Self-supporting film, electrode, and power storage device

Abstract

The purpose of the present invention is to provide a thick and flexible self-supporting film containing 80 mass % or more of a graphene material, an electrode including the self-supporting film, and a power storage device.　This self-supporting film 21 is produced containing 80 mass % or more of a graphene material, and a polytetrafluoroethylene resin, and configured to have a film density of 0.4-1 g / cm3 and a film thickness of 80-1000 µm. The self-supporting film 21 is laminated on one surface or both surfaces of a collector 22 as an electrode film to form electrodes 20a, 20b. The power storage device is constituted using the electrode 20a or the electrode 20b.

Description

Self-supporting membrane, electrode, and energy storage device

[0001] This invention relates to a self-supporting film using graphene material, an electrode using this self-supporting film as an electrode film, and an energy storage device.

[0002] Graphene is sp ２ Graphene is a sheet-like material with a two-dimensional network structure in which carbon atoms are bonded in hexagons. Due to its high conductivity, high strength, and excellent heat resistance, it is attracting attention in various fields, including electronics such as electronic materials, as well as biomedical and aerospace materials. In particular, in energy storage devices with high energy density, such as lithium-ion capacitors and lithium-ion batteries, electrodes using various types of graphene have been proposed with the aim of increasing capacity, improving voltage resistance, and further improving energy density and power density.

[0003] Electrode films using graphene material are formed, for example, by coating a dispersion of graphene material in a solvent onto a substrate (see Patent Document 1). Furthermore, a method has been proposed to manufacture self-supporting electrode films by preparing an aqueous dispersion of an active material and a binder polymer powder mixture, kneading the slurry obtained by evaporating the aqueous liquid to convert it into a malleable material, extruding the resulting malleable material, and then calendering it (see Patent Document 2).

[0004] Japanese Patent Publication No. 2023-16064, Japanese Patent Publication No. 2023-549248

[0005] Energy storage devices such as electric double-layer capacitors and lithium-ion capacitors are required to have higher capacity and higher output. Possible methods for increasing the capacity of energy storage devices include, for example, (1) increasing the density of the electrode film by increasing the content of the electrode active material, (2) making the electrode film thicker to increase the amount of electrode active material contained, and (3) increasing the specific surface area of ​​the electrode film to increase the amount of ions adsorbed.

[0006] However, in the conventional manufacturing method described in Patent Document 1, it is difficult to produce electrode films with a thickness of 100 μm or more because the electrode film is manufactured by a coating method. Furthermore, in the electrode film manufacturing method described in Patent Document 2, the compound (malleable material) is extruded as is without sizing, so the directionality of the fibrous formation of the binder polymer that occurs during compounding remains in the manufactured electrode film, which may lead to deterioration of mechanical strength and electrical properties. In addition, if the shape of the compound (malleable material) is irregular, there is a risk that voids may form inside the electrode film after calendering, or that cracks may occur at the edges.

[0007] Therefore, the present invention aims to provide a self-supporting film that contains 80% by mass or more of graphene material, is thick, flexible, and has a self-supporting film, as well as an electrode and an energy storage device equipped with this self-supporting film.

[0008] The self-supporting membrane according to the present invention contains 80% by mass or more of graphene material and polytetrafluoroethylene resin, and has a membrane density of 0.4 to 1 g / cm³. ３ The film thickness is 60 to 1000 μm. As the graphene material, for example, a composite of graphene and carbon nanotubes or aggregates thereof can be used. The content of the polytetrafluoroethylene resin is, for example, 0.1 to 10% by mass. The self-supporting film of the present invention may further contain 10% by mass or less of a conductive additive. The self-supporting film of the present invention may also contain 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ４ The contact angle with respect to ) can be set from 0° to 100°.

[0009] The electrode according to the present invention comprises the aforementioned self-supporting film and a current collector.

[0010] The energy storage device according to the present invention includes the electrodes described above.

[0011] According to the present invention, it is possible to realize a self-supporting film that contains 80% by mass or more of graphene material, has a thickness of 60 μm or more, and is flexible.

[0012] This is a schematic diagram showing an example structure of a graphene and carbon nanotube composite. This is a flowchart showing the manufacturing process of a self-supporting film according to the first embodiment of the present invention. This is a schematic diagram showing the state in which the graphene material is bound by a polytetrafluoroethylene resin. A and B are schematic diagrams showing an example configuration of an electrode according to the second embodiment of the present invention. This is a schematic diagram showing an example configuration of an energy storage device according to the third embodiment of the present invention. This is a schematic diagram showing a method for evaluating the flexibility of a self-supporting film.

[0013] The embodiments for carrying out the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described below.

[0014] (First Embodiment) First, a self-supporting membrane according to the first embodiment of the present invention will be described. The self-supporting membrane of this embodiment contains 80% by mass or more of graphene material and polytetrafluoroethylene (PTFE) resin, and has a membrane density of 0.4 to 1 g / cm³. ３ The film thickness is 60 to 1000 μm.

[0015] [Graphene Material] Graphene material is the main raw material constituting the self-supporting film of this embodiment, and acts as an electrode active material when the self-supporting film of this embodiment is used as an electrode film. However, if the amount of graphene material contained in the self-supporting film is less than 80% by mass, the capacity of the energy storage device will be insufficient when used as an electrode film. Therefore, in the self-supporting film of this embodiment, the content of graphene material is set to 80% by mass or more.

[0016] Examples of graphene materials incorporated into the self-supporting film of this embodiment include graphene produced by various methods such as mechanical exfoliation, chemical vapor deposition (CVD), lamination on a silicon carbide (SiC) substrate, and chemical oxidation-reduction, composites of graphene and other carbon materials, and composites of graphene and metals. However, from the viewpoint of improving energy density, composites of graphene and carbon nanotubes (hereinafter also referred to as graphene / CNT composites) or aggregates thereof are preferred.

[0017] <Graphene / CNT Composite> Figure 1 is a schematic diagram showing an example of the structure of a graphene and carbon nanotube composite. The graphene / CNT composite used in the self-supporting membrane of this embodiment can be any composite of graphene and carbon nanotubes (CNTs), and its structure is not particularly limited. For example, as shown in Figure 1, the graphene / CNT composite 10 can be used, 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 Figure 1, the graphene 1 is arranged regularly at equal intervals and parallel to each other, but the present invention is not limited to this, and the graphene 1 may be arranged randomly. Similarly, in Figure 1, the CNT 2 is arranged parallel to each other in the in-plane direction, but the present invention is not limited to this, and it may be arranged randomly between the layers of graphene 1.

[0019] Furthermore, the type of CNT2 is not particularly limited and may be single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), or multi-walled carbon nanotubes (MWCNTs). The size of CNT2 is not particularly limited, but from the viewpoint of promoting uniform dispersion of CNTs in graphene and more efficiently compounding 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 is prone to aggregation due to π-π stacking, but in the graphene / CNT composite 10, the CNTs 2 present between the layers of the single-layer or multi-layer stacked graphene 1 function as spacers, preventing re-stacking and ensuring a high specific surface area. Furthermore, when processed into electrodes, electrolyte flows into the gaps between the layers of graphene 1, making it easier for electrolyte ions to be adsorbed onto the graphene surface. In addition, the graphene / CNT composite 10 has high electrical conductivity in the thickness direction because the highly electrically conductive CNTs 2 are present between the layers of graphene 1.

[0021] [PTFE Resin] The PTFE resin is a binder that binds the graphene material and also has the effect of imparting flexibility to the self-supporting film. The content of the PTFE resin is not particularly limited, but is preferably 0.1 to 10% by mass, more preferably 1 to 8% by mass. By setting the content of the PTFE resin within this range, it is possible to form a self-supporting film that is excellent in self-supporting property and flexibility while maintaining the target active material content.

[0022] [Film density: 0.4 to 1 g / cm ３ ] The self-supporting film of this embodiment has a film density of 0.4 to 1 g / cm ３ . When the film density is less than 0.4 g / cm ３ , the content of the active material decreases, so when applied to the electrode film of the power storage device, the capacity is insufficient or the energy density decreases. Further, when the film density exceeds 1 g / cm ３ , it becomes difficult for the electrolytic solution to penetrate when used as an electrode film, and the performance as a power storage device deteriorates. From the viewpoints of improving the performance of the power storage device and the flexibility of the self-supporting film, the film density of the self-supporting film is preferably 0.5 to 0.9 g / cm ３ .

[0023] [Film thickness: 80 to 1000 μm] The self-supporting film of this embodiment has a film thickness of 80 to 1000 μm. When the film thickness is less than 80 μm, it becomes difficult to form a film that satisfies other requirements and has self-supporting property. Further, when the film thickness exceeds 1000 μm, the film density decreases, and when applied to the electrode film of the power storage device, the capacity is insufficient or the energy density decreases. From the viewpoint of ensuring flexibility, the film thickness of the self-supporting film is preferably 80 to 300 μm.

[0024] [Conductive Aid] The self-supporting film of this embodiment may contain a conductive aid in addition to the graphene material. The conductive aid incorporated in the self-supporting film of this embodiment is not particularly limited, and known conductive materials 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 amount of conductive additive added is not particularly limited, but is preferably 10% by mass or less, and more preferably 1 to 8% by mass. This makes it possible to reduce the resistance of the self-supporting film while maintaining the desired active material content and the flexibility of the self-supporting film.

[0026] [EMI-BF] ４ Contact angle relative to [ ] The self-supporting film of this embodiment is, for example, the electrolyte EMI-BF ４ The contact angle can be set from 0° to 100°. Here, a contact angle of 0° refers to EMI-BF ４ It is not a state where it is wetted and spread on the surface of the self-supporting membrane, but EMI-BF ４ This refers to a state in which the EMI-BF penetrates into the self-supporting film, making it impossible to measure the contact angle. Thus, in the self-supporting film of this embodiment, ４ The contact angle with respect to the EMI-BF is influenced by the properties of the self-supporting film surface (wettability), as well as the EMI-BF to the voids within the self-supporting film. ４ The ease with which it penetrates also plays a role.

[0027] EMI-BF ４ When the contact angle with respect to exceeds 100°, the electrolyte becomes less permeable, and when used as an electrode film, the contact area with the active material decreases, which may result in insufficient charge / discharge capacity. On the other hand, EMI-BF ４ Self-supporting films with a contact angle in the range of 0° to 100° allow for easy penetration of the electrolyte, thus enabling the creation of electrode films with high charge and discharge capacities. The contact angle specified here is the value measured by the droplet method.

[0028] [Manufacturing Method] Next, the manufacturing method of the self-supporting film of this embodiment will be described. Figure 2 is a flowchart of the manufacturing process of the self-supporting film of this embodiment. As shown in Figure 2, the self-supporting film of this embodiment can be manufactured, for example, by performing the following mixing step S1, kneading step S2, molding step S3, and rolling step S4.

[0029] Also, in the method for manufacturing the self-supporting film of the present embodiment, a sizing process S12 may be performed after the kneading process S2. In that case, after the sizing process S12 is performed, a molding process S13 and a rolling process 14 are performed. Furthermore, in the method for manufacturing the self-supporting film of the present embodiment, drying processes S5 and S15 can be performed after the rolling processes S4 and S14 as needed.

[0030] <Mixing Process S1> In the mixing process S1, a predetermined amount of PTFE resin is added to the graphene material and mixed using a mixer or the like. At that time, the addition amount of the PTFE resin is adjusted so that the content of the graphene material becomes 80% by mass or more when it becomes a self-supporting film. When the self-supporting film is an electrode film, a conductive auxiliary agent can be blended in the range of 10% by mass or less with respect to the mass of the self-supporting film. In that case, it is preferable to add the PTFE resin after mixing the graphene material and the conductive auxiliary agent.

[0031] Also, in the method for manufacturing the self-supporting film of the present embodiment, an auxiliary agent for a binder may be added to and mixed with each of the above-described raw materials. Examples of the auxiliary agent for a binder used in the present embodiment include alcohols such as isopropyl alcohol (IPA), ethanol, and methanol, ethers, ketones, and water.

[0032] <Kneading Process S2> In the kneading process S2, the raw material composition obtained by blending and mixing in the above-described mixing process S1 is kneaded. The kneading method is not particularly limited as long as a shearing force can be applied to the raw material composition. For example, a method of applying a shearing force by grinding using a mortar and a pestle, a method of applying a shearing force by passing the raw material composition between a plurality of rollers arranged opposite to each other, and a method of applying a shearing force by rotating a blade while applying pressure using a kneader (mixer) can be mentioned.

[0033] FIG. 3 is a schematic diagram showing a state in which the graphene material is bound by the PTFE resin. By the above-described kneading process S2, the PTFE resin 13 is stretched in a fibrous shape, and as shown in FIG. 3, the PTFE resin 13 enters between the particles of the graphene material 11 and the conductive auxiliary agent 12 and binds each particle. Thereby, it becomes possible to form a self-supporting film.

[0034] <Granulation Step S12> The granulation step S12 is carried out as necessary to make the particle sizes of the kneaded material obtained in the kneading step S2 uniform. The method for granulating the kneaded material is not particularly limited, and for example, a method of sieving to make the particle sizes uniform or a method of pulverizing by applying it to a mixer can be applied.

[0035] <Forming Steps S3, S13> In the forming steps S3, S13, pressure is applied to the kneaded material obtained in the kneading step S2 or the granulated material obtained in the granulation step S12 respectively to form it into a predetermined shape. The forming method in the forming steps S3, S13 is not particularly limited. For example, when manufacturing a sheet-shaped self-supporting film, calendar forming in which the kneaded material is passed between two rollers can be applied. <​​​​​​​​​​(Second Embodiment) Next, an electrode according to the first embodiment of the present invention will be described. Figures 4A and 4B are schematic diagrams showing examples of the configuration of the electrode of this embodiment, where Figure 4A shows a configuration in which an electrode film is provided on one side of the current collector, and Figure 4B shows a configuration in which an electrode film is provided on both sides of the current collector. As shown in Figures 4A and 4B, in the electrodes 20a and 20b of this embodiment, the self-supporting film 21 of the first embodiment described above is laminated on one or both sides of the current collector 22.

[0040] The material of the current collector 22 is not particularly limited, and known metal materials for current collectors can be used, but aluminum is particularly preferred. The thickness, size, and shape of the current collector 22 can be appropriately set according to the application.

[0041] The electrodes 20a and 20b of this embodiment described above can be manufactured, for example, by bonding the self-supporting film 21 of the first embodiment to a current collector 22 made of aluminum foil using an adhesive (bonding process). Furthermore, if the self-supporting film 21 or the adhesive contains liquid components, a drying process may be performed after the bonding process to remove the liquid components contained in the self-supporting film 21 and the adhesive.

[0042] The electrode of this embodiment uses the self-supporting film of the first embodiment, which contains graphene material at high density, is a thick film, and has a large specific surface area, as the electrode film, thus enabling the realization of a high-density, high-performance electrode. Furthermore, because the self-supporting film of the first embodiment is flexible, it can be adapted to various shapes of energy storage devices by using metal foil as the current collector. The configuration and effects of this embodiment other than those described above are the same as those of the first embodiment described above.

[0043] (Third Embodiment) Next, a power storage device according to a third embodiment of the present invention will be described. The power storage device of this embodiment is equipped with the electrodes of the second embodiment described above. Figure 5 is a schematic diagram showing an example of the configuration of the power storage device of this embodiment. For example, if the power storage device of this embodiment is an electric double-layer capacitor 30 as shown in Figure 5, an electrolyte 31 is filled between electrodes 20 that are arranged at predetermined intervals.

[0044] The energy storage device of this embodiment uses the self-supporting film of the first embodiment, which contains graphene material at high density, is a thick film, and has a large specific surface area, as the electrode film, thereby enabling the realization of a high-capacity, high-output energy storage device. It should be noted that the energy storage device of the present invention is not limited to the electric double-layer capacitor described above, but can be used in various energy storage devices, in which case the same effects can be obtained. Furthermore, the configuration and effects of this embodiment other than those described above are the same as those of the first and second embodiments described above.

[0045] The effects of the present invention will be specifically described below with reference to examples and comparative examples.

[0046] [First Embodiment] First, as the first embodiment of the present invention, an electric double-layer capacitor was fabricated using electrode films (self-supporting films) No. 1 to 6 shown in Table 1 below, and its performance was evaluated. In this embodiment, a "composite of graphene and carbon nanotubes" was used as the graphene material, and "Ketjenblack," a conductive carbon black, was used as the conductive additive. Furthermore, the volume resistivity shown in Table 1 below was measured using the 4-probe method with the HIOKI E.E. CORPORATION RM2610 electrode resistance measurement system.

[0047]

[0048] <Cell Fabrication> Electrode films No. 1 to 6 were attached to one side of a 20 μm thick aluminum foil to create a sheet-like electrode. From this sheet-like electrode, positive and negative electrodes measuring 30 mm x 30 mm were cut out and stacked with separators in between. After placing separators at the top and bottom, the four sides were secured with tape to obtain an electrode stacking unit.

[0049] Next, aluminum terminals were superimposed on the terminal welds of the positive and negative electrode current collectors of the electrode stacking unit and ultrasonically welded. After drying at 120°C for 12 hours, the ends of the electrode terminals were pulled out of the outer laminate film pouch, the second side was folded back, and the first and second sides of the terminal portion of the outer laminate film were heat-sealed. Subsequently, EMI-BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) was vacuum-impregnated as the electrolyte, and the open fourth side was heat-sealed under reduced pressure to obtain a film-type capacitor cell.

[0050] <Cell Characterization> The electrical characteristics of each cell fabricated using the method described above were measured using a multi-channel potentiostat galvanostat (VMP-300, Bio-Logic). Specifically, the cells were charged for 30 minutes with a constant current of 0.2 A per unit mass (g) of the electrode film active material (cell voltage 3.7 V), and then discharged with a constant current of 0.2 A per unit mass (g) of the electrode film active material until the cell voltage reached 0 V.

[0051] The specific capacitance Cs (F / g) was then calculated from Equation 1 below. In Equation 1 below, I (A) is the constant current, m (g) is the total mass of the two electrode active materials, and dV / dt (V / s) is the slope obtained by linearly fitting the discharge curve between the voltage Vmax at the start of discharge and 0V.

[0052]

[0053] Furthermore, the electrode mass energy density Ecell (Wh / kg) was calculated from equation 2 below, and the electrode volume energy density Ecell (Wh / L) was determined from this electrode mass energy density Ecell (Wh / kg) and the density of the electrode film.

[0054]

[0055] Furthermore, the circuit was opened immediately after reaching the rated voltage, and the resistance value was calculated from the voltage change and charging current after 1 second. The results are summarized in Table 2 below.

[0056]

[0057] As shown in Table 2 above, cell No. 5, which is a comparative example of the present invention, has an electrode film density of 1 g / cm³. ３ Because it exceeded this, the electrolyte had difficulty penetrating the electrode film, resulting in inferior performance as an energy storage device. In addition, cell No. 6, a comparative example of the present invention, had an electrode film density of 0.4 g / cm³. ３ Because it was below that level, its performance as an energy storage device was inferior.

[0058] In contrast, the cells No. 1 to 4, which are embodiments of the present invention, contain 80% or more by mass of graphene material and PTFE resin as electrode films, with a film density of 0.4 to 1 g / cm³. ３ Furthermore, because a self-supporting film with a thickness of 80 to 1000 μm is used, the specific capacity of the electrode film is high, and it was possible to increase the energy density (especially the volumetric energy density) while suppressing the increase in resistance. From these results, it was confirmed that a high-capacity and high-output energy storage device can be realized according to the present invention.

[0059] [Second Example] Next, as a second example of the present invention, the flexibility of the self-supporting film was evaluated. In this example, a "composite of graphene and carbon nanotubes" was used as the graphene material, and "Ketjenbrak" was used as the conductive additive. Self-supporting films with different compositions, film thicknesses, and densities were fabricated and their flexibility was evaluated. Details and evaluation results of each self-supporting film are summarized in Table 3 below.

[0060]

[0061] <Evaluation of Flexibility> Figure 5 schematically shows the method for evaluating the flexibility of the self-supporting membranes. As shown in Figure 5, the flexibility of each self-supporting membrane was evaluated by wrapping a sheet-like self-supporting membrane 21 with a width of 1.5 cm and a length of 8 cm around round rods 40 with diameters of 5 mm (φ5), 10 mm (φ10), 20 mm (φ20), and 30 mm (φ30), and then clamping both ends with clips 41 weighing 1.17 g. Then, each self-supporting membrane was visually inspected, and those that showed damage such as fracture or cracking in the bending test described above were marked with ×, and those that showed no damage were marked with ○.

[0062] As shown in Table 3 above, the self-supporting films No. 10 to 18, which are embodiments of the present invention, did not show any damage in the φ30 bending test and were confirmed to be flexible. In particular, films with a thickness of 100 to 300 μm and a film density of 0.5 to 0.9 g / cm³ were found to be flexible. ３ Furthermore, the self-supporting films No. 10 to 12, which contained 10% by mass or less of the conductive additive, showed no damage in the bending test at φ5 and exhibited excellent flexibility. From these results, it was confirmed that the self-supporting films of the present invention are flexible.

[0063] [Third Example] Next, as a third example of the present invention, the wettability of the self-supporting film was evaluated. In this example, a "composite of graphene and carbon nanotubes" was used as the graphene material, and "Ketjenblack" was used as the conductive additive. Self-supporting films with the same composition as No. 1 described above, but with different film thickness and density, were prepared and the electrolyte was EMI-BF ４ The contact angle was measured.

[0064] Contact angle measurements were performed using the solid-liquid interface droplet method with an automatic contact angle meter SImage AUTO 100 manufactured by Excima Corporation. All measurements were performed under a temperature of 25°C, and each sample (self-supporting film) was treated with EMI-BF. ４ 10 μL of the solution was dropped onto the sample, and the contact angle was measured after 5 seconds. To ensure data accuracy, five measurements were taken for each sample, and the average value was used as the final measurement result. Details of each self-supporting film and the contact angle measurement results are summarized in Table 4 below.

[0065]

[0066] As shown in Table 4 above, the lower the membrane density, the better the electrolyte (EMI-BF ４ The self-supporting film No. 23 exhibited good wettability and high film density, with a contact angle exceeding 100°. This confirmed that self-supporting films with high film density are less permeable to electrolyte.

[0067] 1 Graphene 2 Carbon nanotubes (CNTs) 10 Graphene / CNT composite 11 Graphene material 12 Conductive additive 13 Polytetrafluoroethylene (PTFE) resin 20, 20a, 20b Electrodes 21 Self-supporting film (electrode film) 22 Current collector 30 Electric double-layer capacitor 31 Electrolyte 40 Round rod 41 Clip

Claims

1. Contains 80% or more by mass of graphene material and polytetrafluoroethylene resin, with a film density of 0.4 to 1 g / cm³. ３ It is a self-supporting film with a thickness of 80 to 1000 μm.

2. The self-supporting membrane according to claim 1, wherein the graphene material comprises at least a composite of graphene and carbon nanotubes or an aggregate thereof.

3. The self-supporting membrane according to claim 1 or 2, wherein the content of the polytetrafluoroethylene resin is 0.1 to 10% by mass.

4. The self-supporting film according to any one of claims 1 to 3, further comprising 10% by mass or less of a conductive additive.

5. 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ４ A self-supporting film according to any one of claims 1 to 4, wherein the contact angle with respect to ) is 0° to 100°.

6. An electrode comprising a self-supporting film according to any one of claims 1 to 5, and a current collector.

7. An energy storage device comprising the electrode described in claim 6.

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