Graphene dispersions and lithium-ion battery cathodes

Abstract

To provide a graphene dispersion having excellent dispersibility and fluidity and in which graphene is uniformly mixable when being mixed with a cathode active substance, and to provide a lithium ion battery cathode improved in a battery life thereby.SOLUTION: A graphene dispersion comprises: graphene; a hydroxy group-containing (meth)acrylic polymer; and a dispersant. The average thickness of the graphene is 0.3 or more to 10 nm or less, the content of the hydroxyl group-containing (meth)acrylic polymer to 100 pts.wt. of the graphene is 10 or more to 300 pts.wt. or less, the temperature of the hydroxy group-containing (meth)acrylic polymer is 25°C, and viscosity in a shear rate of 1.0 s-1 is 0.1 Pa s or more to 100 Pa s or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a graphene dispersion and a lithium-ion battery positive electrode. [Background technology]

[0002] In recent years, research and development of lithium-ion batteries has been actively conducted for various applications, such as mobile devices such as smartphones and mobile phones, hybrid vehicles, electric vehicles, and home energy storage. Lithium-ion batteries used in these fields are required to suppress the decrease in battery capacity caused by repeated charging and discharging and to improve battery life.

[0003] As one of the means, conductive additives such as carbon nanotubes and graphene are used. As a technique for improving the dispersibility of conductive additives, a conductive material containing a carbon material and a polymer dispersant, which uses N-methyl-2-pyrrolidone as a dispersion medium, has been proposed. The polymer dispersant has an amine value of 25 to 75 mg KOH / g and a solubility parameter (SP value) of 10.0 to 15.0 (cal / cm). 3 ) 1 / 2and a nanocarbon aqueous dispersion liquid containing at least a nanocarbon material, an aqueous solvent, and a polymer dispersant, wherein the AB block copolymer is composed of 90 mass % or more of methacrylic monomers, and the A block of the AB block copolymer contains at least 70 mass % or more of methacrylic monomers having an aromatic skeleton, and the acid value is 0 to 30 mgKOH / g, and the polystyrene equivalent number in gel permeation chromatography is A nanocarbon aqueous dispersion has been reported in which the average molecular weight is 1,000 to 5,000, the weight average molecular weight / number average molecular weight (molecular weight distribution) is 1.3 or less, the B block of the AB block copolymer contains at least a methacrylic monomer having an acidic group as a constituent component, the acid value is 100 to 300 mgKOH / g, the molecular weight obtained by subtracting the number average molecular weight of the A block from the number average molecular weight of the AB block copolymer is 1,000 to 10,000, and the molecular weight distribution of the AB block copolymer is 1.6 or less (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-45820 A [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-75795 Summary of the Invention [Problem to be solved by the invention]

[0005] To improve the battery life of lithium-ion batteries, it is important to prevent the deterioration of the conductive paths caused by repeated charging and discharging. To achieve this, it is important that the conductive additive that forms the conductive paths is mixed uniformly with materials such as the positive electrode active material to form a homogeneous and stable coating film.

[0006] However, the carbon conductive material slurry described in Patent Document 1 has problems such as insufficient dispersion of the carbon conductive material and insufficient fluidity. Furthermore, the carbon conductive material aggregates reduce the uniformity of the coating film, and the uneven distribution of the carbon conductive material results in insufficient battery life. Furthermore, the nanocarbon aqueous dispersion described in Patent Document 2 can improve the dispersibility of nanocarbons by using a polymer dispersant, but the fluidity is still insufficient and the resistance value tends to be high, so the effect of improving battery life may be insufficient.

[0007] Therefore, an object of the present invention is to provide a graphene dispersion that has excellent dispersibility and flowability of graphene and allows graphene to be uniformly mixed when mixed with a positive electrode active material, thereby providing a lithium ion battery positive electrode with improved battery life. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention provides a graphene dispersion containing graphene, a (meth)acrylic polymer having a hydroxy group, and a dispersion medium, wherein the graphene has an average thickness of 0.3 nm or more and 10 nm or less, the content of the (meth)acrylic polymer having a hydroxy group is 10 parts by weight or more and 300 parts by weight or less relative to 100 parts by weight of the graphene, and the (meth)acrylic polymer having a hydroxy group is dispersed at a temperature of 25°C and a shear rate of 1.0 s -1 The present invention also provides a graphene dispersion having a viscosity of 0.1 Pa·s or more and 100 Pa·s or less at a temperature of 25°C and a shear rate of 1.0 s. The present invention also provides a lithium ion battery positive electrode having, on a current collector, a mixture layer containing a positive electrode active material, graphene, and a (meth)acrylic polymer having a hydroxy group, wherein the graphene has an average thickness of 0.3 nm or more and 10 nm or less, the content of the (meth)acrylic polymer having a hydroxy group is 10 parts by weight or more and 300 parts by weight or less per 100 parts by weight of the graphene, and the (meth)acrylic polymer is dispersed in a solution of 0.1 Pa·s or more and 100 Pa·s or less at a temperature of 25°C and a shear rate of 1.0 s. -1 The present invention relates to a lithium ion battery positive electrode having a viscosity of 0.1 Pa·s or more and 100 Pa·s or less at 100°C. [Effects of the Invention]

[0009] The graphene dispersion of the present invention has excellent dispersibility and flowability of graphene, and when mixed with a positive electrode active material, has excellent uniformity of graphene. The positive electrode of a lithium ion battery of the present invention can improve battery life. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, the graphene dispersion of the present invention will be described. The graphene dispersion of the present invention contains graphene, a (meth)acrylic polymer having hydroxyl groups, and a dispersion medium. The graphene has an average thickness of 0.3 nm or more and 10 nm or less, and the (meth)acrylic polymer having hydroxyl groups is dispersed at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity is between 0.1 Pa·s and 100 Pa·s.

[0011] Thin graphene with an average thickness of 0.3 nm to 10 nm is flexible and conforms well to the surface of the object being coated, easily forming a conductive path. However, because thin graphene is prone to aggregation, it has traditionally been difficult to maintain its dispersion in graphene dispersions. Furthermore, the graphene tends to become highly viscous, resulting in insufficient fluidity. Furthermore, when such dispersions are used in lithium-ion battery positive electrodes, the graphene aggregates cause uneven mixing with the positive electrode active material, resulting in reduced battery life.

[0012] Therefore, in the present invention, together with such thin graphene, a temperature of 25°C and a shear rate of 1.0 s -1 The (meth)acrylic polymer has a specific viscosity at 2000 kJ / cm2 and has a hydroxy group. The (meth)acrylic polymer functions as a dispersant that enhances the dispersibility of graphene in the graphene dispersion, and can improve the dispersibility and fluidity of the graphene dispersion. Therefore, when the graphene dispersion of the present invention is used in a lithium-ion battery positive electrode, the positive electrode active material and graphene are easily mixed uniformly, an effective conductive path is formed, and the battery life can be improved.

[0013] <Graphene> Graphene is useful as a conductive additive because it has a thin layer shape, has many conductive paths per unit weight, and easily forms a good conductive network within the electrode. In a narrow sense, graphene is a single-atom-thick sp 2 The term "graphene" refers to a sheet of bonded carbon atoms (single-layer graphene), but in this specification, the term "graphene" also refers to a thin flake-like structure formed by stacking single-layer graphene. Similarly, the term "graphene oxide" also refers to a thin flake-like structure formed by stacking single-layer graphene.

[0014] In this specification, graphene oxide is referred to as graphene when the O / C ratio, which is the atomic ratio of oxygen atoms to carbon atoms measured by X-ray photoelectron spectroscopy (XPS), exceeds 0.4, and graphene is referred to as graphene when the O / C ratio is 0.4 or less. 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.

[0015] Furthermore, graphene and graphene oxide may be subjected to a surface treatment for the purpose of improving dispersibility, etc., and in this specification, graphene or graphene oxide to which such a surface treatment agent is attached will also be referred to as "graphene" or "graphene oxide."

[0016] The average thickness of the graphene is 0.3 nm or more and 10 nm or less. By using thin graphene having an average thickness within this range, the graphene dispersion of the present invention can enhance the interaction with the (meth)acrylic polymer described below, thereby improving dispersibility and flowability. Furthermore, the graphene dispersion can improve its conformability to the surface of the positive electrode active material while maintaining conductivity, facilitating the formation of conductive paths, thereby improving battery life. If the average thickness of the graphene is less than 0.3 nm, defects are likely to occur, resulting in reduced conductivity and a shortened battery life. On the other hand, if the average thickness of the graphene exceeds 10 nm, the dispersibility and conformability to the surface of the positive electrode active material are reduced, resulting in insufficient formation of conductive paths and a shortened battery life. From the viewpoint of more effectively forming conductive paths and further improving battery life, the average thickness of the graphene is preferably 8 nm or less, 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, observing it using an atomic force microscope with a magnified field of view of approximately 1 to 10 μm square so that the graphene can be properly observed, measuring the thickness of each of 10 randomly selected graphene particles, and calculating the arithmetic mean value of the thickness measurements at five randomly selected points on each graphene particle.

[0017] The size of the graphene in a 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, from the viewpoints of increasing the contact area with the positive electrode active material, more effectively forming a conductive path, and further improving battery life. On the other hand, the size of the graphene in a 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, from the viewpoints of improving the dispersibility and fluidity of the graphene dispersion, more uniformly mixing the positive electrode active material and graphene, and further improving battery life. Here, the size of graphene in the graphene dispersion in the direction parallel to the graphene layer can be calculated by collecting graphene from the graphene dispersion, observing it using an electron microscope at a magnification of 1,500 to 50,000 times so that the graphene fits properly within the field of view, measuring the length of the longest part (major axis) and the length of the shortest part (minor axis) in the direction parallel to the graphene layer for 10 randomly selected graphene particles, and calculating the arithmetic mean value of the values ​​obtained by (major axis + minor axis) / 2. The size of graphene in the direction parallel to the graphene layer can be easily adjusted to within the aforementioned range by micronizing graphene oxide or reduced graphene using the method described below. Alternatively, commercially available graphene oxide or graphene of the desired size may be used.

[0018] The oxygen-to-carbon elemental ratio (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, from the viewpoint of further improving dispersibility and fluidity by residual functional groups. On the other hand, from the viewpoint of further improving the fluidity of the graphene dispersion, and from the viewpoint of further increasing conductivity by restoring the π-electron conjugated structure by reduction, 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 subjecting it to X-ray photoelectron spectroscopy (XPS). The main C1s peak due to carbon atoms is assigned to 284.3 eV, and the O1s peak due to oxygen atoms is assigned to a peak around 533 eV. The O / C ratio is calculated from the area ratio of each peak, and the resulting value is rounded to two decimal places. Note that, for example, when using a chemical exfoliation method, the O / C ratio of graphene can be easily adjusted to the aforementioned range by adjusting the oxidation degree of the graphene oxide raw material or the reduction degree by adjusting the reduction reaction conditions. Alternatively, commercially available graphene oxide or graphene having the desired O / C ratio may be used.

[0019] As described above, graphene and graphene oxide may be surface-treated, and in particular, surface treatment agents containing nitrogen atoms tend to increase the dispersibility of graphene. Furthermore, the surface treatment agent enhances the interaction with the (meth)acrylic polymer described below, further enhancing the effect of improving dispersibility, and can further improve the binding strength when used in a lithium-ion battery positive electrode.

[0020] When graphene is treated with a surface treatment agent containing nitrogen atoms, the amount of the surface treatment agent attached 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 viewpoints of further improving the dispersibility and fluidity of the graphene dispersion and further improving battery life, the N / C ratio of 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, further increasing the conductivity, and further improving battery life, the N / C ratio of 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 graphene in the graphene dispersion can be measured by collecting graphene from the graphene dispersion and subjecting it to X-ray photoelectron spectroscopy (XPS). The main C1s peak due to carbon atoms is assigned to 284.3 eV, and the N1s peak due to nitrogen atoms is assigned to a peak around 402 eV. The N / C ratio is calculated from the area ratio of each peak, and the obtained value is rounded to three decimal places. The N / C ratio of graphene can be easily adjusted to the aforementioned range by, for example, the amount of the surface treatment agent described below.

[0021] The surface treatment agent adheres to the graphene surface and enhances the dispersibility of the graphene. In this specification, graphene with such a surface treatment agent attached thereto is referred to as "surface-treated graphene." Here, in the present invention, "the surface treatment agent is attached to the graphene" means that the surface treatment agent remains in the surface-treated graphene after a washing process of dispersing the surface-treated graphene in 100 times its mass of water and filtering the water is repeated five or more times, followed by drying by a method such as freeze-drying or spray-drying. The presence of the surface treatment agent refers to the detection of the surface treatment agent molecules in the form of protonated molecules in the positive secondary ion spectrum when the dried surface-treated graphene is measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS). However, when the surface treatment agent is a neutralized salt, it can be detected in the form of protonated molecules of the surface treatment agent molecules from which anions have been removed. The chemical structure of the surface treatment agent contained in the surface-treated graphene can be identified by TOF-SIMS. The surface treatment agent is quantified using a sample obtained by repeating a washing process of dispersing the surface-treated graphene in 100 times the mass of water and filtering it five or more times, and then freeze-drying it.

[0022] As the surface treatment agent, a compound having an aromatic ring is preferred from the viewpoint of ease of adsorption onto the graphene surface.

[0023] The surface treatment agent preferably has an acidic group and / or a basic group.

[0024] The acidic group is preferably a hydroxy group, a phenolic hydroxy group, a nitro group, a carboxyl group, or a carbonyl group, and may have two or more of these groups. Of these, a phenolic hydroxy group is preferred.

[0025] Examples of compounds having a phenolic hydroxy group and an aromatic ring include phenol, nitrophenol, cresol, and catechol. Some of the hydrogen atoms in these compounds may be substituted. Among these, catechol and its derivatives are preferred from the viewpoints of adhesion to graphene and dispersibility in a dispersion medium. Examples of preferred compounds include 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.

[0026] The basic group is preferably an amino group.

[0027] Examples of compounds having an amino group and an aromatic ring include benzylamine, phenylethylamine, and salts thereof. Some of the hydrogen atoms in these compounds may be substituted.

[0028] Compounds having an acidic group, a basic group and an aromatic ring are also preferred, such as dopamine hydrochloride.

[0029] 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. Alternatively, commercially available graphene oxide may be purchased.

[0030] The chemical exfoliation method preferably includes, in this order, a step of oxidatively exfoliating graphite to obtain graphene oxide (graphite exfoliation step) and a step of reducing (reduction step). If necessary, a step of attaching a surface treatment agent to graphene (surface treatment step) and / or a step of adjusting the size of graphene in a direction parallel to the graphene layer (micronization step) may be performed between the graphite exfoliation step and the reduction step. When surface-treated graphene is used, the surface treatment agent may be attached to graphene, or the surface-treated graphene may be obtained by attaching the surface treatment agent to graphene oxide and then performing a reduction treatment. Furthermore, when micronizing graphene, the graphene oxide may be micronized, or the graphene after reduction may be micronized. From the viewpoint of uniformity of the reduction reaction, it is preferable to perform the reduction step in a micronized state of graphene oxide, and it is preferable to perform the micronization step before or during the reduction step. Therefore, it is preferable to include the graphite exfoliation step, surface treatment step, micronization step, and reduction step in this order.

[0031] [Graphite peeling process] First, graphene oxide is obtained by oxidative exfoliation of graphite. The oxidation degree of graphene oxide can be adjusted by changing the amount of oxidizing agent used in the oxidation reaction of graphite. Specifically, the greater the amount of sodium nitrate and potassium permanganate used relative to graphite in the oxidation reaction, the higher the oxidation degree, and the smaller the amount, the lower the oxidation degree. The weight ratio of sodium nitrate to graphite is preferably 0.200 or more and 0.800 or less. The ratio of potassium permanganate to graphite is preferably 1.0 or more and 4.0 or less.

[0032] [Surface treatment process] Next, graphene oxide and a surface treatment agent are mixed together to adhere the surface treatment agent to the graphene. Examples of the mixing method include mixing using a mixer or kneader such as an automatic mortar, a triple roll mill, a bead mill, a planetary ball mill, a homogenizer, a homodisper, a homomixer, a planetary mixer, or a twin-screw kneader.

[0033] [Refining process] Next, the graphene oxide is micronized. Examples of micronization methods include colliding a pressurized dispersion against a single ceramic ball, using a liquid-liquid shear wet jet mill in which pressured dispersions are collided with each other to disperse the particles, and applying ultrasonic waves to the dispersion. In the micronization process, the graphene oxide or graphene tends to be micronized as the processing pressure and output increase, and as the processing time increases. The size of the reduced graphene can be controlled by adjusting the type, processing conditions, and processing time of the micronization process. To adjust the size parallel to the graphene layer within the aforementioned range, the solids concentration of the graphene oxide or graphene in the micronization process is preferably 0.01 wt% or more and 2 wt% or less. Furthermore, when performing ultrasonic treatment, the ultrasonic output is preferably 100 W or more and 3,000 W or less.

[0034] [Reduction process] Next, the finely divided graphene oxide is reduced. Chemical reduction is a preferred reduction method. In the case of chemical reduction, the reducing agent may be an organic reducing agent or an inorganic reducing agent, but an inorganic reducing agent is more preferred because of the ease of cleaning after reduction.

[0035] Examples of organic reducing agents include aldehyde reducing agents, hydrazine derivative reducing agents, and alcohol reducing agents. Among them, alcohol reducing agents are particularly suitable because they can reduce relatively gently. Examples of alcohol reducing agents include methanol, ethanol, propanol, isopropyl alcohol, butanol, benzyl alcohol, phenol, ethanolamine, ethylene glycol, propylene glycol, and diethylene glycol.

[0036] Examples of inorganic reducing agents include sodium dithionite, potassium dithionite, phosphorous acid, sodium borohydride, hydrazine, etc. Among them, sodium dithionite and potassium dithionite are preferably used because they can produce graphene that is highly dispersible in a dispersion medium, since they can reduce the graphene while relatively retaining the acidic groups.

[0037] After the reduction step is completed, the purity of the graphene can be improved by preferably carrying out a washing step of diluting with water and filtering.

[0038] <(Meth)acrylic polymer> In this specification, acrylic polymers and methacrylic polymers are collectively referred to as "(meth)acrylic polymers." The same applies to similar expressions such as (meth)acrylic acid.

[0039] As described above, in the present invention, thin graphene is used at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity at 1000 kJ / min is within a specific range, and a (meth)acrylic polymer having a hydroxy group is used. Interactions such as hydrogen bonding between the hydroxyl groups on the (meth)acrylic polymer and the oxygen-containing functional groups on the graphene and / or the functional groups on the surface treatment agent improve the dispersibility of graphene and also improve the binding strength between the graphene and the (meth)acrylic polymer having a hydroxy group. Therefore, in the present invention, it is important that the (meth)acrylic polymer has a hydroxy group. Hereinafter, the (meth)acrylic polymer having a hydroxy group may be referred to as a "(meth)acrylic polymer" unit.

[0040] The (meth)acrylic polymer in the present invention is -1 The viscosity at such a temperature and shear rate is 0.1 Pa s or more and 100 Pa s or less. The inventors' investigations have revealed that the viscosity at such a temperature and shear rate tends to correlate with the viscosity of the graphene dispersion, and affects the dispersibility and fluidity of the graphene dispersion. Therefore, in the present invention, the viscosity at a temperature of 25°C and a shear rate of 1.0 s is used as an index of viscosity. -1 The viscosity was measured at a temperature of 25°C and a shear rate of 1.0 s -1When the viscosity is within this range, the (meth)acrylic polymer is more uniformly mixed in the graphene dispersion, and the interaction with graphene improves dispersibility and fluidity. If the viscosity of the (meth)acrylic polymer is less than 0.1 Pa·s, the dispersibility improvement effect is insufficient, resulting in a shortened battery life. On the other hand, if the viscosity of the (meth)acrylic polymer exceeds 100 Pa·s, the fluidity of the graphene dispersion decreases, resulting in non-uniform mixing of the positive electrode active material and graphene. Furthermore, when used in a lithium-ion battery positive electrode (described below), the formation of a conductive path becomes insufficient, resulting in a shortened battery life. From the viewpoint of further improving the fluidity and dispersibility of the graphene dispersion and further improving the battery life, the viscosity of the (meth)acrylic polymer is more preferably 0.5 Pa·s or more, even more preferably 1.0 Pa·s or more, and even more preferably 2.0 Pa·s or more. On the other hand, from the viewpoint of further improving the fluidity of the graphene dispersion and further improving the battery life, the viscosity of the (meth)acrylic polymer is more preferably 80 Pa·s or less, and even more preferably 60 Pa·s or less.

[0041] Here, the viscosity of the (meth)acrylic polymer was measured using a viscoelasticity measuring device Physica MCR301 (manufactured by Anton Paar) at a temperature of 25°C and a shear rate of 1.0 s -1 Measurements can be performed by rotation measurement using a 43 mmφ parallel plate as a measurement jig under the conditions of an angular frequency of 0.018 rad / s and a sample thickness of 0.5 mm.

[0042] The hydroxyl value of the (meth)acrylic polymer is preferably 10 mgKOH / g or more, more preferably 15 mgKOH / g or more, and even more preferably 20 mgKOH / g or more, from the viewpoint of further improving dispersibility through interaction with graphene and further improving battery life. On the other hand, from the viewpoint of improving the solubility of the (meth)acrylic polymer in the dispersion medium and further improving fluidity by suppressing the formation of network bonds through the hydroxy groups of the (meth)acrylic polymer, the hydroxyl value of the (meth)acrylic polymer is preferably 300 mgKOH / g or less, more preferably 250 mgKOH / g or less, and even more preferably 200 mgKOH / g or less. Here, the hydroxyl value of the (meth)acrylic polymer can be determined in accordance with JIS K0070-1992.

[0043] Contains hydroxyl groups, temperature 25°C, shear rate 1.0 s -1 Examples of (meth)acrylic polymers having a viscosity of 0.1 Pa·s or more and 100 Pa·s or less at 2000°C include UH-2000, UH-2041, UH-2170, and UH-2190 under the trade name "ARUFON" (registered trademark) (manufactured by Toagosei Co., Ltd.), and UMM-1001 and UT-1001 under the trade name "Actflow" (registered trademark) (manufactured by Soken Chemical & Engineering Co., Ltd.).

[0044] The graphene dispersion of the present invention may contain two or more kinds of (meth)acrylic polymers. In such a case, it is preferable that the viscosity and hydroxyl value of the two or more kinds of (meth)acrylic polymers as a whole are within the above-mentioned ranges.

[0045] A (meth)acrylic polymer having a hydroxy group can also be obtained by polymerizing an α,β-ethylenically unsaturated monomer having a hydroxy group. Two or more types of α,β-ethylenically unsaturated monomers having a hydroxy group may be used, or other monomers may be copolymerized as necessary. The types and ratios of the monomers are preferably selected so that the glass transition temperature of the resulting (meth)acrylic polymer is less than 25°C.

[0046] The α,β-ethylenically unsaturated monomer having a hydroxy group is preferably one having a structure represented by the following general formula (a).

[0047] [ka]

[0048] In the general formula (a), X represents hydrogen or a methyl group, Y represents O or NH, and R represents a divalent organic group having 1 to 12 carbon atoms.

[0049] In general formula (a), the number of carbon atoms in R is preferably 2 or more, and more preferably 3 or more. On the other hand, the number of carbon atoms in R is preferably 10 or less, more preferably 8 or less, and even more preferably 6 or less.

[0050] Examples of α,β-ethylenically unsaturated monomers having a hydroxy group include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 5-hydroxypentyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, (4-hydroxymethyl)cyclohexylmethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 2-hydroxy-3-phenoxypropyl (meth)acrylate. propyl, 3-chloro-2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl 2-hydroxyethyl phthalate, 2,2-dimethyl-2-hydroxyethyl (meth)acrylate, N-methylol (meth)acrylamide, N-hydroxyethyl (meth)acrylamide, 2-hydroxyethoxymonoethylene glycol (meth)acrylate, 2-hydroxyethoxydiethylene glycol (meth)acrylate, 2-hydroxyethoxymonopropylene glycol (meth)acrylate, 2-hydroxyethoxydipropylene glycol (meth)acrylate, and the like.

[0051] Preferred other monomers include compounds having a carboxyl group such as (meth)acrylic acid, (meth)acrylic acid esters, aromatic vinyl compounds, etc. Two or more of these may be used.

[0052] Examples of compounds having a carboxyl group include (meth)acrylic acid, crotonic acid, maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, etc. Among these, (meth)acrylic acid is preferred in terms of copolymerization reactivity.

[0053] Examples of (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, phenyl (meth)acrylate, benzyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, stearyl (meth)acrylate, and ethylene oxide adducts of (meth)acrylic acid. Among these, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and benzyl (meth)acrylate are preferred.

[0054] Examples of aromatic vinyl compounds include styrene, α-methylstyrene, p-methylstyrene, m-methylstyrene, o-methylstyrene, p-ethylstyrene, m-ethylstyrene, o-ethylstyrene, tert-butylstyrene, chlorostyrene, p-hydroxystyrene, tert-butoxystyrene, 2-vinylnaphthalene, etc. Among these, styrene is preferred.

[0055] Examples of methods for producing (meth)acrylic polymers include polymerizing an α,β-ethylenically unsaturated bond monomer having a hydroxy group and, if necessary, other monomers by a known radical polymerization reaction. The reaction may be carried out without a solvent, but from the viewpoints of synthesis stability and handling, it is preferable to use a solvent. In addition, a radical polymerization initiator (polymerization initiator) or a chain transfer agent may be added to adjust the molecular weight.

[0056] Examples of the solvent include ethyl acetate, methanol, ethanol, propanol, 2-propanol, butanol, tert-butanol, 3-methyl-3-pentanol, 3,7-dimethyl-3-octanol, ethyl acetate, butyl acetate, etc. Two or more of these may be used.

[0057] Examples of the polymerization initiator include organic peroxides such as benzoyl peroxide, tert-butyl perbenzoate, cumene hydroperoxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di(2-ethoxyethyl)peroxydicarbonate, tert-butylperoxy-2-ethylhexanoate, tert-butylperoxyneodecanoate, tert-butylperoxypivalate, (3,5,5-trimethylhexanoyl)peroxide, dipropionyl peroxide, and diacetyl peroxide; and 2,2' Examples of suitable azo compounds include 1,1'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), dimethyl 2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2-hydroxymethylpropionitrile), and 2,2'-azobis[2-(2-imidazolin-2-yl)propane]. Two or more of these may be used. The amount of polymerization initiator added is preferably 3 parts by mass or less per 100 parts by mass of all monomers constituting the (meth)acrylic polymer.

[0058] Examples of chain transfer agents include alkyl mercaptans such as octyl mercaptan, nonyl mercaptan, decyl mercaptan, and dodecyl mercaptan; thioglycolic acid esters such as octyl thioglycolate, nonyl thioglycolate, and 2-ethylhexyl thioglycolate; 2,4-diphenyl-4-methyl-1-pentene, 1-methyl-4-isopropylidene-1-cyclohexene, α-pinene, and β-pinene. Two or more of these may be used. Among these, from the viewpoint of reducing the odor of the (meth)acrylic polymer, thioglycolic acid esters, 2,4-diphenyl-4-methyl-1-pentene, 1-methyl-4-isopropylidene-1-cyclohexene, α-pinene, and β-pinene are preferred. The amount of chain transfer agent added is preferably 3 parts by mass or less per 100 parts by mass of all monomers constituting the (meth)acrylic polymer.

[0059] The graphene dispersion of the present invention contains 10 to 300 parts by weight of the (meth)acrylic polymer per 100 parts by weight of the graphene. If the (meth)acrylic polymer content is less than 10 parts by weight, the dispersibility-improving effect of the (meth)acrylic polymer is not sufficiently obtained, the fluidity of the graphene dispersion decreases, and battery life decreases. The (meth)acrylic polymer content is preferably 15 parts by weight or more, more preferably 20 parts by weight or more. On the other hand, if the (meth)acrylic polymer content exceeds 300 parts by weight, the viscosity of the graphene dispersion increases and the fluidity decreases. Furthermore, when mixed with a positive electrode active material, the (meth)acrylic polymer inhibits the formation of a conductive path, causing resistance and therefore reducing battery life. The (meth)acrylic polymer content is preferably 200 parts by weight or less, more preferably 100 parts by weight or less.

[0060] The graphene dispersion of the present invention further contains a dispersion medium. As the dispersion medium, a polar solvent is preferred from the viewpoint of excellent solubility of the (meth)acrylic polymer. In particular, in lithium ion battery applications, 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, it is more preferred to contain N-methylpyrrolidone from the viewpoint of more effectively exerting the dispersibility-enhancing effect of the surface treatment agent. Dispersibility is further improved by N-methylpyrrolidone solvating the surface treatment agent attached to the graphene.

[0061] The graphene dispersion of the present invention preferably contains 0.10 parts by weight or more and 5 parts by weight or less of the above-mentioned graphene relative to 100 parts by weight of the dispersion medium. By setting the graphene content to 0.10 parts by weight or more, a conductive path is easily formed in the positive electrode of a lithium ion battery, thereby further improving the battery life. The graphene content is preferably 0.15 parts by weight or more, and more preferably 0.20 parts by weight or more. On the other hand, by setting the graphene content to 5 parts by weight or less, the fluidity of the graphene dispersion can be further improved, thereby further improving the battery life. The graphene content is preferably 4.5 parts by weight or less, and more preferably 4.0 parts by weight or less.

[0062] Examples of the method for producing the graphene dispersion of the present invention include a method in which a graphene powder or a dispersion is mixed with a (meth)acrylic polymer dissolved in the dispersion medium. From the viewpoint of further suppressing aggregation of graphene, it is preferable to use a graphene dispersion.

[0063] The contents of graphene, (meth)acrylic polymer, and dispersion medium in the graphene dispersion of the present invention can be determined by the following method. First, graphene and (meth)acrylic polymer are separated by filtration, the filter cake is thoroughly washed with the dispersion medium, and the filter cake (containing graphene) is then dried, thereby determining the graphene content. Alternatively, the dispersion medium can be distilled off from the filtrate (containing the (meth)acrylic polymer) and then dried, thereby determining the (meth)acrylic polymer content. The weight obtained by subtracting the graphene content and the (meth)acrylic polymer content from the weight of the graphene dispersion is the dispersion medium content. However, if the composition of raw materials used in the graphene dispersion is known, the dispersion medium content can also be determined from the raw material composition.

[0064] The mixing device for mixing the (meth)acrylic polymer solution and the graphene powder or dispersion is preferably a device capable of applying shear force, and examples of devices that can be used include a planetary mixer, "FILMICS" (registered trademark) (Primix Corporation), a planetary mixer, a planetary ball mill, and a three-roll mill.

[0065] A strong stirring step may be performed using a high-shear mixer at a shear rate of 5,000 to 50,000 rpm. By exfoliating graphene using the high-shear mixer during the strong stirring step, stacked graphene particles can be dissolved, allowing the average thickness of the graphene to be adjusted. Preferred high-shear mixers are those employing a thin film rotation method, a rotor / stator method, or a media mill method. Examples include the "Filmix" (registered trademark) 30-30 model (Primix Corporation), the "Clearmix" (registered trademark) CLM-0.8S (M Technique Co., Ltd.), the "Labostar" (registered trademark) Mini LMZ015 (Ashizawa Finetech Co., Ltd.), and the Super Shear Mixer SDRT0.35-0.75 (Satake Chemical Machinery Co., Ltd.).

[0066] As described above, the shear rate in the strong stirring step is preferably 5,000 to 50,000 per second. By setting the shear rate to 5,000 per second or higher, exfoliation of graphene is promoted, and the average thickness of graphene can be easily adjusted to the above-mentioned range. In addition, the treatment time for the strong stirring step is preferably 15 seconds to 30 minutes.

[0067] Next, the lithium ion battery positive electrode of the present invention will be described. The lithium ion battery positive electrode of the present invention has a composite layer containing a positive electrode active material, graphene, and a hydroxyl-containing (meth)acrylic polymer on a current collector. The average thickness of the graphene is 0.3 nm or more and 10 nm or less, and the (meth)acrylic polymer is heated at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity is 0.1 Pa·s or more and 100 Pa·s or less. It is preferable that a dry film of the positive electrode paste is formed on the current collector. If necessary, the positive electrode paste may further contain a conductive additive other than graphene, which will be described later.

[0068] The positive electrode active material is a material that can electrochemically absorb and release lithium ions. Examples include lithium manganese oxide (LiMn2O4) with a spinel structure, lithium manganese oxide (LiMnO2) with a rock salt structure, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and ternary systems in which nickel is partially replaced by manganese and cobalt (LiNi x Mn y Co 1-x-y O2), ternary system partially substituted with cobalt and aluminum (LiNi x Co y Al 1-x-y Examples of such active materials include metal oxide active materials such as O2, V2O5, metal compound active materials such as TiS2, MoS2, and NbSe2, lithium iron phosphate (LiFePO4) with an olivine structure, lithium manganese phosphate (LiMnPO4), and solid solution active materials. Two or more of these may be used. Among these, active materials containing lithium and nickel are preferred. Examples of active materials containing lithium and nickel include lithium nickel oxide (LiNiO2), and ternary systems in which nickel is partially substituted with manganese and cobalt (LiNi x Mny Co 1-x-y O2), ternary system partially substituted with cobalt and aluminum (LiNi x Co y Al 1-x-y O2) is preferred, as it can improve the energy density.

[0069] Furthermore, when a granular positive electrode active material is used, the graphene tends to follow the uneven shape of the surface of the positive electrode active material and come into contact with it, so the effect of the present invention is particularly remarkable. Granular material means particles obtained by granulating a slurry containing dispersed powder into a spherical shape by spray drying or the like. The positive electrode active material used as a granular material includes a ternary system (LiNi x Mn y Co 1-x-y O2) and LiNi x Co y Al 1-x-y O2, etc., and since secondary particles are formed by aggregation of primary particles, the surface tends to be uneven, and it is necessary to increase the contact area between the positive electrode active material and the conductive additive, so the effects of the present invention are particularly pronounced.

[0070] From the viewpoint of ease of forming a conductive path by the graphene, the particle diameter of the positive electrode active material is preferably 20 μm or less. In this specification, the particle diameter is defined as the median diameter (D 50 ) The median diameter can be measured using a laser scattering particle size distribution analyzer (for example, Microtrac HRAX-100 manufactured by Nikkiso Co., Ltd.). In addition, in this specification, the "particle diameter of the positive electrode active material" refers to the secondary particle diameter when the positive electrode active material is a granule.

[0071] Examples of graphene include those exemplified as materials for the graphene dispersion. The average thickness of graphene, the size in the direction parallel to the graphene layer, the O / C ratio, and the N / C ratio can be determined by extracting graphene from the positive electrode of a lithium-ion battery and using the methods described above.

[0072] Examples of the (meth)acrylic polymer include those exemplified as the material for the graphene dispersion.

[0073] The lithium ion battery positive electrode of the present invention may further contain a binder, a conductive aid other than graphene, and other additives.

[0074] Examples of binders include fluoropolymers 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, etc. Two or more of these may be contained.

[0075] The content of the binder is preferably 0.2 parts by weight or more and 2 parts by weight or less relative 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, it is possible to further improve the battery life. On the other hand, by setting the content of the binder to 2 parts by weight or less, it is possible to further improve the fluidity of the positive electrode paste and further improve the battery life. Note that the graphene dispersion of the present invention does not need to contain a binder because it has the characteristics of forming a free-standing film and retaining the positive electrode active material.

[0076] The conductive additive other than graphene preferably has high electronic conductivity, and examples thereof include carbon materials such as carbon fiber, carbon black, acetylene black, carbon nanofiber, carbon nanotube, and "VGCF" (registered trademark)-H (manufactured by Showa Denko K.K.), and metal materials such as copper, nickel, aluminum, and silver. Two or more of these may be contained. Among these, fibrous carbon nanofiber, carbon nanotube, and "VGCF" (registered trademark)-H (manufactured by Showa Denko K.K.) are preferred, as they can improve the conductivity of the electrode in the thickness direction.

[0077] The content of the conductive additive other than graphene is preferably 0.1 to 2 parts by weight per 100 parts by weight of the positive electrode active material. By setting the content of the conductive additive 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 additive 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 can be further increased.

[0078] The lithium-ion battery positive electrode of the present invention contains 10 to 300 parts by weight of the (meth)acrylic polymer described above per 100 parts by weight of the graphene described above. If the (meth)acrylic polymer content is less than 10 parts by weight, the dispersibility-improving effect of the (meth)acrylic polymer is not sufficiently obtained, the fluidity of the graphene dispersion is reduced, and the battery life is shortened. The (meth)acrylic polymer content is preferably 15 parts by weight or more, more preferably 20 parts by weight or more. On the other hand, if the (meth)acrylic polymer content exceeds 300 parts by weight, the content of the positive electrode active material and graphene is relatively reduced, and the resistance due to the (meth)acrylic polymer is likely to increase, resulting in a shortened battery life. Therefore, the (meth)acrylic polymer content is preferably 200 parts by weight or less, more preferably 100 parts by weight or less.

[0079] The lithium ion battery positive electrode of the present invention preferably contains 0.05 parts by weight or more and 2.5 parts by weight or less of the graphene described above relative to 100 parts by weight of the positive electrode active material. By setting the graphene content to 0.05 parts by weight or more, the positive electrode paste solids content can be increased, and battery life can be further improved. The graphene content 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 graphene content to 2.5 parts by weight or less, conductive paths can be easily formed, and battery life can be further improved.

[0080] The graphene content and various physical properties and content of the positive electrode active material in a lithium-ion battery can be measured as follows. First, the battery is disassembled in an Ar glove box, the electrodes are washed with dimethyl carbonate, and vacuum dried for 1 hour in the side box of the glove box. Next, the lithium-ion battery positive electrode layer is peeled from the current collector using a spatula. The resulting powder is dissolved in a solvent such as N-methylpyrrolidone or water, and filtered to separate the residue (positive electrode active material, conductive additive, and solvent) and the filtrate (solvent and other components). The resulting filtrate is dried and redissolved in a heavy solvent and analyzed using NMR to identify the binder. The resulting residue is then dried to remove the solvent, and the total weight of the positive electrode active material and conductive additive is determined. The composition ratio of the positive electrode active material can be determined by X-ray diffraction measurement of the resulting powder. When two or more positive electrode active materials are contained, the powder can be further analyzed by energy dispersive X-ray spectroscopy or inductively coupled plasma mass spectrometry (ICP-MS) to determine the mixing ratio of the positive electrode active materials. However, if the raw material composition of the positive electrode active material is known, it can be calculated from the raw material composition. Furthermore, the positive electrode active material is dissolved using acids such as hydrochloric acid and nitric acid, and then filtered to separate the residue (conductive additive) and the filtrate (dissolved electrode active material, water). The residue is washed with water, dried, and weighed to determine the conductive additive content. The positive electrode active material content can also be calculated from the total weight of the positive electrode active material and conductive additive and the weight of the conductive additive. If the conductive additive contains graphene and other materials, the size of each conductive additive can be determined from SEM images of the powder, and the graphene content can be determined by using a sieve to filter or capture only the graphene. If multiple conductive additives are of similar size and sieving is difficult, the content of each additive can be determined from the ratio of cross-sectional areas in SEM surface images of the powder. However, if the raw material composition of the lithium-ion battery positive electrode is known, it can also be calculated from the raw material composition.

[0081] The method for producing the positive electrode of the lithium ion battery of the present invention may include a method in which the positive electrode paste is applied to a current collector and then dried.

[0082] The positive electrode paste contains the positive electrode active material, graphene, and a hydroxylated (meth)acrylic polymer. The graphene has an average thickness of 0.3 nm to 10 nm, and the (meth)acrylic polymer is heated at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity is 0.1 Pa·s or more and 100 Pa·s or less. If necessary, the composition may further contain a binder, a conductive additive other than graphene, and other additives.

[0083] Examples of the positive electrode active material include those exemplified as materials for the positive electrode of a lithium ion battery.

[0084] Examples of graphene include those exemplified as materials for the graphene dispersion. The average thickness of graphene, the size in the direction parallel to the graphene layer, the O / C ratio, and the N / C ratio can be determined by extracting graphene from the positive electrode paste and using the methods described above.

[0085] Examples of the (meth)acrylic polymer include those exemplified as the material for the graphene dispersion.

[0086] The positive electrode paste preferably contains 0.05 to 2.5 parts by weight of the graphene described above relative to 100 parts by weight of the positive electrode active material. By setting the graphene content to 0.05 parts by weight or more, the solid content of the positive electrode paste can be increased, and battery life can be further improved. The graphene content 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 graphene content to 2.5 parts by weight or less, conductive paths can be easily formed, and battery life can be further improved.

[0087] The content of the (meth)acrylic polymer is preferably 10 parts by weight or more and 300 parts by weight or less relative to 100 parts by weight of the graphene described above. The content of the (meth)acrylic polymer is preferably 15 parts by weight or more and more preferably 20 parts by weight or more, from the viewpoints of easily obtaining a dispersibility-improving effect, further improving the fluidity of the graphene dispersion, and further improving battery life. On the other hand, the content of the (meth)acrylic polymer is preferably 200 parts by weight or less and more preferably 100 parts by weight or less, from the viewpoints of suppressing an increase in viscosity of the graphene dispersion, further improving fluidity, easily forming a conductive path when mixed with the positive electrode active material, suppressing an increase in resistance, and further improving battery life.

[0088] The contents of the positive electrode active material, graphene, and (meth)acrylic polymer in the positive electrode paste 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 total weight of the positive electrode active material and conductive additive is determined from the dried powder. The content of the (meth)acrylic polymer can also be determined by distilling off the dispersion medium from the filtrate (containing the (meth)acrylic polymer) and then drying it. The positive electrode active material is then dissolved using an acid such as hydrochloric acid or nitric acid, and the conductive additive is separated by filtration. The resulting solution is then washed with water, dried, and weighed to determine the content of the conductive additive. The content of the positive electrode active material can also be determined from the total weight of the positive electrode active material and conductive additive and the weight of the conductive additive.

[0089] Examples of conductive additives other than binders and graphene include those exemplified as materials for the positive electrode of a lithium ion battery.

[0090] The content of the binder is preferably 0.2 parts by weight or more and 2 parts by weight or less per 100 parts by weight 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 battery life can be further improved. Note that the positive electrode paste does not need to contain a binder because it has the characteristic of forming a self-standing film and holding the positive electrode active material.

[0091] The content of the conductive additive other than graphene is preferably 0.1 parts by weight or more and 2 parts by weight or less relative to 100 parts by weight of the positive electrode active material. By setting the content of the conductive additive 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 additive other than graphene to 2 parts by weight or less, the fluidity of the positive electrode paste can be further improved, and the battery life can be further improved.

[0092] To analyze the constituent materials and composition ratio of the positive electrode paste, the solids are collected from the positive electrode paste by filtration, washed with a solvent, and then dried. The powder is then subjected to X-ray diffraction analysis to identify the type of positive electrode active material. If two or more types of positive electrode active materials are mixed, the powder can be further analyzed by energy dispersive X-ray spectroscopy or inductively coupled plasma mass spectrometry (ICP-MS) to determine the mixture ratio of the positive electrode active materials. However, if the composition of the raw materials used in the positive electrode paste is known, the ratio can also be determined from the raw material composition.

[0093] The filtrate is measured by FT-IR, and if CF absorption due to PVDF is observed in the resulting spectrum, it can be determined that PVDF is contained as a binder. The binder content in the composite layer can also be measured by drying the filtrate and measuring its weight. The dried filtrate can be redissolved in a heavy solvent and analyzed using NMR (nuclear magnetic resonance spectroscopy) to identify other binders.

[0094] From the viewpoint of coatability, the viscosity of the positive electrode paste at 25°C is preferably 1,800 mPa·s or more and 2,200 mPa·s or less. It is preferable to mix a dispersion medium to achieve the desired viscosity. Here, the viscosity of the positive electrode paste at 25°C can be measured using a Brookfield viscometer LVDV-E under conditions of spindle No. 34 and 60 rpm.

[0095] In this specification, the solids content of the positive electrode paste can be measured from the viscosity at 25°C using a Brookfield viscometer LVDV-E with a spindle No. 34 and 60 rpm. After adjusting the positive electrode paste to 1,800 mPa s or more and 2,200 mPa s or less using the above measurement method, 1 g of the positive electrode paste is placed on a glass slide and heated and dried in a vacuum oven at 120°C for 5 hours, and the solids content refers to the value obtained by dividing the weight after drying by the weight before drying.

[0096] The solid content of the positive electrode paste is preferably 70% by weight or more from the viewpoint of forming a conductive path and improving battery life. High fluidity of the graphene dispersion improves the mixing state of the materials in the positive electrode paste, reduces the amount of dispersion medium required to adjust the viscosity, and increases the solid content of the positive electrode paste.

[0097] A method for producing a positive electrode paste includes, for example, mixing the graphene dispersion of the present invention, a positive electrode active material, and a binder or binder solution in a desired ratio, measuring the viscosity using the method described above, adding a dispersion medium so that the viscosity is 1,800 mPa·s or more and 2,200 mPa·s or less, and then mixing again. Examples of the dispersion medium include those exemplified as the dispersion medium for the graphene dispersion. Before adjusting the viscosity, a conductive additive other than graphene and other additives may be added.

[0098] As a mixing device for the positive electrode paste, for example, the devices exemplified as the mixing device for the (meth)acrylic polymer and the graphene powder or dispersion liquid can be mentioned.

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

[0100] Examples of methods for applying the positive electrode paste onto the current collector include methods using a doctor blade, a die coater, a comma coater, a spray, or the like.

[0101] After applying the positive electrode paste of the present invention to a current collector, the dispersion medium is preferably removed by a drying process. The method for removing the dispersion medium is preferably drying using an oven or a vacuum oven. The atmosphere for removing the dispersion medium may be air, an inert gas, or a vacuum. The temperature for removing the dispersion medium is preferably 60°C or higher and 250°C or lower.

[0102] In addition, it is preferable to have a step of pressing the current collector coated with the positive electrode paste in order to increase the density of the coating film after drying. [Example]

[0103] The present invention will be described below using examples. First, the evaluation methods used in each example and comparative example will be described.

[0104] [Measurement example 1: Average thickness of graphene] The graphene dispersion prepared in each example and comparative example was diluted to 0.002 wt % with N-methylpyrrolidone. Surface-treated graphene was treated for 60 seconds using a "Filmix" (registered trademark) 30-30 model (Primix Corporation) at a rotation speed of 40 m / s (shear rate: 20,000 per second). The diluted solution was dropped onto a mica substrate, dried, and adhered to the substrate. The graphene on the substrate was observed using an atomic force microscope (Dimension Icon; Bruker) with a magnified field of view of approximately 1 to 10 μm square, and the thickness of each of 10 randomly selected graphene particles was measured. The thickness of each graphene particle was calculated as the arithmetic mean of the thickness measurements at five randomly selected points on each graphene particle. The average thickness of the 10 graphene particles was calculated by calculating the arithmetic mean of the thicknesses. Because the thickness of graphene does not change in the graphene dispersion, positive electrode paste, or lithium-ion battery positive electrode, it was measured using only the graphene dispersion.

[0105] [Measurement example 2: Size of graphene in the direction parallel to the graphene layer] The graphene dispersions prepared in each example and comparative example were diluted to 0.002 wt % with N-methylpyrrolidone. Surface-treated graphene was treated for 60 seconds using a "Filmix" (registered trademark) 30-30 model (Primix Corporation) at a rotation speed of 40 m / s (shear rate: 20,000 kJ / s). The diluted solution was dropped onto a mica substrate, dried, and adhered to the substrate. The graphene on the substrate was observed at 30,000x magnification using an S-5500 electron microscope (Hitachi High-Technologies Corporation). Ten randomly selected graphene samples were measured for the longest length (major axis) and shortest length (minor axis) parallel to the graphene layers. The size of the plane parallel to the graphene layers was calculated by calculating the arithmetic mean of the value calculated by (major axis + minor axis) / 2.

[0106] [Measurement Example 3: Measurement of O / C and N / C ratios using X-ray photoelectron spectroscopy] The graphene dispersions prepared in each example and comparative example were filtered using a suction filter, diluted to 0.5% by mass with water, and then subjected to suction filtration. This washing process was repeated five times, and the graphene was then freeze-dried to obtain surface-treated graphene powder. The photoelectron spectrum of the obtained surface-treated graphene powder was measured using a Quantera SXM X-ray photoelectron spectrometer (manufactured by PHI). The excitation X-rays were monochromatic Al K α1,2 The X-ray beam was set to a 1486.6 eV, with an X-ray diameter of 200 μm and a photoelectron escape angle of 45°. The main C1s peak due to carbon atoms was assigned to 284.3 eV, the O1s peak due to oxygen atoms was assigned to a peak near 533 eV, and the N1s peak due to nitrogen atoms was assigned to a peak near 402 eV. The O / C ratio was calculated from the area ratio of the O1s peak to the C1s peak, and the obtained value was rounded to two decimal places. The N / C ratio was also calculated from the area ratio of the N1s peak to the C1s peak, and the obtained value was rounded to three decimal places.

[0107] [Measurement Example 4: Hydroxyl value of (meth)acrylic polymer] The hydroxyl value of the (meth)acrylic polymer used in each of the examples and comparative examples was measured in accordance with JIS K0070-1992.

[0108] [Measurement Example 5: Viscosity of (meth)acrylic polymer] Approximately 1 g of the (meth)acrylic polymer used in each example and comparative example was placed on the glass plate of a viscoelasticity measuring device, Physica MCR301 (manufactured by Anton Paar), and the temperature was 25°C, the shear rate was 1.0 s, and a 43 mmφ parallel plate was used as the measuring jig. -1 The viscosity was measured by rotational measurement 3 minutes after the measurement jig was moved under the conditions of an angular frequency of 0.018 rad / s and a sample thickness of 0.5 mm.

[0109] [Measurement Example 6: Viscosity of graphene dispersion] Approximately 15 g of the graphene dispersion prepared in each of the Examples and Comparative Examples was placed in the sample chamber of a Brookfield viscometer LVDV-E so as to cover the spindle head, and the viscosity was measured 3 minutes after the rotor was started using a No. 34 spindle under the conditions of a temperature of 25°C and a rotation speed of 3 rpm.

[0110] [Measurement Example 7: Dispersibility of graphene dispersion] 0.2 g of the graphene dispersion prepared in each Example and Comparative Example was poured into the deep end of the groove of a 0-25 μm steel grind gauge (length 180 mm, width 60 mm, thickness 12 mm), and a steel scraper was placed so that the long side was parallel to the width direction of the gauge and the cutting edge was in contact with the deep end of the gauge groove. While holding the scraper perpendicular to the surface of the gauge, it was pulled perpendicular to the long side of the groove at a speed of 10 cm / s until the groove disappeared. Within 3 seconds after the scraper was stopped, the depth of the groove at a point containing 5 to 10 particles in a 3 mm wide band along the gauge groove was measured. When the depth of the graphene dispersion in a 3 mm wide band along the gauge groove was less than 1 μm, it was evaluated as ◎, when it was 1 μm or more but less than 5 μm, it was evaluated as ◯, when it was 5 μm or more but less than 10 μm, it was evaluated as △, and when it was 10 μm or more.

[0111] [Measurement Example 8: Fluidity of graphene dispersion] 1 g of the graphene dispersion prepared in each example and comparative example was dropped in a circle with a diameter of approximately 1 cm onto one end of the non-glossy surface of a clean, flat aluminum foil measuring 5 cm in width and 15 cm in length. The aluminum foil was grasped by the side on which the graphene dispersion was placed, pulled up, and stood upright without vibration. After leaving it to stand for 10 minutes, the distance that the graphene dispersion dripped due to its own weight was measured. The dripping distance of the graphene dispersion was measured by measuring the distance to the edge of the graphene dispersion in the direction in which gravity acts when the aluminum foil is stood upright before and after the graphene dispersion dripped. A dripping distance of 10 cm or more was evaluated as ◯, a dripping distance of 3 cm or more but less than 10 cm was evaluated as △, and a dripping distance of less than 3 cm was evaluated as ×.

[0112] [Measurement example 9: Battery life (battery capacity maintenance rate)] For the 2032-type coin batteries produced in each example and comparative example, charge / discharge measurements were performed three times each at rates of 0.1C, 1C, and 5C with an upper limit voltage of 4.2V and a lower limit voltage of 3.0V, and then 191 more times at 2C, for a total of 200 charge / discharge measurements.The battery capacity at the 200th charge / discharge was measured, and the ratio (percentage) of the battery capacity to the first charge / discharge was calculated.

[0113] [Synthesis Example 1: Preparation of graphene oxide] Using 1500-mesh natural graphite powder (Shanghai Yifan Graphite Co., Ltd.), 10 g of natural graphite powder was placed in an ice bath and mixed with 220 ml of 98% concentrated sulfuric acid, 5 g of sodium nitrate, and 30 g of potassium permanganate. The mixture was mechanically stirred for 1 hour while maintaining the temperature below 20°C. The mixture was removed from the ice bath and stirred in a 35°C water bath for 4 hours. 500 ml of ion-exchanged water was then added, and the resulting suspension was stirred at 90°C for an additional 15 minutes. Finally, 600 ml of ion-exchanged water and 50 ml of hydrogen peroxide were added, and the mixture was stirred for 5 minutes to obtain a graphene oxide dispersion. The mixture was filtered while still hot, and the metal ions were washed off with dilute hydrochloric acid, followed by acid washing with ion-exchanged water. Graphene oxide was obtained by repeated washing until the pH reached 7. The oxygen to carbon atom ratio (O / C ratio) of the prepared graphene oxide was 0.53, as measured by X-ray photoelectron spectroscopy.

[0114] [Synthesis Example 2: Preparation of graphene oxide] Graphene oxide was prepared in the same manner as in Synthesis Example 1, except that 1500 mesh natural graphite powder (Shanghai Yifan Graphite Co., Ltd.) was replaced with AGB-32 (manufactured by Ito Graphite Industries Co., Ltd.) The elemental ratio of oxygen atoms to carbon atoms (O / C ratio) of the prepared graphene oxide measured by X-ray photoelectron spectroscopy was 0.51.

[0115] [Synthesis Example 3: Synthesis of (meth)acrylic polymer-1] A reactor equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet tube was charged with 35 g of butyl acetate as a solvent, 11.7 g of 2-hydroxyethyl methacrylate (HEMA), 7.30 g of methyl methacrylate (MMA), 0.86 g of 2-ethylhexyl acrylate (EHA), 0.20 g of acrylic acid (AA), and 0.20 g of 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator. The reactor was then quenched under a nitrogen atmosphere. The contents were stirred while introducing nitrogen gas and heated to 65°C using a mantle heater. The temperature in the reactor was maintained at 65°C while stirring at 300 rpm, and polymerization was carried out for 3 hours. The resulting polymer solution was then poured into 500 mL of a 10:1 (weight ratio) hexane:ethanol solution, and the polymer was precipitated. The precipitated polymer was dissolved again in 30 g of butyl acetate and poured into 500 mL of a 10:1 hexane:ethanol solution to precipitate the polymer. This process was repeated three times. The resulting polymer was freeze-pulverized in liquid nitrogen and then dried in a vacuum oven at 40°C for 8 hours to obtain (meth)acrylic polymer-1. The resulting (meth)acrylic polymer-1 was dried at a temperature of 25°C and a shear rate of 1.0 s. -1 The viscosity was 16 Pa·s and the hydroxyl value was 252 mg KOH / g.

[0116] [Synthesis Example 4: Synthesis of (meth)acrylic polymer-2] (Meth)acrylic polymer-2 was obtained in the same manner as in Synthesis Example 3, except that the amount of HEMA added was changed to 1.50 g and the amount of MMA added was changed to 17.3 g. The obtained (meth)acrylic polymer-2 was subjected to polymerization at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity was 14 Pa·s and the hydroxyl value was 30 mgKOH / g.

[0117] [Synthesis Example 5: Synthesis of (meth)acrylic polymer-3] (Meth)acrylic polymer-3 was obtained in the same manner as in Synthesis Example 3, except that 5.85 g of N-hydroxyethylacrylamide was used instead of 11.7 g of HEMA and the amount of MMA added was changed to 13.2 g. The obtained (meth)acrylic polymer-3 was subjected to polymerization at a temperature of 25°C and a shear rate of 1.0 s -1The viscosity was 80 Pa·s and the hydroxyl value was 110 mg KOH / g.

[0118] [Synthesis Example 6: Synthesis of (meth)acrylic polymer-4] (Meth)acrylic polymer-4 was obtained in the same manner as in Synthesis Example 3, except that the amount of HEMA added was changed to 2.9 g, the amount of MMA added was changed to 13.2 g, and 2.9 g of N-hydroxyethylacrylamide was added. The obtained (meth)acrylic polymer-4 was subjected to a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity was 56 Pa·s and the hydroxyl value was 124 mg KOH / g.

[0119] [Synthesis Example 7: Synthesis of (meth)acrylic polymer-5] (Meth)acrylic polymer-5 was obtained in the same manner as in Synthesis Example 3, except that the amounts of HEMA, MMA, and AIBN were changed to 5.85 g, 13.2 g, and 0.50 g, respectively, and 0.50 g of octyl mercaptan was added as a chain transfer agent. The obtained (meth)acrylic polymer-5 was subjected to polymerization at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity was 1.2 Pa·s and the hydroxyl value was 115 mg KOH / g.

[0120] [Synthesis Example 8: Synthesis of (meth)acrylic polymer-6] (Meth)acrylic polymer-6 was obtained in the same manner as in Synthesis Example 3, except that the amounts of HEMA, MMA, and AIBN were changed to 5.85 g, 13.2 g, and 0.10 g, respectively, and the polymerization time was changed to 8 hours. The obtained (meth)acrylic polymer-6 was solid and had a hydroxyl value of 112 mgKOH / g.

[0121] [Synthesis Example 9: Synthesis of (meth)acrylic polymer-7] (Meth)acrylic polymer-7 was obtained in the same manner as in Synthesis Example 3, except that the amount of HEMA added was changed to 3.0 g and the amount of EHA added was changed to 3.0 g. The obtained (meth)acrylic polymer-7 was subjected to a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity was 41 Pa·s and the hydroxyl value was 52 mgKOH / g.

[0122] [Synthesis Example 10: Synthesis of (meth)acrylic polymer-8] (Meth)acrylic polymer-8 was obtained in the same manner as in Synthesis Example 3, except that the added amounts of HEMA, MMA, and EHA were changed to 4.5 g, 15.3 g, and 2.0 g, respectively. The obtained (meth)acrylic polymer-8 was subjected to polymerization at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity was 22 Pa·s and the hydroxyl value was 81 mgKOH / g.

[0123] [Synthesis Example 11: Synthesis of (meth)acrylic polymer-9] (Meth)acrylic polymer-9 was obtained in the same manner as in Synthesis Example 3, except that the added amounts of HEMA, MMA, and EHA were changed to 8.0 g, 4.0 g, and 6.0 g, respectively. The obtained (meth)acrylic polymer-9 was subjected to polymerization at a temperature of 25°C and a shear rate of 1.0 s -1 The viscosity was 35 Pa·s and the hydroxyl value was 175 mg KOH / g.

[0124] [Synthesis Example 12: Synthesis of (meth)acrylic polymer-10] A polymer solution was obtained in the same manner as in Synthesis Example 3, except that 1.0 g of HEMA, 13.2 g of NOF Corporation's "Blenmer" (registered trademark) PME-200, 0.86 g of EHA, and 0.20 g of AA were used as monomers, and 0.30 g of AIBN was used as a polymerization initiator. The precipitated polymer was dissolved in 30 g of chloroform and poured into 500 mL of a hexane:ethanol = 10:1 solution to precipitate the polymer. This procedure was repeated three times. The obtained polymer was freeze-pulverized and dried in the same manner as in Example 1, to obtain (meth)acrylic polymer-10. The obtained (meth)acrylic polymer-10 was freeze-pulverized at a temperature of 25°C, a shear rate of 1.0 s -1 The viscosity was 0.7 Pa·s and the hydroxyl value was 22 mg KOH / g.

[0125] [Example 1] (Preparation of surface-treated graphene N-methylpyrrolidone dispersion paste) Graphene oxide prepared in Synthesis Example 1 was diluted with ion-exchanged water to a concentration of 30 mg / ml and treated for 30 minutes at 3,000 rpm using a Homodisper 2.5 (Primix Corporation) to obtain a uniform graphene oxide dispersion. 20 ml of the resulting graphene oxide dispersion was mixed with 0.3 g of dopamine hydrochloride as a surface treatment agent and treated for 60 minutes at 3,000 rpm using a Homodisper 2.5 (Primix Corporation). The treated graphene oxide dispersion was subjected to ultrasonic irradiation at 300 W for 30 minutes (micronization process) using an ultrasonicator UP400S (Hielscher). The graphene oxide dispersion after the micronization process was diluted to 5 mg / ml with ion-exchanged water. 20 ml of the diluted dispersion was added with 0.3 g of sodium dithionite and stirred for 1 hour at 3,000 rpm using a Homodisper 2.5 (Primix Corporation) in a water bath at 40 °C. The mixture was then filtered using a vacuum suction filter. This washing process, in which the mixture was diluted to 0.5 wt % with water and then suction filtered, was repeated five times to obtain a graphene-water wet cake (2.9 wt %) after suction filtration. N-methylpyrrolidone was added to the resulting graphene-water wet cake to a concentration of 0.5 wt %. The resulting graphene-water wet cake was then processed using a "Filmix"® 30-30 (Primix Corporation) at a rotation speed of 40 m / s (shear rate: 20,000 m / s) for 60 seconds. After processing, the solvent was removed by vacuum suction filtration. To further remove water, N-methylpyrrolidone was added until the graphene concentration reached 0.5 wt %, and the mixture was diluted by treating it at a rotation speed of 3,000 rpm for 30 minutes using a Homodisper 2.5 (Primix Corporation). This process was then repeated twice to filter the mixture under reduced pressure until the filtrate no longer fell out, yielding an N-methylpyrrolidone dispersion paste containing 5.0 wt % of surface-treated graphene as the filtered residue.

[0126] (Preparation of (meth)acrylic polymer solution) N-methylpyrrolidone 95% by weight, (meth)acrylic polymer "ARUFON" (registered trademark) UH-2041 (manufactured by Toagosei Co., Ltd., hydroxyl value 121 mg KOH / g, temperature 25 °C, shear rate 1.0 s -15% by weight of "ARUFON" UH-2041 (viscosity at 11 Pa·s) was added and thoroughly mixed with "ARUFON" UH-2041 in a sealed container under magnetic stirring to obtain a 5% by weight "ARUFON" UH-2041 / N-methylpyrrolidone solution.

[0127] (Preparation of graphene dispersion) To 10 g of N-methylpyrrolidone dispersion paste containing 5.0 wt% surface-treated graphene, 2.5 g of 5 wt% "ARUFON" UH-2041 / N-methylpyrrolidone and 4.2 g of N-methylpyrrolidone were added, and the mixture was stirred for 15 minutes (strong stirring step) at a rotation speed of 40 m / s (shear rate: 20,000 per second) using a "Filmix" (registered trademark) 30-30 model (Primix Corporation) to obtain a graphene dispersion. The graphene solids concentration of the obtained graphene dispersion was 3 wt%, and the (meth)acrylic polymer content was 25 parts by weight per 100 parts by weight of graphene.

[0128] For 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. In addition, the O / C ratio and N / C ratio were measured according to Measurement Example 3, the viscosity of the graphene dispersion was measured according to Measurement Example 6, and the dispersibility and flowability of the graphene dispersion were evaluated according to Measurement Examples 7 and 8.

[0129] (Coin battery construction) LiNi as the positive electrode active material 0.5 Co 0.2 Mn 0.320g of O2, 5g of a 3wt% graphene dispersion as a conductive additive, and 2g of a 10wt% PVDF / N-methylpyrrolidone solution as a binder were mixed for 15 minutes at 2,000 rpm using a planetary centrifugal mixer. The viscosity was measured using a Brookfield LVDV-E viscometer with a spindle No. 34 at 60 rpm and 25°C. Additional N-methylpyrrolidone was added until the viscosity reached 2,200 mPa·s. The amount of N-methylpyrrolidone added was adjusted to achieve a positive electrode paste viscosity of 2,200 mPa·s. This mixture was then mixed again for 15 minutes at 2,000 rpm using a planetary centrifugal mixer to obtain the positive electrode paste.

[0130] The obtained positive electrode paste was applied to an aluminum foil (thickness 18 μm) so that the amount of the positive electrode paste applied after drying was 18 mg / cm 2 The coating was applied using a doctor blade so that the coating was as follows: the coating was dried at 80°C for 15 minutes, and then vacuum dried at 120°C for 2 hours to obtain an electrode plate.

[0131] The prepared electrode plate was cut to a diameter of 15.9 mm to serve as the positive electrode, and a negative electrode consisting of 98 parts by weight of graphite formed on copper foil, 1 part by weight of sodium carboxymethyl cellulose, and 1 part by weight of an SBR aqueous dispersion was cut to a diameter of 16.1 mm to serve as the counter electrode. A 2032-type coin battery was prepared using a separator made of Celgard #2400 (manufactured by Celgard) cut to a diameter of 17 mm and an electrolyte solution of ethylene carbonate:diethyl carbonate = 7:3 containing 1 mol / L of LiPF6. The battery life (battery capacity retention rate) of the obtained coin battery was measured according to Measurement Example 9.

[0132] [Example 2] A graphene dispersion 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, a positive electrode paste and a 2032-type coin battery were produced in the same manner as in Example 1.

[0133] [Example 3] A graphene dispersion was obtained in the same manner as in Example 1, except that the micronization step was extended to 120 minutes. 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.

[0134] [Example 4] 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-type coin battery were produced in the same manner as in Example 1.

[0135] [Example 5] A graphene dispersion was obtained in the same manner as in Example 1, except that dopamine hydrochloride was changed to catechol. A positive electrode paste and a 2032-type coin battery were produced using the obtained graphene dispersion in the same manner as in Example 1.

[0136] [Example 6] 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-type coin battery were produced in the same manner as in Example 1.

[0137] [Example 7] A graphene dispersion was obtained in the same manner as in Example 1, except that 1.0 g of a 5 wt % "ARUFON" UH-2041 / N-methylpyrrolidone solution was added to 10 g of an N-methylpyrrolidone dispersion paste containing 5.0 wt % of surface-treated graphene, and 5.7 g of N-methylpyrrolidone was added. 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.

[0138] [Example 8] In the same manner as in Example 1, a paste of surface-treated graphene dispersed in N-methylpyrrolidone was prepared.

[0139] 20% by weight of (meth)acrylic polymer "ARUFON" UH-2041 was added to 80% by weight of N-methylpyrrolidone, and the mixture was heated to 90°C in a sealed container while stirring with a magnetic stirrer until the "ARUFON" UH-2041 was thoroughly mixed, yielding a 20% by weight "ARUFON" UH-2041 / N-methylpyrrolidone solution. To prepare the graphene dispersion, 2.5 g of 20 wt% "ARUFON" UH-2041 / N-methylpyrrolidone was added to 10 g of N-methylpyrrolidone-dispersed paste containing 5.0 wt% surface-treated graphene, and 4.2 g of N-methylpyrrolidone was added. The mixture was then stirred for 15 minutes (strong stirring step) at a rotation speed of 40 m / s (shear rate: 20,000 per second) using a "Filmix" (registered trademark) 30-30 type (Primix Corporation) to obtain the graphene dispersion. 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.

[0140] [Example 9] A graphene dispersion was obtained in the same manner as in Example 1, except that 6.25 g of 20 wt % "ARUFON" UH-2041 / N-methylpyrrolidone was added to 10 g of N-methylpyrrolidone-dispersed paste containing 5.0 wt % surface-treated graphene, and 0.45 g of N-methylpyrrolidone was further added. 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.

[0141] [Example 10] In preparing the graphene dispersion, the (meth)acrylic polymer was dispersed in an "ARUFON" UH-2000 (manufactured by Toagosei Co., Ltd., hydroxyl value 19 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / min was 13 Pa s. 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.

[0142] [Example 11] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-1 (hydroxyl value 252 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / cm2 was 16 Pa s. 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.

[0143] [Example 12] In preparing the graphene dispersion, the (meth)acrylic polymer was used as (meth)acrylic polymer-2 (hydroxyl value 30 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / cm2 was 14 Pa s. 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.

[0144] [Example 13] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-3 (hydroxyl value 110 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / cm2 was 80 Pa s. 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.

[0145] [Example 14] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-4 (hydroxyl value 124 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / min was adjusted to 56 Pa s. A positive electrode paste and a 2032-type coin battery were produced using the obtained graphene dispersion in the same manner as in Example 1.

[0146] [Example 15] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-5 (hydroxyl value 115 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / cm2 was adjusted to 1.2 Pa s. 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.

[0147] [Example 16] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-7 (hydroxyl value 52 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / min was 41 Pa s. 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.

[0148] [Example 17] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-8 (hydroxyl value 81 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / min was adjusted to 22 Pa s. 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 18] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-9 (hydroxyl value 175 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / min was adjusted to 35 Pa s. 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 19] In preparing the graphene dispersion, the (meth)acrylic polymer was (meth)acrylic polymer-10 (hydroxyl value 22 mg KOH / g, temperature 25 °C, shear rate 1.0 s -1 A graphene dispersion was obtained in the same manner as in Example 1, except that the viscosity at 1000 kJ / min was adjusted to 0.7 Pa s. 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.

[0151] [Example 20] 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 added was changed to 0.1 g. 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.

[0152] [Example 21] 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 added was changed to 0.2 g. 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.

[0153] [Example 22] A graphene dispersion was obtained in the same manner as in Example 1, except that dopamine hydrochloride was changed to phenylethylamine hydrochloride, the amount added was changed to 0.5 g, and the temperature during addition was changed to 60° C. 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.

[0154] [Example 23] A graphene dispersion was obtained in the same manner as in Example 1, except that the strong stirring step was shortened to 7 minutes. 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.

[0155] [Comparative Example 1] Except for not using a (meth)acrylic polymer, 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 battery were produced in the same manner as in Example 1.

[0156] Comparative Example 2 A graphene dispersion was obtained in the same manner as in Example 1, except that in preparing the graphene dispersion, the (meth)acrylic polymer was changed to "ARUFON" (registered trademark) UC-3510 (manufactured by Toagosei Co., Ltd., no hydroxyl group, viscosity 6.7 Pa s). 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.

[0157] Comparative Example 3 A graphene dispersion was obtained in the same manner as in Example 1, except that in the preparation of the graphene dispersion, the (meth)acrylic polymer was changed to (meth)acrylic polymer-6 (hydroxyl value 112 mgKOH / g, solid). 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.

[0158] Comparative Example 4 A graphene dispersion was obtained in the same manner as in Example 1, except that 0.5 g of 5 wt % "ARUFON" UH-2041 / N-methylpyrrolidone was added to 10 g of N-methylpyrrolidone-dispersed paste containing 5.0 wt % surface-treated graphene, and 6.2 g of N-methylpyrrolidone was further added. 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.

[0159] Table 1 shows the compositions and evaluation results of each of the examples and comparative examples.

[0160] Comparative Example 5 In preparing the surface-treated graphene N-methylpyrrolidone dispersion paste of Example 1, graphite nanoplatelets (model number M-5, manufactured by XG Sciences) were diluted with ion-exchanged water to a concentration of 0.5 wt %, and the diluted solution was treated at 3,000 rpm for 30 minutes using a Homo Disper 2.5 (Primix Corporation). A graphene dispersion was obtained in the same manner as in Example 1. A positive electrode paste and a 2032-type coin battery were fabricated using the obtained graphene dispersion in the same manner as in Example 1.

[0161] Table 1 shows the composition and evaluation results of the graphene dispersions of the examples and comparative examples.

[0162] [Table 1]

Claims

1. A graphene dispersion containing graphene, a (meth)acrylic polymer having a hydroxy group, and a dispersion medium, wherein the graphene has an average thickness of 0.3 nm or more and 10 nm or less, the content of the (meth)acrylic polymer having a hydroxy group is 10 parts by weight or more and 300 parts by weight or less relative to 100 parts by weight of the graphene, and the (meth)acrylic polymer having a hydroxy group is dispersed at a temperature of 25° C. and a shear rate of 1.0 s －１ Graphene dispersion having a viscosity of 0.1 Pa·s or more and 100 Pa·s or less at 0.1 Pa·s or more.

2. 2. The graphene dispersion according to claim 1, wherein the (meth)acrylic polymer having a hydroxy group has a hydroxyl value of 10 mgKOH / g or more and 300 mgKOH / g or less.

3. 3. The graphene dispersion according to claim 1, wherein the content of the graphene relative to 100 parts by weight of the dispersion medium is 0.10 parts by weight or more and 5 parts by weight or less.

4. The graphene dispersion according to any one of claims 1 to 3, wherein the graphene has a size of 0.1 µm or more and 100 µm or less in a direction parallel to the graphene layer.

5. 5. The graphene dispersion according to claim 1, wherein the graphene has an elemental ratio of oxygen to carbon (O / C ratio) of 0.05 or more and 0.35 or less, as measured by X-ray photoelectron spectroscopy.

6. 6. The graphene dispersion according to claim 1, wherein the graphene has an elemental ratio of nitrogen to carbon (N / C ratio) of 0.005 or more and 0.020 or less, as measured by X-ray photoelectron spectroscopy.

7. The graphene dispersion according to any one of claims 1 to 6, containing N,N-dimethylformamide, N-methylpyrrolidone and / or N,N-dimethylacetamide as the dispersion medium.

8. A lithium ion battery positive electrode having a mixture layer containing a positive electrode active material, graphene, and a (meth)acrylic polymer having a hydroxy group on a current collector, wherein the graphene has an average thickness of 0.3 nm or more and 10 nm or less, the content of the (meth)acrylic polymer having a hydroxy group is 10 parts by weight or more and 300 parts by weight or less per 100 parts by weight of the graphene, and the (meth)acrylic polymer having a hydroxy group is heated at a temperature of 25°C and a shear rate of 1.0 s －１ 1. A lithium ion battery positive electrode, wherein the viscosity at 1000 kJ / cm2 is 0.1 Pa·s or more and 100 Pa·s or less.

9. 9. The positive electrode for a lithium ion battery according to claim 8, wherein the (meth)acrylic polymer having a hydroxy group has a hydroxyl value of 10 mgKOH / g or more and 300 mgKOH / g or less.

10. The lithium ion battery positive electrode according to claim 8 or 9, wherein the content of the graphene relative to 100 parts by weight of the positive electrode active material is 0.05 parts by weight or more and 2.5 parts by weight or less.

11. The lithium ion battery positive electrode according to any one of claims 8 to 10, wherein the size of the graphene in a direction parallel to the graphene layer is 0.1 μm or more and 100 μm or less.

12. The lithium ion battery positive electrode according to any one of claims 8 to 11, wherein the graphene has an oxygen to carbon element ratio (O / C ratio) measured by X-ray photoelectron spectroscopy of 0.05 to 0.

35.

13. The lithium ion battery positive electrode according to any one of claims 8 to 12, wherein the graphene has an elemental ratio of nitrogen to carbon (N / C ratio) measured by X-ray photoelectron spectroscopy of 0.005 or more and 0.020 or less.

14. The lithium ion battery positive electrode according to any one of claims 8 to 13, wherein the positive electrode active material contains lithium and nickel.

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