Vinylidene fluoride copolymer, 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, hardly causes gelation when preparing an electrode mixture, and can be produced with favorable yield. This vinylidene fluoride copolymer for achieving the purpose is a particulate vinylidene fluoride copolymer containing a vinylidene fluoride-derived constitutional unit and a chlorotrifluoroethylene-derived constitutional unit. The proportion Xα of the amount of chlorine atoms to the amount of all elements present on the particle surface, as measured by X-ray electron spectroscopy, is at most 1.3 at%, and with respect to all of the constitutional units of the vinylidene fluoride copolymer, the proportion Wc of the chlorotrifluoroethylene-derived constitutional unit is at least 3.1 mass% and the proportion Wf of the vinylidene fluoride-derived constitutional unit is at least 35.5 mass%.

Description

vinylidene fluoride copolymer, electrode binder composition, electrode mixture, electrode, and secondary battery

[0001] This invention relates to vinylidene fluoride copolymer, electrode binder composition, electrode mixture, electrode, and secondary battery.

[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 a good yield. The present invention also aims to provide an electrode binder composition, an electrode mixture, an electrode, and a secondary battery containing the above vinylidene fluoride copolymer.

[0007] The present invention provides vinylidene fluoride copolymers as described in [1] to [9] below. [1] A particulate vinylidene fluoride copolymer comprising a constituent unit derived from vinylidene fluoride and a constituent unit derived from chlorotrifluoroethylene, wherein the ratio of chlorine atoms to the total amount of elements present on the particle surface, as measured by X-ray electron spectroscopy, is X α The ratio of the constituent units derived from chlorotrifluoroethylene to the total constituent units of the vinylidene fluoride copolymer is 1.3 atomic percent or less. ｃ The ratio W of the constituent units derived from vinylidene fluoride to the total constituent units of the vinylidene fluoride copolymer is 3.1% by mass or more. ｆ A vinylidene fluoride copolymer having 35.5% by mass or more of the chlorine atoms. [2] The proportion of the chlorine atoms X α And the proportion W of constituent units derived from the chlorotrifluoroethylene. ｃ The ratio (X α / W ｃ [1] A vinylidene fluoride copolymer according to [1], wherein the ratio of the acidity to (In general formula (1), R １ , R２ , and each 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)) [7] The vinylidene fluoride copolymer according to any one of [1] to [6], wherein an 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 any one of [1] to [7], wherein a content of primary particles having a circularity of 0.85 or less, specified by the following method, is 80% or less. (Method for Specifying Circularity) (i) The vinylidene fluoride copolymer is immersed in an ethanol solution having a surfactant concentration of 5% by mass. (ii) The vinylidene fluoride copolymer is arranged so as not to overlap, 5000 particles are photographed with an optical microscope at a scale of 100 µm, a number of decimal places of 10 digits, and a particle resolution of 100, and a projected area S and a perimeter C of each particle are obtained. (iii) For each of the particles, the projected area S and the perimeter C are substituted into the following formula to obtain the circularity φ of each particle. Circularity φ=(4πS / C ２ ) (iv) A frequency graph is prepared from the circularity φ of each particle, and the content of primary particles having a circularity of 0.85 or less is obtained. [9] The vinylidene fluoride copolymer according to any one of [1] to [7], wherein a proportion of heterogeneous bonds is 4.0 mol% or more.

[0008] The present invention provides an electrode binder composition, an electrode mixture, an electrode, and a secondary battery according to

[10] to

[13] .

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

[11] An electrode mixture comprising the vinylidene fluoride copolymer according to any one of [1] to [9] above, an electrode active material, and water and / or a non-aqueous solvent.

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

[11] , the electrode mixture layer being disposed on the current collector.

[13] A secondary battery comprising the electrode according to

[12] .

[0009] The particulate vinylidene fluoride copolymer of the present invention is less likely to gel when an electrode mixture is prepared. Furthermore, the particulate vinylidene fluoride copolymer can be manufactured with a high yield. The present invention also provides an electrode binder composition, an electrode mixture, an electrode, and a secondary battery containing the above vinylidene fluoride copolymer.

[0010] 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.

[0011] 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 yield during manufacturing. In contrast, through diligent research by the inventors, it was discovered that there is a correlation between the amount of chlorine atoms on the surface of the vinylidene fluoride copolymer and its adhesion to stainless steel and the like. Furthermore, it was found that, using the manufacturing method described later, a vinylidene fluoride copolymer can be obtained in which the amount of chlorine atoms present on the particle surface is below a certain value, even if the proportion of constituent units derived from chlorotrifluoroethylene in the vinylidene fluoride copolymer is high, leading to the present invention. This vinylidene fluoride copolymer not only does not adhere easily to stainless steel and the like during manufacturing, but also fully exhibits the function of suppressing gelation when used as an electrode mixture.

[0012] The following describes the chemical composition of the vinylidene fluoride copolymer of the present invention, followed by a description of the amount of chlorine atoms on the surface of the vinylidene fluoride copolymer, the average particle size, and various physical properties.

[0013] As described above, the vinylidene fluoride copolymer of the present invention may contain constituent units derived from vinylidene fluoride and constituent units derived from chlorotrifluoroethylene, and the vinylidene fluoride copolymer may be composed of only these. On the other hand, the vinylidene fluoride copolymer may also contain constituent units derived from other compounds in addition to the constituent units derived from vinylidene fluoride and chlorotrifluoroethylene, for example, it may further contain constituent units derived from compounds having acidic functional groups as described later.

[0014] 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.

[0015] 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 to be 3.1% 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. ｃHowever, if the amount is 3.1% by mass or more, as described above, when vinylidene fluoride copolymer is used as an electrode mixture, thickening will be less likely to occur. 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 1.7 mol% or more, more preferably 1.7 mol% to 20.0 mol%, even more preferably 2.2 mol% to 11 mol%, and particularly preferably 2.8 mol% to 6.0 mol%. ｃ Ya M ｃ teeth, １９ It can be determined from F-NMR spectroscopy or ion chromatography.

[0016] 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, with the carboxyl group being preferred. When the vinylidene fluoride copolymer contains structural units derived from compounds having acidic functional groups, the amount (percentage) 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 2.00% 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 (percentage) 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＝Ｏ This 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.

[0017] 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 , R ２ , and R ３These are more preferably a hydrogen atom or a methyl group, independently of each other.

[0018] 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 preferably a divalent atomic group with a molecular weight of 500 or less that contains either an oxygen atom or a nitrogen atom and has 1 to 10 atoms in the main chain.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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, 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.

[0024] Of the compounds represented by the general formula (1) above, acrylic acid, acryloyloxyethyl succinic acid, acryloyloxypropyl succinic acid, 2-carboxyethyl acrylate, and carboxymethyl acrylate are more preferred from the viewpoint of availability and reactivity with vinylidene fluoride.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Furthermore, the vinylidene fluoride copolymer of the present invention preferably has a heterogeneous bond ratio of 4.0 mol% or more. A vinylidene fluoride copolymer with a heterogeneous bond ratio of 4.0 mol% or more is easily obtained by polymerization at a temperature above the supercritical temperature of vinylidene fluoride, as described later. In such a vinylidene fluoride copolymer, there are fewer particle-breaking components, and it is less likely to adhere to manufacturing equipment (e.g., stainless steel) during production. In this specification, heterogeneous bond refers to a structure in which vinylidene fluorides are bonded to each other in the vinylidene fluoride copolymer, where "CF ２ " and "CH ２This 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.

[0029] 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)

[0030] 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 adhesion strength of the vinylidene fluoride copolymer to the active material and current collector tends to increase. 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.

[0031] 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.

[0032] 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).

[0033] The ratio of chlorine atoms to the total amount of elements present on the particle surface, as measured by X-ray electron spectroscopy (hereinafter also referred to as "XPS method") of the aforementioned vinylidene fluoride copolymer (particles). α The amount is 1.3 atomic percent or less, preferably 1.0 atomic percent or less, and more preferably less than 0.7 atomic percent. The above proportion X αThe less X the amount, the better, and it may be 0 atomic percent, but it is often greater than 0 atomic percent. As mentioned above, the ratio X of chlorine atoms to the total amount of elements present on the particle surface. α If the amount is 1.3 atomic percent or less, the vinylidene fluoride copolymer will not adhere easily to the manufacturing equipment (such as stainless steel) during production.

[0034] The proportion of chlorine atoms present on the surface of vinylidene fluoride copolymer (particles) X α This value is measured by the following method. First, the vinylidene fluoride copolymer was fixed in particle form to the XPS measurement sample stage using indium foil. Then, the sample was measured using an XPS instrument (for example, JEOL's PS9010MC) under the following conditions. The elemental composition analysis of the sample surface was measured using a narrow scan (pass energy 50 eV, step size 0.1 eV), and the percentage of chlorine atoms X was obtained by dividing the Cl 2p3 / 2 orbital peak area obtained by the narrow scan by the manufacturer's recommended sensitivity coefficient, and then dividing this value by the sum of the values ​​obtained by dividing the orbital peak area of ​​each element by the manufacturer's recommended sensitivity coefficient. α The following is calculated. <XPS measurement conditions> Radiation source: 30W (10kV x 5mA) Al-Kα radiation Neutralizing gun: 4W (2kV x 2mA) electrons supplemented Photoelectron detection angle: 90 degrees

[0035] On the other hand, the proportion of chlorine atoms present on the particle surface of the vinylidene fluoride copolymer X α And the ratio W of constituent units derived from chlorotrifluoroethylene to the total constituent units of the vinylidene fluoride copolymer. ｃ The ratio (X α / W ｃ The ratio is preferably 0.30 or less, more preferably 0.20 or less, and even more preferably 0.15 or less. When the ratio is 0.3 or less, when vinylidene fluoride copolymer is used as an electrode mixture, it becomes less likely to adhere to manufacturing equipment (e.g., stainless steel) during manufacturing.

[0036] 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 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 suspension polymerization as described later, and suspension polymerization makes it easier for the average primary particle diameter 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 dry sieving according to JIS K0069-3.1. Then, the 50% cumulative value (D50) in the particle size cumulative distribution is determined by the log-normal distribution method. This is then taken as the average primary particle diameter.

[0037] Furthermore, it is preferable that the content of primary particles with a circularity of 0.85 or less in the vinylidene fluoride copolymer is 80% or less. When the above content is 80% or less, the vinylidene fluoride copolymer is less likely to adhere to the manufacturing equipment (stainless steel, etc.) 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 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.

[0038] Furthermore, the content of primary particles with a circularity of 0.85 or less may be 80% or less, preferably 60% or less, more preferably 50% or less, even more preferably 40% or less, and particularly preferably 30% or less. The content of primary particles with a circularity of 0.85 or less is achieved by synthesizing vinylidene fluoride copolymer by suspension polymerization.

[0039] (Method for synthesizing vinylidene fluoride copolymer) The proportion of chlorine atoms present on the particle surface as described above X α A vinylidene fluoride copolymer having 1.3 atomic percent or less can be obtained, for example, by copolymerizing vinylidene fluoride, chlorotrifluoroethylene, and optionally a compound having an acidic functional group or other compounds by suspension polymerization.

[0040] Suspension polymerization is a well-known method for synthesizing vinylidene fluoride copolymers having the preferred average primary particle size described above. When producing particulate vinylidene fluoride copolymers 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 copolymers containing chlorotrifluoroethylene as a constituent unit using a general suspension polymerization method, from the viewpoint of reactivity ratio, the vinylidene fluoride copolymer initially contains a large amount of chlorotrifluoroethylene. Therefore, the proportion of chlorine atoms present on the particle surface X α It is difficult to reduce.

[0041] In contrast, the following steps are performed: a first step in which a monomer composition containing vinylidene fluoride is mixed with a dispersion medium, etc., heated to a polymerization temperature T, and suspended polymerized at polymerization temperature T for a certain period of time as needed; and a second step in which chlorotrifluoroethylene is added to the monomer composition after the first step and suspended polymerized at polymerization temperature T or higher, and the amount of chlorotrifluoroethylene in the monomer composition in the first step is set to a certain amount or less relative to the total amount of monomer used in the first and second steps, thereby reducing the proportion of chlorine atoms present on the particle surface X αA vinylidene fluoride copolymer having a chlorine content of 1.3 atomic percent or less is obtained. In this method, in the first step, a monomer composition containing vinylidene fluoride is subjected to suspension polymerization with a small amount of chlorotrifluoroethylene. The monomer used in the first step is the monomer added until the polymerization temperature T is reached. As described above, in suspension polymerization using vinylidene fluoride as a monomer, the outer shell of the particles is formed first, and then the interior of the particles is formed. Therefore, the proportion of chlorine atoms on the surface of the obtained vinylidene fluoride copolymer X α The amount is 1.3 atomic percent or less. Furthermore, after the first step, chlorotrifluoroethylene is added in the second step, and suspension polymerization is carried out at polymerization temperature T or higher, thereby obtaining a vinylidene fluoride copolymer containing a sufficient amount of constituent units derived from chlorotrifluoroethylene. The method will be described in detail below.

[0042] ・First step In the first step described above, a monomer composition containing vinylidene fluoride is mixed with a dispersion medium, etc., heated to the polymerization temperature T, and suspended polymerized at the polymerization temperature T for a certain period of time if necessary. 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 and 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. Furthermore, it is preferable that the amount of chlorotrifluoroethylene in the monomer composition be 1.75 mol% or less, relative to 100.0 mol% of the total number of moles of monomers ultimately copolymerized (total number of moles of vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds). More preferably, the amount of chlorotrifluoroethylene is 1.2 mol% or less, and even more preferably less than 0.81 mol%.

[0043] 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.

[0044] 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.

[0045] In suspension polymerization using water as the dispersion medium, it is preferable to use 0.005 to 1.0 parts by mass of a suspending agent such as methylcellulose, methoxylated methylcellulose, propoxylated methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, polyvinyl alcohol, polyethylene oxide, or gelatin, 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, and more preferably 0.01 to 0.4 parts by mass.

[0046] 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).

[0047] 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 5.0 parts by mass or less, and more preferably 3.0 parts by mass or less, per 100.0 parts by mass of the total monomers (vinylidene fluoride, chlorotrifluoroethylene, compounds having acidic functional groups, and other compounds) used in the final copolymerization.

[0048] In the first step, for example, the temperature is raised to a predetermined polymerization temperature T over a certain period of time. In the first step, the addition of monomers to be used in the first step is completed just before reaching the polymerization temperature T. Alternatively, all monomers to be used in the first step may be added at the start of polymerization. The polymerization temperature T is the 10-hour half-life temperature T of the polymerization initiator. 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 is appropriately selected within the ranges of 29.6°C ≤ T ≤ 79.6°C and 15.5°C ≤ T ≤ 65.5°C, respectively. To increase the heterogeneity of the vinylidene fluoride copolymer, it is more preferable that the polymerization temperature T be above the critical temperature of vinylidene fluoride (30.1°C).

[0049] • Second step In the second step, chlorotrifluoroethylene is added to the monomer composition after the first step at polymerization temperature T, and this is further subjected to suspension polymerization at the polymerization temperature T or higher. It is preferable to start the second step when the reaction rate is 10% or more and 60% or less. The reaction rate here is the value obtained by (mass of polymer formed in the first step / mass of all monomers used in the first step) × 100. If the reaction rate is 10% or more, the proportion of chlorine atoms present on the particle surface X α A vinylidene fluoride copolymer with a concentration of 1.3 atomic percent or less is easily obtained. In the second step, chlorotrifluoroethylene may be added all at once, or in multiple portions, i.e., intermittently or continuously. At this time, vinylidene fluoride or compounds having acidic functional groups may be added as needed.

[0050] In the second step, suspension polymerization is carried out at the polymerization temperature T or higher for a certain period of time until the desired vinylidene fluoride copolymer particles are obtained. In the second step, suspension polymerization may be carried out at the polymerization temperature T or higher for a predetermined time, and the temperature may be increased continuously or stepwise from the polymerization temperature T. Chlorotrifluoroethylene may be added after the temperature has been increased. The polymerization time is not particularly limited, but it is preferably 100 hours or less considering productivity, etc. The polymerization pressure is usually carried out under pressure, and is preferably 2.0 to 10.0 MPa-G.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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 and other metal oxides. In addition, the negative electrode active material may be one obtained by coating the surface of the above carbon material, metal / alloy material, metal oxide or the like. Note that the negative electrode active material may be a commercially available product.

[0059] 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 (0 < x ≦ 1) and other compounds of general formula LiMY ２ (wherein M is one or more transition metals selected from Co, Ni, Fe, Mn, Cr, Ti and V, and Y is a chalcogen element selected from O and S); composite metal chalcogen compounds represented thereby; LiMn ２ O ４ and other composite metal oxides having a spinel structure; LiFePO ４ and LiFeMnPO ４ and other olivine-type lithium compounds; LiNi ｘ Co ｙ M ｚ O ２ (wherein M represents Mn or Al, x, y, and z respectively satisfy 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1) and other lithium metal oxides represented thereby. Examples of sodium-based positive electrode active materials include NaFePO ４ and Na ３ V ２ (PO ４ ) ３ and other polyanion compounds; Na ｘ MO ２ (0 < x < 1, M (transition metal) = Mn, Fe, Cr, Ni) and other layered oxides; 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.

[0060] 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.

[0061] 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 nanotubes, and carbon fibers.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] • 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. Alternatively, a layer containing carbon black or the like may be formed on the surface of another medium, or the above-mentioned metal foils or metal meshes may be applied to it.

[0071] The composite layer is a layer formed by coating the above-mentioned electrode composite onto the current collector and allowing it to solidify. That is, the composite layer contains at least the vinylidene fluoride copolymer, which is the solid component of the above-mentioned electrode binder composition, and the electrode active material. The composite layer may be formed on only one side of the current collector, or it may be arranged on both sides.

[0072] The composite layer contains at least the components of the electrode composite described above, namely the solid content (vinylidene fluoride copolymer) and electrode active material derived from the electrode binder composition, and optionally further contains various additives such as conductive additives, dispersants, adhesion aids, and thickeners. These are the same as those described for the electrode composite.

[0073] 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, 100 g / m ２ or more and 500 g / m ２ or less is more preferable.

[0074] The mixture layer can be formed by carrying out the step of applying the aforementioned electrode mixture onto a current collector and the step of solidifying the applied mixture.

[0075] The method for applying the electrode mixture is not particularly limited, and doctor blade method, reverse roll method, comma bar method, gravure method, air knife method, die coating method, dip coating method, and the like can be applied.

[0076] In addition, after applying the electrode mixture, the solvent is dried by heating at any temperature. In one example, the drying temperature is preferably 60°C or higher and 500°C or lower, and more preferably 80°C or higher and 200°C or lower. Heating may be performed multiple times at different temperatures. The solvent in the mixture may be dried under atmospheric pressure, increased pressure, or reduced pressure, or may be dried in an environment such as air, nitrogen, or argon. Additional heat treatment may be performed after drying.

[0077] After application and drying of the electrode mixture, pressing treatment may be further performed. Electrode density can be improved by performing the pressing treatment. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

[0078] 5. Secondary Battery As mentioned above, the electrode binder composition and electrode mixture described above can be used for electrodes of various non-aqueous electrolyte secondary batteries, and may also be used for forming other layers of non-aqueous electrolyte secondary batteries. They may also be used for secondary batteries other than non-aqueous electrolyte secondary batteries.

[0079] Hereinafter, specific examples of the present invention will be described together with comparative examples, but the present invention is not limited thereto.

[0080] 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).

[0081] [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 of 12.2 μm was used (NCA), 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 weight of the mixture was 75.5% by mass.

[0082] [Electrode Manufacturing] 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.

[0083] (Example 2) Before heating, 4.4 g of CTFE (1.0% by mass, 0.56 mol%) was added along with VDF, deionized water, suspension agent, polymerization initiator, and chain transfer agent, 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 powder of a VDF-CTFE copolymer (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.

[0084] (Example 3) In a 2-liter autoclave, 1130 g of deionized water was added as a dispersion medium, 0.21 g of Metroze SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) as a cellulose-based suspension agent, 2.8 g of 50 wt% diisopropyl peroxydicarbonate-HFE-347pc-f solution as a polymerization initiator, 2.5 g of ethyl acetate as a chain transfer agent, 12.8 g of CTFE (3.0% by mass (1.70 mol%) of the total amount of VDF and CTFE to be used in polymerization), and 400 g of VDF were charged. The temperature was raised to 26°C over 60 minutes. While maintaining the temperature at 26°C, 12.8 g of CTFE was continuously added. From the point when the pressure dropped from the end of the heating to 0.2 MPaG, the temperature was raised to 40°C over 60 minutes. The reaction was continued until the pressure dropped to 1.5 MPa. Subsequently, a powder of a copolymer of VDF and CTFE (mass ratio = 93.8:6.2, molar ratio = 96.5:3.5) was obtained 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.

[0085] (Example 4) 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 55°C over 100 minutes. While maintaining the temperature at 55°C, 16.7 g of CTFE was continuously added. The reaction was continued until the pressure in the system decreased to 2.0 MPa. Subsequently, a polymer of VDF and CTFE (mass ratio = 95.7:4.3, molar ratio = 97.6:2.4) 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.

[0086] (Example 5) A polymer of VDF and CTFE (mass ratio = 91.4:8.6, molar ratio = 95.1:4.9) was obtained in the same manner as in Example 4, except that the temperature was raised to 38°C over 60 minutes, and 33.3 g of CTFE was continuously added while maintaining the temperature at 38°C, and the reaction was continued until the pressure in the system decreased to 1.5 MPa. 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 produced in the same manner as in Example 1.

[0087] (Example 6) In a 2-liter autoclave, 1163 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, 1.9 g of 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). 420 g of vinylidene fluoride (VDF) was added, and the temperature was raised to 45°C over 1.5 hours. Thereafter, while maintaining the temperature at 45°C, 1.9 g of 5 wt% aqueous AA solution (solute equivalent) was continuously added, along with 21.0 g of CTFE. The reaction was continued until the pressure in the system decreased to 2.0 MPa. Then, a polymer of VDF, CTFE, and AA (mass ratio = 92.8:6.6:0.6, molar ratio = 95.7:3.8:0.6) was obtained in the same manner as in Example 1 (yield 90.2%). 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.

[0088] (Example 7) 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 6 (yield 91.4%), except that 2.3 g of a 5 wt% aqueous APS solution was continuously added. 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.

[0089] (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.

[0090] (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% by mass (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.

[0091] (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.

[0092] (Comparative Example 4) A polymer of VDF, CTFE, and AA (mass ratio = 93.1:6.6:0.3, molar ratio = 96.0:3.7:0.3) was obtained in the same manner as in Example 6, except that 25.2 g of CTFE (5.6% by mass (3.18 mol%) relative to the total amount of VDF, CTFE, and AA to be ultimately used in polymerization) was added together with VDF, deionized water, suspension agent, polymerization initiator, chain transfer agent, and AA before heating, and no CTFE was added after heating. (Yield 87.2%). 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.

[0093] 2. Evaluation (Methods for measuring and evaluating physical properties) Inherent viscosity of each vinylidene fluoride copolymer, and the ratio W of VDF constituent units in the vinylidene fluoride copolymer. ｆ , the proportion of CTFE constituent units W ｃ and the proportion of constituent units of the carboxyl-containing vinyl compound W a , the proportion of chlorine atoms on the particle surface X α , the proportion of chlorine atoms X α / Percentage of CTFE constituent units W ｃ The absorbance ratio, heterogeneity bonding rate, average primary particle diameter, particle content with a circularity of 0.85 or less, amount of adhesion to SUS304, viscosity (slurry viscosity) of the electrode mixture, and peel strength were determined by the following methods. The results are shown in Tables 1 and 2.

[0094] - Measurement of Inherent Viscosity The inherent viscosity of vinylidene fluoride copolymer was measured as follows. First, 80 mg of 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) of vinylidene fluoride copolymer was calculated based on the following formula. ｉ ) was calculated. ｉ =(1 / C)・ln(η / η ０ ) In the above formula, η is the viscosity of the solution, η ０ is the viscosity of the solvent, N,N-dimethylformamide, alone, and C is the concentration of the vinylidene fluoride polymer in the solution, i.e., 0.4 g / dl.

[0095] • Percentage of CTFE constituent units in vinylidene fluoride copolymer W ｃ The specific vinylidene fluoride copolymer was burned under an oxygen atmosphere, and the combustion gas was absorbed into an absorbent solution. Subsequently, the chlorine content (wt%) was measured using ion chromatography, and the ratio of CTFE constituent units to the total constituent units was determined. ｃ The (mass %) was calculated.

[0096] • The proportion of carboxyl-containing vinyl compound constituent units in vinylidene fluoride copolymer (W) ａ Wa was calculated by dividing the amount of specific carboxyl group-containing vinyl compounds added by the total amount of monomers (VDF, CTFE, carboxyl group-containing vinyl compounds) added multiplied by the yield.

[0097] • 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.

[0098] • Chlorine atom ratio on the particle surface X α The specific vinylidene fluoride copolymer was fixed in particle form to an XPS measurement sample stage using indium foil. The XPS measurement was performed using a JEOL PS9010MC under the following conditions: <XPS Measurement Conditions> Radiation source: 30W (10kV x 5mA) Al-Kα radiation Neutralizing gun: 4W (2kV x 2mA) electrons Photoelectron detection angle: 90 degrees

[0099] The bond energies of each element were measured using a wide scan (pass energy 10 eV, step size 0.1 eV), and peaks originating from the F1s orbital were confirmed in the range of 676–696 eV, peaks originating from the Cl 2p3 / 2 orbital in the range of 195–210 eV, and peaks originating from the C1s orbital in the range of 278–298 eV. Elemental composition analysis of the sample surface was performed using a narrow scan (pass energy 50 eV, step size 0.1 eV), with the number of scans being 16 for F1s, 64 for C1s, and 128 for Cl 2p3 / 2. The chlorine atom ratio X was calculated by dividing the Cl 2p3 / 2 orbital peak area obtained by the narrow scan by the manufacturer's recommended sensitivity coefficient, and then dividing this value by the sum of the values ​​obtained by dividing the orbital peak areas of each element by the manufacturer's recommended sensitivity coefficient. α It was calculated as follows. Also, the value X α and the proportion W of the constituent units derived from the CTFE mentioned above. ｃ From these ratios (X α / W ｃ ) was sought.

[0100] - 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

[0101] - 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.

[0102] The circularity of the specific vinylidene fluoride copolymer was calculated as follows: (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. Data representing the equivalent diameter of a circle (hereinafter referred to as primary particle diameter D) and data representing the circumference (hereinafter referred to as circumference C) were calculated assuming that each captured particle was circular. (iii) Area S and circumference C were given by the formula (φ = 4πS / C ２ The circularity φ was calculated by substituting the values ​​into the formula. (iv) A graph was obtained showing the frequency of circularity φ in the measured particles from 0 to 1 in increments of 0.01, and the proportion of particles with a circularity of 0.85 or less was determined from this graph.

[0103] - 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))

[0104] - 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.

[0105] - The electrode mixtures prepared in the peel strength measurement examples and comparative examples were coated onto 15 μm thick aluminum foil using a bar coater and dried at 110°C for 30 minutes, resulting in a basis weight of 300 g / m². ２ A single-sided coated electrode was obtained. The obtained single-sided coated electrode was cut to a length of 50 mm and a width of 20 mm, and the peel strength between the aluminum foil and the composite layer was evaluated. Specifically, the upper surface of the formed composite layer was bonded to a thick plastic plate (made of acrylic resin, 5 mm thick), and a 90° peel test was performed using a tensile testing machine (Single-column type material testing machine STA-1150, manufactured by Orientec Co., Ltd.) 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.

[0106]

[0107]

[0108] As shown in Table 1 above, the ratio X of chlorine atoms to the total amount of elements present on the particle surface α The proportion of CTFE-derived constituent units to the total constituent units of the vinylidene fluoride copolymer is 1.3 atomic percent or less. ｃ The proportion of constituent units derived from vinylidene fluoride is 3.1% by mass or more, and W ｆWhen the concentration was 35.5% by mass or more, the amount adhering to SUS was 53% by mass or less, and even after storing the electrode mixture for 7 days, its viscosity ratio remained 1.26 or less (Examples 1 to 7).

[0109] In contrast, as shown in Table 2 above, the ratio X of chlorine atoms to the total amount of elements present on the particle surface α When the amount was greater than 1.3 atomic%, the amount of adhesion to SUS was very high (Comparative Examples 1, 2, and 4). Furthermore, the ratio of chlorine atoms to the total amount of elements present on the particle surface X α Even if the CTFE content was 1.3 atomic percent or less, if the amount of CTFE was excessively low, the viscosity of the electrode mixture doubled during storage (Comparative Example 3).

[0110] This application claims priority under Japanese Patent Application No. 2025-056375, filed on 28 March 2025. All provisions of the said application are incorporated herein by reference.

[0111] 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 containing constituent units derived from vinylidene fluoride and constituent units derived from chlorotrifluoroethylene, wherein the ratio of chlorine atoms to the total amount of elements present on the particle surface, as measured by X-ray electron spectroscopy, is X α The ratio of constituent units derived from chlorotrifluoroethylene to the total constituent units of the vinylidene fluoride copolymer is 1.3 atomic percent or less. ｃ The ratio W of the constituent units derived from vinylidene fluoride to the total constituent units of the vinylidene fluoride copolymer is 3.1% by mass or more. ｆ A vinylidene fluoride copolymer having a content of 35.5% by mass or more.

2. The proportion of chlorine atoms X α And the proportion W of constituent units derived from the chlorotrifluoroethylene. ｃ The ratio (X α / W ｃ The vinylidene fluoride copolymer according to claim 1, wherein the ratio is 0.30 or less.

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 3, 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).) 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 content of primary particles with a circularity of 0.85 or less, as identified by the method described below, is 80% or less. (Method for identifying 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.

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

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

11. An electrode mixture comprising a vinylidene fluoride copolymer according to any one of claims 1 to 9, an electrode active material, and water and / or a non-aqueous solvent.

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

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

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