Electrode film, electrode, and power storage device

Abstract

Provided are: an electrode film which is capable of achieving an electrode having a high energy density per unit volume and a power storage device that is compact and exhibits high output density; an electrode; and a power storage device.　The electrode film has a density of 0.6-1.2 g / cm3 and is formed from a composition that contains at least a binder resin 21b and an aggregate 20 of a composite 10 composed of graphene and carbon nanotubes. The particle diameter of the aggregate 20 is 1-20 μm. The power storage device is formed by using the electrode in which said electrode film is provided on a collector.

Description

Electrode film, electrode, and electricity storage device

[0001] The present invention relates to an electrode film using graphene, and an electrode and an electricity storage device including this electrode film.

[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 it has 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 electrodes using graphene have been proposed with the aim of increasing capacity, improving voltage resistance, and further improving energy density and power density (see Patent Documents 1 to 3).

[0003] For example, Patent Document 1 describes a capacitance of 0.01 to 1.7 g / cm when measured in a dry state without an electrolyte. ３ Physical density, 50 to 3,300 m ２ and a supercapacitor electrode using a solid graphene foam having a specific surface area of ​​1000 W / mK / g, a thermal conductivity of at least 200 W / mK per unit specific gravity, and / or an electrical conductivity of 2,000 S / cm or more per unit specific gravity. Also, Patent Document 2 describes a capacitor electrode using a graphene porous carbon sheet containing a graphene porous carbon material and carbon nanotubes.

[0004] On the other hand, Patent Document 3 describes an electrode using graphene aggregates having a particle size of 0.1 μm or more and less than 100 μm and having a graphene basic skeleton in which fibrous materials such as carbon nanotubes are located between graphene layers.

[0005] International Publication No. WO 2017 / 123463 International Publication No. WO 2020 / 080520 International Publication No. WO 2019 / 065004

[0006] However, since graphene materials have a strong tendency to be re-laminate during the electrode production process, electrodes using the porous graphene materials described in Patent Documents 1 and 2 have the problem 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 resulting in a decrease in capacity. On the other hand, such a problem does not occur with electrodes using graphene aggregates in which fibrous materials exist between the graphene layers described in Patent Document 3, but further improvement in electrode performance is required for recent electricity storage devices.

[0007] Therefore, an object of the present invention is to provide an electrode having a high energy density per unit volume, and an electrode film, an electrode, and an electricity storage device that can realize a small-sized electricity storage device having a high output density.

[0008] The electrode film according to the present invention comprises an aggregate of a composite of graphene and carbon nanotubes and a binder resin, the particle diameter of the aggregate being 1 to 20 μm, and the film density being 0.6 to 1.2 g / cm ３ In the electrode film of the present invention, the binder may also be present inside the aggregates. The composite of graphene and carbon nanotubes that constitutes the aggregates is, for example, a graphene laminate in which carbon nanotubes are present between graphene layers. The electrode film of the present invention may further contain a conductive material.

[0009] The electrode according to the present invention comprises the above-described electrode film.

[0010] The electricity storage device according to the present invention includes the above-described electrode. The electricity storage device according to the present invention is, for example, an electric double layer capacitor, and has an ΩF value of less than 2.

[0011] According to the present invention, an electrode having a high energy density per unit volume can be obtained, and therefore a small-sized electricity storage device having a high output density can be realized.

[0012] FIG. 1 is an enlarged cross-sectional view schematically showing an electrode film according to a first embodiment of the present invention. FIG. 2 is a view schematically showing an aspect of an aggregate 20. FIG. 3 is a view schematically showing an example of the structure of a graphene / CNT composite 10. FIG. 4 is a flowchart showing a manufacturing process of an electrode film 30. FIG. 5 is a cross-sectional view schematically showing an example of the structure of an electrode according to a second embodiment of the present invention. FIG. 6 is a schematic view showing an example of the structure of an electricity storage device according to a third embodiment of the present invention.

[0013] 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.

[0014] First Embodiment First, an electrode film according to a first embodiment of the present invention will be described. FIG. 1 is an enlarged cross-sectional view schematically showing the electrode film of this embodiment. As shown in FIG. 1, an electrode film 30 of this embodiment contains aggregates 20 of composites 10 of graphene and carbon nanotubes (hereinafter referred to as graphene / CNT composites 10) and a binder resin. The aggregates 20 contained in the electrode film 30 of this embodiment have a particle diameter of 1 to 20 μm, and the film density of the electrode film 30 is 0.6 to 1.2 g / cm. ３ is.

[0015] [Aggregate 20] Fig. 2 is a diagram schematically illustrating an aspect of aggregate 20. As shown in Fig. 2, aggregate 20 is formed by aggregating graphene / CNT composites 10 into a substantially spherical shape while maintaining their original shape, and binder resin 21a is present inside. By using graphene / CNT composite 10 in the form of aggregate 20, the excellent conductivity and electrolyte ion adsorption performance of graphene / CNT composite 10 can be maintained, and when used in power storage devices such as electric double layer capacitors, lithium ion capacitors, and lithium ion batteries, the energy density and power density of these devices can be improved.

[0016] <Particle diameter: 1 to 20 μm> For the same volume filling rate, the smaller the particle diameter of the material, the higher the viscosity of the slurry. Therefore, using aggregates with a particle diameter of less than 1 μm for the electrode film 30 makes it difficult to increase the solid content, making coating more difficult and making it difficult to produce a uniform electrode film. On the other hand, as the particle diameter of the material increases, larger voids are more likely to form. Therefore, using aggregates 20 with a particle diameter greater than 20 μm makes it impossible to increase the density of the electrode film and improve the volumetric energy density of the cell. Furthermore, this affects the diffusion of electrolyte ions and affects rapid charge and discharge behavior. Therefore, the electrode film 30 of this embodiment uses aggregates 20 with a particle diameter of 1 to 20 μm. Note that the particle diameter of the aggregates 20 here is a value measured by laser diffraction / scattering.

[0017] <Graphene / CNT composite 10> Fig. 3 is a diagram schematically illustrating an example of the structure of the graphene / CNT composite 10. As the graphene / CNT composite 10, for example, a graphene laminate in which single-layer graphene 1 and carbon nanotubes (CNTs) 2 are alternately stacked, with the carbon nanotubes 2 present between the layers of graphene 1, as shown in Fig. 3, can be used. Note that the graphene / CNT composite 10 constituting the aggregate 20 is not limited to the structure shown in Fig. 3, and may be any composite in which graphene 1 and carbon nanotubes (CNTs) 2 are combined.

[0018] Specifically, in the graphene / CNT composite 10 shown in Fig. 3, the graphene 1 is regularly arranged at equal intervals and parallel to one another, but the graphene 1 may also be arranged randomly. Also, in the graphene / CNT composite 10 shown in Fig. 3, the carbon nanotubes (CNTs) 2 are arranged parallel to one another in the in-plane direction, but the present invention is not limited to this, and the carbon nanotubes (CNTs) 2 may also be arranged randomly between the layers of the graphene 1.

[0019] The type of carbon nanotubes (CNT) 2 in the graphene / CNT composite 10 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 the carbon nanotubes (CNT) 2 is also not particularly limited, and from the viewpoint of promoting uniform dispersion of the carbon nanotubes (CNT) 2 in the graphene 1 and more efficiently combining them with the graphene 1, the length is preferably 1 to 20 μm, and the average outer diameter is preferably 0.4 to 5.0 nm, and 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, carbon nanotubes (CNTs) 2 present between the layers of single-layer graphene 1 function as spacers, preventing stacking and ensuring a high specific surface area. Furthermore, when processed into an electrode, electrolyte flows into the gaps between the layers of single-layer graphene 1, making it easier for electrolyte ions to be adsorbed onto the graphene surface. Furthermore, since carbon nanotubes 2, which have high electrical conductivity, are present between the layers of graphene 1, graphene / CNT composite 10 also has high electrical conductivity in the thickness direction.

[0021] [Binder Resin] The binder resin can be appropriately selected from organic solvent-based binders and aqueous binders that are commonly used in electrode films. Specifically, examples of organic solvent-based binders include polytetrafluoroethylene resin (PTFE), its modified polytetrafluoroethylene resin, and polyvinylidene fluoride (PVDF). Examples of aqueous binders include sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR). These can be used alone or in combination.

[0022] Among these resins, it is particularly preferable to use a combination of aqueous binders, CMC and SBR. Furthermore, the binder resin 21a present inside the aggregate 20 and the binder resin 21b present outside the aggregate 20 and binding the aggregates 20 together may be the same or different.

[0023] [Film Density] The electrode film 30 of this embodiment has a film density of 0.6 to 1.2 g / cm ３ and preferably 0.6 to 1.0 g / cm ３ The film density is 0.6 g / cm ３ If the film density of the electrode film 30 is less than 1.2 g / cm, the conductivity of the electrode film 30 will be low and it will be impossible to improve the volume energy density of the electrode. ３ If the temperature exceeds this value, the electrolyte will not penetrate easily, which will affect the performance of the cell.

[0024] [Conductive Material] The conductive material added to the electrode film 30 of the present embodiment is not particularly limited, and may be any material that is generally used as a conductive material in electrode films. However, from the viewpoint of affinity with graphene, carbon materials such as carbon black, acetylene black, channel black, furnace black, ketjen black, and carbon nanotubes (CNTs) are preferred.

[0025] [Method for Manufacturing Electrode Film 30] Fig. 4 is a flowchart showing the manufacturing process of the electrode film 30 of this embodiment. The electrode film 30 of this embodiment can be manufactured, for example, by carrying out a granulation step S1 of forming aggregates 20 of graphene / CNT composites 10 and a film-forming step S2 of forming the electrode film 30 using the aggregates 20, as shown in Fig. 4 .

[0026] <Step S1: Granulation Step> In the granulation step S1, for example, the graphene / CNT composites 10 are dispersed together with a binder resin 21a in a lower alcohol having 1 to 5 carbon atoms or a mixed solution of such a lower alcohol and water to form aggregates 20 of the graphene / CNT composites 10. By adding the binder resin in the granulation step S1, the binder resin 21a penetrates into the interior of the formed aggregates 20, improving the structural stability of the aggregates 20. The binder resin 21a used here may be the same as or different from the binder resin 21b used in the film-forming step S2 described below.

[0027] <Step S2: Film Forming Step> In the film forming step S2, the electrode film 30 is formed using the aggregate 20 obtained in the granulation step S1. The film forming step S2 may be performed by either a wet method or a dry method. In the case of the wet method, for example, the binder resin 21b, and optionally a conductive material and a solvent such as water are added to the aggregate 20, and the mixture is thoroughly mixed to form a slurry, which is then applied using a roll coater or the like and dried.

[0028] In the case of a dry method, for example, the aggregate 20 and, if necessary, a conductive material are kneaded into the binder resin 21b using a kneader or the like, and then molded to a predetermined thickness by extrusion molding or press molding. In the dry method, the blending amount of the binder resin 21b can be, for example, 0.5 to 10 mass% based on the total mass of the electrode film. When a conductive material is added, the amount can be 1 to 20 mass% based on the total mass of the electrode film.

[0029] In the above-described film-forming step S1, loads are applied during mixing and kneading, which may destroy part of the aggregate 20 and result in insufficient performance of the electrode film 30. On the other hand, the electrode film 30 of this embodiment uses the aggregate 20 whose structural stability is improved by the binder resin 21a. Therefore, even when mixing and kneading are performed in the film-forming step S1, the aggregate 20 is not destroyed, and an electrode film 30 with excellent performance can be stably produced. As a result, the durability, cycle performance, and rapid charge and discharge performance of an electricity storage device produced using this electrode film 30 can be improved.

[0030] As described above in detail, the electrode film of this embodiment contains an aggregate of graphene / CNT composites and a binder resin, the particle diameter of the aggregate is 1 to 20 μm, and the film density is 0.6 to 1.2 g / cm ３ This allows for an electrode with a high energy density per unit volume, making it possible to realize a small-sized electricity storage device with a high output density.

[0031] The graphene / CNT composite used in the electrode film of this embodiment has a layered structure, and therefore has a faster ion absorption rate and a larger amount of absorbed ions than the activated carbon used in conventional catalyst films. Therefore, the electrode film of this embodiment has excellent ion absorption performance and is suitable for use not only in the aforementioned power storage device but also as a catalyst film.

[0032] Second Embodiment Next, an electrode according to a second embodiment of this embodiment will be described. The electrode of this embodiment includes the electrode film 30 of the first embodiment described above. Fig. 5 is a cross-sectional view schematically showing an example of the structure of the electrode of this embodiment. The electrode 32 of this embodiment can have a structure in which the electrode film 30 is provided on a current collector 31, for example, as shown in Fig. 5.

[0033] [Current Collector 31] The material of the current collector 31 is not particularly limited, but for example, when the application is a lithium ion capacitor or a lithium ion secondary battery, aluminum foil, carbon-coated aluminum foil, perforated aluminum foil, etched aluminum foil, etc. The thickness of the current collector 31 can be set appropriately depending on the application and required performance, but is preferably 10 to 30 μm, for example.

[0034] [Manufacturing Method] The electrode 32 of this embodiment can be formed, for example, by adding a solvent such as water to the graphene / CNT composite 10, a conductive material, and a binder resin, thoroughly mixing the mixture to form a slurry, and then coating the slurry on both sides of a metal foil current collector 31 made of, for example, etched aluminum foil using a roll coater or the like to form the electrode film 30, and then drying the mixture. Alternatively, the electrode 32 can be formed by kneading the graphene / CNT composite 10, a conductive material, and a binder resin, and providing the electrode film 30 on both sides of a metal foil current collector 31 made of, for example, etched aluminum foil by extrusion molding, press molding, or the like.

[0035] [Applications] The electrode 32 of the present embodiment can be used for a variety of applications, including various capacitors such as lithium ion capacitors, various secondary batteries such as lithium ion secondary batteries, and other electricity storage devices, as well as fuel cells and electrodes for various reactions.

[0036] The electrode of this embodiment contains an aggregate of graphene / CNT composites and a binder resin on one or both sides of a current collector, the particle diameter of the aggregate being 1 to 20 μm, and the film density being 0.6 to 1.2 g / cm ３ Since the electrode film of this embodiment is provided, the energy density per unit volume is high. Therefore, by using the electrode of this embodiment, a small-sized electricity storage device with high output density can be realized.

[0037] Third Embodiment Next, a description will be given of an electricity storage device according to a third embodiment of the present invention. The electricity storage device according to this embodiment includes the electrodes according to the second embodiment described above. FIG. 6 is a schematic diagram showing a structural example of the electricity storage device according to this embodiment. For example, when the electricity storage device according to this embodiment is an electric double layer capacitor 40 shown in FIG. 6, a positive electrode (cathode) 41 and a negative electrode (anode) 42 are disposed opposite each other and spaced apart from each other with a spacer 43 interposed therebetween, and a Li-ion electrolyte 44 is filled between these electrodes.

[0038] As the electrolyte 44, for example, an ionic liquid such as 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) can be used.

[0039] In this electric double layer capacitor 40, the electrode 32 of the second embodiment described above is used as the positive electrode (cathode) 41, or as both the positive electrode (cathode) 41 and the negative electrode (anode) 42. When the electricity storage device of this embodiment is the electric double layer capacitor 40 having the structure shown in Fig. 6, the ΩF value is preferably 2 or less, which allows rapid charging and discharging.

[0040] The electricity storage device of this embodiment is not limited to the electric double layer capacitor described above, but can be applied to various electricity storage devices such as various capacitors such as lithium ion capacitors, various secondary batteries such as lithium ion secondary batteries, and fuel cells.

[0041] The electricity storage device of this embodiment contains aggregates of graphene / CNT composites and a binder resin on one or both sides of a current collector, the particle diameter of the aggregates being 1 to 20 μm, and the film density being 0.6 to 1.2 g / cm ３ Since the electrode has an electrode film, the energy density per unit volume is high, making it possible to achieve a compact size and high output density.

[0042] The effects of the present invention will be specifically described below with reference to examples and comparative examples.

[0043] Comparative Example 1 An electrode for Comparative Example 1 was fabricated by the following method. A slurry was prepared by mixing 87 parts by mass of graphene / CNT composite aggregates (powder with a particle size of 5 μm), 5 parts by mass of acetylene black powder as a conductive material, 4 parts by mass of an acrylic binder as a binder resin, 4 parts by mass of carboxymethyl cellulose, and 310 parts by mass of water. A 22 μm-thick aluminum etched foil was used as a current collector, and the above-described slurry was applied to both sides of the current collector using a roll coater to form an electrode film. After vacuum drying, an electrode with a thickness of 175 μm (total thickness of the current collector and the electrode film on both sides) was obtained.

[0044] Example 1 An electrode of Example 1 having a thickness of 51 μm was produced in the same manner as in Comparative Example 1, except that press molding was performed.

[0045] Example 2 An electrode of Example 2 having a thickness of 45 μm was produced in the same manner as in Example 1.

[0046] Example 3 An electrode of Example 3 having a thickness of 37 μm was produced in the same manner as in Example 1.

[0047] Example 4 An electrode of Example 4 having a thickness of 34 μm was produced in the same manner as in Example 1.

[0048] Comparative Example 2 An electrode of Comparative Example 2 having a thickness of 31 μm was produced in the same manner as in Example 1.

[0049] Comparative Example 3 An electrode of Comparative Example 3 having a thickness of 27 μm was produced in the same manner as in Example 1.

[0050] [Resistance Measurement] For each of the electrodes of the Examples and Comparative Examples prepared by the above-described method, the resistance per unit volume was measured by the four-probe method using an electrode resistance measurement system RM2610 manufactured by Hioki E.E. Corporation.

[0051] [Preparation of Evaluation Cells] Each electrode (positive electrode and negative electrode) of the Examples and Comparative Examples prepared by the above-described method was cut into a size of 30 mm length x 30 mm width. Five positive electrodes and six negative electrodes were stacked with separators interposed between them. Separators were placed on the top and bottom layers, and the four sides were taped to obtain electrode stack units of positive and negative electrodes. Aluminum terminals (5 mm width, 0.1 mm thickness) were placed on the terminal welding portions (10 mm width) of the positive and negative current collectors of these electrode stack units and ultrasonically welded. The welding area was (Yw: 3 mm) x (Xw: 3 mm).

[0052] Each electrode laminate unit was dried at a temperature of 120°C for 12 hours, and then the second side was folded back with the end of the electrode terminal pulled out of the exterior laminate film pouch, and the first and second sides of the terminal portion of the exterior laminate film were heat-sealed with a sealing width of 2 mm. ４ After vacuum impregnation with 1-ethyl-3-methylimidazolium tetrafluoroborate, the remaining fourth side was heat-sealed under reduced pressure with a sealing width of 2 mm, followed by vacuum sealing to assemble a film-type capacitor cell. Finally, the second and fourth sides were folded once to prepare an evaluation cell.

[0053] [Evaluation of Cell Characteristics] Electrochemical measurements were performed on the evaluation cells prepared by the method described above using a multi-channel potentiostat galvanostat (VMP-300, manufactured by Bio-Logic). Specifically, the cells were charged at a constant current of 0.2 A / g until the cell voltage reached 3.7 V, and then discharged at a constant current of 0.2 A / g until the cell voltage reached 0 V. The initial capacitance was then determined from the discharge curve between the voltage Vmax at the start of discharge and 0 V.

[0054] The results are shown in Table 1 below.

[0055]

[0056] As shown in Table 1 above, the electrode of Comparative Example 1 has a large mass specific capacitance of 175 F / g, but the density of the electrode film is 0.22 g / cm ３ Because the cell density was so small, the resistance per unit volume was high at 0.52 Ω cm. As a result, the resulting battery had a high mass energy density of 83 Wh / kg, but a volume energy density of only 16 Wh / L, a high cell DC resistance of 0.47 Ω, and an ΩF value of 5.6, making it unsuitable for rapid charging and discharging.

[0057] The electrodes of Comparative Examples 2 and 3 have an increased electrode film density compared to the electrode of Comparative Example 1, which reduces the resistance of the electrode film. However, this makes it difficult for the electrolyte to diffuse within the electrode, resulting in a decrease in the specific capacity of the cell, a decrease in energy density, and an increase in the DC resistance of the cell. Specifically, the electrode of Comparative Example 2 had a specific capacity of 131 F / g and an ΩF value of 5.2, while the electrode of Comparative Example 3 had a specific capacity of 86 F / g and an ΩF value of 5.3. Therefore, the electrodes of Comparative Examples 2 and 3 were also unsuitable for rapid charging and discharging.

[0058] In contrast, the electrodes of Examples 1 to 4 produced within the scope of the present invention had an electrode film density of 0.68 g / cm ３ (Example 1) to 1.01 g / cm ３Although the electrode film density was increased to 0.1 Ωcm (Example 1) (Example 4), the resistance per unit volume of the electrode film decreased from 0.1 Ωcm (Example 1) to 0.009 Ωcm (Example 4). Furthermore, the specific capacitance remained around 170 F / g. Therefore, the cells fabricated using the electrodes of Examples 1 to 4 had a high mass energy density, and as the electrode density increased, the volumetric energy density also increased from 48 Wh / L (Example 1) to 68 Wh / L (Example 4). Within this film density range, the increase in electrode film density did not affect the diffusion of electrolyte within the electrode film, and the resulting batteries still had good DC resistance, with an ΩF value of 1.3 to 1.8. Thus, the electrodes of Examples 1 to 4 were suitable for rapid charging and discharging.

[0059] From the above results, it was confirmed that the present invention can realize an electrode with a high energy density per unit volume and a small-sized electricity storage device with a high output density.

[0060] REFERENCE SIGNS LIST 1 Graphene 2 Carbon nanotube (CNT) 10 Graphene / CNT composite 20 Aggregate 21a, 21b Binder resin 30 Electrode film 31 Current collector 32 Electrode 40 Electric double layer capacitor 41 Positive electrode (cathode) 42 Negative electrode (anode) 43 Spacer 44 Electrolyte

Claims

1. A composite aggregate of graphene and carbon nanotubes and a binder resin, wherein the aggregate has a particle diameter of 1 to 20 μm and a film density of 0.6 to 1.2 g / cm ３ The electrode film.

2. The electrode film according to claim 1, wherein the binder is also present inside the aggregates.

3. The electrode film according to claim 1 or 2, wherein the composite of graphene and carbon nanotubes constituting the aggregate is a graphene laminate in which carbon nanotubes are present between graphene layers.

4. The electrode film according to any one of claims 1 to 3, further comprising a conductive material.

5. An electrode comprising the electrode film according to any one of claims 1 to 4.

6. An electricity storage device comprising the electrode according to claim 5.

7. The electricity storage device according to claim 6, which is an electric double layer capacitor and has an ΩF value of less than 2.

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