JPH097898A - Electrolyte for electric double-layer capacitor - Google Patents

Abstract

PURPOSE: To enhance electrolyte in electrical conductivity and electrochemical stability by a method wherein quarternary ammonium salt is dissolved as solute into non-aqueous solvent which contains aliphatic monocarboxylic acid ester whose total number of carbon atoms is four and propylene carbonate. CONSTITUTION: A mixed solvent is composed of 10 to 50wt.% aliphatic monocarboxylic acid ester and 90 to 50wt.% propylene carbonate. Quarternary ammonium salt such as triethylmethylammonium borofluoride or triethylmethylammonium phosphorofluoride is used, and either or a mixture of them is made to serve as solute. Quarternary ammonium salt as solute is set to 0.5 to 2.0 in molar concentration. By this setup, electrolyte of this constitution is enhanced in electric conductivity.

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 for an electric double layer capacitor used for memory backup of various electronic devices and for power of electric vehicles requiring a large current.

[0002]

2. Description of the Related Art An electric double layer capacitor is an element for accumulating charges in an electric double layer formed at the interface between a polarizable electrode and an electrolytic solution (Japanese Patent Laid-Open No. 7-74059). Usually, an electric double layer capacitor is formed by press-molding activated carbon particles, coating a mixture with an appropriate binder on a collector metal, or plasma-spraying aluminum on activated carbon fibers with a polarizable electrode. , The two polarizable electrodes are opposed to the electrolytic solution through the separator,
It has a sealed structure in the case.

The electrolytic solution used in this type of electric double layer capacitor includes an aqueous electrolytic solution such as sulfuric acid or potassium hydroxide aqueous solution and a non-aqueous electrolytic solution obtained by dissolving a quaternary ammonium salt in an organic solvent such as propylene carbonate. Is disclosed in JP-B-55-41015. Aqueous electrolyte has high electric conductivity, but its decomposition voltage is low, so it is necessary to make a series connection in order to obtain an element with high withstand voltage.
There was a problem in miniaturization. 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 electrolyte for an electric double layer capacitor, a quaternary ammonium borofluoride salt which is a solute in a propylene carbonate solvent (Tanabashi et al., Electrochemical, 56
Vol., Page 892, 1988) or quaternary phosphonium borofluoride salts (Hiratsuka et al., Electrochemistry, vol. 59, page 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.

[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 provides an electrolytic solution used in a capacitor that utilizes an electric double layer formed at the interface between a polarizable electrode and the electrolytic solution, in which the total number of carbon atoms represented by formula (I) is 4 aliphatic monocarboxylic acid ester (a) 1
0-50% by weight

## STR2 ## R ¹ COOR ² (I) (wherein, R ¹ represents a hydrogen or carbon number is 1 or 2 saturated alkyl group, R ² is a straight chain or branched chain of 1 to 3 carbon atoms A saturated alkyl group) and 90 to 50% by weight of propylene carbonate (b) in a non-aqueous solvent containing a solute quaternary ammonium salt dissolved therein. is there.

[0007]

[Function] By mixing propylene carbonate, which has a high dielectric constant, with aliphatic monocarboxylic acid ester, which has a low viscosity, to improve the ion mobility without significantly lowering the ion dissociation degree of the solute quaternary ammonium salt. Therefore, an electrolytic solution having high electric conductivity can be obtained.

(Outline of the Invention) Aliphatic monocarboxylic acid ester (a) Specific examples of the aliphatic monocarboxylic acid ester (a) represented by the above formula (I) used as a solvent include propyl formate, isopropyl formate, Examples thereof include ethyl acetate and methyl propionate. These may be used alone or in combination of two or more. Mixed solvent The mixed solvent is 10 to 50% by weight, preferably 20 to 40% by weight of the aliphatic monocarboxylic acid ester (a) and 90 to 5% by weight.
0% by weight, preferably 80-60% by weight of propylene carbonate (b). When the amount of the aliphatic monocarboxylic acid ester in the mixed solvent is less than 10% by weight, improvement in electric conductivity cannot be expected, and when it exceeds 50% by weight, solute precipitation is observed.

[0009] quaternary ammonium salts used as a solute solute has borofluoride triethylmethylammonium and Rinfu' of triethyl methyl ammonium, which may be used alone or in combination two or more. The amount of solute quaternary ammonium salt dissolved in the electrolytic solution is 0.5 to 2.0.
It is a molar concentration (mol / l).

[0010]

The present invention will be described more specifically with reference to the following examples. Example 1 1 molar concentration of triethylmethylammonium borofluoride was dissolved in a mixed solvent of 40% by weight of methyl propionate and 60% by weight of propylene carbonate, 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. is 15.5 mS / c.
m. Also, using a glassy carbon electrode,
At a scanning potential velocity of 5 mV / sec, the decomposition potential when a current of 1 mA / cm ² flows at the time of polarization measurement is −3.0 V on the reducing side and +3.4 V on the oxidizing side with respect to the saturated calomel reference electrode (SCE). there were.

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 propylene carbonate was used alone instead of the mixed solvent. Comparative Example 2 An electrolytic solution was prepared in the same manner as in Example 1 except that tetraethylammonium borofluoride was used instead of triethylmethylammonium borofluoride, but the solute was not completely dissolved.

Example 2 FIG. 1 shows changes in the electric conductivity of the electrolytic solution in Example 1 when the mixing ratio of methyl propionate and propylene carbonate was changed. Example 3 An electrolytic solution having the physical properties shown in Table 1 was obtained in the same manner as in Example 1 except that triethylmethylammonium borofluoride was used instead of triethylmethylammonium borofluoride.

Examples 4 to 5 An electrolytic solution having the physical properties shown in Table 1 was obtained in the same manner as in Example 1 except that ethyl acetate or propyl formate was used instead of methyl propionate.

[0014]

[Table 1]

[0015]

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

[Brief description of drawings]

FIG. 1 is a diagram showing the correlation between the electrical conductivity and the ratio of methyl propionate in a mixed solvent (methyl propionate and propylene carbonate) in an electrolytic solution.

Claims (3)

[Claims] 1. An electrolytic solution used in a capacitor, which uses an electric double layer formed at an interface between a polarizable electrode and an electrolytic solution, wherein an aliphatic monocarboxylic acid represented by the formula (I) and having a total carbon number of 4 is used. 10 to 50% by weight of ester (a) R ¹ COOR ² (I) (wherein R ¹ is hydrogen or a saturated alkyl group having 1 or 2 carbon atoms, and R ² is 1 carbon atom). No. 3 to a linear or branched saturated alkyl group) and a nonaqueous solvent containing 90 to 50% by weight of propylene carbonate (b), and an electric double layer obtained by dissolving a solute quaternary ammonium salt. Electrolyte for capacitors. 2. The electrolytic solution for an electric double layer capacitor according to claim 1, wherein the quaternary ammonium salt is selected from triethylmethylammonium borofluoride and triethylmethylammonium phosphofluoride. 3. 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.