Method for providing conductive material composite particles

Abstract

To provide a conductive material and a method for producing the same, the conductive material being in a dried powder form effective in reducing environmental impact and improving long-term stability, and at the same time having high dispersibility and uniformity in an electrode coating that achieves improvement in battery performance.SOLUTION: Conductive material composite particles are provided, the particles containing at least a conductive material and a dispersant, wherein the particles have a particle size distribution D50 of 20 μm or more and a sieved particle size of 150 μm or less, the conductive material has a DBP oil absorption of 550 ml / 100 g or less, and the dispersant is contained in an amount of 1 to 10 pts.wt. relative to 100 pts.wt. of the conductive material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing conductive material composite particles that have high dispersibility when mixed with an electrode active material and a binder (binding agent), high uniformity in an electrode coating film, do not contain coarse particles or foreign matter that could cause a short circuit during operation, and have been freed from dispersion media such as water and organic solvents, and can be suitably used for electricity storage devices such as batteries and capacitors. [Background technology]

[0002] Current lithium-ion secondary batteries use positive and negative electrodes in which electrode active material is applied to strip-shaped metal foil, which are then wound together with a separator and housed in a battery can. Of these, the positive electrode uses a lithium transition metal composite oxide or the like as the electrode active material. Because these electrode active materials used in the positive electrode alone have poor electronic conductivity, i.e., electrical conductivity, a carbon material such as conductive carbon black, which has a highly developed structure, or graphite, whose crystals show significant anisotropy, is added as a conductive material to impart electrical conductivity, and this is dispersed together with a binder in a non-aqueous solvent such as N-methyl-2-pyrrolidone to prepare an electrode paste, which is then applied to a metal foil and dried to form an electrode coating, thereby producing the positive electrode. In the negative electrode, low-conductivity materials such as silicon and carbon-based materials such as graphite are mainly used as electrode active materials. Although carbon-based materials are conductive, depending on their size, gaps may form between the active materials, which may not be enough to form an electron conduction path by themselves. Therefore, as with the positive electrode, it is effective to use a conductive material to improve the conductivity. As such, conductive materials play an important role in lithium-ion secondary batteries. To improve battery performance, such as high charge / discharge capacity and long-term stability of charge / discharge ability, conductive materials with high dispersibility and uniformity are required, which can disperse easily and spread evenly around the active material to impart conductivity.

[0003] However, the conductive carbon materials primarily used as conductive materials are fine powders with small primary particle diameters, which have strong cohesion and are extremely difficult to disperse uniformly. Therefore, when mixing them with materials such as electrode active materials to prepare electrode pastes, the conductive material must be thoroughly loosened by prolonged stirring or the like, otherwise it will be unevenly distributed within the electrode coating. This results in localized areas of poor conductivity within the positive electrode plate, which prevents sufficient electron movement and ineffective utilization of the electrode active material, ultimately resulting in low charge / discharge capacity, a drawback of which has been cited (Patent Document 1).

[0004] Therefore, conductive material dispersion pastes have been proposed in which a conductive material is dispersed in a solvent in advance to form a liquid or paste-like conductive material, thereby improving dispersibility (Patent Documents 2 to 10, etc.). Furthermore, with the aim of reducing environmental impact, a technology has been proposed in which a conductive material dispersion paste is prepared and then the solvent is removed by drying to produce conductive fine particles with reduced organic solvent usage, and this powder is used to produce lithium ion secondary batteries (Patent Document 11). Additionally, Patent Document 12 proposes a powdered carbon black composition prepared by filtering and drying the prepared conductive material dispersion paste to remove the solvent, followed by crushing in an agate mortar and passing the resulting powder through a sieve with an opening diameter of 45 μm, and a lithium ion secondary battery electrode using this carbon black composition. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-308845 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-70908 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-113821 [Patent Document 4] Patent No. 4235788 [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-238575 [Patent Document 6] Japanese Patent Application Laid-Open No. 2011-192020 [Patent Document 7] Japanese Patent Application Laid-Open No. 2007-335175 [Patent Document 8] Japanese Patent Application Laid-Open No. 2004-281096 [Patent Document 9] Japanese Patent Application Laid-Open No. 2009-252683 [Patent Document 10] WO2014 / 042266 [Patent Document 11] Japanese Patent Application Laid-Open No. 2012-9227 [Patent Document 12] Japanese Patent Application Publication No. 2018-62545 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the techniques for preparing conductive material dispersion pastes, as exemplified by Patent Documents 2 to 10, have drawbacks, such as the need to use large amounts of environmentally hazardous organic solvents during production and poor long-term stability due to sedimentation and re-aggregation. Furthermore, the technique disclosed in Patent Document 11 succeeds in reducing the environmental impact, improving long-term stability, and improving handling, such as storage and transportation, by removing the solvent to produce a powder. However, the conductive material is not uniform enough within the electrode paste and electrode coating, and the electronic conductivity imparted to the active material is insufficient, preventing full utilization of its capabilities, limiting its effectiveness in improving battery performance. The inventors' investigations of the carbon black composition disclosed in Patent Document 12 also revealed that the electronic conductivity imparted to the active material is insufficient, and lithium-ion secondary batteries using this carbon black composition do not exhibit sufficient battery performance.

[0007] The object of the present invention is to solve the problems found in the prior art and overcome the challenges, namely to provide a new conductive material that is in the form of a dry powder that is effective in reducing environmental impact and improving long-term stability, yet has high dispersibility and uniformity in an electrode coating film that improves battery performance, and a method for producing the same. [Means for solving the problem]

[0008] As a result of extensive research, the inventors have discovered that the above-mentioned problems can be solved by producing conductive material composite particles that contain at least a conductive material and a dispersant and exhibit predetermined physical properties, and have thus arrived at the present invention.

[0009] That is, the present invention is (1) Conductive material composite particles containing at least a conductive material and a dispersant, characterized in that the particles have a particle size distribution D50 of 15 μm or more and a particle diameter determined by sieving of 150 μm or less, the DBP oil absorption of the conductive material is 550 ml / 100 g or less, and the dispersant is contained in an amount of 1 to 10 parts by weight per 100 parts by weight of the conductive material. (2) Conductive material composite particles containing at least a conductive material and a dispersant, characterized in that the particles have a particle size distribution D50 of 15 μm or more and an upper particle diameter limit of 300 μm or less, the DBP oil absorption of the conductive material is 550 ml / 100 g or less, and the dispersant is contained in an amount of 1 to 10 parts by weight per 100 parts by weight of the conductive material. (3) The conductive material composite particles according to either (1) or (2) above, characterized in that the OD value in dispersion evaluation is 3.0 or more. (4) The conductive material composite particles according to (1) or (2) above, characterized in that the dispersant is a nonionic dispersant. (5) The conductive material composite particles according to (4) above, characterized in that the weight average molecular weight of the nonionic dispersant is 1,000 to 1,000,000. (6) The conductive material composite particles according to (1) or (2) above, characterized in that the purity of the conductive material is 99.9% or more. (7) The conductive material composite particles according to (1) or (2) above, characterized in that the average primary particle diameter of the conductive material is 10 nm or more and 50 nm or less. (8) The conductive material composite particles according to (1) or (2) above, which are a conductive material for battery electrodes.

[0010] (9) A method for producing conductive material composite particles, comprising the steps of preparing a conductive material dispersion paste containing at least a conductive material, a dispersant, and a dispersion medium, and removing the dispersion medium from the conductive material dispersion paste, wherein the particle diameter of the conductive material composite particles in the conductive material dispersion paste is 50 μm or less. (10) The method for producing conductive material composite particles according to (9) above, wherein the conductive material dispersion paste does not contain foreign matter having a particle diameter exceeding 50 μm. (11) The method for producing conductive material composite particles according to (9) or (10), characterized in that it includes a drying step of heating the conductive material dispersion paste at 80°C or higher and 300°C or lower. (12) A method for producing an electrode, comprising mixing the conductive material composite particles according to (1) or (2) above with at least an active material and a binder, and applying the mixture to a substrate. (13) A method for producing an electrode, comprising mixing the conductive material composite particles obtained by the method according to (9) or (10) above with at least an active material and a binder, and applying the mixture to a substrate. (14) A lithium ion secondary battery using an electrode obtained by the method according to (12) above. (15) A power storage device using the conductive material composite particles according to (1) or (2) above. (16) An electricity storage device using an electrode obtained by the method according to (12) above. is located. [Effects of the Invention]

[0011] The conductive material composite particles obtained by the present invention have a particle size distribution adjusted so that secondary agglomerations are easily broken down during the kneading process when preparing an electrode paste, and because the particles contain a dispersant, they exhibit high dispersibility in solvents. Therefore, long periods of kneading are not required when preparing an electrode paste, and there is no risk of a decrease in conductivity due to re-agglomeration caused by excessive micronization. Furthermore, the conductive material composite particles of the present invention easily form conductive paths within the electrode coating film, resulting in excellent conductivity and dramatically improving battery performance such as charge / discharge capacity. In addition, because coarse particles and foreign matter are removed, the conductive material is dispersed without uneven distribution within the paste, and is distributed uniformly around the active material, providing sufficient electronic conductivity, enabling the formation of an electrode coating film that achieves excellent charge / discharge capacity. Furthermore, because localized current flow and short circuits caused by uneven distribution of coarse particles and conductive material are less likely to occur, problems such as thermal runaway and early decline in charge / discharge capacity of the battery are suppressed, and battery performance is thought to be maintained over a long period of time. Furthermore, the conductive material composite particles obtained by the present invention are dry powders that are substantially free of solvents, and therefore, compared to liquid or paste-like conductive materials, there is no risk of deterioration over time due to sedimentation or solidification, and they are stable enough to be stored at room temperature for long periods of time.In addition, the amount of organic solvent used during production can be reduced, thereby mitigating the burden on the environment.

[0012] Therefore, according to the present invention, conductive material composite particles with excellent long-term stability can be produced while minimizing the burden on the environment. Furthermore, by using the conductive material composite particles of the present invention as an electrode paste to produce positive and negative electrode plates, a lithium ion secondary battery with excellent battery performance, such as high charge / discharge capacity and long-term stability of charge / discharge ability, can be manufactured. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a flowchart illustrating the method for producing the conductive material composite particles of the present invention. [Figure 2] FIG. 2 is a view showing an SEM (scanning electron microscope) photograph of a cross section of the coating film produced in Example 1. [Figure 3] FIG. 3 is a diagram showing an EDS (energy dispersive X-ray spectrometry) analysis image of the cross section of the coating film produced in Example 1. [Figure 4] FIG. 4 is a view showing an SEM photograph of a cross section of the coating film produced in Example 2. [Figure 5] FIG. 5 is a diagram showing an EDS analysis image of a cross section of the coating film produced in Example 2. [Figure 6] FIG. 6 is a view showing an SEM photograph of a cross section of the coating film produced in Example 3. [Figure 7] FIG. 7 is a diagram showing an EDS analysis image of a cross section of the coating film produced in Example 3. [Figure 8] FIG. 8 is a view showing an SEM photograph of a cross section of the coating film produced in Example 4. [Figure 9] FIG. 9 is a diagram showing an EDS analysis image of a cross section of the coating film produced in Example 4. [Figure 10] FIG. 10 is a view showing an SEM photograph of a cross section of the coating film produced in Comparative Example 1. [Figure 11] FIG. 11 is a diagram showing an EDS analysis image of a cross section of the coating film produced in Comparative Example 1. [Figure 12] FIG. 12 is a view showing an SEM photograph of a cross section of the coating film produced in Comparative Example 2. [Figure 13] FIG. 13 is a diagram showing an EDS analysis image of a cross section of the coating film produced in Comparative Example 2. [Figure 14] FIG. 14 is a view showing an SEM photograph of a cross section of the coating film produced in Comparative Example 3. [Figure 15] FIG. 15 is a diagram showing an EDS analysis image of a cross section of the coating film produced in Comparative Example 3. [Figure 16] FIG. 16 is a view showing an SEM photograph of a cross section of the coating film produced in Comparative Example 4. [Figure 17] FIG. 17 is a diagram showing an EDS analysis image of a cross section of the coating film produced in Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0014] The conductive material composite particles of the present invention refer to particles containing at least a conductive material and a dispersant.

[0015] [Conductive material] Suitable conductive materials for use in the present invention include carbon black, carbon nanotubes, carbon nanofibers, graphite, graphene, and hard carbon. Among these, carbon black is preferred because its structure efficiently forms conductive paths within the electrode, improving conductivity. Examples of carbon black that can be used include ketjen black, furnace black, acetylene black, and thermal black. Acetylene black is particularly preferred because of its high conductivity, low impurity content, and excellent heavy overload characteristics. These conductive materials can be used alone or in combination of two or more.

[0016] The average primary particle size of the conductive material is preferably 10 nm or more and 50 nm or less, more preferably 45 nm or less, and even more preferably 40 nm or less. Also, it is more preferably 10 nm or more, and even more preferably 15 nm or more. If the average primary particle size of the conductive material is greater than 50 nm, the conductivity of the coating film obtained from the electrode paste may decrease. Furthermore, if it is less than 10 nm, the viscosity of the conductive material dispersion paste and the electrode paste may increase, which may make it difficult to disperse the conductive material depending on the equipment used, and may also deteriorate handling properties. The average primary particle size referred to here refers to the arithmetic mean particle size measured using a transmission electron microscope in accordance with ASTM: D3849-14.

[0017] The DBP oil absorption of the conductive material is characterized by being 550 ml / 100 g or less, preferably 170 to 240 ml / 100 g, and most preferably 170 to 230 ml / 100 g. If the DBP oil absorption of the conductive material is greater than 550 ml / g, the viscosity of the conductive material dispersion paste and electrode paste increases, making dispersion difficult and leading to uneven distribution of the conductive material, which may result in insufficient conductivity. The DBP oil absorption of the conductive material may be measured in accordance with JIS6217-4.

[0018] The purity of the conductive material is preferably 99.9% or more, and more preferably 99.95 to 100% by mass. By keeping the purity of the conductive material within the above range, the occurrence of battery short circuits due to impurities can be prevented, and the defective rate can be reduced. The purity of carbon black can be calculated based on the amount of impurities, with the ash content measured in accordance with JIS K1469 or JIS K6218 being considered as an impurity.

[0019] Specific examples of suitable conductive materials that satisfy the above conditions include carbon black such as Denka Black powder, Denka Black granules, Denka Black FX-35, Denka Black HS-100, Denka Black Li Li-100, Denka Black Li Li-250, Denka Black Li Li-400, and Denka Black Li Li-435 (all trade names, manufactured by Denka Company Ltd.), LITX 50, LITX 66, LITX 60R, LITX 200, LITX 300, and LITX-HP (all trade names, manufactured by Cabot Specialty Chemicals, Inc.), and SUPER P Li, C-NERGY SUPER C45, and C-NERGY SUPER C65 (all trade names, manufactured by Imerys Graphite & Carbon Co.). Among these, preferred are Denka Black powder, Denka Black granules, Denka Black FX-35, Denka Black HS-100, Denka Black Li Li-100, Denka Black Li Li-250, Denka Black Li Li-400, and Denka Black Li Li-435, more preferred are Denka Black granules, Denka Black FX-35, Denka Black Li Li-100, and Denka Black Li Li-435, and particularly preferred are Denka Black granules and Denka Black FX-35.

[0020] [Dispersant] The dispersant referred to in the present invention refers to an additive used to uniformly disperse inorganic or organic pigments in a dispersion medium to prepare a stable dispersion. Dispersants are broadly classified into anionic, cationic, nonionic, and amphoteric dispersants according to their ionicity. Any dispersant can be used in the present invention as long as it prevents re-aggregation of conductive particles after loosening the agglomeration of the conductive material and uniformly distributes the conductive particles in the solvent. Among these, nonionic dispersants without ionic functional groups are suitable because they do not inhibit the migration of lithium ions. As the nonionic dispersant, one that acts as a binder after film formation and does not affect the electrical properties, or one that has a low decomposition temperature so that it can be removed by heating during electrode fabrication, is preferably used. Dispersants with these characteristics include polyvinylpyrrolidone, polyvinyl butyral, polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, polyvinyl alcohol, polyvinyl acetal, polyvinyl ether, polyether, polyhydric alcohol ester, cellulose acetate, cellulose acetate butyrate, methylcellulose, ethylcellulose, hydroxyethylcellulose, ethylhydroxyethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose. Among these, methylcellulose or polyvinylpyrrolidone is most preferred.

[0021] The nonionic dispersant preferably has a weight-average molecular weight of 1,000 or more and 1,000,000 or less. It is more preferably 5,000 or more and 300,000 or less, and even more preferably 5,000 or more and 200,000 or less. If the weight-average molecular weight exceeds 1,000,000, the viscosity of the conductive material dispersion paste increases, resulting in a loss of fluidity and poor dischargeability, which deteriorates handleability. On the other hand, if the weight-average molecular weight is less than 1,000, dispersibility is poor, making it difficult to produce a conductive material dispersion paste. These nonionic dispersants can be used alone or in combination of two or more types. The weight-average molecular weight can be measured using gel permeation chromatography (GPC). Examples of measurement conditions for gel permeation chromatography are shown below. Apparatus: High-performance liquid chromatograph (Shimadzu Corporation, Prominence) Column: Shodex OHpakSB-802.5HQ Shodex Corporation, OHpakSB-804HQ Detector: RI Eluent: 0.5M NaCl aqueous solution Flow rate: 1.0ml / min Sample concentration: 0.2 wt / vol% Column temperature: 40℃

[0022] [Method of manufacturing composite particles] Although the method for producing the composite particles of the present invention is not particularly limited, as an example of a preferred embodiment disclosed herein, the above materials are mixed with a dispersion medium and an additive, the mixture is subjected to a wet grinding process to produce a conductive material dispersion paste, and the dispersion medium of the conductive material dispersion paste is removed by drying to produce the conductive material composite particles. Note that the conductive material dispersion paste in the present invention refers to a state in which the conductive material is disintegrated in a liquid dispersion medium and is uniformly stabilized.

[0023] [Dispersion medium] The dispersion medium used in the conductive material dispersion paste is not particularly limited as long as it can disperse the conductive material and can be removed in the subsequent drying step, but it is preferable to use a dispersion medium that can uniformly dissolve the dispersant used. Examples of solvents that can uniformly dissolve the dispersant used in the present invention include water, methanol, ethanol, N-methyl-2-pyrrolidone, and methyl ethyl ketone. When used for lithium ion secondary batteries, water or N-methyl-2-pyrrolidone is generally selected, but water is preferred from the perspectives of safety and convenience in the subsequent drying step and environmental considerations. The amount of the dispersion medium is preferably in the range of 99.0 to 50.0% by mass, more preferably 99.0 to 60.0% by mass, and even more preferably 99.0 to 70.0% by mass. If the amount of the dispersion medium is less than 50% by mass, the conductive material dispersion paste will have poor fluidity, and it may be difficult to achieve the maximum particle size and viscosity within the preferred ranges in the wet grinding process.

[0024] [Additives] When preparing the conductive material dispersion paste, additives can be selected and added as needed. Examples of additives include pH adjusters, binders, solvents, thickeners, antifoaming agents, surfactants, preservatives, and antifungal agents. Any amount of additives may be added depending on the performance requirements of the electrode, the amount of dispersion medium, and the like, as long as the effects of the present invention are not impaired.

[0025] Examples of pH adjusters include potassium hydroxide, sodium hydroxide, and triethanolamine. The pH adjusters may be contained alone or in combination of two or more. The content of the pH adjuster can be appropriately adjusted according to the desired pH.

[0026] Examples of binders include water-soluble polymers and emulsion resins. The binder may be natural, semi-synthetic, or synthetic. Specifically, cellulose, starch, and modified forms thereof, natural rubber, rosin and modified forms thereof, polyvinyl alcohols, acrylic resins, epoxy resins, urethane resins, melamine resins, and the like may be used as binders. The binders may be contained alone or in combination of two or more.

[0027] In order to adjust the drying property and the film-forming property of the coating film, a solvent may be added as an additive. The solvent here is a substance different from the dispersion medium used for the wet grinding treatment, and refers to the following solvents used as adjusters: Examples of the solvent that may be used include alcohols, alkyl ether alcohols, glycols, and diols. Examples of the alcohols include methanol, ethanol, and isopropyl alcohol. Examples of the alkyl ether alcohols include ethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and propylene glycol monobutyl ether. Examples of the glycols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, and polyethylene glycol having a number average molecular weight of 2,000 or less. Examples of the diols include glycerin. The above solvents may be contained alone or in combination of two or more.

[0028] Examples of thickeners include natural polysaccharides and synthetic polymer thickeners. Examples of natural polysaccharides include guar gum, locust bean gum, galactomannan, pectin and its derivatives, psyllium pseudogum, tamarind gum, microbial xanthan gum, rheosan gum, rhamsan gum, welan gum, gellan gum, carrageenan, a seaweed polysaccharide, alginic acid and its derivatives, talagant gum, a resin polysaccharide, and cellulose and its derivatives. Examples of synthetic polymer thickeners include water-based surfactant-emulsified emulsions, water-based self-emulsifying emulsions, and water-based core-shell emulsions of polyacrylic acid and its crosslinked copolymers, polyvinyl alcohol, polyvinylpyrrolidone and its derivatives, polyvinyl methyl ether and its derivatives, polyether acrylic resins, and silicone acrylic resins. The above thickeners may be used alone or in combination.

[0029] [Mixing and stirring of materials] First, the dispersant is weighed so that the weight ratio to the conductive material is as described below, and added to the dispersion medium and stirred to dissolve thoroughly. If an additive is added, it is added simultaneously with the dispersant to an extent that does not inhibit the function of each material depending on the application. There are no particular restrictions on the method for stirring the materials, and commercially available stirrers, kneaders, mixers, etc. can be used. Next, the conductive material is added to this dispersant solution and stirred to obtain a conductive material mixture.

[0030] [Wet grinding process] The conductive material mixture is subjected to a wet-pulverization process, and is pulverized and dispersed until the desired viscosity and maximum particle size are achieved, thereby obtaining a desired conductive material dispersion paste, as described below. Commercially available wet-pulverizing and wet-dispersing devices can be used for the wet-pulverization process. There are no limitations on the type or type of device, as long as it is capable of pulverizing to the desired viscosity and maximum particle size. Examples of devices that can be selected include wet-media dispersers such as ball mills, sand grinders, dyno mills, spike mills, DCP mills, basket mills, and paint conditioners, as well as medialess dispersers such as nanomizers, ultimizers, ultrasonic dispersers, thin-film rotary high-speed mixers, roll mills, colloid mills, high-pressure dispersers, homogenizers, and in-line mixers.

[0031] The particle size of the carbon black in the conductive material dispersion paste is preferably 50 μm or less, more preferably less than 50 μm, more preferably 40 μm or less, and particularly preferably 30 μm or less, as the maximum particle size. If the maximum particle size of the paste exceeds 50 μm, the distribution of the active material and conductive material in the battery electrode coating may become uneven, which may impair battery performance. The maximum particle size can be measured using a grind gauge in accordance with JIS K5600-2-5.

[0032] The viscosity of the conductive material dispersion paste is preferably 3000 mPa s or less, more preferably 2000 mPa s or less, and even more preferably 1000 mPa s or less. By keeping the viscosity of the paste within the above range, the fluidity is improved, and handling properties such as delivery and discharge are improved.

[0033] [Foreign matter removal process] Because measuring the maximum particle size using a grind gauge cannot completely remove particles larger than 50 μm, it is preferable to include a step of removing foreign matter and coarse particles larger than 50 μm after preparing the conductive material dispersion paste. Here, "foreign matter" refers to any substance, including conductive material, larger than 50 μm contained in the dispersion paste. Removing foreign matter and coarse particles prevents coarse particles from contacting each other on the electrode coating when used as an electrode paste, forming a short circuit, thereby reducing the risk of internal short circuits in the battery. While the method for removing foreign matter is not particularly limited, foreign matter larger than a certain size can be removed by pumping the conductive material dispersion paste using a pump or the like and passing it through a filter or magnetic separator installed along the path.

[0034] [Dispersion medium removal process] The conductive material dispersion paste prepared by the above method is dried to remove part or all of the dispersion medium, and the conductive material composite particles of the present invention are prepared by granulating the paste. The method for removing the dispersion medium is not particularly limited, and commercially available dryers such as freeze dryers, spray dryers, flash dryers, thermal dryers, and fluidized bed dryers can be used.

[0035] [Spray drying] Among these drying methods, spray drying is particularly preferred because it allows particle formation and particle size adjustment simultaneously with drying, thereby shortening the process and simplifying the equipment. Various spray dryers can be used for the spray drying method, including centrifugal atomization and spray atomization, but any method can be used as long as it involves spraying a liquid or a mixture of a solid and a liquid into a gas and drying it. When using a spray dryer, the droplet size can be reduced by increasing the amount of conductive material dispersion paste (material), the amount of compressed air supplied, or the disk rotation speed. This operation allows the particle size after drying to be adjusted to within the specified range described below, thereby obtaining the conductive material composite particles of the present invention.

[0036] [Temperature adjustment during drying] When drying the conductive material dispersion paste, increasing the heating temperature can increase the drying speed and further reduce the moisture content after drying. The heating temperature during drying can be finely adjusted as desired based on the temperature at which each material dries, but for all conductive materials, if the temperature is below 80°C, moisture is likely to remain in the powder after drying, and if it exceeds 300°C, the material is likely to deteriorate and decompose. Therefore, a temperature of 80 to 300°C is preferable, and a temperature of 100 to 150°C is more preferable.

[0037] [Moisture content after drying] When removing the dispersant, the moisture content of the conductive composite particles after drying is preferably 4% by weight or less, and more preferably 1% by weight or less. Residual dispersant, such as water, in the conductive composite particles reduces the solubility of polyvinylidene fluoride, the most commonly used binder in electrode paste production. This prevents the formation of a uniform coating, resulting in reduced strength within the composite layer and reduced adhesion between the composite layer and the metal foil. Furthermore, when used in lithium-ion secondary batteries, the remaining water decomposes during the initial charge, generating hydrogen and oxygen that can cause deterioration of the battery's internal components. If the moisture content exceeds 4% by weight, these effects become significant, potentially resulting in reduced battery performance. After drying, the moisture content is measured, and if a high moisture content is found, the drying time is extended until the moisture content drops below the specified value. Moisture content can be measured using a commercially available moisture meter, such as a halogen lamp-heated moisture meter (MA-120, manufactured by Shinko Denshi Co., Ltd.).

[0038] [Crushing process] When a drying method other than spray drying is used in the dispersion medium removal step, the dried conductive material dispersion paste is pulverized until a powder having a predetermined particle size described below is obtained. The pulverization method is not particularly limited, and a commonly used method such as a hammer mill, crusher, mixer, mortar, or ball mill can be suitably selected depending on the equipment conditions and the production amount.

[0039] [Sifting] The conductive material composite particles from which the dispersion medium has been removed can be classified by sieving to obtain conductive material composite particles having a specific particle size range. The screening procedure is not particularly limited, but can be carried out as follows. Several sieves with different nominal mesh sizes conforming to JIS8801-1 are stacked in order with the larger mesh size on top, and the conductive composite particles are placed on the top sieve. After that, the sieve is shaken for 30 minutes in an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to classify the conductive composite particles by particle size. By removing the particles that have passed through the desired mesh size and the particles that have not, conductive composite particles classified to the specified particle size can be obtained.

[0040] The conductive material composite particles obtained by these operations have the following characteristics and preferable properties.

[0041] The conductive material composite particles of the present invention are characterized by a particle size distribution D50 of 15 μm or more, preferably 20 μm or more, and particularly preferably 25 μm or more. As a result of the inventors' investigations, it was found that when the particle size distribution D50 is smaller than 15 μm, the battery charge / discharge capacity and cycle characteristics are reduced. It was also found that when mixed with the active material to prepare an electrode paste, kneading for 10 minutes or more significantly reduces the electrical conductivity. Although the mechanism is not completely clear, the inventors speculate as follows. As mentioned above, conductive materials connect the electrode and active material, and between active materials themselves, within the electrode coating, forming conductive paths, thereby contributing to improved conductivity and battery performance. Here, when kneading the electrode paste, a force is also applied to the conductive material, but the effect of this force varies depending on whether the particle size is large or small. Specifically, for example, when the conductive material is carbon black, carbon black generally forms a structure in which primary particles are connected by chemical bonds, and these structures further aggregate together to form secondary aggregates (agglomerates) of tens to hundreds of microns in size through van der Waals bonds or simple adhesion or entanglement. Even within the conductive material composite particles, carbon black exists primarily in the form of secondary aggregates. When force is applied to the composite particles during kneading during electrode paste preparation, this force naturally also acts on the secondary aggregates within the composite particles. However, in the case of large particles, most of the force is used to break up the secondary aggregates, and after breaking up the aggregates, it is not used to further break the bonds between the structures. Therefore, even after kneading, the bonds between the structures are maintained to a certain extent when dispersed within the paste, facilitating the formation of conductive paths and improving conductivity and battery performance. However, in the case of small particles, secondary agglomerates do not develop much and do not require a large force to break them up, so it is thought that the force applied by kneading is also used to break the bonds between the structures after breaking up the agglomerations.As a result, single structures with broken bonds or small agglomerates with only a few structures bonded together are dispersed in the paste, making it difficult to form conductive paths and preventing high conductivity and battery performance.

[0042] As a result of the above-mentioned action, the bonds between structures can be maintained even after forming an electrode paste, and as an index showing the range of particle diameters that can exhibit high conductivity and battery performance, in addition to particle size distribution D50, particle size distribution D10, particle size distribution D90, and average particle diameter can also be used. When particle size distribution D10 is used as an index, it is preferably 10 μm or more, and particularly preferably 15 μm or more. When particle size distribution D90 is used as an index, it is preferably 30 μm or more, and particularly preferably 40 μm or more.

[0043] In the present invention, particle size distributions D10, D50, and D90 refer to values ​​corresponding to particle diameters at which the cumulative volume of a cumulative particle size distribution curve obtained by photographing a dispersed particle group using a scanning electron microscope and measuring the maximum diameter passing through the center point of each particle is 10%, 50%, and 90% for D10, D50, and D90, respectively, when plotting the curve from the smaller diameter side. The particle size distributions D10, D50 and D90 of the conductive material composite particles can be measured by the following method, but the measurement method is not limited to this as long as the same results are obtained. First, a conductive sample stage is used to prevent static electricity, and conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, a scanning electron microscope (FlexSEM1000, manufactured by Hitachi High-Tech Corporation) and AZtec (particle analysis software, manufactured by Oxford Instruments) are used to analyze more than 300 particles, and the diameter can be calculated from the numerical value.

[0044] In the first invention of the present application, the conductive material composite particles preferably have a particle size determined by sieving of 150 μm or less, more preferably 100 μm or less, and particularly preferably 75 μm or less. By sieving the particles to a size of 150 μm or less, coarse particles are removed, improving battery performance. If the size of the particles sieved is larger than 150 μm, the conductive material will be unevenly distributed in the electrode coating, and sufficient electronic conductivity will not be imparted to the active material. The particle size determined by sieving here refers to the smallest nominal opening of a sieve conforming to JIS 8801-1 through which the target particles can pass. There are no particular limitations on the sieving operation as long as the sieve conforms to JIS 8801-1, but by using the method disclosed in 0039, particles can be passed through sieves of different sizes in a single measurement, and particle size measurement, particle classification, and extraction of particles of the desired particle size can be performed simultaneously.

[0045] In the second invention of the present application, the conductive material composite particles are characterized in that the upper limit of particle diameter is 300 μm or less. Preferably, the upper limit of particle diameter is 200 μm or less, and particularly preferably, 100 μm or less. By setting the upper limit of particle diameter to 300 μm or less, coarse particles are removed, and the composite particles can obtain sufficient dispersibility. As a result, when the conductive material is made into an electrode paste and applied, it is uniformly dispersed in the paste and the electrode coating, which can impart sufficient conductivity to the active material and improve the charge / discharge capacity of the battery. If the upper limit of particle diameter is greater than 300 μm, the composite particles are too large compared to the size of common active materials, which is several μm to several tens of μm. Therefore, the composite particles cannot obtain sufficient dispersibility during kneading when making the electrode paste, resulting in uneven distribution of the conductive material in the electrode coating, and insufficient electronic conductivity being imparted to the active material. In the present invention, the upper limit of particle size refers to a measurement value determined by the following method, however, the measurement method is not limited to this as long as the same result is obtained. First, a conductive sample stage is used to prevent static buildup, and the conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, a scanning electron microscope (FlexSEM1000, Hitachi High-Tech Corporation) and AZtec (particle analysis software, Oxford Instruments) are used to measure the maximum diameter passing through the center of approximately 300 particles, and the particle diameter can be calculated as the largest of these measurements.

[0046] The conductive material composite particles of the present invention preferably have an average particle size of 20 μm to 80 μm, more preferably 25 μm to 60 μm, and most preferably 25 μm to 40 μm. By setting the average particle diameter within the above range, uneven distribution of the conductive material is less likely to occur and handling is improved. Furthermore, the aforementioned action facilitates the formation of conductive paths, contributing to improved conductivity and battery performance. The average particle diameter here is the arithmetic mean of the largest diameters passing through the center of each particle, and can be determined as follows, but the measurement method is not limited to this as long as the same results are obtained. First, a conductive sample stage is used to prevent static buildup, and conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, a scanning electron microscope (FlexSEM1000, Hitachi High-Tech Corporation) and AZtec (particle analysis software, Oxford Instruments) are used to analyze more than 300 particles, and the diameter can be calculated from the diameter values.

[0047] The conductive material composite particles of the present invention may contain 9% or more particles having an aspect ratio of less than 1.2. Compared to particles containing less than 9% of particles having an aspect ratio of less than 1.2, such conductive material composite particles have excellent handling properties during powder transfer, storage, etc., and excellent dispersibility when made into a conductive paste. Furthermore, the conductive material composite particles may contain 55% or more particles having an aspect ratio of less than 1.2 and 85% or more particles having an aspect ratio of less than 1.5. Particles containing the above proportions of particles having aspect ratios of less than 1.2 and less than 1.5 are highly preferred because they not only have particularly improved dispersibility when made into an electrode paste, but also improved handling properties during powder transfer, storage, etc. In particular, by forming particles by the aforementioned spray drying, conductive material composite particles having an aspect ratio close to 1 and excellent handleability and dispersibility can be obtained. The aspect ratio of the conductive composite particles of the present invention refers to the ratio (a / b) of the maximum diameter (a) passing through the center point of each particle to the minimum diameter (b), and can be determined as follows, but the measurement method is not limited to this as long as the same result is obtained. First, a conductive sample stage is used to prevent static buildup, and the conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, a scanning electron microscope (FlexSEM1000, Hitachi High-Tech Corporation) and AZtec (particle analysis software, Oxford Instruments) are used to measure the aspect ratio of each particle, which can be calculated from measurements of more than 300 particles. By using the materials and manufacturing methods described above, the aspect ratio of the conductive composite particles can be set within the above range, but by using spray drying in particular during drying, it is possible to produce more particles with an aspect ratio close to 1.

[0048] The conductive material composite particles of the present invention preferably have an angle of repose of less than 45°, more preferably less than 38°. By setting the angle of repose within this range, the fluidity is good, and since crosslinking does not occur during storage and stirring or shaking before use is not required, handling during production and use is excellent. The angle of repose referred to here can be determined in accordance with the measurement method specified in Japanese Industrial Standard JIS 9301-2-:2:1999 by gently piling powder on a horizontal surface and reading the angle between the naturally formed slope of the pile and the horizontal plane, but the measurement method is not limited to this as long as the same result is obtained.

[0049] The conductive material composite particles are characterized by containing 1 to 10 parts by weight of a dispersant per 100 parts by weight of the conductive material. The dispersant content is more preferably 4 to 10 parts by weight, and most preferably 6 to 10 parts by weight. By using a dispersant, the conductive material can be uniformly crushed and dispersed in the dispersion medium during the wet-milling process. Furthermore, the presence of the dispersant in the conductive material composite particles after drying improves their dispersibility in the solvent. If the dispersant content is less than 1 part by weight, there is a problem that the dispersant is not sufficiently effective in dispersing the particles in the dispersion medium during the wet-milling process. Furthermore, if the dispersant content exceeds 10 parts by weight, there is a problem that the resistance value increases, which can adversely affect battery performance when the particles are used as an electrode coating. With the manufacturing method disclosed above, the decomposition temperature of the dispersant is not exceeded, and even after the material mixing, wet dispersion, dispersant removal, and pulverization processes, the dispersant is hardly removed, or is removed while being coated by the conductive material, maintaining its ratio to the conductive material.Therefore, by keeping the weight ratio of the dispersant added during material mixing and stirring within the above-mentioned range, the weight ratio in the conductive material composite particles can also be kept within that range.

[0050] However, since the dispersant ratio may change depending on the equipment used and the type of dispersant, the dispersant content in the conductive material composite particles after production can be measured and confirmed as necessary using the following method. The conductive material composite particles are heated at a temperature at which the dispersant is thermally decomposed but the weight of the conductive material does not change, for a time at which the dispersant can be completely decomposed, and the weight of the dispersant can be determined from the difference in weight before and after heating. The weight of the dispersant is converted into the amount of active ingredient if the dispersant is dissolved or dispersed in another solvent, etc. The content ratio of each component in the composite particles can be easily determined by measuring the thermogravimetric change of the dispersant using a TG-DTA (thermogravimetric-differential thermal analyzer) while continuously heating it within a temperature range where the dispersant decomposes but the conductive material does not. As a specific example, the dispersant content can be measured using a TG-DTA (manufactured by Bruker Japan Co., Ltd.) by holding the conductive material composite particles at 100°C for 1 hour to completely remove moisture, then raising the temperature to 300°C at a rate of 5°C per minute, and holding the particles for another hour to decompose the dispersant. The weight after holding at 300°C is the weight of the conductive material alone, and the difference between the weight after holding at 100°C and the weight after holding at 300°C is the amount of dispersant in the conductive material composite particles.

[0051] The bulk density of the conductive composite particles is preferably 0.04 g / ml or more, more preferably 0.10 g / ml or more, and even more preferably 0.20 g / ml or more. By setting the bulk density at this value or more, the particles are less likely to scatter within the production factory, and dust collection equipment is not required, which reduces costs related to transportation and storage. The bulk density can be measured by measuring the initial bulk density in accordance with R-1627-1997.

[0052] To evaluate the dispersibility of the conductive material composite particles, the arithmetic mean roughness (Ra) of the coated film is preferably 0.4 or less. It is more preferably 0.3 or less, and even more preferably 0.2 or less. The arithmetic mean roughness (Ra) is a value obtained by measuring the unevenness of the surface in the height direction over a certain reference length (section), setting the average value as a reference line, and calculating the average of the absolute values ​​of the distances from the reference line within that section. A high arithmetic mean roughness (Ra) indicates the presence of many unevennesses, i.e., the high possibility of the presence of aggregation or uneven distribution of the conductive material, coarse particles, or foreign matter, which are the main causes of unevenness on the coated film. If the arithmetic mean roughness is greater than 0.4, these effects become greater, potentially hindering uniform dispersion of the conductive material in the electrode coating film. In the present invention, the arithmetic mean roughness refers to a value determined by the following method, but the method is not limited to this as long as the same results are obtained. First, a commercially available polyvinylidene fluoride binder (KF Polymer W#7200, manufactured by Kureha Corporation) is diluted to 8.0% with N-methyl-2-pyrrolidone as a solvent. Conductive composite particles are weighed out and added to the solution so that they account for 8.3% of the total, and the mixture is kneaded at 2000 rpm for 1 minute using a planetary mixer (Thinky Corporation, Awatori Rentaro) to prepare a paste. This paste is applied to a glass plate using an applicator to a dry film thickness of 7.5 to 8.5 μm, and then dried in a hot air dryer at 100°C for 30 minutes to remove the solvent, yielding a coating film. The resulting coating film is referred to as the "coating film for dispersibility evaluation." The surface roughness of the coating film for dispersibility evaluation is measured using a contact-type surface roughness meter (Tokyo Seimitsu Co., Ltd., Surfcom 130A), and the arithmetic mean roughness is calculated.

[0053] In the third invention of the present application, the conductive material composite particles are characterized in that the optical density OD value during dispersion evaluation is 3.0 or more. It is more preferably 3.5 or more, and even more preferably 4.0 or more. The OD value during dispersion evaluation here is a value determined by measuring the aforementioned dispersibility evaluation coating film using a spectrodensitometer (X-rite, manufactured by Videojet X-Rite Inc.), but the measurement method is not limited to this as long as the same results are obtained. The OD value is a logarithmic representation of the light absorption measured by measuring the spectral transmittance of a coating film, and it is known that the lower the light transmitted through the coating film, the higher the value. As a result of the inventors' studies, it was found that there is a correlation between the OD value and electrical properties, and that by adjusting the OD value of conductive composite particles when coated within a predetermined range, excellent performance as a battery material can be obtained. More specifically, it was found that conductive composite particles with an OD value of 3.0 or more when coated result in uniform dispersion of the conductive material when coated into an electrode film, resulting in increased charge / discharge capacity. It was also found that when conductive composite particles with an OD value of less than 3.0 when coated are used to form a battery, the conductive material tends to be less uniformly dispersed, which can easily reduce charge / discharge capacity. The possible reasons for this are as follows: If the conductive material is not dispersed uniformly enough and a large amount of it aggregates and is unevenly distributed in the coating film, localized areas where the conductive material is absent will occur unevenly. Because these areas do not absorb light, the amount of light transmitted through the coating film as a whole increases, resulting in a decrease in the OD value. If the conductive material is not dispersed uniformly in the coating film, it will not be possible to impart sufficient electronic conductivity to the active material, which will lead to a decrease in charge / discharge capacity when the battery is made, and a premature decrease in capacity due to localized current flow. Therefore, a decrease in the OD value can be considered to indicate insufficient dispersion of the conductive material, resulting in a decrease in performance when the battery is made.

[0054] The conductive material composite particles of the present invention, which satisfy the above-mentioned properties, exhibit the following performance and effects. Compared to powder-like conductive materials that do not contain dispersants, etc., the shear viscosity when formed into an electrode paste is low, resulting in excellent coatability. The moderate viscosity gradient between low and high shear suppresses sedimentation of the active material, preventing uneven distribution of the conductive material when formed into a coating film. Furthermore, because agglomerations are easily broken down during the kneading process when forming the electrode paste, the film thickness becomes consistent when formed into a coating film, and areas with locally thin film thickness or no coating are less likely to occur, resulting in increased yield. Furthermore, when forming the electrode paste, the paste becomes uniform with shorter mixing times compared to powder-like conductive materials, resulting in cost benefits due to shorter process times. In addition, compared to conventional conductive material composite particles, the conductive material is less likely to be unevenly distributed in the coating film, and therefore the conductive material is more uniformly dispersed among a larger number of active materials in the electrode coating film, imparting electronic conductivity and increasing charge / discharge capacity, and it is thought that this will reduce the occurrence of early capacity reduction due to local current concentration in the positive and negative electrodes. Furthermore, since the conductive material composite particles do not contain a solvent, the amount of organic solvent used during production can be reduced compared to a slurry-type conductive material dispersion paste, reducing the burden on the environment. In addition, the conductive material composite particles can be used regardless of the type or presence of a solvent used when making the electrode paste.

[0055] [Method for manufacturing lithium ion secondary battery] The lithium-ion secondary battery of the present invention can be fabricated by winding a positive electrode, which is prepared by dispersing the conductive composite particles of the present invention together with an electrode active material and a binder in a non-aqueous solvent such as N-methyl-2-pyrrolidone, onto a metal substrate and drying the resulting electrode paste, or a negative electrode, which is prepared by dispersing the conductive composite particles of the present invention together with an electrode active material and a binder in a solvent such as water, onto a metal substrate and drying the resulting electrode paste, together with a separator, and then storing the resulting electrode paste and an electrolyte in a battery can. To prevent internal pressure buildup and overcharge / discharge, a fuse, an overcurrent protection element such as a PTC element, an expanded metal, a lead plate, or the like may be provided as needed. The battery can may have any shape, such as a coin, button, sheet, cylindrical, rectangular, or flat.

[0056] [Use in other electricity storage devices] The characteristics of the conductive material composite particles of the present invention, such as uniformity during dispersion and the fact that they are substantially free of solvent, are not limited to lithium ion secondary batteries, and are effective in improving charge / discharge performance, reducing costs, and reducing environmental impact when used in other energy storage devices. In particular, in electricity storage devices that use carbon materials such as carbon black, carbon nanotubes, graphite, and slurries using these as conductive materials, the conductive material composite particles of the present invention can be suitably used as a substitute for these existing conductive materials. For example, it can be suitably used in primary batteries such as alkaline manganese dry batteries and nickel manganese batteries, secondary batteries such as nickel-metal hydride batteries, nickel-cadmium batteries, sodium-sulfur batteries and sodium-ion batteries, and other electrochemical elements such as capacitors. When used in these electricity storage devices, the conductive material already used can be replaced with the conductive material composite particles of the present invention. If the existing conductive material is a powder-like conductive material such as carbon black, it can be replaced as is. If the conductive material is a slurry-like conductive material such as a carbon black dispersion, it can be used by replacing the conductive material with the conductive material composite particles of the present invention and adjusting the solid content of the electrode paste to achieve an appropriate viscosity. [Example]

[0057] The present invention will now be described in more detail with reference to examples.

[0058] Example 1 To 84 parts by weight of ion-exchanged water used as a dispersion medium, 1 part by weight of methylcellulose polymer (weight-average molecular weight 35,000 as measured by GPC) was added as a dispersant, and the mixture was thoroughly dissolved using a commercially available stirrer to obtain a dispersant solution. Next, 15 parts by weight of acetylene black (manufactured by Denka Co., Ltd., "Denka Black Granules") having an average primary particle diameter of 35 nm as measured by a method conforming to ASTM: D3849-14 was added as a conductive material to 85 parts by weight of the obtained dispersant solution, and the mixture was mixed and stirred using a commercially available stirrer to obtain a conductive material mixture. The obtained conductive material mixture was subjected to a wet grinding process using a commercially available horizontal bead mill to further refine the conductive material until there were no particles larger than 50 μm as evaluated using a grind gauge, thereby obtaining a conductive material dispersion paste. The obtained conductive material dispersion paste was filtered through a mesh with 50 μm openings to remove foreign matter, and this was named Conductive Material Dispersion Paste 1. The conductive material dispersion paste 1 was dried using a commercially available spray nozzle type spray dryer at an inlet temperature of 160°C and a spray pressure of 0.03 MPa, and then classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 1 with particle diameters of 75 μm or less and 45 μm or more.

[0059] Example 2 To 84 parts by weight of ion-exchanged water used as a dispersion medium, 1 part by weight of polyvinylpyrrolidone (weight average molecular weight 66,800 as measured by GPC) as a dispersant was added, and the mixture was thoroughly dissolved using a commercially available stirrer.The same operations as in Example 1 were then carried out to obtain a conductive material dispersion paste 2. Except for using this conductive material dispersion paste 2, the subsequent operations were the same as in Example 1, and conductive material composite particles 2 with particle diameters of 75 μm or less and 45 μm or more were obtained by classification using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.).

[0060] Example 3 Conductive material dispersion paste 1 obtained by the same operation as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer, and the resulting dried product was pulverized using a commercially available cutter mixer and then classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 3 having a particle size of 75 μm or less and 45 μm or more after sieving.

[0061] Example 4 Conductive material dispersion paste 1 obtained by the same operation as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer, and the resulting dried product was pulverized using a commercially available cutter mixer and then classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 4 having a particle size of 150 μm or less and 75 μm or more after sieving.

[0062] Comparative Example 1 Conductive material dispersion paste 1 obtained by the same operation as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer, and the resulting dried product was pulverized using a commercially available cutter mixer and then classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 5 having a particle size of 250 μm or less and 150 μm or more after sieving.

[0063] Comparative Example 2 Conductive material dispersion paste 1 obtained by the same operation as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer, and the resulting dried product was pulverized using a commercially available cutter mixer and then classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 6 having a particle size of 500 μm or less and 250 μm or more after sieving.

[0064] Comparative Example 3 Conductive material dispersion paste 1 obtained by the same operation as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer, and the resulting dried product was pulverized using a commercially available cutter mixer and then classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 7 having a particle diameter of 45 μm or less after sieving.

[0065] Comparative Example 4 Commercially available acetylene black (manufactured by Denka Co., Ltd., "Denka Black Powder") was used as the conductive material raw material 1. This acetylene black is the same type as the acetylene black used as the conductive material in each of the examples and comparative examples.

[0066] [Physical property measurement and performance evaluation] The dispersion pastes, conductive composite particles, and conductive base materials prepared by the methods disclosed in the above Examples and Comparative Examples were subjected to the following physical property measurements and performance evaluations.

[0067] The DBP oil absorption of the acetylene black used as the conductive material in each of the examples and comparative examples was measured by a method in accordance with JIS 6217-4, and the measured value was 360 ml / 100 g. The viscosity of the conductive material dispersion pastes 1 and 2 was measured using a B-type viscometer (TVB10M type viscometer manufactured by Toki Sangyo Co., Ltd.). The moisture content of the conductive material composite particles 1, 2, 3, 4, 5, 6, and 7 and the conductive material base material 1 was measured using a halogen lamp heating moisture meter (MA-120, manufactured by Shinko Denshi Co., Ltd.). For conductive composite particles 1, 2, 3, 4, 5, 6, and 7, the upper limit of particle size, average particle size, lower limit of particle size, particle size distribution (D10, D50, D90, D95), and aspect ratio were measured and calculated using a scanning electron microscope (FlexSEM1000, Hitachi High-Tech Corporation) and AZtec (particle analysis software, Oxford Instruments Ltd.). The angle of repose was also measured in accordance with JIS 9301-2-:2:1999. The measurement results are shown in Table 1.

[0068] The arithmetic mean roughness of conductive composite particles 1, 2, 3, 4, 5, 6, and 7 and conductive base material 1 was measured using the following method. A commercially available polyvinylidene fluoride binder (KF Polymer W#7200, manufactured by Kureha Corporation) was diluted to 8.0% with N-methyl-2-pyrrolidone as a solvent. Conductive composite particles or conductive base material were weighed and mixed to account for 8.3% of the total, and the mixture was kneaded for 1 minute in a planetary centrifugal mixer (Thinky Corporation, Awatori Rentaro) at 2000 rpm to prepare a paste. This paste was applied to a glass plate using an applicator to a dry film thickness of 7.5 to 8.5 μm, and the paste was dried in a hot air dryer at 100°C for 30 minutes to remove the solvent, yielding a coating film. The coating film obtained here is referred to as the “coating film for dispersibility evaluation.” The arithmetic mean roughness (Ra) and arithmetic mean height (Sa) of this coating film for dispersibility evaluation were measured and calculated using a contact-type surface roughness meter (Tokyo Seimitsu Co., Ltd., Surfcom130A). Furthermore, the OD value of the coating film for dispersibility evaluation was measured using a spectrodensitometer (x-rite, manufactured by Videojet X-Rite Inc.).

[0069] Conductive composite particles 1, 2, 3, 4, 5, 6, and 7 and conductive base material 1 (referred to as A) were tested using commercially available active materials such as LiNi 1 / 3 C o 1 / 3 Mn 1 / 3 The composition of O2, HED NCM111 1100 (manufactured by BASF Toda Battery Materials LLC) (referred to as B), and the binder PVdF (Solvay Japan Solef5130) (referred to as C) were weighed out so that the weight ratio of the solids was 2 parts by weight of A, 97 parts by weight of B, and 1 part by weight of C, and then diluted with NMP so that the total solids of A, B, and C was 65% by mass. This was then kneaded at 2000 rpm for 2 minutes in a planetary centrifugal mixer to obtain "electrode paste A for evaluation." Furthermore, "electrode paste B for evaluation" was obtained in the same manner as the coating film evaluation paste A, except that the kneading time in the planetary centrifugal mixer was changed to 10 minutes. In addition, for conductive composite particles 1, 2, 3, 4, 5, 6, and 7, the dispersant contained in the composite particles acts as a binder on the electrode coating film, so here the amount of dispersant is regarded as the amount of binder, and in calculating the blending ratio, the amount of dispersant contained in the conductive composite particles is divided by A, and this amount of dispersant is added to C. The conductive composite particles, active material, and binder were then weighed so that A, B, and C were 2 parts by weight, 97 parts by weight, and 1 part by weight, respectively. The above-mentioned electrode paste A for evaluation and electrode paste B for evaluation were applied to a PET film using an applicator, and then dried at 100°C for 30 minutes using a hot air dryer to remove the NMP, yielding coating films with a film thickness in the range of 100µm to 120µm. The coating films obtained here are referred to as "electrode paste evaluation coating film A" and "electrode paste evaluation coating film B," respectively. Of these, a cross section of Coating Film A for electrode paste evaluation was exposed using a cross-section cutter for preparing SEM samples, and the dispersion state of the conductive material in the coating film was photographed at 500x magnification using a scanning electron microscope (FlexSEM1000, manufactured by Hitachi High-Tech Corporation) and an energy dispersive X-ray spectrometer (AZtecEnergy x-act, manufactured by Oxford Instruments). In addition, the coating resistance of both electrode paste evaluation coatings A and B was measured by the following method. First, the coating film was cut into a piece 2 cm wide and 3 cm long, and the volume resistivity of the coating film was measured at an applied voltage of 10 V using a resistivity meter (Loresta-GP MCP-T610, manufactured by Nitto Seiko Analytech Co., Ltd.). The results of the above measurement tests are shown in Table 1.

[0070] [Battery performance evaluation] Next, using the conductive composite particles 1, 2, 3, 4, 5, 6, and 7, the conductive base material 1, and the following materials, a CR2032 type coin-type secondary battery (diameter 20 mm, height 3.2 mm) was produced by the following method, and its performance was evaluated. The secondary battery and the method for producing the same disclosed below are merely examples for evaluating the conductive material composite particles of the present invention, and do not limit the use method and embodiments of the conductive material composite particles of the present invention in any way. The conductive material composite particles of the present invention can be suitably used as an electrode material for a wide range of power storage devices, including lithium ion secondary batteries, without being limited by the materials, battery production method, battery form, and other conditions disclosed below.

[0071] [Preparation of Positive Electrode of Example 1] A positive electrode active material (HED NCM111 1100 manufactured by BASF Toda Battery Materials LLC, theoretical capacity: 160 mAh / g), a binder (PVdF (Solvay Japan Solef5130)) and a conductive material (conductive material composite particle 1) were weighed in a weight ratio of 97:1:2. The binder was dissolved in a solvent (N-methyl-2-pyrrolidone), and the positive electrode active material and the conductive material were mixed. The coating solution was diluted with the solvent so that the solid content was 70 to 75% by mass, forming an electrode paste. This electrode paste was applied to a 20 μm-thick aluminum foil so that the battery capacity per unit area was 3.0 to 4.0 mAh / cm. 2The positive electrode coated plate was then dried in an oven at 100°C for 30 minutes to obtain a positive electrode coated plate. In the positive electrode coated plate obtained here, the portion on the aluminum foil current collector where the electrode paste had dried and formed a film was designated the "positive electrode composite layer." This positive electrode coated plate was then subjected to a test to obtain a positive electrode coated plate having an electrode density (positive electrode composite layer mass / positive electrode composite layer volume) of 2.9 to 3.4 g / cm. 3 After pressing to form a positive electrode, the electrode was punched out to a diameter of 14 mm using a coin punching machine. In calculating the electrode density, the mass of the positive electrode composite layer was obtained by subtracting the mass of the aluminum foil from the mass of the positive electrode coated plate, and the volume of the positive electrode composite layer was obtained by subtracting the thickness of the aluminum foil from the thickness of the positive electrode coated plate to obtain the height, and then using Φ14 mm as the base area.

[0072] [Production and Evaluation of Evaluation Battery of Example 1] A separator ("Seillon P2010", material: polypropylene, Φ17mm, manufactured by CS Tech) was inserted between the positive electrode obtained as described above and a negative electrode (lithium foil, thickness 200μm, φ16mm, manufactured by Honjo Metals Co., Ltd.), and a CR2032 coin cell filled with an electrolyte (1.0M LiPF6 EC:DEC (1:1 v / v%), manufactured by Kishida Chemical Co., Ltd.) was assembled to prepare a battery for evaluation. The obtained evaluation battery was subjected to charge / discharge and actual discharge capacity measurements as follows using a charge / discharge tester (580-type high performance charge / discharge system, manufactured by Scribner Associates). The charge / discharge procedure was performed at room temperature (25°C) using a constant current and voltage of 1C, with the upper limit voltage set to 4.3V, followed by discharging to 3.0V at a constant current of 1C. This cycle was repeated 30 times, and the actual discharge capacity at the 30th cycle was measured. A 10-minute rest period was provided between the completion of full charge and the start of discharge during each cycle, as well as between cycles. Based on the actual discharge capacity (mAh) obtained, the initial effective capacity, effective capacity at the 30th cycle, and capacity retention were calculated. The results are shown in Table 1. Here, the effective capacity refers to the ratio of the actual discharge capacity when the theoretical capacity (mass (g) of the positive electrode composite layer × active material blending ratio × theoretical capacity per 1 g of active material) is taken as 100%. The effective capacity at the first cycle was taken as the initial effective capacity. The capacity retention rate is a value showing the ratio of the effective capacity at the 30th cycle to the initial effective capacity, which is taken as 100%.

[0073] [Production and Evaluation of Batteries in Examples 2 to 4 and Comparative Examples 1 to 4] Batteries were fabricated in the same manner as in Example 1, except that the conductive material was changed to each composite particle and conductive material raw material shown in Table 1, and their performance was evaluated. The results are shown in Table 1.

[0074] [Table 1]

[0075] From the results shown in Table 1, the conductive material composite particles of the examples have an OD value of 3.0 or more after coating, which indicates that the conductive material is uniformly dispersed in the coating. In particular, in Example 1, the optical density of the coating exceeded 4.0, indicating excellent uniformity. Comparing Examples 3 and 4 with Comparative Examples 1 and 2, which differ only in the upper particle size limit, the comparative examples, which have larger composite particle sizes, show a lower optical density and a decrease in uniformity. Furthermore, the surfaces of Comparative Examples 1 and 2 are rougher than those of the examples, indicating the presence of many coarse particles in the coating film. Comparative Example 4 has a low OD value and the surface roughness of the coating film is not good.

[0076] In the SEM images of Figures 2, 4, 6, 8, 10, 12, 14, and 16, the white spherical substances are the active material, and the gray mist surrounding them is the conductive material. In the EDS analysis images of Figures 3, 5, 7, 9, 11, 13, 15, and 17, the conductive carbon is visualized as white. These figures show that the conductive material is uniformly distributed throughout the active material in the examples, whereas in the comparative examples, there are areas where it is concentrated and areas where it is almost absent, indicating uneven distribution within the coating film. Furthermore, the EDS analysis images of comparative examples 1, 2, and 4 reveal the presence of coarse particles or agglomerations (white lumps in the images). While no large agglomerates are visible in Figure 15, there is also very little white haze. This is thought to indicate that secondary agglomerates are excessively loosened during the paste mixing process, resulting in the severing of bonds between the structures. In this state, conductive paths are difficult to form, and high battery performance cannot be achieved. As described above, it is clear that by using the composite particles produced according to the present invention, an electrode coating film having better dispersibility than the comparative example and in which the conductive material is uniformly dispersed can be obtained.

[0077] The evaluation of volume resistance showed that the electrode coating films using the conductive composite particles of each Example generally had lower resistance than the Comparative Examples. In Comparative Example 3, when the kneading time for pasting was 2 minutes, the resistance was relatively low, but when the kneading time was 10 minutes, the resistance was significantly higher. This is thought to be because there were more small composite particles compared to the Examples, and therefore the bonds between the structures were easily broken by kneading, making it difficult to form conductive paths.

[0078] In the evaluation of battery performance, the batteries using the conductive material composite particles of each Example showed high battery performance. Comparative Examples 1, 2, and 3 showed low effective capacity and capacity retention. Furthermore, Comparative Example 4 showed a significant decrease in capacity retention at the 30th cycle.

[0079] As described above, in the electrode coating film using the conductive material composite particles of the present invention shown in the examples, the conductive material is uniformly dispersed among the active materials in the coating film, and sufficient electronic conductivity can be imparted to many active materials, thereby improving the charge / discharge capacity when the battery is made. Furthermore, since there is no uneven distribution or aggregation of the conductive material and there are no coarse particles or foreign matter, there is a lower possibility of localized current flow or short circuiting, which can cause thermal runaway in the battery or a premature decrease in charge / discharge capacity, compared to conventional techniques, and battery performance can be maintained for a long period of time. [Industrial Applicability]

[0080] By using the conductive material composite particles of the present invention, high-quality positive and negative electrodes that improve the performance of lithium-ion secondary batteries can be produced. Lithium-ion secondary batteries using the present invention can be suitably used as power sources for electric motors installed in electric vehicles, etc. Furthermore, the conductive material composite particles of the present invention can be stored at room temperature for long periods of time, and the amount of solvent used during production is reduced, thereby reducing the burden on the environment and production costs. Furthermore, by using the conductive material composite particles of the present invention as a substitute for existing conductive materials such as carbon black, they can contribute to performance improvement and cost reduction not only in lithium ion secondary batteries but also when used in other power storage devices.

Claims

1. Conductive material composite particles are characterized in that they contain at least a conductive material and a dispersant, have a particle size distribution D50 of 15 μm or more and a particle diameter determined by sieving of 150 μm or less, the DBP oil absorption of the conductive material is 550 ml / 100 g or less, and contain 1 to 10 parts by weight of the dispersant per 100 parts by weight of the conductive material.

2. Conductive material composite particles characterized in that the particles contain at least a conductive material and a dispersant, the particles having a particle size distribution D50 of 15 μm or more and an upper particle diameter limit of 300 μm or less, the DBP oil absorption of the conductive material being 550 ml / 100 g or less, and the conductive material contains 1 to 10 parts by weight of the dispersant per 100 parts by weight of the conductive material.

3. 3. The conductive material composite particle according to claim 1, which is a particle containing at least a conductive material and a dispersant.

4. 3. The conductive material composite particle according to claim 1, wherein the dispersant is a nonionic dispersant.

5. 5. The conductive material composite particle according to claim 4, wherein the weight average molecular weight of the nonionic dispersant is 1,000 or more and 1,000,000 or less.

6. 3. The conductive material composite particle according to claim 1, wherein the purity of the conductive material is 99.9% or more.

7. 3. The conductive material composite particle according to claim 1, wherein the average primary particle size of the conductive material is 10 nm or more and 50 nm or less.

8. The conductive material composite particle according to claim 1 or 2, which is a conductive material for a battery electrode.

9. A method for producing conductive material composite particles, comprising the steps of preparing a conductive material dispersion paste containing at least a conductive material, a dispersant, and a dispersion medium, and removing the dispersion medium from the conductive material dispersion paste, wherein the particle diameter of the conductive material composite particles in the conductive material dispersion paste is 50 μm or less.

10. The method for producing conductive material composite particles according to claim 9, wherein the conductive material dispersion paste does not contain foreign matter having a size exceeding 50 μm.

11. 11. The method for producing conductive material composite particles according to claim 9, further comprising a drying step of heating the conductive material dispersion paste at a temperature of 80°C or higher and 300°C or lower.

12. A method for producing an electrode, comprising mixing the conductive composite particles according to claim 1 or 2 with at least an active material and a binder, and applying the mixture to a substrate.

13. A method for producing an electrode, comprising mixing the conductive material composite particles obtained by the method according to claim 9 or 10 with at least an active material and a binder, and applying the mixture to a substrate.

14. A lithium ion secondary battery using an electrode obtained by the method according to claim 12.

15. A power storage device using the conductive material composite particles according to claim 1 or 2.

16. An electricity storage device using an electrode obtained by the method according to claim 12.

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