Method for producing flaked graphite dispersion and flaked graphite dispersion

Abstract

The present invention addresses the problem of providing a method for producing a flaked graphite dispersion having high dispersibility of flaked graphite. This method for producing a flaked graphite dispersion comprises: a first step in which a material to be treated, which contains one selected from the group consisting of graphite and a graphite compound, and a first organic solvent other than an amide-based solvent, a ketone-based solvent, or a sulfoxide-based solvent, is subjected to a wet jet mill treatment to obtain flaked graphite; and a second step in which the product obtained in step 1 is mixed with at least one specific solvent selected from the group consisting of an amide-based solvent, a ketone-based solvent, and a sulfoxide-based solvent to obtain a flaked graphite dispersion.

Description

Method for producing exfoliated graphite dispersion and exfoliated graphite dispersion

[0001] The present invention relates to a method for producing an exfoliated graphite dispersion. The present invention also relates to an exfoliated graphite dispersion.

[0002] In recent years, exfoliated graphite, which has a small number of graphene stacks in graphite, has been attracting attention. A known method for producing exfoliated graphite is to produce a graphite intercalation compound (GIC) from graphite and then subject the compound to an exfoliation treatment. Various methods have been proposed for producing the GIC, including a gas-phase method and a solution method. Among these, a solution method for producing a GIC has attracted attention because it allows GIC to be produced more easily than a gas-phase method.

[0003] A method for producing exfoliated graphite by subjecting GIC to an exfoliation treatment is disclosed, for example, in Patent Document 1. Specifically, a method for producing exfoliated graphite by adding a polar aprotic solvent to GIC and subjecting the resulting mixture to ultrasonic treatment is disclosed. The above procedure results in an exfoliated graphite dispersion in which exfoliated graphite is dispersed in a solution.

[0004] JP 2015-105200 A

[0005] The present inventors produced an exfoliated graphite dispersion using the production method described in Patent Document 1, and found that the dispersibility of exfoliated graphite in the resulting exfoliated graphite dispersion was difficult to increase.

[0006] Therefore, an object of the present invention is to provide a method for producing an exfoliated graphite dispersion liquid that increases the dispersibility of exfoliated graphite. Another object of the present invention is to provide an exfoliated graphite dispersion liquid.

[0007] As a result of extensive research into solving the above problems, the present inventors have found that the above problems can be solved by the following configuration.

[0008] [1] A method for producing an exfoliated graphite dispersion, comprising: Step 1 of subjecting a workpiece containing one selected from the group consisting of graphite and a graphite compound and a first organic solvent other than an amide-based solvent, a ketone-based solvent, and a sulfoxide-based solvent to wet jet milling to obtain exfoliated graphite; and Step 2 of mixing the product obtained in Step 1 with at least one specific solvent selected from the group consisting of an amide-based solvent, a ketone-based solvent, and a sulfoxide-based solvent to obtain an exfoliated graphite dispersion. [2] A method for producing an exfoliated graphite dispersion according to [1], using the graphite compound in Step 1. [3] A method for producing an exfoliated graphite dispersion according to [2], wherein the graphite compound is a graphite compound in which an alkali metal source and graphite are mixed in a second organic solvent other than an amide-based solvent, a ketone-based solvent, and a sulfoxide-based solvent, and the alkali metal is intercalated between graphene layers in the graphite. [4] The method for producing an exfoliated graphite dispersion according to any one of [1] to [3], wherein in the step 2, the mass ratio of the specific solvent to the product is 0.5 to 100. [5] The method for producing an exfoliated graphite dispersion according to any one of [1] to [4], wherein the specific solvent is at least one solvent selected from the group consisting of N,N-dimethylformamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide, cyclohexanone, and dimethyl sulfoxide. [6] The method for producing an exfoliated graphite dispersion according to any one of [1] to [5], wherein the first organic solvent includes an ether-based solvent. [7] An exfoliated graphite dispersion comprising exfoliated graphite, a first organic solvent other than an amide solvent, a ketone solvent, or a sulfoxide solvent, and at least one specific solvent selected from the group consisting of an amide solvent, a ketone solvent, and a sulfoxide solvent, wherein the content of the first organic solvent is 0.1 mass% or more and less than 50 mass% with respect to the total mass of the exfoliated graphite dispersion, and the content of the specific solvent is 50 mass% or more with respect to the total mass of the exfoliated graphite dispersion.[8] The exfoliated graphite dispersion liquid according to [7], wherein the specific solvent is at least one solvent selected from the group consisting of N,N-dimethylformamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide, cyclohexanone, and dimethyl sulfoxide. [9] The exfoliated graphite dispersion liquid according to [7] or [8], wherein the first organic solvent is an ether-based solvent.

[0009] According to the present invention, a method for producing an exfoliated graphite dispersion liquid in which the dispersibility of exfoliated graphite is increased can be provided. Also, according to the present invention, an exfoliated graphite dispersion liquid can be provided.

[0010] FIG. 2 is a schematic diagram illustrating a distribution unit.

[0011] The present invention will be described in detail below. The following description of the components may be based on representative embodiments of the present invention, but the present invention is not limited to such embodiments.

[0012] The meaning of each description in this specification is as follows: In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0013] In this specification, exfoliated graphite is obtained by exfoliating original graphite, and the number of stacked graphene layers (graphene sheets) in the exfoliated graphite may be smaller than that of the original graphite. Examples of exfoliated graphite include graphene and graphene stacks. When the exfoliated graphite is a graphene stack, the number of stacked graphene layers in the stack is not particularly limited, but is preferably 2 or more. There is no particular upper limit, but it is preferably 100 layers or less, and more preferably 30 layers or less.

[0014] <Method for Producing Exfoliated Graphite Dispersion> The method for producing an exfoliated graphite dispersion of the present invention includes Step 1 of subjecting a workpiece containing one selected from the group consisting of graphite and graphite compounds and a first organic solvent other than amide solvents, ketone solvents, and sulfoxide solvents to wet jet mill treatment to obtain exfoliated graphite. The method for producing an exfoliated graphite dispersion of the present invention also includes Step 2 of mixing the product obtained in Step 1 with a specific solvent containing one selected from the group consisting of amide solvents, ketone solvents, and sulfoxide solvents to obtain an exfoliated graphite dispersion.

[0015] The reason why the dispersibility of exfoliated graphite in the exfoliated graphite dispersion liquid is increased by the exfoliated graphite dispersion liquid production method of the present invention (hereinafter also referred to as "the present production method") is not necessarily clear, but the inventors speculate as follows: In the above step 1, when wet jet mill treatment is performed in a first organic solvent that is not the above specific solvent, graphite or a graphite compound is likely to be exfoliated. Furthermore, it is thought that in the above step 2, when the obtained product is mixed with the above specific solvent, exfoliated graphite is likely to be dispersed in the specific solvent, and a highly dispersible exfoliated graphite dispersion liquid is obtained.

[0016] The present manufacturing method will be described below. Note that the present manufacturing method may include steps other than step 1 and step 2 described below.

[0017] [Step 1] In step 1, a workpiece containing one selected from the group consisting of graphite and graphite compounds and a first organic solvent other than an amide solvent, a ketone solvent, or a sulfoxide solvent (hereinafter also referred to as "solvent X") is subjected to a wet jet mill treatment to obtain exfoliated graphite. Step 1 will be described in detail below.

[0018] (Graphite and graphite compound) In step 1, the material to be treated contains one selected from the group consisting of graphite and graphite compounds. The graphite is not particularly limited as long as it is a compound having a structure in which graphene is stacked. Examples of graphite include natural graphite, synthetic graphite (artificial graphite), highly oriented pyrolytic graphite, and graphite fiber. Of these, natural graphite is preferred.

[0019] Examples of graphite compounds include graphite intercalation compounds in which other compounds are inserted between the graphene layers of graphite. Examples of other compounds contained in graphite compounds (graphite intercalation compounds) include, for example, one or more compounds selected from the group consisting of alkali metals, organic compounds, inorganic compounds, and ions thereof. The other compounds may form complexes. More specifically, examples include alkali metals, ether-based solvents, aromatic compounds, and inorganic ions. The graphite compound is preferably a graphite compound in which an alkali metal is intercalated between the graphene layers of graphite. The graphite compound of this embodiment can be obtained, for example, by mixing an alkali metal source and graphite in a second organic solvent (solvent Y) described below. A method for producing the graphite compound of this embodiment will be described in detail later. The graphite compound may also be so-called expanded graphite. Expanded graphite refers to a graphite compound in which sulfate ions, nitrate ions, etc. are inserted between the graphene layers. Expanded graphite can be obtained, for example, by immersing graphite in a sulfuric acid aqueous solution containing an oxidizing agent.

[0020] (First Organic Solvent (Solvent X)) In step 1, the material to be treated contains a first organic solvent (solvent X) other than an amide-based solvent, a ketone-based solvent, or a sulfoxide-based solvent. Amide-based solvents, ketone-based solvents, and sulfoxide-based solvents will be described in detail later. Examples of the solvent X include hydrocarbon-based solvents, ether-based solvents, and ester-based solvents. Among these, ether-based solvents are preferred because they more easily exfoliate the graphite or graphite compounds contained in the material to be treated.

[0021] A hydrocarbon solvent refers to a solvent of a compound consisting only of C and H. The hydrocarbon solvent may have a cyclic structure. Examples of hydrocarbon solvents include pentane, hexane, heptane, octane, benzene, toluene, xylene, styrene, cyclopentane, cyclohexane, and methylcyclohexane.

[0022] The ether solvent is not particularly limited as long as it has an ether bond (—O—), and the number of ether bonds in the molecule is not limited. The ether solvent may also have a cyclic structure. Examples of ether solvents include dimethyl ether, diethyl ether, 1,2-dimethoxyethane (DME or glyme), diethylene glycol dimethyl ether (diglyme), furan, tetrahydrofuran (THF), 1,3-dioxolane (DOL), tetraethylene glycol dimethyl ether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), tetraethylene glycol dibutyl ether (DEGDBE), bis(2-ethoxyethyl) ether, and dihydrolevoglucosenone (Cyrene (registered trademark)). Among these, 1,2-dimethoxyethane or tetrahydrofuran is preferred because they more easily exfoliate the graphite or graphite compounds contained in the workpiece.

[0023] The ester solvent is not particularly limited as long as it has an ester bond (—O(CO)—), and the number of ester bonds in the molecule is not particularly limited. The ester solvent may have a cyclic structure. Examples of the ester solvent include methyl acetate, ethyl acetate, butyl acetate, isobutyl acetate, normal propyl acetate, 2-methoxy-1-methylethyl acetate (propylene glycol monomethyl ether acetate), and 3-methoxy-3-methylbutyl acetate.

[0024] The solvent X may be used alone or in combination of two or more.

[0025] (Wet Jet Mill Treatment) In step 1, a wet jet mill treatment is performed on a workpiece containing one selected from the group consisting of graphite and graphite compounds and the solvent X. The workpiece and the wet jet mill treatment will be described below.

[0026] The material to be treated is subjected to wet jet mill treatment, and in order to efficiently obtain exfoliated graphite, the solid content concentration and viscosity are preferably within the ranges detailed below.

[0027] From the viewpoint of efficiently obtaining exfoliated graphite, the solid content concentration of the material to be treated is preferably 0.1 to 10 mass %, more preferably 0.5 to 5 mass %, relative to the total mass of the material to be treated. The solid content in the material to be treated refers to the components excluding the solvent in the material to be treated.

[0028] The viscosity of the material to be treated is preferably 1 to 20,000 mPa·s, since it can be treated using a wet jet mill. Viscosity can be measured by any known method, such as the method of JIS Z 8803:2011.

[0029] The solids concentration and viscosity of the material to be treated can be adjusted, for example, by the content of solvent X in the material to be treated.

[0030] The material to be treated may contain one selected from the group consisting of graphite and graphite compounds, as well as components other than the organic solvent (other components). Examples of other components include surfactants. Other components also include components used to produce the graphite compound.

[0031] Wet jet milling is advantageous in that it applies shear forces to the workpiece, allowing for efficient production of exfoliated graphite without excessively pulverizing the graphite or graphite compound. There are no limitations on the wet jet milling method, as long as it allows a slurry containing a solvent and powder to flow at high speed, resulting in pulverization and / or disintegration of the powder. Examples of wet jet milling methods include a method in which a high-speed flowing fluid is caused to collide with a fluid collision section that is approximately perpendicular to the direction of the fluid flow, a method in which a high-speed flowing fluid is passed through a constricted section, a method in which multiple high-speed flowing fluids are caused to collide against each other, a method in which turbulence is generated and a fluid is caused to collide against the wall of a pipe or the like through which the fluid flows, and a method in which a shear force is applied to a fluid by a cavitation jet generated by, for example, depressurization. Wet jet milling may be a combination of these methods. Among these, a method in which multiple high-speed flowing fluids are caused to collide against each other.

[0032] An example of a wet jet mill process in which multiple fluids collide against each other in a counter-flow manner is a process that includes the following steps A, B, and C in this order: Step A: A step of causing objects to collide against each other in a counter-flow manner; Step B: A step of causing the collided objects to flow in a direction different from the direction in Step A; and Step C: A step of separating and flowing the objects to be processed.

[0033] While the method for performing the above steps A to C is not particularly limited, it is preferable to use a dispersion unit shown in FIG. 1 . The dispersion unit shown in FIG. 1 will be described below. The dispersion unit 10 shown in FIG. 1 is a unit disposed in an apparatus for performing wet jet mill processing. As described below, steps A to C can be performed by passing the material to be processed through the flow path in this dispersion unit 10. The dispersion unit 10 includes an inlet-side disk 12, an intermediate disk 14, and a discharge-side disk 16. The inlet-side disk 12, the intermediate disk 14, and the discharge-side disk 16 are disk-shaped and have approximately the same diameter. The intermediate disk 14 is disposed downstream of the inlet-side disk 12 in close contact with the center axis C1. The discharge-side disk 16 is disposed downstream of the intermediate disk 14 in close contact with the center axis C1. The inlet-side disk 12, the intermediate disk 14, and the discharge-side disk 16 are made of wear-resistant materials such as ceramics, cemented carbide, and diamond, and are formed with approximately the same diameter. The lead-in side disk 12 and the discharge-side disk 16 are formed to have approximately the same thickness, and the intermediate disk 14 is formed to have a thickness thinner than the lead-in side disk 12 and the discharge-side disk 16 .

[0034] The lead-side disk 12 has a first through hole 12A and a second through hole 12B. The first through hole 12A and the second through hole 12B have approximately the same diameter and are arranged in approximately symmetrical positions with respect to the center of the lead-side disk 12. An lead-side groove-like passage 18 (first flow path) having a width smaller than the diameters of the first through hole 12A and the second through hole 12B is arranged in a straight line on the surface of the lead-side disk 12 facing the intermediate disk 14. The first through hole 12A and the second through hole 12B are connected via the lead-side groove-like passage 18. The intermediate disk 14 has a third through hole 14A (second flow path) arranged in its center. The discharge-side disk 16 has a fourth through hole 16A and a fifth through hole 16B with approximately the same diameter and arranged in symmetrical positions with respect to the center of the discharge-side disk 16. A discharge-side groove-like passage 20 (third flow path) formed with a width smaller than the diameters of the fourth through hole 16A and the fifth through hole 16B is disposed on the surface of the discharge-side disk 16 facing the intermediate disk 14. The fourth through hole 16A and the fifth through hole 16B are communicated with each other via the discharge-side groove-like passage 20.

[0035] Next, the flow of steps A to C using the dispersion unit will be described. First, the material to be processed is pressurized and introduced into the dispersion unit 10 as an ultra-high-speed fluid. The pressure is preferably between 100 MPa and 250 MPa. Upon reaching the inlet-side disk 12, the introduced material L branches off and flows through the first through-hole 12A and the second through-hole 12B. After passing through the first through-hole 12A and the second through-hole 12B, the branched material L is forced to change direction within the inlet-side groove-like passage 18 toward the center of the inlet-side disk 12 while colliding with the intermediate disk 14. The material L then accelerates and flows in opposing directions along a straight line, colliding with each other. This completes step A. Next, the flow direction of the material L that collides and rejoins is changed to a substantially vertical direction, and the material L is guided into the third through-hole 14A of the intermediate disk 14. This releases some of the collision energy and reduces wear at the center of the inlet-side groove-like passage 18 of the inlet-side disk 12. The turbulence generated by the collision is maintained in this state. In this manner, step B is carried out. Next, the workpiece L that has passed through the third through hole 14A further collides with the discharge-side disk 16, and then branches again and flows through the discharge-side grooved passage 20 toward the outer periphery of the discharge-side disk 16. In this manner, the workpiece L that has passed through the fourth through hole 16A and the fifth through hole 16B is discharged from the discharge-side disk 16, reunites, and is discharged from the dispersion unit 10. In this manner, step C is carried out.

[0036] Examples of apparatuses for carrying out the wet jet mill treatment comprising the above steps A, B, and C include "NAGS20," "NAGS100," "NAGS500," and "NAGS1000" manufactured by Joko Co., Ltd.

[0037] The wet jet mill treatment is preferably carried out multiple times. The number of passes is not particularly limited, but 2 to 100 passes is preferred. 2 to 20 passes is common. The wet jet mill treatment may also be carried out at two or more different liquid feed pressures. For example, wet jet mill treatment may be carried out at a low liquid feed pressure, followed by wet jet mill treatment at a high liquid feed pressure. Each treatment at each pressure may be carried out multiple times.

[0038] The wet jet mill treatment is preferably carried out in an inert gas atmosphere inside the apparatus. Examples of inert gas include nitrogen gas and argon gas. Furthermore, since the wet jet mill treatment generates heat due to compression, etc., it is also preferable to carry out the treatment while cooling. A known cooling method can be used, and examples include a method of heat exchange in which a refrigerant is brought into direct or indirect contact with a heat-generating portion.

[0039] [Step 2] In Step 2, the product obtained in Step 1 is mixed with at least one specific solvent selected from the group consisting of amide solvents, ketone solvents, and sulfoxide solvents. Step 2 provides an exfoliated graphite dispersion. Step 2 will be described below.

[0040] (Specific Solvent) A specific solvent is used in step 2. The specific solvent is at least one solvent selected from the group consisting of amide-based solvents, ketone-based solvents, and sulfoxide-based solvents.

[0041] The amide solvent refers to a solvent made of a compound having an amide structure in the molecule. Specifically, the amide solvent refers to a solvent made of a compound represented by the following formula (a):

[0042]

[0043] In formula (a), R １ , R ２ and R ３ each independently represents an alkyl group which may have a substituent or a hydrogen atom. １ and R ３ may be bonded to each other to form a ring, or R ２ and R ３ and may be bonded to each other to form a ring. １ , R ２ and R ３ The number of carbon atoms in the alkyl group moiety of the alkyl group represented by R which may have a substituent is preferably 1 to 8, and more preferably 1 to 3. １ , R ２ and R ３The alkyl group represented by R which may have a substituent is also preferably an alkyl group which does not have a substituent. ２ and R ３ Each of R preferably represents an alkyl group which may have a substituent (more preferably, an alkyl group which does not have a substituent). That is, the amide solvent is preferably a solvent made of a tertiary amide compound. １ represents a hydrogen atom or an unsubstituted alkyl group having 1 to 3 carbon atoms, and R ２ and R ３ However, an embodiment in which the alkyl group is an unsubstituted alkyl group having 1 to 3 carbon atoms is also preferred.

[0044] Examples of amide solvents include N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), tetramethylurea, and 1,3-dimethyl-2-imidazolidinone. Of these, N,N-dimethylformamide, N-methyl-2-pyrrolidone, and N,N-dimethylacetamide are preferred as amide solvents.

[0045] A ketone solvent refers to a solvent consisting of a compound that has a carbonyl group (-CO-) in the molecule and a carbon atom bonded to the carbon atom of the carbonyl group. In other words, compounds that have a ketone structure such as an ester structure (-COO-) or an amide structure are not included in the ketone solvent. Specifically, a ketone solvent refers to a solvent consisting of a compound represented by the following formula (b):

[0046]

[0047] In formula (b), R ４ and R ５ represents an alkyl group which may have a substituent. ４ and R ５ The alkyl groups represented by the formula (I) may be bonded to each other to form a ring. ４ and R ５The number of carbon atoms in the alkyl group moiety of the alkyl group represented by R which may have a substituent is preferably 1 to 8, more preferably 1 to 3. ４ and R ５ The optionally substituted alkyl group represented by the formula (I) is preferably an alkyl group having no substituent.

[0048] Examples of ketone solvents include acetone, methyl ethyl ketone (MEK), diethyl ketone, methyl isobutyl ketone (MIBK), diisobutyl ketone (DIBK), cyclopentanone, and cyclohexanone. Of these, cyclohexanone is preferred as the ketone solvent.

[0049] A sulfoxide solvent is a solvent consisting of a compound having a sulfinyl group (—SO—) in the molecule, with a carbon atom bonded to the sulfur atom of the sulfinyl group. Specifically, a sulfoxide solvent is a solvent consisting of a compound represented by the following formula (c):

[0050]

[0051] In formula (c), R ６ and R ７ represents an alkyl group which may have a substituent. ６ and R ７ The alkyl groups represented by the formula (I) may be bonded to each other to form a ring. ６ and R ７ The number of carbon atoms in the alkyl group moiety of the alkyl group represented by R which may have a substituent is preferably 1 to 8, more preferably 1 to 3. ６ and R ７ The optionally substituted alkyl group represented by the formula (I) is preferably an alkyl group having no substituent.

[0052] Examples of sulfoxide solvents include dimethyl sulfoxide, di-n-butyl sulfoxide, tert-butyl methyl sulfoxide, tetramethylene sulfoxide, etc. Among these, dimethyl sulfoxide is preferred as the sulfoxide solvent.

[0053] The specific solvent may be used alone or in combination of two or more.

[0054] The SP value (solubility parameter) of the specific solvent is 9.0 (cal / cm ３ ) １／２ More than 9.2 (cal / cm ３ ) １／２ More preferably, 9.5 (cal / cm ３ ) １／２ The SP value of the specific solvent is more preferably 18.0 (cal / cm ３ ) １／２ Preferably, 15.0 (cal / cm ３ ) １／２ More preferably, 13.0 (cal / cm ３ ) １／２ The following is more preferable: The SP value is also called the Hildebrand parameter, and literature values ​​can be used.

[0055] In step 2, the product obtained in step 1 is mixed with the specific solvent. There are no particular limitations on the mixing ratio of the product to the specific solvent, but the mass ratio of the specific solvent to the product is preferably 0.5 or more, more preferably 1 or more, even more preferably 2 or more, and particularly preferably 5 or more. In addition, the ratio is often 200 or less, preferably 100 or less, more preferably 50 or less, and even more preferably 15 or less.

[0056] The mixing method in step 2 is not particularly limited, and known mixing methods can be used. For example, a method of stirring the solution with a rotor, a method of generating convection with a pump, and a method of vibrating a container containing the material to be treated can be mentioned. Specific examples include a mixer, a magnetic stirrer, a mechanical stirrer, and a shaker. It is also preferable that the mixing in step 2 is performed in an inert atmosphere.

[0057] [Step 3] When the material to be treated in Step 1 contains a graphite compound, the present production method may include Step 3, which is performed before Step 1, to obtain a graphite compound (graphite intercalation compound). Specifically, Step 3 involves mixing an alkali metal source and graphite in a second organic solvent (hereinafter also referred to as "solvent Y") to produce a graphite intercalation compound in which the alkali metal is intercalated between the graphene layers in the graphite. Note that, in Step 3, components other than the above components may be added. Step 3 will be described below.

[0058] (Second Organic Solvent (Solvent Y)) The second organic solvent (solvent Y) used in Step 3 is not particularly limited, but preferred examples include the first organic solvent (solvent X) used in Step 1. Of these, ether-based solvents are preferred as solvent Y. Examples and preferred examples of ether-based solvents are as described above in the section on Step 1.

[0059] (Alkali Metal Source) The alkali metal source used in step 3 is not particularly limited as long as it contains an alkali metal, and examples thereof include an alkali metal element (a zero-valent alkali metal) and a salt containing an alkali metal. Examples of alkali metals contained in the alkali metal source include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). The alkali metal is preferably Li, Na, or K, more preferably Li or K, and even more preferably Li. The alkali metal source is preferably lithium metal, potassium metal, or sodium metal, more preferably lithium metal or sodium metal, and even more preferably lithium metal. Only one type of alkali metal source may be used, or two or more types may be used.

[0060] (Graphite) The graphite used in step 3 is not particularly limited as long as it is a compound having a structure in which graphene is stacked. Examples and preferred examples of graphite are as described above in step 1.

[0061] (Aromatic Hydrocarbon) In step 3, it is preferable to mix an aromatic hydrocarbon with the alkali metal source and graphite. Mixing the aromatic hydrocarbon with the above components promotes dissolution of the alkali metal in the alkali metal source into the solvent, and also facilitates intercalation of the alkali metal into the graphite due to the difference in electron affinity between the aromatic hydrocarbon and graphite. Examples of aromatic hydrocarbons include benzene, naphthalene, phenanthrene, anthracene, and pyrene. Of these, naphthalene is preferred.

[0062] (Mixing Method and Conditions) The method for mixing the various components in step 3 is not particularly limited as long as it allows mixing of solvent Y, the alkali metal source, graphite, and any optional components.

[0063] The order of mixing the components used in step 3 is not particularly limited. For example, various components (such as alkali metal and graphite) may be added sequentially to solvent Y, or various components may be added simultaneously to solvent Y. It is particularly preferred to add an aromatic hydrocarbon to solvent Y, and then add the alkali metal and graphite to solvent Y in this order. The amounts of the various components used in step 3 are not particularly limited and can be adjusted as appropriate. The amount of solvent Y used is preferably 50 to 99.9 mass%, more preferably 60 to 99 mass%, and even more preferably 70 to 95 mass%, based on the total amount of the components used in step 3. Examples of the components used in step 3 include solvent Y, an alkali metal source, graphite, and optional components. When two or more types of solvent Y are used as a mixture, the total mass of solvent Y is preferably within the above range. The amount of the alkali metal source used is not particularly limited. It is preferably 2.5 to 300 mass%, more preferably 4 to 150 mass%, based on the amount of graphite used. The amount of graphite used is not particularly limited, and is preferably 0.1 to 10 mass %, more preferably 1 to 5 mass %, based on the total mass of solvent Y. The amount of aromatic hydrocarbon used is not particularly limited, and is preferably 50 to 500 mass %, more preferably 100 to 500 mass %, based on the amount of graphite used.

[0064] The above components can be mixed by known methods, such as stirring the solution with a rotor, generating convection with a pump, and vibrating a container containing the material to be treated. Specific examples include a magnetic stirrer, a mechanical stirrer, and a shaker.

[0065] The mixing time in step 3 is not particularly limited, and is preferably 1 minute to 300 hours, more preferably 30 minutes to 200 hours, and even more preferably 1 to 100 hours. The temperature during mixing in step 3 is not particularly limited, and is preferably 10 to 50°C, and more preferably 20 to 30°C. In step 3, the mixture may be allowed to stand after mixing. The time for standing may be, for example, 1 to 200 hours, and preferably 8 to 150 hours. When the mixture is left to stand, the graphite intercalation compound settles, and solid-liquid separation in step 4, which will be described later, can be easily carried out.

[0066] The atmosphere in which step 3 is performed is not particularly limited, but step 3 is preferably performed in an inert gas atmosphere. Examples of inert gas include nitrogen gas and argon gas. The atmosphere in which step 3 is performed preferably has a low water vapor content. The dew point of the atmosphere in which step 3 is performed is preferably 0°C or lower, more preferably -20°C or lower, even more preferably -40°C or lower, and particularly preferably -50°C or lower. The lower limit of the dew point of the atmosphere in which step 3 is performed is not particularly limited, but is usually -100°C or higher.

[0067] (Graphite Intercalation Compound) In step 3, a graphite intercalation compound is produced in which an alkali metal is intercalated between graphene layers in graphite. The graphite intercalation compound may be a binary system of an alkali metal and graphene, a ternary system of an alkali metal, solvent Y, and graphene, or a quaternary or higher system containing additives in addition to the above ternary system. For example, the alkali metal may form a complex with the solvent THF, and the complex may be intercalated. The formation of the graphite intercalation compound can be confirmed by X-ray diffraction. Specifically, this can be confirmed by the disappearance of diffraction peaks derived from the graphite structure and the appearance of diffraction peaks corresponding to the extension of the distance between the graphene layers due to intercalation.

[0068] [Step 4] When the material to be treated in Step 1 contains a graphite compound, the present production method may include Step 4, in which the graphite compound (graphite intercalation compound) is recovered after Step 3, in which the graphite compound is obtained. The recovery method is not particularly limited, and known methods may be used. Examples include solid-liquid separation and vaporizing the liquid from a solid-liquid mixture. Specific examples include filtration (including pressure filtration and vacuum filtration), decantation, centrifugation, natural drying, vacuum drying, freeze-drying, and spray-drying. Among these, decantation or filtration is preferred. Decantation refers to a method in which a solid-liquid mixture is allowed to stand and the supernatant liquid is removed to recover the solids. The above methods may be performed alone or in combination. Furthermore, in the recovery, complete solid-liquid separation is not required, and the graphite compound and a solvent component (e.g., solvent Y) may still be present.

[0069] [Step 5] In the present production method, a coarse particle removal treatment may be performed on the exfoliated graphite dispersion obtained by carrying out step 2. As the coarse particle removal treatment, a known treatment method (specifically, a classification treatment method) can be applied, and examples thereof include filtering, wet cyclone treatment, and centrifugal separation treatment.

[0070] <Exfoliated Graphite Dispersion> The exfoliated graphite dispersion of the present invention contains exfoliated graphite, a first organic solvent other than amide solvents, ketone solvents, and sulfoxide solvents, and at least one specific solvent selected from the group consisting of amide solvents, ketone solvents, and sulfoxide solvents. Furthermore, the content of the first organic solvent is 0.1% by mass or more and less than 50% by mass relative to the total mass of the exfoliated graphite dispersion, and the content of the specific solvent is 50% by mass or more relative to the total mass of the exfoliated graphite dispersion. The exfoliated graphite dispersion of the present invention is obtained by the method for producing exfoliated graphite dispersion of the present invention.

[0071] Examples of the first organic solvent in the exfoliated graphite dispersion of the present invention include those similar to the solvent X described above, and preferred embodiments are also similar to those of solvent X. Furthermore, examples of the specific solvent in the exfoliated graphite dispersion of the present invention include those similar to the specific solvent described above, and preferred embodiments are also similar to those of the specific solvent described above. Furthermore, the exfoliated graphite dispersion of the present invention may contain components used in each step of the above-mentioned production method. For example, the exfoliated graphite dispersion of the present invention may contain an aromatic hydrocarbon.

[0072] Furthermore, with regard to the exfoliated graphite dispersion of the present invention, the absorbance at a wavelength of 660 nm of the diluted solution obtained by diluting the exfoliated graphite dispersion of the present invention 50 times is preferably 0.25 or more, more preferably 0.30 or more, even more preferably 0.40 or more, and particularly preferably 0.50 or more. There are no particular restrictions on the upper limit of the absorbance, but examples include 50.0 or less, and it is often 30.0 or less, and 20.0 or less is preferred. When the absorbance at a wavelength of 660 nm is within the above range, it is considered that the dispersibility of exfoliated graphite is high in the exfoliated graphite dispersion of the present invention.

[0073] In the present invention, the absorbance is measured using a spectrophotometer V-770 (manufactured by JASCO Corporation). First, the exfoliated graphite dispersion of the present invention is diluted 50 times using the specific solvent contained therein to obtain a diluted solution. Next, the absorbance at 660 nm is measured using the spectrophotometer. Note that if the absorbance is 1 or more, the diluted solution is further diluted so that the absorbance becomes 1 or less, and the absorbance is measured, and the absorbance of 1 or less is converted to the absorbance before dilution, and this is used as the absorbance of the exfoliated graphite dispersion of the present invention.

[0074] In the exfoliated graphite dispersion of the present invention, the content of exfoliated graphite is often 0.05% by mass or more, and preferably 0.1% by mass or more, relative to the total mass of the exfoliated graphite dispersion. The content of exfoliated graphite is often 25% by mass or less, and preferably 15% by mass or less, and more preferably 10% by mass or less, relative to the total mass of the exfoliated graphite dispersion.

[0075] The content of the first organic solvent other than the amide solvent, ketone solvent, and sulfoxide solvent in the exfoliated graphite dispersion of the present invention is 0.1% by mass or more, often 1% by mass or more, preferably 3% by mass or more, and more preferably 5% by mass or more, relative to the total mass of the exfoliated graphite dispersion. Furthermore, the content of the first organic solvent is less than 50% by mass, preferably 30% by mass or less, and more preferably 15% by mass or less, relative to the total mass of the exfoliated graphite dispersion. Furthermore, the content of the first organic solvent may be 10% by mass or less, relative to the total mass of the exfoliated graphite dispersion.

[0076] The content of the specific solvent in the exfoliated graphite dispersion of the present invention is 50% by mass or more relative to the total mass of the exfoliated graphite dispersion, and is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, in order to further increase the dispersion stability of exfoliated graphite. Furthermore, the content of the specific solvent is often 99.8% by mass or less relative to the total mass of the exfoliated graphite dispersion, preferably 99% by mass or less, and more preferably 95% by mass or less. Dispersion stability refers to little change in dispersibility over time, and is evaluated by the method described later.

[0077] In the exfoliated graphite dispersion of the present invention, the mass ratio of the content of the specific solvent to the content of the first organic solvent is preferably 0.5 or more, more preferably 1 or more, even more preferably 2 or more, and particularly preferably 5 or more. The mass ratio may be 8 or more. Furthermore, the mass ratio is often 200 or less, preferably 100 or less, more preferably 50 or less, and even more preferably 15 or less.

[0078] <Uses of Exfoliated Graphite> The use of the exfoliated graphite contained in the exfoliated graphite dispersion of the present invention is not particularly limited, and it can be used, for example, as a functional filler. Exfoliated graphite is known to have excellent toughness, electrical conductivity, and thermal conductivity. By using exfoliated graphite as a functional filler and compounding it, a composite imparted with the above functions can be obtained. Examples of the base material to be compounded include resin, ceramic, and metal. Furthermore, because exfoliated graphite has a large specific surface area, it can effectively impart functions such as toughness with the addition of a small amount. Furthermore, because exfoliated graphite has electrical conductivity and a large specific surface area, it can also be used as an electrode material for secondary batteries, electrochemical capacitors, and the like. For example, exfoliated graphite is preferably used as a negative electrode material for secondary batteries.

[0079] When exfoliated graphite is used for the above applications, for example, a slurry containing the exfoliated graphite dispersion of the present invention and other components appropriate for each of the above applications may be prepared, the slurry may be applied in a desired form, and the solvent components contained in the slurry (for example, the first organic solvent and the specific solvent) may be removed. The exfoliated graphite dispersion of the present invention has excellent dispersibility, so that exfoliated graphite is easily dispersed uniformly in each application, and desired properties are easily obtained.

[0080] The present invention will be described in more detail below with reference to examples. The materials, amounts used, ratios, processing details, equipment, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the examples shown below.

[0081] Example 1 Synthesis of Graphite Intercalation Compound (Step 3) The synthesis of the graphite intercalation compound (Step 3) was carried out in a glove box filled with high-purity Ar (purity 99.999%). The dew point of the glove box was −55°C. First, 100 mL of tetrahydrofuran (THF, special grade) was placed in a 100 mL glass screw-top bottle, and 7.5 g of naphthalene (special grade) was dissolved therein. 0.5 g of metallic lithium (purity 99.5%) was added to this solution, and the solution was stirred to dissolve the metallic lithium. 2.5 g of graphite particles (Sigma-Aldrich, 808091-2.5KG, average particle size: 150 μm) were added to the lithium solution, and the mixture was stirred at 25°C for 24 hours to obtain a mixture containing the graphite intercalation compound. The resulting mixture containing the graphite intercalation compound was dried and subjected to XRD measurement, revealing peaks at 2θ = 6.5°, 13°, 21°, 28°, 36°, 43°, and 51°, which corresponded to (001), (002), (003), (004), (005), (006), and (007) of stage-1 of Li-THF-GIC in phase A. This result corresponds to a structure in which a complex in which THF molecules are coordinated to lithium intercalates into the graphite layers, and indicates that the spacing between the graphite layers is approximately 1.10 nm.

[0082] [Step 1] 200 mL of THF (tetrahydrofuran, corresponding to the above-mentioned solvent X) was added to the mixture containing the graphite intercalation compound obtained by the above procedure, to obtain a material to be treated, which was then subjected to wet jet mill treatment. The wet jet mill treatment was performed using a "NAGS20 (AC 200V specification)" manufactured by Joko Corporation. The equipment conditions were as follows: - Normal nozzle - Nozzle diameter: φ0.15 mm - Chiller set temperature: 0°C - Liquid delivery pressure: 200 MPa Under the above equipment conditions, the wet jet mill treatment was performed five times to obtain a dispersion A of exfoliated graphite.

[0083] [Step 2] 10 mL of the product (dispersion liquid A) obtained in step 1 was mixed with 90 mL of NMP (N-methyl-2-pyrrolidone, a specific solvent) to obtain an exfoliated graphite dispersion liquid.

[0084] [Step 5] After obtaining the exfoliated graphite dispersion liquid obtained in step 2, the exfoliated graphite dispersion liquid was centrifuged at 2000 rpm for 2 minutes using a centrifuge (Optima XE-90 manufactured by Beckman) and the supernatant liquid was collected. By the above treatment, coarse particles were removed, and exfoliated graphite dispersion liquid 1 was obtained. Note that if coarse particles were found to be floating on the surface of the exfoliated graphite dispersion liquid after the above treatment, an intermediate liquid was collected.

[0085] [Measurement of absorbance] The obtained exfoliated graphite dispersion 1 was diluted 50 times with NMP to obtain a diluted solution, and the absorbance at 660 nm was measured using a spectrophotometer V-770 (JASCO Corporation). The measurement method was as described above.

[0086] [Evaluation of Dispersion Stability] The exfoliated graphite dispersion 1 was allowed to stand for 7 days, and then the absorbance was measured in the same manner as above. The standing was carried out in an environment of 20°C.

[0087] Examples 2 to 10 In the procedure for obtaining exfoliated graphite dispersion 1 in Example 1 above, exfoliated graphite dispersions 2 to 10 were obtained in the same manner as in Example 1, except that the dilution ratio and the type of specific solvent used in step 2 were changed as shown in the table below. In addition, absorbance was measured in the same manner as in Example 1. Note that for Examples 2 to 4, dispersion stability was evaluated in the same manner as in Example 1.

[0088] Comparative Examples 1 to 7 Exfoliated graphite dispersions C1 to C7 were obtained in the same manner as in Example 1, except that in the procedure for obtaining exfoliated graphite dispersion 1 in Example 1 above, the solvents shown in the table below were used instead of the specific solvent used in step 2.

[0089] Example 21 Synthesis of Graphite Intercalation Compound (Steps 3 and 4) The synthesis of the graphite intercalation compound (Step 3) was carried out in a glove box filled with high-purity Ar (purity 99.999%). The dew point of the glove box was −55°C. First, 100 mL of tetrahydrofuran (THF, special grade) was placed in a 100 mL glass screw-top bottle, and 7.5 g of naphthalene (special grade) was dissolved therein. 0.5 g of metallic lithium (purity 99.5%) was added to this solution, and the solution was stirred to dissolve the metallic lithium. 2.5 g of graphite particles (manufactured by Nippon Graphite Co., Ltd., particle size: 500 μm to 1 mm) were added to the lithium solution, and the mixture was stirred at 25°C for 48 hours to obtain a mixture containing the graphite intercalation compound. This mixture was allowed to stand for 120 hours, and the supernatant was removed for solid-liquid separation to obtain the graphite intercalation compound (Step 4). XRD measurement of the obtained graphite intercalation compound confirmed peaks at 2θ = 6.5°, 13°, 21°, 28°, 36°, 43°, and 51°, which corresponded to (001), (002), (003), (004), (005), (006), and (007) of stage-1 of Li-THF-GIC in phase A. This result corresponds to a structure in which a complex in which THF molecules are coordinated to lithium is intercalated into the graphite layers, and indicates that the distance between the graphite layers is expanded to approximately 1.10 nm.

[0090] [Step 1] 22 g of the obtained graphite intercalation compound was mixed with 88 mL of THF and subjected to wet jet milling. The wet jet milling was performed using a "NAGS20 (AC 200V specification)" manufactured by Joko Co., Ltd. The equipment conditions were as follows: Straight nozzle (model number: J01502S), nozzle diameter: φ0.15 mm, chiller temperature setting: 0°C, discharge rate: 38.5 × 10 ４ ±1×10 ４ Liquid sending pressure: 120 MPa Under the above conditions of the device, the wet jet mill treatment was carried out for 22 passes, and a dispersion B of exfoliated graphite was obtained.

[0091] [Step 2] 10 mL of the product (dispersion liquid B) obtained in step 1 and 90 mL of NMP (corresponding to a specific solvent) were mixed to obtain exfoliated graphite dispersion liquid 21.

[0092] [Measurement of Absorbance] The absorbance of exfoliated graphite dispersion 21 of Example 21 was measured in the same manner as in exfoliated graphite dispersion 1 of Example 1.

[0093] Examples 22 to 25 In the procedure for obtaining exfoliated graphite dispersion 21 of Example 21 above, except that the dilution ratio and the type of specific solvent used in step 2 were changed as shown in the table below, exfoliated graphite dispersions 22 to 25 were obtained in the same manner as in Example 21. In addition, absorbance was measured in the same manner as in Example 21.

[0094] Comparative Examples 21 and 22 Exfoliated graphite dispersions C21 and C22 were obtained in the same manner as in Example 1, except that in the procedure for obtaining exfoliated graphite dispersion 21 of Example 21, the solvent shown in the table below was used instead of the specific solvent used in step 2.

[0095] <Results> The type and dilution ratio of the solvent used in step 2 of each Example and Comparative Example, as well as the results of absorbance measurement, are shown in Tables 1 and 2. In Tables 1 and 2, the abbreviations in the column "Solvent used in step 2" represent the following solvents: MNP: N-methyl-2-pyrrolidone DMF: N,N-dimethylformamide DMAc: N,N-dimethylacetamide DMSO: dimethyl sulfoxide MEK: methyl ethyl ketone THF: tetrahydrofuran MeOH: methanol EtOH: ethanol IPA: isopropyl alcohol nBtOH: n-butanol In addition, in the column "Dilution ratio" in Tables 1 and 2, for example, the notation "1:9" indicates that the specific solvent or the like was mixed in step 2 at a volume ratio such that 9 parts by volume of the product obtained in step 1 was mixed with 1 part by volume of the product obtained in step 1.

[0096]

[0097]

[0098] From the results shown in Tables 1 and 2, it was confirmed that in each Example in which the above-mentioned specific solvent was used as the solvent used in Step 2, the dispersibility of exfoliated graphite in the obtained exfoliated graphite dispersion was higher. On the other hand, in the Comparative Example in which the specific solvent was not used as the solvent used in Step 2, the dispersibility of exfoliated graphite was not higher than in each Example. From a comparison between Examples 1 and 5 to 8 and Examples 9 and 10, it was confirmed that the dispersibility of exfoliated graphite was higher when the specific solvent used in Step 2 was at least one solvent selected from the group consisting of N,N-dimethylformamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide, cyclohexanone, and dimethyl sulfoxide.

[0099] When the exfoliated graphite in the exfoliated graphite dispersion was collected and analyzed by Raman spectroscopy, it was found that the exfoliated graphite was composed of an average of 2 to 3 graphene layers.

[0100] REFERENCE SIGNS LIST 10 Dispersion unit 12 Inlet-side disk 12A First through-hole 12B Second through-hole 14 Intermediate disk 14A Third through-hole 16 Discharge-side disk 16A Fourth through-hole 16B Fifth through-hole 18 Inlet-side groove-like passage 20 Discharge-side groove-like passage C1 Central axis

Claims

1. Step 1 of obtaining exfoliated graphite by subjecting an object to be treated, which contains one selected from the group consisting of graphite and graphite compounds and a first organic solvent other than amide solvents, ketone solvents, and sulfoxide solvents, to wet jet milling treatment; and step 2 of mixing the product obtained in step 1 with at least one specific solvent selected from the group consisting of amide solvents, ketone solvents, and sulfoxide solvents to obtain an exfoliated graphite dispersion. A method for producing an exfoliated graphite dispersion having these steps.

2. The method for producing an exfoliated graphite dispersion according to claim 1, wherein in step 1, the graphite compound is used.

3. The method for producing an exfoliated graphite dispersion according to claim 2, wherein the graphite compound is a graphite compound obtained by mixing an alkali metal source and graphite in a second organic solvent other than amide solvents, ketone solvents, and sulfoxide solvents, and intercalating the alkali metal between the graphene layers in the graphite.

4. The method for producing an exfoliated graphite dispersion according to any one of claims 1 to 3, wherein in step 2, the mass ratio of the specific solvent to the product is 0.5 to 100.

5. The method for producing an exfoliated graphite dispersion according to any one of claims 1 to 3, wherein the specific solvent is at least one solvent selected from the group consisting of N,N-dimethylformamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide, cyclohexanone, and dimethyl sulfoxide.

6. The method for producing an exfoliated graphite dispersion according to any one of claims 1 to 3, wherein the first organic solvent is an ether solvent.

7. An exfoliated graphite dispersion containing exfoliated graphite, a first organic solvent other than amide solvents, ketone solvents, and sulfoxide solvents, and at least one specific solvent selected from the group consisting of amide solvents, ketone solvents, and sulfoxide solvents, wherein the content of the first organic solvent is 0.1% by mass or more and less than 50% by mass based on the total mass of the exfoliated graphite dispersion, and the content of the specific solvent is 50% by mass or more based on the total mass of the exfoliated graphite dispersion.

8. The exfoliated graphite dispersion according to claim 7, wherein the specific solvent is at least one solvent selected from the group consisting of N,N-dimethylformamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide, cyclohexanone, and dimethyl sulfoxide.

9. The exfoliated graphite dispersion according to claim 7 or 8, wherein the first organic solvent contains an ether-based solvent.

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