Binder for secondary battery, electrode mixture, electrode, and non-aqueous electrolyte secondary battery

Abstract

Provided are a binder for a secondary battery that has exceptional adhesiveness to a current collector when an electrode is formed, an electrode mixture that contains the binder, an electrode in which the electrode mixture is used, and a non-aqueous electrolyte secondary battery comprising the electrode.　This binder for a secondary battery contains a vinylidene-fluoride-based copolymer (A) containing structural units (a1) derived from vinylidene fluoride and structural units (a2) derived from a compound represented by formula (1). (In the formula, RH represents a halogen atom, a hydroxy group, a carboxy group, an alkyl group that may have a substituent (excluding a butyl group having no substituent), or a phenyl group having a substituent.)

Description

Binder for secondary battery, electrode mixture, electrode, and non-aqueous electrolyte secondary battery

[0001] The present invention relates to a binder for a secondary battery, an electrode mixture, an electrode, and a non-aqueous electrolyte secondary battery.

[0002] The positive electrode of a lithium-ion battery is generally manufactured by coating a current collector foil with a positive electrode mixture slurry containing a positive electrode active material, a solvent, a binder, and other additives, followed by drying. Since peeling or falling off of this positive electrode coating film during the manufacturing process reduces yield, the coating film is required to have strong adhesiveness.

[0003] For example, Patent Document 1 discloses a compound containing vinylidene fluoride and a compound of the following formula (3): It is disclosed that an electrode produced using a binder containing a copolymer with a compound represented by the formula (I) has high adhesive strength.

[0004] Patent No. 5797206

[0005] However, there is a demand for a positive electrode coating film to have stronger adhesiveness than that achieved by using the binder disclosed in Patent Document 1.

[0006] The present invention has been made in view of the above-mentioned problems, and has an object to provide a binder for a secondary battery that has excellent adhesion to a current collector when an electrode is formed, an electrode mixture containing the binder, an electrode using the electrode mixture, and a nonaqueous electrolyte secondary battery including the electrode.

[0007] As a result of extensive research into solving the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by using a binder containing a vinylidene fluoride copolymer including a structural unit derived from a maleimide comonomer, and have thus completed the present invention.

[0008] Aspects of the present invention relate to the following secondary battery binder, electrode mixture, electrode, and nonaqueous electrolyte secondary battery.

[0009] [1] A binder for a secondary battery, comprising a vinylidene fluoride copolymer (A) including a structural unit (a1) derived from vinylidene fluoride and a structural unit (a2) derived from a compound represented by the following formula (1): (In the formula, R Ｈ represents a halogen atom, a hydroxy group, a carboxy group, an alkyl group which may have a substituent (excluding an unsubstituted butyl group), or a phenyl group which has a substituent. [2] In the compound represented by formula (1), R Ｈ is a hydroxy group, a carboxy group, an alkyl group having a hydroxy group, a phenyl group having a hydroxy group, an alkyl group having a carboxy group, or a phenyl group having a carboxy group. [3] The binder for a secondary battery according to [1] or [2], wherein the compound represented by formula (1) is a compound represented by the following formula (2): (wherein R is an atomic group having 1 to 5 carbon atoms.) [4] The binder for secondary batteries according to any one of [1] to [3], which contains 0.05 mol % or more and 1 mol % or less of the structural units (a2) derived from the compound represented by formula (1), relative to a total of 100 mol % of the structural units (a1) derived from vinylidene fluoride. [5] An electrode mixture comprising the binder according to any one of [1] to [4], an active material (B), and a conductive additive (C). [6] An electrode having, on a current collector, an electrode mixture layer comprising the electrode mixture according to [5]. [7] A nonaqueous electrolyte secondary battery comprising the electrode according to [6].

[0010] According to the present invention, it is possible to provide a binder for a secondary battery that exhibits excellent adhesion to a current collector when an electrode is formed, an electrode mixture containing the binder, an electrode using the electrode mixture, and a nonaqueous electrolyte secondary battery including the electrode.

[0011] <Binder> The binder is used for secondary batteries. The binder is preferably used for non-aqueous electrolyte secondary batteries. The binder contains a vinylidene fluoride copolymer (A) including a structural unit (a1) derived from vinylidene fluoride and a structural unit (a2) derived from a compound represented by formula (1) described below. By including the structural unit (a2) in the binder containing the vinylidene fluoride copolymer (A) including the structural unit (a1), an electrode formed using the binder exhibits excellent adhesion to a current collector.

[0012] Essential and optional components contained in the binder will be described below.

[0013] <Vinylidene fluoride copolymer (A)> As described above, the vinylidene fluoride copolymer (A) contains a structural unit (a1) derived from vinylidene fluoride and a structural unit (a2) derived from a compound represented by the following formula (1):

[0014] (Structural unit (a1) derived from vinylidene fluoride) The structural unit (a1) derived from vinylidene fluoride is a main structural unit of the vinylidene fluoride copolymer (A). From the viewpoint of excellent adhesion to a current collector when an electrode is formed using a binder, the content of the structural unit (a1) derived from vinylidene fluoride is preferably 98.0 parts by mass or more and 99.9 parts by mass or less, more preferably 98.5 parts by mass or more and 99.8 parts by mass or less, and even more preferably 98.8 parts by mass or more and 99.7 parts by mass or less, relative to 100 parts by mass of all structural units of the vinylidene fluoride copolymer (A).

[0015] (Structural Unit (a2) Derived from a Compound Represented by Formula (1)) The structural unit (a2) is derived from a compound represented by the following formula (1).

[0016] In the above formula (1), R Ｈ represents a halogen atom, a hydroxy group, a carboxy group, an alkyl group which may have a substituent (excluding an unsubstituted butyl group), or a phenyl group which has a substituent.

[0017] R Ｈ The alkyl group in the formula (I) is preferably an alkyl group having 1 to 5 carbon atoms. As the alkyl group having 1 to 5 carbon atoms, a linear or branched alkyl group is preferred, and specific examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a tert-butyl group, a pentyl group, an isopentyl group, and a neopentyl group.

[0018] R ＨExamples of the substituent that the alkyl group in the formula (I) may have include a hydroxy group, a carboxy group, an amide group, a boronic acid group, a nitro group, a cyano group, a carbonyl group, etc. Among these, from the viewpoint of excellent adhesion to a current collector when an electrode is formed using the binder, a hydroxy group and a carboxy group are preferred, and a carboxy group is more preferred.

[0019] R Ｈ The substituents that the phenyl group in Ｈ Examples of the substituents include the same substituents that the alkyl group in the above may have.

[0020] In the compound represented by the above formula (1), R Ｈ is preferably a hydroxy group, a carboxy group, an alkyl group having a hydroxy group, a phenyl group having a hydroxy group, an alkyl group having a carboxy group, or a phenyl group having a carboxy group.

[0021] The compound represented by the above formula (1) is preferably a compound represented by the following formula (2).

[0022] In the above formula (2), R is an atomic group having 1 to 5 carbon atoms. The number of carbon atoms in the atomic group represented by R means the number of carbon atoms in the shortest chain constituting the chain from one end of R bonded to the nitrogen atom to the other end of R bonded to the carboxy group. R may be linear or branched.

[0023] Examples of the atomic group having 1 to 5 carbon atoms include a methylene group, an ethylene group, an n-propylene group, an isopropylene group, an n-butylene group, an isobutylene group, an n-pentylene group, and an isopentylene group.

[0024] From the viewpoint of excellent adhesion to a current collector when an electrode is formed using a binder, the content of the structural unit (a2) derived from the compound represented by formula (1) is preferably 0.1 parts by mass or more and 2.0 parts by mass or less, more preferably 0.2 parts by mass or more and 1.5 parts by mass or less, and even more preferably 0.3 parts by mass or more and 1.2 parts by mass or less, relative to 100 parts by mass of all structural units of the vinylidene fluoride copolymer (A).

[0025] From the viewpoint of excellent adhesion to a current collector when an electrode is formed using the binder, the content of the structural unit (a2) derived from the compound represented by formula (1) is preferably 0.05 mol % or more and 1 mol % or less, more preferably 0.07 mol % or more and 0.7 mol % or less, more preferably 0.1 mol % or more and 0.4 mol % or less, and even more preferably 0.1 mol % or more and 0.25 mol % or less, relative to a total of 100 mol % of the structural unit (a1) derived from vinylidene fluoride.

[0026] The reason why an electrode formed using a binder containing the vinylidene fluoride copolymer (A) has superior adhesion to a current collector is unclear, but it is thought that the maleimide skeleton in the binder is opened by the base or heat generated in the process of preparing the active material slurry, generating a new carboxyl group, and this newly generated carboxyl group interacts with R Ｈ It is presumed that the excellent adhesive properties are exhibited because the adhesive functional groups contained in the copolymer are concentrated in one comonomer unit.

[0027] For example, in a binder containing a vinylidene fluoride copolymer (A) including a structural unit (a2) derived from 3-maleimidopropionic acid, which corresponds to the compound represented by the above formula (2), it is presumed that the maleimide skeleton undergoes ring-opening as shown in the following formula, thereby generating the above-mentioned new carboxy group.

[0028] The main chain of vinylidene fluoride copolymer (A) may contain other structural units than the structural unit (a1) and the structural unit (a2), as long as it does not impair the effect of the present invention.Other structural units include, for example, vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, fluoroalkyl vinyl ether, perfluoroalkyl vinyl ether represented by perfluoromethyl vinyl ether, (meth)acrylic acid, (meth)acrylic acid ester represented by methyl (meth)acrylate, acryloyloxyethyl succinic acid, acryloyloxypropyl succinic acid, carboxyethyl acrylate, etc.These compounds may be referred to as "other monomers" below.

[0029] <Physical Properties of Vinylidene Fluoride Copolymer (A)> (Inherent Viscosity) The inherent viscosity of the vinylidene fluoride copolymer (A) is preferably 1.5 dL / g or more and 4.0 dL / g or less, and more preferably 2.0 dL / g or more and 3.5 dL / g or less. When the inherent viscosity is 2.0 dL / g or more, the adhesive strength between the binder and the active material or current collector is increased. On the other hand, when the inherent viscosity is 3.0 dL / g or less, the viscosity of the slurry does not become too high when an electrode slurry is prepared, and workability is excellent.

[0030] The inherent viscosity (ηi) is calculated, for example, as follows. A polymer solution is prepared by dissolving 80 mg of the vinylidene fluoride copolymer in 20 mL of N,N-dimethylformamide. The viscosity η of the prepared polymer solution is measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. The inherent viscosity (ηi) is then calculated using the following formula:

[0031] ηi=(1 / C)·ln(η / η0) In the above formula, η0 is the viscosity of the solvent N,N-dimethylformamide, and C is the concentration of vinylidene fluoride polymer in the prepared polymer solution (0.4 g / dL).

[0032] <<Method for Producing Vinylidene Fluoride Copolymer (A)>> The method for producing the vinylidene fluoride copolymer (A) is not particularly limited, and is usually carried out by methods such as suspension polymerization, emulsion polymerization, solution polymerization, etc. From the viewpoint of ease of post-treatment, etc., aqueous suspension polymerization and emulsion polymerization are preferred, and aqueous suspension polymerization is more preferred.

[0033] The aqueous suspension polymerization method is not particularly limited, and examples thereof include a method in which all monomers to be used in copolymerization (vinylidene fluoride, the compound represented by the above formula (1), and, if necessary, the other monomers) are copolymerized in an aqueous medium in the presence of a suspending agent and a polymerization initiator.

[0034] The suspending agent is not particularly limited, and examples thereof include methyl cellulose, methoxylated methyl cellulose, propoxylated methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, polyvinyl alcohol, polyethylene oxide, gelatin, etc. The amount of the suspending agent used is not particularly limited, and is preferably 0.005 parts by mass or more and 1.0 parts by mass or less, and more preferably 0.01 parts by mass or more and 0.4 parts by mass or less, relative to 100 parts by mass of all monomers used in the copolymerization.

[0035] The polymerization initiator is not particularly limited, and examples thereof include diisopropyl peroxydicarbonate, di-normal propyl peroxydicarbonate, di-normal heptafluoropropyl peroxydicarbonate, isobutyryl peroxide, di(chlorofluoroacyl)peroxide, di(perfluoroacyl)peroxide, t-butyl peroxypivalate, etc. The amount of the polymerization initiator used is not particularly limited, and for example, is preferably 0.05 parts by mass or more and 5 parts by mass or less, and more preferably 0.15 parts by mass or more and 2 parts by mass or less, relative to 100 parts by mass of all monomers used in the copolymerization.

[0036] The total amount of monomers used in copolymerization, expressed as the mass ratio of total monomers to water, is typically 1:1 to 1:10, preferably 1:2 to 1:5. The polymerization conditions, such as polymerization temperature and polymerization time, used in suspension polymerization are not particularly limited; for example, known polymerization conditions may be employed. The polymerization temperature T is appropriately selected depending on the 10-hour half-life temperature T10 of the polymerization initiator, and is typically selected within the range of T10 - 25°C ≦ T ≦ T10 + 25°C. For example, the T10 values ​​of t-butyl peroxypivalate and diisopropyl peroxydicarbonate are 54.6°C and 40.5°C, respectively (see NOF Corporation product catalog). Therefore, in polymerizations using t-butyl peroxypivalate and diisopropyl peroxydicarbonate as polymerization initiators, the polymerization temperature T is appropriately selected within the ranges of 29.6°C ≦ T ≦ 79.6°C and 15.5°C ≦ T ≦ 65.5°C, respectively. The polymerization time is not particularly limited, but is preferably 1 to 24 hours in consideration of productivity and the like.

[0037] <Other Components> The binder may contain components other than the vinylidene fluoride copolymer (A) (hereinafter also referred to as "other components") as long as the effects of the present invention are not impaired. As the other components, any known additives can be used, and examples thereof include a non-aqueous solvent (S), a plasticizer, a dispersant, etc.

[0038] Examples of the non-aqueous solvent (S) include N-methyl-2-pyrrolidone (hereinafter also referred to as NMP), dimethylformamide, N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, hexamethylphosphoamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate, acetone, cyclohexanone, methyl ethyl ketone, and tetrahydrofuran. These may be used alone or in combination of two or more.

[0039] <Method of Producing Binder> The binder can be prepared by mixing the vinylidene fluoride copolymer (A) and, if necessary, other components.

[0040] <Electrode Mix> The electrode mix refers to an electrode mix for a positive electrode. The electrode mix preferably contains the binder of the above-described embodiment, an active material (B), and a conductive additive (C). By adding a binder containing the vinylidene fluoride copolymer (A) containing the structural unit (a2) derived from the compound represented by formula (1) to the electrode mix containing the active material (B), an electrode having excellent adhesion to the current collector can be formed.

[0041] Essential and optional components contained in the electrode mixture will be described below.

[0042] <Binder> The content of the binder is not particularly limited, but from the viewpoint of battery performance and adhesion to the current collector, it is preferably 0.2 parts by mass or more and 15 parts by mass or less, and more preferably 0.5 parts by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the active material (B) described below.

[0043] <Active Material (B)> The type of active material (B) is not particularly limited, and may be a positive electrode active material or a negative electrode active material. In particular, the active material (B) is preferably a positive electrode active material.

[0044] Examples of positive electrode active materials include LiMnO ２ , LiMn ２ O ４ , LiCoO ２ , LiNiO ２ , LiNi ｘ Co １－ｘ O ２ (0<x<1), LiNi ｘ Co ｙ Mn １－ｘ－ｙ O ２ (0<x<1, 0<y<1), LiNi ｘ Co ｙ Al １－ｘ－ｙ O ２ (0<x<1, 0<y<1), LiMaPO ４ (wherein Ma is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr), Li ２ MnO ３ -LiMbO ２Lithium-excess solid solution cathode represented by (Mb=Mn, Co, Ni), lithium titanate (Li ４ Ti ５ O １２ ), titanium oxide (TiO ２ ), Li ｓ Ni ｔ Co ｕ Al ｖ O ２ (0.9<s<1.3, 0.9<t+u+v<1.1), etc.

[0045] From the viewpoint of increasing the charging potential of the secondary battery and further improving the cycle characteristics, the positive electrode compound is preferably a lithium metal oxide (ternary lithium metal oxide) represented by the following formula (4).

[0046] LiNi ｘ Co ｙ M ｚ O ２ ...(4)

[0047] In formula (4), M is Mn or Al, 0<x<1, 0<y<1, 0<z<1, and x+y+z=1.

[0048] Examples of ternary lithium metal oxides include Li １．００ Ni ０．３５ Co ０．３４ Mn ０．３４ O ２ (NCM111), Li １．００ Ni ０．５ Co ０．２ Mn ０．３ O ２ (NCM523), Li １．００ Ni ０．５０ Co ０．３０ Mn ０．２０ O ２ (NCM532), Li １．００ Ni ０．６０ Co ０．２０ Mn ０．２０ O ２ (NCM622), Li １．００ Ni ０．８０ Co ０．１０ Mn ０．1０ O ２ (NCM811), and Li １．００ Ni ０．８０ Co ０．１５Al ０．０５ O ２ (NCA811) and the like.

[0049] Among the positive electrode active materials exemplified above, Li １．００ Ni ０．８０ Co ０．１０ Mn ０．1０ O ２ (NCM811), and LiNi ０．８０ Co ０．１５ Al ０．０５ O ２ (NCA811) is preferred.

[0050] The content of the active material (B) is not particularly limited, but is preferably 40 parts by mass or more and 99.9 parts by mass or less, more preferably 80 parts by mass or more and 99.5 parts by mass or less, and even more preferably 90 parts by mass or more and 99.5 parts by mass or less, relative to 100 parts by mass of the electrode mixture.

[0051] The positive electrode active material can be synthesized by a known method, for example, by mixing compounds of each constituent element, such as a Li-containing compound, a Ni-containing compound, a Co-containing compound, a Mn-containing compound, and an Al-containing compound, and then firing the mixture.

[0052] Examples of the negative electrode active material include carbon materials, silicon materials, metal oxides, and lithium alloys.

[0053] <Conductive Aid (C)> Examples of the conductive aid (C) include carbon black, carbon nanotubes, etc. These may be used alone or in combination of two or more.

[0054] The content of the conductive additive (C) is not particularly limited, but is preferably 0.1 parts by mass or more and 5 parts by mass or less, and more preferably 0.2 parts by mass or more and 4 parts by mass or less, relative to 100 parts by mass of the active material (B).

[0055] <Other Components> The electrode mixture may contain components (hereinafter also referred to as "other components") other than the binder, active material (B), and conductive additive (C) of the above-described embodiment, as long as the effects of the present invention are not impaired. As the other components, any known additives can be used, for example, a non-aqueous solvent (S), an insulating inorganic filler such as alumina, magnesia, or silica, an insulating organic filler such as polytetrafluoroethylene, polyimide, or polyacrylonitrile, a plasticizer, a dispersant, a flame retardant, etc.

[0056] Examples of the non-aqueous solvent (S) include N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP"), dimethylformamide, N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, hexamethylphosphoamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate, acetone, cyclohexanone, methyl ethyl ketone, and tetrahydrofuran. These may be used alone or in combination of two or more.

[0057] <Method for producing electrode mixture> The electrode mixture can be prepared by mixing the binder of the above-described embodiment, the active material (B), the conductive additive (C), and other components as necessary. The mixing method is not particularly limited, and the components can be mixed by a known method.

[0058] <Electrode> The electrode refers to a positive electrode. The electrode preferably has an electrode mixture layer containing the electrode mixture of the above-described embodiment on a current collector. The electrode mixture layer contains the vinylidene fluoride copolymer (A) containing the structural unit (a2) derived from the compound represented by formula (1) in the binder contained in the electrode mixture. By containing such vinylidene fluoride copolymer (A), the electrode exhibits the effect of excellent adhesion to the current collector.

[0059] <Current Collector> The current collector is a terminal for extracting electricity. The material of the current collector is not particularly limited, and metal foil or metal mesh of aluminum, copper, iron, stainless steel, steel, nickel, titanium, etc. can be used. Alternatively, the current collector may be formed by applying the above metal foil or metal mesh to the surface of another medium. Alternatively, the current collector may be formed by applying a surface treatment to the metal foil or metal mesh using a non-metallic substance such as a polymer or conductive carbon.

[0060] <Electrode Mixture Layer> The electrode mixture layer is obtained by applying the electrode mixture of the above-described embodiment onto the current collector and drying it.

[0061] The coating method is not particularly limited, and for example, a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, a die coating method, a dip coating method, or the like can be applied.

[0062] The drying temperature is, for example, preferably 80°C or higher and 300°C or lower, more preferably 90°C or higher and 250°C or lower, and even more preferably 100°C or higher and 200°C or lower. The drying time is, for example, preferably 10 seconds or higher and 300 minutes or lower, more preferably 1 minute or higher and 200 minutes or lower, and even more preferably 10 minutes or higher and 100 minutes or lower. Drying may be performed multiple times at different temperatures. Pressure may be applied during drying.

[0063] <<Non-aqueous electrolyte secondary battery>> A non-aqueous electrolyte secondary battery preferably includes the electrode of the above-described embodiment. As a non-aqueous electrolyte secondary battery, conventionally known components other than the positive electrode, such as a negative electrode and a separator, can be used.

[0064] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0065] <<Preparation Examples of Binders>> <Examples 1 to 3 and Comparative Examples 1 and 3> In Examples 1 to 3 and Comparative Examples 1 and 2, the following vinylidene fluoride copolymers A1 to A5 were prepared as the vinylidene fluoride copolymer (A) according to the following method. The vinylidene fluoride copolymers (A) A1 to A5 were used as binders in the preparation of electrode mixtures and electrodes described below. The inherent viscosity of the obtained vinylidene fluoride copolymers (A) was measured according to the following method. In Comparative Example 3, polyvinylidene fluoride homopolymer A6 was used as the binder. Hereinafter, the above polyvinylidene fluoride homopolymer A6 may also be referred to as "vinylidene fluoride copolymer (A)" for convenience.

[0066] [Inherent Viscosity] First, 80 mg of vinylidene fluoride copolymer (A) was dissolved in 20 mL of N,N-dimethylformamide, and the viscosity was measured using an Ubbelohde viscometer in a thermostatic bath at 30° C. From the obtained value, the inherent viscosity (η ｉ ) was calculated. ｉ =(1 / C)・ln(η / η ０ In the above formula, η is the viscosity of the solution, η ０ is the viscosity of the solvent N,N-dimethylformamide alone, and C is the concentration of vinylidene fluoride copolymer (A) in the solution, that is, 0.4 g / dL.

[0067] Example 1 Preparation of Vinylidene Fluoride Copolymer A1 (VDF / MImPrA) 1,213 g of ion-exchanged water, 27.6 g of a 1.45 mass % aqueous solution of Metolose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.2 g of a 50 mass % diisopropyl peroxydicarbonate-Flon 225cb solution, 400 g of vinylidene fluoride, and 4.0 g of MImPrA (3-maleimidopropionic acid) were charged into a 2-liter autoclave and heated to 45°C. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer A1. Vinylidene fluoride copolymer A1 contained 99.3 mass % of VDF-derived structural units and 0.7 mass % of MImPrA-derived structural units relative to all structural units. Inherent viscosity (η) of vinylidene fluoride copolymer A1ｉ In the NMR measurement, a signal derived from a carbonyl group was observed around 12.4 ppm, and a signal derived from the main chain of the MImPrA structural unit was observed around 3.6 ppm.

[0068] Example 2: Preparation of vinylidene fluoride copolymer A2 (VDF / MImPrA) A 2-liter autoclave was charged with 1,194 g of ion-exchanged water, 27.6 g of a 1.45 mass % aqueous solution of Metolose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.4 g of a 50 mass % diisopropyl peroxydicarbonate-Flon 225cb solution, 400 g of vinylidene fluoride, and 20.0 g of a 4 mass % aqueous solution of MImPrA (3-maleimidopropionic acid), and heated to 45°C. Next, while maintaining the temperature at 45°C, a 4 mass % aqueous solution of MImPrA was continuously fed into the reaction vessel at a flow rate of 0.5 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer A2 (VDF / MImPrA). A total of 4.0 g of MImPrA was added, including the amount added initially. The vinylidene fluoride copolymer A2 contained 99.6 mass% of VDF-derived structural units and 0.4 mass% of MImPrA-derived structural units relative to all structural units. The inherent viscosity (η ｉ ) was 2.76 dL / g.

[0069] Example 3: Preparation of vinylidene fluoride copolymer A3 (VDF / MImPrA) A 2-liter autoclave was charged with 1,175 g of ion-exchanged water, 27.6 g of a 1.45 mass % aqueous solution of Metolose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.4 g of a 50 mass % diisopropyl peroxydicarbonate-Flon 225cb solution, 400 g of vinylidene fluoride, and 40.0 g of a 4 mass % aqueous solution of MImPrA (3-maleimidopropionic acid), and heated to 45°C. Next, while maintaining the temperature at 45°C, a 4 mass % aqueous solution of MImPrA was continuously fed into the reaction vessel at a flow rate of 0.5 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer A3 (VDF / MImPrA). A total of 6.0 g of MImPrA was added, including the amount added initially. The vinylidene fluoride copolymer A3 contained 99.1 mass% of VDF-derived structural units and 0.9 mass% of MImPrA-derived structural units relative to all structural units. The inherent viscosity (η ｉ ) was 2.75 dL / g.

[0070] Comparative Example 1: Preparation of vinylidene fluoride copolymer A4 (VDF / MIm) A 2-liter autoclave was charged with 1,205 g of ion-exchanged water, 27.6 g of a 1.45 mass % Metolose SM-100 (Shin-Etsu Chemical Co., Ltd.) aqueous solution, 2.4 g of a 50 mass % diisopropyl peroxydicarbonate-Flon 225cb solution, 400 g of vinylidene fluoride, and 8.0 g of a 5 mass % MIm (maleimide) aqueous solution, and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5 mass % MIm aqueous solution was continuously fed into the reaction vessel at a flow rate of 0.5 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer A4. A total of 4.0 g of MIm was added, including the amount added initially. The vinylidene fluoride copolymer A4 contained 99.5% by mass of structural units derived from VDF and 0.5% by mass of structural units derived from MIm, based on the total structural units. ｉ ) was 2.42 dL / g.

[0071] Comparative Example 2: Preparation of vinylidene fluoride copolymer A6 (VDF / AA) A 2-liter autoclave was charged with 1,248 g of ion-exchanged water, 0.4 g of Metrose 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.0 g of a 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.2 g of acrylic acid (AA), and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by mass aqueous AA solution was continuously fed into the reaction vessel at a flow rate of 0.5 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer A5. A total of 4.0 g of AA was added, including the amount added initially. The vinylidene fluoride copolymer A5 contained 99% by mass of structural units derived from VDF and 1% by mass of structural units derived from AA, based on the total structural units. ｉ ) was 2.5 dL / g.

[0072] Comparative Example 3: Preparation of polyvinylidene fluoride homopolymer A7 (PVDF) A 2-liter autoclave was charged with 1000 g of ion-exchanged water, 0.2 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.8 g of a 50% by mass isopropyl peroxide-Flon 225cb solution, and 400 g of vinylidene fluoride, and the mixture was heated to 26°C over 1 hour. Next, while maintaining the temperature at 26°C, polymerization was carried out for a total of 20 hours from the start of the temperature increase. After completion of the polymerization, the polymer slurry was heat-treated at 95°C for 60 minutes, dehydrated, washed with water, and further dried at 80°C for 20 hours to obtain polyvinylidene fluoride homopolymer A6 (VDF polymer). Polyvinylidene fluoride homopolymer A6 contained 100% by mass of structural units derived from VDF relative to all structural units. Inherent viscosity (η) of polyvinylidene fluoride homopolymer A6 ｉ ) was 3.1 dL / g.

[0073] <<Preparation Examples of Electrode Mixtures>> <Examples 1A to 3A and Comparative Examples 1A to 3A>> In preparing the electrode mixtures, the following B1 was used as the active material (B). B1: NCA811 (LiNi ０．８ Co ０．１５ Al ０．０５ O ２)

[0074] In preparing the electrode mixture, the following C1 was used as the conductive additive (C). C1: Carbon black (SP: SuperP (registered trademark) manufactured by Timcal Japan, average particle diameter: 40 nm, specific surface area: 60 m ２ / g)

[0075] In preparing the electrode mixture, the following S1 was used as the non-aqueous solvent (S): S1: N-methyl-2-pyrrolidone (NMP)

[0076] Each binder obtained above in Examples 1 to 3 and Comparative Examples 1 to 3 was dissolved in a nonaqueous solvent (S) to prepare a binder solution containing 6% by mass of binder. Then, the active material (B), the conductive additive (C), and the nonaqueous solvent (S) were added and kneaded. Specifically, the binder solution and the conductive additive (C) were subjected to primary kneading at 2000 rpm for 1 minute. Next, the active material (B) was further added and secondary kneading was performed at 2000 rpm for 2 minutes. The nonaqueous solvent (S) was further added to adjust the solids concentration to 75% by mass, and tertiary kneading was performed at 2000 rpm for 3 minutes to obtain electrode mixtures of Examples 1A to 3A and Comparative Examples 1A to 3A. The mass ratio of the active material (B), conductive additive (C), and binder in the obtained electrode mixture, in this order, was 100:1.5:1.5.

[0077] <Electrode Preparation Examples> <Examples 1B to 3B and Comparative Examples 1B to 3B> Using the obtained electrode mixtures of Examples 1A to 3A and Comparative Examples 1A to 3A, positive electrodes of Examples 1B to 3B and Comparative Examples 1B to 3B were prepared, and the peel strength of the electrodes was measured. Specifically, each of the electrode mixtures was applied to an aluminum foil having a thickness of 15 μm using a bar coater, and then this was subjected to primary drying in a thermostatic chamber under a nitrogen atmosphere at 110°C for 30 minutes. Next, it was subjected to secondary drying under a nitrogen atmosphere at 130°C for 2 hours, resulting in a coating weight of approximately 250 g / m ２ The electrode was obtained.

[0078] <Evaluation> [Electrode Peel Strength] The obtained electrodes of Examples 1B to 3B and Comparative Examples 1B to 3B were cut into a length of 100 mm and a width of 20 mm. Then, a 90° peel test was performed at a head speed of 10 mm / min using a tensile tester (ORIENTE CHSIA-1150 manufactured by UNIVERSAL TESTING MACHINE) in accordance with JIS F6854-1 to measure the peel strength. The results are shown in Table 1.

[0079]

[0080] As can be seen from Table 1, when electrodes were formed using binders containing the vinylidene fluoride copolymers (A) of Comparative Examples 1 to 3 that did not contain the structural unit (a2), the maximum peel strength was only 7.1 gf / mm, as in Comparative Example 1. On the other hand, it can be seen that when binders containing the vinylidene fluoride copolymers (A) of Examples 1 to 3 in which the structural unit (a2) was introduced into the vinylidene fluoride copolymer (A) were used, the peel strength increased.

Claims

1. A binder for a secondary battery, comprising a vinylidene fluoride-based copolymer (A) including a structural unit (a1) derived from vinylidene fluoride and a structural unit (a2) derived from a compound represented by the following formula (1): (In the formula, R Ｈ represents a halogen atom, a hydroxy group, a carboxy group, an alkyl group which may have a substituent (excluding a butyl group which has no substituent), or a phenyl group which has a substituent.

2. In the compound represented by the formula (1), R Ｈ The binder for a secondary battery according to claim 1 , wherein is a hydroxy group, a carboxy group, an alkyl group having a hydroxy group, a phenyl group having a hydroxy group, an alkyl group having a carboxy group, or a phenyl group having a carboxy group.

3. The binder for a secondary battery according to claim 1, wherein the compound represented by formula (1) is a compound represented by the following formula (2): (In the formula, R is an atomic group having 1 to 5 carbon atoms.) 4. The binder for secondary batteries according to claim 1, comprising 0.05 mol % or more and 1 mol % or less of the structural unit (a2) derived from the compound represented by formula (1), relative to a total of 100 mol % of the structural unit (a1) derived from vinylidene fluoride.

5. An electrode mixture comprising the binder according to any one of claims 1 to 4, an active material (B), and a conductive assistant (C).

6. An electrode having an electrode mixture layer containing the electrode mixture according to claim 5 on a current collector.

7. A non-aqueous electrolyte secondary battery comprising the electrode according to claim 6.

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