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 having a carboxy group, and the slurry viscosity ratio used in a specific method is not more than 100%.

Description

Technical Field

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

Background Art

[0002] Conventionally, extensive studies have been conducted on utilizing non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries as in-vehicle batteries. In-vehicle secondary batteries are required to have a high capacity. Therefore, reducing the amount of the binder in each electrode of a lithium-ion secondary battery and increasing the proportion of the active material are considered important for increasing the capacity. Generally, polyvinylidene fluoride (PVDF) is used as a binder for the positive electrode of a lithium-ion secondary battery, and various studies have been conducted to further enhance the adhesiveness of the binder.

[0003] For example, Patent Document 1 discloses a vinylidene fluoride-based polymer that is superior in adhesiveness to a metal foil compared to conventional vinylidene fluoride-based polymers.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, even with the vinylidene fluoride polymer described in Patent Document 1 above, adhesion can be insufficient, and further improvements are needed. In addition, when the positive electrode active material is combined with conventional vinylidene fluoride polymers, there is a problem that the electrode mixture tends to gel. In recent years, in order to increase the capacity of batteries, the use of ternary compounds with a high nickel content as positive electrode active materials has been considered, but when such positive electrode active materials are used, the progression of gelation is further accelerated.

[0006] Therefore, the present invention aims to provide a binder that does not easily cause gelation of the electrode mixture even when mixed with a positive electrode active material, and that has sufficient adhesion even in small amounts, an electrode mixture containing the same, an electrode, and a battery. [Means for solving the problem]

[0007] The present invention provides a binder for the positive electrode of a lithium-ion secondary battery, comprising a vinylidene fluoride polymer, wherein the vinylidene fluoride polymer comprises a constituent unit derived from vinylidene fluoride and two or more constituent units having a carboxyl group, and the viscosity ratio of the slurry, determined by the following method, is 100% or less. [How to determine the viscosity ratio of a slurry] (1) A slurry is prepared consisting only of the electrode active material NCA811, carbon black, the vinylidene fluoride polymer in the binder, and N-methyl-2-pyrrolidone, wherein the mass ratio of the electrode active material, the carbon black, and the vinylidene fluoride polymer in the binder is 100:2:2, and the solid content concentration is 75% by mass. (2) The viscosity of the slurry is determined using an E-type viscometer at 25°C and a shear rate of 2s. -1 Measure it using this method. (3) Store the slurry at 25°C under a nitrogen atmosphere for two weeks. (4) The viscosity of the slurry after storage was measured using an E-type viscometer at 25°C and a shear rate of 2s. -1 Measure it using this method. (5) The viscosity ratio of the slurry is calculated based on the following formula. Viscosity ratio (%) = (Slurry viscosity after storage) / (Slurry viscosity immediately after preparation) × 100

[0008] The present invention also provides an electrode mixture comprising the above-mentioned binder and positive electrode active material.

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

[0010] The binder of the present invention is less prone to gelation when mixed with a nickel-containing positive electrode active material and exhibits sufficient adhesion even in small amounts. Therefore, it is possible to provide an electrode mixture that can be used stably over a long period of time, and to provide high-capacity lithium-ion secondary batteries and electrodes used therein. [Modes for carrying out 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 adhesive properties. In addition, there has been a problem 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: The positive electrode active material contains a base, and positive electrode active materials with a high nickel ratio contain a particularly large amount of base. Therefore, when vinylidene fluoride comes into contact with the positive electrode active material, the base accelerates the degradation of the binder. Then, the degraded binder forms a cross-linking structure in the slurry-like electrode mixture (hereinafter also called electrode mixture slurry), causing the electrode mixture slurry to gel.

[0013] In contrast, the binder of the present invention contains a vinylidene fluoride polymer comprising a constituent unit derived from vinylidene fluoride and two or more constituent units having carboxyl groups. Furthermore, the viscosity ratio of the slurry containing the vinylidene fluoride polymer, as measured by a specific method described later, is 100% or less. Here, monomers copolymerized with vinylidene fluoride have different copolymerizability with vinylidene fluoride depending on their structure. Therefore, a vinylidene fluoride polymer copolymerized from two or more different monomers has a complex constituent unit arrangement that differs from a polymer using only one type and a polymer blended from two polymers using only one type. Because this complex constituent unit arrangement suppresses the degradation of the binder, it is considered that the electrode mixture is less likely to gel even when the vinylidene fluoride polymer and the positive electrode active material are mixed. Furthermore, the vinylidene fluoride polymer contains carboxyl groups derived from constituent units having carboxyl groups. These carboxyl groups can bond to polar groups present on the surface of the active material and current collector. Therefore, even in small amounts, the binder containing the vinylidene fluoride polymer exhibits high adhesive strength to the active material and current collector. In other words, this binder is extremely useful as a material for the electrode mixture layer of lithium-ion secondary batteries. The vinylidene fluoride polymer and other components contained in the binder are described below. Note that the binder may contain only one of the vinylidene fluoride polymers described below, or it may contain two or more.

[0014] • Polyvinylidene fluoride polymers The vinylidene fluoride polymer contains a constituent unit derived from vinylidene fluoride and two or more constituent units having the carboxyl group described below. The vinylidene fluoride polymer may contain only two of the constituent units having the carboxyl group described below, but it is preferable to contain three or more from the viewpoint of suppressing gelation.

[0015] The above-mentioned constituent unit having a carboxyl group may be, for example, a constituent unit represented by formula (1). [ka] In the above general formula (1), R 1 ~R3 Each of these independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 4 carbon atoms, which may have substituents. The alkyl group may be linear or branched. Specific examples of alkyl groups having 1 to 4 carbon atoms include methyl, ethyl, propyl, and butyl groups. Examples of substituents on the alkyl group include halogen atoms. However, from the viewpoint of minimizing steric hindrance during polymerization with vinylidene fluoride, it is preferable that the alkyl group does not contain substituents. 1 ~R 3 A hydrogen atom or a methyl group is more preferable, and a hydrogen atom is particularly preferred.

[0016] On the other hand, in the above general formula (1), X represents an alkylene group having 1 to 4 carbon atoms, which may have substituents. The alkylene group having 1 to 4 carbon atoms may be linear or branched. Specific examples of alkylene groups include methylene, ethylene, propylene, and butylene groups. Examples of substituents include halogen atoms. From the viewpoint of minimizing inhibition of the adhesion of carboxyl groups due to steric hindrance, it is preferable that the alkyl chain does not contain substituents. Furthermore, if one or more of the two or more carboxyl group-containing structural units constituting the vinylidene fluoride polymer are structural units represented by general formula (1), it is more preferable from the viewpoint of suppressing gelation of the electrode mixture that at least one of the structural units represented by general formula (1) has X as a methylene group or an ethylene group. Furthermore, if the vinylidene fluoride polymer contains two or more structural units represented by general formula (1), it is particularly preferable that all of the structural units represented by general formula (1) have X as a methylene group or an ethylene group.

[0017] Furthermore, in the above general formula (1), n ​​is an integer between 0 and 5, and is more preferably 0 or 1. In particular, when one or more of the two or more carboxyl group-containing structural units constituting the vinylidene fluoride polymer are structural units represented by general formula (1), it is especially preferable from the viewpoint of suppressing gelation of the electrode mixture (from the viewpoint of satisfying the viscosity ratio of the slurry described later) that n of at least one structural unit represented by general formula (1) is 0 or 1. In addition, it is especially preferable that n of at least one structural unit represented by general formula (1) is 1.

[0018] Here, the molecular weight of the constituent unit having the carboxyl group is preferably 70 to 600, more preferably 100 to 400, and even more preferably 100 to 250. When the molecular weight is within this range, it is possible to maintain high adhesive strength and suppress gelation.

[0019] The above-mentioned constituent units having a carboxyl group can be, for example, constituent units 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)ethanol)oxy)ethaneic acid, 2-((((2-(acryloyloxy)ethanol)oxy)ethanol)oxy)ethaneic acid, 3-((3-(acryloyloxy)propanoyl)oxy)propanoic acid, 3-((((3-(acryloyloxy)propanoyl)oxy)propanoic acid, etc.

[0020] Among these, constituent units derived from carboxymethyl methacrylate, carboxymethyl acrylate, carboxyethyl methacrylate, carboxyethyl acrylate, carboxypropyl methacrylate, carboxypropyl acrylate, carboxybutyl methacrylate, carboxybutyl acrylate, etc., can be preferably used.

[0021] The proportion of the constituent units having the carboxyl group in the vinylidene fluoride polymer is not particularly limited. However, the total amount of the constituent units having the carboxyl group is preferably 0.1% to 10% by mass, more preferably 0.5% to 5% by mass, and even more preferably 0.5% to 2% by mass, relative to the total amount of constituent units constituting the vinylidene fluoride polymer. When the proportion of the total amount of constituent units having the carboxyl group in the vinylidene fluoride polymer is 10% by mass or less, the crystallinity of the vinylidene fluoride polymer increases, and the adhesion strength between the binder (vinylidene fluoride polymer) and the active material or current collector tends to increase. On the other hand, when the proportion of the total amount of constituent units having the carboxyl group is 0.1% by mass or more, a complex arrangement of constituent units is sufficiently formed, suppressing the deterioration of the binder. Therefore, when the binder and the positive electrode active material are mixed, gelation is less likely to occur, and the viscosity ratio of the slurry described later tends to fall within the desired range.

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

[0023] On the one hand, the amount of the structural unit derived from vinylidene fluoride with respect to all the 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 the structural unit derived from vinylidene fluoride is 90% by mass or more, the physical properties peculiar to vinylidene fluoride are easily obtained. On the other hand, when the amount of the structural unit derived from vinylidene fluoride is 99.9% by mass or less, the total amount of the structural units having the carboxy group becomes relatively sufficient, and the adhesive strength between the binder (vinylidene fluoride-based polymer) and the active material and the current collector increases. The amount of the structural unit derived from vinylidene fluoride in the vinylidene fluoride-based polymer can be specified, for example 19 by analysis by 19F-NMR or the like.

[0024] In addition, the vinylidene fluoride-based polymer may contain a structural unit derived from vinylidene fluoride and a structural unit other than the structural unit having a carboxy group (hereinafter, also referred to as "structural unit derived from other compounds") as long as the object and effect of the present invention are not impaired. The vinylidene fluoride-based polymer may contain only one kind of the structural unit derived from other compounds, or may contain two or more kinds. However, the total amount of the structural unit derived from vinylidene fluoride and the structural unit having a carboxy group with respect to all the structural units of the vinylidene fluoride-based polymer is preferably 90% by mass or more, and more preferably 95% by mass or more.

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

[0026] Here, the vinylidene fluoride polymer may be obtained by block polymerization of vinylidene fluoride and two or more precursors of the carboxyl group-containing structural units, but random polymerization is more preferable. When the vinylidene fluoride polymer is prepared by random polymerization, the uniformity of the polymer chain is improved, and the adhesive properties exhibited by the carboxyl group-containing structural units are enhanced.

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

[0028] The inherent viscosity of the above 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 adhesion 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 slurry viscosity does not become too high when preparing the electrode slurry, resulting in excellent workability. Inherent viscosity (η i The viscosity 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 30°C constant temperature bath. 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, i.e., 0.4 g / dL.

[0029] Here, the vinylidene fluoride polymer can be prepared by copolymerizing vinylidene fluoride with two or more precursors of the constituent units represented in (1) above, and other compounds as needed, using known methods. Examples of methods for copolymerizing these include suspension polymerization, emulsion polymerization, and solution polymerization, but suspension polymerization is preferred from the viewpoint of easily obtaining a binder with high adhesive strength and having fewer impurities.

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

[0031] If the binder contains a non-aqueous solvent, it is possible to dissolve or disperse the vinylidene fluoride polymer, thereby making the binder liquid.

[0032] Examples of non-aqueous solvents include, for example, polar solvents (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 non-aqueous solvent in the binder is preferably 400 parts by mass or more and 5000 parts by mass or less per 100 parts by mass of the vinylidene fluoride polymer, and more preferably 500 parts by mass or more and 5000 parts by mass or less. When the amount of non-aqueous solvent in the binder is within this range, it is possible to uniformly disperse or dissolve the vinylidene fluoride polymer in the non-aqueous solvent.

[0034] Other ingredients The binder may further contain other resins, such as acrylic resin, fillers, inorganic fillers, and various additives, to the extent that it does not impair the purpose and effects of the present invention.

[0035] • Viscosity ratio when the vinylidene fluoride polymer in the binder is used as a slurry. Furthermore, the binder described above has a viscosity ratio of 100% or less of the slurry obtained by the following method, preferably 80% or less, and more preferably 50% or less. When the viscosity ratio is 100% or less, the electrode mixture described later becomes stable. The viscosity ratio depends largely on the type and structure of the vinylidene fluoride polymer described above.

[0036] The viscosity ratio test is performed on the vinylidene fluoride polymer isolated from the binder. In other words, if the binder contains a non-aqueous solvent, resins other than vinylidene fluoride polymers, fillers, various additives, etc., these should be removed before performing the test described below. If the binder contains multiple vinylidene fluoride polymers, the test should be performed with all of them mixed together.

[0037] [How to determine the viscosity ratio of a slurry] (1) Prepare a slurry consisting only of the electrode active material NCA811, carbon black, a vinylidene fluoride polymer in the binder, and N-methyl-2-pyrrolidone, wherein the mass ratio of the electrode active material, the carbon black, and the vinylidene fluoride polymer in the binder is 100:2:2 and the solid content concentration is 75% by mass. (2) The viscosity of the slurry was measured using an E-type viscometer at 25°C and a shear rate of 2s.-1 Measure it using this method. (3) Store the slurry at 25°C under a nitrogen atmosphere for two weeks. (4) The viscosity of the slurry after storage was measured using an E-type viscometer at 25°C and a shear rate of 2s. -1 Measure it using this method. (5) The viscosity ratio of the slurry is calculated based on the following formula. Viscosity ratio (%) = (Slurry viscosity after storage) / (Slurry viscosity immediately after preparation) × 100

[0038] Furthermore, in order to reduce variations in the viscosity ratio of the slurry due to the materials and manufacturing conditions used to prepare the slurry, it is preferable to prepare the slurry using the following materials and conditions, respectively.

[0039] (i) Water content of N-methyl-2-pyrrolidone: less than 500 ppm (ii) Median average particle size (D50) of an aqueous dispersion of NCA811 measured by laser diffraction / scattering: 8 μm to 13 μm (iii) Specific surface area measured by the BET1 method of NCA811: 0.2~0.5m² 2 / g (iv) For NCA811, the pH when using the extraction method compliant with JIS K5101-16-2 (details below) was 11.5~12.0 (v) Median average particle size (D50) measured by electron microscopy analysis of carbon black: 30 nm to 50 nm (vi) Specific surface area of ​​carbon black measured by the BET1 point method: 50-70 m² 2 / g (vii) Slurry preparation conditions: Mix in two stages using a rotational / revolving mixer according to the procedure described below. (viii) Slurry preparation environment: 22°C, dew temperature -30°C to -40°C (iX) Storage environment and viscosity measurement environment for slurry: 25°C, dew temperature -30°C to -40°C

[0040] (Method for measuring the pH of electrode active materials) The electrode active material (NCA811) is placed in ultrapure water in an amount 50 times its mass. Then, a magnetic stirrer is used at a rotation speed of 600 rpm for 10 minutes. The pH of the solution is measured using a Horiba pH meter MODEL F-21, and the measured pH is taken as the pH of NCA811.

[0041] (Method for preparing slurry) Prepare a powder mixture of NCA811 and carbon black, and a vinylidene fluoride polymer-containing solution obtained by dissolving the vinylidene fluoride polymer in N-methyl-2-pyrrolidone. Add the vinylidene fluoride polymer-containing solution to the powder mixture so that the solid content concentration is 81.5% by mass, and perform primary mixing at 2000 rpm for 2.5 minutes using a rotation-orbit mixer. After allowing the slurry to cool to below 40°C, add the remaining vinylidene fluoride polymer and perform secondary mixing at 2000 rpm for 3 minutes using a rotation-orbit mixer.

[0042] 2. Electrode mixture The above-mentioned binder and the positive electrode active material can be mixed to form an electrode mixture for manufacturing the positive electrode of a lithium-ion secondary battery. The electrode mixture may further contain conductive additives, solvents, and other additives.

[0043] The type of positive electrode active material is not particularly limited, and general lithium-based positive electrode active materials containing lithium can be used. For example, the following general formula (2) is used for lithium-based positive electrode active materials. LiM x O2···(2) Examples of lithium metal oxides represented by [formula] include [formula]. In general formula (2), M represents at least one metallic element including Ni, and the metallic element other than Ni is preferably selected from the group consisting of Co, Al, Fe, Mn, Cr, and V. In addition to Ni, it is more preferable to further contain one or more elements selected from the group consisting of Co, Mn, and Al. Furthermore, in the lithium metal oxide represented by the above formula (2), when the total amount of metallic elements constituting M is taken as 100 mol%, it is preferable to contain 55 mol% or more of Ni, and more preferably 70% or more of Ni. In the above general formula (2), 0.5 ≤ x ≤ 1.5, and more preferably 0.7 ≤ x ≤ 1.3.

[0044] The positive electrode active material may be a compound with a coating applied to its surface. Furthermore, the positive electrode active material may be a commercially available product.

[0045] Examples of the composition of lithium-based cathode active materials represented by the above general formula (2), and other lithium-based cathode 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 0.05This includes O2 (NCA811), LiCoO2 (LCO), and LiFePO4 (LFP), among others.

[0046] Furthermore, the positive electrode active material may contain multiple different types of lithium-based positive electrode active materials, for example, LiNi with different compositions from among the above lithium-based positive electrode active materials. x Co y Mn Z It may contain multiple O2 molecules (where x, y, and z are the same as in the specific example above), 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 Al z It may also contain O2 (where x, y, and z are as shown in the specific example above).

[0047] The amount of positive electrode active material contained in the electrode mixture is appropriately selected depending on the application of the electrode mixture, but it is preferably 40% by mass or more and 99.9% by mass or less of the total amount of solids derived from the binder, positive electrode active material, and conductive additive. When the amount of positive electrode active material is within this range, for example, sufficient charge and discharge capacity can be obtained, and the battery performance tends to be good.

[0048] On the other hand, the above binder does not easily gel even when mixed with the above-mentioned positive electrode active material, particularly a positive electrode active material containing a relatively large amount of Ni. Furthermore, the above binder exhibits high adhesion to the above-mentioned positive electrode active material, etc. Therefore, the ratio of binder-derived solids (total amount excluding components that volatilize during curing), active material, and conductive additive to the total amount of binder-derived solids can be set to, for example, 0.2% by mass or more and 20% by mass or less. The amount of binder-derived solids is more preferably 0.2% by mass or more and 10% by mass or less, and even more preferably 0.2% by mass or more and 4% by mass or less.

[0049] Furthermore, the conductive additive included in the electrode mixture is not particularly limited as long as it is a compound that can further enhance the conductivity between the positive electrode active materials or between the positive electrode active materials and the current collector. Examples of conductive additives include acetylene black, Ketjen black, carbon black, graphite powder, graphene, carbon nanofibers, carbon nanotubes, and carbon fibers.

[0050] The amount of conductive additive contained in the electrode mixture is appropriately selected depending on its type and other factors. From the viewpoint of improving both conductivity and the dispersibility of the conductive additive, it is preferable that the amount is 0.1% by mass or less, 15% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, and even more preferably 0.1% by mass or more and 5% by mass or less, relative to the total amount of solid content derived from the binder, positive electrode active material, and conductive additive.

[0051] The electrode mixture may contain a solvent other than the non-aqueous solvent contained in the binder described above. This solvent can be selected from among the non-aqueous solvents that the binder may contain.

[0052] 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 generally preferably 20 parts by mass or more and 150 parts by mass or less per 100 parts by mass of the active material described above.

[0053] The electrode mixture may further contain a dispersant, an adhesion aid, a thickener, etc., and known compounds can be used for 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 it is preferable that they be 15% by mass or less relative to the total amount of solids derived from the binder and the active material.

[0054] Furthermore, the electrode mixture 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 polymers mentioned above, resins such as polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and polyacrylonitrile (PAN); etc. These are not particularly limited as long as they do not impair the purpose and effects of the present invention, but it is preferable that they be 15% by mass or less of the total amount of solids derived from the binder and positive electrode active material.

[0055] The above electrode mixture 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.

[0056] The viscosity of the electrode mixture is not particularly limited, as long as it prevents dripping, uneven coating, and delayed drying after coating when applying the electrode mixture to form the electrode mixture layer, and provides good workability and applicability during electrode mixture layer preparation. Typically, the viscosity (slurry viscosity) measured with a B-type 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. In this specification, the viscosity (slurry viscosity) of the electrode mixture is the value measured 2 minutes after the start of rotation with the above-mentioned B-type rotational viscometer.

[0057] 3. Electrode The electrode (positive electrode) of the lithium-ion secondary battery of the present invention may include an electrode mixture layer made of the above-described electrode mixture, and may be, for example, a structure having a current collector and the above-described electrode mixture layer disposed on the current collector.

[0058] • Current collector A current collector is a terminal 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, a layer containing carbon black or the like may be formed on the surface of another medium, or the above-mentioned metal foils or metal meshes may be applied to it.

[0059] • Electrode mixture layer The electrode mixture layer is a layer formed by coating a composition containing a binder and a positive electrode active material (for example, the electrode mixture described above) onto a current collector and drying it. The electrode mixture layer may be formed on only one side of the current collector, or it may be arranged on both sides.

[0060] The electrode mixture layer contains at least the solid components (vinylidene fluoride polymer) derived from the binder mentioned above and the active material, and further contains various additives such as conductive additives, dispersants, adhesion aids, and thickeners as needed. These are the same as those described for the electrode mixture.

[0061] Here, the thickness of the electrode mixture layer is not particularly limited, but in one example, it is preferably 1 μm to 1000 μm. Also, the basis weight of the electrode mixture layer formed on one side of the current collector is not particularly limited and can be any basis weight, but in one example, it is 50 g / m². 2 More than 1000g / m 2 The following is preferable, 100g / m 2 More than 500g / m 2 The following are preferable.

[0062] The electrode mixture layer described above can be formed by the steps of applying the electrode mixture described above onto a current collector and drying it.

[0063] 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.

[0064] Furthermore, after applying the electrode mixture, the non-aqueous solvent is dried by heating at an arbitrary temperature. In one example, the drying temperature is preferably 60°C to 500°C, and more preferably 80°C to 200°C. Heating may be performed multiple times at different temperatures. The solvent in the mixture may also be dried under atmospheric pressure, under pressure, or under reduced pressure. Further heat treatment may be performed after drying.

[0065] 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.

[0066] 4. Lithium-ion rechargeable batteries As described above, the binders and electrode mixtures can be used to form electrodes (positive electrodes) in lithium-ion secondary batteries, but they may also be used to form other layers of lithium-ion secondary batteries. [Examples]

[0067] The following describes specific embodiments of the present invention along with comparative examples, but the present invention is not limited to these.

[0068] 1. Methods for measuring and evaluating physical properties In the examples and comparative examples described later, the inherent viscosity of the vinylidene fluoride polymer was measured using the following method. Furthermore, electrode peel strength tests and viscosity ratio tests were performed according to the following procedure.

[0069] 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 30°C constant temperature bath. Then, from the obtained value, the inherent viscosity (η) of the vinylidene fluoride polymer was calculated based on the following formula. 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, i.e., 0.4 g / dL.

[0070] • Electrode peel strength test The electrodes prepared in the examples and comparative examples described below were cut to a length of 100 mm and a width of 20 mm. Then, a 90° peel test was performed using a tensile testing machine (ORIENTE CHSIA-1150 UNIVERSAL TESTING MACHINE) in accordance with JIS F6854-1, at a head speed of 10 mm / min, and the peel strength was measured.

[0071] • Viscosity ratio test (1) Preparation of slurry The electrode active material NCA811(Li) described below 1.00 Ni 0.85 Co 0.15 Al 0.05 O2) and carbon black (SP: SuperP (registered trademark) manufactured by Timcal Japan, average particle size: 40 nm, specific surface area: 60 m²) 2 The powder was mixed with ( / g) and the other ingredients. On the other hand, the vinylidene fluoride polymer in the binder used in each example or comparative example was dissolved in N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") as described later, and a solution containing 6% by mass of the binder's resin component (vinylidene fluoride polymer) was prepared. Then, the vinylidene fluoride polymer solution was added to the mixture of NCA811 and carbon black and kneaded. Specifically, the above solution was added so that the solid content concentration was 81.5% by mass, and primary kneading was performed at 2000 rpm for 2.5 minutes. Next, the remaining above solution and NMP were added to make the solid content concentration 75% by mass. Then, secondary kneading was performed at 2000 rpm for 3 minutes to obtain a slurry. After primary and secondary kneading, the slurry was allowed to cool until the slurry temperature reached 40°C. The above slurry preparation was carried out in an environment of 22°C with a dew point of -30°C to -40°C. The mass ratio of the electrode active material, carbon black, and vinylidene fluoride polymer in the obtained slurry is 100:2:2, in that order.

[0072] (2) Measurement of viscosity The obtained slurry was measured using an E-type viscometer at 25°C and under conditions of dew point between -30°C and -40°C, with a shear rate of 2s. -1 The measurements were performed using the following method. Viscosity was measured by placing the slurry in the measuring device, waiting 60 seconds, and then rotating the rotor. The viscosity of the slurry was defined as the value 300 seconds after the start of rotor rotation.

[0073] (3) Storage of slurry Each slurry was stored at 25°C under a nitrogen atmosphere for two weeks. The dew point of the storage environment was maintained between -30°C and -40°C.

[0074] (4) Measurement of viscosity The viscosity of the slurry after 2 weeks of storage was measured using an E-type viscometer at 25°C and a dew point between -30°C and -40°C, with a shear rate of 2s. -1 The measurement was performed using the same method as described above for measuring viscosity.

[0075] (5) Calculation of viscosity ratio The viscosity ratio was calculated from the viscosity measured in (2) above and the viscosity measured in (4) above, based on the following formula. Viscosity ratio (%) = (Slurry viscosity after storage) / (Slurry viscosity immediately after preparation) × 100

[0076] 2.Raw materials The following materials were used to prepare the vinylidene fluoride polymer described later. 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 succinate

[0077] The following components were used for the preparation of electrodes and the slurry described above. • NCA811 (Average particle size measured by laser diffraction-diffusion method of aqueous dispersion: 12.5 μm, specific surface area measured by BET single-point method: 0.24 m²) 2 (pH: 11.6, measured by the method described above) • N-methyl-2-pyrrolidone (moisture content: less than 500 ppm) • Carbon black: SuperP (registered trademark) manufactured by Timcal Japan, average particle size measured by electron microscopy: 40 nm, specific surface area measured by BET single-point method: 60 m² 2 / g

[0078] 3. Preparation of binder and electrodes (1) Preparation of vinylidene fluoride polymers [Preparation of polyvinylidene fluoride polymer A] In a 2-liter autoclave, 1240 g of deionized water, 0.4 g of Metroze 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.2 g of 50% by mass 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)) were charged and heated to 45°C. Next, while maintaining 45°C, a 5% by mass aqueous solution of monomer mixture A was continuously supplied to the reaction vessel at an average flow rate of 0.75 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride-based polymer A (VDF / CEA / APOPA / APOPOPA). A total of 4.0 g of monomer mixture A was added, including the amount added initially.

[0079] [Preparation of polyvinylidene fluoride polymer B] In a 2-liter autoclave, 1248 g of deionized water, 0.5 g of Metroze 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 0.48 g of 50% by mass 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)) were charged and heated to 45°C. Next, while maintaining 45°C, a 5% by mass aqueous solution of monomer mixture B was continuously supplied to the reaction vessel at a flow rate of 0.3 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride-based polymer B (VDF / AA / CEA / APOPA / APOPOPA). A total of 4.0 g of monomer mixture B was added, including the amount added initially.

[0080] [Preparation of polyvinylidene fluoride polymer C] In a 2-liter autoclave, 1248 g of deionized water, 0.4 g of Metroze 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.0 g of 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.2 g of AA were charged and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by mass aqueous solution of AA was continuously supplied to the reaction vessel at a flow rate of 0.5 g / min. The resulting polymer slurry was dehydrated and dried to obtain the vinylidene fluoride-based polymer C (VDF / AA). A total of 4.0 g of AA was added, including the amount added initially.

[0081] [Preparation of polyvinylidene fluoride polymer D] In a 2-liter autoclave, 1248 g of deionized water, 0.4 g of Metroze 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.2 g of 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of CEA were charged and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by mass aqueous solution of CEA was continuously supplied to the reaction vessel at a flow rate of 0.8 g / min. The resulting polymer slurry was dehydrated and dried to obtain vinylidene fluoride-based polymer D (VDF / CEA). A total of 4.0 g of CEA was added, including the amount added initially.

[0082] [Preparation of polyvinylidene fluoride polymer E] In a 2-liter autoclave, 1224 g of deionized water, 0.4 g of Metroze 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.2 g of 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of APS were charged and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by mass aqueous solution of monomer mixture F (APS / AA = 0.7 / 0.2 (mass ratio)) was continuously supplied to the reaction vessel 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).

[0083] [Preparation of polyvinylidene fluoride polymer F] In a 2-liter autoclave, 1212 g of deionized water, 0.4 g of Metroze 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.4 g of 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, 0.05 g of CEA, and 0.1 g of CMA were charged and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by mass aqueous solution of monomer mixture G (CEA / CMA = 1 / 2 (mass ratio)) was continuously supplied to 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).

[0084] [Preparation of polyvinylidene fluoride polymer G] In a 2-liter autoclave, 1222 g of deionized water, 0.4 g of Metroze 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.2 g of 50% by mass diisopropyl peroxydicarbonate-Flon 225cb solution, 396 g of vinylidene fluoride, and 0.4 g of CMA were charged and heated to 45°C. Next, while maintaining the temperature at 45°C, a 5% by mass aqueous solution of CMA was continuously supplied to the reaction vessel 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).

[0085] (2) Preparation of electrode mixture and fabrication of electrodes (Example 1) The electrode active material NCA811 is combined with carbon black (SP: SuperP®, manufactured by Timcal Japan, average particle size: 40 nm, specific surface area: 60 m²) as a conductive additive. 2 Add ( / g) and mix the powder.

[0086] The above-mentioned vinylidene fluoride polymer A (binder) was dissolved in NMP to prepare a vinylidene fluoride polymer solution containing 8% by mass of vinylidene fluoride polymer A. Then, NMP was added to a mixture of NCA811 and carbon black and kneaded. Specifically, the vinylidene fluoride polymer solution was added to achieve a solid content concentration of 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 achieve a solid content concentration of 73.5% by mass, and secondary kneading was performed at 2000 rpm for 3 minutes to obtain an electrode mixture.

[0087] The resulting electrode mixture was coated onto a 15 μm thick aluminum foil current collector using a bar coater, and then primary dried in a constant temperature oven under a nitrogen atmosphere at 110°C for 30 minutes. Subsequently, secondary drying was performed under a nitrogen atmosphere at 130°C for 2 hours, resulting in a basis weight of approximately 250 g / m². 2 An electrode (electrode peeling measurement sample) was obtained. The mass ratio of the electrode active material, carbon black, and vinylidene fluoride polymer in the obtained electrode mixture was 100:2:1.5, in that order.

[0088] (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 vinylidene fluoride-based polymer B.

[0089] (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 a vinylidene fluoride polymer F.

[0090] (Comparative Example 1) An electrode mixture and electrode were obtained in the same manner as in Example 1, except that the binder was changed to KF#7300 manufactured by Kureha Corporation.

[0091] (Comparative Example 2) An electrode mixture and 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.

[0092] (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 a vinylidene fluoride polymer C.

[0093] (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.

[0094] (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.

[0095] (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.

[0096] [Table 1]

[0097] As shown in Table 1 above, when a vinylidene fluoride-based polymer containing a constituent unit derived from vinylidene fluoride and two or more constituent units having a carboxyl group was included as a binder, the peel strength was high and the gelation resistance was good (Examples 1-3).

[0098] On the other hand, when a homopolymer of vinylidene fluoride was used as a binder, peel strength and gelation resistance were low (Comparative Example 1). Furthermore, when a vinylidene fluoride polymer containing vinylidene fluoride-derived structural units and only one type of structural unit having a carboxyl group was included as a binder, peel resistance improved, but the viscosity ratio exceeded 100%, resulting in low gelation resistance (Comparative Examples 3-5). Moreover, when two types of vinylidene fluoride polymers containing vinylidene fluoride-derived structural units and only one type of structural unit having a carboxyl group were mixed, gelation resistance was also low (Comparative Examples 2 and 6).

[0099] This application claims priority under Japanese Patent Application No. 2022-049941, filed on 25 March 2022. All provisions of the said application are incorporated herein by reference. [Industrial applicability]

[0100] The binder of the present invention exhibits minimal degradation or thickening even when mixed with positive electrode active materials containing a high amount of nickel. Furthermore, the binder provides excellent adhesive strength even in small quantities. Therefore, this binder, as well as electrode mixtures and electrodes containing it, are extremely useful in the manufacture of lithium-ion secondary batteries.

Claims

1. A binder for the positive electrode of a lithium-ion secondary battery, containing a vinylidene fluoride polymer, The vinylidene fluoride polymer comprises a constituent unit derived from vinylidene fluoride and two or more constituent units having a carboxyl group. Of the constituent units having a carboxyl group, at least two have a structure represented by the following general formula (1): 【Chemistry 1】 (In general formula (1), R1 to R3 each independently represent a hydrogen atom, a chlorine atom, or an alkyl group having 1 to 4 carbon atoms without substituents. X represents an alkylene group having 1 to 4 carbon atoms, which may have substituents. n represents an integer between 0 and 5 (inclusive). A binder whose slurry viscosity ratio, determined by the following method, is 100% or less. [How to determine the viscosity ratio of a slurry] (1) Prepare a slurry consisting only of the electrode active material NCA811, carbon black, the vinylidene fluoride polymer in the binder, and N-methyl-2-pyrrolidone, wherein the mass ratio of the electrode active material, the carbon black, and the vinylidene fluoride polymer in the binder is 100:2:2 and the solid content concentration is 75% by mass. (2) The viscosity of the slurry is measured using an E-type viscometer at 25°C and a shear rate of 2s. －１ Measure it using this method. (3) Store the slurry at 25°C under a nitrogen atmosphere for two weeks. (4) The viscosity of the slurry after storage was measured using an E-type viscometer at 25°C and a shear rate of 2s. －１ Measure it using this method. (5) The viscosity ratio of the slurry is calculated based on the following formula. Viscosity ratio (%) = (Slurry viscosity after storage) / (Slurry viscosity immediately after preparation) × 100

2. The vinylidene fluoride polymer comprises a constituent unit derived from vinylidene fluoride and three or more constituent units having a carboxyl group. The binder according to claim 1.

3. The vinylidene fluoride polymer has a total amount of carboxyl group-containing constituent units of 0.1% by mass or more and 2% by mass or less, when the total constituent units are considered to be 100% by mass. The binder according to claim 1 or 2.

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

5. An electrode mixture comprising the binder described in claim 3 and a positive electrode active material.

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

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

8. An electrode in which an electrode mixture layer containing the electrode mixture described in claim 6 is provided on a current collector.

9. An electrode in which an electrode mixture layer containing the electrode mixture described in claim 7 is provided on a current collector.

10. The electrode included in claim 8, Lithium-ion rechargeable battery.

11. The electrode included in claim 9, Lithium-ion rechargeable battery.

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