JPH08298230A - Electrolyte for electric double-layer capacitor - Google Patents

Abstract

PURPOSE: To obtain an electrolyte having a high electric conductivity by a method wherein a quaternary ammonium salt, as a solute, is dissolved into a non-aqueous mixed solvent which is formed by mixing a sulfolane compound and a chain-like carbonate. CONSTITUTION: This electric double-layer capacitor electrolyte, utilizing an electric double-layer which is formed on the interface between a polarized electrode and an electrolyte, is formed by chain-like carbonate of 10 to 80wt.% and quaternary ammonium salt of solute dissolved into a non-aqueous solvent of 20 to 90wt.% containing sulfolane compound, indicated by formula I (R in the formula is a methyl radical). Also, a sulfolane compound, containing one or more kinds selected from sulfolane and 3-methyl sulfolane, is contained in the electrolyte. Besides, the quaternary ammonium salt, containing one or more kinds selected from borofluoric triethylmethyl ammonium, borofluoric tetraethyl ammonium, phosphofluoride triethylmethyl ammonium and phosphofluoride tetraethyl ammonium, is contained in the electrolyte.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic solution having a high electric conductivity which is used inside an electric double layer capacitor used for memory backup of various electronic devices and for power of electric vehicles requiring a large current. Regarding

[0002]

2. Description of the Related Art An electric double layer capacitor is an element for accumulating charges in an electric double layer generated at the interface between a polarizable electrode and an electrolytic solution. Usually, an electric double layer capacitor is obtained by press-molding activated carbon particles, coating a mixture with an appropriate binder on a current collector metal, or plasma-spraying aluminum on activated carbon fibers with a polarizable electrode. The two polarizable electrodes are opposed to each other via the electrolytic solution via the separator and are hermetically sealed in the case.

The electrolytic solution used in this type of electric double layer capacitor includes an aqueous electrolytic solution such as an aqueous solution of sulfuric acid or potassium hydroxide and a non-aqueous electrolytic solution obtained by dissolving a quaternary ammonium salt in an organic solvent such as propylene carbonate. The liquid is disclosed in JP-B-55-41015. Although the aqueous electrolytic solution has a high electric conductivity, it has a low decomposition voltage. Therefore, in order to obtain hydrogen having a high withstand voltage, it is necessary to make a series connection in a laminated manner, and there is a difficulty in downsizing. On the other hand, the non-aqueous electrolyte has an advantage that it can be miniaturized because of its high decomposition voltage, but it has a drawback that the internal resistance of the element becomes high and a large current cannot be taken out because of its low electric conductivity.

Further, as a non-aqueous electrolytic solution for an electric double layer capacitor, a quaternary ammonium borofluoride salt in propylene carbonate solvent (Tanabashi et al., Electrochemistry, Vol. 56, 89)
2 (1988) or quaternary phosphonium borofluoride salt (Hiratsuka et al., Electrochemistry, Vol. 59, 209, 1991).
Has been put into practical use. However, the electric conductivity and electrochemical stability of these electrolytes are not yet sufficient, and development of an electrolyte having high electric conductivity and electrochemical stability is desired, and further, sulfolane or 3-methyl is used as a solvent. An electric double layer capacitor using sulfolane and a quaternary phosphonium salt as a solvent has been proposed (Sanada et al., Electrochemistry, Vol. 61, pp. 448, 1993), but has a problem that the electric conductivity of the electrolytic solution is low. was there.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an electrolytic solution for an electric double layer capacitor which exhibits high electric conductivity and electrochemical stability.

[0006]

The present invention relates to an electrolytic solution for use in a capacitor that utilizes an electric double layer formed at the interface between a polarizable electrode and the electrolytic solution. %, And 90 to 20% by weight of the sulfolane compound (b) represented by the formula (I)

[0007]

Embedded image

[In the formula, R is H or a methyl group. ]
The present invention provides an electrolytic solution for an electric double layer capacitor in which a solute quaternary ammonium salt is dissolved in a non-aqueous solvent containing.

[0009]

By mixing a sulfolane compound which is solid at room temperature but has a high dielectric constant with a chain carbonate having a low viscosity, a mixed solvent which is liquid at room temperature can be obtained. By using this mixed solvent, without significantly lowering the degree of ionic dissociation of the quaternary ammonium salt of the solute,
By improving the ion mobility, an electrolytic solution having high electric conductivity can be obtained.

(Summary of the Invention) Chain Carbonate Preferred chain carbonates are dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate, which may be used alone or in combination of two or more. Sulfolane compound Sulfolane is used as a sulfolane compound (melting point: 28
C.) and 3-methylsulfolane (melting point: 6.degree. C.) are used alone or in combination of two or more.

Mixed solvent The mixed solvent is 10 to 80% by weight of chain carbonate and 90%.
-20% by weight of sulfolane compound. If the amount of the chain carbonate in the mixed solvent is less than 10% by weight, improvement in electric conductivity cannot be expected, and if it exceeds 80% by weight, solute precipitation is observed.

Solutes The quaternary ammonium salts used as solutes include triethylmethylammonium borofluoride, tetraethylammonium borofluoride, triethylmethylammonium phosphorofluoride and tetraethylammonium phosphorofluoride, which may be used alone or in admixture of two or more. Used. The amount of solute quaternary ammonium salt dissolved in the electrolytic solution is 0.5 to 2.0 molar concentration (mol / 1)
Is.

[0013]

EXAMPLES The present invention will be described more specifically below with reference to examples. Example 1 1 molar concentration of triethylmethylammonium borofluoride was dissolved in a mixed solvent of 60% by weight of dimethyl carbonate and 40% by weight of sulfolane, and dehydrated (30 ppm or less) by heating under reduced pressure to obtain an electrolytic solution. The electric conductivity of this electrolytic solution at 25 ° C. was 10.0 mS / cm. Also, using a glassy carbon electrode, 5 mV / s
1 mA / c when polarization was measured at a scanning potential velocity of ec
The decomposition potential when a current of m ² flows was −3.1 V on the reduction side and +3.3 on the oxidation side with respect to the saturated calomel reference electrode (SCE).
It was 4V.

Comparative Example 1 An electrolytic solution having the physical properties shown in Table 1 was obtained in the same manner as in Example 1 except that sulfolane was used alone instead of the mixed solvent. Examples 2 and 3 In Example 1, instead of dimethyl carbonate,
An electrolytic solution having the physical properties shown in Table 1 was obtained in the same manner except that ethyl methyl carbonate or diethyl carbonate was used.

Example 4 FIG. 1 shows the change in electric conductivity of the electrolytic solution in Example 1 when the mixing ratio of dimethyl carbonate and sulfolane was changed. Example 5 An electrolytic solution having physical properties shown in Table 1 was obtained in the same manner as in Example 1 except that tetraethylammonium phosphofluoride was used instead of triethylmethylammonium borofluoride.

Comparative Example 2 An electrolytic solution having the physical properties shown in Table 1 was obtained in the same manner as in Example 5, except that sulfolane was used alone instead of the mixed solvent. Example 6 An electrolytic solution having the physical properties shown in Table 1 was obtained in the same manner as in Example 1 except that 3-methylsulfolane was used instead of sulfolane. Comparative Example 3 An electrolytic solution having the physical properties shown in Table 1 was obtained in the same manner as in Example 6 except that 3-methylsulfolane was used alone instead of the mixed solvent.

[0017]

[Table 1]

[0018]

EFFECT OF THE INVENTION An electrolytic solution showing high electric conductivity.

[Brief description of drawings]

FIG. 1 is a diagram showing a change in electric conductivity when a ratio of a mixed solvent is changed.

Claims (4)

[Claims] 1. An electrolytic solution used in a capacitor that utilizes an electric double layer formed at an interface between a polarizable electrode and an electrolytic solution, wherein the chain carbonate (a) is 10 to 80% by weight.
And a sulfolane compound (b) represented by the formula (I) 90-
20% by weight [In the formula, R is H or a methyl group. ] An electrolytic solution for an electric double layer capacitor in which a solute quaternary ammonium salt is dissolved in a non-aqueous solvent containing. 2. The electrolytic solution according to claim 1, wherein the sulfolane compound is one or more selected from sulfolane and 3-methylsulfolane. 3. The electrolytic solution according to claim 1, wherein the quaternary ammonium salt is at least one selected from triethylmethylammonium borofluoride, tetraethylammonium borofluoride, triethylmethylammonium phosphorofluoride and tetraethylammonium phosphorofluoride. 4. The amount of solute quaternary ammonium salt dissolved in the electrolytic solution is 0.5 to 2.0 molar.
The electrolytic solution for an electric double layer capacitor described.