JPH11176702A - Electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a higher withstand voltage and higher energy density by using an electrolyte consisting of a particular organic solvent and a quaternary onium salt. SOLUTION: A mixture consisting of coconut shell active carbon of 80 weight%, polytetrafluoroethylene of 10 weight%, and carbon black of 10 weight% is added with ethanol and is kneaded, and after it is molded into a sheet shape, is stamped out into a disk shape to form a positive and a negative electrodes. These disk-shaped positive and negative electrodes are bonded inside the positive and negative electrode sides of a stainless steel case respectively using electrically conductive graphite bonding agent, and are heat-treated together with the stainless steel case under a reduced pressure condition to remove water content, etc. Then, an electrolyte is formed by dissolving (C2 H5 )3 (CH3 )NBF4 of 1.5 mol/l in a mixed solvent of 1,3-propane sultone and methyl carbonate with a mixing volume ratio of 80:20, and the positive and negative electrodes are impregnated with this electrolyte. Then, the positive and negative electrodes are counterposed with a separator being sandwiched between them, and the stainless steel case is caulked via a gasket to seal the opening thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electric double layer capacitor, and more particularly to an electric double layer capacitor having a high withstand voltage and a high energy density.

[0002]

2. Description of the Related Art A conventional electric double layer capacitor has a shape in which a separator formed between a pair of polarizable electrodes mainly composed of activated carbon formed on a current collector is placed in a metal case together with an electrolytic solution. A coin type sealed and sealed with a metal lid via a gasket, or an element formed by winding a pair of sheet-shaped polarizable electrodes with a separator interposed therebetween is accommodated in a metal case together with an electrolytic solution in an opening of the case. There is a wound type that is sealed so that the electrolyte does not evaporate from the part.

[0003] For large current charging and discharging and large capacity,
A multilayer electric double layer capacitor in which an element formed by laminating a number of sheet-shaped polarizable electrodes with a separator interposed therebetween has been proposed (Japanese Patent Laid-Open No. 4-154106,
JP-A-3-203331, JP-A-4-286108).
In this case, a plurality of sheet-shaped polarizable electrodes formed in a rectangular shape are used as a positive electrode and a negative electrode, and are alternately stacked via a separator to form an element. The elements are housed in a case connected by caulking, and the element is impregnated with an electrolytic solution and sealed with a lid.

As a conventional electrolyte for an electric double layer capacitor, various non-aqueous electrolytes are used in addition to an aqueous electrolyte containing a mineral acid such as sulfuric acid or an alkali metal salt. Known solvents for the non-aqueous electrolyte include propylene carbonate, γ-butyrolactone, acetonitrile, dimethylformamide (JP-A-49-68254), and sulfolane derivatives (JP-A-62-237715).

When the withstand voltage is compared, the aqueous electrolyte is 0.8 V
On the other hand, the non-aqueous electrolyte is 2.5 to 3.3 V, and the electrostatic energy of the capacitor is proportional to the square of the withstand voltage. Therefore, the non-aqueous electrolyte is more advantageous in terms of electrostatic energy. . However, even when the sulfolane derivative having the highest withstand voltage is used as the solvent of the electrolytic solution, the withstand voltage of the capacitor is 3.3 V.
Therefore, higher withstand voltage is desired.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art and to provide an electric double layer capacitor having high withstand voltage, high energy density and excellent reliability.

[0007]

According to the present invention, there is provided an electric double layer capacitor comprising an electrode mainly composed of a carbon material as a positive electrode and a negative electrode, and an electrolytic solution forming an electric double layer at an interface with the electrodes. And an electric double layer capacitor, wherein the electrolyte comprises an organic solvent containing 1,3-propane sultone and a quaternary onium salt.

The 1,3-propane sultone used in the electrolytic solution of the electric double layer capacitor of the present invention is an electrochemically stable solvent and can increase the withstand voltage. In addition, since the quaternary onium salt, which has a high dielectric constant and is a solute, can be dissolved at a high concentration, the electric conductivity of the electrolytic solution can be increased. But 1,
3-Propane sultone has a high melting point of 31 ° C., and its electric conductivity sharply decreases at a low temperature. Therefore, when used at a low temperature, there is a problem that the capacitor characteristics are deteriorated as compared with room temperature or the like. Therefore, when used at a low temperature, it is preferable to use a mixed solvent of 1,3-propane sultone and a chain carbonate.

The chain carbonate is represented by the general formula R ¹ OC (=
O) is represented by OR ^2, it has low freezing point less increase in viscosity even at low temperatures, which is electrochemically stable solvent. Particularly, in the case of an asymmetric chain carbonate in which R ¹ and R ² are different monovalent organic groups, the concentration of the electrolyte in the electrolytic solution can be increased, and the electric conductivity at a low temperature is further improved. In the above general formula of the chain carbonate, R ¹ and R ² are appropriately selected from monovalent organic groups such as an alkyl group and an aryl group. This monovalent organic group may be a halogen-substituted organic group.

In consideration of the melting point, viscosity and dielectric constant of the obtained mixed solvent, the organic groups R ¹ and R ² are each independently a methyl group, an ethyl group, an n-propyl group, an isopropyl group or a 2,2,2-triol. It is preferably a fluoroethyl group, and particularly preferably, R ¹ and R ² are different. Specifically, methyl ethyl carbonate, methyl isopropyl carbonate, ethyl isopropyl carbonate, 2,
Examples thereof include 2,2-trifluoroethyl-methyl carbonate and the like, and methyl ethyl carbonate is particularly preferred.

The solute used for the electrolytic solution of the electric double layer capacitor of the present invention is a quaternary onium salt. Preferred cations of the quaternary onium salt are R ³ R ⁴ R ⁵ R ⁶
N ⁺ or expressed by ^{R 3 R 4 R 5 R 6+} . In the present invention, R ³ , R ⁴ , R ⁵ , and R ⁶ in the above formula have 1 to 1 carbon atoms.
More preferably, it is 5 alkyl groups. R ³ ,
R ⁴ , R ⁵ and R ⁶ may all be the same or different, but preferably not all.

A quaternary onium salt having a cation wherein R ³ , R ⁴ , R ⁵ and R ⁶ are composed of two or more alkyl groups is
Compared to a quaternary onium salt in which R ³ , R ⁴ , R ⁵ , and R ⁶ are all the same, the solubility in the solvent used in the present invention is large, and the concentration of the solute in the electrolyte can be increased. The concentration is preferable because the electric conductivity of the electrolytic solution increases.

R ³ , R ⁴ , R ⁵ and R ⁶ each independently represent a methyl group (—CH ₃ ) or an ethyl group (—C ₂ H
_5), propyl (-C ₃ H ₇₎ or a butyl group (-C ₄
H ₉ ). Specific examples of preferred cations of the quaternary onium salt in the present invention include:
(C ₂ H ₅ ) ₃ (CH ₃ ) N ⁺ , (C ₂ H ₅ ) ₂ (CH
₃ ) ₂ N ⁺ , (C ₂ H ₅ ) (CH ₃ ) ₃ N ⁺ , (C ₃ H ₇
) ₃ (CH ₃ ) N ⁺ , (C ₃ H ₇ ) ₂ (CH ₃ ) ₂ N ⁺
, (C ₃ H ₇ ) (CH ₃ ) ₃ N ⁺ , (C ₃ H ₇ ) ₃
(C ₂ H ₅ ) N ⁺ , (C ₃ H ₇ ) ₂ (C ₂ H ₅ ) ₂ N
⁺ , (C ₃ H ₇ ) (C ₂ H ₅ ) ₃ N ⁺ , (C ₂ H ₅ ) ₃
(CH ₃ ) P ⁺ , (C ₂ H ₅ ) ₂ (CH ₃ ) ₂ P ⁺ ,
(C ₂ H ₅ ) (CH ₃ ) ₃ P ⁺ . However,
Here, the propyl group (—C ₃ H ₇ ) may be an n-propyl group or an isopropyl group, and the butyl group (—C ₃ H ₇ )
₄ H ₉ ) may be an n-butyl group, an isobutyl group, a sec-butyl group or a tert-butyl group.

[0014] solutes used for the electrolytic solution, a quaternary onium cation of _{^{the, BF 4 -, PF 6 -}} , Cl -, CF 3 S
O ₃ ⁻ , AsF ₆ ⁻ , N (SO ₂ CF ₃ ) ₂ ⁻ , NO ₃ ⁻ , Cl
It is preferably a salt comprising an anion such as O ₄ ⁻ , Br ⁻ , SO ₄ ²⁻ or the like. (C ₂ H ₅ ) ₃ (CH ₂ ) in terms of solubility in a solvent, electric conductivity of a solution, and electrochemical stability.
₃ ) NBF ₄ is most preferred.

The electric conductivity of the electrolytic solution is such that the concentration of the solute is 1.
It increases in proportion to the concentration up to around 5 mol / l,
At 1.5 mol / l or more, it approaches a constant value. Therefore, the concentration of the solute is preferably set to 1.2 to 1.8 mol / l.

The positive electrode and the negative electrode used in the electric double layer capacitor of the present invention are both electrodes mainly composed of a carbon material, and the carbon material has a high specific surface area of about 800 to 3500 m ² / g. Specific examples include activated carbon, carbon black, and polyacene. The positive electrode and the negative electrode are obtained, for example, by kneading a mixture of the above-mentioned carbon material powder and a binder such as polytetrafluoroethylene with a liquid lubricant, rolling, and forming into a sheet. Also,
It may be obtained by dispersing a carbon material powder in a solution in which a binder such as polyvinylidene fluoride is dissolved in an organic solvent, and applying the solution to a metal current collector.

[0017]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples (Examples 1 to 5) and Comparative Examples (Examples 6 and 7), but the present invention is not limited thereto. Example 1 Ethanol was added to a mixture of 80% by weight of activated carbon activated with steam having a specific surface area of 1800 m ² / g, 10% by weight of polytetrafluoroethylene and 10% by weight of carbon black, and kneaded to form a sheet. After the molding, the resultant was roll-rolled to a thickness of 0.6 mm, and the obtained electrode sheet was punched into a disk having a diameter of 12 mm to obtain a positive electrode and a negative electrode.

The disc-shaped positive and negative electrodes were adhered to the inside of the positive and negative electrode sides of a stainless steel case used as a current collector and a housing member of a coin type cell, respectively, using a graphite-based conductive adhesive. . Next, the entire stainless steel case was subjected to a heat treatment under reduced pressure to remove moisture and the like.

In a mixed solvent of 1,3-propane sultone and methyl ethyl carbonate in a volume ratio of 80:20, 1.
A solution in which 5 mol / l of (C ₂ H ₅ ) ₃ (CH ₃ ) NBF ₄ was dissolved was used as an electrolytic solution, and the positive electrode and the negative electrode were impregnated.
17. A stainless steel case is caulked and closed via a gasket, which is an insulator, with a separator made of nonwoven polypropylene fiber sandwiched between the positive electrode and the negative electrode.
A coin-type electric double layer capacitor having a thickness of 4 mm and a thickness of 2.0 mm was obtained.

Example 2 As an electrolytic solution, the volume ratio of 1,3-propane sultone to ethyl propyl carbonate was 7
1.0 mol / l (C ₂ H ₅ ) in a 5:25 mixed solvent
_A coin-type electric double layer capacitor was obtained in the same manner as in Example 1 except that a solution in which ₄ NBF ₄ was dissolved was used.

Example 3 Specific surface area 2 activated by molten KOH
80% by weight of phenolic resin-based activated carbon of 2,000 m ² / g,
Ethanol is added to a mixture consisting of 10% by weight of polytetrafluoroethylene and 10% by weight of carbon black, kneaded, formed into a sheet, and roll-rolled to a thickness of 0.2 mm. The obtained electrode sheet is a conductive adhesive. And bonded to an aluminum foil whose surface was etched through to obtain two electrode bodies. Next, heat treatment under reduced pressure to remove moisture etc.,
It was wound around a core having a diameter of 2 mm so that a glass fiber separator was interposed between the two electrode bodies to obtain a wound element body having a diameter of 7 mm.

In a mixed solvent of 1,3-propane sultone and methyl ethyl carbonate at a volume ratio of 70:30, 1.
An electrolytic solution in which 5 mol / l of (C ₂ H ₅ ) ₃ (CH ₃ ) PBF ₄ is dissolved is impregnated into the above element body, accommodated in a bottomed cylindrical aluminum case, and a sealing lid made of butyl rubber is used. And sealed with a caulking machine to obtain a wound type electric double layer capacitor.

Example 4 As an electrolytic solution, the volume ratio of 1,3-propane sultone to methyl ethyl carbonate was 8
1.0 mol / l (C ₂ H ₅ ) in a 0:20 mixed solvent
Except for using a solution of ₄ NBF ₄ was obtained electric double layer capacitor of the wound type in the same manner as in Example 3.

[Example 5] [0024] Instead of activated carbon activated by steam, a resole resin was fired at 650 ° C in a nitrogen atmosphere and a KOH-activated carbon material having a specific surface area of 2000 m ² / g was used. An electrode was produced in the same manner as in Example 1.

Using the above electrodes, the volume ratio of 1,3-propane sultone to methyl ethyl carbonate is 90:10.
1.5 mol / l of (C ₂ H ₅ ) ₃ (CH
₃ ) A coin-type electric double layer capacitor was obtained in the same manner as in Example 1 except that the solution in which NBF ₄ was dissolved was used as the electrolytic solution.

Example 6 As an electrolytic solution, 1.5 mol / l of (C ₂ H ₅ ) ₃ (CH ₃ ) NB was mixed in a mixed solvent of sulfolane and methyl ethyl carbonate in a volume ratio of 80:20.
Except for using a solution prepared by dissolving F ₄ in the same manner as Example 1 to obtain a coin type electric double layer capacitor.

[Example 7] As the electrolyte, the volume ratio of propylene carbonate to methyl ethyl carbonate was 80:
1.0 mol / l of (C ₂ H ₅ ) ₄ N was added to 20 mixed solvents.
A wound electric double layer capacitor was obtained in the same manner as in Example 4 except that a solution in which BF ₄ was dissolved was used.

For the electric double layer capacitors of Examples 1 to 7,
Each voltage shown in Table 1 was applied, and the initial capacitance and internal resistance were measured. Further, the rate of change in capacity after holding at 70 ° C. for 1000 hours was measured. Table 1 shows the results.

Table 1 shows that the electric double layer capacitor of the present invention has a large capacitance and a small rate of change in capacitance even when it is held at a high temperature.

[0030]

[Table 1]

[0031]

The electric double layer capacitor according to the present invention has the following features.
It has excellent characteristics of a large capacitance and a high withstand voltage of 3.3 V or more.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Katsuharu Ikeda 1150 Hazawacho, Kanagawa-ku, Yokohama-shi, Kanagawa-ken Asahi Glass Co., Ltd.

Claims (5)

[Claims] 1. An electric double layer capacitor comprising an electrode mainly composed of a carbon material as a positive electrode and a negative electrode, and an electrolytic solution forming an electric double layer at an interface with the electrode. Organic solvent containing 3-propane sultone and fourth
An electric double layer capacitor comprising a high-grade onium salt. 2. A method according to claim 1, wherein the solvent of the electrolytic solution is 1,3-propane sultone and a chain carbonate ester represented by the general formula R ¹ OC (ＯO) OR ² (where R ¹ and R ² are each monovalent) 2. The electric double layer capacitor according to claim 1, wherein the solvent is a mixed solvent containing an organic group, which may be the same or different. (3) R ¹ and R ² each independently represent a methyl group,
Ethyl group, n-propyl group, isopropyl group, or 2,
A 2,2-trifluoroethyl group, and R ¹ and R ²
3. The electric double layer capacitor according to claim 2, wherein 4. The electric double layer capacitor according to claim 2, wherein the solvent of the electrolyte contains 60 to 95% by volume of 1,3-propane sultone and 5 to 40% by volume of a chain carbonate. 5. The quaternary onium salt is R ³ R ⁴ R ⁵ R ⁶ N
⁺ Or R ³ R ⁴ R ⁵ R ⁶ P ⁺ (provided that R ³ , R ⁴ , R
⁵ and R ⁶ are each an alkyl group having 1 to 5 carbon atoms, which may be the same or different. 5. The electric double layer capacitor according to claim 1, which is a salt having a cation represented by the formula: