Method for manufacturing carbon material dispersions for secondary batteries, method for dehydrating carbon material dispersions for secondary batteries

Abstract

To provide a method for producing a carbon material dispersion for secondary batteries, and a method for dehydrating a carbon material dispersion for secondary batteries, which has excellent dispersion stability in a low-polarity solvent and, when an electrode film is formed, allows the carbon materials to form a secondary structure, enabling high conductivity even when the amount of carbon material is small. [Solution] The above problem is solved by a method for producing a carbon dispersion for secondary batteries comprising a low-polarity solvent, a carbon material, and a dispersant, the method comprising the step of applying mechanical energy to a mixture comprising a low-polarity solvent, a carbon material, and a dispersant under conditions where the water content is 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material becomes 3.0 μm or less.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a carbon material dispersion for a secondary battery and a method for dehydrating the carbon material dispersion for a secondary battery.

Background Art

[0002] With the miniaturization and weight reduction of notebook computers, the high performance of smart devices, and the spread of electric vehicles, there is a demand for batteries with high capacity in a limited space, and lithium ion secondary batteries (LIBs) with characteristics such as high energy density and high voltage are used as batteries for many devices. Among them, in mobility applications where the cruising range, charging time, and battery life are issues, the development of next-generation batteries such as all-solid-state lithium ion secondary batteries (all-solid LIBs) using solid electrolytes as ion conductors is actively underway. All-solid LIBs can reduce the risk of ignition and improve safety because they do not use a flammable electrolyte, in addition to battery characteristics such as high capacity, rapid charging, and high cycle characteristics compared to LIBs using conventional liquid electrolytes. The practical application of electric vehicles equipped with all-solid LIBs is expected due to these advantages.

[0003] In all-solid LIBs, since the electrolyte, which is a conductor of lithium ions, is solid, it is important to increase the contact between solid electrolytes (formation of a solid interface) in order to enhance the ionic conductivity in the electrode. On the other hand, since the active material, solid electrolyte, binder, etc. contained in the electrode are usually insulating materials, it is essential to add a conductive material in order to ensure electron conduction in the electrode required for the charge and discharge reaction.

[0004] Regarding the conductive paste for the above all-solid LIB, For example, Patent Document 1 relates to a method for removing water from a carbonaceous material dispersion using a low-polarity or non-polar solvent, and describes a method in which a dry inert gas is blown into a dispersion heated to 40-60°C, and the water in the dispersion is evaporated by bringing the dispersion into contact with the dry inert gas. Patent Document 2 discloses a method for producing a conductive paste with excellent viscosity and storage properties by using an acrylic resin, which is a polymerizable unsaturated monomer polymer having polar groups, as a dispersant, and further dispersing it in an ultrasonic homogenizer for 2 minutes. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-186527 [Patent Document 2] Japanese Patent Publication No. 2020-102421 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, the method described in Patent Document 1 has the problem that, because the dispersion is heated when it is brought into contact with a dry inert gas, the dispersant adsorbed during the dispersion process is likely to be desorbed, and the stability of the dispersion is likely to decrease. In addition, the short-time ultrasonic treatment described in Patent Document 2 makes it difficult to sufficiently dissolve conductive carbon, and thus it is difficult to achieve excellent dispersion stability.

[0007] Therefore, the problem that the present invention aims to solve is to provide a method for producing a carbon material dispersion for secondary batteries, and a method for dehydrating a carbon material dispersion for secondary batteries, which has excellent dispersion stability in a low-polarity solvent, and in which the carbon materials form a secondary structure when an electrode film is formed, thereby exhibiting high conductivity even when the amount of carbon material is small. [Means for solving the problem]

[0008] As a result of diligent research to solve the above problems, we have found that the above problems can be solved by the embodiments shown below, and have completed the present invention.

[0009] [1] The present invention relates to a method for producing a carbon dispersion for secondary batteries comprising a low-polarity solvent, a carbon material, and a dispersant, comprising the step of applying mechanical energy to a mixture comprising a low-polarity solvent, a carbon material, and a dispersant under conditions in which the water content is 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less.

[0010] [2] The present invention relates to a method for producing the carbon material dispersion for secondary batteries described in [1], wherein the water content is 600 ppm or less.

[0011] [3] The present invention relates to a method for producing a carbon material dispersion for secondary batteries according to [1] or [2], wherein the low-polarity solvent comprises at least one selected from the group consisting of ester solvents having an alkyl group having 4 or more carbon atoms, aromatic hydrocarbon solvents, and aliphatic hydrocarbon solvents.

[0012] [4] The present invention relates to a method for producing a carbon material dispersion for secondary batteries according to any one of [1] to [3], wherein the carbon material comprises at least one selected from the group consisting of carbon black and fibrous carbon.

[0013] [5] The present invention relates to a method for producing a carbon dispersion for secondary batteries comprising a low-polarity solvent, a carbon material, and a dispersant, comprising the steps of: dehydrating a mixture comprising a low-polarity solvent, a carbon material, and a dispersant to reduce the water content to 1,000 ppm or less; and applying mechanical energy to the mixture after dehydration to disperse the carbon material so that the dispersed particle size (D50) is 3.0 μm or less.

[0014] [6] The present invention relates to a method for producing a carbon dispersion for secondary batteries comprising a low-polarity solvent, a carbon material, and a dispersant, comprising the step of applying mechanical energy to a mixture comprising a low-polarity solvent, a carbon material, and a dispersant while dehydrating it so that the water content becomes 1,000 ppm or less, and dispersing the carbon material so that the dispersed particle size (D50) is 3.0 μm or less.

[0015] [7] The present invention relates to a method for producing a carbon material dispersion for secondary batteries according to [5] or [6], wherein the water content is 600 ppm or less.

[0016] [8] The present invention relates to a method for producing a carbon material dispersion for secondary batteries according to any one of [5] to [7], wherein the low-polarity solvent comprises at least one selected from the group consisting of ester solvents having an alkyl group having 4 or more carbon atoms, aromatic hydrocarbon solvents, and aliphatic hydrocarbon solvents.

[0017] [9] The present invention relates to a method for producing a carbon material dispersion for secondary batteries according to any one of [5] to [8], wherein the carbon material comprises at least one selected from the group consisting of carbon black and fibrous carbon.

[0018]

[10] The present invention relates to a method for producing an electrode slurry for an all-solid-state secondary battery, comprising the step of blending an active material, a solid electrolyte, and a binder into a carbon material dispersion for a secondary battery produced by the method for producing a carbon material dispersion for a secondary battery described in any one of [1] to [9].

[0019]

[11] The present invention relates to a method for manufacturing an electrode for an all-solid-state secondary battery having at least a current collector and an electrode film, comprising the step of applying and drying an electrode slurry for an all-solid-state secondary battery manufactured by the method for manufacturing an electrode slurry for an all-solid-state secondary battery described in

[10] onto a current collector to form an electrode film.

[0020]

[12] The present invention relates to a method for manufacturing an all-solid-state secondary battery having at least a positive electrode, a negative electrode, and a separator layer, comprising the step of using an electrode for an all-solid-state secondary battery manufactured by the method for manufacturing an electrode for an all-solid-state secondary battery described in

[11] as being used in at least one of the positive electrode and the negative electrode. [Effects of the Invention]

[0021] The present invention provides a method for producing a carbon material dispersion for secondary batteries, and a method for dehydrating a carbon material dispersion for secondary batteries, which has excellent dispersion stability in low-polarity solvents and, when an electrode film is formed, the carbon materials form a secondary structure with each other, enabling high conductivity even when the amount of carbon material is small. [Modes for carrying out the invention]

[0022] The present invention relates to a method for producing a carbon dispersion for secondary batteries comprising a low-polarity solvent, a carbon material, and a dispersant, characterized by comprising the step of applying mechanical energy to a mixture comprising the low-polarity solvent, the carbon material, and the dispersant under conditions in which the water content is 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material becomes 3.0 μm or less.

[0023] Generally, proper dispersion requires appropriate wetting and disintegration in the solvent. To disperse carbon materials with strong cohesive forces, it is necessary to apply mechanical energy to disintegrate the aggregates of secondary particles. On the other hand, when dispersing carbon materials in low-polarity solvents, water, which does not readily mix with low-polarity solvents, is stabilized on the surface of the carbon materials in the dispersion. Therefore, even if mechanical energy is applied to disintegrate the carbon materials, there is a problem in that the adsorption of the dispersant to the surface of the carbon materials does not proceed easily. As a result, dispersion of carbon materials in low-polarity solvents often results in insufficient adsorption of the dispersant before disintegration proceeds, leading to overdispersion or excessive re-aggregation, making it difficult to achieve excellent dispersion stability. However, the carbon material dispersion produced by the present invention is dispersed by applying mechanical energy under conditions where the water content is 1,000 ppm or less, and further dispersed so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less. This suppresses the adsorption inhibition of the dispersant, as well as separation and sedimentation, and exhibits excellent dispersion stability in low-polarity solvents. Furthermore, the coating film (electrode film) formed from the electrode slurry containing the above carbon material dispersion forms a secondary structural structure among the carbon materials, resulting in high conductivity. Therefore, the carbon material dispersion produced by the present invention is extremely useful for all-solid-state secondary battery applications.

[0024] The following describes in detail an embodiment of the present invention: a method for producing a carbon material dispersion liquid for secondary batteries. The present invention is not limited to the following embodiments, and includes embodiments that are implemented without changing the essence of the invention. In this specification, carbon black may be referred to as "CB," carbon nanotubes as "CNT," polyvinyl alcohol as "PVA," and polyvinyl butyral as "PVB." Furthermore, carbon material dispersions for secondary batteries may be referred to as "electrode composition" or "composition," electrode slurries for secondary batteries as "electrode slurry," and electrodes for secondary batteries as "electrodes."

[0025] <Carbon materials> The carbon material can be any material that functions as a conductive material within the electrode, and known carbon materials can be used. Examples of conductive carbon materials include carbon black, fibrous carbon, graphene, graphite, and fullerene. These conductive carbon materials may be used individually or in combination of two or more. Among these, from the viewpoint of conductivity, it is preferable that the conductive carbon material includes at least one selected from the group consisting of carbon black and fibrous carbon. Furthermore, the use of fibrous carbon is preferred from the viewpoint of conductivity.

[0026] [Carbon Black] Various types of carbon black can be used individually or in combination of two or more, including furnace black, which is produced by continuously thermally decomposing gaseous or liquid raw materials in a reactor; Ketjenblack, especially that made from ethylene heavy oil; channel black, which is produced by burning raw material gas and rapidly cooling the bottom surface of channel steel with the flame; and thermal black, which is obtained by periodically repeating combustion and thermal decomposition using gas as a raw material; especially acetylene black, especially that made from acetylene gas. In addition, conventionally treated oxidized carbon black and hollow carbon can also be used.

[0027] Oxidation of carbon involves treating carbon at high temperatures in air, or secondarily treating it with nitric acid, nitrogen dioxide, ozone, etc., to directly introduce (covalently bond) oxygen-containing polar functional groups such as phenol groups, quinone groups, carboxyl groups, and carbonyl groups to the carbon surface. This is a common procedure to improve the dispersibility of carbon. However, since the conductivity of carbon generally decreases as the amount of functional groups introduced increases, it is preferable to use carbon that has not undergone oxidation treatment.

[0028] The smaller the primary particle diameter (hereinafter also called the average primary particle diameter) of carbon black, the greater the number of particles contained per unit mass, and the more contact points there are between carbon black particles, which is advantageous for lowering the internal resistance of electrodes. Therefore, the carbon black raw material used in the production of the carbon material dispersion in this embodiment preferably has a primary particle size of 1 nm or more, more preferably 10 nm or more, and even more preferably 20 nm or more, from the viewpoint of conductivity and availability. It is also preferably 100 nm or less, more preferably 80 nm or less, and even more preferably 70 nm or less. That is, the primary particle size of the carbon black raw material may be, for example, 1 to 100 nm, 10 to 80 nm, or 20 to 70 nm. In this disclosure, the primary particle diameter refers to the spherical particles that form aggregates (primary aggregates), and is the average value of the particle diameters measured by an electron microscope. The average primary particle diameter of the carbon black raw material can be determined as follows: First, the carbon black raw material is observed and imaged using a transmission electron microscope. Next, 100 arbitrary spherical carbon black primary particles are selected from the observation image, and their outer diameters are measured. Then, the average primary particle diameter (nm) of the carbon black is calculated as the number average of the outer diameters.

[0029] Carbon black forms agglomerates (secondary aggregates) in a carbon material dispersion, which are composed of aggregates (primary aggregates). When the size of the secondary aggregates is larger than a certain threshold, it facilitates the formation of a conductive network, which is advantageous for reducing the internal resistance of electrodes.

[0030] [fibrous carbon] Fibrous carbon may be obtained by calcining petroleum-derived raw materials, or it may be obtained by calcining plant-derived raw materials. Examples of fibrous carbon include carbon nanotubes.

[0031] Carbon nanotubes have a cylindrical shape formed by winding planar graphite, and may be single-walled, double-walled, or multi-walled, or a mixture of these. Single-walled carbon nanotubes have a structure in which one layer of graphite is wound. Double-walled or multi-walled carbon nanotubes have a structure in which two or three or more layers of graphite are wound. Furthermore, the sidewalls of carbon nanotubes do not have to be of a graphite structure. For example, carbon nanotubes with sidewalls having an amorphous structure can also be used as carbon nanotubes.

[0032] The shape of the fibrous carbon is not limited. Examples of such shapes include needle-shaped, cylindrical tubular, fishbone-shaped (fishbone or cup-stacked), playing card-shaped (platelet), and coil-shaped. Among these, needle-shaped or cylindrical tubular shapes are preferred. The fibrous carbon may be used alone in one shape, or in combination of two or more shapes.

[0033] Examples of fibrous carbon forms, such as carbon nanotubes, include graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. Fibrous carbon may be used alone in one form, or in combination of two or more forms. In some embodiments, carbon material dispersions can be obtained using fibrous carbon raw materials. The fibrous carbon raw material has an average outer diameter of preferably 1 nm or more, more preferably 5 nm or more. It is also preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 13 nm or less. The average outer diameter of the fibrous carbon raw material can be determined as follows: First, the fibrous carbon raw material is observed and imaged using a transmission electron microscope. Next, 300 arbitrary fibrous carbons are selected from the observation image, and the outer diameter of each is measured. Then, the average outer diameter (nm) of the fibrous carbon is calculated as the number average of the outer diameters.

[0034] The carbon material may consist of two or more types of fibrous carbon with different average outer diameters. When used in combination, the average outer diameter of the first fibrous carbon raw material may be 1 nm or more and less than 5 nm, and the average outer diameter of the second fibrous carbon raw material may be 5 nm or more and 30 nm or less, or 20 nm or less. Furthermore, when used in combination, the mass ratio of the first fibrous carbon raw material to the second fibrous carbon raw material may be 1:10 to 1:100, or 1:10 to 1:50.

[0035] From the viewpoint of forming a conductive network, the fibrous carbon raw material preferably has an average fiber length of 0.5 μm or more, more preferably 0.8 μm or more, and even more preferably 1.0 μm or more. It is also preferably 20 μm or less, and more preferably 10 μm or less. The average fiber length of fibrous carbon raw material can be determined as follows: First, the fibrous carbon raw material is observed and imaged using a transmission electron microscope. Next, 300 arbitrary fibrous carbons are selected from the observed images, and the fiber length of each is measured. Then, the average fiber length (μm) of the fibrous carbon is calculated as the number average of the fiber lengths.

[0036] The aspect ratio is obtained by dividing the average fiber length by the average outer diameter. The higher the aspect ratio of the fibrous carbon raw material used, the higher the conductivity that can be achieved when electrodes are formed. From the viewpoint of conductivity, the aspect ratio of the fibrous carbon raw material is preferably 30 or more, more preferably 50 or more, and even more preferably 80 or more. It is also preferably 10,000 or less, more preferably 3,000 or less, and even more preferably 1,000 or less.

[0037] Generally, the larger the specific surface area of ​​a conductive carbon material, the smaller the primary particle diameter of the conductive carbon material. This increases the number of contact points between particles, thus lowering the internal resistance of the electrode. Therefore, from the perspective of conductivity, coating suitability, and electrode adhesion, a BET specific surface area of ​​20 m² obtained from the nitrogen adsorption method is desirable. 2 It is desirable to use conductive carbon material raw materials of a value of / g or higher. If the carbon material (a) is fibrous carbon, the specific surface area is preferably 50 m². 2 / g or more, comfortably 100m 2 It is 1,200 m or more. 2 Less than / g, more preferably 1,000m 2 It is less than or equal to / g. In some embodiments, the specific surface area is, for example, 100 to 1,200 m². 2 / g, 150~1,000m 2 / g is also acceptable. If the carbon material is carbon black, the specific surface area is more preferably 30 m².2 above / g and 70 m 2 below / g is preferred, and 60 m 2 below / g is more preferred. In some embodiments, the specific surface area is 20 - 70 m 2 / g, 30 - 60 m 2 / g, 39 - 58 m 2 / g may also be used. The nitrogen adsorption method is a method of measuring the adsorption isotherm by adsorbing and desorbing nitrogen as an adsorbing molecule on an adsorbent, analyzing the measured data, and calculating the specific surface area, pore volume, and pore diameter. In the present disclosure, the specific surface area is determined by the BET method.

[0038] From the perspective of handling properties, the bulk density of the conductive carbon material raw material is preferably 0.03 - 0.2 g / cm 3 When the carbon material (a) is fibrous carbon, the bulk density is preferably 0.03 g / cm 3 or more, more preferably 0.04 g / cm 3 or more. Also preferably 0.2 g / cm 3 or less, more preferably 0.15 g / cm 3 or less, 0.1 g / cm 3 or less. In some embodiments, the bulk density is, for example, 0.03 - 0.15 g / cm 3 , 0.04 - 0.15 g / cm 3 , 0.03 - 0.1 g / cm 3 may also be used. When the carbon material is carbon black, the bulk density is preferably 0.03 g / cm 3 or more, more preferably 0.05 g / cm 3 or more. Also preferably 0.2 g / cm 3 or less, and may be 0.15 g / cm 3 or less. In some embodiments, the bulk density is 0.03 - 0.2 g / cm 3 , 0.05 - 0.2 g / cm 3 , 0.05 - 0.15 g / cm 3 , 0.08 - 0.15 g / cm 3 may also be used. In this disclosure, bulk density can be determined by placing carbon material powder into a 30 mL stainless steel cylindrical container by free-falling, leveling off the portion that has risen on the upper surface of the container, determining the mass of the carbon material powder, and dividing it by the volume of the container. The carbon material powder used is prepared by crushing aggregates formed during storage and passing it through a 0.5 mm sieve.

[0039] The content of conductive carbon material is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 0.8% by mass or more, based on the total amount of the carbon material dispersion for secondary batteries. It is also preferably 20% by mass or less, and more preferably 10% by mass or less. By keeping it within the above range, the conductive carbon material can be well contained while maintaining its structure. More preferably it is 0.1 to 20% by mass, and even more preferably 0.5 to 10% by mass. Furthermore, it is preferable to appropriately adjust the content of conductive carbon material so that a carbon material dispersion with appropriate fluidity or viscosity is obtained, depending on the specific surface area of ​​the conductive carbon material, its affinity to the dispersion medium, the dispersibility of the dispersant, etc.

[0040] The carbon content in a carbon material dispersion can be appropriately adjusted depending on the type of carbon material used, its specific surface area, the amount of surface functional groups, and other physical properties specific to that carbon material. For example, if the carbon material contains carbon black, the carbon black content is preferably 1% by mass or more, more preferably 5% by mass or more, based on the mass of the carbon material dispersion. It is also preferably 50% by mass or less, more preferably 30% by mass or less, and may be, for example, 1 to 50% by mass or 5 to 50% by mass. When the carbon material contains fibrous carbon, the carbon material content is preferably 0.1% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, based on the mass of the carbon material dispersion. It is also preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. For example, it may be 1 to 10% by mass or 1 to 5% by mass. When the carbon material content is within the above range, the dispersion exhibits excellent dispersibility, the viscosity of the dispersion is within an appropriate range, and production efficiency and handling are superior.

[0041] <Low polarity solvent> Low-polarity solvents are solvents that have low polarity and do not react easily with sulfur-based solid electrolytes, and include non-polar solvents. Examples of low-polarity solvents include solvents with a dielectric constant of less than 10 and / or solvents with a solubility of less than 1 g in 100 g of water at 20°C. Preferably, the solvent has a dielectric constant of less than 10.0, more preferably 8.0 or less, and even more preferably 6.0 or less. The relative permittivity in this specification is a value measured at 20-25°C, and can be measured, for example, by measuring the double cylindrical tube current at 10 kHz using a liquid dielectric constant meter Model 871 (manufactured by Sanyo Trading Co., Ltd.).

[0042] When using two or more solvents, the relative permittivity of the mixed solvent can be calculated as the weighted average of the relative permittivity of the individual solvents, based on the volume of each solvent used. That is, if the relative permittivity of solvent A is εr A The relative permittivity of solvent B is εr B The volume of solvent A is V A (ml), the volume of solvent B is V B When the volume is (ml), the weighted average relative permittivity of the mixed solvent can be calculated using the following formula. Weighted average relative permittivity = (εr A ×V A +εr B ×V B ) / (V A +V B )

[0043] Based on the total mass of the low-polarity solvent, the proportion of solvents with a relative permittivity of less than 10, and / or solvents with a solubility of less than 1 g in 100 g of water at 20°C is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass.

[0044] Examples of such low-polarity solvents include ester solvents having four or more C4 alkyl groups, ether solvents having four or more C4 alkyl groups, ketone solvents having four or more C4 alkyl groups, aromatic hydrocarbons, and aliphatic hydrocarbons. Because these solvents are highly hydrophobic, they can prevent the degradation of sulfide-based solid electrolytes that is a concern when using polar solvents such as water, alcohol, or N-methyl-2-pyrrolidone. Furthermore, by combining them with dispersants described later, the dispersibility of carbon materials can be improved.

[0045] Examples of ester solvents having alkyl groups with 4 or more carbon atoms include butyl butyrate, pentyl butyrate, hexyl butyrate, butyl acetate, pentyl acetate, hexyl acetate, and butyl propionate. Examples of ether solvents having alkyl groups with 4 or more carbon atoms include dibutyl ether, ethyl butyl ether, tert-butyl methyl ether, tert-butyl ethyl ether, and tetrahydrofuran. Examples of ketone solvents having alkyl groups with 4 or more carbon atoms include diisobutyl ketone. Examples of aromatic hydrocarbons include toluene, xylene, mesitylene, and tetralin. Examples of aliphatic hydrocarbons include n-hexane, n-heptane, n-octane, and cyclohexane. More preferably, the solution contains at least one selected from the group consisting of ester solvents having an alkyl group with 4 or more carbon atoms, aromatic hydrocarbons, and aliphatic hydrocarbons. Butyl butyrate may be suitably used as the ester solvent having an alkyl group with 4 or more carbon atoms.

[0046] The carbon material dispersion may contain a polar solvent, to the extent that it does not impair the effects of this embodiment. The content of the low-polarity solvent is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, and may be 100% by mass, based on the total mass of the solvent. These solvents may be used individually or in combination of two or more.

[0047] <Dispersant> The carbon material dispersion for secondary batteries of the present invention contains a dispersant. The dispersant is not particularly limited as long as it can disperse and stabilize the conductive carbon material in the carbon material dispersion for secondary batteries, and for example, at least one of a resin-type dispersant and a surfactant can be used. Although either a resin-type dispersant or a surfactant can be used as the dispersant, a resin-type dispersant is preferred because it has a strong adsorption force to the conductive carbon material and provides good dispersion stability. The type and amount of dispersant can be adjusted as appropriate according to the properties required for dispersing conductive carbon materials. A single dispersant may be used, or two or more may be used in combination.

[0048] [Resin-type dispersant] Examples of resin-type dispersants include polymers derived from ethylenically unsaturated hydrocarbons, cellulosic derivatives, and copolymers thereof.

[0049] (Ethylene-unsaturated hydrocarbon-derived polymers) Examples of polymers derived from ethylenically unsaturated hydrocarbons include (meth)acrylic polymers, polyvinyl alcohol resins, polyvinylpyrrolidone resins, polyacrylonitrile resins, and rubbers.

[0050] Examples of (meth)acrylic resins include methyl polyacrylate, ethyl polyacrylate, butyl polyacrylate, polymethyl methacrylate, polyethyl methacrylate, polytart-butyl methacrylate, copolymers of ethylene-methyl acrylate and ethylene-ethyl acrylate.

[0051] Examples of polyvinyl alcohol-based resins include polyvinyl alcohol, modified polyvinyl alcohol having functional groups other than hydroxyl groups (e.g., acetyl groups, sulfo groups, carboxyl groups, carbonyl groups, amino groups), polyvinyl alcohol modified with various salts, other anionic or cationic modified polyvinyl alcohol, and polyvinyl acetals modified with aldehydes (acetoacetal modification, butyral modification) (e.g., polyvinyl acetal, polyvinyl butyral).

[0052] Examples of polyvinylpyrrolidone resins include Luvitec K17, K30, K80, K85, K90, and K90HM manufactured by BASF Japan, K15, K30, K90, and K120 manufactured by Ashland, and polyvinylpyrrolidone K30, K85, and K90 manufactured by Nippon Shokubai Co., Ltd.

[0053] Examples of polyacrylonitrile resins include polyacrylonitrile homopolymers, polyacrylonitrile copolymers, and modified versions thereof. More preferably, the polyacrylonitrile resin has an alkyl group in its side chain. Examples of the alkyl group include alkyl groups derived from (meth)acrylate esters and α-olefins, and as such a polyacrylonitrile resin having an alkyl group in its side chain, for example, the acrylonitrile copolymer described in Japanese Patent Application Publication No. 2020-163362 can be used.

[0054] Examples of rubbers include acrylonitrile butadiene rubber, hydrogenated acrylonitrile butadiene rubber, styrene-based elastomers, and hydrogenated styrene-based elastomers. Examples of styrene-based elastomers and hydrogenated styrene-based elastomers include modified versions of any of the following: styrene-butadiene rubber (SBR), styrene-ethylene-butylene block copolymer (SEB), styrene-ethylene-propylene block copolymer (SEP), styrene-ethylene-butylene-styrene block copolymer (SEBS), styrene-ethylene-propylene-styrene block copolymer (SEPS), styrene-ethylene-butylene-styrene-styrene block copolymer (SEBSS), styrene-isobutylene-styrene block copolymer (SIBS), and styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS).

[0055] (Cellulose derivatives) Examples of cellulose derivatives include cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethylcellulose, ethyl hydroxyethylcellulose, nitrocellulose, methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, carboxymethylcellulose, and copolymers thereof.

[0056] The dispersant may be a polymer in which other substituents have been introduced into a portion of these polymers, or a modified polymer. Among these, polyvinyl acetal, acrylonitrile butadiene rubber, hydrogenated acrylonitrile butadiene rubber, styrene-based elastomers, and hydrogenated styrene-based elastomers are preferred from the viewpoint of solubility in low-polarity solvents and dispersion stability.

[0057] The weight-average molecular weight (Mw) of the resin-type dispersant is preferably 500,000 or less, more preferably 300,000 or less, from the viewpoint of the affinity balance between the dispersed material and the dispersion medium. It is also preferably 3,000 or more, more preferably 5,000 or more. The resin-type dispersant may be used alone or in combination of two or more types.

[0058] From the viewpoint of dispersion stability and battery performance, the dispersant content is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, based on the total amount of conductive carbon material. Furthermore, it is preferably 300% by mass or less, more preferably 200% by mass or less, and even more preferably 100% by mass or less. That is, the dispersant content may be 5 to 300% by mass, 10 to 200% by mass, or 15 to 100% by mass. The dispersant content is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, based on the total mass of the carbon material dispersion. It is also preferably 10% by mass or less, more preferably 5% by mass or less, and may be, for example, 0.1 to 10% by mass or 0.5 to 5% by mass.

[0059] <Manufacturing of carbon material dispersions for secondary batteries> In the manufacturing method of the present disclosure, the carbon material dispersion includes a step of dispersing a mixture containing a low-polarity solvent, a carbon material, and a dispersant by applying mechanical energy under conditions where the water content is 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less (hereinafter referred to as the dispersion step). A carbon material dispersion may be manufactured by finely dispersing a mixture containing a carbon material, a low-polarity solvent, and a dispersant, with a water content of 1,000 ppm or less, using a dispersion apparatus capable of applying mechanical energy, so that the dispersed particle size (D50) is 3.0 μm or less. The water content does not need to be 1,000 ppm or less for the entire duration of the dispersion process; for example, it may be 1,000 ppm or less for the latter half of the dispersion process. The dispersion process may involve adjusting the timing of material addition as needed, and may consist of two or more multi-stage processes.

[0060] In this specification, mechanical energy refers to the energy applied when impact or shear force is applied to break up the agglomeration of carbon materials. Examples of dispersion devices that apply mechanical energy include kneaders, two-roll mills, three-roll mills, paint shakers, ball mills, horizontal sand mills, vertical sand mills, annular bead mills, attritors, colloid mills, high-shear mixers, high-pressure homogenizers, and ultrasonic homogenizers. Among these, colloid mills, high-shear mixers, bead mills, and high-pressure homogenizers are preferred from the viewpoint of efficiently breaking up agglomerated carbon materials.

[0061] The manufacturing method of the present invention may include a step of dispersion using a dispersion device that does not apply mechanical energy (hereinafter also referred to as a pretreatment step) before the dispersion step. Examples of dispersion devices that do not apply mechanical energy include propeller-type mixers, dispersers, turbine blades, and container-rotating type rotating / revolving mixers. Performing a pretreatment step promotes wetting of the carbon material, allows for better dissolution of coarse particles, and improves the uniformity of the particle size distribution and productivity.

[0062] The manufacturing method of the present invention is characterized by dispersion under conditions where the moisture content is 1,000 ppm or less. To achieve these conditions, dispersion may be performed using a mixture of raw materials with low moisture content, the mixture may be dehydrated before dispersion, the mixture may be dispersed while being dehydrated, or a combination of these methods may be used. Specifically, for example, the following methods (I) and (II) can be cited. (I) A method for producing a carbon material dispersion for secondary batteries, comprising the steps of (I) dehydrating a mixture containing a low-polarity solvent, a carbon material, and a dispersant to reduce the water content to 1,000 ppm or less, and applying mechanical energy to the dehydrated mixture to disperse the carbon material so that the dispersed particle size (D50) is 3.0 μm or less. (II) A method for producing a carbon material dispersion for secondary batteries, comprising the step of applying mechanical energy to a mixture containing a low-polarity solvent, a carbon material, and a dispersant while dehydrating it to a water content of 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less.

[0063] The dehydration method is not particularly limited, and may include, for example, dehydrating agents such as molecular sieves, vacuum distillation, bubbling of dry gas, or dehydration membranes. For example, the moisture content of raw materials or mixtures can be reduced by dehydrating agents or drying processes. As a method of dehydration during dispersion, for example, a mixture containing a low-polarity solvent, a carbon material, and a dispersant is dispersed by applying mechanical energy so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less, and during this process, the mixture is brought into contact with a dehydrating agent to reduce the moisture content to 1,000 ppm or less. As a method of contacting with a dehydrating agent, for example, the dehydrating agent is passed through the line of a circulating disperser to bring it into contact, and dry gas is bubbling into the dispersion tank during dispersion to bring it into contact.

[0064] The water content of a mixture can be measured by the Karl Fischer titration method. The Karl Fischer titration method is a technique for quantifying water content in a sample by utilizing the chemical reaction in which iodine and water react in a 1:1 molar ratio. There are two methods: volumetric titration and coulometric titration, which can be appropriately selected depending on the water content of the mixture. The moisture content in the dispersion process is preferably 800 ppm or less, and more preferably 600 ppm or less.

[0065] The particle size (D50) of the dispersed carbon material is the 50% cumulative distribution diameter (median diameter) based on volume, measured by dynamic light scattering using laser light, and can be measured using a particle size analyzer (NANOTRAC WAVE, manufactured by Microtrac-Bell Co., Ltd.). For measurement, the particle refractive index of the dispersed carbon material should be 1.8, the shape should be non-spherical, and the concentration of the dispersed carbon material should be diluted with a solvent so that the loading index value is in the range of 0.8 to 1.2. The present invention is characterized by dispersing the dispersed particles such that the dispersed particle size (D50) is 3.0 μm or less. Having a size of 3.0 μm or less suppresses separation and sedimentation in low-polarity solvents. The dispersed particle size (D50) is preferably 2.5 μm or less, more preferably 2.0 μm or less, and even more preferably 1.0 μm or less. It is also preferably 0.2 μm or more, and may be, for example, 0.2 to 3.0 μm, 0.2 to 2.5 μm, 0.2 to 2.0 μm, or 0.2 to 1.0 μm. By keeping the range as described above, the amount of aggregated carbon material is reduced, and the carbon material is not excessively refined. As a result, a dispersion with a good dispersion state is achieved, enabling the formation of excellent dispersion stability and an efficient conductive network.

[0066] The carbon material dispersion for secondary batteries produced by the manufacturing method of the present invention may contain optional components such as wetting agents, pH adjusters, wetting and penetrating agents, leveling agents, film-forming aids, defoaming agents, preservatives, and viscosity modifiers, as well as other conductive materials other than carbon materials, polymer components (binders), etc., to the extent that the effects of the present invention are not impaired. The description of binders can be referenced from the section on [Binders] in the section on <Electrode Slurry for Secondary Batteries>. These optional components can be added at any step and at any timing in the manufacturing method of the present disclosure, such as during mixture preparation, dispersion, after dispersion, or in combination thereof.

[0067] The carbon material dispersion for secondary batteries has a viscosity of 10 mPa·s or higher, as measured using an E-type viscometer at 25°C and 60 rpm. Preferably, it is 10,000 mPa·s or less, more preferably 2,000 mPa·s or less, and even more preferably 1,000 mPa·s or less. For example, it may be between 10 mPa·s and 10,000 mPa·s, between 10 mPa·s and 2,000 mPa·s, or between 10 mPa·s and 1,000 mPa·s.

[0068] The TI value of a carbon material dispersion for secondary batteries can be calculated from the viscosity (mPa·s) at 6 rpm measured at 25°C using an E-type viscometer, divided by the viscosity (mPa·s) at 60 rpm. The TI value is preferably 1.0 or higher, preferably 10.0 or lower, more preferably 5.0 or lower, and even more preferably 3.0 or lower. For example, it may be between 1.0 and 10.0, between 1.0 and 5.0, or between 1.0 and 3.0. A higher TI value indicates greater structural viscosity due to entanglement of carbon material, dispersant, and other resin components, or their intermolecular forces, while a lower TI value indicates lower structural viscosity. By setting the TI value within the above range, it is possible to suppress entanglement of carbon material, dispersant, and other resin components while allowing these intermolecular forces to act appropriately.

[0069] The solid content of the carbon material dispersion for secondary batteries is preferably 0.2% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more. It is also preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 20% by mass or less, for example, 0.2 to 40% by mass, 0.3 to 30% by mass, or 0.5 to 20% by mass.

[0070] <Electrode slurry for secondary batteries> The present disclosure provides a method for producing an electrode slurry for an all-solid-state secondary battery, comprising the step of blending an active material, a solid electrolyte, and a binder into a carbon material dispersion for a secondary battery produced by the method for producing a carbon material dispersion for a secondary battery according to the present disclosure. That is, an electrode slurry for a secondary battery can be produced by blending the active material, a solid electrolyte, and a binder into the carbon material dispersion obtained above. The electrode slurry may further contain the above-mentioned low-polarity solvent depending on the required physical properties.

[0071] [Active material] The active material (hereinafter also referred to as the electrode active material) is a substance that causes a battery reaction necessary to extract electrical energy, and is not particularly limited. As the positive electrode active material, for example, metal oxides capable of reversibly doping or intercalating lithium ions, and metal compounds such as metal sulfides can be used. Specific examples of such positive electrode active materials include, for example, lithium manganese composite oxides (for example, Li x Mn2O4 or Li x MnO2), lithium nickel composite oxides (for example, LiNiO2), lithium cobalt composite oxides (LixCoO2), lithium nickel cobalt composite oxides (for example, Li x Ni 1-y Co y O2), lithium manganese cobalt composite oxides (for example, Li x Mn y Co 1-y O2), lithium nickel manganese cobalt composite oxides (for example, Li x Ni y Co z Mn 1-y-z O2), spinel-type lithium manganese nickel composite oxides (for example, Li x Mn 2-y Ni y O4), etc., composite oxide powders of lithium and transition metals, lithium phosphate powders having an olivine structure (for example, Li x FePO4, Li x Fe 1-y Mn y PO4, Li x CoPO4), transition metal oxide powders such as manganese oxide, iron oxide, copper oxide, nickel oxide, vanadium oxide (for example, V2O5, V6O 13 ), titanium oxide, etc., transition metal sulfide powders such as iron sulfate (Fe2(SO4)3), TiS2, and FeS. However, x, y, and z are numbers, where 0 < x < 1, 0 < y < 1, 0 < z < 1, and 0 < y + z < 1. These positive electrode active materials may be used alone or in combination of two or more.

[0072] As the negative electrode active material, for example, metal Li capable of reversibly doping or intercalating lithium ions, or its alloy (such as metal In), tin alloy, silicon alloy negative electrode, Li X TiO2, Li XFe2O3, Li X Fe3O4, Li X Metal oxide systems such as WO2, conductive polymers such as polyacetylene and poly-p-phenylene, artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite, and resin-fired carbon materials. However, x is a number and 0 < x < 1. These negative electrode active materials may be used alone or in combination of two or more. In particular, when using a silicon alloy negative electrode, although the theoretical capacity is large, the volume expansion is extremely large. Therefore, it is preferably used in combination with artificial graphite such as highly graphitized carbon materials, carbonaceous powders such as natural graphite, resin-fired carbon materials, etc.

[0073] [Solid electrolyte] As the solid electrolyte, a sulfide-based solid electrolyte can be used, and an oxide solid electrolyte or a halide solid electrolyte may be used in part. The sulfide-based solid electrolyte is not particularly limited. The element contains S and may further contain Li. Also, crystals, non-crystals (glass), glass ceramics obtained by crystallizing glass, or those in which only a part is crystallized may be used. Examples of the sulfide-based solid electrolyte include, for example, Li 9.54 Si 1.74 P 1.44 S 11.7 C l0.3 , Li 10 GeP2S 12 , Li6PS5C l And those produced by mixing raw materials in an arbitrary molar ratio as shown in the following examples may also be used. Example) Li2S-P2S5, Li2S-P2S5-LiCl, Li2S-LiI-P2S5, Li2S-LiI-Li2O-P2S5, Li2S-LiBr-P2S5, Li2S-Li2O-P2S5, Li2S-Li3PO4-P2S5, Li2S-P2S5-P2O5, Li2S-P2S5-SiS2, Li2S-P2S5-SiS2-LiCl, Li2S-P2S5-SnS, Li2S-P2S5-Al2S3, Li2S-Al2S3, Li2S-SiS2, Li2S-Si S2-Al2S3, Li2S-SiS2-P2S5, Li2S-SiS2-LiI, Li2S-SiS2-P2S5-LiI, Li2S-SiS2-Li4SiO4, Li2S-SiS2-P2O5, Li2S-B2S3, Li2S-B2 S3-Li3PO4, Li2S-GeS2, Li2S-Ga2S3, Li2S-GeS2-Ga2S3, Li2S-GeS2-P2S5, Li2S-GeS2-Sb2S5, Li2S-GeS2-ZnS, Li2S-GeS2-Al2S3. These may be used individually or in combination of two or more types.

[0074] [binder] The binder plays a role in bonding the materials contained in the electrode and the materials to the substrate, and there are no particular restrictions on the binder resin; it can be appropriately selected according to the purpose. Examples of binder resins include polymers or copolymers containing ethylene, propylene, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, acrylonitrile, styrene, vinyl butyral, vinyl acetal, vinylpyrrolidone, etc. as constituent units; polyurethane resins, polyester resins, phenolic resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, fluororesins; cellulose resins; elastomers such as styrene-butadiene rubber and fluororubber; and conductive resins such as polyaniline and polyacetylene. The binder may also be a modified form or mixture of these resins, or a copolymer.

[0075] The binder content in the electrode slurry for secondary batteries is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, based on the mass of the active material. It is also preferably 20% by mass or less, more preferably 10% by mass or less.

[0076] <Electrode for secondary batteries> The present disclosure provides a method for manufacturing electrodes for all-solid-state secondary batteries, which includes the step of applying and drying an electrode slurry for all-solid-state secondary batteries, manufactured by the method for manufacturing electrode slurry for all-solid-state secondary batteries, to a current collector to form an electrode film. The electrode for the secondary battery includes at least one of the electrode films formed from the above-described electrode slurry, and may further include a current collector. The electrode film can be obtained, for example, by coating the electrode slurry onto a substrate such as a current collector and drying the volatile components. The material and shape of the current collector are not particularly limited, and can be appropriately selected to suit various types of secondary batteries. Examples of current collector materials include metals and alloys such as aluminum, copper, nickel, titanium, and stainless steel. As for the shape of the current collector, a flat foil shape is generally used, but current collectors with a roughened surface, a perforated foil shape, and a mesh shape may also be used. The current collector may have a coating layer on its surface. Examples of coating layers include a carbon layer containing a conductive carbon material or binder to improve adhesion to the current collector and conductivity. The thickness of the current collector is preferably about 0.5 to 30 μm.

[0077] The method for applying the electrode slurry onto the current collector is not particularly limited and includes, for example, die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic coating. As for drying methods, for example, in addition to drying by standing, drying using equipment such as a forced-air dryer, hot-air dryer, infrared heater, and far-infrared heater may be used.

[0078] The electrodes may be subjected to a pressing process after the electrode slurry has been applied and dried. Examples of pressing methods include a flatbed press and a roll press such as a calender roll, and heating may be used during the pressing process.

[0079] <All-solid-state secondary battery> An all-solid-state secondary battery comprises at least a positive electrode, a negative electrode, and a separator layer as components of the drive unit. The all-solid-state secondary battery is stacked so that the positive electrode and the negative electrode face each other with a separator layer in between, and at least one of the positive electrode and the negative electrode is provided with the electrode of this disclosure. The all-solid-state secondary battery in this disclosure can be manufactured by using an electrode manufactured by the method for manufacturing electrodes for all-solid-state secondary batteries in this disclosure as at least one of the positive electrode and the negative electrode. The positive electrode and / or the negative electrode are the electrode films, and the separator layer placed between the positive electrode and the negative electrode consists of a solid electrolyte and may contain a binder. There are no particular limitations on the method of stacking the components of the drive unit (positive electrode, negative electrode, and separator layer) in an all-solid-state secondary battery. For example, the positive electrode, negative electrode, and separator layer may be formed separately and then stacked, or a separator layer may be formed on the surface of the positive electrode and / or negative electrode, and then the paired positive or negative electrodes may be stacked. Furthermore, a pressing process may be performed during stacking, and heating may be added during pressing. The pressing process may be performed after all components have been stacked, or at each stage in which the components have been stacked. The all-solid-state battery may further include an exterior such as a laminate film or metal case, connection terminals, etc.

[0080] The carbon material dispersion produced by the manufacturing method of the present invention has excellent dispersion stability in low-polarity solvents, and when an electrode film is formed, the carbon materials form a secondary structure with each other, enabling high conductivity even with a small amount of compounding. Therefore, it can be suitably used in all-solid-state secondary batteries installed in electric vehicles such as electric cars and hybrid cars; and portable electronic devices such as personal computers and smartphones. [Examples]

[0081] The present invention will be specifically described below with reference to examples. The present invention is not limited to the following examples unless it exceeds the gist of the invention. In the examples and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" unless otherwise specified.

[0082] <Measurement of weight-average molecular weight (Mw)> The weight-average molecular weight (Mw) was measured by gel permeation chromatography (GPC) equipped with a radioisotope detector. An HLC-8320GPC (manufactured by Tosoh Corporation) was used, with three separation columns connected in series. The packing materials used were, in order, Tosoh Corporation's "TSK-GEL SUPER AW-4000," "AW-3000," and "AW-2500." The oven temperature was 40°C, and the eluent was a 30 mM triethylamine and 10 mM LiBr N,N-dimethylformamide solution. The measurement was performed at a flow rate of 0.6 mL / min. The measurement sample was diluted to a 1% concentration using tetrahydrofuran (THF) and injected in 20 microliters. The weight-average molecular weight is expressed in polystyrene equivalent.

[0083] Furthermore, when separating and recovering a resin-type dispersant from a carbon material dispersion and measuring its weight-average molecular weight, the following method can be used. The carbon material is separated from the carbon material dispersion by centrifugation, and the supernatant is collected. The obtained supernatant is added dropwise to purified water to precipitate the resin-type dispersant, etc., and the precipitate is collected by filtration using a Buchner funnel. The precipitate is rinsed by sprinkling purified water over it in the Buchner funnel, and then dissolved in tetrahydrofuran (THF) to obtain a solution. The obtained solution is added dropwise again to purified water, and the filtration and washing steps using purified water are repeated to redissolve the precipitate in THF, which is used as a measurement sample, and the weight-average molecular weight is measured using the method described above.

[0084] <Synthesis of Dispersant 1> In a stainless steel polymerization reactor, 30 parts acrylonitrile, 70 parts 1,3-butadiene, 3 parts potassium soap oleate, 0.3 parts azobisisobutyronitrile, 0.6 parts t-dodecyl mercaptan, and 200 parts deionized water were added. Polymerization was carried out at 45°C for 20 hours under a nitrogen atmosphere with stirring, and the polymerization was terminated when the conversion rate reached 90%. Unreacted monomers were removed by vacuum stripping to obtain an acrylonitrile-conjugated diene rubber latex with a solid content of approximately 30%. Subsequently, deionized water was added to the latex to adjust the total solid content to 12%, and the mixture was placed in a 1 L autoclave with a stirrer. Dissolved oxygen in the contents was removed by flowing nitrogen gas through the mixture for 10 minutes. A catalyst solution prepared by dissolving 75 mg of palladium acetate as a hydrogenation catalyst in 180 mL of deionized water with 4 molar amounts of nitric acid relative to the palladium was added to the autoclave. After purging the autoclave twice with hydrogen gas, the contents of the autoclave were heated to 50°C under a hydrogen gas pressure of 3 MPa and a hydrogenation reaction was carried out for 6 hours. Afterward, the contents were returned to room temperature, the autoclave was subjected to a nitrogen atmosphere, and the solids were dried to recover copolymer 1. Copolymer 1 had a hydrogenation rate of 99.5%, and its weight-average molecular weight (Mw) was 200,000. In the acrylonitrile-conjugated diene rubber, the content of conjugated diene monomer units was 80% and the content of nitrile group-containing monomer units was 20%, based on the mass of the acrylonitrile-conjugated diene rubber. The content of these monomer units and structural units was determined from the amount of monomer used.

[0085] <Preparation of carbon material dispersions> (Example 1-1) 96.5 parts of butyl butyrate, a low-polarity solvent, was stirred in a mixer, and 1.0 part of polyvinyl butyral (Mowital B16H: manufactured by Kuraray Co., Ltd.) was added as a dispersant and stirred until dissolved. Next, 2.5 parts of multi-walled carbon nanotubes (100T: manufactured by KUMUHO Co., Ltd.) were added in small amounts while circulating through a colloid mill (Magic Lab), and after the entire amount had been added, circulating dispersion was carried out for 5 minutes. Molecular sieves 4A were brought into contact with the prepared dispersion as a dehydrating agent to remove water. The water content of the dispersion after water removal was 380 ppm. Subsequently, in order to prevent absorption of moisture from the atmosphere, a circulating dispersion treatment was carried out using a colloid mill under sealed conditions for a residence time of 60 minutes to obtain carbon material dispersion 1.

[0086] (Examples 1-2) 96.5 parts of butyl butyrate were added as a solvent in a mixer and stirred, while 1.0 part of polyvinyl butyral (Mowital B16H: manufactured by Kuraray Co., Ltd.) was added as a dispersant and stirred until dissolved. Next, 2.5 parts of multi-walled carbon nanotubes (100T: manufactured by KUMUHO Co., Ltd.) were added little by little while circulating in a colloid mill (Magic Lab) as the carbon material. After the entire amount had been added, circulating dispersion was carried out for 5 minutes to prepare the dispersion. Subsequently, to prevent absorption of moisture from the atmosphere, a moisture trap filled with molecular sieves 4A as a dehydrating agent was installed in the circulation line under sealed conditions, and a circulating dispersion treatment was carried out using a colloid mill with a residence time of 60 minutes to obtain carbon material dispersion 2. The moisture content at the start of the dispersion process was 1240 ppm, and the moisture content at a residence time of 5 minutes was 860 ppm.

[0087] (Examples 1-3) 96.5 parts of butyl butyrate (dehydrated grade) was added as a solvent in a mixer and stirred, while 1.0 part of polyvinyl butyral (Mowital B16H: manufactured by Kuraray Co., Ltd.) was added as a dispersant and stirred until dissolved. Next, 2.5 parts of multi-walled carbon nanotubes (100T: manufactured by KUMUHO Co., Ltd.) were added little by little while circulating in a colloid mill (Magic Lab) as the carbon material. After the entire amount had been added, circulating dispersion was performed for 5 minutes to prepare the dispersion. The water content of the prepared dispersion was 470 ppm. Subsequently, in order to prevent absorption of moisture from the atmosphere, a circulating dispersion treatment was performed using a colloid mill under sealed conditions for a residence time of 60 minutes to obtain carbon material dispersion 3.

[0088] (Examples 1-4) 96.5 parts of butyl butyrate (dehydrated grade) as a solvent, 1.0 part of polyvinyl butyral (Mowital B16H: manufactured by Kuraray Co., Ltd.) as a dispersant, and 2.5 parts of multi-walled carbon nanotubes (100T: manufactured by KUMUHO Co., Ltd.) as a carbon material were weighed and mixed. The water content of the prepared pretreatment solution was 430 ppm. Subsequently, 150 parts of zirconia beads were added, the mixture was sealed to prevent moisture from entering from the air, and shaken in a paint shaker for 2 hours. After that, the mixture was filtered to remove the zirconia beads and obtain carbon material dispersion 4.

[0089] (Examples 1-5 to 1-13) Carbon material dispersions 5 to 13 were obtained in the same manner as in Example 1-1, except that the materials and composition were changed as shown in Table 1, and the contact time with molecular sieves 4A was changed to adjust the water content at the start of dispersion as shown in Table 1.

[0090] (Comparative Example 1-1) A carbon material dispersion 14 was prepared in the same manner as in Example 1-1, except that the materials and composition were changed as shown in Table 1, and the dehydration treatment of the pretreatment solution was omitted.

[0091] (Comparative Example 1-2) 96.5 parts of butyl butyrate were added as a solvent in a mixer and stirred, while 1.0 part of polyvinyl butyral (Mowital B16H: manufactured by Kuraray Co., Ltd.) was added as a dispersant and stirred until dissolved. Next, 2.5 parts of multi-walled carbon nanotubes (100T: manufactured by KUMUHO Co., Ltd.) were added as the carbon material, and then molecular sieves 4A were brought into contact with the mixture as a dehydrating agent to remove the water. The water content in the liquid after water removal was 480 ppm. Next, the mixture was dispersed at 2000 rpm for 5 minutes using a rotation-orbit mixer (Awatori Rentaro, ARE-310, manufactured by Thinky) to obtain a carbon material dispersion 15. Note that the rotation-orbit mixer is a dispersion device that does not apply mechanical energy.

[0092] (Comparative Examples 1-3) 96.5 parts of butyl butyrate (dehydrated grade) was added as a solvent and stirred in a disperser. 1.0 part of polyvinyl butyral (Mowital B16H: manufactured by Kuraray Co., Ltd.) was added as a dispersant and stirred until dissolved. Next, 2.5 parts of multi-walled carbon nanotubes (100T: manufactured by KUMUHO Co., Ltd.) were added in small increments. After adding the entire amount, the mixture was dispersed using a disperser at 3,000 rpm for 15 minutes under sealed conditions to prevent moisture from entering from the atmosphere, thereby preparing carbon material dispersion 16. The moisture content of the processing solution at the end of adding the carbon material was 480 ppm. Note that the disperser is a dispersion device that does not apply mechanical energy.

[0093] (Comparative Examples 1-4) A carbon material dispersion 17 was prepared in the same manner as in Example 1-11, except that the materials and composition were changed as shown in Table 1, and the dehydration treatment of the pretreatment solution was omitted.

[0094] <Evaluation of carbon material dispersions> The following measurements and evaluations were performed on the obtained carbon material dispersions. The results, along with a list of the carbon material dispersions, are shown in Table 1.

[0095] (moisture content) The water content of the carbon material dispersion was determined using a Karl Fischer moisture meter (MKC-710 model: manufactured by Kyoto Electronics Manufacturing Co., Ltd.) equipped with a water vaporizer. The sample was heated to 150°C under a flow of 200 mL / min of nitrogen gas to vaporize the water, which was then collected in an electrolyte solution and subjected to Karl Fischer titration (coulometric titration). The water content (ppm) was calculated by dividing the amount of water obtained by the measurement by the total mass of the carbon material dispersion used for the measurement.

[0096] (Dispersed particle size) The dispersed particle size was determined using a particle size analyzer (Microtrac-Bell, Nanotrac Wave) to obtain the particle size at 50% of the cumulative frequency based on volume. For the measurements, the particle refractive index of the carbon material dispersion was set to 1.8, the shape to be non-spherical, and the concentration of the carbon material dispersion was diluted so that the loading index value was in the range of 0.8 to 1.2. The same solvent used for dilution was used for the dispersion itself.

[0097] (Initial viscosity) The initial viscosity was measured using an E-type viscometer (TV-100 model: manufactured by Toki Sangyo Co., Ltd.) under the conditions of a carbon material dispersion temperature of 25°C and a rotor rotation speed of 50 rpm (mPa·s).

[0098] (dispersion stability) The obtained carbon material dispersion was stored at 40°C for 7 days, and then its viscosity was measured in the same manner as described above for (initial viscosity), and the rate of change from the initial viscosity was calculated. The appearance was also observed visually. Based on the rate of change in viscosity and the appearance results, the following criteria were used for evaluation. A: The rate of change is within 30%, and no separation is observed. B: The rate of change is between 30% and 40%, and no separation is observed. C: The rate of change exceeds 40%, or separation is observed.

[0099] [Table 1]

[0100] The abbreviations used in Table 1 have the following meanings: <Carbon materials> • CNT: Multiwall carbon nanotube K-Nanos-100T (manufactured by KUMUHO PETROCHEMICAL, bulk density 0.1 g / cm³) 3 , average outer diameter 10~15nm, specific surface area 190m 2 / g) AB-1: Acetylene Black Denka Black Li-250 (manufactured by Denka Co., Ltd., bulk density 0.08 g / cm³) 3 , average primary particle diameter 37nm, specific surface area 58m 2 / g) AB-2: Acetylene Black Denka Black Li-400 (manufactured by Denka Co., Ltd., bulk density 0.15 g / cm³) 3 , average primary particle diameter 48nm, specific surface area 39m 2 / g) <Dispersant> • PVB: Polyvinyl butyral Mowital B16H (manufactured by Kuraray Co., Ltd.) • Vinyl chloride-vinyl acetate copolymer resin: Solvine A (manufactured by Nisshin Chemical Co., Ltd.) • ECo: Ethyl Cellulose (manufactured by Nisshin Kasei Co., Ltd.)

[0101] <Fabrication of positive electrode> (Example 2-1) In a plastic container, 4.7 parts of a 15% styrene-based elastomer resin solution dissolved in the same solvent as the carbon material dispersion (butyl butyrate) as the binder, 8.4 parts of electrode active material NMC, 10.5 parts of solid electrolyte LPS, 17.5 parts of carbon material dispersion 1, and 8.9 parts of the same solvent as the carbon material dispersion (butyl butyrate) were measured out. Using a rotation / revolution mixer (Sinky Awatori Rentaro, ARE-310), the mixture was stirred at 2,000 rpm for 3 minutes to obtain a positive electrode slurry. The obtained positive electrode slurry was coated onto a 20 μm thick aluminum foil, which would serve as the current collector, using an applicator, and then dried on a hot plate at 150°C ± 5°C for 25 minutes to obtain a basis weight of 20 mg / cm² per unit area of ​​the electrode. 2The mixture was adjusted to achieve the desired result. Subsequently, a heat press was used to perform a pressurized treatment at 120°C to obtain the positive electrode 1. The carbon material dispersion, resin solution, and solvent used in the electrode slurry were prepared by contacting them with molecular sieves 4A as a dehydrating agent beforehand to reduce their moisture content to 50 ppm or less. The pre-dehydration, electrode slurry preparation, and electrode film preparation were carried out in a glove box maintained in an argon atmosphere with a dew point of -60°C or lower.

[0102] (Examples 2-2 to 2-9, Comparative Examples 2-1 to 2-3) Positive electrodes 2-9 and 14-16 were obtained in the same manner as in Example 2-1, except that carbon material dispersion 1 was replaced with carbon material dispersions 2-9 and 14-16. The same solvent as that used for the carbon material dispersions was used.

[0103] (Examples 2-10) In a plastic container, 4.7 parts of a 15% styrene-based elastomer resin solution dissolved in the same solvent (butyl butyrate) as the carbon material dispersion was measured out. 7.4 parts of NMC (electrode active material), 10.5 parts of LPS (solid electrolyte), 17.5 parts of carbon material dispersion 10, and 9.9 parts of butyl butyrate were then mixed at 2,000 rpm for 3 minutes using a rotation / revolution mixer (Sinky Awatori Rentaro, ARE-310) to obtain a positive electrode slurry. The obtained positive electrode slurry was coated onto a 20 μm thick aluminum foil, which would serve as the current collector, using an applicator. After drying on a hot plate at 150°C ± 5°C for 25 minutes, the basis weight per unit area of ​​the electrode was 20 mg / cm². 2 The mixture was adjusted to achieve the desired result. Subsequently, a heat press was used to perform a pressurized treatment at 120°C to obtain the positive electrode 10. The carbon material dispersion, resin solution, and solvent used in the electrode slurry were prepared by contacting them with molecular sieves 4A as a dehydrating agent beforehand to reduce their moisture content to 50 ppm or less. The pre-dehydration, electrode slurry preparation, and electrode film preparation were carried out in a glove box maintained in an argon atmosphere with a dew point of -60°C or lower.

[0104] (Examples 2-11) In a plastic container, 4.7 parts of a 15% styrene-based elastomer resin solution dissolved in the same solvent (butyl butyrate) as the carbon material dispersion was measured out. 7.4 parts of NMC (electrode active material), 10.5 parts of LPS (solid electrolyte), 11.6 parts of carbon material dispersion 11, and 15.8 parts of butyl butyrate were then mixed at 2,000 rpm for 3 minutes using a rotation / revolution mixer (Sinky Awatori Rentaro, ARE-310) to obtain a positive electrode slurry. The obtained positive electrode slurry was coated onto a 20 μm thick aluminum foil, which would serve as the current collector, using an applicator. After drying on a hot plate at 150°C ± 5°C for 25 minutes, the basis weight per unit area of ​​the electrode was 20 mg / cm². 2 The mixture was adjusted to achieve the desired result. Subsequently, a heat press was used to perform a pressurized treatment at 120°C to obtain the positive electrode 11. The carbon material dispersion, resin solution, and solvent used in the electrode slurry were prepared by contacting them with molecular sieves 4A as a dehydrating agent beforehand to reduce their moisture content to 50 ppm or less. The pre-dehydration, electrode slurry preparation, and electrode film preparation were carried out in a glove box maintained in an argon atmosphere with a dew point of -60°C or lower.

[0105] (Examples 2-12, 2-13, Comparative Example 2-4) Positive electrodes 12, 13, and 17 were obtained in the same manner as in Example 2-11, except that carbon material dispersion 11 was replaced with carbon material dispersions 12, 13, and 17. The same solvent as used for the carbon material dispersions was used.

[0106] <Evaluation of the positive electrode> The following evaluations were performed using the obtained electrodes. The results are shown in Table 2.

[0107] (electrode resistance) The volume resistivity of the positive electrode was measured using the four-probe method with a Rolester GP (manufactured by Nitto Seikou Analytech Co., Ltd.) in accordance with JIS-K7194. For resistance measurement, measurement electrodes were used in which the coating substrate during the manufacture of each electrode was changed from aluminum foil to a PET substrate. The positive electrodes 1-9, 15, and 16 were evaluated based on the following criteria, with the relative value (%) determined using the volume resistivity of positive electrode 14, which was prepared in Comparative Example 2-1, as the reference. S: Less than 50% (Excellent) A: 50% to less than 70% (Good) B: 70% to less than 80% (usable) C: 80% to less than 100% (defective) D: 100% or more (extremely poor)

[0108] The positive electrodes 10-13 were evaluated based on the following criteria, with the relative value (%) determined using the volume resistivity of the electrode film 17 prepared in Comparative Example 2-4 as the reference. S: Less than 50% (Excellent) A: 50% to less than 70% (Good) B: 70% to less than 80% (usable) C: 80% to less than 100% (defective) D: 100% or more (extremely poor)

[0109] (Cycle characteristics) First, the all-solid-state battery evaluation cell was assembled as follows. Positive electrodes 1-17 were punched out to a diameter of φ10 mm to serve as working electrodes. An LPS layer was created on the working electrode by sequentially placing the working electrode and 50 mg of LPS powder into the cylindrical container of the all-solid-state battery evaluation cell and pressurizing it at 50 MPa. On the opposite side of the working electrode, with the LPS layer in between, metallic indium foil and metallic lithium foil were sequentially placed as counter electrodes. Next, the cell was assembled and fixed with bolts, and then tightened using a torque wrench to the specified pressure to obtain positive electrode evaluation cells 1-17. The assembly of the evaluation cells was carried out in a glove box maintained in an argon atmosphere with a dew point of -60°C or lower.

[0110] The fabricated all-solid-state secondary battery positive electrode evaluation cell was placed in a constant temperature room at 25°C, and charge / discharge measurements were performed using a charge / discharge device (Hokuto Denko Co., Ltd., SM-8). Constant current constant voltage charging (cutoff current: 0.02C current) was performed at a charge rate of 0.2C with a charge termination voltage of 4.2V, followed by constant current discharge at a discharge rate of 0.2C with a discharge termination voltage of 2.5V. This operation was repeated 250 times. 1C was defined as the current value required to charge or discharge the theoretical capacity of the positive electrode in 1 hour. The cycle characteristics can be expressed by the ratio of the 0.2C discharge capacity at the 3rd cycle to the 0.2C discharge capacity at the 250th cycle, as shown in Equation 2 below. (Equation 2) Cycle characteristics = 0.2C discharge capacity at 250 cycles / 0.2C discharge capacity at 3 cycles × 100 (%)

[0111] The cycle characteristics of positive electrode evaluation cells 1-9, 15, and 16 were evaluated by calculating relative values ​​(%) based on the cycle characteristics of positive electrode evaluation cell 14, and using the following criteria. S: Over 200% (Excellent) A: Over 150% but under 200% (Good) B: Over 100 and under 150% (usable) C: 100% or less (defective)

[0112] The cycle characteristics of positive electrode evaluation cells 10-13 were evaluated by calculating relative values ​​(%) based on the cycle characteristics of positive electrode evaluation cell 17, according to the following criteria. S: Over 200% (Excellent) A: Over 150% but under 200% (Good) B: Over 100 and under 150% (usable) C: 100% or less (defective)

[0113] [Table 2]

[0114] According to Table 1, the carbon material dispersion obtained by the manufacturing method of the present invention, which includes a step of dispersing a mixture containing a low-polarity solvent, a carbon material, and a dispersant by applying mechanical energy under conditions where the water content is 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less, exhibited low initial viscosity and excellent dispersion stability. This is presumed to be because the low water content in the dispersion made it less susceptible to dispersion inhibition by water, allowing the dispersant to be properly adsorbed onto the carbon material surface and maintain a stable dispersion state. In particular, dispersions with a water content of 600 ppm or less exhibited superior dispersion stability. On the other hand, in Comparative Examples 1-1 and 1-4, it is presumed that the large amount of water present in the dispersion inhibited the adsorption of the dispersant to the carbon material, preventing the formation of a stable dispersion state and resulting in reduced dispersion stability. In Comparative Examples 1-2 and 1-3, it is presumed that sufficient mechanical energy was not applied to dissolve the carbon material with strong cohesive force, preventing the maintenance of a stable dispersion state and resulting in reduced stability.

[0115] According to Table 2, electrodes formed from an electrode slurry containing a carbon material dispersion obtained by the manufacturing method of the present invention showed higher electrode resistance and superior cycle characteristics compared to the comparative example. This is presumed to be because the carbon material contained in the composition maintained its structure, resulting in the uniform formation of conductive paths within the electrode, reducing electrode resistance, and further improving cycle characteristics. In particular, electrodes using a dispersion with a water content of 600 ppm or less showed superior cycle characteristics. Furthermore, electrodes using a dispersion with a dispersion particle size (D50) of 1.0 μm or less showed superior conductivity. From the above, it can be concluded that the carbon material dispersion of the present invention can exhibit high conductivity even at low concentrations. On the other hand, in the comparative example, it is presumed that the formation of conductive paths within the electrode became uneven due to structural collapse caused by aggregation or overdispersion, or excessive aggregation, leading to an increase in electrode resistance and deterioration of cycle characteristics.

Claims

1. A method for producing a carbon dispersion for secondary batteries, comprising a low-polarity solvent, a carbon material, and a dispersant, A method for producing a carbon material dispersion for secondary batteries, comprising the step of applying mechanical energy to a mixture containing a low-polarity solvent, a carbon material, and a dispersant under conditions where the water content is 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less.

2. A method for producing a carbon material dispersion for a secondary battery according to claim 1, wherein the water content is 600 ppm or less.

3. The method for producing a carbon material dispersion for a secondary battery according to claim 1, wherein the low-polarity solvent includes at least one selected from the group consisting of ester solvents having an alkyl group having 4 or more carbon atoms, aromatic hydrocarbon solvents, and aliphatic hydrocarbon solvents.

4. A method for producing a carbon material dispersion for a secondary battery according to claim 1, wherein the carbon material comprises at least one selected from the group consisting of carbon black and fibrous carbon.

5. A method for producing a carbon dispersion for secondary batteries, comprising a low-polarity solvent, a carbon material, and a dispersant, A method for producing a carbon material dispersion for secondary batteries, comprising the steps of: dehydrating a mixture containing a low-polarity solvent, a carbon material, and a dispersant to reduce the water content to 1,000 ppm or less; and applying mechanical energy to the dehydrated mixture to disperse the carbon material so that the dispersed particle size (D50) is 3.0 μm or less.

6. A method for producing a carbon dispersion for secondary batteries, comprising a low-polarity solvent, a carbon material, and a dispersant, A method for producing a carbon material dispersion for secondary batteries, comprising the step of applying mechanical energy to a mixture containing a low-polarity solvent, a carbon material, and a dispersant while dehydrating it to a water content of 1,000 ppm or less, so that the dispersed particle size (D50) of the carbon material is 3.0 μm or less.

7. A method for producing an electrode slurry for an all-solid-state secondary battery, comprising the step of blending an active material, a solid electrolyte, and a binder into a carbon material dispersion for a secondary battery produced by the method for producing a carbon material dispersion for a secondary battery described in any one of claims 1 to 6.

8. A method for manufacturing an electrode for an all-solid-state secondary battery having at least a current collector and an electrode film, comprising the step of applying and drying an electrode slurry for an all-solid-state secondary battery manufactured by the method for manufacturing an electrode slurry for an all-solid-state secondary battery according to claim 7 onto a current collector to form an electrode film.

9. A method for manufacturing an all-solid-state secondary battery having at least a positive electrode, a negative electrode, and a separator layer, comprising the step of using an electrode for an all-solid-state secondary battery manufactured by the method for manufacturing an electrode for an all-solid-state secondary battery described in claim 8, in addition to the positive electrode and the negative electrode.

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