Electrode materials, electrodes, and capacitors

Abstract

To provide an electrode material that can realize a lithium ion capacitor with high capacity and excellent durability, and an electrode and a capacitor using this electrode material.SOLUTION: In a composite 10 of graphene and carbon nanotubes, an electrode is formed by using an electrode material including those whose carbon atom to oxygen atom ratio (C / O) measured by X-ray photoelectron spectroscopy is 7 or more and carbon nanotube content is less than 20 mass% (excluding 0 mass%), and the electrode is used, for example, as a positive electrode to configure a lithium ion capacitor.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electrode material using graphene, an electrode formed using this electrode material, and a capacitor.

Background Art

[0002] Graphene is a sheet-like substance 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 of electronic materials, as well as biological and medical materials and aerospace materials. In particular, single-layer graphene has a large specific surface area and can be expected to have a high capacity, so it has been studied as an electrode material for batteries and capacitors (see, for example, Patent Documents 1 to 3). 2

[0003] Patent Document 1 describes graphene oxide in which the ratio (C / O) of carbon atoms to oxygen atoms measured by X-ray photoelectron spectroscopy is made 2.5 to 4 to improve the dispersibility in a solvent and the electrical conductivity. Further, Patent Document 2 proposes a graphene powder having a specific surface area measured by the BET measurement method of 80 to 250 m 2 / g and an elemental ratio of oxygen to carbon (O / C ratio) measured by X-ray photoelectron spectroscopy of 0.09 to 0.30.

[0004] On the other hand, Patent Document 3 proposes a lithium ion capacitor in which a cathode is formed of a composite of graphene and carbon nanotubes, an anode is formed of a composite of Li-doped graphene and carbon nanotubes, and the mass ratio of the anode to the cathode is greater than 0 and less than 1.0. The composite of graphene and carbon nanotubes described in this Patent Document 3 has a structure in which single-layer graphene is laminated with single-layer carbon nanotubes as spacers.

Prior Art Documents

Patent Documents

[0005] ​[Patent Document 1] Japanese Patent Publication No. 2015-143162 [Patent Document 2] International Publication No. 2016 / 056557 [Patent Document 3] Patent No. 6732302 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the graphene oxide described in Patent Document 1 has the problem of having low conductivity due to the presence of numerous oxygen-containing functional groups, which affects its electrochemical performance. Furthermore, the graphene powder described in Patent Document 2 has the problem of being easily stacked between graphene sheets. When actually constructing an energy storage device, electrode materials are mixed with conductive materials and binders and processed into a film. However, when the graphene powder of Patent Document 2 is mixed with such conductive materials and binders, these adsorb onto the surface of the graphene, causing stacking between graphene sheets, which can affect the penetration and diffusion of electrolyte ions and potentially degrade the energy characteristics of the energy storage device.

[0007] On the other hand, the lithium-ion capacitor described in Patent Document 3 uses a composite of graphene and carbon nanotubes as the electrode material, which allows for improvements in specific capacity and energy density. However, in order to expand the range of applications for lithium-ion capacitors, there is a need for improved durability and even higher capacity.

[0008] Therefore, the present invention provides an electrode material that can realize a lithium-ion capacitor with high capacity and excellent durability, an electrode using this electrode material, and a capacitor. [Means for solving the problem]

[0009] The electrode material according to the present invention is an electrode material comprising a composite of graphene and carbon nanotubes, wherein the ratio of carbon atoms to oxygen atoms (C / O) of the composite, as measured by X-ray photoelectron spectroscopy,12.6 The above results indicate that the carbon nanotube content is less than 20% by mass (0% by mass is not included). As the composite, for example, a graphene laminate containing carbon nanotubes between the layers can be used. The composite may constitute a substantially spherical aggregate. In that case, a polymer layer may be formed on the surface of the aggregate.

[0010] The electrode according to the present invention is formed using the electrode material described above, and in addition to the electrode material described above, it contains, for example, a conductive material and a binder.

[0011] The capacitor according to the present invention is equipped with the electrodes described above. If the capacitor of the present invention is a lithium-ion capacitor, the electrode can be used as the positive electrode.

[0012] Another electrode material according to the present invention contains graphene powder having a carbon-to-oxygen atom ratio (C / O) of 12 or more, as measured by X-ray photoelectron spectroscopy.

[0013] The carbon-to-oxygen ratio (C / O) defined in this invention is a value calculated from the amounts of carbon atoms (C) and oxygen atoms (O) measured by X-ray photoelectron spectroscopy (XPS), and the same applies to the following explanation. [Effects of the Invention]

[0014] According to the present invention, a lithium-ion capacitor with high capacity and excellent durability can be realized. [Brief explanation of the drawing]

[0015] [Figure 1] This diagram schematically shows an example structure of a graphene and carbon nanotube composite. [Figure 2] This diagram schematically shows the configuration of aggregates of graphene and carbon nanotube composites. [Figure 3]It is a schematic diagram showing a structural example of a lithium-ion capacitor. [Figure 4] It is a diagram showing the performance of the capacitor of the present invention with the output density on the horizontal axis and the energy density on the vertical axis.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, embodiments 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.

[0017] (First Embodiment) First, the electrode material according to the first embodiment of the present invention will be described. The electrode material of this embodiment is composed of a composite of graphene and carbon nanotubes (CNT) (hereinafter also referred to as a graphene / CNT composite), or contains a graphene / CNT composite as a main raw material. The graphene / CNT composite used in the electrode material of this embodiment has a ratio of carbon atoms to oxygen atoms (C / O) measured by X-ray photoelectron spectroscopy (XPS) of 7 or more, and the content of carbon nanotubes is less than 20% by mass (0% by mass is not included).

[0018] [Structure of Graphene / CNT Composite] FIG. 1 is a diagram schematically showing a structural example of a graphene / CNT composite. The graphene / CNT composite used in the electrode material of this embodiment may be a composite of graphene and CNT, and its structure is not particularly limited. For example, a structure in which single-layer graphene 1 and carbon nanotubes 2 are alternately laminated like the graphene / CNT composite 10 shown in FIG. 1 can be used.

[0019] 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, the CNT 2 is arranged parallel to each other in the in-plane direction, but the present invention is not limited to this, and the CNT 2 may be arranged randomly between the layers of graphene 1.

[0020] 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 the CNT2 is not particularly limited, but from the viewpoint of promoting uniform dispersion of CNTs in graphene and more efficient composite formation 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.

[0021] 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 single-layer graphene 1 function as spacers, preventing stacking and ensuring a high specific surface area. Furthermore, when processed into electrodes, electrolyte flows into the gaps between the layers of single-layer graphene 1, making it easier for electrolyte ions to adsorb onto the graphene surface. In addition, the graphene / CNT composite 10 also has high electrical conductivity in the thickness direction because carbon nanotubes 2, which have high electrical conductivity, are present between the layers of graphene 1.

[0022] The graphene / CNT composite used in the electrode material of this embodiment may form substantially spherical aggregates, in which case a polymer layer may be formed on the surface of the aggregates. Figure 2 is a schematic diagram showing the form of the aggregate of the graphene / CNT composite 10. The aggregate 20 shown in Figure 2 is formed by the aggregation of the graphene / CNT basic framework, and the shape of the graphene / CNT composite 10 is maintained. Therefore, while maintaining the excellent conductivity and electrolyte ion adsorption performance of the graphene / CNT composite 10, it is possible to improve the energy density and power density when used in energy storage devices such as electric double-layer capacitors, lithium-ion capacitors, and lithium-ion batteries.

[0023] Here, the substantially spherical aggregates 20 of the graphene / CNT composite 10 can be formed, for example, by dispersing the graphene / CNT composite 10, in which CNTs are located between layers of graphene, in a lower alcohol having 1 to 5 carbon atoms or a mixture of such a lower alcohol and water.

[0024] [Graphene / CNT complex C / O ratio of 7 or higher] The graphene / CNT composite used in the electrode material of this embodiment has a carbon-to-oxygen atom ratio (C / O) of 7 or higher, as measured by X-ray photoelectron spectroscopy (XPS). If the C / O ratio of the graphene / CNT composite is less than 7, sufficient durability cannot be obtained when used as an electrode, and the capacity retention rate decreases with repeated use. The C / O ratio of the graphene / CNT composite is 12.6 The above is preferable, and as a result, it is possible to realize capacitor electrodes that can maintain high capacitance over a wide temperature range for a long period of time.

[0025] Here, the C / O ratio of the graphene / CNT composite can be adjusted, for example, by changing the C / O ratio of the graphene constituting the composite. The graphene constituting the graphene / CNT composite is prepared by reducing graphene oxide, and the C / O ratio of the resulting graphene can be adjusted by changing the reduction time of graphene oxide, the reduction temperature, or the concentration of the reducing agent.

[0026] [CNT content of graphene / CNT composite: less than 20% by mass] The graphene / CNT composite used in the electrode material of this embodiment has a CNT content of less than 20% by mass. If the CNT content is increased to 20% by mass or more, the graphene content decreases relatively, and using such a graphene / CNT composite as an electrode material will degrade the energy characteristics of the energy storage device. In this embodiment, CNTs are an essential component, so a CNT content of 0% by mass is not included.

[0027] As described in detail above, the electrode material of this embodiment contains a graphene / CNT composite in which the ratio of carbon atoms to oxygen atoms (C / O) is 7 or more and the CNT content is less than 20% by mass. Therefore, it is possible to realize a capacitor electrode that has excellent durability and can maintain a high cell capacity even at high temperatures.

[0028] (Second embodiment) Next, an electrode according to the second embodiment of this embodiment will be described. The electrode of this embodiment is formed from the electrode material of the first embodiment described above, and contains at least a graphene / CNT composite, and may further contain a conductive material and a binder.

[0029] The conductive material used in the electrodes of this embodiment is not particularly limited and can be any conductive material used in ordinary electrodes. However, from the viewpoint of affinity with graphene, carbon materials such as carbon black, acetylene black, channel black, furnace black, and Ketjen black are preferred.

[0030] Furthermore, the binder can be appropriately selected from organic solvent-based binders and aqueous binders commonly used in electrodes. Specifically, examples of organic solvent-based binders include tetrafluoroethylene resin (PTFE), modified tetrafluoroethylene resin, and polyvinylidene fluoride (PVDF), while examples of aqueous binders include sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR). Among these, it is particularly preferable to use a combination of the aqueous binders CMC and SBR.

[0031] The electrode of this embodiment can be formed, for example, by adding a solvent such as water to a graphene / CNT composite, a conductive material, and a binder, thoroughly mixing the mixture to form a slurry, and then coating both sides of a metal foil current collector made of aluminum etched foil or the like using a roll coater to form an electrode layer, which is then dried. Furthermore, the electrode of this embodiment can be used in a variety of applications, including various capacitors such as lithium-ion capacitors, various secondary batteries such as lithium-ion secondary batteries, and other energy storage devices, as well as fuel cells and electrodes for various reactions.

[0032] The electrodes of this embodiment are formed from an electrode material containing a graphene / CNT composite having a carbon-to-oxygen atom ratio (C / O) of 7 or more and a CNT content of less than 20% by mass. Therefore, it is possible to realize a capacitor that is highly durable and can maintain a high cell capacity even at high temperatures.

[0033] (Third embodiment) Next, a capacitor according to the third embodiment of this embodiment will be described. The capacitor of this embodiment is equipped with the electrodes of the second embodiment described above.

[0034] Figure 3 is a schematic diagram showing the structure of a lithium-ion capacitor (LIC). For example, if the capacitor in this embodiment is the lithium-ion capacitor 30 shown in Figure 3, the positive electrode (cathode) 31 and the negative electrode (anode) 32 are arranged opposite each other, spaced apart by a spacer 33, and a Li-ion electrolyte 34 is filled between these electrodes. In this lithium-ion capacitor, at least the electrode from the second embodiment described above is used as the positive electrode (cathode) 31.

[0035] The electrode material used in the lithium-ion capacitor of this embodiment has, for example, an energy density of 177 Wh / kg or more and a power density of 108 W / kg or more, thus enabling the creation of a capacitor with higher power output and higher capacity compared to conventional products.

[0036] The capacitor of this embodiment has a higher capacity and superior durability compared to conventional products, because at least the positive electrode is formed of a graphene / CNT composite or electrode material containing this composite, where the ratio of carbon atoms to oxygen atoms (C / O) is 7 or more and the CNT content is less than 20% by mass. Specifically, while the electrode material used in conventional lithium-ion capacitors has a capacity of 70-80 F / g and an operating voltage of about 2.2-3.8 V per unit mass, the electrode material used in the lithium-ion capacitor of this embodiment has a capacity of about 160 F / g and an operating voltage of 2.2-4.3 V per unit mass, resulting in an energy density about three times higher than that of conventional lithium-ion capacitors.

[0037] Figure 4 shows the performance of various capacitors, with power density on the horizontal axis and energy density on the vertical axis. As shown in Figure 4, the lithium-ion capacitor of the present invention, which uses a graphene / CNT composite as the electrode material, having a carbon-to-oxygen atom ratio (C / O) of 7 or more and a CNT content of less than 20 mass%, can significantly improve the power density compared to conventional electric double-layer capacitors, making it possible to design energy storage devices that surpass lithium-ion secondary batteries.

[0038] (Fourth embodiment) Next, an electrode material according to the fourth embodiment of this embodiment will be described. The electrode material of the first embodiment described above uses a graphene / CNT composite, but the present invention is not limited thereto, and graphene can also be used alone without being composited with CNTs. Specifically, the electrode material of this embodiment is graphene powder with a carbon-to-oxygen atom ratio (C / O) of 12 or more as measured by XPS.

[0039] The electrode material of this embodiment contains graphene powder with a carbon-to-oxygen atom ratio (C / O) of 12 or more, thus enabling the realization of a capacitor electrode that is highly durable and can maintain a high cell capacity even at high temperatures. [Examples]

[0040] The effects of the present invention will be specifically described below with reference to examples and comparative examples.

[0041] <First Example> As a first embodiment of the present invention, electrodes were fabricated using graphene / CNT composites with different carbon-to-oxygen atom ratios (C / O), and the characteristics of a lithium-ion capacitor using these electrodes as the positive electrode were evaluated.

[0042] [Cell performance] Cell performance was evaluated using the following methods and criteria.

[0043] (1) Manufacturing of the positive electrode A slurry for the positive electrode was obtained by thoroughly mixing 87 parts by mass of graphene / CNT composite powder with 5 parts by mass of acetylene black powder, 4 parts by mass of acrylic binder, 4 parts by mass of carboxymethylcellulose, and 210 parts by mass of water. A 31 μm thick aluminum through-foil was used as the current collector for the positive electrode. The aforementioned slurry for the positive electrode was then applied to both sides of the current collector using a roll coater to form the positive electrode layer, which was then vacuum-dried. The total thickness of the resulting positive electrode (sum of the thickness of the positive electrode layers on both sides and the thickness of the current collector) was 195 μm.

[0044] (2) Manufacturing of the negative electrode A slurry for the negative electrode was obtained by thoroughly mixing 88 parts by mass of graphite with a particle size (D50) of 5 ± 0.5 μm with 5 parts by mass of acetylene black powder, 3 parts by mass of SBR (styrene-butadiene rubber) binder, 4 parts by mass of carboxymethylcellulose, and 210 parts by mass of water. A copper foil with a thickness of 21 μm was used as the current collector for the negative electrode. The aforementioned slurry for the negative electrode was then applied to both sides of the current collector using a roll coater to form the negative electrode layer, and then vacuum-dried. The total thickness of the obtained negative electrode (sum of the thickness of the negative electrode layers on both sides and the thickness of the current collector for the negative electrode) was 66 μm.

[0045] (3) Measurement of capacitance per unit mass of the positive electrode Two positive electrodes, each measuring 3.0 cm x 3.0 cm, were cut out using the method described above to serve as evaluation electrodes. Terminals were ultrasonically fused to each of these two evaluation electrodes, and they were then placed opposite each other with a 25 μm thick cellulose separator in between. These electrodes were then housed in an outer casing made of a laminate film composed of polypropylene, aluminum, and nylon. An electrolyte solution (1M LiPF6 / EC (Ethylene carbonate):DEC (Diethyl carbonate) = 1:1 (v / v%) mixed solvent) was injected into the outer casing, and the outer casing was heat-sealed with the ends of the electrode terminals extended outside the casing to seal it and obtain an evaluation laminate cell.

[0046] Next, using an evaluation laminate cell, measurements were taken at room temperature in the potential range of 0 to 2.7 V, and the capacitance C (F / g) per unit weight was calculated based on Equation 1 below. In Equation 1 below, 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 linearly fitting the discharge curve between Vmax (voltage at discharge initiation) and 1 / 2Vmax.

[0047]

number

[0048] (4) Measurement of capacitance per unit mass of the negative electrode The negative electrode prepared using the method described above was cut to a size of 3.0 cm × 3.0 cm and used as the evaluation electrode. A half-cell was fabricated using a 3.0 cm × 3.0 cm lithium metal electrode with a thickness of 100 μm as the counter electrode, and a 50 μm thick microporous polypropylene membrane as the separator. In this case, lithium metal was used as the reference electrode. A 1M LiPF6 / EC:DEC = 1:1 (v / v%) electrolyte was used.

[0049] The charging current density was set to 50 mA / g, and lithium ions equivalent to 500 mAh / g were charged relative to the weight of the negative electrode active material. Subsequently, the electrode was discharged at 50 mA / g down to 3V. The capacitance per unit weight of the negative electrode was calculated from the discharge time during which the potential of the negative electrode changed by 0.2V from the potential 1 minute after the start of discharge, and was found to be 14000 F / g.

[0050] (5) Cell creation Ten positive electrodes measuring 2.8 cm x 2.8 cm were cut out, and nine negative electrodes measuring 3.0 cm x 3.0 cm were cut out. Each of these was stacked with a separator in between, and dried at 120°C for 12 hours. After that, separators were placed at the top and bottom layers, the four sides were taped together, and one sheet of lithium metal pressed onto copper lath (copper mesh material) was placed on the outermost layer opposite the positive electrode to obtain an electrode stacking unit.

[0051] An aluminum positive electrode terminal was superimposed on the terminal weld of the positive electrode current collector of the electrode stacked unit fabricated using the method described above and ultrasonically welded. Similarly, a nickel negative electrode terminal was superimposed on the terminal weld of the copper lath to which the negative electrode current collector and lithium metal foil were pressed and ultrasonically welded. Then, with the ends of the electrode terminals extended outside the outer laminate film pouch (9.8 mm x 9.8 mm x 2.9 mm), three sides were heat-sealed, and after vacuum impregnation with 1 M LiPF6 / EC:DEC = 1:1 (v / v%) as the electrolyte, the remaining side was heat-sealed under reduced pressure to create a vacuum seal, thereby assembling a film-type capacitor cell.

[0052] (6) Characterization of cells After leaving the cells assembled using the method described above for 14 days, the cell voltage was measured and found to be above 2.7V, indicating that the lithium-ion batteries had been pre-charged. Therefore, the cells were first charged with a constant current of 100mA until the voltage reached 4.3V, and then discharged with a constant current of 100mA until the voltage reached 2.2V. The initial capacitance was evaluated from this 4.3V-2.2V cycle.

[0053] [Durability] Durability was evaluated in the following three stages by simulating the capacitance and capacitance retention rate after 2000 hours at an ambient temperature of 65°C with an applied cell voltage of 4.2V. ×: Products with a capacity retention rate of less than 90%. ○: Products with a capacity retention rate of 90-95%. ◎: Products with a capacity retention rate exceeding 95%.

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

[0055] [Table 1]

[0056] As shown in Table 1 above, the ratio of carbon atoms to oxygen atoms (C / O) is 12.6 No. 1-3 using graphene / CNT complexes of less than 3 ,6 The sample had inferior energy density, power density, or durability. In contrast, the ratio of carbon atoms to oxygen atoms (C / O) was 12.6 No. 4 using the graphene / CNT complex described above. ,5,7 Samples ~12 exhibited high energy density and power density, as well as excellent durability.

[0057] <Second Example> As a second embodiment of the present invention, electrodes were fabricated using graphene powders with different carbon-to-oxygen atom ratios (C / O), and the characteristics of a lithium-ion capacitor using these electrodes as the positive electrode were evaluated in the same manner as in the first embodiment described above. The evaluation results are shown in Table 2 below.

[0058] [Table 2]

[0059] As shown in Table 2 above, samples No. 21, 22, and 28, which used graphene powder with a carbon-to-oxygen atom ratio (C / O) of less than 12, exhibited low power density and poor durability. Furthermore, samples No. 26 and 27, which used graphene oxide, were unsuitable as electrodes as they could not be measured. In contrast, samples No. 23-25, 29, and 30, which used graphene powder with a carbon-to-oxygen atom ratio (C / O) of 12 or greater, showed high energy density, high power density, and excellent durability.

[0060] From the above results, it has been confirmed that the present invention makes it possible to realize a lithium-ion capacitor that is both high-capacity and highly durable. [Explanation of Symbols]

[0061] 1. Graphene 2. Carbon nanotubes (CNTs) 10 Graphene / CNT complex 20 aggregates 21 Polymer layer 30 Li-ion capacitors 31. Positive electrode (cathode) 32. Negative electrode (anode) 33 Spacers 34 Li-ion electrolytes

Claims

1. An electrode material comprising a composite of graphene and carbon nanotubes, The aforementioned composite is The ratio of carbon atoms to oxygen atoms (C / O) measured by X-ray photoelectron spectroscopy is 12.6 or higher. An electrode material having a carbon nanotube content of less than 20% by mass (excluding 0% by mass).

2. The electrode material according to claim 1, wherein the composite is a graphene laminate in which carbon nanotubes are present between the layers.

3. The electrode material according to claim 1, wherein the composite constitutes a substantially spherical aggregate.

4. The electrode material according to claim 3, wherein a polymer layer is formed on the surface of the aggregate.

5. An electrode formed using the electrode material described in any one of claims 1 to 4.

6. The electrode according to claim 5, further comprising a conductive material and a binder.

7. A capacitor comprising the electrodes described in claim 5.

8. The capacitor according to claim 7, wherein the electrode is used as the positive electrode, which is a lithium-ion capacitor.

Images