JP2005097010A - Carbon material, production method therefor and its application - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a secondary battery excellent in properties such as discharge capacity and Coulomb efficiency, to provide a negative electrode material for the secondary battery, and to provide a carbon material suitable for the negative electrode material. <P>SOLUTION: The subject method for producing a carbon material for a battery electrode comprises: a stage where natural graphite grains in which the average grain size is controlled to 0.1 to 100 μm is subjected to immersing treatment with a hydrogen fluoride-containing solution; and a stage where heat treatment is performed at 2,400 to 3,300°C, and in which the total content of elements other than carbon is controlled to ≤800 ppm. Thus, the negative electrode material large in discharge capacity and excellent in Coulomb efficiency, and a secondary battery using the negative electrode material can be obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrode material for a non-aqueous electrolyte secondary battery having a large charge / discharge capacity, excellent charge / discharge cycle characteristics and large current load characteristics, an electrode using the same, and a non-aqueous electrolyte secondary battery, in particular, lithium secondary battery. The present invention relates to a negative electrode material for a secondary battery, a negative electrode using the negative electrode material, and a lithium secondary battery.

  As portable devices become smaller and lighter and have higher performance, there is an increasing demand for higher capacity lithium ion secondary batteries having high energy density, that is, lithium ion secondary batteries. As a negative electrode material used for a lithium ion secondary battery, graphite fine powder which is a material capable of inserting lithium ions between layers is mainly used. Since graphite having higher crystallinity exhibits a higher discharge capacity, studies have been made on the use of a material with high crystallinity centered on natural graphite for the negative electrode material of a lithium ion secondary battery. The theoretical capacity of the discharge capacity when using a graphite material is 372 mAh / g, but in recent years, a material close to the theoretical capacity of 350 to 360 mAh / g in the practical range has been developed. It is difficult to produce a theoretical capacity for artificial graphite because of crystallinity, and even if it is realized, the cost becomes high. Compared with this, natural graphite is cheaper, and a discharge capacity close to the theoretical capacity can be easily obtained.

  However, in general, natural graphite has a large content of impurities due to its origin, which has been a cause of deterioration of various symptoms of the product. In particular, clay minerals mainly composed of silica, alumina, iron oxide, etc. have a high content in natural graphite, and are completely satisfied to the extent that they satisfy the required characteristics of Li secondary batteries that cause performance degradation even in the presence of very small amounts of impurities. It is difficult to remove. In particular, lithium ion secondary batteries in recent years are required to have very high initial charge / discharge efficiency in addition to high capacity, and it is not easy to satisfy them.

  As a method for solving such a problem, Japanese Patent No. 3188032 (Patent Document 1) discloses a method of performing a heat treatment at a temperature of 2400 ° C. or higher. However, this method cannot sufficiently remove silica, alumina, etc., and it is difficult to satisfy the initial charge / discharge efficiency.

  In JP-A-10-284080 (Patent Document 2) and JP-A 2000-306582 (Patent Document 3), it is said that various battery characteristics are improved by surface-treating graphite with acid or alkali. This is the level of surface treatment, and impurities cannot be removed sufficiently. In particular, in Patent Document 2, heat treatment at 2000 ° C. or lower is performed, but the heat treatment is performed with an acidic solution or the like, and this makes it difficult to sufficiently remove moisture that has entered the graphite. In addition, the temperature of the heat treatment is low and good impurity removal cannot be expected, and it is difficult to obtain an initial efficiency that meets the current requirements.

  Japanese Patent Application Laid-Open No. 11-199322 (Patent Document 4) and Japanese Patent Application Laid-Open No. 2000-1000043 (Patent Document 5) generate surface functional groups by treating graphite with sulfuric acid or a mixture of sulfuric acid and nitric acid and then heating. It is described that the bonding property with the binder and the rapid charge / discharge characteristics are improved by generating minute voids inside the particles. However, in the method using sulfuric acid or nitric acid as the acid treatment, it is contained in natural graphite. It is difficult to sufficiently remove impurities such as silica and alumina.

  JP-A-5-299074 (Patent Document 6) discloses a method of heating a carbon material after alkali treatment, but for the purpose of surface treatment, only combining an acid and an alkali, As a result, it is difficult to sufficiently remove impurities from natural graphite. Further, in this method, the acid and alkali treated surfaces are exposed on the surface, and they remain after the heat treatment. These are not preferable because they may cause side reactions at the time of gas generation or initial charge when they are made into batteries.

  Further, there is a method of purifying the carbon material by flowing a fluorocarbon gas or the like while heating it (Japanese Patent Laid-Open No. 11-236205 (Patent Document 7)), but it is actually carried out from the viewpoint of environmental problems and cost problems. Not right.

Japanese Patent No. 3188032 JP-A-10-284080 JP 2000-306582 A Japanese Patent Application Laid-Open No. 11-199322 Japanese Patent Application Laid-Open No. 2000-1000043 JP-A-5-299074 JP-A-11-236205

  The present invention provides a negative electrode material for a lithium ion secondary battery having a large discharge capacity, excellent coulomb efficiency and cycle characteristics, and a small irreversible capacity by sufficiently removing impurities in natural graphite by a relatively simple method. For the purpose.

  As a result of intensive studies to solve the above problems, the inventors of the present invention, after appropriately pulverizing natural graphite, treating with an acid centered on hydrofluoric acid, and heat-treating at 2400-3300 ° C. By using different methods, preferably by treating with alkali before treatment with acid centered on hydrofluoric acid, more preferably by treating the particles with resin before heat treatment, which is surprisingly very high The inventors have found that a negative electrode material for a lithium ion secondary battery having initial charge / discharge efficiency, a large discharge capacity, and excellent cycle characteristics can be obtained, thereby completing the present invention.

That is, the present invention provides the following carbon material for a battery electrode, a method for producing the same, an electrode using the carbon material, and a battery.
[1] A step of immersing natural graphite particles having an average particle size of 0.1 to 100 μm with a hydrogen fluoride-containing solution and a step of heat-treating the immersed natural graphite particles at a temperature of 2400 ° C. or higher and lower than 3300 ° C. A method for producing a carbon material, characterized in that the total content of elements other than carbon is 800 ppm or less.
[2] The method for producing a carbon material as described in 1 above, wherein a step of immersing with an alkaline aqueous solution is provided before the step of immersing with the hydrogen fluoride-containing solution.
[3] The above 1 or 2 wherein a step of granulating natural graphite particles immersed in a hydrogen fluoride-containing solution by a binder force and / or a van der Waals force between particle interfaces due to shear is provided before the heat treatment step. The manufacturing method of the carbon material as described in 2.
[4] In the granulation step, natural graphite particles having an average particle diameter of 0.1 to 50 μm which have been dipped in a hydrogen fluoride-containing solution have an average circularity of 0.85 to 0.99 and an average particle diameter of 0.5. 4. The method for producing a carbon material as described in 3 above, which is a step of granulating to 200 μm.
[5] The method for producing a carbon material according to any one of 1 to 4, wherein a step of coating at least part of the particle surface with a resin is provided before the heat treatment step.
[6] The method for producing a carbon material as described in 1 above, wherein the hydrogen fluoride-containing solution has a hydrogen fluoride concentration (HF concentration) of 10 to 80%.
[7] The carbon material as described in 6 above, wherein the hydrogen fluoride-containing solution is an aqueous solution in which at least one selected from nitric acid, sulfuric acid, hydrochloric acid, bromic acid, iodic acid, trichloroacetic acid, trifluoroacetic acid and oxalic acid is mixed. Manufacturing method.
[8] A polymer in which the resin covering at least a part of the particle surface includes at least one selected from the group consisting of a phenol resin, a polyvinyl alcohol resin, a furan resin, a cellulose resin, a polystyrene resin, a polyimide resin, and an epoxy resin. 6. The method for producing a carbon material as described in 5 above.
[9] The method for producing a carbon material as described in 5 above, wherein the resin covering at least a part of the particle surface is a phenol resin.
[10] The method for producing a carbon material as described in 8 above, wherein the amount of the resin covering at least a part of the particle surface is 2 to 80% by mass of the particle mass.
[11] The method for producing a carbon material as described in 9 above, wherein the phenol resin is prepared by adding a drying oil or a fatty acid thereof during the reaction of phenols and formaldehyde.
[12] The method for producing a carbon material as described in 9 above, wherein vapor-grown carbon fiber is present during the reaction of phenols and formaldehyde, and vapor-grown carbon fiber is adhered to the particle surface by the generated phenol resin.
[13] The method for producing a carbon material as described in 12 above, wherein the amount of vapor grown carbon fiber adhered to the particles is 0.01 to 20% by mass of the mass of the particles.
[14] A carbon material comprising natural graphite particles having a total content of elements other than carbon of 800 ppm or less.
[15] The carbon material as described in 14 above, wherein at least a part of the particle surface is coated with a carbonaceous material obtained by carbonizing and firing a resin.
[16] The carbon material as described in 15 above, wherein the resin is a phenol resin.
[17] The above-mentioned 15, wherein a vapor-grown carbon fiber having a hollow structure inside and having an outer diameter of 2 to 1000 nm and an aspect ratio of 10 to 15000 is attached to the surface of at least some of the particles via a carbonaceous material The carbon material described.
[18] The carbon material as described in 17 above, wherein the vapor grown carbon fiber comprises carbon having an average interplanar spacing (d ₀₀₂ ) of 0.344 nm or less (d ₀₀₂ ) by X-ray diffraction.
[19] The carbon material as described in 14 above, containing 1 to 800 ppm of boron.
[20] The carbon material as described in any one of 14 to 19, which satisfies the following requirements (1) to (3):
(1) An average circularity of 0.85 to 0.99,
(2) a specific surface area of 0.2 to ⁵ m ² / g,
(3) The average particle size is 10 to 40 μm.
[21] A carbon material obtained by the production method according to any one of 1 to 13 above.
[22] An electrode paste comprising the carbon material according to any one of 14 to 21 and a binder.
[23] An electrode comprising a molded body of the electrode paste as described in 22 above.
[24] A secondary battery including the electrode according to 23 as a constituent element.
[25] A non-aqueous electrolyte and an electrolyte, wherein the non-aqueous electrolyte is at least one selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, vinylene carbonate, γ-butyrolactone, and propylene carbonate. 25. The secondary battery as described in 24 above.

[Natural graphite]
There is no particular limitation on natural graphite can be used in the present invention is an average spacing (d ₀₀₂₎ thereof is preferably less 0.340 nm, more preferably below 0.337nm in (002) plane.
Natural graphite is adjusted to a predetermined particle size as necessary before various treatments. Adjustment of the particle size is usually performed by pulverization and classification. There is no limitation on the particle shape after the particle size adjustment (pulverization), and for example, it may have any shape such as a lump shape, a scale shape, a spherical shape, and a fibrous shape, but a spherical shape that is a suitable shape as a carbon material for battery electrodes. Is preferred. In particular, it is preferable that the average circularity measured by a flow type particle image analyzer (refer to the section of Examples described later for the calculation method) is 0.85 to 0.99. When the average circularity is less than 0.85, the packing density at the time of forming the electrode does not increase, so that the discharge capacity per volume decreases. When the average circularity is larger than 0.99, the fine powder portion has a low circularity, and therefore the fine powder portion is hardly included, and the discharge capacity at the time of forming the electrode does not increase. The circularity value is preferably controlled so that the content of particles having a particle size of less than 0.90 is in the range of 2 to 20% by number. The average circularity of the powder can be adjusted using, for example, a particle shape control device such as a mechanofusion (surface fusion) process.

  The natural graphite particles are adjusted so that the average particle diameter by laser diffraction method is in the range of 0.1 to 100 μm. Preferably it is 1-80 micrometers, More preferably, it is 5-40 micrometers, More preferably, it is 10-30 micrometers. Moreover, the particle size distribution which does not substantially contain particles of 1 μm or less and / or 80 μm or more is good. This is because, when the particle size is large, fine particles are generated by the charge / discharge reaction, and the cycle characteristics deteriorate. On the other hand, when the particle size is small, the particles cannot efficiently participate in the electrochemical reaction with lithium ions, and the capacity and cycle characteristics are lowered.

  In order to adjust the particle size distribution, known pulverization methods and classification methods can be used. Specific examples of the pulverizer include a hammer mill, a jaw crusher, and a collision pulverizer. As a classification method, airflow classification or classification using a sieve is possible. Examples of the air classifier include a turbo classifier and a turboplex.

[Acid treatment]
The natural graphite particles having a controlled particle size are immersed in a hydrogen fluoride-containing solution. By this immersion treatment, impurities such as silica, silicate, alumina and iron oxide are dissolved and removed from the graphite particles.
The hydrogen fluoride-containing solution used in the present invention may be any of 80% HF concentration, 46% HF concentration, 35.37% aqueous solution which is an azeotrope with water, and mixed aqueous solution with other acids. Can also be used. The concentration of hydrofluoric acid is not limited to the concentration of the azeotrope, and a concentration lower than that, for example, in the range of 10 to 35% can be used. Examples of other acids used for mixing include inorganic acids such as nitric acid, sulfuric acid, hydrochloric acid, bromic acid and iodic acid, and organic acids such as trichloroacetic acid, trifluoroacetic acid and succinic acid. If treatment time and treatment temperature are variously set, the concentration of hydrofluoric acid, the concentration of other acids, and the mixing ratio of hydrofluoric acid and other acids can be set over a wide range, and the treatment conditions are particularly limited. However, for example, a mixed aqueous solution having an HF concentration of 18 to 23% and an inorganic acid concentration such as sulfuric acid and nitric acid of 20 to 40% is preferable. By using such an aqueous solution, usefulness similar to or higher than that of hydrofluoric acid (alone) is recognized. Thereby, there exists an effect which suppresses the density | concentration of hydrofluoric acid low.
The processing time varies depending on various processing conditions such as the degree of pulverization of natural graphite, the concentration of the processing solution and the amount of the processing solution, and temperature, but it is difficult to define it uniquely. Minutes to hours are often sufficient.

[Alkali treatment]
In the present invention, it is preferable to immerse the natural graphite particles in an alkaline aqueous solution prior to the acid treatment. By immersing with an aqueous alkali solution, silicic acid that is present in the graphite particles and hardly soluble in acid is first eluted with alkali, and then the remaining silicate is removed by immersing with an aqueous solution containing hydrogen fluoride. It becomes possible.
Known alkaline aqueous solutions can be used, and there is no particular limitation, and the concentration is not particularly limited, but those which dissolve silicic acid well such as sodium hydroxide and potassium hydroxide are preferable.
As with the acid treatment described above, the treatment time varies depending on various treatment conditions such as the degree of pulverization of natural graphite, the concentration of the treatment liquid and the quantity of the treatment liquid, and the temperature. It takes several minutes to several hours.

[Granulation]
Natural graphite particles immersed in a hydrogen fluoride-containing solution are granulated as necessary. Natural graphite impurities removal treatment by immersion in hydrogen fluoride-containing solution or aqueous alkali solution will increase the time required for the treatment liquid to penetrate into the particles if the natural graphite particles to be treated are large. become longer. Therefore, it is preferable from the viewpoint of the removal efficiency of impurities, etc., that after immersing the small natural graphite particles, it is granulated to have a desired particle size as the carbon material for battery electrodes.

  Specifically, natural graphite particles for immersion treatment have an average particle size of 0.1 to 50 μm, preferably 0.1 to 20 μm, more preferably 0.5 to 10 μm, and even more preferably 1 to 8 μm by laser diffraction. After the immersion treatment, the average particle size is 0.5 to 200 μm, preferably 0.5 to 100 μm, more preferably 1 to 80 μm, still more preferably 5 to 40 μm, and particularly preferably 10 to 30 μm. Granulate so that

  Since the particle shape after granulation can be the shape of the carbon material for battery electrodes, the average circularity measured by the flow type particle image analyzer is adjusted to 0.85 to 0.99 as described above. It is preferable that the content of particles having a circularity of less than 0.90 is controlled to be in the range of 2 to 20% by number. The particle shape before the immersion treatment is not particularly limited, and may have any shape such as a lump shape, a scale shape, a spherical shape, and a fibrous shape, but the spherical shape or the lump shape is easily granulated into particles having the above average circularity. This is preferable.

  Granulation is performed by binding with a binder force and / or van der Waals force between particle interfaces due to shearing. Specifically, there are a method of granulating by adding a phenol resin as a binder using a Redige mixer, a Spartan Luzer, or a method of granulating by binding using a mechanofusion, a hybridizer or the like.

[surface treatment]
In the present invention, particles before heat treatment, that is, natural graphite particles immersed in a hydrogen fluoride-containing solution or particles obtained by granulating the particles are treated with a resin as necessary. Here, the treatment with a resin refers to a treatment for adhering a resin to the particle surface and / or a treatment for infiltrating the resin into the inside of the particle.
If the graphite particles are treated with resin, and the resin is carbonized and fired by a subsequent heat treatment, when used as an electrode for a secondary battery, the Coulomb efficiency ("Discharge capacity / Charge capacity for the first time") )), An increase in irreversible capacity, and a decrease in cycle characteristics can be suppressed.

  As the resin to be used, those capable of forming a dense carbon layer by carbonization / firing treatment are preferable, and those having adhesion to graphite particles and / or vapor grown carbon fibers described later are preferable. Adhesive resin refers to a covalent bond, van der Waals force, hydrogen bond, etc. by interposing between both objects in order to keep graphite particles and vapor grown carbon fiber in contact with each other so as not to leave Both objects are integrated, including physical adhesion such as chemical bonding and anchor effect. In the processing such as mixing, stirring, solvent removal, and heat treatment, any material can be used as long as it exhibits resistance to forces such as compression, bending, peeling, impact, pulling, and tearing to such an extent that peeling does not occur.

  Specific examples of such a resin include at least one selected from the group consisting of phenol resin, polyvinyl alcohol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin, and epoxy resin, preferably Phenol resin and polyvinyl alcohol resin, and phenol resin is particularly preferable.

  The phenol resin is produced by a reaction between phenols and aldehydes, and unmodified phenol resins such as novolac type and resol type and partially modified phenol resins can be used. Moreover, rubbers, such as a nitrile rubber, can be mixed and used for a phenol resin as needed. Examples of the raw material phenols include phenol, cresol, xylenol, and alkylphenol having a C20 or lower alkyl group.

  Among the phenol resins, a so-called modified phenol resin in which a drying oil or a fatty acid thereof is mixed is preferable. By mixing the drying oil or its fatty acid, a denser carbonaceous material is formed after the heat treatment. This is because the portion of the unsaturated fat bond in the phenolic resin and the drying oil causes a chemical reaction to become a so-called drying oil-modified phenolic resin, which eases decomposition in the heat treatment (or baking) process and prevents foaming. Guessed. In addition, drying oil does not just have double bonds, but has rather long alkyl groups and ester bonds, which are also involved in terms of ease of gas release during the firing process. Conceivable.

  As a phenol resin mixed with drying oil or its fatty acid, phenol and drying oil are first added in the presence of a strong acid catalyst, then a basic catalyst is added to make the system basic, and formalin is added. Or a product obtained by reacting phenols with formalin and then adding a drying oil or a fatty acid thereof.

  Dry oil is a vegetable oil that has the property of solidifying and drying in a relatively short time when left in the air as a thin film, such as commonly known tung oil, linseed oil, dehydrated castor oil, soybean oil, cashew nut oil, etc. May be its fatty acid.

  The ratio of the drying oil or its fatty acid to the phenol resin is suitably 5 to 50 parts by mass of the drying oil or its fatty acid with respect to 100 parts by mass of the phenol and formalin condensate, for example. When the amount is more than 50 parts by mass, the adhesion to graphite particles and vapor grown carbon fibers as nuclei decreases.

  As a method of coating at least a part of the particle surface with a resin, for example, there is a method of adhering and / or penetrating a resin to graphite particles, a method of mixing graphite particles in a solution in which the resin itself is dissolved, or a resin raw material And a method in which graphite particles are mixed in a solution in which the monomer component, catalyst, additive and the like are dissolved, and then polymerized.

  In the method of mixing graphite particles into a solution in which the resin itself is dissolved, the resin solution is prepared by diluting the resin with water, acetone, ethanol, toluene, etc. to adjust the viscosity, and mixing the solution with the graphite particles. Stir. The atmosphere at the time of mixing may be any of atmospheric pressure, pressurization, and reduced pressure, but is preferably under reduced pressure because the affinity between the graphite particles and the resin solution is improved.

  The solvent used in the polymerization method after mixing graphite particles in a solution in which the monomer component, catalyst, additive, etc. as the resin raw material are dissolved is not particularly limited as long as the polymer raw material is dissolved and / or dispersed. Are water, acetone, ethanol, acetonitrile, ethyl acetate and the like.

  After stirring, it is filtered and washed, and the solvent is removed by drying by a known method such as hot air drying or vacuum drying. The drying temperature depends on the boiling point, vapor pressure and the like of the solvent used, but is specifically 50 ° C. or higher, preferably 100 ° C. or higher and 1000 ° C. or lower, more preferably 150 ° C. or higher and 500 ° C. or lower.

In order to sufficiently infiltrate the resin raw material or the solution thereof into the voids inside the graphite particles, it is preferable to perform evacuation once to dozens of times before or during stirring. By evacuating, the air remaining in the fine voids of the graphite particles can be extracted. However, since the resin raw material may be volatilized by evacuation, after mixing the graphite particles and the solvent, evacuation is performed and the pressure is returned to normal pressure, and then the resin raw material can be added and mixed. The lower the degree of vacuum, the better, but about 13 kPa to 0.13 kPa (about 100 torr to 1 torr) is preferable.
The atmosphere at the time of adhesion and / or infiltration may be any of atmospheric pressure, pressure and reduced pressure, but the method of adhering under reduced pressure is preferred because the affinity between the graphite particles and the resin raw material is improved. .

  As for the usage-amount of a monomer component, 4-500 mass parts is preferable with respect to 100 mass parts of graphite particles, More preferably, it is 100-500 mass parts. If the amount used is too small, sufficient performance cannot be obtained, and if it is too large, the particles aggregate, which is not preferable.

  The conditions for the polymerization after the above treatment are not particularly limited as long as the polymerization reaction proceeds, but it is usually performed by heating. The heating temperature varies depending on the polymer raw material to be used, and cannot be defined unconditionally. For example, the heating temperature can be in the range of 100 to 500 ° C.

Next, a method for attaching and / or penetrating graphite particles using a phenol resin raw material will be specifically described.
First, phenols, formaldehydes, a reaction catalyst and graphite particles are added to a reaction vessel and stirred. At this time, it is preferable that at least a stirrable amount of water is present as a solvent. The mixing ratio of phenols and formaldehydes is preferably set within a range of formaldehydes 1 to 3.5 with respect to phenols 1 in terms of molar ratio. Further, the graphite particles are preferably set within a range of 5 to 3000 parts by mass with respect to 100 parts by mass of phenols.
Prior to and / or during agitation, evacuation can be performed from one to a dozen times as described above. However, since many phenols and formaldehyde are volatilized when evacuated, the carbonaceous particles can be mixed with water before evacuation, and after returning to normal pressure, phenols and formaldehyde can be added and mixed. .

By the above stirring treatment, the resin raw material is sufficiently adhered and permeated into the graphite particles, and then polymerized. Polymerization can be carried out by heat treatment under conditions equivalent to general phenol resin production conditions, for example, 100 to 500 ° C.
When mixing phenols, formaldehydes, catalysts, graphite particles and water, the reaction system has a viscosity of about mayonnaise at the beginning of the reaction, but gradually the condensation reaction between phenols containing graphite particles and formaldehydes Things begin to separate from the water in the reaction system. After the reaction has progressed to the desired level, stirring is stopped and cooling produces black particles as a precipitate. This is washed and filtered, and the solvent can be used after sufficiently drying by a known method such as hot air drying or vacuum drying.

  The amount of the deposited resin can be increased by increasing the concentration of phenols and formaldehydes in the reaction system, and can be decreased by decreasing the concentration. Therefore, the control can be performed by adjusting the amount of water and by adjusting phenols and formaldehydes. These can be adjusted before the reaction, or can be adjusted by dropping into the system during the reaction.

[Vapor grown carbon fiber]
In the present invention, the vapor grown carbon fiber used by adhering to the graphite particles needs to be excellent in electrical conductivity, so that a high crystallinity is desirable. Also, when the carbon material is made into an electrode and incorporated in a lithium ion secondary battery, it is necessary to quickly pass a current through the entire negative electrode, so the crystal growth direction of vapor grown carbon fiber fibers is parallel to the fiber axis, It is preferable that the fiber is branched (branched). Moreover, if it is a branched fiber, it will become easy to electrically join between carbon particles with a fiber, and electroconductivity will improve.

The vapor grown carbon fiber can be produced, for example, by blowing an organic compound gasified with iron serving as a catalyst in a high temperature atmosphere.
Vapor grown carbon fiber can be used as it is, for example, heat treated at 800-1500 ° C., eg, graphitized at 2000-3000 ° C., but heat treated at about 1500 ° C. Is more preferred.

Further, as a preferred form of vapor grown carbon fiber, there is a branched fiber, but the branched portion may have a hollow structure where the entire fiber including that portion is in communication with each other. Therefore, the carbon layer which comprises the cylindrical part of a fiber is continuing. A hollow structure is a structure in which a carbon layer is wound in a cylindrical shape, and includes a structure that is not a complete cylinder, a structure that has a partial cut portion, and a structure in which two stacked carbon layers are bonded to one layer. . Further, the cross section of the cylinder is not limited to a perfect circle, but includes an ellipse or a polygon. Note that the interplanar spacing d ₀₀₂ of the carbon layer is not limited for the crystallinity of the carbon layer. Incidentally, it is preferable that the average interplanar distance d ₀₀₂ by the X-ray diffraction method is 0.344 nm or less, preferably 0.339 nm or less, more preferably 0.338 nm or less, and the thickness Lc in the C-axis direction of the crystal is 40 nm or less.

  The vapor grown carbon fiber is a carbon fiber having a fiber outer diameter of 2 to 1000 nm and an aspect ratio of 10 to 15000, preferably a fiber outer diameter of 10 to 500 nm, a fiber length of 1 to 100 μm (aspect ratio of 2 to 2000), or a fiber. The outer diameter is 2 to 50 nm and the fiber length is 0.5 to 50 μm (aspect ratio 10 to 25000).

  The vapor grown carbon fiber can be further heat-treated at 2000 ° C. or higher after its production to further increase the crystallinity and increase the conductivity. Also in this case, it is effective to add boron or the like having a function of promoting the degree of graphitization before the heat treatment.

  The content of the vapor grown carbon fiber is preferably in the range of 0.01 to 20% by mass of the carbon material for electrodes, preferably 0.1 to 15% by mass, and more preferably 0.5 to 10% by mass. When the content exceeds 20% by mass, the electric capacity decreases, and when the content is less than 0.01% by mass, the value of internal resistance at a low temperature (for example, −35 ° C.) increases.

  Vapor-grown carbon fibers often have irregularities and disturbances on the fiber surface, but their adhesion to the carbonaceous particles that form the core improves, and as a negative electrode active material and a conductive auxiliary agent even after repeated charge and discharge The vapor grown carbon fiber which also serves as a role can be kept in a close contact state without dissociating, the electron conductivity can be maintained, and the cycle characteristics can be improved.

In addition, when the vapor grown carbon fiber contains a lot of branched fibers, a network can be formed efficiently, and high electronic conductivity and thermal conductivity are easily obtained. Further, the active material can be dispersed so as to be wrapped, the strength of the negative electrode is increased, and the contact between the particles can be maintained well.
In addition, since the vapor grown carbon fiber enters between the particles, the liquid retaining property of the electrolytic solution is increased, and lithium ions are smoothly doped and undoped even in a low temperature environment.

The vapor-grown carbon fiber can be attached to the graphite particles via a resin that adheres and permeates the graphite particles.
Specifically, the vapor grown carbon fiber and graphite particles are mixed and stirred in a solution in which the resin itself is dissolved, or the vapor grown carbon fiber is dissolved in a solution in which monomer components, catalysts, additives, and the like as resin raw materials are dissolved. And graphite particles can be mixed and stirred and then polymerized.

  As the solvent to be used, any resin and gas phase grown carbon fiber or those having good affinity with the resin raw material and gas phase grown carbon fiber can be used, for example, water, alcohols, ketones, aromatic hydrocarbons. And esters. Preferably, water, methanol, ethanol, butanol, acetone, methyl ethyl ketone, toluene, ethyl acetate, butyl acetate and the like are preferable.

Although the stirring method is not particularly limited, for example, apparatuses such as a ribbon mixer, a screw type kneader, a spartan luzer, a redige mixer, a planetary mixer, and a universal mixer can be used.
In addition, it can carry out similarly to the above-mentioned resin adhesion / penetration treatment except using vapor grown carbon fiber.

[Heat treatment]
In the present invention, the acid-treated graphite particles, particles obtained by granulating the particles, or particles obtained by treating the particles with a resin and attaching vapor grown carbon fibers to the surface as necessary are heat-treated at 2400 to 3300 ° C. It is attached to. By performing the heat treatment, the resin or the like present on the surface is graphitized, and the discharge capacity is increased as a whole, and an improvement in initial efficiency due to a decrease in active sites can be expected. When the graphite particles are treated with a resin, the resin adhered to the particles becomes a carbonaceous material that is carbonized and fired by this heat treatment.

  In order to remove impurities and improve the crystallinity of carbon, the heat treatment temperature is preferably 2500 ° C. or higher, more preferably 2700 ° C. or higher.

Boron or a boron compound can be added before the heat treatment. By adding boron or a boron compound, crystallization by heat treatment can be promoted. Examples of the boron compound include boron carbide (B ₄ C), boron oxide (B ₂ O ₃ ), elemental boron, boric acid (H ₃ BO ₃ ), borate, and the like. Boron is added, for example, in the range of 50 to 2000 ppm with respect to the carbon material, and the particles may be contained in an amount of 1 to 800 ppm, preferably 1 to 600 ppm.

  The temperature increase rate for the heat treatment does not particularly affect the performance within the range of the maximum temperature increase rate and the minimum temperature increase rate in a known apparatus. However, since it is a powder, there is almost no problem of cracking as in the case of a molded material or the like, so that the heating rate should be fast from the viewpoint of cost. The arrival time from room temperature to the maximum temperature is preferably 12 hours or less, more preferably 6 hours or less, and particularly preferably 2 hours or less.

  As the heat treatment apparatus, a known apparatus such as an Atchison furnace or a direct current heating furnace can be used. These devices are also advantageous in terms of cost. However, since the presence of nitrogen gas may decrease the resistance of the powder or the strength of the carbonaceous material may decrease due to oxidation by oxygen, the furnace atmosphere can be preferably maintained in an inert gas such as argon or helium. A batch furnace having such a structure, for example, a batch furnace capable of gas replacement after evacuating the container itself, a batch furnace or a continuous furnace in which the atmosphere in the furnace can be controlled with a tubular furnace is preferable.

[Carbon material for battery electrode]
The battery electrode carbon material of the present invention produced by the above method is a carbon material with few impurities, in which the total content of elements other than carbon is reduced to 800 ppm or less. By adjusting the production conditions, the total content of elements other than carbon can be 700 ppm or less, and further 350 ppm or less.
The particles constituting the carbon material for battery electrodes of the present invention preferably have an average circularity of 0.85 to 0.99 as measured by a flow particle image analyzer. When the average circularity is less than 0.85, the packing density at the time of forming the electrode does not increase, so that the discharge capacity per volume decreases. On the other hand, when the average circularity is larger than 0.99, the fine powder portion has a low circularity, so that the fine powder portion is hardly included, and the discharge capacity at the time of forming the electrode does not increase. Furthermore, it is preferable that the content rate of particles having a circularity value of less than 0.90 is controlled in the range of 2 to 20% by number.

As for the particle size of the particles constituting the carbon material for battery electrodes of the present invention, the average particle size by laser diffraction method is preferably about 1 to 80 μm. More preferably, it is 10-40 micrometers, More preferably, it is 10-30 micrometers.
When the average particle size is smaller than 1 μm, the aspect ratio tends to increase and the specific surface area tends to increase. Further, for example, when producing an electrode of a battery, a method is generally employed in which a negative electrode material is made into a paste with a binder and applied. When the average particle diameter of the negative electrode material is less than 1 μm, fine powder smaller than 1 μm is contained considerably, and the viscosity of the paste increases and the applicability also deteriorates.
Further, when large particles having an average particle size of 80 μm or more are mixed, irregularities are increased on the electrode surface, which may cause damage to the separator used in the battery. For example, those substantially free of particles of 1 μm or less and particles of 80 μm or more can be suitably used.

The specific surface area of the particles constituting the carbon material for battery electrodes of the present invention is preferably as small as possible, and the BET specific surface area is preferably 0.2 to ⁵ m ² / g. More preferably, it is 0.2-3 m < ² > / g. If this value is exceeded, the surface activity of the particles increases, and the Coulomb efficiency decreases due to decomposition of the electrolyte and the like.

[Production of secondary battery]
The carbon material for battery electrodes of the present invention can be suitably used, for example, as a negative electrode material for lithium ion secondary batteries. The lithium ion secondary battery using the carbon material for battery electrodes of the present invention can be produced by a known method.

The electrode can be produced by diluting a binder (binder) with a solvent and kneading it with a negative electrode material as usual and applying it to a current collector (base material).
As the binder, known polymers such as a fluorine-based polymer such as polyvinylidene fluoride and polytetrafluoroethylene, and a rubber-based material such as SBR (styrene butadiene rubber) can be used. As the solvent, a known solvent suitable for each binder, for example, toluene, N-methylpyrrolidone or the like in the case of a fluorine-based polymer, and water or the like in the case of SBR can be used.

When the negative electrode material is 100 parts by mass, the amount of the binder used is suitably 1 to 30 parts by mass, and particularly preferably about 3 to 20 parts by mass.
For kneading the negative electrode material and the binder, a known apparatus such as a ribbon mixer, a screw type kneader, a Spartan rewinder, a Redige mixer, a planetary mixer, a universal mixer can be used.

Application to the current collector after kneading can be carried out by a known method. For example, a method of forming by a roll press or the like after applying with a doctor blade or a bar coater can be mentioned.
As the current collector, known materials such as copper, aluminum, stainless steel, nickel, and alloys thereof can be used.
Although a well-known thing can be used for a separator, especially polyethylene and a polypropylene nonwoven fabric are preferable.

  As the electrolyte and electrolyte in the lithium secondary battery of the present invention, known organic electrolytes, inorganic solid electrolytes, and polymer solid electrolytes can be used. Preferably, an organic electrolyte is preferable from the viewpoint of electrical conductivity.

  Examples of organic electrolytes include diethyl ether, dibutyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol dimethyl ether, and ethylene glycol phenyl ether. Ether; formamide, N-methylformamide, N, N-dimethylformamide, N-ethylformamide, N, N-diethylformamide, N-methylacetamide, N, N-dimethylacetamide, N-ethylacetamide, N, N-diethyl Acetamide, N, N-dimethylpropionamide, hexamethylphosphorylamide Amides such as dimethyl sulfoxide, sulfolane, etc .; dialkyl ketones such as methyl ethyl ketone, methyl isobutyl ketone; ethylene oxide, propylene oxide, tetrahydrofuran, 2-methoxytetrahydrofuran, 1,2-dimethoxyethane, 1,3-dioxolane, etc. Cyclic ethers; carbonates such as ethylene carbonate and propylene carbonate; γ-butyrolactone; N-methylpyrrolidone; solutions of organic solvents such as acetonitrile and nitromethane are preferred. Furthermore, preferably ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, vinylene carbonate, esters such as γ-butyrolactone, ethers such as dioxolane, diethyl ether, diethoxyethane, dimethyl sulfoxide, acetonitrile, tetrahydrofuran, etc. Particularly preferred are carbonate-based non-aqueous solvents such as ethylene carbonate and propylene carbonate. These solvents can be used alone or in admixture of two or more.

Lithium salts are used as solutes (electrolytes) for these solvents. Commonly known lithium salts include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCl, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiN (CF ₃ SO ₂ ) _{2 and the} like. is there.

  Examples of the polymer solid electrolyte include a polyethylene oxide derivative and a polymer containing the derivative, a polypropylene oxide derivative and a polymer containing the derivative, a phosphate ester polymer, a polycarbonate derivative and a polymer containing the derivative.

In the lithium secondary battery using the negative electrode material in the present invention, the positive electrode material used is a lithium-containing transition metal oxide. Preferably, an oxide mainly containing at least one transition metal element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Mo and W and lithium, wherein the molar ratio of lithium to the transition metal Is a compound of 0.3 to 2.2. More preferably, the oxide mainly contains at least one transition metal element selected from V, Cr, Mn, Fe, Co, and Ni and lithium, and the molar ratio of lithium to the transition metal is 0.3 to 0.3. The compound of 2.2. In addition, Al, Ga, In, Ge, Sn, Pb, Sb, Bi, Si, P, B, or the like may be contained in a range of less than 30 mole percent with respect to the transition metal present mainly. Among the above positive electrode active materials, the general formula Li _x MO ₂ (M is at least one of Co, Ni, Fe and Mn, x = 0 to 1.2), or Li _y N ₂ O ₄ (N is It is preferable to use at least one material having a spinel structure represented by at least Mn and y = 0-2.

Further, the positive electrode active material _{Li y M a D 1-a} O 2 (M is Co, Ni, Fe, at least one of Mn, D is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag , W, Ga, In, Sn, Pb, Sb, Sr, B, P, at least one type other than M, y = 0 to 1.2, a = 0.5 to 1.), or _{li z (N b E 1-} b) 2 O 4 (N is Mn, E is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag, W, Ga, in, Sn, Pb, Sb, It is particularly preferable to use at least one material having a spinel structure represented by at least one of Sr, B, and P, b = 1 to 0.2, and z = 0 to 2.

_{Specifically, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Co b V 1-b O z, Li x Co b Fe 1-b _{O 2, Li x Mn 2 O} 4, Li x Mn c Co 2-c O 4, Li x Mn c Ni 2-c O 4, Li x Mn c V 2-c O 4, Li x Mn c Fe 2- _c O ₄ (where x = 0.02 to 1.2, a = 0.1 to 0.9, b = 0.8 to 0.98, c = 1.6 to 1.96, z = 2. 01-2.3.). The most preferred lithium-containing transition metal _{oxides, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Mn 2 O 4, Li x Co b V 1 _-b O _z (x = 0.02-1.2, a = 0.1-0.9, b = 0.9-0.98, z = 2.01-2.3). In addition, the value of x is a value before the start of charging / discharging, and increases / decreases by charging / discharging.

The average particle size of the positive electrode active material is not particularly limited, but is preferably 0.1 to 50 μm. It is preferable that the volume of particles of 0.5 to 30 μm is 95% or more. More preferably, the volume occupied by a particle group having a particle size of 3 μm or less is 18% or less of the total volume, and the volume occupied by a particle group of 15 μm or more and 25 μm or less is 18% or less of the total volume. Although the specific surface area is not particularly limited, but is preferably 0.01 to 50 m ² / g by the BET method, particularly preferably 0.2m ² / g~1m ² / g. The pH of the supernatant when 5 g of the positive electrode active material is dissolved in 100 ml of distilled water is preferably 7 or more and 12 or less.

  There are no restrictions on the selection of members necessary for battery configuration other than those described above.

  The carbon material for battery electrodes of the present invention sufficiently removes impurities in natural graphite, has a large discharge capacity as a negative electrode material for lithium ion secondary batteries, and has excellent coulomb efficiency. In addition, the carbon material production method of the present invention uses inexpensive natural graphite having excellent availability as a raw material, is relatively easy to operate, and can be said to be excellent in economy and mass productivity.

The present invention will be described in more detail below with typical examples. Note that these are merely illustrative examples, and the present invention is not limited thereto.
The physical properties used in the following examples were measured by the following methods.

Average circularity:
The average circularity of the carbon material was measured as follows using a flow particle image analyzer FPIA-2100 (manufactured by Sysmex Corporation).
The sample for measurement was purified by removing fine dust through a 106 μm filter. A sample dispersion for measurement was prepared by adding 0.1 g of the sample into 20 ml of ion-exchanged water and uniformly dispersing 0.1 to 0.5% by mass of an anionic / nonionic surfactant. Dispersion was performed by treating for 5 minutes using an ultrasonic cleaner UT-105S (manufactured by Sharp Manufacturing System).
The outline of the measurement principle and the like is described in “Powder and Industry”, VOL. 32, No. 2, 2000, Japanese Patent Laid-Open No. 8-136439, and the like.
When the dispersion liquid of the measurement sample passes through the flow path of a flat and transparent flow cell (thickness: about 200 μm), strobe light is irradiated at 1/30 second intervals and imaged with a CCD camera. A certain number of the still images were taken and analyzed, and calculated according to the following formula.
Circularity = (circle circumference obtained from equivalent circle diameter) / (perimeter of particle projection image)
The equivalent circle diameter is the diameter of a true circle having the same projected area as the circumference of the actually imaged particle, and the circumference of the circle obtained from this equivalent circle diameter is divided by the circumference of the actually imaged particle. Value. For example, the value is 1 for a perfect circle, and the value becomes smaller as the shape becomes more complicated. The average circularity is an average value of circularity of each measured particle.

Average particle size:
It measured using the laser diffraction scattering type particle size distribution measuring apparatus Microtrac HRA (made by Nikkiso Co., Ltd.).

Specific surface area:
Using a specific surface area measuring device NOVA-1200 (manufactured by Yuasa Ionics Co., Ltd.), the measurement was performed by the BET method, which is a general method for measuring the specific surface area.
C ₀ : Determined by X-ray diffraction method.
Impurity amount: Measured by fluorescent X-ray analysis.

Battery evaluation method:
(1) Preparation of paste 0.1 mass part of KF polymer L1320 (N-methylpyrrolidone (NMP) solution product containing 12 mass% of polyvinylidene fluoride (PVDF)) made by Kureha Chemical Co., Ltd. was added to 1 mass part of the negative electrode material. The mixture was kneaded with a Lee mixer to obtain a main agent stock solution.

(2) Electrode preparation After adding NMP to the main agent stock solution and adjusting the viscosity, it was applied on a high-purity copper foil to a thickness of 250 μm using a doctor blade. This was vacuum-dried at 120 ° C. for 1 hour and punched out to 18 mmφ. Further, the punched electrode is sandwiched between super steel press plates, and the press pressure is about 1 × 10 ^{2 to} 3 × 10 ² N / mm ² (1 × 10 ^{3 to} 3 × 10 ³ kg / cm ² ) with respect to the electrode. It pressed so that it might become. Then, it dried at 120 degreeC and 12 hours with the vacuum dryer, and was set as the electrode for evaluation.

(3) Battery preparation A triode cell was prepared as follows. The following operation was carried out in a dry argon atmosphere with a dew point of -80 ° C or lower.
In a cell (with an inner diameter of about 18 mm) with a screw-in type lid made of polypropylene, the carbon electrode with copper foil (positive electrode) and metal lithium foil (negative electrode) produced in (2) above were separated by a separator (polypropylene microporous film (Selga -2400)). Further, metallic lithium for reference was laminated in the same manner. An electrolytic solution was added thereto to obtain a test cell.

(4) Electrolyte (i) EC system: A mixture of 8 parts by mass of EC (ethylene carbonate) and 12 parts by mass of DEC (diethyl carbonate), and 1 mol / liter of LiPF ₆ was dissolved as an electrolyte.

(5) Charging / discharging cycle test A constant current low voltage charging / discharging test was conducted at a current density of 0.2 mA / cm ² (equivalent to 0.1 C).
Charging (insertion of lithium into carbon) was performed by CC (constant current: constant current) at 0.2 mA / cm ² from the rest potential to 0.002V. Next, it switched to CV (constant volt: constant voltage) charging at 0.002 V, and stopped when the current value decreased to 25.4 μA.
The discharge (release from carbon) was CC discharge at 0.2 mA / cm ² (equivalent to 0.1 C), and cut off at a voltage of 1.5 V.

Example 1: Acid treatment Natural graphite made in China (ash content 16.5% by mass, see Table 1 for details of ingredients) was pulverized with a hammer mill (Hosokawa Micron Bantam Mill), and airflow classifier (Nisshin Engineering Turbo Classifier) To adjust the average particle size to 5 μm to produce natural graphite fine powder.
An acidic aqueous solution (acid A) in which 50 parts by mass of 46% hydrofluoric acid and 50 parts by mass of distilled water are mixed, and an acidic aqueous solution (acid B) in which 50 parts by mass of reagent grade 97% sulfuric acid and 50 parts by mass of distilled water are mixed. Further, an acidic aqueous solution (acid C) in which 50 parts by mass of acid A and 50 parts by mass of acid B were mixed was prepared.
300 parts by weight of acid C was added to 100 parts by weight of natural graphite fine powder, and the mixture was stirred and mixed at 25 ° C. for 6 hours. Then, it filtered, washed with water, and dried at 120 degreeC for 5 hours.
This was subjected to spheronization treatment with Hosokawa Micron Mechano-Fusion to obtain an average particle size of 20 μm, then packed in a graphite crucible and heated to 2800 ° C. in an argon atmosphere, and the resulting fine powder was used as a negative electrode material.

Example 2: Alkali treatment + acid treatment To 100 parts by mass of natural graphite fine powder prepared in Example 1, 300 parts by mass of a 1 mol / liter sodium hydroxide aqueous solution was added and stirred and mixed at 25 ° C. for 6 hours. This was filtered and washed with water, and 300 parts by mass of acid C was added to the total amount, followed by stirring and mixing at 25 ° C. for 6 hours. Then, it filtered, washed with water, and dried at 120 degreeC for 5 hours.
This was subjected to spheronization treatment with Hosokawa Micron Mechano-Fusion to obtain an average particle size of 20 μm, then packed in a graphite crucible and heated to 2800 ° C. in an argon atmosphere, and the resulting fine powder was used as a negative electrode material.

Example 3 Alkali Treatment + Acid Treatment + Resin Treatment 100 parts by weight of natural graphite fine powder prepared in Example 1 and 300 parts by weight of a sodium hydroxide aqueous solution having a concentration of 1 mol / liter were added and mixed with stirring at 25 ° C. for 6 hours. This was filtered and washed with water, and 300 parts by mass of acid C was added to the total amount, followed by stirring and mixing at 25 ° C. for 6 hours. Then, it filtered, washed with water, and dried at 120 degreeC for 5 hours.
This was spheroidized with Mechanofusion manufactured by Hosokawa Micron to obtain an average particle size of 20 μm. 500 parts by mass of this powder, 398 parts by mass of phenol, 466 parts by mass of 37% formalin, 38 parts by mass of hexamethylenetetramine as a reaction catalyst, and 385 parts by mass of water were added. This was stirred at 60 rpm for 20 minutes. Next, while stirring, the container was evacuated to 0.4 kPa (3 Torr) and held for 5 minutes, and the operation of returning to atmospheric pressure was repeated three times to impregnate the liquid into the granulated product. Further, the mixture was heated and maintained at 150 ° C. while stirring was continued. The contents initially had a mayonnaise-like fluidity, but gradually, a reaction product of phenol and formaldehyde containing graphite and a water-based layer began to separate, and after about 15 minutes from graphite and phenolic resin. As a result, the black granular material was dispersed in the reaction vessel. After that, after further stirring for 60 minutes at 150 ° C., the contents of the reaction vessel were cooled to 30 ° C. and stirring was stopped. The black granular material obtained by filtering the contents of the container is washed with water, further filtered, and dried using a fluidized bed dryer at a hot air temperature of 55 ° C. for 5 hours, whereby graphite / phenolic resin granular material is obtained. Obtained.
Next, this graphite phenol resin granular material was pulverized with a Henschel mixer at 1800 rpm for 5 minutes. This was put into a heating furnace, the inside of the furnace was replaced with a vacuum to make it under an argon atmosphere, and then the temperature was raised while flowing argon gas. It was kept at 2800 ° C. for 10 minutes and then cooled. After cooling to room temperature, the obtained heat-treated product was sieved with a sieve having an aperture of 63 μm, and the fine powder under the sieve was used as a negative electrode material.

Comparative Example 1: Sulfuric Acid Treatment 100 parts by mass of natural graphite fine powder prepared in Example 1 and 300 parts by mass of acid B were stirred and mixed at 25 ° C. for 6 hours. Thereafter, it was filtered, washed with water, and dried at 120 ° C. for 5 hours.
This was subjected to spheronization treatment with Hosokawa Micron Mechano-Fusion to obtain an average particle size of 20 μm, then packed in a graphite crucible and heated to 2800 ° C. in an argon atmosphere, and the resulting fine powder was used as a negative electrode material.

Comparative Example 2: Sulfuric acid treatment + resin treatment 100 parts by mass of natural graphite fine powder prepared in Example 1 and 300 parts by mass of acid B were stirred and mixed at 25 ° C for 6 hours. Thereafter, it was filtered, washed with water, and dried at 120 ° C. for 5 hours.
This was spheroidized with Mechanofusion manufactured by Hosokawa Micron to obtain an average particle size of 20 μm, and then the same coating treatment as in Example 3 was performed. This was packed in a graphite crucible and heated to 2800 ° C. in an argon atmosphere, and the resulting fine powder was used as a negative electrode material.

Comparative Example 3: Sulfuric acid treatment + resin treatment (heat treatment temperature 1000 ° C.)
In Comparative Example 2, instead of heating at 2800 ° C., heating was performed at 1000 ° C., and other operations were performed in the same manner as in Comparative Example 2 to obtain a fine powder, which was used as a negative electrode material.

Comparative Example 4: Resin treatment (no acid treatment)
The natural graphite fine powder prepared in Example 1 was subjected to spheronization treatment with Meso-Fuso made by Hosokawa Micron to obtain an average particle size of 20 μm, then packed in a graphite crucible and heated to 2800 ° C. in an argon atmosphere. Fine powder was used as the negative electrode material.

  About the negative electrode material (fine powder) manufactured by Examples 1-3 and Comparative Examples 1-4, the capacity | capacitance efficiency of the 1st charge / discharge cycle test and the capacity | capacitance of 50th cycle were investigated. The results are shown in Table 2.

Claims (25)

  Including a step of immersing natural graphite particles having an average particle size of 0.1 to 100 μm with a hydrogen fluoride-containing solution and a step of heat-treating the immersed natural graphite particles at a temperature of 2400 ° C. or higher and lower than 3300 ° C., A method for producing a carbon material, wherein the total content of elements other than the above is 800 ppm or less.   The method for producing a carbon material according to claim 1, wherein a step of immersing with an alkaline aqueous solution is provided before the step of immersing with the hydrogen fluoride-containing solution.   The natural graphite particles immersed in the hydrogen fluoride-containing solution are granulated by a binder force and / or a van der Waals force between particle interfaces due to shear before the heat treatment step. Carbon material manufacturing method.   In the granulation step, natural graphite particles having an average particle diameter of 0.1 to 50 μm that have been immersed in a hydrogen fluoride-containing solution have an average circularity of 0.85 to 0.99 and an average particle diameter of 0.5 to 200 μm. The method for producing a carbon material according to claim 3, which is a step of performing granulation as described above.   The method for producing a carbon material according to any one of claims 1 to 4, wherein a step of coating at least a part of the particle surface with a resin is provided before the heat treatment step.   The method for producing a carbon material according to claim 1, wherein the hydrogen fluoride concentration (HF concentration) of the hydrogen fluoride-containing solution is 10 to 80%.   The carbon material production according to claim 6, wherein the hydrogen fluoride-containing solution is an aqueous solution in which at least one selected from nitric acid, sulfuric acid, hydrochloric acid, bromic acid, iodic acid, trichloroacetic acid, trifluoroacetic acid and oxalic acid is mixed. Method.   The resin covering at least a part of the particle surface is a polymer containing at least one selected from the group consisting of phenol resin, polyvinyl alcohol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin, and epoxy resin. Item 6. A method for producing a carbon material according to Item 5.   The method for producing a carbon material according to claim 5, wherein the resin covering at least a part of the particle surface is a phenol resin.   The method for producing a carbon material according to claim 8, wherein the amount of the resin covering at least a part of the particle surface is 2 to 80% by mass of the particle mass.   The method for producing a carbon material according to claim 9, wherein the phenol resin is prepared by adding a drying oil or a fatty acid thereof during the reaction of phenols and formaldehydes.   The method for producing a carbon material according to claim 9, wherein vapor-grown carbon fibers are present during the reaction of phenols and formaldehyde, and vapor-grown carbon fibers are adhered to the particle surfaces by the generated phenol resin.   The method for producing a carbon material according to claim 12, wherein the amount of vapor grown carbon fiber attached to the particles is 0.01 to 20% by mass of the particle mass.   A carbon material comprising natural graphite particles having a total content of elements other than carbon of 800 ppm or less.   The carbon material according to claim 14, wherein at least a part of the particle surface is coated with a carbonaceous material obtained by carbonizing and firing a resin.   The carbon material according to claim 15, wherein the resin is a phenol resin.   The vapor grown carbon fiber having a hollow structure inside and having an outer diameter of 2 to 1000 nm and an aspect ratio of 10 to 15000 is attached to the surface of at least some of the particles via a carbonaceous material. Carbon material. The carbon material according to claim 17, wherein the vapor grown carbon fiber is made of carbon having an average interplanar spacing (d ₀₀₂ ) of 0.344 nm or less (d ₀₀₂ ) by X-ray diffraction.   The carbon material according to claim 14 containing 1 to 800 ppm of boron. The carbon material according to any one of claims 14 to 19, which satisfies the following requirements (1) to (3):
(1) An average circularity of 0.85 to 0.99,
(2) a specific surface area of 0.2 to ⁵ m ² / g,
(3) The average particle size is 10 to 40 μm.   A carbon material obtained by the production method according to claim 1.   An electrode paste comprising the carbon material according to any one of claims 14 to 21 and a binder.   An electrode comprising a molded body of the electrode paste according to claim 22.   A secondary battery comprising the electrode according to claim 23 as a constituent element.   A non-aqueous electrolyte and an electrolyte are used, and the non-aqueous electrolyte is at least one selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, vinylene carbonate, γ-butyrolactone and propylene carbonate. 24. The secondary battery according to 24.