Graphene material, its manufacturing method and its use

Abstract

To provide a method for producing graphene with excellent physical properties such as electrical properties without using a reducing agent in a production step.SOLUTION: The method for producing a graphene material according to the present invention comprises the steps of: preparing graphene oxide; heat-treating the graphene oxide at 300°C to 700°C; dispersing the heat-treated graphene oxide in a dispersion medium to prepare a thermally reduced graphene oxide dispersion; and hydrothermally treating the thermally reduced graphene oxide dispersion at 150°C to 250°C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a graphene material, a method for producing the same, and its uses. [Background technology]

[0002] Graphene is a material in which carbon atoms are arranged in a hexagonal planar structure. The carbon atoms in graphene are sp 2 They are bonded together and take the form of planar sheets that are a single atom thick.

[0003] Graphene has excellent electrical conductivity, thermal conductivity, and mechanical strength, and research is underway to utilize it in a variety of fields, including battery materials, energy storage materials, electronic devices, as well as automobiles, aerospace, and medicine.

[0004] Graphene can be produced by mechanical exfoliation, chemical vapor deposition (CVD), lamination onto a silicon carbide (SiC) substrate, chemical oxidation-reduction, and the like.

[0005] Among these, the chemical redox method is a method in which graphene oxide is obtained by oxidizing natural graphite and then a reduction reaction is performed to produce graphene (see, for example, Patent Document 1). The chemical redox method is promising as an industrial production method because it allows for mass production of graphene. However, this method requires the use of a reducing agent such as hydrazine for the deoxidation reaction of graphene oxide. Many of these reducing agents have hazards such as high corrosivity, explosiveness, toxicity to the human body, and environmental harm, which strongly limits their industrial use. In addition, there are concerns that the produced graphene may contain impurities, resulting in low conductivity.

[0006] Therefore, there is a demand for a method for producing graphene with excellent physical properties such as electrical properties through a low-risk process. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-104269 Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above, an object of the present invention is to provide a method for producing graphene having excellent physical properties such as electrical properties, without using a reducing agent in the production process. Another object of the present invention is to provide a graphene material produced by such a production method and uses of the graphene material. [Means for solving the problem]

[0009] A method for producing a graphene material according to the present invention includes the steps of preparing graphene oxide, heat-treating the graphene oxide at 300°C to 700°C, dispersing the heat-treated graphene oxide in a dispersion medium to prepare a thermally reduced graphene oxide dispersion, and hydrothermally treating the thermally reduced graphene oxide dispersion at 150°C to 250°C, thereby solving the above-mentioned problems. The time for the heat treatment step may be 1 minute or less, and the time for the hydrothermal treatment step may be in the range of 10 hours to 36 hours. In the step of preparing the thermally reduced graphene oxide dispersion, the dispersion medium may be water or ethanol. The heat-treating step may include heat-treating the graphene oxide in a muffle furnace at 350°C to 500°C for one minute or less, and the step of preparing the thermally reduced graphene oxide dispersion may include hydrothermally treating the thermally reduced graphene oxide dispersion in an autoclave at 160°C to 220°C for 12 hours to 24 hours.

[0010] The graphene material according to the present invention is substantially free of aggregation of graphene sheets and has an oxygen content of 7.0% or less as determined by XPS analysis, thereby solving the above-mentioned problems. In the graphene material of the present invention, the number of layers of the graphene sheets measured from a high-resolution transmission electron microscope (HRTEM) image may be 7 or less.

[0011] The graphene electrode according to the present invention contains the above-mentioned graphene material, thereby solving the above-mentioned problems. The graphene electrode of the present invention may further contain a conductive material and a binder. The graphene electrode of the present invention may be used for an electric double layer capacitor. [Effects of the Invention]

[0012] The method for producing a graphene material of the present invention includes the steps of heat-treating graphene oxide under predetermined conditions and hydrothermal-treating a thermally reduced graphene oxide dispersion, in which the heat-treated graphene oxide is dispersed in a predetermined dispersion medium, under predetermined conditions. Combining the heat-treatment step and the hydrothermal-treatment step in this order progressively removes oxygen-containing functional groups on the graphene sheets constituting the graphene oxide, thereby maximizing the effects of the thermal reduction and hydrothermal reduction while suppressing structural destruction and defects. Therefore, the method for producing a graphene material of the present invention can produce a graphene material with excellent physical properties, such as electrical properties, even through a production process that does not use a reducing agent. The method for producing a graphene material of the present invention not only involves low-risk processes, but also does not require skilled techniques or expensive equipment, making it economical and efficient, and is environmentally friendly, making it suitable for mass production.

[0013] The graphene material of the present invention is produced by the above-described production method, and is a graphene material with a small number of layers, in which aggregation due to interactions between graphene molecules is suppressed, and with a low oxygen content. Therefore, the graphene material of the present invention is a graphene material that does not use a reducing agent in its production process and has excellent physical properties such as electrical properties. By using the graphene material of the present invention, it is possible to provide graphene electrodes for power storage devices such as electric double layer capacitors and lithium ion batteries. Furthermore, the graphene material of the present invention has properties comparable to or superior to those of graphene produced by conventional chemical oxidation-reduction methods, and therefore can be applied to various fields other than electrode materials. [Brief explanation of the drawings]

[0014] [Figure 1] Flowchart showing the manufacturing process of the graphene material of the present invention [Figure 2] Schematic diagram showing an electric double layer capacitor of the present invention. [Figure 3] (a) to (e) are TEM and HRTEM images of the graphene materials of Examples 1 to 5. [Figure 4A] Figure showing the XPS spectrum of the graphene material in Example 1 [Figure 4B] XPS spectrum of graphene material from Example 2 [Figure 4C] Figure 1 shows the XPS spectrum of the graphene material of Example 3. [Figure 4D] Figure 1 shows the XPS spectrum of the graphene material of Example 4 [Figure 4E] 1 shows the XPS spectrum of the graphene material of Example 5. [Figure 5] 1A and 1B show (a) constant current charge / discharge curves (GCD curves), (b) rate characteristics, (c) electrochemical impedance spectra (EIS), and (d) cycle characteristics of coin cells fabricated using the graphene materials of Examples 1 to 5 as electrode materials. [Figure 6]FIG. 1 shows (a) the specific capacity-voltage curve (CV curve), (b) the constant current charge-discharge curve (GCD curve), (c) the rate characteristics, and (d) the electrochemical impedance spectrum (EIS) of a laminated cell fabricated using the graphene material of Example 4 as an electrode material. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are designated by like numbers and their description will be omitted.

[0016] (Embodiment 1) In the first embodiment, a method for producing a graphene material of the present invention will be described. FIG. 1 is a flowchart showing the steps for producing the graphene material of the present invention.

[0017] Step S110: Prepare graphene oxide.

[0018] The graphene oxide (GO) may be a commonly available material, or step S110 may further include a step of preparing graphene oxide.

[0019] Graphene oxide may be produced by known production methods, for example, graphene oxide prepared from natural graphite using a modified Hummers method.

[0020] Step S120: The graphene oxide is heat-treated at 300°C to 700°C.

[0021] In step S120, graphene oxide is heat-treated under predetermined conditions to thermally reduce the graphene oxide. The conditions for heat-treating graphene oxide are not particularly limited as long as the above-mentioned temperature conditions are met, but a shorter heat treatment time is preferable from the viewpoints of production efficiency and reducing environmental impact. Specifically, for example, a graphene oxide aqueous dispersion in which a predetermined amount of graphene oxide is dispersed in water is freeze-dried for several days (approximately 2 days) in a freeze dryer, and the resulting graphene oxide solid is then heat-treated in a muffle furnace at 300°C to 700°C for less than 1 minute and quickly removed, thereby obtaining the desired thermally reduced graphene oxide (TRGO).

[0022] More preferably, the solid graphene oxide is heat-treated in a muffle furnace at 350° C. to 500° C. for 1 minute or less, thereby enabling the desired thermally reduced graphene oxide to be obtained more efficiently.

[0023] Step S130: The heat-treated graphene oxide is dispersed in a dispersion medium to prepare a thermally reduced graphene oxide dispersion.

[0024] The dispersion medium for preparing the thermally reduced graphene oxide dispersion is not particularly limited, and examples thereof include water and ethanol.

[0025] The concentration of the thermally reduced graphene oxide dispersion is not particularly limited, and may be appropriately selected so that the thermally reduced graphene oxide can be well dispersed in the dispersion medium. Specifically, the concentration of the thermally reduced graphene oxide dispersion may be in the range of 0.1 mg / mL to 1.0 mg / mL, for example.

[0026] In order to uniformly disperse the thermally reduced graphene oxide in the dispersion medium, a mixing / dispersing device such as an ultrasonic homogenizer may be used.

[0027] Step S140: The thermally reduced graphene oxide dispersion is subjected to a hydrothermal treatment at 150°C to 250°C.

[0028] In step S140, the thermally reduced graphene oxide is hydrothermally treated under predetermined conditions to remove functional groups (especially oxygen-containing functional groups) remaining on the graphene sheets after the thermal reduction in step S120, thereby obtaining a graphene material that is highly reduced and in which aggregation of the graphene sheets is suppressed. The conditions for hydrothermal treatment of the thermally reduced graphene oxide are not particularly limited as long as the above-mentioned temperature conditions are met, but a shorter treatment time is preferable from the perspectives of production efficiency and reducing environmental impact. Specifically, for example, the thermally reduced graphene oxide dispersion obtained in step S130 can be hydrothermally treated in an autoclave at 150°C to 250°C for 10 to 36 hours to obtain the desired graphene material.

[0029] More preferably, the thermally reduced graphene oxide dispersion is subjected to hydrothermal treatment in an autoclave at 160° C. to 220° C. for 12 to 24 hours, thereby enabling the desired graphene material to be obtained more efficiently.

[0030] As described above, the graphene material production method of the present invention is characterized in that, when graphene oxide is viewed as a starting material, a two-stage reduction treatment combining heat treatment step S120 and hydrothermal treatment step S140 removes oxygen-containing functional groups on the graphene sheets that constitute graphene oxide, thereby producing graphene with excellent physical properties such as electrical properties while suppressing structural destruction and defects. Furthermore, the order of these steps is to first perform reduction by heat treatment and then reduction by hydrothermal treatment, which improves the effect of removing the functional groups described above and enables the production of a graphene material with excellent physical properties.

[0031] (Embodiment 2) In the second embodiment, the graphene material of the present invention will be described.

[0032] The graphene material of the present invention is preferably produced by the production method described in the first embodiment. The graphene material of the present invention is substantially free of aggregation of graphene sheets, and has an oxygen content of 7.0% or less as determined by XPS analysis. In addition, the graphene material of the present invention preferably has a number of graphene sheets of 7 or less as measured from a high-resolution transmission electron microscope (HRTEM) image.

[0033] As used herein, the phrase "graphene material is substantially free of aggregation of graphene sheets" means that structures resulting from aggregation of graphene sheets are not substantially observed in a transmission electron microscope (TEM) image of the material. In other words, if graphene sheet aggregation is present, a state in which sheet-like structures are gathered together or a wrinkled pattern is observed as a high-contrast area in the transmission electron microscope (TEM) image, but the graphene material of the present invention is characterized by being substantially free of such aggregate structures.

[0034] The oxygen content of the graphene material of the present invention can be measured from a spectrum obtained using an X-ray photoelectron spectroscopy (XPS). The graphene material of the present invention has an oxygen content of 7.0% or less, preferably 6.0% or less, and more preferably 5.5% or less, as determined by XPS analysis. This gives the graphene material of the present invention excellent physical properties such as electrical characteristics, making it suitable for use as an electrode material for electricity storage devices such as electric double layer capacitors and lithium ion batteries. Furthermore, the graphene material of the present invention can also be used suitably for various applications other than as an electrode material.

[0035] The number of graphene sheet layers constituting the graphene material of the present invention can be measured from a high-resolution transmission electron microscope (HRTEM) image. The graphene material of the present invention preferably has 7 or less graphene sheet layers, more preferably 5 or less, as determined by the HRTEM image. This makes the graphene material of the present invention excellent in physical properties such as electrical properties, and more suitable as an electrode material for the above-mentioned electricity storage device or as a material used for various applications other than an electrode material.

[0036] (Embodiment 3) In the third embodiment, applications of the graphene material of the present invention described in the second embodiment will be described.

[0037] The graphene material of the present invention can be suitably used as an electrode material. In particular, the graphene material of the present invention is suitably used as an electrode material for an electricity storage device. Hereinafter, an electric double layer capacitor will be described as an example of an electricity storage device using the graphene material of the present invention as an electrode material.

[0038] FIG. 2 is a schematic diagram showing the electric double layer capacitor of the present invention.

[0039] The electric double layer capacitor of the present invention includes at least electrodes and an electrolyte. The electric double layer capacitor 200 of FIG. 2 includes a positive electrode 210 and a negative electrode 220 immersed in an electrolyte 230. The positive electrode 210 and the negative electrode 220 are electrodes containing the graphene material described in the second embodiment. The electrolyte 230 is, for example, an ionic liquid selected from the group consisting of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazolium borofluoride (EMI-BF4), and 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPp-TFSI), or M'OH (M' is an alkali metal).

[0040] The positive electrode 210 and the negative electrode 220 may further contain a conductive material and a binder in addition to the graphene material described in the second embodiment. This results in a film-like electrode with a smooth surface. In this case, the contents of the graphene material, conductive material, and binder are not particularly limited, but may be mixed so as to satisfy, for example, a weight ratio of graphene material:conductive material:binder = 80 to 95:0 to 10:1 to 10. Mixing at such a weight ratio can achieve higher power density and energy density when applied to a capacitor. Note that in the above ratios, 80 to 95 means 80 or more and 95 or less, 0 to 10 means more than 0 and 10 or less, and 1 to 10 means 1 or more and 10 or less, and the total of the graphene material, conductive material, and binder is prepared to be 100 parts by weight.

[0041] The conductive material is not particularly limited as long as it is a conductive material used in ordinary electrodes. However, taking into consideration dispersibility with the graphene material, a carbon material selected from the group consisting of carbon black, acetylene black, channel black, furnace black, and ketjen black is preferred, for example.

[0042] There are no particular restrictions on the binder as long as it is one that is used as a binder in ordinary electrodes, but typical examples include organic solvent-based binders and water-based binders. Organic solvent-based binders include polytetrafluoroethylene (PTFE), its modified polytetrafluoroethylene resin, polyvinylidene fluoride (PVDF), etc. Water-based binders include sodium carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), etc. In particular, it is recommended to use a combination of CMC and SBR for water-based binders.

[0043] The electric double layer capacitor 200 further includes a separator 240 between the positive electrode 210 and the negative electrode 220 to separate the positive electrode 210 and the negative electrode 220 .

[0044] The material of separator 240 is, for example, a material selected from fluorine-based polymers, polyethers such as polyethylene oxide and polypropylene oxide, polyolefins such as polyethylene and polypropylene, polyacrylonitrile, polyvinylidene chloride, polymethyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, polyurethane-based polymers and derivatives thereof, cellulose, paper, and nonwoven fabric.

[0045] In the electric double layer capacitor 200, the above-mentioned positive electrode 210, negative electrode 220, electrolyte 230, and separator 240 are housed in a cell 250. Furthermore, the positive electrode 210 and the negative electrode 220 each have an existing current collector.

[0046] Such an electric double layer capacitor 200 may be a chip type, coin type, mold type, pouch type, laminate type, cylindrical type, square type, or the like capacitor, and may further be used in a module in which multiple of these are connected together.

[0047] Next, the operation of the electric double layer capacitor 200 shown in FIG. 2 will be described.

[0048] When a voltage is applied to the electric double layer capacitor 200, electrolyte ions (anions) of the electrolyte 230 are adsorbed to the positive electrode 210, and electrolyte ions (cations) of the electrolyte 230 are adsorbed to the negative electrode 220. As a result, an electric double layer is formed in each of the positive electrode 210 and the negative electrode 220, and charging occurs. Here, since the positive electrode 210 and the negative electrode 220 are formed from the graphene material described in the second embodiment, adsorption and diffusion of cations and anions by the graphene is facilitated, thereby achieving high rate characteristics. Furthermore, since the positive electrode 210 and the negative electrode 220 are formed from the graphene material described in the second embodiment, many electrolyte ions are adsorbed not only on the surface of the graphene but also inside the graphene, forming an electric double layer. As a result, electron exchange between the graphene and the electrolyte ions is increased, and high energy density can be achieved.

[0049] When the charged electric double layer capacitor 200 is connected to a circuit such as a resistor, the anions and cations adsorbed to the positive electrode 210 and the negative electrode 220, respectively, are desorbed and discharged. Here again, the positive electrode 210 and the negative electrode 220 are formed from the graphene material described in the second embodiment, which facilitates desorption and diffusion of electrolyte ions, thereby achieving high rate characteristics and energy density. Furthermore, because of their excellent electrical conductivity, the ease of desorption and diffusion can also lead to improved power density.

[0050] As described above, the electric double layer capacitor 200 of the present invention can fully utilize the properties of graphene in the electrodes, enabling rapid charging and achieving high energy density and high power density. Furthermore, since the formation of an electric double layer is utilized for charging and discharging, the electric double layer capacitor 200 of the present invention is excellent for repeated use. The electric double layer capacitor 200 of the present invention can be used in wind power generation, electric vehicles, etc.

[0051] Although the electric double layer capacitor has been described as an example here, it goes without saying that the graphene material of the present invention can be applied as an electrode material for electricity storage devices such as lithium ion batteries in addition to electric double layer capacitors.

[0052] The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples. [Example]

[0053] [Production of graphene materials] (Examples 1 to 5) Graphene oxide (GO) was prepared from natural graphite using a modified Hummers method (step S110 in Figure 1).

[0054] Next, a reduction treatment of the graphene oxide was carried out according to the manufacturing steps shown in Table 1 below.

[0055] In Example 1, graphene oxide aqueous dispersion (0.5 mg / mL) prepared by dispersing graphene oxide in water was freeze-dried for 2 days in a freeze dryer, and the resulting sponge-like graphene oxide was then heated at 400°C in a muffle furnace for 1 minute or less and quickly removed (step S120 in FIG. 1).

[0056] In Example 2, the above-mentioned graphene oxide aqueous dispersion was subjected to hydrothermal treatment in an autoclave at 160° C. for 12 hours (hydrothermal reduction).

[0057] In Example 3, after hydrothermal treatment under the same conditions as in Example 2, the hydrothermally treated graphene oxide was heated in a muffle furnace at 400°C for less than 1 minute and quickly removed (thermal reduction).

[0058] In Example 4, graphene oxide obtained in the same manner as in Example 1 was heated in a muffle furnace at 400° C. for 1 minute or less and quickly removed (Step S120 in FIG. 1). Next, the heat-treated graphene oxide was dispersed in water to prepare a thermally reduced graphene oxide dispersion (0.5 mg / mL) (step S130 in FIG. 1). The thermally reduced graphene oxide dispersion was then hydrothermally treated in an autoclave at 160° C. for 12 hours (step S140 in FIG. 1).

[0059] In Example 5, graphene oxide obtained in the same manner as in Example 1 was reduced with hydrazine (hydrazine reduction).

[0060] [Analysis of physical properties] 3(a) to 3(e) show the results of observation of the graphene materials obtained in Examples 1 to 5, respectively, using a transmission electron microscope (TEM, JEOL Ltd., field emission electron microscope: JEM-2100F). The scale bar in each figure is 500 nm. In addition, a high-resolution transmission electron microscope (HRTEM) image is shown in the upper right corner of each figure. The scale bar in the HRTEM image is 10 nm.

[0061] Figure 3(a) shows that the graphene material of Example 1 has a small number of layers and is in the form of a thin sheet. However, as indicated by the arrows in the HRTEM image, there are amorphous areas with low crystallinity, and the material has defects in the crystal structure. 3(b) and (c) show that the graphene materials of Examples 2 and 3 both have a large number of layers, and the graphene sheets are aggregated in areas with high contrast. Furthermore, Figure 3(e) shows that the graphene material of Example 5 has a large number of layers, is partially wrinkled as indicated by the arrows in the figure, and has areas where the graphene sheets are aggregated (areas with strong contrast).

[0062] In contrast, Figure 3(d) shows that the graphene material of Example 4 has a smaller number of layers, and the surface of the edge region where the graphene sheets are not folded is almost transparent. Furthermore, the HRTEM image clearly shows a single graphene sheet. The number of layers was evaluated from the HRTEM image, and the number of graphene sheets in this frame was approximately 3 to 4.

[0063] These results demonstrate that the method for producing a graphene material of the present invention can provide a graphene material that has a small number of layers, is in the form of a thin sheet, and is substantially free of aggregation of graphene sheets.

[0064] The specific surface area and oxygen content of the graphene materials obtained in Examples 1 to 5 were measured. The results are shown in Table 1. The specific surface area was evaluated from the nitrogen adsorption-desorption isotherm by the BET method. The oxygen content was evaluated from the spectrum obtained using an X-ray photoelectron spectrometer (XPS: PHI Quantera SXM manufactured by ULVAC-PHI, X-ray source: Al Kα, analyzer: hemispherical analyzer).

[0065] 4A to 4E are diagrams showing XPS spectra of the graphene materials of Examples 1 to 5, respectively. Each diagram shows raw data (Raw) that has not been subjected to analytical processing, data (Fitting) that has been subjected to fitting processing on the raw data, and data on four components (CC, CO, COOH, π-π) obtained by peak separation (waveform separation).

[0066] [Table 1]

[0067] According to Table 1, it was found that the graphene material of Example 4 had a larger specific surface area and a lower oxygen content than the graphene material of Example 5 (i.e., conventional graphene oxide reduced with hydrazine). In other words, it was found that the method for producing a graphene material of the present invention can effectively remove oxygen-containing functional groups on the graphene sheets constituting graphene oxide, and produce a graphene material with physical properties equivalent to or superior to those of chemically reduced graphene oxide, by a production process that does not use a hazardous chemical substance such as hydrazine and does not use a reducing agent.

[0068] On the other hand, the graphene material of Example 1 has a larger specific surface area than the graphene material of Example 4, but has a higher oxygen content, and it is clear that the reduction of graphene oxide is insufficient. The graphene materials of Examples 2 and 3 have a smaller specific surface area and a higher oxygen content than the graphene material of Example 4. These results indicate that in the method for producing a graphene material of the present invention, oxygen-containing functional groups on the graphene sheets constituting graphene oxide are effectively removed by a two-step reduction treatment that combines a heat treatment step and a hydrothermal treatment step, and that the order of these steps, in which reduction by heat treatment is carried out first and then reduction by hydrothermal treatment, improves the effect of removing the functional groups and enables the production of a graphene material with excellent physical properties.

[0069] [Analysis of electrical characteristics] Next, to evaluate the electrical properties of the graphene materials, electric double-layer capacitors (CR2032 coin cells) were fabricated using these as electrodes.

[0070] Specifically, the graphene materials of Examples 1 to 5 were dispersed in a CMC (carbomexymethylcellulose) aqueous dispersion and mixed with conductive carbon black as a conductive material and SBR (styrene butadiene rubber) as a binder to obtain a slurry. This slurry was applied to an Al (aluminum) current collector and dried in a vacuum at 120°C for 24 hours to obtain an electrode film. Next, a separator (glass fiber) was placed between the electrodes in a stainless steel cell, and an ionic liquid (EMI-BF4) was filled as an electrolyte to prepare a coin cell. The coin cell was assembled in a glove box filled with Ar gas.

[0071] Electrochemical measurements of the coin cells were performed using a multi-channel potentiostat / galvanostat (Bio-Logic, VMP-300). Specific capacitance-voltage measurements (CV measurements) and galvanostatic charge / discharge measurements were performed at room temperature in the potential range of 0 V to 3.5 V. Electrochemical impedance measurements were also performed.

[0072] The specific capacitance Cs (F / g) was calculated according to the formula Cs = 4I / (mdV / dt), where I (A) is the constant current, m (g) is the total mass of the two electrodes, and dV / dt (V / s) is the slope obtained by linear fitting of the discharge curve between Vmax (the voltage at the start of discharge) and 1 / 2Vmax. The energy density E cell (Wh / kg) in the formula E cell =CsV 2 The power density P cell (W / kg) into the formula P cell =E cell / t (where t is the discharge time).

[0073] Figures 5(a) to 5(d) show the constant current charge / discharge curves (GCD curves), rate characteristics, electrochemical impedance spectra (EIS), and cycle characteristics of the prototype coin cells. In Figures 5(b) to 5(d), the plots corresponding to Examples 1 to 5 are circles, squares, downward-pointing triangles, upward-pointing triangles, and diamonds, respectively. The GCD curve shown in FIG. 5(a) was measured at a current density of 0.2 A / g. The rate characteristics shown in FIG. 5(b) are the results obtained in the current density range of 0.1 A / g to 5.0 A / g. The EIS shown in Figure 5(c) was fitted using an equivalent circuit model. The cycle characteristics shown in FIG. 5(d) indicate the capacitance retention rate up to 10,000 cycles.

[0074] Table 2 below shows a summary of the electrical properties of the graphene materials of Examples 1 to 5 obtained from the measurement results.

[0075] [Table 2]

[0076] As shown in Table 2, the graphene material of Example 4 exhibited a specific capacitance of 154 F / g at 0.1 A / g and maintained a specific capacitance of 128 F / g even at a higher current of 5.0 A / g, demonstrating a rate capability of over 80%. Furthermore, the graphene material exhibited a capacitance retention rate of over 90% even after 10,000 cycles, demonstrating excellent longevity. Furthermore, the energy density and power density reached 66 Wh / kg and 172 kW / kg, respectively, at a current density of 0.1 A / g. Furthermore, the graphene material of Example 4 exhibited the lowest equivalent resistance (3.3 Ω) (Figure 5(c)). These results are superior to those of the graphene material of Example 5 (i.e., graphene oxide reduced by conventional hydrazine treatment), confirming that the graphene material produced by the production method of the present invention can exhibit excellent performance as an electrode material for energy storage devices.

[0077] On the other hand, the graphene materials of Examples 1 to 3 partially exhibit performance equivalent to that of the graphene material of Example 4, but it cannot be said that the electrical properties inherent to graphene are fully exhibited.

[0078] Here, a laminate-type electric double layer capacitor was fabricated using the graphene material of Example 4 for the electrodes, and the electrical properties were further evaluated. The structure of this electric double layer capacitor was the same as that shown in Figure 2. Specifically, two electrodes were placed on each side, each measuring 3x3cm. 2 The positive and negative electrodes were ultrasonically fused to a positive electrode terminal and a negative electrode terminal, respectively, and the electrodes were placed facing each other with a 25 μm thick cellulose separator sandwiched between them, and then housed in an exterior body made of a laminate film of polypropylene, aluminum, and nylon. An ionic liquid (EMI-BF4) was injected into the exterior body as an electrolyte, and the exterior body was heat-sealed to encapsulate the positive and negative electrode terminals with the ends of the terminals pulled out of the exterior body, thereby assembling a laminate cell.

[0079] 6(a) to 6(d) are graphs showing the specific capacity-voltage curve (CV curve), constant current charge / discharge curve (GCD curve), rate characteristics, and electrochemical impedance spectrum (EIS) of the prototype laminated cell, respectively. Here, the CV curve shown in FIG. 6(a) was measured in the potential range of 0 V to 3.7 V at a sweep rate of 10 mV / s. The GCD curve shown in Figure 6(b) was measured at a current density of 0.2 A / g. The rate characteristics shown in FIG. 6(c) are the results obtained in the current density range of 0.1 A / g to 5.0 A / g. The EIS shown in Figure 6(d) was fitted using an equivalent circuit model.

[0080] As shown in Figure 6(a), the graphene material of Example 4 exhibited a rectangular CV curve, which is indicative of an ideal electric double layer capacitor. Furthermore, as shown in Figures 6(b) and 6(c), it exhibited a specific capacitance of 150 F / g at 0.1 A / g, and maintained a specific capacitance of 132 F / g even at a higher current of 5.0 A / g, demonstrating a rate capability of over 80%. Additionally, the graphene material of Example 4 exhibited a low equivalent resistance (0.8 Ω), similar to that of the coin cell described above (Figure 6(d)).

[0081] [Analysis of the electrical properties of graphene materials produced under different hydrothermal treatment conditions] Next, in the graphene material production process of Example 4, graphene materials were produced by varying the hydrothermal treatment conditions (conditions of step S140 in FIG. 1) in various ways, and their electrical properties were evaluated.

[0082] (Examples 6 to 10) In Examples 6 to 10, graphene materials were produced under temperature conditions varied from 120° C. to 220° C. as shown in Table 3 below. Using the graphene material as an electrode, an electric double layer capacitor (CR2032 coin cell) similar to that described above was fabricated, and electrochemical measurements were performed. Specific capacitance-voltage measurements (CV measurements) and galvanostatic charge-discharge measurements were performed at room temperature in the potential range of 0 V to 3.5 V. Table 3 shows the results of the specific capacity and rate characteristics obtained in the current density range of 0.1 A / g to 5.0 A / g.

[0083] [Table 3]

[0084] According to Table 3, when the hydrothermal treatment temperature was 140°C, the rate characteristic was 80% or more, while the specific capacity at 0.1 A / g was 126 F / g (Example 7). In contrast, when the hydrothermal treatment temperature was 160°C, the specific capacity at 0.1 A / g was 155 F / g, and the rate characteristic was 81% (Example 8). Furthermore, when the hydrothermal treatment temperatures were 180°C and 220°C, the specific capacities at 0.1 A / g were 154 F / g and 157 F / g, and the rate characteristic was 83% (Examples 9 and 10). This suggests that when the hydrothermal treatment temperature is higher than 140°C, the specific capacity at 0.1 A / g is 150 F / g or more, and the rate characteristic can be 80% or more. From these results, it was found that the temperature condition for the hydrothermal treatment is preferably 150°C or higher, and more preferably 160°C or higher.

[0085] (Examples 11 to 15) In Examples 11 to 15, graphene materials were produced by varying the time conditions from 2 hours to 24 hours as shown in Table 4 below. Using the graphene material as an electrode, an electric double layer capacitor (CR2032 coin cell) similar to that described above was fabricated, and electrochemical measurements were performed. Specific capacitance-voltage measurements (CV measurements) and galvanostatic charge-discharge measurements were performed at room temperature in the potential range of 0 V to 3.5 V. Table 4 shows the results of the specific capacity and rate characteristics obtained in the current density range of 0.1 A / g to 5.0 A / g.

[0086] [Table 4]

[0087] According to Table 4, when the hydrothermal treatment time was 8 hours, a specific capacity of 150 F / g was observed at 0.1 A / g, while the rate characteristic was 77% (Example 14). In contrast, when the hydrothermal treatment time was 12 hours, a specific capacity of 155 F / g was observed at 0.1 A / g, while the rate characteristic was 81% (Example 8). Furthermore, when the hydrothermal treatment time was 24 hours, a specific capacity of 158 F / g was observed at 0.1 A / g, while the rate characteristic was 83% (Example 15). This suggests that when the hydrothermal treatment time is longer than 8 hours, a specific capacity of 150 F / g or more at 0.1 A / g and a rate characteristic of 80% or more can be observed. From these results, it was found that the hydrothermal treatment time condition is preferably 10 hours or longer, and more preferably 12 hours or longer.

[0088] (Example 16, Example 17) In Examples 16 and 17, graphene materials were produced using ethanol and a mixed solvent of ethanol:N,N-dimethylformamide (DMF):water=90:9:1 (volume ratio) as the dispersion medium for the thermally reduced graphene oxide dispersion, respectively, as shown in Table 5 below. Using the graphene material as an electrode, an electric double layer capacitor (CR2032 coin cell) similar to that described above was fabricated, and electrochemical measurements were performed. Specific capacitance-voltage measurements (CV measurements) and galvanostatic charge-discharge measurements were performed at room temperature in the potential range of 0 V to 3.5 V. Table 5 shows the results of the specific capacity and rate characteristics obtained in the current density range of 0.1 A / g to 1.0 A / g.

[0089] [Table 5]

[0090] According to Table 5, when the dispersion medium was water or ethanol, the specific capacity was 155 F / g or more at 0.1 A / g, and the rate characteristic was 85% or more (Examples 8 and 16). In contrast, when the dispersion medium was the above-mentioned mixed solvent, the specific capacity was 130 F / g at 0.1 A / g, and the rate characteristic was 61% (Example 17). These results demonstrate that water or ethanol is preferable as the dispersion medium for the thermally reduced graphene oxide dispersion to be subjected to hydrothermal treatment (the dispersion medium used in step S130 in FIG. 1). Furthermore, considering the results in Tables 3 and 4, water is considered to be a more preferable dispersion medium. [Industrial Applicability]

[0091] The method for producing a graphene material of the present invention can produce a graphene material with excellent physical properties, such as electrical properties, through a production process that does not use a reducing agent. In other words, it is a method that can produce a graphene material economically and efficiently through a low-risk process, and can be easily scaled up for mass production. The graphene material of the present invention produced by such a production method can effectively exhibit the electrical properties inherent to graphene, and is expected to be applied to various fields, including as an electrode material for electricity storage devices (particularly for electric double layer capacitors). [Explanation of symbols]

[0092] 200 Electric double layer capacitor 210 Positive electrode 220 Negative electrode 230 Electrolytes 240 Separator 250 cells

Claims

1. 1. A method for producing graphene material, comprising: providing graphene oxide; heat-treating the graphene oxide at 300°C to 700°C; dispersing the heat-treated graphene oxide in a dispersion medium to prepare a thermally reduced graphene oxide dispersion; subjecting the thermally reduced graphene oxide dispersion to a hydrothermal treatment at 150°C to 250°C; A method comprising:

2. 2. The method of claim 1, wherein the duration of the heat treating step is 1 minute or less, and the duration of the hydrothermal treating step is in the range of 10 hours to 36 hours.

3. The method according to claim 1 or 2, wherein in the step of preparing the thermally reduced graphene oxide dispersion, the dispersion medium is water or ethanol.

4. The heat-treating step includes heat-treating the graphene oxide in a muffle furnace at 350°C to 500°C for 1 minute or less; In the step of preparing the thermally reduced graphene oxide dispersion, the dispersion medium is water; the hydrothermal treatment step comprises hydrothermally treating the thermally reduced graphene oxide dispersion in an autoclave at 160°C to 220°C for 12 hours to 24 hours; The method according to any one of claims 1 to 3.

5. A graphene material in which aggregation of graphene sheets is substantially absent and the oxygen content determined by XPS analysis is 7.0% or less.

6. 6. The graphene material according to claim 5, wherein the number of layers of the graphene sheets measured from a high-resolution transmission electron microscope (HRTEM) image is 7 or less.

7. A graphene electrode comprising the graphene material according to claim 5 or 6.

8. 8. The graphene electrode of claim 7, further comprising a conductive material and a binder.

9. The graphene electrode according to claim 7 or 8, which is for an electric double layer capacitor.

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