Binder composition, electrode mixture, electrode, and non-aqueous electrolyte secondary battery

Abstract

Provided is a binder composition that suppresses thickening of an electrode mixture slurry while maintaining high peel strength between a current collector and an electrode mixture layer.

Description

[Technical Field]

[0001] The present invention relates to a binder composition, an electrode mixture, and uses thereof, and more particularly to a binder composition, and an electrode mixture, an electrode, and a nonaqueous electrolyte secondary battery obtained using the binder composition. [Background technology]

[0002] In recent years, electronic technology has developed remarkably, and small portable devices are becoming increasingly sophisticated. Therefore, the power sources used in these devices are required to be smaller and lighter, i.e., have higher energy densities. Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are widely used as batteries with high energy densities.

[0003] An electrode for a non-aqueous electrolyte secondary battery has a structure including a current collector and an electrode mixture layer formed on the current collector. The electrode mixture layer is generally formed by applying an electrode mixture containing an electrode active material and a binder composition in a slurry state dispersed in a suitable solvent or dispersion medium onto the current collector, and then volatilizing the solvent or dispersion medium.

[0004] From the viewpoint of improving the safety and performance of batteries, binders for non-aqueous electrolyte secondary batteries are required to have high adhesiveness to electrode active materials and current collectors. Due to their high adhesiveness, vinylidene fluoride polymers containing mainly repeating units derived from vinylidene fluoride (VDF) are mainly used as binders (binding agents) in binder compositions.

[0005] As an example of such a binder, Patent Document 1 describes a resin mixture in which two or more types of vinylidene fluoride homopolymers having different weight-average molecular weights are mixed.

[0006] Furthermore, Patent Document 2 describes a binder containing 50% by weight or more of polyvinylidene fluoride having a weight-average molecular weight of 300,000 to 400,000 based on the total amount of the binder.

[0007] Patent Document 3 discloses a binder for forming electrodes of non-aqueous electrochemical elements, which comprises a vinylidene fluoride homopolymer (A) having an inherent viscosity of 0.5 to 1.5 dl / g and a vinylidene fluoride polymer (B) having an inherent viscosity 1.4 times or more that of the polymer (A), and the proportion of the polymer (A) to the total amount of the polymers (A) and (B) is in the range of 60 to 98% by weight. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-243643 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-107753 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-310747 Summary of the Invention [Problem to be solved by the invention]

[0009] However, as a result of investigations by the present inventors, it was found that when an active material with a small particle size is used as the positive electrode active material, the electrode mixture slurry becomes thicker.

[0010] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a binder composition that can suppress thickening of an electrode mixture slurry while maintaining high adhesiveness, even when an active material with a small particle size is used as the positive electrode active material. [Means for solving the problem]

[0011] In order to solve the above problems, a binder composition according to one aspect of the present invention is a binder composition containing a vinylidene fluoride polymer, wherein the vinylidene fluoride polymer comprises a vinylidene fluoride homopolymer (A) and a vinylidene fluoride homopolymer (B) different from the vinylidene fluoride homopolymer (A), wherein a proportion of the vinylidene fluoride homopolymer (A) in the total amount of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) in the binder composition is 25% by weight or more and 55% by weight or less, and the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) are polymers that satisfy the following conditions (i) to (iii): (i) The 5% by weight solution of the vinylidene fluoride homopolymer (A) in N-methyl-2-pyrrolidone was heated at 25°C and a shear rate of 3.83 s -1 The solution viscosity is 30 mPa·s or more and 500 mPa·s or less. (ii) The 5 wt % N-methyl-2-pyrrolidone solution of the vinylidene fluoride homopolymer (B) was heated at 25°C and a shear rate of 3.83 s -1 The solution viscosity is 800 mPa·s or more and 30,000 mPa·s or less, and (iii) A 5 wt % N-methyl-2-pyrrolidone solution of a mixture of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B), which are mixed in the same ratio as in the binder composition, has a turbidity of 1 or less at 25°C. [Effects of the Invention]

[0012] The binder composition according to the present invention can suppress thickening of the electrode mixture slurry while maintaining high adhesiveness, even when an active material with a small particle size is used as the positive electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the binder composition according to the present invention and its use will be described.

[0014] [Binder Composition] The binder composition according to this embodiment is a composition used to bind an electrode active material to a current collector in an electrode in which an electrode mixture layer containing an electrode active material is formed on a current collector. The binder composition according to this embodiment contains a vinylidene fluoride polymer. The binder composition according to this embodiment may further contain a conductive assistant.

[0015] (vinylidene fluoride polymer) The binder composition according to this embodiment contains, as vinylidene fluoride polymers, a vinylidene fluoride homopolymer (A) having a low solution viscosity and a vinylidene fluoride homopolymer (B) having a high solution viscosity. Specifically, the vinylidene fluoride homopolymer (A) is a vinylidene fluoride homopolymer that, when added to N-methyl-2-pyrrolidone (hereinafter, NMP) to a concentration of 5 wt %, and then heated and stirred at 50°C to prepare an NMP solution, has a solution viscosity of 30 mPa·s to 500 mPa·s at 25°C. On the other hand, the vinylidene fluoride homopolymer (B) is a vinylidene fluoride homopolymer that, when added to NMP to a concentration of 5 wt %, and then prepared an NMP solution, has a solution viscosity of 800 mPa·s to 30,000 mPa·s at 25°C. Here, the solution viscosity was measured using an E-type viscometer at 25°C and a shear rate of 3.83 s -1 This is the value measured at

[0016] Hereinafter, the solution viscosity of an NMP solution of each polymer or a mixture described below, measured in this manner, will be referred to as "solution viscosity of vinylidene fluoride homopolymer (A)," "solution viscosity of vinylidene fluoride homopolymer (B)," "solution viscosity of the mixture," etc.

[0017] The solution viscosity of the vinylidene fluoride homopolymer (A) may be 30 mPa·s or more and 500 mPa·s or less, but from the viewpoint of improving electrode adhesion, it is preferably 40 mPa·s or more, more preferably 60 mPa·s or more. When the solution viscosity of the vinylidene fluoride homopolymer (A) is 30 mPa·s or more, the tensile strength of the electrode obtained using this binder composition is increased, and high electrode adhesion can be obtained. Furthermore, from the viewpoint of suppressing thickening of the slurry, the solution viscosity is preferably 450 mPa·s or less, more preferably 200 mPa·s or less.

[0018] Such vinylidene fluoride homopolymer (A) may be a commercially available product or may be newly synthesized.

[0019] The solution viscosity of the vinylidene fluoride homopolymer (B) may be 800 mPa·s or more and 30,000 mPa·s or less, but from the viewpoint of improving electrode adhesion, it is preferably 1,000 mPa·s or more, more preferably 2,000 mPa·s or more, and from the viewpoint of suppressing an increase in slurry viscosity, it is preferably 25,000 mPa·s or less, more preferably 10,000 mPa·s or less, and even more preferably 7,000 mPa·s or less.

[0020] Such vinylidene fluoride homopolymer (B) may be a commercially available product or may be newly synthesized.

[0021] In the binder composition of this embodiment, the proportion of vinylidene fluoride homopolymer (A) in the total amount of vinylidene fluoride homopolymer (A) and vinylidene fluoride homopolymer (B) in the binder composition is 25% by weight or more and 55% by weight or less. Having a proportion of vinylidene fluoride homopolymer (A) of 25% by weight or more can provide an effect of suppressing slurry thickening. Having a proportion of vinylidene fluoride homopolymer (A) of 55% by weight or less can provide an effect of suppressing slurry thickening, improve adhesion, and prevent cracking of the resulting electrode. From the viewpoint of suppressing slurry thickening, the proportion of vinylidene fluoride homopolymer (A) is preferably 30% by weight or more, more preferably 40% by weight or more. From the viewpoint of improving adhesion, the proportion is preferably 50% by weight or less, more preferably less than 50% by weight, and even more preferably 45% by weight or less.

[0022] In a preferred embodiment of the binder composition of this embodiment, the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) are used in such a combination that, when a separate mixture of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) is prepared in the same ratio as that of the binder composition, the mixture is added to NMP to a concentration of 5 wt % and the resulting NMP solution has a solution viscosity at 25°C of 400 mPa·s to 20,000 mPa·s. The solution viscosity of the mixture is preferably 400 mPa·s to 7,000 mPa·s, more preferably 500 mPa·s to 2,500 mPa·s. By including the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) in the binder composition at a ratio such that the solution viscosity of the mixture falls within this range, the binder composition can achieve high adhesiveness.

[0023] Furthermore, the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) are used in such a combination that when a mixture of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) is separately prepared in the same ratio as the ratio of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) in the binder composition, the turbidity of the NMP solution obtained by adding the mixture to NMP so that the concentration of the mixture becomes 5% by weight is 1 or less at 25°C.

[0024] Here, turbidity is measured as follows: First, a polymer mixture is added to NMP so that the concentration of the polymer mixture becomes 5% by weight, and the mixture is heated and stirred at 50°C to prepare a vinylidene fluoride solution. The turbidity of the obtained vinylidene fluoride solution is measured by integrating sphere photoelectric photometry using a turbidity meter.

[0025] By using a combination of vinylidene fluoride homopolymer (A) and vinylidene fluoride homopolymer (B) such that the turbidity of the solution viscosity of the mixture at a predetermined mixing ratio is 1 or less, it is possible to achieve high adhesiveness as a binder composition and prevent the resulting electrode mixture slurry from becoming thicker.

[0026] In this specification, "preventing the electrode mixture slurry from thickening" means that the electrode mixture is allowed to stand for 48 hours after preparation, and the viscosity of the slurry after 48 hours (measured with a Brookfield viscometer at 25°C) is 45,000 mPa s or less.

[0027] It is preferable that the binder composition does not substantially contain any polymer other than the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B).

[0028] (Conductive additive) The binder composition according to this embodiment may contain a conductive aid for the purpose of improving the conductivity of the resulting electrode mixture layer. Examples of conductive aids that can be used include carbonaceous materials such as carbon nanotubes, carbon black, graphite fine powder, and graphite fiber, graphene, and metal fine powders or fibers such as nickel and aluminum. Of these, carbon nanotubes are preferred as the conductive aid. By using carbon nanotubes as the conductive aid, it is possible to obtain the effect of suppressing the thickening of the slurry while maintaining high adhesiveness.

[0029] (solvent) The binder composition may contain a solvent. The solvent may be the same as the solvent in the electrode mixture, and may be water or a non-aqueous solvent. Examples of non-aqueous solvents include NMP, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N,N-dimethyl sulfoxide, hexamethylphosphoramide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate, acetone, ethyl acetate, n-butyl acetate, n-butanol, dipropylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, cyclohexanone, cyclohexane, and methyl ethyl ketone.

[0030] (Preparation of Binder Composition) To prepare the binder composition, vinylidene fluoride homopolymer (A) and vinylidene fluoride homopolymer (B) that satisfy the solution viscosity requirements for each polymer are selected. The turbidity of the mixture at the desired ratio is then measured, and a combination of vinylidene fluoride homopolymer (A) and vinylidene fluoride homopolymer (B) that satisfies the turbidity requirement is selected to determine the vinylidene fluoride homopolymer (A) and vinylidene fluoride homopolymer (B) to be used in the binder composition. The binder composition can then be prepared by mixing the vinylidene fluoride homopolymer (A) and vinylidene fluoride homopolymer (B) in the desired ratio. Furthermore, when the binder composition contains a solvent and a conductive additive, the order of mixing is not particularly limited. For example, a mixture of vinylidene fluoride homopolymer (A) and vinylidene fluoride homopolymer (B) may be prepared, and then a solvent may be added to the mixture. Alternatively, one vinylidene fluoride homopolymer may be added to a solvent, and then the other vinylidene fluoride homopolymer may be added thereto.

[0031] [Electrode mixture] The electrode mixture according to this embodiment contains the binder composition according to this embodiment and an electrode active material. The electrode mixture may further contain a solvent.

[0032] The electrode mixture according to this embodiment can be used as an electrode mixture for a positive electrode or a negative electrode by changing the type of active material according to the type of current collector to be coated. The binder composition according to this embodiment can solve the problem of thickening of the electrode mixture slurry that occurs when a positive electrode active material with a small particle size is used as the active material. Therefore, the electrode mixture according to this embodiment is suitable for use as an electrode mixture for a positive electrode.

[0033] (electrode active material) Lithium composite metal oxides are typically used as the positive electrode active material. Examples of lithium composite metal oxides include LiMnO2, LiMn2O4, LiCoO2, LiNiO2, and LiNi x Co 1-x O2(0 <x<1)、LiNix Co y Mn 1-x-y O2(0 < x < 1, 0 < y < 1), LiNi x Co y AlZO2(0.55 ≤ x < 1, 0 < y2 < 0.55, 0 < z < 0.55, and x / (x + y + z) ≥ 0.55), LiFePO4, etc. can be mentioned.

[0034] The binder composition according to this embodiment can solve the problem of thickening in the electrode binder slurry that occurs when using a cathode active material with a small particle size as the active material. More specifically, these problems can occur when the primary particle size is 50 nm or more and 600 nm or less, and D10 in the volume-based particle size distribution is 1 μm or less and D50 is 1.6 μm or less. Therefore, in the electrode binder of this embodiment, a cathode active material with a primary particle size of 50 nm or more and 600 nm, and D10 in the volume-based particle size distribution of 1 μm or less and D50 of 1.6 μm or less is preferably used. Note that the particle size of the cathode active material can vary depending on the preparation method even for the same active material.

[0035] The primary particle size of the cathode active material is the average particle size in the major axis diameter measured by observation using a scanning electron microscope (SEM). Also, the volume-based particle size distribution (D10 and D50) may be measured using the laser diffraction scattering method.

[0036] As the anode active material, conventionally known materials such as carbon materials, metal / alloy materials, and metal oxides can be used. For example, as carbon materials, artificial graphite, natural graphite, graphitizable carbon, and easily graphitizable carbon can be mentioned.

[0037] (solvent) The solvent may be water or a non-aqueous solvent. Examples of non-aqueous solvents include NMP, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N,N-dimethyl sulfoxide, hexamethylphosphoamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate, acetone, ethyl acetate, n-butyl acetate, n-butanol, dipropylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, cyclohexanone, cyclohexane, and methyl ethyl ketone. One or more of these solvents may be contained in the electrode mixture. The solvent may be added to the binder composition or may be added separately from the binder composition.

[0038] (Other ingredients) The electrode mixture in this embodiment may contain other components within the scope of not impairing the effects of the present invention. Examples of other components include a pigment dispersant such as polyvinylpyrrolidone, a dispersion stabilizer, and the like.

[0039] (Preparation of electrode mixture) The electrode mixture may be prepared by, for example, mixing the binder composition of this embodiment, the electrode active material, and the solvent to form a uniform slurry, and the order of mixing is not particularly limited. Furthermore, when a binder composition containing a solvent is used as the binder composition, the electrode active material may be added before adding the solvent to the binder composition. For example, the electrode active material may be added to the binder composition, and then the solvent may be added, followed by stirring and mixing to obtain the electrode mixture. Alternatively, the electrode active material may be dispersed in a solvent, and the binder composition may be added thereto, followed by stirring and mixing to obtain the electrode mixture. Alternatively, the electrode active material may be added to a binder composition containing a solvent as the binder composition, followed by stirring and mixing to obtain the electrode mixture. Furthermore, the kneading may be performed multiple times, for example, secondary kneading may be performed.

[0040] In the electrode mixture, when the amount of the electrode active material is 100 parts by weight, the total amount of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) is preferably 1 to 5 parts by weight, more preferably 2 to 3 parts by weight.

[0041] 〔electrode〕 The electrode of this embodiment includes an electrode mixture layer formed from the electrode mixture on a current collector. Unless otherwise specified, the term "electrode" in this specification and the like refers to a battery electrode in which an electrode mixture layer formed from the electrode mixture of this embodiment is formed on a current collector.

[0042] (current collector) The current collector is the base material of the electrode and a terminal for extracting electricity. Examples of materials for the current collector include iron, stainless steel, steel, copper, aluminum, nickel, and titanium. The current collector is preferably in the form of a foil or mesh. When the electrode is a positive electrode, the current collector is preferably a carbon-coated aluminum foil. The thickness of the current collector is preferably 5 μm to 100 μm, and more preferably 5 to 20 μm.

[0043] (Electrode mixture layer) The electrode mixture layer is a layer obtained by applying a slurry of the electrode mixture to a current collector and drying it. The electrode mixture can be applied by any method known in the art, including methods using a bar coater, die coater, or comma coater. The drying temperature for forming the electrode mixture layer is preferably 50°C to 170°C, and more preferably 50°C to 150°C. The electrode mixture layer may be formed on both sides of the current collector, or on only one side.

[0044] The thickness of the electrode mixture layer is usually 20 to 600 μm, preferably 20 to 350 μm per side. The electrode mixture layer may be pressed to increase its density. The weight of the electrode mixture layer on both sides is usually 20 to 700 g / m 2 and preferably 30 to 500 g / m2 is.

[0045] (Electrode properties) The electrode serves as a positive electrode when an electrode mixture layer is obtained using an electrode mixture for a positive electrode, and serves as a negative electrode when an electrode mixture layer is obtained using an electrode mixture for a negative electrode. The electrode according to this embodiment can be suitably used as a positive electrode for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery.

[0046] [Nonaqueous electrolyte secondary battery] The nonaqueous electrolyte secondary battery of this embodiment includes the electrode of this embodiment. Other components of the nonaqueous electrolyte secondary battery are not particularly limited, and for example, conventionally used components can be used. One aspect of the nonaqueous electrolyte secondary battery has a cylindrical battery structure in which a power generation element, formed by stacking a positive electrode and a negative electrode with a separator disposed between them and winding the stacked elements into a spiral, is housed in a metal casing. Another aspect of the nonaqueous electrolyte secondary battery may be a nonaqueous electrolyte secondary battery of other shapes, such as a coin shape, a prismatic shape, or a paper shape.

[0047] A non-aqueous electrolyte secondary battery can be produced, for example, by stacking a negative electrode layer and a positive electrode layer with a separator interposed therebetween, placing the stacked layers in a battery container, injecting an electrolyte into the battery container, and sealing the container.

[0048] 〔summary〕 The binder composition according to the present embodiment is a binder composition containing a vinylidene fluoride polymer, and the vinylidene fluoride polymer contains a vinylidene fluoride homopolymer (A) and a vinylidene fluoride homopolymer (B) different from the vinylidene fluoride homopolymer (A), the proportion of the vinylidene fluoride homopolymer (A) in the total amount of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) in the binder composition is 25% by weight or more and 55% by weight or less, and the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) are polymers that satisfy the following conditions (i) to (iii): (i) The 5% by weight solution of the vinylidene fluoride homopolymer (A) in N-methyl-2-pyrrolidone was heated at 25°C and a shear rate of 3.83 s -1 The solution viscosity is 30 mPa·s or more and 500 mPa·s or less. (ii) The 5 wt % N-methyl-2-pyrrolidone solution of the vinylidene fluoride homopolymer (B) was heated at 25°C and a shear rate of 3.83 s -1 The solution viscosity is 800 mPa·s or more and 30,000 mPa·s or less, and (iii) A 5 wt % N-methyl-2-pyrrolidone solution of a mixture of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B), which are mixed in the same ratio as in the binder composition, has a turbidity of 1 or less at 25°C.

[0049] In the binder composition according to the present embodiment, the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) preferably further satisfy the following condition (iv): (iv) A 5% by weight solution of the mixture in N-methyl-2-pyrrolidone was heated at 25°C and a shear rate of 3.83 s -1 The solution viscosity is 400 mPa·s or more and 25,000 mPa·s or less.

[0050] In addition, the binder composition according to this embodiment preferably further contains carbon nanotubes.

[0051] The electrode mixture according to this embodiment is an electrode mixture comprising the binder composition described above and a positive electrode active material having a primary particle diameter of 50 nm or more and 600 nm or less, and having a D10 of 1 μm or less and a D50 of 1.6 μm or less in a volume-based particle size distribution.

[0052] The electrode according to this embodiment is an electrode including an electrode mixture layer formed from the above-described electrode mixture on a current collector.

[0053] The nonaqueous electrolyte secondary battery according to this embodiment is a nonaqueous electrolyte secondary battery including the above-described electrodes.

[0054] The following examples are provided to further explain the embodiments of the present invention. It goes without saying that the present invention is not limited to the following examples, and various modifications are possible in detail. Furthermore, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed herein are also included in the technical scope of the present invention. Furthermore, all of the documents cited in this specification are incorporated by reference. [Example]

[0055] [Measurement of particle diameter of positive electrode active material] The primary particle size of LiFePO4 (hereinafter referred to as LFP), which is the positive electrode active material used in the following examples, was measured by observation using an SEM.

[0056] The particle size distribution of LFP was determined as follows. First, 0.1 g of a dispersant (cationic surfactant "SN Wet 366" (manufactured by San Nopco)) was added to 0.01 g of LFP sample, and the dispersant was allowed to soak into the sample. Next, 20 mL of pure water was added, and the mixture was dispersed in an ultrasonic cleaner for approximately 5 minutes. The particle size distribution in the particle size range of 0.1 to 1000 μm was then determined using a particle size distribution analyzer (manufactured by Microtrac: MT3300EXII). The dispersion medium was pure water, and the refractive index of the dispersion medium was 1.333. From the obtained particle size distribution, the particle sizes at which the cumulative frequency was 10% and 50% on a volume basis were calculated and designated D10 and D50, respectively.

[0057] Example 1 (Preparation of electrode mixture) A polymer mixture was prepared by mixing a vinylidene fluoride homopolymer "KF#850 (manufactured by Kureha Corporation)" (hereinafter referred to as polymer (A1)) with a different vinylidene fluoride homopolymer (hereinafter referred to as polymer (B1)) so that the mixing ratio of polymer (A1) to polymer (B1) (polymer (A1):polymer (B1), weight ratio) was 25:75.

[0058] Polymer B1 is a vinylidene fluoride homopolymer prepared as follows. First, 950 g of ion-exchanged water, 0.2 g of methyl cellulose, 0.4 g of diisopropyl peroxydicarbonate (IPP), 1.3 g of ethyl acetate, and 440 g of vinylidene fluoride were charged into a 2-liter autoclave and heated to 26°C to initiate polymerization. The pressure was then reduced by 0.5 MPa from the maximum pressure, and the temperature was raised to 40°C. Polymerization was completed 14 hours after reaching 26°C. The resulting polymer slurry was heated at 95°C for 60 minutes, dehydrated, washed with water, and dried at 80°C for 20 hours. Powdered vinylidene fluoride polymer was obtained in a 91% yield. The inherent viscosity of the resulting vinylidene fluoride polymer was 3.9 dL / g, and the viscosity of a 5 wt% solution in N-methylpyrrolidone was 6800 mPa·s.

[0059] The polymer mixture was added to NMP to a concentration of 5% by weight, and the mixture was heated and stirred at 50°C for 3 hours to prepare a polymer mixture NMP solution. The polymer mixture NMP solution was added to a dispersion of carbon nanotubes (CNTs) as a conductive additive ("LB107" manufactured by Cnano, CNT diameter: 13-25 nm), and kneaded at 2000 rpm for 3 minutes. LFP (primary particle diameter: 100-200 nm, D10: 0.7 μm, D50: 1.4 μm, specific surface area: 19.9 m) was added to the mixture so that the solid content concentration was 57% by weight. 2 1 / g) and NMP were added. Secondary kneading was carried out at 2000 rpm for 5 minutes to obtain an electrode mixture. The weight ratio of the electrode active material, conductive additive, and polymer mixture in the obtained electrode mixture was 100:2:2, in this order.

[0060] Example 2 An electrode mixture was prepared in the same manner as in Example 1, except that polymer (A1) was a vinylidene fluoride homopolymer "KF#1100 (manufactured by Kureha Corporation)" (hereinafter referred to as polymer (A2)), and the mixing ratio of polymer (A2) to polymer (B1) was 33:67.

[0061] Example 3 A polymer mixture was prepared by mixing polymer (A2) with a vinylidene fluoride homopolymer "KF#4300 (manufactured by Kureha Corporation)" (hereinafter referred to as polymer (B2)) so that the mixing ratio of polymer (A2) to polymer (B2) was 33:67.

[0062] The polymer mixture was added to NMP to a concentration of 5 wt %, and the mixture was heated and stirred at 50°C for 3 hours to prepare a polymer mixture NMP solution. Carbon black (SuperP (registered trademark) manufactured by Timcal) was added as a conductive additive to the same LFP used in Example 1, and powder mixing was performed. The prepared polymer mixture NMP solution was added to this powder mixture, and kneaded at 2000 rpm for 3 minutes. Additional NMP was added to the mixture so that the solids concentration was 57 wt %. A secondary kneading was performed at 2000 rpm for 5 minutes to obtain an electrode mixture. The weight ratio of the electrode active material, conductive additive, and polymer mixture in the obtained electrode mixture, in this order, was 100:2:2.

[0063] Example 4 An electrode mixture was prepared in the same manner as in Example 2, except that the polymer (B1) was replaced with the polymer (B2).

[0064] Example 5 An electrode mixture was prepared in the same manner as in Example 3, except that the mixing ratio of polymer (A2) and polymer (B2) was 50:50.

[0065] Example 6 An electrode mixture was prepared in the same manner as in Example 4, except that the mixing ratio of polymer (A2) and polymer (B2) was 50:50.

[0066] Comparative Example 1 An electrode mixture was prepared in the same manner as in Example 1, except that polymer (A1) was replaced with polymer (A2) and polymer (B1) was replaced with a vinylidene fluoride homopolymer, "kynar (registered trademark) HSV900 (manufactured by Arkema)" (hereinafter referred to as polymer (B3)).

[0067] Comparative Example 2 An electrode mixture was prepared in the same manner as in Example 1, except that only one type of vinylidene fluoride homopolymer (polymer (B2)) was used instead of a mixture of two types of vinylidene fluoride homopolymer.

[0068] Comparative Example 3 An electrode mixture was prepared in the same manner as in Example 3, except that the mixing ratio of polymer (A2) and polymer (B2) was 80:20.

[0069] Comparative Example 4 An electrode mixture was prepared in the same manner as in Example 6, except that polymer (B2) was replaced with polymer (B4).

[0070] Polymer (B4) was a copolymer of vinylidene fluoride, hexafluoropyrene, and monomethyl maleate prepared as follows. First, 1300 g of ion-exchanged water, 0.4 g of methyl cellulose, 0.8 g of IPP, 380 g of vinylidene fluoride, 15 g of hexafluoropropylene, and 1.2 g of monomethyl maleate were charged into a 2-L autoclave, and the temperature was raised to 35°C to initiate polymerization. The polymerization was then terminated when the pressure dropped by 4.0 MPa from the maximum pressure. The polymerization time was 18 h. The resulting polymer slurry was heated at 95°C for 60 minutes, dehydrated, washed with water, and dried at 80°C for 20 hours. Powdered vinylidene fluoride polymer was obtained in a 90% yield. The inherent viscosity of the resulting vinylidene fluoride polymer was 3.0 dL / g, and the viscosity of a 5 wt% solution in N-methylpyrrolidone was 2200 mPa·s.

[0071] Comparative Example 5 An electrode mixture was prepared in the same manner as in Example 1, except that polymer (A1) was a vinylidene fluoride homopolymer "KF#1300 (manufactured by Kureha Corporation)" (hereinafter referred to as polymer (A3)), and the mixing ratio of polymer (A3) to polymer (B1) was 20:80.

[0072] [Evaluation Example 1. Solution Viscosity Measurement] The vinylidene fluoride polymer used in each Example or Comparative Example, or the polymer mixture obtained in each Example or Comparative Example, was added to NMP to make a 5 wt% NMP solution. The 5 wt% NMP solution was measured at 25°C and a shear rate of 3.83 s using a Toki Sangyo Co., Ltd. E-type viscometer "RE-215". -1 The viscosity of the solution was measured by waiting for 60 seconds after the prepared NMP solution was charged into the measuring device, and then the rotor was rotated, and the value after 300 seconds was taken as the viscosity of the solution. Table 1 shows the measurement results of the solution viscosity of each polymer and polymer mixture (polymer blend).

[0073] [Evaluation Example 2: Turbidity Measurement] The polymer mixture prepared in each Example and Comparative Example was added to NMP to a concentration of 5 wt %, and the mixture was heated and stirred at 50°C to prepare a 5 wt % NMP solution. The prepared 5 wt % NMP solution was stirred at 25°C, and immediately after stirring was stopped, the turbidity was measured at 25°C using a turbidity meter (NDH2000, manufactured by Nippon Denshoku Industries Co., Ltd.). The turbidity measurement results are shown in Table 1.

[0074] [Evaluation Example 3: Measurement of Slurry Viscosity] The slurry viscosity of the electrode composite was measured at 25°C using a Brookfield DVS+ viscometer. A fixed amount of the slurry was placed in a 1.2 cm diameter plastic tube and allowed to stand at 25°C and humidity of 10% or less to serve as the measurement sample. A No. 64 spindle was used for this measurement, and the slurry viscosity was measured after pre-stirring at 0.5 rpm for 1 minute, followed by 1 rpm for 2 minutes, and then stirring at 6 rpm for 30 seconds. Viscosity measurements were taken immediately after sample preparation and 48 hours after the start of static storage. The results of the slurry viscosity measurements are shown in Table 1. In Table 1, "Initial" refers to the slurry viscosity immediately after sample preparation.

[0075] [Evaluation Example 4. Peel strength measurement] The electrode mixture obtained in each example and comparative example was applied to an Al foil having a thickness of 15 μm using a bar coater, and then dried by heating at 110° C. for 30 minutes. The mixture was then further heat-treated at 130° C. for 1 hour, resulting in a coating weight of 150 g / m on one side. 2 A single-sided coated electrode was prepared.

[0076] The resulting single-sided coated electrodes were cut into pieces 50 mm long and 20 mm wide, and a 90-degree peel test was performed in accordance with JIS K6854-1 using a tensile tester (AND Corporation, "STB-1225S") at a head speed of 10 mm / min to measure the peel strength (gf / mm). In this measurement, a peel strength of 0.7 gf / mm or more was considered to indicate good adhesion, and a peel strength of less than 0.7 gf / mm was considered to indicate insufficient adhesion. The peel strength measurement results are shown in Table 1.

[0077] [Table 1] As shown in Table 1, in the electrode mixtures of Examples 1 to 6, although the electrode active material was an active material with a small particle size, the resulting electrodes maintained high adhesiveness and were able to suppress the increase in viscosity of the electrode mixture slurry after standing for 48 hours.

[0078] On the other hand, in the electrode mixture of Comparative Example 1, which had a turbidity of more than 1, the slurry viscosity increased after standing for 48 hours. Furthermore, the electrode using the electrode mixture of Comparative Example 1 had insufficient adhesion. Furthermore, the electrode mixture of Comparative Example 2, which was not a mixture of two types of vinylidene fluoride homopolymer, had an increased slurry viscosity after standing for 48 hours. Furthermore, the electrode using the electrode mixture of Comparative Example 3, in which the proportion of low-viscosity vinylidene fluoride homopolymer exceeded 55% by weight, had insufficient adhesion and even developed cracks. Furthermore, in the electrode mixture of Comparative Example 4, in which the high-viscosity vinylidene fluoride polymer was a copolymer rather than a homopolymer of vinylidene fluoride, the slurry viscosity increased to an unmeasurable level after standing for 48 hours. Similarly, the electrode mixture of Comparative Example 5, in which the proportion of low-viscosity vinylidene fluoride homopolymer was less than 25% by weight, also had an increased slurry viscosity after standing for 48 hours. [Industrial Applicability]

[0079] The present invention can be used in non-aqueous electrolyte secondary batteries.

Claims

1. 1. A binder composition comprising a vinylidene fluoride polymer, The vinylidene fluoride polymer includes a vinylidene fluoride homopolymer (A) and a vinylidene fluoride homopolymer (B) different from the vinylidene fluoride homopolymer (A), a proportion of the vinylidene fluoride homopolymer (A) in the total amount of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) in the binder composition is 25% by weight or more and 55% by weight or less; The binder composition, wherein the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) are polymers that satisfy the following conditions (i) to (iii): (i) The 5 wt % N-methyl-2-pyrrolidone solution of the vinylidene fluoride homopolymer (A) was heated at 25°C and a shear rate of 3.83 s －１ The viscosity of the solution is 30 mPa·s or more and 500 mPa·s or less. (ii) The 5 wt % N-methyl-2-pyrrolidone solution of the vinylidene fluoride homopolymer (B) was heated at 25°C and a shear rate of 3.83 s －１ The solution viscosity is 800 mPa s or more and 30,000 mPa s or less, and (iii) A 5 wt % N-methyl-2-pyrrolidone solution of a mixture of the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B), which are mixed in the same ratio as in the binder composition, has a turbidity of 1 or less at 25°C.

2. 2. The binder composition according to claim 1, wherein the vinylidene fluoride homopolymer (A) and the vinylidene fluoride homopolymer (B) are vinylidene fluoride homopolymers that further satisfy the following condition (iv): (iv) A 5% by weight solution of the mixture in N-methyl-2-pyrrolidone was heated at 25°C and a shear rate of 3.83 s －１ The viscosity of the solution is 400 mPa·s or more and 25,000 mPa·s or less.

3. The binder composition of claim 1 or 2, further comprising carbon nanotubes.

4. The binder composition according to claim 1 or 2; and a positive electrode active material having a primary particle diameter of 50 nm or more and 600 nm or less, and having a D10 of 1 μm or less and a D50 of 1.6 μm or less in a volumetric particle size distribution.

5. A binder composition according to claim 3, and a positive electrode active material having a primary particle diameter of 50 nm or more and 600 nm or less, and having a D10 of 1 μm or less and a D50 of 1.6 μm or less in a volumetric particle size distribution.

6. An electrode comprising an electrode mixture layer formed from the electrode mixture according to claim 4 on a current collector.

7. An electrode having an electrode mixture layer formed from the electrode mixture described in claim 5 on a current collector.

8. A non-aqueous electrolyte secondary battery comprising the electrode according to claim 6.

9. A non-aqueous electrolyte secondary battery comprising the electrode described in claim 7.

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