JP2009032876A - Electric double-layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric double-layer capacitor with little self-discharge, usable in a wide temperature range. <P>SOLUTION: The electric double-layer capacitor comprises a polarizing electrode and an organic electrolyte. The organic electrolyte contains 0.01-1.0 wt.% of a cyanoethylated starch as an additive. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an improvement of an electric double layer capacitor, and more particularly to an electric double layer capacitor using an organic electrolyte that suppresses self-discharge.

  In an electric double layer capacitor, an electrode solution is held in a capacitor element in which a polarizable electrode made of activated carbon and a separator are wound. In such an electric double layer capacitor, an electric double layer is formed on the surface of the polarizable electrode by the electrolytic solution held in the separator. Thereby, the electric double layer capacitor has a large capacitance of F (farad) unit. The electrolytic solution used for this type of electric double layer capacitor is a liquid electrolytic solution, and is classified into two systems, an aqueous solution system and a non-aqueous system. The former is known as an aqueous electrolyte solution of sulfuric acid or potassium hydroxide, and the latter is a solution in which tetraethylammonium borofluoride or hexafluorophosphate is dissolved as a solute in an organic solvent such as propylene carbonate or γ-butyrolactone. An aqueous electrolyte (organic electrolyte) is known.

  Furthermore, a typical organic electrolyte composition includes a propylene carbonate solution of tetraethylammonium tetrafluoroborate. However, when these organic electrolytes are used, there is a drawback that self-discharge is large.

  Various polymer electrolytes have been studied for such problems. Known polymer electrolytes include composites of polyethylene oxide and electrolyte salts, and random copolymers of ethylene oxide and propylene oxide and electrolyte salts. (Patent Document 1).

JP-A-62-249361

  In the polymer electrolyte as described above, the self-discharge characteristics are improved, but the ionic conductivity at room temperature or lower is small. This is because below the room temperature, the viscosity of the electrolyte solution of the polymer electrolyte becomes high and ions are difficult to diffuse.

  When such a polymer electrolyte having a low ionic conductivity at room temperature or lower is used as the electrolyte of an electric double layer capacitor, the capacitance at a low temperature below room temperature is extremely low, and a high discharge current value cannot be obtained even near room temperature. There was a drawback of (high internal resistance).

  The present invention has been made in view of the above technical problem, and an object of the present invention is to provide an electric double layer capacitor that can be used in a wide temperature range with little self-discharge.

  In order to achieve the above object, the present inventors have conceived that the above-mentioned problems can be overcome by dissolving cyanoethylated starch in an organic electrolyte.

  The specific invention based on this idea is as follows.

  That is, the electric double layer capacitor according to the present invention is an electric double layer capacitor using an organic electrolyte as an electrolyte, and the organic electrolyte contains cyanoethylated starch.

  It is preferable that content of the said cyanoethylated starch is 0.01-1.0 wt% with respect to the whole organic electrolyte solution.

  In the present invention, a known electrolyte can be used for the organic electrolyte, and is selected based on the electrochemical stability, the use of an electric double layer capacitor and the required withstand voltage. This electrolyte is composed of an anion and a cation. Specific examples of the anion and cation include the following.

(1) Examples of anions include perchloric acid, tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroarsenic acid, hexafluoroantimonic acid, perfluoroalkane (2 to 12 carbon atoms) sulfonic acid, perfluoroalkanes (having 2 to 12 carbon atoms) sulfonyl imide, hydroiodic acid, (the number of carbon atoms in the alkyl group 2-8) tetraalkyl borate [e.g. _{B (n-C 4 H 9} ) 4 - ], a tetrahalide Aluminum halide (AlCl ₄ ^— etc.) and the like can be mentioned.

  (2) Examples of the inorganic cation include alkali metal ions such as lithium ion, sodium ion and potassium ion, and alkaline earth metal ions such as calcium ion and magnesium ion.

  (3) Examples of organic cations include tetraalkylammonium ions having 1 to 8 carbon atoms in the alkyl group, such as tetramethylammonium, tetraethylammonium, tetrabutylammonium, dimethyldiethylammonium, and methyltriethylammonium, tetramethylphosphonium, Tetraalkylphosphonium ions having 1 to 8 carbon atoms in the alkyl group such as tetraethylphosphonium, tetrabutylphosphonium, dimethyldiethylphosphonium, methyltriethylphosphonium, 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, C 1-8 imidazolium ions such as 1,2,3-trimethylimidazolium, 1,2,3-trimethylimidazolinium, 1,2,3,4-teto Imidazolinium ions having 1 to 8 carbon atoms in the alkyl group, such as methylimidazolinium and 1-ethyl-2,3-dimethylimidazolinium, and 1,3-dimethyl-1,4,5,6- Tetrahydropyrimidinium, 1,2,3-trimethyl-1,4,5,6-tetrahydropyrimidinium, 1-methyl-1,8-diazabicyclo [5.4.0] undecene-7, 1-methyl- Examples thereof include pyrimidinium ions having 1 to 18 carbon atoms in the substituted alkyl group such as 1,5-diazabicyclo [4.3.0] nonene-5.

  The electrolyte is not particularly limited, but is preferably a quaternary ammonium salt, a quaternary phosphonium salt, a quaternary amidium salt, and a mixture thereof. Two or more electrolytes may be used in combination, and the weight ratio in that case is not particularly limited.

  Examples of the quaternary ammonium salt include tetraalkylammonium tetrafluoroborate having 1 to 8 carbon atoms such as tetraethylammonium tetrafluoroborate and methyltriethylammonium tetrafluoroborate, and examples of the quaternary phosphonium salt include tetraethylphosphonium tetra Examples of tetraalkylphosphonium tetrafluoroborates having 1 to 8 carbon atoms, such as fluoroborate and methyltriethylphosphonium tetrafluoroborate, and quaternary amidinium salts include 1,3-dimethylimidazolium tetrafluoroborate, 1-ethyl- C 1-18 imidazoles of substituted alkyl groups such as 3-methylimidazolium tetrafluoroborate and 1,2,3-trimethylimidazolium tetrafluoroborate Um tetrafluoroborate, 1,2,3-trimethylimidazolinium tetrafluoroborate, 1,2,3,4-tetramethylimidazolinium tetrafluoroborate, 1-ethyl-2,3-dimethylimidazolinium tetra Imidazolinium tetrafluoroborate having 1 to 18 carbon atoms of a substituted alkyl group such as fluoroborate, 1,3-dimethyl-1,4,5,6-tetrahydropyrimidinium tetrafluoroborate, 1,2,3 -Trimethyl-1,4,5,6-tetrahydropyrimidinium tetrafluoroborate, 1-methyl-1,8-diazabicyclo [5,4,0] undecene-7 tetrafluoroborate, 1-methyl-1,5- Substituted alkyl groups such as diazabicyclo [4,3,0] nonene-5 tetrafluoroborate Pyrimidinium tetrafluoroborates 1 to 18 carbon atoms can be mentioned.

Further, examples of the anion of these salts include those obtained by replacing tetrafluoroborate ions with ClO ₄ ⁻ , PF ₆ ⁻ , trifluoromethanesulfonate, trifluoromethanesulfonylimide, or the like.

  As the solvent used in the organic electrolytic solution in the present invention, a known solvent can be used, and it is appropriately selected from the solubility and electrochemical stability of the electrolyte. Specific examples include the following. Two or more of these can be used in combination.

(1) Water (2) Protic polar organic solvent Examples of alcohols include carbon such as methanol, ethanol, n-, iso-propanol, n-, sec-, i-, t-butanol, and ethylene glycol monomethyl ether. 1 to 4 monohydric alcohols, n-, iso-pentanol, benzyl alcohol, diethylene glycol monomethyl ether and other monohydric alcohols having 5 to 8 carbon atoms, ethylene glycol, glycerin and other 2 to 3 or more valences And polyhydric alcohols. Examples of the amides include N-propylformamide having 4 to 8 carbon atoms such as formamide having 1 to 3 carbon atoms and N-methylformamide.
Examples of the phenols include [phenol having 6 to 8 carbon atoms, p-butylphenol having 9 to 12 carbon atoms, and the like.
(3) Aprotic polar organic solvent Examples of ethers include chain ethers having 2 to 6 carbon atoms such as diethyl ether, methyl isopropyl ether, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether, diethylene glycol diethyl ether, and triethylene glycol dimethyl ether. The number of carbons such as a chain ether having 7 to 12 carbon atoms such as tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, etc., a cyclic ether having 2 to 4 carbon atoms, 4-butyldioxolane, crown ether, etc. Examples thereof include 5 to 18 cyclic ethers. Examples of amides include N, N-dimethylformamide, N, N-dimethylacetamide, N, N-dimethylpropionamide, hexamethylphosphorylamide, N-methylpyrrolidone and the like. Examples of lactones include γ-butyrolactone, α-acetyl-γ-butyrolactone, β-butyrolactone, γ-valerolactone, and δ-valerolactone. Examples of nitriles include acetonitrile and acrylonitrile. Examples of carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, and diethyl carbonate. Examples of the sulfoxides include dimethyl sulfoxide, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane.
(4) Other organic solvents Examples of the heterocyclic solvent include N-methyl-2-oxazolidinone, 3,5-dimethyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, and the like. . Of these, an aprotic polar organic solvent is preferable.

  The concentration of the solute containing tetraethylammonium with respect to the organic solvent is preferably in the range of 0.5 to 3.0 mol / L, and more preferably 1 mol / L.

  In the present invention, by dissolving cyanoethylated starch in the organic electrolytic solution, the viscosity of the organic electrolytic solution is increased, and the movement of ions in the organic electrolytic solution can be suppressed to suppress the diffusion of ions. In addition, the electrolytic solution can be sufficiently permeated into the pores of the electrode activated carbon. As a result, it is possible to suppress self-discharge while the room temperature has an internal resistance equivalent to that of a conventional electric double layer capacitor. Further, since the electrolytic solution can be sufficiently permeated into the pores of the electrode activated carbon even at a low temperature of room temperature or lower, a capacitance equivalent to that of a conventional electric double layer capacitor can be obtained.

  Examples of the present invention will be described below.

Example 1
The electric double layer capacitor according to Example 1 is obtained by impregnating an organic electrolyte into a capacitor element in which a polarizable electrode made of coconut shell activated carbon and a separator are wound.

  The organic electrolyte solution uses tetraethylammonium tetrafluoroborate 1.0 mol / L as a solute and propylene carbonate as a solvent. Cyanoethylated starch is dissolved in this organic electrolyte. The weight of cyanoethylated starch was adjusted by reducing the amount of solvent.

  In Example 1, the content of cyanoethylated starch is set to 0.01 wt% with respect to the total amount of the organic electrolyte.

(Example 2)
In Example 2, an electric double layer capacitor was produced in the same manner as in Example 1 except that the content of cyanoethylated starch in the organic electrolyte was 0.50 wt% with respect to the total amount of the organic electrolyte. .

(Example 3)
In Example 3, an electric double layer capacitor was produced in the same manner as in Example 1 except that the content of cyanoethylated starch in the organic electrolyte was 1.0 wt% with respect to the total amount of the organic electrolyte. .

Example 4
In Example 4, an electric double layer capacitor was produced in the same manner as in Example 1 except that the content of cyanoethylated starch in the organic electrolyte was 0.005 wt% with respect to the total amount of the organic electrolyte. .

(Example 5)
In Example 5, an electric double layer capacitor was produced in the same manner as in Example 1 except that the content of cyanoethylated starch in the organic electrolyte was 1.5 wt% with respect to the total amount of the organic electrolyte. .

(Conventional example 1)
In the conventional example, an electric double layer capacitor was produced in the same manner as in Example 1 except that the organic electrolyte did not contain cyanoethylated starch.

  Table 1 shows the internal compositions of the electrolytic solutions of Examples 1 to 5 and Conventional Example 1.

  Table 2 shows the capacitance (−25 ° C., 25) of the electric double layer capacitors (rated: 2.5 V−22 F, size: 16 × 31.5 mm) according to the above Examples 1 to 5 and Conventional Example 1. , 75 ° C.), internal resistance (series resistance) at 25 ° C. and self-discharge (voltage change rate between the initial voltage of a fully charged electric double layer capacitor and the voltage after standing for 24 hours). . In addition, the measurement made 10 electric double layer capacitors each as a sample, and the value in the table | surface has shown the average value.

  As is clear from Table 2, in Examples 1 to 3, self-discharge is less than that in Conventional Example 1, and the capacitance at each temperature shows the same characteristics. Therefore, it turns out that content of the cyanoethylated starch with respect to organic electrolyte solution is good in the range of 0.01-1.0 wt%. In other words, for example, when the amount of cyanoethylated starch is less than 0.01 wt% as in Example 4, the internal resistance does not increase, but the self-discharge suppressing effect is not observed, which is not preferable. On the other hand, when the added amount of cyanoethylated starch exceeds 1.0 wt% as in Example 5, although the self-discharge suppressing effect is observed, the viscosity of the electrolytic solution becomes too high and the internal resistance becomes high, which is not preferable.

  In addition, this invention is not limited to the said Example.

  For example, in the above-described examples, the organic electrolyte was described as an example in which the solvent was propylene carbonate and the solute was tetraethylammonium tetrafluoroborate 1 mol / L, but other solvents include ethylene carbonate, dimethyl carbonate, Acetonitrile or the like may be used. Alternatively, an organic electrolytic solution in which tetraethylammonium borofluoride or hexafluorophosphate is dissolved in an organic solvent such as propylene carbonate or γ-butyrolactone may be used. That is, the organic electrolytic solution may be one obtained by dissolving a solute containing tetraethylammonium in an organic solvent (the concentration of the solute containing tetraethylammonium with respect to the organic solvent is preferably 0.5 to 3.0 mol / L). Thus, it is predicted that the organic electrolyte contains cyanoethylated starch, thereby suppressing ion diffusion and reducing the internal current used for self-discharge compared to the case where cyanoethylated starch is not contained. The effect of suppressing discharge can be expected.

  It goes without saying that various design changes and modifications can be made within the scope of the claims attached to this specification.

  The present invention is useful as an electric double layer capacitor because self-discharge is reduced and it can be used in a wide temperature range.

Claims (2)

In an electric double layer capacitor using an organic electrolyte as an electrolyte,
The electric double layer capacitor, wherein the organic electrolyte contains cyanoethylated starch.   The electric double layer capacitor according to claim 1, wherein the content of the cyanoethylated starch is 0.01 to 1.0 wt%.