Carbon black dispersion composition for batteries, composite paste for positive electrode, positive electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

To provide a carbon black dispersion composition for a battery which improves the dispersibility of carbon black, continues to maintain dispersibility immediately after dispersion, and uses N-methyl-2-pyrrolidone as a dispersion medium, which has excellent storage stability without requiring much labor or time.SOLUTION: A carbon black dispersion composition for a battery includes carbon black, dispersant and N-methyl-2-pyrrolidone, and further includes 0.0001 to 5 parts by mass of a hindered phenol compound per 100 parts by mass of carbon black.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbon black dispersion composition for batteries, a composite paste for positive electrodes, a positive electrode for lithium ion secondary batteries, and a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are characterized by their high energy density, and the market has expanded due to the rapid spread of mobile devices such as mobile phones and laptop computers, which has led to a dramatic improvement in performance. In recent years, as part of efforts to realize a sustainable society, progress has been made in electrifying automobiles and improving the efficiency of power storage systems, and the market for lithium-ion secondary batteries is expected to continue expanding.

[0003] The positive electrode of a lithium-ion secondary battery primarily comprises a positive electrode active material, a conductive additive, a binder, and a current collector. Because the capacity of a lithium-ion secondary battery is primarily determined by the amount of active material, minimizing the amount of materials other than the active material, including the conductive additive, is desirable for achieving high battery capacity. To achieve high conductivity with a smaller amount of additive, conductive additives are being developed with a higher specific surface area and a more structured structure. However, these higher specific surface areas and structures increase the cohesion of the conductive additive, making it difficult to achieve uniform dispersion. This can result in uneven conductive paths within the electrode, increasing internal resistance, and shortening the battery's lifespan due to uneven load distribution.

[0004] Therefore, in recent years, a commonly used method for preventing aggregation of the conductive additive and producing an electrode in which the conductive additive is uniformly dispersed is to prepare a conductive additive dispersion liquid by uniformly dispersing the conductive additive in advance in a dispersion medium such as an organic solvent using a polymer dispersant, and then mixing the obtained conductive additive dispersion liquid with a positive electrode active material and a binder to prepare a positive electrode composite slurry.

[0005] The above-mentioned conductive additive dispersion is required to have the conductive additive in a uniform and well-dispersed state, and in addition, the dispersion state of the conductive additive and the viscosity of the dispersion are required to be stable and not change during the storage period from the production of the dispersion until the preparation of the electrode mixture slurry.

[0006] This is because if the conductive additive aggregates due to poor dispersion, the viscosity of the dispersion increases, which deteriorates the coatability of the positive electrode composite slurry produced using the dispersion and prevents a smooth composite coating from being obtained. This, together with the non-uniformity of the conductive path within the electrode, ultimately leads to a deterioration in performance.

[0007] Furthermore, the dispersion storage stability reflects the change over time in the dispersion state of the conductive additive, and if the change in the dispersion state due to storage is large, the quality variation in the manufactured positive electrodes increases.

[0008] Generally, carbon black is used as the conductive additive, and a dispersion liquid containing carbon black as the conductive additive and having an acidic compound such as carboxylic acid added thereto as a further additive is known as a method for improving the dispersibility and storage stability of the dispersion liquid (JP 2016-046188 A (Patent Document 1)).

[0009] In addition, as a method of not using additional additives in a conductive additive dispersion, a dispersion is known in which the viscosity characteristics of the dispersion are controlled within a specific range to improve dispersibility and storage stability (Patent Publication No. 2020-021632 (Patent Document 2)). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-046188 [Patent Document 2] Japanese Patent Publication No. 2020-021632 Summary of the Invention [Problem to be solved by the invention]

[0011] Although Patent Document 1 does not explain the mechanism by which the acidic compound affects the dispersibility of carbon black, it is speculated that the dispersibility of carbon black is improved by controlling the pH environment on the surface of the carbon black particles. However, there is a concern that acidic compounds may act as a factor in chemically modifying the composition of the electrode mixture, so their addition to carbon black dispersions was not desirable. Furthermore, the method described in Patent Document 2 requires that the dispersion process be continued until a dispersion liquid having the target viscosity characteristics is obtained, which often requires a lot of labor and time, and may make it difficult to meet the recent demands from the battery market for reduced production costs and increased production volume.

[0012] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a carbon black dispersion composition for batteries containing N-methyl-2-pyrrolidone as a dispersion medium, which enhances the dispersibility of carbon black, maintains the dispersibility immediately after dispersion, and has excellent storage stability without requiring much labor or time; a positive electrode composite paste using the carbon black dispersion composition for batteries; a positive electrode for a lithium ion secondary battery; and a lithium ion secondary battery. [Means for solving the problem]

[0013]

[0006] In order to achieve the above object, the present inventors conducted research and found that trace amounts of radical species generated during storage of N-methyl-2-pyrrolidone and during storage of a dispersion are significantly involved in the deterioration of carbon black dispersibility. Furthermore, they found that adding a hindered phenol compound, which functions as a radical scavenger, at a specific concentration range to eliminate the effects of these radical species is effective in improving the dispersibility and long-term storage stability of carbon black in a dispersion. Based on this finding, the present inventors conducted extensive research and completed the present invention.

[0014] That is, the present invention provides the following carbon black dispersion composition for a battery, a composite paste for a positive electrode, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery. 1. A carbon black dispersion composition for a battery, comprising carbon black, a dispersant, and N-methyl-2-pyrrolidone, further comprising a hindered phenol compound per 100 parts by mass of the carbon black. 0.018 ~ 0.3 parts by mass of a carbon black dispersion composition for a battery. 2. 2. The carbon black dispersion composition for batteries according to 1, wherein the carbon black content is 5 to 20 mass %. 3. The BET specific surface area of ​​the carbon black is 30 to 1500 m 2 3. The carbon black dispersion composition for a battery according to 1 or 2, wherein the carbon black dispersion composition for a battery has a molecular weight of 1. / g. 4. 4. The carbon black dispersion composition for batteries according to any one of 1 to 3, wherein the amount of the dispersant added is 1 to 20 parts by mass per 100 parts by mass of carbon black. 5. 6. The carbon black dispersion composition for a battery according to any one of 1 to 5, wherein the dispersant is at least one selected from methyl cellulose, hydroxypropyl methyl cellulose, polyvinyl alcohol, and polyvinyl pyrrolidone. 6. The hindered phenol compound is 2,6-di-tert-butyl-p-cresol, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoic acid][oxalylbis(azanediyl)]bis(ethane-2,1-diyl), 2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)mesi 6. The carbon black dispersion composition for a battery according to any one of 1 to 5, wherein the carbon black dispersion composition is at least one selected from the group consisting of 4-[[4,6-bis(n-octylthio)-1,3,5-triazin-2-yl]amino]-2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methoxyphenol, and 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione. 7. 7. A positive electrode mixture paste comprising the carbon black dispersion composition for a battery according to any one of 1 to 6, a positive electrode active material, and a binder. 8. 8. A positive electrode for a lithium ion secondary battery comprising a current collector and a positive electrode mixture layer formed on the current collector, the positive electrode mixture paste being a coated and dried film of the positive electrode mixture paste according to 7. 9. 9. A lithium ion secondary battery comprising the positive electrode for lithium ion secondary batteries according to 8, a negative electrode, an electrolyte, and a separator. [Effects of the Invention]

[0015] The present invention provides a carbon black dispersion composition for batteries that exhibits excellent dispersibility and storage stability of carbon black. Furthermore, use of the carbon black dispersion composition for batteries is expected to reduce the defective rate and production costs in the production of lithium-ion secondary batteries. DETAILED DESCRIPTION OF THE INVENTION

[0016] [Carbon black dispersion composition for batteries] The carbon black dispersion composition for batteries according to the present invention will be described below. The carbon black dispersion composition for batteries according to the present invention is characterized in that the carbon black dispersion composition for batteries contains carbon black, a dispersant, and N-methyl-2-pyrrolidone, and further contains 0.0001 to 5 parts by mass of a hindered phenol compound relative to 100 parts by mass of the carbon black.

[0017] (carbon black) Carbon black is added as a conductive aid to enhance the electrical conductivity of an electrode (specifically, a positive electrode in a lithium-ion secondary battery, which will be described later) formed from the carbon black dispersion composition for batteries, and examples thereof include acetylene black, furnace black, thermal black, ketjen black, etc. Among these, acetylene black and ketjen black are preferred from the viewpoint of electrical conductivity.

[0018] Carbon black used in electrodes is being developed to have a high specific surface area in order to exhibit good conductivity with a small amount of addition. The BET specific surface area (specific surface area measured by the BET method) of carbon black suitable for use in electrodes is 30 to 1500 m 2 / g, and 40 to 1200m 2 / g, and more preferably 60 to 1000m 2 / g is particularly preferred.

[0019] From the viewpoint of the fluidity and operability of the dispersion composition to be prepared, the carbon black content is preferably 5 to 20 mass %, and more preferably 6 to 18 mass % (i.e., preferably 5 to 20 parts by mass, and more preferably 6 to 18 parts by mass, per 100 parts by mass of the total amount of the carbon black dispersion composition for batteries (preferably the total amount of carbon black, dispersant, N-methyl-2-pyrrolidone, and hindered phenol compound)).

[0020] Other carbon materials, such as carbon nanotubes, graphene, and graphite, may be blended together with carbon black as a conductive aid. When blended together, the amount of the other carbon materials added is preferably 5 to 20 parts by mass, more preferably 6 to 18 parts by mass, per 100 parts by mass of the total amount of the carbon black dispersion composition for batteries (preferably the total amount of carbon black, dispersant, N-methyl-2-pyrrolidone, and hindered phenol compound).

[0021] (dispersant) The dispersant used in the present invention is not particularly limited as long as it improves the dispersibility of carbon black, and is, for example, at least one selected from methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), polyvinyl alcohol (PVA), and polyvinyl pyrrolidone (PVP). Of these, methyl cellulose and polyvinyl alcohol are preferred, and methyl cellulose is more preferred, from the viewpoint of exhibiting good dispersibility with a small addition amount.

[0022] The degree of substitution (DS) of methoxy groups in methylcellulose is preferably 1.60 to 2.10, more preferably 1.70 to 2.00. The degree of substitution (DS) of methoxy groups in methylcellulose can be determined by converting a value measured by the method for analyzing the degree of substitution of methylcellulose in the 18th Edition of the Japanese Pharmacopoeia (the same applies hereinafter in the Examples). From the viewpoints of dispersibility and stability, the viscosity of a 2% by mass aqueous solution of methylcellulose at 20°C is preferably 2.0 to 100.0 mPa·s, and more preferably 3.0 to 50.0 mPa·s. The viscosity of a 2% by mass aqueous solution of methylcellulose at 20°C can be the viscosity value measured at 20°C using an Ubbelohde viscometer specified in JIS K2283-1993 (the same applies hereinafter in the examples).

[0023] The degree of substitution (DS) of methoxy groups in hydroxypropyl methylcellulose is not particularly limited, but is preferably 1.60 to 2.10, more preferably 1.70 to 2.00, and the molar substitution number (MS) of hydroxypropoxy groups is preferably 0.10 to 0.30, more preferably 0.13 to 0.27. The degree of substitution (DS) of methoxy groups and the molar substitution number (MS) of hydroxypropoxy groups in hydroxypropyl methylcellulose can be determined by converting values ​​measured by the method for analyzing the degree of substitution of hypromellose (hydroxypropyl methylcellulose) in the 18th Edition of the Japanese Pharmacopoeia. From the viewpoints of dispersibility and stability, the viscosity of a 2% by mass aqueous solution of hydroxypropyl methylcellulose at 20°C is preferably 2.0 to 100.0 mPa·s, and more preferably 3.0 to 50.0 mPa·s. The viscosity of a 2% by mass aqueous solution of hydroxypropyl methylcellulose at 20°C can be the viscosity value measured at 20°C using an Ubbelohde viscometer specified in JIS K2283-1993.

[0024] From the viewpoint of solubility in N-methyl-2-pyrrolidone, the degree of saponification of polyvinyl alcohol is preferably 78.0 mol % or more, more preferably 85.0 to 99.5 mol %. The degree of saponification of polyvinyl alcohol can be measured according to the saponification degree measurement method described in JIS K6726. From the viewpoints of dispersibility and stability, the viscosity of a 4 mass % aqueous solution of polyvinyl alcohol at 20° C. is preferably 2.0 to 100.0 mPa s, and more preferably 3.0 to 50.0 mPa s. The viscosity of a 4 mass % aqueous solution of polyvinyl alcohol at 20° C. can be measured according to the measurement method described in JIS K6726.

[0025] The K value of polyvinylpyrrolidone is not particularly limited, but is preferably 10 to 120, and more preferably 15 to 100. The K value of polyvinylpyrrolidone can be measured by the K value measurement method described in the section on "Povidone" in the 18th Edition of the Japanese Pharmacopoeia.

[0026] The amount of dispersant added is preferably 1 to 20 parts by mass, more preferably 5 to 13 parts by mass, per 100 parts by mass of carbon black in the carbon black dispersion composition for batteries, from the viewpoints of dispersibility of carbon black and the electrical properties of an electrode formed from the carbon black dispersion composition for batteries. The content of the dispersant is preferably 0.05 to 4% by mass, more preferably 0.25 to 2% by mass, from the viewpoints of dispersibility of carbon black and the electrical properties of an electrode formed from the carbon black dispersion composition for batteries (i.e., preferably 0.05 to 4 parts by mass, more preferably 0.25 to 2 parts by mass, per 100 parts by mass of the total amount of the carbon black dispersion composition for batteries (preferably the total amount of carbon black, dispersant, N-methyl-2-pyrrolidone, and hindered phenol compound)).

[0027] (N-methyl-2-pyrrolidone) N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") is an organic solvent used in the production of positive electrodes of lithium ion batteries, and is also added to the carbon black dispersion composition for batteries of the present invention as a dispersion medium for carbon black. The content of NMP is not particularly limited, but is preferably 80 to 95 mass %, more preferably 85 to 93 mass %, from the viewpoint of ease of handling of the carbon black dispersion composition for batteries. (That is, the content of NMP is preferably 80 to 95 parts by mass, more preferably 85 to 93 parts by mass, per 100 parts by mass of the total amount of the carbon black dispersion composition for batteries (preferably the total amount of carbon black, dispersant, N-methyl-2-pyrrolidone, and hindered phenol compound). From the viewpoint of improving the affinity of each component (component of the battery carbon dispersion liquid and the cathode composite paste), one or more other solvents may be used in combination, and examples of solvents other than N-methyl-2-pyrrolidone include water, N-ethyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropionamide, N,N-diethylacetamide, 4-acetylmorpholine, etc. In the present invention, it is preferable to use NMP alone as the solvent.

[0028] (Hindered phenol compounds) The present invention is characterized in that the carbon black dispersion composition for batteries containing the above-mentioned components further contains a hindered phenol compound as a radical scavenger, which can remove radical species that are generated during storage and that adversely affect the stability of the dispersion composition.

[0029] Examples of hindered phenol compounds include 2,6-di-tert-butyl-p-cresol, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoic acid][oxalylbis(azanediyl)]bis(ethane-2,1-diyl), 2,4,6-tris(3',5'-di-tert-butyl)propionate, ... butyl-4'-hydroxybenzyl)mesitylene, 4-[[4,6-bis(n-octylthio)-1,3,5-triazin-2-yl]amino]-2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methoxyphenol, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, etc. Among these, 2,6-di-tert-butyl-p-cresol and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] are particularly preferred from the viewpoint of not adversely affecting battery performance.

[0030] The amount of the hindered phenol compound added is 0.0001 to 5 parts by mass, preferably 0.001 to 1 part by mass, and more preferably 0.01 to 0.3 parts by mass, per 100 parts by mass of carbon black. If the amount is less than 0.0001 part by mass, the amount is too small, and radical species that adversely affect the stability of the composition cannot be sufficiently removed, resulting in failure to achieve the expected effect. On the other hand, if the amount exceeds 5 parts by mass, the viscosity of the composition decreases during storage, resulting in variations in the viscosity of the positive electrode composite paste produced in the subsequent process, which can cause dripping when the paste is applied to the current collector, and a non-uniform composite layer due to instability of the coating film. For the same reasons as above, the content of the hindered phenol compound is preferably 0.000001 to 1 mass%, more preferably 0.00001 to 0.2 mass%, and even more preferably 0.0001 to 0.07 mass% (that is, preferably 0.000001 to 1 part by mass, more preferably 0.00001 to 0.2 parts by mass, and even more preferably 0.0001 to 0.07 parts by mass, relative to 100 parts by mass of the total amount of the carbon black dispersion composition for batteries (preferably the total amount of carbon black, dispersant, N-methyl-2-pyrrolidone, and hindered phenol compound)).

[0031] (Other ingredients) The carbon black dispersion composition for a battery of the present invention may contain other components within the scope of the object of the present invention or to the extent that the dispersibility of the carbon black dispersion composition or the performance of the carbon black dispersion composition when formed into an electrode are not impaired. Examples of other components include surfactants, antifoaming agents, pH adjusters, and viscosity adjusters.

[0032] The method for producing the carbon black dispersion composition for batteries of the present invention is not particularly limited as long as the above components are uniformly mixed and carbon black can be dispersed, but the composition can be prepared, for example, by a dispersion step in which carbon black, a dispersant, a hindered phenol compound, NMP, etc. are dispersed using a dispersion device. In this case, the powder raw materials, carbon black, dispersant, and hindered phenol compound, may be mixed in advance and then this mixture may be dispersed in NMP, or the dispersant may be dissolved in a solvent such as NMP in advance and then mixed with the carbon black and hindered phenol compound.

[0033] Dispersing devices that can be used include a homogenizer, a homodisper, a planetary mixer, a paint conditioner, a bead mill, and a thin film rotary high-speed mixer.

[0034] In order to further improve the dispersibility of carbon black, the dispersion process described above may be carried out in stages using multiple dispersing devices. For example, powder raw materials (carbon black, dispersant, hindered phenol compound) and a dispersion medium (NMP) may be uniformly mixed using a homodisper or planetary mixer, and then the carbon black may be dispersed as fine particles using a bead mill or thin-film rotary high-speed mixer.

[0035] [Positive electrode mixture paste] The positive electrode mixture paste of the present invention contains the above-described carbon black dispersion composition for a battery of the present invention, a positive electrode active material, and a binder, and can be used to produce a positive electrode for a lithium ion secondary battery.

[0036] There are no particular limitations on the positive electrode active material as long as it is a material that can be used as a positive electrode for a lithium-ion secondary battery. Examples include transition metal oxides containing lithium, or transition metals in which a portion of the transition metal element contained in the transition metal oxide has been substituted with a different element. Specific examples include lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, ternary (NCM) active materials, and NCA active materials. Furthermore, unlike the oxide-based active materials described above, the active materials contain phosphate ions (PO4 3- ) and silicate ions (SiO4 4- Polyanion-based positive electrode active materials can also be used, in which the ionicity of the transition metal is enhanced by introducing polyvalent anions such as lithium iron phosphate and lithium iron silicate.

[0037] The content of the positive electrode active material is 35 to 80 mass %, preferably 40 to 70 mass % (35 to 80 mass parts, preferably 40 to 70 mass parts per 100 mass parts of the positive electrode composite paste). If the content of the positive electrode active material is less than 35 mass %, the energy density when assembled into a battery will be poor, and if it exceeds 80 mass %, the positive electrode composite paste will harden, which may make the process of applying the paste to the current collector difficult.

[0038] As the binder, a fluorine-based polymer material is preferred from the viewpoint of durability of the formed positive electrode composite layer, and examples thereof include polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene. Furthermore, from the viewpoint of the binding property and coatability of the positive electrode composite paste, the weight-average molecular weight of the polymer material used as the binder is preferably 200,000 to 1,200,000, and more preferably 600,000 to 1,000,000. The weight average molecular weight of the polymer material used as the binder can be measured by a known method such as gel permeation chromatography.

[0039] The binder content is 0.3 to 10 mass%, preferably 0.5 to 6 mass% (i.e., 0.3 to 10 mass parts, preferably 0.5 to 6 mass parts, per 100 mass parts of the positive electrode composite paste). If the binder content is less than 0.3 mass%, the positive electrode composite layer will not have enough strength, and peeling or cracking of the composite layer may occur. If the binder content exceeds 10 mass%, the electrical resistance inside the electrode may increase.

[0040] The positive electrode composite paste of the present invention can be prepared by mixing the above-described carbon black dispersion composition for a battery of the present invention, a positive electrode active material, and a binder, and the dispersing device shown in the above-described method for producing the carbon black dispersion composition for a battery can be used as the mixing device.

[0041] When producing the positive electrode composite paste, a solvent may be added to adjust the viscosity, and the solvents (e.g., NMP) described above for the carbon black dispersion composition for a battery of the present invention can be used as the solvent. The amount of the solvent (NMP) contained in the positive electrode composite paste (i.e., the total amount of NMP contained in the carbon black dispersion composition for batteries and the amount of NMP added) is 10 to 70 mass %, and preferably 20 to 60 mass %.

[0042] The procedure for adding materials during mixing is not particularly limited, and all materials may be added and mixed at the same time, or the mixing process may be performed in stages, such as by mixing the binder, the positive electrode active material, and the solvent, and then adding the carbon black dispersion composition for a battery and further mixing. Note that, from the viewpoint of preventing foreign matter such as undissolved binder from being mixed into the positive electrode composite paste, it is recommended that the binder be dissolved in the solvent to be used in advance to prepare a solution and then add it.

[0043] The positive electrode composite paste of the present invention is suitable for producing a positive electrode for a lithium ion secondary battery. Specifically, the positive electrode for a lithium ion secondary battery can be produced by applying the positive electrode composite paste to a current collector to a desired thickness to form a coating film, drying the coating film, and using the resulting dried coating film as a positive electrode composite layer.

[0044] The current collector may be a film foil of a metal or alloy such as iron, stainless steel, copper, aluminum, or nickel, with aluminum being particularly preferred from the viewpoint of potential stability at the positive electrode. The current collector may be subjected to a surface treatment such as carbon coating in order to reduce interface resistance.

[0045] The apparatus used for coating the positive electrode composite paste is not particularly limited, and examples thereof include a knife coater, a comma coater, a die coater, and a gravure coater. In order to improve the electrical resistance of the electrode and the adhesiveness of the positive electrode composite layer, the positive electrode composite paste coated on the current collector or the coated, dried film of the positive electrode composite paste may be rolled using a roll press or the like.

[0046] The coating thickness of the positive electrode mixture paste is preferably 50 to 1000 μm, and more preferably 100 to 500 μm.

[0047] The coated film of the positive electrode mixture paste is dried in a drying oven, preferably at a temperature of 50 to 180° C. for 0.5 to 1200 minutes, more preferably at 60 to 140° C. for 1 to 600 minutes.

[0048] This results in a positive electrode for a lithium ion secondary battery, which includes a current collector and a positive electrode composite layer that is a coated and dried film of the positive electrode composite paste of the present invention. The thickness of the positive electrode composite layer is preferably 10 to 800 μm, and more preferably 30 to 400 μm.

[0049] [Lithium-ion secondary battery] The lithium ion secondary battery of the present invention includes a positive electrode for a lithium ion secondary battery including the above-described positive electrode mixture layer of the present invention, a negative electrode, an electrolyte, and a separator, and has a structure in which the separator is sandwiched between the positive electrode for a lithium ion secondary battery and the negative electrode and is impregnated with the electrolyte in a sealed state.

[0050] Here, the negative electrode of the lithium ion secondary battery has a negative electrode mixture layer on one or both sides of a negative electrode current collector.

[0051] The negative electrode current collector is a metal or alloy film foil, similar to the current collector in the positive electrode of a lithium ion secondary battery, but copper foil or nickel foil is preferred from the viewpoint of potential stability at the negative electrode.

[0052] The negative electrode mixture layer contains one or more negative electrode active materials capable of absorbing and releasing lithium ions, and may contain a negative electrode binder, a negative electrode conductive additive, and a negative electrode dispersant as necessary.

[0053] The negative electrode active material is not particularly limited as long as it is a material that can be used as a negative electrode active material in a lithium ion secondary battery. Specific examples include carbon-based negative electrode materials such as natural graphite and artificial graphite, oxide-based negative electrode materials such as lithium titanate, and Si-based negative electrode materials such as nanosilicon, silicon alloys, and silicon monoxide.

[0054] The negative electrode binder may be, for example, one or more of polymer materials, synthetic rubber, etc. Examples of polymer materials include polyvinylidene fluoride, polyimide, polyamideimide, aramid, polyacrylic acid, lithium polyacrylate, sodium carboxymethyl cellulose, etc. Examples of synthetic rubbers include styrene-butadiene rubber, fluorine-based rubber, ethylene propylene diene, etc.

[0055] As the conductive additive for the negative electrode, for example, one or more of carbon materials such as acetylene black, ketjen black, graphite, carbon nanotubes, and carbon nanofibers can be used.

[0056] As the dispersant for the negative electrode, for example, one or more of methyl cellulose, hydroxypropyl methyl cellulose, ethyl cellulose, sodium carboxymethyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidone, and polyurethane can be used.

[0057] The separator electronically insulates the positive and negative electrodes of a lithium-ion secondary battery, preventing current short circuits due to contact between the electrodes, while allowing lithium ions to pass through by being impregnated with an electrolyte. This separator is formed, for example, from a porous membrane made of synthetic resin or ceramic, and may have a laminate structure in which two or more types of porous membranes are laminated. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene, and examples of ceramics include alumina.

[0058] The electrolyte is a composition that mediates ionic conduction between the positive and negative electrodes. The electrolyte is prepared by dissolving a lithium salt such as lithium hexafluorophosphate in a non-aqueous solvent, for example, a mixture of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate. An additive may be added to the electrolyte for the purposes of improving stability, safety, and reducing resistance.

[0059] As described above, the carbon black dispersion composition for batteries of the present invention has excellent dispersibility of carbon black and storage stability. Use of the carbon black dispersion composition for batteries makes it possible to reduce the defective rate and production costs in the production of lithium ion secondary batteries. [Example]

[0060] EXAMPLES The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples. The BET specific surface area is a value measured by the BET method.

[0061] [Test material] The materials for the carbon black dispersion compositions for batteries used in the examples and comparative examples are shown below. <Carbon black> Acetylene black: DENKA BLACK Li-435 (hereinafter referred to as "Li-435"), manufactured by Denka Co., Ltd., BET specific surface area 136 m 2 / g Acetylene black: DENKA BLACK Li-100 (hereinafter referred to as "Li-100"), manufactured by Denka Co., Ltd., BET specific surface area 68 m 2 / g Ketjenblack: Carbon ECP (hereinafter referred to as "ECP"), manufactured by Lion Specialty Chemicals Co., Ltd., BET specific surface area 782 m 2 / g

[0062] <Dispersant> Methylcellulose-1 (hereinafter referred to as "MC-1"): manufactured by Shin-Etsu Chemical Co., Ltd., 2% by weight aqueous solution viscosity (20°C) 4 mPa·s, degree of methoxy group substitution (DS) 1.8 Methylcellulose-2 (hereinafter referred to as "MC-2"): manufactured by Shin-Etsu Chemical Co., Ltd., 2% by weight aqueous solution viscosity (20°C) 15 mPa·s, degree of methoxy group substitution (DS) 1.8

[0063] <Radical scavengers and comparative additives> [Table 1]

[0064] <Preparation of Carbon Black Dispersion Composition for Batteries> [Example 1] A carbon black dispersion composition for batteries was prepared by the following procedure. A powder mixture of 1.3 g of acetylene black ("Li-435") as carbon black, 0.12 g of methylcellulose-1 (MC-1) as a dispersant (9 parts by mass per 100 parts by mass of carbon black), and 0.0008 g of hindered phenol (1a) (0.06 parts by mass per 100 parts by mass of carbon black) was prepared. 14.8 g of N-methyl-2-pyrrolidone (NMP) was added to the mixture, and the mixture was pre-mixed for 5 minutes at 2,000 rpm using a degassing mixer (Thinky Corporation, "ARV-310"), and the resulting dispersion was further stirred for 30 seconds at 10,000 rpm using a thin-film swirling high-speed mixer (Primix Corporation, "Filmix 30-L") to obtain a carbon black dispersion composition for batteries.

[0065] [Example 2] A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that the amount of hindered phenol (1a) added was changed to 0.00024 g.

[0066] [Example 3] A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that the hindered phenol (1a) in Example 1 was changed to hindered phenol (2a).

[0067] [Comparative Example 1] A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that the hindered phenol (1a) was not added.

[0068] Comparative Example 2 A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that phenol (1c) was used instead of the hindered phenol (1a).

[0069] Comparative Example 3 A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that a phosphorus-based antioxidant (1d) was used instead of the hindered phenol (1a).

[0070] Comparative Example 4 A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that a sulfur-based antioxidant (1e) was used instead of the hindered phenol (1a).

[0071] Comparative Example 5 A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that the amount of hindered phenol (1a) added was changed to 0.00000013 g (0.00001 part by mass per 100 parts by mass of carbon black).

[0072] Comparative Example 6 A carbon black dispersion composition for batteries was obtained in the same manner as in Example 1, except that the amount of hindered phenol (1a) added was changed to 0.078 g (6 parts by mass per 100 parts by mass of carbon black).

[0073] [Example 4] A powder mixture of 1.3 g of Ketjen black (ECP) as carbon black, 0.13 g of methylcellulose-2 (MC-2) as a dispersant (10 parts by mass per 100 parts by mass of carbon black), and 0.00186 g of hindered phenol (1a) (0.14 parts by mass per 100 parts by mass of carbon black) was prepared, to which 17.1 g of NMP was added. The mixture was pre-mixed for 5 minutes at a rotation speed of 2,000 rpm using a degassing mixer (ARV-310, manufactured by Thinky Corporation). The resulting dispersion was further stirred for 30 seconds at a rotation speed of 10,000 rpm using a thin-film rotary high-speed mixer (Filmix 30-L, manufactured by Primix Corporation), yielding a carbon black dispersion composition for batteries.

[0074] Comparative Example 7 A carbon black dispersion composition for batteries was obtained in the same manner as in Example 4, except that the hindered phenol (1a) was not added.

[0075] [Example 5] A powder mixture of 1.3 g of Ketjen black (ECP) as carbon black, 0.17 g of methylcellulose-1 (MC-1) as a dispersant (13 parts by mass per 100 parts by mass of carbon black), and 0.0039 g of hindered phenol (1a) (0.3 parts by mass per 100 parts by mass of carbon black) was prepared, to which 20.2 g of NMP was added. The mixture was pre-mixed for 5 minutes at a rotation speed of 2,000 rpm using a degassing mixer (ARV-310, manufactured by Thinky Corporation). The resulting dispersion was further stirred for 30 seconds at a rotation speed of 10,000 rpm using a thin-film rotary high-speed mixer (Filmix 30-L, manufactured by Primix Corporation), yielding a carbon black dispersion composition for batteries.

[0076] [Comparative Example 8] A carbon black dispersion composition for batteries was obtained in the same manner as in Example 5, except that the hindered phenol (1a) was not added.

[0077] [Example 6] A powder mixture of 3.2 g of acetylene black ("Li-100") as carbon black, 0.22 g of methylcellulose-1 (MC-1) as a dispersant (7 parts by weight per 100 parts by weight of carbon black), and 0.0013 g of hindered phenol (1a) (0.04 parts by weight per 100 parts by weight of carbon black) was mixed with 11.85 g of NMP and pre-mixed for 5 minutes at 2,000 rpm using a degassing mixer (Thinky Corporation, "ARV-310"), followed by 5 minutes of mixing with 5.92 g of NMP. The resulting dispersion was further stirred for 30 seconds at 10,000 rpm using a thin-film swirling high-speed mixer (Primix Corporation, "Filmix 30-L") to obtain a carbon black dispersion composition for batteries.

[0078] Comparative Example 9 A carbon black dispersion composition for batteries was obtained in the same manner as in Example 6, except that the hindered phenol (1a) was not added.

[0079] (Evaluation method) The dispersibility of the carbon black dispersion composition for batteries obtained as described above was evaluated using a rotational rheometer. Specifically, a rotational rheometer ("HAAKE MARS" manufactured by Thermo Fisher Scientific) was used, with a cone-plate measuring jig, a set temperature of 25°C, and a shear rate of 20 m / s to measure the shear viscosity. The lower the shear viscosity, the better the dispersibility of the carbon black. Here, the dispersibility of the carbon black was determined to be good if the initial viscosity was 250 mPa·s or less. The storage stability was evaluated based on the change in shear viscosity (viscosity after one-week storage) from the initial viscosity after allowing the carbon black dispersion composition for batteries to stand at 35°C for one week. A dispersion with a smaller change in viscosity has better storage stability. Here, a carbon black dispersion composition for batteries was determined to have good storage stability if the change in viscosity from the initial viscosity after one week of storage was +200 mPa s or less. The results are shown in Table 2.

[0080] [Table 2]

[0081] As shown in Examples 1 to 3, the carbon black dispersion compositions for batteries to which an appropriate amount of a hindered phenol compound was added as a radical scavenger showed a smaller change in viscosity after standing for one week and improved storage stability compared to Comparative Example 1 to which no hindered phenol compound was added. Similarly, the results of Examples 4 to 6 and Comparative Examples 7 to 9 also showed that the carbon black dispersion compositions for batteries of Examples 4 to 6 to which an appropriate amount of a hindered phenol compound was added as a radical scavenger had improved storage stability. Furthermore, in Comparative Example 2, in which a normal phenol without radical scavenging ability was added instead of the hindered phenol compound, and in Comparative Examples 3 and 4, in which a phosphorus-based or sulfur-based antioxidant was added, the storage stability of the composition was not improved. Furthermore, Comparative Examples 5 and 6 showed that when the amount of the hindered phenol compound, which is a radical scavenger, added was less than 0.0001 parts by mass or more than 5 parts by mass relative to 100 parts by mass of carbon black, the change in viscosity after standing for one week was greater than in Example 1.

[0082] <Preparation of Positive Electrode Composite Paste> The carbon black dispersion compositions for batteries obtained in Examples 1 to 6 were used to prepare positive electrode mixture pastes according to the following procedure. 12.5 g of the carbon black dispersion composition for batteries of Example 1 and 13.3 g of an NMP solution (solution concentration: 7% by mass) of polyvinylidene fluoride (weight average molecular weight: 630,000) were added to a stirring vessel and kneaded for 3 minutes at a rotation speed of 2000 rpm using a degassing kneader ("ARV-310" manufactured by Thinky Corporation). To this mixture, 48.0 g of a ternary positive electrode active material (NCM622) and 2.65 g of N-methyl-2-pyrrolidone (NMP) were added, and the mixture was kneaded for 3 minutes at a rotation speed of 2000 rpm using the degassing kneader, thereby obtaining a positive electrode composite paste (solid concentration: 64% by mass). Positive electrode mixture pastes were obtained in the same manner as above for the carbon black dispersion compositions for batteries of Examples 2 to 6. All of the positive electrode mixture pastes were uniformly dispersed and showed good coatability.

[0083] <Preparation of positive electrodes for lithium-ion secondary batteries> A positive electrode of a lithium ion secondary battery was produced using the above-described positive electrode composite paste by the following procedure. The positive electrode composite paste was coated and dried on a current collector made of aluminum foil (thickness: 20 μm, width: 14 mm) using an electrode coater dryer ("LiB-W140" manufactured by Clean Technology Co., Ltd.) The coating and drying conditions were a coating thickness of 180 μm, a conveying speed of 0.2 m / min, and an oven temperature of 115 to 120°C in a drying oven (oven length: 50 cm). The coated and dried film obtained by the coating and drying process was then rolled together with a current collector at a linear pressure of 200 kg / cm using a tabletop roll press (Tester Sangyo Co., Ltd., model SA-602) to obtain a positive electrode for a lithium-ion secondary battery. The positive electrodes for lithium-ion secondary batteries were successfully produced using any of the positive electrode composite pastes described above.

[0084] Although the present invention has been described above using the above-mentioned embodiments, the present invention is not limited to these embodiments, and can be modified within the scope of what a person skilled in the art can conceive, such as other embodiments, additions, changes, deletions, etc., and any aspect is included in the scope of the present invention as long as it achieves the effects of the present invention.

Claims

1. The carbon black dispersion composition for batteries contains carbon black, a dispersant, and N-methyl-2-pyrrolidone, and further contains 0.018 to 0.3 parts by mass of a hindered phenol compound per 100 parts by mass of the carbon black.

2. 2. The carbon black dispersion composition for batteries according to claim 1, wherein the carbon black content is 5 to 20 mass %.

3. The BET specific surface area of ​​the carbon black is 30 to 1500 m 2 2. The carbon black dispersion composition for a battery according to claim 1, wherein the carbon black dispersion composition has a molecular weight of 1.001 or more.

4. 2. The carbon black dispersion composition for batteries according to claim 1, wherein the dispersant is added in an amount of 1 to 20 parts by mass per 100 parts by mass of carbon black.

5. 2. The carbon black dispersion composition for batteries according to claim 1, wherein the dispersant is at least one selected from the group consisting of methyl cellulose, hydroxypropyl methyl cellulose, polyvinyl alcohol, and polyvinyl pyrrolidone.

6. The hindered phenol compound is 2,6-di-tert-butyl-p-cresol, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoic acid][oxalylbis(azanediyl)]bis(ethane-2,1-diyl), 2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)methyl 2. The carbon black dispersion composition for batteries according to claim 1, wherein the carbon black dispersion composition is at least one selected from the group consisting of styrene, 4-[[4,6-bis(n-octylthio)-1,3,5-triazin-2-yl]amino]-2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methoxyphenol, and 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione.

7. A positive electrode mixture paste comprising the carbon black dispersion composition for a battery according to any one of claims 1 to 6, a positive electrode active material, and a binder.

8. A positive electrode for a lithium ion secondary battery, comprising: a current collector; and a positive electrode mixture layer formed on the current collector as a coated and dried film of the positive electrode mixture paste according to claim 7.

9. A lithium ion secondary battery comprising the positive electrode for lithium ion secondary batteries according to claim 8, a negative electrode, an electrolyte, and a separator.

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