WO2021095651A1

WO2021095651A1 - Graphene dispersion, positive electrode paste, and lithium ion battery positive electrode

Info

Publication number: WO2021095651A1
Application number: PCT/JP2020/041519
Authority: WO
Inventors: 加藤智博; 竹内孝; 片瀬郁也; 玉木栄一郎
Original assignee: 東レ株式会社
Priority date: 2019-11-15
Filing date: 2020-11-06
Publication date: 2021-05-20
Also published as: TW202135365A; JPWO2021095651A1

Abstract

A graphene dispersion containing graphene and polyvinyl alcohol, wherein the average thickness of the graphene is 0.3 to 10 nm, the polyvinyl alcohol saponification rate is 70 to 100%, and the graphene dispersion contains 10 to 300 weight parts of the polyvinyl alcohol per 100 weight parts of the graphene. Provided are a graphene dispersion and a positive electrode paste that allow graphene to be mixed uniformly when being mixed with a positive electrode active substance, thereby providing a lithium ion battery positive electrode having improved battery life.

Description

Graphene dispersion, positive electrode paste and lithium ion battery positive electrode

The present invention relates to a graphene dispersion, a method for producing the same, a positive electrode paste, and a positive electrode of a lithium ion battery.

In recent years, research and development of lithium-ion batteries have been actively carried out in various applications such as mobile devices such as smartphones and mobile phones, hybrid vehicles, electric vehicles, and household storage batteries. Lithium-ion batteries used in these fields are required to suppress a decrease in battery capacity due to repeated charging and discharging to improve battery life.

As one of the means, conductive auxiliary agents such as carbon nanotubes and graphene are used. As a technique using a conductive auxiliary agent, a dispersion liquid containing a dispersion liquid medium, a polymer dispersion auxiliary agent, and carbon nanotubes dispersed in the dispersion liquid medium, wherein the carbon nanotubes have a specific aggregated form is dispersed. An electrode for a secondary battery having a liquid (see, for example, Patent Document 1), an active material for a secondary battery, and a mixture layer containing graphene, and the content of graphene in the mixture layer and the void ratio of the mixture layer. (For example, see Patent Document 2) and the like have been proposed.

Special Table 2016-514080 Japanese Unexamined Patent Publication No. 2018-174134

In order to improve the battery life of lithium-ion batteries, it is important to suppress deterioration of the conductive path due to repeated charging and discharging. For that purpose, it is considered important that the conductive auxiliary agent forming the conductive path is uniformly mixed with the material such as the positive electrode active material to form a homogeneous and stable coating film.

However, in the dispersion liquid described in Patent Document 1, the dispersion of carbon nanotubes is insufficient, the positive electrode paste becomes non-uniform due to the aggregates of carbon nanotubes, voids in the coating film are likely to occur, and charging and discharging are repeated. There is a problem that the battery capacity is lowered, that is, the battery life is insufficient. Further, in the electrode for a secondary battery described in Patent Document 2, it is possible to prevent the formation of voids by using graphene. However, in recent years, further improvement in battery life has been required.

Therefore, the present invention provides a graphene dispersion in which graphene can be uniformly mixed when mixed with a positive electrode active material and a positive electrode paste using the same, thereby providing a lithium ion battery positive electrode having an improved battery life. Make it an issue.

In order to solve the above problems, the present invention is a graphene dispersion containing graphene and polyvinyl alcohol, in which the average thickness of the graphene is 0.3 nm or more and 10 nm or less, and the saponification rate of the polyvinyl alcohol is 70. A graphene dispersion containing 10 parts by weight or more and 300 parts by weight or less of polyvinyl alcohol having a saponification rate of 70% or more and 100% or less with respect to 100 parts by weight of the graphene.

Another aspect of the present invention is a positive electrode paste containing a positive electrode active material, graphene and polyvinyl alcohol, in which the average thickness of the graphene is 0.3 nm or more and 10 nm or less, and the saponification rate of the polyvinyl alcohol is 70. A positive electrode paste containing 10 parts by weight or more and 300 parts by weight or less of the polyvinyl alcohol with respect to 100 parts by weight of the graphene.

Further, another aspect of the present invention is a lithium ion battery positive electrode containing a positive electrode active material, graphene and polyvinyl alcohol, wherein the graphene has an average thickness of 0.3 nm or more and 10 nm or less, and the polyvinyl alcohol is saponified. A lithium ion battery positive electrode having a rate of 70% or more and 100% or less and containing 10 parts by weight or more and 300 parts by weight or less of the polyvinyl alcohol with respect to 100 parts by weight of the graphene.

The graphene dispersion of the present invention has excellent fluidity and excellent graphene uniformity when mixed with the positive electrode active material. The positive electrode paste of the present invention has excellent coating film uniformity, can increase the solid content, and can improve battery life.

First, the graphene dispersion of the present invention will be described. The graphene dispersion of the present invention contains graphene having an average thickness of 0.3 nm or more and 10 nm or less and polyvinyl alcohol having a saponification rate of 70% or more and 100% or less. Thin graphene with an average thickness of 0.3 nm or more and 10 nm or less is flexible and follows the surface of the positive electrode active material well, and easily forms a conductive path. On the other hand, since thin graphene tends to cause aggregation, it is conventionally difficult to maintain dispersibility in the graphene dispersion and the positive electrode paste when such thin graphene is used, and the fluidity of the dispersion becomes poor. It was inadequate. Therefore, when such a thin graphene is used, there are problems that the coating film uniformity and battery life of the positive electrode paste are lowered, and it is difficult to increase the solid content ratio of the positive electrode paste. Therefore, in the present invention, polyvinyl alcohol having a specific saponification rate is used together with such thin graphene. Such polyvinyl alcohol functions as a dispersant for enhancing the dispersibility of graphene in the graphene dispersion liquid, and at the same time, functions as a structural material for forming a uniform coating film at the time of forming a coating film. Therefore, when the graphene dispersion of the present invention is used for a positive electrode paste or a positive electrode of a lithium ion battery, it is easy to obtain a uniform coating film in which a positive electrode active material and a highly fluid graphene dispersion are uniformly mixed, and the positive electrode paste can be used. The solid content ratio can be increased. Further, since the binding of the positive electrode of the lithium ion battery is strengthened, deterioration of the conductive path due to repeated charging and discharging can be suppressed, and the battery life can be improved.

<Graphene>
Graphene is useful as a conductive auxiliary agent because it has a thin layer shape and has many conductive paths per unit weight, and it is easy to form a good conductive network in the electrode. Graphene, in a narrow sense, ^{refers to a sheet of sp 2-} bonded carbon atoms (single-layer graphene) having a thickness of one atom, but in the present specification, it also includes those having a flaky form in which single-layer graphene is laminated. It is called graphene. Similarly, graphene oxide is also referred to as a name including those having a laminated flaky form.

Further, in the present specification, graphene oxide having an O / C ratio of more than 0.4, which is the ratio of oxygen atoms to carbon atoms measured by X-ray photoelectron spectroscopy (XPS), is 0.4 or less. The thing is called graphene. Further, reduced graphene obtained by reducing graphene oxide and having an O / C ratio of 0.4 or less is also referred to as graphene.

Further, graphene and graphene oxide may be surface-treated for the purpose of improving dispersibility, etc., but in the present specification, graphene or graphene oxide to which such a surface treatment agent is attached is also included in "graphene". It shall be referred to as "graphene" or "graphene oxide".

The average thickness of graphene used in the graphene dispersion of the present invention is 0.3 nm or more and 10 nm or less. The graphene dispersion of the present invention uses a thin graphene in a range in which the average thickness is applied, thereby improving the followability of the graphene to the surface of the positive electrode active material while maintaining the conductivity, and facilitating the formation of a conductive path. Can be done. If the average thickness of graphene is less than 0.3 nm, defects are likely to occur, so that the conductivity is lowered and the battery life is shortened. On the other hand, when the average thickness of graphene exceeds 10 nm, the dispersibility is lowered and the coating film uniformity is lowered. Further, since the followability to the surface of the positive electrode active material is lowered, the formation of the conductive path becomes insufficient and the battery life is shortened. The average thickness of graphene is 8 nm or less from the viewpoint of making it easier to increase the solid content of the positive electrode paste, further improving the uniformity of the coating film, and more effectively forming the conductive path and further improving the battery life. It is preferably 6 nm or less, and more preferably 6 nm or less. Here, the average thickness of graphene in the graphene dispersion is observed by collecting graphene from the graphene dispersion and magnifying it to a viewing range of about 1 to 10 μm square using an atomic force microscope so that the graphene can be observed appropriately. However, it can be calculated by measuring the thickness of each of 10 randomly selected graphenes and calculating the arithmetic mean value thereof. The thickness of each graphene shall be the arithmetic mean value of the measured values of the thicknesses of five randomly selected points in each graphene.

The size of graphene in the direction parallel to the graphene layer is preferably 0.1 μm or more from the viewpoint of increasing the uniformity of the coating film of the positive electrode paste, increasing the contact area with the positive electrode active material, and further improving the battery life. , 0.5 μm or more is more preferable, and 1 μm or more is further preferable. On the other hand, the size of graphene in the direction parallel to the graphene layer further improves the dispersibility, improves the fluidity of the positive electrode paste, makes it easier to increase the solid content ratio, and further improves the uniformity of the coating film. Therefore, 100 μm or less is preferable, 50 μm or less is more preferable, and 20 μm or less is further preferable. Here, the size of graphene in the graphene dispersion in the direction parallel to the graphene layer is such that graphene is sampled from the graphene dispersion and an electron microscope is used so that the graphene is appropriately within the field of view. For 10 graphenes randomly selected by magnifying 500 to 50,000 times, 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 were determined. It can be calculated by measuring each and obtaining the arithmetic average value of the numerical 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 the above-mentioned range by refining graphene oxide or graphene after reduction by the method described later. Further, commercially available graphene oxide or graphene having a desired size may be used.

The element ratio (O / C ratio) of oxygen to carbon measured by X-ray photoelectron spectroscopy of graphene is from the viewpoint of further improving the dispersibility by the residual functional group and further improving the coating uniformity of the positive electrode paste. , 0.05 or more, more preferably 0.07 or more, still more preferably 0.08 or more. On the other hand, the O / C ratio is determined from the viewpoint of further improving the fluidity of the graphene dispersion liquid and from the viewpoint of restoring the π-electron conjugated structure by reduction to further improve the conductivity and further improving the coating film uniformity and the battery life. It is preferably 0.35 or less, more preferably 0.20 or less, and even more preferably 0.15 or less. Here, the O / C ratio of graphene in the graphene dispersion can be measured by collecting graphene from the graphene dispersion and using X-ray photoelectron spectroscopy (XPS). The peak near 284.3 eV was assigned to the C1s main peak based on carbon atoms, the peak near 533 eV was assigned to the O1s peak based on oxygen atoms, and the O / C ratio was calculated from the area ratio of each peak. Round off the third decimal place of the value to obtain the second decimal place. The O / C ratio of graphene can be easily adjusted to the above range by, for example, when the chemical stripping method is used, the degree of oxidation of graphene oxide as a raw material and the degree of reduction according to the reduction reaction conditions are adjusted. Can be done. Further, commercially available graphene oxide or graphene having a desired O / C ratio may be used.

As mentioned above, graphene and graphene oxide may be surface-treated, and in particular, surface-treating agents containing nitrogen atoms tend to enhance the dispersibility of graphene. Further, the surface treatment agent can enhance the interaction with polyvinyl alcohol, which will be described later, further enhance the effect of improving the dispersibility, and can further improve the binding force when used for the positive electrode of a lithium ion battery.

When graphene is treated with a surface treatment agent containing nitrogen atoms, the amount of surface treatment agent adhering to graphene is the atomic ratio of nitrogen to carbon (N / C ratio) measured using X-ray photoelectron spectroscopy. Can be obtained from. The N / C ratio of graphene is preferably 0.005 or more from the viewpoint of further improving the dispersibility, further improving the fluidity of the graphene dispersion liquid and the coating film uniformity of the positive electrode paste, and further improving the battery life. 0.006 or more is more preferable, and 0.008 or more is further preferable. On the other hand, the N / C ratio of graphene is preferably 0.020 or less from the viewpoint of further improving the fluidity of the graphene dispersion and further improving the conductivity and the battery life and the uniformity of the coating film, and is 0. .018 or less is more preferable, and 0.016 or less is further preferable. Here, the N / C ratio of graphene in the graphene dispersion can be measured by collecting graphene from the graphene dispersion and measuring it by X-ray photoelectron spectroscopy (XPS). The peak near 284.3 eV was assigned to the C1s main peak based on carbon atoms, the peak near 402 eV was assigned to the N1s peak based on nitrogen atoms, and the N / C ratio was calculated from the area ratio of each peak. Round the 4th decimal place of the value to the 3rd decimal place. The N / C ratio of graphene can be easily adjusted to the above-mentioned range by, for example, the amount of the surface treatment agent adhered to be described later.

The surface treatment agent is present on the surface of graphene, so that it exerts the effect of further enhancing the dispersibility of graphene. In the present specification, graphene in a state where such a surface treatment agent is attached is referred to as "surface treatment graphene". Here, in the present invention, the fact that the surface treatment agent is present on graphene means that the cleaning step of dispersing the surface treatment graphene in water having a mass ratio of 100 times and filtering it is repeated 5 times or more, and then freeze-dried. It means that the surface treatment agent remains in the surface treatment graphene after being dried by a method such as drying or spray drying. The residual surface treatment agent means that when the surface treatment graphene after drying is measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS), the surface treatment agent molecules are protonated in the positive secondary ion spectrum. It means that it can be detected in the form of a molecule. However, when the surface treatment agent is a neutralizing salt, it can be detected in the form of protons added to the surface treatment agent molecules from which the anion molecules have been removed. The chemical structure of the surface treatment agent contained in the surface treatment graphene can be specified by TOF-SIMS. The quantification of the surface treatment agent is carried out using a sample obtained by repeating the washing step of dispersing the surface treatment graphene in water having a mass ratio of 100 times and filtering it 5 times or more, and then freeze-drying.

As the surface treatment agent, a compound having an aromatic ring is preferable from the viewpoint of easily adsorbing on the graphene surface.

Further, the surface treatment agent preferably has an acidic group and / or a basic group.

As the acidic group, a group selected from a hydroxy group, a phenolic hydroxy group, a nitro group, a carboxyl group and a carbonyl group is preferable, and two or more of these may be present. Of these, phenolic hydroxy groups are preferred.

Examples of the compound having a phenolic hydroxy group and an aromatic ring include phenol, nitrophenol, cresol, and catechol. Some of the hydrogen in these compounds may be substituted. Among these, catechol and its derivatives are preferable from the viewpoint of adhesion to graphene and dispersibility in a dispersion medium, for example, catechol, dopamine hydrochloride, 3- (3,4-dihydroxyphenyl) -L-alanine, and the like. 4- (1-Hydroxy-2-aminoethyl) catechol, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, caffeic acid, 4-methylcatechol and 4-tert-butylpyrocatechol are preferable.

As the basic group, an amino group is preferable.

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

Compounds having an acidic group, a basic group and an aromatic ring are also preferable, and for example, dopamine hydrochloride and the like are preferable.

The graphene used in the present invention may be manufactured by a physical stripping method or may be manufactured by a chemical stripping method. When produced by the chemical stripping method, the method for producing graphene oxide is not particularly limited, and a known method such as the Hammers method can be used. Alternatively, commercially available graphene oxide may be purchased.

The chemical stripping method preferably includes a step of oxidatively stripping graphite to obtain graphene oxide (graphite stripping step) and a step of reducing (reducing step) in this order. If necessary, between the graphite stripping step and the reduction step, a step of adhering 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 (fine). (Chemicalization step) may be performed. When the surface-treated graphene is attached to graphene, the surface-treating agent may be attached to graphene after the reduction step, or may be subjected to the reduction treatment after being attached to graphene oxide. Further, when graphene is miniaturized, graphene oxide may be miniaturized, or graphene after reduction may be miniaturized. From the viewpoint of the uniformity of the reduction reaction, it is preferable to carry out the reduction step in a state where graphene oxide is refined, and it is preferable to carry out the reduction step before or during the reduction step. Therefore, it is preferable to include the graphite stripping step, the surface treatment step, the miniaturization step, and the reduction step in this order.

[Graphite peeling process]
First, graphite is oxidatively exfoliated to obtain graphene oxide. The degree of oxidation of graphene oxide can be adjusted by changing the amount of the oxidizing agent used in the oxidation reaction of graphite. Specifically, sodium nitrate and potassium permanganate can be used as the oxidizing agent. The larger the amount of the oxidizing agent for graphite used in the oxidation reaction, the higher the degree of oxidation, and the smaller the amount, the lower the degree of oxidation. The weight ratio of sodium nitrate to graphite is preferably 0.200 or more and 0.800 or less. The weight ratio of potassium permanganate to graphite is preferably 1.00 or more and 4.00 or less.

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

[Miniaturization process]
Next, graphene oxide is refined. Examples of the miniaturization method include a method of colliding a pressure-applied dispersion with a single ceramic ball, and a method of using a liquid-liquid shear type wet jet mill in which pressure-applied dispersions collide with each other to disperse. , A method of applying ultrasonic waves to the dispersion liquid and the like. In the miniaturization step, graphene oxide or graphene tends to be miniaturized as the treatment pressure and output are higher and the treatment time is longer. It is possible to adjust the size of graphene after reduction depending on the type of miniaturization treatment, treatment conditions, and treatment time in the miniaturization step. In order to adjust the size parallel to the graphene layer to the above range, the solid content concentration of graphene oxide or graphene in the miniaturization step is preferably 0.01% by weight or more and 2% by weight or less. Further, when performing ultrasonic treatment, the ultrasonic output is preferably 100 W or more and 3000 W or less.

[Reduction process]
Next, the finely divided graphene oxide is reduced. As the reduction method, chemical reduction is preferable. In the case of chemical reduction, examples of the reducing agent include an organic reducing agent and an inorganic reducing agent, but the inorganic reducing agent is more preferable because of the ease of cleaning after reduction.

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

Examples of the inorganic reducing agent include sodium dithionite, potassium dithionite, phosphorous acid, sodium borohydride, hydrazine and the like. Among them, sodium dithionite or potassium dithionite can be reduced while relatively retaining an acidic group, so graphene having high dispersibility in a solvent can be produced and is preferably used.

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

<Polyvinyl alcohol>
As described above, in the present invention, polyvinyl alcohol having a specific saponification rate is used together with thin graphene. The dispersibility of graphene is improved by the interaction between the hydroxyl group on polyvinyl alcohol and the oxygen-containing functional group on graphene and / or the functional group on the surface treatment agent, and the dispersibility of graphene is improved, and the dispersibility of graphene and polyvinyl alcohol is improved. The binding force is improved. Therefore, in the present invention, the hydroxyl group content of polyvinyl alcohol, that is, the saponification rate is important.

The saponification rate of polyvinyl alcohol used in the graphene dispersion of the present invention is 70% or more and 100% or less. By setting the saponification rate within such a range, the dispersibility can be improved by the interaction with graphene. If the saponification rate of polyvinyl alcohol is less than 70%, the interaction with graphene is insufficient, and the effect of improving dispersibility is insufficient, so that the battery life is shortened. From the viewpoint of further improving the dispersibility of graphene and the fluidity of the graphene dispersion liquid and further improving the battery life, the saponification rate of polyvinyl alcohol is preferably 75% or more, more preferably 80% or more, and 85% or more. More preferred. On the other hand, from the viewpoint of improving the solubility of polyvinyl alcohol in an organic solvent, the saponification rate of polyvinyl alcohol is preferably 99.9% or less, more preferably 98% or less, still more preferably 95% or less. Here, the saponification rate of polyvinyl alcohol can be determined according to JIS K6726-1994. In addition,% in the saponification rate means mol%.

The polyvinyl alcohol may be unmodified polyvinyl alcohol or modified polyvinyl alcohol.

Examples of the unmodified polyvinyl alcohol include the product name "Kuraray Poval" (registered trademark) "(Kuraray Co., Ltd.), the product name" Gosenol "(registered trademark)" (Mitsubishi Chemical Co., Ltd.), and the product name "Denka". Examples include "Poval" (registered trademark) "(Denka Co., Ltd.) and the product name" J-Poval "(Japan Vam & Poval Co., Ltd.).

Examples of the modified polyvinyl alcohol include those having a group selected from a carboxyl group, a sulfonic acid group, a cationic group (quaternary ammonium salt) and an ethylene oxide group in the side chain. Specifically, for example, the product name "Gosenex" (registered trademark) "(Mitsubishi Chemical Corporation) L, T, WO series and the like can be mentioned. Of these, modified polyvinyl alcohol having a carboxyl group or a sulfonic acid group in the side chain is more preferable from the viewpoint of further improving the dispersibility of graphene and the fluidity of the graphene dispersion and further improving the battery life. Specific examples thereof include the trade name "Gosenex" (registered trademark) "(Mitsubishi Chemical Corporation) T series or L series. Further, those having a sulfonic acid group are more preferable, and examples thereof include the trade name "Gosenex" (registered trademark) "(Mitsubishi Chemical Corporation) L series.

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

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

The graphene dispersion of the present invention contains 10 parts by weight or more and 300 parts by weight or less of the above-mentioned polyvinyl alcohol with respect to 100 parts by weight of the above-mentioned graphene. If the content of polyvinyl alcohol is less than 10 parts by weight, the effect of improving the dispersibility of polyvinyl alcohol cannot be sufficiently obtained, the fluidity of the graphene dispersion is lowered, and the uniformity of the coating film of the positive electrode paste and the battery life are lowered. To do. The content of polyvinyl alcohol is preferably 15 parts by weight or more, more preferably 20 parts by weight or more. On the other hand, if the content of polyvinyl alcohol exceeds 300 parts by weight, the electric resistance becomes high when the coating film is formed, so that the battery life is shortened. In addition, since the fluidity of the graphene dispersion is reduced, the solid content of the positive electrode paste and the uniformity of the coating film are reduced. The content of polyvinyl alcohol is preferably 200 parts by weight or less, more preferably 100 parts by weight or less.

The content of graphene and polyvinyl alcohol in the graphene dispersion of the present invention can be determined by the following method. First, graphene and polyvinyl alcohol are separated by filtration. The graphene content can be determined by thoroughly washing the graphene-containing filter medium with a solvent and then drying the filter medium. Further, the content of polyvinyl alcohol can be determined by distilling off the solvent from the filtrate (including polyvinyl alcohol), drying the mixture, and measuring the weight. However, if the raw material composition used for the graphene dispersion is known, it can be calculated from the raw material composition.

The graphene dispersion of the present invention preferably further contains a solvent. As the solvent, a polar solvent is preferable from the viewpoint of excellent solubility of polyvinyl alcohol. In particular, in lithium ion battery applications, a solvent selected from N, N-dimethylformamide, N-methylpyrrolidone and N, N-dimethylacetamide is preferable from the viewpoint of affinity with the binder polymer solution. Two or more of these may be contained. Among these, it is more preferable to contain N-methylpyrrolidone from the viewpoint of more effectively exerting the effect of improving the dispersibility of the surface treatment agent. Dispersibility is further enhanced by solvating N-methylpyrrolidone with the surface treatment agent attached to graphene.

The graphene dispersion of the present invention preferably has fluidity. In the present specification, the term “fluid” means that 1 g of graphene dispersion is dropped on one end of a clean and flat aluminum foil having a width of 5 cm and a length of 15 cm in a circular shape having a diameter of about 1 cm. By grasping the side where the graphene dispersion liquid is installed and pulling it up, the aluminum foil is erected vertically, held without vibration, and after standing for 10 minutes, the distance that the graphene dispersion liquid drips due to its own weight is 3 cm or more. Refers to something. The distance at which the graphene dispersion drips can be determined by measuring the distance before and after the graphene dispersion drips at the end of the graphene dispersion in the direction in which gravity is applied when the aluminum foil is erected vertically. The greater the dripping distance of the graphene dispersion, the higher the fluidity. From the viewpoint of facilitating the mixing of each material of the positive electrode paste and further improving the battery life, the distance that the graphene dispersion drips due to its own weight is more preferably 10 cm or more.

Next, the method for producing the graphene dispersion of the present invention will be described. Examples of the method for producing the graphene dispersion include a method in which a graphene powder or a graphene dispersion is mixed with a solution of polyvinyl alcohol in the solvent. From the viewpoint of further suppressing graphene aggregation, it is preferable to use a graphene dispersion.

As a mixing device for the polyvinyl alcohol solution and the graphene powder or graphene dispersion, a device capable of applying a shearing force is preferable. For example, a planetary mixer, "Fillmix" (registered trademark) (Primix Corporation), a self-revolving mixer. , A planetary ball mill, a three-roll mill, or the like can be used.

A high shear mixer may be used to perform a strong stirring step in which the shearing process is performed at a shear rate of 5,000 to 50,000 per second. By peeling the graphene with a high shear mixer in the strong stirring step, the stack of graphene can be eliminated and the average thickness of the graphene can be adjusted. As the high shear mixer, a thin film swirl system, a rotor / stator system, or a media mill system is preferable. Specifically, for example, "Filmix" (registered trademark) 30-30 type (Primix), "Clearmix" (registered trademark) CLM-0.8S (M Technique), "Laboster" (registered trademark). ) Mini LMZ015 (Ashizawa Finetech), Super Share Mixer SDRT0.35-0.75 (Satake Chemical Machinery Co., Ltd.) and the like.

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 or more per second, the peeling of graphene can be promoted, and the average thickness of graphene can be easily adjusted to the above range. The processing time of the strong stirring step is preferably 15 seconds to 300 seconds.

A graphene-containing film can be formed by applying the above graphene dispersion liquid onto a substrate. Examples of the graphene dispersion coating method include a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush coating method, a spray coating method, an inkjet method, and a flexographic method. Be done. Among these, the spray method or the coater method is preferable from the viewpoint of ease of application to the positive electrode paste and the positive electrode of the lithium ion battery.

Additives may be further mixed with the graphene dispersion of the present invention. Examples of the additive include a positive electrode active material, a binder, a cross-linking agent, a deterioration inhibitor, an inorganic filler and the like.

Next, the positive electrode paste of the present invention will be described. The positive electrode paste of the present invention contains a positive electrode active material, graphene having an average thickness of 0.3 nm or more and 10 nm or less, and polyvinyl alcohol having a saponification rate of 70% or more and 100% or less. Further, if necessary, a conductive auxiliary agent other than graphene may be contained.

Examples of graphene include those exemplified as the material of the graphene dispersion liquid. 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 obtained by collecting graphene from the positive electrode paste and using the method described above.

Examples of polyvinyl alcohol include those exemplified as the material of the graphene dispersion liquid.

The positive electrode active material is a material capable of electrochemically occluding and releasing lithium ions. For example, spinel-type structure lithium manganate (LiMn ₂ O ₄ ), rock salt-type structure lithium manganate (LiMnO ₂ ), lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), nickel with manganese cobalt. some substituted ternary _{_{_{(LiNi x Mn y Co 1-}}} x-y O 2), a portion of a cobalt-aluminum-substituted ternary _{_{_{(LiNi x Co y Al 1-}}} x-y O 2), V 2 O metal oxide such as ₅ active material and _TiS _2, MoS 2, metal compound-based active material such as NbSe _2, lithium iron phosphate (LiFePO ₄₎ of olivine structure, lithium manganese phosphate (LiMnPO _4), a solid solution system Examples include active materials. Two or more of these may be used. Among these, an active material containing lithium and nickel is preferable. Examples of the active material containing lithium and nickel include lithium nickelate (LiNiO ₂ ), a ternary system in which nickel is partially replaced with manganese and cobalt (LiNi _x Mn _y Co _1-x-y O ₂ ), and cobalt. aluminum some substituted ternary _{_{_{(LiNi x Co y Al 1-}}} x-y O 2) , etc. are preferred, it is possible to improve the energy density.

Further, when the positive electrode active material of the granulated product is used, graphene tends to come into contact with the surface while following the uneven shape of the surface of the positive electrode active material, so that the effect of the present invention is particularly remarkable. The granulated material means spherical particles obtained by spray-drying a slurry in which powder is dispersed. Positive electrode active materials used as granules include a ternary system (LiNi _x Mn _y Co _1-x-y O ₂ ) and LiNi _x Co _y Al _1-x-y O ₂ . In the granulated material, since the primary particles are aggregated to form the secondary particles, the surface tends to have an uneven shape, and it is necessary to increase the contact surface between the positive electrode active material and the conductive auxiliary agent. The effect is noticeable.

The particle size of the positive electrode active material is preferably 20 μm or less from the viewpoint of ease of forming a conductive path with graphene described above. In this specification, the particle size means the median diameter (D ₅₀ ). The median diameter can be measured by a laser scattering particle size distribution measuring device (for example, Microtrack HRX-100 manufactured by Nikkiso Co., Ltd.). Further, in the present specification, the "particle size of the positive electrode active material" means the secondary particle size when the positive electrode active material is a granulated material.

The positive electrode paste of the present invention contains 10 parts by weight or more and 300 parts by weight or less of the above-mentioned polyvinyl alcohol with respect to 100 parts by weight of the above-mentioned graphene. If the content of polyvinyl alcohol is less than 10 parts by weight, the effect of improving the dispersibility of polyvinyl alcohol cannot be sufficiently obtained, the fluidity of the positive electrode paste is lowered, and the uniformity of the coating film and the battery life are lowered. In addition, it becomes difficult to increase the solid content of the positive electrode paste. The content of polyvinyl alcohol is preferably 15 parts by weight or more, more preferably 20 parts by weight or more. On the other hand, when the content of polyvinyl alcohol exceeds 300 parts by weight, the content of the positive electrode active material and graphene is relatively lowered, and the electric resistance is likely to increase, so that the battery life is shortened. In addition, the solid content of the positive electrode paste is lowered, and the uniformity of the coating film is lowered. The content of polyvinyl alcohol is preferably 200 parts by weight or less, more preferably 100 parts by weight or less. Here, the solid content of the positive electrode paste of the present invention is a value obtained by placing 1 g of the positive electrode paste on a slide glass, heating and drying in a vacuum oven at 120 ° C. for 5 hours, and dividing the weight after drying by the weight before drying. Say.

The positive electrode paste of the present invention preferably contains the above-mentioned graphene in an amount of 0.05 parts by weight or more and 2.5 parts by weight or less with respect 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. The graphene content is preferably 0.1 parts by weight or more, 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, it is easy to form a conductive path, and the battery life can be further improved.

The content of the positive electrode active material, graphene and polyvinyl alcohol in the positive electrode paste of the present invention can be determined by the following method. The solid content is collected from the positive electrode paste by filtration, washed with a solvent, and then the weight of the dried powder is measured to determine the total weight of the positive electrode active material and the conductive additive. Further, the positive electrode active material in the solid content is dissolved with an acid such as hydrochloric acid and nitric acid, and the conductive auxiliary agent is separated by filtering. The content of the conductive additive can be measured by washing the filter with water, drying it, and measuring the weight. Further, the weight of the positive electrode active material can be obtained from the total weight of the positive electrode active material and the conductive auxiliary agent and the weight of the conductive auxiliary agent. If the conductive auxiliary agent contains graphene and other materials, determine the size of each conductive auxiliary agent from the SEM image of the powder, and sieve the sieve so that only graphene passes through or is captured. The content of graphene alone can be determined by using and recovering. When the sizes of the plurality of conductive auxiliaries are about the same and sieving is difficult, the content of each can be obtained from the ratio of the cross-sectional areas of the surface SEM images of the powder. However, if the raw material composition used for the positive electrode paste is known, it can be calculated from the raw material composition.

The positive electrode paste of the present invention may further contain a binder, a conductive auxiliary agent other than graphene, and other additives.

Examples of the binder 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 resin, polyamideimide resin, polyamide resin, polyacrylic acid, sodium polyacrylate, acrylic resin, polyacrylonitrile and the like can be mentioned. Two or more of these may be contained.

The content of the binder is preferably 0.2 parts by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the content of the positive electrode active material. By setting the content of the binder to 0.2 parts by weight or more, the battery life can be further improved. On the other hand, by setting the content of the binder to 2 parts by weight or less, the fluidity of the positive electrode paste can be further improved and the solid content ratio can be further increased. Since the graphene dispersion liquid and the positive electrode paste of the present invention have the characteristics of forming a self-supporting film and retaining the positive electrode active material, they do not have to contain a binder.

The conductive auxiliary agent other than graphene preferably has high electron conductivity. For example, carbon fiber, carbon black, acetylene black, carbon nanofiber, carbon nanotube, "VGCF" (registered trademark) -H (Showa Denko Co., Ltd.) (Manufactured); carbon materials such as copper, nickel, aluminum, silver and the like. Two or more of these may be contained. Among these, fiber-shaped carbon nanofibers, carbon nanotubes, or "VGCF" (registered trademark) -H (manufactured by Showa Denko Co., Ltd.) are preferable, and the conductivity in the thickness direction of the electrode can be improved.

The content of the conductive additive other than graphene is preferably 0.1 part by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the content of the positive electrode active material. By setting the content of the conductive auxiliary agent 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 auxiliary agent other than graphene to 2 parts by weight or less, the fluidity of the positive electrode paste can be further improved and the solid content ratio can be further improved.

As a method for analyzing the constituent materials and composition ratio of the positive electrode paste, the solid content is collected from the positive electrode paste by filtration, washed with a solvent, and then dried, and the powder is measured by X-ray diffraction to measure the type of positive electrode active material. Can be identified. When two or more types of positive electrode active materials are mixed, the mixing ratio of the positive electrode active materials is obtained by further analyzing the powder by energy dispersion X-ray spectroscopy or ICP-MS (inductively coupled plasma mass spectrometer). be able to. However, if the raw material composition used for the positive electrode paste is known, it can be calculated from the raw material composition.

Further, when the filtrate obtained by the above filtration is measured by FT-IR and CF absorption derived from PVDF is observed from the obtained spectrum, it can be determined that PVDF is contained as a binder. Further, the content of the binder in the positive electrode paste can be measured by drying the filtrate and measuring the weight. In addition, other binders can be identified by redissolving the dried filtrate in a heavy solvent and analyzing it using NMR (nuclear magnetic resonance spectroscopy).

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

In the present specification, the solid content of the positive electrode paste is a slide in the positive electrode paste after the viscosity measured by the above measurement method is adjusted to be 1800 mPa · s or more and 2,200 mPa · s or less. It refers to a value obtained by placing 1 g of positive electrode paste on glass, heating and drying in a vacuum oven at 120 ° C. for 5 hours, and dividing the weight after drying by the weight before drying.

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 the battery life. When the fluidity of the graphene dispersion is high, the mixed state of each material in the positive electrode paste is improved, the amount of the solvent required for adjusting the viscosity is reduced, and the solid content ratio of the positive electrode paste can be increased.

As a method for producing the positive electrode paste of the present invention, for example, the above-mentioned graphene dispersion of the present invention, the positive electrode active material, and a binder or a binder solution are mixed in a desired ratio, and then the viscosity is measured by the above-mentioned method. Then, a method of adding a solvent so as to be 1,800 mPa · s or more and 2,000 mPa · s or less and then mixing again can be mentioned. As another embodiment of the method for producing a positive electrode paste, for example, a graphene dispersion liquid containing no polyvinyl alcohol, a positive electrode active material, a binder or a binder solution, and a polyvinyl alcohol solution are mixed in a desired ratio, and then described above. Another method is to measure the viscosity by a method, add a solvent so that the viscosity is 1800 mPa · s or more and 2,000 mPa · s or less, and then mix again. Examples of the solvent include those exemplified as the solvent of the graphene dispersion liquid. Conductive aids other than graphene and other additives may be added before adjusting the viscosity.

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

Next, the positive electrode of the lithium ion battery of the present invention will be described. The positive electrode of the lithium ion battery of the present invention contains a positive electrode active material, graphene having an average thickness of 0.3 nm or more and 10 nm or less, and polyvinyl alcohol having a saponification rate of 70% or more and 100% or less. The positive electrode of the lithium ion battery is preferably one in which a dry film of the positive electrode paste is formed on the current collecting foil.

Examples of graphene include those exemplified as the material of the graphene dispersion liquid. 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 obtained by collecting graphene from the positive electrode of the lithium ion battery and using the method described above.

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

Examples of the method for manufacturing the positive electrode of the lithium ion battery of the present invention include a method of applying the positive electrode paste on the current collecting foil and drying it.

Examples of the method of applying the positive electrode paste on the current collector foil include a method of applying the positive electrode paste using a doctor blade, a die coater, a comma coater, a spray, or the like.

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

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

The content of graphene in the positive electrode of the lithium-ion battery and various physical properties and contents of the positive electrode active material can be measured as follows. First, the battery is disassembled in the Ar glove box, the electrodes are washed with dimethyl carbonate, and then vacuum dried in the side box of the inert glove box for 1 hour. Next, a spatula is used to peel off the positive electrode layer of the lithium ion battery from the current collector foil. The obtained powder of the positive electrode layer is dissolved in a solvent such as N-methylpyrrolidone or water and filtered to form a filtrate (positive electrode active material, conductive aid, solvent) and filtrate (solvent, etc.). To separate. The binder can be identified by drying the obtained filtrate, redissolving it in a heavy solvent, and analyzing it using NMR. Further, the solvent is removed by drying the obtained filter medium, and the total weight of the positive electrode active material and the conductive auxiliary agent is obtained by measuring the weight. The composition ratio of the positive electrode active material in the obtained powder can be analyzed in the same manner as in the case of the positive electrode paste. Further, the positive electrode active material is dissolved by using an acid such as hydrochloric acid and nitric acid, and the filtrate (conductive aid) and the filtrate (dissolved electrode active material, water) are separated by filtration. The content of the conductive additive can be measured by washing the filter with water, drying it, and measuring the weight. Further, the weight of the positive electrode active material can be obtained from the total weight of the positive electrode active material and the conductive auxiliary agent and the weight of the conductive auxiliary agent. The obtained conductive auxiliary agent can be analyzed in the same manner as in the case of the positive electrode paste described above.

The present invention will be described below with reference to examples. First, the evaluation method in each Example and Comparative Example will be described.

[Measurement example 1: Graphene thickness]
The graphene dispersion prepared in each Example and Comparative Example was diluted to 0.002% by weight with N-methylpyrrolidone. At this time, the surface-treated graphene was treated with "Filmix" (registered trademark) 30-30 type (Primix Corporation) at a rotation speed of 40 m / s (shear velocity: 20000 per second) for 60 seconds. The diluted solution was dropped onto a mica substrate, dried, and graphene was adhered onto the substrate. Graphene on the substrate was magnified and observed using an atomic force microscope (Dimension Icon; Bruker) in a field of view of about 1 to 10 μm square, and the thickness of each of 10 randomly selected graphenes was measured. .. The thickness of each graphene was taken as the arithmetic mean value of the measured values of the thicknesses at five randomly selected points in each graphene. The graphene thickness was calculated by obtaining the arithmetic mean value of the thicknesses of 10 graphenes. Since the thickness of graphene does not change in the graphene dispersion, the positive electrode paste, and the positive electrode of the lithium ion battery, it was measured using only the graphene dispersion.

[Measurement example 2: The size of graphene in the plane direction parallel to the graphene layer]
The graphene dispersion prepared in each Example and Comparative Example was diluted to 0.002% by weight with N-methylpyrrolidone. At this time, the surface-treated graphene was treated with "Filmix" (registered trademark) 30-30 type (Primix Corporation) at a rotation speed of 40 m / s (shear velocity: 20000 per second) for 60 seconds. The diluted solution was dropped onto a mica substrate, dried, and graphene was adhered onto the substrate. Graphene on the substrate was magnified and observed at a magnification of 30,000 times using an electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation), and 10 randomly selected graphenes were observed in a plane parallel to the graphene layer. The graphene layer is obtained by measuring the length (major axis) of the longest part in the direction and the length (minor axis) of the shortest part, respectively, and obtaining the arithmetic average value of the numerical value obtained by (major axis + minor axis) / 2. The size of the plane parallel to is calculated.

[Measurement Example 3: Measurement of O / C ratio and N / C ratio by X-ray photoelectron spectroscopy]
The reduced surface-treated graphene dispersion prepared in each Example and Comparative Example is filtered using a suction filter, then diluted to 0.5% by mass with water and suction-filtered, and the washing step is repeated 5 times. It was washed and then freeze-dried to obtain a surface-treated graphene powder. The obtained surface-treated graphene powder was subjected to photoelectron spectrum measurement using an X-ray photoelectron spectroscopic analyzer Quantera SXM (manufactured by PHI). The excited X-rays were monochromatic Al K _{α1 and 2} rays (1486.6 eV), the X-ray diameter was 200 μm, and the photoelectron escape angle was 45 °. The peak near 284.3 eV was assigned to the C1s main peak based on carbon atoms, the peak near 533 eV was assigned to the O1s peak based on oxygen atoms, and the peak near 402 eV was assigned to the N1s peak based on nitrogen atoms. The O / C ratio was calculated from the area ratio of the O1s peak and the C1s peak, and the third decimal place of the obtained value was rounded off to obtain the second decimal place. Further, N / C was calculated from the area ratio of the N1s peak and the C1s peak, and the fourth decimal place of the obtained value was rounded off to obtain the third decimal place.

[Measurement Example 4: Fluidity of Graphene Dispersion Liquid]
1 g of the graphene dispersion liquid prepared in each Example and Comparative Example was dropped onto one end of a clean and flat aluminum foil having a width of 5 cm and a length of 15 cm in a circular shape having a diameter of about 1 cm. By grasping the side where the graphene dispersion liquid of aluminum foil is installed and pulling it up, the aluminum foil stands vertically, holds it without vibration, and measures the distance that the graphene dispersion liquid drips due to its own weight after standing for 10 minutes. did. The distance at which the graphene dispersion drips was measured by measuring the distance between the end of the graphene dispersion in the direction in which gravity is applied when the aluminum foil is erected vertically, before and after the graphene dispersion drips. .. The case where the graphene dispersion was dripped was 10 cm or more was designated as A, the case where the graphene dispersion was 3 cm or more and less than 10 cm was designated as B, and the case where the graphene dispersion was less than 3 cm was designated as C.

[Measurement Example 5: Solidity Percentage of Positive Electrode Paste]
1 g of the positive electrode paste prepared in each Example and Comparative Example was weighed, placed on a slide glass, and heated and dried in a vacuum oven at 120 ° C. for 5 hours. The weight after drying was measured, and the value obtained by rounding off the first decimal place of the value divided by the weight before drying to obtain an integer was defined as the positive paste solid content.

[Measurement example 6: Coating film uniformity]
5 g of the positive electrode paste prepared in each Example and Comparative Example was applied to an aluminum foil (thickness 18 μm) using a doctor blade (300 μm), dried at 80 ° C. for 15 minutes, vacuum dried at 120 ° C. for 2 hours, and coated. A membrane was prepared. The number of places where defects such as faintness, cracks, foamy defects and hangnail of the coating film were found by visually inspecting the appearance of 1 cm square per 10 locations randomly selected from the coating film. Was ranked based on the following indicators.
A: No defects are seen, B: 1 or 2 defects, C: 3 to 5 defects, D: 6 or more defects.

[Measurement example 7: Battery life (battery capacity retention rate)]
For the 2032 type coin batteries produced in each Example and Comparative Example, charge / discharge measurements were performed three times each in the order of rates 0.1C, 1C, and 5C at an upper limit voltage of 4.2 V and a lower limit voltage of 3.0 V, and then 2C. The charge / discharge measurement was further performed 291 times, a total of 300 times, the battery capacity of the 300th time was measured, and the ratio (percentage) to the battery capacity of the first time was calculated, which was used as the battery capacity retention rate.

[Synthesis Example 1: Preparation of Graphene Oxide]
The raw material was 1500 mesh natural graphite powder (Shanghai Ichiho Ishikumi Co., Ltd.). 220 ml of 98% concentrated sulfuric acid, 5 g of sodium nitrate, and 30 g of potassium permanganate were added to 10 g of natural graphite powder in an ice bath, and the mixture was mechanically stirred for 1 hour while maintaining the temperature of the mixture at 20 ° C. or lower. The mixture was removed from the ice bath and stirred in a 35 ° C. water bath for 4 hours. Then, 500 ml of ion-exchanged water was added, and the obtained suspension was stirred at 90 ° C. for another 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 obtained graphene oxide dispersion was filtered while hot, and the filtrate was washed with a dilute hydrochloric acid solution to remove metal ions, and then washed with ion-exchanged water to remove the acid. Graphene oxide was prepared by repeating washing with ion-exchanged water until the pH reached 7. The element ratio (O / C ratio) of the prepared graphene oxide to the carbon atom of the oxygen atom measured by X-ray photoelectron spectroscopy was 0.53.

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

[Example 1]
(Preparation of surface-treated graphene N-methylpyrrolidone dispersion paste)
Graphene oxide prepared in Synthesis Example 1 is diluted to a concentration of 30 mg / ml with ion-exchanged water and treated with Homo Disper 2.5 type (Primix Corporation) at a rotation speed of 3,000 rpm for 30 minutes to make it uniform. A graphene oxide dispersion was obtained. 20 ml of the obtained graphene oxide dispersion and 0.3 g of dopamine hydrochloride as a surface treatment agent were mixed and treated with Homo Disper 2.5 type (Primix Corporation) at a rotation speed of 3,000 rpm for 60 minutes. The treated graphene oxide dispersion was applied with ultrasonic waves at an output of 300 W for 30 minutes (miniaturization step) using an ultrasonic device UP400S (heelscher). The graphene oxide dispersion liquid that has undergone the micronization step is diluted to a concentration of 5 mg / ml using ion-exchanged water, 0.3 g of sodium dithionite is added to 20 ml of the diluted dispersion liquid, and homogen is used in a water bath at 40 ° C. Using a Disper 2.5 type (Primix Co., Ltd.), the mixture was stirred at a rotation speed of 3,000 rpm for 1 hour. Then, the mixture was filtered using a vacuum suction filter, and water was further added to the filtrate to dilute it to 0.5% by weight, and the washing step of suction filtration was repeated 5 times to obtain a graphene aqueous dispersion. .. N-Methylpyrrolidone (hereinafter, NMP) was added to the obtained aqueous graphene dispersion so that the graphene concentration was 0.5% by weight, and "Filmix" (registered trademark) 30-30 type (Primix Corporation) was added. Was treated at a rotation speed of 40 m / s (shear velocity: 20,000 per second) for 60 seconds. After the treatment, the solvent was removed by suction filtration under reduced pressure. In order to further remove water, NMP was added to the filter medium so that the graphene concentration was 0.5% by weight, and the mixture was treated using Homo Disper 2.5 type (Primix Corporation) at a rotation speed of 3000 rpm for 30 minutes. The steps of diluting and suction-filtering under reduced pressure until the filtrate did not drop were repeated twice to obtain an NMP-dispersed paste containing 5.0% by weight of surface-treated graphene as a filtrate.

(Preparation of polyvinyl alcohol solution)
To 95% by weight of NMP, 5% by weight of polyvinyl alcohol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., saponification rate 88%, degree of polymerization 500) was added, and heated to 90 ° C. with stirring of a magnetic stirrer in a closed container. Then, polyvinyl alcohol was completely dissolved to obtain a 5 wt% polyvinyl alcohol / NMP solution.

(Preparation of graphene dispersion)
After adding 5 g of a 5 wt% polyvinyl alcohol / NMP solution to 20 g of the NMP-dispersed paste containing 5.0 wt% of the surface-treated graphene obtained as described above, “Filmix” (registered trademark) 30-30 Using a mold (Primix Corporation), the mixture was stirred at a rotation speed of 40 m / s (shear velocity: 20,000 per second) for 15 minutes (strong stirring step) to obtain a graphene dispersion. The solid content concentration of the obtained graphene dispersion was 4% by weight, and the polyvinyl alcohol content was 25 parts by weight with respect to 100 parts by weight of graphene.

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. Further, the O / C ratio and the N / C ratio were measured according to Measurement Example 3, and the fluidity of the graphene dispersion was evaluated according to Measurement Example 4. The results are shown in Table 1.

(Preparation of positive electrode paste)
_{LiNi 0.5} Co _0.2 Mn _0.3 O ₂ 20 g as the positive electrode active material, 5 g of the 4 wt% graphene dispersion as the conductive auxiliary agent, and 2 g of the 10 wt% PVDF / NMP solution as the binder. The mixture was mixed at a rotation speed of 2000 rpm for 15 minutes. NMP was added to the resulting mixture. Here, using the Brookfield viscometer LVDVII +, the rotor No. The amount of NMP to be added was adjusted so that the viscosity of the mixture whose viscosity was measured under the conditions of 6, 60 rpm and 25 ° C. was 2,000 mPa · s. This mixture was mixed again using a rotation mixer at a rotation speed of 2,000 rpm for 15 minutes to obtain a positive electrode paste.

With respect to the obtained positive electrode paste, the solid content ratio of the positive electrode paste was measured according to Measurement Example 5, and the uniformity of the coating film was evaluated according to Measurement Example 6. The results are shown in Table 3.

(Making coin batteries)
The obtained positive electrode paste was applied onto an aluminum foil (thickness 18 μm) ^{using a doctor blade so that the amount of positive electrode paste after drying was 18 mg / cm 2} , dried at 80 ° C. for 15 minutes, and then 120 ° C. Vacuum drying was carried out for 2 hours to obtain an electrode plate.

The prepared electrode plate was cut out into a circle having a diameter of 15.9 mm and used as a positive electrode. As a counter electrode, a coating film composed of 98 parts by weight of graphite, 1 part by weight of sodium carboxymethyl cellulose and 1 part by weight of SBR aqueous dispersion was formed on a copper foil, and cut into a circle having a diameter of 16.1 mm to form a negative electrode. Cellguard # 2400 (manufactured by Cellguard) cut into a circle with a diameter of 17 mm was used as a separator. A solvent of ethylene carbonate: diethyl carbonate = 7: 3 containing 1 M of _{LiPF 6 was used as the electrolytic solution.} A 2032 type coin battery was produced by sandwiching the separator and the electrolytic solution between the positive electrode and the negative electrode, adding 3 mL of the electrolytic solution, and caulking. The battery life (battery capacity retention rate) of the obtained coin battery was measured according to Measurement Example 7.

[Example 2]
A graphene dispersion was obtained in the same manner as in Example 1 except that the treatment time of the strong stirring step was extended to 30 minutes. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 3]
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.

[Example 4]
A graphene dispersion was obtained in the same manner as in Example 1 except that the miniaturization step was extended to 120 minutes. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 5]
A graphene dispersion was obtained in the same manner as in Example 1 except that the miniaturization step was extended to 90 minutes. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 6]
A graphene dispersion was obtained in the same manner as in Example 1 except that the miniaturization step was shortened to 10 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.

[Example 7]
Instead of the graphene oxide prepared in Synthesis Example 1, the graphene oxide prepared in Synthesis Example 2 was used, and a graphene dispersion was obtained in the same manner as in Example 1 except that the miniaturization step was not performed. 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.

[Example 8]
A graphene dispersion was obtained in the same manner as in Example 1 except that the amount of sodium dithionite used was reduced to 0.1 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 9]
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.

[Example 10]
A graphene dispersion was obtained in the same manner as in Example 1 except that the amount of sodium dithionite used was reduced to 0.01 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 11]
A graphene dispersion was obtained in the same manner as in Example 1 except that the dopamine hydrochloride was changed to catechol. 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.

[Example 12]
A graphene dispersion was obtained in the same manner as in Example 1 except that the amount of dopamine hydrochloride was changed to benzylamine hydrochloride and the amount used was reduced to 0.1 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 13]
A graphene dispersion was obtained in the same manner as in Example 1 except that the amount of dopamine hydrochloride was changed to phenylethylamine hydrochloride and the amount used was reduced to 0.2 g. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 14]
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.

[Example 15]
In the preparation of the graphene dispersion, the same procedure as in Example 1 except that 2 g of 5 wt% polyvinyl alcohol / NMP solution was added to 20 g of NMP dispersion paste containing 5.0 wt% of surface-treated graphene and 3 g of NMP was added. A graphene dispersion was obtained. 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.

[Example 16]
A graphene dispersion was prepared in the same manner as in Example 1.

In the preparation of the polyvinyl alcohol solution, 4 g of polyvinyl alcohol and 16 g of NMP are heated to 90 ° C. under stirring of a magnetic stirrer in a closed container to partially dissolve the polyvinyl alcohol to obtain a 20 wt% polyvinyl alcohol / NMP mixture. It was.

In the preparation of the graphene dispersion, 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene was added to 5 g of the obtained 20 wt% polyvinyl alcohol / NMP solution, and the mixture was heated again at 90 ° C. for 8 hours, and then the spatula. The whole is blended with, and treated with "Fillmix" (registered trademark) 30-30 type (Primix) at a rotation speed of 40 m / s (shear speed: 20,000 per second) for 60 minutes to obtain a graphene dispersion. It was. 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.

[Example 17]
A graphene dispersion was prepared in the same manner as in Example 1.

In the preparation of the polyvinyl alcohol solution, 10 g of polyvinyl alcohol and 10 g of NMP are heated to 90 ° C. under stirring of a magnetic stirrer in a closed container to partially dissolve the polyvinyl alcohol to obtain a 50 wt% polyvinyl alcohol / NMP mixture. It was.

In the preparation of the graphene dispersion, 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene was added to 5 g of the obtained 50 wt% polyvinyl alcohol / NMP mixture, and the mixture was heated again at 90 ° C. for 8 hours, and then the spatula. The whole is blended with, and treated with "Filmix" (registered trademark) 30-30 type (Primix) at a rotation speed of 40 m / s (shear speed: 20,000 per second) for 60 minutes to obtain a graphene dispersion. It was. 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.

[Example 18]
In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 75% and a degree of polymerization of 500 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). 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.

[Example 19]
In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 98% and a degree of polymerization of 500 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). 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.

[Example 20]
In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 88% and a degree of polymerization of 1500 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). 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.

[Example 21]
In the preparation of polyvinyl alcohol, a graphene dispersion was obtained in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 88% and a degree of polymerization of 3500 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). 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.

[Example 22]
In the preparation of polyvinyl alcohol, graphene was prepared in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 72% and a degree of polymerization of 500 (manufactured by Japan Vam & Poval Co., Ltd., product name “JR-05”). A dispersion was obtained. 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.

[Example 23]
In the preparation of the graphene dispersion, 5% by weight polyvinyl alcohol / NMP 2 g and 3 g of NMP were added to 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene, and the polyvinyl alcohol content was adjusted to 100 parts by weight of graphene. A graphene dispersion was obtained in the same manner as in Example 22 except that the amount was 10 parts by weight. 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.

[Example 24]
In the preparation of polyvinyl alcohol, graphene was prepared in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 82% and a degree of polymerization of 250 (manufactured by Japan Vam & Poval Co., Ltd., product name “JMR-10H”). A dispersion was obtained. 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.

[Example 25]
In the preparation of the graphene dispersion, 5% by weight polyvinyl alcohol / NMP 2 g and 3 g of NMP were added to 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene, and the polyvinyl alcohol content was adjusted to 100 parts by weight of graphene. A graphene dispersion was obtained in the same manner as in Example 24 except that the amount was 10 parts by weight. 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.

[Example 26]
In the preparation of polyvinyl alcohol, graphene was prepared in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 94% and a degree of polymerization of 500 (manufactured by Japan Vam & Poval Co., Ltd., product name “JT-05”). A dispersion was obtained. 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.

[Example 27]
In the preparation of the graphene dispersion, 5% by weight polyvinyl alcohol / NMP 2 g and 3 g of NMP were added to 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene, and the polyvinyl alcohol content was adjusted to 100 parts by weight of graphene. A graphene dispersion was obtained in the same manner as in Example 26 except that the amount was 10 parts by weight. 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.

[Example 28]
In the preparation of polyvinyl alcohol, the same as in Example 1 except that the polyvinyl alcohol has a saponification rate of 98.5% and a degree of polymerization of 500 (manufactured by Japan Vam & Poval Co., Ltd., product name “JF-05”). A graphene dispersion was obtained. 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.

[Example 29]
In the preparation of the graphene dispersion, 5% by weight polyvinyl alcohol / NMP 2 g and 3 g of NMP were added to 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene, and the polyvinyl alcohol content was adjusted to 100 parts by weight of graphene. A graphene dispersion was obtained in the same manner as in Example 28 except that the amount was 10 parts by weight. 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.

[Example 30]
In the preparation of polyvinyl alcohol, the same as in Example 1 except that the polyvinyl alcohol has a saponification rate of 99.3% and a degree of polymerization of 240 (manufactured by Japan Vam & Poval Co., Ltd., product name “JMR-10HH”). A graphene dispersion was obtained. 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.

[Example 31]
In the preparation of the graphene dispersion, 5% by weight polyvinyl alcohol / NMP 2 g and 3 g of NMP were added to 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene, and the polyvinyl alcohol content was adjusted to 100 parts by weight of graphene. A graphene dispersion was obtained in the same manner as in Example 30 except that the amount was 10 parts by weight. 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.

[Example 32]
In the preparation of polyvinyl alcohol, except that polyvinyl alcohol was an anion-modified polyvinyl alcohol having a saponification rate of 87.8% and a degree of polymerization of 200 (manufactured by Mitsubishi Chemical Corporation, trade name "Gosenex" (registered trademark) L-3266 "). 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.

[Example 33]
In the preparation of the graphene dispersion, 5% by weight polyvinyl alcohol / NMP 2 g and 3 g of NMP were added to 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene, and the polyvinyl alcohol content was adjusted to 100 parts by weight of graphene. A graphene dispersion was obtained in the same manner as in Example 32 except that the amount was 10 parts by weight. 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.

[Example 34]
In the preparation of the graphene dispersion, a graphene dispersion was prepared without using the polyvinyl alcohol / NMP solution, and in the preparation of the positive electrode paste, 1.25 g of the polyvinyl alcohol / NMP of Example 1 was added in the same manner as in Example 1. A positive electrode paste was prepared. Using the obtained positive electrode paste, a 2032 type coin battery was produced in the same manner as in Example 1.

[Example 35]
In the preparation of the positive electrode paste, 1.25 g of polyvinyl alcohol / NMP of Example 32 was added to prepare a positive electrode paste in the same manner. Using the obtained positive electrode paste, a 2032 type coin battery was produced in the same manner as in Example 1.

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

[Comparative Example 2]
A graphene dispersion was obtained in the same manner as in Example 1 except that polyvinyl alcohol was changed to polyvinylpyrrolidone K-60 (manufactured by Tokyo Chemical Industry Co., Ltd.). Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Comparative Example 3]
In the preparation of the graphene dispersion liquid, the same procedure as in Example 1 was carried out except that 5% by weight polyvinyl alcohol / NMP 1 g was added to 20 g of the NMP dispersion paste containing 5.0% by weight of the surface-treated graphene, and 12.3 g of NMP was added. A graphene dispersion was obtained. 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.

[Comparative Example 4]
A graphene dispersion was prepared in the same manner as in Example 1.

In the preparation of the graphene dispersion, 20 g of NMP dispersion paste containing 5.0% by weight of surface-treated graphene was added to 5 g of polyvinyl alcohol powder, mixed at a rotation speed of 2000 rpm for 15 minutes using a rotation speed mixer, and 16 at 90 ° C. After heating for an hour, the whole is blended with a spatula and treated with "Fillmix" (registered trademark) 30-30 type (Primix) at a rotation speed of 40 m / s (shear speed: 20,000 per second) for 60 minutes. , Graphene dispersion was obtained. 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.

[Comparative Example 5]
A graphene dispersion was obtained in the same manner as in Example 1 except that the strong stirring step was not performed in the preparation of the graphene dispersion. Using the obtained graphene dispersion, a positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Comparative Example 6]
In the preparation of polyvinyl alcohol, graphene was prepared in the same manner as in Example 1 except that the polyvinyl alcohol had a saponification rate of 5% and a degree of polymerization of 110 (manufactured by Japan Vam & Poval Co., Ltd., product name “JMR-3L”). A dispersion was obtained. 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.

[Comparative Example 7]
In the preparation of polyvinyl alcohol, the same as in Example 1 except that the polyvinyl alcohol had a saponification rate of 9.9% and a degree of polymerization of 230 (manufactured by Japan Vam & Poval Co., Ltd., product name “JMR-10LL”). A graphene dispersion was obtained. 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.

[Comparative Example 8]
In the preparation of polyvinyl alcohol, the same as in Example 1 except that the polyvinyl alcohol has a saponification rate of 37.8% and a degree of polymerization of 400 (manufactured by Japan Vam & Poval Co., Ltd., product name “JMR-20L”). A graphene dispersion was obtained. 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.

[Comparative Example 9]
In the preparation of polyvinyl alcohol, the same as in Example 1 except that the polyvinyl alcohol has a saponification rate of 65.4% and a degree of polymerization of 230 (manufactured by Japan Vam & Poval Co., Ltd., product name “JMR-20M”). A graphene dispersion was obtained. 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.

The compositions of each example and comparative example are shown in Tables 1 and 2, and the evaluation results are shown in Table 3.

[Example 36]
In the preparation of the positive electrode paste, the positive electrode paste was prepared in the same manner as in Example 1 except that the amount of the graphene dispersion used was reduced to 0.15 g (the surface-treated graphene content was 0.03 parts by weight with respect to 100 parts by weight of the positive electrode active material). And a 2032 type coin battery was manufactured.

[Example 37]
In the preparation of the positive electrode paste, the positive electrode paste was prepared in the same manner as in Example 1 except that the amount of the graphene dispersion used was reduced to 0.5 g (the surface-treated graphene content was 0.1 part by weight with respect to 100 parts by weight of the positive electrode active material). And a 2032 type coin battery was manufactured.

[Example 38]
A graphene dispersion was prepared in the same manner as in Example 1.

In the preparation of the surface-treated graphene NMP dispersion paste, in the second step of suction-filtering under reduced pressure after diluting with NMP, suction filtration was continued for 15 minutes even after the filtrate did not fall, and 6.1 weight of surface-treated graphene was used as a filter. An NMP-dispersed paste containing% was obtained.

In the preparation of the graphene dispersion, 20 g of NMP dispersion paste containing 6.1% by weight of surface-treated graphene was added to 0.3 g of polyvinyl alcohol powder, mixed at a rotation speed of 2000 rpm for 15 minutes using a rotation speed mixer, and mixed at 90 ° C. After heating for 16 hours in, blend the whole with a spatula, and use "Fillmix" (registered trademark) 30-30 type (Primix) at a rotation speed of 40 m / s (shear speed: 20,000 per second) for 60 minutes. The treatment was performed to obtain a graphene dispersion. The graphene solid content concentration of the obtained graphene dispersion was 6.0% by weight, and the polyvinyl alcohol content was 25 parts by weight with respect to 100 parts by weight of graphene.

In the preparation of the positive electrode paste, the amount of the graphene dispersion having a solid content concentration of 6.0% by weight of graphene was set to 6.7 g (2 parts by weight of the surface-treated graphene content with respect to 100 parts by weight of the positive electrode active material). A positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1.

[Example 39]
In the preparation of the positive electrode paste, the amount of the graphene dispersion having a solid content concentration of 6.0% by weight was increased to 10 g, and 0.2 g of PVDF was added as a powder (surface-treated graphene content 3 with respect to 100 parts by weight of the positive electrode active material). A positive electrode paste and a 2032 type coin battery were produced in the same manner as in Example 1 except for the part by weight.

Table 4 shows the main compositions and evaluation results of the positive electrode pastes of Examples 36 to 39.

Claims

A graphene dispersion containing graphene and polyvinyl alcohol, wherein the average thickness of the graphene is 0.3 nm or more and 10 nm or less, the saponification rate of the polyvinyl alcohol is 70% or more and 100% or less, and the graphene 100 A graphene dispersion containing 10 parts by weight or more and 300 parts by weight or less of the polyvinyl alcohol with respect to parts by weight.
The graphene dispersion according to claim 1, wherein the element ratio (O / C ratio) of oxygen to carbon measured by X-ray photoelectron spectroscopy of the graphene is 0.05 or more and 0.35 or less.
The graphene dispersion according to claim 1 or 2, wherein the element ratio (N / C ratio) of nitrogen to carbon measured by X-ray photoelectron spectroscopy of the graphene is 0.005 or more and 0.020 or less.
The graphene dispersion according to any one of claims 1 to 3, further comprising a solvent selected from the group consisting of N, N-dimethylformamide, N-methylpyrrolidone and N, N-dimethylacetamide.
A positive electrode paste containing a positive electrode active material, graphene and polyvinyl alcohol, wherein the average thickness of the graphene is 0.3 nm or more and 10 nm or less, the saponification rate of the polyvinyl alcohol is 70% or more and 100% or less, and A positive electrode paste containing 10 parts by weight or more and 300 parts by weight or less of the polyvinyl alcohol with respect to 100 parts by weight of the graphene.
The positive electrode paste according to claim 5, wherein the graphene is contained in an amount of 0.05 parts by weight or more and 2.5 parts by weight or less with respect to 100 parts by weight of the positive electrode active material.
The positive electrode paste according to claim 5 or 6, wherein the element ratio (O / C ratio) of oxygen to carbon measured by X-ray photoelectron spectroscopy of graphene is 0.05 or more and 0.35 or less.
The positive electrode paste according to any one of claims 5 to 7, wherein the elemental ratio (N / C ratio) of nitrogen to carbon measured by X-ray photoelectron spectroscopy of graphene is 0.005 or more and 0.020 or less. ..
The positive electrode paste according to any one of claims 5 to 8, wherein the positive electrode active material contains lithium and nickel.
A lithium ion battery positive electrode containing a positive electrode active material, graphene and polyvinyl alcohol, wherein the average thickness of the graphene is 0.3 nm or more and 10 nm or less, and the saponification rate of the polyvinyl alcohol is 70% or more and 100% or less. A positive electrode of a lithium ion battery containing 10 parts by weight or more and 300 parts by weight or less of the polyvinyl alcohol with respect to 100 parts by weight of the graphene.
The lithium ion battery positive electrode according to claim 10, wherein the graphene is contained in an amount of 0.05 parts by weight or more and 2.5 parts by weight or less with respect to 100 parts by weight of the positive electrode active material.
The lithium ion battery positive electrode according to claim 10 or 11, wherein the element ratio (O / C ratio) of oxygen to carbon measured by X-ray photoelectron spectroscopy of graphene is 0.05 or more and 0.35 or less.
The lithium ion according to any one of claims 10 to 12, wherein the element ratio (N / C ratio) of nitrogen to carbon measured by X-ray photoelectron spectroscopy of graphene is 0.005 or more and 0.020 or less. Battery positive electrode.
The lithium ion battery positive electrode according to any one of claims 10 to 13, wherein the positive electrode active material contains lithium and nickel.