Patent Application
20250218697
The present invention relates to an electrode material using graphene, and an electrode and a capacitor formed using this electrode material.
Graphene is a sheet-like material having a two-dimensional network structure in which sp2 carbon atoms are bonded hexagonally, and has high electrical conductivity and high strength and is also excellent in heat resistance, thus, has attracted attention in various fields such as electronics such as electronic materials, and additionally, biomedical materials, aerospace materials, and the like. In particular, single-layer graphene has a large specific surface area and is expected to have a high capacity, so it is being studied as an electrode material for batteries and capacitors (see, for example, Patent Literatures 1 to 3).
Patent Literature 1 describes graphene oxide having improved dispersibility in a solvent and improved electrical conductivity by adjusting the ratio of carbon atoms to oxygen atoms (C/O) as measured by X-ray photoelectron spectroscopy to 2.5 to 4. Further, Patent Literature 2 proposes a graphene powder having a specific surface area of 80 to 250 m2/g as measured by BET measurement method and an elemental ratio of oxygen to carbon (O/C ratio) of 0.09 to 0.30 as measured by X-ray photoelectron spectroscopy.
On the other hand, Patent Literature 3 proposes a lithium ion capacitor in which a cathode is formed from a composite of graphene and carbon nanotubes, an anode is formed from 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 Patent Literature 3 has a structure in which single-layer graphene is laminated with single-layer carbon nanotubes as a spacer.
However, the graphene oxide described in the above-mentioned Patent Literature 1 has a problem that it has low electrical conductivity and affects electrochemical performance because it contains many oxygen-containing functional groups. In addition, the graphene powder described in Patent Literature 2 has a problem that it is easy to stack between graphene sheets. When actually constructing an electricity storage device, the electrode material is mixed with a conductive material and a binder (a binding agent) and processed into a film, and when the graphene powder of Patent Literature 2 is mixed with such a conductive material and binder, these are adsorbed on the surface of the graphene, causing stacking between the graphene sheets, which may affect the penetration and diffusion of electrolyte ions and degrade the energy characteristics of the electricity storage device.
On the other hand, the lithium ion capacitor described in Patent Literature 3 uses a composite of graphene and carbon nanotubes as an electrode material, and therefore can improve the specific capacitance and energy density, but, in order to expand the range of application of lithium ion capacitors, there is a demand for improved durability and further increased capacity.
Therefore, the present invention provides an electrode material capable of realizing a lithium ion capacitor having high capacity and excellent durability, and an electrode and a capacitor using this electrode material.
The electrode material according to the present invention is an electrode material comprising a composite of graphene and carbon nanotubes, wherein the composite has a ratio of carbon atoms to oxygen atoms (C/O) of 7 or more as measured by X-ray photoelectron spectroscopy, and a carbon nanotube content of less than 20 mass % (excluding 0 mass %).
As the composite, for example, a graphene laminate having carbon nanotubes present between layers can be used.
The composite may have a ratio of carbon atoms to oxygen atoms (C/O) of 12 or more as measured by X-ray photoelectron spectroscopy.
The composite may be in the form of an approximately spherical aggregate.
In this case, a polymer layer may be formed on the surface of the aggregate.
The electrode according to the present invention is formed using the above-mentioned electrode material, and contains, in addition to the above-mentioned electrode material, for example, a conductive material and a binder.
The capacitor according to the present invention includes the above-described electrode.
When the capacitor of the present invention is a lithium ion capacitor, the above electrode can be used as a positive electrode.
Another electrode material according to the present invention contains graphene powder having a ratio of carbon atoms to oxygen atoms (C/O) of 12 or more as measured by X-ray photoelectron spectroscopy.
The ratio of carbon atoms to oxygen atoms (C/O) specified in the present 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 in the following descriptions.
According to the present invention, a lithium ion capacitor having high capacity and excellent durability can be realized.
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.
First, an electrode material according to a first embodiment of the present invention will be described. The electrode material according to this embodiment is composed of a composite of graphene and carbon nanotubes (CNTs) (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 according to this embodiment has a ratio of carbon atoms to oxygen atoms (C/O) of 7 or more as measured by X-ray photoelectron spectroscopy (XPS), and a carbon nanotube content of less than 20 mass % (excluding 0 mass %).
In the graphene/CNT composite 10 shown in
Furthermore, the type of CNT2 is not particularly limited, and may be any of single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT). The size of CNT2 is not particularly limited, but from the viewpoint of promoting uniform dispersion of CNT in graphene and more efficiently making the composite with 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.
Graphene 1 has a characteristic of being easily aggregated due to T-x stacking, but in the graphene/CNT composite 10, CNTs 2 present between the layers of the single-layer graphene 1 function as spacers, and therefore, stacking is prevented and a high specific surface area can be ensured. Furthermore, when processed into an electrode, an electrolyte flows into the gaps between the layers of the single-layer graphene 1, and electrolyte ions are easily adsorbed onto the graphene surface. Furthermore, since carbon nanotubes 2 with high electrical conductivity are present between the layers of the graphene 1, the graphene/CNT composite 10 also has high electrical conductivity in the thickness direction.
The graphene/CNT composite used in the electrode material of this embodiment may constitute an approximately spherical aggregate, and in that case, a polymer layer may be formed on the surface of the aggregate.
Here, the approximately spherical aggregate 20 of the graphene/CNT composites 10 can be formed, for example, by dispersing the graphene/CNT composites 10 in which the CNT is located between the graphene layers in a lower alcohol having 1 to 5 carbon atoms or a mixed liquid of such a lower alcohol and water.
The graphene/CNT composite used in the electrode material of this embodiment has a ratio of carbon atoms to oxygen atoms (C/O) of 7 or more 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 it is made into an electrode, and the capacity retention rate decreases when it is used repeatedly. The C/O ratio of the graphene/CNT composite is preferably 12 or more, and this makes it possible to realize a capacitor electrode that can retain a high capacity for a long period of time over a wide temperature range.
Here, the C/O value 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 in this case, the C/O ratio of the obtained graphene can be adjusted by changing the graphene oxide reduction time, reduction temperature, or reducing agent concentration.
[Graphene/CNT Composite, CNT Content: Less than 20 Mass %]
The graphene/CNT composite used in the electrode material of this embodiment has a CNT content of less than 20 mass %. When the CNT content is 20 mass % or more, the graphene content is relatively reduced, and therefore, when such a graphene/CNT composite is used as an electrode material, the energy characteristics of the electricity storage device are degraded. Note that, since CNT is an essential component in the electrode material of this embodiment, the CNT content does not include 0 mass %.
As described above in detail, the electrode material of the present embodiment contains a graphene/CNT composite having a ratio of a carbon atom to oxygen atom (C/O) of 7 or more and a CNT content of less than 20 mass %, and therefore can realize a capacitor electrode that is excellent in durability and can maintain a high cell capacity even at high temperatures.
Next, an electrode according to a second embodiment of the present invention will be described. The electrode according to the present embodiment is formed from the electrode material according to the first embodiment described above, and contains at least a graphene/CNT composite, and may further contain a conductive material, a binder, and the like.
The conductive material used in the electrode of the present embodiment is not particularly limited and may be any material that is used as a conductive material in a normal electrode, and from the viewpoint of affinity with graphene, carbon materials such as carbon black, acetylene black, channel black, furnace black, and ketjen black are preferred.
The binder can also be appropriately selected from organic solvent-based binders and aqueous binders used in normal electrodes. Specifically, examples of organic solvent-based binders include polytetrafluoroethylene (PTFE) resin, its modified polytetrafluoroethylene resin, polyvinylidene fluoride (PVDF), etc., and examples of aqueous binders include sodium carboxymethylcellulose (CMC), styrene butadiene rubber (SBR), etc. Among these, it is particularly preferable to use a combination of the aqueous binders CMC and SBR.
The electrode of this embodiment can be formed, for example, by adding a solvent such as water to the graphene/CNT composite, the conductive material, and the binder, thoroughly mixing the mixture to form a slurry, coating both sides of a metal foil collector made of an aluminum etched foil or the like with the slurry using a roll coater or the like to form an electrode layer, and drying the resulting layer. The electrode of this embodiment can be used for various applications, such as 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.
The electrode of the present embodiment is formed from an electrode material containing 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 mass %, and therefore it is possible to realize a capacitor that is excellent in durability and can maintain a high cell capacity even at high temperatures.
Next, a capacitor according to a third embodiment of the present invention will be described. The capacitor according to the present embodiment includes the electrodes according to the second embodiment described above.
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 an power density of 108 W/kg or more, so that a capacitor with higher power and capacity can be obtained compared to conventional products.
In the capacitor of the present embodiment, at least the positive electrode is formed of a graphene/CNT composite or an electrode material containing this 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 mass %, and therefore the capacitor has a higher capacity and is more durable than conventional products. Specifically, the electrode material used in conventional lithium ion capacitors has a capacity per unit mass of 70 to 80 F/g and an operating voltage of about 2.2 to 3.8 V, but the electrode material used in the lithium ion capacitor of the present embodiment has a capacity per unit mass of about 160 F/g and an operating voltage of 2.2 to 4.3 V, and thus, the energy density is about three times that of conventional lithium ion capacitors.
Next, an electrode material according to a fourth embodiment of this embodiment will be described. Although the electrode material according to the first embodiment described above uses a graphene/CNT composite, the present invention is not limited thereto, and graphene can be used alone without being combined with CNT. Specifically, the electrode material according to this embodiment is graphene powder having a ratio of carbon atoms to oxygen atoms (C/O) of 12 or more as measured by XPS.
The electrode material of the present embodiment contains graphene powder having a ratio of carbon atoms to oxygen atoms (C/O) of 12 or more, and therefore can realize a capacitor electrode that is excellent in durability and can maintain a high cell capacity even at high temperatures.
The effects of the present invention will be specifically described below with reference to examples and comparative examples.
As a first example of the present invention, electrodes were fabricated using graphene/CNT composites having different ratios of carbon atoms to oxygen atoms (C/O), and the characteristics of lithium ion capacitors using these electrodes as positive electrodes were evaluated.
The cell performance was evaluated according to the following methods and criteria.
Eighty seven parts by mass of graphene/CNT composite powder was mixed with 5 parts by mass of acetylene black powder, 4 parts by mass of acrylic binder, 4 parts by mass of carboxymethyl cellulose, and 210 parts by mass of water, and thoroughly mixed to obtain a positive electrode slurry. In addition, an aluminum through-foil having a thickness of 31 μm was used as the positive electrode current collector. Then, the above-mentioned positive electrode slurry was applied to both sides of the positive electrode current collector by a roll coater to form a positive electrode layer, and then vacuum dried. The total thickness of the obtained positive electrode (the sum of the thickness of the positive electrode layer and the thickness of the positive electrode current collector on both sides) was 195 μm.
Eighty eight parts by mass of graphite having a particle diameter (D50) of 5±0.5 μm was mixed with 5 parts by mass of acetylene black powder, 3 parts by mass of SBR (styrene butadiene rubber)-based binder, 4 parts by mass of carboxymethyl cellulose, and 210 parts by mass of water, and thoroughly mixed to obtain a negative electrode slurry. A copper foil having a thickness of 21 μm was used as the negative electrode current collector. The above-mentioned negative electrode slurry was then applied to both sides of the negative electrode current collector by a roll coater to form a negative electrode layer, and then vacuum dried. The total thickness of the obtained negative electrode (the sum of the thickness of the negative electrode layer and the thickness of the negative electrode current collector on both sides) was 66 μm.
The positive electrode fabricated by the above-mentioned method was cut into two pieces with a size of 3.0 cm×3.0 cm to be used as electrodes for evaluation. After ultrasonic welding of terminals to each of the two electrodes for evaluation, they were placed facing each other with a cellulose separator having a thickness of 25 μm sandwiched therebetween, and were housed in an exterior body made of a laminate film in which polypropylene, aluminum, and nylon were laminated. Then, an electrolyte (1M LiPF6/EC (Ethylene carbonate):DEC (Diethyl carbonate)=1:1 (v/v %) mixed solvent) was injected into the exterior body, and the exterior body was heat-sealed to encapsulate the electrode terminal with the electrode terminal end pulled out of the exterior body, to obtain an evaluation laminate cell.
Next, using the evaluation laminate cell, measurements were performed at room temperature in a potential range of 0 to 2.7 V, and the electrostatic capacity per unit weight C (F/g) was calculated based on the following formula 1. In the following formula 1, I (A) is a constant current, m (g) is the total mass of the two electrodes, and dV/dt (V/s) is a slope obtained by linear fitting of the discharge curve between Vmax (the voltage at the start of discharge) and ½Vmax.
The negative electrode fabricated by the above-mentioned method was cut into a size of 3.0 cm×3.0 cm to be used as an electrode for evaluation. In addition, a lithium metal having a size of 3.0 cm×3.0 cm and a thickness of 100 μm was used as a counter electrode of this electrode for evaluation, and a microporous membrane made of polypropylene having a thickness of 50 μm was used as a separator to fabricate a half cell. At that time, lithium metal was used as a reference electrode. 1M LiPF6/EC:DEC=1:1 (v/v %) was used as the electrolyte.
The charging current density was set to 50 mA/g, and lithium ions were charged at 500 mAh/g relative to the weight of the negative electrode active material, and then discharged at 50 mA/g to 3 V. The electrostatic capacity per unit weight of the negative electrode was calculated to be 14,000 F/g from the discharge time during which the potential of the negative electrode changed by 0.2 V from the potential of the negative electrode 1 minute after the start of discharge.
The positive electrode was cut into 10 pieces measuring 2.8 cm×2.8 cm, and the negative electrode was cut into 9 pieces measuring 3.0 cm×3.0 cm, which were then laminated one by one with a separator between them and dried for 12 hours at 120° C. Thereafter, separators were placed on the top and bottom layers, the four sides were taped, and one piece of lithium metal pressed onto copper lath (copper mesh material) was placed on the outermost layer so as to face the positive electrode, to obtain an electrode laminate unit.
An aluminum positive electrode terminal was superimposed on the terminal welded part of the positive electrode current collector of the electrode laminate unit fabricated by the above-mentioned method and ultrasonically welded. In addition, a nickel negative electrode terminal was superimposed on the terminal welded part of the copper lath to which the negative electrode current collector and lithium metal foil were pressed, and ultrasonically welded. Then, the end of the electrode terminal was pulled out of the exterior laminate film pouch (9.8 mm×9.8 mm×2.9 mm) and three sides were heat-sealed, and 1M LiPF6/EC:DEC=1:1 (v/v %) was vacuum-impregnated as an electrolyte, and the remaining side was heat-sealed under reduced pressure to perform vacuum sealing, thereby assembling a film-type capacitor cell.
The cell assembled by the above method was left for 14 days, and then the cell voltage was measured and found to be 2.7 V or higher, so it was determined that the lithium ions had been pre-dopped. Therefore, the cell was first charged at a constant current of 100 mA until the cell voltage reached 4.3 V, and then discharged at a constant current of 100 mA until the cell voltage reached 2.2 V. The initial electrostatic capacity was evaluated from this 4.3 V-2.2 V cycle.
The durability was evaluated on the following three-level scale by simulating the electrostatic capacity and electrostatic capacity retention rate after 2000 hours in a state where a cell voltage of 4.2 V was applied at a temperature of 65° C.
The results are shown in Table 1 below.
As shown in Table 1 above, samples Nos. 1 to 3 using graphene/CNT composites with a ratio of a carbon atom to oxygen atom (C/O) of less than 7 were poor in energy density, power density, or durability. In contrast, samples Nos. 4 to 12 using graphene/CNT composites with a ratio of a carbon atom to oxygen atom (C/O) of 7 or more were high in energy density and power density, and also excellent in durability.
As a second example of the present invention, electrodes were fabricated using graphene powders with different ratios of carbon atoms to oxygen atoms (C/O), and the characteristics of a lithium ion capacitor using this electrode as a positive electrode were evaluated in the same manner as in the first example described above. The evaluation results are shown in Table 2 below.
As shown in Table 2 above, samples Nos. 21, 22, and 28, which used graphene powder with a ratio of a carbon atom to oxygen atom (C/O) of less than 12, had low power density and also poor durability. Samples Nos. 26 and 27, which used graphene oxide, could not be measured and were unsuitable as electrodes. In contrast, samples Nos. 23 to 25, 29, and 30, which used graphene powder with a ratio of a carbon atom to oxygen atom (C/O) of 12 or more, had high energy density and power density and were also excellent in durability.
From the above results, it was confirmed that the present invention can realize a lithium ion capacitor having high capacity and also excellent durability.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-082604 | May 2022 | JP | national |
The present application is a National Phase of International Application No. PCT/JP2023/018663 filed May 18, 2023, which claims the benefit of priority from the prior Japanese patent application No. 2022-082604 filed on May 19, 2022.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/018663 | 5/18/2023 | WO |
