Vinylidene fluoride copolymer and method for producing same, binder composition for electrode, electrode mixture, electrode, and secondary battery

Abstract

The purpose of the present invention is to provide a particulate vinylidene fluoride copolymer which contains vinylidene fluoride and chlorotrifluoroethylene, said copolymer being less likely to cause gelation when an electrode mixture is prepared using the same, and can be produced with a high yield. A vinylidene fluoride copolymer that solves the above problem is a particulate vinylidene fluoride copolymer which contains a constituent unit derived from vinylidene fluoride and a constituent unit derived from chlorotrifluoroethylene, wherein: the ratio Wc of the amount of the constituent unit derived from chlorotrifluoroethylene to all constituent units of the vinylidene fluoride copolymer is 2.2 mass% or more; the amount Wf of the constituent unit derived from vinylidene fluoride to all constituent units of the vinylidene fluoride copolymer is 35.5 mass% or more; and the content of primary particles having a circularity of 0.85 or less as specified by a specific method is 20% or less.

Description

vinylidene fluoride copolymer and its manufacturing method, electrode binder composition, electrode mixture, electrode, and secondary battery

[0001] This invention relates to vinylidene fluoride copolymers, methods for producing the same, electrode binder compositions, electrode mixtures, electrodes, and secondary batteries.

[0002] In non-aqueous electrolyte secondary batteries, vinylidene fluoride polymer is primarily used as a binder (binding agent) for the electrodes. In recent years, the use of ternary materials such as nickel-cobalt-aluminate lithium (NCA) and nickel-cobalt-manganate lithium (NCM) has been investigated as positive electrode active materials for non-aqueous electrolyte secondary batteries, as they are expected to increase density and capacity. With these active materials, increasing the Ni ratio is desirable from the standpoint of performance and cost.

[0003] However, generally, active materials with a high nickel ratio contain many bases. When such a nickel-rich active material is mixed with vinylidene fluoride polymer to form an electrode mixture (slurry) for the electrode mixture layer, a problem arises in that gelation is likely to occur. Furthermore, this type of gelation of electrode mixtures has also been prone to occurring in electrode mixtures for sodium-ion batteries, which have been under development in recent years. Conventionally, it has been proposed to suppress the gelation of electrode mixtures by using a copolymer of vinylidene fluoride and chlorotrifluoroethylene as a binder for the electrode mixture (for example, Patent Document 1).

[0004] International Publication No. 2018 / 092675

[0005] However, after diligent research by the inventors, it became clear that the more chlorotrifluoroethylene is present in the vinylidene fluoride copolymer, the more likely it is to adhere to manufacturing equipment (such as stainless steel) during production, posing challenges in terms of production yield.

[0006] The present invention has been made in view of the above problems. The present invention aims to provide a particulate vinylidene fluoride copolymer containing vinylidene fluoride and chlorotrifluoroethylene, which is less likely to gel when an electrode mixture is prepared and can be manufactured with good yield. The present invention also aims to provide an electrode binder composition containing the above vinylidene fluoride copolymer, a method for producing the same, an electrode mixture, an electrode, and a secondary battery.

[0007] The present invention provides the following vinylidene fluoride copolymers [1] to [8]: [1] A particulate vinylidene fluoride copolymer comprising constituent units derived from vinylidene fluoride and constituent units derived from chlorotrifluoroethylene, wherein the ratio W of the amount of constituent units derived from chlorotrifluoroethylene to the total constituent units of the vinylidene fluoride copolymer ｃ The amount is 2.2% by mass or more, and the amount of constituent units derived from vinylidene fluoride relative to the total constituent units of the vinylidene fluoride copolymer is W. ｆ A vinylidene fluoride copolymer having a content of 35.5% by mass or more, and a content of primary particles with a circularity of 0.85 or less, as determined by the method described below, of 20% or less. (Method for determining circularity) (i) Immerse the vinylidene fluoride copolymer in a 5% by mass ethanol solution containing a surfactant. (ii) Arrange the vinylidene fluoride copolymers so that they do not overlap, and take 5000 images using an optical microscope with a scale of 100 μm, 10 decimal places, and a particle separation degree of 100, to obtain the projected area S and perimeter C of each particle. (iii) For each particle, substitute the projected area S and perimeter C into the following formula to determine the circularity φ of each particle. Circularity φ = (4πS / C) ２ (iv) Create a graph showing the frequency from the circularity φ of each particle and determine the content of primary particles with a circularity of 0.85 or less. [2] Bulk density of 0.35 g / cm³ ３The vinylidene fluoride copolymer according to [1], wherein the above is true. [3] The vinylidene fluoride copolymer according to [1] or [2], further containing a constituent unit derived from a compound having an acidic functional group. [4] The vinylidene fluoride copolymer according to [3], wherein the ratio of the intensity of the peak derived from C=O stretching to the intensity of the peak derived from C-H stretching, as measured by infrared spectroscopy, is 0.10 or more. [5] The vinylidene fluoride copolymer according to [3], wherein the acidic functional group is a carboxyl group. [6] The vinylidene fluoride copolymer according to [5], wherein the compound having the acidic functional group is a compound represented by the following general formula (1). (In general formula (1), R １ , R ２ , and R ３ (Each represents independently a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms, and X represents a hydroxyl group or a functional group represented by -Y-COOH (Y represents an atomic group)) [7] A vinylidene fluoride copolymer according to any one of [1] to [6], wherein the average primary particle diameter measured by a sieving method in accordance with JIS K0069-3.1 is 5 μm or more and 1000 μm or less. [8] A vinylidene fluoride copolymer according to any one of [1] to [7], wherein the proportion of heterogeneous bonds is 4.0 mol% or more.

[0008] The present invention provides electrode binder compositions, electrode mixtures, electrodes, and secondary batteries according to [9] to

[12] . [9] An electrode binder composition comprising a vinylidene fluoride copolymer according to any one of [1] to [8] above, and water and / or a non-aqueous solvent.

[10] An electrode mixture comprising a vinylidene fluoride copolymer according to any one of [1] to [8] above, an electrode active material, and water and / or a non-aqueous solvent.

[11] An electrode comprising a current collector and an electrode mixture layer obtained from the electrode mixture according to

[10] disposed on the current collector.

[12] A secondary battery comprising the electrode according to

[11] .

[0009] The present invention provides a method for producing vinylidene fluoride copolymers as described in

[13] to

[17] below.

[13] Amount W of constituent units derived from vinylidene fluoride relative to the total constituent units ｆis 35.5% by mass or more, and based on all constituent units, the proportion W of the amount of constituent units derived from chlorotrifluoroethylene ｃ is 2.2% by mass or more. The present invention relates to a method for producing a particulate vinylidene fluoride copolymer, comprising: a first step of mixing a monomer composition containing vinylidene fluoride and a dispersion medium and raising the temperature to a polymerization temperature; and a second step of adding chlorotrifluoroethylene to the monomer composition obtained after the first step at said polymerization temperature to perform suspension polymerization, wherein the amount of chlorotrifluoroethylene in the monomer composition in the first step is 3.1% by mass or less based on the total amount of monomers fed to the first step and the second step.

[14] The method for producing a vinylidene fluoride copolymer according to

[13] , wherein the polymerization temperature is not lower than the critical temperature of the vinylidene fluoride.

[15] The method for producing a vinylidene fluoride copolymer according to

[13] , wherein the monomer composition in the first step comprises a compound having an acidic functional group.

[16] The method for producing a vinylidene fluoride copolymer according to

[15] , wherein the acidic functional group is a carboxy group.

[17] The method for producing a vinylidene fluoride copolymer according to

[15] , wherein the compound having an acidic functional group 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 a functional group represented by -Y-COOH (Y represents an atomic group))

[0010] The particulate vinylidene fluoride copolymer of the present invention is a vinylidene fluoride copolymer containing vinylidene fluoride and chlorotrifluoroethylene, and is less likely to cause gelation when an electrode mixture is prepared. Furthermore, the particulate vinylidene fluoride copolymer can be produced with good yield. Also, according to the present invention, there are also provided an electrode binder composition comprising the above vinylidene fluoride copolymer, an electrode mixture, an electrode, and a non-aqueous electrolyte secondary battery.

[0011] 1. Vinylidene Fluoride Copolymer The vinylide copolymer of the present invention is a particulate polymer containing constituent units derived from vinylidene fluoride and constituent units derived from chlorotrifluoroethylene.

[0012] As mentioned above, with conventional vinylidene fluoride copolymers, the yield during manufacturing tends to be low when the proportion of constituent units derived from chlorotrifluoroethylene is high, making it difficult to achieve both gelation suppression when used as an electrode mixture and a good manufacturing yield. In response to this, the inventors of the present invention conducted diligent research and found that even with a vinylidene fluoride copolymer containing 2.2% by mass or more of constituent units derived from chlorotrifluoroethylene, by limiting the content of primary particles with a circularity of 0.85 or less to 20% or less, it is possible to obtain the effect of not only being less likely to adhere to stainless steel and other materials during manufacturing, but also exhibiting sufficient gelation suppression when used as an electrode mixture, leading to the present invention.

[0013] The following describes the chemical composition of the vinylidene fluoride copolymer of the present invention, followed by a description of the chlorine atom content, average particle size, and various physical properties of the vinylidene fluoride copolymer surface.

[0014] As described above, the vinylidene fluoride copolymer of the present invention may contain structural units derived from vinylidene fluoride and structural units derived from chlorotrifluoroethylene, and the vinylidene fluoride copolymer may consist only of these. On the other hand, the vinylidene fluoride copolymer may also contain structural units derived from other compounds in addition to the structural units derived from vinylidene fluoride and structural units derived from chlorotrifluoroethylene, for example, structural units derived from compounds having acidic functional groups as described later.

[0015] Here, the amount (percentage) of constituent units derived from vinylidene fluoride in the vinylidene fluoride copolymer is preferably 35.5% by mass or more relative to the total constituent units of the vinylidene fluoride copolymer (100.0% by mass), but is preferably 68.7% by mass or more and 96.4% by mass or less, and more preferably 83.2% by mass or more and 94.7% by mass or less. When the amount of constituent units derived from vinylidene fluoride is 35.5% by mass or more, the physical properties characteristic of vinylidene fluoride are more easily obtained. Furthermore, the amount (percentage) of constituent units derived from vinylidene fluoride in the vinylidene fluoride copolymer is preferably 50.0 mol% or more relative to the total constituent units of the vinylidene fluoride copolymer (100.0 mol%), is more preferably 80.0 mol% or more and 98.0 mol% or less, and is even more preferably 90.0 mol% or more and 97.0 mol% or less. The amount of constituent units derived from vinylidene fluoride in a vinylidene fluoride copolymer can be calculated, for example, by identifying the amount of constituent units derived from compounds other than vinylidene fluoride that the vinylidene fluoride copolymer contains.

[0016] On the other hand, the amount (percentage) of constituent units derived from chlorotrifluoroethylene in the vinylidene fluoride copolymer is W. ｃ The amount of chlorotrifluoroethylene-derived constituent units W is sufficient if it is 2.2% by mass or more relative to the total constituent units (100.0% by mass) of the vinylidene fluoride copolymer, but is preferably 3.1% by mass or more and 31.3% by mass or less, more preferably 3.9% by mass or more and 18.4% by mass or less, and even more preferably 5.0% by mass or more and 10.4% by mass or less. ｃ However, if the amount is 2.2% by mass or more, thickening will not occur easily when vinylidene fluoride copolymer is used as an electrode mixture. Also, the amount (percentage) of constituent units derived from chlorotrifluoroethylene in the vinylidene fluoride copolymer M ｃ The amount of chlorotrifluoroethylene-derived constituent units in the vinylidene fluoride copolymer is preferably 1.2 mol% or more, more preferably 1.7 mol% to 20.0 mol%, even more preferably 2.2 mol% to 11.0 mol%, and particularly preferably 2.8 mol% to 6.0 mol%. ｃ and M ｃ teeth, １９It can be determined from F-NMR spectroscopy or ion chromatography.

[0017] Furthermore, the vinylidene fluoride copolymer may further contain structural units derived from compounds having acidic functional groups. The vinylidene fluoride copolymer may contain only one structural unit derived from compounds having acidic functional groups, or it may contain two or more. In this specification, an acidic functional group refers to a proton-donating group such as a carboxyl group, a phosphonic acid group, a sulfonic acid group, and a phenolic hydroxyl group, and is preferably a carboxyl group. When the vinylidene fluoride copolymer contains structural units derived from compounds having acidic functional groups, the amount is preferably 0.01% by mass or more and 10.00% by mass or less, more preferably 0.05% by mass or more and 3.00% by mass or less, even more preferably 0.10% by mass or more and 1.50% by mass or less, and particularly preferably 0.20% by mass or more and 1.50% by mass or less, relative to the total structural units of the vinylidene fluoride copolymer (100.0% by mass). Furthermore, if the vinylidene fluoride copolymer contains constituent units derived from compounds having acidic functional groups, the amount is preferably 0.01 mol% to 10.00 mol%, more preferably 0.05 mol% to 3.00 mol%, and even more preferably 0.10 mol% to 1.50 mol%, relative to the total constituent units of the vinylidene fluoride copolymer (100.0 mol%). When the vinylidene fluoride copolymer contains compounds having acidic functional groups, the adhesion strength between the vinylidene fluoride copolymer and the active material or current collector tends to increase further when the vinylidene fluoride copolymer is used in the electrode mixture layer. The amount of constituent units derived from compounds having acidic functional groups in the vinylidene fluoride copolymer is １９ F-NMR spectrum, １ It can be determined from the 1H-NMR spectrum, neutralization titration, and the amount and yield of the raw materials used. Furthermore, if the compound with an acidic functional group is acrylic acid, the amount may be determined by calculating the IR absorbance ratio (the ratio of the peak intensity originating from C=O stretching to the peak intensity originating from C-H stretching) from the absorption spectrum measured by Fourier transform infrared spectroscopy using the following formula: IR absorbance ratio = A Ｃ＝Ｏ / A Ｃ－Ｈ (In the formula, A Ｃ＝ＯThis represents the absorbance of the peak originating from the C=O stretching, and A Ｃ－Ｈ (This represents the absorbance of the peak originating from C-H stretching.) As specified above, A Ｃ＝Ｏ This represents the relative amount of comonomers containing carbonyl groups (including those that are carboxyl groups) in the vinylidene fluoride copolymer. On the other hand, A Ｃ－Ｈ This relatively indicates the amount of C-H bonds in the vinylidene fluoride copolymer, i.e., the amount of constituent units derived from vinylidene fluoride in the vinylidene fluoride copolymer. In other words, the amount of comonomers having carbonyl groups (e.g., acrylic acid) can be determined from the IR absorbance ratio specified above using a calibration curve or the like. The above IR absorbance is preferably 0.1 or higher, more preferably 0.15 or higher and 5.00 or lower, even more preferably 0.15 or higher and 3.00 or lower, even more preferably 0.15 or higher and 1.00 or lower, even more preferably 0.15 or higher and 0.80 or lower, and particularly preferably 0.20 or higher and 0.75 or lower. When it is within this range, the amount of constituent units derived from compounds having acidic functional groups tends to fall within the above range.

[0018] The structure of the compound having an acidic functional group is not particularly limited, but for example, a compound represented by the following general formula (1) is preferred. When the vinylidene fluoride copolymer contains constituent units derived from the compound represented by the following general formula (1), the adhesion between the vinylidene fluoride copolymer and the electrode active material or current collector is further enhanced when the vinylidene fluoride copolymer is used in the electrode mixture layer. In the above general formula (1), R １ , R ２ , R ３ Each of these independently represents a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms. Examples of alkyl groups having 1 to 5 carbon atoms include linear or branched alkyl groups. Specific examples include methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, isobutyl, t-butyl, and pentyl groups, and from the viewpoint of availability, methyl, ethyl, or butyl groups are preferred. In particular, from the viewpoint of minimizing steric hindrance during polymerization with vinylidene fluoride, R １ , R２ , and R ３ These are more preferably a hydrogen atom or a methyl group, independently of each other.

[0019] Furthermore, in the above general formula (1), X is a hydroxyl group or a group represented by -Y-COOH. Here, Y represents an atomic group, and Y is preferably an atomic group containing either an oxygen atom or a nitrogen atom, more preferably an atomic group containing either an oxygen atom or a nitrogen atom and having 1 to 10 atoms in the main chain, and even more preferably a divalent atomic group with a molecular weight of 500 or less, containing either an oxygen atom or a nitrogen atom and having 1 to 10 atoms in the main chain.

[0020] The atomic group (Y) may be linear, branched, or cyclic, or a combination thereof. Among these, the atomic group (Y) is preferably linear or branched, from the viewpoint of minimizing steric hindrance during polymerization with vinylidene fluoride.

[0021] The number of atoms in the main chain of the atomic group (Y) may be between 1 and 10, and preferably between 2 and 8. In this specification, the main chain of the atomic group (Y) refers to the longest chain among the chains connecting the carbonyl group of general formula (1) and the carboxyl group of -Y-COOH.

[0022] Here, the atomic group (Y) preferably contains either or both of oxygen atoms and nitrogen atoms (hereinafter collectively referred to as "heteroatoms"). The number of heteroatoms in the atomic group (Y) is preferably 1 to 10, and more preferably 1 to 5. If the atomic group (Y) contains two or more heteroatoms, they may be of the same type or different types. 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, carboxyl groups, amide groups, and hydroxyl groups. Among these, ether bonds, ester groups, carbonyl groups, carboxyl groups, amide groups, and hydroxyl groups are preferred.

[0023] The structure of atomic group (Y) is not particularly limited, and may be a structure in which a hydrocarbon group such as an alkylene group or alkyl group is bonded to a structure (functional group) containing the above-mentioned heteroatom. The molecular weight of atomic group (Y) should be 500 or less, and is preferably 30 to 200 from the viewpoint of polymerization reactivity.

[0024] Specific examples of compounds represented by the above general formula (1) include (meth)acrylic acid, (meth)acryloyloxyethyl succinic acid, (meth)acryloyloxypropyl succinic acid, 2-carboxyethyl (meth)acrylate, 2-carboxymethyl (meth)acrylate, (meth)acryloyloxyethyl phthalic acid, and (meth)acrylamide compounds such as N-carboxyethyl (meth)acrylamide. In this specification, (meth)acrylic refers to methacrylic, acrylic, or mixtures thereof; (meth)acrylate refers to methacrylate, acrylate, or mixtures thereof; and (meth)acryloyl refers to methacryloyl, acryloyl, or mixtures thereof.

[0025] Of the compounds represented by the above general formula (1), acrylic acid, acryloyloxyethyl succinic acid, acryloyloxypropyl succinic acid, 2-carboxyethyl acrylate, and 2-carboxymethyl acrylate are more preferred from the viewpoint of availability and reactivity with vinylidene fluoride.

[0026] Examples of compounds having acidic functional groups other than the compound represented by the general formula (1) above 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. The vinylidene fluoride copolymer may contain constituent units derived from these compounds.

[0027] Furthermore, the vinylidene fluoride copolymer may contain, to the extent that it does not impair the objectives and effects of the present invention, some constituent units derived from compounds other than vinylidene fluoride, chlorotrifluoroethylene, and the above-mentioned acidic functional group compounds (other compounds). The vinylidene fluoride copolymer may contain only one constituent unit derived from other compounds, or it may contain two or more. However, the total amount of constituent units derived from other compounds relative to the total constituent units (100.0% by mass) of the vinylidene fluoride copolymer is preferably 10.0% by mass or less, and more preferably 5.0% by mass or less. Also, the total amount of constituent units derived from other compounds relative to the total constituent units (100.0 mol%) of the vinylidene fluoride copolymer is preferably 10.0 mol% or less, and more preferably 5.0 mol% or less. These amounts are １９ Obtained from the F-NMR spectrum, １ 1H-NMR spectroscopy may be used as an aid.

[0028] Other examples of compounds include fluorinated vinyl compounds that have a vinyl group and a fluorine atom or a fluorinated alkyl group in one molecule. Examples of fluorinated vinyl compounds include vinyl fluoride; trifluoroethylene; tetrafluoroethylene; hexafluoropropylene; and perfluoroalkyl vinyl ethers, such as perfluoromethyl vinyl ether. Other examples of compounds include compounds that have a vinyl group but do not contain fluorine. Examples of these include unsaturated hydrocarbon compounds such as ethylene and propylene.

[0029] Furthermore, the vinylidene fluoride copolymer of the present invention preferably has a heterogeneous bond ratio of 4.0 mol% or more. The heterogeneous bond ratio is mainly controlled by the polymerization temperature of the vinylidene fluoride copolymer, as described later. In particular, the ratio can be increased by polymerization above the critical temperature of vinylidene fluoride. Such a vinylidene fluoride copolymer has fewer particle-breaking components and is less likely to adhere to manufacturing equipment (e.g., stainless steel) during production. As a result, it becomes easier to further reduce the amount of components with low circularity, as described later.

[0030] In this specification, heterogeneous bonding refers to a structure in which vinylidene fluorides are bonded together in a vinylidene fluoride copolymer, where "CF ２ " and "CH ２ This refers to the part where two "CF" molecules are bonded together. In the bond between vinylidene fluoride molecules, it is usually "CF ２ " and "CH ２ " and are joined alternately, but in some cases, "CF ２ " and "CH ２ " bond with each other. In this specification, "CF" is defined as the total number of bonds in the portion where vinylidene fluoride is bonded to each other. ２ " and "CH ２ The ratio of the number of bonds formed between two or more molecules is called the ratio of heterogeneous bonds. The ratio of heterogeneous bonds is more preferably 4.0 mol% to 5.5 mol%, and even more preferably 4.0 mol% to 5.0 mol%. The ratio of heterogeneous bonds can be adjusted by the polymerization temperature of the vinylidene fluoride copolymer.

[0031] The above proportion of heterogeneous bonds is for vinylidene fluoride copolymer. １９ It can be determined from F-NMR measurement. Specifically, 40 mg of vinylidene fluoride copolymer is dissolved in 0.8 ml of deuterium dimethyl sulfoxide (DMSO-d6) and measured at room temperature. １９ F-NMR is measured. The peak areas (A, B, C) below are determined, and the heterogeneity ratio is calculated using the following formula (I): A: Peak area from -89 ppm to -97 ppm B: Peak area around -114 ppm C: Peak area around -116 ppm Heterogeneity ratio = (((B + C) / 2) / (A + B + C)) × 100 (I)

[0032] On the other hand, the weight-average molecular weight Mw of the vinylidene fluoride copolymer is preferably 100,000 to 5,000,000, more preferably 300,000 to 4,000,000, and even more preferably 300,000 to 3,500,000. When the weight-average molecular weight Mw of the vinylidene fluoride copolymer is 300,000 or more, the heat resistance and strength of the electrode (electrode mixture layer) containing the vinylidene fluoride copolymer tend to be good. In this specification, the weight-average molecular weight Mw of the vinylidene fluoride copolymer is a polystyrene-equivalent value measured by gel permeation chromatography (GPC). At this time, the eluent is N,N-dimethylacetamide, and the weight-average molecular weight is determined by a differential refractive index (RI) detector.

[0033] Furthermore, 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 1.0 dL / g or more and 5.0 dL / g or less, and even more preferably 1.0 dL / g or more and 4.5 dL / g or less. When the inherent viscosity is 1.0 dL / g or more, the adhesion strength with the active material and current collector tends to increase when the vinylidene fluoride copolymer is used in the electrode mixture layer. On the other hand, when the inherent viscosity is 5.0 dL / g or less, the initial viscosity of the electrode mixture containing the vinylidene fluoride copolymer does not become too high, and workability is particularly good. Inherent viscosity (η ｉ The value shown represents 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 30°C constant temperature bath. Then, the value obtained is calculated 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, i.e., 0.4 g / dl.

[0034] Furthermore, the melting point of the vinylidene fluoride copolymer is preferably 100°C to 175°C, more preferably 130°C to 175°C, and particularly preferably 150°C to 170°C. When the melting point of the vinylidene fluoride copolymer is 150°C or higher, it is less likely to swell in the electrolyte when used as an electrode, and the resulting battery tends to have good performance. On the other hand, when the melting point is 175°C or lower, the flexibility of the electrode when it is formed tends to be good. The melting point of the vinylidene fluoride copolymer can be determined by calorimetry using a differential scanning calorimetry (DSC).

[0035] Here, the vinylidene fluoride copolymer (particles) of the present invention has a content of primary particles with a circularity of 0.85 or less, as determined by the following method, of 20% or less. When the above content is 20% or less, as described above, the vinylidene fluoride copolymer is less likely to adhere to manufacturing equipment (such as stainless steel) during manufacturing. (Method for determining circularity) (i) Immerse the vinylidene fluoride copolymer in a 5% by mass ethanol solution containing a surfactant. (ii) Arrange the vinylidene fluoride copolymers so that they do not overlap, and take 5000 images using an optical microscope (e.g., VHX X-1 manufactured by Keyence Corporation) with a scale of 100 μm, 10 decimal places, and a particle separation degree of 100, to obtain the projected area S and perimeter C of each particle. (iii) For each particle, substitute the projected area S and the perimeter C into the following formula to determine the circularity φ of each particle. Circularity φ = (4πS / C) ２ (iv) Create a graph showing the frequency of each particle from 0 to 1 in increments of 0.01 based on the circularity φ of each particle, and determine the content of primary particles with a circularity of 0.85 or less.

[0036] Furthermore, the content of primary particles with a circularity of 0.85 or less may be 20% or less, preferably 18% or less, and more preferably 15% or less. The content of primary particles with a circularity of 0.85 or less is achieved by synthesizing the vinylidene fluoride copolymer using the method described later.

[0037] Here, the average primary particle diameter of the vinylidene fluoride copolymer is preferably 5 μm to 1000 μm, more preferably 50 μm to 1000 μm, and even more preferably 80 μm to 800 μm. When the average primary particle diameter of the vinylidene fluoride copolymer is within this range, the handling properties of the powder are improved. The vinylidene fluoride copolymer of the present invention can be manufactured by the method described later (suspension polymerization), and with suspension polymerization, the average primary particle diameter tends to fall within the above range. In this specification, the average primary particle diameter of the vinylidene fluoride copolymer is a value obtained by the following method. First, using a sieve shaker (for example, a Type D rotap-type II sieve shaker manufactured by Hirako Seisakusho), the particle size distribution is determined by the dry sieving method in accordance with JIS K0069-3.1. Then, the 50% cumulative value (D50) of the particle size cumulative distribution is determined by the log-normal distribution method. This is then defined as the average primary particle diameter.

[0038] The bulk density of the powder containing primary particles of vinylidene fluoride copolymer is 0.35 g / cm³. ３ The above is preferable, and 0.36 g / cm³ ３ The above is more preferable, 0.38 g / cm³ ３ The above is even more preferable. If the bulk density is excessively low, it may lead to poor efficiency in the drying process during manufacturing and low powder flowability, which can cause a decrease in yield. In this specification, bulk density is a value measured in accordance with the measurement method of JIS K 6721-3.3 "Bulk Specific Gravity". Specifically, 2 mL of a 5% ethanol solution of surfactant is added to 100 g of sample, and after stirring thoroughly with a spatula, it is left to stand for 10 minutes. After 120 mL of the sample thus destaticized is placed in a funnel into which the damper of the bulk specific gravity measuring device is inserted, the damper is quickly withdrawn and the sample is dropped into a receiver. After scraping off the sample that has risen from the receiver with a glass rod, the mass of the receiver containing the sample is accurately weighed to 0.1 g and calculated from the formula (s = (c - a) / b). s is the bulk density (g / cm³). ３ a) is the mass of the receiver (g), b) is the internal volume of the receiver (cm³) ３ ), c represents the mass (g) of the receiver containing the sample.

[0039] (Synthesis Method (Manufacturing Method) of Vinylidene Fluoride Copolymer) The above-mentioned method for manufacturing the Vinylidene Fluoride copolymer is described below. Suspension polymerization is a known method for synthesizing Vinylidene Fluoride copolymer. When producing particulate Vinylidene Fluoride copolymer by suspension polymerization, the outer shell of the particle is formed first, followed by the formation of the interior of the particle. However, when synthesizing Vinylidene Fluoride copolymer containing chlorotrifluoroethylene as a constituent unit using a general suspension polymerization method, from the viewpoint of reactivity ratio, a Vinylidene Fluoride copolymer with a large amount of chlorotrifluoroethylene is introduced in the initial stages of polymerization. In that case, the copolymer structure of chlorotrifluoroethylene and Vinylidene Fluoride has a relatively low elastic modulus. If such a copolymer structure of chlorotrifluoroethylene and Vinylidene Fluoride is formed first, it is thought that the circularity of the final particles will be low.

[0040] In contrast, the present invention involves a first step of mixing a monomer composition containing vinylidene fluoride with a dispersion medium, raising the temperature to polymerization temperature T, and, if necessary, performing suspension polymerization at polymerization temperature T for a certain period of time; and a second step of adding chlorotrifluoroethylene to the monomer composition after the first step at polymerization temperature T and performing suspension polymerization. Furthermore, the amount of chlorotrifluoroethylene in the monomer composition in the first step is set to 3.1% by mass (1.75 mol%) or less relative to the total amount of monomer used in the first and second steps. In other words, by keeping the amount of chlorotrifluoroethylene relatively low in the first step and reducing the copolymer structure of chlorotrifluoroethylene and vinylidene fluoride, the circularity of the resulting particles can be increased. The monomer used in the first step refers to the monomer added before reaching the polymerization temperature T. Furthermore, in the second step, chlorotrifluoroethylene is added after the first step, and suspension polymerization is carried out at polymerization temperature T, thereby obtaining a vinylidene fluoride copolymer containing a sufficient amount of constituent units derived from chlorotrifluoroethylene. It is preferable that the polymerization temperature T be above the critical temperature of vinylidene fluoride. The method will be described in detail below.

[0041] ・First step In the first step described above, a monomer composition containing vinylidene fluoride is mixed with a dispersion medium, etc., and the mixture is heated to the polymerization temperature T. If necessary, suspension polymerization is carried out at the polymerization temperature T for a certain period of time. Here, the monomer composition refers to a composition consisting of monomers for polymerizing the vinylidene fluoride copolymer. The monomer composition only needs to contain at least vinylidene fluoride, and may consist only of vinylidene fluoride. It may also consist of vinylidene fluoride and chlorotrifluoroethylene. Furthermore, the monomer composition may also contain the above-mentioned compounds having acidic functional groups or other compounds. However, the amount of chlorotrifluoroethylene in the monomer composition shall be 3.1% by mass or less based on 100.0% by mass of the total mass of the monomers ultimately copolymerized (total mass of vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds). The amount of chlorotrifluoroethylene is preferably 2.2% by mass or less, more preferably 1.5% by mass or less, and particularly preferably 1.0% by mass or less.

[0042] In the first step, the monomer composition (such as vinylidene fluoride and, if necessary, chlorotrifluoroethylene) is typically mixed with a dispersion medium, a suspension agent, a polymerization initiator, and a chain transfer agent.

[0043] For example, water can be used as a dispersion medium. The amount of water is adjusted so that the total amount of monomers used for copolymerization (vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds) is typically 1:1 to 1:10, preferably 1:2 to 1:5, in terms of the mass ratio of total monomers to water.

[0044] In suspension polymerization using water as the dispersion medium, it is preferable to use a suspending agent such as methylcellulose, methoxylated methylcellulose, propoxylated methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, polyvinyl alcohol, polyethylene oxide, or gelatin in an amount of 0.005 to 1.0 parts by mass, and more preferably 0.01 to 0.4 parts by mass, per 100.0 parts by mass of the total monomers (vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds) ultimately used in copolymerization. Furthermore, it is preferable that the amount of chlorotrifluoroethylene in the monomer composition be 1.75 mol% or less, per 100.0 mol% of the total number of moles of the monomers ultimately copolymerized (total number of moles of vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds). The amount of chlorotrifluoroethylene is more preferably 1.2 mol% or less, and even more preferably less than 0.81 mol%.

[0045] Suitable polymerization initiators include diisopropyl peroxydicarbonate, dinormalpropyl peroxydicarbonate, dinormalheptafluoropropyl peroxydicarbonate, isobutyryl peroxide, di(chlorofluoroacyl)peroxide, di(perfluoroacyl)peroxide, and t-butyl peroxypivalate. The amount used is preferably 0.05 to 10.0 parts by mass, and more preferably 0.15 to 5.0 parts by mass, per 100.0 parts by mass of the total monomers ultimately used in copolymerization (vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds).

[0046] Furthermore, it is possible to adjust the degree of polymerization of the resulting vinylidene fluoride copolymer by adding chain transfer agents such as ethyl acetate, methyl acetate, diethyl carbonate, acetone, ethanol, n-propanol, acetaldehyde, propylaldehyde, ethyl propionate, and carbon tetrachloride. When using a chain transfer agent, the amount used is usually preferably 0.01 to 5.0 parts by mass, and more preferably 0.01 to 3.0 parts by mass, per 100.0 parts by mass of the total monomers (vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds) ultimately used in copolymerization.

[0047] In the first step, the temperature is raised to a predetermined polymerization temperature T over a certain period of time. The polymerization temperature T should be above the critical temperature of vinylidene fluoride (30.1°C), and the polymerization initiator should have a 10-hour half-life T. １０ Selected as appropriate, usually T １０ -25℃ ≤ T ≤ T １０ The temperature range is selected to be +25°C. For example, t-butyl peroxypivalate and diisopropyl peroxydicarbonate. １０ These are 54.6°C and 40.5°C, respectively (see NOF Corporation product catalog). Therefore, in polymerization using t-butyl peroxypivalate and diisopropyl peroxydicarbonate as polymerization initiators, the polymerization temperature T can be appropriately selected within the ranges of 30.1°C ≤ T ≤ 79.6°C and 30.1°C ≤ T ≤ 65.5°C, respectively.

[0048] ・Second step In the second step, chlorotrifluoroethylene is added to the monomer composition after the first step, i.e., the reaction solution, at a polymerization temperature T (above the critical temperature of vinylidene fluoride), and this is further subjected to suspension polymerization. It is preferable that the second step be carried out when the reaction rate is between 10% and 60%. The reaction rate here is the value obtained by (mass of polymer formed in the first step / total amount of monomer used in the first step) × 100. When the reaction rate is 10% or more, a vinylidene fluoride copolymer with high circularity is easily obtained. In the second step, chlorotrifluoroethylene may be added all at once, or it may be added in multiple steps, i.e., intermittently or continuously. At this time, vinylidene fluoride or compounds having acidic functional groups may be added as needed.

[0049] In the second step, suspension polymerization is carried out at the polymerization temperature T described above until the desired vinylidene fluoride copolymer particles are obtained. In the second step, suspension polymerization may be carried out at the polymerization temperature T for a predetermined time, and the temperature may be further increased if necessary. The polymerization time is not particularly limited, but it is preferably 100 hours or less considering productivity, etc. The polymerization is usually carried out under pressure, and is preferably 2.0 to 10.0 MPa-G.

[0050] 2. Electrode Binder Composition The electrode binder composition of the present invention may contain the vinylidene fluoride copolymer described above, as well as water and / or a non-aqueous solvent (hereinafter, these will be collectively referred to as "solvent"). The electrode binder composition may contain only one solvent, or it may contain two or more solvents.

[0051] The above non-aqueous solvent may be a polar solvent. Examples of polar solvents 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 and 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.

[0052] The amount of solvent in the electrode binder composition is preferably 50 parts by mass or more and 10,000 parts by mass or less, and more preferably 100 parts by mass or more and 5,000 parts by mass or less, per 100 parts by mass of the vinylidene fluoride copolymer. When the amount of solvent in the electrode binder composition is within this range, it is possible to uniformly disperse or dissolve the vinylidene fluoride copolymer in the solvent.

[0053] The electrode binder composition may further contain, to the extent that it does not impair the objectives and effects of the present invention, other resins such as acrylic resins, fillers such as inorganic fillers, and various additives such as dispersants and emulsifiers.

[0054] 3. Electrode mixture The above-described electrode binder composition and electrode active material (positive electrode active material or negative electrode active material) can be mixed to produce an electrode mixture (slurry) for manufacturing electrodes of secondary batteries. The electrode mixture may further contain conductive additives, additional solvents, and other additives.

[0055] The amount of solids derived from the electrode binder composition (vinylidene fluoride copolymer) relative to the total amount of solids derived from the electrode binder composition (total amount excluding volatile components), electrode active material, and conductive additive 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 7% by mass or less.

[0056] Furthermore, the vinylidene fluoride copolymer contained in the electrode binder composition described above is less likely to cause thickening or gelation of the electrode mixture. Therefore, various materials can be used as the electrode active material in the electrode mixture. The electrode active material is not particularly limited, and either a known negative electrode active material or a positive electrode active material can be used.

[0057] Examples of negative electrode active materials include carbon materials such as artificial graphite, natural graphite, non-graphitizable carbon, easily graphitizable carbon, activated carbon, or carbonized phenolic resin and pitch; metallic and alloy materials such as Cu, Li, Mg, B, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, Cd, Ag, Zn, Hf, Zr, and Y; and GeO, GeO ２ SnO, SnO ２ , PbO, PbO ２ This includes metal oxides such as those mentioned above. Furthermore, the negative electrode active material may be a material in which the surface of the above-mentioned carbon material, metal / alloy material, or metal oxide is coated. The negative electrode active material may also be a commercially available product.

[0058] On the other hand, examples of positive electrode active materials include lithium-based positive electrode active materials containing lithium and sodium-based positive electrode active materials containing sodium. Examples of lithium-based positive electrode active materials include LiCoO ２ LiNi ｘ Co １－ｘ O ２ General formulas such as (0 < x ≤ 1) LiMY ２ A composite metal chalcogen compound represented by (M being one or more transition metals such as Co, Ni, Fe, Mn, Cr, Ti, and V, and Y being a chalcogen element such as O and S); LiMn ２ O ４Composite metal oxides that take on a spinel structure, such as LiFePO ４ yaLiFeMnPO ４ Olivine-type lithium compounds such as LiNi ｘ Co ｙ M ｚ O ２ This includes lithium metal oxides represented as (where M represents Mn or Al, and x, y, and z satisfy 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1, respectively). An example of a sodium-based cathode active material is NaFePO ４ or Na ３ V ２ (PO ４ ) ３ Polyanionic compounds such as Na ｘ MO ２ Layered oxides such as (0 < x < 1, M (transition metal) = Mn, Fe, Cr, Ni); Na ｘ M １ [M ２ (CN)6]y・nH ２ O(M) １ M ２ (Transition metals) = Prussian blue compounds such as Fe, Co, Mn, Ni are included. Furthermore, the positive electrode active material may be a material in which the surface of the above compound has been coated. In addition, the positive electrode active material may be a commercially available product.

[0059] The amount of electrode active material contained in the electrode mixture is appropriately selected depending on the application of the electrode mixture, but it is preferably 50% by mass or more and 99.9% by mass or less of the total amount of solids derived from the electrode binder composition, electrode active material, and conductive additive. When the amount of electrode active material is within this range, for example, sufficient charge and discharge capacity can be obtained, and the battery performance tends to be good.

[0060] Furthermore, the conductive additive included in the electrode mixture is not particularly limited as long as it is a compound that can further enhance the conductivity between the electrode active materials or between the electrode active materials and the current collector. Examples of conductive additives include acetylene black, Ketjen black, carbon black, graphite powder, carbon nanofibers, carbon nanotubes, and carbon fibers.

[0061] The amount of conductive additive contained in the electrode mixture is appropriately selected depending on its type and other factors. From the viewpoint of improving both 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 derived from the electrode binder composition, electrode active material, and conductive additive.

[0062] The electrode mixture may contain a solvent different from the solvent that the electrode binder composition may contain. This solvent can be selected from among the solvents that the electrode binder composition may contain.

[0063] The total amount of solvent in the electrode mixture (including the amount of solvent in the electrode binder composition) is not particularly limited, but is generally preferably 10 to 150 parts by mass per 100 parts by mass of the electrode active material.

[0064] The electrode mixture may further contain a dispersant, an adhesion aid, a thickener, etc., and known compounds can be used for these. Examples of the dispersant include polyvinylpyrrolidone, methylcellulose, methoxylated methylcellulose, propoxylated methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, polyvinyl alcohol, polyethylene oxide, polypropylene oxide, and gelatin. Examples of the adhesion aid include poly(meth)acrylic acid, poly(meth)acrylic acid metal salts such as sodium poly(meth)acrylate, and carboxymethylcellulose. The amounts of these are not particularly limited as long as they do not impair the purpose and effects of the present invention, but it is preferable that they be 15% by mass or less of the total amount of solids derived from the electrode binder composition and electrode active material.

[0065] Furthermore, the electrode mixture may further contain additives such as phosphorus compounds; sulfur compounds; amine compounds and nitrogen compounds such as ammonium compounds; organic acids; organic esters; various silane-based, titanium-based and aluminum-based coupling agents; vinylidene fluoride polymers other than the vinylidene fluoride copolymers mentioned above; polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and resins such as polyacrylonitrile (PAN). These are not particularly limited as long as they do not impair the purpose and effects of the present invention, but it is preferable that they be 15% by mass or less of the total amount of solids derived from the electrode binder composition and the electrode active material.

[0066] The above electrode mixture may be prepared by mixing all components simultaneously, or by mixing some components first and then mixing the remaining components later.

[0067] The viscosity of the electrode mixture is not particularly limited, as long as it prevents dripping, uneven coating, and delayed drying after coating when applying the electrode mixture to form a mixture layer, and provides good workability and applicability during the preparation of the mixture layer. Typically, the viscosity (slurry viscosity) measured with a B-type viscometer at 20°C and a rotation speed of 12 rpm is preferably 100 mPa·s or more and 50,000 mPa·s or less, more preferably 1,000 mPa·s or more and 30,000 mPa·s or less, and most preferably 5,000 mPa·s or more and 20,000 mPa·s or less. In this specification, the viscosity (slurry viscosity) of the electrode mixture is the value measured 2 minutes after the start of rotation with the above-mentioned B-type rotational viscometer.

[0068] 4. Electrodes The electrode mixture described above can be used to form the electrode mixture layer of various non-aqueous electrolyte secondary batteries. The electrode of a non-aqueous electrolyte secondary battery includes, for example, a current collector and an electrode mixture layer disposed on the current collector. The electrode mixture described above can be used to form the electrode mixture layer.

[0069] ・Current Collector The current collector is a terminal for extracting electricity. The material of the current collector is not particularly limited, and metal foils or metal meshes such as aluminum, copper, iron, stainless steel, steel, nickel, and titanium can be used. Further, it may be one in which a layer containing carbon black or the like is formed on the surface of another medium, or one on which the above-mentioned metal foil or metal mesh or the like is applied.

[0070] ・Mixture Layer The mixture layer is a layer obtained by applying the above-mentioned electrode mixture onto a current collector and solidifying the same. That is, the mixture layer contains at least the vinylidene fluoride copolymer which is a solid component of the above-mentioned binder composition for electrodes, and an electrode active material. The mixture layer may be formed on only one surface of the current collector, or may be disposed on both surfaces thereof.

[0071] The mixture layer contains at least the components contained in the above-mentioned electrode mixture, that is, the solid content derived from the binder composition for electrodes (vinylidene fluoride copolymer) and the electrode active material, and further contains various additives such as a conductive aid, a dispersant, an adhesion aid, and a thickener as necessary. These are the same as those described for the electrode mixture.

[0072] Here, the thickness of the mixture layer is not particularly limited, but in one example, it is preferably 1 μm or more and 1000 μm or less. In addition, the basis weight of the mixture layer formed on one surface of the current collector is not particularly limited and may be any basis weight, but in one example, it is 50 g / m ２ or more and 1000 g / m ２ or less is preferable, and 100 g / m ２ or more and 500 g / m ２ or less is more preferable.

[0073] The mixture layer can be formed by performing a step of applying the above-mentioned electrode mixture onto a current collector and a step of solidifying the same.

[0074] The method for applying the electrode mixture is not particularly limited, and a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, a die coating method, a dip coating method, and the like can be applied.

[0075] Furthermore, after applying the electrode mixture, the solvent is dried by heating at an arbitrary temperature. In one example, the drying temperature is preferably 60°C to 500°C, and more preferably 80°C to 200°C. Heating may be performed multiple times at different temperatures. The solvent in the mixture may be dried under atmospheric pressure, under pressure, under reduced pressure, or in an environment such as air, nitrogen, or argon. Further heat treatment may be performed after drying.

[0076] After coating and drying the electrode mixture described above, a pressing treatment may be performed. Pressing can improve the electrode density. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

[0077] 5. Secondary Batteries The electrode binder compositions and electrode mixtures described above can be used as electrodes for various non-aqueous electrolyte secondary batteries, as described above, but they may also be used to form other layers of non-aqueous electrolyte secondary batteries. They may also be used in secondary batteries other than non-aqueous electrolyte secondary batteries.

[0078] The following describes specific embodiments of the present invention along with comparative examples, but the present invention is not limited to these.

[0079] 1. Preparation of vinylidene fluoride copolymer and fabrication of electrodes (Example 1) [Preparation of vinylidene fluoride copolymer] 1298 g of deionized water as a dispersion medium, 0.4 g of metholose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 2.2 g of 50 wt% diisopropyl peroxydicarbonate-HFE-347pc-f solution as a polymerization initiator, 2.6 g of ethyl acetate as a chain transfer agent, and 414 g of vinylidene fluoride (VDF) were charged into a 2 liter autoclave and the temperature was raised to 38°C over 1.5 hours. Thereafter, while maintaining the temperature at 38°C, 26.4 g of chlorotrifluoroethylene (CTFE) was continuously added. The reaction was continued until the pressure in the system decreased to 1.5 MPa. After polymerization was complete, the polymer slurry was heat-treated at 95°C for 60 minutes, then dehydrated, washed with water, and further dried at 80°C for 20 hours to obtain a powder of vinylidene fluoride copolymer (mass ratio = 93.5:6.5, molar ratio = 96.3:3.7), which is a copolymer of vinylidene fluoride (VDF) and chlorotrifluoroethylene (CTFE).

[0080] [Preparation of Electrode Mixture] Nickel-cobalt-aluminum ternary lithium composite metal oxide (Ni content 78%, Co content 19%, Al content 3%, specific surface area 0.38 m²) is used as the electrode active material. ２ A polyvinylidene fluoride copolymer (NCA) with an average particle size D50: 12.2 μm was used, carbon black (Timcal Super-P, SP) was used as a conductive additive, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium. The amount of NMP was adjusted so that the slurry viscosity was 5,000 to 20,000 mPa·s when measured for 2 minutes at 20°C and 12 rpm using a B-type viscometer. The above vinylidene fluoride copolymer, NCA, and SP were mixed to prepare the electrode mixture. The mass ratio of NCA, SP, and vinylidene fluoride copolymer in the obtained electrode mixture was 100:1:1.5, and the solid content concentration relative to the total mass of this mixture was 75.5% by mass.

[0081] [Manufacture of Electrode Mixture] The obtained electrode mixture was applied to the current collector using a bar coater. This was then dried in a constant temperature bath under a nitrogen atmosphere at 110°C for 30 minutes until the dry mixture weight was 300 g / m². ２ We fabricated the electrodes.

[0082] (Example 2) Before heating, 4.4 g of CTFE was added together with VDF, deionized water, suspension agent, polymerization initiator, and chain transfer agent (1.0% by mass (0.56 mol%) relative to the total amount of VDF and CTFE used for polymerization), and the temperature was raised to 38°C over 1.5 hours. Then, while maintaining the temperature at 38°C, 22.0 g of CTFE was continuously added, except that a polymer powder of VDF and CTFE (mass ratio = 93.9:6.1, molar ratio = 96.6:3.4) was obtained in the same manner as in Example 1. The physical properties of the vinylidene fluoride copolymer and its adhesion to SUS304 are shown in Table 1. Then, using the vinylidene fluoride copolymer, an electrode mixture was prepared in the same manner as in Example 1.

[0083] (Example 3) In a 2-liter autoclave, 1250 g of deionized water was added as a dispersion medium, 0.4 g of Metroze SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 0.83 g of a 50 wt% diisopropyl peroxydicarbonate-HFE-347pc-f solution as a polymerization initiator, and 400 g of VDF. The temperature was raised to 38°C over 60 minutes. While maintaining the temperature at 38°C, 33.3 g of CTFE was continuously added. The reaction was continued until the pressure in the system decreased to 1.5 MPa. Subsequently, a polymer of VDF and CTFE (mass ratio = 91.4:8.6, molar ratio = 95.1:4.9) was obtained in powder form using the same method as in Example 1. The physical properties of the vinylidene fluoride copolymer and its adhesion to SUS304 are shown in Table 1. Then, using the vinylidene fluoride copolymer, an electrode mixture was prepared in the same manner as in Example 1.

[0084] (Example 4) 1163 g of deionized water was placed in a 2-liter autoclave as a dispersion medium, 0.4 g of Metroze SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 1.9 g of a 50 wt% diisopropyl peroxydicarbonate-HFE-347pc-f solution as a polymerization initiator, 0.6 g of ethyl acetate, 0.4 g of acrylic acid (AA), and 4.2 g of CTFE (0.9% by mass (0.53 mol%) of the total amount of VDF, CTFE, and AA to be ultimately used in polymerization) and 420 g of vinylidene fluoride (VDF) were charged, and the temperature was raised to 45°C over 1.5 hours. Thereafter, while maintaining the temperature at 45°C, 1.9 g of a 5 wt% aqueous solution of AA was continuously added in solute terms, and 21.0 g of CTFE was continuously added in addition. The reaction was continued until the pressure in the system decreased to 2.0 MPa. Then, a polymer powder of VDF, CTFE, and AA (mass ratio = 92.8:6.6:0.6, molar ratio = 95.7:3.8:0.6) was obtained using the same method as in Example 1. Table 1 shows the physical properties of the vinylidene fluoride copolymer and its adhesion to SUS304. Then, using the vinylidene fluoride copolymer, an electrode mixture was prepared in the same manner as in Example 1.

[0085] (Example 5) Before heating, 1.89 g of a chain transfer agent, 0.4 g of acryloyloxypropyl succinic acid (APS), and 4.2 g of CTFE (0.9% by mass (0.53 mol%) relative to the total amount of VDF, CTFE, and APS to be ultimately used in polymerization) were added together with VDF, deionized water, suspension agent, and polymerization initiator. A copolymer powder of VDF, CTFE, and APS (mass ratio = 93.0:6.5:0.5, molar ratio = 96.2:3.7:0.2) was obtained in the same manner as in Example 4, except that 2.3 g of a 5 wt% aqueous APS solution was continuously added in solute terms. The physical properties of the vinylidene fluoride copolymer and its adhesion to SUS304 are shown in Table 1. An electrode mixture was then prepared using the vinylidene fluoride copolymer in the same manner as in Example 1.

[0086] (Comparative Example 1) Before raising the temperature, 26.4 g of CTFE (6.0% by mass (3.38 mol%) of VDF and CTFE used for polymerization) was added together with VDF, deionized water, suspension agent, polymerization initiator, and chain transfer agent. The temperature was raised to 38°C, and no further CTFE was added thereafter. The same method as in Example 1 was used to obtain a powder of a VDF-CTFE copolymer (mass ratio = 93.6:6.4, molar ratio = 96.4:3.6). The physical properties of the vinylidene fluoride copolymer and its adhesion to SUS304 are shown in Table 1. An electrode mixture was then prepared using the vinylidene fluoride copolymer in the same manner as in Example 1.

[0087] (Comparative Example 2) A polymer of VDF and CTFE (mass ratio = 95.7:4.3, molar ratio = 97.6:2.4) was obtained in the same manner as in Comparative Example 1, except that 422 g of VDF and 17.6 g of CTFE (4.0 mol% (2.24 mol%) relative to the total amount of VDF and CTFE used for polymerization) were added before heating. The physical properties of the vinylidene fluoride copolymer and its adhesion to SUS304 are shown in Table 1. An electrode mixture was then prepared using the vinylidene fluoride copolymer in the same manner as in Example 1.

[0088] (Comparative Example 3) A polymer powder of VDF and CTFE (mass ratio = 97.8:2.2, molar ratio = 98.8:1.2) was obtained in the same manner as in Comparative Example 1, except that 431 g of VDF and 8.8 g of CTFE (2.0% by mass (1.11 mol%) relative to the total amount of VDF and CTFE used for polymerization) were added before heating. The physical properties of the vinylidene fluoride copolymer and its adhesion to SUS304 are shown in Table 1. Then, using the vinylidene fluoride copolymer, an electrode mixture was prepared in the same manner as in Example 1.

[0089] (Comparative Example 4) Before temperature increase, 25.2 g of CTFE (5.6 mass% (3.18 mol%) based on the total amount of VDF, CTFE and AA finally used in the polymerization) was added together with VDF, ion-exchanged water, a suspending agent, a polymerization initiator, a chain transfer agent and AA. Except that no CTFE was added after temperature increase, a powder of a copolymer of VDF, CTFE and AA (mass ratio=93.1:6.6:0.3, molar ratio=96.0:3.7:0.3) was obtained by the same method as in Example 4. The physical properties of the vinylidene fluoride and adhesion to SUS304 are shown in Table 1. Then, using the vinylidene fluoride copolymer, an electrode mixture was produced in the same manner as in Example 1 above.

[0090] 2. Evaluation (Measurement method and evaluation method of physical properties) The inherent viscosity of each of the above vinylidene fluoride copolymers, the proportion W of VDF constitutional units in the vinylidene fluoride copolymer ｆ and the proportion W of CTFE constitutional units ｃ and the proportion W of constitutional units of the carboxy-containing vinyl compound ａ , absorbance ratio, heterogeneous bond ratio, content of particles with circularity of 0.85 or less, average primary particle diameter, bulk density, adhesion rate to SUS304, viscosity of the electrode mixture (slurry viscosity) and peel strength were determined by the following methods. The results are shown in Table 1 and Table 2.

[0091] • Measurement of inherent viscosity The inherent viscosity of the vinylidene fluoride copolymer was measured as follows. First, 80 mg of the vinylidene fluoride copolymer was dissolved in 20 ml of N,N-dimethylformamide, and the viscosity was measured using an Ubbelohde viscometer in a constant temperature bath at 30°C. Then, from the obtained value, the inherent viscosity (η ｉ ) of the vinylidene fluoride copolymer was calculated 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 N,N-dimethylformamide alone as the solvent, and C is the concentration of the vinylidene fluoride copolymer in the solution, that is, 0.4 g / dl.

[0092] • Proportion W of CTFE constitutional units in the vinylidene fluoride copolymer ｃThe specific vinylidene fluoride copolymer is burned under an oxygen atmosphere, and the combustion gas is absorbed into an absorbent solution. Subsequently, the chlorine content (wt%) is measured using ion chromatography, and the ratio of CTFE constituent units to the total constituent units is determined. ｃ The (mass %) was calculated.

[0093] • The proportion of carboxyl-containing vinyl compound constituent units in vinylidene fluoride copolymer (W) ａ Wa was calculated by dividing the amount of specific carbonyl group-containing vinyl compounds added by the total amount of monomers (VDF, CTFE, carboxyl group-containing vinyl compounds) added multiplied by the yield.

[0094] • Ratio of VDF constituent units in vinylidene fluoride copolymer W ｆ The proportion of VDF-derived constituent units in the specific vinylidene fluoride copolymer W ｆ (Mass %) is (100 - W ｃ -W ａ ) was sought from this.

[0095] To calculate the specific absorbance ratio, a 30 mm x 30 mm press sheet was prepared by hot-pressing vinylidene fluoride copolymer at 230°C. The IR spectrum of the prepared press sheet was measured at 1500 cm using an infrared spectrophotometer FT-730 (manufactured by Horiba, Ltd.). －１ ~4000 －１ Measurements were taken within the specified range. The absorbance ratio was calculated using the following formula: Absorbance ratio = A （１７００－１８００） / A （３０２３） In the above formula, A （１７００－１８００） 1700-1800cm －１ This absorbance originates from the stretching vibration of the carbonyl group detected within the range A （３０２３） 3023cm －１ This absorbance originates from the stretching vibrations of CH detected in the vicinity.

[0096] - The ratio of heterogeneous bonding was determined by dissolving 40 mg of vinylidene fluoride copolymer in 0.8 ml of deuterium dimethyl sulfoxide (DMSO-d6) and measuring 19F-NMR at room temperature. The peak areas (A, B, C) below were determined, and the ratio of heterogeneous bonding was calculated using the following formula: A: Peak area from -89 ppm to -97 ppm B: Peak area around -114 ppm C: Peak area around -116 ppm Ratio of heterogeneous bonding = (((B + C) / 2) / (A + B + C)) × 100

[0097] - Measurement of average primary particle size: The average primary particle size of vinylidene fluoride copolymer was determined by dry sieving using a sieve shaker (Hirako Seisakusho Type II Ro-Tap Sieve Shaker Type D) in accordance with JIS K0069-3.1. The 50% cumulative value (D50) of the cumulative particle size distribution was then determined using the log-normal distribution method.

[0098] The average primary particle diameter and circularity of the specific vinylidene fluoride copolymer were calculated using a circularity calculation device (Keyence VHX-X-1) as follows. The circularity was determined by the following method: (i) The vinylidene fluoride copolymer was immersed in a 5% by mass ethanol solution containing a surfactant. (ii) The vinylidene fluoride copolymers were arranged so as not to overlap, and 5000 images were taken using an optical microscope (Keyence VHX-X-1) with a scale of 100 μm, 10 decimal places, and a particle separation degree of 100, and the projected area S and perimeter C of each particle were obtained. (iii) For each particle, the projected area S and perimeter C were substituted into the following formula to determine the circularity φ of each particle. Circularity φ = (4πS / C) ２ (iv) A graph was created showing the frequency of the circularity φ of each particle from 0 to 1 in increments of 0.01, and the content of primary particles with a circularity of 0.85 or less was determined.

[0099] - Measurement of bulk density The bulk density was measured in accordance with the measurement method for JIS K 6721-3.3 "Bulk density". Specifically, 2 mL of a 5% ethanol solution of surfactant was added to 100 g of sample, and after thorough stirring with a spatula, it was left to stand for 10 minutes. 120 mL of the sample, thus destaticized, was placed in a funnel into which the damper of the bulk density measuring device was inserted. The damper was then quickly removed, and the sample was dropped into a receiver. The sample that rose from the receiver was scraped off with a glass rod. The mass of the receiver containing the sample was then accurately measured to 0.1 g and calculated using the formula (s = (c - a) / b). s is the bulk density (g / cm³). ３ a) is the mass of the receiver (g), b) is the internal volume of the receiver (cm³) ３ ), c represents the mass (g) of the receiver containing the sample.

[0100] - Measurement of adhesion to SUS304 The adhesion of vinylidene fluoride copolymer to SUS was measured as follows: (i) A 10 cm (length) x 5 cm (width) piece of SUS304 (Ra (arithmetic mean roughness): 0.452 μm, Rz (maximum height roughness): 0.047 μm, hereafter referred to as SUS) was weighed. (ii) The vinylidene fluoride copolymer was weighed (approximately 1.3 g) and spread evenly on the SUS. (iii) N ２ (iv) The sample was heated and stored at 110°C for 1 hour under controlled conditions. (v) After heating and storage, it was cooled at room temperature for 15 minutes. (v) The SUS was inverted, and the vinylidene fluoride copolymer that fell by gravity was removed. (vi) The amount of vinylidene fluoride copolymer remaining on the SUS was weighed. (vii) The adhesion rate to the SUS was calculated using the following formula: Adhesion rate (%) = 100 × (Total weight after heating and inversion (vi) - Weight of SUS (i)) / (Amount of vinylidene fluoride copolymer before heating (ii))

[0101] - Measurement of Electrode Mixture (Slurry) Viscosity The slurry viscosity of the electrode mixtures prepared in the examples and comparative examples was measured as follows. The slurry viscosity was measured using a Type B viscometer (Toki Sangyo Co., Ltd., TVB-10M, spindle No. M4) at 20°C and a rotation speed of 12 rpm, with the viscosity measured 2 minutes after the start of rotation. The slurry viscosity was measured immediately after the manufacture of the electrode mixture and 7 days after manufacture. Specifically, the viscosity of the electrode mixture was measured immediately after preparation. The electrode mixture was then stored for 7 days at room temperature of 20°C and a dew point of -30°C or lower. After that, the slurry viscosity of the electrode mixture was measured. The ratio of the slurry viscosity immediately after manufacture to the slurry viscosity after storage (slurry viscosity after storage / slurry viscosity immediately after manufacture) was defined as the slurry viscosity ratio.

[0102] - The electrode mixtures prepared in the examples and comparative examples for measuring peel strength were coated onto 15 μm thick aluminum foil using a bar coater and dried at 110°C for 30 minutes to obtain single-sided coated electrodes with a basis weight of 300 g / m². The obtained single-sided coated electrodes were cut to a length of 50 mm and a width of 20 mm, and the peel strength between the aluminum foil and the mixture layer was evaluated. Specifically, the upper surface of the formed mixture layer was bonded to a thick plastic plate (acrylic resin, 5 mm thick), and a 90° peel test was performed using a tensile testing machine (Orientec Co., Ltd. single-column type material testing machine STA-1150) in accordance with JIS K6854-1, at a head speed of 10 mm / min. The measurement environment was controlled at a temperature of 25°C and a dew point of -20°C.

[0103]

[0104]

[0105] As shown in Table 1 above, the amount of constituent units W derived from CTFE relative to the total amount of constituent units of the vinylidene fluoride copolymer. ｃ The amount is 2.2% by mass or more, and the amount of constituent units W derived from vinylidene fluoride is also 2.2% by mass or more. ｆWhen the content of primary particles with a circularity of 0.85 or less was 35.5% by mass or more, and the content of primary particles with a circularity of 0.85 or less was 20% or less, the adhesion rate to SUS was small, and even after storing the electrode mixture for 7 days, the viscosity change rate was 1.26 or less (Examples 1 to 5).

[0106] In contrast, as shown in Table 2 above, when the content of primary particles with a circularity of 0.85 or less exceeded 20%, the adhesion rate to SUS was very high (Comparative Examples 1, 2, and 4). Furthermore, even when the content of primary particles with a circularity of 0.85 or less was 20% or less, the viscosity change rate was high when the amount of CTFE was low (Comparative Example 3).

[0107] This application claims priority under Japanese Patent Application No. 2025-057031, filed on 28 March 2025. All provisions of the said application are incorporated herein by reference.

[0108] The present invention provides a particulate vinylidene fluoride copolymer containing vinylidene fluoride and chlorotrifluoroethylene, which is less prone to gelation when preparing electrode mixtures and can be manufactured with a high yield. This vinylidene fluoride copolymer is extremely useful in the field of manufacturing various types of batteries.

Claims

1. A particulate vinylidene fluoride copolymer comprising constituent units derived from vinylidene fluoride and constituent units derived from chlorotrifluoroethylene, wherein the ratio W of the amount of constituent units derived from chlorotrifluoroethylene to the total constituent units of the vinylidene fluoride copolymer is... ｃ The amount of the constituent units derived from vinylidene fluoride is 2.2% by mass or more, and the amount of the constituent units derived from vinylidene fluoride relative to the total constituent units of the vinylidene fluoride copolymer is W. ｆ A vinylidene fluoride copolymer having a content of 35.5% by mass or more, and a content of primary particles with a circularity of 0.85 or less, as determined by the method described below, of 20% or less. (Method for determining circularity) (i) Immerse the vinylidene fluoride copolymer in a 5% by mass ethanol solution containing a surfactant. (ii) Arrange the vinylidene fluoride copolymers so that they do not overlap, and take 5000 images using an optical microscope with a scale of 100 μm, 10 decimal places, and a particle separation degree of 100, to obtain the projected area S and perimeter C of each particle. (iii) For each particle, substitute the projected area S and perimeter C into the following formula to determine the circularity φ of each particle. Circularity φ = (4πS / C) ２ (iv) Create a frequency graph from the circularity φ of each particle and determine the content of primary particles with a circularity of 0.85 or less.

2. Bulk density is 0.35 g / cm³ ３ The vinylidene fluoride copolymer according to claim 1 is as described above.

3. The vinylidene fluoride copolymer according to claim 1, further comprising a constituent unit derived from a compound having an acidic functional group.

4. The vinylidene fluoride copolymer according to claim 3, wherein the ratio of the intensity of the peak originating from C=O stretching to the intensity of the peak originating from C-H stretching, as measured by infrared spectroscopy, is 0.10 or more.

5. The vinylidene fluoride copolymer according to claim 3, wherein the acidic functional group is a carboxyl group.

6. The vinylidene fluoride copolymer according to claim 5, wherein the compound having the acidic functional group is a compound represented by the following general formula (1). (In general formula (1), R １ , R ２ , and R ３ Each of the following independently represents a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms, and X represents a hydroxyl group or a functional group represented as -Y-COOH (where Y represents an atomic group).

7. The vinylidene fluoride copolymer according to claim 1, wherein the average primary particle diameter measured by a sieving method in accordance with JIS K0069-3.1 is 5 μm or more and 1000 μm or less.

8. The vinylidene fluoride copolymer according to claim 1, wherein the proportion of heterogeneous bonds is 4.0 mol% or more.

9. An electrode binder composition comprising a vinylidene fluoride copolymer according to any one of claims 1 to 8, and water and / or a non-aqueous solvent.

10. An electrode mixture comprising a vinylidene fluoride copolymer according to any one of claims 1 to 8, an electrode active material, and water and / or a non-aqueous solvent.

11. An electrode comprising: a current collector; and an electrode mixture layer obtained from the electrode mixture according to claim 10, disposed on the current collector.

12. A secondary battery comprising the electrode described in claim 11.

13. Wherein, based on all constituent units, the amount W of constituent units derived from vinylidene fluoride ｆ is 35.5% by mass or more, and based on all constituent units, the proportion W of the amount of constituent units derived from chlorotrifluoroethylene ｃ is 2.2% by mass or more, which is a method for producing a particulate vinylidene fluoride copolymer, comprising: a first step of mixing a monomer composition containing vinylidene fluoride and a dispersion medium, and raising the temperature to a polymerization temperature; and a second step of adding chlorotrifluoroethylene to the monomer composition obtained after the first step at said polymerization temperature to perform suspension polymerization, wherein the amount of chlorotrifluoroethylene in the monomer composition in the first step is 3.1% by mass or less based on the total amount of monomers supplied to the first step and the second step. A method for producing a vinylidene fluoride copolymer.

14. The method for producing a vinylidene fluoride copolymer according to claim 13, wherein the polymerization temperature is equal to or greater than the critical temperature of vinylidene fluoride.

15. The method for producing a vinylidene fluoride copolymer according to claim 13, wherein the monomer composition in the first step comprises a compound having an acidic functional group.

16. The method for producing a vinylidene fluoride copolymer according to claim 15, wherein the acidic functional group is a carboxyl group.

17. The method for producing a vinylidene fluoride copolymer according to claim 15, wherein the compound having the acidic functional group is a compound represented by the following general formula (1). (In general formula (1), R １ , R ２ , and R ３ Each of the following independently represents a hydrogen atom, a halogen atom, or an alkyl group having 1 to 5 carbon atoms, and X represents a hydroxyl group or a functional group represented as -Y-COOH (where Y represents an atomic group).

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