Power storage device and method for manufacturing same

Abstract

Provided are a high-density and high-capacity power storage device and a method for manufacturing the same.　In manufacturing a power storage device comprising an electrode film using a graphene material as an active material, an electrode body provided with an electrode film and an electrolyte are sealed with an exterior material, and then a high-voltage treatment for applying a voltage higher than the upper limit of the working voltage for a certain period of time is performed in a state in which volume expansion is suppressed. The electrode density of the electrode film is at least 0.4 g / cm3, and the interlayer distance d in the C-axis direction of the graphene in the electrode film as measured by the X-ray diffraction method is at least 0.38 nm.

Description

Electricity storage device and manufacturing method thereof

[0001] The present invention relates to an electricity storage device using a graphene material for electrodes, and a method for manufacturing the same.

[0002] Graphene is sp ２ Graphene is a sheet-like material with a two-dimensional network structure in which carbon atoms are bonded hexagonally, and because of its high conductivity, high strength, and excellent heat resistance, it has attracted attention in various fields, including the electronics field such as electronic materials, as well as biomedical materials and aerospace materials. In particular, for high-energy-density power storage devices such as lithium-ion capacitors and lithium-ion batteries, various types of graphene-based electrodes have been proposed with the aim of increasing capacity, improving voltage resistance, and further improving energy density and power density (see Patent Documents 1 and 2).

[0003] For example, Patent Document 1 describes a capacitor electrode using a graphene porous carbon sheet formed from a graphene porous carbon material such as graphene meso-sponge and carbon nanotubes. Patent Document 2 describes an electrode formed using an electrode material that includes a graphene composite in which carbon nanotubes exist between graphene layers, has a carbon atom to oxygen atom ratio (C / O) of 7 or more as measured by X-ray photoelectron spectroscopy, and contains less than 20% by mass of carbon nanotubes (excluding 0% by mass).

[0004] Furthermore, conventionally, for the purpose of improving energy density, a peak A derived from the G band is observed in the Raman spectrum by Raman spectroscopy at least in the active material of the negative electrode, and the half width of the peak A is 50 cm －１ As described above, a diffraction peak B derived from the (002) plane is observed near 2θ=26° in a diffraction profile obtained by X-ray diffraction (XRD), and a capacitor using graphene in which the interlayer distance derived from this diffraction peak B is 0.330 nm or more and 0.360 nm or less has been proposed (see Patent Document 3).

[0005] International Publication No. 2020 / 080520 JP 2023-170689 A JP 2023-131624 A

[0006] However, since graphene materials have a strong tendency to be re-laminate during the electrode production process, electrodes using the porous graphene material described in Patent Document 1 have a problem in that, particularly when attempting to form a high-density electrode film, the pores of the porous graphene are blocked by the re-lamination, weakening the effect of promoting electrolyte transport and reducing capacity. Similarly, the graphene material described in Patent Document 3 is also prone to change in its lamination state during the electrode production process, and when incorporated into a capacitor, the interlayer distance is reduced, and the expected energy density may not be obtained.

[0007] In contrast, the electrode material described in Patent Document 2 has carbon nanotubes present between graphene layers, and therefore can maintain a constant interlayer distance even during the electrode production process. However, in recent electricity storage devices, further improvements in the performance of electrode materials are required.

[0008] Therefore, an object of the present invention is to provide a high-density, high-capacity electricity storage device and a method for manufacturing the same.

[0009] In order to solve the above-mentioned problems, the present inventors conducted extensive experimental studies and found that, by performing a specific treatment after manufacturing an electricity storage device, it is possible to expand the spaces between layers of graphene material constituting an electrode, thereby increasing the capacity of the electricity storage device, and thus arrived at the present invention.

[0010] That is, the electricity storage device according to the present invention has an electrode film using a graphene material as an active material, and the electrode film has a density of 0.4 g / cm ３ The graphene has an interlayer distance of 0.38 nm or more in the C-axis direction as measured by X-ray diffraction. The graphene material is, for example, a composite of graphene and carbon nanotubes. The electrode film may contain a binder resin and may also contain a conductive material. The power storage device of the present invention is, for example, a lithium ion capacitor or an electric double layer capacitor.

[0011] The method for producing an electricity storage device according to the present invention is a method for producing an electricity storage device including an electrode film using a graphene material as an active material, and includes a high-voltage treatment step of sealing an electrode body including the electrode film and an electrolyte with an exterior material, and then applying a voltage higher than an upper limit of a working voltage for a certain period of time in a state in which volume expansion is suppressed, and the electrode film after the high-voltage treatment step has an electrode density of 0.4 g / cm ３ or more, and the interlayer distance in the C-axis direction of the graphene measured by X-ray diffraction is 0.38 nm or more. In the method for producing an electricity storage device of the present invention, a composite of graphene and carbon nanotubes may be used as the graphene material. When the electricity storage device is a lithium ion capacitor, a voltage of 4.5 V or more may be applied for 30 minutes or more in the high-voltage treatment step. When the electricity storage device is an electric double layer capacitor, a voltage of 3.5 V or more may be applied for 30 minutes or more in the high-voltage treatment step. In the high-voltage treatment step, charging and discharging at a high voltage may be repeated multiple times. The high-voltage treatment step may be performed in an environment of 10°C or less.

[0012] According to the present invention, the capacity of an electricity storage device can be increased without reducing the volumetric energy density of the electrode.

[0013] 1 is a schematic diagram showing an example of the structure of a composite of graphene and carbon nanotubes, and FIG. 2 is a schematic diagram showing a state in which the interlayer distance is expanded by high voltage treatment, and FIG. 3 is a result of X-ray diffraction measurement of the electrode films of Example 2, Comparative Example 1, and Reference Example 1.

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments.

[0015] In the electricity storage device according to the embodiment of the present invention, a graphene material is used as the active material, and the density of the electrode film (electrode density) is 0.4 g / cm ３ The graphene in the electrode film has an interlayer distance in the C-axis direction of 0.38 nm or more as measured by X-ray diffraction (XRD).

[0016] [Graphene Material] In the electricity storage device of this embodiment, examples of the graphene material used as the active material include graphene produced by various methods such as a mechanical exfoliation method, a chemical vapor deposition method (CVD method), a method of laminating on a silicon carbide (SiC) substrate, and a chemical oxidation-reduction method, a composite of graphene and other carbon materials, and a composite of graphene and metal. From the viewpoint of improving energy density, a composite of graphene and carbon nanotubes (hereinafter also referred to as a graphene / CNT composite) is preferred.

[0017] <Graphene / CNT Composite> Fig. 1 is a schematic diagram showing an example of the structure of a composite of graphene and carbon nanotubes. The graphene / CNT composite used in the electricity storage device of this embodiment may be any composite of graphene and carbon nanotubes (CNTs), and its structure is not particularly limited. For example, a graphene / CNT composite 10 shown in Fig. 1 may have a structure in which a single layer or several layers of graphene 1 are stacked via CNTs 2.

[0018] In the graphene / CNT composite 10 shown in Fig. 1, the graphenes 1 are regularly arranged at equal intervals and parallel to one another, but the present invention is not limited to this, and the graphenes 1 may be randomly arranged. Similarly, in Fig. 1, the CNTs 2 are arranged parallel to one another in the in-plane direction, but the present invention is not limited to this, and the graphenes 1 may be randomly arranged between the layers of the graphene 1.

[0019] Furthermore, the type of CNT2 is not particularly limited, and may be any of single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT). The size of CNT2 is not particularly limited, but from the viewpoint of promoting uniform dispersion of CNTs in graphene and more efficiently combining with graphene 1, the length is preferably 1 to 20 μm, and the average outer diameter is preferably 0.4 to 5.0 nm, more preferably 1.0 to 3.0 nm.

[0020] Graphene 1 has the characteristic of being prone to aggregation due to π-π stacking, but in graphene / CNT composite 10, CNT2 present between layers of graphene 1 in a single-layer or several-layer stack structure functions as a spacer, preventing restacking and ensuring a high specific surface area. Furthermore, when processed into an electrode, electrolyte flows into the gaps between the layers of graphene 1, making it easier for electrolyte ions to be adsorbed onto the graphene surface. Furthermore, graphene / CNT composite 10 also has high electrical conductivity in the thickness direction because CNT2, which has high electrical conductivity, is present between the layers of graphene 1.

[0021] [Electrode Density] The electrode film of the electricity storage device of this embodiment has a density (electrode density) of 0.4 g / cm ３ The density of the electrode film is 0.4 g / cm ３ If it is less than this, the volume energy density of the electrode will be low, and the performance as an electricity storage device will be insufficient.

[0022] [Graphene Interlayer Distance d] In the power storage device of this embodiment, the graphene contained in the electrode film has an interlayer distance in the C-axis direction of 0.38 nm or more, as measured by X-ray diffraction (XRD). The interlayer distance of graphene is generally about 0.33 to 0.35 nm, and is a maximum of about 0.36 nm even after ions are doped between the graphene layers during charging and discharging.

[0023] In contrast, in the electricity storage device of this embodiment, the interlayer distance d of the graphene contained in the electrode film is 0.38 nm or more, which is longer than that of conventional graphene materials, thereby enabling the realization of a high-capacity electricity storage device.

[0024] [Other Components of Electrode Film] The electrode film of the electricity storage device of this embodiment may contain a conductive material and a binder resin in addition to the graphene material as an active material. The conductive material used in the electrode film of the electricity storage device of this embodiment is not particularly limited, and any known conductive material can be used. However, from the viewpoint of affinity with the graphene material, carbon materials such as carbon black, acetylene black, channel black, furnace black, and ketjen black are preferred.

[0025] The binder resin can also be appropriately selected from organic solvent-based binders and aqueous binders used in ordinary electrodes. Specifically, examples of organic solvent-based binders include polytetrafluoroethylene (PTFE), its modified polytetrafluoroethylene resin, and polyvinylidene fluoride (PVDF). Examples of aqueous binders include sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR). Among these, it is particularly preferable to use a combination of aqueous binders, such as CMC and SBR.

[0026] The thickness of the electrode film is not particularly limited, but is, for example, 50 μm or less.

[0027] [Structure and Type of Electricity Storage Device] The type and structure of the electricity storage device of this embodiment are not particularly limited, and it is sufficient that the device has two or more electrode bodies and an electrolyte, and the above-mentioned electrode film is provided on at least one of the electrode bodies. For example, if the electricity storage device of this embodiment is a lithium ion capacitor, it has a configuration in which a positive electrode (cathode) and a negative electrode (anode) are arranged spaced apart from each other, and a Li ion electrolyte is filled between these electrodes. In this lithium ion capacitor, the above-mentioned electrode film is used for at least the positive electrode.

[0028] When the electricity storage device of this embodiment is an electric double layer capacitor, for example, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazolium borofluoride (EMI-BF ４ The electric double layer capacitor has a configuration in which a positive electrode (cathode) and a negative electrode (anode) are disposed apart from each other in an electrolyte such as an ionic liquid such as 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPp-TFSI) or M'OH (M' is an alkali metal), and a separator is disposed between the positive and negative electrodes. In this electric double layer capacitor, the above-mentioned electrode film is used for both the positive and negative electrodes.

[0029] [Manufacturing Method] Next, a method for manufacturing the above-described electricity storage device will be described. The electricity storage device of this embodiment can be manufactured, for example, by sealing the electrode assembly and the electrolyte with an exterior material, and then performing a high-voltage treatment step in a state in which volume expansion is suppressed.

[0030] <High Voltage Treatment> High voltage treatment is a treatment in which a voltage higher than the upper limit of the working voltage of the electricity storage device is applied for a certain period of time. For example, if the electricity storage device is a lithium ion capacitor, a voltage of 4.5 V or higher may be applied for 30 minutes or more, and if the electricity storage device is an electric double layer capacitor, a voltage of 3.5 V or higher may be applied for 30 minutes or more.

[0031] 2 is a schematic diagram showing a state in which the interlayer distance d has been increased by high-voltage treatment. As shown in FIG. 2, high-voltage treatment allows ions to penetrate between the graphene layers, thereby increasing the interlayer distance d of the graphene. For example, in the case of a graphene / CNT composite 10 having an interlayer distance d of 0.36 nm, high-voltage treatment can increase the interlayer distance d to approximately 0.385 nm. When the interlayer distance d of graphene is increased in this way, the amount of ion storage increases, making it possible to increase the capacity of an electricity storage device.

[0032] The higher the voltage of the high-voltage treatment and the longer the treatment time (high-voltage application time), the greater the effect of expanding the interlayer distance of graphene, but on the other hand, the electrode expands, reducing the electrode volumetric energy density and increasing the resistance value due to poor contact between the active materials. Therefore, in the manufacturing method for an electricity storage device of this embodiment, during the high-voltage treatment, pressure is applied to the periphery of the exterior material with a jig or the like to suppress the reduction in volumetric density due to the expansion of the electrode.

[0033] Depending on the conditions of the high-voltage treatment, the electrolyte may be decomposed to produce gas or polymers, which may result in a decrease in capacity or an increase in resistance. Therefore, the high-voltage treatment is preferably carried out in an environment of 10° C. or less, and more preferably in an environment of 0° C. or less. This can prevent the production of gas or polymers due to decomposition of the electrolyte.

[0034] In the high-voltage treatment step, charging and discharging at a high voltage may be repeated multiple times, thereby suppressing decomposition of the electrolyte solution and enhancing the expansion of the electrode and the interlayer expansion effect of the graphene.

[0035] As described above in detail, in the electricity storage device of this embodiment, the interlayer distance of the graphene material constituting the electrodes is set to 0.38 nm or more by high-voltage treatment, so that the amount of ions stored is increased compared to conventional electricity storage devices, and an electricity storage device with high density and high capacity can be realized. The configuration of this embodiment is particularly suitable for lithium ion capacitors and electric double layer capacitors.

[0036] The effects of the present invention will be specifically described below with reference to examples and comparative examples.

[0037] First Example First, as a first example of the present invention, an electric double layer capacitor using electrodes having electrode films using a graphene material on the positive and negative electrodes was subjected to a high voltage treatment by the method and conditions shown below, and the interlayer distance, capacitance, and resistance value of the graphene were measured.

[0038] <Fabrication of Electrode> Raw materials containing 87 parts by mass of graphene / CNT composite powder, 5 parts by mass of acetylene black powder, 4 parts by mass of acrylic binder, 4 parts by weight of carboxymethyl cellulose, and 310 parts by mass of water were thoroughly mixed to obtain a slurry. This slurry was coated on both sides of a current collector made of carbon-coated aluminum foil with a thickness of 15 μm using a roll coater to form an electrode film. This was then vacuum dried to obtain an electrode with a total thickness (the sum of the thickness of the electrode film on both sides and the thickness of the current collector) of 85 μm (electrode film density: 0.62 g / cm ３ ) was obtained.

[0039] <Preparation of Electric Double Layer Capacitor Cell> Ten positive electrodes measuring 30 mm x 30 mm were cut out from the electrodes prepared by the method described above, and eleven negative electrodes measuring 30 mm x 30 mm were cut out. These positive and negative electrodes were alternately stacked with separators interposed between them. Separators were placed at the top and bottom of the stack, and the four sides were taped to obtain an electrode stack unit. Aluminum terminals (width 5 mm, thickness 0.1 mm) were then placed on the terminal welds (width 10 mm) of the positive and negative current collectors of this electrode stack unit and ultrasonically welded. The welding area was Yw = 3 mm, Xw = 3 mm.

[0040] This was dried at a temperature of 120°C for 12 hours, and then one side of the electrode terminal was folded back with the end of the electrode terminal pulled out of the exterior laminate film pouch, and the folded back side was heat-sealed to one side of the terminal part of the exterior laminate film with a sealing width of 2 mm. ４ The remaining sides were then heat sealed under reduced pressure with a sealing width of 2 mm, followed by vacuum sealing to assemble a film-type capacitor cell.

[0041] <High Voltage Treatment> (1) Example 1 The cell was clamped in a jig to suppress cell expansion, and in a thermostatic chamber at 10°C, the cell was charged at a current of 0.2 A / g (0.2 A per 1 g of active material). When the voltage reached 3.7 V, constant current charging was initiated. In this state, voltage was applied for 30 minutes, and then current discharge was carried out at 0.2 A / g (0.2 A per 1 g of active material). This procedure was repeated three times to obtain the electricity storage device (electric double layer capacitor) of Example 1.

[0042] (2) Example 2 An electricity storage device (electric double layer capacitor) of Example 2 was obtained in the same manner and under the same conditions as in Example 1, except that the voltage was set to 4.0V.

[0043] (3) Comparative Example 1 Comparative Example 1 was an electricity storage device (electric double layer capacitor) that was not subjected to high voltage treatment.

[0044] (4) Comparative Example 2 Without suppressing expansion with a jig, the cell was charged at a current of 0.2 A / g (0.2 A per 1 g of active material) in a thermostatic chamber at 10°C, and when the voltage reached 3.7 V, constant current charging was initiated. In this state, voltage was applied for 6 hours, and then current discharge was performed at 0.2 A / g (0.2 A per 1 g of active material). This operation was repeated three times to obtain an electricity storage device (electric double layer capacitor) of Comparative Example 2.

[0045] <Measurement of electrode film density> The density of the electrode film in the cell was measured after each of the electricity storage devices of the Examples and Comparative Examples was completely discharged, the cell was disassembled, and the electrode film was removed from the exterior material. The electrode film was washed with ethanol to remove adsorbed electrolyte, and then vacuum dried at 120°C for 12 hours.

[0046] <Measurement of graphene interlayer distance> The graphene interlayer distance in the electrode film in the cell was determined from the results of X-ray diffraction measurement of each electrode film that had been removed from the cell, washed with ethanol, and then vacuum-dried at 120°C for 12 hours.

[0047] <Measurement of Capacitance> The capacitance of the electric double layer capacitor cell was measured at room temperature in a potential range of 0 to 3.2 V, and the capacitance per unit weight C (F / g) was calculated based on the following formula 1. In the following formula 1, 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 ½Vmax.

[0048]

[0049] The results are shown in Table 1. The "electrode volume energy density" shown in Table 1 is the amount of energy per 1 L of electrode volume, and the "electrode volume" here is the combined volume of the positive electrode and negative electrode incorporated in the cell.

[0050]

[0051] 3 shows the results of X-ray diffraction measurements of the electrode films of Example 2, Comparative Example 1, and Reference Example 1. As shown in FIG. 3, for the electrode film (Reference Example 1) before being incorporated into the cell, a diffraction peak corresponding to the C-axis (002) plane of graphene was observed near 2θ = 24.7°, and the calculated interlayer distance in the C-axis direction was 0.360 nm. In contrast, for the electrode film of Example 2, which was subjected to high-voltage treatment, a diffraction peak corresponding to the C-axis (002) plane of graphene was observed near 2θ = 23.1°, and the calculated interlayer distance in the C-axis direction was 0.385 nm. On the other hand, for the electrode film of Comparative Example 1, which was not subjected to high-voltage treatment, a diffraction peak corresponding to the C-axis (002) plane of graphene of the electrode film was observed near 2θ = 23.8°, and the calculated interlayer distance in the C-axis direction was 0.373 nm.

[0052] Furthermore, as shown in Table 1 above, the electricity storage devices of Examples 1 and 2, which underwent high-voltage treatment while suppressing volume expansion, had a higher specific capacity than the electricity storage device of Comparative Example 1, which was not subjected to high-voltage treatment, confirming that the capacity was increased. Furthermore, the electricity storage devices of Examples 1 and 2 did not show an increase in resistance value and still had good DC resistance. As a result, the electrode volume energy density of the electricity storage devices of Examples 1 and 2 increased to 16 Wh / L (Example 1) to 19 Wh / L (Example 2), and the time constant (ΩF value) was 0.78 to 1.1.

[0053] On the other hand, the electricity storage device of Comparative Example 2, which was subjected to high voltage treatment without suppressing volume expansion, had a specific capacitance of 142 F / g and an electrode film density of 0.12 g / cm ３ The electrode volume energy density was reduced to 4.9 Wh / L, the resistance was high, and the time constant (ΩF value) was 7.7, which was inferior in performance to the electricity storage devices of Examples 1 and 2.

[0054] Second Example Next, as a second example of the present invention, a lithium ion capacitor using an electrode having an electrode film using a graphene material in the positive electrode was subjected to high voltage treatment by the method and conditions shown below, and the interlayer distance, capacitance, and resistance value of the graphene were measured.

[0055] <Preparation of Electrode> (1) Positive Electrode A mixture of raw materials containing 87 parts by mass of graphene / CNT composite powder, 5 parts by mass of acetylene black powder, 4 parts by mass of acrylic binder, 4 parts by weight of carboxymethyl cellulose, and 310 parts by mass of water was thoroughly mixed to obtain a slurry. This slurry was coated on both sides of a 31 μm-thick current collector made of aluminum perforated foil using a roll coater to form a positive electrode film. This was then vacuum dried to obtain an electrode having a total thickness (the sum of the thickness of the positive electrode film on both sides and the thickness of the positive electrode current collector) of 85 μm (electrode film density: 0.68 g / cm ３ ) was obtained.

[0056] (2) Negative electrode: A mixture of 88 parts by weight of Nikabeads P5B-GP powder, 5 parts by weight of acetylene black powder, 3 parts by weight of SBR (styrene butadiene rubber) binder, 4 parts by weight of carboxymethyl cellulose, and 210 parts by weight of water was thoroughly mixed to obtain a slurry. This slurry was then coated on both sides of a 21 μm thick copper foil current collector using a roll coater to form a negative electrode film. This was then vacuum dried to obtain an electrode with a total thickness (the sum of the thickness of the negative electrode film on both sides and the thickness of the negative electrode current collector) of 77 μm.

[0057] <Preparation of Lithium Ion Capacitor Cell> Ten pieces measuring 29 mm × 29 mm were cut out from the positive electrode prepared by the method described above, and eleven pieces measuring 30 mm × 30 mm were cut out from the negative electrode, and these positive and negative electrodes were alternately stacked with separators interposed therebetween and dried for 12 hours at a temperature of 120° C. Thereafter, separators were placed on the top and bottom and taped along all four sides, and one lithium metal laminated onto a copper lath was placed on the outermost side of the electrode laminate unit so as to face the positive electrode, thereby obtaining an electrode laminate unit.

[0058] An aluminum positive electrode terminal (5 mm wide) was then placed on the terminal weld (10 mm wide) of the positive electrode current collector of this electrode laminate unit and ultrasonically welded. A nickel negative electrode terminal (5 mm wide) was placed on the terminal weld (10 mm wide) of the copper lath crimped together with the negative electrode current collector and lithium metal foil and ultrasonically welded. The weld area was Yw = 3 mm, Xw = 3 mm. One side of the electrode terminal was folded back with the end of the electrode terminal pulled out of the exterior laminate film pouch, and the folded back side was heat-sealed to one side of the terminal portion of the exterior laminate film with a sealing width of 2 mm.

[0059] The electrolyte was 1M LiPF ６ The negative electrode was vacuum-impregnated with EC (Ethylene Carbonate):DEC (DEC:Diethyl Carbonate) = 1:1 (volume ratio). The remaining edges were then heat-sealed under reduced pressure with a sealing width of 2 mm, and a lithium ion capacitor cell was assembled by vacuum sealing. The cell was left for 14 days after assembly, and the cell voltage was measured to be 2.4 V or higher, indicating that lithium ions had been pre-doped into the negative electrode.

[0060] <High Voltage Treatment> (1) Example 3 The cell was clamped in a jig to suppress expansion of the cell, and in a constant temperature bath at 0° C., the cell was charged at a current of 0.2 A / g (0.2 A per 1 g of active material), and constant current charging was initiated when the voltage reached 4.5 V. In this state, voltage was applied for 6 hours, and then current discharge was carried out at 0.2 A / g (0.2 A per 1 g of active material), to obtain an electricity storage device (lithium ion capacitor) of Example 3.

[0061] (2) Example 4 An electricity storage device (lithium ion capacitor) of Example 4 was obtained in the same manner and under the same conditions as in Example 3, except that the voltage was set to 4.8V.

[0062] (3) Example 5 An electricity storage device (lithium ion capacitor) of Example 5 was obtained in the same manner and under the same conditions as in Example 3, except that the voltage was set to 5.0V.

[0063] (4) Comparative Example 3 Comparative Example 3 was an electricity storage device (lithium ion capacitor) that was not subjected to high voltage treatment.

[0064] <Measurement of Capacitance> The capacitance of the lithium ion capacitor cell was measured at room temperature in a potential range of 2.2 to 4.2 V. Specifically, the cell was charged at a constant current of 0.2 A / g (0.2 A per 1 g of active material) until the cell voltage reached 4.2 V, and then discharged at a constant current of 0.2 A / g (0.2 A per 1 g of active material) until the cell voltage reached 2.2 V. The capacitance was evaluated from this 4.2 V-2.2 V cycle.

[0065] The results are shown in Table 2. The "electrode volume energy density" shown in Table 2 is the amount of energy per 1 L of electrode volume, and the "electrode volume" here is the combined volume of the positive electrode and negative electrode incorporated in the cell.

[0066]

[0067] As shown in Table 2 above, the energy storage devices of Examples 3 to 5, which had undergone high-voltage treatment, had a higher specific capacity than the energy storage device of Comparative Example 3, which had not undergone low-temperature high-voltage treatment, confirming that the capacity had increased. Furthermore, the energy storage devices of Examples 3 to 5 did not show an increase in resistance value and still had good DC resistance. As a result, the electrode volume energy density of the energy storage devices of Examples 3 to 5 increased from 50 Wh / L (Example 3) to 66 Wh / L (Example 5), and the time constant (ΩF value) was 1.3 to 1.7.

[0068] From the above results, it was confirmed that according to the present invention, it is possible to increase the capacity of an electricity storage device without reducing the volumetric energy density of the electrode.

Claims

1. An electrode film using a graphene material as an active material, wherein the electrode film has a density of 0.4 g / cm ３ and an interlayer distance in the C-axis direction of the graphene measured by X-ray diffraction is 0.38 nm or more.

2. The electricity storage device according to claim 1, wherein the graphene material is a composite of graphene and carbon nanotubes.

3. The electricity storage device according to claim 1 or 2, wherein the electrode film contains a binder resin.

4. The electricity storage device according to any one of claims 1 to 3, wherein the electrode film further contains a conductive material.

5. The electricity storage device according to any one of claims 1 to 4, which is a lithium ion capacitor or an electric double layer capacitor.

6. A method for manufacturing an electricity storage device including an electrode film using a graphene material as an active material, the method comprising: a high-voltage treatment step of sealing an electrode body including the electrode film and an electrolyte with an exterior material, and then applying a voltage higher than the upper limit of a working voltage for a certain period of time while suppressing volume expansion; and the electrode film after the high-voltage treatment step has an electrode density of 0.4 g / cm. ３ The method for manufacturing an electricity storage device, wherein the interlayer distance in the C-axis direction of graphene measured by X-ray diffraction is 0.38 nm or more.

7. The method for producing an electricity storage device according to claim 6, wherein the graphene material is a composite of graphene and carbon nanotubes.

8. The method for manufacturing an electricity storage device according to claim 6 or 7, wherein the electricity storage device is a lithium ion capacitor, and the high voltage treatment step applies a voltage of 4.5 V or more for 30 minutes or more.

9. The method for manufacturing an electricity storage device according to claim 6 or 7, wherein the electricity storage device is an electric double layer capacitor, and the high voltage treatment step applies a voltage of 3.5 V or more for 30 minutes or more.

10. The method for producing an electricity storage device according to any one of claims 6 to 9, wherein in the high voltage treatment step, charging and discharging at a high voltage are repeated multiple times.

11. The method for manufacturing an electricity storage device according to any one of claims 6 to 10, wherein the high voltage treatment step is carried out in an environment of 10°C or less.

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