JP6878273B2 - Copolymers, binders for electrodes of secondary batteries, compositions for electrodes of secondary batteries, electrodes for secondary batteries - Google Patents

Description

The present invention relates to a copolymer, a binder for an electrode of a secondary battery, a composition for an electrode of 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.

Secondary batteries are used as batteries for consumer devices such as mobile phones, and as in-vehicle batteries for hybrid electric vehicles, plug-in hybrid electric vehicles, electric vehicles, and the like. Among them, 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 contains an active material containing a lithium transition metal oxide such as lithium cobalt oxide, a conductive auxiliary agent 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 a material of the active material layer such as an active material is dispersed in a solvent in which a binder is dissolved to prepare an active material solution, which is applied onto a current collector.

At present, polyvinylidene fluoride (PVDF), which is a fluorine-containing resin, is mainly used as a binder in the positive electrode of a lithium ion battery. However, PVDF has insufficient binding property with a current collector.
Further, in recent years, it has been desired to apply water as a solvent for an electrode binder of a secondary battery in order to deal with environmental problems in the future. However, PVDF is only soluble in special protic solvents such as N-methyl-2-pyrrolidone (NMP). Therefore, 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 describes fluororubber containing a copolymer of hexafluoropropylene and vinylidene fluoride as a main component, or trifluorochloroethylene and fluoride. It has been proposed to use fluororubber whose main component is a copolymer with vinylidene.
Further, Patent Document _{2, -CH 2 -CF 2 -,} - CF (CF 3) -CF 2 -, - CF 2 -CF 2 - a more composed mainly fluoropolymer copolymer, as a binder It has been proposed to use it.
Further, Patent Document 3 proposes to use a polymer having an N-vinylformamide unit as a binder.

Japanese Unexamined Patent Publication No. 4-95363 Special Fair 8-4007 Gazette International Publication No. 2012/176895

However, the conventional binder for electrodes of a secondary battery has insufficient binding force to a current collector. Therefore, in the electrodes of the conventional secondary battery, the active material layer may be peeled off from the current collector. In the electrodes of a secondary battery, the peeling of the active material layer has a great influence on the performance life and safety of the secondary battery.

Further, in the electrode of the conventional secondary battery, there is a problem that the binder for the electrode contained in the active material layer swells due to being immersed in the electrolytic solution. When the electrode binder contained in the active material layer swells, the active material layer may be peeled off from the current collector, or the internal resistance of the active material layer may increase, resulting in deterioration of the performance of the secondary battery. is there.

Further, the conventional binder for electrodes of a secondary battery has insufficient ability to disperse a conductive auxiliary agent such as carbon black. Therefore, when forming the active material layer, it is necessary to contain a dispersant in addition to the binder in the active material solution. The dispersant may be a factor that increases the internal resistance in the secondary battery. Therefore, it is desirable not to contain it.

The present invention has been made in view of the above circumstances, and has solubility in water, solubility in NMP usually used when forming an electrode of a secondary battery, binding property to a current collector, and conductivity. An object of the present invention is to provide a copolymer having excellent dispersibility of an auxiliary agent and suppressing swelling when immersed in an electrolytic solution, which is suitable as a material for a binder for an electrode of a secondary battery. ..
Another object of the present invention is to provide a binder for an electrode of a secondary battery containing the above-mentioned copolymer, a composition for an electrode of a secondary battery, and an electrode for a secondary battery.

In order to solve the above problems, the present inventor has focused on N-vinylacetamide, which is a water-soluble monomer, and diligently studied it.
As a result, 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. A copolymer of a seed monomer and N-vinylacetamide, which contains a sufficient amount of constituent units derived from N-vinylacetamide, may be used as a binder for an electrode of a secondary battery. The heading 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 the monomer of the seed and N-vinylacetamide, and the copolymer contains the number of moles of the structural unit derived from N-vinylacetamide and the other structural unit derived from the N-vinylacetamide. A copolymer having a ratio of 1.00: 0.010 to 1.00: 0.250 with the number of moles.
(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 the fluorine-containing resin are contained, and the fluorine-containing resin is contained in a total amount of the copolymer and the fluorine-containing resin. Binder for electrodes of secondary batteries of 90% by mass or less.

(8) A composition for an electrode of a secondary battery, which comprises the copolymer according to any one of (1) to (6), a solvent, an active material, and a conductive auxiliary agent.
(9) It has a current collector and an active material layer formed on the current collector, and the active material layer is either an active material, a conductive auxiliary agent, or any of (1) to (6). An electrode for a secondary battery containing the above-mentioned copolymer.

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 to a current collector, and dispersibility of a conductive auxiliary agent. , The swelling when immersed in the electrolytic solution is suppressed. Therefore, the copolymer of the present invention is suitable as a binder for electrodes of secondary batteries.

In addition, the composition for electrodes of the secondary battery of the present invention contains the copolymer of the present invention, which is excellent in solubility in water and NMP and also excellent in dispersibility of the conductive auxiliary agent. Therefore, the electrode composition of the secondary battery of the present invention can be easily produced without using a dispersant by dissolving the copolymer in a solvent and dispersing the active material and the conductive auxiliary agent in the copolymer. it can. Further, the electrode composition of the secondary battery of the present invention is excellent in binding property to the current collector by applying it on the current collector and drying it, and swelling when immersed in the electrolytic solution is achieved. A suppressed active material layer can be formed.

Further, in the electrode for a secondary battery of the present invention, the active material layer contains the copolymer of the present invention. Therefore, the active material layer has an excellent binding force between the active material layer and the current collector, and has an active material layer in which swelling is suppressed when immersed in the electrolytic solution. Therefore, the electrode for the secondary battery of the present invention can suppress the deterioration of the secondary battery.

It is a figure which shows the shape of the test piece which performed the tensile test, and the state of the tensile test.

Hereinafter, the copolymer of the present embodiment, the binder for the electrode of the secondary battery, the composition for the electrode of the secondary battery, and the electrode for the secondary battery will be described in detail.
"Copolymer, binder for electrodes of secondary batteries"
The copolymer of the present 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, may be referred to as “N-vinylacetamide copolymer”).

Other monomers of N-vinylacetamide used for the polymerization of the N-vinylacetamide copolymer (hereinafter, may be referred to as "other monomers") are unsaturated carboxylic acid monomers and unsaturated carboxylics. It is at least one monomer selected from the group consisting of salts of acid monomers, unsaturated carboxylic acid ester monomers, vinyl ester monomers, and unsaturated nitrile monomers.
As another monomer, the SP value δ (solubility parameter) is on the non-polar side as compared with 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 a monomer having an SP value of δ of N-vinylacetamide or less (13 (cal / cm ³ ) ^1/2 or less) is used as the other monomer, the SP value of the N-vinylacetamide copolymer is δ. It is close to NMP (SP value δ (feedors estimation method) 11.3 (cal / cm ³ ) ^1/2). An N-vinylacetamide copolymer having an SP value δ close to NMP is preferable because it has excellent solubility in NMP.

Examples of the monomer having an SP value δ on the non-polar side as compared with N-vinylacetamide and suitable as another monomer used for the polymerization of the N-vinylacetamide copolymer are shown in Table 1. Examples include compounds. Table 1 shows the SP value δ of the monomer that can be used as another monomer, N-vinylacetamide, and NMP calculated by using the fedors estimation method.

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 one in which the hydrogen atom of the carboxyl group contained in the above-mentioned unsaturated carboxylic acid monomer is replaced 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 contained as another monomer of N-vinylacetamide, the polymerizable unsaturated group of the monomer is contained. Can be copolymerized with N-vinylacetamide. Further, the N-vinylacetamide copolymer obtained after the polymerization has a structure containing a carboxyl group which is a polar group. Therefore, a high hydrogen bond force with the metal surface can be obtained. Therefore, when the active material layer is provided on the current collector made of metal by using the binder for electrodes containing this copolymer, the active material layer having excellent binding property to the current collector can be obtained.

Specific examples of the unsaturated carboxylic acid monomer and / or its salt used for other monomers include acrylic acid, methacrylic acid, crotonic acid, and ammonium salts, organic amine salts, and monovalent metal salts thereof. A divalent metal salt is preferable. Among these, it is particularly 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 swelling when immersed in the electrolytic solution. As for the salt, sodium salt and ammonium salt are preferable from the viewpoint of stability.

As used herein, the term "(meth) acrylic acid" means acrylic acid or methacrylic acid.

The unsaturated carboxylic acid ester monomer used for the polymerization of the N-vinylacetamide copolymer preferably has an SP value of 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). ) Methyl 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 the cohesive force can be expected to be improved by the interaction with N-vinylacetamide.

In the polymerization of the N-vinylacetamide copolymer, when an unsaturated carboxylic acid ester monomer is contained as another monomer, the N-vinylacetamide copolymer obtained after the polymerization has an affinity for NMP rather than an amide group. Contains highly potent ester moieties. Therefore, it becomes an N-vinylacetamide copolymer having excellent solubility in NMP. When an unsaturated carboxylic acid ester monomer is contained as another monomer, N obtained after polymerization can be obtained by adjusting the ratio of the blending amount of N-vinylacetamide and the unsaturated carboxylic acid ester monomer. -The affinity of the vinylacetamide copolymer for the electrolytic solution can be adjusted.

The vinyl ester monomer used for the polymerization of the N-vinylacetamide copolymer preferably has an SP value of δ of 13 (cal / cm ³ ) ^1/2 or less. Examples of such vinyl ester monomers include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl laurate, vinyl decanoate, vinyl stearate, vinyl hexanoate, vinyl octanate, and vinyl palmitate. And so on. Among these vinyl ester monomers, vinyl acetate is particularly preferable 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 contained 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, similar to N-vinylacetamide.

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

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

The N-vinylacetamide copolymer is the ratio of the number of moles of the structural unit derived from N-vinylacetamide to the number of moles of the other structural units derived from the N-vinylacetamide (constituent unit derived from N-vinylacetamide: other). The structural unit of the above is 1.00: 0.010 to 1.00: 0.250.

When the constituent unit derived from N-vinylacetamide is 1.00, the ratio of the number of moles of the other constituent units derived from N-vinylacetamide to the N-vinylacetamide copolymer exceeds 0.250. And, the solubility in water is insufficient. Further, the N-vinylacetamide copolymer in which the ratio of the number of moles of the above-mentioned constituent units exceeds 0.250 is an electrode for a secondary battery in which an active material layer containing this copolymer is provided on a current collector. When formed, the electrolytic solution greatly swells the active material layer. Therefore, the active material layer may be peeled off from the current collector, or the internal resistance of the active material layer may increase, resulting in deterioration of the performance of the secondary battery. The N-vinylacetamide copolymer preferably has a molar ratio of the above-mentioned structural units of 0.150 or less, and more preferably 0.100 or less.

Further, the N-vinylacetamide copolymer in which the ratio of the number of moles of the above-mentioned structural units is less than 0.010 is insufficient in solubility in NMP which is usually used when forming an electrode of a secondary battery. Further, the N-vinylacetamide copolymer in which the ratio of the number of moles of the above-mentioned structural units is less than 0.010 has insufficient compatibility with the fluorine-containing resin. From these facts, it is assumed that the ratio of the number of moles of the above-mentioned constituent units of the N-vinylacetamide copolymer is 0.010 or more, and preferably 0.050 or more.

The weight average molecular weight of the N-vinylacetamide copolymer is the ease of adjusting the viscosity when producing an electrode composition containing the N-vinylacetamide copolymer, the dispersibility of the conductive auxiliary agent, and the case where an active material layer containing the same is formed. From the viewpoint of binding property to the current collector, the amount 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 electrodes of a secondary battery in which swelling is suppressed when immersed in an electrolytic solution. 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, it is easy to obtain an electrode composition having a viscosity that is easy to apply.

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

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

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

When the binder for the electrode of the secondary battery of the present embodiment contains a fluorine-containing resin, the content of the fluorine-containing resin with respect to the total amount of the N-vinylacetamide copolymer and the fluorine-containing resin is 90% by mass or less. It is preferable, more preferably 70% by mass or less, further preferably 50% by mass or less, and preferably 1% by mass or more. The smaller the content of the fluorine-containing resin in the electrode binder, the more remarkable the effect of the N-vinylacetamide copolymer 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 can be sufficiently obtained, and the dispersibility of the conductive auxiliary agent 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 an electrode for a secondary battery having an active material layer containing the binder for the electrode is formed, a sufficient binding force between the active material layer and the current collector is obtained. Be done.

Further, the binder for electrodes of the secondary battery of the present embodiment preferably has an electrolytic solution immersion swelling rate of 10% or less measured by a method described later. When the electrolytic solution immersion swelling rate is 10% or less, when a secondary battery having an electrode for a secondary battery having an active material layer containing the binder for the electrode is formed, the active material and conductivity in the active material layer are formed. The distance from the auxiliary agent can be maintained properly. Further, when the electrolytic solution immersion swelling rate is 10% or less, it is possible to prevent the binder from dissolving into the electrolytic solution due to the swelling of the active material layer in the active material layer containing the electrode binder. As a result, when the electrolytic solution immersion swelling rate is 10% or less, deterioration of the performance of the secondary battery can be prevented.

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

"Manufacturing method of copolymer, manufacturing method of 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 electrodes of a secondary battery will be described.
To produce the electrode binder for the secondary battery of the present embodiment, first, an N-vinylacetamide copolymer is produced by the method shown below.

In the reactor, the N-vinylacetamide copolymer is composed 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 carboxylic acid monomer. It can be produced by subjecting at least one monomer (another monomer) selected from the group consisting of nitrile monomers and N-vinylacetamide to a polymerization reaction in the presence of a polymerization initiator.

The ratio of N-vinylacetamide to the total amount of monomers used for the polymerization of the N-vinylacetamide copolymer (total amount of N-vinylacetamide and other monomers) is 85.0 to 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 N-vinylacetamide-derived The ratio of other structural units to the number of moles (constituent unit derived from N-vinylacetamide: other structural units) is 1.00: 0.010 to 1.00: 0.250 for both N-vinylacetamide. A polymer can be easily obtained.

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

When producing the N-vinylacetamide copolymer, all 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 is being polymerized. It may be supplied to the reaction vessel. When a part of the monomer used for polymerization is supplied to the reaction vessel being polymerized, it is preferable to supply the monomer dissolved in the solvent to the reaction vessel.

In particular, when the ratio 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 appropriately control the difference in reaction rate between N-vinylacetamide and other monomers. When producing an N-vinylacetamide copolymer in which the monomer component is localized (block structure), all the monomers used for polymerization are previously dissolved and mixed in a solvent. It is preferable to use the method of keeping.

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

As the solvent used for the polymerization reaction of the N-vinylacetamide copolymer, it is preferable to use a solvent in which the above-mentioned monomer used for the polymerization of the N-vinylacetamide copolymer is dissolved and the produced copolymer is precipitated. .. Further, when carrying out the polymerization reaction, a solvent in which the above-mentioned monomer is easily dissolved is used, and after the copolymer is synthesized, the copolymer is precipitated by using another solvent in which the copolymer is easily precipitated. You may.

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

In the present embodiment, an N-vinylacetamide copolymer having a weight average molecular weight of 0.1 to 1.5 million can be easily obtained by producing by a polymerization method using an organic solvent as a solvent. Further, by producing by a polymerization method using water as a solvent, an N-vinylacetamide copolymer having a weight average molecular weight of about 5 to 3 million can be easily obtained.

As the 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, sodium peracetero, peroxygen compounds such as sodium percarbonate, azobisisobutyronitrile, 2 , 2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [N- (carboxyethyl) -2-methylpropionamide], 2,2'-azobis {2- [N- (2) -Carboxyethyl) amidino] propane}, azo compounds such as dimethyl 2,2'-azobis (2-methylpropionate), 2,2'-azobis (2-methylpropionic acid) and the like can be mentioned.

Among the above-mentioned polymerization initiators, it is particularly preferable to use azobisisobutyronitrile or dimethyl-2,2'-azobis (2-methylpropionate) which is soluble in an organic solvent. Further, it is most preferable to use dimethyl-2,2'-azobis (2-methylpropionate) which does not contain a nitrile group and a halogen as the polymerization initiator.
The amount of the polymerization initiator used may be in the range of 0.005 to 5.0 parts by mass with respect to 100 parts by mass of the total of the monomers used for the polymerization, as long as the polymerization reaction can be started and proceeded. It is not specified in particular.

The reaction temperature for producing the N-vinylacetamide copolymer is preferably 30 to 120 ° C. By setting the reaction temperature within the above range, the N-vinylacetamide copolymer can be polymerized at a reaction rate suitable for polymerization. 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 a radical polymerization and is preferably carried out in a nitrogen gas atmosphere because it is greatly affected by oxygen.
In the present embodiment, after the reaction product containing the N-vinylacetamide copolymer is obtained by the polymerization reaction, the reaction product may be washed with a solvent if necessary.

The N-vinylacetamide copolymer thus obtained can be used alone as a binder for electrodes of a secondary battery.
When the electrode binder of the secondary battery of the present embodiment contains an N-vinylacetamide copolymer and one or more types of fluorine-containing resin, N obtained by the above production method. A mixture of a vinylacetamide copolymer and one or more fluorine-containing resins in the above-mentioned ratio can be used as an electrode binder for a secondary battery.

"Composition for electrodes of secondary batteries"
The composition for electrodes of the secondary battery of the present embodiment contains the N-vinylacetamide copolymer of the present embodiment, a solvent, an active material, and a conductive auxiliary agent.
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 dissolves 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 cobalt oxide, can be used depending on the application of the secondary battery and the like.
As the conductive auxiliary agent, for example, carbon black, acetylene black, graphite, and the like, which are conventionally known and used as the conductive auxiliary agent, can be used depending on the use of the secondary battery and the like.

The electrode composition of the secondary battery of the present embodiment contains one or more fluorine-containing resins in addition to the N-vinylacetamide copolymer, the solvent, the active material, and the conductive additive, if necessary. It may be one containing a conventionally known additive.

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 the present embodiment and one or more fluorine-containing resins contained as necessary are dissolved in a solvent. Next, the active material, the conductive auxiliary agent, and the additive contained as necessary are dispersed in a solvent in which the N-vinylacetamide copolymer (or the N-vinylacetamide copolymer and the fluorine-containing resin) is dissolved. Obtained by the method of causing.

"Electrodes for secondary batteries"
The electrode for a secondary battery of the present 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 having good adhesion to the electrode binder of the secondary battery of the present embodiment.
The active material layer contains an active material, a conductive auxiliary agent, and the N-vinylacetamide copolymer of the present embodiment. The active material layer is produced 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 the present embodiment, the ratio of the number of moles of the constituent units derived from N-vinylacetamide to the number of moles of the other constituent units derived from the N-vinylacetamide is 1.00: 0. It is 1.001 to 1.00: 0.250. Therefore, the N-vinylacetamide copolymer of the present embodiment is excellent in solubility in water and NMP, binding property to a current collector, and dispersibility of a conductive auxiliary agent, and swelling when immersed in an electrolytic solution is achieved. It is suppressed.

Moreover, since the N-vinylacetamide copolymer of the present embodiment has a thickening viscosity with respect to NMP and water, it is easy to apply and is suitable as a binder for an electrode of a secondary battery. Further, the solution in which the N-vinylacetamide copolymer of the present embodiment is dissolved in NMP or water easily thickens and a high binding force can be obtained. Therefore, when producing the electrode composition containing the N-vinylacetamide copolymer of the present embodiment, it is not necessary to perform the kneading step for improving the viscosity and the binding force, and the kneading step is performed. Even in this case, it can be simplified as compared with the case where PVDF is used, for example.

Further, when the binder for the electrode of the secondary battery of the present embodiment does not contain a fluorine-containing resin, the corrosive acid component is not generated even if the secondary battery manufactured by using the binder contains a high temperature due to thermal runaway or the like. Therefore, it is preferable.

In addition, the electrode composition of the present embodiment contains the N-vinylacetamide copolymer of the present embodiment, which has excellent solubility in NMP and water and also has excellent dispersibility of the conductive auxiliary agent. Therefore, the electrode composition of the present embodiment can be easily prepared by dissolving the N-vinylacetamide copolymer in a solvent and dispersing the active material and the conductive auxiliary agent in the solvent without using a dispersant. Can be manufactured. Further, the electrode composition of the present embodiment was excellent in binding property to the current collector by applying it on the current collector and drying it, and swelling when immersed in the electrolytic solution was suppressed. An active material layer can be formed.

Further, in the electrode for a secondary battery of the present embodiment, the active material layer contains the N-vinylacetamide copolymer of the present embodiment. Therefore, the active material layer has an excellent binding force between the active material layer and the current collector, and has an active material layer in which swelling is suppressed when immersed in the electrolytic solution. Therefore, according to the electrode for the secondary battery of the present embodiment, deterioration of the secondary battery can be prevented.

Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples.

( Reference Example 1, Examples 2 to 8, Comparative Example 2)
N-vinylacetamide and the other monomers shown in Table 2 are used in the mass ratio (N-vinylacetamide / other monomers) shown in Table 2 and dimethyl-2,2'-as a polymerization initiator. The content (mass) of azobis (2-methylpropionate) (trade name: V-601 (oil-soluble azo polymerization initiator) manufactured by Wako Pure Chemical Industries, Ltd.) is shown in Table 3 with respect to a total of 100 parts by mass of the monomers. Part) was used and polymerized by the method shown below to obtain the copolymers (binders) of Reference Example 1, Examples 2 to 8 and Comparative Example 2. Table 2 shows the polymer forms of the binders of Reference Example 1, Examples 2 to 8, and Comparative Example 2.

(Comparative Example 1)
Only N-vinylacetamide was used as the monomer used for the polymerization, and the polymerization initiator shown in Table 3 was used in the content (parts by mass) shown in Table 3 with respect to 100 parts by mass of N-vinylacetamide. Polymerization was carried out 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 Reference Example 1, Examples 2, 4 to 6 and 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, the initial charged ethyl acetate and N-vinylacetamide having a mass ratio of 20% with respect to ethyl acetate were placed in the reaction vessel, and the reaction vessel was heated to the polymerization initiation temperature shown in Table 3 under nitrogen inflow with stirring. The temperature inside was raised. Table 3 shows the volume ratio of the initial charged ethyl acetate to the reaction vessel.
Subsequently, while maintaining the inside of the reaction vessel at the polymerization initiation temperature, other monomers shown in Table 2 were added into the reaction vessel so as to have the mass ratio shown in Table 2, and after a certain period of time, the polymerization initiator was added. It was added at the ratio shown in Table 3.

Then, the stirring speed was gradually increased while dropping ethyl acetate, and the polymerization reaction was carried out at the polymerization time shown in Table 3. The reaction product was precipitated as a powder. Then, the reaction product was suction-filtered, 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 for the fact that the volume ratio (50%) of the initial charged ethyl acetate to the reaction vessel shown in Table 3 was set, the polymerization start temperature was set to 62 ° C, and that other monomers and the polymerization initiator were added at the same time. Was polymerized in the same manner as in Reference Example 1, Examples 2, 4 to 6, and 8.

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

"Method for Producing Copolymer (Binder) of Comparative Example 2"
In the same manner as in Reference Example 1 and Examples 2, 4 to 6 and 8, the polymerization initiator is added at the ratio shown in Table 3 after performing the steps before adding the other monomers into the reaction vessel. did. Then, another monomer diluted to 20% by mass with ethyl acetate was added dropwise to the reaction vessel so as to have the mass ratio shown in Table 2, and the polymerization reaction was carried out at the polymerization time shown in Table 3. The reaction product was suction filtered in the same manner as in Reference Example 1 and Examples 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. By doing so, a binder was obtained.

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

Next, with respect to the binders of Reference Examples 1, Examples 2 to 8 and Comparative Examples 1 and 2, the residual monomer was measured by the HPLC (High Performance Liquid Chromatography) method, and the polymerization reaction was completed. It was confirmed.

In addition, the structural states of the binders of Reference Examples 1, Examples 2 to 8 and Comparative Examples 1 and 2 were confirmed by NMR (nuclear magnetic resonance) measurement. Specifically, methylene / methine derived from N-vinylacetamide and other monomers in each of the binders (copolymers) of Reference Example 1, Examples 2 to 8 and Comparative Examples 1 and 2 in 1H-NMR. The content was determined by the cumulative ratio of / methyl protons and the cumulative ratio of C = O derived from each monomer by 13C-NMR. Using this, the ratio (molar ratio) of N-vinylacetamide-derived and other constituent units was determined. In Example 8 containing two types of monomers as other monomers, the ratios (molar ratios) of the other two types of constituent units derived from N-vinylacetamide were calculated and totaled. The value was taken as the ratio (molar ratio) of other constituent units derived from N-vinylacetamide. The results are shown in Table 4.

Next, the following items were examined for the binders of Reference Example 1, Examples 2 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 seal, and a binder was added so that the concentration of the binder was 5% by mass, and the bottle was sealed. Then, the glass bottle with a tight stopper was manually shaken up and down 20 times to mix the NMP and the binder. Then, the glass bottle with a tight stopper was allowed to stand in an incubator at 20 ° C., and the dissolved state of the binder was visually confirmed to measure the time required for complete dissolution.
Further, it takes to completely dissolve the binder in the same manner as the above-mentioned solubility of the binder in NMP, except that pure water is put in a glass bottle with a tight stopper and the binder is added so that the concentration of the binder becomes 10% by mass. The time was measured.

(Solution viscosity)
A binder solution in which the binder was dissolved in NMP at a concentration of 5% by mass was placed in a container. The container was allowed to stand in a circulating type constant temperature water tank whose temperature was adjusted to 20 ° C. After that, it was confirmed that the temperature of the binder solution put in the container was adjusted to 20 ° C., and the viscosity was measured under the following conditions.
Further, 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 above binder was dissolved in NMP.

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

(Electrolytic solution immersion swelling rate)
The binder was dissolved in NMP to a concentration of 5% by mass, placed in a petri dish, and dried in a vacuum at 150 ° C. for 4 hours to prepare a 1 mm thick film. The obtained film was cut into a square having a length of 1 cm and a width of 1 cm, and the mass was precisely weighed.
Next, the finely weighed film was immersed in an electrolytic solution (ethyl carbonate / dimethyl carbonate / propylene carbonate = 1/1/1 (volume ratio)) in a test tube. Then, the test tube containing the film and the electrolytic solution was placed in a warm bath adjusted to 95 to 98 ° C., and 30 minutes later, the test tube was taken out of the warm bath. Then, the film was taken out from the electrolytic solution. The electrolytic solution adhering to the film was wiped off and the mass of the film was precisely weighed.

Then, the electrolytic solution immersion swelling rate was calculated from the mass of the film before and after immersion in the electrolytic solution using the following formula.
Electrolyte immersion swelling rate (%) = (mass of film after immersion-mass of film before immersion) / mass of film before immersion) x 100

(Peeling strength)
1.5 g of a solution prepared by dissolving the binder in NMP so as to have a concentration of 5% by mass was uniformly applied onto an aluminum foil test piece (3 cm × 10 cm) using a bar coater. Next, another aluminum foil test piece was laminated and bonded onto the aluminum foil test piece on the side to which the solution was applied, and the aluminum foil test pieces were brought into close contact with each other with a roller. Then, it was vacuum dried at 150 ° C. for 6 hours to prepare a test piece.

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

(Dispersibility of conductive auxiliary agent)
A binder was added to pure water which was stirred by a stirrer at room temperature at 1000 rpm so as to have a concentration of 1% by mass. It was visually confirmed that the added binder was uniformly dissolved, and an aqueous binder solution was obtained. Next, carbon black (trade name: VGCF, manufactured by Showa Denko KK) was added to the binder aqueous solution so as to be 5% by mass as a conductive auxiliary agent. After 20 minutes, stirring was stopped 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 in a dryer. The state on the glass plate after drying was displayed on the screen of the display device using a microscope and visually observed, and the dispersed state of carbon black was evaluated as follows. When it was observed that carbon black agglomerates were present on the glass plate, the diameter of the agglomerates was measured using a scale on the screen of the display device.
⊚: No agglomerates, uniform dispersion state ○: Fine lumps (aggregates) with a diameter of less than 200 μm are observed Δ: Medium agglomerates with a diameter of 200 μm to less than 1 mm are observed ×: Large agglomerates with a diameter of 1 mm or more Things can be seen

(Weight average molecular weight)
The binder was dissolved in distilled water at a concentration of 1% by mass, and measured by a 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 prepared from the measurement results 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)
Analytical device: SIC4802 Data Station (manufactured by Shimadzu Corporation)
Column: SB806 x 2 (manufactured by SHODEX)
Eluent: DW (distilled water)
Flow rate: 0.7 ml / min

(Compatibility with fluorine-containing resin)
The binder was dissolved in NMP to a concentration of 5% by mass (dissolution solution 1). Polyvinylidene fluoride (PVDF) was dissolved in NMP to a concentration of 5% by mass (dissolution solution 2). The solution 1 and the solution 2 were mixed at a ratio of 1: 1 (mass ratio) to prepare a mixed solution, and the mixture was allowed to stand at room temperature for 24 hours.
Then, with respect to the mixed liquid, the presence or absence of turbidity and the presence or absence of separation between the dissolution liquid 1 and the dissolution liquid 2 were visually evaluated according to the criteria shown below.

"Evaluation of turbidity"
○: Clear ×: There is turbidity “Presence / absence of separation”
○: No separation ×: With separation

As shown in Table 5, the binders (copolymers) of Reference Examples 1 and Examples 2 to 8 are completely NMP-satisfactory as compared with the binder of Comparative Example 1 using only N-vinylacetamide as a monomer. It takes a short time to dissolve in NMP and has excellent solubility in NMP. Moreover, the binders of Reference Examples 1 and Examples 2 to 8 take longer to completely dissolve in water than the binders (copolymers) of Comparative Example 2 in which the number of constituent units derived from N-vinylacetamide is small. Is short and has excellent solubility in water.
Further, as shown in Table 5, the binders of Reference Examples 1 and Examples 2 to 8 have a sufficiently high solution viscosity, and the solution dissolved in NMP or water has a sufficient viscosity increase and is applied. It was preferable to.

Further, the binders of Reference Examples 1 and Examples 2 to 8 had an electrolytic solution immersion swelling rate of 10% or less, and swelling when immersed in the electrolytic solution was suppressed.
Further, the binders of Reference Examples 1 and Examples 2 to 8 have a peel strength of 0.015 N / m or more, and are excellent in binding property to a current collector.
Further, the binders of Reference Examples 1 and Examples 2 to 8 have an evaluation of the dispersibility of the conductive auxiliary agent of ⊚ or ◯, and are excellent in the dispersibility of the conductive auxiliary agent.
In addition, the binders of Reference Examples 1 and Examples 2 to 8 had a turbidity evaluation of ◯, no separation, and good compatibility with the fluorine-containing resin.
In addition, the binders of Reference Examples 1 and Examples 2 to 8 had an appropriate weight average molecular weight.

On the other hand, Comparative Example 1 in which only N-vinylacetamide was used as the monomer had insufficient solubility in NMP. Further, the binder of Comparative Example 1 was inferior in compatibility with the fluorine-containing resin.

Further, the binder of Comparative Example 2 having a small number of constituent units derived from N-vinylacetamide has lower solubility in water than the binders of Reference Example 1 and Examples 2 to 8. Further, the binder of Comparative Example 2 has an electrolytic solution immersion swelling rate of more than 10%, and the swelling suppressing effect when immersed in the electrolytic solution is lower than that of the binders of Reference Examples 1 and Examples 2 to 8. ..

Further, 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, in the binder of Comparative Example 3, the evaluation of the dispersibility of the conductive auxiliary agent was x. Further, the binder of Comparative Example 3 had a peel strength of less than 0.015 N / m, and the peel strength was insufficient.

( Reference Example 9, Reference Example 10, Examples 11 to 13)
The binder (copolymer) of Reference Example 1 or Example 2 and the binder of Comparative Example 3 which is a fluorine-containing resin were mixed. Ratio of the binder of Reference Example 1 or Example 2 to the total mass of the binder of Reference Example 1 or Example 2 and the fluorine-containing resin {[(Reference Example 1 or Example 2) / ( Reference Example 1 or Example 2 +) Comparative Example 3) × 100 (mass%)] was mixed so as to have the ratio shown in Table 5. The mixed binder was dissolved in NMP so as to have a concentration of 5% by mass to prepare a solution, and the mixture 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 prepare a solution, and the mixture was allowed to stand at 20 ° C. for 12 hours.

The solutions of Reference Example 9, Reference Example 10, Examples 11 to 13, and Comparative Example 4 thus obtained were placed in a petri dish, and the electrolytic solution immersion swelling rate was measured by the same method as described above. Further, the peel strength was measured by the same method as described above using the solutions of Reference Example 9, Reference Example 10, Examples 11 to 13, and Comparative Example 4. Table 6 shows the results of the electrolytic solution immersion swelling rate and the peel strength.
In addition, the dispersibility of the conductive auxiliary agent was evaluated by the methods shown below using the solutions of Reference Example 9, Reference Example 10, Examples 11 to 13, and Comparative Example 4, respectively. The results are shown in Table 6.

(Dispersibility of conductive auxiliary agent)
Comparative Example 3 which is a binder (copolymer) of Reference Example 1 or Example 2 and a fluorine-containing resin while charging 300 g of NMP in a 500 ml separable flask at room temperature and stirring at 150 rpm with a stirrer equipped with a stirring blade. Binder and was added. These were added at the ratio (mass%) shown in Table 6 so that the total mass of the binder of Reference Example 1 or Example 2 and the binder of Comparative Example 3 had a concentration of 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 obtained NMP containing the binder so as to be 5% by mass as a conductive auxiliary agent, 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 in a dryer. Then, the state on the glass plate after drying was evaluated in the same manner as the dispersibility of the conductive auxiliary agent of Reference Example 1.

As shown in Table 6, in Reference Example 9, Reference Example 10, and Examples 11 to 13 (binder for electrodes), the binding property to the current collector is compared with that of Comparative Example 4 in which only the fluorine-containing resin is used. It was confirmed that the dispersibility of the conductive auxiliary agent was excellent and the swelling when immersed in the electrolytic solution was suppressed.

Claims (2)

The copolymer and the fluorine-containing resin are contained, and the fluorine-containing resin is 90% by mass or less with respect to the total amount of the copolymer and the fluorine-containing resin, and the copolymer is N-vinyl. It is a copolymer consisting of acetoamide and other monomers only, and the ratio of the number of moles of the constituent unit derived from N-vinylacetamide to the number of moles of the constituent unit derived from the other monomer is 1. A secondary battery having a value of 0.00: 0.010 to 1.00: 0.250, wherein the other monomer is at least one selected from the group consisting of (meth) acrylic acid ester, vinyl acetate, and acrylonitrile. Binder for electrodes. The binder for electrodes of a secondary battery according to claim 1, wherein the other monomer has a solubility parameter of 13 (cal / cm ³ ) ^{1/2 or less.}

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