JPH11297580A - Electric double-layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the durability of an electric double-layer capacitor which exhibits a high breakdown voltage. SOLUTION: An electric double-layer capacitor is provided with a nonaqueous electrolytic solution and active carbon electrodes for both the poles and its applied voltage is 3.35 V or higher. In this case, the active carbon electrodes do not contain a binder material having carbon-fluorine combination. Also, the solute of the electrolytic solution is a nonaqueous electrolytic solution, containing a salt of combination of quarternary onium cation chosen from among a group of R4 N<+> , R4 P<+> (where R is an alkyl group represented by the expression Cn H2n+1 ) and triethylmethyl ammomnium ion and anion containing at least one of BF4 <-> , PF6 <-> , ClO4 <-> , SbF6 <-> and CF3 CO3 <-> or a lithium salt, in which the cation is lithium ion. Also, the rest potential of the active carbon electrodes in the electrolytic solution is in the range of 1.7 V or higher to -2.5 V or lower, when its counter electrode is set as Li/Li<+> .

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 which has a high withstand voltage, a high energy density, can be rapidly charged and discharged, and has excellent durability.

[0002]

2. Description of the Related Art Electric double layer capacitors which can be charged and discharged with a large current are promising for applications such as electric vehicles and auxiliary power supplies. Therefore, it is desired to realize an electric double layer capacitor having a high energy density, capable of rapid charge / discharge, and having excellent durability when a high voltage is applied and charge / discharge cycle durability. The energy stored in the cell of the capacitor is calculated as ・ · C · V ² , where C is the capacity per cell (F) and V is the voltage (V) that can be applied to the cell. Since the square of the value of the applicable voltage V is reflected in the energy, it is effective to increase the voltage applied to the capacitor to improve the energy density. However, at a large voltage, the decomposition of the electrolytic solution occurs.

Therefore, in the conventional electric double layer capacitor, the withstand voltage per unit cell is about 2.4 V in the case of the non-aqueous electrolyte electric double layer capacitor, depending on the type of solvent and solute of the electrolytic solution used. (Japanese Unexamined Patent Publication (Kokai) No. 7-145001), when used at a high voltage of 2.5 V or more, an increase in internal series resistance or a decrease in capacitance occurs in a short time. Therefore, an active carbon, a separator, an electrolytic solution, a container, and the like, which are main materials on the positive and negative sides, are examined, and attempts have been made to improve the withstand voltage. For example, reduction of metal impurities in activated carbon, reduction of impurities in activated carbon by activating raw materials such as phenolic resin and furan resin containing less metal impurities (JP-A-8-162375)
JP-A-8-339941), a method of reducing the oxygen-containing functional groups by heat-treating activated carbon after activation and improving the durability by using porous aluminum as a current collector of a capacitor
Japanese Patent Application Laid-Open No. 9-205041), and attempts to use an electrolytic solution using 2-methylsulfolane having a high decomposition voltage as a solvent have been made. With such an attempt, the withstand voltage was improved to 2.8 V, but has reached a plateau here.

[0004] The main method of producing an activated carbon electrode is to add a conductive substance such as acetylene black and a Teflon resin such as polytetrafluoroethylene and tetrafluoroethylene resin to activated carbon powder and mix them. Press molding methods (JP-A-62-200715, JP-A-62-17311, JP-A-5-121269,
JP-A-5-283287). These Teflon resin binders have a feature that they have a binder effect in a relatively small amount as compared with other binder substances, and thus tend to be used favorably.

[0005]

In recent years, the present inventors have
In Japanese Patent Application No. 9-183670, the potential at the time of charging is adjusted to be equal to or lower than the substantial decomposition starting voltage on the high potential side (oxidation side) of the electrolyte by arbitrarily adjusting the natural potential of the carbonaceous electrode. It proposes that the decomposition of the electrolytic solution is suppressed, and the applicable voltage and durability of the electric double layer capacitor can be improved. This will be described briefly. When an electrode substantially composed of a carbonaceous substance of a propylene carbonate solution of a quaternary alkylammonium salt, which is a typical non-aqueous electrolyte, is used, the decomposition start voltage on the oxidation side of the electrolyte is 4.4 V
It is said to be near (to Li / Li ⁺ ). On the other hand, the natural potential of a normal activated carbon electrode is around 3 V (vs. Li / Li ⁺ ), and when the voltage applied to the capacitor is 2.8 V, the polarization on the positive electrode side after charging is about 1.4 V, and the potential on the oxidation side Is 4.4V (vs.
Li / Li ⁺ ) or more, and it is considered that electrochemical decomposition of the electrolytic solution occurs. As a result, when a conventional activated carbon electrode is used, the capacity is reduced due to a gas or the like generated by the decomposition of the electrolytic solution, and thus there has been a problem of durability when used for a long period of time. Therefore, in the invention of Japanese Patent Application No. Hei 9-183670, by lowering the natural potential of the activated carbon electrode so that the potential on the positive electrode side after charging is equal to or lower than the oxidative decomposition starting voltage of the electrolytic solution, the practically applicable voltage of the capacitor is reduced. It proposes that the energy density can be greatly increased and the energy density can be improved. However, in the invention of Japanese Patent Application No. 9-183670, when a high voltage of 3.35 V or more is applied, depending on the type of electrode, a problem such as swelling of a capacitor cell may occur.

[0006]

Means for Solving the Problems Accordingly, the present inventors have:
As a result of intensive studies to study the above issues, the applied voltage
When the voltage exceeds 3.35V, the negative electrode side⁺Potential is 0.6
~ 0.9V (vs. Li / Li ⁺Enter the lower potential area
Can cause reductive decomposition reaction
They found that this was the cause and arrived at the present invention. Sand
That is, the object of the present invention is to provide a high durability when applying a high voltage of 3.35 V or more.
Excellent resistance and charge / discharge durability, low resistance and energy density
To provide large electric double layer capacitors.
For this purpose, an activated carbon electrode is used for the non-aqueous electrolyte and both electrodes.
Electric double layer capacitor with applied voltage of 3.35V or more
In the above, a bag having a carbon-fluorine bond in the activated carbon electrode
Containing no inder substance, and the solute of the electrolytic solution is R
_FourN⁺, R_FourP⁺(However, R is C_nH_{2n + 1}Indicated by
Alkyl group), triethylmethylammonium ion
Quaternary oni containing at least one selected from the group consisting of
Um cation and BF_Four ^-, PF₆ ^-, ClO_Four ^-, SbF₆ ^-, CF_ThreeS
O_Three ^-Containing at least one selected from the group consisting of
Salt in combination with on, or the cation is lithium
Non-aqueous electrolyte containing lithium ion
And the natural potential of the activated carbon electrode body in the electrolyte is Li
/ Li⁺1.7V or more and 2.5V or less when
This is easily achieved.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The greatest feature of the present invention is that, in a non-aqueous electrolyte solution electric double layer capacitor, the activated carbon electrode body does not contain a substance having a carbon-fluorine bond, and is formed of a binder substance having substantially only a carbon-carbon bond. By using the activated carbon molded body, high energy density and low internal resistance are shown when a high voltage of 3.35 V or more is applied, and durability and cycle durability when a long-time voltage is applied are greatly improved. is there.

More specifically, the present invention relates to an electric double layer capacitor using a non-aqueous electrolyte and an electrode body substantially containing activated carbon on both electrodes, wherein the activated carbon electrode body contains a substance having a carbon-fluorine bond. An activated carbon molded body molded with a binder material having substantially only a carbon-carbon bond without using the same was used, and the electrolyte was R ₄ N ⁺ , R ₄ P ⁺ (where R is
C _n H _{2n + 1} alkyl groups represented by) quaternary containing at least one selected from the group consisting of triethyl-ammonium ion onium cation and BF ₄ ^-, PF ₆ ^-,
A non-aqueous electrolyte in which a salt in combination with an anion containing at least one selected from the group consisting of ClO ₄ ⁻ , SbF ₆ ⁻ , and CF ₃ SO ₃ ⁻ is a solute, and the electrolyte of an activated carbon electrode body When the natural potential inside is Li / Li ⁺ as the counter electrode, 1.7V
An electric double layer capacitor characterized by having a voltage of not less than 2.5 V or less, and in particular, an alkali metal such as lithium, an alkaline earth metal, or a rare earth It contains metal.

The electrode mainly composed of activated carbon is composed of activated carbon powder, a conductive agent and a binder. Fluorine-containing polymer resins such as polytetrafluoroethylene and tetrafluoroethylene, which are generally widely used as binders, cannot be used for the activated carbon electrode of the present invention because they contain carbon-fluorine bonds. It is considered that the reason why the carbon-fluorine bond is particularly problematic is that, apart from the breakage of the carbon-fluorine bond, ions or molecules in the electrolytic solution react with fluorine to generate decomposition products. Examples of the binder substance mainly containing a carbon-carbon bond include polycyclic aromatic compounds such as coal tar pitch, petroleum pitch, phenol resin, and furan resin. However, for the reason that volatile components such as oxygen-containing functional groups and nitrogen-containing functional groups, which are often contained in these polycyclic aromatic compounds, are partially soluble in electrolyte solvents such as propylene carbonate. When used directly as a binder, the durability of the electric double layer capacitor is reduced. Therefore, after molding a mixture of the polycyclic aromatic compound and the activated carbon powder, the molded body is heat-treated in an inert gas such as nitrogen or argon at 700 ° C. to 1300 ° C., which is higher than the thermal decomposition temperature of the polycyclic aromatic compound. Then, by removing the volatile components, only carbonaceous material becomes a binder material and can be suitably used as an activated carbon electrode of the electric double layer capacitor of the present invention. When the heat treatment is performed at 1300 ° C. or more, the specific surface area is reduced due to the contraction of the pores of the activated carbon, and the capacity of the electric double layer capacitor is reduced. The heat treatment of the molded body may be performed under pressure using a hot press or the like to increase the bulk density of the molding to increase the capacity of the electric double layer capacitor. The amount of these polycyclic aromatic compounds varies depending on the volatile composition, the softening point, and the like.
The content is preferably from 0% by weight to 70% by weight. If the blending amount is large, the strength of the molded body increases, but the content of activated carbon in the molded body decreases, and the capacity of the electric double layer capacitor decreases. It is preferable to use a high softening point pitch having a softening point of 220 ° C. or higher for the coal tar pitch and petroleum pitch because the softened pitch does not block the pores of the activated carbon during the heat treatment. For the same reason, in the case of phenolic resin,
Thermosetting types are preferred over thermoplastic types.

In order to increase the adhesiveness between the activated carbon particles and the binder at the time of molding, polyethylene glycol,
1 to 30% by weight aqueous solution of thermoplastic resin such as polyvinyl alcohol, ethylene glycol, propylene glycol, carboxymethyl cellulose, or 2 to 40% by weight of thermally decomposable resin such as polyolefin polymer, acrylic resin, polystyrene resin and polyethylene resin Including,
2 to 2 in organic solvents such as acetone and dimethyl sulfoxide
A solution in which 40% by weight is dissolved may be added in an amount of 10 to 40% by weight based on the total weight of the activated carbon powder and the organic binder, and then heat-treated to obtain an activated carbon molded body.

The activated carbon powder has an average particle diameter of 5 to 5.
It is preferably about 40 μm. With an average particle size of 40 μm or more,
Although the amount of the binder can be reduced, the internal resistance of the electric double layer capacitor increases due to factors such as a decrease in the impregnation of the electrolyte into the activated carbon particles. Average particle size is 3μ
If it is less than m, it is necessary to add about 70% by weight or more of a binder in order to obtain a sufficient strength of the molded body, and the capacity of the electric double layer capacitor is reduced. In addition, an activated carbon molded body produced without using a binder by a method such as thermal sintering or spark sintering only activated carbon, or spraying electrochemically stable aluminum onto activated carbon is also referred to as an electrode body for an electric double layer capacitor. It is possible to The electrode may be a sheet-shaped or plate-shaped formed body, or a plate-shaped formed body made of a composite.

[0012] The activated carbon molded body obtained by the above molding method does not contain an insulating substance such as polytetrafluoroethylene,
In addition, since it is heat-treated at high temperature, its electrical conductivity is relatively high, but in order to further improve conductivity, carbon black such as acetylene black and Ketjen black, natural graphite, and thermal expansion graphite are used as conductive agents. And at least one conductive agent selected from the group consisting of carbon fibers, metal fibers such as ruthenium oxide, titanium oxide, aluminum and nickel. Acetylene black and Ketjen black are particularly preferred from the viewpoint that conductivity is effectively improved with a small amount, and the blending amount thereof is preferably 10 to 30% by weight based on the total weight of the activated carbon powder and the organic binder. The current collector is not particularly limited as long as it has electrochemical and chemical corrosion resistance.Examples include, but are not limited to, stainless steel, aluminum, titanium, and tantalum for a positive electrode, and stainless steel, nickel, and copper for a negative electrode. Used for

In the present invention, the measurement of the spontaneous potential of the activated carbon electrode body is performed by a usual electrochemical technique. In the measurement of potential in a non-aqueous system, a potential reference such as a standard hydrogen electrode in an aqueous solution is not strictly defined, but in practice, silver-
It is generally widely used using electrodes such as a silver chloride electrode, a platinum electrode, and a lithium electrode. In the present invention, it can be measured by a similar method.

[0014] Only activated carbon, which is substantially the main material of the polarizable electrode body used in the present invention, has a natural potential of 1.7 V or more and 2.7 V or more.
Since the voltage does not fall within the range of 5 V or less (vs. Li / Li ⁺ ), some adjustment is required. The method of adjusting the natural potential of the activated carbon electrode to 1.7 V or more and 2.5 V or less (vs. Li / Li ⁺ ) is not particularly limited, but at least one selected from alkali metals, alkaline earth metals, and rare earth metals. It is preferable to add the above substances to the electrode body by an electrochemical method, a chemical method, a physical method, or the like. For example, as one of the simple methods, there is a lithium-containing electrode made of a substance containing lithium or lithium, which is a very noble metal, a carbonaceous electrode mainly composed of activated carbon, a separator and a non-aqueous electrolyte. In a chemical cell, lithium can be introduced into an activated carbon electrode by short-circuiting a lithium-containing electrode and a carbonaceous electrode or charging the lithium-containing electrode as a positive electrode and the carbonaceous electrode as a negative electrode. The material containing lithium is not particularly limited, for example,
Lithium-aluminum alloys, lithium-containing alloys such as lithium-magnesium alloys, lithium intermetallic compounds,
At least one selected from composite oxides such as manganese oxides, cobalt oxides, nickel oxides, and vanadium oxides containing lithium, titanium sulfide containing lithium, niobium selenide, chalcogenite such as molybdenum sulfide, and carbon containing lithium. It is preferable to use the above substances. As a metal having a low potential, in addition to lithium, an alkali metal such as sodium and potassium, an alkaline earth metal such as calcium and magnesium, a rare earth metal such as yttrium and neodymium, or a substance containing these metals is referred to as lithium. Similarly, it may be used as a substance that lowers the natural potential. If the electrode potential is excessively lowered to be less than 1.7 V, when the applied voltage is 3.35 V or more, decomposition of the electrolytic solution occurs, which may lower the initial energy density and durability, which is not preferable.

An electric double layer capacitor is assembled using the activated carbon electrodes whose natural potential has been adjusted in this way for both electrodes.
If the difference in the natural potential between the two electrodes of the activated carbon electrode is too large, the operation becomes unstable, which is not preferable. The content of lithium in the electrode slightly varies depending on the bulk density, specific surface area, surface properties and the like of the activated carbon, but is about 0.02% by weight or more and 2% by weight or less. As the activated carbon before adjusting the natural potential, it is preferable to use activated carbon having a large specific surface area in order to increase the capacity of the electric double layer capacitor. If the specific surface area of the activated carbon is too large, the bulk density is reduced and the energy density is reduced.
0 to 3000 m ² / g is preferred, and more preferably 300 to 2300
m ² / g. Raw materials for activated carbon include plant-based wood, sawdust, coconut husk, pulp waste liquor, fossil fuel-based coal, heavy petroleum oil, or coal obtained by thermally decomposing them, and fibers spun from petroleum-based pitch and tar pitch. , Synthetic polymers, phenolic resins, furan resins, polyvinyl chloride resins, polyvinylidene chloride resins, polyimide resins, polyamide resins, liquid crystal polymers, plastic waste, waste tires and many other uses. These carbonized materials are activated after carbonization. Activation methods are roughly classified into gas activation and chemical activation. In the gas activation method, chemical activation is chemical activation, whereas physical activation is also called physical activation, and the carbonized raw material is contact-reacted with steam, carbon dioxide, oxygen, and other oxidizing gases at high temperatures. Then, activated carbon is obtained. The chemical activation method is a method in which a raw material is uniformly impregnated with an activation chemical, heated in an inert gas atmosphere, and activated carbon is obtained by a dehydration and oxidation reaction of the chemical. The chemicals used are zinc chloride, phosphoric acid,
Examples include sodium phosphate, calcium chloride, potassium sulfide, potassium hydroxide, sodium hydroxide, potassium carbonate, sodium carbonate, sodium sulfate, potassium sulfate, calcium carbonate, and the like. Various methods for producing activated carbon have been described above, but are not particularly limited. The activated carbon has various shapes such as crushing, granulation, granule, fiber, felt, woven fabric and sheet shape, and any of them can be used in the present invention.
Among these activated carbons, activated carbons obtained by chemical activation using KOH are particularly preferable because they tend to have larger capacities than steam activated products. Activated carbon after activation treatment,
Heat treatment at 500 to 2500 ° C, preferably 700 to 1500 ° C in an inert atmosphere of nitrogen, argon, helium, xenon, etc. to remove unnecessary surface functional groups or to develop carbon crystallinity May be increased.

The electrolyte is preferably a non-aqueous electrolyte. Non-aqueous
The solute of the electrolyte is R_FourN⁺, R_FourP ⁺(However, R is C
_nH_{2n + 1}), Triethylmethyla
At least one selected from the group consisting of ammonium ions, etc.
A quaternary onium cation containing one and BF_Four ^-, P
F₆ ^-, ClO_Four ^-, SbF₆ ^-, CF_ThreeSO_Three ^-Selected from the group consisting of
Combined with an anion containing at least one of
Salt or lithium whose cation is lithium ion
Use salt. Lithium salt is LiBF_Four, LiCl
O_Four, LiPF₆, LiSbF₆, LiAsF₆, Li
CF_ThreeSO_Three, LiC (CF _ThreeSO_Two)_Three, LiB (C
₆H_Five)_Four, LiC_FourF₉SO_Three, LiC₈F₁₇S
O _Three, LiB (C₆H_Five)_Four, LiN (CF_ThreeSO_Two)
_TwoOne or more substances selected from are preferred. In particular, electricity
In terms of conductivity, stability, and low cost,
Is R_FourN⁺(However, R is C_nH_{2n + 1}A shown by
Alkyl group) and triethylmethylammonium ion,
The anion is BF_Four ^-, PF₆ ^-, ClO_Four ^-, And SbF₆ ^-Group
Combined salts are preferred.

The solute concentration in these non-aqueous electrolytes is adjusted so that the characteristics of the electric double layer capacitor can be sufficiently obtained.
~ 2.1 mol / l is preferred. Although the solvent of the non-aqueous electrolyte is not particularly limited, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, sulfolane, methyl sulfolane, γ
Preferred is an organic solvent comprising at least one selected from -butyrolactone, γ-valerolactone, N-methyloxazolidinone, dimethylsulfoxide, and trimethylsulfoxide. At least one selected from propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, sulfolane, methyl sulfolane, and γ-butyrolactone from the viewpoint of excellent electrochemical and chemical stability and electric conductivity. Is especially preferred.
However, a high melting point solvent such as ethylene carbonate alone cannot be used because it becomes a solid at a low temperature and must be a mixed solvent with a low melting point solvent such as propylene carbonate. The water content in the non-aqueous electrolyte is preferably 200 ppm or less, more preferably 50 ppm or less, so as to obtain a high withstand voltage.

[0018]

EXAMPLES Hereinafter, the present invention will be described with reference to specific examples, but the present invention is not limited to the following examples. [Example 1] Potato steam activated carbon powder (specific surface area)
1680m ² / g, average particle size 28μm, purity 99.8%) 7g and coal tar pitch powder (softening point 300 ° C, average particle size 30μm)
3 g of the mixture, plus 2 parts of polyethylene glycol
The mixture was thoroughly mixed in a mortar while dropping 2 ml of a weight% aqueous solution little by little. A part of the obtained mixture was press-formed at a pressure of 400 kgf / cm ^{2 to} a diameter of 10 mm and a thickness of 0.5 mm by a hydraulic press using a tablet press made by JASCO to form a disc-shaped molded body. Two were obtained. After heating these molded bodies from room temperature to 850 ° C. at a heating rate of 3 ° C./min in a tubular heating furnace through which nitrogen gas was passed, and maintaining the temperature at 850 ° C. for 2 hours,
It was allowed to cool to room temperature to obtain two activated carbon molded articles used for the electric double layer capacitor of the present invention. Next, the two activated carbon molded bodies were dried at 300 ° C. for 3 hours in a vacuum of 0.1 torr or less, and then transferred to a glove box under an argon atmosphere. A polyethylene separator was sandwiched between the two activated carbon moldings, and the entire body was sandwiched between two platinum plates used as current collectors outside the separator. Then the current collector,
An open-cell capacitor was assembled by sandwiching between two Teflon plates having a thickness of 5 mm and four bolt holes from the outside so that the activated carbon electrode and the separator were in good contact with each other. The thus obtained open cell type capacitor and a lithium electrode produced by pressing metal lithium foil on the tip of a platinum plate were connected to a 1 mol / liter Li in a beaker.
BF ₄ was immersed in a propylene carbonate solution. Next, connect the lithium electrode and the activated carbon electrode with a lead wire.
Short-circuited for hours. After short-circuiting, the natural potential of the activated carbon electrodes measured by connecting a voltmeter between the activated carbon electrode and the lithium electrode was 2.18 V (vs. Li / Li ⁺ ) for both of them. Thereafter, the electrode portion was disassembled and two activated carbon electrode bodies were taken out.

The propylene carbonate of triethylmethylammonium tetrafluoroborate at a concentration of 1.3 mol / liter + ethylene carbonate (volume mixing ratio of 1+
1) An electric double layer capacitor was obtained by assembling a coin-type cell as shown in FIG. 1 with a solution impregnated sufficiently as a positive electrode and a negative electrode, respectively, and a polyethylene separator disposed between both electrodes. The internal resistance of the obtained electric double layer capacitor was 1.90 Ω, and a voltage of 3.8 V was applied at room temperature to 1 using a charging / discharging device “HJ201-B” manufactured by Hokuto Denko.
After the application for a period of time, the initial capacitance obtained by discharging at a constant current of 1.0 V at 1.16 mA was 1.29 F, and the initial energy density was 5.01 J. At room temperature, to evaluate the long-term operation reliability of the capacitor under voltage application conditions,
The rate of change in energy density after applying a voltage of 3.8 V for 800 hours was -9%.

[Example 2] Activated carbon powder was converted to KO using coal pitch.
H was obtained by activation (specific surface area 550 m ² / g, average particle diameter 18 μm), and the adjusted natural potential of the activated carbon electrode body was 2.
An electric double layer capacitor similar to that of Example 1 was configured except that the voltage was set to 11V. The initial internal resistance of the obtained electric double layer capacitor was 2.1 Ω, and after applying a voltage of 3.8 V for 1 hour, a constant current discharge at 1.16 mA to 1.0 V resulted in an initial capacitance of 1.81. F, the initial energy density was 6.80 J. The rate of change in energy density after applying a voltage of 3.8 V for 800 hours at room temperature was -10%.

Example 3 Activated carbon powder 7.5 of Example 2
g and 2.5 g of petroleum pitch powder (softening point: 230 ° C., average particle size: 40 μm) to which 1.5 ml of a 2% by weight aqueous solution of polyethylene glycol was added. An electric double-layer capacitor similar to that of Example 1 was formed except that the above-mentioned was adopted. The initial internal resistance of the obtained electric double layer capacitor was 2.2 Ω, and after applying a voltage of 3.8 V for 1 hour, 1.0 V was applied at 1.16 mA.
The initial capacitance was 1.82 F, and the initial energy density was 6.76 J, obtained by constant-current discharge until the discharge. The rate of change in energy density after applying a voltage of 3.8 V for 800 hours at room temperature was -10%.

Example 4 Activated carbon powder was converted to petroleum coke.
It was obtained by KOH activation (specific surface area 1095m ² / g, average particle size 25μm), and the adjusted natural potential of the activated carbon electrode body was
An electric double layer capacitor similar to that of Example 1 was configured except that the voltage was changed to 2.12 V. The initial internal resistance of the obtained electric double layer capacitor was 2.3Ω, and after applying a voltage of 3.8 V for one hour, a constant current discharge at 1.16 mA to 1.0 V resulted in an initial capacitance of 1.58. F, the initial energy density was 5.99J. The rate of change of the energy density after applying a voltage of 3.8 V for 800 hours at room temperature was -11%.

[Comparative Example 1] A mixture comprising 80% by weight of activated carbon powder, 10% by weight of acetylene black and 10% by weight of polytetrafluoroethylene of Example 1 was kneaded, and then hydraulically pressed using a tablet press made by JASCO. With diameter 10.5mm, thickness
Example 1 was the same as Example 1 except that two molded bodies obtained by pressure molding at a pressure of 50 kgf / cm ² so as to be 0.5 mm were used as activated carbon electrodes and the natural potential of the activated carbon electrodes was 2.18 V. A similar electric double layer capacitor was constructed. The initial internal resistance of the obtained electric double layer capacitor was 4.9Ω, and 3.8
After applying a voltage of V for 1 hour, the initial capacitance was 1.28 F and the initial energy density was 4.91 J, which was obtained by discharging at a constant current of 1.16 mA to 1.0 V. At room temperature, the rate of change in energy density after applying a voltage of 3.8 V for 800 hours is-
40%.

Comparative Example 2 Two activated carbon electrodes were obtained using the activated carbon powder of Example 2 in the same manner as in Comparative Example 1, and the natural potential of the activated carbon electrode was 2.22 V. Formed an electric double layer capacitor similar to that of Example 2. The initial internal resistance of the obtained electric double layer capacitor is 5.1Ω, and after applying a voltage of 3.8 V for 1 hour,
The initial capacitance obtained by discharging at a constant current of 1.0 V at 1.16 mA was 1.77 F, and the initial energy density was 6.45 J.
At room temperature, the rate of change of the energy density after applying a voltage of 3.8 V for 800 hours was -45%.

Comparative Example 3 In Example 1, the natural potential was set to 2.90 by short-circuiting the lithium electrode and the activated carbon electrode for a short time.
An electric double layer capacitor was formed in the same manner as in Example 1 except that the voltage was adjusted to V. The initial internal resistance of the obtained electric double layer capacitor was 1.9 Ω, and after applying a voltage of 3.8 V for 1 hour, a constant current discharge at 1.16 mA to 1.0 V resulted in an initial capacitance of 1.30. F, the initial energy density was 4.95J. At room temperature, the rate of change of the energy density after applying a voltage of 3.8 V for 800 hours was -35%.

[0026]

According to the present invention, it is possible to obtain an electric double layer capacitor that operates without any problem even when a high voltage of 3.35 V or more is applied.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a coin cell used in an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Case of stainless steel container 2 Activated carbon molding 3 Gasket 4 Separator 5 Activated carbon molding 6 Stainless steel container top lid

Claims (6)

[Claims] 1. An electric double layer capacitor in which a non-aqueous electrolyte and an activated carbon electrode are used for both electrodes and an applied voltage of 3.35 V or more does not contain a binder substance having a carbon-fluorine bond in the activated carbon electrode, and The solute of the electrolyte is R ₄ N
⁺ , R ₄ P ⁺ (where R is an alkyl group represented by C _n H _{2n + 1} ), a quaternary onium cation containing at least one selected from the group consisting of triethylmethylammonium ions, and BF ₄ ⁻ , PF ^{_{6 -, ClO 4 -, SbF}} 6 -, CF 3 SO 3 -
Or a non-aqueous electrolyte containing a lithium salt whose cation is a lithium ion, and the activated carbon electrode body in the electrolyte is a salt in combination with an anion containing at least one selected from the group consisting of: Natural potential is Li / Li ⁺
An electric double layer capacitor characterized by having a voltage of 1.7 V or more and 2.5 V or less, when. 2. The electric double layer capacitor according to claim 1, wherein the concentration of the solute in the electrolyte is not less than 1 mol / l and not more than 2.1 mol / l. 3. The electric double layer capacitor according to claim 1, wherein the activated carbon electrode contains an activated carbon powder and a binder material having a carbon-carbon bond. 4. An activated carbon electrode body comprising at least one selected from an alkali metal, an alkaline earth metal, and a rare earth metal.
The electric double layer capacitor according to claim 1, comprising at least one substance. 5. The electric double layer capacitor according to claim 1, wherein the activated carbon electrode body contains lithium. 6. The activated carbon electrode body is formed by molding a mixture of activated carbon powder and at least one binder substance selected from coal pitch and petroleum pitch having a softening point of 220 ° C. or higher, and then 700 ° C. to 1300 ° C. The electric double layer capacitor according to any one of claims 1 to 5, wherein the electric double layer capacitor is obtained by heat treatment as follows.