Electric double layer capacitor and method for manufacturing the same

Abstract

To provide an electric double layer capacitor which can keep a leakage current low even in a high-temperature region, has a high reliability in a high-temperature region, and can increase the life of a device, and a method for manufacturing the electric double layer capacitor.SOLUTION: There are disclosed an electric double layer capacitor which uses an aqueous electrolyte solution including a water-soluble electrolyte in which the Hammett acidity function H0 at the temperature of 25°C is at least -2.8 and the vapor pressure at the temperature of 100°C is 400 mmHg or smaller, and a method for manufacturing the capacitor.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to an electric double-layer capacitor and a method for manufacturing the same. [Background technology]

[0002] Electric double-layer capacitors utilize an electric double layer, approximately a few nanometers thick, formed at the interface between a charged solid and the electrolyte in contact with it, as the dielectric. Electric double-layer capacitors exhibit minimal capacity degradation with charge-discharge cycles and can instantly deliver a large output compared to conventional batteries, making them suitable for use as backups for IC memory and actuators.

[0003] Electric double-layer capacitors can be classified into two types based on the type of electrolyte used: those using non-aqueous (organic) electrolytes and those using aqueous electrolytes. Electric double-layer capacitors using non-aqueous electrolytes generally have the advantage of high voltage resistance, but tend to have low electrical conductivity.

[0004] Patent Document 1 discloses an electric double-layer capacitor that uses an electrolyte containing sulfuric acid as an aqueous electrolyte. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 4942116 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Electric double-layer capacitors using sulfuric acid as the electrolyte, while possessing high conductivity, sometimes struggled to maintain low leakage current (LC) values ​​over extended periods in high-temperature ranges (e.g., 85-105°C). Furthermore, the strong acidity of sulfuric acid placed a significant burden on electrodes and separators, leading to rapid device degradation.

[0007] This disclosure is made in light of the above background and aims to provide an electric double-layer capacitor and a method for manufacturing the same that can maintain low leakage current for a long period of time even in high-temperature ranges, have high reliability in high-temperature ranges, and enable a longer lifespan for the device. [Means for solving the problem]

[0008] The electric double-layer capacitor according to this disclosure uses an aqueous electrolyte containing a water-soluble electrolyte whose Hammett acidity function H0 at 25°C is -2.8 or higher, and whose vapor pressure at 100°C is 400 mmHg or lower. The electric double-layer capacitor may have a concentration of the water-soluble electrolyte in the aqueous electrolyte solution of 65 to 77% by mass. In any of the above-mentioned electric double-layer capacitors, the mass ratio of activated carbon contained in the polarizing electrode to the aqueous electrolyte may be 1:0.5 to 1:3. In any of the above-described electric double-layer capacitors, the aqueous electrolyte does not necessarily have to contain sulfuric acid. The method for manufacturing an electric double-layer capacitor according to this disclosure involves applying a paste, which is a mixture of activated carbon and an electrolyte, to a gasket and drying it to produce a polarizing electrode containing the dried electrolyte. The dried electrolyte is an aqueous electrolyte containing a water-soluble electrolyte whose Hammett acidity function H0 at 25°C is -2.8 or higher and whose vapor pressure at 100°C is 400 mmHg or lower. The drying conditions are a temperature of 20-30°C and a relative humidity of 30-60%. In the method for manufacturing the electric double-layer capacitor, the vapor pressure of the water-soluble electrolyte contained in the electrolyte solution used in the paste before drying may be 8 to 20 mmHg at a temperature of 20 to 30°C. In any of the above methods for manufacturing an electric double-layer capacitor, the mass ratio of the activated carbon to the electrolyte in the paste before drying may be 1:1 to 1:4. In any of the above methods for manufacturing an electric double-layer capacitor, the mass ratio of the activated carbon contained in the polarizing electrode to the electrolyte may be 1:0.5 to 1:3. In any of the above methods for manufacturing an electric double-layer capacitor, the concentration of the water-soluble electrolyte in the dried electrolyte may be 65 to 77% by mass. In any of the above methods for manufacturing an electric double-layer capacitor, the electrolyte does not need to contain sulfuric acid. [Effects of the Invention]

[0009] According to this disclosure, it is possible to provide an electric double-layer capacitor and a method for manufacturing the same that can maintain low leakage current for a long period of time even in high-temperature ranges, have high reliability in high-temperature ranges, and enable a longer lifespan for the device. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view showing an example of a unit cell of an electric double-layer capacitor related to this disclosure. [Figure 2] This is a schematic cross-sectional view showing one embodiment of an electric double-layer capacitor according to the present disclosure. [Figure 3] This is a schematic diagram of the circuit used to measure leakage current. [Figure 4] This is a schematic diagram of the circuit used to measure capacitance. [Figure 5] This graph shows an example of the time-dependent change in leakage current (LC) at 25°C in an electric double-layer capacitor, using sulfuric acid and phosphoric acid as the water-soluble electrolytes, respectively. [Figure 6] This graph shows an example of the time-dependent change in leakage current (LC) at 70°C in an electric double-layer capacitor, using sulfuric acid and phosphoric acid as the water-soluble electrolytes, respectively. [Figure 7] This graph shows an example of the time-dependent change in leakage current (LC) at 85°C in an electric double-layer capacitor, using sulfuric acid and phosphoric acid as the water-soluble electrolytes, respectively. [Figure 8]A graph showing an example of the change over time of the rate of decrease in capacitance at a temperature of 85°C and a potential difference of 5.5V in an electric double layer capacitor when sulfuric acid and phosphoric acid are used as water-soluble electrolytes, respectively. [Figure 9] A graph showing an example of the change over time of the rate of decrease in capacitance at a temperature of 95°C and a potential difference of 5.5V in an electric double layer capacitor when sulfuric acid and phosphoric acid are used as water-soluble electrolytes, respectively. [Figure 10] A graph showing an example of the change over time of the rate of decrease in capacitance at a temperature of 105°C and a potential difference of 5.5V in an electric double layer capacitor when sulfuric acid and phosphoric acid are used as water-soluble electrolytes, respectively. [Figure 11] A graph showing an example of the change over time of the rate of decrease from the initial value of capacitance at each temperature of an electric double layer capacitor when phosphoric acid is used as a water-soluble electrolyte.

Embodiments for Carrying Out the Invention

[0011] In an electric double layer capacitor using sulfuric acid as an electrolyte, although a high conductivity can be obtained, since sulfuric acid is a strong acid, the device tends to deteriorate rapidly, and long-term quality assurance at high temperatures (for example, 85 to 105°C) may be insufficient. The upper limit temperature for use of a conventional electric double layer capacitor using a sulfuric acid electrolyte is usually 85°C (product life: about 2000 hours), and the LC value at 85°C can be about 150 times the LC value at room temperature (25°C).

[0012] On the other hand, the electric double-layer capacitor according to this disclosure (hereinafter also referred to as "this capacitor") uses an aqueous electrolyte containing a water-soluble electrolyte in which the Hammett acidity function H0 at 25°C is -2.8 or higher and the vapor pressure at 100°C is 400 mmHg or lower. In this capacitor using the water-soluble electrolyte, damage to components such as electrodes and separators is minimized while maintaining appropriate conductivity, and evaporation of the electrolyte can be appropriately suppressed even in high-temperature ranges. As a result, this capacitor can maintain a low LC value for a long period of time even in high-temperature ranges (for example, a product life of 6,000 to 9,000 hours at 85°C), and can be used as a supercapacitor in which quality can be guaranteed for a long period of time (for example, 2,000 hours or more) even in ultra-high temperature ranges (for example, 105°C).

[0013] The following describes an example of an embodiment to which this disclosure applies. This disclosure is not limited to the embodiments described below and may be modified as appropriate without departing from the spirit of the invention. Unless otherwise specified, the "~" symbol indicating a numerical range includes both the lower and upper limits.

[0014] <Electric double layer capacitor> This capacitor may have a pair of polarizable electrodes and an aqueous electrolyte. This capacitor uses an aqueous electrolyte containing a water-soluble electrolyte having a Hammett acidity function H0 of -2.8 or higher at 25°C and a vapor pressure of 400 mmHg or less at 100°C. Here, in this specification, "water-soluble electrolyte" may mean a solution containing a solvent (e.g., water) or a solution without a solvent. For example, when phosphoric acid, as described later, is used as the electrolyte, the "water-soluble electrolyte" may mean 100% by mass phosphoric acid or an aqueous solution of phosphoric acid at a specific concentration.

[0015] Hammett's acidity function H0 is a numerical value proposed by Lewis Hammett that quantitatively represents the strength of the acidity of a medium such as a solution. It should be noted that Hammett's acidity function is a value specific to the type, composition, and concentration of the solution in question, and it changes with temperature. The larger the negative value of Hammett's acidity function H0, the stronger the acidity. That is, in this capacitor, when a solvent such as water is used in the aqueous electrolyte, the "Hammett's acidity function H0 at 25°C" for the "water-soluble electrolyte" can be the value of the water-soluble electrolyte dissolved in this solvent (for example, a water-soluble electrolyte solution (e.g., an aqueous phosphoric acid solution)).

[0016] If the Hammett acidity function H0 of a water-soluble electrolyte at 25°C is -2.8 or higher, it has an appropriate acidity, which can reduce damage to components such as electrodes. Furthermore, water-soluble electrolytes with an H0 of -2.8 or higher have appropriate reactivity, which can suppress the reduction in liquid volume due to the decomposition of the electrolyte. Moreover, from the viewpoint of reactivity and high-temperature reliability, the Hammett acidity function H0 of a water-soluble electrolyte at 25°C is preferably -2.5 or higher, and more preferably -2.2 or higher. Furthermore, from the viewpoint of conductivity, the Hammett acidity function H0 of a water-soluble electrolyte at 25°C is preferably 0 or less, and more preferably -1.0 or less.

[0017] The sulfuric acid concentration in the sulfuric acid electrolyte typically used in electric double-layer capacitors is 45-60% by mass, and the Hammett acidity function H0 at 25°C is -4.46 to -2.85. Therefore, it can be seen that the sulfuric acid aqueous solution conventionally used in electric double-layer capacitors is more acidic than the aqueous electrolyte containing water-soluble electrolytes used in this capacitor.

[0018] The Hammett acidity function H0 of a water-soluble electrolyte at a temperature of 25°C can be measured by the following method: A small amount of a nitroaniline neutral base (B) with a known dissociation constant (e.g., p-nitroaniline) is added to the sample (the acidic solution (HA) for which the acidity function is to be determined). The concentration ratio of the protonated and unprotonated forms in the solution is determined by the integral of the NMR spectrum or by spectrophotometric analysis, and the Hammett acidity function H0 is calculated using the following formula. H0 = pKBH + - log[BH + ] / [B] Here is pKBH + is BH + (Proton(H) + The acid dissociation constant of the )-formed base B) [BH + ] is BH + The molar concentration of [B] represents the molar concentration of base B. To determine the Hammett acidity function H0 of the water-soluble electrolyte in the electrolyte of an electric double-layer capacitor at a temperature of 25°C, first, the electrolyte contained in the electric double-layer capacitor is identified. Then, based on the concentration and composition ratio of the identified water-soluble electrolyte, the above H0 of the water-soluble electrolyte is determined.

[0019] Furthermore, if the vapor pressure of the water-soluble electrolyte at 100°C is 400 mmHg or less, evaporation of the electrolyte at high temperatures can be appropriately suppressed. From the viewpoint of suppressing evaporation of the electrolyte, the vapor pressure at 100°C is preferably 370 mmHg or less, and more preferably 350 mmHg or less. While a lower vapor pressure of the water-soluble electrolyte at 100°C is advantageous for product characteristics, it is preferably 50 mmHg or higher. If the vapor pressure of the water-soluble electrolyte at 100°C is 50 mmHg or higher, it is easier to maintain an appropriate concentration of the water-soluble electrolyte in the aqueous electrolyte during manufacturing, and it is easier to maintain an appropriate viscosity of the aqueous electrolyte in the manufacturing environment (temperature: 20-30°C). As a result, squeegeeing of the prepared electrode paste can be easily performed, improving ease of manufacturing. From a similar viewpoint, it is more preferable that the vapor pressure of the water-soluble electrolyte at 100°C is 100 mmHg or higher.

[0020] The vapor pressure of a water-soluble electrolyte at 100°C can be measured by the static method. Specifically, the sample is sealed in a container equipped with a pressure gauge, the inside is completely evacuated to contain only the sample and its vapor, the container is left to stand in a constant temperature bath at 100°C, and the pressure exerted by the vapor at that time is measured. In this capacitor, if a solvent such as water is used in the aqueous electrolyte, the "vapor pressure at 100°C" of the "water-soluble electrolyte" can be the value of the water-soluble electrolyte dissolved in this solvent (for example, a water-soluble electrolyte solution (for example, an aqueous phosphoric acid solution)).

[0021] Examples of water-soluble electrolytes that have a Hammett acidity function H0 of -2.8 or higher at 25°C and a vapor pressure of 400 mmHg or less at 100°C include: acetic acid, boric acid, phosphoric acid, oxalic acid, butyric acid, and dichloroacetic acid. Other acids can be used without restriction as long as they satisfy the above conditions for acidity function and vapor pressure. However, among these, from the viewpoint of conductivity and the ease of manufacture mentioned above, it is preferable to use an acid selected from acetic acid, oxalic acid, and phosphoric acid as the water-soluble electrolyte. Furthermore, from the same viewpoint, it is particularly preferable to use phosphoric acid as the water-soluble electrolyte.

[0022] The concentration of each water-soluble electrolyte (the concentration in the aqueous electrolyte contained in this capacitor) can be appropriately set within the range that satisfies the above conditions, namely, that Hammett's acidity function H0 at 25°C is -2.8 or higher, and that the vapor pressure at 100°C is 400 mmHg or lower. For example, the phosphoric acid concentration in the aqueous electrolyte is preferably 77% by mass or less from the viewpoint of adjusting Hammett's acidity function H0 to -2.8 or higher, and more preferably 75% by mass or less from the viewpoint of conductivity. Furthermore, it is preferably 65% ​​by mass or more from the viewpoint of adjusting the vapor pressure at 100°C to 400 mmHg or less, and more preferably 70% by mass or more from the viewpoint of suppressing evaporation of the electrolyte. For example, an aqueous phosphoric acid solution with a concentration of 65-75% by mass has a freezing point of -20°C or lower. Therefore, it can be used without freezing even at low temperatures below -20°C, providing an electric double-layer capacitor that can be used in a very wide temperature range from high to low temperatures.

[0023] In this capacitor, the above-mentioned water-soluble electrolyte may be used alone or in combination of multiple types. However, when multiple types of water-soluble electrolytes are used, the overall properties of these water-soluble electrolytes (for example, the properties of the entire aqueous electrolyte) must satisfy the above-mentioned H0 and vapor pressure conditions.

[0024] The aqueous electrolyte may contain other components in addition to the water-soluble electrolyte and water, which serves as the solvent for the electrolyte, to the extent that the effects of this disclosure are obtained. Examples of these other components include: water-soluble organic solvents such as glycerin, butanediol, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, polyvinyl alcohol, and sulfolane. Furthermore, if other components are included in the aqueous electrolyte, it is preferable that the entire aqueous electrolyte satisfies the various conditions mentioned above, such as H0 and vapor pressure at 100°C. The aqueous electrolyte may also be composed of an aqueous solution of the water-soluble electrolyte mentioned above.

[0025] Furthermore, since this capacitor was invented to solve the problems of electric double-layer capacitors using sulfuric acid electrolyte, it is preferable that the aqueous electrolyte does not contain sulfuric acid in order to better demonstrate the excellent effects of this disclosure.

[0026] As described above, in addition to the above aqueous electrolyte, this capacitor can include a pair of polarized electrodes. This capacitor can have, for example, a unit cell 10 as shown in FIG. 1. The unit cell 10 includes a positive electrode current collector 1 and a polarized electrode 3 as a positive electrode provided thereon, and a negative electrode current collector 2 and a polarized electrode 4 as a negative electrode provided thereon. The polarized electrodes 3 and 4 are portions that form an electric double layer. Note that FIG. 1 is a schematic cross-sectional view showing an example of a unit cell of an electric double layer capacitor according to the present disclosure.

[0027] Here, the capacitance of the electric double layer capacitor is several tens of μF per 1 cm 2 However, by using activated carbon with a surface area reaching several thousand m 2 as the main component of the electrode, an extremely large capacitance of several hundred to several thousand F can be obtained. The main component means the component that is most contained among the components contained in a certain object (here, the electrode). Therefore, the polarized electrodes of this capacitor can be appropriately selected from known materials, but it is preferable that at least one (for example, the positive electrode) or both of the pair of polarized electrodes contain activated carbon. Hereinafter, an embodiment of this capacitor will be shown, but the present disclosure is not limited to this embodiment.

[0028] The polarized electrodes 3 and 4 contain activated carbon powder and become paste-like by mixing with an aqueous electrolyte. As the activated carbon, synthetic resin-based activated carbon such as phenol, rayon, acrylic, and polyvinyl chloride; natural material-based activated carbon such as coconut shell and hardwood; and coal and petroleum-based activated carbon such as pitch and coke can be used. Among these, as the activated carbon, phenol resin-based activated carbon, coconut shell activated carbon, or petroleum coke-based activated carbon can be preferably used.

[0029] Here, in this capacitor, the mass ratio of the activated carbon contained in the polarized electrode to the aqueous electrolyte (including a solute (for example, phosphoric acid) and a solvent (for example, water)) is preferably 1:0.5 to 1:3 from the viewpoint of the capacitance per unit volume. The blending ratio of the activated carbon can be appropriately selected according to the shape and particle size of the activated carbon used.

[0030] In the electric double-layer capacitor shown in Figure 1, the positive polarity electrode 3 and the negative polarity electrode 4 are arranged opposite each other via a separator 5. The separator 5 is made of polypropylene nonwoven fabric, polyethylene nonwoven fabric, or polypropylene microporous membrane, and can prevent short circuits between the polarity electrodes 3 and 4.

[0031] The positive electrode current collector 1 and the negative electrode current collector 2 are larger in plan view than the polarity electrodes 3 and 4, and a frame region exists between them in plan view. An insulating gasket 6 (electrode gasket) is placed at the outer edges of the frame region of the positive electrode current collector 1 and the negative electrode current collector 2. That is, the gasket 6 is placed at the outer edges of the polarity electrodes 3 and 4 and the separator 5, and the positive electrode current collector 1 and the negative electrode current collector 2 are positioned opposite each other via the gasket 6, the polarity electrodes 3 and 4, and the separator 5. The gasket 6 prevents short circuits between the positive electrode current collector 1 and the negative electrode current collector 2.

[0032] The positive electrode current collector 1, the negative electrode current collector 2, and the gasket 6 can be appropriately selected considering their resistance to aqueous electrolytes, conductivity, non-conductivity, etc. For example, conductive butyl rubber can be used for the positive electrode current collector 1 and the negative electrode current collector 2, and insulating butyl rubber can be used for the gasket 6. The gasket 6, positive electrode current collector 1, and negative electrode current collector 2 form a sealed structure for the unit cell 10, preventing leakage of the aqueous electrolyte filled inside the container to the outside.

[0033] The unit cell 10 is typically housed in a container formed by an outer casing (not shown). The positive electrode current collector 1 is connected to a positive electrode tab (not shown), and the negative electrode current collector 2 is connected to a negative electrode tab (not shown). These tabs are extended outside the container. That is, the positive electrode current collector 1 and the negative electrode current collector 2 can act as a medium to electrically connect the outside of the cell to the polarity electrodes 3 and 4.

[0034] The aqueous electrolyte 7 is a solution containing a water-soluble electrolyte that has electrical conductivity and satisfies the above-mentioned conditions. It can be impregnated into the polarizing electrodes 3 and 4 as a paste mixed with activated carbon powder, and can also be impregnated into the separator 5. An electrical double layer is formed by the aqueous electrolyte 7 and the polarizing electrodes 3 and 4.

[0035] An electric double-layer capacitor may have a structure in which unit cells are stacked. The shape of the unit cell 10 can take various forms, such as box-shaped, cylindrical, or sheet-shaped.

[0036] <Manufacturing method for electric double layer capacitors> The manufacturing method for an electric double-layer capacitor (capacitor element) according to this disclosure (hereinafter also referred to as "this manufacturing method") involves applying a paste, which is a mixture of activated carbon and an electrolyte, to a gasket, drying it, and then producing a polarizing electrode containing the electrolyte after drying, while satisfying the following conditions. Specifically, the electrolyte after drying is an aqueous electrolyte containing a water-soluble electrolyte, where the Hammett acidity function H0 at 25°C is -2.8 or higher, and the vapor pressure at 100°C is 400 mmHg or lower. Furthermore, the drying conditions are a temperature of 20-30°C and a relative humidity of 30-60%. Note that even if this manufacturing method is not used, any manufacturing method that can produce the capacitor described above may be used as appropriate. Specifically, in this manufacturing method, in order to provide fluidity, a paste with an excess of electrolyte in the mixing ratio is prepared, applied to the gasket, and then the final amount and concentration of electrolyte are adjusted by vapor-liquid equilibrium (specifically, by evaporating the electrolyte), using a paste method. However, to obtain this capacitor, it is also possible to prepare dry electrodes and electrolyte separately and then drop the electrolyte onto the electrodes.

[0037] This manufacturing method may more specifically include the following steps: • The process of preparing the electrolyte (hereinafter also referred to as the raw material electrolyte) (electrolyte preparation process). • The process of preparing the gasket (gasket preparation process). • A process of mixing activated carbon and electrolyte (raw material electrolyte) to prepare a paste (electrode paste) (paste preparation process). - A step of squeegeeing the paste onto the gasket (squeegeeing step). - A step of drying the squeegeeed paste to obtain unit cells (drying step). • The process of fabricating an electric double-layer capacitor using the obtained unit cell (capacitor fabrication process). These processes may be carried out sequentially, or multiple processes (for example, the electrolyte preparation process and the gasket preparation process) may be carried out in parallel. Furthermore, these processes may be performed multiple times. These processes are explained in detail below.

[0038] First, an aqueous electrolyte (e.g., an aqueous phosphoric acid solution) containing a predetermined concentration of a water-soluble electrolyte (e.g., phosphoric acid) is prepared (electrolyte preparation step). The water-soluble electrolyte and its concentration can be set without particular limitations, as long as the Hammett acidity function and vapor pressure at 100°C of the electrolyte after drying satisfy the above conditions. That is, the Hammett acidity function H0 of the raw electrolyte (water-soluble electrolyte) at 25°C may be -2.8 or higher, or it may not be -2.8 or higher. Furthermore, the vapor pressure of the raw electrolyte (water-soluble electrolyte) at 100°C may be 400 mmHg or lower, or it may not be 400 mmHg or lower. However, from the viewpoint of producing a more stable paste electrode and performing the drying step described later appropriately, it is preferable that the vapor pressure of the raw electrolyte (electrolyte before drying) at 20-30°C (temperature in the manufacturing environment (room temperature)) is 8-20 mmHg. More specifically, if the vapor pressure of the raw material electrolyte (water-soluble electrolyte) at the temperature is 8 mmHg or higher, it is easier to keep the amount of moisture absorbed by the paste within an appropriate range during the drying process, and it is easier to keep the mass ratio of activated carbon to electrolyte in the paste within an appropriate range. Furthermore, if the vapor pressure of the raw material electrolyte (water-soluble electrolyte) at the temperature is 20 mmHg or lower, it is easier to keep the amount of evaporation of the electrolyte within an appropriate range, and it is easier to keep the mass ratio of activated carbon to electrolyte in the paste within an appropriate range.

[0039] For example, the vapor pressure of a water-soluble electrolyte (or a water-soluble electrolyte solution if the aqueous electrolyte contains a solvent) at a temperature of 25°C can be set to 15 mmHg or less, or to 10 mmHg or more, from the viewpoint of workability during electrode formation.

[0040] Next, the activated carbon and the raw electrolyte are mixed to form a paste to produce an electrode paste (paste preparation step). The mass ratio of activated carbon to raw electrolyte (containing a solute (e.g., phosphoric acid) and a solvent (e.g., water)) to be mixed into the paste is adjusted as appropriate depending on the shape, specific surface area, and type of activated carbon. However, from the viewpoint of volume per unit area, the mass ratio of activated carbon to raw electrolyte in the paste before drying is preferably 1:1 to 1:4.

[0041] Next, a hollow, circular gasket 6 is prepared (gasket preparation step). Then, the gasket 6 is bonded to the disc-shaped positive electrode current collector 1 and negative electrode current collector 2, and a PE (polyethylene) or PET (polyethylene terephthalate) sheet with punched holes the same size as the diameter of the gasket 6 is bonded to the gasket 6 to mask the surface of the gasket 6 so that the electrode paste does not adhere to it. The electrode paste is then dropped into the gasket 6 and squeegeeed to ensure a uniform amount of electrode paste is applied (squeegeeing step). After the electrode paste is applied, the bonded PE / PET sheet is removed, and the moisture in the raw electrolyte is dried to form one side of the unit cell (drying step). In this process, the electrode paste is dried under conditions of 20°C to 30°C (room temperature) and 30 to 60% RH relative humidity until it reaches equilibrium. By drying under these conditions until equilibrium is reached, the electrolyte concentration can be easily and stably controlled. At this time, the concentration of water-soluble electrolyte in the electrolyte after drying is preferably 65 to 77% by mass, as described above. Also, as described above, the mass ratio of activated carbon contained in the polarizing electrode to the electrolyte is preferably 1:0.5 to 1:3. Furthermore, in this manufacturing method, from the viewpoint of further demonstrating the effects of this disclosure, it is preferable that the electrolyte does not contain sulfuric acid.

[0042] Similarly, the other half of the unit cell is prepared. Then, by placing these opposite each other with the separator 5 in between, the polarizing electrodes 3 and 4 containing the aqueous electrolyte 7 are sealed. During sealing, the pressure is reduced to below 100 Pa to remove the internal air and vacuum sealing is performed. The sealed unit cell has a pressure of 6 kg / cm². -2 The cells are pressurized to a certain extent and vulcanized at 120°C for 30-60 minutes to ensure tight bonding. Through these steps, a unit cell 10 can be manufactured. If necessary, the unit cells 10 are stacked to create an electric double-layer capacitor (capacitor manufacturing process). Electric double-layer capacitors can achieve their capacity through the physical adsorption of ions from an aqueous electrolyte onto polarized electrodes during charging and discharging. [Examples]

[0043] The present disclosure will be described in detail below with reference to examples and comparative examples. These descriptions are not intended to limit the present disclosure.

[0044] [Example 1] An electric double-layer capacitor containing an aqueous electrolyte (phosphate aqueous solution) with phosphoric acid (PA) at a concentration of 70% by mass (concentration after the drying process) was fabricated according to the following procedure. The Hammett acidity function H0 of the phosphate aqueous solution (water-soluble electrolyte aqueous solution) at 25°C after the drying process was -2.29, and the vapor pressure at 100°C after the drying process was 340 mmHg. Furthermore, the vapor pressure of the raw material electrolyte at 25°C before the drying process, i.e., during paste preparation, was 8.5 mmHg. The mass ratio of activated carbon to raw material electrolyte during paste preparation was 1:1.5. The mass ratio of activated carbon to electrolyte in the polarizing electrode after the drying process was 1:1.3. The manufacturing procedure is described in more detail below. First, a 60% by mass aqueous phosphoric acid solution was added to phenol resin-based activated carbon with a particle size of 4-14 μm and thoroughly kneaded to form a paste. Next, a sheet (electrode coating area: thickness 0.5 mm, diameter 6.6 mm) was prepared, consisting of a gasket made of non-conductive butyl rubber and a current collector made of conductive butyl rubber. The paste was applied to the electrode forming area of ​​this sheet and dried to produce electrodes. At that time, the drying conditions were set to a temperature of 20-30°C and a relative humidity of 30-60% RH so that the paste could be dried appropriately. Next, a pair of the above sheets with electrodes formed on them was prepared and stacked together via a porous separator made of polytetrafluoroethylene with a thickness of 0.05 mm, a diameter of 8.0 mm, and 55% pores. Then, by promoting the vulcanization of butyl rubber through thermocompression bonding, single cell sheets were fabricated by sealing gaskets made of non-conductive butyl rubber with each other and with a current collector made of conductive butyl rubber. Six of these single cell sheets were then stacked to create a unit cell laminate. An electric double-layer capacitor 11 having the structure shown in Figure 2 was fabricated using a unit cell stack. The lower current collector of the unit cell stack 12 in Figure 2 was directly connected to the connection terminal 13b, while the upper current collector in Figure 2 was in contact with the bottom of a bottomed cylindrical case 15 made of metal such as stainless steel, with the edge of the opening bent inward and crimped, and connected to the connection terminal 13a which was arranged via an insulator 14.

[0045] [Comparative Example 1] An electric double-layer capacitor was fabricated in the same manner as in Example 1, except that an aqueous electrolyte (sulfuric acid aqueous solution) containing sulfuric acid (SA) at a concentration of 50% by mass (concentration after the drying process) was used as the water-soluble electrolyte. The Hammett acidity function H0 of the sulfuric acid aqueous solution at 25°C after the drying process was -3.38, and the vapor pressure at 100°C was 325 mmHg.

[0046] [Leakage current] Tables 1 to 3 show the time-dependent changes in leakage current (LC) of electric double-layer capacitors at various temperatures (25°C, 70°C, and 85°C) obtained for each example, and Figures 5 to 7 show graphs based on these results. The LC values ​​of the electric double-layer capacitor at each temperature were measured using the following method. Specifically, in the circuit shown in Figure 3, the rated voltage E0 was applied to the electric double-layer capacitor C by turning on the switch SW, and the voltage VR across the series resistor Rc was measured. The leakage current was then calculated using the following formula. Current: I=VR / Rc In the formula, I represents the leakage current [A], VR represents the voltage across the resistor [V], and Rc represents the resistance [Ω].

[0047] [Table 1]

[0048] [Table 2]

[0049] [Table 3]

[0050] [Capacitance] Tables 4 to 6 show the time-dependent decrease in capacitance from the initial value at each temperature (85°C, 95°C, and 105°C) for the electric double-layer capacitors obtained in each example, and Figures 8 to 10 show graphs based on these results. The capacitance of the electric double-layer capacitor at each temperature was measured using the following constant-current discharge method. Specifically, in the circuit shown in Figure 4, the electric double-layer capacitor was charged for 30 minutes after the terminal voltage reached the maximum operating voltage. Next, using a constant-current load device, the capacitor was discharged with a current of 1mA per 1F of rated capacity, and the time it took for the terminal voltage to drop from 60% to 50% of the rated voltage was measured. The capacitance was then calculated using the following formula. In Figure 4, V represents a voltmeter, A represents an ammeter, R represents a variable resistor, C represents a capacitor, and SW represents a switch. Capacitance: C = I × (T2 - T1) / (V1 - V2) In the formula, C is capacitance [F], I is discharge current [A], V1 is the voltage when the terminal voltage is 60% of the rated voltage [V], V2 is the voltage when the terminal voltage is 50% of the rated voltage [V], T1 is the time [seconds] when the terminal voltage reaches 60% of the rated voltage, and T2 is the time [seconds] when the terminal voltage reaches 50% of the rated voltage.

[0051] [Table 4]

[0052] [Table 5]

[0053] [Table 6]

[0054] Furthermore, Figure 11 summarizes the time-dependent change in the rate of decrease from the initial value of the capacitance of the electric double-layer capacitor according to Example 1 at each temperature (85°C, 95°C, and 105°C). Referring to Figure 11, it can be seen that the electric double-layer capacitor of Example 1, which used an aqueous phosphoric acid solution, took more than 9,000 hours (predicted value) at 85°C, more than 3,500 hours at 95°C, and more than 2,000 hours at 105°C for the capacitance to reach -30% of the initial value, indicating that quality can be guaranteed for a very long period. On the other hand, the electric double-layer capacitor of Comparative Example 1, which used an aqueous sulfuric acid solution, took approximately 2,500 hours at 85°C, approximately 1,000 hours at 95°C, and approximately 500 hours at 105°C for the capacitance to reach -30% of the initial value, resulting in a shorter product life compared to the electric double-layer capacitor of Example 1.

[0055] Thus, the electric double-layer capacitor according to this disclosure, using a specific water-soluble electrolyte, was able to maintain low leakage current for a long period of time, even at high temperatures (85°C), demonstrating high reliability at high temperatures and enabling a longer lifespan for the device. Furthermore, it was found that the electric double-layer capacitor according to this disclosure can be used as a supercapacitor that can guarantee high quality for a long period of time, even in extremely high-temperature environments of 105°C. [Explanation of Symbols]

[0056] 1 Positive electrode current collector 2 Negative electrode current collector 3, 4 polarized electrodes 5 Separators 6 Gasket 7 Aqueous electrolyte 10 unit cells 11 Electric double-layer capacitor 12-unit cell stack 13a, 13b Connection terminals 14 Insulator 15 cases

Claims

1. Hammett acidity function H at a temperature of 25°C ０ An electric double-layer capacitor characterized by using an aqueous electrolyte containing a water-soluble electrolyte whose pH is -2.8 or higher and whose vapor pressure at 100°C is 400 mmHg or less.

2. The electric double-layer capacitor according to claim 1, wherein the concentration of the water-soluble electrolyte in the aqueous electrolyte is 65 to 77% by mass.

3. The electric double-layer capacitor according to claim 1, wherein the mass ratio of activated carbon contained in the polarizing electrode to the aqueous electrolyte is 1:0.5 to 1:

3.

4. The electric double layer capacitor according to any one of claims 1 to 3, wherein the aqueous electrolyte does not contain sulfuric acid.

5. When applying a paste, which is a mixture of activated carbon and electrolyte, to a gasket and drying it, to produce a polarizing electrode containing the electrolyte after drying, The electrolyte after drying has Hammett's acidity function H at a temperature of 25°C. ０ An aqueous electrolyte containing a water-soluble electrolyte whose pH is -2.8 or higher and whose vapor pressure at 100°C is 400 mmHg or less. A method for manufacturing an electric double-layer capacitor, characterized in that the drying conditions are a temperature of 20 to 30°C and a relative humidity of 30 to 60%.

6. The method for manufacturing an electric double layer capacitor according to claim 5, wherein the vapor pressure of the water-soluble electrolyte contained in the electrolyte solution used in the paste before drying is 8 to 20 mmHg at a temperature of 20 to 30°C.

7. The method for manufacturing an electric double-layer capacitor according to claim 5, wherein the mass ratio of the activated carbon to the electrolyte in the paste before drying is 1:1 to 1:

4.

8. A method for manufacturing an electric double-layer capacitor according to claim 5, wherein the mass ratio of the activated carbon contained in the polarizing electrode to the electrolyte is 1:0.5 to 1:

3.

9. The method for manufacturing an electric double-layer capacitor according to claim 5, wherein the concentration of the water-soluble electrolyte in the electrolyte after drying is 65 to 77% by mass.

10. A method for manufacturing an electric double-layer capacitor according to any one of claims 5 to 9, wherein the electrolyte does not contain sulfuric acid.

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