JP6863099B2 - Graphene / Organic Solvent Dispersion, Graphene-Active Material Complex Particle Production Method and Electrode Paste Production Method - Google Patents

Description

The present invention relates to a graphene / organic solvent dispersion, a method for producing graphene-active material composite particles using the same, and a method for producing an electrode paste.

Graphene is a two-dimensional crystal consisting of carbon atoms, and has been a material that has received much attention since it was discovered in 2004. Graphene has excellent electrical, thermal, optical, and mechanical properties, and is expected to have a wide range of applications in areas such as battery materials, energy storage materials, electronic devices, and composite materials.

In order to realize such an application of graphene, it is indispensable to improve the efficiency and dispersibility of the production method for cost reduction.

Examples of the graphene production method include a mechanical peeling method, a CVD (Chemical Vapor Deposition) method, and a CEG (Crystal Epitaxy Growth) method, but these methods have low productivity and are not suitable for mass production. On the other hand, the redox method (a method of obtaining graphene oxide or graphene by oxidation treatment of natural graphite and then producing graphene by a reduction reaction) enables mass synthesis of graphene, and is suitable for practical use of graphene. This is a very important technique.

Graphene has high conductive performance and has a flaky structure, so that it can increase the number of conductive paths. Therefore, graphene has a high potential as a conductive auxiliary agent especially for batteries. However, since graphene is nanocarbon, it easily aggregates, and even if graphene is simply produced by the redox method, it cannot be appropriately dispersed in the matrix and its potential cannot be exhibited.

In Patent Document 1, graphene powder is produced by chemically reducing graphene oxide and freeze-drying it.

In Patent Document 2, a graphene powder is produced by drying a dispersion medium from a graphene dispersion obtained by applying ultrasonic waves to a mixed solution containing graphite particles.

In Patent Document 3, graphene oxide is chemically reduced in the presence of a water-soluble compound having a 9,9-bis (substituted aryl) fluorene skeleton, and an organic solvent is mixed with the obtained aqueous graphene dispersion, followed by centrifugation. An organic solvent is further added to the recovered graphene to prepare a graphene dispersion.

Special Table 2009-511415 Japanese Unexamined Patent Publication No. 2011-219318 Japanese Unexamined Patent Publication No. 2015-59079

In order for graphene to function effectively as a conductive auxiliary agent, it must be thin and have high dispersibility. However, graphene produced by the heat expansion / reduction method as in Patent Document 1 has an excessively large specific surface area, induces aggregation, and deteriorates dispersibility.

In the method of Patent Document 2, since the dispersion medium is dried from the mixed solution containing the atomized graphite powder to obtain the graphene powder, it is not possible to prevent the graphene powder from being excessively aggregated by the drying.

Further, in the method of Patent Document 3, since the water-soluble compound having a 9,9-bis (substituted aryl) fluorene skeleton is removed in the dispersion liquid preparation step, the effect of suppressing graphene aggregation is insufficient, and graphene in the dispersion liquid is insufficient. Could not prevent the agglomeration of.

As described above, graphene is very easy to aggregate, and when it is produced by the redox method, sufficient dispersibility cannot be obtained, so that its potential cannot be exhibited. An object of the present invention is to provide graphene in a form having high dispersibility and capable of maintaining high conductivity and ionic conductivity when used as a raw material for producing an electrode material.

In order to solve the above problems, the present invention applies a graphene / organic solvent dispersion in which graphene is dispersed in an organic solvent, and the solid content is adjusted to 0.005 or more and 0.012 or less. A graphene / organic solvent dispersion having a surface roughness of 1.0 μm or less when the graphene film formed is measured with a laser microscope having a wavelength of 408 nm using a 50x objective lens at a measurement pitch of 0.100 μm. ..

In the graphene / organic solvent dispersion liquid in the present invention, graphene that is sufficiently thin to function as a conductive auxiliary agent is dispersed in the organic solvent, and aggregation is suppressed. By using such a graphene / organic solvent dispersion, the dispersibility of graphene in the resin or the electrode paste is improved. In addition, graphene can be easily adsorbed on the surface of the active material, and the contact resistance between graphenes is sufficiently reduced, so that high electron conductivity and ion conductivity can be maintained for a long period of time when the electrodes are formed.

Therefore, by using the graphene / organic solvent dispersion of the present invention together with the binder and the electrode active material to form a conductive network in the electrode matrix, it is possible to provide an electrode for a lithium ion battery having excellent discharge characteristics. It becomes.

<Graphene dispersion>
The graphene / organic solvent dispersion liquid of the present invention (hereinafter, may be simply referred to as “graphene dispersion liquid”) is obtained by dispersing graphene in an organic solvent. ^{Graphene generally refers to a sheet of sp 2-} bonded carbon atoms (single-layer graphene) having a thickness of one atom, but in the present specification, it also includes a substance having a flaky form in which single-layer graphene is laminated. Called graphene. Similarly, graphene oxide, which will be described later, is also referred to as a name including a substance having a laminated flaky form.

In the graphene dispersion of the present invention, the surface roughness of the graphene film formed by applying the graphene / organic solvent dispersion whose solid content is adjusted to 0.005 or more and 0.012 or less using an organic solvent has a wavelength of 408 nm. The surface roughness is 1.0 μm or less when measured with a laser microscope in a range of 50 × objective lens, measurement pitch 0.100 μm, and 250 × 200 μm.

In the present invention, the solid content ratio is a value obtained by placing 1 g of graphene dispersion on a slide glass, heating and drying on a hot plate at 120 ° C. for 1 hour, and dividing the weight after drying by the weight before drying.

The solid content of graphene is adjusted as follows. First, the solid content of the graphene / organic solvent dispersion is measured by the above method. When the solid content is higher than 0.012, add an amount of organic solvent having a solid content of 0.005 or more and 0.012 or less and mix at 2000 rpm for 1 minute using a mixer to obtain a solid content of 0.005 or more and 0. Prepare a graphene dispersion of 0.012 or less. When the solid content is lower than 0.005, it is concentrated by suction filtration to a solid content of 0.015 or more and 0.060 or less, and after adding an amount of organic solvent having a solid content of 0.005 or more and 0.012 or less. , Mix in the same way using a mixer and redisperse.

Next, 1 g of the graphene dispersion having the adjusted solid content was placed on an aluminum foil having a thickness of 20 μm, the coating thickness of the film-forming applicator was adjusted to 50 μm, and the coater was coated at a coating speed of 16.7 mm / s. Further, it is dried at 80 ° C. for 1 hour to obtain a graphene film. As the film-forming applicator, one having a flat shape at a portion in contact with the dispersion liquid is used. A dispersion having a graphene film having a surface roughness of more than 1.0 μm has a low degree of exfoliation of the graphene contained and dispersibility in an organic solvent, and forms and maintains a good conductive path in a resin or an electrode paste. I can't. The surface roughness is preferably 0.8 μm or less, more preferably 0.6 μm or less. A dispersion having a very small surface roughness of the graphene film formed tends to have a strong interaction between graphenes, and it may be difficult to adhere to other materials. The surface roughness of the graphene film formed is preferably 0.05 μm or more, more preferably 0.2 μm or more, and further preferably 0.3 μm or more.

The graphene contained in the graphene / organic solvent dispersion of the present invention has an element ratio (O / C ratio) of oxygen to carbon measured by X-ray photoelectron spectroscopy to be 0.08 or more and 0.30 or less. Is preferable. The oxygen atom on the graphene surface is an oxygen atom contained in an acidic group bonded to graphene itself or an acidic group contained in a surface treatment agent adhering to the graphene surface. Such oxygen atoms have the effect of improving the dispersed state of graphene, and if the amount of oxygen atoms on the graphene surface is too small, the dispersibility deteriorates. The O / C ratio of graphene is more preferably 0.10 or more. On the other hand, if there are too many oxygen atoms on the graphene surface, the conductivity will decrease. The O / C ratio of graphene is more preferably 0.20 or less, still more preferably 0.15 or less.

In X-ray photoelectron spectroscopy, the graphene dispersion is pre-dried with a vacuum dryer or freeze dryer, and then the dried sample is introduced into a measurement chamber equipped with a high vacuum chamber and softened on the sample surface placed in an ultra-high vacuum. It irradiates X-rays and detects photoelectrons emitted from the surface with an analyzer. Elemental information on the surface of a substance can be obtained by measuring these photoelectrons by wide scan and narrow scan and obtaining the binding energy value of bound electrons in the substance. Specifically, it can be measured by the method described in Measurement Example 4 described later.

When graphene is measured by X-ray photoelectron spectroscopy, a C1s peak derived from carbon is detected near 284 eV, but it is known that when carbon is bound to oxygen, it shifts to the high energy side. Specifically, peaks based on CC bonds, C = C double bonds, and C—H bonds in which carbon is not bound to oxygen are detected near 284 eV without shifting, and in the case of CO single bonds 286. It shifts to around .5 eV, to around 287.5 eV in the case of C = O double bond, and to around 288.5 eV in the case of COO bond. Therefore, the signal derived from carbon is detected in the form of superimposing the peaks in the vicinity of 284 eV, in the vicinity of 286.5 eV, in the vicinity of 287.5 eV, and in the vicinity of 288.5 eV. At the same time, a nitrogen-derived N1s peak is detected near 402 eV, and an oxygen-derived O1s peak is detected near 533 eV. Further, the O / C ratio can be obtained from the peak areas of the C1s peak and the O1s peak.

The O / C ratio of graphene can be controlled by changing the degree of oxidation of graphene oxide as a raw material or changing the amount of surface treatment agent. For example, the higher the degree of oxidation of graphene oxide, the larger the amount of oxygen remaining after reduction, and the lower the degree of oxidation, the smaller the amount of oxygen atoms after reduction. In addition, the amount of oxygen atoms can be increased by increasing the amount of the surface treatment agent having an acidic group attached.

The elemental ratio (N / C ratio) of nitrogen to carbon measured by X-ray photoelectron spectroscopy of graphene powder is preferably 0.005 or more and 0.020 or less, preferably 0.008 or more and 0.015. The following is preferable. The nitrogen atom on the surface of graphene powder is derived from nitrogen contained in the surface treatment agent of the present invention. When the N / C ratio of graphene exceeds 0.020, nitrogen atoms replace the graphene conjugated structure, so that low conductivity tends to occur. The N / C ratio can be obtained from the peak areas of the C1s peak and the N1s peak, similarly to the O / C ratio.

The graphene dispersion of the present invention preferably contains a surface treatment agent having an acidic group (hereinafter, may be simply referred to as "surface treatment agent"). The surface treatment agent having an acidic group exhibits an effect of enhancing the dispersibility of graphene because at least a part thereof is attached to the surface of graphene and exists. Here, the acidic group is a hydroxy group, a phenolic hydroxy group, a nitro group, a carboxyl group or a carbonyl group. The surface treatment agent is not particularly limited as long as it is a compound having an acidic group, and may be a high molecular weight compound or a low molecular weight compound.

Examples of the polymer compound having an acidic group include polyvinylpyrrolidone, polyvinyl alcohol, and polymethyl vinyl ether. As the low molecular weight compound, a compound having an aromatic ring is preferable from the viewpoint of affinity with the graphene surface. From the viewpoint of increasing the conductivity of graphene, a low molecular weight compound is preferable to a high molecular weight compound.

Among them, a compound having a catechol group is preferable as a surface treatment agent because it has high adhesiveness to graphene and dispersibility in a solvent. Compounds having a catechol group include catechol, dopamine hydrochloride, 4- (1-hydroxy-2-aminoethyl) catechol, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, caffeic acid, 4- Methylcatechol and and 4-tert-butylpyrocatechol can be mentioned.

As the acidic group of the surface treatment agent, a phenolic hydroxy group is preferable. Examples of the compound having a phenolic hydroxy group include phenol, nitrophenol, cresol, catechol, and a compound having a structure in which a part thereof is substituted.

Further, a surfactant having an acidic group is also preferably used as a surface treatment agent. As the surfactant, any of a cationic surfactant, an anionic surfactant, a nonionic surfactant and the like can be used, but since the anion and the cation themselves may be involved in the electrochemical reaction, the battery When used as a material, a non-ionized nonionic surfactant is suitable.

Further, the surface treatment agent may have a basic group in addition to the acidic group, and the dispersibility is improved by having an amino group in particular. Therefore, a compound having both a catechol group and an amino group is particularly preferable as a surface treatment agent. Dopamine hydrochloride is exemplified as such a compound.

The specific surface area of graphene contained in the graphene / organic solvent dispersion of the present invention measured by the BET measurement method (hereinafter, may be simply referred to as “specific surface area”) is 80 m ² / g or more and 250 m ² / g or less. Is preferable. The specific surface area of graphene reflects the thickness of graphene and the degree of exfoliation of graphene, and the larger the value, the thinner the graphene and the higher the degree of exfoliation. If the specific surface area of graphene ^{is less than 80 m 2} / g, it tends to be difficult to form a conductive work when applied to a lithium ion battery, and if it is ^{larger than 250 m 2} / g, the dispersibility tends to decrease. is there. The specific surface area of graphene ^{is more preferably 100 m 2} / g or more, and more preferably 130 m ² / g or more. Similarly, it is preferably 200 meters ² / g or less, more preferably 180 m ² / g or less. The BET measurement method is performed by the method described in JIS Z8830: 2013 for a dried sample after pre-drying the graphene dispersion with a vacuum dryer or a freeze dryer, and the method for measuring the amount of adsorbed gas is carrier gas. By the method, the analysis of adsorption data shall be performed by the one-point method.

The solid content of the graphene / organic solvent dispersion liquid of the present invention is preferably 0.003 or more and 0.400 or less. When the solid content exceeds 0.400, graphene stacks are likely to occur in the dispersion. On the other hand, if the solid content is less than 0.003, when used in the production of the electrode paste, the solid content of the electrode paste is lowered by the solvent in the dispersion liquid and the viscosity is lowered, so that the coatability is deteriorated. Tend. The solid content ratio is more preferably 0.200 or less, further preferably 0.100 or less, further preferably 0.070 or less, and particularly preferably 0.050 or less. When the solid content ratio is 0.050 or less, fluidity is easily obtained and handleability is excellent. Further, the solid content ratio is more preferably 0.007 or more, and further preferably 0.010 or more.

<Organic solvent>
The organic solvent used in the graphene dispersion of the present invention is not limited, but a highly polar organic solvent is preferable. As the organic solvent having high polarity, an organic solvent having a dipole moment of 3.0 Debye or more is preferable. Examples of such an organic solvent include N-methylpyrrolidone (NMP), γ-butyrolactone, dimethylacetamide, dimethylformamide, dimethyl sulfoxide, acetonitrile, and mixtures thereof. As the organic solvent, NMP is particularly preferably used.

<Manufacturing method of graphene dispersion>
As an example, the graphene dispersion of the present invention is prepared by mixing graphene oxide and a surface treatment agent having an acidic group in a solvent, performing a micronization step of refining graphene oxide, and then performing a reduction treatment. Further, it can be produced by a production method in which the solvent is replaced with an organic solvent such as NMP after stirring with a high shear mixer.

[Graphene oxide]
The method for producing graphene oxide is not particularly limited, and a known method such as the Hammers method can be used. You may also purchase commercially available graphene oxide. The case where the Hammers method is used as a method for producing graphene oxide is illustrated below.

While stirring graphite (stone ink powder) and sodium nitrate in concentrated sulfuric acid, gradually add potassium permanganate so that the temperature does not rise, and stir at 25 to 50 ° C for 0.2 to 5 hours. .. Then, ion-exchanged water is added to dilute the suspension, and the mixture is reacted at 80 to 100 ° C. for 5 to 50 minutes. Finally, hydrogen peroxide and deionized water are added and reacted for 1 to 30 minutes to obtain a graphene oxide dispersion. The obtained graphene oxide dispersion is filtered and washed to obtain a graphene oxide gel. The graphene oxide gel may be diluted and mixed with a surface treatment agent or reduced.

The graphite used as a raw material for graphene oxide may be either artificial graphite or natural graphite, but natural graphite is preferably used. The number of meshes of graphite used as a raw material is preferably 20000 or less, and more preferably 5000 or less.

As an example, the ratio of each reaction product is 150 to 300 ml of concentrated sulfuric acid, 2 to 8 g of sodium nitrate, 10 to 40 g of potassium permanganate, and 40 to 80 g of hydrogen peroxide with respect to 10 g of graphite. When adding sodium nitrate and potassium permanganate, use an ice bath to control the temperature. When hydrogen peroxide and deionized water are added, the mass of deionized water is 10 to 20 times the mass of hydrogen peroxide. As the concentrated sulfuric acid, it is preferable to use one having a mass content of 70% or more, and it is more preferable to use one having a mass content of 97% or more.

Although graphene oxide has high dispersibility, it is itself insulating and cannot be used as a conductive additive or the like. If the degree of oxidation of graphene oxide is too high, the conductivity of the graphene powder obtained by reduction may deteriorate. Therefore, the ratio of carbon atoms to oxygen atoms of graphene oxide as measured by X-ray photoelectron spectroscopy is determined. It is preferably 0.5 or more. When graphene oxide is measured by X-ray photoelectron spectroscopy, the solvent is sufficiently dried.

Further, if graphite is not oxidized to the inside, it is difficult to obtain flaky graphene powder when reduced. Therefore, it is desirable that graphene oxide does not have a peak peculiar to graphite when it is dried and X-ray diffraction measurement is performed.

The degree of oxidation of graphene oxide can be adjusted by changing the amount of the oxidizing agent used in the oxidation reaction of graphite. Specifically, the larger the amount of sodium nitrate and potassium permanganate used in the oxidation reaction with respect to graphite, the higher the degree of oxidation, and the smaller the amount, the lower the degree of oxidation. The weight ratio of sodium nitrate to graphite is not particularly limited, but is preferably 0.200 or more and 0.800 or less, more preferably 0.250 or more and 0.500 or less, and 0.275 or more. It is more preferably 0.425 or less. The ratio of potassium permanganate to graphite is not particularly limited, but is preferably 1.00 or more, more preferably 1.40 or more, and even more preferably 1.65 or more. Further, it is preferably 4.00 or less, more preferably 3.00 or less, and further preferably 2.55 or less.

[Surface treatment agent mixing process]
Graphene oxide is then mixed with a surface treatment agent having an acidic group, if necessary. As the surface treatment agent, the above-mentioned ones can be used.

In order to mix graphene oxide and the surface treatment agent well, it is preferable to mix graphene oxide and the surface treatment agent in a state of being dispersed in the aqueous solution. At this time, it is preferable that both graphene oxide and the surface treatment agent are completely dissolved, but a part of the graphene oxide and the surface treatment agent may not be dissolved and may be dispersed as a solid. A solvent other than water may be partially contained in the aqueous solution, and the solvent other than water is not particularly limited, but a polar solvent is preferable, and ethanol, methanol, 1-propanol, 2-propanol, etc. Examples thereof include NMP, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone and the like.

[Miniaturization process]
Next, a miniaturization step of refining graphene oxide in a solvent is performed. As the solvent in the miniaturization step, the same solvent as that described in the surface treatment agent mixing step can be used. The method for miniaturization is not particularly limited, and examples thereof include a bead mill, a planetary ball mill, ultrasonic processing, and a jet mill.

[Reduction process]
Next, a reduction step of reducing graphene oxide in a solvent is performed. As the solvent in the reduction step, the same solvent as that described in the surface treatment agent mixing step can be used. When the above-mentioned surface treatment agent mixing step is performed in a solvent, the micronization step is performed in the state after the completion of the step and then the reduction step is performed as it is, or the solvent used in the surface treatment agent mixing step is performed. It is preferable to dilute with the same solvent as above and reduce.

The reducing agent that reduces graphene oxide is not particularly limited, and various organic reducing agents and inorganic reducing agents can be used. Of these, an inorganic reducing agent is more preferable because of the ease of cleaning after reduction.

Examples of the organic reducing agent include an aldehyde-based reducing agent, a hydrazine derivative reducing agent, and an alcohol-based reducing agent. Among them, the alcohol-based reducing agent is particularly suitable because it can be reduced relatively gently. Examples of the alcohol-based reducing agent include methanol, ethanol, propanol, isopropyl alcohol, butanol, benzyl alcohol, phenol, ethanolamine, ethylene glycol, propylene glycol, diethylene glycol, and the like.

Examples of the inorganic reducing agent include sodium dithionite, potassium dithionite, phosphorous acid, sodium borohydride, hydrazine, etc. Among them, sodium dithionite and potassium dithionite relatively retain acidic groups. Since it can be reduced, graphene having high dispersibility in a solvent can be produced and is preferably used.

[Solvent replacement step]
When the organic solvent used in the graphene dispersion of the present invention is replaced with a solvent different from the solvent used in the reduction step, a solvent replacement step is performed. Hereinafter, a case of substituting with NMP, which is generally used as an electrode paste, will be described as an example.

The solvent replacement step is a step of mixing the solvent dispersion of graphene that has completed the reduction step with NMP, and then removing a part of the solvent by suction filtration or distillation. Solvent removing means that exert a strong force on graphene contained in the dispersion, such as pressure filtration and centrifugation, tends to cause graphene to stack, which is not preferable. As the suction filtration, it is preferable to perform vacuum suction filtration. Further, this operation may be repeated a plurality of times in order to reduce the residual ratio of the solvent used in the reduction step. Distillation is applicable when the boiling point of the solvent used in the reduction step is lower than NMP. The pressure for distillation is not limited, but it is preferable to distill at a low pressure of 3.0 kPa or less because the solvent can be removed efficiently. Further, a washing step of diluting the graphene dispersion with water and filtering it may be performed after the reduction step and before the solvent replacement step for the purpose of removing the reducing agent.

The mixing ratio when mixing the graphene solvent dispersion after the reduction step with NMP is not particularly limited, but when water or NMP is used as the solvent in the reduction step, a low-viscosity dispersion that is easy to handle is obtained. On the other hand, from the viewpoint of suppressing the amount of the dispersion liquid to be processed, the NMP is preferably 10 to 3000 parts by mass, more preferably 20 to 2000 parts by mass with respect to 100 parts by mass of the graphene aqueous dispersion or graphene NMP dispersion after the reduction step. , More preferably 50 to 1500 parts by mass is mixed.

The method for mixing the graphene dispersion and NMP is not particularly limited, but a planetary mixer, Homo Disper (Primix), Philmix (registered trademark) (Primix) and the like can be used.

[Strong stirring process]
In order to produce the graphene dispersion of the present invention, it is preferable to carry out a step of stirring the graphene solvent dispersion with a high shear mixer (strong stirring step) after the reduction step. By peeling the graphene with a high shear mixer in the strong stirring step, the stack of graphene can be eliminated. When the solvent replacement step is performed, the strong stirring step is used for the graphene solvent dispersion liquid after the reduction step and finally the graphene dispersion liquid at any stage before the final solvent removal in the step. It is preferable to carry out the process in a state of being mixed with an organic solvent. When the solvent replacement step is not performed, a strong stirring step may be directly performed on the graphene solvent dispersion liquid after the reduction step.

The shear rate in the strong stirring step is preferably 5,000 to 30,000 per second. If the shear rate is too low, graphene is less likely to peel off and the degree of graphene peeling is low. On the other hand, if the shear rate is too high, the degree of exfoliation of graphene becomes too high and the dispersibility is lowered. The shear rate is preferably 10,000 or more per second, more preferably 15,000 or more per second. Similarly, it is preferably 28,000 or less per second, and more preferably 25,000 or less per second. The processing time of the strong stirring step is preferably 15 seconds to 300 seconds, more preferably 20 seconds to 120 seconds, and even more preferably 30 seconds to 80 seconds.

As the high shear mixer used in the strong stirring step, a thin film swirling method, a rotor / stator type, and a media mill type are preferable. Examples of such a mixer include Fillmix (registered trademark) 40-40 type (Primix), Claremix (registered trademark) CLM-0.8S (M-Technique), and Labostar (registered trademark) Mini LMZ015 (registered trademark). Ashizawa Finetech Co., Ltd.), Super Share Mixer SDRT0.35-0.75 (Satake Chemical Machinery Co., Ltd.) and the like.

<Method of manufacturing graphene-electrode active material complex particles>
The use of the graphene dispersion liquid of the present invention is not limited, but as an example, it is usefully used when combining graphene with electrode active material particles such as lithium ion battery electrode active material particles. Here, the compounding means that the graphene is maintained in contact with the surface of the electrode active material particles. The composite of graphene and the electrode active material particles is carried out by a production method including mixing the graphene dispersion of the present invention with the electrode active material particles and then drying the particles. Examples of the compounding mode include those in which graphene and electrode active material particles are integrally granulated, and those in which graphene is adhered to the surface of the electrode active material particles.

When applied to the production of graphene-electrode active material composite particles, the active material may be either a positive electrode active material or a negative electrode active material. That is, the graphene dispersion of the present invention can be used for both the production of a positive electrode and the production of a negative electrode. When applied to the electrode active material particles of a lithium ion battery, the positive electrode active material is not particularly limited, but lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), spinel-type lithium manganate (LiMn ₂ O ₄ ), Alternatively, cobalt ternary system was replaced partially with nickel-manganese _{(LiMn x Ni y Co 1-} x-y O 2), a composite oxide of lithium and a transition metal such as spinel-type lithium manganese oxide (LiMn ₂ O ₄₎ things, olivine such as lithium iron phosphate (LiFePO ₄₎ (phosphoric acid) active _material, metal oxides or _TiS 2, such as V 2 _{O 5,} MoS _2, metal compounds such as NbSe _2, and the like. The negative electrode active material is not particularly limited, but is a carbon-based material such as natural graphite, artificial graphite, or hard carbon, a silicon compound containing SiO, SiC, SiOC, or the like as basic constituent elements, and lithium titanate (Li ₄ Ti ₅ O _12). ), Metal oxides such as manganese oxide (MnO) and cobalt oxide (CoO) that can convert with lithium ions.

The graphene-electrode active material composite particles can be produced by mixing the graphene dispersion of the present invention and the active material particles and then drying them by a method such as spray drying or freeze-drying. Examples of the method of mixing the graphene dispersion liquid and the active material particles include a method using a three-roll, a wet bead mill, a wet planetary ball mill, a homogenizer, a planetary mixer, a twin-screw kneader and the like.

<Manufacturing method of electrode paste>
The graphene dispersion of the present invention can also be used in the production of electrode pastes used in the production of electrodes for lithium ion batteries and the like. That is, the electrode paste can be prepared by mixing the graphene dispersion of the present invention as a conductive additive with the electrode active material and the binder after adding an appropriate amount of solvent as needed. ..

As the electrode active material when applied to the production of the electrode paste of the lithium ion battery, the same active material as described above can be used.

The binder is not particularly limited, but is a fluorine-based polymer such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), rubber such as styrene-butadiene rubber (SBR) or natural rubber, and polysaccharides such as carboxymethyl cellulose. , Polyimide precursor and / or polyimide resin, polyamideimide resin, polyamide resin, polyacrylic acid, sodium polyacrylic acid, acrylic resin, polyacrylonitrile and the like. These may be used as a mixture of two or more kinds.

The conductive auxiliary agent may be only graphene contained in the graphene dispersion of the present invention, or an additional conductive auxiliary agent may be added separately. The additional conductive auxiliary agent is not particularly limited, and is, for example, carbon blacks such as furnace black, Ketjen black (registered trademark), acetylene black, natural graphite (scaly graphite, etc.), graphite such as artificial graphite, and the like. Examples include conductive fibers such as carbon fibers and metal fibers, and metal powders such as copper, nickel, aluminum and silver. Examples of the solvent additionally used include NMP, γ-butyrolactone, water, dimethylacetamide and the like.

[Measurement example 1: Solid content]
1 g of graphene dispersion was adhered onto a slide glass having a known weight, the weight was measured, and the solvent was volatilized by heating on a hot plate whose temperature was adjusted to 120 ° C. for 1 hour. The solid content of the graphene dispersion was repeatedly measured three times from the amount of adhered graphene dispersion before heating and the amount of solvent volatilized calculated from the weight difference before and after heating, and the average was calculated.

[Measurement example 2: Surface roughness]
Using a filter paper of Whatman 50 (GE Healthcare Japan Co., Ltd.) and a diagram pump (ULVAC DTC-41), the graphene dispersion was concentrated by suction filtration to obtain a high-concentration solution having a solid content of 0.020 or more. N-Methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation, grade for electronic materials) was added to the obtained high-concentration graphene dispersion so as to have a solid content of 0.010, and further a mixer (kakuhunter, SK300s, Photochemical Corporation) was added. ) Was used to uniformly mix at 2000 rpm for 1 minute. The obtained graphene / organic solvent paste is applied to an aluminum foil (thickness 20 μm) using a doctor blade type applicator (SA-204, manufactured by Tester Sangyo Co., Ltd., thickness 50 μm setting) at a coating speed of 16.7 mm / s. Then, it was further dried in an oven at 80 ° C. for 1 hour to obtain a coating film. The obtained coating film was subjected to an arithmetic mean roughness in a 250 × 200 μm field of view under the conditions of a 50x objective lens of a laser microscope (VK-X250, Keyence), a laser wavelength of 408 nm, and a measurement pitch of 0.100 μm. This was randomly measured at three points, and the average value was taken as the surface roughness (Ra).

[Measurement example 3: Battery performance evaluation]
The graphene dispersion prepared in each Example / Comparative Example was 1.5 parts by weight as a graphene solid content, 100 parts by weight of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ as an electrode active material, and additional conductivity aid. An electrode paste was obtained by mixing 1.5 parts by weight of acetylene black as an agent, 5 parts by weight of polyvinylidene fluoride as a binder, and 100 parts by weight of NMP as a solvent with a planetary mixer. This electrode paste was applied to an aluminum foil (thickness 18 μm) using a doctor blade (300 μm), dried at 80 ° C. for 15 minutes, and then vacuum dried to obtain an electrode plate.

The prepared electrode plate was cut out to a diameter of 15.9 mm to serve as a positive electrode, and a negative electrode composed of 98 parts by weight of graphite, 1 part by weight of sodium carboxymethyl cellulose and 1 part by weight of SBR aqueous dispersion was cut out to a diameter of 16.1 mm and used. A 2042 type coin battery was produced using Celgard # 2400 (manufactured by Cellguard) cut out to a diameter of 17 mm as a separator and a solvent of ethylene carbonate: diethyl carbonate = 7: 3 containing 1 M of _{LiPF 6 as an electrolytic solution.} Charge / discharge measurements were performed 3 times each in the order of rates 0.1C, 1C, and 5C at an upper limit voltage of 4.2 V and a lower limit voltage of 3.0 V, and then charged / discharged measurements were performed 491 times at 1C, for a total of 500 times. .. The discharge capacities of the third rate 1C, the third rate 5C, and the subsequent 491th rate 1C (500 cycles) were measured.

[Measurement example 4: X-ray photoelectron measurement]
The X-ray photoelectron measurement of each sample was measured using Quantera SXM (manufactured by PHI). The excited X-rays are monochromatic Al Kα1, 2 lines (1486.6 eV), the X-ray diameter is 200 μm, and the photoelectron escape angle is 45 °. The C1s main peak based on carbon atoms is 284.3 eV, the O1s peak based on oxygen atoms is assigned to the peak near 533 eV, and the N1s peak based on nitrogen atoms is assigned to the peak near 402 eV. , And the N / C ratio were determined. For the measurement, the reduced graphene aqueous dispersion prepared in the following example is filtered with a suction filter, diluted with water to 0.5% by mass, suction-filtered, and the washing step is repeated 5 times, and then freeze-dried. This was performed on the graphene powder obtained in the above process.

[Measurement Example 5: Evaluation of Specific Surface Area]
The specific surface area of graphene was measured using HM Model-1210 (manufactured by Macsorb). The measurement was performed in accordance with JIS Z8830: 2013, the method for measuring the amount of adsorbed gas was the carrier gas method, and the analysis of the adsorbed data was performed by the one-point method. The degassing condition was 100 ° C. × 180 minutes. For the measurement, the reduced graphene aqueous dispersion prepared in the following example is filtered with a suction filter, diluted with water to 0.5% by mass, suction-filtered, and the washing step is repeated 5 times, and then freeze-dried. This was performed on the graphene powder obtained in the above process.

[Synthesis Example 1: Method for adjusting graphene oxide gel]
Using 1500 mesh natural graphite powder (Shanghai Ichiho Ishikumi Co., Ltd.) as a raw material, 220 ml of 98% concentrated sulfuric acid, 5 g of sodium nitrate, and 30 g of potassium permanganate were added to 10 g of natural graphite powder in an ice bath. The mixture was mechanically stirred for 1 hour, and the temperature of the mixed solution was maintained at 20 ° C. or lower. This mixed solution was taken out from the ice bath, stirred and reacted in a water bath at 35 ° C. for 4 hours, and then the suspension obtained by adding 500 ml of ion-exchanged water was reacted at 90 ° C. for another 15 minutes. Finally, 600 ml of ion-exchanged water and 50 ml of hydrogen peroxide were added, and the reaction was carried out for 5 minutes to obtain a graphene oxide dispersion. This was filtered while it was hot, the metal ions were washed with a dilute hydrochloric acid solution, the acid was washed with ion-exchanged water, and the washing was repeated until the pH reached 7, to prepare a graphene oxide gel. The element ratio of the oxygen atom to the carbon atom of the prepared graphene oxide gel measured by X-ray photoelectron spectroscopy was 0.53.

[Example 1]
The graphene oxide gel prepared in Synthesis Example 1 was diluted with ion-exchanged water to a concentration of 30 mg / ml and treated with an ultrasonic cleaner for 30 minutes to obtain a uniform graphene oxide dispersion.

20 ml of the graphene oxide dispersion and 0.3 g of dopamine hydrochloride as a surface treatment agent are mixed, and using Homo Disper 2.5 type (Primix Corporation), the rotation speed is 3000 rpm (shear velocity: 80 per second) for 60 minutes. Processed. The treated graphene oxide dispersion was applied with ultrasonic waves at an output of 300 W for 30 minutes (miniaturization step) using an ultrasonic device UP400S (heelscher). After the treatment, the graphene oxide dispersion was diluted with ion-exchanged water to 5 mg / ml, 0.3 g of sodium dithionite was added to 20 ml of the diluted dispersion, and the mixture was kept at 40 ° C. for a reduction reaction for 1 hour. .. Then, the washing step of filtering with a vacuum suction filter, further diluting with ion-exchanged water to 0.5% by mass and suction filtration was repeated 5 times for washing. After cleaning, dilute to 0.5% by mass with NMP having a dipole moment of 4.1 Debye, and use Philmix® 40-40 type (Primix) at a rotation speed of 40 m / s (shear velocity: 20000 per second) to 60. It was subjected to a second treatment (strong stirring step) and suction-filtered under reduced pressure. After filtration, dilute to 0.5% by mass concentration with NMP, treat with Homo Disper 2.5 type (Primix Corporation) at a rotation speed of 3000 rpm for 30 minutes, and repeat the process of vacuum suction filtration twice (solvent replacement). Step), a graphene dispersion was obtained.

[Example 2]
The graphene dispersion was prepared in the same manner as in Example 1 except that the rotation speed of the fill mix was changed to 20 m / s (shear velocity: 10000 per second) in the strong stirring step.

[Example 3]
A graphene dispersion was prepared in the same manner as in Example 1 except that the dopamine hydrochloride was changed to 0.1 g.

[Example 4]
A graphene dispersion was prepared in the same manner as in Example 1 except that the surface treatment agent was changed to 0.3 g of antipyrine.

[Example 5]
A graphene dispersion was prepared in the same manner as in Example 1 except that the surface treatment agent, dobamine hydrochloride, was not added.

[Example 6]
A graphene dispersion was prepared in the same manner as in Example 1 except that 0.3 g of sodium borohydride was used instead of the reducing agent sodium dithionite.

[Example 7]
A graphene dispersion was prepared in the same manner as in Example 1 except that the number of repetitions of vacuum suction and dilution in the solvent replacement step was changed to 5.

[Example 8]
The process was the same as in Example 1 until the strong stirring step, and then the graphene dispersion obtained was distilled with a rotary evaporator (Tokyo Rika Kikai Co., Ltd., EYELA N1110 type). As the distillation conditions, the vacuum pressure was set to 2.5 kPa, the temperature was set to 85 ° C., and the treatment time was set to 3.0 h. After distillation, suction filtration was performed under reduced pressure to obtain a graphene dispersion (solvent replacement step).

[Example 9]
A graphene dispersion was prepared in the same manner as in Example 1 except that the number of repetitions of vacuum suction filtration and dilution in the solvent replacement step was changed to 15.

[Comparative Example 1]
In Example 1, instead of the fill mix treatment, a homodisper 2.5 type (Primix Corporation) having a weaker shearing force than the fill mix was used for 30 minutes at a rotation speed of 3000 rpm (shearing speed: 80 per second). All other operations were carried out in the same manner as in Example 1 to obtain a graphene / organic solvent dispersion.

[Comparative Example 2]
The reduction / washing step was carried out in the same manner as in Example 1 to obtain a graphene aqueous dispersion. Graphene powder was obtained by diluting with water to 0.5% by mass and lyophilizing.

NMP was added to this graphene powder so as to be 0.5% by mass, and the graphene powder was treated using Homo Disper 2.5 type (Primix Corporation) at a rotation speed of 3000 rpm (shear velocity: 80 per second) for 30 minutes. Then, it was suction-filtered under reduced pressure to obtain a graphene dispersion having a solid content of 0.043.

[Comparative Example 3]
The reduction / washing step was carried out in the same manner as in Example 1 to obtain a graphene aqueous dispersion. Graphene powder was obtained by diluting with water to 0.5% by mass and lyophilizing.

NMP is added to this graphene powder so as to be 0.5% by mass, and it is treated with Philmix (registered trademark) 40-40 type (Primix Corporation) at a rotation speed of 40 m / s (shear velocity: 20000 per second) for 60 seconds (strong stirring). Process). Then, it was suction-filtered under reduced pressure to obtain a graphene dispersion having a solid content of 0.043.

[Comparative Example 4]
The graphene oxide gel prepared in Synthesis Example 1 was vacuum dried at 80 ° C. for 6 hours to obtain graphene oxide powder. The obtained graphene oxide powder was heated at 700 ° C. for 6 hours in an argon atmosphere to obtain a heat-reduced graphene powder.

NMP is added to this graphene powder so as to be 0.5% by mass, and it is treated with Philmix (registered trademark) 40-40 type (Primix Corporation) at a rotation speed of 40 m / s (shear velocity: 40,000 per second) for 60 seconds (strong stirring). Process). Then, it was suction-filtered under reduced pressure to obtain a graphene dispersion having a solid content of 0.042.

[Comparative Example 5]
Graphene dispersion (SC Pas1001, solid content 5.4% by mass) purchased from SuperC Technology was added with NMP so as to be 0.5% by mass, and Philmix (registered trademark) 40-40 type (Primix) was used. The treatment (strong stirring step) was performed at a rotation speed of 40 m / s (shear velocity: 20000 per second) for 60 seconds. Other than that, the same operation as in Experiment 1 was carried out to obtain a graphene dispersion.

For the graphene / organic solvent dispersions prepared in each Example and Comparative Example, the solid content ratio was measured according to Measurement Example 1, the surface roughness was measured according to Measurement Example 2, and the discharge capacity was measured according to Measurement Example 3.

For analysis, the obtained graphene / organic solvent dispersion was diluted 3-fold with water and suction filtration was performed, and then dilution and suction filtration were repeated twice to achieve 0.5% by mass of graphene / water. After obtaining the dispersion liquid, it was freeze-dried to obtain a graphene powder. With respect to the obtained graphene powder, the O / C ratio and the N / C ratio were measured according to Measurement Example 4, and the specific surface area was measured according to Measurement Example 5. Table 1 shows the results of these physical property evaluations and battery performance evaluations.

Claims (10)

A graphene / organic solvent dispersion in which graphene is dispersed in an organic solvent, and a graphene film formed by applying a graphene / organic solvent dispersion having a solid content adjusted to 0.005 or more and 0.012 or less is applied to a graphene film having a wavelength of 408 nm. A graphene / organic solvent dispersion having a surface roughness of 1.0 μm or less when measured with a laser microscope using a 50x objective lens at a measurement pitch of 0.100 μm. The graphene / organic solvent dispersion according to claim 1, wherein the graphene film has a surface roughness of 0.05 μm or more. The graphene / organic solvent dispersion according to claim 1 or 2, wherein the element ratio (O / C ratio) of oxygen to carbon measured by X-ray photoelectron spectroscopy of the graphene is 0.08 or more and 0.30 or less. liquid. The graphene / according to any one of claims 1 to 3, wherein the element ratio (N / C ratio) of nitrogen to carbon measured by X-ray photoelectron spectroscopy of the graphene is 0.005 or more and 0.020 or less. Organic solvent dispersion. The graphene / organic solvent dispersion according to any one of claims 1 to 4, wherein the graphene has a specific surface area of 80 m ² / g or more and 250 m ^{2 / g or less as measured by a BET measurement method.} The graphene / organic solvent dispersion liquid according to any one of claims 1 to 5, wherein the solid content ratio is 0.003 or more and 0.400 or less. The graphene / organic solvent dispersion liquid according to any one of claims 1 to 6, wherein the dipole moment of the organic solvent is 3.0 Debye or more. The graphene / organic solvent dispersion according to claim 7, wherein the organic solvent is N-methylpyrrolidone. A method for producing graphene-electrode active material composite particles, which comprises mixing the graphene / organic solvent dispersion according to any one of claims 1 to 8 with the electrode active material particles and then drying the particles. A method for producing an electrode paste, which comprises mixing an electrode active material, a binder, and the graphene / organic solvent dispersion liquid according to any one of claims 1 to 8.