Binder for electrode, electrode mixture, electrode, and battery

Abstract

The present invention addresses the problem of providing a binder for an electrode, which exhibits excellent adhesion to a conductor or the like and contains a vinylidene fluoride copolymer, and with which an electrode mixture is unlikely to thicken even if lithium iron oxide is used as an electrode active substance. This binder for an electrode, which solves the abovementioned problem, contains a vinylidene fluoride copolymer having a constituent unit derived from vinylidene fluoride and a constituent unit derived from a carboxyl group-containing vinyl compound. The weight average molecular weight Mwa of the vinylidene fluoride copolymer is 50,000 or more. When the vinylidene fluoride copolymer is modified with a labeling substance composed of 1-bromomethylpyrene and the weight average molecular weight Mwc of the carboxy group-containing vinylidene fluoride copolymer that absorbs light having a wavelength of 345 nm is specified, the Mwc is less than 0.90 with respect to the Mwa.

Description

Electrode binder, electrode mixture, electrode, and battery

[0001] The present invention relates to an electrode binder, an electrode mixture, an electrode, and a battery.

[0002] In electrodes of non-aqueous electrolyte secondary batteries, vinylidene fluoride polymers are mainly used as binders (binding agents). High adhesiveness is required for binders because they serve to bond active materials to current collectors. For example, Patent Document 1 describes introducing functional groups such as carboxyl groups into fluorine-based resins to improve adhesion to current collectors and solubility in solvents. Patent Document 2 describes a binder composition containing a copolymer (vinylidene fluoride copolymer) of vinylidene fluoride and a compound with a specific structure, and describes that the vinylidene fluoride copolymer exhibits high adhesiveness to current collectors.

[0003] On the other hand, in order to improve battery characteristics such as improving rate characteristics and reducing internal resistance, electrodes using active materials that have been subjected to carbon coating treatment or reduced particle size have been investigated.

[0004] JP-A-6-172452 Patent No. 5797206

[0005] In order to improve adhesion to a current collector or the like, it is conceivable to use the vinylidene fluoride copolymers described in Patent Documents 1 and 2 as electrode binders even for active materials that have been carbon-coated or particle-reduced as described above. However, after extensive research, the inventors have found that when an active material having numerous surface functional groups and a vinylidene fluoride copolymer having carboxy groups are mixed together through carbon coating, the carboxy groups of the vinylidene fluoride copolymer interact with the functional groups on the surface of the active material to form a strong network. Furthermore, the use of an active material with a small particle size tends to increase the viscosity of the slurry. In other words, the accumulation of these particles tends to make the slurry very viscous, resulting in poor handleability.

[0006] The present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide an electrode binder containing a vinylidene fluoride copolymer that has excellent adhesion to a current collector and is resistant to thickening of the electrode mixture even when an electrode active material having many surface functional groups is used. Another object of the present invention is to provide an electrode mixture, an electrode, and a battery that contain the electrode binder.

[0007] [1] The present invention provides an electrode binder containing a vinylidene fluoride copolymer having structural units derived from vinylidene fluoride and structural units derived from a vinyl compound having a carboxy group, wherein the vinylidene fluoride copolymer has a weight-average molecular weight Mwa of 50,000 or more, and when the vinylidene fluoride copolymer is modified with a labeling substance consisting of 1-bromomethylpyrene to determine the weight-average molecular weight Mwc of the carboxy group-containing vinylidene fluoride copolymer that absorbs light at a wavelength of 345 nm, the ratio of the Mwc to the Mwa is less than 0.90. [2] The present invention provides the electrode binder according to [1], wherein the ratio of the Mwc to the Mwa is 0.25 or more. [3] The present invention provides the electrode binder according to [1] or [2], wherein the amount of the vinylidene fluoride-derived structural units is 90.0 mol % or more and the amount of the vinyl compound-derived structural units is 0.01 mol % or more and 0.80 mol % or less, relative to 100.0 mol % of all structural units of the vinylidene fluoride copolymer. [4] The present invention provides the electrode binder according to any one of [1] to [3], wherein the vinyl compound is a compound represented by the following general formula (1): (In general formula (1), R １ , R ２ , R ３ each independently represents a hydrogen atom, a halogen atom, or an alkyl group having from 1 to 5 carbon atoms, and X represents a hydroxy group or —Y—COOH (wherein Y represents an atomic group). [5] The present invention provides the electrode binder according to any one of [1] to [3], wherein the vinyl compound is a compound represented by the following general formula (1): (In general formula (1), R １ , R ２ , R ３each independently represents a hydrogen atom, a halogen atom, or an alkyl group having from 1 to 5 carbon atoms, and X represents a hydroxy group or -Y-COOH (wherein Y represents a divalent atomic group containing either an oxygen atom or a nitrogen atom and having from 1 to 10 atoms in its main chain and a molecular weight of 500 or less). [6] The present invention provides an electrode mixture comprising the electrode binder according to any one of [1] to [5] above and an electrode active material. [7] The present invention provides an electrode comprising the solid content of the electrode binder according to any one of [1] to [5] above. [8] A battery comprising the electrode according to [7] above.

[0008] According to the present invention, there is provided an electrode binder containing a vinylidene fluoride copolymer that has excellent adhesion to a current collector and is resistant to thickening of the electrode mixture even when an electrode active material having many surface functional groups is used. Further, there are also provided an electrode mixture, an electrode, and a battery that contain the electrode binder.

[0009] 1. Electrode Binder The electrode binder of the present invention may contain a vinylidene fluoride copolymer having a constituent unit derived from vinylidene fluoride and a constituent unit derived from a vinyl compound having a carboxy group. The electrode binder may be in a solid form, such as a powder, or in a liquid form. The electrode binder may be composed solely of the vinylidene fluoride copolymer, or may contain a solvent as needed. When the electrode binder contains a solvent, the vinylidene fluoride copolymer may be dissolved in the solvent or may be dispersed in the solvent.

[0010] In the past, when an electrode mixture (slurry) was prepared by mixing a vinylidene fluoride copolymer containing carboxyl groups with an active material having a large number of functional groups on its surface and a small particle size, such as LFP (lithium iron phosphate), the viscosity was likely to become very high. In contrast, the electrode binder of the present invention is unlikely to cause an excessive increase in the slurry viscosity, even when mixed with an active material having a large number of functional groups and a small particle size, such as LFP. The reason for this is believed to be as follows.

[0011] The vinylidene fluoride copolymer contained in the electrode binder of the present invention is a copolymer of at least vinylidene fluoride and a vinyl compound having a carboxy group, and is an aggregate of numerous polymers. The weight-average molecular weight Mwa of the vinylidene fluoride copolymer is 50,000 or more. The vinylidene fluoride copolymer includes a polymer containing structural units derived from vinylidene fluoride and structural units derived from a vinyl compound (referred to herein as a "carboxy group-containing vinylidene fluoride copolymer" or "carboxy group-containing copolymer"), and a polymer composed mainly of structural units derived from vinylidene fluoride and not containing structural units derived from a vinyl compound having a carboxy group. The weight-average molecular weight Mwc of the carboxy group-containing copolymer, as determined by the method described below, is less than 0.90 relative to the Mwa. In other words, the structural units derived from the vinyl compound are mainly introduced into a polymer with a relatively low molecular weight. As described above, when a typical vinylidene fluoride copolymer having a carboxy group is mixed with an active material, it forms a strong network originating from the active material. This results in a very high viscosity of the electrode mixture (slurry). In contrast, the vinylidene fluoride copolymer contained in the electrode binder of the present invention has a relatively small weight average molecular weight of the carboxy group-containing copolymer that interacts with the active material. Therefore, even if the carboxy group-containing copolymer interacts with the active material, it is difficult for a strong network to form, and the viscosity of the slurry is unlikely to increase excessively. Below, the vinylidene fluoride copolymer contained in the electrode binder of the present invention, its physical properties, and the solvent, etc., are described in detail.

[0012] (Vinylidene fluoride copolymer) As described above, the vinylidene fluoride copolymer is a copolymer obtained by copolymerizing at least vinylidene fluoride and a vinyl compound having a carboxy group. The amount of vinylidene fluoride-derived structural units in the vinylidene fluoride copolymer is 90.00 mol% or more, preferably 95.00 mol% or more and 99.98 mol% or less, relative to 100.00 mol% of all structural units of the vinylidene fluoride copolymer. When the amount of vinylidene fluoride-derived structural units is 90.00 mol% or more, physical properties specific to vinylidene fluoride are easily obtained. This value can be calculated, for example, by specifying the amount of structural units derived from compounds other than vinylidene fluoride contained in the vinylidene fluoride copolymer.

[0013] On the other hand, the amount of the structural units derived from a vinyl compound having a carboxy group in the vinylidene fluoride copolymer is preferably 0.01 mol% or more and 0.80 mol% or less, and more preferably 0.02 mol% or more and 0.50 mol% or less, relative to 100.0 mol% of all structural units of the vinylidene fluoride copolymer. If the amount of the structural units derived from the vinyl compound is 0.01 mol% or more, when an electrode is formed using the electrode binder, the adhesion of the vinylidene fluoride copolymer to the current collector is likely to be further improved. On the other hand, if the amount of the structural units derived from the vinyl compound is 0.80 mol% or less, it is more likely to suppress an increase in slurry viscosity when the electrode binder is mixed with an active material. The amount of the structural units derived from the vinyl compound can be determined by neutralization titration or １ It can be determined from H-NMR spectrum or the like.

[0014] Here, the structure of the vinyl compound is not particularly limited, as long as it has a vinyl group polymerizable with vinylidene fluoride and a carboxy group. The number of carboxy groups contained in the vinyl compound may be one or two or more. In this specification, an acid anhydride structure composed of two carboxy groups is also considered to be a type of carboxy group. The vinylidene fluoride copolymer may contain only one type of structural unit derived from the vinyl compound, or may contain two or more types. Examples of preferred vinyl compounds include compounds represented by the following general formula (1): In general formula (1), R１ , R ２ , R ３ each independently represents a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms. Examples of the alkyl group having 1 to 5 carbon atoms include linear or branched alkyl groups, specific examples of which include a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a t-butyl group, and a pentyl group. Among these, a methyl group, an ethyl group, or a butyl group is preferred from the viewpoint of availability and the like. In particular, from the viewpoint of less steric hindrance during polymerization with vinylidene fluoride, R １ , R ２ , and R ３ are each independently a hydrogen atom or a methyl group.

[0015] In the general formula (1), X is a hydroxy group or a group represented by -Y-COOH. Here, Y represents an atomic group, and Y is preferably an atomic group containing either an oxygen atom or a nitrogen atom, more preferably an atomic group containing either an oxygen atom or a nitrogen atom and having 1 to 10 atoms in the main chain, and more preferably a divalent atomic group containing either an oxygen atom or a nitrogen atom, having 1 to 10 atoms in the main chain, and having a molecular weight of 500 or less.

[0016] The atomic group (Y) may be linear, branched, or cyclic, or may be a combination thereof. Among these, the atomic group (Y) is preferably linear or branched, from the viewpoint of less occurrence of steric hindrance during polymerization with vinylidene fluoride.

[0017] The number of atoms in the main chain of the atomic group (Y) may be from 1 to 10, and preferably from 2 to 8. In this specification, the main chain of the atomic group (Y) refers to the longest chain among the chains connecting the carbonyl group in general formula (1) and the carboxy group of -Y-COOH.

[0018] Here, the atomic group (Y) contains either one or both of an oxygen atom and a nitrogen atom (hereinafter, these are also collectively referred to as "heteroatoms"). The number of heteroatoms in the atomic group (Y) is preferably 1 to 10, more preferably 1 to 5. When the atomic group (Y) contains two or more heteroatoms, these may be the same type of atom or different types of atoms. The heteroatom may be contained in any structure (functional group) and may be located at any position within the atomic group (Y). Examples of structures (functional groups) containing these heteroatoms include an ether bond, an ester bond, a carbonyl group, a carboxy group, an amide group, a hydroxy group, etc. Among these, an ether bond, an ester group, a carbonyl group, a carboxy group, an amide group, and a hydroxy group are preferred.

[0019] The structure of the atomic group (Y) is not particularly limited, and can be, for example, a structure in which a hydrocarbon group such as an alkylene group or an alkyl group is bonded to the above-mentioned heteroatom-containing structure (functional group). The molecular weight of the atomic group (Y) is sufficient as long as it is 500 or less, and is preferably 30 to 200 from the viewpoint of polymerization reactivity.

[0020] Specific examples of the compound represented by the general formula (1) include (meth)acrylic acid, (meth)acryloyloxyethyl succinate, (meth)acryloyloxypropyl succinate, 2-carboxyethyl (meth)acrylate, 2-carboxymethyl (meth)acrylate, (meth)acryloyloxyethyl phthalate, and (meth)acrylamide-based compounds such as N-carboxyethyl (meth)acrylamide. In this specification, (meth)acrylic represents methacrylic, acrylic, or a mixture thereof; (meth)acrylate represents methacrylate, acrylate, or a mixture thereof; and (meth)acryloyl represents methacryloyl, acryloyl, or a mixture thereof.

[0021] The compound represented by the general formula (1) is more preferably acrylic acid, acryloyloxyethyl succinic acid, acryloyloxypropyl succinic acid, 2-carboxyethyl acrylate, or 2-carboxymethyl acrylate, from the viewpoints of availability and reactivity with vinylidene fluoride.

[0022] However, the vinyl compound having a carboxy group is not limited to the compound represented by general formula (1). Examples of vinyl compounds other than the compound represented by general formula (1) include unsaturated dibasic acids such as maleic acid, fumaric acid, and itaconic acid; unsaturated dibasic acid anhydrides such as maleic anhydride and itaconic anhydride; and unsaturated dibasic acid monoesters such as monomethyl fumarate, monoethyl fumarate, monomethyl maleate, monoethyl maleate, monomethyl citraconic acid, monoethyl citraconic acid, monomethyl phthalate, monoethyl phthalate, monomethyl itaconate, and monoethyl itaconate.

[0023] Furthermore, the vinylidene fluoride copolymer may partially contain structural units derived from compounds (other compounds) other than vinylidene fluoride and the vinyl compound having a carboxy group, as long as the purpose and effect of the present invention are not impaired. The vinylidene fluoride copolymer may contain only one type of structural unit derived from the other compounds, or may contain two or more types. However, the total amount of structural units derived from the other compounds relative to 100.0 mol% of all structural units of the vinylidene fluoride copolymer is preferably 10.0 mol% or less, more preferably 5.0 mol% or less. These amounts are １９ F-NMR spectrum and １ It is identified from H-NMR spectrum, etc.

[0024] Examples of other compounds include fluorine-based vinyl compounds having a vinyl group and a fluorine atom or a fluorine-containing alkyl group in one molecule. Examples of fluorine-based vinyl compounds include vinyl fluoride; trifluoroethylene; tetrafluoroethylene; chlorotrifluoroethylene; hexafluoropropylene; perfluoroalkyl vinyl ethers such as perfluoromethyl vinyl ether. Examples of other compounds also include compounds having a vinyl group but not containing fluorine. Examples of such compounds include unsaturated hydrocarbon compounds such as ethylene and propylene.

[0025] Here, the weight-average molecular weight Mwa of the vinylidene fluoride copolymer may be 50,000 or more, as described above, preferably 100,000 to 5,000,000, more preferably 200,000 to 4,000,000, and particularly preferably 250,000 to 3,000,000. When the weight-average molecular weight Mwa of the vinylidene fluoride copolymer is 50,000 or more, the adhesive strength of the vinylidene fluoride copolymer with the active material and the current collector tends to be increased. On the other hand, when Mwa is 5,000,000 or less, the slurry viscosity does not become too high when the electrode mixture (slurry) is prepared, and workability is particularly excellent. In this specification, the weight-average molecular weight Mwa of the vinylidene fluoride copolymer is a polystyrene-equivalent value measured by gel permeation chromatography (GPC). In this case, the eluent is N,N-dimethylacetamide, and the weight-average molecular weight is determined using a refractive index (RI) detector.

[0026] On the other hand, the weight-average molecular weight Mwc of the carboxyl group-containing copolymer in the vinylidene fluoride copolymer may be less than 0.90 relative to the weight-average molecular weight Mwa of the vinylidene fluoride copolymer, i.e., Mwc / Mwa<0.90, but Mwc / Mwa is preferably 0.25 or more and less than 0.90, more preferably 0.25 or more and less than 0.8, and even more preferably 0.25 or more and less than 0.7. As described above, if the ratio is less than 0.90, when the vinylidene fluoride copolymer and the active material are mixed to form an electrode mixture (slurry), the slurry viscosity is unlikely to be excessively increased.

[0027] Here, the specific weight average molecular weight Mwc of the carboxy group-containing copolymer is preferably 10,000 or more, more preferably 25,000 or more and 4,700,000 or less, even more preferably 50,000 or more and 3,800,000 or less, and particularly preferably 63,000 or more and 2,800,000 or less. When the weight average molecular weight Mwc of the carboxy group-containing copolymer is within this range, the ratio (Mwc / Mwa) tends to fall within the desired range.

[0028] In this specification, the weight-average molecular weight Mwc of the carboxy group-containing copolymer is a value measured as follows. A labeling substance (1-bromomethylpyrene) for labeling the carboxy groups and potassium carbonate are mixed with a vinylidene fluoride copolymer to esterify the carboxy groups, thereby obtaining a pyrene-modified vinylidene fluoride copolymer. GPC is then performed using N,N-dimethylacetamide as the eluent, and the value is the polystyrene-equivalent value obtained using an ultraviolet-visible (UV-vis) detector (detection wavelength: 345 nm). The pyrene structure has characteristic absorption in the ultraviolet-visible region, strongly absorbing light at a wavelength of 345 nm. In other words, the weight-average molecular weight Mwc of the carboxy group-containing copolymer is determined by esterifying the carboxy groups with 1-bromomethylpyrene.

[0029] Here, the inherent viscosity of the vinylidene fluoride copolymer is preferably 0.3 dL / g or more and 6.0 dL / g or less, more preferably 0.5 dL / g or more and 5.0 dL / g or less, and even more preferably 0.8 dL / g or more and 4.5 dL / g or less. When the inherent viscosity is 0.5 dL / g or more, the adhesive strength of the vinylidene fluoride copolymer to the active material or the current collector is particularly likely to be increased. On the other hand, when the inherent viscosity is 6.0 dL / g or less, the viscosity of the slurry does not become too high when an electrode mixture (slurry) is prepared, and workability is particularly good. Inherent viscosity (η ｉ ) indicates logarithmic viscosity. First, 80 mg of vinylidene fluoride copolymer is dissolved in 20 ml of N,N-dimethylformamide, and the viscosity is measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. Then, the viscosity is calculated from the obtained value based on the following formula: η 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 in the solution, ie, 0.4 g / dl.

[0030] The melting point of the vinylidene fluoride copolymer is preferably 150°C or higher and 180°C or lower, more preferably 155°C or higher and 175°C or lower, and particularly preferably 160°C or higher and 175°C or lower. When the vinylidene fluoride copolymer has a melting point of 150°C or higher, when an electrode binder (vinylidene fluoride copolymer) is used in an electrode, the copolymer is less likely to swell with an electrolyte, and the performance of the resulting battery is likely to be good. On the other hand, when the melting point is 180°C or lower, the flexibility of the formed electrode is likely to be good. The melting point of the vinylidene fluoride copolymer can be determined by calorimetry using a differential scanning calorimeter (DSC).

[0031] The vinylidene fluoride copolymer can be prepared by copolymerizing vinylidene fluoride with the vinyl compound having a carboxy group, and optionally other compounds. Examples of the copolymerization method include suspension polymerization, emulsion polymerization, solution polymerization, etc., but suspension polymerization is preferred from the viewpoint of reducing impurities.

[0032] In suspension polymerization using water as a dispersion medium, a suspending agent such as methyl cellulose, propoxylated methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, gelatin, etc. is added in an amount of 0.005 to 1.0 part by mass, preferably 0.01 to 0.4 part by mass, per 100 parts by mass of all monomers used in the copolymerization (vinylidene fluoride, vinyl compounds having carboxy groups, and other monomers).

[0033] Examples of polymerization initiators that can be used include diisopropyl peroxydicarbonate, di-normal propyl peroxydicarbonate, di-normal heptafluoropropyl peroxydicarbonate, isobutyryl peroxide, di(chlorofluoroacyl)peroxide, di(perfluoroacyl)peroxide, and t-butyl peroxypivalate. The amount used is 0.05 to 10 parts by mass, preferably 0.15 to 5 parts by mass, based on 100 parts by mass of all monomers used in copolymerization (vinylidene fluoride, vinyl compound having a carboxy group, and other monomers as needed).

[0034] It is also possible to adjust the degree of polymerization of the resulting vinylidene fluoride copolymer by adding a chain transfer agent such as ethyl acetate, methyl acetate, diethyl carbonate, acetone, ethanol, n-propanol, acetaldehyde, propylaldehyde, ethyl propionate, carbon tetrachloride, etc. When a chain transfer agent is used, the amount used is usually 0.01 to 5 parts by mass, preferably 0.01 to 3 parts by mass, per 100 parts by mass of all monomers used in the copolymerization (vinylidene fluoride, vinyl compound having a carboxy group, and any other monomers).

[0035] The amount of all monomers (vinylidene fluoride, a vinyl compound having a carboxy group, and other monomers) used in the copolymerization is usually 1:1 to 1:10, preferably 1:2 to 1:5, in terms of the mass ratio of all monomers to water.

[0036] The polymerization temperature T is the 10-hour half-life temperature T of the polymerization initiator. １０ is selected appropriately depending on the １０ -25℃≦T≦T １０ +25°C. For example, the T １０ 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 100 hours or less in consideration of productivity and the like. The polymerization is usually carried out under increased pressure, preferably 2.0 to 10.0 MPa-G.

[0037] Here, examples of methods for making the weight-average molecular weight Mwc of the carboxyl group-containing copolymer less than 0.90 relative to the weight-average molecular weight Mwa of the vinylidene fluoride copolymer include the following two methods. However, these methods are not limited to these. The first method is to mix vinylidene fluoride (and other compounds) with the entire amount of the vinyl compound having a carboxyl group and then initiate polymerization. The second method is to polymerize vinylidene fluoride (and other compounds) to a certain extent, then add the entire amount of the vinyl compound having a carboxyl group to the reaction system in a short period of time and further polymerize. An example of short-term addition includes setting the addition rate of the vinyl compound having a carboxyl group to 0.2 parts by mass / hour or more per 100 parts by mass of all monomers used in copolymerization. This allows the weight-average molecular weight Mwc to be within the desired range. The weight-average molecular weight Mwa of the vinylidene fluoride copolymer can be adjusted by the amount of polymerization initiator, the amount of chain transfer agent, the polymerization temperature, etc.

[0038] (Solvent) As described above, the electrode binder may contain a solvent. The solvent may be a non-aqueous solvent or water. Furthermore, the non-aqueous solvent may be a solvent having polarity (polar solvent). Examples of the polar solvent include amide compounds such as N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; alcohol compounds such as methanol, ethanol, isopropyl alcohol, 2-ethyl-1-hexanol, 1-nonanol, lauryl alcohol, and tripropylene glycol; amine compounds such as o-toluidine, m-toluidine, and p-toluidine; imide compounds such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; lactone compounds such as γ-butyrolactone and δ-butyrolactone; sulfoxide / sulfone compounds such as dimethyl sulfoxide and sulfolane; ether compounds such as tetrahydrofuran, diethyl ether, 1,4-dioxane, and diethylene glycol dimethyl ether; and ketone compounds such as acetone, 2-butanone, methyl isobutyl ketone, and cyclohexanone. The electrode binder may contain only one of the above solvents, or may contain two or more of them.

[0039] When the electrode binder contains a solvent, the amount of the solvent in the electrode binder is preferably 50 parts by mass or more and 10,000 parts by mass or less, and more preferably 100 parts by mass or more and 5,000 parts by mass or less, relative to 100 parts by mass of the vinylidene fluoride copolymer. When the amount of the solvent in the electrode binder is within this range, the vinylidene fluoride copolymer can be uniformly dispersed or dissolved in the solvent.

[0040] (Others) The electrode binder may further contain various additives, such as other resins such as acrylic resins, fillers such as inorganic fillers, dispersants, emulsifiers, etc., within the scope of not impairing the object and effect of the present invention.

[0041] 2. Electrode Mixture The electrode binder described above can be mixed with an active material (positive electrode active material or negative electrode active material) to prepare an electrode mixture (slurry) for producing an electrode for a non-aqueous electrolyte secondary battery. The electrode mixture may further contain a conductive aid, a solvent, other additives, etc.

[0042] The amount of the solid content (vinylidene fluoride copolymer) derived from the electrode binder relative to the total amount of the solid content derived from the electrode binder (total amount excluding volatile components), the active material, and the conductive additive is preferably 0.2 mass% or more and 20 mass% or less, more preferably 0.2 mass% or more and 10 mass% or less, and even more preferably 0.2 mass% or more and 7 mass% or less.

[0043] Furthermore, even when the vinylidene fluoride copolymer contained in the electrode binder is mixed with an active material that is carbon-coated and has many surface functional groups or has a small particle size (e.g., LFP or LFMP (lithium iron manganese phosphate)), thickening or gelation of the electrode mixture is unlikely to occur. For example, LFP has an average particle size of 15 μm or less, typically about 0.1 μm or more and 15 μm or less, as measured by particle size distribution measurement using laser diffraction / scattering method with pure water as a dispersion medium. Even when the electrode binder is mixed with such an active material, thickening of the electrode mixture is unlikely to occur. However, the average particle size of the active material is not limited to the above range.

[0044] In the present invention, various active materials can be used as the active material of the electrode mixture. The active material is not particularly limited, and any known active material for a negative electrode (negative electrode active material) or active material for a positive electrode (positive electrode active material) can be used.

[0045] Examples of negative electrode active materials include carbon materials such as artificial graphite, natural graphite, non-graphitizable carbon, easily graphitizable carbon, activated carbon, or phenolic resin and pitch that have been fired and carbonized; metal and alloy materials such as Cu, Li, Mg, B, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, Cd, Ag, Zn, Hf, Zr, and Y; and GeO, GeO ２ , SnO, SnO ２ , PbO, PbO ２ The negative electrode active material may be a material in which a coating is applied to the surface of the above-mentioned carbon material, metal / alloy material, metal oxide, etc. The negative electrode active material may be a commercially available product.

[0046] On the other hand, examples of the positive electrode active material include lithium-based positive electrode active materials containing lithium. Examples of the lithium-based positive electrode active material include LiCoO ２ , LiNiO ２ , 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), LiNi ｘ Co １－ｘ O ２ (0<x≦1) ２ (M is one or more transition metals such as Co, Ni, Fe, Mn, Cr, Ti, and V, and Y is a chalcogen element such as O or S); LiMnO ２ , LiMn ２ O ４ composite metal oxides having a spinel structure such as LiFePO ４ and LiFeMnPO ４ LiMaPO etc. 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 ２ Lithium-excess solid solution positive electrode 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); LiNi ｘ Co ｙ M ｚ O ２ (where M represents Mn or Al, and x, y, and z satisfy 0<x<1, 0<y<1, 0<z<1, and x+y+z=1, respectively). Among these, LiFePO 4 , which has a small particle size and an active material surface coated with carbon and has many surface functional groups, is particularly preferred. ４ (LFP), LiFeMnPO ４ (LFMP) is preferred from the viewpoint of easily achieving the effects of the present invention. The positive electrode active material may be a compound having a surface coated thereon. Furthermore, the positive electrode active material may be a commercially available product.

[0047] The amount of active material contained in the electrode mixture is appropriately selected depending on the application of the electrode mixture, etc., but is preferably 50% by mass or more and 99.9% by mass or less of the total amount of the solid content derived from the electrode binder, the active material, and the conductive additive. When the amount of active material is within this range, for example, sufficient charge / discharge capacity can be obtained, and battery performance is likely to be good.

[0048] The conductive additive contained in the electrode mixture is not particularly limited as long as it is a compound that can further increase the conductivity between the active materials or between the active material and the current collector. Examples of the conductive additive include acetylene black, ketjen black, carbon black, graphite powder, carbon nanofiber, carbon nanotube, and carbon fiber.

[0049] The amount of the conductive additive contained in the electrode mixture is appropriately selected depending on the type of the conductive additive, etc. From the viewpoint of improving both the conductivity and the dispersibility of the conductive additive, the amount is preferably 0.1% by mass to 15% by mass or less, more preferably 0.1% by mass to 7% by mass or less, and even more preferably 0.1% by mass to 5% by mass or less, based on the total amount of the solid content derived from the electrode binder, the active material, and the conductive additive.

[0050] The electrode mixture may contain a solvent different from the solvent that the electrode binder may contain, etc. The solvent can be selected from the solvents that the binder may contain.

[0051] The total amount of solvent in the electrode mixture (including the amount of solvent in the electrode binder) is not particularly limited, but is usually preferably 10 parts by mass or more and 150 parts by mass or less per 100 parts by mass of the above-mentioned active material.

[0052] The electrode mixture may further contain a dispersant, an adhesive aid, a thickener, etc., and known compounds can be used for these. Examples of the dispersant include polyvinylpyrrolidone, methyl cellulose, propoxylated methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, polypropylene oxide, gelatin, etc. Examples of the adhesive aid include poly(meth)acrylic acid, metal poly(meth)acrylates such as sodium poly(meth)acrylate, and carboxymethyl cellulose. The amount of these additives is not particularly limited as long as it does not impair the object and effect of the present invention, but is preferably 15% by mass or less based on the total amount of the solid content derived from the electrode binder and the active material.

[0053] The electrode mixture may further contain additives such as phosphorus compounds, sulfur compounds, nitrogen compounds such as amine compounds and ammonium compounds, organic acids, organic esters, various silane-based, titanium-based and aluminum-based coupling agents, vinylidene fluoride polymers other than the above-mentioned vinylidene fluoride copolymers, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), and other resins. These are not particularly limited as long as they do not impair the objects and effects of the present invention, but are preferably 15 mass% or less based on the total amount of the solid content derived from the electrode binder and the active material.

[0054] The electrode mixture may be prepared by mixing all of the components simultaneously, or by first mixing some of the components and then mixing the remaining components.

[0055] The viscosity of the electrode mixture is not particularly limited as long as it can prevent dripping, uneven coating, and delayed drying after coating when applying the electrode mixture to form a mixture layer, and has good workability and applicability when preparing the mixture layer. The viscosity is a value measured at 25°C using an E-type viscometer, and is measured at a shear rate of 1 s after an incubation period of 60 seconds at 25°C using an E-type viscometer. －１ The rotor is rotated at 120 s, and the value is measured 120 seconds after the rotor starts rotating.

[0056] 3. Electrode The above-described electrode mixture can be used to form a mixture layer of an electrode of various non-aqueous electrolyte secondary batteries. An electrode of a non-aqueous electrolyte secondary battery includes, for example, a current collector and a mixture layer disposed on the current collector. The above-described electrode mixture can be used to form the mixture layer.

[0057] 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., or a layer containing carbon black or the like formed on the surface thereof can be used. Alternatively, the current collector may be a medium having a layer containing carbon black or the like formed on the surface thereof, or a medium having the above-mentioned metal foil or metal mesh applied thereto.

[0058] Mixture Layer The mixture layer is a layer formed by applying the electrode mixture described above onto a current collector and solidifying it. That is, the mixture layer contains at least the vinylidene fluoride compound, which is the solid content of the electrode binder described above, and an active material. The mixture layer may be formed on only one surface of the current collector, or may be disposed on both surfaces.

[0059] The mixture layer contains at least the components contained in the electrode mixture described above, i.e., the solid content (vinylidene fluoride copolymer) derived from the electrode binder and the active material, and may further contain various additives such as a conductive aid, a dispersant, an adhesive aid, a thickener, etc. as needed, which are the same as those described for the electrode mixture.

[0060] Here, the thickness of the mixture layer is not particularly limited, but in one example, it is preferably 1 μm or more and 1000 μm or less. The weight per unit area of ​​the mixture layer formed on one surface of the current collector is not particularly limited, and can be any weight per unit area. In one example, it is 50 g / m ２ More than 1000g / m ２ Preferably, 100 g / m or less ２ More than 500g / m ２ The following is more preferred:

[0061] The mixture layer can be formed by carrying out a step of applying the electrode mixture onto a current collector and a step of solidifying the applied mixture.

[0062] The method for applying the electrode mixture is not particularly limited, and methods such as a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, a die coating method, and a dip coating method can be used.

[0063] After application of the electrode mixture, the mixture is heated at an arbitrary temperature to dry the solvent. In one example, the drying temperature is preferably 60°C or higher and 500°C or lower, and more preferably 80°C or higher and 200°C or lower. Heating may be performed multiple times at different temperatures. The solvent in the mixture may be dried under atmospheric pressure, pressure, or reduced pressure, or may be dried in an environment such as air, nitrogen, or argon. After drying, a heat treatment may be further performed.

[0064] After the electrode mixture is applied and dried, a pressing process may be further performed. The pressing process can improve the electrode density. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

[0065] 4. Batteries The electrode binders and electrode mixtures described above can be used in electrodes of various non-aqueous electrolyte secondary batteries, as described above, but may also be used to form other layers of non-aqueous electrolyte secondary batteries.

[0066] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0067] (Methods for measuring and evaluating physical properties) In the examples and comparative examples described later, the amount of vinyl compound having a carboxy group in the vinylidene fluoride copolymer was determined by the following method. １ The inherent viscosity of the vinylidene fluoride copolymer, the weight average molecular weight Mwa of the vinylidene fluoride copolymer, and the weight average molecular weight Mwc of the carboxy group-containing copolymer were determined by H-NMR measurement and neutralization titration. The slurry viscosity and peel strength of the electrode mixture were determined by the following methods.

[0068] - Identifying the amount of vinyl compound with a carboxy group introduced ( １ H-NMR measurement) １ From the spectrum obtained by H-NMR, among the peaks derived from the constituent units of the vinylidene fluoride polymer, the integrated intensity of the peak derived from vinylidene fluoride and the integrated intensity of the peak derived from the vinyl compound having a carboxy group were each determined. Then, the proportion (mol %) of the vinyl compound having a carboxy group in all constituent units of the vinylidene fluoride polymer was calculated from the ratio of the integrated intensity of the peak derived from the vinyl compound having a carboxy group to all constituent units of the vinylidene fluoride polymer (the sum of the integrated intensities of the peaks derived from the vinyl compound and the peaks derived from vinylidene fluoride). The measurement device used was a nuclear magnetic resonance spectrometer (NMR, JEOL, JNM-ECZ600R / S1, frequency 600 MHz), and the solvent used was DMSO-d6.

[0069] Determining the amount of vinyl compound with a carboxy group introduced (neutralization titration) 0.50 g of accurately weighed polymer was added to 100 mL of acetone while stirring. The mixture was heated and stirred at 50°C. After dissolution was confirmed, 5 mL of water was slowly added while heating and stirring. Approximately five drops of phenolphthalein ethanol solution were then added as an indicator. Using a burette, a 0.1 N aqueous sodium hydroxide solution with a known concentration was added to the solution to be titrated while stirring, and titration was performed. This was performed three times, and the mole fraction of vinyl compound was calculated from the average value.

[0070] Inherent Viscosity The inherent viscosity of the vinylidene fluoride copolymer was measured as follows. First, 80 mg of the vinylidene fluoride copolymer 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. Then, 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 the vinylidene fluoride polymer in the solution, ie, 0.4 g / dl.

[0071] Weight-average molecular weight Mwa of vinylidene fluoride copolymer The weight-average molecular weight Mwa of the vinylidene fluoride copolymer prepared in each example and comparative example was measured using a differential refractometer (RI). Specifically, GPC (gel permeation chromatography) was performed under the following conditions, and Mwa was determined using a differential refractive index detector. This was performed three times, and the average value was taken as Mwa. Separation column: Shodex KD-807, KD-806M Detector: JASCO RI-4030 (differential refractive index detector) Eluent: 10 mM LiBr—N,N-dimethylacetamide (DMAc) solution Eluent flow rate: 0.5 mL / min Column temperature: 40°C Standard polymer for calibration curve: TSK standard POLY(STYRENE) (standard polystyrene) (Tosoh Corporation)

[0072] Weight-average molecular weight Mwc of carboxy group-containing copolymer: 10 mg of the vinylidene fluoride copolymer prepared in each Example and Comparative Example, 2 mg of 1-bromomethylpyrene, and 2 mg of potassium carbonate were added to 10 mg of DMAc, and the mixture was dissolved and reacted for 3 hours while stirring in a thermostatic shaking bath at 50°C. The potassium carbonate was then removed using a 0.45 μm filter. As a result, the carboxy group of the carboxy group-containing copolymer in the vinylidene fluoride copolymer was labeled with 1-bromomethylpyrene.

[0073] The weight-average molecular weight Mwc of the carboxyl group-containing copolymer (a polymer that absorbs light at a wavelength of 345 nm) contained in the vinylidene fluoride copolymer labeled with 1-bromomethylpyrene was determined using a UV-vis detector (UV-4075 manufactured by JASCO Corporation). Measurement conditions were the same as for the above Mwa, except for the detector. The detection wavelength was 345 nm, and the weight-average molecular weight Mwc of the carboxyl group-containing copolymer was determined. This was performed three times, and the average value was taken as Mwc.

[0074] Slurry viscosity The slurry viscosity of the electrode mixture prepared in the examples and comparative examples was measured immediately after the production of the electrode mixture. Specifically, the viscosity of the electrode mixture immediately after the preparation was measured. The electrode mixture immediately after preparation was placed in an E-type viscometer (RE-215R type viscometer manufactured by Toki Sangyo Co., Ltd., rotor 3° x R14). After a 60-second incubation period at 25°C in the device, the electrode mixture was slurried at a shear rate of 1 s －１ The rotor was rotated at 120 s, and the viscosity measured 120 seconds after the start of rotor rotation was determined as the slurry viscosity.

[0075] Peel strength: The electrode mixtures prepared in the examples and comparative examples were applied to aluminum foil with a thickness of 16 μm using a bar coater and dried at 110° C. for 30 minutes, resulting in a coating weight of 160 g / cm ２A single-sided coated electrode was obtained. The obtained single-sided coated electrode was cut into a length of 70 mm and a width of 20 mm, and the peel strength between the aluminum foil and the mixture layer was evaluated. Specifically, the upper surface of the formed mixture layer was bonded to a thick plastic plate (made of acrylic resin, thickness 5 mm), and a 90° peel test was performed at a head speed of 10 mm / min using a tensile tester (single column type material testing machine STA-1150 manufactured by Orientec Co., Ltd.) in accordance with JIS K6854-1. The measurement environment was a controlled temperature of 25°C and a dew point of -20°C.

[0076] Example 1 Preparation of Vinylidene Fluoride Copolymer (Electrode Binder) A 2-liter autoclave was charged with 1,240 g of ion-exchanged water as a dispersion medium, 0.4 g of Metolose SM-100 (Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspending agent, 2.0 g of acryloyloxypropyl succinic acid (APS), 0.6 g of a diisopropyl peroxydicarbonate-HFE-347pc-f solution having a polymerization initiator concentration of 50 wt%, 0.72 g of ethyl acetate as a chain transfer agent, and 400 g of vinylidene fluoride, and the temperature was raised to 45°C over 2 hours. While maintaining the temperature at 45°C, the reaction was continued until the pressure in the system decreased to 1.5 MPaG. After completion of the polymerization, the polymer slurry was heat-treated at 95°C for 60 minutes. Thereafter, the mixture was dehydrated, washed with water, and further dried at 80°C for 20 hours to obtain a powder of vinylidene fluoride copolymer (electrode binder), which is a copolymer of vinylidene fluoride (VDF) and APS. The amount of the vinyl compound having a carboxy group introduced was １ The amount of the vinyl compound having a carboxy group introduced into the vinylidene fluoride copolymer, the inherent viscosity, the weight-average molecular weight Mwa, and the weight-average molecular weight Mwc of the carboxy group-containing copolymer contained therein are shown in Table 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 4% by mass, and used as a binder solution.

[0077] [Preparation of electrode mixture] LFP (carbon coating amount: 1.4%, average particle diameter D50: 4 μm) was used as the electrode active material, carbon nanotubes (CNT: LB107-44 manufactured by Cnao Corporation) were used as the conductive additive, and NMP was used as the dispersion medium. Then, using an E-type viscometer, the amount of NMP was adjusted so that the slurry viscosity was 8000 to 30,000 mPa sec at 25 ° C. and a shear rate of 1 / sec. The binder solution, LFP, and CNT were mixed to prepare an electrode mixture. In the obtained electrode mixture, the mass ratio of LFP, CNT, and PVDF was 100:2:2.5, and the solids concentration was 52.0 mass%. The slurry viscosity at this time is shown in Table 1.

[0078] [Production of Electrode] The obtained electrode mixture was applied to a current collector (a 16 μm thick carbon-coated aluminum foil) using a bar coater. This was dried in a thermostatic oven at 110° C. for 30 minutes under a nitrogen atmosphere to give a dry mixture basis weight of 160 g / m. ２ The electrodes were fabricated.

[0079] Example 2 A 2-liter autoclave was charged with 1,034 g of ion-exchanged water as a dispersion medium, 0.47 g of Metolose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 1.65 g of diisopropyl peroxydicarbonate-HFE-347pc-f solution having a polymerization initiator concentration of 50 wt%, and 470 g of vinylidene fluoride, and the mixture was heated to 26°C over 55 minutes. While maintaining the temperature at 26°C, the temperature was raised to 40°C over 63 minutes from the point when the pressure had dropped by 0.2 MPaG from the pressure at the end of the temperature rise. During this time, 1.18 g of an aqueous solution of APS with a concentration of 5 wt% was added in solute equivalent. While maintaining the temperature at 40°C, the reaction was continued until the pressure in the system dropped to 1.58 MPaG. The obtained polymer slurry was treated in the same manner as in Example 1 to obtain a powder of vinylidene fluoride copolymer (electrode binder). The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 4 mass%, and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 1 above.

[0080] Comparative Example 1 A 2-liter autoclave was charged with 1,143 g of ion-exchanged water as a dispersion medium, 0.21 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspending agent, 0.13 g of APS, 2.10 g of a 50 wt % diisopropyl peroxydicarbonate-HFE-347pc-f solution as a polymerization initiator, and 420 g of vinylidene fluoride, and the mixture was heated to 26°C over 55 minutes. While maintaining the temperature at 26°C, 2 hours after the start of the temperature increase, 0.92 g of an aqueous solution of APS with a concentration of 5 wt % was added over 4.3 hours in terms of solute. From the point when the pressure had dropped by 0.2 MPaG from the pressure at the end of the temperature increase, the mixture was heated to 40°C over 120 minutes. While maintaining the temperature at 40°C, the reaction was continued until the pressure in the system decreased to 1.3 MPaG. The obtained polymer slurry was treated in the same manner as in Example 1 to obtain a powder of vinylidene fluoride copolymer (electrode binder). The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 4 mass%, and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 1. However, the viscosity of the electrode mixture slurry using the vinylidene fluoride copolymer was high, and poor dispersion caused lumps to form during electrode production, making uniform coating difficult.

[0081] Example 3 Polymerization and post-polymerization treatment were carried out in the same manner as in Example 1, except that 1.40 g of a diisopropyl peroxydicarbonate-HFE-347pc-f solution with a polymerization initiator concentration of 50 wt % and 0.80 g of ethyl acetate were charged, thereby obtaining a powder of vinylidene fluoride copolymer (electrode binder). Evaluation of the vinylidene fluoride copolymer was carried out in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 5 mass %, and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 1, except that the solids concentration of the electrode mixture was 55.0 mass %.

[0082] Comparative Example 2 Polymerization and post-polymerization treatment were carried out in the same manner as in Comparative Example 1, except that 0.07 g of APS, 3.15 g of a diisopropyl peroxydicarbonate-HFE-347pc-f solution with a polymerization initiator concentration of 50 wt % were added before the start of the temperature increase, and 0.77 g (solute equivalent) of an APS aqueous solution with a concentration of 5 wt % was added over 4.3 hours starting 2 hours after the start of the temperature increase, to obtain a powder of vinylidene fluoride copolymer (electrode binder). The obtained polymer slurry was treated in the same manner as in Example 1, to obtain a powder of vinylidene fluoride copolymer (electrode binder). The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 5% by mass and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 3 above.

[0083] Example 4 Polymerization and post-polymerization treatment were performed in the same manner as in Example 3, except that the amount of APS charged was 2.12 g and the amount of ethyl acetate charged was 1.60 g, to obtain a powder of vinylidene fluoride copolymer (electrode binder). The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 6 mass%, and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 1, except that the solids concentration of the electrode mixture was 58.5 mass%.

[0084] (Comparative Example 3) A 2-liter autoclave was charged with 1085 g of ion-exchanged water as a dispersion medium, 0.21 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 0.13 g of APS, 2.12 g of diisopropyl peroxydicarbonate-HFE-347pc-f solution having a 50 wt% concentration of polymerization initiator, 1.70 g of ethyl acetate as a chain transfer agent, and 4248 g of vinylidene fluoride, and the temperature was raised to 26 ° C. over 55 minutes. While maintaining the temperature at 26 ° C., 2 hours after the start of the temperature rise, 12.12 g of an aqueous solution of APS having a concentration of 5 wt% was added in solute equivalent over 12 hours. From the point when the pressure had dropped by 0.2 MPaG from the pressure at the end of the temperature rise, the temperature was raised to 40 ° C. over 180 minutes. While maintaining the temperature at 40 ° C., the reaction was continued until the pressure in the system dropped to 1.3 MPaG. The obtained polymer slurry was treated in the same manner as in Example 1 to obtain a powder of vinylidene fluoride copolymer (electrode binder). The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 6 mass%, and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 4 above. However, the viscosity of the electrode mixture slurry using the vinylidene fluoride copolymer was high, making viscosity measurement impossible.

[0085] Example 5 A 2-liter autoclave was charged with 1024 g of ion-exchanged water as a dispersion medium, 0.6 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspending agent, 0.04 g of APS, 4.80 g of a di-normal propyl peroxydicarbonate-methanol solution having a 50 wt% concentration of a polymerization initiator, 1.96 g of ethyl acetate as a chain transfer agent, and 400 g of vinylidene fluoride, and the temperature was raised to 26°C over 55 minutes. The temperature was maintained at 26°C, and from the point at which the pressure had decreased by 0.2 MPaG from the pressure at the end of the temperature increase, the temperature was raised to 55°C over 60 minutes. During this time, 3.96 g of an aqueous APS solution having a concentration of 5 wt% was added in solute equivalent. While maintaining the temperature at 55°C, the reaction was continued until the pressure in the system decreased to 1.3 MPaG. The obtained polymer slurry was treated in the same manner as in Example 1 to obtain a powder of vinylidene fluoride copolymer (electrode binder). The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 13 mass% and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 1 above, except that the mass ratio of LFP, CNT, and PVDF in the electrode mixture was 100:2:4 and the solids concentration was 59.0 mass%.

[0086] Example 6 A 2-liter autoclave was charged with 1034 g of ion-exchanged water as a dispersion medium, 0.47 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 6.58 g of diisopropyl peroxydicarbonate-HFE-347pc-f solution having a polymerization initiator concentration of 50 wt%, 15.09 g of ethyl acetate as a chain transfer agent, and 470 g of vinylidene fluoride, and the temperature was raised to 26°C over 55 minutes. The temperature was maintained at 26°C, and from the point when the pressure at the end of the temperature increase had decreased by 0.2 MPaG, the temperature was raised to 45°C over 127 minutes. During this time, 2.35 g of an aqueous solution of APS having a concentration of 5 wt% was added in solute equivalent. While maintaining the temperature at 45°C, the reaction was continued until the pressure in the system decreased to 1.3 MPaG. The obtained polymer slurry was treated in the same manner as in Example 1 to obtain a powder of vinylidene fluoride copolymer (electrode binder). The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 13 mass% and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 5 above.

[0087] Example 7 Polymerization and post-polymerization treatment were carried out in the same manner as in Example 4, except that 1.20 g of acrylic acid (AA) was used instead of APS, 2.48 g of a diisopropyl peroxydicarbonate-HFE-347pc-f solution with a 50 wt% concentration as the polymerization initiator, and 2.60 g of ethyl acetate as the chain transfer agent were used, to obtain a powder of vinylidene fluoride copolymer (electrode binder). The obtained polymer slurry was treated in the same manner as in Example 1, to obtain a powder of vinylidene fluoride copolymer (electrode binder). Evaluation of the vinylidene fluoride copolymer was carried out in the same manner as in Example 1, except that the amount of vinyl compound introduced was calculated by neutralization titration measurement. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 13% by mass and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 5 above.

[0088] (Comparative Example 4) Before the start of the temperature increase, 0.21 g of APS, 5.09 g of diisopropyl peroxydicarbonate-HFE-347pc-f solution having a concentration of 50 wt% as a polymerization initiator, and 9.33 g of ethyl acetate as a chain transfer agent were added. Two hours after the start of the temperature increase, 4.03 g of an APS aqueous solution having a concentration of 5 wt% was added over 18 hours in solute equivalent. Polymerization and post-polymerization treatment were carried out in the same manner as in Comparative Example 3, and a powder of vinylidene fluoride copolymer (electrode binder) was obtained. The vinylidene fluoride copolymer was evaluated in the same manner as in Example 1. The vinylidene fluoride copolymer was then dissolved in NMP to a concentration of 13% by mass and used as a binder solution. Using the vinylidene fluoride copolymer, an electrode mixture and an electrode were produced in the same manner as in Example 5 above.

[0089] (result)

[0090]

[0091] As shown in Tables 1 and 2 above, when the weight average molecular weight Mwa of the vinylidene fluoride copolymer was 50,000 or more and the weight average molecular weight Mwc of the carboxyl group-containing copolymer was less than 0.90 relative to Mwa, the slurry viscosity was lower than that of the comparative example having the same slurry composition and solid concentration, and the peel strength was at the same level.

[0092] This application claims priority from Japanese Patent Application No. 2024-056534, filed March 29, 2024, the entire contents of which are incorporated herein by reference.

[0093] According to the present invention, there is provided an electrode binder containing a vinylidene fluoride copolymer that has excellent adhesion to a current collector and is resistant to thickening of the electrode mixture even when lithium iron phosphate is used as the electrode active material, etc. The electrode binder is extremely useful in the field of manufacturing various batteries.

Claims

1. An electrode binder containing a vinylidene fluoride copolymer having structural units derived from vinylidene fluoride and structural units derived from a vinyl compound having a carboxy group, wherein the weight average molecular weight Mwa of the vinylidene fluoride copolymer is 50,000 or more, and when the vinylidene fluoride copolymer is modified with a labeling substance consisting of 1-bromomethylpyrene to determine the weight average molecular weight Mwc of the carboxy group-containing vinylidene fluoride copolymer that absorbs light at a wavelength of 345 nm, the Mwc is less than 0.90 relative to the Mwa.

2. The electrode binder according to claim 1, wherein the Mwc is 0.25 or more relative to the Mwa.

3. The electrode binder according to claim 1, wherein the amount of structural units derived from vinylidene fluoride is 90.0 mol% or more, and the amount of structural units derived from vinyl compounds is 0.01 mol% or more and 0.80 mol% or less, relative to 100.0 mol% of all structural units of the vinylidene fluoride copolymer.

4. The electrode binder according to claim 1, wherein the vinyl compound is a compound represented by the following general formula (1): (In the general formula (1), R １ , R ２ , R ３ each independently represents a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms, and X represents a hydroxy group or -Y-COOH (wherein Y represents an atomic group).

5. The electrode binder according to claim 1, wherein the vinyl compound is a compound represented by the following general formula (1): (In the general formula (1), R １ , R ２ , R ３ each independently represents a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms, X represents a hydroxy group or -Y-COOH (Y represents a divalent atomic group having a molecular weight of 500 or less, containing either an oxygen atom or a nitrogen atom and having 1 to 10 atoms in the main chain), 6. An electrode mixture comprising the electrode binder according to any one of claims 1 to 5 and an electrode active material.

7. An electrode comprising the solid content of the electrode binder according to any one of claims 1 to 5.

8. A battery comprising the electrode of claim 7.

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