JP2014196206A - Method for manufacturing graphene agglomerate and cathode carbon material for lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide, at a low cost, a carbonaceous cathode material having a novel structure for constructing a lithium ion battery having a high input density.SOLUTION: Graphene 6 is electrochemically peeled within an electrolytic solution 2 by using a carbon material including a graphene laminate as an anode 3, and an agglomerate 7 of graphene dissociated from the anode is recovered. The graphene agglomerate of a high density (0.5 g/cmor higher) having a relatively large average particle diameter can be obtained at this time by adjusting the voltage, and the carbon material thus manufactured is suitable as a cathode material for a high-input-density lithium ion battery.

Description

  The present invention relates to a method for producing graphene aggregates, a negative electrode carbon material for a lithium ion battery using the same, and a lithium ion battery including the carbon material.

  Lithium ion batteries (LIBs) have come to be installed in modern portable electronic devices such as notebook computers, mobile phones, smartphones, and tablet terminals, as well as electric vehicles (EV) and plug-in hybrid electricity. With the arrival of interest in automobiles (PHEV), there is an increasing demand for development of LIBs capable of high input performance mounted on EVs and PHEVs.

  As a negative electrode material for LIBs, a graphite-based negative electrode material such as graphite capable of reversible lithium ions is used.

  Conventional graphite-based negative electrode materials are difficult to provide high input density LIBs due to the slow diffusivity of lithium ions into the bulk electrode material.

On the other hand, the graphene sheet is expected to improve lithium ion diffusivity by having an interlayer wider than the interlayer distance d ₀₀₂ = 0.3354 nm of graphite.

  For example, Patent Document 1 states that conventional graphite-based negative electrode materials are not sufficient in terms of characteristics such as cycle life, reversible capacity, irreversible capacity, and the like. Nano graphene platelets (NGPs), which are sheets or a stack of graphene sheets, have been proposed.

  Conventional graphene sheets are manufactured by thermally or mechanically treating graphite oxide to peel it off to a graphene oxide sheet and chemically reducing it.

  In Patent Document 2, graphite is immersed in an acidic electrolyte aqueous solution, the graphite is used as a working electrode (anode), and a direct current in a range of 0.6 to 0.8 V is provided between the reference electrode and a natural electrode in addition to a natural potential. A method for producing expanded graphite in which anions (nitrate anions) are intercalated between layers by an electrochemical treatment in which a voltage is applied for 48 hours or more and 1000 hours or less is disclosed. Further, it is described that graphene and exfoliated graphite can be obtained by easily exfoliating the expanded graphite by applying mechanical exfoliation or ultrasonic peeling force.

Special table 2011-503804 gazette (WO2009 / 061685) JP 2012-131691 A

  However, the conventional method for producing a graphene sheet is a method in which the film is once oxidized and peeled and then subjected to a reduction treatment, and the productivity is very poor. Moreover, once oxidized, even if it performs a reduction process, it becomes lower than the conductivity of the original graphite or the like.

  In the electrochemical treatment of Patent Document 2, since the treatment is performed using graphite as an anode, it is subjected to anodization to become graphene oxide, which has the same problem as the above-described chemical redox technique. In addition, it is necessary to carry out at a low potential in order to avoid oxidation more than necessary. For this reason, the efficiency of anion intercalation is poor and a long time is required.

  In any case, since peeling is performed by mechanical or thermal impact force, the peeled graphene is finely pulverized and has a small particle diameter (secondary particle diameter) and a low density even when aggregated.

  For this reason, a carbon-based negative electrode material having a new structure and a low cost has been demanded in order to construct a high input density LIBs.

  As a result of intensive studies on the production of a graphene sheet that does not pass through graphene oxide, the present inventors have electrochemically exfoliated carbon material as a cathode in an electrolyte solution, thereby providing a graphene aggregate having a high density and a large particle size. It was found that such graphene aggregates have excellent characteristics as a negative electrode material for lithium ion batteries.

That is, according to one embodiment of the present invention,
There is provided a method for producing a graphene aggregate, in which an graphene is electrochemically exfoliated in an electrolytic solution using a carbon material containing a graphene laminate as a cathode, and the graphene aggregate detached from the cathode is recovered.

Also, according to another embodiment of the present invention,
A graphene aggregate satisfying the following conditions is provided.
(1) The particle diameter is 300 μm or less and the average particle diameter D ₅₀ is 1 to 20 μm,
(2) True density is 0.5 g / cm ³ or more,
(3) The surface area by the BET method is 300 m ² / g or less.

  According to still another embodiment of the present invention, a lithium ion battery using the graphene aggregate as a negative electrode material is provided. When used as a negative electrode material for a lithium ion battery, pores having a diameter of 0.5 to 500 nm are preferably formed on the surface of the graphene aggregate, and are preferably modified with a lithium storage material. Further, these negative electrode materials are preferably coated with amorphous carbon.

  According to one embodiment of the present invention, a graphene aggregate can be obtained from a carbon material containing a graphene laminate without undergoing an oxidation treatment, and thus a highly conductive negative electrode material for a lithium ion battery can be provided, and input / output characteristics can be provided. Can provide a lithium ion battery with improved performance.

It is a conceptual diagram explaining the structure of the electrochemical cell used for the cathode peeling which concerns on one Embodiment of this invention. It is a conceptual diagram explaining the mechanism of the cathode peeling which concerns on one Embodiment of this invention. It is a conceptual diagram explaining the mechanism of the cathode peeling which concerns on another embodiment of this invention. The X-ray diffraction (XRD) figure of the graphene aggregate obtained in Example 1 is shown with the conventional graphite. The SEM image (500 times) of the graphene aggregate obtained in Example 1 is shown. The SEM image (10,000 times) of the graphene aggregate obtained in Example 1 is shown.

  Hereinafter, embodiments of the present invention will be specifically described, but the present invention is not limited to these embodiments.

[1. (Method for producing graphene aggregate)
In the method for producing graphene aggregates in the present invention, graphene is exfoliated electrochemically (hereinafter referred to as cathode exfoliation) using a carbon material containing a graphene laminate in an electrolyte solution as a cathode, and the aggregated graphene aggregates are desorbed from the cathode. Collect the collection.

(1A. Carbon material)
The carbon material to be used for cathode separation of the present invention includes a graphene laminate, a highly crystalline carbon material such as natural or artificial graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon. And low crystalline carbon materials such as (hard carbon). Depending on the carbon material used, the characteristics of the graphene aggregate obtained are different. Details will be described later. These carbon materials are used after being formed into an electrode shape. The electrode shape may be any shape such as a rod shape (bar shape) or a plate shape (plate shape), and is not particularly limited.

(1B. Cathode peeling)
The cathode peeling of the carbon material is performed using, for example, an electrochemical cell as shown in FIG. In FIG. 1, an electrolytic solution 2 is placed in a casing 1 of an electrochemical cell. In the electrolytic solution 2, a carbon material 3 as a working electrode (cathode) and a noble metal electrode resistant to oxidation as a control electrode (anode) The insoluble anode 4 such as a noble metal oxide electrode is used, and a predetermined voltage is applied from the power source 5 to perform the cathode peeling of the carbon material 3. Peeling is started by setting the voltage to a level at which the graphene 6 peels on the carbon material 3 of the cathode. The peeling speed can be controlled by adjusting the voltage. The size of the aggregate (secondary particle diameter) can be controlled in the range of 0.1 to 500 μm depending on the voltage and the bath composition. When used as a negative electrode material for a lithium ion battery, the thickness is preferably 300 μm or less. The higher the voltage, the faster the exfoliation, the larger the density, and the graphene aggregate 7 having a large particle size can be produced. The longer the time and the lower the voltage, the lower the density and the smaller particle size graphene aggregate 7 can be produced. In the electrochemical cell, it is preferable to provide a partition between the anode and the cathode so that the gas generated on each electrode is not mixed.

(1C. Electrolytic solution)
The electrolytic solution used for the cathode stripping of the present invention contains a cation intercalating between graphene layers, and contains an ionic species larger than the interlayer distance between graphenes (about 0.34 nm for graphite). As such ionic species, a solvate of an alkali metal ion and an organic solvent is preferable, and in particular, a solvate of sodium ion and dimethyl sulfoxide (DMSO) (sometimes referred to as Na ⁺ : DMSO) is more preferable. preferable. For example, a desired solvate can be obtained by mixing NaCl in DMSO at a volume ratio of 1:50 to 1: 1. Furthermore, the obtained solvate can be mixed with water to obtain an electrolytic solution. It adjusts so that the density | concentration of Na ^{<+> in} electrolyte solution may be set to 0.1M-1M. Moreover, an ionic liquid can also be used as an electrolytic solution as it is. The ionic liquid is preferably in the form of a liquid at room temperature, and examples thereof include imidazolium salts, pyrrolidinium salts, piperidinium salts, pyridinium salts, and ammonium / phosphonium / sulfonium salts.

Here, the mechanism of cathode peeling will be described with reference to a conceptual diagram. FIG. 2 shows an example in which the cathode was stripped from graphite using a solvate of Na ⁺ and DMSO. First, FIG. 2A shows the graphite 11 in which the graphene 12 is stacked in layers. As shown in FIG. 2B, when the cathode exfoliation using the solvate 13 of Na ⁺ and DMSO is started, the graphene 12 The solvate 13 intercalates between the layers. Separation occurs when the voltage applied between the positive and negative electrodes becomes 3 V or more when the interlayer of the graphene 12 is expanded by the stress of the solvate 13 to be intercalated and the cathode generation gas (for example, hydrogen gas). The exfoliated graphene 12 is detached from the cathode and aggregated in the electrolytic solution to become a graphene aggregate 14 (FIG. 2C). Since the oxidation reaction does not occur in the cathode peeling according to the present invention, this is a new and low-cost method for producing a single-layer or several-layer graphene material having good conductivity. In the case of a low crystalline carbon material such as hard carbon or soft carbon, for example, soft carbon 21 shown in FIG. 3, the size of the cluster 22 of graphene 12 ′ is small (FIG. 3A), and the solvate 13 is intercalated. When the separation voltage is further exceeded (FIG. 3B), the bond between the clusters is weak, so that the separation of the graphene 12 ′ occurs along with the separation between the clusters 22 (FIG. 3C). As a result, it is easy to obtain a graphene aggregate 14 'having a small particle size as compared with the case of a highly crystalline carbon material such as graphite (FIG. 3 (d)). Note that exfoliation does not necessarily occur between all graphenes, and some exfoliate as multi-layer graphene. Usually, aggregates are formed as a mixture of single-layer graphene and multi-layer graphene. In some cases, the clusters may be separated while being connected.

(1D. Structure of graphene aggregate)
Single-layer or several-layer graphene synthesized by a conventional chemical oxidation-reduction method can only be obtained with a density of about 0.01 to 0.5 g / cm ³ even when an aggregate is formed. On the other hand, the graphene aggregate according to the present invention can provide a higher density (0.5-2 g / cm ³ ) product than before by adjusting the cathode peeling condition. Such a high density product is suitable for a negative electrode material of a lithium ion battery. Further, the graphene aggregate according to the present invention has better conductivity (<500 S / m) than conventional graphene (100-300 S / m) obtained by chemical reduction from graphene oxide. Cathodic stripping can be adjusted to a single layer or multi-layer graphene (2-200 layers) of 5 nm-50 μm size (Lc (002)) with the raw carbon material, stripping time and set voltage. The specific surface area of the graphene aggregate can be 1-500 m ² / g depending on the size of the aggregate. It is suitable for the negative electrode material of a lithium ion battery that it is 300 m < ² > / g or less. Carboxyl groups, hydroxyl groups and other functional groups that are indicative of oxidation can be characterized by FTIR. The graphene aggregate of the present invention has few functional groups that serve as an index of oxidation as compared with a reduced product from conventional graphene oxide. The graphene aggregate according to the present invention includes multilayer graphene having a lattice spacing (d ₀₀₂ : 0.3355-0.4 nm) larger than that of graphite (0.3354 nm).

[2. (Anode material for lithium ion battery)
The negative electrode material for a lithium ion battery according to the present invention contains the above graphene aggregate. Although it can be used as it is, it is preferable to perform pore formation (activation treatment), modification with various lithium storage materials, coating treatment, and the like.

(2A. Activation treatment)
As the carbon material activation treatment, gas activation and chemical activation are known. However, in the graphene aggregate according to the present invention, chemical activation, particularly nanocrystals (KOH, NaOH, ZrO or other hydroxides or oxidations) is known. The activation treatment using the product can be preferably used. A pore of 0.5 to 500 nm can be formed by the activation treatment. Thus, the pores on the carbon material due to activation shorten the diffusion path of lithium ions and provide more active sites for lithium ion retention. Since the size of the pores depends on the size of the nanocrystals, these precipitation conditions (for example, the concentration of KOH solution and the drying time) can be appropriately changed and adjusted to a certain range. In particular, it is preferable to form pores of 20 to 100 nm. When multi-layer graphene is included, the pores are not limited to the surface layer graphene, and may be formed over two or more layers, but the pore diameter here is observed on the surface layer graphene, particularly the surface of the aggregate Indicates the size of the pores.

  The heating temperature during the activation treatment is not particularly limited, but the lower limit is usually 500 ° C., preferably 600 ° C., and the upper limit is usually 1200 ° C., preferably 900 ° C., particularly preferably 800 ° C. The activation process is performed in an inert gas atmosphere, for example, in a nitrogen atmosphere.

(2B. Modification with lithium storage material)
Since the graphene aggregate of the present invention has a larger interlayer distance than graphite, a capacity larger than 372 mAh / g of the theoretical capacity of graphite (LiC ₆ ) can be obtained, but is modified with a lithium storage material. Thus, the capacity value can be further increased. As such a lithium occlusion substance, a known material can be used, for example, a lithium ion active transition metal oxide such as SiOx (x <2), TiO ₂ , Co ₃ O ₄ , SnO ₂ or Si, Ge, Sn. , Pb, Al, Ga, In, Mg, and other metals capable of forming an alloy with lithium.

Such modification with a lithium storage material can be performed by an in-situ method. For example, for lithium ion active transition metal oxides, two different types of solutions (Na ₂ SO ₄ and X (CH ₃ COO) ₂ , XCl ₂ , XSO ₄ , or XNO ₃ , where X is Sn, Ti, Co, etc.). Modification can also be done by CVD, PVD, magnetron sputtering. The metal can be modified by coating the graphene aggregate by magnetron sputtering or electroplating.

  The content of the lithium storage material is preferably 0.1 to 30% by mass with respect to the graphene aggregate according to the present invention. If the content is too small, there is no sufficient content effect. If the content is too large, the influence of volume expansion / contraction during charging / discharging of the metal or metal oxide is large, and the graphene aggregate is likely to deteriorate.

(2C. Coating treatment)
The graphene aggregate subjected to the above activation treatment and / or modification treatment can be coated with an amorphous carbon film having a thickness of 1 to 500 nm. Coating process of amorphous carbon film is CVD method, sol-gel method, ball mill method, in-situ polymerization, dry mixing, spray pyrolysis, wet mixing following thermal decomposition, thermal evaporation, rheological phase reaction, liquid system synthesis, hydrothermal It depends on the law. By performing such a coating treatment, the reactivity between the graphene aggregate and the nonaqueous solvent used for the electrolyte is suppressed, and the range of usable solvents is expanded. Moreover, charging / discharging efficiency improves and reaction capacity increases.

(2D. Material properties)
Graphene aggregates obtained from a highly crystalline carbon material such as graphite are obtained in a relatively narrow range with an average particle diameter D _{50 of 10} to 20 μm and become high-density aggregates. On the other hand, a graphene aggregate obtained from a low crystalline carbon material such as soft carbon can be obtained in a relatively wide range of an average particle diameter D ₅₀ of 1 to 20 μm. In addition, comparing the two, graphene aggregates obtained from highly crystalline carbon materials are advantageous for providing lithium ion batteries with high discharge capacity, and graphene aggregates obtained from low crystalline carbon materials have charge rate characteristics. It is advantageous to provide an excellent lithium ion battery. The graphene aggregate covered with the activation treatment, the modification treatment, and the amorphous carbon is advantageous for providing a lithium ion battery having further excellent capacity. These graphene aggregates can be used individually by 1 type or in combination of 2 or more types from which a particle size differs. It is also possible to use a combination of graphene aggregates of different raw carbon materials.

[3. Lithium ion battery)
The lithium ion battery by embodiment of this invention contains the negative electrode containing the said negative electrode material for lithium ion batteries, a positive electrode, and electrolyte. The lithium ion battery according to the present invention can be used mainly as a secondary battery.

(3A. Negative electrode)
The negative electrode material for a lithium ion battery according to the present embodiment can be applied to a negative electrode active material of a lithium ion battery. By using this negative electrode material as a negative electrode active material, a lithium ion battery having particularly improved input characteristics is provided. be able to.

  A negative electrode for a lithium ion battery can be produced, for example, by forming a negative electrode active material containing the negative electrode material and a negative electrode active material layer containing a binder on the negative electrode current collector. You may add well-known negative electrode active materials other than the negative electrode material which concerns on this invention to a negative electrode active material as needed. This negative electrode active material layer can be formed by a general slurry coating method. Specifically, a negative electrode can be obtained by preparing a slurry containing a negative electrode active material, a binder, and a solvent, applying the slurry onto a negative electrode current collector, drying, and pressing as necessary. . Examples of the method for applying the negative electrode slurry include a doctor blade method, a die coater method, and a dip coating method. A negative electrode can also be obtained by forming a negative electrode active material layer in advance and then forming a metal thin film as a current collector by vapor deposition, sputtering, or the like.

  The binder for the negative electrode is not particularly limited, but polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene. Copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, isoprene rubber, butadiene rubber, fluorine rubber Can be mentioned. As the slurry solvent, N-methyl-2-pyrrolidone (NMP) or water can be used. When water is used as a solvent, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, and polyvinyl alcohol can be used as a thickener.

  The content of the binder for the negative electrode is preferably in the range of 0.1 to 30 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoint of the binding force and energy density that are in a trade-off relationship. The range of 0.5-25 mass parts is more preferable, and the range of 1-20 mass parts is more preferable.

  The negative electrode current collector is not particularly limited, but copper, nickel, stainless steel, molybdenum, tungsten, tantalum, and alloys containing two or more of these are preferable from the viewpoint of electrochemical stability. Examples of the shape include foil, flat plate, and mesh.

(3B. Positive electrode)
For the positive electrode, for example, a slurry containing a positive electrode active material, a binder, and a solvent (and a conductive auxiliary material if necessary) is prepared, applied to the positive electrode current collector, dried, and pressurized as necessary. Thus, a positive electrode active material layer can be formed on the positive electrode current collector. After forming the positive electrode active material layer in the same manner as the negative electrode, a thin film for the current collector may be formed.

Although it does not restrict | limit especially as a positive electrode active material, For example, lithium complex oxide, lithium iron phosphate, etc. can be used. Examples of the lithium composite oxide include lithium manganate (LiMn ₂ O ₄ ); lithium cobaltate (LiCoO ₂ ); lithium nickelate (LiNiO ₂ ); and at least part of the manganese, cobalt, and nickel portions of these lithium compounds. Replaced with other metal elements such as aluminum, magnesium, titanium, zinc; nickel-substituted lithium manganate in which part of manganese in lithium manganate is replaced with at least nickel; part of nickel in lithium nickelate is replaced with at least cobalt Cobalt-substituted lithium nickelate; a part of manganese of nickel-substituted lithium manganate substituted with another metal (for example, at least one of aluminum, magnesium, titanium, and zinc); one nickel of cobalt-substituted lithium nickelate Other metal elements (e.g. aluminum, magnesium, titanium, at least one zinc) include those substituted with. These lithium composite oxides may be used individually by 1 type, and 2 or more types may be mixed and used for them. Regarding the average particle diameter of the positive electrode active material, for example, a positive electrode active material having an average particle diameter in the range of 0.1 to 50 μm can be used from the viewpoint of reactivity with the electrolytic solution, rate characteristics, and the like. A positive electrode active material having a particle diameter in the range of 1 to 30 μm, more preferably an average particle diameter in the range of 5 to 25 μm can be used. Here, the average particle diameter means the particle diameter (median diameter: D ₅₀ ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.

  Although it does not restrict | limit especially as a binder for positive electrodes, The thing similar to the binder for negative electrodes can be used. Among these, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost. The content of the binder for the positive electrode is preferably in the range of 1 to 25 parts by mass with respect to 100 parts by mass of the positive electrode active material, from the viewpoint of the binding force and energy density in a trade-off relationship, and 2 to 20 parts by mass. The range of 2-10 mass parts is more preferable. As binders other than polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, Examples include polyethylene, polyimide, and polyamideimide. As the slurry solvent, N-methyl-2-pyrrolidone (NMP) can be used.

  The positive electrode current collector is not particularly limited, but from the viewpoint of electrochemical stability, for example, aluminum, nickel, titanium, tantalum, stainless steel (SUS), other valve metals, or their Alloys can be used. Examples of the shape include foil, flat plate, and mesh. In particular, an aluminum foil can be suitably used.

  In the production of the positive electrode, a conductive auxiliary material may be added for the purpose of reducing the impedance. Examples of the conductive auxiliary material include carbonaceous fine particles such as graphite, carbon black, and acetylene black.

(3C. Electrolyte)
As the electrolyte, a non-aqueous electrolyte solution in which a lithium salt is dissolved in one or two or more non-aqueous solvents can be used. The non-aqueous solvent is not particularly limited. For example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); dimethyl carbonate (DMC) Chain carbonates such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; γ- such as γ-butyrolactone Lactones; chain ethers such as 1,2-ethoxyethane (DEE) and ethoxymethoxyethane (EME); and cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran. Other non-aqueous solvents include dimethyl sulfoxide, 1,3-dioxolane, dioxolane derivatives, formamide, acetamide, dimethylformamide, acetonitrile, propionitrile, nitromethane, ethyl monoglyme, phosphate triester, trimethoxymethane, sulfolane, methyl Non-protons such as sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone An organic solvent can also be used.

Examples of the lithium salt dissolved in the nonaqueous solvent, is not particularly limited, for _{example, LiPF 6, LiAsF 6, LiAlCl} 4, LiClO 4, LiBF 4, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Examples thereof include Li (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , lithium bisoxalatoborate, and the like. These lithium salts can be used individually by 1 type or in combination of 2 or more types. Further, a polymer electrolyte may be used instead of the non-aqueous electrolyte solution.

(3D. Battery configuration)
The positive electrode and the negative electrode described above can constitute a battery by making each active material layer face each other and filling the electrolyte therebetween. A separator can be provided between the positive electrode and the negative electrode. As this separator, a porous film, a woven fabric, or a nonwoven fabric made of a polyolefin such as polypropylene or polyethylene, a fluororesin such as polyvinylidene fluoride, polyimide, or the like can be used.

  Examples of the battery shape include a cylindrical shape, a square shape, a coin shape, a button shape, and a laminate shape. In the case of a laminate type, it is preferable to use a laminate film as an exterior body that accommodates a positive electrode, a separator, a negative electrode, and an electrolyte. The laminate film includes a resin base material, a metal foil layer, and a heat seal layer (sealant). Examples of the resin base material include polyester and polyamide (nylon), and examples of the metal foil layer include aluminum, aluminum alloy, and titanium foil. Examples of the material for the heat welding layer include thermoplastic polymer materials such as polyethylene, polypropylene, and polyethylene terephthalate. Moreover, the resin base material layer and the metal foil layer are not limited to one layer, and may be two or more layers. From the viewpoint of versatility and cost, an aluminum laminate film is preferable.

  The positive electrode, the negative electrode, and the separator disposed between them are accommodated in an outer container made of a laminate film or the like, and when a nonaqueous electrolytic solution is used, the electrolytic solution is further injected and sealed. A structure in which an electrode group in which a plurality of electrode pairs are stacked can be accommodated.

  Hereinafter, an example is given and it demonstrates concretely. In addition, this invention is not limited only to these Examples.

Various measurement methods are as follows.
・ Density (true density)
The measurement was performed according to JIS K2151.
Specific surface area According to the BET method (ISO 9277: 2010, JIS Z8830).
-Crystallite diameter (Lc), lattice spacing (d ₀₀₂ )
It is calculated | required by the method based on the Gakushin method based on the [002] main peak by XRD measurement.
・ Maximum particle size, average particle size (D ₅₀ )
The maximum value in the particle size distribution (volume basis) by the laser diffraction scattering method and the particle size (median diameter: D ₅₀ ) at an integrated value of 50% were measured.

Example 1 (Production of graphene aggregates from graphite)
An electrochemical cell as shown in FIG. 1 was prepared, and a commercially available graphite rod (Niraco graphite rod C-072561) was used as the cathode (working electrode), and a platinum electrode was used as the anode (control electrode). As an electrolytic solution, 1 g of sodium chloride was dissolved in 20 ml of DMSO and then added to 5 ml of deionized water. Cathode stripping was performed at a voltage of 15 V and a current density of 10 mA / cm ² . After the treatment for 24 hours, the graphene aggregates settled at the bottom of the cell were collected by filtration. After washing with deionized water, it was dried in a vacuum desiccator at 80 ° C. for 4 hours.

  Various physical properties of the obtained graphene aggregate were measured. The results are shown in Table 1. Moreover, the XRD analysis result of the obtained graphene aggregate is shown in FIG. 4 with untreated graphite. Further, SEM images of the obtained graphene aggregates are shown in FIG. 5 (500 times) and FIG. 6 (10,000 times).

Example 2 (Production of graphene aggregates from soft carbon)
A graphene aggregate was obtained in the same manner as in Example 1 except that a soft carbon (pitch coke) packed in a mesh bag as a cathode and a stainless bar inserted as a terminal was prepared and the cathode was peeled off. Table 1 shows the measurement results of various physical properties.

Example 3
A negative electrode active material for a lithium ion battery in which the graphene aggregate obtained in Example 1 was activated using KOH was prepared. Specifically, graphene aggregates are first immersed in a 7M KOH aqueous solution for 12 hours, separated by vacuum filtration, heat treated at 800 ° C. for 1 hour in a nitrogen atmosphere, washed with water, dried at 70 ° C. for 24 hours, and activated. A treated negative electrode active material was prepared.

Example 4
A negative electrode active material for a lithium ion battery in which tin oxide was modified on the graphene aggregate obtained in Example 1 was prepared. Specifically, first, 0.1-0.5 M tin chloride, 20 ml ethanol, 5 ml surfactant (Triton X100) was added to a solution in which graphene aggregates were dispersed in 200 ml water, and the mixture was heated at 220 ° C. for 4 hours. Hydrothermal synthesis for hours. This was filtered through a PTFE filter, and the resulting slurry was dried in vacuum at 80 ° C. for 12 hours. The dried powder was heat treated in vacuum at 800 ° C. for 3 hours and then washed with deionized water until the pH reached 7. Then, it dried at 80 degreeC for 4 hours in vacuum, and prepared the negative electrode active material which modified the tin oxide.

Example 5
A negative electrode active material for a lithium ion battery in which the graphene aggregate obtained in Example 1 was coated with an amorphous carbon film was prepared. Specifically, graphene aggregates were first dispersed in a 0.1 M sucrose solution and hydrothermally synthesized at 220 ° C. for 6 hours. This was filtered through a PTFE filter, and the resulting slurry was dried in a vacuum at 80 ° C. for 12 hours. The dried powder was heat treated in vacuum at 800 ° C. for 3 hours and then washed with deionized water until the pH reached 7. Then, it dried at 80 degreeC in vacuum for 4 hours, and prepared the negative electrode active material which coat | covered the amorphous carbon.

Comparative Example A graphite powder having an average particle size of 10 μm and a specific surface area of 7 m ² / g was prepared and used as it was as a negative electrode material.

(Charge / discharge test)
The negative electrode active material, conductive agent (carbon black), and binder (PVdF) of Examples 1 to 5 and Comparative Example were mixed at a mass ratio of negative electrode active material: conductive agent: binder = 92: 1: 7. The slurry was dispersed in NMP. The slurry was applied on a copper foil, dried and rolled, and then cut into 22 × 25 mm to obtain an electrode. This electrode was used as a working electrode, and a laminate was obtained by combining with a counter electrode Li foil across a separator. This laminate and an electrolytic solution (a mixed solution of EC and DEC containing 1M LiPF ₆ and a volume ratio EC / DEC = 3/7) were sealed in an aluminum laminate container to produce a battery.

  At a predetermined current value, the potential of the working electrode with respect to the counter electrode was charged to 0 V (Li was inserted into the working electrode) and discharged to 1.5 V (Li was desorbed from the working electrode). The current value at the time of charging / discharging is such that the current value for flowing the discharge capacity of the working electrode in 1 hour is 1C, and charging / discharging in the first and second cycles is 0.1C charging-0.1C discharging. Was 0.5 C charge-0.1 C discharge.

  As the charge / discharge characteristics, an initial discharge capacity (discharge capacity at the first cycle) and a charge rate characteristic (discharge capacity at the third cycle / discharge capacity at the second cycle) were obtained. The results are shown in Table 2.

DESCRIPTION OF SYMBOLS 1 Electrochemical cell housing | casing 2 Electrolyte 3 Carbon material 4 Insoluble anode 5 Power supply 6 Graphene 7 Graphene aggregate 11 Graphite 21 Soft carbon 12, 12 'Graphene 13 Solvate 14, 14' Graphene aggregate 22 Cluster

Claims (13)

  A method for producing a graphene aggregate, wherein a graphene aggregate is electrochemically exfoliated in an electrolytic solution using a carbon material containing a graphene laminate as a cathode, and the graphene aggregate desorbed from the cathode is recovered.   The method for producing a graphene aggregate according to claim 1, wherein the electrolytic solution contains a solvated cation.   The method for producing a graphene aggregate according to claim 2, wherein the solvated cation is a sodium ion solvated with dimethyl sulfoxide.   The method for producing a graphene aggregate according to any one of claims 1 to 3, wherein a voltage during the electrochemical stripping is 3 V or more.   The method for producing a graphene aggregate according to any one of claims 1 to 4, wherein the carbon material used as the cathode contains natural graphite or artificial graphite.   The method for producing a graphene aggregate according to any one of claims 1 to 4, wherein the carbon material used as the cathode contains non-graphitizable or graphitizable carbon. Graphene aggregate that satisfies the following conditions.
(1) The particle diameter is 300 μm or less and the average particle diameter D ₅₀ is 1 to 20 μm,
(2) Density is 0.5 g / cm ³ or more,
(3) Specific surface area by BET method is 300 m < ² > / g or less. The graphene aggregate is a single layer graphene having a crystallite diameter Lc (002) in the range of 0.005 to 50 μm, and a multilayer in which the graphene having the crystallite diameter is stacked with a lattice spacing (d ₀₀₂ ) of 0.3355 nm or more. The graphene aggregate according to claim 7, which is formed by aggregation of graphene or a mixture thereof.   The graphene aggregate according to claim 7 or 8, wherein the graphene aggregate is obtained by the method according to any one of claims 1 to 6.   The lithium ion battery which used the graphene aggregate of any one of Claims 7-9 for the negative electrode material.   The lithium ion battery according to claim 10, wherein the graphene aggregate has pores having a diameter of 0.5 to 500 nm on a surface.   The lithium ion battery according to claim 10, wherein the graphene aggregate is modified with a lithium storage material.   The lithium ion battery according to claim 10, wherein a surface of the graphene aggregate is coated with an amorphous carbon film.