Electrode sheet for aqueous electric double layer capacitor, and aqueous electric double layer capacitor

Abstract

The present invention addresses one problem of enhancing the energy density of an electric double layer capacitor by improving the conductivity in the thickness direction of an electrode sheet and then increasing the thickness of the electrode sheet. Moreover, the purpose of the present invention is also to provide an electric double layer capacitor which is safe, has a wide potential range, and has an improved amount of energy that can be stored.　The present invention for solving the problem pertains to an electrode sheet for an aqueous electric double layer capacitor, the electrode sheet being characterized by comprising a conductive auxiliary agent and porous carbon, wherein carbon nanotubes are contained as the conductive auxiliary agent, and the length of the carbon nanotubes is 5-90% of the thickness of the electrode sheet.

Description

Electrode sheets for aqueous electric double layer capacitors, and aqueous electric double layer capacitors

[0001] The present invention relates to an electrode sheet for a water-based electric double-layer capacitor and a water-based electric double-layer capacitor.

[0002] An electric double-layer capacitor is an energy storage device composed of electrodes, an electrolyte, and a current collector. When a voltage is applied, it extracts electricity from the electric double layer formed at the electrode interface. While electric double-layer capacitors excel in high power density and low performance degradation (long lifespan), they have a lower energy density than typical rechargeable batteries.

[0003] To improve the energy density of the aforementioned electric double-layer capacitor, research is underway on constituent materials, including electrodes and electrolytes. For example, Patent Document 1 (Japanese Patent Application Publication No. 2019-145414) discloses an aqueous secondary battery (a type of electric double-layer capacitor) that includes an electrode sheet containing carbon black (Ketjenblack) and uses an aqueous electrolyte. Patent Document 2 (Japanese Patent Application Publication No. 2020-141060) describes a technology that finds a correlation between the pore size of porous carbon and the increase in capacity, and enables an increase in the capacity of an electric double-layer capacitor by using porous carbon with a specific pore volume. However, the above prior art has issues with the thickness of the electrodes and the driving voltage as electric double-layer capacitors, so further improvements are needed, including output characteristics, cycle characteristics, energy density, and cost reduction by reducing the number of components through thicker electrodes.

[0004] Japanese Patent Publication No. 2019-145414 Japanese Patent Publication No. 2020-141060

[0005] The inventors of the present invention have considered increasing the thickness of the electrode sheet in order to improve the energy density of an electric double-layer capacitor. However, increasing the thickness of the electrode sheet reduces the conductivity in the thickness direction, which is the conductive direction (resistance increases), resulting in the disadvantage that energy is consumed within the charging sheet during charging and discharging. Therefore, one of the objectives of this disclosure is to improve the energy density of an electric double-layer capacitor by increasing the thickness of the electrode sheet while improving the conductivity in the thickness direction of the electrode sheet.

[0006] Furthermore, the present invention also aims to provide an electric double layer capacitor that is safe, has a wide potential range as a water-based electric double layer capacitor, and has an improved amount of energy that can be stored.

[0007] In the electric double-layer capacitor described in Patent Document 1, developed by the present inventors, carbon black aggregates (primary aggregates) within the electrode sheet play a major role as conduction paths. Here, the electrode sheet refers to the portion consisting only of the composite layer, without the current collector. However, the shape of the aggregates is irregular, and the length of the conduction paths created by the aggregates is thought to be on the order of several hundred nanometers. Therefore, the transfer of electrons and positive charges in the thickness direction of the electrode sheet, starting from the current collector, needs to be carried out repeatedly from one irregularly shaped aggregate to another. Thus, it was found that there is room to improve conductivity in the thickness direction within the electrode sheet of the electric double-layer capacitor described in Patent Document 1. Therefore, the inventors have diligently conducted research to solve the aforementioned problems and have found that by mixing a small amount of long carbon nanotubes (CNTs) into the electrode material of an electric double-layer capacitor, a long-distance, one-dimensional conduction path derived from the nanotubes and a short-distance, three-dimensional conduction path derived from the carbon black combine within the thick electrode, thereby constructing a good conductive path and improving conductivity in the thickness direction of the electrode. They have also found that the effective potential range can be broadened and the amount of stored energy can be improved by changing the elemental composition and chemical modification of the carbon material used in the electrode. In other words, the gist of the present invention for solving the aforementioned problems is as follows.

[0008] [1] An electrode sheet for an aqueous electric double layer capacitor, comprising a conductive additive and porous carbon, wherein the conductive additive comprises carbon nanotubes, and the length of the carbon nanotubes is 5% to 90% of the thickness of the electrode sheet. [2] The electrode sheet according to [1], further comprising carbon black as the conductive additive. [3] The electrode sheet according to [1] or [2], further comprising PTFE as a binder. [4] The electrode sheet according to [1] or [2], wherein the carbon nanotube content is 0.1% to 50% of the porous carbon content. [5] The electrode sheet according to [1] or [2], wherein the carbon component of the porous carbon is 65% or more and 95% or less in CHN analysis. [6] The electrode sheet according to claim 1 or 2, further comprising an electrolyte, wherein the electrolyte contains 0.1 M / L or more of sodium ions. [7] An aqueous electric double layer capacitor comprising the electrode sheet according to [1] or [2]. [8] The aqueous electric double layer capacitor according to [7], further comprising an electrolyte, wherein the electrolyte contains 0.1 M / L or more of sodium ions. [9] An electrode sheet for an aqueous electric double layer capacitor, comprising a conductive additive and porous carbon, wherein the conductive additive includes carbon nanofibers, and the length of the carbon nanofibers is 5% to 90% of the thickness of the electrode sheet.

[0009] The electrode sheet for aqueous electric double-layer capacitors of the present invention incorporates long carbon nanotubes as a conductive additive, thereby creating a good conductive path within the electrode where long, one-dimensional conduction paths derived from nanotubes and short, three-dimensional conduction paths derived from carbon black combine. As a result, the output characteristics do not deteriorate even when the thickness of the electrode sheet is increased. Therefore, by using the electrode sheet for aqueous electric double-layer capacitors of the present invention, it is possible to improve the energy density of the electric double-layer capacitor. Furthermore, by adjusting the elemental composition and chemical modifications of the carbon material used in the electrode, the effective potential range can be broadened and the amount of stored energy can be improved.

[0010] This figure shows the output characteristics of an aqueous electric double layer capacitor using the electrode sheet of the present invention. This figure shows the output characteristics of an aqueous electric double layer capacitor using the electrode sheet of the present invention (examination of CNT length). This figure shows the output characteristics of an aqueous electric double layer capacitor using the electrode sheet of the present invention (examination of carbon material). This figure shows the results of a comparative study of carbon materials as electrode materials for aqueous electric double layer capacitors. This figure shows the cycle characteristics of an aqueous electric double layer capacitor using activated carbon 1 as the carbon material. This figure shows the charge-discharge characteristics of the aqueous electric double layer capacitor at 100 cycles in Figure 5. This figure shows the charge-discharge characteristics of the aqueous electric double layer capacitor at 4,000 cycles in Figure 5. This figure shows the charge-discharge characteristics of the aqueous electric double layer capacitor at 10,000 cycles in Figure 5. This figure shows the charge-discharge curve and capacity retention rate of an aqueous electric double layer capacitor. This figure shows the charge-discharge curve and capacity retention rate of an aqueous electric double layer capacitor. This figure shows the surface functional groups of activated carbon as an electrode material for aqueous electric double layer capacitors measured by the temperature-controlled desorption method (TPD method). This figure shows the output characteristics of an aqueous electric double layer capacitor using the electrode sheet of the present invention (verification of electrode sheet thickness).

[0011] The electrode sheet for aqueous electric double layer capacitors and the aqueous electric double layer capacitor equipped therewith of the present invention will be described in detail below.

[0012] <Electrode Sheet> The electrode sheet of the present invention is an electrode sheet for an aqueous electric double layer capacitor, and is characterized in that it contains a conductive additive and porous carbon, and the conductive additive contains carbon nanotubes.

[0013] (Conductive additive) A conductive additive is a material added to improve the conductivity of an electrode. The electrode sheet of the present invention contains carbon nanotubes (CNT) as a conductive additive.

[0014] In the present invention, the diameter (wire diameter) of the carbon nanotube used as a conductive additive is not particularly limited, but may be in the range of 2 nm to 1000 nm, preferably 5 nm to 100 nm, and more preferably 10 nm to 60 nm. Furthermore, the length of the carbon nanotube is preferably longer than that used in conventional electrodes, may be in the range of 1% to 99% of the electrode sheet thickness, preferably 2% to 95%, more preferably 5% to 90%, and even more preferably 10% to 80%. Considering the thickness of an electrode sheet for a typical electric double-layer capacitor, the length of the carbon nanotube may be in the range of 0.01 mm to 10 mm, preferably 0.1 mm to 5 mm, more preferably 0.2 mm to 3 mm, even more preferably 0.3 mm to 2 mm, and particularly preferably 0.5 mm to 1.5 mm. In addition, carbon nanotubes of different lengths may be mixed and used. The length of the carbon nanotube here refers to the length from the tip to the end of the tube in the axial direction, and is the average length before it is incorporated into the electrode after the bundle has been unbundled.

[0015] In this invention, the carbon nanotubes used as conductive additives may be single-walled, double-walled, or multi-walled. Multi-walled carbon nanotubes are preferred.

[0016] The electrode sheet of the present invention may further contain, in addition to carbon nanotubes, highly conductive graphite; carbon black (acetylene black, Ketjen black, other furnace blacks, channel blacks, thermal lamp blacks, etc.); and pyrolytic graphite, which is obtained by gas-phase thermal decomposition of hydrocarbons such as methane, propane, and acetylene and depositing them as a thin film on a graphite substrate. Among these, carbon black (Ketjen black and acetylene black, in particular, have relatively small particle sizes and relatively good conductivity) is preferred because it has a low content of metal impurities and can ensure high conductivity. In the electrode sheet of the present invention, it is preferable to use a combination of carbon nanotubes and carbon black so that the carbon nanotubes and carbon black become composite, developing three-dimensional conductive paths and significantly improving conductivity.

[0017] The amount (content) of carbon nanotubes is 0.1 to 50 parts by mass, preferably 0.2 to 8 parts by mass, and more preferably 0.4 to 5 parts by mass, when the total mass of the main components of the electrode material, the carbon material described later (porous carbon particles, etc.), other conductive additives, and binders is 100 parts by mass. The total mass is the dry mass after removing the molding additive used when blending the main components. Furthermore, the amount (content) of carbon nanotubes in the electrode is in the range of 0.05% to 60% by mass of the porous carbon content described later, preferably 0.1% to 50% by mass, more preferably 0.2% to 30% by mass, particularly preferably 0.2% to 10% by mass, and most preferably 0.2% to 3% by mass.

[0018] The total amount of conductive additive is preferably 50 parts by mass or less, based on a total mass of 100 parts by mass of the aforementioned carbon material (porous carbon particles, etc.), conductive additive, and binder, which are the main components of the electrode material.

[0019] The electrode sheet of the present invention may use carbon nanofibers (CNF) instead of the carbon nanotubes (CNT) described above. In this case, the carbon nanofibers have a large aspect ratio and specifically have the following characteristics.

[0020] In the present invention, the diameter (wire diameter) of the carbon nanofiber used as a conductive additive is not particularly limited, but may be in the range of 2 nm to 1000 nm, preferably 5 nm to 100 nm, and more preferably 10 nm to 60 nm. Furthermore, the length of the carbon nanofiber is preferably longer than that used in conventional electrodes, may be in the range of 1% to 99% of the electrode sheet thickness, preferably 2% to 95%, more preferably 5% to 90%, and even more preferably 10% to 80%. Considering the thickness of an electrode sheet for a typical electric double-layer capacitor, the length of the carbon nanofiber may be in the range of 0.01 mm to 10 mm, preferably 0.1 mm to 5 mm, more preferably 0.2 mm to 3 mm, even more preferably 0.3 mm to 2 mm, and particularly preferably 0.5 mm to 1.5 mm. The length of the carbon nanofiber here refers to the length from the tip to the end in the axial direction of the fiber, and is the average length before being kneaded into the electrode after the bundle unbundling process.

[0021] In the electrode sheet of the present invention, similar to the case in which carbon nanotubes are used, a combination of carbon nanofibers and carbon black is used, which allows the carbon nanofibers and carbon black to become composite, develop three-dimensional conductive paths, and significantly improve conductivity, which is preferable.

[0022] When the total mass of the carbon material (such as porous carbon particles) described later, which is the main component of the electrode material, other conductive aids, and the binder is 100 parts by mass, the blending amount (content) of the carbon nanofibers is 0.1 to 50 parts by mass, preferably 0.2 to 8 parts by mass, and more preferably 0.4 to 5 parts by mass. The total mass is the dry mass after removing the molding aid used when blending the main components. Also, the blending amount (content) of the carbon nanofibers in the electrode is in the range of 0.05% by mass to 60% by mass of the content of the porous carbon described later, preferably 0.1% by mass to 50% by mass, more preferably 0.2% by mass to 30% by mass, particularly preferably 0.2% by mass to 10% by mass, and even more particularly preferably 0.2% by mass to 3% by mass.

[0023] (Porous Carbon) As the porous carbon as the carbon material of the electrode sheet of the present invention, activated carbon can be used. This activated carbon is an aggregate of condensed polycyclic aromatic compounds and can be used as both the electrode material on the negative electrode side and the electrode material on the positive electrode side.

[0024] The porous carbon as a carbon material suitable for the electrode (positive electrode and negative electrode) preferably has many pores of 5 nm or less, preferably 2 nm or less. Also, the BET specific surface area is 200 m ２ or more, 500 m ２ or more, 1,000 m ２ or more, 1,500 m ２ or more, 2,000 m ２ or more, 3,000 m ２ or more. The BET specific surface area is usually in the range of 200 m ２ / g to 5000 m ２ / g, and more preferably 1,000 m ２ / g to 5,000 m ２ / g, even more preferably 1,500 m ２ / g to 5,000 m ２ / g, particularly preferably 1,500 m ２ / g to 4,000 m ２ / g.

[0025] Suitable carbon materials for electrodes (positive and negative electrodes) are those with a carbon content of 60% by mass or more, more preferably 65% ​​to 95% by mass, even more preferably 80% to 94% by mass, and particularly preferably 90% to 93% by mass, according to CHN elemental analysis. By setting the carbon content of the carbon material to the above range according to CHN elemental analysis, the energy density of the electric double-layer capacitor using it can be improved.

[0026] As a carbon material suitable for the electrodes (positive and negative electrodes) of the aqueous electric double-layer capacitor of the present invention, in addition to the above-mentioned properties, the D / G band value measured by Raman spectroscopy is preferably 3 or less, more preferably 2 or less, more preferably 1 or less, and even more preferably 0.8 or less. Of the Raman spectrum, the 1590 cm⁻¹ value is preferred. －１ The spectrum in this region is a G-band originating from the graphite structure, at 1350 cm⁻¹. －１ The spectral distribution in the vicinity is a D-band, originating from the defect structure. A smaller D-band peak indicates higher purity. Raman spectroscopy can be performed according to conventional methods known to those skilled in the art.

[0027] There are no particular restrictions on the raw materials for activated carbon; various materials can be used, including plant-based wood, coconut shells, pulp wastewater, fossil fuel-based coal, petroleum heavy oil, coal obtained by thermal decomposition of these, petroleum pitch, coke, and synthetic resins such as phenolic resin, furan resin, polyvinyl chloride resin, and polyvinylidene chloride resin.

[0028] The carbon material of the electrode sheet of the present invention strongly exhibits properties as an electron donor, H ＋To selectively adsorb ions, it is thought that the area near the electrode should be made alkaline. This effect can widen the potential window in the negative direction, making it effective for use as a negative electrode. To further improve this effect, it is preferable that the carbon material has many functional groups or specific structures that further create an alkaline atmosphere near the electrode. Examples of such alkaline functional groups or specific structures include amino groups, amide groups, ether groups, carbonyl groups, pyrones, lactam structures, pyridine rings, nitrile structures, imide structures, and imine structures. Various methods such as gas activation and chemical activation can be applied to make the carbon material have even more of these functional groups or specific structures.

[0029] On the other hand, if carbon materials such as activated carbon contain many oxygen-containing functional groups or specific oxygen-containing structures such as carboxyl groups, hydroxyl groups, sulfo groups, phosphate groups, and lactone structures, or if these groups or structures are further introduced into conventional carbon materials, the atmosphere near the electrode becomes more acidic, which can broaden the potential window in the positive direction, making them effective as positive electrodes. Various methods such as gas activation and chemical activation can be applied to make carbon materials further possess these functional groups and specific structures, for example, hydrogen peroxide (H ２ O ２ ) Processing or other similar actions may be considered.

[0030] The amount (content) of porous carbon is 75 to 99 parts by mass, preferably 80 to 95 parts by mass, and more preferably 82 to 90 parts by mass, when the total mass of the main components of the electrode material, namely the carbon material (including porous carbon particles), conductive additive, and binder, is 100 parts by mass. The total mass is the dry mass after removing the molding additive used when blending the main components.

[0031] (Binding Agent) The electrodes (positive or negative electrode) in the aqueous electric double-layer capacitor of the present invention may contain a binding agent (binder), etc. The binding agent included in the electrode can be selected and used if it does not decompose in the potential range in which it is used and is suitable for the application. Examples include polytetrafluoroethylene (PTFE), polyvinylidene fluoride, carboxymethylcellulose, styrene-butadiene rubber, polyacrylic acid, polyimide resin, polyamide resin, fluororubber, etc.

[0032] The amount of binder is preferably 1 to 30 parts by mass, more preferably 3 to 20 parts by mass, based on a total mass of 100 parts by mass of porous carbon particles, conductive additives, and binder, which are the main components of the electrode material. The total mass is the dry mass after removing the molding aids used when blending these main components. If the amount of binder is less than 1 part by mass, it becomes difficult to retain the porous carbon particles in the polarizing electrode material. Conversely, if the amount exceeds 30 parts by mass, the energy density of the electric double-layer capacitor decreases and the internal resistance increases.

[0033] The binders contained in the positive electrode and the negative electrode may be a single type or a combination of two or more types.

[0034] (Additives) By adding powder such as Silica to the carbon material contained in the electrodes of the aqueous electric double layer capacitor of the present invention, an electric double layer capacitor with high energy density and less degradation of capacitance and resistance over time, i.e., excellent long-term reliability, can be obtained. By including powder such as Silica, moisture and decomposition products can be adsorbed and removed, thereby preventing blockage of the pores of porous carbon particles.

[0035] The addition amount of powders such as silica is preferably in the ratio of 2 parts by mass or more and 40 parts by mass or less, more preferably 4 parts by mass or more and 30 parts by mass or less, with the total mass of the carbon material, conductive assistant, and binder, which are the main components of the electrode material, being 100 parts by mass (the total mass is the dry mass after removing the molding assistant used when blending these main components). When the addition amount is less than 2 parts by mass, the effect of adsorbing and removing moisture and decomposition products is insufficient and the effect is not sufficient. On the other hand, as the addition amount of silica or the like increases, the ratio of the carbon material per unit volume of the electrode material decreases, so the initial capacitance of the capacitor cell decreases. When the addition amount is 4 parts by mass or more and 30 parts by mass or less, the effect of adsorbing and removing moisture and decomposition products is remarkable and at the same time the initial capacitance can be ensured, so it is more preferable.

[0036] (Aqueous electrolyte) The electrodes of the aqueous electric double layer capacitor of the present invention may be impregnated with an aqueous electrolyte. As the aqueous electrolyte, an aqueous electrolyte in which a neutral electrolyte is dissolved can be used, and it is preferably one containing water and at least one kind of neutral electrolyte. As the neutral electrolyte, it is more preferably at least one kind of salt selected from the group consisting of salts of alkali metal elements and salts of alkaline earth metal elements, and even more preferably at least one kind of salt selected from the group consisting of sodium salts, magnesium salts, calcium salts, lithium salts, potassium salts, and beryllium salts. It is particularly preferably a sodium salt, magnesium salt, lithium salt, or potassium salt, and most preferably a sodium salt.

[0037] The type of anion contained in the neutral electrolyte is not particularly limited. Examples of the anion include halide ions, sulfate ions, nitrate ions, phosphate ions, tetrafluoroborate ions, etc. The halide ions are specifically chloride ions, bromide ions, iodide ions, etc.

[0038] The neutral electrolyte is preferably a neutral salt at 25°C. Among them, at least one neutral sodium salt selected from the group consisting of sodium chloride, sodium bromide, sodium iodide, sodium sulfate, sodium nitrate, sodium tetrafluoroborate, etc.; at least one neutral magnesium salt selected from the group consisting of magnesium chloride, magnesium bromide, magnesium iodide, magnesium sulfate, magnesium nitrate, magnesium tetrafluoroborate, etc.; at least one neutral lithium salt selected from the group consisting of lithium chloride, lithium bromide, lithium tetrafluoroborate, etc.; at least one neutral potassium salt selected from the group consisting of potassium chloride, potassium bromide, potassium iodide, potassium tetrafluoroborate, etc. are more preferable, sodium chloride, sodium sulfate, magnesium chloride, etc. are still more preferable, and sodium chloride is particularly preferable.

[0039] The concentration of the neutral salt in the aqueous electrolyte is appropriately selected according to the type of the neutral salt and the like. The concentration of the neutral salt in the aqueous electrolyte varies in solubility depending on the temperature and solute. For example, when the aqueous electrolyte is at 20°C, if the water-soluble salt is sodium chloride, it is preferably 0.1 M / L or more (including 0.1 M / L or more of sodium ions), and more preferably in the range of 0.1 M / L to 6.1 M / L (including 0.1 M / L to 6.1 M / L of sodium ions). If the water-soluble salt is magnesium chloride, it is preferably 0.1 M / L or more (including 0.1 M / L or more of magnesium ions), and more preferably in the range of 0.1 M / L to 5.7 M / L (including 0.1 M / L to 5.7 M / L of magnesium ions). The concentration of the neutral salt in the aqueous electrolyte may be below the saturation solubility, and the higher the concentration, the better.

[0040] (Method for Manufacturing Electrode Sheets) The electrode sheets in the aqueous electric double-layer capacitor of the present invention can be manufactured in layers by a sheet manufacturing method, a coating method, etc., to form electrode layers. The sheet manufacturing method is a method in which an electrode material sheet (electrode layer) is formed in advance and then adhered or installed on a metal foil (aluminum foil, copper foil, etc.), while the coating method is a method in which the electrode material is coated onto a metal foil (aluminum foil, copper foil, etc.) which is a current collector to form an electrode layer. When the same activated carbon is used, the sheet manufacturing method is preferred over the coating method in that it can produce a higher capacitance.

[0041] In the aforementioned sheet manufacturing method, for example, the above-mentioned carbon material can be processed to have an average particle size D50 of approximately 5 to 200 μm, then mixed with a conductive additive, silica, and a binder, and rolled to form a sheet. During the mixing process, liquid additives such as water, ethanol, and acetonitrile may be used individually or in combination as appropriate. Alternatively, the electrode sheet may be impregnated with the above-mentioned aqueous electrolyte after preparation.

[0042] To improve the output of the aqueous electric double-layer capacitor of the present invention, it is preferable to support a gelling agent such as carboxymethylcellulose (CMC) gel on the carbon material contained in the electrodes. For example, it is preferable to support 0.5 to 2.0% by mass of CMC on activated carbon, which is the carbon material of the electrodes. Specifically, the CMC can be supported on the activated carbon by coating the activated carbon with a mixture of CMC, ethanol, and water, and heating it on a hot plate or the like until the solution evaporates.

[0043] The electrode sheet technology of the present invention described above can also be applied to electrode sheets in desalination capacitors.

[0044] <Aqueous Electric Double Layer Capacitor> The aqueous electric double layer capacitor of the present invention is characterized by comprising the electrode sheet of the present invention described above. In addition to the electrode sheet, the aqueous electric double layer capacitor of the present invention comprises an aqueous electrolyte, a separator, and a current collector. Each of these will be described below.

[0045] (Electrode Sheet) The electrode sheet provided in the aqueous electric double-layer capacitor of the present invention is the electrode sheet of the present invention as described above. For specific details, the explanation in the "Electrode Sheet" section can be applied as is.

[0046] (Aqueous Electrolyte) The electrolyte in the aqueous electric double-layer capacitor of the present invention is an aqueous electrolyte in which a neutral electrolyte is dissolved, and contains water and at least one neutral electrolyte. The neutral electrolyte is preferably at least one salt selected from the group consisting of alkali metal element salts and alkaline earth metal element salts, more preferably at least one salt selected from the group consisting of sodium salt, magnesium salt, calcium salt, lithium salt, potassium salt, and beryllium salt, even more preferably sodium salt, magnesium salt, lithium salt, and potassium salt, and particularly preferably sodium salt.

[0047] The types of anions contained in neutral electrolytes are not particularly limited. Examples of anions include halide ions, sulfate ions, nitrate ions, phosphate ions, and tetrafluoroborate ions. Specifically, halide ions include chloride ions, bromide ions, and iodide ions.

[0048] The neutral electrolyte is preferably a neutral salt at 25°C. In particular, at least one neutral sodium salt selected from the group consisting of sodium chloride, sodium bromide, sodium iodide, sodium sulfate, sodium nitrate, sodium tetrafluoroborate, etc.; at least one neutral magnesium salt selected from the group consisting of magnesium chloride, magnesium bromide, magnesium iodide, magnesium sulfate, magnesium nitrate, magnesium tetrafluoroborate, etc.; at least one neutral lithium salt selected from the group consisting of lithium chloride, lithium bromide, lithium tetrafluoroborate, etc.; at least one neutral potassium salt selected from the group consisting of potassium chloride, potassium bromide, potassium iodide, potassium tetrafluoroborate, etc., is more preferred, with sodium chloride being even more preferred, and sodium chloride being particularly preferred.

[0049] The concentration of the neutral salt in the aqueous electrolyte is appropriately selected depending on the type of neutral salt, etc. The solubility of the neutral salt in the aqueous electrolyte varies depending on the temperature and solute, but for example, when the aqueous electrolyte is at 20°C, if the water-soluble salt is sodium chloride, it is preferable that the concentration is 0.1 M / L or more (containing 0.1 M / L or more sodium ions), and more preferably in the range of 0.1 M / L to 6.1 M / L (containing 0.1 M / L to 6.1 M / L sodium ions). If the water-soluble salt is magnesium chloride, it is preferable that the concentration is 0.1 M / L or more (containing 0.1 M / L or more magnesium ions), and more preferably in the range of 0.1 M / L to 5.7 M / L (containing 0.1 M / L to 5.7 M / L magnesium ions). The concentration of the neutral salt in the aqueous electrolyte should be below the saturation solubility, and a higher concentration is preferable.

[0050] The aqueous electrolyte may contain a water-soluble organic solvent. Examples of organic solvents include acetonitrile and acetone.

[0051] If the aqueous electrolyte contains an organic solvent, its content is preferably greater than 0% by mass and less than or equal to 50% by mass relative to the water, and more preferably greater than 0% by mass and less than or equal to 10% by mass.

[0052] The aqueous electrolyte may contain various additives as needed. Examples of additives include sodium sulfite and gelling agents such as carboxymethylcellulose.

[0053] The aqueous electrolyte preferably has a dissolved oxygen content of 7.3 ppm or less. A dissolved oxygen content of 7.3 ppm or less tends to improve the cycle characteristics of the secondary battery. A more preferable dissolved oxygen content is 5 ppm or less, and the most preferable dissolved oxygen content is 4 ppm or less.

[0054] Generally, the saturated dissolved oxygen content of aqueous electrolytes at room temperature (22°C to 23°C) is 8.2 to 8.6 ppm, and the dissolved oxygen content can be reduced to 7.3 ppm or less by commonly used procedures. For example, the dissolved oxygen content can be maintained within a desired range by degassing at least once during battery manufacturing, or by providing a packing during battery manufacturing to suppress the increase in oxygen content. The degassing method can be appropriately selected from commonly used methods, for example, by reducing the pressure or heating.

[0055] In the electric double-layer capacitor of the present invention, the aqueous electrolyte also includes a gel-like solution made using a gelling agent. The gel-like aqueous electrolyte in the present invention is a liquid aqueous electrolyte made into a gel, and can be obtained by adding a gelling agent to a liquid aqueous electrolyte. In the present invention, "gel-like" refers to a type of dispersion system, which is a colloid of a liquid dispersion medium such as a sol, but has high viscosity due to the network of dispersed particles, loses fluidity, and the system as a whole becomes solid. The aqueous electrolyte may be sealed in the internal space of a storage case, or it may be used by impregnating an electrode sheet or the like. The gel-like aqueous electrolyte can be prepared, for example, by putting a liquid electrolyte into a snap cup, adding an appropriate amount of carboxymethyl cellulose, crushing it with a spatula, and stirring it at room temperature at 10,000 rpm for about 5 minutes using a homomixer.

[0056] Furthermore, in open-system batteries, the amount of dissolved oxygen can be reduced by installing an insertion tube in the aqueous electrolyte and continuously bubbling nitrogen gas through it.

[0057] (Separator) The separator in the aqueous electric double-layer capacitor of the present invention is arranged to separate the positive and negative electrodes and is required to allow ions to pass through and prevent short circuits between the positive and negative electrodes. The separator is not particularly limited and conventionally known materials can be used. For example, polyolefin fibrous nonwoven fabrics, polyolefin microporous membranes, glass filters, and porous ceramic materials can be used.

[0058] (Current Collector) The current collectors (positive electrode current collector and negative electrode current collector) of the aqueous electric double-layer capacitor of the present invention are made of materials that do not cause side reactions at the potentials of the positive and negative electrodes, respectively. Specifically, the positive electrode current collector and negative electrode current collector should be made of corrosion-resistant materials that do not cause reactions such as dissolution at the potentials of the positive and negative electrodes. For example, metal materials, alloys, carbon materials, inorganic conductive oxide materials, etc., can be used as materials for the positive electrode current collector and negative electrode current collector. Examples of metal materials include copper, nickel, brass, zinc, aluminum, stainless steel, tungsten, gold, platinum, etc. Among these, stainless steel and aluminum are preferred. Examples of alloys include SUS. Examples of carbon materials include graphite, hard carbon, glassy carbon, etc. Alternatively, a material coated with a carbon material on a metal material may also be used.

[0059] (Method for manufacturing an aqueous electric double-layer capacitor) The aqueous electric double-layer capacitor of the present invention has a structure in which a pair of electrodes (positive electrode and negative electrode) are opposed to each other via a separator. Each electrode is joined to a current collector and impregnated with an aqueous electric field solution. The electric double-layer capacitor is an energy storage element that utilizes the capacitance of the electric double layer formed by the adsorption / desorption of electrolyte ions at the interface between the electrodes and the electrolyte solution.

[0060] The aqueous electric double-layer capacitor of the present invention may take any shape. For example, cylindrical, coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flattened, and rectangular shapes are examples. It may also be applied to large-scale capacitors used in electric vehicles and the like.

[0061] The present invention will now be described in detail with reference to examples, but the present invention is not intended to be limited to these examples.

[0062] <Investigation of Electric Double-Layer Capacitor Characteristics> (Test 1) Investigation of the effect of carbon nanotube (CNT) addition Activated carbon (activated carbon 1) as the carbon material used for the electrode, carbon black (CB) as a conductive additive, and carbon nanotubes (CNT: multi-wall nanotubes, length 1.0 mm), and polytetrafluoroethylene (PTFE) as a binder were mixed and kneaded, and rolled with a roller press to prepare electrode sheets with thicknesses of 1.2 mm and 2 mm. Carbon nanotubes (CNT) were not added in the comparative example.

[0063] The obtained sheets were cut to create electrodes as shown in the table below, and these were used in each test. The mass ratios of each component of the electrodes are also shown.

[0064] Comparative example 1: Activated carbon 1 / CB / PTFE=84 / 7 / 9 (1.2mm)

[0065]

[0066] Comparative example 2: Activated carbon 1 / CB / PTFE=84 / 7 / 9 (2mm)

[0067]

[0068] Example 1: Activated carbon 1 / CB / CNT / PTFE = 84 / 5.25 / 1.75 / 9 (1.2 mm)

[0069]

[0070] Example 2: Activated carbon 1 / CB / CNT / PTFE = 84 / 5.25 / 1.75 / 9 (2 mm)

[0071]

[0072] The electrode sheets obtained from measurements in a beaker cell were cut out with an 8 mm diameter punch to obtain the positive and negative electrodes. Each was then attached to a platinum mesh to which a platinum wire was mounted. A 3 M NaCl aqueous electrolyte was placed in a container, ensuring that the electrodes were fully submerged, and the electrodes were impregnated with the electrolyte under a vacuum atmosphere.

[0073] A discharge test was performed using the electric double-layer capacitor of the beaker cell prepared as described above. The results are shown in Table 5 and Figure 1.

[0074]

[0075] As shown in Table 5 and Figure 1, when carbon nanotubes (CNTs) were incorporated as a conductive additive in addition to carbon black (CB), the output characteristics were equivalent to those of the 1.2 mm electrode thickness, even when the electrode thickness was 2 mm. On the other hand, in the comparative example without carbon nanotubes (CNTs), as the electrode thickness increased, the resistance within the electrode increased significantly, and the output characteristics deteriorated significantly. This is thought to be the result of the effective operation of one-dimensional, long-distance conduction paths of carbon nanotubes (CNTs) in the thickness direction within the electrode.

[0076] (Test 2) Examination of the length of carbon nanotubes (CNTs) to be incorporated Similar to Test 1, activated carbon as the carbon material to be used for the electrode, carbon black (CB) and carbon nanotubes (CNT(S): single-wall nanotubes, length 0.3 mm / CNT(M): multi-wall nanotubes, length 1.0 mm), and polytetrafluoroethylene (PTFE) as a binder were mixed and kneaded, and rolled with a roller press to prepare electrode sheets with thicknesses of 1.2 mm and 2 mm.

[0077] The obtained sheets were cut to create electrodes as shown in the table below, and these were used in each test. The mass ratios of each component of the electrodes are also shown.

[0078] Example 3: Activated carbon 1 / CB / CNT(S) / PTFE = 84 / 5.25 / 1.75 / 9 (1.2 mm)

[0079]

[0080] Example 4: Activated carbon 1 / CB / CNT(S) / PTFE = 84 / 5.25 / 1.75 / 9 (2 mm)

[0081]

[0082] Example 5: Activated carbon 1 / CB / CNT(M) / PTFE = 84 / 5.25 / 1.75 / 9 (1.2 mm)

[0083]

[0084] Example 6: Activated carbon 1 / CB / CNT(M) / PTFE = 84 / 5.25 / 1.75 / 9 (2 mm)

[0085]

[0086] A double-layer electric capacitor was manufactured and discharged in the same manner as in Test Example 1. The results are shown in Table 10 and Figure 2. The capacitor was charged to 1.5V with a current of 20mA and discharged at 20mA.

[0087]

[0088] As shown in Table 10 and Figure 2, when the electrode thickness was 1.2 mm, there was almost no difference in the output characteristics between the case where 0.3 mm long carbon nanotubes (CNT(S)) were incorporated and the case where 1.0 mm long carbon nanotubes (CNT(M)) were incorporated. On the other hand, when the electrode thickness was 2.0 mm, the output characteristics were superior to those of the 1.2 mm electrode thickness case, regardless of the length of CNT used, but the results were particularly superior when 1.0 mm long carbon nanotubes (CNT(M)) were incorporated.

[0089] (Test 3) Investigation of activated carbon with different CHN ratios Similar to Test 1, activated carbon 1 or activated carbon 2 as the carbon material to be used for the electrode, carbon black (CB) and carbon nanotubes (CNT(M): multi-wall nanotubes, length 1.0 mm) as conductive additives, and polytetrafluoroethylene (PTFE) as a binder were mixed and kneaded, and rolled with a roller press to prepare an electrode sheet with a thickness of 2 mm.

[0090] The obtained sheet is cut, and electrodes are prepared as shown in the table below. The mass ratio of each component of the electrodes used in each test is also shown.

[0091] Example 7: Activated carbon 1 / CB / CNT(S) / PTFE = 84 / 5.25 / 1.75 / 9 (2 mm)

[0092]

[0093] Example 8: Activated carbon 2 / CB / CNT(S) / PTFE = 90 / 5.5 / 0.5 / 4 (2 mm)

[0094]

[0095] A double-layer electric capacitor was manufactured and discharged in the same manner as in Test Example 1. It was charged to 1.5V with a current of 20mA and discharged at 20mA. The results are shown in Table 13 and Figure 3.

[0096]

[0097] As shown in Table 13 and Figure 3, the output characteristics when using activated carbon 2 were superior to those when using activated carbon 1. The results of the elemental analysis of each activated carbon are shown in Table 14 below. The elemental analysis ratio of C in activated carbon 1 was 93.14%, and in activated carbon 2 it was 89.65%.

[0098] <Examination of carbon materials to be used for electrodes> (Test 4) Comparative test of carbon materials Activated carbons 1 to 4 with different CHN ratios were prepared. The analysis results of each activated carbon are shown in Table 14 below.

[0099]

[0100] The activated carbon 1-3 used as the carbon material for the electrode, carbon black (CB) as a conduction aid, and polytetrafluoroethylene (PTFE) as a binder were mixed and kneaded, and then rolled with a roller press to prepare an electrode sheet with a thickness of 1.2 mm.

[0101] The obtained sheets were cut to create electrodes as shown in the table below, and these were used in each test. The mass ratios of each component of the electrodes are also shown.

[0102] Activated carbon 1 / CB / PTFE=84 / 7 / 9 (1.2mm)

[0103]

[0104] Activated carbon 2 / CB / PTFE=86 / 8 / 6 (1.2mm)

[0105]

[0106] Activated carbon 3 / CB / PTFE=94 / 2 / 4 (1.2mm)

[0107]

[0108] A double-layer electric capacitor was manufactured and discharged in the same manner as in Test Example 1. It was charged to 1.5V with a current of 5mA and discharged at 5mA. The results are shown in Table 18 and Figure 4.

[0109]

[0110] As shown in Table 18 and Figure 4, the power characteristics were best in the order of activated carbon 2, activated carbon 1, and activated carbon 3. Elemental analysis of the activated carbons revealed that using activated carbon with a certain number of functional groups resulted in lower resistance and higher power output.

[0111] (Test 5) Confirmation of cycle characteristics of laminate cell type electric double layer capacitor using activated carbon 1 Activated carbon 1, carbon black (CB) as a conductive additive, and polytetrafluoroethylene (PTFE) as a binder were mixed and kneaded in a ratio of 84:7:9, and rolled into a sheet using a roller press. The obtained sheet was cut out with a 20 mmφ punch. The obtained positive and negative electrodes (the electrode sheet composition is the same for both electrodes: Table 19) were placed on the current collector inside the laminate cell. A separator was placed between the positive and negative electrodes, and an aqueous NaCl electrolyte was placed therein. In addition, before assembling the laminate cell, the electrodes were immersed in the electrolyte and impregnated with the electrolyte under a vacuum atmosphere. The obtained laminate cell was subjected to more than 100,000 charge-discharge cycles at 1.5 V and a current of 2 mA. The cycle characteristics are shown in Table 20 and Figure 5. Furthermore, the charge-discharge characteristics at the 100th, 4,000th, and 10,000th cycles are shown in Figures 6, 7, and 8, respectively.

[0112] Activated carbon 1 / CB / PTFE = 84 / 7 / 9 (0.9 mm)

[0113]

[0114]

[0115] As shown in Table 20 and Figures 5-8, the electric double-layer capacitor fabricated using activated carbon 1 as the carbon material retained sufficiently excellent charge-discharge characteristics even after 10,000 cycles. From the above, it was found that the laminate cell can stably perform charging and discharging at 1.5V.

[0116] (Test 6) Two-electrode measurement: Activated carbon 4, carbon black (CB) as a conductive additive, and polytetrafluoroethylene (PTFE) as a binder were mixed in a mass ratio of 1:0.01:0.5 with an equal amount of 3M NaCl aqueous electrolyte to the activated carbon, mixed and kneaded, and rolled into a sheet using a roller press. The obtained sheet was cut out with a 12 mmφ punch. The obtained positive and negative electrodes (the electrode sheet composition was the same for both electrodes) were placed on the current collector inside the coin cell. A separator was placed between the positive and negative electrodes, and in some cases the electrodes were immersed in the electrolyte and impregnated with the electrolyte under a vacuum atmosphere. The obtained coin cell was charged and discharged at 1.5 V. The charge-discharge characteristics after 10 cycles are shown in Figure 9. Furthermore, charge-discharge was performed at 1.8 V, and the charge-discharge characteristics after 10 cycles are shown in Figure 10.

[0117] As shown in Figures 9 and 10, it was found that the coin cell maintained sufficiently excellent charge and discharge characteristics even after 10 cycles. From the above, it was found that the coin cell can stably perform charge and discharge at 1.5V and 1.8V.

[0118] (Test 7) ​​Evaluation of surface functional groups of activated carbon by temperature-controlled desorption (TPD method) To investigate the surface functional groups contained in activated carbon 1, activated carbon 2, and activated carbon 3, measurements were performed using the temperature-controlled desorption (TPD method). The samples were stored under room temperature and vacuum conditions, and approximately 50 mg was weighed and introduced into a sample tube. After circulating Ar gas at 100°C for 2 hours, the temperature was increased from 100°C to 1000°C at a rate of 10°C / min (90 min) under Ar gas circulation, and the outlet gas was measured. The results are shown in Figure 11.

[0119] Results from the thermal desorption method (TPD method) and elemental analysis of activated carbon revealed that the higher the amount of functional groups, the lower the internal resistance. Furthermore, from the thermal desorption method (TPD method) results shown in Figure 11, the functional groups commonly found in activated carbon 1, activated carbon 2, and activated carbon 3 were carboxyl groups, lactones, carbonyl groups, and quinones. In addition, activated carbon 2 is thought to contain acid anhydrides and ethers, in addition to carboxyl groups, lactones, carbonyl groups, and quinones.

[0120] (Test 8) Test on the thickness of the electrode sheet Similar to Test 1, activated carbon as the carbon material to be used for the electrode, carbon black (CB) and carbon nanotubes as conductive additives, and polytetrafluoroethylene (PTFE) as a binder were mixed and kneaded, and then rolled with a roller press to prepare electrode sheets with thicknesses of 1.2 mm, 2 mm, and 3 mm.

[0121] The obtained sheets were cut to create electrodes approximately 3 mm thick, as shown in the table below, which were used in each test. The mass ratios of each component of the electrodes are also shown. For electrodes with a thickness of 1.2 mm and 2 mm, the same electrodes as in Test Example 1 were used.

[0122] Comparative example 3: Activated carbon 1 / CB / PTFE=84 / 7 / 9 (3.0mm)

[0123]

[0124] Example 9: Activated carbon 1 / CB / CNT / PTFE = 84 / 5.25 / 1.75 / 9 (3.0 mm)

[0125]

[0126] The obtained electrode sheets were cut out with an 8 mm diameter punch to obtain the positive and negative electrodes. Each was attached to a platinum mesh to which a platinum wire was mounted. A 3 M NaCl aqueous electrolyte was placed in a container, ensuring that the electrodes were fully submerged, and the electrodes were impregnated with the electrolyte under a vacuum atmosphere. A discharge test was performed using an electric double-layer capacitor in a beaker cell. The results are shown in Table 23 and Figure 12.

[0127]

[0128] As shown in Table 23 and Figure 12, when carbon nanotubes (CNTs) were incorporated as a conductive additive in addition to carbon black (CB), the output characteristics were relatively good, although slightly inferior to those of 1.2 mm and 2.0 mm, even with an electrode thickness of 3 mm. On the other hand, in the comparative example without carbon nanotubes (CNTs), the resistance within the electrode increased significantly as the electrode thickness increased, and the output characteristics deteriorated significantly. This is thought to be a result of the effective operation of one-dimensional, long-distance conduction paths of carbon nanotubes (CNTs) in the thickness direction within the electrode.

[0129] The electrode sheet for aqueous electric double-layer capacitors of the present invention incorporates long carbon nanotubes as a conductive additive. This allows for the combination of long-distance, one-dimensional electric buses derived from the nanotubes and short-distance, three-dimensional electric buses derived from the carbon black within the electrode, creating a good conductive path. As a result, the output characteristics do not deteriorate even when the thickness of the electrode sheet is increased. Therefore, by using the electrode sheet for aqueous electric double-layer capacitors of the present invention, it is possible to improve the energy density of the electric double-layer capacitor. Furthermore, by modifying the elemental composition and chemical modification of the carbon material used in the electrode, the effective potential range can be broadened and the amount of stored energy can be improved.

Claims

1. An electrode sheet for an aqueous electric double-layer capacitor, comprising a conductive additive and porous carbon, wherein the conductive additive includes carbon nanotubes, and the length of the carbon nanotubes is 5% to 90% of the thickness of the electrode sheet.

2. The electrode sheet according to claim 1, further comprising carbon black as the conductive additive.

3. The electrode sheet according to claim 1 or 2, further containing PTFE as a binder.

4. The electrode sheet according to claim 1 or 2, wherein the carbon nanotube content is 0.1% to 50% of the porous carbon content.

5. The electrode sheet according to claim 1 or 2, wherein the carbon component of the porous carbon is 65% or more and 95% or less in the CHN analysis value.

6. The electrode sheet according to claim 1 or 2, further comprising an electrolyte, wherein the electrolyte contains 0.1 M / L or more of sodium ions.

7. A water-based electric double-layer capacitor comprising the electrode sheet described in claim 1 or 2.

8. The aqueous electric double layer capacitor according to claim 7, further comprising an electrolyte, wherein the electrolyte contains 0.1 M / L or more of sodium ions.

9. An electrode sheet for an aqueous electric double-layer capacitor, comprising a conductive additive and porous carbon, wherein the conductive additive includes carbon nanofibers, and the length of the carbon nanofibers is 5% to 90% of the thickness of the electrode sheet.

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