JP2000331887A - Electrolyte solution for electrochemical capacitor and electrochemical capacitor using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a superior withstanding voltage and a long type reliability by specifying the glycol content in an electrochemical capacitor electrolyte soln. SOLUTION: A mixture of coconut husk type activated carbon powder obtained by activating a carbonaceous substance with steam, acetylene black, and polytetrafluoroethylene is kneaded, pressure formed into a disc compact to be a polarizable electrode 2. This forming step is repeated to obtain another polarizable electrode identical in compsn. and shape, the obtained two compacts are dried and left cold and impregnated with an electrolyte soln. A separator 3 is sandwiched between the two polarizable electrodes 2 impregnated with the electrolyte soln., and all are caulked and sealed through a gasket in a case to obtain an electrochemical capacitor. The propylene glycol content of the electrolyte soln. contg. trimethyl-ethyl ammonium tetrafluoroborate dissolved in a propylene carbonate solvent is 100 ppm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrolytic solution for an electrochemical capacitor and an electrochemical capacitor using the same. More specifically, the present invention relates to an electrolytic solution for an electrochemical capacitor in which a glycol contained as an impurity in a nonaqueous electrolytic solution is reduced to an extremely small amount, and an electrochemical capacitor using the same. INDUSTRIAL APPLICABILITY The electrochemical capacitor of the present invention is useful for memory backup of various electronic devices and power for electric vehicles and the like that require a large current.

[0002]

2. Description of the Related Art In addition to the conventional electric double layer capacitor using only a polarizable electrode and an electric double layer formed in an electrolytic solution, an electrochemical capacitor also includes a pseudo capacitance due to oxidation and reduction of electrodes together with an electric double layer capacitance. Supercapacitors incorporated as power storage elements are also included (BE Conway, J.A.
Electrochem. Soc. , 183, 153
9, 1991). A normal electric double layer capacitor is
Pressed activated carbon particles, kneaded with a suitable binder and coated on the current collector metal, or activated carbon fiber with plasma radiation of aluminum as the polarizable electrode, these two polarizable It has a structure in which electrodes are opposed to an electrolytic solution via a separator and sealed in a case. On the other hand, it has been proposed to use an oxide such as nickel or ruthenium or a conductive polymer such as polypyrrole or polythiophene as an electrode for a supercapacitor using a pseudo capacitance (A. Rudge).
Et al., Electrochim. Acta, 39, 27
3, p. 1994). FIG. 1 is a cross-sectional view of a general electrochemical capacitor, which is a preferred embodiment of the present invention. In FIG. 1, 1 is an electrode, 2 is a current collector, and 3 is a separator. The electrolyte is impregnated in the electrodes and the separator.

An electrolytic solution used for this type of electrochemical 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 or the like in an organic solvent such as propylene carbonate. A liquid is known (JP-B-55-41015). Electrochemical capacitors using non-aqueous electrolytes can increase withstand voltage,
There is a feature that the energy density can be higher than that of an electrochemical capacitor using an aqueous electrolyte. These are rapidly spreading as backup power supplies for consumer electronic devices. Particularly, in recent years, electrochemical capacitors for power use such as electric vehicles having a capacitance of 50 F or more, which have attracted attention, include:
Those using a non-aqueous electrolyte are suitable. As a non-aqueous electrolyte for an electrochemical capacitor, a quaternary ammonium borofluoride salt (Tanahashi et al., Vol. 56, p. 892, 1988) or a quaternary phosphonium fluorinated salt (Hiratsuka et al. Electrochemistry, Vol. 59,
209 (1991, 1991).

[0004]

However, electrochemical capacitors using these electrolytes often lack their withstand voltage and long-term reliability. An object of the present invention is to provide an electrolytic solution for an electrochemical capacitor having excellent withstand voltage and long-term reliability, and an electrochemical capacitor using the same.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in view of such circumstances, and as a result, the cause is that impurities, particularly glycols, are contained in the electrolytic solution. As a result, it has been found that a reduction in the withstand voltage and a deterioration in the capacitance of the capacitor can be suppressed, and the present invention has been completed. It has not been known that such impurities affect the performance of an electrochemical capacitor, particularly an electric double layer capacitor.

As an electrolyte for an electrochemical capacitor, cyclic carbonates such as propylene carbonate and ethylene carbonate are preferably used because of their high electric conductivity and decomposition voltage and wide usable temperature range. However, these cyclic carbonates usually contain dihydric alcohols (glycols) such as propylene glycol, ethylene glycol, and diethylene glycol as impurities, and an electrolytic solution obtained by dissolving a solute in a non-aqueous solvent containing cyclic carbonate as a main component. Will naturally contain glycol. Cyclic carbonates are generally synthesized from oxides or glycols, such as 1) reacting oxides with carbon dioxide at high temperature and pressure, and 2) reacting glycols with cyclic carbonates under catalytic conditions. For example, if propylene carbonate is used, propylene glycol is used. In the case of ethylene carbonate, ethylene glycol and diethylene glycol are included as inevitable impurities. It is desirable that such a glycol can be removed as much as possible in the production process of the cyclic carbonate. However, in the production method described above, a considerable amount (about 200 to 20,000 ppm) of the glycol is contained in the product. Glycol has two hydroxyl groups in one molecule, but the hydroxyl groups are electrochemically unstable, causing a decrease in withstand voltage and a decrease in long-term reliability.

The gist of the present invention is as follows. In the electrolytic solution for an electrochemical capacitor, in which an electrolyte is dissolved in a non-aqueous solvent containing at least a cyclic carbonate as a main component, the glycol content is 100 ppm or less. Using the electrolytic solution for the electrochemical capacitor described,
And at least one of the positive electrode and the negative electrode is a polarizable electrode containing a carbonaceous substance as a main component.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (Electrolytic Solution for Electrochemical Capacitor) The electrolytic solution of the present invention must have a glycol content of 100 ppm or less in the electrolytic solution. 100 ppm glycol content
In the case where it exceeds, the withstand voltage may decrease or the capacity may deteriorate. The glycol content in the electrolyte is preferably 50 ppm or less, more preferably 20 ppm or less,
In particular, it is preferably at most 10 ppm, more preferably at most 5 ppm. In order to reduce the amount of glycol in the electrolytic solution to the above range, for example, a method of reducing the amount of glycol in the cyclic carbonate used for the solvent, the amount of water in the electrolytic solution is reduced, and the amount of glycol in the aqueous solution is reduced by hydrolysis of the cyclic carbonate. There is a method to suppress the increase. These methods may be performed alone or in combination.

As a method for reducing the glycol in the cyclic carbonate, there are, for example, a method of precision distillation of the cyclic carbonate to be used, and a method of performing an adsorption treatment with an adsorbent such as silica gel, activated carbon, activated alumina, and special molecular sieve. To perform precision distillation, setting conditions such as a reflux ratio, a rectification temperature, and a degree of reduced pressure are important. On the other hand, the method of performing the adsorption treatment is preferable in that the operation is easy. The extent to which the glycol content can be reduced by the adsorption treatment depends on the type of adsorbent used and the treatment conditions. The method of precision distillation and the method of adsorption treatment may be performed alone or in combination. Further, as a method for reducing the water content in the electrolyte, for example, a method of dissolving a solute that has been sufficiently dried in advance in a nonaqueous solvent that has been sufficiently dehydrated in advance,
A method in which a solution obtained by dissolving a solute in a non-aqueous solvent is heated under reduced pressure to evaporate and remove a trace amount of water contained therein,
A method in which a solution obtained by dissolving a solute in a non-aqueous solvent is subjected to an adsorption treatment with an adsorbent such as molecular sieve to remove water, and the like. These methods may be performed alone or in combination.

[0010] The non-aqueous solvent of the electrolytic solution according to the present invention contains at least a cyclic carbonate as a main component. In order to make use of the characteristics of the cyclic carbonate described above, it is preferable that the entire amount of the non-aqueous solvent is a cyclic carbonate, but it may be mixed with another non-aqueous solvent. As a specific example of the cyclic carbonate,
Examples include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Particularly preferred are ethylene carbonate and propylene carbonate. These may be used alone or in combination of two or more. The non-aqueous solvent to be mixed with the cyclic carbonate is not particularly limited, but specific examples thereof include chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, and aliphatic monocarbonates such as methyl acetate and methyl propionate. Carboxylic acid esters, γ-butyrolactone,
γ-valerolactone, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile, N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidinone, N-methyloxazolidinone, N, N '-Dimethylimidazolidinone, nitromethane, nitroethane,
Sulfolane, dimethyl sulfoxide, trimethyl phosphate and the like can be mentioned. Two or more of these may be used.

Further, as a solute of the electrolytic solution according to the present invention,
For example, conventionally known BF ₄ ⁻ , PF ₆ ⁻ , As
F ₆ ⁻ , SbF ₆ ⁻ , etc., fluoride ion, N (CF ₃ S
O ₃ ) ²⁻ , ClO ₄ ⁻ , RfSO ₃ ⁻ (Rf has 1 carbon atom)
To 8 fluoroalkyl groups), anions such as C (CF ₃ SO ₃ ) ^3- and quaternary ammonium ions, quaternary phosphonium ions, imidazolinium ions, imidazolium ions, pyrrolidinium ions, inorganic acids and organic acids A salt formed by combining with a cation such as an alkali metal ion or an alkaline earth metal ion or an ammonium ion of an acid,
They can be used alone or in combination of two or more, and are not limited thereto. Specific examples of the anionic component include boric acid, carbonic acid, silicic acid, phosphoric acid, phosphorous acid, hypophosphorous acid, nitric acid, sulfuric acid, sulfurous acid, thiocyanic acid, cyanic acid, borofluoric acid, hydrophosphoric acid, and arsenic fluoride. Hydrogen acid, antimony hydrofluoric acid, inorganic acids such as perchloric acid, and anions of organic acids such as methanesulfonic acid, benzenesulfonic acid, and trifluoromethanesulfonic acid can be exemplified. Hydrofluoric acid, phosphoric hydrofluoric acid, arsenic hydrofluoric acid, and antimony hydrofluoric acid are particularly preferable, and borohydrofluoric acid and phosphoric hydrofluoric acid are particularly preferable. On the other hand, specific examples of the cation component include lithium, sodium, potassium, a quaternary ammonium ion represented by the following general formula (I), and a quaternary phosphonium ion represented by the following general formula (II). be able to.

[0012]

Embedded image

(In the formula, R ¹ , R ² , R ³ and R ⁴ each independently represent a C 1 -C 10 which may have a substituent.
Represents a hydrocarbon group, or may be bonded to each other directly or via a nitrogen atom to form a ring)

[0014]

Embedded image

(Wherein R ¹ , R ² , R ³ and R ⁴ each independently represent a hydrocarbon group having 1 to 10 carbon atoms, or may combine with each other to form a ring) )

In the formula (I), the hydrocarbon group which may have a substituent has 1 to 10, preferably 1 to 4, carbon atoms. Specific examples thereof include a methyl group, an ethyl group and a propyl group. Butyl group, benzyl group and the like. Specific examples of the ring include, for example, a cyclohexyl group, a piperidyl group, a pyrrolidyl group, a pyridyl group, an imidazolyl group, and the like.

Specific examples of the group represented by the formula (I) include, for example, tetramethylammonium, triethylmethylammonium, diethyldimethylammonium, ethyltrimethylammonium, tetraethylammonium, tetrabutylammonium, N, N-dimethylpyrrolidone. Dinium, N-ethyl-N-methylpyrrolidinium, N, N-dimethylpiperidinium, benzyltrimethylammonium, N-ethylpyridinium,
N'-dimethylimidazolium and the like. Of these, triethylmethylammonium and tetraethylammonium are preferred.

In the formula (II), the hydrocarbon group has 1 to 10, preferably 1 to 4, carbon atoms. Specific examples thereof include a methyl group, an ethyl group, a propyl group, a butyl group and a hexyl group. And the like. Specific examples of the group represented by the formula (II) include, for example, tetramethylphosphonium, triethylmethylphosphonium, tetraethylphosphonium, tetrapropylphosphonium, tetrabutylphosphonium ion and the like.

In the combination of the above-mentioned solvent and solute, the cyclic carbonate solvent used alone or in the mixed solvent used in the present invention dissolves only a small amount of an alkali metal salt or an ammonium salt. It is preferable to use quaternary ammonium salts which are easily available. The content of the solute in the electrolyte is suitably from 0.1 to 3 mol / l, and particularly preferably from 0.5 to 2 mol / l. If the concentration is too low, the internal resistance increases due to the low conductivity of the electrolytic solution.
On the other hand, if the temperature is too high, salts may precipitate at low temperatures, which may cause problems. The water content in the electrolyte is 300ppm
Or less, preferably 100 ppm or less, more preferably 3 ppm or less.
It is 0 ppm or less. If the water content exceeds 300 ppm, the electrochemical stability decreases.

(Electrochemical capacitor) Electricity according to the present invention
The main component of the polarizable electrode of a chemical capacitor is
Is electrochemically inert and has moderate electrical conductivity.
Carbonaceous materials are preferred because
BET by nitrogen adsorption method
The specific surface area obtained by the method is 10 m^Two/ G or more of porous charcoal
It is preferable to use a substance. Of porous carbonaceous material
Specific surface area is the capacitance per unit area due to carbonaceous species
(F / m^Two), Such as a decrease in bulk density accompanied by an increase in specific surface area
BET method using nitrogen adsorption method
The specific surface area determined by 30 to 2500 m ^Two/ G is preferred
In particular, the specific surface area is 300 to 2300 m^Two/ G activity
Sex charcoal has a large capacitance per volume and is preferable. granular
In the case of carbonaceous materials, the bulk density of the electrode is improved,
In terms of reduction, the average particle diameter is preferably 30 μm or less.
No.

The polarizable electrode body mainly composed of a carbonaceous substance is
Both are composed of a carbonaceous material, a conductive agent and a binder material. The electrode body can be formed by a conventionally known method. For example, it can be obtained by adding and mixing polytetrafluoroethylene to a mixture of a carbonaceous substance and acetylene black, followed by press molding. Also,
After a carbonaceous material and a binder material such as pitch, tar, and phenol resin are mixed and molded, a heat treatment is performed in an inert atmosphere to obtain a sintered body. Furthermore, it is also possible to obtain a polarizable electrode by sintering only a carbonaceous substance without using a conductive agent and a binder. Alternatively, a polarizable electrode can be obtained by sintering a carbonaceous substance and a binder without using a conductive agent. The electrode is a thin coating film, a sheet-like or plate-like molded body,
Further, it may be any of plate-like molded bodies made of a composite.

The conductive agent used in the electrode body is a group consisting of carbon black such as acetylene black and Ketjen black, natural graphite, thermally expanded graphite, carbon fiber, and metal fiber such as ruthenium oxide, titanium oxide, aluminum and nickel. At least one kind of conductive agent selected from the above is preferable. Acetylene black and Ketjen black are particularly preferred in that the conductivity is effectively improved with a small amount.For example, when the carbonaceous substance is activated carbon, the compounding amount with activated carbon differs depending on the bulk density of activated carbon but is too large. Because the ratio of activated carbon decreases and the capacity decreases, the weight of activated carbon is reduced by 5%.
~ 50%, particularly preferably about 10-30%.

Examples of the binder material include polytetrafluoroethylene, polyvinylidene fluoride, carboxymethylcellulose, fluoroolefin copolymer crosslinked polymer, polyvinyl alcohol, polyacrylic acid, polyimide, petroleum pitch, coal pitch, and phenol resin.
It is preferable to use at least one kind. The current collector of the electrochemical capacitor according to the present invention is not particularly limited as long as it has electrochemical and chemical corrosion resistance, and is not particularly limited.For example, the positive electrode includes stainless steel, aluminum, titanium, and tantalum, and the negative electrode includes , Stainless steel, nickel, copper and the like are preferably used. The separator of the electrochemical capacitor according to the present invention is preferably a material having a small thickness, a high electronic insulating property and a high ion permeability, and is not particularly limited. For example, a nonwoven fabric such as a polyethylene or polypropylene material is preferable. Used for

[0024]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the invention. The content (by weight) of glycol in the electrolyte was measured by gas chromatography.

Example 1 1.5 mol / mol of purified propylene carbonate
The propylene glycol content of the electrolytic solution obtained by dissolving 1 liter of triethylmethylammonium tetrafluoroborate was 3 ppm. About this electrolyte,
To evaluate the withstand voltage, platinum was used for the working electrode and the counter electrode, and a voltage scan was performed at a sweep speed of 5 mV / sec using silver / silver perchlorate as the reference electrode.
The voltage at the time when A / cm ² current flows is the oxidation potential,
The reduction potential was used. The larger the difference, the higher the withstand voltage. Table 1 shows the results. On the other hand, in order to evaluate the performance as an electrochemical capacitor, it was manufactured as follows. 80% by weight of coconut shell-based activated carbon powder (specific surface area: 1700 m ² / g, average particle diameter: 10 μm) obtained by steam activation of a carbonaceous substance, acetylene black 10
After kneading a mixture consisting of 10% by weight of polytetrafluoroethylene and pressurizing the mixture at a pressure of 50 kgf / cm ² , a disk-shaped molded body having a diameter of 10 mm and a thickness of 0.5 mm was obtained. Polar electrodes were used. By repeating this molding operation, one more polarizable electrode having the same composition and shape was obtained. The obtained two 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 in a nitrogen gas atmosphere. The above-mentioned electrolytic solution was impregnated under reduced pressure into the two polarizable electrode bodies (activated carbon molded bodies) after cooling. An electrochemical capacitor as shown in FIG. 1 was obtained by sandwiching a polyethylene separator between these two polarizable electrodes impregnated with the electrolyte and caulking it in a stainless steel case via a polypropylene gasket. . The initial capacitance was determined by applying a voltage of 2.8 V to the obtained electrochemical capacitor at 25 ° C., and then discharging at a constant current of 1.16 mA. As the durability evaluation of the electrochemical capacitor, after holding at 70 ° C. for 1000 hours while applying a voltage of 2.8 V,
The capacitance after discharging at a constant current of 1.16 mA was measured, and the capacitance change rate obtained by dividing the value by the initial capacitance was adopted. Table 1 shows the results.

Example 2 In Example 1, the content of propylene glycol was 7
Table 1 shows the results obtained for the same electrolytic solution except that the concentration was ppm. Example 3 In Example 1, the content of propylene glycol was 1
Table 1 shows the results obtained for the same electrolytic solution except that the concentration was 5 ppm. Example 4 In Example 1, the content of propylene glycol was 3
Table 1 shows the results obtained for the same electrolytic solution except that it was 0 ppm. Comparative Example 1 Table 1 shows the results obtained in Example 1, except that the content of propylene glycol was 0.1%.

Comparative Example 2 In Example 1, the content of propylene glycol was 1
%, The results obtained for the same electrolyte solution except that
Shown in Example 5 In Example 1, a solvent in which propylene carbonate and ethylene carbonate were mixed at a weight ratio of 1: 1 was used instead of the propylene carbonate solvent, and the content of propylene glycol was 4 ppm, and the content of ethylene glycol was 3 ppm. Table 1 shows the results obtained with respect to the same electrolytic solution except for the presence. Comparative Example 3 In Example 5, the content of propylene glycol was 5
00 ppm, ethylene glycol content 400 pp
Table 1 shows the results obtained for the same electrolyte solution except that
Shown in

Comparative Example 4 Table 1 shows the results obtained for the same electrolytic solution in Example 5 except that the content of propylene glycol was 0.5% and the content of ethylene glycol was 0.4%. . Example 6 Table 1 shows the results obtained in Example 2, except that tetraethylammonium tetrafluoroborate was used instead of triethylmethylammonium tetrafluoroborate. Comparative Example 5 In Example 6, the content of propylene glycol was 0.1%.
Table 1 shows the results obtained for the same electrolytic solution except that it was 1%.

[0029]

[Table 1]

[0030]

The electrolytic solution of the present invention has excellent withstand voltage and long-term reliability due to the low content of glycol, which is an impurity. Electrochemical capacitors using the same can be used for memory backup of various electronic devices. It is suitable for use in electric vehicles and the like that require a large current.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an electric double layer capacitor.

[Explanation of symbols]

 Reference Signs 1 electrode 2 current collector 3 separator

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yasushi Yaura 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture Inside the Tsukuba Research Laboratory Mitsubishi Chemical Corporation (72) Inventor Masahiro Takehara 8-Chome, Ami-cho, Inashiki-gun, Ibaraki Prefecture No.3-1 Inside the Tsukuba Research Laboratory, Mitsubishi Chemical Corporation (72) Inventor Akiko Torikai 3-1-1, Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture F-term in the Mitsubishi Chemical Corporation Tsukuba Research Laboratory FH529 AJ02 AJ05 AK06 AK08 AL06 AL08 AM02 AM03 AM04 AM05 AM07 HJ01 HJ10

Claims (6)

[Claims] An electrolytic solution for an electrochemical capacitor in which an electrolyte is dissolved in a non-aqueous solvent containing at least a cyclic carbonate as a main component has a glycol content of 100 p.
pm or less, the electrolytic solution for an electrochemical capacitor. 2. The electrolytic solution for an electrochemical capacitor according to claim 1, wherein the electrolyte is a quaternary ammonium salt. 3. The electrolytic solution for an electrochemical capacitor according to claim 1, wherein the electrolyte is a quaternary phosphonium salt. 4. An electrode for an electrochemical capacitor according to claim 1, wherein at least one of a positive electrode and a negative electrode is a polarizable electrode mainly composed of a carbonaceous substance. Electrochemical capacitor. 5. The electrochemical capacitor according to claim 4, wherein the carbonaceous substance is activated carbon. 6. An electric double layer capacitor comprising the electrolytic solution for an electrochemical capacitor according to claim 1. Description: