Electrode film, electrode, and power storage device

Abstract

To provide an electrode film capable of achieving an electrode having high energy density per unit volume and a compact power storage device having high output density, an electrode, and a power storage device.SOLUTION: An electrode film with film density of 0.6 to 1.2 g / cm3 is formed by a composition which includes at least an aggregate 20 of a composite 10 of graphene and carbon nanotubes and binder resin 21b and in which a particle size of the aggregate 20 is 1 to 20 μm. A power storage device is configured using the electrode in which the electrode film is provided on a current collector.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Graphene is a sheet-like material having a two-dimensional network structure in which carbon atoms are bonded in a hexagonal shape. Since it has high conductivity, high strength, and excellent heat resistance, it has attracted attention in various fields such as the electronics field such as electronic materials, as well as biological / medical materials and aerospace materials. In particular, in high-energy-density power storage devices such as lithium-ion capacitors and lithium-ion batteries, various graphene-based electrodes have been proposed for the purpose of increasing the capacity, improving the withstand voltage, and further improving the energy density and output density (see Patent Documents 1 to 3). 2

[0003]

[0004] 3 2 For example, Patent Document 1 describes a supercapacitor electrode using a solid graphene foam having a physical density of 0.01 to 1.7 g / cm

[0005]

Prior Art Documents

[0005]

Patent Document 1

[0006] However, since graphene materials tend to be re-stacked during the production process of electrodes, in the case of electrodes using the porous graphene materials described in Patent Documents 1 and 2, especially when trying to form a high-density electrode film, the pores of the porous graphene are blocked by re-stacking, and the effect of promoting electrolyte transport is weakened, resulting in a problem of reduced capacity. On the other hand, such a problem does not occur in the electrodes using graphene aggregates in which fibrous substances exist between the layers of graphene described in Patent Document 3. However, in recent energy storage devices, further improvement in electrode performance is required.

[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 energy storage device capable of realizing a small-sized and high output density energy storage device. [Means for Solving the Problems]

[0008] The electrode film according to the present invention has an aggregate of a composite of graphene and carbon nanotubes 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 3. In the electrode film of the present invention, the binder may also be present inside the aggregate. The composite of graphene and carbon nanotubes constituting the aggregate is, for example, a graphene laminate in which carbon nanotubes exist between the layers of graphene. The electrode film of the present invention may further contain a conductive material.

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

[0010] The power storage device according to the present invention includes the above-described electrode. The power storage device of the present invention is, for example, a lithium ion capacitor, and has an ΩF value of less than 2.

Advantages of the Invention

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

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

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Figure 5

Figure 6

Modes for Carrying Out the Invention

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

[0014] (First Embodiment) First, the electrode film according to the 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, the electrode film 30 of this embodiment contains an aggregate 20 of a composite 10 of graphene and carbon nanotubes (hereinafter referred to as graphene / CNT composite 10) and a binder resin. And the aggregate 20 contained in the electrode film 30 of this embodiment has a particle size 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 showing the mode of the aggregate 20. As shown in FIG. 2, the aggregate 20 is formed by aggregating the graphene / CNT composite 10 in a substantially spherical shape while maintaining its shape, and a binder resin 21a is present inside thereof. By using the graphene / CNT composite 10 in the state of the aggregate 20, while maintaining the excellent conductivity and electrolyte ion adsorption performance of the graphene / CNT composite 10, when used in energy storage devices such as electric double layer capacitors, lithium ion capacitors and lithium ion batteries, these energy density and output density can be improved.

[0016] <Particle size: 1 to 20 μm> When the volume filling rate is the same, the smaller the particle size of the material, the higher the viscosity of the slurry. Therefore, if the aggregate with a particle size of less than 1 μm is used for the electrode film 30, the solid content cannot be increased, and the coating difficulty becomes high, so it is difficult to produce a uniform electrode film. On the other hand, when the particle size of the material becomes larger, larger voids are more likely to be formed. Therefore, if the aggregate 20 with a size exceeding 20 μm is used, the density of the electrode film cannot be increased to improve the volume energy density of the cell, and furthermore, it affects the diffusion of electrolyte ions and the rapid charge and discharge behavior. Therefore, in the electrode film 30 of this embodiment, the aggregate 20 with a particle size of 1 to 20 μm is used. The particle size of the aggregate 20 referred to here is the value measured by the laser diffraction / scattering method.

[0017] [Graphene / CNT composite 10] FIG. 3 is a diagram schematically showing an example of the structure of the graphene / CNT composite 10. As the graphene / CNT composite 10, for example, a graphene laminate in which a single-layer graphene 1 and carbon nanotubes (CNTs) 2 are alternately laminated as shown in FIG. 3 and the carbon nanotubes 2 are present between the layers of the graphene 1 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 any composite of the graphene 1 and the carbon nanotubes (CNTs) 2 may be used.

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

[0019] The type of the carbon nanotubes (CNTs) 2 in the graphene / CNT composite 10 is not particularly limited, and any of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs) may be used. The size of the carbon nanotubes (CNTs) 2 is also not particularly limited, and from the viewpoint of promoting the uniform dispersion of the carbon nanotubes (CNTs) 2 in the graphene 1 and more efficiently performing the composite 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, more preferably 1.0 to 3.0 nm.

[0020] Graphene 1 has the characteristic of being prone to aggregation by π-π stacking. However, in the graphene / CNT composite 10, the carbon nanotubes (CNTs) 2 present between the layers of monolayer graphene 1 function as spacers, preventing stacking and enabling the securing of a high specific surface area. Also, when processed into an electrode, the electrolyte flows into the gaps between the layers of monolayer graphene 1, making it easier for electrolyte ions to adsorb onto the graphene surface. Furthermore, in the graphene / CNT composite 10, since the carbon nanotubes 2 with high electrical conductivity are present between the layers of graphene 1, the electrical conductivity in the thickness direction is also high.

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

[0022] Among these resins, in particular, it is preferable to use a combination of the 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 bonding 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 - 1.2 g / cm 3 and preferably 0.6 - 1.0 g / cm 3 . When the film density is less than 0.6 g / cm 3 , the conductivity of the electrode film 30 decreases, and it becomes impossible to improve the volumetric energy density of the electrode. On the other hand, when the film density of the electrode film 30 exceeds 1.2 g / cm 3 , it becomes difficult for the electrolyte to penetrate, affecting 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 as long as it is generally used as a conductive material in the electrode film. From the perspective of affinity with graphene, carbon materials such as carbon black, acetylene black, channel black, furnace black, ketjen black, and carbon nanotubes (CNT) are preferable.

[0025] [Manufacturing method of electrode film 30] FIG. 4 is a flowchart showing the manufacturing process of the electrode film 30 of the present embodiment. The electrode film 30 of the present embodiment can be manufactured, for example, by performing a granulation step S1 of forming aggregates 20 of the graphene / CNT composite 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 composite 10 is dispersed together with the 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 composite 10. In the granulation step S1, by adding the binder resin, the binder resin 21a enters the inside of the formed aggregates 20, and the structural stability of the aggregates 20 is improved. 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 later.

[0027] [Step S2: Film-forming step] In the film-forming step S2, the electrode film 30 is formed using the aggregates 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 aggregates 20 are added with the binder resin 21b, a conductive material if necessary, and a solvent such as water, and thoroughly mixed to form a slurry, which is then applied using a roll coater or the like and dried.

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

[0029] In the film-forming step S1 described above, a load is applied during mixing or kneading, and a part of the aggregate 20 may be broken, resulting in insufficient performance of the electrode film 30. On the other hand, in the electrode film 30 of the present embodiment, since the aggregate 20 with improved structural stability by the binder resin 21a is used, the aggregate 20 will not be broken even when mixing or kneading is performed in the film-forming step S1, and an electrode film 30 with excellent performance can be stably manufactured. As a result, the durability, cycle performance, rapid charging, and discharging performance of the power storage device manufactured using this electrode film 30 can be improved.

[0030] As described in detail above, the electrode film of the present embodiment contains an aggregate of a graphene / CNT composite 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 Therefore, an electrode with a high energy density per unit volume can be obtained, and a small-sized and high output density power storage device can be realized.

[0031] Since the graphene / CNT composite used in the electrode film of the present embodiment has a layered structure, the ion adsorption rate is faster than that of the activated carbon used in the conventional catalyst film, and the complete release amount of the adsorbed ions is also larger. For this reason, the electrode film of the present embodiment is also excellent in ion adsorption performance, and is preferably used not only for the power storage device described above but also as a catalyst film.

[0032] (Second Embodiment) Next, the electrode according to the second embodiment of the present embodiment will be described. The electrode of the present embodiment includes the electrode film 30 of the first embodiment described above. FIG. 5 is a cross-sectional view schematically showing a structural example of the electrode of the present embodiment. The electrode 32 of the present embodiment can have a structure in which the electrode film 30 is provided on the 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. For example, in the case of a lithium ion capacitor or a lithium ion secondary battery, aluminum foil, carbon-coated aluminum foil, aluminum through foil, etched aluminum foil, etc. can be used. Further, the thickness of the current collector 31 can be appropriately set according to the application and the required performance, but it is preferably 10 to 30 μm, for example.

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

[0035] [Applications] The electrode 32 of the present embodiment can be used in various applications such as various capacitors such as lithium ion capacitors, various secondary batteries such as lithium ion secondary batteries, and other power storage devices, as well as fuel cells and various reaction electrodes.

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

[0037] (Third Embodiment) Next, the power storage device according to the third embodiment of the present embodiment will be described. The power storage device of the present embodiment includes the electrode of the second embodiment described above. FIG. 6 is a schematic diagram showing a structural example of the power storage device of the present embodiment. For example, when the power storage device of the present embodiment is the electric double layer capacitor 40 shown in FIG. 6, the positive electrode (cathode) 41 and the negative electrode (anode) 42 are arranged to face 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, ionic liquids such as 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), and 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPp-TFSI), or M'OH (M' is an alkali metal) can be used.

[0039] And 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 both the positive electrode (cathode) 41 and the negative electrode (anode) 42. When the power storage device of the present embodiment is the electric double layer capacitor 40 having the structure shown in FIG. 6, the ΩF value is preferably 2 or less, and thereby rapid charge and discharge can be performed.

[0040] The power storage device of the present embodiment is not limited to the electric double layer capacitor described above, and can be applied to various power 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 energy storage device of this embodiment uses an electrode provided with an electrode film containing an aggregate of a graphene / CNT composite and a binder resin on one or both sides of a current collector, where the particle size of the aggregate is 1 to 20 μm and the film density is 0.6 to 1.2 g / cm 3 Therefore, since it has a high energy density per unit volume, it can achieve miniaturization and high output density.

Examples

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

[0043] (Comparative Example 1) The electrode of Comparative Example 1 was fabricated by the method shown below. 87 parts by mass of an aggregate of a graphene / CNT composite (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 were mixed to prepare a slurry. An aluminum etching foil with a thickness of 22 μm was used as the current collector, and the aforementioned slurry was coated on both sides thereof with a roll coater to form an electrode film. After vacuum drying, an electrode with a thickness of 175 μm (the total thickness of the current collector and the electrode films on both sides) was obtained.

[0044] (Example 1) The electrode of Example 1 with a thickness of 51 μm was fabricated in the same manner as Comparative Example 1, except that it was press-molded.

[0045] (Example 2) The electrode of Example 2 with a thickness of 45 μm was fabricated in the same manner as Example 1.

[0046] (Example 3) The electrode of Example 3 with a thickness of 37 μm was fabricated in the same manner as Example 1.

[0047] (Example 4) The electrode of Example 4 with a thickness of 34 μm was fabricated in the same manner as Example 1.

[0048] (Comparative Example 2) In the same manner as in Example 1, an electrode of Comparative Example 2 with a thickness of 31 μm was fabricated.

[0049] (Comparative Example 3) In the same manner as in Example 1, an electrode of Comparative Example 3 with a thickness of 27 μm was fabricated.

[0050] 〔Resistance Measurement〕 For each of the electrodes of the examples and comparative examples fabricated by the method described above, using the electrode resistance measurement system RM2610 manufactured by Hioki E.E. Corporation, the resistance per unit volume was measured by the four-probe method.

[0051] 〔Fabrication of Evaluation Cell〕 Each of the electrodes (positive electrode and negative electrode) of the examples and comparative examples fabricated by the method described above was cut out to a size of 30 mm in length × 30 mm in width. For 5 positive electrodes and 6 negative electrodes, they were laminated through separators respectively, and after placing separators on the topmost layer and the bottommost layer, the four sides were taped to obtain an electrode laminated unit of the positive electrode and the negative electrode. Aluminum terminals (width 5 mm, thickness 0.1 mm) were overlapped and ultrasonically welded to the terminal welding parts (width 10 mm) of the positive electrode current collector and the negative electrode current collector of these electrode laminated units. The welding area at that time was set to (Yw: 3 mm) × (Xw: 3 mm).

[0052] After each electrode laminated unit was dried at a temperature of 120 °C for 12 hours, with the end of the electrode terminal drawn out outside the outer packaging laminate film pouch, the second side was folded back, and the first side and the second side of the terminal part of the outer packaging laminate film were heat-sealed with a sealing width of 2 mm. Next, after impregnating with EMI-BF 4 (1-ethyl-3-methylimidazolium tetrafluoroborate) under vacuum, the remaining fourth side was heat-sealed with a sealing width of 2 mm under reduced pressure, and then a film-type capacitor cell was assembled by performing vacuum sealing. Finally, the second side and the fourth side were bent once to obtain an evaluation cell.

[0053] 〔Characteristic Evaluation of Cell〕 Electrochemical measurements of the evaluation cells prepared by the method described above were carried out 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. Then, the initial electrostatic capacitance was determined from the discharge curve between the voltage Vmax at the start of discharge and 0 V.

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

[0055]

Table 1

[0056] As shown in Table 1 above, the electrode of Comparative Example 1 had a large mass specific capacitance of 175 F / g, but since the density of the electrode film was as low as 0.22 g / cm 3 , the resistance per unit volume was as large as 0.52 Ωcm. Therefore, the obtained battery had a large mass energy density of 83 Wh / kg, but the volume energy density was only 16 Wh / L, the cell DC resistance was as large as 0.47 Ω, the ΩF value was 5.6, and it was not suitable for rapid charge and discharge.

[0057] The electrodes of Comparative Examples 2 and 3 had a decreased resistance of the electrode film because the density of the electrode film increased compared to the electrode of Comparative Example 1. However, the diffusion of the electrolyte in the electrode became difficult, the specific capacitance of the cell decreased, the energy density decreased, and the DC resistance of the cell also increased. Specifically, the electrode of Comparative Example 2 had a specific capacitance of 131 F / g and an ΩF value of 5.2, and Comparative Example 3 had a specific capacitance of 86 F / g and an ΩF value of 5.3. Therefore, the electrodes of Comparative Examples 2 and 3 were also not suitable for rapid charge and discharge.

[0058] On the other hand, the electrodes of Examples 1 to 4 prepared 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 3(Example 4) Although it has been increased, the resistance per unit volume of the electrode film decreased from 0.1 Ωcm (Example 1) to 0.009 Ωcm (Example 4). Also, the specific capacitance was maintained at 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 volume energy density also increased from 48 Wh / L (Example 1) to 68 Wh / L (Example 4). Within this film density range, the increase in the electrode film density did not affect the diffusion of the electrolyte within the electrode film, and the resulting battery still had a good DC resistance, with the ΩF value being 1.3 to 1.8. Thus, the electrodes of Examples 1 to 4 were suitable for rapid charge and discharge.

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

Explanation of Symbols

[0060] 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. An aggregate of a composite of graphene and carbon nanotubes, a binder resin, and having, wherein the aggregate has a particle diameter of 1 to 20 μm, An electrode film having a film density of 0.6 to 1.2 g / cm ３ .

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

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

4. Furthermore, the electrode film according to claim 1, further comprising a conductive material.

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

6. A power storage device comprising the electrode according to claim 5.

7. The power storage device according to claim 6, which is a lithium ion capacitor and has an ΩF value of less than 2.

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