JP6645260B2 - Nanographene, nanographene-electrode active material composite particles, paste for lithium ion battery electrode, and lithium ion battery electrode - Google Patents

Description

  The present invention relates to nanographene, nanographene-electrode active material composite particles using the same, a paste for a lithium ion battery electrode, and a lithium ion battery electrode.

  Graphite has long been known as a carbon material with many excellent properties such as high electrical conductivity, heat resistance, light weight, low thermal expansion, high thermal conductivity, and self-lubricating properties, and is used in many applications. . In recent years, as a new carbon-based material similar to graphite, graphite sheets obtained by carbonizing and firing resin materials, fullerenes called carbon nanomaterials, and carbon nanotubes have been newly developed as materials with many excellent properties similar to graphite. It has been found and is being used for a wide range of applications. Very recently, a method for producing graphene with a structure in which a number of nanometer-order benzene ring structures that form part of graphite are spread in the plane direction has been discovered, and research and development have been conducted to use them as new materials. It is becoming active.

  Graphene is a material with many excellent physical properties, such as high strength / high elasticity, high mobility, high gas barrier properties, and high flexibility, in addition to having high electrical conductivity and high thermal conductivity like graphite. Because it is chemically stable, it is expected to be widely applied in fields such as battery materials, energy storage materials, electronic devices, and composite materials.

  Examples of a method for producing graphene include a mechanical exfoliation method, a chemical vapor deposition (CVD) method, and a crystal epitaxy growth (CEG) method. However, these methods have low productivity and are not suitable for mass production. In contrast, the oxidation-reduction method (a method of producing graphene by a reduction reaction after obtaining graphite oxide or graphite oxide by oxidizing natural graphite) enables large-scale synthesis of graphene, and commercializes graphene. This is a very important technique to do.

  Graphene has a high potential as a conductive material for a conductive resin or a battery electrode because it has high conductive performance and has a flaky shape, so that the number of conductive paths can be increased. Particularly in lithium ion batteries, the conductivity of the surface of the electrode active material particles is important, and a conductive material having a flaky and flexible structure, such as graphene, adheres to the surface of the active material particles to increase the surface conductivity. There is hope that it can be improved.

  In order to increase the capacity of a lithium ion battery, it is essential to increase the capacity density of the positive electrode. In recent years, many active materials, such as olivine-based positive electrode active materials and solid solution-based active materials, which have not been put to practical use due to low conductivity despite high capacity, have been studied. In order to put these positive electrode active materials into practical use, a technique for imparting conductivity to the positive electrode is required. If a highly conductive conductive agent such as graphene is applied to a lithium ion battery, there is an expectation that the capacity of the lithium ion battery can be increased. Further, even for lithium cobalt oxide having high conductivity and a ternary positive electrode active material, improving the surface conductivity leads to improvement in rate characteristics and cycle characteristics.

  However, since graphene has a high specific surface area, it tends to aggregate due to the interaction between graphenes. In addition, since it is flaky and flexible, it is easily folded, and self-aggregation occurs, making it difficult to sufficiently exert a potential as a conductive material.

  Patent Document 1 discloses a method of mixing graphene oxide and a positive electrode active material of a lithium ion battery and then reducing the mixture. Patent Literature 2 discloses a method in which a positive electrode paste of a lithium ion battery containing graphene oxide is applied to a current collector and dried, and then the graphene oxide is thermally reduced to form an electrode. Patent Document 3 discloses a method for producing graphene by exfoliating graphite in an ultrasonic wave or a supercritical fluid, and a method for oxidizing and reducing spheroidal graphite.

  Patent Literature 4 and Non-Patent Literature 1 disclose a technique for manufacturing fine graphene oxide by oxidizing carbon nanotubes. Patent Document 5 discloses a technique for attaching nanoscale graphene to an active material.

JP 2012-99467 A JP 2013-030463 A JP-T-2013-516037 JP 2013-069993 A JP 2012-099468 A

J. Luo, et. al. , Journal of the American Chemical Society, 2010, 132, 17667.

  When graphene oxide is kneaded with a lithium ion positive electrode active material without any fine treatment as in Patent Document 1 or Patent Document 2, the graphene oxide is too large, aggregation of the graphene oxides is not suppressed, and the graphene oxide is folded. And a sufficient conductive path cannot be obtained.

  In Patent Literature 3, graphite is exfoliated in an ultrasonic wave or a supercritical fluid. However, graphite has high mechanical strength and cannot be easily exfoliated or crushed and cannot be miniaturized. Even if micrometer-scale spherical graphite is oxidized, only micrometer-scale graphene oxide can be obtained.

  In Patent Literature 4 and Non-Patent Literature 1, fine graphene is produced by oxidizing and reducing fibrous carbon. The oxidation technique is a well-known technique called the Hammers method, and is obtained from this method. Graphene obtained by reducing the obtained graphene oxide has low ionic conductivity because the functionalization ratio of graphene is low.

  Patent Document 5 discloses a method of attaching fine graphene to an active material. However, since the ion conductivity is low, good rate characteristics cannot be obtained even when applied to a lithium ion battery.

The present invention for solving the above problems has a feature that a size in a plane direction is 20 nm or more and 100 nm or less, a thickness is 10 nm or less, and a functionalization ratio measured by X-ray photoelectron spectroscopy is 0.35 or more and 0 or more. .80 Ri der less, nanographene element ratio of nitrogen atoms to carbon atoms (N / C) is Ru der 0.005 or more and 0.05 or less.

  Since the nanographene of the present invention has a sufficiently small structure and an appropriate functional group remains, graphene is less likely to be in contact with each other and is less likely to aggregate. In addition, high conductive performance can be obtained without the graphene itself being folded. By having such properties, nanographene has good dispersibility in a resin or an electrode, and can be easily adsorbed on the surface of inorganic particles. In addition, since nanographene has an appropriate functionalization rate, it has higher ionic conductivity than conventional graphene. By adsorbing such nanographene on the surface of the lithium-ion battery active material, it is possible to achieve both high electron conductivity and high ion conductivity on the surface of the active material. This makes it possible to provide a lithium ion battery electrode having excellent discharge performance.

<Nanographene>
The nanographene of the present invention is a flaky structure in which single-layer graphene is stacked, and has a planar size (a size parallel to the graphene layer) of 20 nm or more and 100 nm or less, and a thickness of 10 nm or less. Point to. In addition, when the powder obtained by drying the solvent is subjected to X-ray diffraction measurement, the nano-graphene does not have a peak at 9.0 ° to 13.0 ° based on the interlayer distance of the graphene layer. Distinct from graphene.

  The size of the nanographene in the plane direction is determined by diluting the nanographene to 0.001 to 0.005% by mass in an N-methylpyrrolidone solvent, dropping it on a highly smooth substrate such as a glass substrate, and drying it. Alternatively, it can be easily measured by observing with a laser microscope. As the laser microscope, for example, VK-X250 manufactured by Keyence Corporation can be used. The size of the nanographene in the plane direction is determined by measuring the length (longest diameter) of the longest part and the length (shortest diameter) of the shortest part of each nanographene small piece observed by the above method, and (long diameter + minor diameter) / 2 In the present invention, it indicates the average value when 50 or more small pieces of nanographene are randomly measured.

  Since the nanographene of the present invention has a very fine structure, self-aggregation due to folding and aggregation due to interaction between graphenes are suppressed. When the size of the nanographene in the plane direction is larger than 100 nm, the nanographene is easily folded, self-aggregates, and easily aggregates due to the interaction between the graphenes. On the other hand, when the size of the nanographene in the plane direction is less than 20 nm, the conductive path is too short to exhibit a potential. The size of the nanographene in the plane direction is preferably from 25 nm to 80 nm, more preferably from 25 nm to 60 nm.

  The thickness of nanographene (the size in the direction perpendicular to the graphene layer) can be measured by an atomic force microscope (AFM). At the time of measurement, the nanographene solution was diluted to 0.001 to 0.005% by mass, dropped and dried on a highly smooth substrate such as a glass substrate or a mica substrate, and observed with an AFM (Dimension Icon, Bruker). I do. Flake graphene usually has its thinnest direction perpendicular to the plane of nanographene. Therefore, in the flaky graphene attached to the substrate, the thickness in the direction perpendicular to the substrate is interpreted as the thickness of nanographene. When the thickness is distributed within one small piece of nanographene, the thickness can be obtained by averaging the integrated value of the thickness in the small piece. In the present invention, the thickness of nanographene refers to the average value when the thickness of 50 or more small pieces of nanographene is randomly measured by the above method.

  Although the thickness of the nanographene in the present invention is 10 nm or less, the lower limit is not particularly limited, and may be a single-layer graphene. When the thickness of the nanographene is greater than 10 nm, the flexibility, which is a characteristic of graphene, is reduced and the nanographene simply becomes an object like a particle, and cannot be attached to the particle surface. The upper limit of the thickness of the nanogflafen is preferably 5 nm or less, more preferably 3 nm or less.

  The functionalization ratio of the nanographene measured by X-ray photoelectron spectroscopy in the present invention is from 0.25 to 0.80. The functional group included in nanographene is typically derived from a hydroxy group, a carboxyl group, a carbonyl group, and the like remaining after reducing graphene oxide.

The functionalization ratio of nanographene is determined by X-ray photoelectron spectroscopy. In X-ray photoelectron spectroscopy, when a sample containing carbon is measured, a peak derived from carbon is detected at around 284 eV. However, when carbon is bonded to oxygen, it is known that the peak shifts to a higher energy side. I have. Specifically, peaks based on C—C bonds, C ＝ C double bonds, and C—H bonds in which carbon is not bonded to oxygen are detected near 284 eV without shifting, and in the case of C—O single bond, 286 It shifts to around 0.5 eV, to around 287.5 eV for a C ＝ O double bond, and to around 288.5 eV for a COO bond. Therefore, a signal derived from carbon is detected in a form in which respective peaks near 284 eV, near 286.5 eV, near 287.5 eV, and near 288.5 eV are superimposed. It is possible to calculate each peak area intensity by subjecting the superposed peaks to peak separation analysis of each component by peak fitting. Signals also appear near 286e and 290.5eV based on the graphite component. This signal is fitted as a component based on CC, C = C and CH bonds. Functionalization rate in the present invention,
Functionalization rate = [(peak area based on C—O single bond) + (C = peak area based on O double bond) + (peak area based on COO bond)] / (C—C, C = C and Peak area based on CH bond)
Is a numerical value defined by

  The degree of functionalization affects the dispersibility and ionic conductivity of nanographene. If the functionalization ratio in the nanographene is too low, both the dispersibility and the ionic conductivity deteriorate. On the other hand, if the functionalization ratio is too high, the graphene is in a state where it cannot be sufficiently reduced, and the conductivity decreases. The functionalization ratio of the nanographene is preferably 0.35 or more, more preferably 0.45 or more. Moreover, about an upper limit, Preferably it is 0.70 or less, More preferably, it is 0.60 or less.

  The functionalization ratio can be controlled by changing the degree of oxidation of graphene oxide, which is a raw material of nanographene, or by changing the amount of a reducing agent used for reducing graphene oxide. The higher the degree of oxidation of graphene oxide, the higher the functionalization rate, and the lower the degree of oxidation, the lower the functionalization rate.

  Further, the nanographene of the present invention preferably has an element ratio of oxygen to carbon (O / C) of 0.10 or more and 0.30 or less. The oxygen atoms in nanographene are based on functional groups in nanographene. The greater the elemental ratio of oxygen to carbon in nanographene, the better the dispersibility and the higher the ionic conductivity, but too much results in a lower conductivity. On the other hand, if the amount is too small, the conductivity increases, but the ionic conductivity and dispersibility deteriorate. Therefore, the element ratio of oxygen to carbon (O / C) is preferably 0.10 or more and 0.30 or less, more preferably 0.10 or more and 0.20 or less, and 0.12 or more and 0.18 or less. It is more preferred that:

  Further, the nanographene of the present invention preferably contains a small amount of nitrogen atoms. Dispersibility is improved by including a nitrogen atom, particularly an amine, in nanographene. The nanographene of the present invention preferably has an element ratio of nitrogen to carbon (N / C) of 0.005 or more and 0.05 or less.

  The nitrogen contained in the nanographene is derived from a nitrogen-containing functional group such as an amino group and a nitro group and a nitrogen-containing heterocycle such as a pyridine group and an imidazole group contained in the surface treatment agent. (In the present specification, the term “nanographene” includes nanographene to which a surface treatment agent is attached.) The presence of such a functional group can improve dispersibility. On the other hand, if the nitrogen content is too large, the ionic conductivity and dispersibility deteriorate. Therefore, the element ratio of oxygen to carbon (N / C) is preferably 0.005 or more and 0.05 or less, more preferably 0.008 or more and 0.04 or less, and 0.01 or more and 0.03 or less. It is more preferred that:

  In the present invention, the elemental ratio of oxygen to carbon (O / C) and the elemental ratio of nitrogen to carbon (N / C) in nanographene are determined by the weight of carbon, the weight of nitrogen, and the weight of oxygen measured by an organic elemental analysis method using a combustion method. Is converted to a molar ratio from the above value. In order to sufficiently dry the water, the nano-graphene oxide and the nano-graphene can be analyzed by vacuum-drying at 80 ° C. for 2 hours and then analyzing the organic components with a combustion-type elemental analyzer. As the elemental analyzer, a fully automatic elemental analyzer, vario MICRO cube (Elementar), is exemplified. However, since the sulfate ions are mixed in the nanographene oxide and the nanographene from the manufacturing process, the oxygen components include those derived from sulfuric acid. Therefore, in the organic element analysis method, sulfur is also measured at the same time, and four times the oxygen atom molar ratio of sulfur is subtracted on the assumption that all detected sulfur is derived from sulfuric acid. Specifically, the elemental ratio of oxygen to carbon (O / C) is calculated from (molar ratio of oxygen−molar ratio of sulfur × 4) / molar ratio of carbon.

<Nanographene-electrode active material composite particles>
The use of the nanographene of the present invention is not limited, but it is advantageously used, for example, by forming a composite with electrode active material particles of a lithium ion battery (hereinafter, sometimes simply referred to as “electrode active material particles”). Here, the term “composite” means that nanographene is kept in contact with the surface of the electrode active material particles. As a mode of the composite, a nanograpene and an electrode active material which are integrally granulated and a nanographene adhered to the surface of the electrode active material may be mentioned. In a lithium ion battery, the conductivity and ionic conductivity on the surface of the electrode active material greatly affect the characteristics. Since the nanographene of the present invention, which is fine and has a high functionalization ratio, has both high conductivity and ionic conductivity, by contacting the surface of the electrode active material, the performance of the lithium ion battery can be greatly improved.

The composite lithium ion battery electrode active material may be either a positive electrode active material or a negative electrode active material. That is, the nanographene of the present invention can be applied to both a positive electrode and a negative electrode. The positive electrode active material is not particularly limited, but lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), spinel-type lithium manganate (LiMn ₂ O ₄ ), or a part of cobalt is nickel manganese substituted ternary _{(LiMn x Ni y Co 1-} x-y O 2), a composite oxide of lithium and a transition metal such as spinel-type lithium manganate (LiMn ₂ O _4), lithium iron phosphate (LiFePO ₄₎ Olivine (phosphoric acid) active materials such as Examples include metal oxides such as V ₂ O ₅ and metal compounds such as TiS ₂ , MoS ₂ , and NbSe ₂ . The negative electrode active material is not particularly limited, but may be a carbon-based material such as natural graphite, artificial graphite, hard carbon, a silicon compound having SiO, SiC, SiOC, or the like as a basic constituent element, or an oxidation capable of performing a conversion reaction with lithium ions. Examples include metal oxides such as manganese (MnO) and cobalt oxide (CoO).

  There is no limitation on the method of combining nanographene and the electrode active material.However, a method of drying a dispersion in which the nanographene and the electrode active material are dispersed by a method such as spray drying or freeze drying, and a method of drying the nanographene powder and the electrode active material. , A method of mixing, a method in which a dispersion in which the graphene oxide and the electrode active material are dispersed is dried by a method such as spray drying or freeze drying, and then reduced, and a method in which the graphene oxide nanoparticle and the electrode active material are mixed. Reduction method.

As a method of mixing nanographene or nanographene oxide and an electrode active material in a dispersion, a method using a three-roll, wet bead mill, wet planetary ball mill, homogenizer, planetary mixer, twin-screw kneader, or the like is used. Examples of the method of mixing the nanographene powder or nanographene oxide powder and the electrode active material include a method using an automatic mortar, a dry bead mill, a dry planetary ball mill, and the like. Ball mills are preferred.

  The nanographene of the present invention can be used in a state of a dispersion liquid dispersed in an organic solvent. In this case, as the organic solvent, N-methylpyrrolidone, γ-butyrolactone, dimethylacetamide, dimethylformamide, dimethylsulfoxide, acetonitrile, or a mixture of the above are preferable, and N-methylpyrrolidone is particularly preferable.

<Lithium ion battery electrode>
In the first embodiment of the lithium ion battery electrode of the present invention, the nanographene of the present invention is included alone as a conductive auxiliary, that is, without being combined with an active material. The lithium ion battery electrode according to the first embodiment includes a current collector, an electrode active material, a binder, and the nanographene of the present invention.

  The current collector is not particularly limited as long as it is a conductive sheet or mesh, but a metal foil or a metal mesh that does not significantly affect the electrochemical reaction is used. The current collector on the positive electrode side is preferably an aluminum foil. As the negative electrode side current collector, a copper foil is preferable. A hole may be formed in a part of the metal foil to increase the electrode density.

  As the electrode active material, the same one as described above as the electrode active material to be composited with nanographene can be used.

  The binder is not particularly limited, but a fluorine-based polymer such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), or a rubber such as styrene-butadiene rubber (SBR) or natural rubber can be used.

  As the conductive additive, only the nanographene of the present invention may be used, or another different conductive additive may be added. The conductive additive to be added is not particularly limited. Examples thereof include carbon blacks such as furnace black, Ketjen Black (registered trademark), and acetylene black; natural graphite (such as flaky graphite); and graphite such as artificial graphite. , Conductive fibers such as carbon fibers and metal fibers, and metal powders such as copper, nickel, aluminum and silver.

  A paste for a lithium ion battery electrode is prepared by mixing these electrode active materials, a conductive auxiliary agent, and a binder with an appropriate amount of a solvent, and the electrode paste for an electrode is coated on a current collector and dried to form a lithium ion battery paste. A battery electrode can be made. Examples of the solvent for the electrode paste include N-methylpyrrolidone, γ-butyrolactone, carboxymethylcellulose, and dimethylacetamide, and N-methylpyrrolidone is particularly preferred because of its high affinity with nanographene.

  Because nanographene is extremely small in size, the quantity per weight is large. Therefore, a conductive path in the lithium ion battery electrode is easily formed. Since the conductive paths in the lithium ion battery electrode can be formed well, the electron conductivity in the electrode is improved, and a high output lithium ion battery can be obtained. In particular, when the cross section of the electrode for a lithium ion battery is observed by SEM, it is preferable that the nanographenes are connected to each other to form a conductive path between the active materials.

  The second embodiment of the lithium ion battery electrode of the present invention includes the above-described nanographene-electrode active material composite particles as an active material. The lithium ion battery electrode according to the second embodiment includes a current collector, the above-described nanographene-electrode active material composite particles, and a binder. As the current collector and the binder, those similar to those described above can be used.

  Further, the lithium ion battery electrode of the present invention may include the above-described nanographene-electrode active material composite particles as an active material, and may further include the nanographene of the present invention as a conductive aid.

  In order to obtain high output in a lithium ion battery, it is particularly preferable to form both conductivity on the surface of the active material for a lithium ion battery and conductivity between the active materials for a lithium ion battery. Therefore, when the cross section of the electrode for a lithium ion battery is observed by SEM, it is preferable that nanographene is attached to the surface of the active material and a conductive path between the active materials is also formed.

<Method for producing nanographene>
The nanographene of the present invention can be produced by, for example, a production method for reducing nanographene oxide. Nano graphene oxide is a kind of graphene oxide, and has a peak at 9.0 ° to 13.0 ° based on the interlayer distance of the graphene layer in X-ray diffraction measurement.

  The size of the nanographene oxide in the plane direction is dropped and dried on a substrate having high smoothness such as a glass substrate by drying an aqueous solution of nanographene oxide diluted to 0.001 to 0.005% by mass, and then using an optical microscope or a laser microscope. It is possible to easily measure by observing with. As the laser microscope, for example, VK-X250 manufactured by Keyence Corporation can be used. The size of the nanographene oxide in the plane direction is obtained by measuring the length (longest diameter) of the longest part and the length (shortest diameter) of the shortest part of each small piece of nanographene oxide observed by the above method, (Diameter) / 2, which in the present invention indicates an average value when 50 or more nanographene oxide small pieces are randomly measured.

  Since nanographene oxide has a very fine structure like nanographene, self-aggregation due to folding and aggregation due to interaction between nanographene oxides are suppressed. In the case of manufacturing nanographene, the size of nanographene reflects the size of nanographene oxide. Therefore, the size in the plane direction is preferably 20 nm or more and 100 nm or less, and more preferably 25 nm or more and 60 nm or less.

  The thickness (the size in a direction perpendicular to the graphene layer) of the nanographene oxide can be measured by an atomic force microscope (AFM). In the measurement, the nano graphene oxide aqueous solution is diluted to 0.001 to 0.005% by mass, dropped on a highly smooth substrate such as a glass substrate or a mica substrate, dried, and observed by AFM. Flake graphene oxide usually has the thinnest direction perpendicular to the plane of nanographene oxide. Therefore, in the graphene oxide attached to the substrate, the thickness in the direction perpendicular to the substrate is interpreted as the thickness of nanographene. When the thickness is distributed within one small piece of nanographene, the thickness can be obtained by averaging the integrated value of the thickness in the small piece. In the present invention, the thickness of nanographene refers to the average value when the thickness of 50 or more small pieces of nanographene is randomly measured by the above method.

  In the present manufacturing method, a nanographene oxide having a thickness of 10 nm or less is used, but the lower limit is not particularly limited, and a single layer of graphene oxide may be used. When the thickness of the nano-graphene oxide is more than 10 nm, the flexibility, which is a characteristic of graphene, is reduced, so that it becomes an object like a particle and cannot be attached to the particle surface. The upper limit of the thickness of the nanofiber guflafen is preferably 5 nm or less, more preferably 3 nm or less.

  In the present production method, it is preferable to use nano-graphene oxide having a functionalization rate of 1.0 or more and 1.5 or less. When the functionalization ratio is 1.0 or less, the dispersibility of the reduced nanographene becomes poor. On the other hand, when the ratio is 1.5 or more, the structure as graphene is difficult to maintain, and the conductivity of the reduced nanographene deteriorates.

  In order to produce the nanographene of the present invention, it is necessary to produce graphene oxide as a raw material under strong oxidation conditions. As an example, a method for further oxidizing graphene oxide manufactured by the Hummers method will be described below.

  First, 15 g of graphite (graphite powder) and 7.5 g of sodium nitrate were placed in 330 ml of concentrated sulfuric acid, and while stirring, 45 g of potassium permanganate was gradually added so that the temperature did not rise. After stirring for hours, the mixture is stirred at 30 ° C to 40 ° C for 2.5 hours. Thereafter, 690 ml of ion-exchanged water is added to dilute the suspension, which is reacted at 80 to 100 ° C. for 15 minutes. Finally, 50 ml of hydrogen peroxide solution and 1020 ml of deionized water are added and reacted for 30 minutes to obtain a graphene oxide dispersion. The obtained graphene oxide dispersion is filtered and washed to obtain a graphene oxide dispersion having a pH of 5 to 7. The graphene oxide dispersion is diluted and the solvent is removed by a freeze-drying method, a spray-drying method, or the like to obtain graphene oxide powder.

  Graphite as a raw material of graphene oxide may be either artificial graphite or natural graphite, but natural graphite is preferably used. The concentrated sulfuric acid preferably has a mass content of 70% or more, and more preferably 97% or more.

  Then, 5 g of the graphene oxide powder and 2.5 g of sodium nitrate prepared in the above procedure were put into 110 ml of concentrated sulfuric acid, and while stirring, 15 g of potassium permanganate was gradually added so that the temperature did not rise. And then stirred at 30 ° C. to 40 ° C. for 2.5 hours. Thereafter, 230 ml of ion-exchanged water is added to dilute the suspension, and the mixture is reacted at 80 to 100 ° C for 15 minutes. Finally, 20 ml of hydrogen peroxide solution and 340 ml of deionized water are added and reacted for 30 minutes to obtain a graphene oxide dispersion. The obtained graphene oxide dispersion is filtered and washed to obtain a graphene oxide dispersion.

  The degree of oxidation of graphene oxide can be adjusted by changing the amount of an oxidizing agent used for the oxidation reaction of graphite. In the above reaction examples, the higher the amount of sodium nitrate and potassium permanganate with respect to graphite used in the oxidation reaction, the higher the oxidation degree, and the smaller the amount, the lower the oxidation degree.

  The graphene oxide dispersion is refined and dispersed by an ultrasonic treatment device, a bead mill, a ball mill, a jet mill, or the like, so that nanographene oxide having a size in a plane direction of 20 nm or more and 100 nm or less and a thickness of 10 nm or less is produced. can do.

  As described above, a defect of the graphite structure is produced until the graphite structure reaches a level of several tens of nanometers by performing the two oxidation steps, and then nano-graphene oxide can be produced by performing miniaturization and dispersion treatment.

  The nanographene of the present invention can be produced by reducing the above-mentioned nanographene oxide. In the reduction step, the size of the nanographene in the plane direction of the graphene can be substantially equal to the size of the nanographene oxide in the plane direction.

In the present invention, the method for reducing the graphene oxide nanoparticle is not limited, but the chemical reduction by a reducing agent such as sodium borohydride (NaBH ₄ ) or hydrazine (N ₂ H ₄ ), laser light, flash light, Reduction methods such as heat reduction using a light source such as ultraviolet rays or microwaves or electromagnetic waves, and heat reduction using an oven or the like can be used. In the case of reduction by heat, carbon dioxide is desorbed from graphene oxide during the reduction reaction, so that carbon tends to escape from the graphene structure and the conductivity tends to decrease. Since the graphene structure is less likely to be broken in the reduction by chemical reduction than the reduction by heat, chemical reduction is preferable as the reduction method.

  Examples of the reducing agent for chemical reduction include an organic reducing agent and an inorganic reducing agent, and an inorganic reducing agent is preferable because of ease of washing after reduction.

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

  Examples of the inorganic reducing agent include sodium dithionite, potassium dithionite, phosphorous acid, sodium borohydride, hydrazine, etc., and among them, sodium dithionite and potassium dithionite have a relatively high functional group. Since it can be reduced, it is preferably used.

  In performing the chemical reduction, it is preferable that the nanographene oxide is well dispersed in the dispersion medium, and it is preferable that the dispersion medium and the nanographene oxide are stirred by various stirring devices. As the stirring device, a mixer such as a rotation and revolution mixer, a planetary mixer, a homomixer, and a film mix (Primix), and a disperser such as an ultrasonic disperser and a jet mill can be used. The dispersion medium is not particularly limited, but it is preferable to use one in which graphene oxide is partially or entirely dissolved. As such a dispersion medium, a polar solvent is preferable, and water / ethanol / methanol, 1-propanol, 2-propanol, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, γ-butyrolactone, and the like are preferable. . Among them, water has a very high affinity and solubility with graphene oxide, and can be said to be the most preferable solvent.

[Measurement Example 1: Element ratio (O / C), (N / C)]
After vacuum drying the nanographene oxide or the nanographene at 80 ° C. for 2 hours to sufficiently remove the solvent, the carbon atoms, the oxygen atoms, and the carbon atoms of the nanographene oxide and the nanographene are measured using a full-automatic elemental analyzer vario MICRO cube (Elementar). Measure the weight of sulfur and nitrogen atoms. After converting the measured weight ratio into a molar ratio, the molar ratio of (oxygen molar ratio−sulfur molar ratio × 4) / carbon molar ratio was calculated to obtain the element ratio of oxygen to carbon. Further, the element ratio of nitrogen to oxygen (N / C) was determined by converting the measured weight ratio to a molar ratio.

[Measurement Example 2: Surface Size of Graphene]
In the case of nanographene, the solvent was diluted to 0.002% by mass using an N-methylpyrrolidone solvent, and in the case of nanographene oxide, water was used. The solution was dropped on a glass substrate, dried, and adhered to the substrate. The nanographene or nanographene oxide on the substrate is observed with a Keyence Corporation laser microscope VK-X250, and the length of the longest part (longest diameter) and the length of the shortest part (shortest diameter) of the small piece of nanographene or nanographene oxide are observed. Were randomly measured, and the numerical value obtained by (major axis + minor axis) / 2 was averaged for 50 elements.

[Measurement Example 3: Graphene thickness]
The nanographene was diluted to 0.002% by mass with an N-methylpyrrolidone solvent, and the nanographene oxide was diluted with water using water, dropped and dried on a mica substrate, and adhered to the substrate. The nanographene or nanographene oxide on the substrate was observed by AFM (Dimension Icon, Bruker), and the thickness of 50 nanographene or nanographene oxide was measured at random, and the average value was obtained. When there was variation in the thickness of one small piece, the area average was determined.

[Measurement Example 4: Functionalization ratio]
Quantara SXM (registered trademark: manufactured by PHI) was used for X-ray photoelectron measurement of nanographene. Excited X-rays are monochromatic Al K _{α1, 2} lines (1486.6 eV), the X-ray diameter is 200 μm, and the photoelectron escape angle is 45 °.

  The peak based on carbon atoms is a peak around 284 eV based on a C = C bond, a CH bond, a peak near 286 eV based on the case of a CO bond, a peak near 287.5 eV based on a C = O bond, a COO The peak was separated into four components, around 288.5 eV based on the bond, and the functionalization ratio was determined from the area ratio of each peak.

[Measurement Example 5: Battery Performance Evaluation]
The discharge capacity was measured as follows, except where otherwise noted. 1.5 parts by weight of nanographene prepared in the following examples, 92 parts by weight of LiMn ₂ O ₄ as an electrode active material, 1.5 parts by weight of acetylene black as a conductive aid, 5 parts by weight of polyvinylidene fluoride as a binder, and a solvent Was added with 100 parts by weight of N-methylpyrrolidone, and mixed with a planetary mixer to obtain a lithium ion battery electrode paste. The electrode paste was applied to an aluminum foil (18 μm in thickness) using a doctor blade (300 μm), dried at 80 ° C. for 15 minutes, and vacuum dried to obtain an electrode plate.

The prepared electrode plate cut into a diameter 15.9mm and a positive electrode, a lithium foil cut to a diameter 16.1mm 0.2mm thick and the negative electrode, as Celgard # 2400 (Celgard Inc.) separator was cut to a diameter of 17 mm, LiPF ₆ Was used as an electrolytic solution with a solvent of ethylene carbonate: diethyl carbonate = 7: 3 containing 1M of the same, and a 2042 type coin battery was manufactured and subjected to electrochemical evaluation. Charge / discharge measurement was performed three times each in the order of 0.1C, 1C, and 5C at an upper limit voltage of 4.3V and a lower limit voltage of 3.0V, for a total of nine times, and a third discharge at a rate 1C and a third discharge at a rate 5C. The respective discharge capacities at the time were evaluated.

(Synthesis example 1)
Starting from 1500 graphite natural graphite powder (Shanghai Ichihan Graphite Co., Ltd.) as raw materials, 15 g of graphite (graphite powder) and 7.5 g of sodium nitrate in an ice bath were placed in 330 ml of 98% concentrated sulfuric acid, and stirred while permanganese was added. 45 g of potassium acid was gradually added so that the temperature became 10 ° C. or lower, and after the addition was completed, the mixture was stirred for 1.5 hours, and then stirred at 35 ° C. for 2.5 hours. Thereafter, 690 ml of ion-exchanged water was added to dilute the suspension, which was reacted at 90 ° C. for 15 minutes. Finally, 50 ml of hydrogen peroxide solution and 1020 ml of deionized water are added and reacted for 30 minutes to obtain a graphene oxide dispersion. The obtained graphene oxide dispersion was filtered and washed until the pH reached 5, to obtain a graphene oxide dispersion.

  The O / C of the graphene oxide was 0.69, the size in the plane direction was 9.8 μm, and the thickness was 6.2 nm.

(Synthesis example 2)
The same treatment as in Synthesis Example 1 was carried out except that sodium nitrate and potassium permanganate were changed to sodium nitrate 5.25 g and potassium permanganate 31.5 g, to obtain a graphene oxide dispersion.

  The O / C of this graphene oxide was 0.62, the size in the plane direction was 11.2 μm, and the thickness was 9.8 nm.

(Synthesis example 3)
The graphene oxide dispersion prepared in Synthesis Example 1 was freeze-dried to obtain a graphene oxide powder.

  5 g of the above-mentioned graphene oxide powder and 2.5 g of sodium nitrate were put into 110 ml of 98% concentrated sulfuric acid, and while stirring, 15 g of potassium permanganate was gradually added so that the temperature became 10 ° C. or lower. After stirring for 5 hours, the mixture was stirred at 35 ° C. for 2.5 hours. Thereafter, 230 ml of ion-exchanged water was added to dilute the suspension, and the mixture was reacted at 90 ° C. for 15 minutes. Finally, 20 ml of aqueous hydrogen peroxide and 340 ml of deionized water were added and reacted for 30 minutes to obtain a nano graphene oxide dispersion.

  The O / C of the nano graphene oxide was 1.05, the size in the plane direction was 28 nm, and the thickness was 3.4 nm.

(Synthesis example 4)
The same treatment as in Synthesis Example 3 was performed except that the graphene oxide dispersion used was prepared in Synthesis Example 2, to obtain a nanographene oxide dispersion.

  The O / C of the nano graphene oxide was 1.05, the size in the plane direction was 28 nm, and the thickness was 3.4 nm.

(Synthesis example 5)
100 ml of a 0.5% graphene oxide dispersion was prepared from the graphene oxide dispersion prepared in Synthesis Example 1, and treated with an ultrasonic homogenizer at 200 W for 1 hour.

  The O / C of the graphene oxide was 0.69, the size in the plane direction was 1.1 μm, and the thickness was 5.8 nm.

[Example 1]
From the nanographene oxide dispersion prepared in Synthesis Example 3, 100 ml of a 0.5% nanographene oxide dispersion was prepared, and 1.5 g of sodium dithionite was added as a reducing agent while stirring with a stirrer. The reduction reaction was performed for 30 minutes. After completion of the reduction, a washing step of filtering with a suction filter, diluting with water to a concentration of 0.5%, and filtering with suction was repeated five times to wash. After the washing was completed, the resultant was freeze-dried to obtain nanographene.

[Example 2]
Nanographene was obtained in the same manner as in Example 1, except that the nanographene oxide dispersion prepared in Synthesis Example 4 was used.

[Example 3]
After freeze-drying the nanographene oxide dispersion prepared in Synthesis Example 3, it was reduced by heat treatment at 200 ° C. for 5 hours in a nitrogen atmosphere to obtain nanographene.

[Example 4]
After freeze-drying the nanographene oxide dispersion prepared in Synthesis Example 3, it was reduced by heat treatment at 600 ° C. for 5 hours in a nitrogen atmosphere to obtain nanographene.

[Example 5]
The nanographene oxide dispersion liquid prepared in Synthesis Example 3 and the electrode active material LiMn ₂ O ₄ were mixed with a homogenizer so that the solid content became 2.25: 100, and the mixture was freeze-dried and then nitrogen atmosphere. The reduction was performed by heat treatment at 200 ° C. for 5 hours to obtain composite particles of nanographene and the electrode active material LiMn ₂ O ₄ (nanographene-electrode active material composite particles). When the carbon component of the composite particles was measured, it was 1.5%.

  The battery performance was evaluated in the same manner as in Measurement Example 5 except that the composite particles were used as an electrode active material (92 parts by weight) and nanographene was not added alone.

[Example 6]
100 ml of a 0.5% nano graphene oxide dispersion was prepared from the nano graphene oxide dispersion prepared in Synthesis Example 3, 250 mg of dopamine hydrochloride was added, and then 1.5 g of dithionite as a reducing agent was added while stirring with a stirrer. Sodium was added and a reduction reaction was performed at a temperature of 40 ° C. for 30 minutes. After completion of the reduction, a washing step of filtering with a suction filter, diluting with water to a concentration of 0.5%, and performing suction filtration was repeated five times to wash. After the washing, freeze drying was performed to obtain nanographene.

[Comparative Example 1]
100 ml of a 0.5% graphene oxide dispersion was prepared from the graphene oxide dispersion prepared in Synthesis Example 1, and 1.5 g of sodium dithionite was added as a reducing agent while stirring with a stirrer. A reduction reaction was performed. After completion of the reduction, a washing step of filtering with a suction filter, diluting with water to a concentration of 0.5%, and filtering with suction was repeated five times to wash. After the washing was completed, the resultant was freeze-dried to obtain nanographene.

[Comparative Example 2]
Graphene was obtained in the same manner as in Comparative Example 1 except that the graphene oxide dispersion prepared in Synthesis Example 2 was used.

[Comparative Example 3]
To 100 ml of the 0.5% graphene oxide dispersion after sonication prepared in Synthesis Example 5 was added 1.5 g of sodium dithionite as a reducing agent while stirring with a stirrer, and a reduction reaction was performed at 40 ° C. for 30 minutes. Was done. After completion of the reduction, a washing step of filtering with a suction filter, diluting with water to a concentration of 0.5%, and performing suction filtration was repeated five times to wash. After the washing was completed, lyophilization was performed to obtain graphene.

Table 1 shows the O / C of graphene, the size in the plane direction, the thickness, and the functional grouping ratio in each example and comparative example, and the results of the battery performance evaluation of the lithium ion battery using each graphene. However, Examples 1 to 5 should be read as Comparative Examples.

Claims (7)

The size of the planar direction is at 20nm or more 100nm or less, a thickness of 10nm or less state, and are functionalized ratio of 0.35 or more 0.80 or less as measured by X-ray photoelectron spectroscopy, nitrogen to carbon atoms element ratio atoms (N / C) is Ru der 0.005 or more and 0.05 or less nanographene. The nanographene according to claim 1, wherein an element ratio (O / C) of oxygen atoms to carbon atoms is 0.10 or more and 0.30 or less. The nanographene according to claim 1, wherein a size in a plane direction is 25 nm or more and 60 nm or less. A nanographene-electrode active material composite particle obtained by combining the nanographene according to any one of claims 1 to 3 with a lithium ion battery electrode active material. A paste for a lithium ion battery electrode comprising the nanographene according to any one of claims 1 to 3 or the nanographene-electrode active material composite particles according to claim 4. A lithium ion battery electrode comprising the nanographene according to any one of claims 1 to 3 or the nanographene-electrode active material composite particle according to claim 4. A nanographene dispersion obtained by dispersing the nanographene according to claim 1 in an organic solvent.