TW201710308A - Copolymer, electrode binder for secondary battery, electrode composition for secondary battery, and electrode for secondary battery - Google Patents

Abstract

The present invention provides a copolymer including at least one monomer selected from an unsaturated carboxylic acid monomer, an unsaturated carboxylate monomer, an unsaturated carboxylic ester monomer, a vinylester monomer, and an unsaturated nitrile monomer, and an N-vinyl acetamide, wherein the ratio of the structural unit derived from the N-vinyl acetamide in moles to the structural unit derived from the at least one monomer in moles is 1.00:0.010 to 1.00:0.250.

Description

Electrode for electrode, electrode for secondary battery, electrode for secondary battery, electrode for secondary battery

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

The present application claims priority based on Japanese Patent Application No. 2015-096330, filed on Jan.

The secondary electrode is used as a battery for a live-machine device such as a mobile phone, a battery for a hybrid electric vehicle, a plug-in hybrid automobile, or an electric vehicle. Among them, lithium ion batteries have been widely used because of their excellent energy density and charge and 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 of a lithium transition metal oxide such as lithium cobaltate or the like, and a conductive aid such as carbon black. Agents, and adhesives. The binder contained in the active material layer serves as an adhesive for fixing the active material layer to the current collector. The active material layer is generally formed by dissolving a material of an active material layer such as an active material in a solvent in which a binder is dissolved, as an active material solution, and applying it to a current collector.

At present, the positive electrode of a lithium ion battery is a fluorine-containing resin of polyvinylidene fluoride (PVDF) as a main binder. However, the adhesion of PVDF to the current collector is insufficient.

Further, in recent years, it has been desired to use water as a solvent for an electrode binder for a secondary battery based on future countermeasures against environmental problems and the like. However, PVDF can only be dissolved in a special highly polar solvent such as N-methyl-2-pyrrolidone (NMP). Therefore, it is desirable to have an adhesive that can be dissolved or dispersed in water.

As a binder for an electrode of a secondary battery excellent in adhesion, for example, Patent Document 1 proposes to use a fluororubber containing a copolymer of hexafluoropropylene and a vinylidene fluoride as a main component or fluorinated with chlorotrifluoroethylene. A fluororubber containing a copolymer of ethylene as a main component.

Further, Patent Document 2 proposes to use a fluorine-based polymer copolymer mainly composed of -CH ₂ -CF ₂ -, -CF(CF ₃ )-CF ₂ -, and -CF ₂ -CF ₂ - as a binder. .

Further, Patent Document 3 proposes to use a polymer having an N-vinylformamide unit as a binder.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 4-95363

[Patent Document 2] Japanese Patent Publication No. 8-4007

[Patent Document 3] International Publication No. 2012/176895

However, the adhesive strength of the electrode for a secondary battery of the prior art and the current collector are insufficient. Therefore, the electrode of the conventional secondary battery has a case where the active material layer is peeled off from the current collector. In the electrode of the secondary battery, the peeling of the active material layer is largely associated with the performance life and safety of the secondary battery.

Further, in the electrode of the conventional secondary battery, the electrode binder contained in the active material layer is swelled by being immersed in the electrolytic solution and becomes a problem. When the electrode binder contained in the active material layer is swollen, the active material layer is promoted from the current collector, and the internal resistance of the active material layer is increased to deteriorate the performance of the secondary battery.

Further, the conventional electrode for an electrode for a secondary battery has insufficient ability to disperse a conductive auxiliary agent such as carbon black. Therefore, when forming an active material layer, it is necessary to contain a dispersing agent other than a binder in the active material solution. The dispersant sometimes becomes a factor of an increase in internal resistance in the secondary battery. Therefore, it is desirable not to contain.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide solubility in water, solubility in NMP which is generally used for forming an electrode for a secondary battery, adhesion to a current collector, and dispersibility of a conductive auxiliary agent. a co-polymer which is inhibited by swelling when immersed in an electrolyte, and is suitable A copolymer used as a material for a binder for an electrode of a secondary battery.

Further, an object of the present invention is to provide an electrode binder for a secondary battery containing the above-described copolymer, an electrode composition for a secondary battery, and an electrode for a secondary battery.

In order to solve the above problems, the present inventors have focused on the active review of N-vinylacetamide which is a water-soluble monomer.

As a result, it has been found that 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 is used. A copolymer of a monomer and N-vinylacetamide and a copolymer containing a constituent unit derived from N-vinylacetamide as an electrode binder for a secondary battery, and thus invention.

That is, the present invention relates to the following matters.

(1) A copolymer comprising a salt selected from the group consisting of an unsaturated carboxylic acid monomer, an unsaturated carboxylic acid monomer, an unsaturated carboxylic acid ester monomer, a vinyl ester monomer, and an unsaturated nitrile monomer. a copolymer of at least one monomer and N-vinylacetamide, a molar amount of the constituent unit derived from N-vinylacetamide of the above-mentioned copolymer and derived from the aforementioned N-vinyl B The molar ratio of constituent units other than guanamine is 1.00:0.010 to 1.00:0.250.

(2) A copolymer according to (1), which is 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 solubility parameter of at least one monomer of the group is 13 (cal/cm ³ ) ^1/2 or less.

(3) The copolymer according to (1), wherein the aforementioned unsaturated carboxylic acid monomer is (meth)acrylic acid.

(4) The copolymer according to (1), wherein the aforementioned unsaturated carboxylic acid ester monomer is (meth) acrylate.

(5) The copolymer according to (1), wherein the aforementioned vinyl ester monomer is vinyl acetate.

(6) The copolymer according to (1), wherein the aforementioned unsaturated nitrile monomer is acrylonitrile.

(7) A binder for an electrode for a secondary battery, comprising the copolymer of any one of (1) to (6) and a fluorine-containing resin, and a total amount of the copolymer and the fluorine-containing resin The fluorine-containing resin is 90% by mass or less.

(8) A composition for an electrode for a secondary battery, comprising the copolymer of any one of (1) to (6), a solvent, an active material, and a conductive auxiliary agent.

(9) An electrode for a secondary battery, comprising: a current collector; and an active material layer formed on the current collector, wherein the active material layer contains an active material, a conductive auxiliary agent, and any one of (1) to (6) The co-polymer of the item.

The copolymer of the present invention is excellent in solubility in water, solubility in NMP which is usually used for forming an electrode for a secondary battery, adhesion to a current collector, dispersibility of a conductive auxiliary agent, and immersion in an electrolyte solution. The swelling is suppressed. Therefore, the copolymer of the present invention is suitable as an electrode binder for secondary batteries.

Moreover, the electrode composition 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 excellent in dispersibility of a conductive auxiliary agent. Therefore, the electrode composition of the secondary battery of the present invention can be easily produced by dissolving the copolymer in a solvent and dispersing the active material and the conductive auxiliary agent therein without using a dispersing agent. Further, the electrode composition for a secondary battery of the present invention is applied to a current collector and dried, whereby an active material layer having excellent adhesion to a current collector and suppressing swelling during immersion in an electrolytic solution can be formed.

Further, the electrode-based active material layer for a secondary battery of the present invention contains the copolymer of the present invention. Therefore, the active material layer having excellent adhesion between the active material layer and the current collector and suppressing swelling during immersion in the electrolytic solution is provided. Therefore, the electrode for a secondary battery of the present invention can suppress deterioration of the secondary battery.

Fig. 1 is a view showing the shape of a test body subjected to a tensile test and the state of a tensile test.

In the following, the copolymer for the electrode of the present embodiment, the electrode for the secondary battery, the electrode composition for the secondary battery, and the secondary battery are used. Detailed description of the electrodes.

[Copolymer for electrode for electrode and secondary battery]

The copolymer of the present embodiment is a group 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 at least one monomer and N-vinylacetamide (hereinafter sometimes referred to as "N-vinylacetamide copolymer").

The monomer other than N-vinylacetamide used in the polymerization of the N-vinylacetamide copolymer (hereinafter sometimes referred to as "other monomer") is selected from the group consisting of unsaturated carboxylic acid monomers and unsaturated. At least one monomer of the group consisting of a salt of a carboxylic acid monomer, an unsaturated carboxylate monomer, a vinyl ester monomer, and an unsaturated nitrile monomer.

As other monomers, it is preferred to use the SP value δ (solubility parameter) compared with N-vinylacetamide (SP value δ (fedors method) 13 (cal/cm ³ ) ^1/2 ) on the non-polar side ( Those with smaller SP values.

The N-vinylacetamide copolymer of the present embodiment contains a sufficient constituent unit derived from N-vinylacetamide. Therefore, when the SP value δ is -vinylacetamide or less (13 (cal/cm ³ ) ^1/2 or less) as the other monomer, the SP value δ of the N-vinylacetamide copolymer becomes close. For NMP (SP value δ (fedors push algorithm) 11.3 (cal/cm ³ ) ^1/2 ). The N-vinylacetamide copolymer having an SP value δ close to NMP is preferred because it is excellent in solubility to NMP.

As the SP value δ compared with N-vinylacetamide compared to non-polar Preferred monomers of the other monomers used in the polymerization of the N-vinylacetamide copolymer are exemplified by the compounds shown in Table 1, for example. In Table 1, the SP value δ calculated using the fedors' estimation algorithm using a monomer which can be used as another monomer and N-vinylacetamide and NMP is shown.

δ =(⊿E/V) ^1/2

V: molar molecular volume of solvent ⊿E: agglutination energy (evaporation energy)

The unsaturated carboxylic acid monomer used in 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 in the polymerization of the N-vinylacetamide copolymer is a metal atom or the like substituted with a hydrogen atom of a carboxyl group contained in the above unsaturated carboxylic acid monomer.

In the polymerization of the N-vinylacetamide copolymer, the monomer other than N-vinylacetamide contains an unsaturated carboxylic acid monomer and/or a salt thereof The copolymerizable unsaturated group having a monomer may be copolymerized with N-vinylacetamide. Further, the N-vinylacetamide copolymer obtained after the polymerization has a structure of a carboxyl group having a polar group. Therefore, a high hydrogen bonding force with a metal surface is obtained. Therefore, when the active material layer is provided on the current collector made of a metal using the electrode binder containing the copolymer, an active material layer excellent in adhesion to the current collector is obtained.

The unsaturated carboxylic acid monomer and/or its salt used as the other monomer is preferably acrylic acid, methacrylic acid, crotonic acid, and the like, the ammonium salt, the organic amine salt, the monovalent metal salt, and the divalent metal. salt. Among these, (meth)acrylic acid and/or its salt is particularly preferably used as the unsaturated carboxylic acid monomer and/or its salt. The unsaturated carboxylic acid monomer and/or its salt is preferably acrylic acid or a salt thereof from the viewpoint of improving the adhesion to the current collector and suppressing swelling at the time of immersion in the electrolytic solution. Regarding the salt, from the viewpoint of stability, a sodium salt or an ammonium salt is preferred.

In the present specification, "(meth)acrylic acid" means acrylic acid or methacrylic acid.

The unsaturated carboxylic acid ester monomer used in the polymerization of the N-vinylacetamide copolymer preferably has an SP value δ of 13 (cal/cm ³ ) ^1/2 or less. As such unsaturated carboxylic acid ester monomers, (meth) acrylate is exemplified. Specific examples are methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isopropyl (meth)acrylate, octyl (meth)acrylate, (meth)acrylic acid 2 Ethylhexyl ester, methoxyethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and the like. Among these, methyl methacrylate or methyl acrylate is preferably used as the unsaturated carboxylic acid ester monomer because the cohesive force can be expected to be improved particularly by interaction with N-vinylacetamide.

In the polymerization of the N-vinylacetamide copolymer, when the unsaturated carboxylate monomer is contained as another monomer, the N-vinylacetamide copolymer obtained after the polymerization contains affinity for NMP. An ester moiety that is higher than the guanamine group. Therefore, it becomes an N-vinyl acetamide copolymer excellent in solubility with respect to NMP. When the unsaturated carboxylate monomer is contained as another monomer, the N-vinylacetamene obtained after the polymerization can be adjusted by adjusting the ratio of the amount of the N-vinylacetamide to the unsaturated carboxylate monomer. Affinity of the amine copolymer to the electrolyte.

The vinyl ester monomer used in the polymerization of the N-vinylacetamide copolymer preferably has an SP value δ of 13 (cal/cm ³ ) ^1/2 or less. Examples of such vinyl ester monomers are, for example, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl laurate, vinyl phthalate, vinyl stearate, vinyl hexanoate. , vinyl octanoate, vinyl palmitate, and the like. Among these vinyl ester monomers, especially vinyl acetate, it is preferably used because the reaction rate and molecular size are similar to those of N-vinylacetamide.

In the polymerization of the N-vinylacetamide copolymer, when the vinyl ester monomer is contained as another monomer, the copolymerization property with N-vinylacetamide is high, and it is preferable. This is because the vinyl ester monomer is a non-conjugated monomer having a Q value (conjugate effect) of 0.2 or less, similarly to N-vinylacetamide.

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

In the polymerization of the N-vinylacetamide copolymer, when an unsaturated nitrile monomer is contained as another monomer, structural interactivity in copolymerization can be expected and electrochemical stability can be improved. This is because the e value (polar effect) of the unsaturated nitrile monomer exceeds 0 and the e value of N-vinylacetamide is -1.57, and the reaction rate of the two is less than one.

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

When the N-vinylacetamide copolymer is set to 1.00, the constituent unit derived from N-vinylacetamide is more than the molar ratio of the constituent units other than the aforementioned N-vinylacetamide At 0.250, the solubility in water is insufficient. Further, the N-vinylacetamide copolymer having a molar ratio of more than 0.250 in the above-mentioned constituent unit is active when an active material layer containing the copolymer is provided on a current collector to form an electrode for a secondary battery. The material layer is greatly swelled by the electrolyte. Therefore, the active material layer is peeled off from the current collector, and the internal resistance of the active material layer is increased to deteriorate the performance of the secondary battery. The N-vinylacetamide copolymer preferably has a molar ratio of 0.150 or less, more preferably 0.100 or less, in the above-mentioned constituent unit.

Moreover, the N-vinylacetamide copolymer having a molar ratio of the above-mentioned constituent units of less than 0.010 is used for forming the electrode of the secondary battery. The solubility of NMP which is usually used is insufficient. Further, the N-vinylacetamide copolymer having a molar ratio of the above-mentioned constituent units of not more than 0.010 is insufficient in compatibility with the fluorine-containing resin. Based on these, the N-vinylacetamide copolymer preferably has a molar ratio of the above constituent unit of 0.010 or more, preferably 0.050 or more.

The weight average molecular weight of the N-vinylacetamide copolymer is based on the ease of adjustment of the viscosity when the electrode composition containing the electrode is used, the dispersibility of the conductive auxiliary agent, and the formation of the active material layer containing the same The viewpoint of the adhesion of the current collector, etc., is preferably from 0.1 to 3,000,000, more preferably from 10 to 1.5 million. When the weight average molecular weight of the N-vinylacetamide copolymer is 0.1 million or more, it is easy to obtain an electrode binder for a secondary battery in which swelling at the time of immersion in an electrolytic solution is suppressed. When the weight average molecular weight of the N-vinylacetamide copolymer is 3,000,000 or less, when the electrode composition containing the copolymer is prepared, it is easy to obtain an electrode composition which is easy to apply.

The weight average molecular weight of the N-vinylacetamide copolymer of the present embodiment is a value calculated by the method shown below. A calibration curve was prepared using the absolute molecular weight measurement results of N-vinylacetamide in each molecular weight band of GPC-MALS (Multi-Angle Light Scattering Detector), GPC from N-vinylacetamide copolymer (condensation) Gel permeation chromatography) The value calculated by the measurement results.

The N-vinylacetamide copolymer of the present embodiment can be used alone as an electrode binder for 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, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), and polyvinyl fluoride (PVF).

The N-vinylacetamide copolymer is excellent in compatibility with a fluorine-containing resin. Therefore, the N-vinylacetamide copolymer and the fluorine-containing resin can be used in combination at any ratio.

In addition to the N-vinylacetamide copolymer, a binder for an electrode containing a fluorine-containing resin is excellent in dispersibility as a conductive auxiliary agent and swelled when immersed in an electrolytic solution, compared with a case where a fluorine-containing resin is used alone. The electrode for suppressing the binder is excellent in adhesion to the current collector when the active material layer containing the same is produced.

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 is preferably 90% by mass or less based on the total amount of the N-vinylacetamide copolymer and the fluorine-containing resin. More preferably, it is 70% by mass or less, more preferably 50% by mass or less, and still more 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 containing the N-vinylacetamide copolymer in the electrode binder. When the content of the fluorine-containing resin is 50% by mass or less based on the total amount, the peel strength described later can be sufficiently obtained, and the dispersibility of the conductive auxiliary agent is excellent.

The peeling strength of the electrode adhesive for a secondary battery of the present embodiment measured by the method described later is preferably 0.015 N/m or more. When the peeling strength is 0.015 N/m or more, when an electrode for a secondary battery having the active material layer containing the electrode binder is formed, the active material layer can be sufficiently obtained. Bonding force with the collector.

In the electrode binder for a secondary battery of the present embodiment, the electrolyte impregnation rate measured by the method described later is preferably 10% or less. When the secondary battery of the secondary battery electrode including the active material layer containing the electrode adhesive is formed, the active material and the conductive auxiliary in the active material layer can be appropriately maintained when the electrolyte solution impregnation rate is 10% or less. The distance of the agent. When the electrolyte impregnation swelling ratio is 10% or less, the active material layer is swollen in the active material layer containing the electrode binder to prevent the binder from being dissolved in the electrolyte. By this, when the electrolyte impregnation swelling ratio is 10% or less, deterioration of the performance of the secondary battery can be prevented.

The electrode adhesive for a secondary battery of the present embodiment can be used for a positive electrode of a secondary battery or for a negative electrode.

"Method for producing a copolymer and a method for producing an electrode for a secondary battery"

Next, a method of producing a copolymer and a method of producing an electrode binder for a secondary battery of the present embodiment will be described.

In the case of producing an electrode binder for a secondary battery of the present embodiment, first, an N-vinylacetamide copolymer is produced by the method described below.

The N-vinylacetamide copolymer can be obtained by a salt selected from the group consisting of an unsaturated carboxylic acid monomer, an unsaturated carboxylic acid monomer, and an unsaturated carboxylic acid ester in the presence of a polymerization initiator in the reaction apparatus. A monomer, a vinyl ester monomer, and an unsaturated nitrile monomer are produced by polymerizing at least one monomer (other monomer) with N-vinylacetamide.

The ratio of the total amount of the monomers used in the polymerization of the N-vinylacetamide copolymer (the total amount of N-vinylacetamide to other monomers) is preferably 85.0. ~99.9 mass%. When the ratio of N-vinylacetamide used in the polymerization of the N-vinylacetamide copolymer is from 85.0 to 99.9% by mass, the constituent unit derived from N-vinylacetamide can be easily obtained. The ratio of the number of ears to the number of moles derived from the constituent units other than the aforementioned N-vinylacetamide (constituting unit derived from N-vinylacetamide: other constituent units) is 1.00:0.010 to 1.00:0.250 N-vinylacetamide copolymer.

The polymerization method for producing the N-vinylacetamide copolymer is not particularly limited. A polymerization method such as solution polymerization, dropping polymerization, reverse phase suspension polymerization, emulsion polymerization, precipitation polymerization or the like can be used. Among these, the polymerization method as the N-vinylacetamide copolymer is particularly preferably a precipitation polymerization method.

When the N-vinylacetamide copolymer is produced, the monomer used for the polymerization may be supplied to the reaction vessel before the start of the polymerization, or a part of the monomer used for the polymerization may be supplied to the reaction vessel in the polymerization. When a part of the monomer used for the polymerization is supplied to the reaction vessel in the polymerization, the monomer dissolved in the solvent is preferably supplied to the reaction vessel.

In particular, when the proportion of the other monomer in the total amount of the monomers used for the polymerization exceeds 5% by mass, a method of supplying a mixture of one of the other monomers used for the polymerization and the solvent to the reaction vessel in the polymerization is preferably used. This is because the reaction speed of N-vinylacetamide with other monomers can be appropriately controlled. Further, as the N-vinylacetamide copolymer, when the monomer component is localized (block structure), it is preferred to use a prepolymer. The monomers used in combination are all dissolved in a solvent.

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 the polymerization, or a part of the solvent used for the polymerization may be supplied to the reaction vessel in the polymerization.

The solvent used for the polymerization of the N-vinylacetamide copolymer is preferably used to dissolve the above-mentioned monomer used for the polymerization of the N-vinylacetamide copolymer and to precipitate the resulting copolymer. Solvent. Further, when the polymerization reaction is carried out, a solvent which easily dissolves the above monomer may be used, and after the copolymer is synthesized, a copolymer may be precipitated using another solvent which easily precipitates the copolymer.

As the solvent used for the polymerization of the N-vinylacetamide copolymer, a solvent which can be used for the polymerization of a general vinyl compound can be used. Specifically, for example, water, ethyl acetate, butyl acetate, acetone, methyl ethyl ketone, methyl isobutyl ketone, benzene, toluene, methanol, ethanol, isopropanol or the like can be exemplified. Among the above solvents, ethyl acetate is particularly preferably used.

In the present embodiment, a N-vinylacetamide copolymer having a weight average molecular weight of 0.1 to 1,500,000 is easily obtained by a polymerization method using an organic solvent as a solvent. Further, it is easy to obtain an N-vinylacetamide copolymer having a weight average molecular weight of about 5 to 3,000,000 by a polymerization method using water as a solvent.

As the polymerization initiator used for the polymerization of the N-vinylacetamide copolymer, a general user in the radical polymerization of a vinyl compound can be used without particular limitation. Specific examples are persulfate such as sodium, potassium and ammonium, benzammonium peroxide, hydrogen peroxide, hexanyl peroxide, sodium peracetate, Peroxy compounds such as sodium percarbonate, azobisisobutyronitrile, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[N-(carboxyl) 2-methylpropionamide], 2,2'-azobis{2-[N-(2-carboxyethyl)indolyl]propane}, 2,2'-azobis(2- An azo compound such as methyl propionate or dimethyl ester or 2,2'-azobis(2-methylpropionic acid).

Among the above polymerization initiators, azobisisobutyronitrile or 2,2'-azobis(2-methylpropionate) dimethyl ester which is soluble in an organic solvent is preferably used. Further, as the polymerization initiator, 2,2'-azobis(2-methylpropionate) dimethyl ester which does not contain a nitrile group and a halogen is preferably used.

In addition, the polymerization initiator is used in an amount of 0.005 to 5.0 parts by mass, for example, in the range of 0.005 to 5.0 parts by mass, based on 100 parts by mass of the monomers used for the polymerization, and is not particularly specified.

The reaction temperature for producing the N-vinylacetamide copolymer is preferably from 30 to 120 °C. By setting the reaction temperature in the above range, it is possible to carry out polymerization at a reaction rate suitable for the polymerization of N-vinylacetamide copolymer. The reaction temperature may be fixed from the beginning to the end of the polymerization, or may be changed in the polymerization reaction.

The polymerization of the N-vinylacetamide copolymer is a radical polymerization, and it is preferably carried out in a nitrogen atmosphere due to the influence of oxygen.

In the present embodiment, after the reaction product containing the N-vinylacetamide copolymer is obtained by a polymerization reaction, the reaction product may be washed with a solvent as needed.

The N-vinylacetamide copolymer thus obtained can be used alone as an electrode binder for a secondary battery.

Further, when the electrode binder for a secondary battery of the present embodiment contains an N-vinylacetamide copolymer and one or more fluorine-containing resins, the N-vinyl B obtained by the above production method When the guanamine copolymer and one or more fluorine-containing resins are mixed in the above ratio, they can be used as an electrode binder for a secondary battery.

[Composition for electrode of secondary battery]

The electrode composition for a secondary battery of the present embodiment contains the N-vinylacetamide copolymer, the solvent, the active material, and the conductive auxiliary agent of the present embodiment.

As the solvent, for example, a solvent for dissolving the N-vinylacetamide copolymer such as NMP, water, methanol, butanol, propylene glycol monomethyl ether, dimethyl hydrazine, or ethylene glycol can be used.

As the active material, a lithium transition metal oxide such as lithium cobalt oxide or the like can be used as an active material in the past, depending on the use of the secondary battery or the like.

As the conductive auxiliary agent, for example, carbon black, acetylene black, graphite or the like can be used as a conductive auxiliary agent depending on the use of the secondary battery or the like.

In addition to the N-vinylacetamide copolymer, the solvent, the active material, and the conductive auxiliary agent, the electrode composition of the secondary battery of the present embodiment may contain one or more kinds of fluorine-containing substances. The resin may also contain a conventionally known additive.

The electrode composition of the secondary battery of the present embodiment can be produced, for example, by the method described below.

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

[Electrode for secondary battery]

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, an aluminum foil having good adhesion to the electrode adhesive for a secondary battery of the present embodiment is preferably used.

The active material layer contains an active material, a conductive auxiliary agent, and an 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 to a current collector and drying it.

The number of moles of the constituent unit derived from N-vinylacetamide of the N-vinylacetamide copolymer of the present embodiment and the number of moles derived from the constituent units other than the aforementioned N-vinylacetamide The ratio is 1.00:0.010~1.00:0.250. Therefore, the N-vinylacetamide copolymer of the present embodiment is excellent in solubility in water and NMP, adhesion to a current collector, and dispersibility of a conductive auxiliary agent, and swelling at the time of immersion in an electrolytic solution is suppressed. By.

Moreover, the N-vinylacetamide copolymerization of the present embodiment The material is easy to apply because it has viscosity-increasing properties to NMP and water, and is suitable as an electrode binder for secondary batteries. Further, the N-vinylacetamide copolymer of the present embodiment is dissolved in a solution of NMP or water to be easily viscous, and a high cohesive force is obtained. Therefore, when the electrode composition containing the N-vinylacetamide copolymer of the present embodiment is produced, the kneading step for improving the viscosity and the cohesive force can be omitted, and even when the kneading step is performed, for example, when PVDF is used, Can be simplified.

Further, when the electrode binder for a secondary battery of the present embodiment does not contain a fluorine-containing resin, it is preferable because a corrosive acid component does not occur even when a secondary battery produced by using the secondary battery is thermally decomposed to a high temperature.

Further, the electrode composition of the present embodiment contains the N-vinylacetamide copolymer of the present embodiment which is excellent in solubility in NMP and water and excellent in dispersibility of a conductive auxiliary agent. Therefore, the electrode composition of the present embodiment can be easily produced by dissolving an N-vinylacetamide copolymer in a solvent and dispersing the active material and the conductive auxiliary agent therein without using a dispersing agent. . Moreover, the electrode composition of the present embodiment can be applied to a current collector and dried, whereby an active material layer having excellent adhesion to a current collector and suppressing swellability when immersed in an electrolytic solution can be formed.

Further, the electrode-based active material layer for a secondary battery of the present embodiment contains the N-vinylacetamide copolymer of the present embodiment. Therefore, the active material layer having excellent adhesion between the active material layer and the current collector and suppressing swellability when immersed in the electrolytic solution is obtained. Therefore, the electrode for a secondary battery according to the embodiment can prevent deterioration of the secondary battery.

[Examples]

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

(Examples 1 to 8 and Comparative Example 2)

N-vinylacetamide and other monomers shown in Table 2 were used in the mass ratio shown in Table 2 (N-vinylacetamide/other monomer) as the polymerization initiator 2,2'- Azobis(2-methylpropionate) dimethyl ester (trade name: V-601 (oil-soluble azo polymerization initiator) and Wako Pure Chemical Industries, Ltd.), based on 100 parts by mass of the total monomers, Table 3 The content (parts by mass) shown and used was polymerized to obtain the copolymers (adhesives) of Examples 1 to 8 and Comparative Example 2. Table 2 shows the polymer forms of the binders of Examples 1 to 8 and Comparative Example 2.

(Comparative Example 1)

As the monomer used for the polymerization, only N-vinylacetamide was used, and the polymerization initiator shown in Table 3 was used in an amount (parts by mass) shown in Table 3 with respect to 100 parts by mass of N-vinylacetamide. The binder of Comparative Example 1 was obtained by polymerization by the following method. The polymer form of the binder of Comparative Example 1 is shown in Table 2.

(Comparative Example 3)

Polyvinylidene fluoride (trade name: Kynar PVDF 760: manufactured by Arkema Co., Ltd.) containing a fluorine resin was used.

[Method for Producing Copolymer (Binder) of Examples 1, 2, 4 to 6, and 8]

A 3-neck separable flask as a reaction vessel was prepared, and a nitrogen insertion tube, a stirrer, a solvent dropping device, and a thermometer were attached. Next, in the reaction vessel, the initial feed-in ethyl acetate and N-vinylacetamide in a mass ratio of 20% to ethyl acetate were placed, and the inside of the reaction vessel was heated under a nitrogen flow while stirring. The polymerization starting temperature shown in Table 3. The volume ratio of the ethyl acetate initially fed to the reaction vessel is shown in Table 3.

Next, the polymerization initiation temperature was maintained in the reaction vessel, and the other monomers shown in Table 2 were placed in the reaction vessel at a mass ratio shown in Table 2. After a certain period of time, the polymerization initiator was introduced in the ratio shown in Table 3. .

Subsequently, the stirring speed was gradually increased while dropwise adding ethyl acetate, and polymerization was carried out at the polymerization time shown in Table 3. The reaction product precipitated in the form of a powder. Subsequently, the reaction product was filtered under suction, washed with ethyl acetate, and dried using nitrogen gas at a drying temperature and a drying time shown in Table 3 to obtain a binder.

[Method for Producing Copolymer (Binder) of Examples 3 and 7]

In addition to the volume ratio (50%) of the ethyl acetate to the reaction vessel initially fed in Table 3, the polymerization initiation temperature was 62 ° C, and other monomers and polymerization initiators were charged, and Examples 1, 2, 4 to 6, and 8 were also polymerized.

[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 added to 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, the polymerization initiator was charged in the proportions shown in Table 3 after the steps in which the other monomers were charged in the reaction vessel. Subsequently, other monomers diluted with ethyl acetate to 20% by mass were dropped into the reaction vessel in a mass ratio shown in Table 2, and polymerization was carried out at the polymerization time shown in Table 3. The reaction product was suction-filtered in the same manner as in Examples 1, 2, 4 to 6, and 8. The mixture was washed with ethyl acetate, and dried using nitrogen gas at a drying temperature and a drying time shown in Table 3 to obtain a binder.

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

Next, the binders of Examples 1 to 8 and Comparative Examples 1 and 2 were subjected to measurement by HPLC (High Speed Liquid Chromatography) to confirm the completion of the polymerization reaction.

Further, the binders of Examples 1 to 8 and Comparative Examples 1 and 2 were measured by NMR (nuclear magnetic resonance) to confirm the state of the structure. Specifically, methylene/A derived from N-vinylacetamide and other monomers in each of the binders (copolymers) of Examples 1 to 8 and Comparative Examples 1 and 2 by ¹ H-NMR The ratio of the base/methyl proton integral is determined by the integral ratio of C=O derived from each monomer by ¹³ C-NMR. Using this, the ratio derived from N-vinylacetamide and other monomers (Morby ratio) was determined. Further, in Example 8, which contains two kinds of monomers as other monomers, the ratio (mol ratio) was calculated for each of the two types of constituent units derived from N-vinylacetamide, and the total value was used as a source. The ratio of constituent units other than N-vinylacetamide (Morby). The results are shown in Table 4.

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

(Solubility of the binder to N-methyl-2-pyrrolidone (NMP) and pure water)

NMP was placed in a glass bottle with a dense plug, and the adhesive was added and embossed so that the binder concentration became 5% by mass. Next, the glass bottle with the tight plug was manually shaken up and down 20 times to mix the NMP with the adhesive. Subsequently, the glass bottle with the dense plug was allowed to stand in a thermostat at 20 ° C, and the dissolved state of the adhesive was visually confirmed, thereby measuring the time required until complete dissolution.

In addition, pure water is placed in a glass bottle with a dense plug, and the binder is added so that the binder concentration is 10% by mass, and the time required until the complete dissolution is measured in the same manner as the solubility of the above-mentioned binder to NMP. .

(solution viscosity)

The 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 placed in a circulating thermostatic water bath adjusted to a temperature of 20 °C. Subsequently, it was confirmed that the temperature of the binder solution fed into the container was adjusted to 20 ° C, and the viscosity was measured under the following conditions.

Further, the viscosity of the binder solution dissolved in pure water at a concentration of 10% by mass of the binder was measured in the same manner as the above-mentioned binder solution in which the binder was dissolved in NMP.

Viscometer: DVE (Brookfield) Viscometer

Rotor: No. 4 rotor

Number of rotations: 50rpm

Temperature: 20 ° C

Measurement time: The rotor of the viscometer was placed in the binder solution, and the value 30 minutes after the start of the rotation of the rotor was used as the measured value.

(electrolyte impregnated swelling ratio)

The binder was dissolved in NMP so as to have a concentration of 5% by mass, placed in a petri dish, and vacuum-dried at 150 ° C for 4 hours to prepare a film having a thickness of 1 mm. The obtained film was cut into a square of 1 cm in width and 1 cm in width and finely weighed.

Next, the film of the fine scale was immersed in the electrolytic solution in the test tube (ethyl carbonate/dimethyl carbonate/propyl propyl carbonate = 1/1/1 (volume ratio)). Next, the test tube in which the film and the electrolyte were placed was placed in a bath adjusted to 95 to 98 ° C, and taken out from the warm bath after 30 minutes. Subsequently, the film was taken out of the electrolyte. Wipe the electrolyte attached to the film and weigh the film.

Subsequently, the infiltration swell ratio of the electrolytic solution was calculated from the film quality before and after the immersion in the electrolytic solution using the following formula.

Electrolyte impregnation swelling ratio (%) = (film quality after impregnation - film quality before impregnation) / (film quality before impregnation) × 100

(peel strength)

1.5 g of a solution in which the binder was dissolved in NMP so as to have a concentration of 5 mass% was uniformly applied to an aluminum foil test piece (3 cm × 10 cm) using a bar coater. Next, the aluminum foil test piece on the side to which the solution was applied was laminated, and another aluminum foil test piece was laminated and bonded, and the roll was adhered. Subsequently, it was vacuum dried at 150 ° C for 6 hours to prepare a test body.

Subsequently, 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 bonding area of the test piece subjected to the tensile test was 21 cm ² (3 cm × 7 cm), and the distance between the jigs was 5 cm. The shape of the test piece and the tensile test in the tensile test are shown in Fig. 1.

The test system was prepared for each of the adhesives. The maximum values of the tensile tests of the three test pieces were averaged as the peel strength of each of the adhesives.

(Dispersibility of conductive additives)

The binder was added in a pure water stirred at 1000 rpm at a normal temperature by a stirrer to have a concentration of 1% by mass. The added adhesive was visually confirmed to be uniformly dissolved to obtain an aqueous binder solution. Next, carbon black (trade name: VGCF, manufactured by Showa Denko Co., Ltd.) as a conductive auxiliary agent was added to the aqueous binder solution in an amount of 5% by mass. Stirring was stopped after 20 minutes to obtain a carbon black dispersion.

2 g of the obtained carbon black dispersion was 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 of the dried glass plate was displayed on the screen of the display device using a microscope, and visually observed, and the dispersion state of the carbon black was evaluated as follows. Further, when the agglomerates of carbon black were observed on the glass plate, the diameter of the aggregate was measured using a ruler on the screen of the display device.

◎: no agglomerate, uniform state of dispersion

○: A small piece (aggregate) having a diameter of less than 200 μm was observed.

△: Agglomerates of a diameter of 200 μm to less than 1 mm were observed.

×: A large agglomerate having a diameter of 1 mm or more was seen.

(weight average molecular weight)

The binder was dissolved in distilled water at a concentration of 1% by mass, and was measured by GPC (gel permeation chromatography) under the following conditions. Using the results, the weight average molecular weight (Mw) was calculated.

The weight average molecular weight was calculated by using the measurement results of the absolute molecular weight (GPC-MALS) of N-vinylacetamide in each molecular weight band.

Detector (RI): RI-201H (manufactured by SHODEX)

Pump: LC-20AD (made by Shimadzu Corporation)

Column oven: AO-30C (made by SHODEX)

Analysis device: SIC4802 Deta Station (made by Shimadzu Corporation)

Column: SB806×2 (made by SHODEX)

Dissolved solution: DW (distilled water)

Flow rate: 0.7ml/min

(Compatibility of fluorine-containing resin)

The binder was dissolved in NMP (dissolved solution 1) so as to have a concentration of 5% by mass. Polyvinylidene fluoride (PVDF) was dissolved in NMP (dissolved solution 2) so as to have a concentration of 5% by mass. Mix the solution 1 and the solution 2 in a ratio of 1:1 (mass ratio) to form a mixed solution, and let stand at room temperature. Set for 24 hours.

Subsequently, the presence or absence of turbidity and the separation of the solution 1 and the solution 2 were evaluated by visual observation on the basis of the mixture.

[turbidity evaluation]

○: Clarification

×: turbidity

[Is there or not]

○: not separated

×: There is separation

As shown in Table 5, the binders (copolymers) of Examples 1 to 8 were compared with the binder of Comparative Example 1 using only N-vinylacetamide as a monomer, and required for complete dissolution in NMP. The time is short and the solubility in NMP is excellent. Further, comparing the binders of Examples 1 to 8 with the binder (copolymer) of Comparative Example 2 derived from N-vinylacetamide, the time required for completion in water until complete dissolution was compared. Short, excellent in solubility in water.

Further, in the adhesives of Examples 1 to 8, as shown in Table 5, the solution viscosity was sufficiently high, and the solution dissolved in NMP or water had sufficient viscosity-increasing property, which was preferable at the time of coating.

Further, in the adhesives of Examples 1 to 8, the swelling ratio of the electrolytic solution impregnation was 10% or less, and the swelling at the time of immersion in the electrolytic solution was suppressed.

Moreover, the adhesives of Examples 1 to 8 have a peel strength of 0.015 N/m. Excellent adhesion to the current collector.

Further, in the adhesives of Examples 1 to 8, the dispersibility of the conductive auxiliary agent was evaluated as ◎ or ○, and the dispersibility of the conductive auxiliary agent was excellent.

Further, the adhesives of Examples 1 to 8 were evaluated for turbidity as ○, and were not separated, and the compatibility with the fluorine-containing resin was good.

Further, the binders of Examples 1 to 8 have an appropriate weight average molecular weight.

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

Further, the binder of Comparative Example 2, which has a small constituent unit derived from N-vinylacetamide, has a low solubility in water as compared with the binders of Examples 1 to 8. Further, in the adhesive of Comparative Example 2, the electrolytic solution impregnation rate was more than 10%, and the swelling suppressing effect when immersed in the electrolytic solution was lower than that of the adhesives of Examples 1 to 8.

Further, Comparative Example 3 of the fluorine-containing resin was insoluble in water. Further, the binder of Comparative Example 3 had a very large impregnation rate of the electrolyte. Further, the dispersibility of the conductive auxiliary agent of the binder of Comparative Example 3 was evaluated as ×. Further, the peel strength of the adhesive of Comparative Example 3 was 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 was mixed with the binder of Comparative Example 3 of the fluorine-containing resin. The adhesive of the embodiment 1 or the embodiment 2 is combined with the fluororesin of the adhesive of the embodiment 1 or the embodiment 2. The ratio of the measured mass [(Example 1 or Example 2) / (Example 1 or Example 2+ Comparative Example 3) × 100 (% by mass)] was mixed in the ratio shown in Table 5. The mixed binder was dissolved in NMP so as to be a solution at a concentration of 5% by mass, and allowed to stand at 20 ° C for 12 hours.

(Comparative Example 4)

The binder of Comparative Example 3 of the fluorine-containing resin was dissolved in NMP so as to have a concentration of 5% by mass, and was allowed to stand at 20 ° C for 12 hours.

The dissolving solutions of Examples 9 to 13 and Comparative Example 4 thus obtained were placed in a Petri dish, and the impregnation rate of the electrolytic solution was measured by the same method as above. Further, using the solutions of Examples 9 to 13 and Comparative Example 4, the peel strength was measured in the same manner as above. The results of the electrolyte impregnation swelling ratio and the peel strength are shown in Table 6.

Further, the dispersibility of the conductive auxiliary agent 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 additives)

300 g of NMP was fed into a 500 ml separable flask at room temperature, and the binder of the first embodiment or the second embodiment (copolymer) and the fluorine-containing resin were added while stirring with a stirrer at 150 rpm. Adhesive. The ratio (% by mass) shown in Table 6 was used to make the total mass of the binder of Example 1 or Example 2 and the binder of Comparative Example 3 Add in a concentration of 1% by mass. It was visually confirmed that the added adhesive was completely dissolved. Then, carbon black (trade name: VGCF, manufactured by Showa Denko Co., Ltd.) as a conductive auxiliary agent was added to NMP containing the obtained binder in an amount of 5% by mass, and stirring was stopped after 20 minutes.

2 g of the obtained carbon black dispersion was 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. Next, the state of the dried glass plate was evaluated in the same manner as the dispersibility of the conductive auxiliary agent of Example 1.

Adhesive ratio (% by mass) = [(Example 1 or Example 2) / (Example 1 or Example 2+ Comparative Example 3)] × 100

As shown in Table 6, in Examples 9 to 13 (adhesive for electrode), it was confirmed that the adhesion to the current collector and the dispersibility of the conductive auxiliary agent were excellent as compared with Comparative Example 4 using only the fluorine-containing resin, and immersed in The swelling of the electrolyte is suppressed.

Claims (9)

a copolymer of 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 monomer and N-vinylacetamide, the number of moles of the constituent unit derived from N-vinylacetamide of the above-mentioned copolymer and other than the aforementioned N-vinylacetamide The ratio of the number of moles of the constituent units is 1.00: 0.010 to 1.00: 0.250. The copolymer of claim 1, wherein the group 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 solubility parameter of at least one of the monomers is 13 (ca1/cm ³ ) ^1/2 or less. The copolymer of claim 1, wherein the aforementioned unsaturated carboxylic acid monomer is (meth)acrylic acid. The copolymer of claim 1, wherein the aforementioned unsaturated carboxylic acid ester monomer is a (meth) acrylate. The copolymer of claim 1 wherein the vinyl ester monomer is vinyl acetate. The copolymer of claim 1, wherein the aforementioned unsaturated nitrile monomer is acrylonitrile. A binder for an electrode for a secondary battery, comprising the copolymer of any one of claims 1 to 6 and a fluorine-containing resin, the fluorine-containing resin in combination with the total amount of the copolymer and the fluorine-containing resin It is 90% by mass or less. A composition for an electrode for a secondary battery, comprising the copolymer, the solvent, the active material, and the conductive auxiliary agent according to any one of claims 1 to 6. An electrode for a secondary battery, comprising: a current collector; and an active material layer formed on the current collector, wherein the active material layer comprises an active material, a conductive auxiliary, and a copolymer of any one of claims 1 to 6. .