JPH08162375A - Electrical double-layer capacitor and manufacture of polarizable electrode used therefor - Google Patents

Abstract

PURPOSE: To provide an electrical double-layer capacitor which is high in dielectric strength, large in capacitance, and high in energy density. CONSTITUTION: A polarizable electrode primarily formed of activated carbon and organic solvent electrolyte solution are combined to form an electrical double-layered capacitor, wherein the above activated carbon is obtained in such a manner that carbon material obtained using one or more kinds of resin selected from phenolic resin, furan resin, and polyacrylonitrile resin as material is processed into activated carbon through an activation process where molten potassium hydroxide is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric double layer capacitor having a large electrostatic capacity and a high withstand voltage, and a method for manufacturing a polarizable electrode for an electric double layer capacitor.

[0002]

2. Description of the Related Art In order to make an electric double layer capacitor small and lightweight, it is necessary to increase the amount of stored energy per unit volume (hereinafter referred to as energy density) that can be stored in the capacitor. The energy stored in the capacitor is proportional to the electrostatic capacity of the capacitor and is also proportional to the square of the applied voltage (withstand voltage or working voltage). Therefore, in order to increase the energy density of the capacitor, the capacitance per unit volume should be increased and the capacitor should have a high withstand voltage.

Conventionally, in order to obtain an electric double layer capacitor having a large electrostatic capacity, activated carbon having a high specific surface area has been used for the polarizable electrode. As the activated carbon having a high specific surface area, activated carbon in which a carbon material is activated by a steam activation treatment method using steam or a chemical activation treatment method using zinc chloride or the like is mainly used. However, these activated carbons are promising in the future because of the capacitance density per unit volume when used as a polarizable electrode and the rapid performance deterioration when a high voltage is applied,
It is not sufficient for applications that require high energy density.

By the way, Japanese Patent Laid-Open No. 63-78513 discloses that
There has been proposed an electric double layer capacitor using activated carbon having a high specific surface area, which is obtained by activating petroleum coke by a high temperature activation treatment method using molten potassium hydroxide, and an organic solvent-based electrolytic solution. This activated carbon is advantageous in that it can have a higher energy density than conventional electric double layer capacitors because of its very large specific surface area. However, since the withstand voltage is about 2.8 V at most, it is expected that the withstand voltage is improved and the energy density is further improved.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above problems in the prior art of electric double layer capacitors, and provides an electric double layer capacitor having a high withstand voltage and a high energy density capable of applying a high voltage. With the goal.

[0006]

The electric double layer capacitor of the present invention is an electric double layer capacitor having a polarizable electrode and an electrolytic solution for forming an electric double layer on the surface of the polarizable electrode. The electrode is mainly composed of activated carbon obtained by activating a carbon material made of one or more resins selected from phenol resin, furan resin and polyacrylonitrile resin by an activation treatment method using potassium hydroxide in a molten state. The electrolytic solution is an organic solvent-based electrolytic solution.

The method for producing a polarizable electrode for an electric double layer capacitor according to the present invention comprises carbonizing one or more resins selected from phenolic resins, furan resins and polyacrylonitrile resins into carbonaceous materials, and watering the carbonaceous materials. By heating the mixture to which potassium oxide is added to 600 to 1000 ° C., the carbon material is activated carbon in the presence of potassium hydroxide in a molten state, and the mixture of the powder of the activated carbon and the binder and the conductive material is collected. It is characterized by being fixed to the body and used as a polarizable electrode.

In the electric double layer capacitor of the present invention, the polarizable electrode is made of a carbon material made of a specific resin as a raw material, that is, a carbon material obtained by carbonizing one or more selected from phenol resin, furan resin and polyacrylonitrile resin. Activated carbon activated by an activation treatment method using molten potassium hydroxide is used.

This activated carbon is heated in a state in which a carbon material obtained by carbonizing the above-mentioned specific resin and potassium hydroxide are mixed, and the carbon material is activated in the presence of molten potassium hydroxide. can get. This activated carbon has a large specific surface area, has a lower concentration of surface functional groups than the conventional activated carbon obtained by activating the petroleum coke by an activation treatment method using molten potassium hydroxide, and impurities of heavy metals derived from the starting material. There is a characteristic that there is little.

The present inventors have obtained the finding that an electric double layer capacitor can be made to have a high withstand voltage by using activated carbon obtained by activating the above-mentioned specific carbon material by a molten potassium hydroxide activation treatment method as a polarizable electrode. , An electric double layer capacitor with high electrostatic capacity and high withstand voltage was realized. It is presumed that the reason why an electric double layer capacitor having a high withstand voltage can be obtained is that the performance deterioration at the time of applying a high voltage due to the presence of surface functional groups of activated carbon and impurities of heavy metals could be reduced as much as possible.

The phenolic resin, furan resin, and polyacrylonitrile resin, which are the starting materials for these carbon materials, all have many double bonds in the molecular structure, so that the carbonization yield after carbonization is high. Common in points. Of these resins, the phenolic resin is the most suitable because the characteristics of the resulting activated carbon are good and the price is low.

The carbonization treatment of these resins is 500 to 12
It is preferably carried out in a non-oxidizing atmosphere at 00 ° C. If the temperature of the carbonization treatment is lower than 500 ° C, the carbonization treatment requires a long time. Also,. When the carbonization temperature is higher than 1200 ° C, it becomes difficult to obtain activated carbon having a large specific surface area.

The activation conditions of the carbon material using the potassium hydroxide in a molten state, which influences the size of the specific surface area and the size of the pores of the activated carbon, are the amount of potassium hydroxide mixed with the carbon material and the activation treatment temperature. is important. In order to obtain activated carbon having preferable characteristics, 2 to 6 parts by weight of potassium hydroxide is added to 1 part by weight of carbon material, and particularly 2.5 to 4.5 parts by weight is added and mixed with 1 part by weight of carbon material. Is preferred.

If the mixing amount of potassium hydroxide to 1 part by weight of the carbon material is 2 parts by weight or more, a good contact state between the carbon material and the molten potassium hydroxide can be ensured during the activation treatment, and the carbon material can be quickly activated. . However, if the amount of potassium hydroxide mixed with a certain amount of the carbon material is too large, the amount of the mixture to be treated increases and the activation treatment capacity of the activation treatment furnace decreases, resulting in a wasteful consumption of potassium hydroxide. Since it will increase, the amount is 6 parts by weight or less.

The activation temperature of the carbon material is preferably 600.
-1000 degreeC, Especially preferably, it is 700-900 degreeC. When the activation treatment temperature is low, the activation treatment time required to obtain activated carbon having a large specific surface area becomes long. Therefore, the activation treatment temperature is preferably 600 ° C. or higher. Further, if the activation treatment temperature is too high, the pores of the activated carbon become large and the electrostatic capacity per unit volume of the polarizable electrode decreases, and the wear of members of the furnace used for the activation treatment increases, and the energy of the activation treatment furnace increases. Consumption will also increase. Therefore, the activation treatment temperature is 10
The temperature is preferably set to 00 ° C or lower.

The larger the specific surface area of activated carbon, the larger the electrostatic capacity of the electric double layer per unit weight of activated carbon. However, on the contrary, if the activation proceeds too much and the specific surface area becomes too large, the pore volume of the activated carbon increases and the apparent density of the electrode decreases. When the apparent density of activated carbon decreases, the electrostatic capacity per unit volume of the polarizable electrode decreases. Therefore, the specific surface area of the activated carbon is preferably 1500 to 2500 m ² /
Control in the range of g. When the specific surface area of the activated carbon is within this range, a polarizable electrode having a large capacitance per unit volume can be obtained.

In order to manufacture a polarizable electrode, it is necessary to mold and fix an electrode material mainly composed of activated carbon on a current collector while maintaining electrical connection. As the method, a conventional method of molding and fixing a polarizable electrode can be adopted.

In the present invention, in order to secure a high withstand voltage of the capacitor, an organic solvent type electrolytic solution having a high decomposition voltage is used as the electrolytic solution. As the electrolyte of the organic solvent-based electrolytic solution, a quaternary ammonium ion represented by R ₄ N ⁺ and a quaternary phosphonium ion represented by R ₄ P ⁺ (wherein R is represented by C _n H _{2n + 1} ) Fourth such as alkyl group)
Grade onium cation and ^{_{BF 4 -, PF 6 -,}} ClO 4 -, CF
_It is preferred to use salts in combination with anions such as ₃ SO ₃ ⁻ .

Of these salts, it is particularly preferable to use a quaternary ammonium salt or a quaternary phosphonium salt. It is preferable to use an organic solvent having a low water content in the organic solvent-based electrolytic solution so that a high decomposition voltage can be obtained.

As the organic solvent used in the electrolytic solution, propylene carbonate, butylene carbonate, diethyl carbonate,
It is preferable to use one or more selected from carbonates such as ethylmethyl carbonate, lactones such as γ-butyrolactone, sulfolane and derivatives thereof.

[0021]

EXAMPLES The present invention will be further described below with reference to Examples (Examples 1 to 13) of the present invention and Comparative Examples (Examples 14 to 19), but the present invention is not limited to these Examples.

Approximately 70 using the resins shown in Table 1 as starting materials
Various carbon materials carbonized at 0 ° C. and having a particle size of about 10 mm (A: phenol resin, B: furan resin, C: polyacrylonitrile resin, D: petroleum coke, E: coconut shell,
F: sawdust) and three types of activation treatment methods (G, G'and G ": molten KOH activation treatment method, H: steam activation treatment method,
I: zinc chloride activation treatment method) was used to obtain activated carbon having a specific surface area (unit: m ² / g) shown in Table 1. The activation treatment conditions G and G'of activated carbon using molten potassium hydroxide
And G "are 3 parts by weight, 2.5 parts by weight and 2.5 parts by weight of potassium hydroxide with respect to 1 part by weight of the carbon material, and the activation temperatures are 800 ° C, 800 ° C and 700 ° C, respectively. .

80% by weight of powder (average particle size is about 10 μm) obtained by crushing each activated carbon obtained by the above activation treatment method and 10% by weight of polytetrafluoroethylene as a binder.
And carbon black 10 which is a conductive material for enhancing conductivity
% By weight. Next, ethanol was added to this mixture, and the mixture was kneaded, formed into a sheet shape, further rolled into a thickness of 0.6 mm, and punched into a circle having a diameter of 12 mm.

The sheet having a diameter of 12 mm was adhered and fixed to the inside of each of the stainless steel case and the lid, which are the collector and container member of the coin-type capacitor cell, by using a carbon-based conductive adhesive, and both were vacuumed. By heat-treating in it, water and the like contained in both polarizable electrodes were removed.

An electrolyte having a concentration (unit: molar concentration) shown in Table 1 in an organic solvent (PC: propylene carbonate, BC: butylene carbonate, SL: sulfolane) having a low water content shown in Table 1 (50 ppm or less in each case). (C
₂ H ₅ ) ₄ NBF ₄ was dissolved in an organic solvent-based electrolytic solution, and the organic solvent-based electrolytic solution was sufficiently impregnated into a bi-polarizable electrode mainly composed of activated carbon, and a separator paper made of polypropylene fiber non-woven fabric was used. Placed between both polarizable electrodes and crimped and sealed in a coin-shaped container made of stainless steel,
A coin-type electric double layer capacitor having a diameter of 18.4 mm and a thickness of 2.0 mm was obtained.

The initial capacitance (unit: F) and internal resistance (unit: Ω) of each of the obtained electric double layer capacitors were measured. Next, each capacitor was placed in a constant temperature bath of 70 ° C., a voltage of 3.0 V was applied, and the electrostatic capacity (unit: F) and internal resistance (unit: after 1000 hours) were applied.
Ω) was measured. The long-term operational reliability of the capacitor under high voltage application conditions was acceleratedly evaluated by comparing the degree of performance change before and after. Table 1 summarizes the evaluation results of the obtained electric double layer capacitors.

[0027]

[Table 1]

[0028]

As can be seen from Table 1, the electric double layer capacitor of the present invention has a high withstand voltage, little deterioration in characteristics after applying a high voltage of 3.0 V for 1000 hours, and a large capacitance. . Therefore, according to the present invention, an electric double layer capacitor having high reliability and high energy density can be provided.

Claims (5)

[Claims] 1. An electric double layer capacitor having a polarizable electrode and an electrolytic solution for forming an electric double layer on the surface of the polarizable electrode, wherein the polarizable electrode comprises phenol resin, furan resin and polyacrylonitrile resin. It is mainly composed of activated carbon obtained by activating a carbon material using one or more selected resins as a raw material by an activation treatment method using potassium hydroxide in a molten state, and the electrolytic solution is an organic solvent-based electrolytic solution. Electric double layer capacitor characterized by. 2. The electric double layer capacitor according to claim 1, wherein the organic solvent-based electrolytic solution contains a quaternary ammonium salt or a quaternary phosphonium salt as an electrolyte. 3. The specific surface area of the activated carbon is 1500 to 250.
It is 0 m < ² > / g, The electric double layer capacitor of Claim 1 or 2. 4. A carbon material obtained by carbonizing one or more resins selected from a phenol resin, a furan resin and a polyacrylonitrile resin, and heating a mixture obtained by adding potassium hydroxide to the carbon material to 600 to 1000 ° C. The carbon material is activated carbon in the presence of molten potassium hydroxide,
A method for producing a polarizable electrode for an electric double layer capacitor, comprising fixing a mixture of a powder of the activated carbon and a binder and a conductive material to a collector to form a polarizable electrode. 5. A weight ratio of potassium hydroxide / carbon material of 2 to 6
The method for producing a polarizable electrode for an electric double layer capacitor according to claim 4, wherein potassium hydroxide is added to and mixed with the carbon material so that