Electrode mixture, electrode, and battery

Abstract

The present invention addresses the problem of providing an electrode mixture containing an electrode active material coated with carbon, and a vinylidene fluoride copolymer having a carboxy group, the electrode mixture having low initial viscosity and excellent handleability. This electrode mixture for addressing the aforementioned problem contains 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, and an electrode active material coated with carbon. The carbon coating amount of the electrode active material is 0.5-3.0 mass% with respect to the mass of the electrode active material, and the weight-average molecular weight Mwa of the vinylidene fluoride copolymer is 50,000 or greater. When the vinylidene fluoride copolymer is modified with a labeling substance composed of 2-bromomethylpyrene and the weight-average molecular weight Mwc of a carboxy-group-containing vinylidene fluoride copolymer that absorbs light having a wavelength of 345 nm is identified, Mwc is less than 0.90 with respect to Mwa.

Description

Electrode mixture, electrode, and battery

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

[0002] To form a mixture layer of an electrode of a nonaqueous electrolyte secondary battery, an electrode mixture containing a binder and an active material is typically used. A vinylidene fluoride polymer is typically used as the binder. High adhesiveness is required for the binder, as it serves to adhere the active material to the current collector. For example, Patent Document 1 describes introducing a functional group such as a carboxyl group into a fluorine-based resin to improve adhesion to the current collector and solubility in a solvent. 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 the current collector.

[0003] On the other hand, in order to reduce the cost of batteries, the use of lithium iron phosphate (LFP) as a positive electrode active material has been investigated, and the use of lithium iron manganese phosphate (LFMP) as a next-generation material has also been investigated.

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

[0005] To improve adhesion to a current collector or the like, it has been considered to use the vinylidene fluoride copolymer described in Patent Documents 1 and 2 as an electrode binder in electrode mixtures (slurries) containing the above-mentioned lithium iron phosphate (LFP) or lithium iron manganese phosphate (LFMP). However, when LFP or LFMP is mixed with a vinylidene fluoride copolymer, which has high adhesive strength to a current collector, the initial viscosity of the electrode mixture increases, and thixotropy tends to increase. Furthermore, it has been very difficult to fabricate electrodes using such electrode mixtures. After extensive research, the inventors have found that the reason for the increased initial viscosity and increased thixotropy in electrode mixtures containing LFP or LFMP is believed to be due to an interaction between the carbon coating present on the surface of the LFP or LFMP and the vinylidene fluoride copolymer having a carboxy group.

[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 mixture that contains a carbon-coated electrode active material and a vinylidene fluoride copolymer having a carboxy group, and that has low initial viscosity and good handleability. Another object of the present invention is to provide an electrode and a battery obtained using the electrode mixture.

[0007] [1] The present invention provides an electrode mixture comprising a vinylidene fluoride copolymer having structural units derived from vinylidene fluoride and structural units derived from a vinyl compound having a carboxy group, and a carbon-coated electrode active material, wherein the amount of carbon coating on the electrode active material is 0.5% by mass or more and 3.0% by mass or less, relative to the mass of the electrode active material, 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 Mwc is less than 0.90 relative to the Mwa. [2] The present invention provides the electrode mixture according to [1], wherein the Mwc is 0.25 or more relative to the Mwa. [3] The present invention provides the electrode mixture according to [1] or [2], wherein the average particle size of the electrode active material is 15 μm or less. [4] The present invention provides the electrode mixture according to any one of [1] to [3], wherein the electrode active material has a carbon coating amount of 2.5 mass% or less. [5] The present invention provides the electrode mixture according to [1] to [4], 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 1 to 5 carbon atoms, and X represents a hydroxy group or -Y-COOH (wherein Y represents an atomic group). [6] The present invention provides an electrode mixture according to any one of [1] to [4], wherein the vinyl compound is a compound represented by the following general formula (1): (In general formula (1), R １ , R２ , 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 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)

[0008] [7] The present invention provides an electrode comprising the solid content of the electrode mixture according to any one of [1] to [6] above. [8] The present invention provides a battery comprising the electrode according to [7] above.

[0009] According to the present invention, there is provided an electrode mixture that contains a carbon-coated electrode active material and a vinylidene fluoride copolymer having a carboxy group, and that has low initial viscosity and good handleability. Further, there are also provided an electrode and a battery obtained using the electrode mixture.

[0010] 1. Electrode Mix The electrode mix of the present invention is a slurry composition containing a vinylidene fluoride copolymer having structural units derived from vinylidene fluoride and structural units derived from a vinyl compound having a carboxy group, and a carbon-coated electrode active material (hereinafter also simply referred to as "active material"). The electrode mix may further contain a conductive aid, a solvent, other additives, etc. When the electrode mix contains a solvent, the vinylidene fluoride copolymer may be dissolved in the solvent or may be dispersed in the solvent.

[0011] As described above, when an electrode mixture (slurry) is prepared by mixing a vinylidene fluoride copolymer having carboxy groups with a carbon-coated active material (e.g., LFP, LFMP), the viscosity tends to be very high, particularly the initial viscosity, and the thixotropy tends to be high. In this specification, the initial viscosity refers to the slurry viscosity immediately after stirring the slurry. Generally, LFP, LFMP, and the like are active materials with small particle sizes and coated with carbon, with large surface areas and many functional groups, such as OH groups, present on the surface. Therefore, the carboxy groups of the vinylidene fluoride copolymer and these functional groups tend to interact with each other. When a typical vinylidene fluoride copolymer is used, a strong network originating from the active material is formed within the electrode mixture, resulting in increased viscosity. In contrast, the electrode mixture of the present invention, despite containing a vinylidene fluoride copolymer having carboxy groups and a carbon-coated electrode active material, tends to have a low initial viscosity and good thixotropy. The reasons for this are as follows:

[0012] The vinylidene fluoride copolymer contained in the electrode mixture 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 also 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 primarily of structural units derived from vinylidene fluoride and not containing structural units derived from a vinyl compound. 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 carboxy group-containing copolymer exists in a relatively low molecular weight range compared to the molecular weight distribution of the vinylidene fluoride copolymer. When the molecular weight of the carboxyl group-containing copolymer is relatively small, it is difficult for the carboxyl group-containing copolymer to interact with the functional groups on the surface of the active material, making it difficult to form a strong network, and it is therefore thought that the viscosity (initial viscosity) of the slurry is unlikely to increase excessively. Below, the vinylidene fluoride copolymer contained in the electrode mixture of the present invention, its physical properties, and the active material and other components will be described in detail.

[0013] (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 90.0 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.

[0014] On the other hand, the amount of the constituent 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 constituent units of the vinylidene fluoride copolymer. If the amount of the constituent units derived from a vinyl compound is 0.01 mol% or more, when an electrode is formed using the electrode mixture, 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 constituent units derived from the vinyl compound is 0.80 mol% or less, the increase in the initial viscosity of the electrode mixture is more likely to be suppressed. The amount of the constituent units derived from a vinyl compound can be determined by neutralization titration or １ It can be determined from H-NMR spectrum or the like.

[0015] 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), 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. 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.

[0016] 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, more preferably an atomic group containing either an oxygen atom or a nitrogen atom and having 1 or more and 10 or less atoms in the main chain, and preferably a divalent atomic group having a molecular weight of 500 or less and having 1 or more and 10 or less atoms in the main chain.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] The compound represented by the general formula (1) is more preferably acrylic acid, acryloyloxyethyl succinate, acryloyloxypropyl succinate, 2-carboxyethyl acrylate, or 2-carboxymethyl acrylate, from the viewpoints of availability and reactivity with vinylidene fluoride.

[0023] 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.

[0024] Furthermore, the vinylidene fluoride copolymer may partially contain structural units derived from compounds (other compounds) other than vinylidene fluoride and the above vinyl compounds, as long as the purpose and effects 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 by H-NMR spectrum, neutralization titration, etc.

[0025] 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.

[0026] Here, the weight-average molecular weight Mwa of the vinylidene fluoride copolymer may be 50,000 or more, as described above, preferably 200,000 or more and 4,000,000 or less, and more preferably 250,000 or more and 3,000,000 or less. When the weight-average molecular weight Mwa of the vinylidene fluoride copolymer is 50,000 or more, the heat resistance and strength of the electrode (electrode mixture layer) obtained using the electrode mixture tend to be good. 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. Mwa is the average value measured three times.

[0027] On the other hand, the weight-average molecular weight Mwc of the carboxyl group-containing copolymer in the vinylidene fluoride copolymer is required to 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 0.85 or less, more preferably 0.30 or more and 0.70 or less, more preferably 0.30 or more and 0.60 or less, and even more preferably 0.30 or more and 0.50 or less. As mentioned above, if the ratio is less than 0.90, the initial viscosity of the electrode mixture is unlikely to be excessively increased.

[0028] 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.

[0029] 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 measurement is then performed using an ultraviolet-visible (UV-visible) detector (detection wavelength: 345 nm) and N,N-dimethylacetamide as the eluent, and the value is calculated in terms of polystyrene. The pyrene structure has characteristic absorption in the ultraviolet-visible region and strongly absorbs light with 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. Mwc is the average value of three measurements.

[0030] Here, the inherent viscosity of the vinylidene fluoride copolymer is preferably 0.5 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 tends to be increased. On the other hand, when the inherent viscosity is 5.0 dL / g or less, the initial viscosity of the electrode mixture does not become too high, and the workability is particularly excellent. The 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.

[0031] The amount of vinylidene fluoride copolymer contained in the solid content of the electrode mixture (components excluding components that volatilize during mixture layer formation) is preferably 0.2% by mass to 20% by mass, more preferably 0.2% by mass to 10% by mass, and even more preferably 0.2% by mass to 5% by mass. When the amount of vinylidene fluoride copolymer is within this range, a mixture layer with high strength is likely to be obtained.

[0032] 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.

[0033] 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).

[0034] 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).

[0035] 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).

[0036] 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.

[0037] 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≦T１０ +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.

[0038] 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, the 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 within a short period of time (e.g., within 2 hours), and further polymerize. 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.

[0039] (Active Material) The active material contained in the electrode mixture of the present invention is a carbon-coated compound, and the carbon coating amount is 0.5% by mass or more and 3.0% by mass or less relative to the mass of the active material. The carbon coating amount is determined by heating and dissolving each positive electrode material in aqua regia, suction filtering with a membrane filter, and calculating the acid-insoluble content (carbon amount) from the amount remaining on the filter. The carbon coating amount is preferably 0.5% by mass or more and 2.7% by mass or less, and more preferably 0.5% by mass or more and 2.5% by mass or less.

[0040] Here, the type of active material is not particularly limited as long as the above-mentioned carbon coating amount is satisfied. Generally, the positive electrode active material is LiFePO ４ (LFP) and LiFeMnPO ４ LiMaPO such as (LFMP) ４ In olivine-type lithium compounds represented by the formula (where Ma is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr), the surface is often coated with carbon to reduce particle resistance, making it easy to satisfy the above coverage. When an electrode mixture is prepared by mixing the carbon-coated active material described above with a general vinylidene fluoride copolymer having a carboxy group, the initial viscosity tends to increase. In contrast, when the electrode mixture is combined with the above-mentioned vinylidene fluoride copolymer, the initial viscosity of the electrode mixture is less likely to increase.

[0041] Furthermore, the LFP and LFMP have relatively small average particle diameters. Even when the average particle diameter of the active material in the electrode mixture is small, the initial viscosity of the electrode mixture is usually likely to increase. However, in the electrode mixture of the present invention, it is difficult for the vinylidene fluoride copolymer to form a strong network structure starting from the active material. Therefore, even if the average particle diameter of LFP or LFMP is small, the initial viscosity of the electrode mixture is unlikely to increase. The average particle diameter of the active material is the particle diameter at which the particle size cumulative ratio is 50% in a volume-based particle size cumulative diagram (based on JIS K 1474). The average particle diameter Dv50 of the active material can be 15 μm or less, for example, 0.1 μm or more and 15 μm or less. However, the average particle diameter of the active material is not limited to this range.

[0042] 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 relative to the total amount of solids in the electrode mixture. When the amount of active material is within this range, for example, sufficient charge / discharge capacity is obtained, and battery performance is likely to be good.

[0043] (Conductive Aid) The electrode mixture may further contain a conductive aid. The conductive aid 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 aid include acetylene black, ketjen black, carbon black, graphite powder, carbon nanofiber, carbon nanotube, and carbon fiber.

[0044] 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, and even more preferably 0.1% by mass to 5% by mass, relative to the total amount of solids in the electrode mixture.

[0045] (Solvent) The electrode mixture may contain a solvent. The solvent may be a non-aqueous solvent or water. The non-aqueous solvent may also be a polar solvent (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 mixture may contain only one of the above solvents, or may contain two or more of them.

[0046] The total amount of the solvent in the electrode mixture 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 active material.

[0047] (Other Components) 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, methoxylated 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 salts of poly(meth)acrylic acid such as sodium poly(meth)acrylate, carboxymethyl cellulose, etc. The amount of these is not particularly limited as long as it does not impair the purpose and effects of the present invention, but is preferably 15 mass% or less based on the total solid content of the electrode mixture.

[0048] 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 of the total solid content of the electrode mixture.

[0049] (Physical Properties of Electrode Mix) The viscosity of the electrode mix is ​​not particularly limited as long as it can prevent dripping, uneven coating, and delayed drying after coating when applying the electrode mix to form a mix layer, and provides good workability and applicability when preparing the mix layer. This is the value measured at 25°C with an E-type viscometer for the electrode mix, and is measured at a shear rate of 1 s after a 60-second incubation period at 25°C with the E-type viscometer. －１ The initial viscosity measured 120 seconds after the start of rotor rotation is preferably 2,000 mPa s or more and 60,000 mPa s or less, and more preferably 5,000 mPa s or more and 30,000 mPa s or less. When the initial viscosity is within this range, the handling property during the formation of the mixture layer tends to be further improved.

[0050] In addition, after a 60-second incubation period at 25°C using an E-type viscometer, the sample was －１ The viscosity measured 30 seconds after the start of rotor rotation is preferably 1000 mPa·s or more and 5000 mPa·s or less, and more preferably 1000 mPa·s or more and 4000 mPa·s or less. －１ The viscosity is specified by the shear rate 1 s －１ The ratio of the viscosity to the viscosity specified by the shear rate 1 s －１ Viscosity / shear rate 40 s －１ The viscosity (specified by) is preferably 2 or more and 12 or less, and more preferably 3 or more and 11 or less. When the ratio is within this range, the handleability of the electrode mixture when forming the mixture layer tends to be further improved.

[0051] (Method of Preparing Electrode Mixture) The electrode mixture may be prepared by mixing all of the components at once, or by first mixing some of the components and then mixing the remaining components.

[0052] 2. 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.

[0053] 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.

[0054] 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 and the active material described above. The mixture layer may be formed on only one surface of the current collector, or may be disposed on both surfaces.

[0055] The mixture layer contains at least the components contained in the electrode mixture described above, i.e., the vinylidene fluoride copolymer and the active material, and further contains 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.

[0056] The thickness of the mixture layer is not particularly limited, but in one example, it is preferably 1 μm or more and 300 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 500g / m ２ Preferably, 100 g / m or less ２ More than 300g / m ２ The following is more preferred:

[0057] 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.

[0058] 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.

[0059] 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 200°C or lower, and more preferably 80°C or higher and 150°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.

[0060] 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.

[0061] 3. Batteries As described above, the electrode mixture described above can be used in electrodes of various non-aqueous electrolyte secondary batteries, etc., but it may also be used to form other layers of non-aqueous electrolyte secondary batteries.

[0062] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0063] 1. Measurement and evaluation methods for various physical properties 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 carboxyl group-containing copolymer are shown below. The methods for measuring the average particle size (Dv50) and carbon coating amount of the electrode active material are shown below. Furthermore, the method for measuring the slurry viscosity of the electrode mixture is shown below.

[0064] (1) Inherent Viscosity of Vinylidene Fluoride Copolymer 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.

[0065] (2) Weight-average molecular weight Mwa of vinylidene fluoride copolymer The weight-average molecular weight Mw1 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)

[0066] (3) 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 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.

[0067] The weight-average molecular weight 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. The measurement was carried out under the same measurement conditions 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 carried out three times, and the average value was taken as Mwc.

[0068] (4) Measurement of Average Particle Diameter Dv50 of Electrode Active Material 0.1 g of dispersant (cationic surfactant "SN Wet 366" (manufactured by San Nopco)) was added to 0.01 g of each electrode active material, and the dispersant was allowed to soak into the sample. Next, 20 mL of pure water was added, and the mixture was dispersed in an ultrasonic cleaner for approximately 5 minutes. The particle size distribution in the particle diameter range of 0.1 to 1000 μm was determined using a particle size distribution measuring device (manufactured by Microtrac: MT3300EXII). The dispersion medium was pure water, and the refractive index of the dispersion medium was 1.333. From the obtained particle size distribution, the particle diameter at which the cumulative frequency was 50% on a volume basis was calculated, and the average particle diameter Dv50 was determined.

[0069] (5) Method for Determining the Amount of Carbon Coating on Electrode Active Material 500 mg of each electrode active material was weighed into a beaker, 40 mL of water and aqua regia (3 mL of nitric acid + 9 mL of hydrochloric acid) were added, and the mixture was dissolved by heating. The mixture was then suction filtered through a membrane filter, and the acid-insoluble portion (amount of carbon coating) was calculated from the remaining amount.

[0070] (6) Slurry viscosity of electrode mixture The slurry viscosity of the electrode mixture prepared in the examples and comparative examples was measured. The electrode mixture was placed in an E-type viscometer (RE-215 type viscometer manufactured by Toki Sangyo Co., Ltd., rotor 3° × R14) immediately after preparation. After keeping the mixture at 25°C for 60 seconds, the mixture was slurried at a shear rate of 1 s －１ The rotor was rotated at a shear rate of 40 s, and the viscosity measured 120 seconds after the rotor started rotating was determined as the slurry viscosity. －１ The rotor was rotated at a shear rate of 1 s , and the viscosity measured 30 seconds after the rotor started rotating was determined as the slurry viscosity. －１ and a shear rate of 40 s －１ Viscosity ratio of the slurry viscosity (shear rate 1 s －１ Slurry viscosity / shear rate 40 s －１ The slurry viscosity was determined.

[0071] 2. Preparation of Materials The following materials were prepared as electrode mixtures to be prepared in the Examples and Comparative Examples.

[0072] (1) Active materials ・LFP-A Carbon coverage: 1.0%, average particle diameter D50: 2 μm) ・LFP-B Carbon coverage: 1.4%, average particle diameter D50: 4 μm) ・LFP-C Carbon coverage: 2.1%, average particle diameter D50: 10 μm) ・LFMP , carbon coverage: 2.0%, average particle diameter D50: 1 μm) ・LCO carbon coverage: 0.0%, average particle diameter D50: 2 μm)

[0073] (2) Vinylidene fluoride copolymers VDF / APS-1: Prepared in Synthesis Example 1 below VDF / APS-2: Prepared in Synthesis Example 2 below VDF / AA-1: Prepared in Synthesis Example 3 below VDF / AA-2: Prepared in Synthesis Example 4 below

[0074] Synthesis Example 1 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 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspending agent, 2.0 g of acryloyloxypropyl succinic acid (APS), 1.40 g of a diisopropyl peroxydicarbonate-HFE-347pc-f solution having a polymerization initiator concentration of 50 wt %, 0.8 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 MPa. After the polymerization was completed, 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 a powder of vinylidene fluoride copolymer (VDF / APS-1), which is a copolymer of vinylidene fluoride (VDF) and acryloyloxypropyl succinic acid (APS). The inherent viscosity, weight-average molecular weight Mwa, and weight-average molecular weight Mwc of the carboxy group-containing copolymer contained in the vinylidene fluoride copolymer are shown in Table 1 below.

[0075] Synthesis Example 2 A 2-liter autoclave was charged with 1,054.5 g of ion-exchanged water as a dispersion medium, 0.23 g of Metolose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspending agent, 0.07 g of APS, 3.50 g of a diisopropyl peroxydicarbonate-HFE-347pc-f solution having a polymerization initiator concentration of 50 wt%, and 400 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, 0.86 g (in terms of solute) of an aqueous solution of APS having a concentration of 5 wt% was added over 4.3 hours. From the point when 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 40 minutes. 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 Synthesis Example 1 to obtain a powder of vinylidene fluoride copolymer (VDF / APS-2), which is a copolymer of vinylidene fluoride (VDF) and acryloyloxypropyl succinic acid (APS). The inherent viscosity, weight average molecular weight Mwa, and weight average molecular weight Mwc of the carboxy group-containing copolymer contained therein are shown in Table 1 below.

[0076] Synthesis Example 3 A 2-liter autoclave was charged with 1,056 g of ion-exchanged water as a dispersion medium, 0.44 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspending agent, 9.37 g of acrylic acid (AA), 5.28 g of a 50 wt % tertiary butyl peroxypivalate-HFE-347pc-f solution as a polymerization initiator, 0.44 g of ethyl acetate as a chain transfer agent, and 440 g of vinylidene fluoride, and the temperature was raised to 52°C over 110 minutes. While maintaining the temperature at 52°C, the reaction was continued until the pressure in the system decreased to 4.7 MPaG. The resulting polymer slurry was treated in the same manner as in Synthesis Example 1, to obtain a powder of vinylidene fluoride copolymer (VDF / AA-1), which is a copolymer of vinylidene fluoride (VDF) and acrylic acid (AA). The inherent viscosity and weight average molecular weight Mwa of the vinylidene fluoride copolymer, and the weight average molecular weight Mwc of the carboxy group-containing copolymer contained therein are shown in Table 1 below.

[0077] Synthesis Example 4 A 2-liter autoclave was charged with 1,056 g of ion-exchanged water as a dispersion medium, 0.44 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 0.22 g of acrylic acid (AA), 3.08 g of a tertiary butyl peroxypivalate-HFE-347pc-f solution having a polymerization initiator concentration of 50 wt %, 0.44 g of isododecane as a chain transfer agent, and 440 g of vinylidene fluoride, and the temperature was raised to 52°C over 110 minutes. While maintaining the temperature at 52°C, 3.74 g (in terms of solute) of a 1 wt % AA aqueous solution was added so as to maintain the pressure immediately after completion of the temperature increase. The reaction was continued until the pressure in the system decreased to 7.18 MPaG. The resulting polymer slurry was treated in the same manner as in Synthesis Example 1 to obtain a powder of vinylidene fluoride copolymer (VDF / AA-2), which is a copolymer of vinylidene fluoride (VDF) and acrylic acid (AA). The inherent viscosity, weight average molecular weight Mwa, and weight average molecular weight Mwc of the carboxy group-containing copolymer contained in the vinylidene fluoride copolymer are shown in Table 1 below.

[0078] (3) Physical Properties of Vinylidene Fluoride Copolymer The physical properties of the vinylidene fluoride copolymer are shown below.

[0079] 3. Preparation of electrode mixtures Each electrode mixture was prepared by the following method.

[0080] Example 1 LFP:A was used as the electrode active material, VDF / APS-1 was used as the vinylidene fluoride copolymer (binder), and a carbon nanotube (CNT) NMP dispersion was used as the conductive additive. These were then mixed using a planetary centrifugal mixer, Thinky Corporation's Awatori Rentaro ARE310, to prepare an electrode mixture. In the obtained electrode mixture, the mass ratio of the electrode active material, conductive additive, and binder was 100:2:2.5, and the solids concentration was 55.0 mass%. The slurry viscosity at this time is shown in Table 2.

[0081] (Examples 2 to 5, Comparative Examples 1 to 5, and Reference Examples 1 and 2) As shown in Table 2, electrode mixtures were prepared in the same manner as in Example 1, except that the type of electrode active material, the type of vinylidene fluoride copolymer, and the solid content concentration in Example 4 and Comparative Example 4 were changed to 52%.

[0082] (result)

[0083] As shown in Table 2 above, when an active material not coated with carbon was used, there was no significant difference due to the difference in the vinylidene fluoride copolymer having a carboxy group (Reference Examples 1 and 2). In contrast, when Example 1 and Comparative Example 1 were compared, it was found that even when the same active material was used, the difference in Mwc / Mwa affected the slurry viscosity (particularly the shear rate s －１ There was a large difference in the viscosity ratio (particularly in the case of shear rate s) and also in the viscosity ratio. The same is true in comparisons between Examples 2 to 5 and Comparative Examples 2 to 5. When the weight average molecular weight Mwa of the vinylidene fluoride copolymer is 50,000 or more and the weight average molecular weight Mwc of the carboxyl group-containing copolymer is less than 0.90 relative to Mwa, the slurry viscosity (particularly in the case of shear rate s －１ ) was 51041 mPa·s or less, and the viscosity ratio was 11.2 or less (Examples 1 to 5). In other words, there was no significant difference from Reference Examples 1 and 2.

[0084] This application claims priority from Japanese Patent Application No. 2024-056545, filed March 29, 2024, the entire contents of which are incorporated herein by reference.

[0085] According to the present invention, there is provided an electrode mixture that contains a carbon-coated electrode active material and a vinylidene fluoride copolymer having a carboxy group, has a low initial viscosity, and is easy to handle. The electrode mixture is very useful in the field of manufacturing various batteries.

Claims

1. An electrode mixture comprising: a vinylidene fluoride copolymer having structural units derived from vinylidene fluoride and structural units derived from a vinyl compound having a carboxy group; and an electrode active material coated with carbon, wherein the amount of carbon coating on the electrode active material is 0.5% by mass or more and 3.0% by mass or less relative to the mass of the electrode active material; 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 2-bromomethylpyrene and the weight-average molecular weight Mwc of the carboxy group-containing vinylidene fluoride copolymer that absorbs light at a wavelength of 345 nm is specified, the Mwc is less than 0.90 relative to the Mwa.

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

3. The electrode mixture according to claim 1, wherein the electrode active material has an average particle size of 15 μm or less.

4. The electrode mixture according to claim 1, wherein the amount of carbon coating on the electrode active material is 2.5 mass % or less.

5. The electrode mixture 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).

6. The electrode mixture 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 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) 7. An electrode comprising the solid content of the electrode mixture according to any one of claims 1 to 6.

8. A battery comprising the electrode of claim 7.

Images