Vinylidene fluoride copolymer, electrode binder, electrode mixture, electrode, and battery

Abstract

The purpose of the present invention is to provide a vinylidene fluoride copolymer that, even when mixed with an electrode active material such as lithium iron phosphate or lithium iron manganese phosphate to prepare an electrode mixture, has a slurry viscosity that tends not to increase excessively, and allows binding of an electrode active material at a high adhesive strength. This vinylidene fluoride copolymer that addresses the above problem contains a structural unit derived from vinylidene fluoride, and has a slurry viscosity as measured by a predetermined method and a binder solution viscosity as measured by a predetermined method that satisfy: the slurry viscosity / the binder solution viscosity < 5. The molar absorption coefficient at a wavelength of 346 nm two days after preparation of an N-methyl-2-pyrrolidone solution having a concentration of the vinylidene fluoride copolymer of 5 mass% is 1,800 M-1cm-1 or more.

Description

Vinylidene fluoride copolymer, electrode binder, electrode mixture, electrode, and battery

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

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

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

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

[0005] Here, lithium iron phosphate (LFP) and lithium iron manganese phosphate (LFMP) have a larger specific surface area than known electrode active materials. Therefore, when binding them to a current collector, a larger amount of binder is required than when using other electrode active materials. However, from the viewpoint of improving battery performance, a smaller amount of binder is preferable.

[0006] Therefore, it has been considered to use a vinylidene fluoride copolymer with high adhesive strength to bind LFP or LFMP with a small amount of binder, as described in the above-mentioned Patent Documents 1 and 2. However, when an electrode mixture is prepared by mixing such a vinylidene fluoride copolymer with LFP or LFMP, the slurry viscosity becomes very high, which poses a problem in terms of handleability.

[0007] The present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide a vinylidene fluoride copolymer that is unlikely to cause an excessive increase in slurry viscosity even when mixed with an electrode active material such as lithium iron phosphate or lithium iron manganese phosphate to prepare an electrode mixture, and that can bind the electrode active material with high adhesive strength. Another object of the present invention is to provide an electrode binder, an electrode mixture, an electrode, and a battery that contain the vinylidene fluoride copolymer.

[0008] [1] The present invention relates to a vinylidene fluoride copolymer containing a structural unit derived from vinylidene fluoride, and is characterized in that it is a lithium iron phosphate copolymer (having an average particle size D50 measured by a laser diffraction / scattering method of 1.5 to 2.0 μm and a BET specific surface area of ​​10 to 12 m). ２ / g), carbon black (BET specific surface area of ​​57 to 67 m ２ / g), carbon nanotubes (average diameter 7 to 11 nm, BET specific surface area 200 to 280 m ２ A mixture of the vinylidene fluoride copolymer and the vinylidene fluoride copolymer in a mass ratio of 100:1:1:2.5 was dispersed in N-methyl-2-pyrrolidone to a solid content of 52 mass %. The slurry was measured at 25°C using an RE80 type viscometer at a shear rate of 1 s －１ The slurry viscosity measured 120 seconds after the start of stirring at 25°C using an RE80 type viscometer at a shear rate of 3.83 s －１ and the binder solution viscosity measured 300 seconds after the start of stirring satisfies the relationship of the slurry viscosity / the binder solution viscosity<5, and the molar absorption coefficient at a wavelength of 346 nm of an N-methyl-2-pyrrolidone solution having a concentration of the vinylidene fluoride copolymer of 5 mass %, which is obtained by dissolving the vinylidene fluoride copolymer in N-methyl-2-pyrrolidone at 70° C. for 5 hours, is 1800 M or less after 2 days from the preparation. －１ cm －１ [2] The present invention provides the vinylidene fluoride copolymer according to [1], which contains a structural unit derived from a compound represented by the following general formula (1) and / or a structural unit derived from a compound represented by the following general formula (2): (In general formulas (1) and (2), R １ and R ４ are each independently a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or R １０ -O-C(=O)- (where R １０ represents an alkyl group having 1 to 5 carbon atoms, and R ２ , R ３ , R ５ and R ６ each independently represents a hydrogen atom or an alkyl group having from 1 to 5 carbon atoms, and X represents a divalent atomic group having from 1 to 20 atoms in its main chain and a molecular weight of 500 or less. [3] The present invention provides a vinylidene fluoride copolymer further containing structural units derived from a fluorine-containing vinyl compound (excluding the vinylidene fluoride) containing a fluorine atom and a vinyl group. [4] The present invention provides the vinylidene fluoride copolymer according to [3], wherein the fluorine-containing vinyl compound is one or more compounds selected from the group consisting of hexafluoropropylene, tetrafluoroethylene, and chlorotrifluoroethylene. [5] The present invention provides the vinylidene fluoride copolymer according to [3] or [4], wherein the amount of structural units derived from the fluorine-containing vinyl compound is 5 molar parts or less per 100 molar parts of the amount of structural units derived from the vinylidene fluoride. [6] The present invention provides an electrode binder comprising the vinylidene fluoride copolymer according to any one of [1] to [5] above. [7] The present invention provides an electrode mixture comprising the electrode binder described in [6] above and an electrode active material. [8] The present invention provides the electrode mixture described in [7] above, wherein the electrode active material is lithium iron phosphate and / or lithium iron manganese phosphate. [9] The present invention provides an electrode comprising the vinylidene fluoride copolymer described in any of [1] to [5] above, and an electrode active material.

[10] The present invention provides the electrode described in [9] above, wherein the electrode active material is lithium iron phosphate and / or lithium iron manganese phosphate.

[11] The present invention provides a battery comprising the electrode described in [9] or

[10] above.

[0009] According to the present invention, there is provided a vinylidene fluoride copolymer that, even when mixed with an electrode active material such as lithium iron phosphate or lithium iron manganese phosphate to prepare an electrode mixture, is unlikely to cause an excessive increase in slurry viscosity and can bind these materials with high adhesive strength. Further, there are also provided an electrode binder, an electrode mixture, an electrode, and a battery each containing the vinylidene fluoride copolymer.

[0010] 1. Vinylidene fluoride copolymer The vinylidene fluoride copolymer of the present invention is a copolymer of vinylidene fluoride and another monomer (hereinafter also referred to as a "comonomer"). The vinylidene fluoride copolymer contains structural units derived from vinylidene fluoride and structural units derived from the other monomer.

[0011] As described above, when a known vinylidene fluoride copolymer containing a carboxyl group or the like is used as a binder for the electrode mixture layer to firmly bind an electrode active material such as LFP or LFMP to a current collector, the viscosity of the electrode mixture slurry tends to increase in order to form the mixture layer. On the other hand, when a vinylidene fluoride copolymer that can reduce the viscosity of the electrode mixture slurry is used, it is difficult to sufficiently increase the binding strength to the current collector.

[0012] The present inventors have conducted extensive research into these problems and have found that the above problems can be solved by a vinylidene fluoride copolymer that satisfies the following (i) and (ii): (i) The slurry viscosity (measurement method will be described later) when LFP, carbon black, carbon nanotubes, and the vinylidene fluoride copolymer are mixed, and the binder solution viscosity (measurement method will be described later) when vinylidene fluoride is dissolved in N-methyl-2-pyrrolidone (NMP) satisfy the following relationship: slurry viscosity / binder solution viscosity<5. (ii) The molar extinction coefficient (measurement method will be described later) at a wavelength of 346 nm, measured two days after dissolving the vinylidene fluoride copolymer in NMP, is 1,800 M －１ cm －１ That's all.

[0013] Generally, when the molecular weight of a vinylidene fluoride copolymer is large, the viscosity of the binder solution increases when the vinylidene fluoride copolymer is dissolved in a solvent. Therefore, the value of the slurry viscosity / binder solution viscosity specified in (i) above decreases. Furthermore, the smaller the amount of functional groups (e.g., carbonyl groups and carboxy groups) that can interact with LFP and the like in the vinylidene fluoride copolymer, the lower the slurry viscosity. Therefore, the value of the slurry viscosity / binder solution viscosity specified in (i) above decreases. Furthermore, since the vinylidene fluoride copolymer of the present invention has a ratio specified in (i) of less than 5, it can be said that the copolymer has a relatively high molecular weight and a small amount of functional groups such as carbonyl groups and carboxy groups.

[0014] On the other hand, the molar absorption coefficient of the vinylidene fluoride copolymer NMP solution two days after preparation, as specified in (ii) above, is proportional to the amount of hydrofluoric acid removed (HF removed) from the vinylidene fluoride copolymer during the two days after preparation. Generally, from the viewpoint of the stability of the vinylidene fluoride copolymer, a smaller amount of HF removed is preferable. In contrast, as a result of extensive studies by the present inventors, it has become clear that when the specification in (i) above is satisfied and the amount of HF removed (ii) is a certain amount or more, the slurry viscosity is unlikely to increase excessively, and when an electrode is formed, the adhesive strength with the current collector, etc. is good.

[0015] From the above (i) and (ii), it can be said that the vinylidene fluoride copolymer of the present invention contains a moderate amount of functional groups (e.g., carbonyl groups) to the extent that the viscosity of the slurry does not excessively increase when it is used as an electrode mixture. Furthermore, since the vinylidene fluoride copolymer has a relatively high molecular weight and further contains the functional groups, when an electrode is formed, the adhesive strength with the current collector is good.

[0016] As described above, the vinylidene fluoride copolymer of the present invention is unlikely to have an excessively high slurry viscosity when used as an electrode mixture. On the other hand, when an electrode is formed, the adhesive strength with the current collector and the like is improved. The vinylidene fluoride copolymer will be described in detail below.

[0017] As described above, the vinylidene fluoride copolymer is a copolymer of vinylidene fluoride and a comonomer. The amount of vinylidene fluoride-derived structural units in the vinylidene fluoride copolymer is preferably 95.000 mol% or more and 99.995 mol% or less, and more preferably 96.800 mol% or more and 99.980 mol% or less, relative to 100 mol% of all structural units of the vinylidene fluoride copolymer. When the amount of vinylidene fluoride-derived structural units is 95.000 mol% or more, physical properties specific to vinylidene fluoride are easily obtained. The amount of vinylidene fluoride-derived structural units is １９ It can be determined from the F-NMR spectrum.

[0018] On the other hand, the comonomer copolymerized with vinylidene fluoride is not particularly limited as long as it can achieve the above-mentioned (i) ratio of slurry viscosity to binder solution viscosity and (ii) molar absorption coefficient. The vinylidene fluoride copolymer may contain only one type of structural unit derived from the comonomer, or may contain two or more types. In the present invention, it is preferable that at least one type of structural unit derived from the comonomer contains a carbonyl group.

[0019] Examples of compounds having a carbonyl group and copolymerizable with vinylidene fluoride include compounds represented by the following general formulas (1) and (2) (hereinafter, these are also collectively referred to as "carbonyl group-containing vinyl compounds"). The vinylidene fluoride copolymer may contain structural units derived from only one of these compounds, or may contain structural units derived from both compounds.

[0020] In the above general formula (1), R １ is a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or R １０ -O-C(=O)- (where R １０ represents a group represented by the formula (I) where I is an alkyl group having 1 to 5 carbon atoms.

[0021] The above R １Examples of the alkyl group having 1 to 5 carbon atoms include linear and branched alkyl groups. Specific examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a t-butyl group, and a pentyl group. １０ Examples of the alkyl group having 1 to 5 carbon atoms, represented by R, include linear or branched alkyl groups, and specific examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a t-butyl group, and a pentyl group. １０ The number of carbon atoms represented by R is preferably 1 or more and 3 or less, and a methyl group is particularly preferred. １ Among these, R １０ -O-C(=O)- is preferred, and CH ３ -O-C(=O)-,C ２ H ５ -O-C(=O)-,C ３ H ７ It is more preferably —O—C(═O)—.

[0022] In addition, R in the above general formula (1) ２ and R ３ R each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. ２ and R ３ Examples of the alkyl group having 1 to 5 carbon atoms include linear or branched alkyl groups, specific examples of which include a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a t-butyl group, a pentyl group, etc. Among these, from the viewpoint of polymerizability with vinylidene fluoride copolymer, R ２ and R ３ is preferably a hydrogen atom, a methyl group, an ethyl group, or a butyl group. In particular, from the viewpoint of preventing steric hindrance during polymerization with vinylidene fluoride, R ２ and R ３ are each independently a hydrogen atom or a methyl group.

[0023] Examples of the compound represented by the general formula (1) include unsaturated dibasic acid monoesters such as (meth)acrylic acid, monomethyl maleate, monoethyl maleate, monomethyl fumarate, monoethyl fumarate, monomethyl citraconic acid, and monoethyl citraconic acid. Among these, monomethyl maleate is preferred because it has good reactivity with vinylidene fluoride and is easy to achieve the above-mentioned (i) ratio of slurry viscosity to binder solution viscosity and (ii) molar absorption coefficient. In this specification, (meth)acrylic refers to methacrylic, acrylic, or a mixture thereof, (meth)acrylate refers to methacrylate, acrylate, or a mixture thereof, and (meth)acryloyl refers to methacryloyl, acryloyl, or a mixture thereof.

[0024] In the above general formula (2), R ４ are each independently a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or R １０ -O-C(=O)- (where R １０ represents a group represented by the formula (2), where R is an alkyl group having 1 to 5 carbon atoms. ４ represents R in general formula (1). １ The R ４ is preferred from the viewpoint that a hydrogen atom or a methyl group is unlikely to cause steric hindrance during copolymerization of vinylidene fluoride with a carbonyl group-containing vinyl compound.

[0025] Also, R ５ and R ６ R each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. ５ and R ６ represents R in the above general formula (1). ２ and R ３ R ５ and R ６ In particular, from the viewpoint of less steric hindrance occurring during polymerization with vinylidene fluoride, R ５ and R ６ are each independently a hydrogen atom or a methyl group.

[0026] In general formula (2), X represents a divalent atomic group having a molecular weight of 500 or less and having 1 to 20 atoms in the main chain. The atomic group (X) may be linear, branched, or cyclic, or may be a combination thereof. Among these, the atomic group (X) is preferably linear or branched, from the viewpoint of reducing the likelihood of steric hindrance during polymerization with vinylidene fluoride.

[0027] The number of atoms in the main chain of the atomic group (X) may be from 1 to 20, and preferably from 1 to 15. In this specification, the main chain of the atomic group (X) refers to the longest chain among the chains connecting the vinyl group in general formula (1) and the carboxy group bonded to the atomic group (X).

[0028] Here, the atomic group (X) may contain a heteroatom such as an oxygen atom or a nitrogen atom. The number of heteroatoms in the atomic group (X) is preferably 1 to 10, more preferably 1 to 7. When the atomic group (X) contains two or more heteroatoms, these may be the same type of atom or different types of atoms. The heteroatom may be contained in any structure (functional group) and may be located at any position within the atomic group (X). Examples of structures (functional groups) containing these heteroatoms include an ether bond, an ester bond, a carbonyl group, a carboxy group, an amide group, a hydroxy group, etc. Among these, an ether bond, an ester group, a carbonyl group, a carboxy group, an amide group, and a hydroxy group are preferred in terms of high stability.

[0029] The structure of the atomic group (X) is not particularly limited, and may be composed mainly of hydrocarbon groups such as alkylene groups or alkyl groups, or may be composed of these groups together with the above-mentioned heteroatom-containing structure (functional group). The molecular weight of the atomic group (X) may be 500 or less, and preferably 300 or less.

[0030] Specific examples of the compound represented by the general formula (2) include (meth)acrylamide-based compounds such as (meth)acryloyloxyethyl succinate, (meth)acryloyloxypropyl succinate, 2-carboxyethyl (meth)acrylate, (meth)acryloyloxyethyl phthalate, and N-carboxyethyl (meth)acrylamide.

[0031] The compound represented by the general formula (2) is more preferably (meth)acryloyloxypropyl succinic acid or (meth)acryloyloxyethyl succinic acid, from the viewpoints of availability and ease of achieving the above-mentioned (i) ratio of slurry viscosity to binder solution viscosity, and (ii) molar absorption coefficient.

[0032] The amount of structural units derived from carbonyl group-containing vinyl compounds (compounds represented by the above general formula (1) and compounds represented by the above general formula (2)) in the vinylidene fluoride copolymer is not particularly limited as long as the above-mentioned (i) ratio of slurry viscosity to binder solution viscosity and (ii) molar extinction coefficient can be achieved. The total amount of structural units derived from carbonyl group-containing vinyl compounds in the vinylidene fluoride copolymer is preferably 0.01 to 0.2 molar parts, more preferably 0.02 to 0.15 molar parts, per 100 molar parts of structural units derived from vinylidene fluoride. When the amount of structural units derived from carbonyl group-containing vinyl compounds is 0.01 molar parts or more, the adhesion of the vinylidene fluoride copolymer to the current collector when an electrode is formed is likely to be further improved. On the other hand, when the amount of structural units derived from carbonyl group-containing vinyl compounds is 0.2 molar parts or less, the increase in slurry viscosity when the vinylidene fluoride copolymer is mixed with an electrode active material is more likely to be suppressed. The amount of the constitutional unit derived from the carbonyl group-containing vinyl compound is １９ F-NMR spectrum and １ It can be determined from H-NMR spectrum or the like.

[0033] The vinylidene fluoride copolymer of the present invention may further contain structural units derived from compounds other than vinylidene fluoride and the carbonyl group-containing vinyl compound. The vinylidene fluoride copolymer may further contain, for example, a fluorine-containing vinyl compound containing a fluorine atom and a vinyl group (excluding those equivalent to vinylidene fluoride). The vinylidene fluoride copolymer may contain only one type of fluorine-containing vinyl compound, or may contain two or more types.

[0034] The structure of the fluorine-containing vinyl compound is not particularly limited as long as it does not impair the objects and effects of the present invention. The number of carbon atoms contained in the fluorine-containing vinyl compound is preferably 2 or more and 10 or less, more preferably 2 or more and 5 or less. The number of fluorine atoms contained in the fluorine-containing vinyl compound (the number of fluorine atoms) is preferably 1 or more and 20 or less, more preferably 1 or more and 10 or less. The fluorine atoms may be directly bonded to the vinyl group, or may be bonded to a carbon chain or the like bonded to the vinyl group. When the number of carbon atoms or the number of fluorine atoms in the fluorine-containing vinyl compound are within the above ranges, the melting point of the vinylidene fluoride copolymer can be easily adjusted to a desired range.

[0035] Specific examples of the fluorine-containing vinyl compound include vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, perfluoromethyl vinyl ether, and the like. Among these, hexafluoropropylene, tetrafluoroethylene, and chlorotrifluoroethylene are preferred from the viewpoints of availability, stability, and the like.

[0036] The amount of the constituent units derived from fluorine-containing vinyl compounds contained in the vinylidene fluoride copolymer is preferably 5 parts by mole or less, more preferably 3.5 parts by mole or less, per 100 parts by mole of the constituent units derived from vinylidene fluoride. When the amount of the constituent units derived from fluorine-containing vinyl compounds is within this range, it becomes easier to adjust the melting point of the vinylidene fluoride copolymer to a desired range. The amount of the constituent units derived from fluorine-containing vinyl compounds is １９ F-NMR spectrum and １ It is identified from H-NMR spectrum, etc.

[0037] The vinylidene fluoride copolymer may further contain structural units derived from other compounds. Examples include structural units derived from unsaturated hydrocarbon compounds such as ethylene and propylene. The amount of these structural units derived from other compounds is preferably 5 parts by mole or less, more preferably 3.5 parts by mole or less, per 100 parts by mole of the structural units derived from vinylidene fluoride.

[0038] The weight-average molecular weight of the vinylidene fluoride copolymer is not particularly limited, but is preferably 1,000,000 or more, more preferably 1,700,000 or more. When the weight-average molecular weight of the vinylidene fluoride copolymer is within this range, the heat resistance and strength of an electrode (mixture layer) containing the vinylidene fluoride copolymer tend to be good. In this specification, the weight-average molecular weight of the vinylidene fluoride copolymer is a polystyrene-equivalent value determined by a refractive index (RI) detector when performing gel permeation chromatography (GPC). The solvent used when performing GPC is dimethylacetamide (DMAc).

[0039] The slurry viscosity specified in (i) above is a value specified by the following method: the average particle diameter D50 measured by the laser diffraction scattering method is 1.5 to 2.0 μm, and the BET specific surface area is 10 to 12 m ２ / g, BET specific surface area of ​​57 to 67 m ２ / g, carbon black with an average diameter of 7 to 11 nm and a BET specific surface area of ​​200 to 280 m ２ Carbon nanotubes having a mass ratio of 100:1:1:2.5 were mixed with a vinylidene fluoride copolymer in a mass ratio of 100:1:1:2.5, and N-methyl-2-pyrrolidone was added as needed to prepare a slurry having a solids concentration of 52 mass%. The obtained slurry was measured at 25°C using an RE80 type viscometer at a shear rate of 1 s －１ In this specification, the slurry viscosity of the vinylidene fluoride copolymer is determined as the viscosity measured 120 seconds after the start of stirring.

[0040] In this specification, the average particle size D50 of lithium iron phosphate is a value measured by laser diffraction / scattering unless otherwise specified. Specifically, the measurement method involves placing lithium iron phosphate in water containing a surfactant and irradiating it with ultrasound for 5 minutes to prepare a sample. Water is used as the solvent during measurement. Particle transmittance during analysis is measured as reflection. D50 refers to the median diameter in the particle size distribution. The specific surface areas of lithium iron phosphate, carbon black, and carbon nanotubes are BET specific surface areas unless otherwise specified. The average diameter of carbon nanotubes is the average value measured by measuring the diameters of 10 or more carbon nanotubes using an electron microscope. The physical properties of the lithium iron phosphate, carbon black, and carbon nanotubes used in measuring the slurry viscosity generally have a certain range, even for commercially available products, and it is not practical to limit them to specific values. Therefore, ranges are used in this specification as well.

[0041] Furthermore, the binder solution viscosity specified in (i) above is a value specified by the following method. A vinylidene fluoride copolymer is dissolved in NMP to prepare an NMP solution with a vinylidene fluoride copolymer concentration of 4.5 mass %. At this time, it is preferable to dissolve the vinylidene fluoride copolymer in NMP at 50°C for 3 hours. Then, 24 hours after the NMP preparation, the viscosity is measured at 25°C using an RE80 viscometer. In this specification, the viscosity is determined at a shear rate of 3.83 s －１ The value measured 300 seconds after the start of stirring is taken as the viscosity of the binder solution.

[0042] As specified in (i) above, the ratio of the slurry viscosity to the binder solution viscosity (slurry viscosity / binder solution viscosity) may be less than 5, preferably 1 or more and 4 or less, and more preferably 1.2 or more and 3.5 or less.

[0043] The molar absorption coefficient specified in (ii) above is a value specified by the following method. A vinylidene fluoride copolymer is dissolved in NMP at 70°C over 5 hours to prepare an NMP solution with a vinylidene fluoride copolymer concentration of 5% by mass. The binder solution is allowed to stand for 2 days. The environment in which the solution is allowed to stand is not particularly limited, but is typically an environment of 15°C or higher and 25°C or lower. The measurement cell is a quartz cell with an optical path length of 1 cm. The absorption intensity at a wavelength of 346 nm is then measured using a known device, and the molar absorption coefficient is calculated from the molar concentration of the solution according to the Lambert-Beer law. The weight-average molecular weight of the vinylidene fluoride copolymer is a value measured by the above method. The molar absorption coefficient, as specified in (ii), is 1800 M －１ cm －１ It would be fine if it was more than 1800M. －１ cm －１ More than 5000M －１ cm －１ Preferably less than 2000M －１ cm －１ Over 4000M －１ cm －１ The following is more preferred:

[0044] The inherent viscosity of the vinylidene fluoride copolymer is preferably 0.5 dL / g or more and 8.0 dL / g or less, more preferably 0.5 dL / g or more and 5.0 dL / g or less, even more preferably 1.0 dL / g or more and 5.0 dL / g or less, and particularly preferably 1.0 dL / g or more and 4.5 dL / g or less. When the inherent viscosity is 0.5 dL / g or more, the adhesive strength of the vinylidene fluoride copolymer to the electrode active material and the current collector tends to be increased. On the other hand, when the inherent viscosity is 5.0 dL / g or less, the viscosity of the slurry does not become too high when an electrode mixture (slurry) is prepared, and workability is particularly good. The inherent viscosity (η ｉ ) indicates logarithmic viscosity. First, 80 mg of vinylidene fluoride copolymer is dissolved in 20 ml of N,N-dimethylformamide, and the viscosity is measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. Then, the viscosity is calculated from the obtained value based on the following formula: η ｉ =(1 / C)・ln(η / η ０In the above formula, η is the viscosity of the solution, η ０ is the viscosity of the solvent N,N-dimethylformamide alone, and C is the concentration of vinylidene fluoride copolymer in the solution, ie, 0.4 g / dl.

[0045] The melting point of the vinylidene fluoride copolymer is preferably 150°C or higher and 180°C or lower, more preferably 155°C or higher and 175°C or lower, and particularly preferably 160°C or higher and 175°C or lower. When the vinylidene fluoride copolymer has a melting point of 150°C or higher, when the vinylidene fluoride copolymer is used in an electrode, the copolymer is less likely to swell with an electrolyte, and the performance of the resulting battery is likely to be good. On the other hand, when the melting point is 180°C or lower, the flexibility of the formed electrode is likely to be good. The melting point of the vinylidene fluoride copolymer can be determined by calorimetry using a differential scanning calorimeter (DSC).

[0046] The vinylidene fluoride copolymer can be prepared by copolymerizing vinylidene fluoride with a carbonyl group-containing vinyl compound or a fluorine-containing vinyl compound, and, if necessary, other compounds. Examples of methods for copolymerizing these include suspension polymerization, emulsion polymerization, solution polymerization, etc., but suspension polymerization is preferred from the viewpoint of reducing impurities. When a compound represented by general formula (2) is used as the carbonyl group-containing vinyl compound, it is preferred to mix the entire amount of the carbonyl group-containing vinyl compound with vinylidene fluoride at once and polymerize them.

[0047] In suspension polymerization using water as a dispersion medium, a suspending agent such as methyl cellulose, methoxylated methyl cellulose, propoxylated methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, or gelatin is added in an amount of 0.005 to 1.0 part by mass, preferably 0.01 to 0.4 part by mass, per 100 parts by mass of all monomers used in the copolymerization (vinylidene fluoride, carbonyl group-containing vinyl compound, fluorine-containing vinyl compound, and other monomers).

[0048] Examples of polymerization initiators that can be used include diisopropyl peroxydicarbonate, di-normal propyl peroxydicarbonate, di-normal heptafluoropropyl peroxydicarbonate, isobutyryl peroxide, di(chlorofluoroacyl)peroxide, di(perfluoroacyl)peroxide, and t-butyl peroxypivalate. The amount used is 0.05 to 10 parts by mass, and preferably 0.15 to 5 parts by mass, based on 100 parts by mass of all monomers used in the copolymerization (vinylidene fluoride, carbonyl group-containing vinyl compound, fluorine-containing vinyl compound, and other monomers).

[0049] It is also possible to adjust the degree of polymerization of the resulting vinylidene fluoride copolymer by adding a chain transfer agent such as ethyl acetate, methyl acetate, diethyl carbonate, acetone, ethanol, n-propanol, acetaldehyde, propylaldehyde, ethyl propionate, carbon tetrachloride, etc. When a chain transfer agent is used, the amount used is usually 0.01 to 5 parts by mass, and preferably 0.01 to 3 parts by mass, per 100 parts by mass of all monomers used in the copolymerization (vinylidene fluoride, carbonyl group-containing vinyl compound, fluorine-containing vinyl compound, and other monomers).

[0050] The amount of all monomers (vinylidene fluoride, carbonyl group-containing vinyl compound, fluorine-containing vinyl compound, and other monomers) used in the copolymerization is usually 1:1 to 1:10, preferably 1:2 to 1:5, in terms of the mass ratio of total monomers to water.

[0051] The polymerization temperature T is the 10-hour half-life temperature T of the polymerization initiator. １０ is selected appropriately depending on the １０ -25℃≦T≦T １０ +25°C. For example, the T １０are 54.6°C and 40.5°C, respectively (see NOF Corporation product catalog). Therefore, in polymerizations using t-butyl peroxypivalate and diisopropyl peroxydicarbonate as polymerization initiators, the polymerization temperature T is appropriately selected within the ranges of 29.6°C≦T≦79.6°C and 15.5°C≦T≦65.5°C, respectively. The polymerization time is not particularly limited, but is preferably 100 hours or less in consideration of productivity and the like. The polymerization is usually carried out under increased pressure, preferably 2.0 to 10.0 MPa-G.

[0052] 2. Electrode Binder The electrode binder of the present invention may contain the vinylidene fluoride copolymer described above. The electrode binder may be in a solid form, such as a powder, or in a liquid form. The electrode binder may be composed solely of the vinylidene fluoride copolymer, or may contain a solvent as needed. When the electrode binder contains a solvent, the vinylidene fluoride copolymer may be dissolved in the solvent or may be dispersed in the solvent.

[0053] The solvent that the electrode binder may contain may be a non-aqueous solvent or water. The non-aqueous solvent may also be a polar solvent (polar solvent). Examples of the polar solvent include amide compounds such as N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; alcohol compounds such as methanol, ethanol, isopropyl alcohol, 2-ethyl-1-hexanol, 1-nonanol, lauryl alcohol, and tripropylene glycol; amine compounds such as o-toluidine, m-toluidine, and p-toluidine; imide compounds such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; lactone compounds such as γ-butyrolactone and δ-butyrolactone; sulfoxide / sulfone compounds such as dimethyl sulfoxide and sulfolane; ether compounds such as tetrahydrofuran, diethyl ether, 1,4-dioxane, and diethylene glycol dimethyl ether; and ketone compounds such as acetone, 2-butanone, methyl isobutyl ketone, and cyclohexanone. The electrode binder may contain only one of the above solvents, or may contain two or more of them.

[0054] When the electrode binder contains a solvent, the amount of the solvent in the electrode binder is preferably 50 parts by mass or more and 10,000 parts by mass or less, and more preferably 100 parts by mass or more and 5,000 parts by mass or less, relative to 100 parts by mass of the vinylidene fluoride copolymer. When the amount of the solvent in the electrode binder is within this range, the vinylidene fluoride copolymer can be uniformly dispersed or dissolved in the solvent.

[0055] The electrode binder may further contain various additives such as other resins such as acrylic resins, fillers such as inorganic fillers, dispersants, emulsifiers, etc., within the scope of not impairing the objects and effects of the present invention.

[0056] 3. Electrode Mixture The electrode binder described above can be mixed with an electrode active material (positive electrode active material or negative electrode active material) to prepare an electrode mixture (slurry) for producing an electrode for a non-aqueous electrolyte secondary battery. The electrode mixture may further contain a conductive aid, a solvent, other additives, etc.

[0057] The amount of the solid content (vinylidene fluoride copolymer) derived from the electrode binder relative to the total amount of the solid content derived from the electrode binder (total amount excluding volatile components), the electrode active material, and the conductive assistant is preferably 0.2 mass% or more and 20 mass% or less, more preferably 0.2 mass% or more and 10 mass% or less, and even more preferably 0.2 mass% or more and 7 mass% or less.

[0058] Furthermore, even when the vinylidene fluoride copolymer contained in the electrode binder is mixed with an electrode active material having a large specific surface area (e.g., iron phosphate (LFP) or lithium iron manganese phosphate (LFMP)), thickening or gelation of the electrode mixture is unlikely to occur. For example, the specific surface areas of LFP and LFMP are measured by the BET method. These specific surface areas are measured at 10 mm ２ / g or more, usually 10 mm ２ / g or more 20mm ２ / g or less. Even when the above-mentioned vinylidene fluoride copolymer (electrode binder) is mixed with such an electrode active material, the thickening of the electrode mixture is unlikely to occur. However, the specific surface area of ​​the electrode active material used in the present invention is not limited to the above range.

[0059] In the present invention, various materials can be used as the electrode active material of the electrode mixture. The electrode active material is not particularly limited, and any known electrode active material for a negative electrode (negative electrode active material) or electrode active material for a positive electrode (positive electrode active material) can be used.

[0060] Examples of the negative electrode active material include carbon materials such as artificial graphite, natural graphite, non-graphitizable carbon, easily graphitizable carbon, activated carbon, or phenolic resin and pitch that have been fired and carbonized; metal and alloy materials such as Cu, Li, Mg, B, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, Cd, Ag, Zn, Hf, Zr, and Y; and GeO, GeO ２ , SnO, SnO ２ , PbO, PbO ２ The negative electrode active material may be a material in which a coating is applied to the surface of the above-mentioned carbon material, metal / alloy material, metal oxide, etc. The negative electrode active material may be a commercially available product.

[0061] On the other hand, examples of the positive electrode active material include lithium-based positive electrode active materials containing lithium. Examples of the lithium-based positive electrode active material include LiCoO ２ , LiNiO ２ , LiNi ｘ Co ｙ Mn １－ｘ－ｙ O ２ (0<x<1, 0<y<1), LiNi ｘ Co ｙ Al １－ｘ－ｙ O ２ (0<x<1, 0<y<1), LiNi ｘ Co １－ｘ O ２ (0<x≦1) ２ (M is one or more transition metals such as Co, Ni, Fe, Mn, Cr, Ti, and V, and Y is a chalcogen element such as O or S); LiMnO ２ , LiMn ２ O ４ composite metal oxides having a spinel structure such as LiFePO ４ (LFP) and LiFeMnPO ４ LiMaPO such as (LFMP) ４(wherein Ma is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr); Li ２ MnO ３ -LiMbO ２ Lithium-excess solid solution positive electrode represented by (Mb=Mn, Co, Ni); lithium titanate (Li ４ Ti ５ O １２ ), titanium oxide (TiO ２ ); Li ｓ Ni ｔ Co ｕ Al ｖ O ２ (0.9<s<1.3, 0.9<t+u+v<1.1); LiNi ｘ Co ｙ M ｚ O ２ (M represents Mn or Al, and x, y, and z satisfy the following conditions: 0<x<1, 0<y<1, 0<z<1, and x+y+z=1), respectively. Among these, particularly preferred are lithium metal oxides such as LiFePO4, as described above. ４ (LFP) and LiFeMnPO ４ (LFMP) is preferred from the viewpoint of easily achieving the effects of the present invention. The positive electrode active material may be a compound having a surface coated thereon. Furthermore, the positive electrode active material may be a commercially available product.

[0062] The amount of the electrode active material contained in the electrode mixture is appropriately selected depending on the application of the electrode mixture, etc., but is preferably 50% by mass or more and 99.9% by mass or less of the total amount of the solid content derived from the electrode binder, the electrode active material, and the conductive additive. When the amount of the electrode active material is within this range, for example, sufficient charge / discharge capacity can be obtained, and battery performance is likely to be good.

[0063] The conductive additive contained in the electrode mixture is not particularly limited as long as it is a compound that can further increase the conductivity between the electrode active materials or between the electrode active material and a current collector. Examples of the conductive additive include acetylene black, ketjen black, carbon black, graphite powder, carbon nanofiber, carbon nanotube, and carbon fiber.

[0064] The amount of the conductive additive contained in the electrode mixture is appropriately selected depending on the type of the conductive additive, etc. From the viewpoint of improving both the conductivity and the dispersibility of the conductive additive, the amount is preferably 0.1% by mass to 15% by mass or less, more preferably 0.1% by mass to 7% by mass, and even more preferably 0.1% by mass to 5% by mass, based on the total amount of the solid content derived from the electrode binder, the electrode active material, and the conductive additive.

[0065] The electrode mixture may contain a solvent different from the solvent that the electrode binder may contain, etc. The solvent can be selected from the solvents that the binder may contain.

[0066] The total amount of solvent in the electrode mixture (including the amount of solvent in the electrode binder) is not particularly limited, but is usually preferably 10 parts by mass or more and 150 parts by mass or less per 100 parts by mass of the above-mentioned electrode active material.

[0067] The electrode mixture may further contain a dispersant, an adhesive aid, a thickener, etc., and known compounds can be used for these. Examples of the dispersant include polyvinylpyrrolidone, methylcellulose, methoxylated methylcellulose, propoxylated methylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, polypropylene oxide, gelatin, etc. Examples of the adhesive aid include poly(meth)acrylic acid, metal salts of poly(meth)acrylic acid such as sodium poly(meth)acrylate, and carboxymethyl cellulose. The amount of these is not particularly limited as long as it does not impair the purpose and effects of the present invention, but is preferably 15% by mass or less based on the total amount of the solid content derived from the electrode binder (vinylidene fluoride copolymer) and the electrode active material.

[0068] The electrode mixture may further contain additives such as phosphorus compounds, sulfur compounds, nitrogen compounds such as amine compounds and ammonium compounds, organic acids, organic esters, various silane-based, titanium-based and aluminum-based coupling agents, vinylidene fluoride polymers other than the above-mentioned vinylidene fluoride copolymers, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), and other resins. These are not particularly limited as long as they do not impair the objects and effects of the present invention, but are preferably 15 mass% or less based on the total amount of the solid content derived from the electrode binder and the electrode active material.

[0069] The electrode mixture may be prepared by mixing all of the components simultaneously, or by first mixing some of the components and then mixing the remaining components.

[0070] The viscosity of the electrode mixture is not particularly limited as long as it can prevent dripping, uneven coating, and delayed drying after coating when applying the electrode mixture to form a mixture layer, and provides good workability and applicability when preparing the mixture layer, but is preferably 50,000 mPa·s or less, and more preferably 20,000 mPa·s or less. The viscosity is a value measured at 25°C using an E-type viscometer, and is measured at a shear rate of 1 s after an incubation period of 60 seconds at 25°C using an E-type viscometer. －１ The rotor is rotated at 1000 kJ / min, and the value is measured 60 seconds after the rotor starts rotating.

[0071] 4. Electrode The above-described electrode mixture can be used to form a mixture layer of an electrode of various nonaqueous electrolyte secondary batteries. An electrode of a nonaqueous electrolyte secondary battery includes, for example, a current collector and a mixture layer disposed on the current collector. The above-described electrode mixture can be used to form the mixture layer.

[0072] Current collector: The current collector is a terminal for extracting electricity. The material of the current collector is not particularly limited, and metal foil or metal mesh of aluminum, copper, iron, stainless steel, steel, nickel, titanium, etc. can be used. Alternatively, the current collector may have a layer containing carbon black or the like formed on the surface of another medium, or may have the above-mentioned metal foil or metal mesh applied thereto.

[0073] Mixture Layer The mixture layer is a layer formed by applying the electrode mixture described above onto a current collector and solidifying it. That is, the mixture layer contains at least the vinylidene fluoride copolymer, which is the solid content of the electrode binder described above, 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.

[0074] The mixture layer contains at least the components contained in the electrode mixture described above, i.e., the solid content (vinylidene fluoride copolymer) derived from the electrode binder and the electrode active material, and may further contain various additives such as a conductive aid, a dispersant, an adhesive aid, a thickener, etc. as needed. These are the same as those described for the electrode mixture.

[0075] Here, the thickness of the mixture layer is not particularly limited, but in one example, it is preferably 1 μm or more and 1000 μm or less. The weight per unit area of ​​the mixture layer formed on one surface of the current collector is not particularly limited, and can be any weight per unit area. In one example, it is 50 g / m ２ More than 1000g / m ２ Preferably, 100 g / m or less ２ More than 500g / m ２ The following is more preferred:

[0076] The mixture layer can be formed by carrying out a step of applying the electrode mixture onto a current collector and a step of solidifying the applied mixture.

[0077] The method for applying the electrode mixture is not particularly limited, and methods such as a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, a die coating method, and a dip coating method can be used.

[0078] After application of the electrode mixture, the mixture is heated at an arbitrary temperature to dry the solvent. In one example, the drying temperature is preferably 60°C or higher and 500°C or lower, and more preferably 80°C or higher and 200°C or lower. Heating may be performed multiple times at different temperatures. The solvent in the mixture may be dried under atmospheric pressure, pressure, or reduced pressure, or may be dried in an environment such as air, nitrogen, or argon. After drying, a heat treatment may be further performed.

[0079] After the electrode mixture is applied and dried, a pressing process may be further performed. The pressing process can improve the electrode density. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

[0080] 5. Batteries The electrode binders and electrode mixtures described above can be used in electrodes of various non-aqueous electrolyte secondary batteries, as described above, but may also be used to form other layers of non-aqueous electrolyte secondary batteries.

[0081] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0082] (Methods for measuring and evaluating physical properties) In the examples and comparative examples described below, the inherent viscosity, slurry viscosity, binder solution viscosity, and molar absorption coefficient of the vinylidene fluoride copolymer were determined by the following methods. The peel strength of the electrode mixture layer was determined by the following method.

[0083] Inherent Viscosity The inherent viscosity of the vinylidene fluoride copolymer was measured as follows. First, 80 mg of the vinylidene fluoride copolymer was dissolved in 20 ml of N,N-dimethylformamide, and the viscosity was measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. Then, from the obtained value, the inherent viscosity (η ｉ ) was calculated. ｉ =(1 / C)・ln(η / η ０ In the above formula, η is the viscosity of the solution, η ０ is the viscosity of the solvent N,N-dimethylformamide alone, and C is the concentration of the vinylidene fluoride polymer in the solution, ie, 0.4 g / dl.

[0084] Slurry viscosity: Lithium iron phosphate (LFP) was DY-5F manufactured by Shenzhen Dynanonic Co., Ltd. (China), with an average particle size (D50 = 1.5 to 2.0 μm (measured by laser diffraction / scattering method) and a BET specific surface area of ​​10 to 12 m. ２ / g) was prepared. An MT3300EXII manufactured by MICROTRAC was used to measure the average particle size. Laser diffraction was used as the measurement method. LFP and a surfactant were added to water, and dispersed by ultrasonic irradiation for 5 minutes to prepare a measurement sample. The refractive index of the LFP was set to 1.74, and the shape was aspherical. D50 was calculated from the particle size distribution based on the number. A MONOSORB manufactured by Quantachrome Instruments was used to measure the specific surface area. The sample was measured after drying at 200°C. A mixed gas of nitrogen (20%) and helium (80%) was used for the measurement. Calculation was performed using the BET single-point method (relative pressure x = 0.2) at liquid nitrogen temperature. Carbon black (Super-P (SP) manufactured by Timical Japan, BET specific surface area 57-67 m) was used as the conductive additive. ２ / g) was prepared. In addition, a carbon nanotube (CNT) dispersion (LB107-44, manufactured by Jiangsu Cnano Technology Co., Ltd. (China)) was prepared as another conductive additive. The dispersion contained multi-walled carbon nanotubes (average diameter 7 to 11 nm, BET specific surface area 200 to 280 m ２ The dispersion liquid was a CNT concentration of 4.3% by mass and a dispersion material concentration of 1.08% by mass. Battery-grade N-methylpyrrolidone manufactured by Nippon Refine Co., Ltd. (water content: 0.03% or less, purity: 99.8% or more, pH of 10% aqueous solution: 6.5 to 8.0, specific gravity (25°C / 4°C): 1.027 to 1.032, absorbance: 0.15 or less) was prepared and used in an environment controlled at a dew point of -40°C.

[0085] LFP, SP, CNT, and a vinylidene fluoride copolymer prepared in an example or comparative example were mixed in a mass ratio of LFP / SP / CNT / vinylidene fluoride copolymer = 100 / 1 / 1 / 2.5, and NMP was added appropriately to prepare a slurry with a nonvolatile content of 52 mass%. More specifically, the entire amount of SP and CNT dispersion was mixed and kneaded for 2 minutes using a Thinky Mixer ARE-310 at 800 rpm and 2000 rpm. LFP and NMP were then added, and the mixture was kneaded for 4 minutes at 800 rpm and 2000 rpm. Further, the NMP solution of vinylidene fluoride copolymer and NMP were added, and the mixture was kneaded for 4 minutes under conditions of rotation at 800 rpm and revolution at 2000 rpm to obtain the desired slurry.

[0086] The viscosity of the resulting slurry was measured using a Toki Sangyo RE80 viscometer. 0.55 mL of slurry was used for the measurement. The measurement temperature was 25°C, and the sample was preheated for 1 minute before measurement. A 3° x R14 cone was used. The measurement program consisted of 120 seconds at 0.1 rpm, 120 seconds at 0.2 rpm, 120 seconds at 0.5 rpm, and 120 seconds at 0.5 rpm, and the values ​​at these times were taken as the slurry viscosity.

[0087] Binder Solution Viscosity The vinylidene fluoride copolymers prepared in the Examples and Comparative Examples were dissolved in NMP at 50°C for 3 hours to prepare an NMP solution with a concentration of 4.5% by mass of the vinylidene fluoride copolymer. This was allowed to stand at room temperature (15-25°C) for 24 hours to obtain a binder solution. The viscosity of the binder solution was measured using an RE80 viscometer manufactured by Toki Sangyo Co., Ltd. 1.1 mL of binder solution was used for the measurement. The measurement temperature was 25°C, and the sample was preheated for 1 minute before measurement. A 1.34° x R24 cone was used. The value after 300 seconds at 1 rpm was used as the viscosity of the binder solution.

[0088] Calculation of Molar Absorption Coefficient The vinylidene fluoride copolymers prepared in the Examples and Comparative Examples were each dissolved in N-methyl-2-pyrrolidone (NMP) at 70°C for 5 hours to obtain a solution with a vinylidene fluoride copolymer concentration of 5% by mass. The solution was allowed to stand at room temperature (15-25°C) for 2 days after preparation, and the UV-visible absorption spectrum was measured. A quartz cell with an optical path length of 1 cm was used for measurement. The molar absorption coefficient was calculated from the absorption intensity at a wavelength of 346 nm and the molar concentration of the solution according to the Lambert-Beer law. The molecular weight of each vinylidene fluoride copolymer was determined using a differential refractometer for GPC, with polystyrene used as a reference, as a relative molecular weight. Dimethylacetamide was used as the solvent for GPC measurement.

[0089] ・90° peel strength 2 x 5 cm ２ A double-sided tape (Nichiban Co., Ltd., Nicetack) was attached to the mixture layer side of the cut electrode, and pressed at 4 MPa for 20 seconds to obtain a test specimen. Measurements were performed using an A&D Co., Ltd., Tensilon RTF-1210, at a tensile speed of 10 mm / min. The measurement environment was a room temperature of 25°C and a dew point of -20°C.

[0090] Example 1 Preparation of vinylidene fluoride copolymer A 2-liter autoclave was charged with 1,200 g of ion-exchanged water, 0.2 g of Metrose (registered trademark) SM-100 (Shin-Etsu Chemical Co., Ltd.), 0.92 g of a 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 400 g of vinylidene fluoride (VDF), and 0.2 g of monomethyl maleate (MMM). The mixture was heated to 45°C, and then maintained at 45°C, allowing the reaction to proceed until the pressure decreased. The resulting polymer slurry of VDF / MMM copolymer was dehydrated, washed with water, dehydrated, and then dried at 80°C for 20 hours to obtain a polymer powder.

[0091] [Preparation of Electrode Mixture] LFP, SP, CNT, and NMP were prepared in the same manner as those used in the above-described method for measuring the slurry viscosity. Then, LFP, SP, CNT, and vinylidene fluoride copolymer were mixed so that the mass ratio of LFP / SP / CNT / vinylidene fluoride copolymer was 100 / 1 / 1 / 2.5 and the nonvolatile content was 52 mass%. More specifically, the entire amount of SP and CNT dispersion was mixed and kneaded for 2 minutes using a Thinky Mixer ARE-310 under conditions of 800 rpm rotation and 2000 rpm revolution. LFP and NMP were added thereto, and the mixture was kneaded for 4 minutes under conditions of 800 rpm rotation and 2000 rpm revolution. Further, an NMP solution (6% by mass) of vinylidene fluoride copolymer was added, and the mixture was kneaded for 4 minutes under conditions of rotation at 800 rpm and revolution at 2000 rpm to obtain the desired mixture slurry.

[0092] [Production of Electrode] The obtained electrode mixture was applied to a carbon-coated Al foil having a thickness of 16 μm using a desktop coater, and dried at 110° C. for 30 minutes. The coating amount of the electrode was 200 g / m ２ It was decided.

[0093] (Examples 2 to 7 and Comparative Examples 1 and 2) A vinylidene fluoride copolymer was prepared in the same manner as in Example 1, except that the type and amount of each monomer used in preparing the vinylidene fluoride copolymer was changed as shown in Table 1. Then, an electrode mixture was prepared in the same manner as in Example 1, and an electrode was obtained. In Table 1, VDF represents vinylidene fluoride, MMM represents monomethyl maleate, HFP represents hexafluoropropylene, and APS represents acryloyloxypropyl succinic acid.

[0094] (Example 8) A vinylidene fluoride homopolymer and a vinylidene fluoride copolymer (VDF / MMM) were each prepared in the same manner as in Example 1, and then mixed in a mass ratio of 1:1. An electrode mixture was then prepared in the same manner as in Example 1, and an electrode was obtained. In Example 8, the molar absorption coefficient was measured for a mixture of two vinylidene fluoride copolymers.

[0095] (result)

[0096] As shown in Table 1 above, when the copolymerization ratio of monomethyl maleate (MMM) in the vinylidene fluoride copolymer increased, the ratio of the slurry viscosity to the binder solution viscosity (slurry viscosity / binder solution viscosity) became 5 or more (Comparative Example 2). In this case, the high slurry viscosity made it difficult to form a mixture layer. On the other hand, even if the ratio of the slurry viscosity to the binder solution viscosity (slurry viscosity / binder solution viscosity) was less than 5, the molar absorption coefficient was 1800 M －1 cm －１ When the bonding strength was less than 100%, the peel strength was low (Comparative Example 1). It is believed that the vinylidene fluoride copolymer did not contain sufficient carbonyl groups, carboxyl groups, etc., making it difficult to obtain sufficient adhesive strength to the current collector.

[0097] In contrast, when the ratio (slurry viscosity / binder solution viscosity) is less than 5 and the molar absorption coefficient is 1800 M －1 cm －１ When the thickness was above this, the mixture layer was easily formed, and the resulting mixture layer had high peel strength from the current collector.

[0098] This application claims priority from Japanese Patent Application No. 2024-056552, filed March 29, 2024, the entire contents of which are incorporated herein by reference.

[0099] According to the present invention, there is provided a vinylidene fluoride copolymer that, even when mixed with an electrode active material such as LFP or LFMP to prepare an electrode mixture, is unlikely to increase in slurry viscosity and can bind the electrode active material with high adhesive strength. The vinylidene fluoride copolymer is very useful in the field of manufacturing various batteries.

Claims

1. A vinylidene fluoride copolymer containing structural units derived from vinylidene fluoride, and lithium iron phosphate (having an average particle size D50 of 1.5 to 2.0 μm as measured by the laser diffraction / scattering method and a BET specific surface area of ​​10 to 12 m) ２ / g), carbon black (BET specific surface area of ​​57 to 67 m ２ / g), carbon nanotubes (average diameter 7 to 11 nm, BET specific surface area 200 to 280 m ２ A mixture of the vinylidene fluoride copolymer and the vinylidene fluoride copolymer in a mass ratio of 100:1:1:2.5 was dispersed in N-methyl-2-pyrrolidone to a solid content of 52 mass %. The slurry was measured at 25°C using an RE80 type viscometer at a shear rate of 1 s －１ and a slurry viscosity measured 120 seconds after the start of stirring at 25°C using an RE80 type viscometer at a shear rate of 3.83 s. The viscosity of the N-methyl-2-pyrrolidone solution having a vinylidene fluoride copolymer concentration of 4.5 mass % was measured 24 hours after the preparation. －１ the binder solution viscosity measured 300 seconds after the start of stirring satisfies the relationship of the slurry viscosity / the binder solution viscosity<5, and the molar absorption coefficient at a wavelength of 346 nm of an N-methyl-2-pyrrolidone solution having a concentration of the vinylidene fluoride copolymer of 5 mass %, which is obtained by dissolving the vinylidene fluoride copolymer in N-methyl-2-pyrrolidone at 70° C. for 5 hours, is 1800 M or less after 2 days from the preparation. －１ cm －１ That is all, vinylidene fluoride copolymer.

2. The vinylidene fluoride copolymer according to claim 1, comprising a structural unit derived from a compound represented by the following general formula (1) and / or a structural unit derived from a compound represented by the following general formula (2): (In the general formulas (1) and (2), R １ and R ４ are each independently a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or R １０ -O-C(=O)- (where R １０ R represents a group represented by the formula (I) ２ , R ３ , R ５ and R ６ each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and X represents a divalent atomic group having a molecular weight of 500 or less and having 1 to 20 atoms in the main chain.

3. The vinylidene fluoride copolymer according to claim 1, further comprising a structural unit derived from a fluorine-containing vinyl compound (excluding said vinylidene fluoride) containing a fluorine atom and a vinyl group.

4. The vinylidene fluoride copolymer according to claim 3, wherein the fluorine-containing vinyl compound is one or more compounds selected from the group consisting of hexafluoropropylene, tetrafluoroethylene, and chlorotrifluoroethylene.

5. The vinylidene fluoride copolymer according to claim 3, wherein the amount of the structural units derived from the fluorine-containing vinyl compound is 5 parts by mole or less per 100 parts by mole of the structural units derived from the vinylidene fluoride.

6. A binder for electrodes, comprising the vinylidene fluoride copolymer according to any one of claims 1 to 5.

7. An electrode mixture comprising the electrode binder according to claim 6 and an electrode active material.

8. The electrode mixture according to claim 7, wherein the electrode active material is lithium iron phosphate and / or lithium iron manganese phosphate.

9. An electrode comprising the vinylidene fluoride copolymer according to any one of claims 1 to 5 and an electrode active material.

10. The electrode of claim 9, wherein the electrode active material is lithium iron phosphate and / or lithium iron manganese phosphate.

11. A battery comprising the electrode of claim 9.

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