Binder, electrode mixture, electrode, and lithium-ion secondary battery

Abstract

The present invention addresses the problem of providing a binder that has sufficient adhesiveness in small quantities and that is not prone to gelling even when mixed with a positive electrode active material including nickel. A binder that solves the above problem contains a vinylidene fluoride polymer, and the vinylidene fluoride polymer includes vinylidene fluoride-derived structural units and two or more structural units represented by a specific structural formula.

Description

[Technical Field]

[0001] The present invention relates to a binder for a positive electrode of a lithium ion secondary battery, an electrode mixture and an electrode using the same, and a lithium ion secondary battery. [Background technology]

[0002] The use of non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, as automotive batteries has been widely considered. High capacity is required for automotive secondary batteries. Therefore, reducing the amount of binder in each electrode of a lithium-ion secondary battery and increasing the proportion of active material is considered important for achieving high capacity. Generally, polyvinylidene fluoride (PVDF) is used as the binder for the positive electrode of a lithium-ion secondary battery, but various research efforts are being conducted to further improve the adhesiveness of the binder.

[0003] For example, Patent Document 1 discloses a vinylidene fluoride polymer that has better adhesiveness to metal foil than conventional vinylidene fluoride polymers. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2012 / 090876 Summary of the Invention [Problem to be solved by the invention]

[0005] However, even the vinylidene fluoride polymers described in Patent Document 1 mentioned above sometimes have insufficient adhesiveness, and further improvements are needed. Furthermore, when a positive electrode active material is combined with a conventional vinylidene fluoride polymer, the electrode mixture is prone to gelation, which is an issue. In recent years, in order to increase the capacity of batteries, the use of ternary compounds with a high nickel ratio as positive electrode active materials has been considered, but the use of such positive electrode active materials further accelerates the progression of gelation.

[0006] Therefore, an object of the present invention is to provide a binder that is unlikely to cause gelation of an electrode mixture even when mixed with a positive electrode active material and that has sufficient adhesiveness even with a small amount, as well as an electrode mixture, electrode, and battery that contain the binder. [Means for solving the problem]

[0007] The present invention provides a binder for a positive electrode of a lithium ion secondary battery, containing a vinylidene fluoride polymer, wherein the vinylidene fluoride polymer contains a constituent unit derived from vinylidene fluoride and two or more constituent units represented by the following general formula (1): [ka] (In general formula (1), R 1 ~R 3 each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 4 carbon atoms which may have a substituent; X represents an alkylene group having 1 to 4 carbon atoms which may have a substituent; and n represents an integer of 0 to 5.

[0008] The present invention also provides an electrode mixture containing the above binder and a positive electrode active material.

[0009] The present invention provides an electrode in which an electrode mixture layer containing the above electrode mixture is provided on a current collector.The present invention also provides a lithium ion secondary battery including the above electrode. [Effects of the Invention]

[0010] The binder of the present invention is less likely to gel when mixed with a positive electrode active material containing nickel, and has sufficient adhesiveness even in small amounts. Therefore, it is possible to provide an electrode mixture that can be used stably for a long period of time, and it is also possible to provide a high-capacity lithium-ion secondary battery and an electrode for use therein. DETAILED DESCRIPTION OF THE INVENTION

[0011] 1. Binder As mentioned above, various vinylidene fluoride polymers have been used as binders for the positive electrodes of lithium-ion secondary batteries, but there has been a need to further improve their adhesiveness. In addition, there has been a problem in that when vinylidene fluoride polymers are mixed with positive electrode active materials, especially positive electrode active materials with a high nickel content, they tend to gel.

[0012] The reason for this is thought to be as follows: Positive electrode active materials contain bases, and positive electrode active materials with a high nickel content contain particularly large amounts of bases. Therefore, when vinylidene fluoride comes into contact with the positive electrode active material, the base accelerates the degradation of the binder. The degraded binder then forms a crosslinked structure in the electrode mixture slurry (hereinafter also referred to as electrode mixture slurry), causing the electrode mixture slurry to gel.

[0013] In contrast, the binder of the present invention contains a vinylidene fluoride-based polymer containing a vinylidene fluoride-derived structural unit and two or more structural units represented by the general formula (1) described below. Here, the copolymerizability of the monomers copolymerized with vinylidene fluoride with vinylidene fluoride varies depending on their structure. Therefore, a vinylidene fluoride-based polymer copolymerized with two or more different monomers has a complex structural unit arrangement that differs from a polymer using only one type of monomer or a polymer blended with two polymers using only one type of monomer. This complex structural unit arrangement suppresses binder degradation, and therefore, even when the vinylidene fluoride-based polymer is mixed with a positive electrode active material, gelation of the electrode mixture is unlikely. Furthermore, the vinylidene fluoride-based polymer contains a carboxyl group derived from the structural unit represented by the general formula (1). The carboxyl group can bond with polar groups present on the surface of the active material or current collector. Therefore, even a small amount of binder containing the vinylidene fluoride polymer has high adhesive strength to the active material and the current collector. In other words, the binder is very useful as a material for the electrode mixture layer of a lithium ion secondary battery. The vinylidene fluoride polymer and other components contained in the binder are described below.

[0014] Vinylidene fluoride polymers The vinylidene fluoride polymer contains a constituent unit derived from vinylidene fluoride and two or more constituent units represented by the following general formula (1): The vinylidene fluoride polymer may contain only two types of constituent units represented by the following general formula (1), but it is preferable that the vinylidene fluoride polymer contains three or more types from the viewpoint of inhibiting gelation.

[0015] [ka] In the above general formula (1), R 1 ~R 3R each independently represent a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 4 carbon atoms and optionally having a substituent. The alkyl group may be linear or branched. Specific examples of alkyl groups having 1 to 4 carbon atoms include a methyl group, an ethyl group, a propyl group, and a butyl group. Examples of substituents on the alkyl group include a halogen atom. However, from the viewpoint of preventing steric hindrance during polymerization with vinylidene fluoride, it is preferable that the alkyl group does not contain a substituent. 1 ~R 3 is more preferably a hydrogen atom or a methyl group, and particularly preferably a hydrogen atom.

[0016] Meanwhile, X in the general formula (1) represents an alkylene group having 1 to 4 carbon atoms, which may have a substituent. The alkylene group having 1 to 4 carbon atoms may be linear or branched. Specific examples of the alkylene group include a methylene group, an ethylene group, a propylene group, and a butylene group. Examples of the substituent include a halogen atom, etc. From the viewpoint of preventing steric hindrance from interfering with the adhesive action of the carboxy group, it is preferable that the alkyl chain does not contain a substituent. Furthermore, from the viewpoint of preventing gelation of the electrode mixture, it is more preferable that X in at least one of the structural units represented by two or more general formulas (1) is a methylene group or an ethylene group, and it is particularly preferable that X in all of the structural units is a methylene group or an ethylene group.

[0017] Furthermore, n in the above general formula (1) is an integer of 0 or more and 5 or less, more preferably 0 or 1. In particular, it is particularly preferable that n of at least one of the two or more structural units represented by general formula (1) is 0 or 1 from the viewpoint of suppressing gelation of the electrode mixture, and it is particularly preferable that n of at least one of the structural units is 1.

[0018] Here, the molecular weight of the constitutional unit represented by the general formula (1) is preferably from 70 to 600, more preferably from 100 to 400, and even more preferably from 100 to 250. When the molecular weight is within this range, high adhesive strength can be maintained and gelation can be suppressed.

[0019] The structural unit represented by the general formula (1) above can be, for example, a structural unit derived from methacrylic acid, acrylic acid, carboxymethyl methacrylate, carboxymethyl acrylate, carboxyethyl methacrylate, carboxyethyl acrylate, carboxypropyl methacrylate, carboxypropyl acrylate, carboxybutyl methacrylate, carboxybutyl acrylate, 2-((2-(acryloyloxy)ethanoyl)oxy)ethanoic acid, 2-((((2-(acryloyloxy)ethanoyl)oxy)ethanoyl)oxy)ethanoic acid, 3-((3-(acryloyloxy)propanoyl)oxy)propanoic acid, 3-((((3-(acryloyloxy)propanoyl)oxy)propanoyl)oxy)propanoic acid, or the like.

[0020] The structural unit represented by the above general formula (1) where n is 1 is, for example, a structural unit derived from carboxymethyl methacrylate, carboxymethyl acrylate, carboxyethyl methacrylate, carboxyethyl acrylate, carboxypropyl methacrylate, carboxypropyl acrylate, carboxybutyl methacrylate, carboxybutyl acrylate, etc.

[0021] The proportion of the structural units represented by the general formula (1) in the vinylidene fluoride polymer is not particularly limited. However, the total amount of the structural units represented by the general formula (1) is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less, and even more preferably 0.5% by mass or more and 2% by mass or less, relative to the total amount of structural units constituting the vinylidene fluoride polymer. When the proportion of the structural units represented by the general formula (1) in the vinylidene fluoride polymer is 10% by mass or less, the crystallinity of the vinylidene fluoride polymer is increased, which tends to increase the adhesive strength between the binder (vinylidene fluoride polymer) and the active material or current collector. On the other hand, when the proportion of the structural units represented by the general formula (1) is 0.1% by mass or more, a complex structural unit arrangement is sufficiently formed, which suppresses binder degradation, and therefore gelation is less likely to occur when the binder is mixed with the positive electrode active material.

[0022] Furthermore, the amount of each of the structural units represented by general formula (1) is preferably 10% by mass or more, more preferably 20% by mass or more, relative to the total amount of the structural units represented by general formula (1). When the proportion of each of the structural units represented by general formula (1) is 10% by mass or more, as described above, gelation is easily suppressed when mixed with a positive electrode active material containing nickel. The total amount of the structural units represented by general formula (1) and the amount of each structural unit are 19 Analysis by F-NMR and 1 It can be identified by H-NMR etc.

[0023] On the other hand, the amount of vinylidene fluoride-derived structural units relative to all structural units in the vinylidene fluoride-based polymer is preferably 90% by mass or more and 99.9% by mass or less, more preferably 95% by mass or more and 99.5% by mass or less, and even more preferably 98% by mass or more and 99.5% by mass or less. When the amount of vinylidene fluoride-derived structural units is 90% by mass or more, physical properties specific to vinylidene fluoride are easily obtained. On the other hand, when the amount of vinylidene fluoride-derived structural units is 99.9% by mass or less, the total amount of structural units represented by the above general formula (1) becomes relatively sufficient, and the adhesive strength between the binder (vinylidene fluoride-based polymer) and the active material or current collector is increased. The amount of vinylidene fluoride-derived structural units in the vinylidene fluoride-based polymer is, for example, 19 It can be identified by F-NMR analysis, etc.

[0024] The vinylidene fluoride polymer may contain structural units derived from vinylidene fluoride and structural units other than those represented by general formula (1) (hereinafter also referred to as "structural units derived from other compounds"), as long as the purpose and effects of the present invention are not impaired. The vinylidene fluoride polymer may contain only one type of structural unit derived from other compounds, or may contain two or more types. However, the total amount of the vinylidene fluoride-derived structural units and the structural units represented by general formula (1) relative to all structural units of the vinylidene fluoride polymer is preferably 90% by mass or more, and more preferably 95% by mass or more.

[0025] Examples of other compounds include fluorine-based monomers copolymerizable with vinylidene fluoride, hydrocarbon monomers such as ethylene and propylene, and monomers copolymerizable with the above general formula (1). Fluorine-based monomers copolymerizable with vinylidene fluoride include vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, fluoroalkyl vinyl ethers, and perfluoroalkyl vinyl ethers such as perfluoromethyl vinyl ether. Monomers copolymerizable with the above general formula (1) include alkyl (meth)acrylate compounds such as methyl (meth)acrylate.

[0026] Here, the vinylidene fluoride polymer may be a block polymer of vinylidene fluoride and two or more precursors of the structural unit represented by the general formula (1), but is more preferably a random polymer. When the vinylidene fluoride polymer is prepared by random polymerization, the uniformity of the polymer chain is improved, and the adhesiveness exhibited by the structural unit represented by the general formula (1) is improved.

[0027] The melting point of the vinylidene fluoride polymer is preferably 160°C or higher, more preferably 165°C or higher. When the vinylidene fluoride polymer has a melting point of 160°C or higher, it is less likely to swell in an electrolyte, and the resulting lithium-ion secondary battery is likely to have good performance. The melting point of the vinylidene fluoride polymer can be determined by calorimetry using a differential scanning calorimeter (DSC). Specifically, the vinylidene fluoride polymer is heated from 30°C to 230°C at a rate of 10°C / min (first heating), cooled from 230°C to 30°C at a rate of 10°C / min (first cooling), and then heated from 30°C to 230°C at a rate of 10°C / min (second heating). The melting peak is then determined by DSC. In this specification, the maximum melting peak temperature observed in the second heating is defined as the melting point of the vinylidene fluoride polymer.

[0028] The inherent viscosity of the vinylidene fluoride polymer is preferably 0.5 dL / g or more and 5.0 dL / g or less, more preferably 1.0 dL / g or more and 4.0 dL / g or less, and most preferably 1.0 dL / g or more and 3.5 dL / g or less. When the inherent viscosity is 0.5 dL / g or more, the adhesive strength between the binder (vinylidene fluoride polymer) and the active material or current collector is increased. On the other hand, when the inherent viscosity is 5.0 or less, the viscosity of the slurry does not become too high when an electrode slurry is prepared, and workability is excellent. The inherent viscosity (η i ) is expressed as logarithmic viscosity. First, 80 mg of vinylidene fluoride polymer is dissolved in 20 mL of N,N-dimethylformamide, and the viscosity is measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. Then, the viscosity is calculated from the obtained value based on the following formula. η i =(1 / C)·ln(η / η0) In the above formula, η is the viscosity of the solution, η0 is the viscosity of the solvent N,N-dimethylformamide alone, and C is the concentration of the vinylidene fluoride polymer in the solution, ie, 0.4 g / dL.

[0029] The vinylidene fluoride polymer can be prepared by copolymerizing vinylidene fluoride, two or more precursors of the structural unit represented by (1) above, and, if necessary, other compounds, by a known method. Examples of the copolymerization method include suspension polymerization, emulsion polymerization, solution polymerization, etc., but suspension polymerization is preferred from the viewpoints that a binder with high adhesive strength can be easily obtained and that the amount of impurities is small.

[0030] Non-aqueous solvents The binder may be composed solely of the vinylidene fluoride polymer described above, but may also contain a non-aqueous solvent as necessary.

[0031] When the binder contains a non-aqueous solvent, the vinylidene fluoride polymer can be dissolved or dispersed therein, and the binder can be made liquid.

[0032] Examples of non-aqueous solvents include polar solvents. Examples of polar 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; and sulfoxide / sulfone compounds such as dimethyl sulfoxide and sulfolane. The binder may contain only one non-aqueous solvent or two or more.

[0033] The amount of the non-aqueous solvent in the binder is preferably 400 parts by mass or more and 5,000 parts by mass or less, and more preferably 500 parts by mass or more and 5,000 parts by mass or less, relative to 100 parts by mass of the vinylidene fluoride polymer. When the amount of the non-aqueous solvent in the binder is within this range, the vinylidene fluoride polymer can be uniformly dispersed or dissolved in the non-aqueous solvent.

[0034] Other ingredients The binder may further contain, for example, other resins such as acrylic resins, fillers such as inorganic fillers, various additives, and the like, within the scope of not impairing the object and effect of the present invention.

[0035] 2. Electrode mixture The binder and the positive electrode active material can be mixed to prepare an electrode mixture for producing a positive electrode of a lithium ion secondary battery. The electrode mixture may further contain a conductive aid, a solvent, other additives, and the like.

[0036] The type of the positive electrode active material is not particularly limited, and a lithium-based positive electrode active material containing general lithium can be used. As the lithium-based positive electrode active material, for example, a lithium-based positive electrode active material represented by the following general formula (2) can be used. LiM x One example is the lithium metal oxide represented by O2···(2). In general formula (2), M represents at least one metal element including Ni, and the metal element other than Ni is preferably selected from the group consisting of Co, Al, Fe, Mn, Cr, and V. It is more preferable that in addition to Ni, one or more elements selected from the group consisting of Co, Mn, and Al are further contained. Furthermore, in the lithium metal oxide represented by formula (2), when the total of the metal elements constituting M is taken as 100 mol %, it is preferable that the lithium metal oxide contains 55 mol % or more of Ni, and more preferably 70 mol % or more of Ni. In the above general formula (2), 0.5≦x≦1.5, and more preferably 0.7≦x≦1.3.

[0037] The positive electrode active material may be a material in which the surface of the above compound is coated. Furthermore, the positive electrode active material may be a commercially available product.

[0038] Examples of the composition of the lithium-based positive electrode active material represented by the general formula (2) and other lithium-based positive electrode active materials include Li 1.0 Ni 0.8 Co 0.2 O2, Li 1.0 Ni 0.5 Mn 0.5 O2, Li 1.00 Ni 0.35 Co 0.34 Mn 0.34 O2(NCM111), Li 1.00 Ni 0.52 Co 0.20 Mn 0.30 O2(NCM523), Li 1.00 Ni 0.50 Co 0.30 Mn 0.20 O2(NCM532), Li 1.00 Ni 0.6 Co 0.2 Mn 0.2 O2(NCM622), Li 1.00 Ni 0.83 Co 0.12 Mn 0.05 O2(NCM811), Li 1.00 Ni 0.85 Co 0.15 Al 0.05 These include O2 (NCA811), LiCoO2 (LCO), and LiFePO4 (LFP).

[0039] Furthermore, the positive electrode active material may contain a plurality of different types of lithium-based positive electrode active materials. For example, the positive electrode active material may contain a plurality of different types of lithium-based positive electrode active materials, such as LiNi x Co y Mn Z O2 (x, y, and z are the same as in the above specific examples), or LiNi x Co y Mn Z O2 (x, y, and z are as shown in the specific example above) and LiNi x Co y Alz O2 (where x, y, and z are as shown in the specific example above).

[0040] The amount of the positive electrode active material contained in the electrode mixture is appropriately selected depending on the application of the electrode mixture, etc., but is preferably 40% by mass or more and 99.9% by mass or less of the total amount of the solid content derived from the binder, the positive electrode active material, and the conductive additive. When the amount of the positive electrode active material is within this range, for example, sufficient charge / discharge capacity can be obtained, and battery performance tends to be good.

[0041] On the other hand, the binder is less likely to gel when mixed with the above-mentioned positive electrode active material, especially a positive electrode active material containing a relatively large amount of Ni. Furthermore, the binder exhibits high adhesiveness to the positive electrode active material. Therefore, the ratio of the binder-derived solid content to the total amount of the binder-derived solid content (total amount excluding components that volatilize during curing), active material, and conductive additive can be set to, for example, 0.2% by mass to 20% by mass. The amount of the binder-derived solid content is more preferably 0.2% by mass to 10% by mass, and even more preferably 0.2% by mass to 4% by mass.

[0042] The conductive additive contained in the electrode mixture is not particularly limited as long as it is a compound that can further increase the conductivity between the positive electrode active materials or between the positive electrode active material and a current collector. Examples of the conductive additive include acetylene black, ketjen black, carbon black, graphite powder, graphene, carbon nanofibers, carbon nanotubes, and carbon fibers.

[0043] The amount of the conductive additive contained in the electrode mixture is appropriately selected depending on the type of the conductive additive, etc. From the viewpoint of improving both the conductivity and the dispersibility of the conductive additive, the amount is preferably 0.1% by mass to 15% by mass or less, more preferably 0.1% by mass to 7% by mass, and even more preferably 0.1% by mass to 5% by mass, based on the total amount of the solid content derived from the binder, the positive electrode active material, and the conductive additive.

[0044] The electrode mixture may contain a solvent different from the non-aqueous solvent contained in the binder. The solvent can be selected from the non-aqueous solvents that can be contained in the binder.

[0045] The total amount of solvent in the electrode mixture (including the amount of non-aqueous solvent in the binder) is not particularly limited, but is usually preferably 20 parts by mass or more and 150 parts by mass or less per 100 parts by mass of the above-mentioned active material.

[0046] The electrode mixture may further contain a dispersant, an adhesive aid, a thickener, etc., and known compounds can be used for these. The amount of these is not particularly limited as long as it does not impair the object and effect of the present invention, but is preferably 15 mass % or less based on the total amount of the solid content derived from the binder and the active material.

[0047] The electrode mixture may further contain additives such as phosphorus compounds, sulfur compounds, organic acids, amine compounds, 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 polymers described above, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and resins such as polyacrylonitrile (PAN). These additives are not particularly limited as long as they do not impair the objectives and effects of the present invention, but are preferably 15% by mass or less of the total amount of the binder-derived solids and the positive electrode active material.

[0048] The electrode mixture may be prepared by mixing all of the components at once, or by first mixing some of the components and then mixing the remaining components.

[0049] The viscosity of the electrode mixture is not particularly limited as long as it can prevent dripping, uneven coating, and delayed drying after coating when applying the electrode mixture to form an electrode mixture layer, and provides good workability and applicability during electrode mixture layer preparation. Typically, the viscosity (slurry viscosity) measured with a Brookfield viscometer at 20°C and a rotation speed of 6 rpm is preferably 100 mPa·s or more and 100,000 mPa·s or less, more preferably 1,000 mPa·s or more and 80,000 mPa·s or less, and particularly preferably 2,000 mPa·s or more and 70,000 mPa·s or less. The viscosity (slurry viscosity) of the electrode mixture in this specification is the value measured 2 minutes after the start of rotation with the Brookfield viscometer.

[0050] 3. Electrode The electrode (positive electrode) of the lithium ion secondary battery of the present invention may include an electrode mixture layer containing the above-described electrode mixture, and is, for example, a structure having a current collector and the above-described electrode mixture layer disposed on the current collector.

[0051] Current collector The current collector is a terminal for extracting electricity. The material of the current collector is not particularly limited, and metal foil or metal mesh of aluminum, copper, iron, stainless steel, steel, nickel, titanium, etc. can be used. Alternatively, the current collector may have a layer containing carbon black or the like formed on the surface of another medium, or may have the above-mentioned metal foil or metal mesh applied thereto.

[0052] Electrode mixture layer The electrode mixture layer is a layer formed by applying a composition containing a binder and a positive electrode active material (for example, the above-mentioned electrode mixture) onto a current collector and drying it. The electrode mixture layer may be formed on only one surface of the current collector, or may be disposed on both surfaces.

[0053] The electrode mixture layer contains at least the solid content (vinylidene fluoride polymer) derived from the binder and the active material, and may further contain various additives such as a conductive aid, a dispersant, an adhesive aid, a thickener, etc. as needed, which are the same as those described for the electrode mixture.

[0054] Here, the thickness of the electrode mixture layer is not particularly limited, but in one example, it is preferably 1 μm or more and 1000 μm or less. The basis weight of the electrode mixture layer formed on one surface of the current collector is not particularly limited, and can be any basis weight, but in one example, it is 50 g / m 2 More than 1000g / m 2 Less than 100 g / m 2 More than 500g / m 2 The following is more preferred:

[0055] The electrode mixture layer can be formed by carrying out a step of applying the electrode mixture onto a current collector and a step of drying the applied mixture.

[0056] The method for applying the electrode mixture is not particularly limited, and methods such as a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, a die coating method, and a dip coating method can be used.

[0057] After application of the electrode mixture, the mixture is heated at a desired temperature to dry the non-aqueous solvent. In one example, the drying temperature is preferably 60°C or higher and 500°C or lower, and more preferably 80°C or higher and 200°C or lower. Heating may be performed multiple times at different temperatures. The solvent in the mixture may be dried under atmospheric pressure, increased pressure, or reduced pressure. After drying, a further heat treatment may be performed.

[0058] After the electrode mixture is applied and dried, a pressing process may be further performed. The pressing process can improve the electrode density. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

[0059] 4. Lithium-ion secondary batteries The binder and electrode mixture described above can be used to form electrodes (positive electrodes) of lithium ion secondary batteries and the like, as described above, but may also be used to form other layers of lithium ion secondary batteries. [Example]

[0060] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0061] 1.Methods for measuring and evaluating physical properties In the examples and comparative examples described later, the inherent viscosity of vinylidene fluoride polymers was measured by the following method. Furthermore, electrode peel strength tests and electrode mixture storage tests were carried out in the following manner.

[0062] Inherent viscosity The inherent viscosity of the vinylidene fluoride polymer was measured as follows. First, 80 mg of the vinylidene fluoride polymer was dissolved in 20 mL of N,N-dimethylformamide, and the viscosity was measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. Then, the inherent viscosity (η i ) was calculated. η i =(1 / C)·ln(η / η0) In the above formula, η is the viscosity of the solution, η0 is the viscosity of the solvent N,N-dimethylformamide alone, and C is the concentration of the vinylidene fluoride polymer in the solution, ie, 0.4 g / dL.

[0063] Electrode peel strength test The electrodes prepared in the examples and comparative examples were cut into a length of 100 mm and a width of 20 mm, and a 90° peel test was performed at a head speed of 10 mm / min using a tensile tester (ORIENTE CHSIA-1150, manufactured by UNIVERSAL TESTING MACHINE) in accordance with JIS F6854-1 to measure the peel strength.

[0064] Storage test of electrode mixture The electrode mixture slurry storage test samples prepared in the examples and comparative examples were stored at 25°C under a nitrogen atmosphere for 2 weeks. Then, the viscosities were measured using an E-type viscometer at 25°C and a shear rate of 2 s -1The viscosity was measured by waiting 60 seconds after loading the slurry (sample) into the measuring device and then rotating the rotor. The viscosity of the sample was measured 300 seconds after the rotor started to rotate. The viscosity ratio was then calculated using the following formula. Viscosity ratio (%) = (slurry viscosity after storage) / (slurry viscosity immediately after preparation) × 100 The results are shown in the table as follows: if the viscosity ratio is less than 50%, it is rated A (no gelation); if it is 50% or more but less than 100%, it is rated B (signs of gelation); if it is 100% or more, it is rated C (gelation has occurred).

[0065] 2.Raw materials The following materials were used to prepare the vinylidene fluoride polymer: VDF: Vinylidene fluoride CEA: Carboxyethyl acrylate CMA: Carboxymethyl acrylate APOPA: 3-((3-(acryloyloxy)propanoyl)oxy)propanoic acid APOPOPA: 3-((3-(acryloyloxy)propanoyl)oxy)propanoyl)oxy)propanoic acid AA: Acrylic acid APS: Acryloyloxypropyl succinic acid

[0066] 3. Binder and Electrode Preparation (1) Preparation of vinylidene fluoride polymers [Preparation of vinylidene fluoride polymer A] A 2-liter autoclave was charged with 1240 g of ion-exchanged water, 0.4 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 2.2 g of a 50% by weight diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of monomer mixture A (CEA / APOPA / APOPOPA = 0.4 / 0.5 / 0.1 (mass ratio)) and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by weight aqueous solution of monomer mixture A was continuously fed into the reactor at an average flow rate of 0.75 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride polymer A (VDF / CEA / APOPA / APOPOPA). A total of 4.0 g of monomer mixture A was added, including the amount initially added.

[0067] [Preparation of vinylidene fluoride polymer B] A 2-liter autoclave was charged with 1248 g of ion-exchanged water, 0.5 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 0.48 g of a 50% by weight diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of monomer mixture B (AA / CEA / APOPA / APOPOPA = 0.2 / 0.3 / 0.4 / 0.1 (mass ratio)). The mixture was heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by weight aqueous solution of monomer mixture B was continuously fed into the reactor at a flow rate of 0.3 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride polymer B (VDF / AA / CEA / APOPA / APOPOPA). A total of 4.0 g of monomer mixture B was added, including the amount initially added.

[0068] [Preparation of vinylidene fluoride polymer C] A 2-liter autoclave was charged with 1248 g of ion-exchanged water, 0.4 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 2.0 g of a 50% by weight diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.2 g of AA, and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by weight aqueous AA solution was continuously fed into the reactor at a flow rate of 0.5 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride polymer C (VDF / AA). A total of 4.0 g of AA was added, including the amount initially added.

[0069] [Preparation of vinylidene fluoride polymer D] A 2-liter autoclave was charged with 1248 g of ion-exchanged water, 0.4 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 2.2 g of a 50% by weight diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of CEA, and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by weight aqueous CEA solution was continuously fed into the reactor at a flow rate of 0.8 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride polymer D (VDF / CEA). A total of 4.0 g of CEA was added, including the amount initially added.

[0070] [Preparation of vinylidene fluoride polymer E] A 2-liter autoclave was charged with 1224 g of ion-exchanged water, 0.4 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 2.2 g of a 50% by weight diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of APS, and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by weight aqueous solution of the monomer mixture F (APS / AA = 0.7 / 0.2 (mass ratio)) was continuously fed into the reactor at a flow rate of 0.45 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer E (VDF / APS / AA).

[0071] [Preparation of vinylidene fluoride polymer F] A 2-liter autoclave was charged with 1212 g of ion-exchanged water, 0.4 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 2.4 g of a 50% by weight diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, 0.05 g of CEA, and 0.1 g of CMA, and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by weight aqueous solution of monomer mixture G (CEA / CMA = 1 / 2 (mass ratio)) was continuously fed into the reaction vessel at a flow rate of 0.5 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer F (VDF / CEA / CMA).

[0072] [Preparation of vinylidene fluoride polymer G] A 2-liter autoclave was charged with 1222 g of ion-exchanged water, 0.4 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 2.2 g of a 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of CMA, and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by mass aqueous CMA solution was continuously fed into the reactor at a flow rate of 0.6 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride copolymer G (VDF / CMA).

[0073] (2) Preparation of binder, electrode mixture, and electrode Example 1 [Preparation of electrode mixture slurry storage test sample] In Example 1, the above-mentioned vinylidene fluoride polymer A was used as a binder. 1.00 Ni 0.85 Co 0.15 Al 0.05 O2) was treated with carbon black (SP: SuperP (registered trademark) manufactured by Timcal Japan, average particle diameter: 40 nm, specific surface area: 60 m) as a conductive additive. 2 / g) was added and powder mixing was carried out.

[0074] The vinylidene fluoride polymer A (binder) was dissolved in N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") to prepare a vinylidene fluoride polymer solution containing 6 mass% of the vinylidene fluoride polymer A. The vinylidene fluoride polymer solution was then added to a mixture of NCA811 and carbon black, followed by kneading. Specifically, the vinylidene fluoride polymer solution was added so that the solid content was 81.5 mass%, and primary kneading was performed at 2000 rpm for 2.5 minutes. The remaining vinylidene fluoride polymer solution and NMP were then added to adjust the solid content to 75 mass%. Secondary kneading was then performed at 2000 rpm for 3 minutes, yielding an electrode mixture slurry storage test sample.

[0075] The mass ratio of the electrode active material, carbon black, and vinylidene fluoride polymer in the obtained sample was 100:2:2, in this order.

[0076] [Preparation of electrode mixture and electrode (electrode peel strength measurement sample)] The electrode active material NCA811 was treated with carbon black (SP: SuperP (registered trademark) manufactured by Timcal Japan, average particle diameter: 40 nm, specific surface area: 60 m) as a conductive additive. 2 / g) was added and powder mixing was carried out.

[0077] The vinylidene fluoride polymer A was dissolved in NMP to prepare a vinylidene fluoride polymer solution containing 8% by mass of the vinylidene fluoride polymer. Then, NMP was added to a mixture of NCA811 and carbon black, followed by kneading. Specifically, the vinylidene fluoride polymer solution was added so that the solids concentration was 83.7% by mass, and primary kneading was performed at 2000 rpm for 4 minutes. Next, the vinylidene fluoride polymer solution was further added to adjust the solids concentration to 73.5% by mass, and secondary kneading was performed at 2000 rpm for 3 minutes to obtain an electrode mixture.

[0078] The obtained electrode mixture was applied to a 15 μm thick aluminum foil current collector using a bar coater, and the resulting film was subjected to primary drying in a thermostatic chamber under a nitrogen atmosphere at 110°C for 30 minutes. This was then subjected to secondary drying under a nitrogen atmosphere at 130°C for 2 hours to obtain a film with a basis weight of approximately 250 g / m. 2 The mass ratio of the electrode active material, carbon black, and vinylidene fluoride polymer in the obtained electrode mixture, in this order, was 100:2:1.5.

[0079] Example 2 An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to vinylidene fluoride polymer B.

[0080] Example 3 An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to vinylidene fluoride polymer F.

[0081] (Comparative Example 1) An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to KF#7300 manufactured by Kureha Corporation.

[0082] (Comparative Example 2) An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to a blend of vinylidene fluoride polymer C and vinylidene fluoride polymer D in a weight ratio of 3:7.

[0083] (Comparative Example 3) An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to vinylidene fluoride polymer C.

[0084] Comparative Example 4 An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to vinylidene fluoride copolymer D.

[0085] (Comparative Example 5) An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to vinylidene fluoride copolymer E.

[0086] (Comparative Example 6) An electrode mixture and an electrode were obtained in the same manner as in Example 1, except that the binder was changed to a blend of vinylidene fluoride polymer D and vinylidene fluoride polymer G in a weight ratio of 4:6.

[0087] [Table 1]

[0088] As shown in Table 1 above, when a vinylidene fluoride-based polymer containing a structural unit derived from vinylidene fluoride and two or more structural units represented by general formula (1) was contained in the binder, the peel strength was high and the gelation resistance was good (Examples 1 to 3).

[0089] On the other hand, when a homopolymer of vinylidene fluoride was used as the binder, the peel strength and gelation resistance were low (Comparative Example 1). Furthermore, when a vinylidene fluoride-based polymer containing a vinylidene fluoride-derived structural unit and only one structural unit represented by general formula (1) was used as the binder, the peel resistance was improved, but the gelation resistance was low (Comparative Examples 3 to 5). Furthermore, when two vinylidene fluoride-based polymers containing a vinylidene fluoride-derived structural unit and only one structural unit represented by general formula (1) were mixed, the gelation resistance was also low (Comparative Examples 2 and 6).

[0090] This application claims priority from Japanese Patent Application No. 2022-049941, filed March 25, 2022, the entire contents of which are incorporated herein by reference. [Industrial Applicability]

[0091] The binder of the present invention shows little deterioration or viscosity increase even when mixed with a positive electrode active material containing a large amount of nickel. Furthermore, the binder exhibits good adhesive strength even in small amounts. Therefore, the binder, as well as electrode mixtures and electrodes containing the binder, are highly useful in the production of lithium-ion secondary batteries.

Claims

1. A binder for a positive electrode of a lithium ion secondary battery, containing a vinylidene fluoride polymer, The binder, wherein the vinylidene fluoride polymer contains a constituent unit derived from vinylidene fluoride and three or more constituent units represented by the following general formula (1): 【Chemistry 1】 (In general formula (1), R １ ~R ３ each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 4 carbon atoms which may have a substituent; X represents an alkylene group having 1 to 4 carbon atoms which may have a substituent; and n represents an integer of 0 to 5.

2. A binder for a positive electrode of a lithium ion secondary battery, comprising a vinylidene fluoride polymer, The vinylidene fluoride polymer contains a structural unit derived from vinylidene fluoride and two or more structural units represented by the following general formula (1): At least one of the structural units represented by the general formula (1) has n=1. binder. 【Chemistry 2】 (In general formula (1), R 1 to R 3 each independently represent a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 4 carbon atoms which may have a substituent; X represents an alkylene group having 1 to 4 carbon atoms which may have a substituent; and n represents an integer of 0 to 5.)

3. X in all of the structural units represented by the general formula (1) is a methylene group or an ethylene group.

3. The binder according to claim 1 or 2.

4. An electrode mixture comprising the binder according to claim 1 or 2 and a positive electrode active material.

5. An electrode mixture comprising the binder according to claim 3 and a positive electrode active material.

6. The electrode mixture according to claim 4 , wherein the positive electrode active material is a compound represented by the following general formula (2): LiM ｘ O ２ ・・・(2) (In general formula (2), M represents at least one metal element including Ni, and when the total of the metal elements represented by M is taken as 100 mol %, the ratio of Ni is 55 mol % or more, and 0.5≦x≦1.5.)

7. The electrode mixture according to claim 5 , wherein the positive electrode active material is a compound represented by the following general formula (2): LiM ｘ O ２ ・・・(2) (In general formula (2), M represents at least one metal element including Ni, and when the total of the metal elements represented by M is taken as 100 mol %, the ratio of Ni is 55 mol % or more, and 0.5≦x≦1.5.)

8. An electrode, comprising an electrode mixture layer comprising the electrode mixture according to claim 6 provided on a current collector.

9. An electrode, comprising an electrode mixture layer comprising the electrode mixture according to claim 7 provided on a current collector.

10. 9. The electrode of claim 8, Lithium-ion secondary battery.

11. 10. The electrode of claim 9, Lithium-ion secondary battery.

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