dried at 80 ° C. for 12 hours with a dryer. The state on the glass plate after drying was displayed on the screen of a display device using a microscope and visually observed, and the dispersion state of carbon black was evaluated as follows. In addition, when it was observed that the aggregate of carbon black existed on a glass brate, the diameter of the aggregate was measured using the scale on the screen of a display apparatus.
◎: No aggregate, uniform dispersion state ○: A small lump (aggregate) with a diameter of less than 200 μm is observed Δ: Medium aggregate with a diameter of 200 μm to less than 1 mm is observed ×: Large aggregation with a diameter of 1 mm or more You can see

(Weight average molecular weight)
The binder was dissolved in distilled water at a concentration of 1% by mass and measured by the GPC (gel permeation chromatography) method under the following conditions. The weight average molecular weight (Mw) was calculated using the result.
For the calculation of the weight average molecular weight, a calibration curve created from the measurement result of the absolute molecular weight (GPC-MALS) of N-vinylacetamide in each molecular weight band was used.

Detector (RI): RI-201H (manufactured by SHODEX)
Pump: LC-20AD (manufactured by Shimadzu Corporation)
Column oven: AO-30C (manufactured by SHODEX)
Analysis device: SIC4802 Data Station (manufactured by Shimadzu Corporation)
Column: SB806 × 2 (manufactured by SHODEX)
Eluent: DW (distilled water)
Flow rate: 0.7ml / min

(Compatibility with fluorine-containing resin)
The binder was dissolved in NMP to a concentration of 5% by mass (Solution 1). Polyvinylidene fluoride (PVDF) was dissolved in NMP so as to have a concentration of 5% by mass (solution 2). Dissolving solution 1 and dissolving solution 2 were mixed at a ratio (mass ratio) of 1: 1 to form a mixed solution, and allowed to stand at room temperature for 24 hours.
Then, about the mixed liquid, the presence or absence of turbidity and the presence or absence of separation of the dissolving liquid 1 and the dissolving liquid 2 were visually evaluated according to the following criteria.

"Evaluation of turbidity"
○: Kiyosumi ×: Turbidity “Presence / absence of separation”
○: No separation ×: With separation

As shown in Table 5, the binders (copolymers) of Examples 1 to 8 were completely dissolved in NMP as compared with the binder of Comparative Example 1 using only N-vinylacetamide as a monomer. The time required for this is short and the solubility in NMP is excellent. In addition, the binders of Examples 1 to 8 are shorter in time required to completely dissolve in water than the binder (copolymer) of Comparative Example 2 having few structural units derived from N-vinylacetamide. Excellent solubility in
In addition, as shown in Table 5, the binders of Examples 1 to 8 have a sufficiently high solution viscosity, and a solution dissolved in NMP or water has a sufficient viscosity, which is preferable when coating is performed. there were.

Moreover, the binder of Examples 1-8 had an electrolyte solution immersion swelling rate of 10% or less, and the swelling when immersed in the electrolyte solution was suppressed.
In addition, the binders of Examples 1 to 8 have a peel strength of 0.015 N / m or more, and are excellent in binding properties with the current collector.
Moreover, the binder of Examples 1-8 is (double-circle) or (circle) in the evaluation of the dispersibility of a conductive support agent, and is excellent in the dispersibility of a conductive support agent.
In addition, the binders of Examples 1 to 8 had a turbidity evaluation of ◯, no separation, and good compatibility with the fluorine-containing resin.
Moreover, the binder of Examples 1-8 was also appropriate in the weight average molecular weight.

  On the other hand, Comparative Example 1 using only N-vinylacetamide as a monomer has insufficient solubility in NMP. Moreover, the binder of the comparative example 1 was inferior in compatibility with fluorine-containing resin.

  Moreover, the binder of the comparative example 2 with few structural units derived from N-vinylacetamide has low solubility with respect to water compared with the binder of Examples 1-8. Furthermore, the binder of Comparative Example 2 has an electrolytic solution immersion swelling ratio exceeding 10%, and the swelling suppressing effect when immersed in the electrolytic solution is low as compared with the binders of Examples 1-8.

  Moreover, the comparative example 3 which is a fluorine-containing resin is insoluble in water. Further, the binder of Comparative Example 3 has a very large electrolytic solution immersion swelling rate. Further, the binder of Comparative Example 3 was evaluated as x for the dispersibility of the conductive additive. Further, the binder of Comparative Example 3 had a peel strength of less than 0.015 N / m, and the peel strength was insufficient.

(Examples 9 to 13)
The binder (copolymer) of Example 1 or Example 2 and the binder of Comparative Example 3 which is a fluorine-containing resin were mixed. Ratio of binder of Example 1 or Example 2 to total mass of binder of Example 1 or Example 2 and fluorine-containing resin {[(Example 1 or Example 2) / (Example 1 or Example 2+ Comparative Example 3) × 100 (mass%)] was mixed so that the ratio shown in Table 5 was obtained. The mixed binder was dissolved in NMP so as to have a concentration of 5% by mass to obtain a solution, which was allowed to stand at 20 ° C. for 12 hours.

(Comparative Example 4)
The binder of Comparative Example 3 which is a fluorine-containing resin was dissolved in NMP so as to have a concentration of 5% by mass to obtain a solution, which was allowed to stand at 20 ° C. for 12 hours.

Thus, the obtained solution of Examples 9-13 and the comparative example 4 was put into the petri dish, and electrolyte solution immersion swelling ratio was measured by the method similar to the above. Moreover, peeling strength was measured by the method similar to the above using the solution of Examples 9-13 and Comparative Example 4. Table 6 shows the results of the electrolytic solution immersion swelling rate and peel strength.
Moreover, the dispersibility of the conductive assistant was evaluated by the methods shown below using the solutions of Examples 9 to 13 and Comparative Example 4, respectively. The results are shown in Table 6.

(Dispersibility of conductive aid)
Comparative Example 3 which is a binder (copolymer) of Example 1 or Example 2 and a fluorine-containing resin while stirring at 150 rpm with a stirrer equipped with a stirring blade with 300 g of NMP at room temperature in a 500 ml separable flask Of binder was added. These were added at a ratio (mass%) shown in Table 6 so that the total mass of the binder of Example 1 or Example 2 and the binder of Comparative Example 3 was 1% by mass. It was visually confirmed that the added binder was completely dissolved. Subsequently, carbon black (trade name: VGCF, manufactured by Showa Denko KK) was added to the NMP containing the obtained binder so as to be 5% by mass as a conductive assistant, and stirring was stopped after 20 minutes.

  2 g of the carbon black dispersion thus obtained was collected, applied to a 0.2 cm × 5 cm × 10 cm glass plate using a bar coater, and dried at 110 ° C. for 2 hours with a dryer. And the state on the glass plate after drying was evaluated similarly to the dispersibility of the conductive support agent of Example 1.

  As shown in Table 6, in Examples 9 to 13 (binders for electrodes), compared with Comparative Example 4 using only the fluorine-containing resin, the binding property to the current collector and the dispersibility of the conductive auxiliary agent were improved. It was excellent and it was confirmed that the swelling when immersed in the electrolyte was suppressed.

Claims (9)

At least one unit selected from the group consisting of unsaturated carboxylic acid monomers, salts of unsaturated carboxylic acid monomers, unsaturated carboxylic acid ester monomers, vinyl ester monomers, and unsaturated nitrile monomers. And a copolymer of N-vinylacetamide,
In the copolymer, the ratio of the number of moles of structural units derived from N-vinylacetamide to the number of moles of other structural units derived from N-vinylacetamide is 1.00: 0.010 to 1.00: A copolymer that is 0.250. At least one unit selected from the group consisting of unsaturated carboxylic acid monomers, salts of unsaturated carboxylic acid monomers, unsaturated carboxylic acid ester monomers, vinyl ester monomers, and unsaturated nitrile monomers. The copolymer according to claim 1, wherein the monomer has a solubility parameter of 13 (cal / cm ³ ) ^1/2 or less.   The copolymer according to claim 1, wherein the unsaturated carboxylic acid monomer is (meth) acrylic acid.   The copolymer according to claim 1, wherein the unsaturated carboxylic acid ester monomer is a (meth) acrylic acid ester.   The copolymer according to claim 1, wherein the vinyl ester monomer is vinyl acetate.   The copolymer according to claim 1, wherein the unsaturated nitrile monomer is acrylonitrile.   The copolymer according to any one of claims 1 to 6 and a fluorine-containing resin, wherein the fluorine-containing resin is 90% by mass with respect to the total amount of the copolymer and the fluorine-containing resin. The binder for electrodes of the secondary battery which is the following.   The composition for electrodes of the secondary battery containing the copolymer as described in any one of Claims 1-6, a solvent, an active material, and a conductive support agent. A current collector and an active material layer formed on the current collector;
The electrode for secondary batteries in which the said active material layer contains an active material, a conductive support agent, and the copolymer as described in any one of Claims 1-6.

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