Electricity: electrical systems and devices – Electrolytic systems or devices – Liquid electrolytic capacitor
Reexamination Certificate
2002-12-10
2004-03-23
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Liquid electrolytic capacitor
C361S502000, C361S503000, C361S523000, C361S528000, C429S129000
Reexamination Certificate
active
06710999
ABSTRACT:
The present invention relates to an electric double layer capacitor, particularly to an electric double layer capacitor having a low resistance, a high withstand voltage and excellent reliability.
As a shape of a conventional electric double layer capacitor, there may be a coin type wherein an element having a separator sandwiched between a pair of polarized electrodes composed mainly of activated carbon formed on current collectors, is accommodated together with an electrolytic solution in a metal casing, which is then sealed by a metal cover via a gasket, or a cylindrical type wherein an element having a pair of polarized sheet electrodes wound with a separator interposed therebetween, is accommodated together with an electrolytic solution in a metal casing, which is then sealed so that the electrolytic solution will not evaporate from an opening of the casing.
Further, as one for a large current and large capacitance, a lamination type electric double layer capacitor has also been proposed wherein an element having many polarized sheet electrodes laminated via a separator disposed therebetween, is incorporated (JP-A-4-154106, JP-A-3-203311, JP-A-4-286108). Namely, rectangular polarized sheet electrodes are used as a positive electrode and a negative electrode, and they are alternately laminated with a separator interposed therebetween, to form an element, which is then accommodated in a casing in such a state that a positive electrode lead member and a negative electrode lead member are connected by caulking to the terminals of the positive and negative electrodes, respectively, then the element is impregnated with an electrolytic solution, and the casing is closed with a cover.
As an electrolytic solution for a conventional electric double layer capacitor, not only an aqueous type electrolytic solution containing a mineral acid such as sulfuric acid, an alkali metal salt or an alkali, but also various non-aqueous electrolytic solutions have been used. As the solvent for such non-aqueous electrolytic solutions, propylene carbonate, &ggr;-butyrolactone, acetonitrile, dimethyl formamide (JP-A-49-068254) or a sulfolane derivative (JP-A-62-237715), has been known. When the withstand voltages are compared, the aqueous type electrolytic solution has a withstand voltage of 0.8 V, while the non-aqueous electrolytic solution has a withstand voltage of from 2.5 to 3.3 V. The electrostatic energy of a capacitor corresponds to the square of the withstand voltage. Accordingly, from the viewpoint of the electrostatic energy, the non-aqueous type electrolyte is more advantageous. However, such a solvent having a high dielectric constant, in which an electrolyte is highly soluble, usually has a high viscosity and has a problem such that high electrical conductivity can not be obtained although an electrolytic solution having a high electrolyte concentration can be obtained. On the other hand, a chain ether, a chain ester or a chain carbonate, which is a low viscosity solvent, has a low dielectric constant and thus has a problem that it is substantially incapable of dissolving an electrolyte, whereby high electrical conductivity can not be obtained.
Propylene carbonate which is commonly used as a solvent for an electrolytic solution for an electric double layer capacitor, has a high dielectric constant and is a solvent, of which the viscosity is not relatively high. Thus, it is preferably employed. However, it has had a problem that propylene carbonate is likely to decompose when a voltage of about 2.9 V or higher is applied. Whereas, sulfolane which is electrochemically stable, has a high withstand voltage and will not be decomposed until a voltage as high as 3.4 V is applied, but it has had problems such that the solubility of an electrolyte is low, and the melting point is high.
Under these circumstances, it is an object of the present invention to study particularly an electrolytic solution to solve the above problems of the prior art and to provide an electric double layer capacitor having a high withstand voltage, a low resistance and excellent reliability.
The present invention provides an electric double layer capacitor having a pair of polarized electrodes and an electrolytic solution capable of forming an electric double layer at the interface with the polarized electrodes, wherein the electrolytic solution comprises a salt of the Formula 1 as the electrolyte and at least dimethyl carbonate as a solvent:
R
1
R
2
R
3
R
4
N
+
X
−
Formula 1
wherein R
1
is a n-propyl group and each of R
2
, R
3
and R
4
which are independent of one another, is a methyl group or an ethyl group, provided that two selected from R
1
to R
4
may together form a tetramethylene group, and X
−
is an anion.
Heretofore, as the cation of an electrolyte for an electrolytic solution for an electric double layer capacitor, (C
2
H
5
)
4
N
+
, (C
2
H
5
)
4
P
+
or the like, which has high electrical conductivity, has been used. Also, (C
2
H
5
)
3
(CH
3
)N
+
, which has a high solubility in a solvent, has been proposed, but no adequate solubility to a solvent having a low dielectric constant and low viscosity, has been obtained. Among solvents having a low dielectric constant and low viscosity, a chain carbonate is electrochemically stable, but dimethyl carbonate, ethylmethyl carbonate or the like has had a problem that it can not substantially dissolve an electrolyte by itself as a single solvent.
The electrolyte to be used for the electrolytic solution for the electric double layer capacitor of the present invention is a chain quaternary ammonium cation represented by R
1
R
2
R
3
R
4
N
+
, wherein R
1
is a n-propyl group, and each of R
2
, R
3
and R
4
which are independent of one another, is a methyl group or an ethyl group. Here, two selected from R
1
to R
4
may together form a tetramethylene group to provide a cyclic pyrrolidinium cation (each of the remaining two groups is the monovalent alkyl group as defined above).
Such a cation is electrochemically stable, and the electrolyte having such a cation has high solubility to dimethyl carbonate as a solvent having a low dielectric constant and a low viscosity. Accordingly, when a solution having such an electrolyte dissolved in dimethyl carbonate or in a solvent mixture containing dimethyl carbonate, is used as an electrolytic solution, it is possible to provide an electric double layer capacitor which is excellent in durability even when used for a long period of time and which thus has high reliability.
Among cations represented by R
1
R
2
R
3
R
4
N
+
in the Formula 1, a cation represented by any one of the following Formulae 2 to 6 may be mentioned as a specific preferred example:
(C
3
H
7
)(C
2
H
5
)
3
N
+
Formula 2
(C
3
H
7
)(C
2
H
5
)
2
(CH
3
)N
+
Formula 3
(C
3
H
7
)(C
2
H
5
)(CH
3
)
2
N
+
Formula 4
Among them, particularly preferred is a cation represented by any one of the Formulae 3 to 6, which contains three types of groups among a tetramethylene group, a n-propyl group, an ethyl group and a methyl group. The salt having a cation containing such three types of groups, has a high solubility to a solvent and can thus be highly concentrated, as compared with a salt having a cation containing two types of groups, and high electrical conductivity can be realized by making the electrolytic solution to be highly concentrated.
Further, if the carbon chain of the group bonded to the nitrogen atom is long, the solubility to a solvent increases, and the concentration can be made high, such being preferred. On the other hand, if the carbon chain of the group bonded to the nitrogen atom is long, the ion radius becomes large, and such ions tend to hardly enter into fine pores of the electrodes, whereby the amount of ions forming an electric double layer per unit volume, will be small, and from such a viewpoint, the shorter the carbon chain, the better. Specifically, particularly preferred is a salt having a cation represented by any one of the Formulae 2
Hiratsuka Kazuya
Ikeda Katsuji
Kawasato Takeshi
Yoshida Naoki
Asahi Glass Company Limited
Ha Nguyen T.
Reichard Dean A.
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