WO2025205922A1 - Electrode mixture, electrode, and battery - Google Patents

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

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

An electrode mixture containing a binder and active material is typically used to form the mixture layer of an electrode in a non-aqueous electrolyte secondary battery. Vinylidene fluoride polymer is typically used as the binder. High adhesiveness is required for the binder, as it serves the purpose of adhering the active material to the current collector. For example, Patent Document 1 describes introducing functional groups such as carboxyl groups into a fluorine-based resin to improve adhesion to the current collector and solubility in solvents. Patent Document 2 describes a binder composition containing a copolymer of vinylidene fluoride and a compound with a specific structure (vinylidene fluoride copolymer), and describes that the vinylidene fluoride copolymer exhibits high adhesion to the current collector.

Meanwhile, with the aim of reducing battery costs, the use of lithium iron phosphate (LFP) as a positive electrode active material is being considered. Furthermore, the use of lithium iron manganese phosphate (LFMP) as a next-generation material as a positive electrode active material is also being considered.

Japanese Patent Application Publication No. 6-172452 Patent No. 5797206

In order to improve adhesion to current collectors, etc., it is conceivable 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 current collectors, the initial viscosity of the electrode mixture increases and thixotropy tends to become stronger. Furthermore, it is extremely difficult to fabricate electrodes using such electrode mixtures. After extensive research, the inventors have determined 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 containing carboxyl groups.

The present invention was made in consideration of the above-mentioned problems. It aims to provide an electrode mixture that contains a carbon-coated electrode active material and a vinylidene fluoride copolymer having a carboxy group, has low initial viscosity, and is easy to handle. It also aims to provide an electrode and a battery obtained using the electrode mixture.

[1] The present invention provides an electrode mixture comprising a vinylidene fluoride copolymer having a structural unit derived from vinylidene fluoride and a structural unit 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 1-bromomethylpyrene to specify 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 amount of carbon coating on the electrode active material is 2.5 mass % or less.
[5] The present invention provides the 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 , R ² , and R ³ each independently represent 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 (Y represents an atomic group).)
[6] The present invention provides the 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 , R ² , and R ³ each independently represent 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 (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] 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 the above [7].

The present invention provides an electrode mixture that contains a carbon-coated electrode active material and a vinylidene fluoride copolymer having a carboxy group, has low initial viscosity, and is easy to handle. Furthermore, electrodes and batteries obtained using the electrode mixture are also provided.

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.

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 or LFMP), the viscosity tends to be very high, particularly the initial viscosity, and the thixotropy tends to be high. In this specification, "initial viscosity" refers to the slurry viscosity immediately after stirring. Generally, LFP and LFMP are small-particle, carbon-coated active materials with large surface areas and numerous functional groups, such as OH groups, present on the surface. Therefore, these functional groups tend to interact with the carboxy groups of the vinylidene fluoride copolymer. When a typical vinylidene fluoride copolymer is used, a strong network originating from the active material forms 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:

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 includes a polymer containing structural units derived from vinylidene fluoride and structural units derived from vinyl compounds (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 vinyl compounds. The weight-average molecular weight Mwc of the carboxy group-containing copolymer, determined by the method described below, is less than 0.90 times 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. If 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 thought that the viscosity (initial viscosity) of the slurry is unlikely to increase excessively. Below, we will explain in detail the vinylidene fluoride copolymer contained in the electrode mixture of the present invention, its physical properties, and the active material and other components.

(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 more likely to be 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.

On the other hand, the amount of 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 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. When the amount of structural 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, when the amount of structural 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 structural units derived from a vinyl compound can be determined by neutralization titration or ^1H -NMR spectroscopy, etc.

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 , and R ³ each independently represent 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, from the viewpoint of ease of availability, etc., a methyl group, an ethyl group, or a butyl group is preferred. Furthermore, from the viewpoint of reducing the likelihood of steric hindrance during polymerization with vinylidene fluoride, it is more preferred that R ¹ , R ² , and R ³ each independently represent a hydrogen atom or a methyl group.

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

The atomic group (Y) may be linear, branched, or cyclic, or may be a combination of these. Among these, it is preferable that the atomic group (Y) be linear or branched, from the viewpoint of being less likely to cause steric hindrance during polymerization with vinylidene fluoride.

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 connecting the carbonyl group in general formula (1) and the carboxy group of -Y-COOH.

Here, the atomic group (Y) contains either 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 heteroatoms 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 ether bonds, ester bonds, carbonyl groups, carboxy groups, amide groups, and hydroxy groups. Among these, ether bonds, ester groups, carbonyl groups, carboxy groups, amide groups, and hydroxy groups are preferred.

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 a structure (functional group) containing the above heteroatom. The molecular weight of the atomic group (Y) may be 500 or less, and from the viewpoint of polymerization reactivity, a molecular weight of 30 or more and 200 or less is preferred.

Specific examples of compounds represented by the above 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 refers to methacrylic, acrylic, or a mixture thereof; (meth)acrylate refers to methacrylate, acrylate, or a mixture thereof; and (meth)acryloyl refers to methacryloyl, acryloyl, or a mixture thereof.

In terms of availability and reactivity with vinylidene fluoride, the compounds represented by the above general formula (1) are more preferably acrylic acid, acryloyloxyethyl succinate, acryloyloxypropyl succinate, 2-carboxyethyl acrylate, and 2-carboxymethyl acrylate.

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 citraconate, monoethyl citraconate, monomethyl phthalate, monoethyl phthalate, monomethyl itaconate, and monoethyl itaconate.

Furthermore, the vinylidene fluoride copolymer may partially contain structural units derived from compounds other than vinylidene fluoride and the above vinyl compounds (other compounds) within a range that does not impair the objects and effects of the present invention. 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 determined from ^19F -NMR spectrum, ^1H -NMR spectrum, neutralization titration, etc.

Examples of other compounds include fluorine-based vinyl compounds that have 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; and perfluoroalkyl vinyl ethers, such as perfluoromethyl vinyl ether. Examples of other compounds also include compounds that have a vinyl group but do not contain fluorine. Examples include unsaturated hydrocarbon compounds such as ethylene and propylene.

Here, as mentioned above, the weight-average molecular weight Mwa of the vinylidene fluoride copolymer should be 50,000 or more, 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). The eluent used is N,N-dimethylacetamide, and the weight-average molecular weight is determined using a refractive index (RI) detector. Mwa is the average value of three measurements.

On the other hand, the weight-average molecular weight Mwc of the carboxyl group-containing copolymer in the vinylidene fluoride copolymer should be less than 0.90 relative to the weight-average molecular weight Mwa of the vinylidene fluoride copolymer, i.e., Mwc/Mwa < 0.90. However, Mwc/Mwa is preferably 0.25 or greater but less than 0.90, more preferably 0.25 or greater but less than 0.85, more preferably 0.30 or greater but less than 0.70, more preferably 0.30 or greater but less than 0.60, and even more preferably 0.30 or greater but less than 0.50. As mentioned above, if this ratio is less than 0.90, the initial viscosity of the electrode mixture is unlikely to increase excessively.

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 above ratio (Mwc/Mwa) tends to fall within the desired range.

In this specification, the weight-average molecular weight Mwc of the carboxyl group-containing copolymer is a value measured as follows. A labeling substance (1-bromomethylpyrene) for labeling the carboxyl groups and potassium carbonate are mixed with a vinylidene fluoride copolymer to esterify the carboxyl groups, resulting in 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, resulting in a polystyrene-equivalent value. 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 carboxyl group-containing copolymer is determined by esterifying the carboxyl groups with 1-bromomethylpyrene. Mwc is the average of three measurements.

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 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 workability is particularly excellent. The inherent viscosity (η _i ) indicates the 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.

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

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

In suspension polymerization using water as the dispersion medium, suspending agents such as methyl cellulose, propoxylated methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, and gelatin are 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 with carboxy groups, and other monomers).

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 the copolymerization (vinylidene fluoride, vinyl compound having a carboxy group, and other monomers as needed).

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, or carbon tetrachloride. When a chain transfer agent is used, the amount used is typically 0.01 to 5 parts by mass, and preferably 0.01 to 3 parts by mass, per 100 parts by mass of all monomers used in the copolymerization (vinylidene fluoride, vinyl compounds having carboxy groups, and any other monomers).

Furthermore, the amount of all monomers (vinylidene fluoride, vinyl compounds having carboxy groups, and other monomers) used in the copolymerization is typically 1:1 to 1:10, preferably 1:2 to 1:5, in terms of the mass ratio of total monomers to water.

The polymerization temperature T is appropriately selected depending on the 10-hour half-life temperature _T10 of the polymerization initiator, and is usually selected within the range of _T10 -25°C≦T≦ _T10 +25°C. For example, the _T10 of t-butyl peroxypivalate and diisopropyl peroxydicarbonate are 54.6°C and 40.5°C, respectively (see NOF Corporation product catalog). Therefore, in polymerizations using t-butyl peroxypivalate and diisopropyl peroxydicarbonate as polymerization initiators, the polymerization temperature T is appropriately selected within the ranges of 29.6°C≦T≦79.6°C and 15.5°C≦T≦65.5°C, respectively. The polymerization time is not particularly limited, but is preferably 100 hours or less in consideration of productivity, etc. The polymerization is usually carried out under increased pressure, preferably 2.0 to 10.0 MPa-G.

Here, the following two methods can be used to set the weight-average molecular weight Mwc of the carboxyl group-containing copolymer to less than 0.90 relative to the weight-average molecular weight Mwa of the vinylidene fluoride copolymer. However, these methods are not limited to these. The first method involves mixing vinylidene fluoride (and other compounds) with the entire amount of the vinyl compound having a carboxyl group and then initiating polymerization. The second method involves polymerizing vinylidene fluoride (and other compounds) to a certain extent, then adding the entire amount of the vinyl compound having a carboxyl group to the reaction system within a short period of time (e.g., within two hours), followed by further polymerization. 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 adjusting the amount of polymerization initiator, the amount of chain transfer agent, the polymerization temperature, etc.

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

Here, the type of active material is not particularly limited as long as the above-mentioned carbon coating amount is satisfied. However, in general, olivine- _type lithium compounds represented by _LiMaPO4 (where Ma is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr), such as LiFePO4 ( _LFP ) and LiFeMnPO4 (LFMP), which are positive electrode active materials, often have their surfaces carbon-coated to reduce particle resistance, making it easy to satisfy the above-mentioned coverage rate. When an electrode mixture is prepared by mixing the carbon-coated active material as described above with a general vinylidene fluoride copolymer having a carboxy group, the initial viscosity tends to increase. In contrast, when combined with the above-mentioned vinylidene fluoride copolymer, the initial viscosity of the electrode mixture is less likely to increase.

Furthermore, the above-mentioned LFP and LFMP have a relatively small average particle diameter. 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.

The amount of active material contained in the electrode mixture is selected appropriately depending on the application of the electrode mixture, but is preferably 50% by mass or more and 99.9% by mass or less of the total solid content of the electrode mixture. 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.

(Conductive additive)
The electrode mixture may further contain a conductive additive. 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.

The amount of conductive additive contained in the electrode mixture is selected appropriately depending on the type of conductive additive. From the perspective of improving both the conductivity and the dispersibility of the conductive additive, the amount is preferably 0.1% to 15% by mass, more preferably 0.1% to 7% by mass, and even more preferably 0.1% to 5% by mass, relative to the total amount of solids in the electrode mixture.

(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.

The total amount of solvent in the electrode mixture is not particularly limited, but is typically preferably 10 to 150 parts by weight per 100 parts by weight of the active material.

(Other ingredients)
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, methylcellulose, methoxylated methylcellulose, propoxylated methylcellulose, 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 object and effect of the present invention, but is preferably 15 mass% or less based on the total solid content of the electrode mixture.

The electrode mixture may also contain additives such as phosphorus compounds; sulfur compounds; nitrogen compounds such as amine compounds and ammonium compounds; organic acids; organic esters; various silane-, titanium-, and aluminum-based coupling agents; vinylidene fluoride polymers other than the vinylidene fluoride copolymers mentioned above, and resins such as polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and polyacrylonitrile (PAN). There are no particular restrictions on these additives as long as they do not impair the objectives and effects of the present invention, but a content of 15% by mass or less of the total solid content of the electrode mixture is preferred.

(Physical properties of electrode mixture)
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 provides good workability and applicability when preparing the mixture layer. The value of the electrode mixture measured at 25 ° C. using an E-type viscometer is measured after a 60-second incubation period at 25 ° C., rotating the rotor at a shear rate of 1 s ⁻¹ , and measuring 120 seconds after the start of rotor rotation. The value is preferably 2000 mPa s or more and 60,000 mPa s or less, more preferably 5,000 mPa s or more and 30,000 mPa s or less. When the initial viscosity is within this range, handling when forming the mixture layer tends to be even better.

Furthermore, after a 60-second incubation period at 25 ° C. using an E-type viscometer, the rotor is rotated at a shear rate of 40 s ^-1 , and the value measured 30 seconds after the start of rotor rotation is preferably 1000 mPa · s or more and 5000 mPa · s or less, more preferably 1000 mPa · s or more and 4000 mPa · s or less. Furthermore, the ratio of the viscosity specified at a shear rate of 40 s ^-1 to the viscosity specified at a shear rate of 1 s ^-1 , i.e., (viscosity specified at a shear rate of 1 s ^-1 / viscosity specified at a shear rate of 40 s ^-1 ), is preferably 2 or more and 12 or less, 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.

(Method for 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.

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.

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.

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.

The mixture layer contains at least the components contained in the electrode mixture described above, namely vinylidene fluoride copolymer and active material, and may further contain various additives such as a conductive aid, dispersant, adhesive aid, and thickener as needed. These are the same as those described for the electrode mixture.

The thickness of the mixture layer is not particularly limited, but is preferably 1 μm or more and 300 μm or less, for example. The basis weight of the mixture layer formed on one surface of the current collector is not particularly limited, and can be any basis weight, but is preferably 50 g/ ^m2 or more and 500 g/m2 or ^less , and more preferably 100 g/m2 or ^more and 300 g/m2 or ^less , for example.

The mixture layer can be formed by applying the electrode mixture described above to a current collector and then solidifying it.

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

Furthermore, after application of the electrode mixture, it is heated at a desired 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, increased pressure, or reduced pressure, or may be dried in an environment such as air, nitrogen, or argon. A further heat treatment may be performed after drying.

After the electrode mixture has been applied and dried, a pressing process may be performed. Pressing can improve electrode density. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

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.

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

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 carboxy group-containing copolymer are shown below. The measurement methods for 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.

(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, the inherent viscosity (η _i ) of the vinylidene fluoride copolymer was calculated from the obtained value according to 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 the vinylidene fluoride polymer in the solution, ie, 0.4 g/dl.

(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) (manufactured by Tosoh Corporation)

(3) Weight average molecular weight Mwc of carboxy group-containing copolymer
To 10 mg of DMAc were added 10 mg of vinylidene fluoride copolymer prepared in each Example and Comparative Example, 2 mg of 1-bromomethylpyrene, and 2 mg of potassium carbonate, and the mixture was dissolved and reacted while stirring in a thermostatic shaking bath at 50°C, and the potassium carbonate was 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.

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 conditions as for Mwa above, 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.

(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.

(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.

(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 a 60-second incubation period at 25°C in the apparatus, the rotor was rotated at a shear rate of 1 s ^-1 , and the viscosity 120 seconds after the start of rotor rotation was determined as the slurry viscosity.
After the device was left to incubate at 25° C. for 60 seconds, the rotor was rotated at a shear rate of 40 s ⁻¹ , and the viscosity 30 seconds after the rotor started to rotate was determined as the slurry viscosity.
Also, the viscosity ratio of the specified slurry viscosity at a shear rate of 1 s ⁻¹ to the slurry viscosity at a shear rate of 40 s ⁻¹ (slurry viscosity at a shear rate of 1 s ⁻¹ /slurry viscosity at a shear rate of 40 s ⁻¹ ) was specified.

2. Preparation of Materials The following materials were prepared as electrode mixtures for the Examples and Comparative Examples.

(1) Active material ・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 coating amount: 2.0%, average particle size D50: 1 μm)
・LCO carbon coverage: 0.0%, average particle diameter D50: 2 μm)

(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

<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 Metrolose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension 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.

<Synthesis Example 2>
A 2-liter autoclave was charged with 1054.5 g of ion-exchanged water as a dispersion medium, 0.23 g of Metrose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspending agent, 0.07 g of APS, 3.50 g of 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 of an aqueous APS solution having a concentration of 5 wt% was added over 4.3 hours in solute equivalent. From the point when the pressure had decreased by 0.2 MPaG from the pressure at the end of the temperature increase, the mixture was heated 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.

<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 suspension agent, 9.37 g of acrylic acid (AA), 5.28 g of a tertiary butyl peroxypivalate-HFE-347pc-f solution having a polymerization initiator concentration of 50 wt %, 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, 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.

<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 of a 1 wt% AA aqueous solution (in terms of solute) 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.

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

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

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 Mixer ARE310, to prepare an electrode mixture. The mass ratio of the electrode active material, conductive additive, and binder in the resulting electrode mixture was 100:2:2.5, and the solids concentration was 55.0 mass%. The slurry viscosity at this time is shown in Table 2.

(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 above, 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%.

(result)

As shown in Table 2 above, when an active material not coated with carbon was used, there was no significant difference depending on the vinylidene fluoride copolymer having a carboxy group (Reference Examples 1 and 2). In contrast, when Example 1 and Comparative Example 1 were compared, even when the same active material was used, there was a significant difference in the slurry viscosity (particularly at a shear rate of s ⁻¹ ) and also in the viscosity ratio depending on the Mwc/Mwa ratio. This was also the case when Examples 2 to 5 and Comparative Examples 2 to 5 were compared. Furthermore, 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 carboxy group-containing copolymer was less than 0.90 relative to Mwa, the slurry viscosity (particularly at a shear rate of s ⁻¹ ) was 51,041 mPa·s or less, and the viscosity ratio was also 11.2 or less (Examples 1 to 5). In other words, there was no significant difference from Reference Examples 1 and 2.

This application claims priority from Japanese Patent Application No. 2024-056545, filed March 29, 2024. The entire contents of the specification of that application are incorporated herein by reference.

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 (8)

a vinylidene fluoride copolymer having a structural unit derived from vinylidene fluoride and a structural unit derived from a vinyl compound having a carboxy group;
a carbon-coated electrode active material;
An electrode mixture comprising:
The amount of carbon coating on the electrode active material is 0.5% by mass or more and 3.0% by mass or less with respect to the mass of the electrode active material,
The weight average molecular weight M of the vinylidene fluoride copolymer is 50,000 or more,
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.
Electrode mixture. The Mwc is 0.25 or more relative to the Mwa.
The electrode mixture according to claim 1 . The average particle size of the electrode active material is 15 μm or less.
The electrode mixture according to claim 1 . The electrode active material has a carbon coating amount of 2.5 mass% or less.
The electrode mixture according to claim 1 . The vinyl compound is a compound represented by the following general formula (1):
The electrode mixture according to claim 1 .
(In general formula (1),
R ¹ , R ² , and R ³ each independently represent a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms;
X represents a hydroxy group or -Y-COOH (wherein Y represents an atomic group). The vinyl compound is a compound represented by the following general formula (1):
The electrode mixture according to claim 1 .
(In general formula (1),
R ¹ , R ² , and R ³ each independently represent 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) An electrode containing the solid content of the electrode mixture described in any one of claims 1 to 6. A battery comprising the electrode of claim 7.