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

Abstract

The purpose of the present invention is to provide a vinylidene fluoride copolymer with which, even in cases where an electrode mixture is prepared by mixing the vinylidene fluoride copolymer with an electrode active material such as lithium iron phosphate or lithium iron manganese phosphate, the slurry viscosity is not likely to excessively increase and the electrode active material can be bound with high bonding strength. This vinylidene fluoride copolymer includes a constituent unit that is derived from vinylidene fluoride, and has an IR absorbance ratio of 0.15 or less, the IR absorbance ratio being calculated by formula (IR absorbance ratio) = AC=O / AC-H (wherein AC=O represents the absorbance of a peak derived from C=O expansion and contraction, and AC-H represents the absorbance of a peak derived from C-H expansion and contraction) from the absorption spectrum determined by Fourier transform infrared spectroscopy. If an N-methyl-2-pyrrolidone solution having a concentration of the vinylidene fluoride copolymer of 5 mass% is prepared, the molar absorption coefficient thereof at a wavelength of 346 nm after standing for two days from the preparation is 2,000 M-1cm-1 to 5,000 M-1cm-1 inclusive.

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 reduce the amount of vinylidene fluoride copolymers having high adhesive strength, as described in the above-mentioned Patent Documents 1 and 2. However, when these vinylidene fluoride copolymers are mixed with LFP or LFMP to prepare an electrode mixture, 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, wherein the IR absorbance ratio calculated from an absorption spectrum measured by Fourier transform infrared spectroscopy using the following formula is 0.15 or less: IR absorbance ratio = A Ｃ＝Ｏ / A Ｃ－Ｈ (In the formula, A Ｃ＝Ｏ represents the absorbance of the peak due to C═O stretching, and A Ｃ－Ｈ represents the absorbance of the peak due to C-H stretching.) The vinylidene fluoride copolymer was dissolved in N-methyl-2-pyrrolidone at 70°C over 5 hours to prepare an N-methyl-2-pyrrolidone solution having a concentration of the vinylidene fluoride copolymer of 5% by mass. After standing for 2 days from the preparation, the molar absorption coefficient at a wavelength of 346 nm was 2000 M －１ cm －１ More than 5000M －１ cm －１ The present invention provides the following vinylidene fluoride copolymer: [2] The present invention provides the vinylidene fluoride copolymer according to [1], which contains a structural unit derived from a carbonyl group-containing vinyl compound represented by the following general formula (1): (In the general formula (1), R １ represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom; R ２ and R ３each independently represent a hydrogen atom, a chlorine atom, a fluorine atom, or an alkyl group having from 1 to 6 carbon atoms which may be substituted with a fluorine atom. [3] The vinylidene fluoride copolymer according to [2], wherein the content of the structural units derived from the carbonyl group-containing vinyl compound is less than 0.1 parts by mole per 100 parts by mole of the total of the structural units derived from the vinylidene fluoride. [4] The vinylidene fluoride copolymer according to [2] or [3], further comprising a structural unit derived from a fluorine-containing vinyl compound (excluding the vinylidene fluoride and the carbonyl group-containing vinyl compound) which contains a fluorine atom and a vinyl group. [5] The vinylidene fluoride copolymer according to [4], wherein the fluorine-containing vinyl compound is one or more compounds selected from the group consisting of hexafluoropropylene, tetrafluoroethylene, and chlorotrifluoroethylene. [6] The present invention provides the vinylidene fluoride copolymer according to [4] or [5], wherein the amount of the structural units derived from the fluorine-containing vinyl compound is 5 molar parts or less per 100 molar parts of the structural units derived from the vinylidene fluoride. [7] The present invention provides an electrode binder comprising the vinylidene fluoride copolymer according to any one of [1] to [6] above. [8] The present invention provides an electrode mixture comprising the electrode binder according to [7] above and an electrode active material. [9] The present invention provides the electrode mixture according to [8], wherein the electrode active material is lithium iron phosphate and / or lithium iron manganese phosphate.

[10] The present invention provides an electrode comprising the vinylidene fluoride copolymer according to any one of [1] to [6] above and an electrode active material.

[11] The present invention provides the electrode according to

[10] , wherein the electrode active material is lithium iron phosphate and / or lithium iron manganese phosphate.

[12] The present invention provides a battery comprising the electrode according to

[10] or

[11] 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 (i) First embodiment A vinylidene fluoride copolymer according to a first embodiment 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 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 an attempt is made to reduce the viscosity of the electrode mixture slurry, 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 making the vinylidene fluoride copolymer satisfy the following characteristics (i) and (ii): (i) The IR absorbance ratio of the vinylidene fluoride copolymer calculated from the absorption spectrum measured by Fourier transform infrared spectroscopy using the following formula is 0.15 or less: IR absorbance ratio = A Ｃ＝Ｏ / A Ｃ－Ｈ (In the formula, A Ｃ＝Ｏ represents the absorbance of the peak due to C═O stretching, and A Ｃ－Ｈ represents the absorbance of the peak due to C-H stretching. (ii) A solution of vinylidene fluoride copolymer in N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") having a concentration of 5% by mass, obtained by dissolving vinylidene fluoride copolymer in N-methyl-2-pyrrolidone at 70°C over 5 hours, has a molar absorption coefficient of 2000 M at a wavelength of 346 nm two days after preparation. －１ cm －１More than 5000M －１ cm －１ The following is the result.

[0013] A, as specified in (i) above Ｃ＝Ｏ represents the relative amount of comonomers containing carbonyl groups (including those that are carboxyl groups) in the vinylidene fluoride copolymer. Ｃ－Ｈ indicates the amount of C—H bonds in the vinylidene fluoride copolymer, i.e., the relative amount of all structural units constituting the vinylidene fluoride copolymer. In other words, the IR absorbance ratio specified in (i) above indicates the extent to which comonomers having carbonyl groups are present in the vinylidene fluoride copolymer. When a large amount of comonomers containing carbonyl groups is contained, the comonomers interact with functional groups on the surface of the electrode active material, such as LFP, and the slurry viscosity tends to increase when the resulting electrode mixture is used. In contrast, since the vinylidene fluoride copolymer of this embodiment has an IR absorbance ratio of 0.15 or less, it can be said that the amount of comonomers containing carbonyl groups is relatively small.

[0014] On the other hand, the molar absorption coefficient two days after preparation of the NMP solution of vinylidene fluoride copolymer 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. However, as a result of extensive research, the inventors have found that when the vinylidene fluoride copolymer satisfies the above (i) and the amount of HF removed is within a certain range (when the above (ii) is satisfied), the slurry viscosity is unlikely to increase excessively, and when an electrode is formed, the adhesive strength with the current collector, etc. is good. The vinylidene fluoride copolymer of this embodiment will be described in detail.

[0015] 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.

[0016] On the other hand, the comonomer copolymerized with vinylidene fluoride is not particularly limited as long as it can achieve the above (i) IR absorbance ratio and (ii) molar absorption coefficient. The vinylidene fluoride copolymer may contain only one type of comonomer-derived structural unit, or may contain two or more types. However, at least one of the comonomer-derived structural units contains a carbonyl group, and more preferably contains a carbonyl group and a carboxy group.

[0017] Examples of comonomers having a carbonyl group include compounds having a carbonyl group and a vinyl group in the molecule. From the viewpoint of achieving the above (i) IR intensity ratio and (ii) molar absorption coefficient, it is preferable that at least a portion of the comonomers be compounds having two or more carbonyl groups in the molecule. The carbonyl group-containing compound may contain three or more carbonyl groups in the molecule, but from the viewpoint of easily achieving the above (i) IR absorbance ratio and (ii) molar absorption coefficient, it is preferable that the number of carbonyl groups is two. Examples of such compounds include carbonyl group-containing vinyl compounds represented by the following general formula (1). The vinylidene fluoride copolymer may contain only one type of structural unit derived from the carbonyl group-containing vinyl compound, or may contain two or more types. In the above general formula (1), R １represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom. Examples of the alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom include a methyl group, an ethyl group, a propyl group, a butyl group, etc. Among these, R １ is preferably a hydrogen atom, a methyl group, or an ethyl group, from the viewpoint that steric hindrance is unlikely to occur during copolymerization of vinylidene fluoride with a carbonyl group-containing vinyl compound.

[0018] In addition, in the general formula (1), R ２ and R ３ each independently represents a hydrogen atom, a chlorine atom, a fluorine atom, or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom. The alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom is the same as the above-mentioned R １ Among these, R ２ and R ３ are preferably a hydrogen atom or a methyl group, respectively, from the viewpoint that steric hindrance is unlikely to occur during copolymerization of vinylidene fluoride with a carbonyl group-containing vinyl compound.

[0019] Specific examples of the carbonyl group-containing vinyl compound include unsaturated dibasic acids such as maleic acid, fumaric acid, and citraconic acid; and unsaturated dibasic acid monoesters such as monomethyl maleate, monoethyl maleate, monomethyl fumarate, monoethyl fumarate, monomethyl citraconic acid, and monoethyl citraconic acid. Among these, maleic acid and monomethyl maleate are preferred from the viewpoints of good reactivity with vinylidene fluoride and ease of achieving the above-mentioned IR absorbance ratio and molar extinction coefficient.

[0020] The amount of the structural units derived from the carbonyl group-containing vinyl compound in the vinylidene fluoride copolymer is not particularly limited as long as the above IR absorbance ratio and molar extinction coefficient can be achieved. The amount of the structural units derived from the carbonyl group-containing vinyl compound in the vinylidene fluoride copolymer is preferably less than 0.1 molar parts per 100 molar parts of the structural units derived from vinylidene fluoride, more preferably less than 0.08 molar parts, and even more preferably 0.02 molar parts or more and 0.05 molar parts or less. When the amount of the structural units derived from the carbonyl group-containing vinyl compound is 0.02 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 the structural units derived from the carbonyl group-containing vinyl compound is less than 0.1 molar parts, particularly less than 0.08 molar parts, the increase in slurry viscosity when the electrode binder is mixed with the electrode active material is more likely to be suppressed. The amount of the structural units 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.

[0021] The vinylidene fluoride copolymer 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 corresponding to vinylidene fluoride and the carbonyl group-containing vinyl compound). The vinylidene fluoride copolymer may contain only one type of fluorine-containing vinyl compound, or may contain two or more types.

[0022] The structure of the fluorine-containing vinyl compound is not particularly limited as long as it does not impair the purpose and effects of this embodiment. 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 and 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The weight-average molecular weight of the vinylidene fluoride copolymer is not particularly limited, but is preferably 1,000,000 or more, and 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 measured by gel permeation chromatography (GPC). This value is determined using a refractive index (RI) detector. The solvent used in the GPC measurement is dimethylacetamide (DMAc).

[0027] In addition, in the vinylidene fluoride copolymer, as specified in (i), the IR absorbance ratio (A Ｃ＝Ｏ / A Ｃ－Ｈ The IR absorbance ratio can be measured by preparing a sample in a conventional manner and using a known infrared spectrophotometer. Ｃ＝Ｏ is the 1630 to 1810 cm of the FT-IR spectrum obtained by measurement with a transmission infrared spectrophotometer. －１ The maximum value in the above A Ｃ－Ｈ is the 2860 to 3320 cm of the FT-IR spectrum －１ The IR absorbance ratio is preferably 0.03 or more and 0.15 or less, and more preferably 0.05 or more and 0.10 or less. The IR absorbance ratio is preferably an average value of the absorbance ratios obtained when FT-IR spectra are acquired multiple times, and is preferably an average value of the absorbance ratios obtained when FT-IR spectra are acquired three or more times, for example.

[0028] Furthermore, in the vinylidene fluoride copolymer, the molar absorption coefficient specified in (ii) is 2000 M －１ cm －１ More than 5000M －１ cm －１The molar absorption coefficient is specified as follows. First, vinylidene fluoride copolymer is dissolved in NMP at 70°C for 5 hours to prepare an NMP solution with a vinylidene fluoride copolymer concentration of 5% by mass. The solution is allowed to stand for 2 days after preparation, and the UV-visible absorption spectrum is measured. The environment in which the solution is allowed to stand is not particularly limited, but is usually 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. Then, the absorption intensity at a wavelength of 346 nm is 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 described above is used to calculate the number of moles of the vinylidene fluoride copolymer. The molar absorption coefficient is 2000 M －１ cm －１ More than 5000M －１ cm －１ Preferably less than 2500M －１ cm －１ Over 4000M －１ cm －１ The following is more preferred:

[0029] 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 or 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 ０ In the above formula, η is the viscosity of the solution, η ０is 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.

[0030] 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).

[0031] (ii) Second Embodiment The vinylidene fluoride copolymer of this embodiment contains structural units derived from vinylidene fluoride and structural units derived from a carbonyl-containing vinyl compound containing two carbonyl groups (including those that serve as carboxy groups) in one molecule, as represented by the general formula (I) described below. The content of the structural units derived from the carbonyl group-containing vinyl compound is less than 0.1 parts by mole per 100 parts by mole of the total of the structural units derived from vinylidene fluoride. The vinylidene fluoride copolymer may further contain structural units derived from other compounds, as necessary.

[0032] As described above, when a known vinylidene fluoride copolymer 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 an attempt is made to reduce the viscosity of the electrode mixture slurry, it is difficult to sufficiently increase the binding strength to the current collector.

[0033] Generally, when an electrode mixture is prepared by mixing a vinylidene fluoride copolymer containing a large amount of a comonomer component containing a carbonyl group (e.g., a carboxy group) with an electrode active material such as LFP, the carbonyl groups in the comonomer tend to interact with the functional groups on the surface of the electrode active material such as LFP, which increases the slurry viscosity of the electrode mixture. On the other hand, reducing the amount of comonomer containing a carbonyl group makes it difficult to obtain sufficient adhesive strength. Furthermore, increasing the amount of comonomer containing a carbonyl group in the vinylidene fluoride copolymer tends to shorten the polymer chain of the vinylidene fluoride copolymer. In other words, it is difficult to include structural units derived from vinyl compounds containing carbonyl groups in high-molecular-weight polymers.

[0034] In contrast, the vinylidene fluoride copolymer of this embodiment contains only a small amount of structural units derived from a specific carbonyl group-containing vinyl compound in the vinylidene fluoride copolymer. The inventors have found that by adjusting the chemical structure of the carbonyl group-containing vinyl compound and its amount in the vinylidene fluoride copolymer, high adhesive strength to a current collector or the like can be obtained without excessively increasing the slurry viscosity. The vinylidene fluoride copolymer of this embodiment will be described in detail below.

[0035] As described above, the vinylidene fluoride copolymer of this embodiment is a copolymer with a carbonyl-containing vinyl compound containing two carbonyl groups (including those that are carboxy groups) in one molecule. 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.

[0036] The vinylidene fluoride copolymer of the present embodiment contains a structural unit derived from a carbonyl group-containing vinyl compound represented by the following general formula (I): The vinylidene fluoride copolymer may contain only one type of structural unit derived from the carbonyl group-containing vinyl compound, or may contain two or more types of structural units derived from the carbonyl group-containing vinyl compound. In the above general formula (I), R １ represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom. Examples of the alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom include a methyl group, an ethyl group, a propyl group, a butyl group, etc. Among these, R １ is preferably a hydrogen atom, a methyl group, or an ethyl group, from the viewpoint that steric hindrance is unlikely to occur during copolymerization of vinylidene fluoride with a carbonyl group-containing vinyl compound.

[0037] In addition, in general formula (I), R ２ and R ３ each independently represents a hydrogen atom, a chlorine atom, a fluorine atom, or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom. The alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom is the same as the above-mentioned R １ Among these, R ２ and R ３ are preferably a hydrogen atom or a methyl group, respectively, from the viewpoint that steric hindrance is unlikely to occur during copolymerization of vinylidene fluoride with a carbonyl group-containing vinyl compound.

[0038] Specific examples of the carbonyl group-containing vinyl compound represented by general formula (I) are the same as the specific examples of the carbonyl group-containing vinyl compound represented by general formula (I) described above. Among these, maleic acid and monomethyl maleate are preferred from the viewpoint of good reactivity with vinylidene fluoride.

[0039] The amount of the structural unit derived from the carbonyl group-containing vinyl compound in the vinylidene fluoride copolymer may be less than 0.1 molar parts relative to 100 molar parts of the structural unit derived from vinylidene fluoride, preferably 0.08 molar parts or less, and more preferably 0.02 molar parts or more and 0.05 molar parts or less. As mentioned above, if the amount of the structural unit derived from the carbonyl group-containing vinyl compound is less than 0.1 molar parts, when mixed with LFP or the like to form an electrode mixture, the slurry viscosity is unlikely to increase excessively. The amount of the structural unit derived from the carbonyl group-containing vinyl compound in the vinylidene fluoride copolymer is １９ F-NMR spectrum and １ It can be determined from H-NMR spectrum or the like.

[0040] The vinylidene fluoride copolymer of this embodiment may further contain structural units derived from compounds other than vinylidene fluoride and the above-mentioned 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 corresponding to vinylidene fluoride and the above-mentioned carbonyl group-containing vinyl compound). The vinylidene fluoride copolymer may contain only one type of fluorine-containing vinyl compound, or may contain two or more types. The structure and content of the fluorine-containing vinyl compound are the same as those of the fluorine-containing vinyl compound of the first embodiment.

[0041] Furthermore, the vinylidene fluoride copolymer may further contain a structural unit derived from another compound, and the structure and content thereof are the same as those explained in the first embodiment.

[0042] The weight-average molecular weight of the vinylidene fluoride copolymer of this embodiment 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. The method for measuring the weight-average molecular weight is the same as the method described in the first embodiment.

[0043] In addition, the IR absorbance ratio (=A) calculated from the absorption spectrum of the vinylidene fluoride copolymer measured by Fourier transform infrared spectroscopy (FT-IR) using the above formula is Ｃ＝Ｏ / A Ｃ－Ｈ ) is preferably 0.15 or less, more preferably 0.03 or more and 0.15 or less, and more preferably 0.05 or more and 0.10 or less. Ｃ＝Ｏ / A Ｃ－Ｈ In the above formula, A Ｃ＝Ｏ and A Ｃ－Ｈ is obtained by the method described in the first embodiment.

[0044] The vinylidene fluoride copolymer is obtained by dissolving the vinylidene fluoride copolymer in N-methyl-2-pyrrolidone at 70° C. for 5 hours. When an N-methyl-2-pyrrolidone solution containing 5% by mass of the vinylidene fluoride copolymer is prepared and the molar absorption coefficient at a wavelength of 346 nm is measured 2 days after the preparation, the molar absorption coefficient is 2000 M －１ cm －１ More than 5000M －１ cm －１ Preferably, it is 2500M or less. －１ cm －１ Over 4000M －１ cm －１ The following is more preferable. From the viewpoint of the stability of the vinylidene fluoride copolymer, the molar absorption coefficient is preferably such that the amount of HF removed is small. On the other hand, when the vinylidene fluoride copolymer satisfies the above IR absorbance ratio and the amount of HF removed is within a certain range, the slurry viscosity is unlikely to increase excessively, and when an electrode is formed, the adhesive strength with the current collector and the like is likely to be good. The molar absorption coefficient is determined by the method described in the first embodiment.

[0045] Here, the inherent viscosity of the vinylidene fluoride copolymer of this embodiment 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 is likely 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. Inherent viscosity (η ｉ ) is obtained by the method described in the first embodiment.

[0046] The melting point of the vinylidene fluoride copolymer of this embodiment 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. If the melting point of the vinylidene fluoride copolymer is 150°C or higher, when an electrode binder (vinylidene fluoride copolymer) is used in an electrode, the copolymer is less likely to swell with an electrolyte solution, and the performance of the resulting battery is likely to be good. On the other hand, if 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 is determined by the method described in the first embodiment.

[0047] (Method for Preparing Vinylidene Fluoride Copolymer) The vinylidene fluoride copolymer of any of the above-mentioned embodiments can be prepared by copolymerizing vinylidene fluoride with the above-mentioned carbonyl group-containing vinyl compound or 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.

[0048] 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).

[0049] 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).

[0050] 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).

[0051] 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.

[0052] 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 １０ are 54.6°C and 40.5°C, respectively (see NOF Corporation product catalog). Therefore, in polymerizations using t-butyl peroxypivalate and diisopropyl peroxydicarbonate as polymerization initiators, the polymerization temperature T is appropriately selected within the ranges of 29.6°C≦T≦79.6°C and 15.5°C≦T≦65.5°C, respectively. The polymerization time is not particularly limited, but is preferably 100 hours or less in consideration of productivity and the like. The polymerization is usually carried out under increased pressure, preferably 2.0 to 10.0 MPa-G.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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., lithium 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.

[0060] 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 active material for a negative electrode (negative electrode active material) or active material for a positive electrode (positive electrode active material) can be used.

[0061] Examples of negative electrode active materials 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.

[0062] 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 and LiFeMnPO ４ LiMaPO etc. 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), etc. Among these, particularly preferred are lithium metal oxides such as LiFePO 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] The electrode mixture may further contain a dispersant, an adhesive aid, a thickener, etc., and known compounds can be used for these. Examples of the dispersant include polyvinylpyrrolidone, methyl cellulose, methoxylated methyl cellulose, propoxylated methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, polypropylene oxide, gelatin, etc. Examples of the adhesive aid include poly(meth)acrylic acid, metal salts of poly(meth)acrylic acid such as sodium poly(meth)acrylate, carboxymethyl cellulose, etc. The amount of these is not particularly limited as long as it does not impair the object and effect of the present invention, but is 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 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.

[0070] 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.

[0071] The viscosity of the electrode mixture slurry 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. However, it is preferably 50,000 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. －１The rotor is rotated at 120 s, and the value is measured 120 seconds after the rotor starts rotating.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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:

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0083] (Methods for measuring and evaluating physical properties) In the examples and comparative examples described below, the inherent viscosity, IR absorbance, and molar absorption coefficient of vinylidene fluoride copolymers were determined by the following methods. The slurry viscosity of the electrode mixture and the peel strength of the mixture layer using the same were also determined by the following methods.

[0084] 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.

[0085] IR absorbance ratio 30 mg of powdered vinylidene fluoride copolymer was sandwiched between aluminum foil coated with a release agent, placed in a press, and preheated at 200°C for 60 seconds. The pressure of the press was then increased to 10 MPa over 30 seconds, and pressed for 20 seconds. The resulting thick film was cooled and used as a sample for IR measurement. Three samples were prepared for each sample. These were measured using a transmission infrared spectrophotometer (Infrared Spectrophotometer 4100 Type A manufactured by JASCO Corporation) to obtain an FT-IR spectrum. The IR absorbance ratio was determined as the average value calculated using the following formula from the FT-IR spectra of each of the three pressed films. The IR absorbance ratio was the average value of three measurements. IR absorbance ratio = A Ｃ＝Ｏ / A Ｃ－Ｈ (In the formula, A Ｃ＝Ｏ is the absorbance of the peak due to C═O stretching (1630-1810 cm －１ represents the maximum value at Ｃ－Ｈ is the absorbance of the peak due to C-H stretching (2860-3320 cm －１ (represents the maximum value in

[0086] Calculation of Molar Absorption Coefficient Each vinylidene fluoride copolymer was dissolved in N-methyl-2-pyrrolidone (NMP) at 70°C for 5 hours to obtain a solution with a concentration of 5% by mass. The solution was allowed to stand at room temperature (15°C to 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.

[0087] Slurry Viscosity The slurry viscosity of the electrode mixtures prepared in the Examples and Comparative Examples 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 R12 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 60 seconds at 1 rpm, and the value at 60 seconds at 1 rpm was used.

[0088] ・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.

[0089] 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.

[0090] [Preparation of electrode mixture] Lithium iron phosphate (LFP) having an average particle diameter D50 of 2.0 to 3.5 μm and a specific surface area of ​​12 m ２ / g was prepared. A MICROTRAC MT3300EXII 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 1.74, and the shape was aspherical. D50 was calculated from the particle size distribution based on the number. A Quantachrome Instruments MONOSORB was used to measure the specific surface area. The sample was dried at 200°C and then measured. A mixed gas of nitrogen (20%) and helium (80%) was used for the measurement. Calculations were 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 Co., Ltd.) was prepared as a conductive additive. A carbon nanotube (CNT) dispersion (LB107-44 manufactured by Jiangsu Cnano Technology Co., Ltd. (China)) was also prepared as another conductive additive. This dispersion was a dispersion of multi-walled carbon nanotubes, with a CNT concentration of 4.3% by mass and a dispersion material content of 1.08% by mass. Battery-grade N-methylpyrrolidone (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) manufactured by Nippon Refine Co., Ltd. was prepared and used in an environment controlled at a dew point of -40°C.

[0091] 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% by 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 rotation and 2000 rpm revolution. LFP and NMP were added thereto and kneaded for 4 minutes at 800 rpm rotation and 2000 rpm revolution. Further, an NMP solution of vinylidene fluoride copolymer and NMP were added and kneaded for 4 minutes at 800 rpm rotation and 2000 rpm revolution to obtain the desired electrode 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 6 and Comparative Examples 1 to 3) A vinylidene fluoride copolymer was prepared in the same manner as in Example 1, except that the amount (mass ratio) of each monomer charged when 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 7) 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 at 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 7, the molar absorption coefficient and IR intensity ratio were measured for a mixture of two vinylidene fluoride copolymers.

[0095]

[0096] As shown in Table 1 above, the absorbance A of the peak due to C═O stretching in the vinylidene fluoride copolymer Ｃ＝Ｏ and the absorbance A of the peak due to C-H stretchingＣ－Ｈ The ratio of (A Ｃ＝Ｏ / A Ｃ－Ｈ When the molar absorption coefficient at a wavelength of 346 nm was 5000 Mcm, the slurry viscosity exceeded 50,000 mPa·s (Comparative Example 3). It is believed that the high proportion of the comonomer containing a carbonyl group in each vinylidene fluoride copolymer facilitated interaction with the electrode active material, resulting in an increase in the slurry viscosity. －１ Even when the molar absorption coefficient was over 2000 Mcm, the slurry viscosity exceeded 50,000 mPa·s (Comparative Example 2). It is considered that the vinylidene fluoride copolymer was prone to excessive dehydrofluorination, resulting in an increase in the slurry viscosity. －１ When the thickness was less than 100 μm, the peel strength was low (Comparative Example 1). This is thought to be because the vinylidene fluoride (co)polymer did not contain a carbonyl group or the like, and therefore was difficult to bond to the current collector.

[0097] In contrast, the absorbance ratio is 0.15 or less, and the molar absorption coefficient at a wavelength of 346 nm is 2000 Mcm －１ More than 5000Mcm －１ When the viscosity of the slurry was 50,000 mPa·s or less, the peel strength was also good (Examples 1 to 7).

[0098] Furthermore, as shown in Table 1 above, when the amount of monomethyl maleate (MMM), which corresponds to the carbonyl group-containing vinyl compound represented by general formula (I) of the present invention, was 0.1 molar parts or more per 100 molar parts of vinylidene fluoride-derived structural units, the slurry viscosity exceeded 50,000 mPa s (Comparative Example 2). Furthermore, when acryloyloxypropyl succinic acid (APS), which does not correspond to the carbonyl group-containing vinyl compound, was used, the slurry viscosity also exceeded 50,000 mPa s (Comparative Example 3). On the other hand, when vinylidene fluoride homopolymer was used, the slurry viscosity was low, but the peel strength was low (Comparative Example 1).

[0099] In contrast, when a carbonyl group-containing vinyl compound (maleic acid or monomethyl maleate) represented by general formula (I) of the present invention was copolymerized with vinylidene fluoride in an amount of less than 0.1 parts by mole per 100 parts by mole of the constituent units derived from vinylidene fluoride, the slurry viscosity was 50,000 mPa s or less, and further, the peel strength was good (Examples 1 to 7).

[0100] This application claims priority based on Japanese Patent Application No. 2024-056519, filed March 29, 2024, and Japanese Patent Application No. 2024-056537, filed March 29, 2024. The contents of those applications are incorporated herein by reference in their entirety.

[0101] 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, in which the IR absorbance ratio calculated from the absorption spectrum measured by Fourier transform infrared spectroscopy using the following formula is 0.15 or less: IR absorbance ratio = A Ｃ＝Ｏ / A Ｃ－Ｈ (In the formula, A Ｃ＝Ｏ represents the absorbance of the peak due to C═O stretching, and A Ｃ－Ｈ represents the absorbance of the peak due to C-H stretching.) The vinylidene fluoride copolymer was dissolved in N-methyl-2-pyrrolidone at 70°C over 5 hours to prepare an N-methyl-2-pyrrolidone solution having a concentration of the vinylidene fluoride copolymer of 5% by mass. After standing for 2 days from the preparation, the molar absorption coefficient at a wavelength of 346 nm was 2000 M －１ cm －１ More than 5000M －１ cm －１ The following are vinylidene fluoride copolymers:

2. The vinylidene fluoride copolymer according to claim 1, which contains a structural unit derived from a carbonyl group-containing vinyl compound represented by the following general formula (1): (In the general formula (1), R １ represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom; R ２ and R ３ each independently represents a hydrogen atom, a chlorine atom, a fluorine atom, or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom.

3. The vinylidene fluoride copolymer according to claim 2, wherein the content of the structural units derived from the carbonyl group-containing vinyl compound is less than 0.1 parts by mole per 100 parts by mole of the total of the structural units derived from vinylidene fluoride.

4. The vinylidene fluoride copolymer according to claim 2, further comprising a structural unit derived from a fluorine-containing vinyl compound (excluding said vinylidene fluoride and said carbonyl group-containing vinyl compound) containing a fluorine atom and a vinyl group.

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

6. The vinylidene fluoride copolymer according to claim 4, 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.

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

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

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

10. An electrode comprising the vinylidene fluoride copolymer according to any one of claims 1 to 6 and an electrode active material.

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

12. A battery comprising the electrode of claim 10.

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