vinylidene fluoride copolymer and method for producing the same, vinylidene fluoride copolymer composition, polymer dispersion, electrode for non-aqueous electrolyte secondary battery, electrolyte layer for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

To provide a vinylidene fluoride copolymer composition having stable dispersibility in a dispersant over a long period of time.SOLUTION: There is provided a vinylidene fluoride copolymer composition comprising a vinylidene fluoride copolymer. The vinylidene fluoride copolymer composition has a melting point of 140°C or less, the reversing heat flow of the vinylidene fluoride copolymer composition has an endothermic peak having a melting enthalpy of 2 J / g or more and the absolute value of the difference between a temperature showing the maximum endothermic peak of the endothermic peaks and a melting point of the vinylidene fluoride copolymer composition is 10°C or less. When a dispersion containing butyl butyrate and the vinylidene fluoride copolymer composition (vinylidene fluoride content is 10 mass%) is allowed to stand for 20 hours, the content rate of the vinylidene fluoride copolymer composition in the upper 20 vol.% of the dispersion after allowed to stand is 4.0 mass% or more and 10 mass% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a vinylidene fluoride copolymer composition and a method for producing the same, a polymer dispersion, an electrode for a non-aqueous electrolyte secondary battery, an electrolyte layer for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Conventionally, vinylidene fluoride polymers containing vinylidene fluoride have been widely used as binders for each layer of non-aqueous electrolyte secondary batteries. In particular, copolymers of vinylidene fluoride and fluorine-containing alkyl vinyl compounds (hereinafter also referred to as "vinylidene fluoride copolymers") are widely known as binders for binding current collectors and electrode active materials.

[0003] Here, Patent Document 1 describes an example in which vinylidene fluoride copolymer is used as a binder for the electrode layer and electrolyte layer of an all-solid-state battery, which is a type of non-aqueous electrolyte secondary battery. In this case, it is common to mix the vinylidene fluoride copolymer with the electrode active material, solid electrolyte, solvent or dispersion medium, etc., to form the above layer. Furthermore, Patent Document 2 describes the use of a solvent with a low dielectric constant in order to suppress the elution of lithium from the solid electrolyte.

[0004] However, since vinylidene fluoride copolymer is an insulator, the ion conduction paths in the above mixture were sometimes interrupted, resulting in insufficient performance of the all-solid-state battery. Patent document 3 describes that mixing a binder into the solid electrolyte layer reduces ion conductivity.

[0005] Therefore, techniques are being investigated for forming electrode layers and electrolyte layers by dispersing vinylidene fluoride copolymer without dissolving it. For example, Patent Documents 4 and 5 describe slurries in which vinylidene fluoride-hexafluoropropylene copolymer is dispersed in butyl butyrate or the like.

[0006] Furthermore, Patent Document 6 states that by uniformly dispersing particulate binder polymer in a dispersion medium, a solid electrolyte can be fixed without locally or entirely coating it. It also indicates that using such a binder dispersion can suppress the increase in interfacial resistance between solid electrolyte particles, between solid electrolyte particles and current collectors, and that the binder dispersion and solid electrolyte particles can be mixed and applied. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 6257698 [Patent Document 2] International Publication No. 2012 / 063827 [Patent Document 3] Japanese Patent Publication No. 2008-103284 [Patent Document 4] Japanese Patent Publication No. 2016-025025 [Patent Document 5] Japanese Patent Publication No. 2016-025027 [Patent Document 6] Japanese Patent Publication No. 2015-159067 [Overview of the project] [Problems that the invention aims to solve]

[0008] As described in Patent Document 3 above, since vinylidene fluoride copolymers are insulators, they sometimes reduce ionic conductivity when used as binders for electrode layers or electrolyte layers. Therefore, as described in Patent Documents 4 to 6, methods have been considered for forming electrode layers or solid electrolyte layers by dispersing vinylidene fluoride copolymers without dissolving them.

[0009] In order to further improve the performance of all-solid-state batteries, a method for more uniformly dispersing a binder (vinylidene fluoride copolymer) in a polymer dispersion for forming an electrode layer or an electrolyte layer has been explored. Here, in the polymer dispersion for forming an electrode layer or an electrolyte layer, it is preferable that the vinylidene fluoride copolymer is difficult to sediment over a long period of time. Therefore, an object of the present invention is to provide a vinylidene fluoride copolymer composition having stable dispersibility in a dispersion medium with a low relative permittivity over a long period of time, and a method for producing the same. Another object is to provide a polymer dispersion containing the vinylidene fluoride copolymer composition, an electrode for a non-aqueous electrolyte secondary battery, an electrolyte layer for a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery, and the like.

Means for Solving the Problems

[0010] The present invention provides the following vinylidene fluoride copolymer composition. That is, the present invention is a vinylidene fluoride copolymer composition containing a vinylidene fluoride copolymer, wherein the vinylidene fluoride copolymer contains a structural unit derived from vinylidene fluoride and a structural unit derived from a fluorine-containing alkyl vinyl compound, the melting point of the vinylidene fluoride copolymer composition is 140 ° C or lower, when the reversing heat flow of the vinylidene fluoride copolymer composition is measured with a temperature-modulated differential scanning calorimeter, it has an endothermic peak with an endothermic enthalpy amount (ΔHm) of 2 J / g or more, and among the endothermic peaks, the absolute value of the difference between the peak top temperature of the largest endothermic peak and the melting point of the vinylidene fluoride copolymer composition is 10 ° C or lower, a dispersion containing butyl butyrate and the vinylidene fluoride copolymer composition, and the content of the vinylidene fluoride copolymer composition is 10% by mass, after stirring at 25 ° C for 30 minutes and standing for 20 hours, the content of the vinylidene fluoride copolymer composition in the upper 20% by volume of the dispersion is 4.0% by mass or more and 10% by mass or less, and a vinylidene fluoride copolymer composition is provided.

[0011] The present invention also provides a polymer dispersion liquid including the vinylidene fluoride copolymer composition and a dispersion medium having a relative permittivity of 15 or less.

[0012] The present invention also provides an electrode for a nonaqueous electrolyte secondary battery including the above vinylidene fluoride copolymer composition. Further, an electrolyte layer for a nonaqueous electrolyte secondary battery including the above vinylidene fluoride copolymer composition is provided. Also, a nonaqueous electrolyte secondary battery including the above vinylidene fluoride copolymer composition is provided.

[0013] The present invention also provides a method for producing the following vinylidene fluoride copolymer composition. [[ID=⑨]] A method for producing a vinylidene fluoride copolymer composition containing a vinylidene fluoride copolymer including a structural unit derived from vinylidene fluoride and a structural unit derived from a fluorinated alkyl vinyl compound, the method including: preparing an emulsion in an aqueous medium in which an untreated vinylidene fluoride copolymer is dispersed; adding a surfactant to the emulsion and stirring to obtain a surfactant-containing emulsion having a surface tension at 25°C of 40 mN / m or less, and the melting point of the vinylidene fluoride copolymer composition being 140°C or less.

[0014] Also, a method for producing a vinylidene fluoride copolymer composition containing a vinylidene fluoride copolymer including a structural unit derived from vinylidene fluoride and a structural unit derived from a fluorinated alkyl vinyl compound, the method including: preparing an emulsion in an aqueous medium in which an untreated vinylidene fluoride copolymer is dispersed; heating the emulsion at a temperature lower than the end set of the highest temperature endothermic peak among the endothermic peaks observed at 185°C or lower when measuring the heat flow of the untreated vinylidene fluoride copolymer with a differential scanning calorimeter and at a temperature of 40°C or higher, and the melting point of the vinylidene fluoride copolymer composition being 140°C or less.

Advantages of the Invention

[0015] The vinylidene fluoride copolymer composition of the present invention exhibits excellent dispersibility in dispersion media with low dielectric constants. Therefore, it is possible to form electrode layers and electrolyte layers uniformly containing the vinylidene fluoride copolymer composition using this vinylidene fluoride copolymer composition. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a graph illustrating a method for identifying the endset of endothermic peaks in vinylidene fluoride copolymer. [Modes for carrying out the invention]

[0017] 1. Vinylidene fluoride copolymer composition The vinylidene fluoride copolymer composition of the present invention is a composition comprising a vinylidene fluoride copolymer having predetermined physical properties. The vinylidene fluoride copolymer composition may consist only of the vinylidene fluoride copolymer, or it may contain the vinylidene fluoride copolymer and other components such as surfactants.

[0018] The vinylidene fluoride copolymer composition of the present invention only needs to have a melting point of 140°C or lower, and is usually solid at 25°C. In this specification, "solid at 25°C" means that the main components of the composition are solid at 25°C, and it is acceptable to include some liquid components as long as the objectives and effects of the present invention are not impaired.

[0019] As mentioned above, when forming the electrode layer or electrolyte layer of a non-aqueous electrolyte secondary battery, it is preferable that the vinylidene fluoride copolymer is dispersed in a dispersion medium (a dispersion medium with a low dielectric constant), and that its dispersibility be maintained over a long period of time. Through diligent research by the present inventors, it has become clear that when a vinylidene fluoride copolymer composition has certain physical properties, it is easily dispersed in a dispersion medium with a low dielectric constant, and furthermore, the dispersion stability of the vinylidene fluoride copolymer composition dispersion is extremely high when left standing.

[0020] Specifically, it was found that when a vinylidene fluoride copolymer composition has a melting point of 140°C or lower, and when the reversing heat flow of the vinylidene fluoride copolymer composition is measured using a temperature-modulated differential scanning calorimeter, it exhibits an endothermic peak with a melting enthalpy (ΔHm) of 2 J / g or more, and the absolute difference between the peak top temperature of the largest endothermic peak and the melting point of the vinylidene fluoride copolymer composition is 10°C or lower, then the dispersibility of the vinylidene fluoride copolymer composition in a dispersion medium with a relative permittivity of 15 or lower is good.

[0021] A melting point of 140°C or lower for a vinylidene fluoride copolymer composition means that the vinylidene fluoride copolymer contains a certain amount of constituent units derived from fluorine-containing alkyl vinyl compounds. Furthermore, it is thought that when a vinylidene fluoride copolymer composition contains constituent units derived from fluorine-containing alkyl vinyl compounds, it tends to have good affinity with dispersion media with low dielectric constants.

[0022] Furthermore, in a vinylidene fluoride copolymer composition containing a vinylidene fluoride copolymer having a certain composition ratio of constituent units derived from vinylidene fluoride and constituent units derived from a fluorine-containing alkyl vinyl compound, the peak top temperature of the largest endothermic peak in the reversing heat flow changes depending on the state of the crystalline region of the vinylidene fluoride copolymer in the vinylidene fluoride copolymer composition. Since the state of the crystalline region of the vinylidene fluoride copolymer changes depending on polymerization conditions and equipment such as thermal history (temperature, time) and monomer reaction rate, the peak top temperature reflects the manufacturing history. On the other hand, in the vinylidene fluoride copolymer composition, the melting point measured by the method described later is less affected by the state of the crystalline region formed during manufacturing. And, when the vinylidene fluoride copolymer is moderately crystallized without impairing its affinity with the dispersion medium with a low dielectric constant, the absolute value of the difference between the peak top temperature and the melting point of the vinylidene fluoride copolymer composition is 10°C or less. In other words, when the absolute value mentioned above is 10°C or lower, the vinylidene fluoride copolymer composition is easily dispersed in a solvent with a low dielectric constant.

[0023] Furthermore, in the present invention, when a dispersion is prepared containing butyl butyrate and the vinylidene fluoride copolymer composition, wherein the content of the vinylidene fluoride copolymer composition is 10% by mass, the content of the vinylidene fluoride copolymer composition in the top 20% by volume of the dispersion after stirring the dispersion at 25°C for 30 minutes and letting it stand for 20 hours is 4.0% by mass or more and 10% by mass or less. When the content of the vinylidene fluoride copolymer composition in the dispersion is within the above range, the vinylidene fluoride copolymer composition is less likely to settle in a dispersion medium with a low dielectric constant, and good dispersibility is maintained over a long period of time.

[0024] The following provides a detailed explanation of the components of the vinylidene fluoride copolymer composition, its physical properties, and manufacturing method.

[0025] • Vinylidene fluoride copolymer The vinylidene fluoride copolymer composition of the present invention contains structural units derived from vinylidene fluoride and structural units derived from fluorine-containing alkyl vinyl compounds. The amount of structural units derived from vinylidene fluoride in the vinylidene fluoride copolymer is preferably 30% to 85% by mass, and more preferably 40% to 80% by mass, based on 100% by mass of structural units of the vinylidene fluoride copolymer. When the mass fraction of structural units derived from vinylidene fluoride is within the above range, the melting point of the vinylidene fluoride copolymer composition tends to fall within the desired range. The mass fraction of structural units derived from vinylidene fluoride is used to describe the vinylidene fluoride copolymer. 19 It can be identified by analysis using F-NMR.

[0026] On the other hand, the fluorinated alkyl vinyl compound may be any compound having one vinyl group and an alkyl group in which one or more hydrogens of the alkyl group are substituted with fluorine, or a compound having one vinyl group to which fluorine is bonded (except for vinylidene fluoride). Examples include vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, hexafluoroethylene, fluoroalkyl vinyl ether, and perfluoromethyl vinyl ether. Among these, tetrafluoroethylene and hexafluoropropylene are preferred, and hexafluoropropylene is particularly preferred, from the viewpoint that they tend to have good affinity with dispersion media with low dielectric constants. The vinylidene fluoride copolymer may contain only one structural unit derived from the fluorinated alkyl vinyl compound, or it may contain two or more.

[0027] The amount of structural units derived from the fluorinated alkyl vinyl compound in the vinylidene fluoride copolymer is not particularly limited as long as it is possible to set the melting point of the vinylidene fluoride copolymer composition to 140°C or lower, as described later. Depending on the type of fluorinated alkyl vinyl compound, the amount of structural units derived from the fluorinated alkyl vinyl compound is preferably 15% to 70% by mass, and more preferably 20% to 60% by mass, based on 100% by mass of the structural units of the vinylidene fluoride copolymer. When the amount of the fluorinated alkyl vinyl compound in the vinylidene fluoride copolymer is within the above range, the melting point of the vinylidene fluoride copolymer composition tends to fall within the desired range. The mass fraction of structural units derived from the fluorinated alkyl vinyl compound is used to define the vinylidene fluoride copolymer. 19 It can be identified by analysis using F-NMR.

[0028] Furthermore, the vinylidene fluoride copolymer may contain, to the extent that it does not impair the objectives and effects of the present invention, some constituent units derived from vinylidene fluoride or other monomers copolymerizable with fluorine-containing alkyl vinyl compounds.

[0029] Other monomers copolymerizable with vinylidene fluoride, etc., include crosslinkable alkylvinyl compounds having one vinyl group and a crosslinkable group. Examples of crosslinkable groups include vinyl groups. That is, a crosslinkable alkylvinyl compound may be a compound having two or more vinyl groups. Furthermore, the crosslinkable alkylvinyl compound may contain a fluorine atom. Examples of such crosslinkable alkylvinyl compounds include perfluorodivinyl ethers and perfluoroalkylenedivinyl ethers. Examples of perfluoroalkylenedivinyl ethers include compounds having a structure in which two vinyl ether groups, all of which have hydrogen atoms replaced by fluorine atoms, are linked by a linear or branched divalent perfluoroalkylene group having 1 to 6 carbon atoms.

[0030] Other examples of monomers include unsaturated dibasic acids or unsaturated dibasic acid monoesters. Unsaturated dibasic acids are unsaturated dicarboxylic acids or their derivatives, and examples include compounds in which two carboxyl groups are linked by linear or branched unsaturated alkylene groups having between 1 and 6 carbon atoms. More specific examples of the above unsaturated dibasic acids include maleic acid, fumaric acid, itaconic acid, and citraconic acid. On the other hand, unsaturated dibasic acid monoesters are monoester compounds derived from the above unsaturated dibasic acids. Examples of the above unsaturated dibasic acid monoesters include monomethyl maleic acid, monoethyl maleic acid, monomethyl citraconic acid, and monoethyl citraconic acid.

[0031] Other examples of monomers include compounds containing vinyl groups and polar groups (hereinafter also referred to as "polar group-containing compounds"). Examples of such polar group-containing compounds include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, (meth)acryloyloxyethyl succinic acid, (meth)acryloyloxypropyl succinic acid, and glycidyl (meth)acrylate.

[0032] The mass fraction of structural units derived from the above-mentioned crosslinkable alkylvinyl compound, unsaturated dibasic acid, unsaturated dibasic acid monoester, or polar group-containing compound in the vinylidene fluoride copolymer can be arbitrarily set within a range that does not impair the objectives and effects of the present invention. For example, the amount of structural units derived from the unsaturated dibasic acid, unsaturated dibasic acid monoester, or polar group-containing compound introduced can be determined by FT-IR analysis of the vinylidene fluoride copolymer.

[0033] Here, the weight-average molecular weight of the vinylidene fluoride copolymer is preferably 100,000 to 10,000,000, more preferably 200,000 to 5,000,000, and even more preferably 300,000 to 2,000,000. When the weight-average molecular weight of the vinylidene fluoride copolymer is within this range, the dispersibility of the vinylidene fluoride copolymer composition in the dispersion medium tends to be good. Furthermore, when the vinylidene fluoride copolymer composition is used as a binder for the electrode layer or the electrolyte layer of an all-solid-state battery, it facilitates the binding of the active material and solid electrolyte. Note that the above weight-average molecular weight is a polystyrene equivalent value measured by gel permeation chromatography (GPC).

[0034] • Surfactants As described above, the vinylidene fluoride copolymer composition may further contain a surfactant in addition to the vinylidene fluoride copolymer.

[0035] Here, the surfactant contained in the vinylidene fluoride copolymer composition may be an ionic surfactant having an anionic group or a cationic group, or it may be a nonionic surfactant. Furthermore, the surfactant may be a non-fluorinated surfactant that does not contain fluorine, or it may be a fluorinated surfactant that contains fluorine (such as a perfluorinated surfactant and a partially fluorinated surfactant).

[0036] The anionic surfactants that can be used are not particularly limited, and conventionally known surfactants can be used. The hydrophilic group of the anionic surfactant preferably includes carboxylates, sulfates, sulfonates, phosphates, etc., and may further include ester bonds, acid amide bonds, and ether bonds. The hydrophobic group preferably includes alkyl chains, alkyl ether chains, perfluoroalkyl chains, perfluoroalkyl ether chains, fluorocarbon chains, or fluoropolyether chains, and these may have a linear or branched structure.

[0037] The cationic surfactants that can be used are not particularly limited, and conventionally known ones can be used. The hydrophilic group of the cationic surfactant preferably includes an aliphatic quaternary ammonium salt, an aliphatic amine salt, a cyclic quaternary ammonium salt, or an amine acetate. The hydrophobic group preferably includes an alkyl chain, an alkyl ether chain, a perfluoroalkyl chain, a perfluoroalkyl ether chain, a fluorocarbon chain, or a fluoropolyether chain, which may have a linear or branched structure.

[0038] Furthermore, the nonionic surfactants that can be used are not particularly limited, and conventionally known ones can be used. The hydrophilic group of the nonionic surfactant preferably has a hydroxyl group, ether bond, acid amide bond, ester bond, etc. within the molecule. The hydrophobic group preferably includes alkyl chains, alkyl ether chains, perfluoroalkyl chains, perfluoroalkyl ether chains, fluorocarbon chains, or fluoropolyether chains, and these may have a linear or branched structure.

[0039] As a surfactant, it is preferable to use one that dissolves or is miscible in the aqueous medium in the emulsion prepared in the method for producing the vinylidene fluoride copolymer composition described later.

[0040] The amount of surfactant contained in the vinylidene fluoride copolymer composition is preferably 0.01% by mass or more and 20% by mass or less, and more preferably 0.02% by mass or more and 15% by mass or less, relative to the total amount of vinylidene fluoride copolymer and surfactant. If the amount of surfactant is excessively high, the adhesion between the vinylidene fluoride copolymer composition and the active material or electrolyte may decrease when the resulting vinylidene fluoride copolymer composition is used as a binder for the electrode layer or electrolyte layer. However, if the amount is 20% by mass or less, the adhesion is less likely to decrease. On the other hand, if the amount of surfactant is 0.01% by mass or more relative to the total amount of the vinylidene fluoride copolymer composition, the dispersion stability when the vinylidene fluoride copolymer composition is dispersed in a dispersion medium is increased.

[0041] <Physical properties of vinylidene fluoride copolymer composition> As described above, the vinylidene fluoride copolymer composition of the present invention may have a melting point of 140°C or lower, but is preferably 135°C or lower, and more preferably 130°C or lower. The melting point of the vinylidene fluoride copolymer composition can be adjusted by the amount of constituent units derived from the fluorine-containing alkyl vinyl compound in the vinylidene fluoride copolymer described above. Furthermore, in this specification, the melting point of the vinylidene fluoride copolymer composition is measured by the following method. First, the powdered vinylidene fluoride copolymer composition is pressed at 200°C to form a film with a thickness of 150 μm. Using a differential scanning calorimeter, the melting point of the pressed film-like vinylidene fluoride copolymer composition is measured in accordance with ASTM D3418. The melting point of the vinylidene fluoride copolymer composition measured by this method is the value measured after the vinylidene fluoride copolymer composition has been melted once. Therefore, it is less susceptible to the influence of the manufacturing history during the polymerization reaction process of the vinylidene fluoride copolymer and the method of producing the vinylidene fluoride copolymer composition, as described later.

[0042] On the other hand, the vinylidene fluoride copolymer composition of the present invention has an endothermic peak with a melting enthalpy (ΔHm) of 2 J / g or more when the reversing heat flow of the vinylidene fluoride copolymer composition is measured with a temperature-modulated differential scanning calorimeter.0 The literature value (M Neidhofer: Polymer volume 45, Issue 5, 2004, 1679-1688) is 104.5 J / g, so the upper limit of the melting enthalpy (ΔHm) of the vinylidene fluoride copolymer is set to 104.5 J / g. Here, the melting enthalpy (ΔHm), the number of peak tops of the endothermic peak, and the peak top temperature of the vinylidene fluoride copolymer composition vary depending on the state of the crystalline region of the vinylidene fluoride copolymer composition (especially the vinylidene fluoride copolymer), that is, the manufacturing method described later. For example, if a surfactant is optionally added to an emulsion containing a vinylidene fluoride copolymer that has one endothermic peak top, and the emulsion is heated at a predetermined temperature, the endothermic peak top of the vinylidene fluoride copolymer will have two or more peak tops. This is because heating the emulsion changes the crystal structure of the vinylidene fluoride copolymer, resulting in a peak that differs from the melting peak originating from the vinylidene fluoride copolymer before heating. In this specification, an endothermic peak with a melting enthalpy (ΔHm) of 2 J / g or more is defined as a peak observed at temperatures above 0°C, and peaks observed at temperatures below 0°C are not included in the definition of an endothermic peak.

[0043] Here, the reversing heat flow of the vinylidene fluoride copolymer composition is determined by a temperature-modulated differential scanning calorimeter. Specifically, a powdered vinylidene fluoride copolymer composition obtained by freeze-drying an emulsion containing the vinylidene fluoride copolymer composition is used as the measurement sample. Then, under heat-only conditions, it is heated at an average heating rate of 5°C / min, a modulation period of 40 seconds, and a modulation amplitude of ±0.531°C to obtain a reversing heat flow with a downward-convex endothermic peak. In the obtained reversing heat flow, a baseline is drawn linearly so as to overlap with the linear heat flow on the higher temperature side of the endset. Then, among the downward-convex endothermic peaks of the reversing heat flow, the point where the distance from the baseline to the reversing heat flow is furthest is defined as the peak top of the largest endothermic peak, and the temperature at which this peak top occurs is determined. Here, the number of minimum values ​​in the downward-convex endothermic peak of the reversing heat flow is defined as the number of peak tops of the endothermic peak. On the other hand, the region enclosed by the baseline and the reversing heat flow is defined as the enthalpy of melting (ΔHm).

[0044] Furthermore, the vinylidene fluoride copolymer composition of the present invention has a minimum absolute value of 10°C between the peak top temperature of the largest endothermic peak among the endothermic peaks of the vinylidene fluoride copolymer with a melting enthalpy (ΔHm) of 2 J / g or more, and the melting point of the vinylidene fluoride copolymer composition. Here, "peak top temperature of the largest endothermic peak" refers to the temperature (point on the reversed heat flow) at which the distance from the baseline to the reversed heat flow is maximum when a line is drawn perpendicularly from the baseline drawn by the method described above toward the reversed heat flow in the endothermic peak with a melting enthalpy (ΔHm) of 2 J / g or more. The absolute value is more preferably 9.5°C or less, and even more preferably 9°C or less. When the absolute value is within this range, the vinylidene fluoride copolymer composition has both affinity and dispersibility to dispersion media with low dielectric constant, as described above. Furthermore, if the above absolute value exceeds 10°C, the crystallinity of the vinylidene fluoride copolymer composition tends to increase excessively, leading to a decrease in dispersibility. This absolute value can be adjusted by the manufacturing method of the vinylidene fluoride copolymer composition described later. For example, if the vinylidene fluoride copolymer is heated after being in a solid (powder) state, the above absolute value is more likely to exceed 10°C.

[0045] Furthermore, when a dispersion is prepared containing butyl butyrate and the vinylidene fluoride copolymer composition of the present invention, and the content of the vinylidene fluoride copolymer composition is 10% by mass, the content of the vinylidene fluoride copolymer composition in the top 20% by volume of the dispersion after standing for 20 hours is 4.0% by mass or more and 10% by mass or less. More preferably, the content of the vinylidene fluoride copolymer composition in the top 20% by volume is 7.0% by mass or more and 10% by mass or less. When the vinylidene fluoride copolymer composition has such dispersion stability with respect to butyl butyrate, the vinylidene fluoride copolymer composition is less likely to settle in polymer dispersions for forming electrode layers, electrolyte layers, etc., and it becomes easier to obtain electrode layers and electrolyte layers with the desired performance. Note that the content of the vinylidene fluoride copolymer composition in the dispersion after standing can be adjusted by the method for producing the vinylidene fluoride copolymer composition described later. For example, this can be adjusted by the amount and type of surfactant mixed with the vinylidene fluoride copolymer, and whether or not the emulsion is heated.

[0046] The content of the vinylidene fluoride copolymer composition after the above standing period is measured as follows: A dispersion (containing 10% by mass of the vinylidene fluoride copolymer composition) is prepared by adding the vinylidene fluoride copolymer composition to butyl butyrate. 20 mL of this dispersion is placed in a 20 mL graduated cylinder, covered with Parafilm, and left to stand for 20 hours. Then, 4 mL is taken from the supernatant of the graduated cylinder, dried at 135°C for 1 hour, and allowed to cool in a desiccator for 1 hour. The content of the vinylidene fluoride copolymer composition in the top 20% by volume of the dispersion is calculated by measuring the weight before and after drying.

[0047] Furthermore, the turbidity of the above dispersion is preferably 5% or higher. If the turbidity of the above dispersion is less than 5%, the vinylidene fluoride copolymer composition is dissolved in the dispersion medium, and if it is 5% or higher, at least a portion of the vinylidene fluoride copolymer composition is dispersed in the dispersion medium without dissolving. The turbidity can be adjusted by the type of surfactant used when preparing the vinylidene fluoride copolymer composition, whether or not heating is performed, etc. The turbidity of the above dispersion is measured by the following method: A butyl butyrate dispersion of the vinylidene fluoride copolymer composition (containing 10% by mass of the vinylidene fluoride copolymer composition) is prepared, left to stand for 20 hours, and then stirred. The turbidity of the dispersion immediately after stirring is measured in accordance with JIS K 7136.

[0048] The shape of the vinylidene fluoride copolymer composition of the present invention is not particularly limited, but it is generally preferable that it be particulate (powdered). Furthermore, the average secondary particle diameter of the vinylidene fluoride copolymer composition is not particularly limited, but it is preferably 1 μm or more and 5000 μm or less, and more preferably 2 μm or more and 3000 μm or less. When the average secondary particle diameter of the vinylidene fluoride copolymer composition is within this range, the vinylidene fluoride copolymer is easier to handle. The average secondary particle diameter is the cumulative average diameter (D50) of the particle size distribution measured on a volume basis by laser diffraction-scattering method.

[0049] <Method for producing a vinylidene fluoride copolymer composition> A vinylidene fluoride copolymer composition satisfying the above-mentioned physical properties can be produced by, for example, the following three methods. However, the methods for producing the vinylidene fluoride copolymer composition described above are not limited to the following three methods.

[0050] • Method 1 The first method for producing a vinylidene fluoride copolymer composition involves a step of preparing an emulsion in which an untreated vinylidene fluoride copolymer is dispersed in an aqueous medium (hereinafter also referred to as the "emulsion preparation step"), and a step of adding an ionic surfactant to the emulsion and stirring to obtain a surfactant-containing emulsion with a surface tension of 40 mN / m or less at 25°C (hereinafter also referred to as the "surfactant addition step").

[0051] Furthermore, after the surfactant addition step, the surfactant-containing emulsion is dried to extract the vinylidene fluoride copolymer (hereinafter also referred to as the "drying step"), thereby obtaining the aforementioned vinylidene fluoride copolymer composition in solid form. In this method, after obtaining the solid vinylidene fluoride copolymer composition, heating at a temperature above the melting point of the vinylidene fluoride copolymer composition is not performed. The following describes each step.

[0052] (Emulsion preparation process) In the emulsion preparation step, an emulsion is prepared in which untreated vinylidene fluoride copolymer is dispersed in an aqueous medium. This emulsion may be a commercially available product. In this specification, untreated vinylidene fluoride copolymer refers to a copolymer of vinylidene fluoride and a fluorine-containing alkyl vinyl compound prepared by a general method, and which has not undergone any heat treatment or mixing with surfactants other than for the purpose of decomposing residual monomers and residual initiators after polymerization of the vinylidene fluoride copolymer. This untreated vinylidene fluoride copolymer has substantially the same composition as the vinylidene fluoride copolymer described above. However, by performing the surfactant addition step described later on the emulsion in which the untreated vinylidene fluoride copolymer is dispersed in an aqueous medium, the dispersibility of the untreated vinylidene fluoride copolymer in a dispersion medium having a dielectric constant of 15 or less changes.

[0053] In this process, the emulsion in which untreated vinylidene fluoride copolymer is dispersed in an aqueous medium may be prepared by suspension polymerization, emulsion polymerization, solution polymerization, or microsuspension polymerization, etc. Alternatively, commercially available emulsions in which untreated vinylidene fluoride copolymer is dispersed in an aqueous medium may be used. Among these, the method of obtaining an emulsion in which untreated vinylidene fluoride copolymer is dispersed in water by emulsion polymerization is preferred.

[0054] The specific emulsion polymerization method involves mixing vinylidene fluoride, a fluorine-containing alkyl vinyl compound, other monomers as needed, an aqueous medium, and an emulsifier in an autoclave. Then, a polymerization initiator soluble in the aqueous medium is added to the mixture, and the vinylidene fluoride, the fluorine-containing alkyl vinyl compound, and other monomers as needed are polymerized.

[0055] The pressure inside the autoclave during polymerization is preferably 0 to 20 MPa, more preferably 0.5 to 15 MPa, and even more preferably 1 to 10 MPa. By adjusting the pressure during polymerization within the above range, untreated vinylidene fluoride copolymer can be obtained stably during production.

[0056] The aqueous medium used for emulsion polymerization is not particularly limited as long as it is a liquid in which the above-mentioned vinylidene fluoride and fluorine-containing alkyl vinyl compound are poorly soluble. It may also contain a solvent that is miscible with water in addition to water, as long as the main component of the liquid is water. The aqueous medium is preferably water.

[0057] On the other hand, the emulsifier is not particularly limited as long as it can form micelles in an aqueous medium and stably disperse the synthesized untreated vinylidene fluoride copolymer in an aqueous medium, for example, known surfactants can be used. The emulsifier may be any of nonionic surfactants, cationic surfactants, anionic surfactants, or amphoteric surfactants, and these may be used in combination. Examples of emulsifiers include perfluorinated surfactants, partially fluorinated surfactants, and nonfluorinated surfactants that have been conventionally used in the polymerization of polyvinylidene fluoride. Among these surfactants, perfluoroalkyl sulfonic acid and its salts, perfluoroalkyl carboxylic acid and its salts, and fluorinated surfactants having fluorocarbon chains or fluoropolyether chains are preferred. As the emulsifier, one or more selected from the above can be used. The amount of emulsifier added is preferably 0.0001 to 22 parts by mass, based on 100 parts by mass of the total amount of monomers used in polymerization.

[0058] The polymerization initiator is not particularly limited as long as it is a compound that is soluble in an aqueous medium and capable of polymerizing monomers. Examples of polymerization initiators include known water-soluble peroxides, water-soluble azo compounds, and redox initiators. Examples of water-soluble peroxides include ammonium persulfate and potassium persulfate. Examples of water-soluble azo compounds include 2,2'-azobis-isobutyronitrile (AIBN) and 2,2'-azobis-2-methylbutyronitrile (AMBN). Examples of redox initiators include ascorbic acid-hydrogen peroxide. Among these, water-soluble peroxides are preferred from the viewpoint of reactivity, etc. These polymerization initiators can be used individually or in combination of two or more. The amount of polymerization initiator added is preferably 0.01 to 5 parts by mass, based on 100 parts by mass of the total amount of monomers used in polymerization.

[0059] On the other hand, the emulsion polymerization method described above may be a soap-free emulsion polymerization method, a miniemulsion polymerization method, a seed emulsion polymerization method, or the like. A soap-free emulsion polymerization method is a method of emulsion polymerization without using the conventional emulsifiers described above. In addition, in a soap-free emulsion polymerization method, a reactive emulsifier having polymerizable double bonds in its molecule can be used as the emulsifier. The reactive emulsifier forms micelles in the system in the initial stages of polymerization, but as polymerization progresses, it is used and consumed as monomers in the polymerization reaction. Therefore, it is hardly present in a free state in the final reaction system. Consequently, there is an advantage that the reactive emulsifier is less likely to bleed out onto the particle surface of the resulting untreated vinylidene fluoride copolymer.

[0060] Examples of reactive emulsifiers include polyoxyalkylene alkenyl ethers, sodium alkylallyl sulfosuccinate, sodium methacryloyloxypolyoxypropylene sulfate, and alkoxy polyethylene glycol methacrylate.

[0061] Furthermore, miniemulsion polymerization is a method in which strong shear force is applied using an ultrasonic oscillator or the like to atomize monomer oil droplets such as vinylidene fluoride or fluorine-containing alkyl vinyl compounds to submicron size and then polymerize them. At this time, a known hydrohop is added to the mixture to stabilize the atomized monomer oil droplets. In miniemulsion polymerization, ideally, polymerization reactions occur only in each monomer oil droplet, and each oil droplet becomes an untreated vinylidene fluoride copolymer (fine particle). Therefore, it is easy to control the particle size and particle size distribution of the resulting untreated vinylidene fluoride copolymer.

[0062] Seed emulsion polymerization is a polymerization method in which fine particles obtained by the polymerization method described above are coated with a polymer consisting of other monomers. The emulsion of fine particles is further polymerized by adding monomers and, if necessary, an aqueous medium, surfactant, polymerization initiator, etc.

[0063] In any of the emulsion polymerization methods described above, a chain transfer agent may be used to adjust the degree of polymerization of the resulting untreated vinylidene fluoride copolymer. Examples of chain transfer agents include ethyl acetate, methyl acetate, diethyl carbonate, acetone, ethanol, n-propanol, acetaldehyde, propylaldehyde, ethyl propionate, and carbon tetrachloride.

[0064] Furthermore, pH adjusters may be used as needed. Examples of pH adjusters include electrolytes with buffering capacity such as sodium dihydrogen phosphate, disodium hydrogen phosphate, and potassium dihydrogen phosphate, as well as basic substances such as sodium hydroxide, potassium hydroxide, barium hydroxide, calcium hydroxide, and ammonia.

[0065] Furthermore, other optional components such as settling inhibitors, dispersion stabilizers, corrosion inhibitors, fungicides, and wetting agents may be used as needed. The amount of these optional components added is preferably 5 ppm to 10 parts by mass, and more preferably 10 ppm to 7 parts by mass, per 100 parts by mass of the total amount of monomers used in polymerization.

[0066] In the polymerization of untreated vinylidene fluoride copolymers, the polymerization temperature can be appropriately selected depending on the type of polymerization initiator, etc., but for example, it can be set to 0 to 120°C, preferably 20 to 110°C, and more preferably 40 to 100°C. The polymerization time is not particularly limited, but considering productivity, it is preferably 1 to 24 hours.

[0067] According to the manufacturing method of the present invention, when the aqueous medium is water, an emulsion is obtained in which untreated vinylidene fluoride copolymer particles are uniformly dispersed in water. Commercial emulsions and the emulsion obtained in this way may be used as is or diluted to any concentration with an aqueous medium before use. The emulsion may be powdered by at least one method selected from salting out, freeze-drying, spray-drying, and the like, and then physically or chemically redispersed in a desired aqueous medium before the surfactant addition step described later is performed. At this time, other optional components (except surfactants) may be mixed at any time, or impurities may be removed from the untreated emulsion using a dialysis membrane or ion exchange resin. Furthermore, the untreated vinylidene fluoride copolymer may be micronized by freeze-drying or classification, mixed with an aqueous medium, and then the surfactant addition step described later is performed. The method of dispersing the untreated vinylidene fluoride copolymer in the aqueous medium is not particularly limited, and known dispersion methods can be applied.

[0068] Here, the content of untreated vinylidene fluoride copolymer in the emulsion is preferably 5% by mass or more and 70% by mass or less, and more preferably 10% by mass or more and 60% by mass or less. The emulsion prepared or purchased by the method described above may be used as is, or it may be diluted with an aqueous medium before use. When the content of untreated vinylidene fluoride copolymer is 5% by mass or more, the vinylidene fluoride copolymer composition can be prepared efficiently. On the other hand, when it is 70% by mass or less, the dispersibility of the emulsion tends to be more stable.

[0069] The average primary particle diameter of the untreated vinylidene fluoride copolymer in the emulsion, as determined by dynamic light scattering, is preferably 5 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less. On the other hand, the above average primary particle diameter is preferably 0.01 μm or more, more preferably 0.05 μm or more, and even more preferably 0.1 μm or more. The average primary particle diameter of the untreated vinylidene fluoride copolymer in the emulsion is calculated by regularization analysis of dynamic light scattering. For example, it can be measured using a DelsaMaxCORE manufactured by BECKMAN COULTER, in accordance with JIS Z8828, with water as the measurement medium and a measurement temperature of 25°C. The largest peak obtained by the regularization analysis is taken as the average primary particle diameter.

[0070] (Surfactant addition process) In the surfactant addition step, an ionic surfactant is added to the emulsion prepared in the emulsion preparation step described above and stirred to obtain a surfactant-containing emulsion with a surface tension of 40 mN / m or less at 25°C. The ionic surfactant added in this step is not particularly limited as long as it is possible to make the surface tension of the emulsion (surfactant-containing emulsion) after addition 40 mN / m or less. Here, the surface tension of the emulsion (surfactant-containing emulsion) after the addition of the ionic surfactant at 25°C is preferably 5 mN / m or more and 40 mN / m or less, and more preferably 10 mN / m or more and 40 mN / m or less.

[0071] The surface tension of the surfactant-containing emulsion is measured using the Wilhelmy method with a surface tensimeter (Sigma701 / 700, KSV Instruments). A platinum plate is used for measurement, and the average value of three surface tension measurements at 25°C is taken as the surface tension value.

[0072] The ionic surfactant added in this process is selected to dissolve or be miscible in the aqueous medium contained in the emulsion, and may be either anionic or cationic surfactant, with anionic surfactants being particularly preferred. Only one type may be added, or two or more types may be added in combination. By adding an ionic surfactant, it can be adsorbed onto the surface of the untreated vinylidene fluoride copolymer contained in the emulsion, improving its dispersibility in a dispersion medium having a dielectric constant of 15 or less.

[0073] The anionic and cationic surfactants that can be used are the same as those exemplified in the surfactants contained in the vinylidene fluoride copolymer composition described above.

[0074] The amount of ionic surfactant added is appropriately selected according to the surface tension of the surfactant-containing emulsion, but is generally preferably 0.01 parts by mass to 20 parts by mass, and more preferably 0.02 parts by mass to 15 parts by mass, relative to the total amount of untreated vinylidene fluoride copolymer in the emulsion. If the amount of ionic surfactant is excessively high, the adhesion between the vinylidene fluoride copolymer composition and the active material or electrolyte may decrease when the resulting vinylidene fluoride copolymer composition is used as a binder for the electrode layer or electrolyte layer. Therefore, it is preferable to add 20 parts by mass or less per 100 parts by mass of the total amount of untreated vinylidene fluoride copolymer. On the other hand, if the amount of ionic surfactant is 0.01 parts by mass or more relative to the total amount of untreated vinylidene fluoride copolymer, the dispersion stability when the vinylidene fluoride copolymer composition is dispersed in a dispersion medium is increased. Excess components of the surfactant added in this step may be removed by a dialysis membrane, ion exchange resin, etc., as needed.

[0075] (drying process) In the drying step, the aqueous medium is removed from the surfactant-containing emulsion. The method for removing the aqueous medium is not particularly limited, but it is preferable to dry the emulsion at a temperature that does not affect the physical properties of the vinylidene fluoride copolymer composition in the surfactant-containing emulsion. The drying step may be carried out under atmospheric pressure or under reduced pressure. By this drying, the solid (powdered) vinylidene fluoride copolymer composition described above is obtained. Furthermore, the apparatus for removing the aqueous medium is not particularly limited, and a shelf dryer, conical dryer, fluidized bed dryer, airflow dryer, spray dryer, freeze dryer, etc., can be used.

[0076] • Second method The second method for producing a vinylidene fluoride copolymer composition involves at least the following steps: preparing an emulsion in which untreated vinylidene fluoride copolymer is dispersed in an aqueous medium (hereinafter also referred to as the "emulsion preparation step"), and heating the emulsion at a temperature lower than the end set of the highest temperature endothermic peak of the untreated vinylidene fluoride copolymer observed at 185°C or below when the heat flow of the untreated vinylidene fluoride copolymer is measured with a differential scanning calorimeter (hereinafter also referred to as the "heating step"). After the emulsion heating step, the emulsion is dried to extract the vinylidene fluoride copolymer (hereinafter also referred to as the "drying step"), thereby obtaining the solid (powdered) vinylidene fluoride copolymer composition described above. In this method as well, after obtaining the solid vinylidene fluoride copolymer composition, heating at a temperature above the melting point of the vinylidene fluoride copolymer composition is not performed. Each step will be described below.

[0077] (Emulsion preparation process) In the emulsion preparation step, an emulsion is prepared in which untreated vinylidene fluoride copolymer is dispersed in an aqueous medium. This emulsion preparation step can be the same as the emulsion preparation step of the first method described above. As mentioned above, a commercially available emulsion may be used.

[0078] (Heating process) In the heating step, the emulsion is heated after the emulsion preparation step. When polymerization is performed in the emulsion preparation step described above, the heating step may be carried out immediately after polymerization, or the polymerization and heating steps may be carried out separately. When heating the emulsion obtained by polymerization, it is preferable to purge and remove at least unreacted vinylidene fluoride and fluorine-containing alkyl vinyl compounds from the system after polymerization and before the heating step. Performing this operation prevents the polymerization reaction for forming the vinylidene fluoride copolymer from starting again during the heating step. It also makes it easier to adjust the state of the crystalline region of the untreated vinylidene fluoride copolymer during the heating step.

[0079] The heating temperature for the emulsion is determined as follows: First, a portion of the untreated vinylidene fluoride copolymer described above is powdered and then formed into a film. The heat flow of this film-like untreated vinylidene fluoride copolymer is measured using a differential scanning calorimeter in accordance with ASTM D3418. The heating temperature is then set to a temperature lower than the endpoint of the highest temperature endothermic peak observed in the vinylidene fluoride copolymer below 185°C. The heating process improves the dispersibility in dispersion media with a dielectric constant of 15 or less.

[0080] The method for determining the heating temperature of the emulsion is more specifically as follows: First, the emulsion is freeze-dried to make the untreated vinylidene fluoride copolymer into a powder. Then, a mold measuring 5 cm x 5 cm x 150 μm in thickness and approximately 1 g of the powdered untreated vinylidene fluoride copolymer are sandwiched between two aluminum foils sprayed with a release agent, and pressed at 200°C to form a film. Then, using a differential scanning calorimeter (METTLER "DSC-1") in accordance with ASTM D3418, the heat flow of the film-like untreated vinylidene fluoride copolymer is obtained. The endset of the highest temperature peak among the endothermic peaks of the untreated vinylidene fluoride copolymer observed below 185°C in the heat flow is identified, and the temperature lower than this endset is set as the heating temperature of the emulsion.

[0081] The endset is identified as follows: A baseline is drawn linearly so as to overlap with the linear heat flow on the high-temperature side of the endothermic peak (e.g., above 185°C). Among the endothermic peaks of the vinylidene fluoride copolymer observed below 185°C in the heat flow, a tangent is drawn to the heat flow in the temperature range higher than the highest peak top and lower than the lowest temperature at which the heat flow and baseline overlap, as shown in Figure 1. The intersection of this tangent and the baseline is defined as the endset of the vinylidene fluoride copolymer.

[0082] The temperature at which the emulsion is heated is preferably 40°C or higher, more preferably 60°C or higher, and even more preferably 80°C or higher, from the viewpoint of appropriately changing the crystal structure of the untreated vinylidene fluoride copolymer in the emulsion.

[0083] When heating at a temperature lower than the endset of the highest temperature endothermic peak of the vinylidene fluoride copolymer observed below 185°C, the crystal structure of the vinylidene fluoride copolymer in the emulsion changes appropriately, making it easier to obtain a vinylidene fluoride copolymer composition that satisfies the above-mentioned physical properties. On the other hand, when heating at a temperature higher than the said endset, the crystal structure of the vinylidene fluoride copolymer in the emulsion changes significantly, reducing the dispersion stability of the emulsion and making it difficult to extract the vinylidene fluoride copolymer composition in powder form.

[0084] When polymerization is performed in the emulsion preparation step, the heating step can be carried out immediately after polymerization. In this case, it is preferable to purge and remove at least unreacted vinylidene fluoride and fluorine-containing alkyl vinyl compounds from the system after polymerization and before the heating step, and then heat at the above temperature.

[0085] When polymerization is performed in the emulsion preparation step, the polymerization and heating steps can be carried out separately. In this case, it is preferable to cool the mixture to a temperature lower than the polymerization temperature before heating. Specifically, it is preferable to cool it to a temperature at least 5°C lower than the polymerization temperature. The method of cooling at this time is not particularly limited. By cooling to this temperature, the heating effect (change in the state of the crystalline region of the untreated vinylidene fluoride copolymer) is more easily obtained, making it easier to obtain a vinylidene fluoride copolymer composition with the desired physical properties.

[0086] On the other hand, if polymerization is not performed in the emulsion preparation step described above, for example, when using a commercially available emulsion, it is sufficient to simply heat it to the above temperature.

[0087] In this specification, heating at the heating temperature means holding the emulsion at the heating temperature for a certain period of time. The heating time is not particularly limited as long as it is within a range in which the state of the crystalline region of the untreated vinylidene fluoride copolymer changes, since the purpose of the heating process is not to remove the initiator remaining in the emulsion. As an example, 10 seconds to 24 hours is preferred, 20 seconds to 12 hours is preferred, and 30 seconds to 6 hours is more preferred. Holding the emulsion at the heating temperature for the above time makes it easier for the state of the crystalline region formed during the polymerization process of the untreated vinylidene fluoride copolymer to change, making it easier to obtain a vinylidene fluoride copolymer composition with desired physical properties.

[0088] The heating method is not particularly limited; the emulsion may be heated without stirring or while stirring, but it is preferable to heat it while stirring from the viewpoint of dispersion stability of the emulsion. The heating device is also not particularly limited. The emulsion may be heated under pressure, under saturated vapor pressure, or under atmospheric pressure using an autoclave or the like.

[0089] (drying process) In the drying step, the aqueous medium is removed from the emulsion. The method for removing the aqueous medium is not particularly limited, but it is preferable to dry it at a temperature that does not affect the physical properties of the vinylidene fluoride copolymer composition in the emulsion. The drying step may be carried out under atmospheric pressure or under reduced pressure. By this drying, the solid (powdered) vinylidene fluoride copolymer composition described above is obtained. The drying method can be the same as the drying step in the first method.

[0090] • Third method In the third method for producing a vinylidene fluoride copolymer composition, at least one step is performed after the emulsion preparation step in the second method and before the heating step in which a surfactant is added to the emulsion (hereinafter, the emulsion with the surfactant added will also be called a "surfactant-containing emulsion") (hereinafter, this is also called the "surfactant addition step"). In other words, the steps are performed in the order of emulsion preparation step, surfactant addition step, and heating step. After the heating step, the emulsion is dried to extract the vinylidene fluoride copolymer (hereinafter, this is also called the "drying step"), thereby obtaining the solid vinylidene fluoride copolymer composition described above. In this method as well, after obtaining the solid vinylidene fluoride copolymer composition, heating at a temperature above the melting point of the vinylidene fluoride copolymer composition is not performed. Each step will be described below.

[0091] (Emulsion preparation process) In the emulsion preparation step, an emulsion is prepared in which untreated vinylidene fluoride copolymer is dispersed in an aqueous medium. This emulsion preparation step can be the same as the emulsion preparation step of the first method described above.

[0092] (Surfactant addition process) In the surfactant addition step, a surfactant is added to the emulsion prepared in the emulsion preparation step described above to prepare a surfactant-containing emulsion. Furthermore, if polymerization is performed in the emulsion preparation step, the emulsion (or surfactant-containing emulsion) may be cooled before, after, or during the addition of the surfactant.

[0093] When polymerization is carried out in the emulsion preparation step described above, it is preferable to cool the emulsion or the surfactant-containing emulsion after surfactant addition to a temperature lower than the polymerization temperature in the emulsion preparation step. Specifically, it is preferable to cool it to a temperature at least 5°C lower than the polymerization temperature. The method of cooling at this time is not particularly limited. By cooling, the effect of the heating step (change in the state of the crystalline region of the untreated vinylidene fluoride copolymer) is more easily obtained, and it becomes easier to obtain a vinylidene fluoride copolymer composition having the desired physical properties.

[0094] Adding a surfactant to the emulsion in this step is preferable because it improves the stability of the emulsion during the heating step. Since it is sufficient that the emulsion is stable during the heating step due to the surfactant addition step, the surfactant addition step can be the same as the surfactant addition step of the first method described above, except that the surface tension of the emulsion after addition (surfactant-containing emulsion) and the amount added are not restricted.

[0095] In the surfactant addition step, the surfactant is added to the emulsion prepared in the emulsion preparation step described above and stirred. At this time, the surface tension of the emulsion after addition (surfactant-containing emulsion) is not particularly limited, but it is preferable that the surface tension at 25°C be 40 mN / m or less, and more preferably 35 mN / m or less.

[0096] The surfactant added in this process is not particularly limited as long as it dissolves or is miscible in the aqueous medium contained in the emulsion described above. For example, in addition to the surfactant added by the first method described above, a nonionic surfactant can also be used. The nonionic surfactants that can be used are the same as those contained in the vinylidene fluoride copolymer composition described above.

[0097] In this process, only one type of surfactant may be added, or two or more types may be added. Any excess surfactant added in this process may be removed by dialysis, ion exchange resin, or other means as needed.

[0098] (Heating process) In the heating step, the surfactant-containing emulsion obtained in the surfactant addition step described above is heated. This heating step can be the same as the heating step of the second method described above.

[0099] Furthermore, after heat treatment, any excess surfactant contained in the surfactant-containing emulsion may be removed by dialysis, ion exchange resin, or the like. For example, dialysis can be performed by injecting the heat-treated surfactant-containing emulsion into a cellulose dialysis membrane, immersing the dialysis membrane together with a tank filled with pure water, and changing the pure water in the tank at regular intervals.

[0100] (drying process) In the drying step, the aqueous medium is removed from the surfactant-containing emulsion. The drying method can be the same as that used in the drying step of the first method.

[0101] 2. Polymer dispersion The vinylidene fluoride copolymer composition described above can be used in the form of a polymer dispersion obtained by mixing the vinylidene fluoride copolymer composition with a dispersion medium, as an electrode mixture for forming an electrode layer or an electrolyte mixture for forming an electrolyte layer, as described later.

[0102] In particular, as mentioned above, the vinylidene fluoride copolymer composition exhibits excellent dispersibility in dispersion media with low dielectric constants and can maintain a stable state over a long period of time. The usable dispersion media is preferably one that can be removed by drying and has a dielectric constant of 15 or less.

[0103] Here, the dispersion medium having a relative permittivity of 15 or less is not particularly limited, but includes, for example, nonpolar solvents and low-polarity solvents. Specifically, examples include hydrocarbon compounds, ether compounds having ether bonds, ketone compounds having ketone groups, and ester compounds having ester bonds. Ester compounds are preferred from the viewpoint of ease of dehydration treatment.

[0104] Hydrocarbon compounds can be any compound composed of carbon atoms and hydrogen atoms, and may have a chain structure, a branched structure, or a cyclic structure. The number of carbon atoms is not particularly limited, and they may have multiple bonds such as double bonds and triple bonds, as well as aromatic structures. Specific examples include pentane, hexane, heptane, octane, nonane, decane, dodecane, hexene, heptene, cyclohexane, cycloheptane, toluene, xylene, mesitylene, tetralin, and the like.

[0105] Specific examples of ether compounds include alkylene glycol alkyl ethers (ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, diethylene glycol monomethyl ether, diethylene glycol, dipropylene glycol, triethylene glycol, polyethylene glycol, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, diethylene glycol monobutyl ether, diethylene glycol monobutyl ether, etc.), dialkyl ethers (dimethyl ether, diethyl ether, diisopropyl ether, dibutyl ether, etc.), and cyclic ethers (tetrahydrofuran, dioxane (including 1,2-, 1,3-, and 1,4- isomers), morpholine, etc.).

[0106] Specific examples of ketone compounds include methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, diisopropyl ketone, cyclohexanone, phorone, acetophenone, and isophorone.

[0107] Specific examples of ester compounds include ethyl acetate, butyl acetate, propyl acetate, isopropyl acetate, isopentyl acetate, benzyl acetate, ethyl butyrate, propyl butyrate, butyl butyrate, isopentyl butyrate, ethyl propionate, butyl pentanoate, methyl lactate, ethyl lactate, butyl lactate, ethylene glycol monoalkyl ether acetate, and the like. Butyl butyrate is preferred from the viewpoint of ease of dehydration.

[0108] Furthermore, the amount of dispersion medium in the polymer dispersion is appropriately selected depending on the application of the polymer dispersion and the type of vinylidene fluoride copolymer composition, but is preferably 50% to 99.9% by mass, and more preferably 75% to 99.9% by mass, based on 100% by mass of the polymer dispersion. When the amount of dispersion medium is within the above range, the dispersibility of the vinylidene fluoride copolymer composition in the polymer dispersion tends to be good.

[0109] Furthermore, the polymer dispersion may contain a solvent in addition to the dispersion medium and the vinylidene fluoride copolymer composition. The solvent is preferably a medium that can be removed by drying, and in addition to nonpolar and low-polarity solvents, polar solvents and ionic liquids are also included.

[0110] Examples of solvents include amide compounds such as dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; alcohols 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; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; lactones such as γ-butyrolactone and δ-butyrolactone; sulfoxide and sulfone compounds such as dimethyl sulfoxide and sulfolane; and ionic liquids such as ethylmethylimidazolium salt and butylmethylimidazolium salt.

[0111] The amount of solvent in the polymer dispersion is not particularly limited as long as it is sufficient to maintain the dispersibility of the vinylidene fluoride copolymer composition in the polymer dispersion, but in one example, 10% by mass or less is preferred.

[0112] 3. Electrode for non-aqueous electrolyte secondary batteries The polymer dispersion described above can be used to form electrode layers in various non-aqueous electrolyte secondary batteries. The electrodes of a non-aqueous electrolyte secondary battery include, for example, a current collector and an electrode layer placed on the current collector. In this case, the polymer dispersion described above can be used to form the electrode layer. The electrode may be used as either a positive or negative electrode.

[0113] (1) Current collector The current collectors for the negative and positive electrodes are terminals for extracting electricity. The material of the current collector is not particularly limited, and metal foils or metal meshes made of aluminum, copper, iron, stainless steel, steel, nickel, titanium, etc., can be used. Alternatively, the metal foils or metal meshes may be applied to the surface of other media.

[0114] (2) Electrode layer The electrode layer can be formed by preparing an electrode mixture by mixing the above-mentioned vinylidene fluoride copolymer composition or polymer dispersion with an active material and, if necessary, a dispersion medium, then coating the electrode mixture onto the current collector and drying it. The electrode layer may be formed on only one side of the current collector or on both sides. The dispersion medium in the electrode mixture is the same as that described for the polymer dispersion above.

[0115] The components in the electrode layer are appropriately selected depending on the type of non-aqueous electrolyte secondary battery. For example, the layer may contain the vinylidene fluoride copolymer composition described above and an active material. Furthermore, in the electrode layer of an all-solid-state battery, it is preferable to have a layer containing the vinylidene fluoride copolymer composition described above, an active material, and a solid electrolyte. The electrode layer may also contain other components as needed. Examples of other components include various additives such as conductive additives, pigment dispersants, adhesion aids, and thickeners.

[0116] The amount of vinylidene fluoride copolymer composition relative to the total amount of the electrode layer is preferably 0.1% to 50% by mass, more preferably 0.2% to 40% by mass, and even more preferably 0.3% to 30% by mass. When the amount of vinylidene fluoride copolymer composition is within this range, good adhesion between the active material, solid electrolyte and other components in the electrode layer and the current collector tends to be achieved.

[0117] The active material contained in the electrode layer is not particularly limited, and for example, conventionally known active materials for negative electrodes (negative electrode active material) or positive electrodes (positive electrode active material) can be used.

[0118] Examples of the above negative electrode active materials include carbon materials such as artificial graphite, natural graphite, graphitization-resistant carbon, graphitization-susceptible carbon, activated carbon, or those obtained by firing and carbonizing phenol resins and pitches; 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 metal oxides such as GeO, GeO2, SnO, SnO2, PbO, and PbO2. Also included are those with a coating applied to the surface of these active materials. Note that the negative electrode active material may be a commercially available product.

[0119] On the other hand, examples of the positive electrode active materials include lithium-containing lithium-based positive electrode active materials. Examples of the lithium-based positive electrode active materials include composite metal chalcogen compounds represented by the general formula LiMY2 (0 < x ≦ 1) such as LiCoO2, LiNi x Co 1-x O2 (M is one or more of transition metals such as Co, Ni, Fe, Mn, Cr, and V, and Y is a chalcogen element such as O and S); composite metal oxides having a spinel structure such as LiMn2O4; and olivine-type lithium compounds such as LiFePO4; etc. Also included are those with a coating applied to the surface of these active materials. Note that the positive electrode active material may be a commercially available product.

[0120] The amount of the active material contained in the electrode layer is appropriately selected according to its type, the function of the electrode, the type of the battery, etc., and is not particularly limited. However, in one example, it is preferably 50% by mass or more and 99.9% by mass or less with respect to the total amount of the active material, the vinylidene fluoride copolymer composition, and the conductive assistant. When the amount of the active material is within this range, for example, a sufficient charge-discharge capacity can be obtained, and the battery performance tends to be good.

[0121] In addition, the conductive additive is not particularly limited as long as it is a compound that can enhance the conductivity between active materials or between an active material and a current collector. When the electrode layer contains a solid electrolyte as another component, in addition to the above, any compound that can enhance the conductivity between the active material and the solid electrolyte, between the solid electrolyte and the current collector, or between solid electrolytes is not particularly limited. Examples of the conductive additive include acetylene black, ketjen black, carbon black, graphite powder, carbon nanofiber, carbon nanotube, and carbon fiber.

[0122] The amount of the conductive additive contained in the electrode layer is appropriately selected according to its type, the function of the electrode, the type of the battery, etc., and is not particularly limited, but can be arbitrarily set according to its type and the type of the battery. From the viewpoint of enhancing both the improvement of conductivity and the dispersibility of the conductive additive, in one example, it is preferably 0.1% by mass or more and 15% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, and still more preferably 0.1% by mass or more and 5% by mass or less with respect to the total amount of the active material, the vinylidene fluoride copolymer composition, and the conductive additive.

[0123] In addition, the solid electrolyte contained in the electrode composite layer is not particularly limited as long as it is a solid compound having ionic conductivity, and conventionally known inorganic solid electrolytes and polymer solid electrolytes can be used. Examples of the inorganic solid electrolyte include oxide-based solid electrolytes, sulfide-based solid electrolytes, nitride-based solid electrolytes, complex hydride solid electrolytes, etc. Examples of the polymer solid electrolyte include gel-based electrolytes and true polymer electrolytes.

[0124] Examples of the oxide-based solid electrolyte include, but are not limited to, perovskite-type LLTO, garnet-type LLZ, NASICON-type compounds, LISICON-type compounds, LIPON-type compounds, β-alumina-type compounds, etc. Specific examples include Li3PO4, Li 0.34 La 0.51 TiO3, Li 1.3 Al 0.3 Ti 1.7 (PO4)3, Li7La3Zr2O 12Li6BaLa2Ta2O 12 Li 2.9 PO 3.3 N 0.46 Li 4.3 Al 0.3 Si 0.7 O4, 50Li4SiO4-50Li3BO3, Li2O-Al2O3-SiO2-P2O5-TiO2, Li 1.5 Al 0.5 Ge 1.5 (PO4)3-0.05Li2O, etc., are included.

[0125] Sulfide-based solid electrolytes include solid electrolytes containing Li, A (where A is at least one of P, Si, Ge, Al, and B), and S, and these sulfide-based solid electrolytes may further contain halogen elements. Other examples include LGPS (Li-Ge-PS) type compounds, argyrodite type compounds, amorphous compounds, and Li-PS type compounds. Specific examples of sulfide-based solid electrolytes include Li2S-P2S5, Li2S-P2S3, Li2S-P2S3-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, LiI-Li2S-SiS2-P2S5, Li2S-SiS2-Li4SiO4, Li2S-SiS2-Li3PO4, Li3PS4-Li4GeS4, and Li 3.4 P 0.6 Si 0.4 S4, Li 3.25 P 0.25 Ge 0.76 S4, Li 3.25 P 0.75 Ge 0.25 S4, Li 10 GeP2S 12 Li 4-x Ge 1-x P x This includes S4, Li6PS5Cl, Li6PS5Br, Li6PS5I, etc.

[0126] Nitride-based solid electrolytes are not limited to these, but specific examples include LiN3.

[0127] Examples of complex hydride solid electrolytes include, but are not limited to, LiBH4.

[0128] While not limited to these, gel electrolytes include specific examples such as Poly(ethylene oxide)8-LiClO4 (ethylene carbonate (EC) + propylene carbonate (PC)), Poly(ethylene oxide)8-LiClO4(PC), Poly(vinylidene fluoride)-LiN(CF3SO2)2(EC+PC), Poly(vinylidene fluoride-co-hexafluoropropylene)-LiPF6 (EC + diethyl carbonate (DEC) + dimethyl carbonate (DMC)), Poly(ethylene glycol acrylate)-LiClO4(PC), Poly(acrylonitrile)-LiClO4(EC+PC), and Poly(methyl methacrylate)-LiClO4(PC).

[0129] Intrinsic polymer electrolytes are not limited to these, but specific examples include Poly(ethylene oxide)8-LiClO4, Poly(oxymethylene)-LiClO4, Poly(propylene oxide)8-LiClO4, Poly(dimethyl siloxane)-LiClO4, Poly(vinylidene fluoride-co-hexafluoropropylene)-LiTFSI, Poly(2,2-dimethoxypropylene carbonate)-LiFSI, Poly[(2-methoxy)ethylglycidyl ether]8-LiClO4, etc.

[0130] The electrode layer may contain only one type of solid electrolyte, or it may contain two or more types.

[0131] When the electrode layer contains a solid electrolyte, the amount of solid electrolyte is appropriately selected according to its type, the function of the electrode, the type of battery, etc., and is not particularly limited. However, in one example, it is preferable that the amount of solid electrolyte be between 1% by mass and 99.9% by mass relative to the total amount of the active material, the vinylidene fluoride copolymer composition, and the solid electrolyte. When the amount of solid electrolyte is within this range, sufficient ionic conductivity can be obtained, and the battery performance tends to be good.

[0132] As described above, the electrode layer may contain pigment dispersants, adhesion aids, thickeners, etc., and known compounds can be used as these. The amounts of these are not particularly limited as long as they do not impair the purpose and effects of the present invention, but in one example, it is preferable that they be 15% by mass or less relative to the active material, the vinylidene fluoride copolymer composition, and the total amount thereof.

[0133] The electrode layer may further contain additives such as phosphorus compounds, sulfur compounds, organic acids, amine compounds, and nitrogen compounds such as ammonium compounds; organic esters, various silane-based, titanium-based, and aluminum-based coupling agents; vinylidene fluoride polymers other than the vinylidene fluoride copolymers mentioned above, and resins such as polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and polyacrylonitrile (PAN). These are not particularly limited as long as they do not impair the purpose and effects of the present invention, but in one example, it is preferable that they be 15% by mass or less of the total amount of the active material, vinylidene fluoride copolymer composition, and additives.

[0134] Here, the thickness of the electrode layer is not particularly limited, but in one example, it is preferably between 1 μm and 1000 μm. Also, the basis weight of the active material contained in the electrode layer is not particularly limited and can be any basis weight, but in one example, it is 50 to 1000 g / m². 2 Preferably, 100-500 g / m 2 This is preferable.

[0135] (Method for forming electrode layers) The electrode layer described above can be formed by the following steps: preparing an electrode mixture by mixing the aforementioned vinylidene fluoride copolymer composition or polymer dispersion with an active material and, if necessary, a solid electrolyte, dispersion medium, solvent, conductive additive, and various other additives; applying the electrode mixture onto a current collector; and drying it.

[0136] The electrode mixture described above may be prepared by mixing all the components at once, or by mixing some of the components first and then mixing the remaining components later. In this case, it is preferable to mix the electrode layer mixture using a mixer equipped with a temperature control device so that the temperature of the electrode layer mixture does not rise excessively.

[0137] Furthermore, the dispersion medium and solvent in the electrode mixture may be any solvent that can uniformly disperse the vinylidene fluoride copolymer composition with the active material, solid electrolyte, conductive additive, etc. The type of dispersion medium to be added is not particularly limited, but it is preferably the same as the dispersion medium contained in the polymer dispersion described above. Also, if a solvent is added, it is preferably the same as the solvent contained in the polymer dispersion described above. The total amount of dispersion medium in the polymer dispersion and the dispersion medium added thereto is not particularly limited and can be any amount from a manufacturing standpoint, but in one example, it is preferably 10 parts by mass or more and 20,000 parts by mass or less per 100 parts by mass of the active material described above. The total amount of solvent in the polymer dispersion and the solvent added thereto is not particularly limited and can be any amount from a manufacturing standpoint, but in one example, it is preferably 2,000 parts by mass or less per 100 parts by mass of the active material described above.

[0138] The viscosity of the electrode mixture is not particularly limited, as long as it prevents dripping, uneven coating, and delayed drying after coating when applying the electrode mixture to obtain the electrode, and provides good workability for electrode fabrication and electrode coating. In one example, a viscosity of 0.1 Pa·s or more and 100 Pa·s or less is preferred. The viscosity of the electrode mixture is measured using an E-type viscometer or the like.

[0139] Furthermore, the method of applying the electrode mixture is not particularly limited, and methods such as the doctor blade method, reverse roll method, comma bar method, gravure method, air knife method, die coat method, and dip coat method can be applied.

[0140] Furthermore, after applying the electrode mixture, the solvent (dispersion medium) is dried by heating at an arbitrary temperature. In one example, the drying temperature is preferably between 30°C and 500°C. Drying may be performed multiple times at different temperatures. During this time, drying may be performed under atmospheric pressure, under pressure, or under reduced pressure. Further heat treatment may be performed after drying.

[0141] After applying and drying the electrode mixture described above, a pressing treatment may be performed. Pressing treatment can improve the electrode density. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

[0142] 4. Electrolyte layer for nonaqueous electrolyte secondary batteries The vinylidene fluoride copolymer composition or polymer dispersion described above can also be used, for example, in the production of an electrolyte layer for a non-aqueous electrolyte secondary battery. The electrolyte layer for a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as the "electrolyte layer") may, for example, consist only of the electrolyte layer. The polymer dispersion described above (vinylidene fluoride copolymer) can be used as a material for the electrolyte layer.

[0143] The electrolyte layer may contain at least the vinylidene fluoride copolymer composition and an electrolyte as described above, and may further contain other components as needed. The electrolyte layer may be a layer for binding the electrode and the electrolyte, a layer for conducting various ions, or a layer that performs these functions simultaneously. Within the electrolyte layer, the vinylidene fluoride copolymer composition may be in the form of parts, a film (including a porous film), or a gel.

[0144] All-solid-state batteries have a structure in which an electrolyte layer is sandwiched between a pair of electrodes (each having a current collector and an electrode layer). The polymer dispersion described above can also be used to form the electrolyte layer of such an all-solid-state battery.

[0145] The electrolyte layer may be a layer prepared by preparing an electrolyte mixture containing, for example, a vinylidene fluoride copolymer composition or polymer dispersion, a solid electrolyte, a dispersion medium if necessary, and other optional components, coating this mixture onto a substrate, and drying it. In this case, the layer may be obtained by peeling the dried layer off the substrate. Alternatively, the electrolyte mixture may be directly coated onto the electrode and dried to form a layer.

[0146] The solid electrolyte contained in the electrolyte layer can be the same compound as that described in the explanation of the electrode layer of the electrode mentioned above. The amount of solid electrolyte relative to the total amount of the electrolyte layer is appropriately selected according to its type, the function of the electrolyte layer, the type of battery, etc., and is not particularly limited, but in one example, it is preferable that it be 10% by mass or more and 99.9% by mass or less relative to the total amount of the electrolyte layer.

[0147] Furthermore, the electrolyte layer may contain components other than the vinylidene fluoride copolymer and the solid electrolyte described above. Examples of such components include pigment dispersants, adhesion aids, thickeners, fillers, and various additives. Known compounds can be used as pigment dispersants, adhesion aids, and thickeners, and may be the same as the additives contained in the electrode layer. The amounts of these components are not particularly limited as long as they do not impair the purpose and effects of the present invention, but in one example, 0.1% to 90% by mass of the total amount of the electrolyte layer is preferred.

[0148] The filler contained in the electrolyte layer may be an inorganic filler or an organic filler. Examples of inorganic fillers include oxides such as silicon dioxide (SiO2), alumina (Al2O3), titanium dioxide (TiO2), calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO), magnesium oxide (MgO), zinc oxide (ZnO), and barium titanate (BaTiO3); hydroxides such as magnesium hydroxide (Mg(OH)2), calcium hydroxide (Ca(OH)2), zinc hydroxide (Zn(OH)2), aluminum hydroxide (Al(OH)3), and aluminum hydroxide oxide (AlO(OH)); carbonates such as calcium carbonate (CaCO3); sulfates such as barium sulfate; nitrides; clay minerals; and boehmite. The filler may contain only one type or two or more types. Furthermore, these amounts are not particularly limited as long as they do not impair the purpose and effects of the present invention, but in one example, 0.1% by mass or more and 90% by mass or less of the total amount of the electrolyte layer is preferred.

[0149] The thickness of the electrolyte layer is selected appropriately according to the function of the electrolyte layer and is not particularly limited, but in one example, 1 μm to 1000 μm is preferred.

[0150] The amount of vinylidene fluoride copolymer composition relative to the total amount of the electrolyte layer is appropriately selected according to its type, the function of the electrolyte layer, the type of battery, etc., and is not particularly limited, but in one example, 0.1% by mass or more and 10% by mass or less is preferred.

[0151] The method for forming the electrolyte layer is not particularly limited, and as described above, it can be formed by applying and drying the electrolyte mixture. The method for applying and drying the electrolyte mixture is the same as the method for forming the electrode layer. In addition, the polymer dispersion described above may be mixed directly with the solid electrolyte as the electrolyte mixture, or a dispersion medium or solvent may be added as needed. The dispersion medium used to form the electrolyte layer is not particularly limited, but it is preferable that it be the same as the dispersion medium contained in the polymer dispersion described above.

[0152] 5.Nonaqueous electrolyte secondary battery As described above, the vinylidene fluoride copolymer composition or polymer dispersion can be used as electrodes or electrolyte layers in various non-aqueous electrolyte secondary batteries, including all-solid-state batteries, but it may also be used to form other layers in non-aqueous electrolyte secondary batteries. [Examples]

[0153] The present invention will be described in more detail below with reference to examples. These examples should not be interpreted as limiting the scope of the present invention.

[0154] <Emulsion Preparation> (1) Preparation of Emulsion 1 280 parts by mass of deionized water were placed in an autoclave and degassed by nitrogen bubbling for 30 minutes. Next, 0.2 parts by mass of disodium hydrogen phosphate and 1.0 part by mass of ammonium perfluorooctanoate (PFOA) were added, and the autoclave was pressurized to 4.5 MPa and nitrogen purged three times. Then, 0.1 parts by mass of ethyl acetate, 11 parts by mass of vinylidene fluoride (VDF), and 24 parts by mass of hexafluoropropylene (HFP) were added to the autoclave. The mixture was heated to 80°C while stirring. Then, a 5% by mass aqueous solution of ammonium persulfate (APS) was added so that the amount of APS was 0.06 parts by mass, and polymerization was started. Immediately after the start of polymerization, 65 parts by mass of VDF were continuously added to maintain the internal pressure of the autoclave at 2.5 MPa. After the addition was complete, polymerization was considered finished when the pressure dropped to 1.5 MPa. After cooling to below 40°C, residual monomers were purged from the autoclave to remove them, and emulsion 1 was obtained in which the untreated vinylidene fluoride copolymer was dispersed in water. The solid content concentration (concentration of vinylidene fluoride copolymer) of emulsion 1 was 21.0% by mass. Furthermore, the untreated vinylidene fluoride copolymer was extracted by freeze-drying, and the endset of the endothermic peak observed below 185°C of the vinylidene fluoride copolymer was measured according to the method described later, and it was found to be 132°C.

[0155] (How to identify the end set) First, the emulsion was freeze-dried to obtain powdered vinylidene fluoride copolymer. Next, a mold measuring 5 cm x 5 cm x 150 μm thick and approximately 1 g of powdered vinylidene fluoride copolymer were placed between two aluminum foils sprayed with a release agent, and pressed at 200°C to produce a film. Then, the heat flow of the vinylidene fluoride copolymer was obtained by measuring it in accordance with ASTM D3418 using a differential scanning calorimeter (METTLER "DSC-1"). In the obtained heat flow, a baseline was drawn linearly so that it overlapped with the linear heat flow in the temperature range higher than the highest temperature peak top of the endothermic peaks of the vinylidene fluoride copolymer observed below 185°C. Then, in the temperature range higher than the highest-temperature peak top among the endothermic peaks of the vinylidene fluoride copolymer observed below 185°C, and lower than the minimum temperature at which the heat flow and baseline overlap, a tangent was drawn to the heat flow, and the intersection of this tangent and the baseline was defined as the endset of the vinylidene fluoride copolymer.

[0156] (2) Preparation of Emulsion 2 Polymerization was carried out in the same manner as the preparation of emulsion 1, except that the amount of VDF added in a single batch to the autoclave before polymerization was changed from 11 parts by mass to 15 parts by mass, and the amount of HFP was changed from 24 parts by mass to 20 parts by mass, to obtain emulsion 2 in which the untreated vinylidene fluoride copolymer was dispersed in water. The solid content concentration (concentration of vinylidene fluoride copolymer) of emulsion 2 was 20.8% by mass. Furthermore, the endset of the highest temperature peak among the endothermic peaks of the vinylidene fluoride copolymer observed below 185°C, which were identified by the same method as for emulsion 1, was 134°C.

[0157] (3) Preparation of Emulsion 3 Polymerization was carried out in the same manner as for emulsion 1, except that the amount of VDF added in a single batch to the autoclave before polymerization was changed from 11 parts by mass to 25 parts by mass, and the amount of HFP was changed from 24 parts by mass to 10 parts by mass, to obtain emulsion 3 in which the untreated vinylidene fluoride copolymer was dispersed in water. The solid content concentration (concentration of vinylidene fluoride copolymer) of emulsion 3 was 21.3% by mass. The endset of the highest temperature peak among the endothermic peaks of the untreated vinylidene fluoride copolymer, identified by the same method as for emulsion 1, was 152°C.

[0158] <Example 1> Emulsion 1 was placed in an autoclave, and then sodium dodecyl sulfate (SDS) was added to the emulsion so that its concentration relative to the water in the emulsion was 1% by mass, thereby obtaining a surfactant-containing emulsion. At this time, the surface tension of the surfactant-containing emulsion was measured at 25°C. The results are shown in Table 1.

[0159] Then, while stirring at 500 rpm, the mixture was heated at 125°C, a temperature lower than the end set temperature, for 1 hour. After that, while continuing to stir, it was air-cooled at room temperature (24°C) until the temperature inside the can dropped to 40°C or below. The emulsion was then freeze-dried to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 174 μm. The freeze-dried product was obtained by freezing the polymerized emulsion with liquid nitrogen and then drying it under reduced pressure at room temperature.

[0160] <Example 2> A powdered vinylidene fluoride copolymer composition was obtained by heat treatment and freeze-drying in the same manner as in Example 1, except that the concentration of SDS in water in emulsion 1 was changed from 1% by mass to 0.5% by mass. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 141 μm.

[0161] <Example 3> Except for changing the surfactant added to emulsion 1 from SDS to Emulgen LS-110 (manufactured by Kao Corporation, polyoxyalkylene alkyl ether), heat treatment and freeze-drying were performed in the same manner as in Example 2 to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 197 μm.

[0162] <Example 4> A powdered vinylidene fluoride copolymer composition was obtained by heat treatment and freeze-drying in the same manner as in Example 2, except that the surfactant added to emulsion 1 was changed from SDS to PFOA (perfluorooctanoic acid). The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 163 μm.

[0163] <Example 5> Except for changing the heating temperature of emulsion 1 from 125°C to 95°C, heat treatment and freeze-drying were performed in the same manner as in Example 2 to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 391 μm.

[0164] <Example 6> Except for changing the heating temperature of emulsion 1 from 125°C to 75°C, heat treatment and freeze-drying were performed in the same manner as in Example 2 to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 364 μm.

[0165] <Example 7> A powdered vinylidene fluoride copolymer composition was obtained by heat treatment and freeze-drying in the same manner as in Example 2, except that the concentration of SDS in water in emulsion 1 was changed from 0.5% by mass to 0% by mass. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 173 μm.

[0166] <Example 8> A powdered vinylidene fluoride copolymer composition was obtained by heat treatment and freeze-drying in the same manner as in Example 6, except that the concentration of SDS in water in emulsion 1 was changed from 0.5% by mass to 0% by mass. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 424 μm.

[0167] <Example 9> Except for changing the emulsion placed in the autoclave from emulsion 1 to emulsion 2 and changing the heating temperature of the emulsion after the surfactant was added (surfactant-containing emulsion) from 125°C to 130°C, heat treatment and freeze-drying were performed in the same manner as in Example 2 to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 52 μm.

[0168] <Example 10> Emulsion 1 was placed in an autoclave, and PFOA was added to the emulsion so that the concentration of PFOA relative to the water in the emulsion was 0.5% by mass. The PFOA was then dissolved in the emulsion by stirring at 500 rpm at room temperature (24°C). The emulsion was freeze-dried to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 160 μm.

[0169] <Example 11> Emulsion 1 was placed in an autoclave, and SDS was added to the emulsion so that its concentration relative to the water in the emulsion was 1% by mass. The mixture was stirred at 500 rpm at room temperature (24°C) to dissolve the SDS in the emulsion. The emulsion was freeze-dried to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 148 μm.

[0170] <Example 12> Dissolution and freeze-drying were carried out in the same manner as in Example 11, except that the concentration of SDS in the emulsion relative to water was changed from 1% by mass to 0.5% by mass, to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 126 μm.

[0171] <Example 13> Dissolution and freeze-drying were carried out in the same manner as in Example 10, except that the concentration of PFOA in the emulsion relative to water was changed from 0.5% by mass to 1% by mass, to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 219 μm.

[0172] <Example 14> Dissolution and freeze-drying were carried out in the same manner as in Example 10, except that the concentration of PFOA in the emulsion relative to water was changed from 0.5% by mass to 0.1% by mass, to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 178 μm.

[0173] <Example 15> Emulsion 1 was placed in an autoclave, and acetamine 86 (manufactured by Kao Corporation, stearylamine acetate) was added so that the concentration of acetamine 86 relative to the water in the emulsion was 0.05% by mass. The mixture was stirred at 500 rpm at room temperature (24°C) to dissolve the acetamine 86 in the emulsion. The emulsion was freeze-dried to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 821 μm.

[0174] <Comparative Example 1> Emulsion 1 was freeze-dried to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 215 μm.

[0175] <Comparative Example 2> The powdered vinylidene fluoride copolymer composition obtained in Example 11 was heat-treated at 125°C for 1 hour to obtain another powdered vinylidene fluoride copolymer composition. Due to the severe aggregation of this vinylidene fluoride copolymer composition, the average secondary particle size could not be measured.

[0176] <Comparative Example 3> The powdered vinylidene fluoride copolymer composition obtained in Comparative Example 1 was heat-treated at 125°C for 1 hour to obtain another powdered vinylidene fluoride copolymer composition. Due to the severe aggregation of this vinylidene fluoride copolymer composition, the average secondary particle size could not be measured.

[0177] <Comparative Example 4> Dissolution and freeze-drying were carried out in the same manner as in Example 10, except that the surfactant added to the emulsion was changed from PFOA to Emulgen LS-110, to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 127 μm.

[0178] <Comparative Example 5> Except for changing the emulsion placed in the autoclave from emulsion 1 to emulsion 3 and changing the heating temperature of the emulsion from 125°C to 150°C, heat treatment and freeze-drying were performed in the same manner as in Example 2 to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of the powdered vinylidene fluoride copolymer composition was 39 μm.

[0179] <Comparative Example 6> To the powdered vinylidene fluoride copolymer composition obtained in Comparative Example 1, sodium dodecyl sulfate (SDS) was added in an amount equivalent to 1% by mass relative to the water in the emulsion before freeze-drying to obtain a powdered vinylidene fluoride copolymer composition. The average secondary particle size of this powdered vinylidene fluoride copolymer composition was 215 μm.

[0180] <Comparative Example 7> The heat treatment was carried out in the same manner as in Example 2, except that the heating temperature of emulsion 1 was changed from 125°C to 180°C. Because the emulsion could not be recovered as a dispersion-stable emulsion during this heating process, a powdered vinylidene fluoride copolymer composition could not be obtained.

[0181] <Comparative Example 8> The heat treatment was carried out in the same manner as in Example 7, except that the heating temperature of emulsion 1 was changed from 125°C to 180°C. Because the emulsion could not be recovered as a dispersion-stable emulsion after this heating process, a powdery vinylidene fluoride copolymer composition could not be obtained.

[0182] <Methods for measuring various physical properties> The physical properties of the above-mentioned vinylidene fluoride copolymer composition and emulsion were measured as follows.

[0183] • Measurement of solid content concentration for each emulsion Approximately 5 g of the obtained emulsion was placed in an aluminum cup and dried at 80°C for 3 hours. The concentration of vinylidene fluoride copolymer (solid content concentration) in the emulsion was calculated by measuring the weight before and after drying.

[0184] • Measurement of the average secondary particle size of vinylidene fluoride copolymer compositions The average secondary particle size of the vinylidene fluoride copolymer composition was measured by volume-based laser diffraction / scattering for the powdered vinylidene fluoride copolymer composition, and the cumulative average diameter (D50) of the particle size distribution was calculated. Specifically, using a Microtrac MT3300EXII manufactured by Microtrac-Bell, approximately 0.5 mg of the powdered vinylidene fluoride copolymer composition was dispersed in water by stirring to prepare the sample for measurement. The measurement medium was water, with a medium refractive index of 1.333, a non-spherical particle shape, a particle refractive index of 1.42, and a measurement time of 30 seconds. The average value of D50 obtained from five measurements in transmission mode was defined as the average secondary particle size.

[0185] • Melting point measurement of vinylidene fluoride copolymer composition The melting point of the vinylidene fluoride copolymer composition was measured in the form of a film prepared by the following method. First, a mold measuring 5 cm x 5 cm x 150 μm in thickness and approximately 1 g of powdered vinylidene fluoride copolymer composition were placed between two sheets of aluminum foil sprayed with a release agent, and pressed at 200°C to obtain a pressed film. The melting point was then measured using a differential scanning calorimeter (METTLER DSC-1) in accordance with ASTM D3418.

[0186] • Temperature-modulated differential scanning calorimetry The peak top and enthalpy of melt of the reversing heat flow (RHF) of the vinylidene fluoride copolymer composition were measured using a temperature-modulated differential scanning calorimeter (Q-100, TA Instruments). Specifically, approximately 5 mg of the vinylidene fluoride copolymer composition, powdered by freeze-drying, was packed into an aluminum pan and used as the measurement sample. The measurement conditions were set to be heat-only conditions, with an average heating rate of 5°C / min, a modulation period of 40 seconds, and a modulation amplitude of ±0.531°C. The obtained reversing heat flow had a downward-convex endothermic peak. In the obtained reversing heat flow, a baseline was drawn linearly so as to overlap with the linear heat flow on the higher temperature side of the endset. Then, among the downward-convex endothermic peaks of the reversing heat flow, the point on the reversing heat flow that was furthest from the baseline when a vertical line was drawn from the baseline toward the reversing heat flow was defined as the peak top of the endothermic peak, and the temperature of the peak top was determined. Furthermore, the number of minimum values ​​of the downwardly convex endothermic peak in the reversing heat flow was defined as the number of peak tops of the endothermic peak. On the other hand, the region enclosed by the baseline and the reversing heat flow was defined as the enthalpy of melting (ΔHm).

[0187] • Measurement of the surface tension of emulsions The surface tension of the emulsion (or the emulsion after the addition of a surfactant, if applicable) was measured using the Wilhelmy method with a surface tensimeter (Sigma701 / 700, KSV Instruments). A platinum plate was used for the measurement, and the average value of three measurements at 25°C was taken as the surface tension value.

[0188] • Measurement of the content of vinylidene fluoride copolymer composition in the supernatant of a butyl butyrate dispersion of vinylidene fluoride copolymer composition. First, 2 g of powdered vinylidene fluoride copolymer composition was added to 18 g of butyl butyrate, and the mixture was stirred on a stirrer at 25°C for 30 minutes to prepare a butyl butyrate dispersion of vinylidene fluoride copolymer composition (containing 10% by mass of vinylidene fluoride copolymer composition). 20 mL of this dispersion was placed in a 20 mL graduated cylinder, covered with Parafilm, and left to stand for 20 hours. 4 mL of the supernatant from the graduated cylinder was pipetted, placed in an aluminum cup, dried at 135°C for 1 hour, and then allowed to cool in a desiccator for 1 hour. The content of vinylidene fluoride copolymer composition in the supernatant of the butyl butyrate dispersion of vinylidene fluoride copolymer composition was determined by measuring the weight before and after drying.

[0189] Turbidity The turbidity of a dispersion of vinylidene fluoride copolymer composition in butyl butyrate was measured by the following method. First, 2 g of powdered vinylidene fluoride copolymer composition was added to 18 g of butyl butyrate and stirred on a stirrer at 25°C for 30 minutes to prepare a butyl butyrate dispersion of vinylidene fluoride copolymer composition (containing 10% by mass of vinylidene fluoride copolymer composition). After the sample was allowed to stand for 20 hours and then stirred again, the turbidity of the dispersion was measured using an NDH2000 (compliant with JIS K 7136) manufactured by Nippon Denshoku Industries. The sample was placed in a quartz cell. The turbidity of the sample was calculated by setting the turbidity of butyl butyrate to 0%.

[0190] [Table 1]

[0191] [Table 2]

[0192] As shown in Table 2 above, the dispersibility of the vinylidene fluoride copolymer composition was good when the melting point of the vinylidene fluoride copolymer composition was 140°C or lower, the vinylidene fluoride copolymer composition had an endothermic peak with a melting enthalpy (ΔHm) of 2 mJ / g in a reversing heat flow, the absolute value of the difference between the maximum endothermic peak temperature (the peak top temperature of the largest endothermic peak) and the melting point of the composition was 10°C or lower, and the content of the vinylidene fluoride copolymer composition in the upper part of a dispersion (containing 10% by mass of the vinylidene fluoride copolymer composition) obtained by dispersing the vinylidene fluoride copolymer composition in butyl butyrate was 4.0% by mass or more and 10% by mass or less after standing for 20 hours (Examples 1 to 15).

[0193] In contrast, when the melting point of the vinylidene fluoride copolymer composition exceeded 140°C, it was very prone to settling in butyl butyrate (Comparative Example 5). Furthermore, when the absolute difference between the maximum endothermic peak temperature (peak top temperature of the largest endothermic peak) of 2 mJ / g and the melting point of the vinylidene fluoride copolymer composition in the reversing heat flow (ΔHm) exceeded 10°C, aggregation was severe (Comparative Examples 2 and 3). Moreover, when the content of the vinylidene fluoride copolymer composition in the upper part of a dispersion of the vinylidene fluoride copolymer composition in butyl butyrate (vinylidene fluoride copolymer composition content: 10% by mass) after standing for 20 hours was 4.0% by mass or more and exceeding 10% by mass, the dispersibility was poor in all cases (Comparative Examples 1-8).

[0194] Furthermore, as shown in Table 1, by adding a certain amount of surfactant to an emulsion (polymer dispersion) of vinylidene fluoride copolymer, or by heating the emulsion, a vinylidene fluoride copolymer composition with desired physical properties was obtained (Examples 1 to 15). In contrast, when no surfactant was added to the emulsion of vinylidene fluoride copolymer, and the emulsion was not heated, the vinylidene fluoride copolymer composition tended to precipitate in butyl butyrate (Comparative Example 1).

[0195] Furthermore, regardless of whether a surfactant was added to the emulsion, when the vinylidene fluoride copolymer composition was heated after being in powder form, the vinylidene fluoride copolymer composition tended to precipitate in butyl butyrate (Comparative Examples 2 and 3).

[0196] Furthermore, even when a non-ionic surfactant was added to the emulsion and the emulsion was not heated, the vinylidene fluoride copolymer composition readily precipitated in butyl butyrate (Comparative Example 4).

[0197] Furthermore, when the vinylidene fluoride copolymer composition was in powder form before the surfactant was added, the vinylidene fluoride copolymer composition tended to precipitate in butyl butyrate (Comparative Example 6).

[0198] Furthermore, when the melting point of the vinylidene fluoride copolymer composition exceeded 140°C, adding a surfactant to the emulsion or heating the surfactant-containing emulsion did not result in obtaining a vinylidene fluoride copolymer composition with the desired physical properties (Comparative Example 5).

[0199] Furthermore, when an emulsion of vinylidene fluoride copolymer was heated at a temperature higher than the end set of the highest temperature endothermic peaks observed below 185°C when the heat flow of the vinylidene fluoride copolymer in the emulsion was measured with a suggestive scanning calorimeter, a vinylidene fluoride copolymer composition with the desired properties could not be obtained, whether or not a surfactant was added to the emulsion (Comparative Examples 7 and 8).

[0200] This application claims priority under Japanese Patent Application No. 2020-198148, filed on November 30, 2020. All provisions of the said application are incorporated herein by reference. [Industrial applicability]

[0201] The vinylidene fluoride copolymer composition of the present invention disperses easily in dispersion media with low dielectric constant, and the dispersion does not settle easily even when stored for a long period of time. Therefore, it is extremely useful for the production of electrode layers and electrolyte layers for non-aqueous electrolyte secondary batteries.

Claims

1. A vinylidene fluoride copolymer, The vinylidene fluoride copolymer comprises a constituent unit derived from vinylidene fluoride and a constituent unit derived from a fluorine-containing alkyl vinyl compound. The melting point of the vinylidene fluoride copolymer is 140°C or lower. The average primary particle size of the vinylidene fluoride copolymer is 5 μm or less. When the reversing heat flow of the vinylidene fluoride copolymer is measured with a temperature-modulated differential scanning calorimeter, the enthalpy of melting (ΔHm) is 2 J / g or more, and the copolymer has an endothermic peak with two or more peak tops. The absolute value of the difference between the peak top temperature of the largest endothermic peak among the aforementioned endothermic peaks and the melting point of the vinylidene fluoride copolymer is 10°C or less. A dispersion containing butyl butyrate and the vinylidene fluoride copolymer, wherein the content of the vinylidene fluoride copolymer is 10% by mass, is stirred at 25°C for 30 minutes and allowed to stand for 20 hours. After this, the content of the vinylidene fluoride copolymer in the upper 20% by volume of the dispersion is 7.0% by mass or more and 10% by mass or less. vinylidene fluoride copolymer.

2. The fluorine-containing alkyl vinyl compound is hexafluoropropylene. The vinylidene fluoride copolymer according to claim 1.

3. The vinylidene fluoride copolymer contains 15% to 70% by mass of the hexafluoropropylene-derived constituent units, with respect to 100% by mass of the constituent units. The vinylidene fluoride copolymer according to claim 2.

4. The vinylidene fluoride copolymer according to Claim 1, containing surfactants, A vinylidene fluoride copolymer composition.

5. A vinylidene fluoride copolymer according to any one of claims 1 to 3, A dispersion medium having a relative permittivity of 15 or less, A polymer dispersion containing the polymer.

6. A vinylidene fluoride copolymer according to any one of claims 1 to 3, Electrode for non-aqueous electrolyte secondary batteries.

7. A vinylidene fluoride copolymer according to any one of claims 1 to 3, Electrolyte layer for non-aqueous electrolyte secondary batteries.

8. A vinylidene fluoride copolymer according to any one of claims 1 to 3, Nonaqueous electrolyte secondary battery.

9. A method for producing a vinylidene fluoride copolymer comprising a constituent unit derived from vinylidene fluoride and a constituent unit derived from a fluorine-containing alkyl vinyl compound, A step of preparing an emulsion in which untreated vinylidene fluoride copolymer is dispersed in an aqueous medium by polymerization, The process involves heating the emulsion at a temperature lower than the endpoint of the highest-temperature endothermic peak observed below 185°C when the heat flow of the untreated vinylidene fluoride copolymer is measured with a differential scanning calorimeter, and at a temperature of 40°C or higher. Includes, The melting point of the vinylidene fluoride copolymer is 140°C or lower. The process further includes, after the step of preparing the emulsion and before the step of heating the emulsion, a step of cooling the emulsion to a temperature at least 5°C lower than the polymerization temperature. A method for producing vinylidene fluoride copolymer.

10. After the step of preparing the emulsion, and before the step of heating the emulsion, The process further includes adding a surfactant to the emulsion. A method for producing a vinylidene fluoride copolymer according to claim 9.

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