Graphene dispersion and positive electrode paste

Abstract

A graphene dispersion liquid comprising graphene and a solvent, wherein the graphene has an average thickness of 0.3 nm to 10 nm inclusive, the solvent has a dissolution parameter δ of 18 MPa0.5 to 28 MPa0.5 inclusive, and the viscosity at a shear rate of 10 sec-1 and a temperature of 25℃ is 10,000m Pa•s or less when the concentration of graphene is adjusted to 3% by weight. Provided is a graphene dispersion liquid which has excellent fluidability and dispersibility and enables the formation of a coating film having excellent film uniformity, and also provided is a positive electrode paste which can improve the uniformity of a coating film and the life of a battery.

Description

Technical Field

[0001] The present invention relates to a graphene dispersion, a method for producing the same, and a positive electrode paste.

Background Art

[0002] In recent years, research on graphene dispersions and graphene-containing films has been actively conducted, and their use in conductive inks, wiring materials, antistatic films, heat conduction films, barrier films, and conductive aids for lithium ion batteries has been investigated.

[0003] In these applications, fluidity is required for the graphene dispersion. However, since the graphene dispersion tends to have a high viscosity, it is necessary to dilute it in order to increase the fluidity, and it has been difficult to increase the solid content ratio. Further, in the graphene dispersion, graphene is likely to aggregate, and the coating film uniformity may become insufficient. Therefore, it is required to further improve the dispersibility of graphene. For example, a nanocarbon dispersion containing a nanocarbon substance, an organic solvent, and a polymer dispersant, in which the nanocarbon substance is dispersed in the organic solvent (see, for example, Patent Document 1) has been proposed. Further, a dispersion containing carbon nanotubes and graphene platelets (see, for example, Patent Document 2) has been proposed.

[0004] On the other hand, lithium ion batteries used in portable devices, electric vehicles, home power storage, etc. are required to suppress a decrease in battery capacity due to repeated charge and discharge and improve battery life.

[0005] As one means, graphene is used as a conductive aid. As a technique using a conductive aid, a secondary battery electrode having a mixture layer containing an active material for a secondary battery and graphene, in which the content of graphene in the mixture layer and the porosity of the mixture layer are defined (see, for example, Patent Document 3) has been proposed.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2019-19155 [Patent Document 2] Japanese Patent Application Publication No. 2014-525981 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2018-174134 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] However, the dispersion liquids described in Patent Documents 1 and 2 had problems in that the fluidity and dispersibility of graphene were still insufficient.

[0008] On the other hand, as described above, an example of an application affected by the problems with the fluidity and dispersibility of the graphene dispersion liquid is a lithium-ion battery. In the positive electrode paste used for manufacturing the positive electrode of a lithium-ion battery, it is preferable to increase the solid content ratio of the positive electrode paste. Therefore, it is important that the conductive assistant also has a viscosity that allows for a high concentration and easy mixing. In addition, in order to improve the battery life of a lithium-ion battery, it is important to suppress the deterioration of the conductive path accompanying repeated charge and discharge. For this purpose, it is considered important that the conductive assistant forming the conductive path is uniformly mixed with other materials constituting the positive electrode paste, such as the positive electrode active material, etc., and forms a homogeneous and stable coating film. From the above, the graphene dispersion liquid used for manufacturing the positive electrode paste is required to have high dispersibility of graphene and a viscosity that is easy to mix.

[0009] The electrode for a secondary battery described in Patent Document 3 can make it difficult to generate voids by using graphene. However, in recent years, further improvement in battery life has been demanded. Also, the viscosity of the graphene dispersion liquid was high, and improvement in fluidity has been demanded.

[0010] Therefore, an object of the present invention is to provide a graphene dispersion liquid having excellent fluidity and dispersibility, and capable of obtaining a coating film having excellent coating film uniformity, and thereby providing a positive electrode paste capable of improving the coating film uniformity and battery life.

Means for Solving the Problems

[0011] In order to solve the above problems, the present invention provides a graphene dispersion liquid containing graphene and a solvent, wherein the average thickness of the graphene is 0.3 nm or more and 10 nm or less, and the solubility parameter δ of the solvent is 18 MPa 0.5 or more and 28 MPa 0.5 or less, and when adjusted to a graphene concentration of 3% by weight, the viscosity at a shear rate of 10 sec -1 and a temperature of 25°C is 10,000 mPa·s or less.

Effects of the Invention

[0012] The graphene dispersion liquid of the present invention has excellent fluidity and excellent dispersibility of graphene. In particular, when used as a conductive auxiliary agent for lithium-ion batteries, the graphene dispersion liquid of the present invention has excellent uniformity of graphene when mixed with a positive electrode active material. In addition, the positive electrode paste of the present invention has excellent coating film uniformity, can increase the solid content rate, and can improve the battery life.

Modes for Carrying Out the Invention

[0013] First, the graphene dispersion liquid of the present invention will be described. The graphene dispersion liquid of the present invention contains graphene having an average thickness of 0.3 nm or more and 10 nm or less and a solvent.

[0014] Thin graphene with an average thickness of 0.3 nm or more and 10 nm or less is flexible and thus can well conform to the surface of the object to be coated, and it is easy to form a coating film with excellent electrical conductivity and thermal conductivity. On the other hand, since thin graphene is prone to aggregation, conventionally, when such thin graphene is used, it is difficult to maintain the dispersibility in the graphene dispersion, and it is also likely to have increased viscosity, resulting in insufficient fluidity of the dispersion, and the coating film uniformity may decrease. Further, when such a dispersion is used for the positive electrode paste, there are problems such as a decrease in battery life due to the decrease in coating film uniformity and difficulty in increasing the solid content ratio of the positive electrode paste.

[0015] In the present invention, together with such thin graphene, a solvent having a solubility parameter δ of 18 MPa 0.5 or more and 28 MPa 0.5 or less is contained, whereby a dispersion excellent in fluidity can be provided, the viscosity of which at a shear rate of 10 sec -1 and a temperature of 25 °C becomes 10,000 mPa·s or less when adjusted to a graphene concentration of 3% by weight.

[0016] The solubility parameter δ is an index of the solubility between a solvent and a solute proposed by Hildebrand, and the smaller the difference in δ between the solvent and the solute, the greater the solubility. When the solubility of graphene is low, graphene settles and the dispersibility decreases, but the viscosity of the dispersion decreases. Conversely, when the solubility is high, the dispersibility of graphene improves, but the viscosity of the dispersion increases. That is, in the graphene dispersion, the viscosity and the dispersibility are in a trade-off relationship. The graphene dispersion adjusted to a graphene concentration of 3% by weight had a clay-like form without fluidity, the viscosity of which at a shear rate of 10 sec -1 and a temperature of 25 °C exceeded 10,000 mPa·s.

[0017] According to the present invention, it is possible to provide a graphene dispersion liquid having excellent fluidity while using thin graphene. When the graphene dispersion liquid of the present invention is used for a positive electrode paste or a positive electrode of a lithium ion battery, a uniform coating film in which a positive electrode active material and a highly fluid graphene dispersion liquid are uniformly mixed can be easily obtained, and the solid content ratio of the positive electrode paste can be increased. Furthermore, since the binding of the positive electrode of the lithium ion battery is strengthened, deterioration of the conductive path accompanying repeated charge and discharge is suppressed, and the battery life can be improved.

[0018] The graphene dispersion liquid of the present invention contains a solvent having a solubility parameter δ of 18 MPa 0.5 or more and 28 MPa 0.5 or less. By containing a solvent having a solubility parameter δ within such a range, the dispersibility and fluidity of graphene can be enhanced. When the solubility parameter δ is less than 18 MPa 0.5 or exceeds 28 MPa 0.5 , the solubility in graphene becomes insufficient, the fluidity and dispersibility decrease, and the coating film uniformity decreases. Also, the solid content ratio of the positive electrode paste decreases, and the battery life decreases. The solubility parameter δ of the solvent is preferably 19 MPa 0.5 or more, and more preferably 20 MPa 0.5 or more. On the other hand, the solubility parameter δ of the solvent is preferably 27 MPa 0.5 or less, and more preferably 26 MPa 0.5 or less.

[0019] In the present invention, the solubility parameter δ of the solvent shall be the value described in Table V in ALLAN F. M. BARTON, Chemical Reviews, 1975, Vol.75, No.6 731 - 753. For solvents not described in the literature, according to the definition of Hildebrand's solubility parameter, from the molar heat of vaporization ΔH, molar volume V, gas constant R, and temperature T (298.15 K with 25 °C) of the solvent, δ = {(ΔH - RT) / V} 0.5 can be obtained.

[0020] The solubility parameter δ is 18 MPa 0.5 or more and 28 MPa 0.5 Examples of solvents with δ of 18 MPa or more and 28 MPa or less include toluene (δ = 18.2), styrene (δ = 19.0), o-xylene (δ = 18.0), ethylbenzene (δ = 18.0), tetrahydronaphthalene (δ = 19.4), dichloromethane (δ = 19.8), chloroform (δ = 19.8), chlorobenzene (δ = 19.4), furan (δ = 19.2), tetrahydrofuran (δ = 18.6), 1,4-dioxane (δ = 20.5), acetone (δ = 20.3), methyl ethyl ketone (δ = 19.0), cyclohexanone (δ = 20.3), diethyl ketone (δ = 18.0), isophorone (δ = 18.6), acetaldehyde (δ = 21.1), furfural (δ = 22.9), benzaldehyde (δ = 19.2), γ-butyrolactone (δ = 25.8), methyl acetate (δ = 19.6), ethyl acetate (δ = 18.6), acetonitrile (δ = 24.3), acrylonitrile (δ = 21.5), nitromethane (δ = 26.0), nitrobenzene (δ = 20.5), pyridine (δ = 21.9), morpholine (δ = 22.1), N-methylpyrrolidone (δ = 23.1), quinoline (δ = 22.1), N,N-dimethylformamide (δ = 24.8), N,N-dimethylacetamide (δ = 22.1), dimethyl sulfoxide (δ = 24.5), ethanol (δ = 26.0), 1-propanol (δ = 24.3), 2-propanol (δ = 23.5), 1-butanol (δ = 23.3), 2-butanol (δ = 22.1), benzyl alcohol (δ = 22.1), ethyl lactate (δ = 20.5), n-butyl lactate (δ = 19.2), etc. Two or more of these may be used. When using two or more solvents, the sum of the products of the molar ratio and δ of each solvent is defined as the δ of the mixed solvent. Among these, the solubility parameter δ is 20 MPa 0.5 or more and 26 MPa 0.5Solvents selected from the following solvents are more preferred: 1,4-dioxane (δ = 20.5), acetone (δ = 20.3), cyclohexanone (δ = 20.3), acetaldehyde (δ = 21.1), furfural (δ = 22.9), γ-butyrolactone (δ = 25.8), acetonitrile (δ = 24.3), acrylonitrile (δ = 21.5), nitromethane (δ = 26.0), nitrobenzene (δ = 20.5), pyridine (δ = 21.9), morpholine (δ = 22.1), N-methylpyrrolidone (δ = 23.1), quinoline (δ = 22.1), N,N-dimethylformamide (δ = 24.8), N,N-dimethylacetamide (δ = 22.1), dimethyl sulfoxide (δ = 24.5), ethanol (δ = 26.0), 1-propanol (δ = 24.3), 2-propanol (δ = 23.5), 1-butanol (δ = 23.3), 2-butanol (δ = 22.1), benzyl alcohol (δ = 22.1) and ethyl lactate (δ = 20.5). Solvents selected from acetaldehyde (δ = 21.1), furfural (δ = 22.9), acetonitrile (δ = 24.3), acrylonitrile (δ = 21.5), nitromethane (δ = 26.0), nitrobenzene (δ = 20.5), pyridine (δ = 21.9), morpholine (δ = 22.1), N-methylpyrrolidone (δ = 23.1), quinoline (δ = 22.1), N,N-dimethylformamide (δ = 24.8), N,N-dimethylacetamide (δ = 22.1), dimethyl sulfoxide (δ = 24.5), 1-propanol (δ = 24.3), 2-propanol (δ = 23.5), 1-butanol (δ = 23.3), 2-butanol (δ = 22.1) and benzyl alcohol (δ = 22.1), which are solvents with a solubility parameter δ of 21 MPa0.5 or more and 25 MPa0.5 or less, are even more preferred.

[0021] In particular, in the application of lithium-ion batteries, solvents selected from N,N-dimethylformamide, N-methylpyrrolidone and N,N-dimethylacetamide are preferred from the viewpoint of affinity with the binder polymer solution. Two or more of these may be contained. Among these, N-methylpyrrolidone is more preferred from the viewpoint of more effectively exhibiting the effect of improving the dispersibility by the surface treatment agent. By solvating the surface treatment agent attached to graphene, the dispersibility and fluidity can be further improved.

[0022] The solvent of the graphene dispersion of the present invention can be easily identified by filtering the dispersion to remove the solid content and analyzing the filtrate by GC-MS.

[0023] From the viewpoint of fluidity, the graphene dispersion of the present invention preferably has a low viscosity. Since the viscosity of the graphene dispersion depends on the graphene concentration, in the present invention, as an index of viscosity, the viscosity when adjusted to a graphene concentration of 3% by weight is selected, and the shear rate is 10 sec, which is the degree of dripping by its own weight. -1 The viscosity at a temperature of 25 °C was measured.

[0024] As described above, thin graphene tends to have an increased viscosity, and a conventional dispersion containing 3% by weight of graphene often has a viscosity exceeding 10,000 mPa·s. At such a viscosity, for example, when used in a positive electrode paste, the mixing of the positive electrode active material and graphene becomes insufficient, and the coating film uniformity, the solid content ratio of the paste, and the battery life decrease. The graphene dispersion of the present invention improves the fluidity by containing the above-mentioned solvent, and the viscosity at a shear rate of 10 sec and a temperature of 25 °C when adjusted to a graphene concentration of 3% by weight is 10,000 mPa·s or less, thereby improving the coating film uniformity and increasing the solid content ratio of the positive electrode paste to improve the battery life. Shear rate 10 sec -1 By setting the viscosity at a temperature of 25 °C to 10,000 mPa·s or less, the coating film uniformity can be improved, the solid content ratio of the positive electrode paste can be increased, and the battery life can be improved. Shear rate 10 sec -1The viscosity of the graphene dispersion is preferably 5,000 mPa·s or less, more preferably 3,000 mPa·s or less, and even more preferably 1,000 mPa·s or less. From the viewpoint of ease of coating, the viscosity of the graphene dispersion is preferably 10 mPa·s or more, more preferably 20 mPa·s or more, and even more preferably 50 mPa·s or more. Here, the viscosity of the graphene dispersion is measured at 25 °C using a Brookfield viscometer LVDVII+ under the conditions of rotor No. 6 and 1 / s = 10.

[0025] Incidentally, the viscosity of the graphene dispersion can be adjusted to the aforementioned range, for example, by using the aforementioned preferred solvents and the polymer additives described below, or by adjusting the N / C ratio of graphene to the preferred range described below.

[0026] <Graphene> Graphene is useful as a conductive aid because it has a thin layer shape and many conductive and heat conduction paths per unit weight, and it easily forms a good conductive and heat conduction network in the coating film. In addition, since graphene is a non-permeable thin layer-shaped molecule, it can reduce the substance permeability in the coating film and is also useful as a barrier film.

[0027] Graphene, in a narrow sense, refers to a sheet of sp 2 bonded carbon atoms with a thickness of one atom (single-layer graphene). However, in this specification, anything having a flaky form in which single-layer graphene is laminated is also referred to as graphene. Similarly, graphene oxide is also referred to as anything having a flaky form in which it is laminated.

[0028] In addition, in this specification, those with an O / C ratio (the atomic ratio of oxygen atoms to carbon atoms measured by X-ray photoelectron spectroscopy (XPS)) exceeding 0.4 are referred to as graphene oxide, and those with 0.4 or less are referred to as graphene. Also, reduced graphene oxide obtained by reducing graphene oxide and having an O / C ratio of 0.4 or less is also referred to as graphene.

[0029] Furthermore, although surface treatment for improving dispersibility or the like may be performed on graphene or graphene oxide, in this specification, graphene or graphene oxide with such a surface treatment agent adhered thereto is also referred to as "graphene" or "graphene oxide".

[0030] The average thickness of the graphene used in the graphene dispersion of the present invention is 0.3 nm or more and 10 nm or less. By using thin graphene within such a range of average thickness in the graphene dispersion of the present invention, it is possible to improve the followability of graphene to the surface of the positive electrode active material while maintaining conductivity, and to easily form a conductive path. An average thickness of 0.3 nm of graphene is the theoretical minimum value of graphene, indicating that it is single-layer graphene. On the other hand, when the average thickness of graphene exceeds 10 nm, the dispersibility decreases and the coating film uniformity decreases. In addition, since the followability to the surface of the positive electrode active material decreases, the formation of the conductive path becomes insufficient and the battery life is shortened. From the viewpoints of further improving the fluidity of the positive electrode paste to easily increase the solid content ratio, further improving the coating film uniformity, and more effectively forming the conductive path and further improving the battery life, the average thickness of graphene is preferably 8 nm or less, and more preferably 6 nm or less. Here, the average thickness of graphene in the graphene dispersion can be calculated by collecting graphene from the graphene dispersion, using an atomic force microscope to perform enlarged observation in a field of view range of about 1 to 10 μm square so that graphene can be appropriately observed, measuring the thickness of 10 randomly selected graphenes, and obtaining the arithmetic average value thereof. In addition, the thickness of each graphene is the arithmetic average value of the measured values of the thicknesses at 5 randomly selected locations in each graphene.

[0031] From the perspective of enhancing the coating film uniformity of the positive electrode paste, increasing the contact area with the positive electrode active material, and further improving the battery life, the size of graphene in the direction parallel to the graphene layer is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1 μm or more. On the other hand, from the perspective of further improving the dispersibility, improving the fluidity of the positive electrode paste to facilitate increasing the solid content ratio, and further improving the coating film uniformity, the size of graphene in the direction parallel to the graphene layer is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 20 μm or less. Here, the size of graphene in the direction parallel to the graphene layer in the graphene dispersion can be calculated by collecting graphene from the graphene dispersion and observing it under an electron microscope at a magnification of 1,500 to 50,000 times so that the graphene is appropriately within the field of view. For 10 randomly selected graphene, measure the length (major axis) of the longest part and the length (minor axis) of the shortest part in the direction parallel to the graphene layer, respectively, and obtain the arithmetic mean value of the numerical values obtained by (major axis + minor axis) / 2. In addition, the size of graphene in the direction parallel to the graphene layer can be easily adjusted to the aforementioned range by refining graphene oxide or reduced graphene by the method described below. Also, commercially available graphene oxide or graphene of a desired size may be used.

[0032] From the perspective of further improving the dispersibility by residual functional groups and further improving the coating film uniformity of the positive electrode paste, the elemental ratio of oxygen to carbon (O / C ratio) of graphene measured by X-ray photoelectron spectroscopy is preferably 0.05 or more, more preferably 0.07 or more, and even more preferably 0.08 or more. On the other hand, from the perspective of further improving the fluidity of the graphene dispersion and restoring the π-electron conjugate structure by reduction to further enhance the conductivity and further improve the coating film uniformity and battery life, the O / C ratio is preferably 0.35 or less, more preferably 0.20 or less, and even more preferably 0.15 or less. Here, the O / C ratio of graphene in the graphene dispersion can be measured by collecting graphene from the graphene dispersion and using X-ray photoelectron spectroscopy (XPS). The peak near 284.3 eV is attributed to the C1s main peak based on carbon atoms, the peak near 533 eV is attributed to the O1s peak based on oxygen atoms, the O / C ratio is calculated from the area ratio of each peak, and the obtained value is rounded to the second decimal place by rounding the third decimal place. Note that the O / C ratio of graphene can be easily adjusted to the above-mentioned range, for example, in the case of using the chemical exfoliation method, by adjusting the degree of oxidation of the starting graphene oxide and the degree of reduction by the reduction reaction conditions. Also, commercially available graphene oxide or graphene having a desired O / C ratio may be used.

[0033] As described above, graphene or graphene oxide may be surface-treated. In particular, a surface treatment agent containing nitrogen atoms has a tendency to enhance the dispersibility of graphene in a solvent in the range of a solubility parameter δ of 18 MPa 0.5 or more and 28 MPa 0.5 or less. Furthermore, the surface treatment agent can enhance the interaction with polyvinyl alcohol described later, further enhance the effect of improving the dispersibility, and further improve the adhesion when used in a lithium-ion battery positive electrode.

[0034] When graphene is treated with a surface treatment agent containing nitrogen atoms, the amount of the surface treatment agent adhering to the graphene can be determined from the atomic ratio of nitrogen to carbon (N / C ratio) measured by X-ray photoelectron spectroscopy. From the viewpoint of further improving the dispersibility, the fluidity of the graphene dispersion, the coating film uniformity of the positive electrode paste, and the battery life, the N / C ratio of the graphene is preferably 0.005 or more, more preferably 0.006 or more, and even more preferably 0.008 or more. On the other hand, from the viewpoints of further improving the fluidity of the graphene dispersion, increasing the conductivity, and further improving the battery life and the coating film uniformity, the N / C ratio of the graphene is preferably 0.020 or less, more preferably 0.018 or less, and even more preferably 0.016 or less. Here, the N / C ratio of the graphene in the graphene dispersion can be measured by X-ray photoelectron spectroscopy (XPS) after collecting the graphene from the graphene dispersion. The peak near 284.3 eV is attributed to the C1s main peak based on carbon atoms, the peak near 402 eV is attributed to the N1s peak based on nitrogen atoms, the N / C ratio is calculated from the area ratio of each peak, and the obtained value is rounded to the third decimal place after rounding the fourth decimal place. Note that the N / C ratio of the graphene can be easily adjusted to the above-mentioned range depending on, for example, the adhesion amount of the surface treatment agent described later.

[0035] The surface treatment agent adheres to the surface of graphene and thus exhibits the effect of further enhancing the dispersibility of graphene. In this specification, graphene in a state where such a surface treatment agent is attached is referred to as "surface-treated graphene". Here, in the present invention, the fact that the surface treatment agent is present attached to graphene means that a washing step of dispersing the surface-treated graphene in water at a mass ratio of 100 times and filtering is repeated 5 or more times, and then after drying by methods such as freeze-drying and spray-drying, the surface treatment agent remains in the surface-treated graphene. The fact that the surface treatment agent remains means that when the dried surface-treated graphene is measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS), the surface treatment agent molecules can be detected in the form of proton-added molecules in the positive secondary ion spectrum. However, in the case where the surface treatment agent is a neutralized salt, it can be detected in the form of a proton added to the surface treatment agent molecule from which the anion molecule has been removed. The chemical structure of the surface treatment agent contained in the surface-treated graphene can be specified by TOF-SIMS. In addition, the quantification of the surface treatment agent is performed using a sample obtained by repeating a washing step of dispersing the surface-treated graphene in water at a mass ratio of 100 times and filtering 5 or more times, and then freeze-drying.

[0036] As the surface treatment agent, from the viewpoint of being easily adsorbed on the graphene surface, a compound having an aromatic ring is preferable.

[0037] In addition, the surface treatment agent preferably has an acidic group and / or a basic group.

[0038] As the acidic group, a group selected from a hydroxy group, a phenolic hydroxy group, a nitro group, a carboxyl group, and a carbonyl group is preferable, and two or more of these may be present. Among these, the phenolic hydroxy group is preferable.

[0039] Examples of the compound having a phenolic hydroxy group and an aromatic ring include phenol, nitrophenol, cresol, catechol, etc. A part of the hydrogen of these compounds may be substituted. Among these, catechol and its derivatives are preferable from the viewpoints of adhesiveness to graphene and dispersibility in a dispersion medium. For example, catechol, dopamine hydrochloride, 3-(3,4-dihydroxyphenyl)-L-alanine, 4-(1-hydroxy-2-aminoethyl)catechol, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, caffeic acid, 4-methylcatechol, and 4-tert-butylpyrocatechol are preferable.

[0040] As the basic group, an amino group is preferable.

[0041] Examples of the compound having an amino group and an aromatic ring include benzylamine, phenylethylamine, and salts thereof. A part of the hydrogen of these compounds may be substituted.

[0042] Compounds having an acidic group, a basic group, and an aromatic ring are also preferable. For example, dopamine hydrochloride is preferable.

[0043] The graphene used in the present invention may be produced by a physical exfoliation method or a chemical exfoliation method. When produced by a chemical exfoliation method, the method for producing graphene oxide is not particularly limited, and known methods such as the Hummers method can be used. Also, commercially available graphene oxide may be purchased.

[0044] The chemical exfoliation method preferably has, in this order, a step of oxidatively exfoliating graphite to obtain graphene oxide (graphite exfoliation step) and a step of performing reduction (reduction step). If necessary, between the graphite exfoliation step and the reduction step, a step of attaching a surface treatment agent to graphene (surface treatment step) and / or a step of adjusting the size of graphene in the direction parallel to the graphene layer (miniaturization step) may be performed. When attaching the surface treatment graphene to graphene, the surface treatment agent may be attached to graphene after the reduction step, or may be subjected to a reduction treatment after being attached to graphene oxide. Also, when miniaturizing graphene, graphene oxide may be miniaturized, or the graphene after reduction may be miniaturized. From the viewpoint of the uniformity of the reduction reaction, it is preferable to perform the reduction step in a state where graphene oxide is miniaturized, and the miniaturization step is preferably performed before the reduction step or during the reduction step. For this reason, it is preferable to include the graphite exfoliation step, the surface treatment step, the miniaturization step, and the reduction step in this order.

[0045] [Graphite exfoliation step] First, graphite is oxidatively exfoliated to obtain graphene oxide. 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, sodium nitrate and potassium permanganate can be used as the oxidizing agent. The greater the amount of the oxidizing agent relative to graphite used during the oxidation reaction, 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 preferably 0.200 or more and 0.800 or less. The weight ratio of potassium permanganate to graphite is preferably 1.00 or more and 4.00 or less.

[0046] [Surface treatment step] Next, graphene oxide and the surface treatment agent are mixed to attach the surface treatment agent to graphene. Examples of the mixing method include a method of mixing using a mixer or kneader such as an automatic mortar, three-roll mill, bead mill, planetary ball mill, homogenizer, homodisper, homomixer, planetary mixer, or twin-screw kneader.

[0047] [Miniaturization step] Next, the graphene oxide is miniaturized. As the miniaturization method, for example, a method of colliding a dispersed liquid to which pressure is applied with a single ceramic ball, a method using a liquid-liquid shear type wet jet mill in which dispersed liquids to which pressure is applied are collided to perform dispersion, a method of applying ultrasonic waves to the dispersed liquid, etc. can be mentioned. In the miniaturization process, the higher the processing pressure and output, and the longer the processing time, the more likely it is that the graphene oxide or graphene will be miniaturized. It is possible to adjust the size of the reduced graphene according to the type, processing conditions, and processing time of the miniaturization treatment in the miniaturization process. In order to adjust the size parallel to the graphene layer to the aforementioned range, the solid content concentration of the graphene oxide or graphene in the miniaturization process is preferably 0.01 wt% or more and 2 wt% or less. Also, when performing ultrasonic treatment, the ultrasonic output is preferably 100 W or more and 3000 W or less.

[0048] [Reduction process] Next, the miniaturized graphene oxide is reduced. As the reduction method, chemical reduction is preferred. In the case of chemical reduction, examples of the reducing agent include organic reducing agents and inorganic reducing agents, but inorganic reducing agents are more preferred due to the ease of washing after reduction.

[0049] Examples of the organic reducing agent include aldehyde-based reducing agents, hydrazine derivative reducing agents, and alcohol-based reducing agents. Among them, alcohol-based reducing agents are particularly suitable because they can reduce 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, etc.

[0050] Examples of the inorganic reducing agent include sodium dithionite, potassium dithionite, phosphorous acid, sodium borohydride, hydrazine, etc. Among them, sodium dithionite or potassium dithionite can be reduced while relatively retaining acidic groups, so graphene with high dispersibility in a solvent can be produced and is preferably used.

[0051] After completing the reduction process, preferably, a washing process of diluting with water and filtering is performed, whereby the purity of graphene can be improved.

[0052] <Polymer additive> The graphene dispersion of the present invention preferably contains a polymer additive that is soluble in a solvent having a solubility parameter δ of 18 MPa 0.5 or more and 28 MPa 0.5 or less. Examples of the polymer additive include polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose. From the viewpoint of enhancing the dispersibility and fluidity of graphene and further improving the coating film uniformity, among these, polymer additives selected from polyvinyl alcohol, polyvinyl pyrrolidone, and hydroxypropyl cellulose are more preferable, and polyvinyl alcohol or polyvinyl pyrrolidone is even more preferable.

[0053] <Polyvinyl alcohol> In the graphene dispersion of the present invention, it is preferable to use polyvinyl alcohol having a specific saponification rate. Due to interactions such as hydrogen bonding between the hydroxyl groups on polyvinyl alcohol and the oxygen-containing functional groups on graphene and / or the functional groups on the surface treatment agent, the dispersibility and fluidity of graphene are further improved, and the adhesion between graphene and polyvinyl alcohol is improved. Therefore, in the present invention, the hydroxyl group content of polyvinyl alcohol, that is, the saponification rate, is important.

[0054] The saponification degree of the polyvinyl alcohol used in the graphene dispersion of the present invention is preferably 70% or more and 100% or less. By setting the saponification degree within such a range, the dispersibility can be further improved by the interaction with graphene. By setting the saponification degree of polyvinyl alcohol to 70% or more, the dispersibility can be further improved by the interaction with graphene, and the fluidity and battery life of the graphene dispersion can be further improved. The saponification degree of polyvinyl alcohol is more preferably 75% or more, and even more preferably 80% or more. On the other hand, from the viewpoint of improving the solubility of polyvinyl alcohol in an organic solvent, the saponification degree of polyvinyl alcohol is preferably 99.9% or less, and more preferably 98% or less. Here, the saponification degree of polyvinyl alcohol can be determined according to JIS K6726-1994. Further, % in the saponification degree means mol%.

[0055] The polyvinyl alcohol may be unmodified polyvinyl alcohol or modified polyvinyl alcohol.

[0056] Examples of the unmodified polyvinyl alcohol include those having a trade name of “Kuraray Poval” (registered trademark) (Kuraray Co., Ltd.), a trade name of “Gosenol” (registered trademark) (Mitsubishi Chemical Corporation), a trade name of “Denka Poval” (registered trademark) (Denka Co., Ltd.), and a trade name of “J-Poval” (Nippon Vinyl Alcohol & Poval Co., Ltd.).

[0057] Examples of the modified polyvinyl alcohol include those having a group selected from a carboxyl group, a sulfonic acid group, a cationic group (quaternary ammonium salt), and an ethylene oxide group in the side chain. Specifically, for example, those having a trade name of “Gosenex” (registered trademark) (Mitsubishi Chemical Corporation), series K, L, T, WO, etc. can be mentioned.

[0058] Further, from the viewpoint of easily obtaining the effect of improving dispersibility, the degree of polymerization of polyvinyl alcohol is preferably 100 or more, more preferably 200 or more, and even more preferably 300 or more. On the other hand, from the viewpoints of further improving the fluidity of the graphene dispersion, increasing the solid content ratio of the positive electrode paste, and further improving the battery life, the degree of polymerization of polyvinyl alcohol is preferably 10,000 or less, more preferably 5,000 or less, and even more preferably 2,000 or less. Here, the degree of polymerization of unmodified polyvinyl alcohol can be determined according to JIS6726-1994.

[0059] Two or more kinds of polyvinyl alcohol may be contained. In such a case, it is preferable that the saponification rate and the degree of polymerization of the two or more kinds of polyvinyl alcohol as a whole are within the above ranges.

[0060] <Polyvinylpyrrolidone> The graphene dispersion of the present invention may contain polyvinylpyrrolidone. Similar to the aforementioned polyvinyl alcohol, polyvinylpyrrolidone can improve the dispersibility of graphene in a solvent by interactions such as hydrogen bonding with graphene.

[0061] As the polyvinylpyrrolidone, those having molecular weight grades such as K-15, K-30, K-60, K-90, and K-120 can be used. From the viewpoint of easily obtaining the effect of improving the dispersibility of graphene, K-15, K-30, and K-60 are more preferable, and K-15 and K-30 are even more preferable.

[0062] The polyvinylpyrrolidone may be a copolymer with an acrylic monomer other than vinylpyrrolidone. The acrylic monomer other than vinylpyrrolidone is not particularly limited, and examples thereof include vinyl acetate, hydroxyethyl methacrylate, acrylic acid, dimethylacrylamide, and butyl acrylate.

[0063] <Cellulose derivative> The graphene dispersion of the present invention may contain cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose. Among these, from the viewpoint of excellent graphene dispersibility improvement effect, hydroxyethyl cellulose and hydroxypropyl cellulose are more preferable, and hydroxypropyl cellulose is more preferable.

[0064] From the viewpoint that the mass average molecular weight (Mw) of the cellulose derivative is likely to obtain the graphene dispersibility improvement effect, 1,000 or more is preferable, and 5,000 or more is more preferable. Also, from the viewpoint of excellent solubility in the solvent, 1,000,000 or less is preferable, and 500,000 or less is more preferable.

[0065] The graphene dispersion of the present invention preferably contains 1 to 300 parts by weight of the aforementioned polymer additive with respect to 100 parts by weight of the aforementioned graphene. If the content of the polymer additive is 1 part by weight or more, due to the dispersion improvement effect of the polymer additive, the fluidity of the graphene dispersion can be further improved, and the coating film uniformity and battery life of the positive electrode paste can be further improved. The content of the polymer additive is more preferably 3 parts by weight or more, more preferably 5 parts by weight or more, more preferably 10 parts by weight or more, even more preferably 15 parts by weight or more, and particularly preferably 20 parts by weight or more. On the other hand, if the content of the polymer additive is 300 parts by weight or less, the electrical resistance when forming the coating film can be suppressed, and the battery life can be further improved. Also, the fluidity of the graphene dispersion can be further improved, and the solid content ratio and coating film uniformity of the positive electrode paste can be further improved. The content of the polymer additive is more preferably 200 parts by weight or less, and even more preferably 100 parts by weight or less.

[0066] The content of graphene and polymer additives in the graphene dispersion of the present invention can be determined by the following method. First, graphene and polymer additives are separated by filtration. The residue containing graphene is washed thoroughly with a solvent and then dried to determine the content of graphene. Also, after distilling off the solvent from the filtrate (containing polymer additives) and drying it, the content of the polymer additive can be determined by measuring the weight. However, when the raw material composition used for the graphene dispersion is known, it can also be determined by calculation from the raw material composition.

[0067] Further, the graphene dispersion of the present invention preferably has a storage modulus and a loss modulus of 0.1 Pa or more and 100 Pa or less at a strain of 10%, a frequency of 10 Hz, and a temperature of 25 °C when adjusted to a graphene concentration of 3% by weight. The storage modulus and the loss modulus are important indicators from the viewpoint of examining in detail the fluidity of the graphene dispersion. If the storage modulus and the loss modulus at a strain of 10% and 10 Hz are within the above range, for example, flow in a pipe becomes possible, and it becomes easy to continuously supply the graphene dispersion of the present invention. Also, graphene having such properties can improve the coating film uniformity, increase the solid content ratio of the positive electrode paste, and improve the battery life. The storage modulus and the loss modulus of the dispersion are preferably 100 Pa or less, more preferably 80 Pa or less, and even more preferably 60 Pa or less. From the viewpoint of ease of coating, 0.1 Pa or more is preferable, 0.2 Pa or more is more preferable, and 0.5 Pa or more is even more preferable.

[0068] As described above, the viscosity of the conventional graphene dispersion increases significantly from a graphene concentration of 2% by weight or more, and in many cases, the graphene dispersion becomes clay-like rather than liquid at a graphene concentration of 3% by weight, making it impossible to measure the viscosity. However, the graphene dispersion of the present invention maintains a liquid state even at a high graphene concentration of 3% by weight. Therefore, the inventors have found that while the graphene dispersion shows low storage modulus and loss modulus at a strain of 10%, when a certain strain or more is applied, for example, a strain of 200%, a peculiar phenomenon occurs in which the storage modulus and loss modulus increase due to collisions between graphene particles. On the other hand, the conventional graphene dispersion that is clay-like at a graphene concentration of 3% by weight shows high storage modulus and loss modulus at a strain of 10%, and as the strain increases, the structure collapses, resulting in a behavior in which the high storage modulus and loss modulus decrease gradually. In other words, the fact that the storage modulus and loss modulus at a strain of 200% are greater than the storage modulus and loss modulus at a strain of 10% means that the graphene dispersion has excellent fluidity. The graphene dispersion of the present invention, when adjusted to a graphene concentration of 3 wt %, preferably has a storage modulus and a loss modulus at a frequency of 10 Hz and a temperature of 25° C. that satisfy the following formula (1) and / or formula (2). Formula (1): G' 200 / G' 10 ≧1 In formula (1), G' 200 is the storage modulus at 200% strain, G' 10 represents the storage modulus at 10% strain; Formula (2):G'' 200 / G'' 10 ≧1 In formula (2), G'' 200 is the loss modulus at 200% strain, G'' 10 represents the loss modulus at a strain of 10%.

[0069] From the viewpoint of excellent fluidity, G' 200 / G' 10 and / or G'' 200 / G'' 10 is preferably 1 or more, more preferably 10 or more, and even more preferably 100 or more.

[0070] Here, the storage modulus and loss modulus of the dispersion are measured in a nitrogen gas stream using a viscoelasticity measuring device ARES-G2 (manufactured by TA Instrument) at a frequency of 10 Hz and a temperature of 25°C, using a geometry of an equilibrium disk type with a diameter of 40 mm.

[0071] Note that the storage modulus and loss modulus of the graphene dispersion can be easily adjusted to the above-mentioned ranges, for example, by using the aforementioned preferred solvent and polyvinyl alcohol, or by adjusting the N / C ratio of graphene to the aforementioned preferred range.

[0072] The graphene dispersion of the present invention preferably has fluidity. In this specification, having fluidity means that 1 g of the graphene dispersion is dropped in a circular shape with a diameter of about 1 cm at one end of the non-glossy surface of a clean and flat aluminum foil with a width of 5 cm and a length of 15 cm, and the side of the aluminum foil where the graphene dispersion is placed is gripped and pulled upward to stand the aluminum foil vertically, held without applying vibration, and after standing for 10 minutes, the distance that the graphene dispersion has sagged due to its own weight is 3 cm or more. The distance that the graphene dispersion has sagged can be obtained by measuring the distance of the end of the graphene dispersion in the direction of gravity when the aluminum foil is stood vertically before and after the graphene dispersion has sagged. The larger the distance that the graphene dispersion has sagged, the higher the fluidity. From the viewpoint of facilitating the mixing of each material of the positive electrode paste and further improving the battery life, the distance that the graphene dispersion has sagged due to its own weight is more preferably 10 cm or more.

[0073] Next, a method for producing the graphene dispersion of the present invention will be described. Examples of the method for producing the graphene dispersion include a method of mixing graphene powder or a graphene dispersion into a solution obtained by dissolving polyvinyl alcohol in the solvent. From the viewpoint of further suppressing the aggregation of graphene, it is preferable to use a graphene dispersion.

[0074] As a mixing device for the solution of the polymer additive and graphene powder or graphene dispersion, a device capable of applying a shearing force is preferable. For example, a planetary mixer, "Filmix" (registered trademark) (Primix Corporation), a self-revolving and revolving mixer, a planetary ball mill, a three-roll mill, etc. can be used.

[0075] A strong stirring step of performing a stirring treatment at a shear rate of 5,000 to 50,000 per second may be carried out using a high-shear mixer. By exfoliating graphene with a high-shear mixer in the strong stirring step, the stack of graphene can be eliminated, and the average thickness of graphene can be adjusted. As the high-shear mixer, those adopting a thin-film swirling method, a rotor / stator type, or a media mill type are preferable. Specifically, for example, "Filmix" (registered trademark) 30-30 type (Primix Corporation), "ClearMix" (registered trademark) CLM-0.8S (M. Technique Co., Ltd.), "Laboster" (registered trademark) mini LMZ015 (Asazawa Fine Tech Co., Ltd.), Super Share Mixer SDRT0.35-0.75 (Satake Chemical Machinery Co., Ltd.), etc. can be mentioned. The shear rate in the strong stirring step is preferably 5,000 to 50,000 per second as described above. By setting the shear rate to 5,000 or more per second, the exfoliation of graphene can be promoted, and the average thickness of graphene can be easily adjusted within the above-mentioned range. Also, the treatment time of the strong stirring step is preferably from 15 seconds to 300 seconds.

[0076] Particularly when the viscosity of the graphene dispersion before adding the polymer additive is high, mixing may be difficult with a high-shear mixer. In such a case, a propellerless type self-revolving and revolving mixer can be used. Examples of the propellerless type self-revolving and revolving mixer include "Avatori Rentaro" (registered trademark) manufactured by Shin-Ki Co., Ltd. and "Kaku Hunter" (registered trademark) manufactured by Shashin Kagaku Co., Ltd. It is preferable to perform the treatment at a rotational speed of 2000 rpm for 5 minutes or more, more preferably 10 minutes or more, and even more preferably 15 minutes or more.

[0077] By applying the above graphene dispersion liquid onto a substrate, a graphene-containing film can be formed. Examples of the coating method for the graphene dispersion liquid include, for example, the doctor blade method, the dip method, the reverse roll method, the direct roll method, the gravure method, the extrusion method, the brush coating method, the spray coating method, the inkjet method, the flexo method, and the like. Among these, from the viewpoint of ease of application to the positive electrode paste and the positive electrode of the lithium-ion battery, the spray method or the coater method is preferable.

[0078] An additive may be further mixed in the graphene dispersion liquid of the present invention. Examples of the additive include, for example, a positive electrode active material, a binder, a crosslinking agent, a deterioration inhibitor, an inorganic filler, and the like.

[0079] Since the graphene dispersion liquid of the present invention is excellent in fluidity and dispersibility of graphene, it can be suitably used, for example, for a conductive film excellent in conductivity, a heat-dissipating resin excellent in heat conductivity, a corrosion-resistant coating film excellent in barrier properties, and the like.

[0080] Next, the positive electrode paste of the present invention will be described. The positive electrode paste of the present invention contains the above-described graphene dispersion liquid and a positive electrode active material. Further, if necessary, a conductive auxiliary agent other than graphene may be contained.

[0081] The positive electrode active material is a material capable of electrochemically occluding and releasing lithium ions. For example, lithium manganese oxide (LiMn2O4) having a spinel structure, lithium manganese oxide (LiMnO2) having a rock salt structure, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), a ternary system in which nickel is partially substituted with manganese and cobalt (LiNi x Mn y Co 1-x-y O2), a ternary system partially substituted with cobalt and aluminum (LiNi x Co y Al 1-x-yMetal oxide active materials such as O2) and V2O5, metal compound-based active materials such as TiS2, MoS2, and NbSe2, olivine-type lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), solid solution-based active materials, etc. may be mentioned. Two or more of these may be used. Among these, active materials containing lithium and nickel are preferable. Examples of active materials containing lithium and nickel include lithium nickelate (LiNiO2), a ternary system in which nickel is partially substituted with manganese and cobalt (LiNi x Mn y Co 1-x-y O2), a ternary system partially substituted with cobalt and aluminum (LiNi x Co y Al 1-x-y O2), etc. are preferable, and the energy density can be improved.

[0082] Furthermore, when using the positive electrode active material of the granulated body, since graphene tends to be in surface contact while following the uneven shape on the surface of the positive electrode active material, the effect of the present invention becomes particularly remarkable. The granulated body means spherical particles obtained by spray-drying a slurry in which powder is dispersed. Examples of the positive electrode active material used as the granulated body include ternary systems (LiNi x Mn y Co 1-x-y O2) and LiNi x Co y Al 1-x-y O2, etc. Since the granulated body has a tendency for the surface to have an uneven shape because the primary particles aggregate to form secondary particles, and it is necessary to increase the contact surface between the positive electrode active material and the conductive assistant, the effect of the present invention is remarkably exhibited.

[0083] From the viewpoint of the ease of forming a conductive path by the aforementioned graphene, the particle diameter of the positive electrode active material is preferably 20 μm or less. In this specification, the particle diameter means the median diameter (D 50 ). The median diameter can be measured by a laser scattering particle size distribution measuring device (for example, Microtrac HRAX-100 manufactured by Nikkiso Co., Ltd.). Also, in this specification, "the particle diameter of the positive electrode active material" means the secondary particle diameter when the positive electrode active material is a granulated body.

[0084] Preferably, the positive electrode paste of the present invention contains 0.05 parts by weight or more and 2.5 parts by weight or less of the aforementioned graphene with respect to 100 parts by weight of the positive electrode active material. By setting the content of graphene to 0.05 parts by weight or more, the solid content ratio of the positive electrode paste can be increased. The content of graphene is preferably 0.1 parts by weight or more, and more preferably 0.2 parts by weight or more. On the other hand, by setting the content of graphene to 2.5 parts by weight or less, it is easier to form a conductive path and the battery life can be further improved.

[0085] The contents of the positive electrode active material, graphene, and polymer additive in the positive electrode paste of the present invention can be determined by the following method. The solid content is collected from the positive electrode paste by filtration, washed with a solvent, and then the weight of the dried powder is measured to obtain the total weight of the positive electrode active material and the conductive assistant. Further, the positive electrode active material in the solid content is dissolved using an acid such as hydrochloric acid and nitric acid, and the conductive assistant is separated by filtration. The content of the conductive assistant can be measured by washing the residue with water, drying it, and measuring the weight. Also, the weight of the positive electrode active material can be obtained from the total weight of the positive electrode active material and the conductive assistant and the weight of the conductive assistant. When the conductive assistant contains graphene and other materials, the size of each conductive assistant is obtained from the SEM image of the powder, and the content of only graphene can be obtained by recovering it using a sieve so as to pass through or capture only graphene. When the sizes of a plurality of conductive assistants are similar and sieving is difficult, the content of each can be obtained from the ratio of the cross-sectional area of the surface SEM image of the powder. However, when the raw material composition used in the positive electrode paste is known, it can also be obtained by calculation from the raw material composition.

[0086] The positive electrode paste of the present invention may further contain a binder, a conductive assistant other than graphene, and other additives.

[0087] Examples of the binder include fluorine-based polymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE); rubbers such as styrene-butadiene rubber (SBR) and natural rubber; polysaccharides such as carboxymethyl cellulose; polyimide precursors and / or polyimide resins, polyamideimide resins, polyamide resins, polyacrylic acid, sodium polyacrylate, acrylic resins, polyacrylonitrile, and the like. Two or more of these may be contained.

[0088] The content of the binder is preferably 0.2 parts by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the content of the positive electrode active material. By setting the content of the binder to 0.2 parts by weight or more, the battery life can be further improved. On the other hand, by setting the content of the binder to 2 parts by weight or less, the fluidity of the positive electrode paste can be further improved, and the solid content ratio can be made higher. Note that since the graphene dispersion liquid and the positive electrode paste of the present invention form a self-supporting film and have the characteristic of holding the positive electrode active material, they may not contain a binder.

[0089] The conductive assistant other than graphene preferably has high electron conductivity. Examples thereof include carbon materials such as carbon fibers, carbon black, acetylene black, carbon nanofibers, carbon nanotubes, and "VGCF" (registered trademark)-H (manufactured by Showa Denko KK); and metal materials such as copper, nickel, aluminum, and silver. Two or more of these may be contained. Among these, fibrous carbon nanofibers, carbon nanotubes, or "VGCF" (registered trademark)-H (manufactured by Showa Denko KK) are preferable, and the conductivity in the thickness direction of the electrode can be improved.

[0090] The content of the conductive assistant other than graphene is preferably 0.1 parts by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the content of the positive electrode active material. By setting the content of the conductive assistant other than graphene to 0.1 parts by weight or more, the battery life can be further improved. On the other hand, by setting the content of the conductive assistant other than graphene to 2 parts by weight or less, the fluidity of the positive electrode paste can be further improved, and the solid content ratio can be further improved.

[0091] As a method for analyzing the constituent materials and composition ratio of the positive electrode paste, solid content is collected from the positive electrode paste by filtration, washed with a solvent, and then dried. By performing X-ray diffraction measurement on the powder, the type of positive electrode active material can be identified. When two or more types of positive electrode active materials are mixed, the mixing ratio of the positive electrode active materials can be determined by further analyzing the powder by energy dispersive X-ray spectroscopy or ICP-MS (inductively coupled plasma mass spectrometer). However, when the raw material composition used in the positive electrode paste is known, it can also be determined by calculation from the raw material composition.

[0092] In addition, when the filtrate obtained by the above filtration is measured by FT-IR and C-F absorption derived from PVDF is observed from the obtained spectrum, it can be determined that PVDF is included as a binder. Also, the filtrate is dried and the weight is measured to measure the content of the binder in the positive electrode paste. Also, the dried filtrate is redissolved in a heavy solvent and analyzed using an NMR (nuclear magnetic resonance spectrometer) to identify other binders.

[0093] The viscosity of the positive electrode paste of the present invention at 25°C is preferably 1,800 mPa·s or more and 2,200 mPa·s or less from the viewpoint of coatability. When the viscosity of the paste is not within this range, it is preferable to mix and adjust the solvent so as to obtain the desired viscosity. Here, the viscosity of the positive electrode paste at 25°C can be measured using a Brookfield viscometer LVDVII+ under the conditions of rotor No. 6 and 60 rpm.

[0094] In this specification, the solid content ratio of the positive electrode paste refers to the value obtained by placing 1 g of the positive electrode paste on a slide glass, heating and drying it in a vacuum oven at 120°C for 5 hours, and dividing the weight after drying by the weight before drying in the positive electrode paste adjusted so that the viscosity measured by the above measurement method is 1,800 mPa·s or more and 2,200 mPa·s or less.

[0095] From the perspective of forming a conductive path and improving battery life, the solid content ratio of the positive electrode paste is preferably 70% by weight or more. If the fluidity of the graphene dispersion is high, the mixing state of each material in the positive electrode paste is improved, the amount of solvent required for viscosity adjustment is reduced, and the solid content ratio of the positive electrode paste can be increased.

[0096] As a method for manufacturing the positive electrode paste of the present invention, for example, after mixing the aforementioned graphene dispersion of the present invention, a positive electrode active material, a binder or a binder solution in a desired ratio, the viscosity is measured by the aforementioned method, and a solvent is added so that the viscosity becomes 1,800 mPa·s or more and 2,000 mPa·s or less, and then mixing is performed again. Examples of the solvent include those exemplified as the solvent of the graphene dispersion. Before adjusting the viscosity, a conductive auxiliary agent other than graphene and other additives may be added.

[0097] Examples of the mixing device for the positive electrode paste include those exemplified as the mixing device for a polyvinyl alcohol solution and graphene powder or dispersion.

[0098] The positive electrode paste of the present invention is suitably used for a positive electrode of a lithium ion battery. It is preferable to have a dry film of the positive electrode paste on the current collector foil.

[0099] The material constituting the current collector foil is preferably aluminum or its alloy. Since aluminum is stable in the positive electrode reaction atmosphere, high-purity aluminum typified by JIS standards 1030, 1050, 1085, 1N90, 1N99, etc. is preferable. The thickness of the current collector foil is preferably 10 μm or more and 100 μm or less. By setting the thickness of the current collector foil to 10 μm or more, breakage of the positive electrode can be suppressed. On the other hand, by setting the thickness of the current collector foil to 100 μm or less, the energy density of the positive electrode can be improved.

[0100] Examples of the method for manufacturing a positive electrode of a lithium ion battery include a method of coating the positive electrode paste on a current collector foil and drying it.

[0101] Examples of the method for applying the positive electrode paste onto the current collector foil include applying it using a doctor blade, a die coater, a comma coater, a spray, or the like.

[0102] After applying the positive electrode paste of the present invention onto the current collector foil, it is preferable to remove the solvent by a drying process. As the method for removing the solvent, drying using an oven or a vacuum oven is preferable. Examples of the atmosphere for removing the solvent include air, an inert gas, and a vacuum state. Further, the temperature for removing the solvent is preferably 60°C or higher and 250°C or lower.

[0103] Further, in order to increase the density of the coating film after drying, it is preferable to have a step of pressing the current collector foil coated with the positive electrode paste.

[0104] The content of graphene in the positive electrode of the lithium ion battery and various physical properties and contents of the positive electrode active material can be measured as follows. First, the battery is disassembled in an Ar glove box, the electrode is washed with dimethyl carbonate, and then vacuum dried in the side box of the inert glove box for 1 hour. Next, using a spatula, the lithium ion battery positive electrode layer is peeled off from the current collector foil. The powder of the obtained positive electrode layer is dissolved in a solvent such as N-methylpyrrolidone or water, and filtered to separate into a residue (positive electrode active material, conductive assistant, solvent) and a filtrate (solvent, others). After drying the obtained filtrate and re-dissolving it in a heavy solvent, the binder can be identified by analyzing it using NMR. Further, the solvent is removed by drying the obtained residue, and the total weight of the positive electrode active material and the conductive assistant is obtained by measuring the weight. The composition ratio of the positive electrode active material in the obtained powder can be analyzed in the same manner as in the case of the positive electrode paste. Further, the positive electrode active material is dissolved by using an acid such as hydrochloric acid and nitric acid, and filtered to separate into a residue (conductive assistant) and a filtrate (dissolved product of the electrode active material, water). After washing the residue with water, drying it, and measuring the weight, the content of the conductive assistant can be measured. Further, the weight of the positive electrode active material can be obtained from the total weight of the positive electrode active material and the conductive assistant and the weight of the conductive assistant. The obtained conductive assistant can be analyzed in the same manner as in the case of the positive electrode paste described above.

Example

[0105] The present invention will be described below using examples. First, the evaluation methods in each example and comparative example will be described.

[0106] [Measurement Example 1: Thickness of Graphene] The graphene dispersion prepared in each example and comparative example was diluted to 0.002% by weight using N-methylpyrrolidone. At this time, the surface-treated graphene was treated for 60 seconds at a rotation speed of 40 m / s (shear rate: 20,000 per second) using "Filmix" (registered trademark) 30-30 type (Primix Corporation). The diluted solution was dropped onto a mica substrate, dried, and graphene was adhered to the substrate. The graphene on the substrate was magnified and observed using an atomic force microscope (Dimension Icon; Bruker Corporation) in a visual field range of about 1 to 10 μm square, and the thicknesses of 10 randomly selected graphenes were measured. The thickness of each graphene was taken as the arithmetic mean value of the measured values of the thicknesses at 5 randomly selected locations in each graphene. The thickness of graphene was calculated by obtaining the arithmetic mean value of the thicknesses of 10 graphenes. Since the thickness of graphene does not change in the graphene dispersion, the positive electrode paste, and the lithium-ion battery positive electrode, it was measured using only the graphene dispersion.

[0107] [Measurement Example 2: Size of Graphene in the Plane Direction Parallel to the Graphene Layer] The graphene dispersions prepared in each of the examples and comparative examples were diluted to 0.002 wt% using N-methylpyrrolidone. At this time, the surface-treated graphene was treated for 60 seconds at a rotational speed of 40 m / s (shearing rate: 20,000 per second) using "Filmix" (registered trademark) Model 30-30 (Primix Corporation). The diluted solution was dropped onto a mica substrate, dried, and graphene was adhered onto the substrate. The graphene on the substrate was observed under an electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation) at a magnification of 30,000 times. For 10 randomly selected graphene samples, the length of the longest part (major axis) and the length of the shortest part (minor axis) in the plane direction parallel to the graphene layer were measured respectively, and the arithmetic mean value of the numerical values obtained by (major axis + minor axis) / 2 was calculated to determine the size of the plane parallel to the graphene layer.

[0108] [Measurement Example 3: Measurement of O / C ratio and N / C ratio by X-ray photoelectron spectroscopy] The reduced surface-treated graphene dispersions prepared in each of the examples and comparative examples were filtered using a suction filter, and then diluted with water to 0.5 mass% and suction filtered. The washing process was repeated 5 times, and then freeze-dried to obtain surface-treated graphene powder. For the obtained surface-treated graphene powder, X-ray photoelectron spectroscopy measurements were performed using an X-ray photoelectron spectroscopy analyzer Quantera SXM (manufactured by PHI). The excitation X-ray was monochromatic Al K α1,2 line (1486.6 eV), the X-ray diameter was 200 μm, and the photoelectron escape angle was 45°. The peak near 284.3 eV was attributed to the C1s main peak based on carbon atoms, the peak near 533 eV was attributed to the O1s peak based on oxygen atoms, and the peak near 402 eV was attributed to the N1s peak based on nitrogen atoms. The O / C ratio was calculated from the area ratio of the O1s peak and the C1s peak, and the obtained value was rounded to the second decimal place by rounding the third decimal place. Also, N / C was calculated from the area ratio of the N1s peak and the C1s peak, and the obtained value was rounded to the third decimal place by rounding the fourth decimal place.

[0109] [Measurement Example 4: Viscosity of graphene dispersion] To the graphene dispersions prepared in each of the examples and comparative examples, the same solvent as the graphene dispersion was added as necessary so that the graphene concentration became 3% by weight, and after dilution by mixing at a rotation speed of 2000 rpm for 15 minutes using a self-rotating mixer, the viscosity was measured using a Brookfield viscometer LVDVII+ under the conditions of rotor No. 6, 1 / s = 10, and 25°C.

[0110] [Measurement Example 5: Fluidity of Graphene Dispersion] 1 g of the graphene dispersion prepared in each of the examples and comparative examples was dropped in a circular shape with a diameter of about 1 cm at one end of the non-glossy surface of a clean and flat aluminum foil with a width of 5 cm and a length of 15 cm. The aluminum foil was vertically erected by gripping the side on which the graphene dispersion was placed and pulling it up, held without applying vibration, and after standing for 10 minutes, the distance by which the graphene dispersion sagged due to its own weight was measured. The distance by which the graphene dispersion sagged was measured for the end of the graphene dispersion in the direction of gravity when the aluminum foil was vertically erected, from before the graphene dispersion sagged to the end after sagging. When the distance by which the graphene dispersion sagged was 10 cm or more, it was designated as A, when it was 3 cm or more and less than 10 cm, it was designated as B, and when it was less than 3 cm, it was designated as C.

[0111] [Measurement Example 6: Solid Content Ratio of Positive Electrode Paste] 1 g of the positive electrode paste prepared in each of the examples and comparative examples was weighed, placed on a slide glass, and dried by heating in a vacuum oven at 120°C for 5 hours. The weight after drying was measured, and the value obtained by dividing by the weight before drying and rounding the first decimal place to an integer was taken as the solid content ratio of the positive electrode paste.

[0112] [Measurement Example 7: Coating Film Uniformity] 5 g of the positive electrode paste prepared in each of the examples and comparative examples was applied to an aluminum foil (thickness: 18 μm) using a doctor blade (300 μm). After drying at 80°C for 15 minutes, vacuum drying was performed at 120°C for 2 hours to prepare a coating film. For 10 locations randomly selected from the coating film, visual inspection of the appearance of 1 cm square for each location was carried out, and the number of locations where defects such as film fading, cracking, bubble-like defects, and cissing were observed was ranked based on the following criteria. A: No defects are observed, B: 1 or 2 defects, C: 3 to 5 defects, D: 6 or more defects.

[0113] [Measurement Example 8: Battery Life (Battery Capacity Retention Rate)] Regarding the 2032 coin-type batteries prepared in each of the examples and comparative examples, charge-discharge measurements were performed 3 times each in the order of rate 0.1C, 1C, and 5C with an upper limit voltage of 4.2V and a lower limit voltage of 3.0V. After that, charge-discharge measurements were further performed 291 times at 2C for a total of 300 times, the battery capacity at the 300th time was measured, and the ratio (percentage) to the battery capacity at the first time was calculated and used as the battery capacity retention rate.

[0114] [Measurement Example 9: Loss Elastic Modulus and Storage Elastic Modulus] To the graphene dispersion prepared in each of the examples and comparative examples, the same solvent as the graphene dispersion was added as necessary so that the graphene concentration became 3% by weight, and it was diluted by mixing at a rotation speed of 2000 rpm for 15 minutes using a self-revolving mixer. At a frequency of 10 Hz and a temperature of 25°C, using a viscoelasticity measuring device ARES-G2 (manufactured by TA Instrument), a geometry of an equilibrium disk type with a diameter of 40 mm was used, and the measurement was performed in a nitrogen gas stream.

[0115] [Synthesis Example 1: Preparation of Graphene Oxide] 1500-mesh natural graphite powder (Shanghai Yifan Graphite Co., Ltd.) was used as the raw material. In an ice bath, 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, and the mixture was mechanically stirred for 1 hour while maintaining the temperature of the mixture below 20°C. This mixture was taken out of the ice bath and stirred in a 35°C water bath for 4 hours. Then, 500 ml of ion-exchanged water was added, and the resulting suspension was further stirred at 90°C for 15 minutes. Finally, 600 ml of ion-exchanged water and 50 ml of hydrogen peroxide were added, and stirring was carried out for 5 minutes to obtain a graphene oxide dispersion. The obtained graphene oxide dispersion was filtered while it was hot, and the filtrate was washed with a dilute hydrochloric acid solution to remove metal ions, and then washed with ion-exchanged water to remove the acid. Washing with ion-exchanged water was repeated until the pH reached 7 to prepare graphene oxide. The element ratio (O / C ratio) of oxygen atoms to carbon atoms measured by X-ray photoelectron spectroscopy of the prepared graphene oxide was 0.53.

[0116] [Synthesis Example 2: Preparation of Graphene Oxide] Graphene oxide was prepared in the same manner as in Synthesis Example 1, except that AGB-32 (manufactured by Ito Graphite Industry Co., Ltd.) was used instead of 1500-mesh natural graphite powder (Shanghai Yifan Graphite Co., Ltd.). The element ratio (O / C ratio) of oxygen atoms to carbon atoms measured by X-ray photoelectron spectroscopy of the prepared graphene oxide was 0.51.

[0117] [Example 1] (Preparation of Surface-Treated Graphene N-Methylpyrrolidone Dispersion Paste) The graphene oxide prepared in Synthesis Example 1 was diluted to a concentration of 30 mg / ml using ion-exchanged water, and treated at 3,000 rpm for 30 minutes using a Homodisper 2.5 type (Primix Corporation) to obtain a uniform graphene oxide dispersion. 20 ml of the obtained graphene oxide dispersion was mixed with 0.3 g of dopamine hydrochloride as a surface treatment agent, and treated at 3,000 rpm for 60 minutes using a Homodisper 2.5 type (Primix Corporation). The treated graphene oxide dispersion was subjected to ultrasonication at an output of 300 W for 30 minutes (fine particle size reduction step) using an ultrasonic device UP400S (Hielscher). The graphene oxide dispersion that had undergone the fine particle size reduction step was diluted to a concentration of 5 mg / ml using ion-exchanged water, 0.3 g of sodium dithionite was added to 20 ml of the diluted dispersion, and it was stirred at 3,000 rpm for 1 hour in a water bath at 40 °C using a Homodisper 2.5 type (Primix Corporation). Thereafter, filtration was performed using a vacuum suction filter, and the washing step of adding water to the filtrate and diluting it to 0.5 wt% and performing suction filtration was repeated 5 times to obtain a graphene aqueous dispersion. To the obtained graphene aqueous dispersion, N-methylpyrrolidone (hereinafter, NMP) was added so that the graphene concentration became 0.5 wt%, and it was treated at a rotational speed of 40 m / s (shear rate: 20,000 per second) for 60 seconds using a "Filmix" (registered trademark) 30-30 type (Primix Corporation). After the treatment, the solvent was removed by vacuum suction filtration. Further, to remove moisture, NMP was added to the filtrate so that the graphene concentration became 0.5 wt%, and it was diluted by treating at 3,000 rpm for 30 minutes using a Homodisper 2.5 type (Primix Corporation), and the step of performing vacuum suction filtration until the filtrate stopped dropping was repeated 2 times to obtain an NMP dispersion paste containing 5.0 wt% of surface-treated graphene as the filtrate.

[0118] (Preparation of polyvinyl alcohol solution) To 95 wt% of NMP, 5 wt% of polyvinyl alcohol (manufactured by Fujifilm Wako Pure Chemical Corporation, saponification rate 88%, degree of polymerization 500) was added, and it was heated to 90 °C with stirring by a magnetic stirrer in a sealed container to completely dissolve the polyvinyl alcohol, and a 5 wt% polyvinyl alcohol / NMP solution was obtained.

[0119] (Preparation of Graphene Dispersion) To 20 g of an NMP dispersion paste containing 5.0 wt% of the surface-treated graphene obtained as described above, 5 g of 5 wt% polyvinyl alcohol / NMP was added, and then it was stirred for 15 minutes at a rotational speed of 40 m / s (shear rate: 20,000 per second) using a “Filmix” (registered trademark) model 30-30 (Primix Corporation) (strong stirring step) to obtain a graphene dispersion. The solid content concentration of the obtained graphene dispersion was 4 wt%, and the polyvinyl alcohol content was 25 parts by weight with respect to 100 parts by weight of graphene.

[0120] Regarding the obtained graphene dispersion, the thickness of graphene and the size in the direction parallel to the graphene layer were measured according to Measurement Examples 1 and 2. Also, the O / C ratio and N / C ratio were measured according to Measurement Example 3, and the viscosity of the graphene dispersion was measured according to Measurement Example 4. Further, the fluidity of the graphene dispersion was evaluated according to Measurement Example 5, and the loss elastic modulus and storage elastic modulus of the graphene dispersion were measured according to Measurement Example 9. The results are shown in Table 3.

[0121] (Preparation of Positive Electrode Paste) As a positive electrode active material, 20 g of LiNi 0.5 Co 0.2 Mn 0.3 O2, 5 g of a 4 wt% graphene dispersion as a conductive assistant, and 2 g of a 10 wt% PVDF / NMP solution as a binder were mixed using a planetary mixer at a rotational speed of 2000 rpm for 15 minutes. NMP was added to the obtained mixture. Here, the amount of NMP to be added was adjusted so that the viscosity of the mixture measured using a Brookfield viscometer LVDVII+ under the conditions of rotor No. 6, 60 rpm, and 25 °C was 2,000 mPa·s. This mixture was mixed again using a planetary mixer at a rotational speed of 2,000 rpm for 15 minutes to obtain a positive electrode paste.

[0122] Regarding the obtained positive electrode paste, the solid content fraction of the positive electrode paste was measured according to Measurement Example 6, and the coating film uniformity was evaluated according to Measurement Example 7. The results are shown in Table 3.

[0123] (Fabrication of Coin Battery) The obtained positive electrode paste was applied onto an aluminum foil (thickness: 18 μm) using a doctor blade so that the coated weight of the positive electrode paste after drying was 18 mg / cm 2 . After drying at 80°C for 15 minutes, vacuum drying was performed at 120°C for 2 hours to obtain an electrode plate.

[0124] The fabricated electrode plate was cut into a circle with a diameter of 15.9 mm to serve as the positive electrode. As the counter electrode, a coating film 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 formed on a copper foil and cut into a circle with a diameter of 16.1 mm to serve as the negative electrode. Celgard #2400 (manufactured by Celgard) cut into a circle with a diameter of 17 mm was used as the separator. A solvent of ethylene carbonate:diethyl carbonate = 7:3 containing 1 M of LiPF6 was used as the electrolyte. The separator and the electrolyte were sandwiched between the positive electrode and the negative electrode, 3 mL of the electrolyte was added, and a 2032-type coin battery was fabricated by caulking. The battery life (battery capacity retention rate) of the obtained coin battery was measured according to Measurement Example 8.

[0125] [Example 2] A graphene dispersion was obtained in the same manner as in Example 1, except that N,N-dimethylacetamide was used instead of NMP in the preparation of the surface-treated graphene NMP dispersion paste, the preparation of the polyvinyl alcohol solution, and the preparation of the graphene dispersion in Example 1. Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin battery were fabricated in the same manner as in Example 1.

[0126] [Example 3] A graphene dispersion was obtained in the same manner as in Example 1, except that methyl ethyl ketone was used instead of NMP in the preparation of the surface-treated graphene NMP dispersion paste, the preparation of the polyvinyl alcohol solution, and the preparation of the graphene dispersion in Example 1. Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin battery were fabricated in the same manner as in Example 1.

[0127] [Example 4] In the preparation of the surface-treated graphene NMP dispersion paste of Example 1, the preparation of the polyvinyl alcohol solution, and the preparation of the graphene dispersion liquid, a graphene dispersion liquid was obtained in the same manner as in Example 1 except that cyclohexanone was used instead of NMP. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0128] [Example 5] In the preparation of the surface-treated graphene NMP dispersion paste of Example 1, the preparation of the polyvinyl alcohol solution, and the preparation of the graphene dispersion liquid, a graphene dispersion liquid was obtained in the same manner as in Example 1 except that nitromethane was used instead of NMP. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0129] [Example 6] A graphene dispersion liquid was obtained in the same manner as in Example 1 except that the treatment time of the strong stirring step was extended to 30 minutes. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0130] [Example 7] A graphene dispersion liquid was obtained in the same manner as in Example 1 except that the strong stirring step was shortened to 5 minutes. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0131] [Example 8] A graphene dispersion liquid was obtained in the same manner as in Example 1 except that the miniaturization step was extended to 120 minutes. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0132] [Example 9] A graphene dispersion liquid was obtained in the same manner as in Example 1 except that the miniaturization step was extended to 90 minutes. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0133] [Example 10] A graphene dispersion was obtained in the same manner as in Example 1, except that the miniaturization process was shortened to 10 minutes. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0134] [Example 11] A graphene dispersion was obtained in the same manner as in Example 1, except that graphene oxide prepared according to Synthesis Example 2 was used instead of the graphene oxide prepared according to Synthesis Example 1 and the miniaturization process was not performed. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0135] [Example 12] A graphene dispersion was obtained in the same manner as in Example 1, except that the amount of sodium dithionite used was reduced to 0.1 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0136] [Example 13] A graphene dispersion was obtained in the same manner as in Example 1, except that the amount of sodium dithionite used was reduced to 0.05 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0137] [Example 14] A graphene dispersion was obtained in the same manner as in Example 1, except that the amount of sodium dithionite used was reduced to 0.01 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0138] [Example 15] A graphene dispersion was obtained in the same manner as in Example 1, except that dopamine hydrochloride was changed to catechol. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0139] [Example 16] A graphene dispersion was obtained in the same manner as in Example 1, except that dopamine hydrochloride was changed to benzylamine hydrochloride and the amount used was reduced to 0.1 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were prepared in the same manner as in Example 1.

[0140] [Example 17] A graphene dispersion was obtained in the same manner as in Example 1, except that dopamine hydrochloride was changed to phenylethylamine hydrochloride and the amount used was reduced to 0.2 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were prepared in the same manner as in Example 1.

[0141] [Example 18] A graphene dispersion was obtained in the same manner as in Example 1, except that the amount of dopamine hydrochloride used in Example 1 was increased to 0.7 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were prepared in the same manner as in Example 1.

[0142] [Example 19] In the preparation of the graphene dispersion, 2 g of a 5 wt% polyvinyl alcohol / NMP solution was added to 20 g of an NMP dispersion paste containing 5.0 wt% of surface-treated graphene, and 3 g of NMP was additionally added. A graphene dispersion was obtained in the same manner as in Example 1. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were prepared in the same manner as in Example 1.

[0143] [Example 20] A graphene dispersion was prepared in the same manner as in Example 1.

[0144] In the preparation of the polyvinyl alcohol solution, 4 g of polyvinyl alcohol and 16 g of NMP were heated to 90 °C in a sealed container under stirring with a magnetic stirrer to partially dissolve the polyvinyl alcohol, and a 20 wt% polyvinyl alcohol / NMP mixture was obtained.

[0145] In the preparation of the graphene dispersion, 20 g of an NMP dispersion paste containing 5.0 wt% of surface-treated graphene was added to 5 g of the obtained 20 wt% polyvinyl alcohol / NMP mixture, and after heating again at 90 °C for 8 hours, the whole was kneaded with a spatula and treated at a rotational speed of 40 m / s (shear rate: 20,000 per second) for 60 minutes using a "Filmix" (registered trademark) model 30-30 (Primix Corporation) to obtain a graphene dispersion. Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0146] [Example 21] A graphene dispersion was prepared in the same manner as in Example 1.

[0147] In the preparation of the polyvinyl alcohol solution, 10 g of polyvinyl alcohol and 10 g of NMP were heated to 90 °C in a sealed container under stirring with a magnetic stirrer to partially dissolve the polyvinyl alcohol and obtain a 50 wt% polyvinyl alcohol / NMP mixture.

[0148] In the preparation of the graphene dispersion, 20 g of an NMP dispersion paste containing 5.0 wt% of surface-treated graphene was added to 5 g of the obtained 50 wt% polyvinyl alcohol / NMP mixture, and after heating again at 90 °C for 8 hours, the whole was kneaded with a spatula and treated at a rotational speed of 40 m / s (shear rate: 20,000 per second) for 60 minutes using a "Filmix" (registered trademark) model 30-30 (Primix Corporation) to obtain a graphene dispersion. Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0149] [Example 22] In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1 except that polyvinyl alcohol having a saponification degree of 75% and a polymerization degree of 500 (manufactured by Fujifilm Wako Pure Chemical Corporation) was used. Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0150] [Example 23] In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1, except that polyvinyl alcohol with a saponification degree of 98% and a polymerization degree of 500 (manufactured by Fujifilm Wako Pure Chemical Corporation) was used. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0151] [Example 24] In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1, except that polyvinyl alcohol with a saponification degree of 88% and a polymerization degree of 1500 (manufactured by Fujifilm Wako Pure Chemical Corporation) was used. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0152] [Example 25] In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1, except that polyvinyl alcohol with a saponification degree of 88% and a polymerization degree of 3500 (manufactured by Fujifilm Wako Pure Chemical Corporation) was used. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0153] [Example 26] In the preparation of polyvinyl alcohol, a polyvinyl alcohol solution was obtained in the same manner as in Example 1, except that polyvinyl alcohol with a saponification degree of 94.2% and a polymerization degree of 500 (manufactured by Nippon Vinyl Poval Co., Ltd., trade name "JT-05") was used. In the preparation of the graphene dispersion, 5 g of 5 wt% polyvinyl alcohol / NMP was added to 20 g of an NMP dispersion paste containing 5.0 wt% of surface-treated graphene, and then stirred at a rotational speed of 2000 rpm for 15 minutes using "Awatori Rentaro" (registered trademark) ARE-310 (manufactured by Shinki Co., Ltd.) to obtain a graphene dispersion. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0154] [Example 27] In the preparation of polyvinyl alcohol, a polyvinyl alcohol solution was obtained in the same manner as in Example 1, except that the polyvinyl alcohol was changed to a modified polyvinyl alcohol having a saponification degree of 87.8% and a polymerization degree of 200 and having a sulfate group (manufactured by Mitsubishi Chemical Corporation, trade name "Gosenex (registered trademark) L-3266"). Using the obtained polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 26. Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin cell were prepared in the same manner as in Example 1.

[0155] [Example 28] A graphene dispersion was obtained in the same manner as in Example 1, except that polyvinyl alcohol was changed to polyvinylpyrrolidone K-60 (manufactured by Tokyo Chemical Industry Co., Ltd.). Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin cell were prepared in the same manner as in Example 1.

[0156] [Example 29] A graphene dispersion was obtained in the same manner as in Example 1, except that polyvinyl alcohol was changed to hydroxypropyl cellulose (manufactured by Sigma-Aldrich, mass average molecular weight (Mw) 80,000). Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin cell were prepared in the same manner as in Example 1.

[0157] [Example 30] In the preparation of the graphene dispersion, 1 g of 5 wt% polyvinyl alcohol / NMP was added to 20 g of an NMP dispersion paste containing 5.0 wt% surface-treated graphene, 12.3 g of NMP was added, and stirring was carried out at a rotation speed of 40 m / s (shear rate: 20,000 per second) for 30 minutes using "Filmix" (registered trademark) 30-30 type (Primix Co., Ltd.) (strong stirring step). A graphene dispersion was obtained in the same manner as in Example 1. Using the obtained graphene dispersion, a positive electrode paste and a 2032-type coin cell were prepared in the same manner as in Example 1.

[0158] [Comparative Example 1] A graphene dispersion was obtained in the same manner as in Example 1, except that polyvinyl alcohol was not used. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0159] [Comparative Example 2] Polyvinyl alcohol was changed to polyvinylpyrrolidone K-60 (manufactured by Tokyo Chemical Industry Co., Ltd.). In the preparation of the graphene dispersion, a graphene dispersion was obtained in the same manner as in Example 1, except that “Filmix” (registered trademark) 30-30 type (Primix Corporation) was used and stirred at a rotation speed of 40 m / s (shearing rate: 20,000 per second) for 10 seconds. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0160] [Comparative Example 3] A graphene dispersion was obtained in the same manner as in Example 30, except that the treatment time in the strong stirring step was changed to 15 minutes. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0161] [Comparative Example 4] A graphene dispersion was obtained in the same manner as in Example 1, except that the strong stirring step was not performed. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0162] [Comparative Example 5] A graphene dispersion was obtained in the same manner as in Example 1, except that isobutyl acetate was used instead of NMP in the preparation of the surface-treated graphene NMP dispersion paste of Comparative Example 1 and the preparation of the graphene dispersion. Using the obtained graphene dispersion, a positive electrode paste and a 2032 coin cell were fabricated in the same manner as in Example 1.

[0163] [Comparative Example 6] In the preparation of the surface-treated graphene NMP dispersion paste and the graphene dispersion liquid of Comparative Example 1, a graphene dispersion liquid was obtained in the same manner as in Example 1, except that ethylene glycol was used instead of NMP. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0164] [Comparative Example 7] In the preparation of the surface-treated graphene NMP dispersion paste of Example 1, instead of the graphene aqueous dispersion, graphite nanoplatelets (model number M-5, manufactured by XG Sciences) were diluted to a concentration of 0.5 wt% using ion-exchanged water, and the resulting solution was treated for 30 minutes at 3,000 rpm using a Homodisper 2.5 (Primix). A graphene dispersion liquid was obtained in the same manner as in Example 1, except for the above. Using the obtained graphene dispersion liquid, a positive electrode paste and a 2032 coin cell were produced in the same manner as in Example 1.

[0165] The compositions of each example and comparative example are shown in Tables 1 to 3, and the evaluation results are shown in Tables 4 and 5.

[0166]

Table 1

[0167]

Table 2

[0168]

Table 3

[0169]

Table 4

[0170]

Table 5

Claims

1. A graphene dispersion containing graphene and a solvent, wherein the average thickness of the graphene is 1.0 nm or more and 10 nm or less, the graphene is graphene to which a surface treatment agent is attached, and the surface treatment agent is a compound having an aromatic ring and having an acidic group and / or a basic group, and the solubility parameter δ of the solvent is 18 MPa ０．５ or more and 28 MPa ０．５ or less, and when adjusted to a graphene concentration of 3% by weight, the viscosity at a shear rate of 10 sec －１ and a temperature of 25 °C is 10,000 mPa·s or less, a graphene dispersion.

2. The graphene dispersion according to claim 1, wherein the surface treatment agent is selected from catechol and its derivatives, and benzylamine, phenylethylamine, and salts thereof.

3. The graphene dispersion according to claim 2, wherein the surface treatment agent is selected from catechol, dopamine hydrochloride, 3-(3,4-dihydroxyphenyl)-L-alanine, 4-(1-hydroxy-2-aminoethyl)catechol, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, caffeic acid, 4-methylcatechol, 4-tert-butylpyrocatechol, benzylamine, phenylethylamine, and salts thereof.

4. When adjusted to a graphene concentration of 3% by weight, the viscosity at a shear rate of 10 sec －１ and a temperature of 25°C is 10 mPa·s or more and 1,000 mPa·s or less. The graphene dispersion according to any one of claims 1 to 3.

5. The graphene dispersion according to any one of claims 1 to 4, wherein the storage modulus and the loss modulus at a strain of 10%, a frequency of 10 Hz, and a temperature of 25 °C when adjusted to a graphene concentration of 3% by weight are both 0.1 Pa or more and 100 Pa or less.

6.

7. The graphene dispersion according to any one of claims 1 to 5, wherein the storage modulus and the loss modulus at a frequency of 10 Hz and a temperature of 25 °C when adjusted to a graphene concentration of 3% by weight satisfy the following formula (1) and / or formula (2);

8. Formula (1): G' ２００ / G' １０ ≥ 1 In formula (1), G' ２００ represents the storage modulus at a strain of 200%, and G' １０ represents the storage modulus at a strain of 10%; Formula (2): G'' ２００ / G'' １０ ≥ 1 In formula (2), G'' ２００ represents the loss modulus at a strain of 200%, and G'' １０ represents the loss modulus at a strain of 10%.

9. The graphene dispersion according to any one of claims 1 to 6, wherein the elemental ratio of oxygen to carbon (O / C ratio) of the graphene measured by X-ray photoelectron spectroscopy is 0.05 or more and 0.35 or less.

10.

11. The graphene dispersion according to any one of claims 1 to 7, wherein the elemental ratio of nitrogen to carbon (N / C ratio) of the graphene measured by X-ray photoelectron spectroscopy is 0.005 or more and 0.020 or less.

12.

13. The graphene dispersion according to any one of claims 1 to 8, wherein the dispersion further contains a polymer selected from polyvinyl alcohol, polyvinylpyrrolidone, and hydroxypropyl cellulose.

14.

15. The graphene dispersion according to claim 9, wherein the saponification degree of the polyvinyl alcohol is 70% or more and 100% or less.

16.

17. The graphene dispersion according to claim 9 or 10, containing 1 part by weight or more and 300 parts by weight or less of polyvinyl alcohol per 100 parts by weight of graphene.

18. ​ The graphene dispersion liquid according to any one of claims 1 to 11, wherein the solvent contains a solvent selected from N,N-dimethylformamide, N-methylpyrrolidone, and N,N-dimethylacetamide.

13. A positive electrode paste comprising the graphene dispersion liquid according to any one of claims 1 to 12 and a positive electrode active material.

14. The positive electrode paste according to claim 13, containing 0.05 to 2.5 parts by weight of graphene having an average thickness of 1.0 nm or more and 10 nm or less with respect to 100 parts by weight of the positive electrode active material.

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