JP2008283161A - Electrochemical capacitor and electrolyte therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a electrochemical capacitor enhanced a capacitance of the entire capacitor by enhancing a capacitance of a positive electrode by studying an electrolyte component. <P>SOLUTION: The electrolyte for the electrochemical capacitor comprising a negative electrode capable of storing and releasing the alkaline metal ions and/or the alkaline earth group metal ions; and a positive electrode capable of adsorbing anions; contains non-aqueous solvent, alkaline metal salts or alkaline earth group metal salts and organic onium salts, the total concentration of alkaline metal salts, alkaline earth group metal salts and organic onium salts in the entire electrolyte is no more than 3.0 mol/dm<SP>3</SP>. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrolytic solution for an electrochemical capacitor, in particular, an electrolytic solution capable of increasing the capacitance of a positive electrode, and relates to a large-capacity electrochemical capacitor.

  As an alternative material for lead-acid batteries and the like, electrochemical capacitors are attracting attention and are being put into practical use, particularly for small power backup. Electrochemical capacitors are a new energy storage device that can be used in a wide temperature range, has the advantage of being pollution-free and environmentally friendly.

  As one of the electrochemical capacitors, an electric double layer capacitor using a carbon electrode such as activated carbon having a large specific surface area is known. An electric double layer capacitor uses a charge / discharge mechanism based only on adsorption / desorption on the electrode surface of the ion pair attracted to the positive electrode and the negative electrode, and does not involve a Faraday reaction. There is no problem such as performance degradation due to.

  One factor that affects the performance of the electric double layer capacitor is the composition of the electrolyte. There are aqueous and non-aqueous electrolytes, but the non-aqueous electrolyte has a higher withstand voltage. As a typical nonaqueous electrolytic solution, a solution in which a quaternary ammonium salt such as tetraalkylammonium tetrafluoroborate, which is an electrolyte having a high degree of dissociation, is dissolved in a carbonate solvent is known (for example, Patent Document 1). However, the energy density of electric double layer capacitors is much smaller than that of lead-acid batteries, and there is still room for improvement.

  On the other hand, a lithium ion capacitor using an electrode capable of occluding and desorbing lithium ions and using a lithium salt as an electrolyte is also known, and this lithium ion capacitor has a higher energy density than an electric double layer capacitor, It has been clarified that a high withstand voltage is exhibited (for example, Patent Document 2).

However, the capacitance C of the capacitor as a whole is affected by the positive electrode capacitance C ₊ at the same ratio as the negative electrode capacitance C ₋ , as shown by the following equation.

That is, the capacitance C of the entire capacitor cannot be increased unless the capacitance of the positive electrode is increased.
JP 2007-43015 A (Background Art) JP 2000-306609 A (0009)

  Thus, the present invention has been made to increase the electrostatic capacity of the positive electrode by examining the composition of the electrolytic solution, and to increase the electrostatic capacity C of the entire capacitor.

The present invention that has solved the above-mentioned problems is provided for an electrochemical capacitor comprising a negative electrode capable of occluding and releasing alkali metal ions and / or alkaline earth metal ions, and a positive electrode capable of adsorbing anions. An electrolytic solution comprising a non-aqueous solvent, an alkali metal salt or an alkaline earth metal salt, and an organic onium salt, wherein the total concentration of the alkali metal salt, alkaline earth metal salt and organic onium salt is based on the entire electrolytic solution, It is characterized in that it is 3.0 mol / dm ³ or less.

  It is preferable that the alkali metal ion is a lithium ion and the alkali metal salt is a lithium salt. In particular, the configuration in which the organic onium salt is a quaternary ammonium salt is the most preferred embodiment of the present invention.

  The present invention includes an anode capable of occluding and releasing alkali metal ions and / or alkaline earth metal ions, a positive electrode capable of adsorbing anions, and an electrochemical capacitor containing the above electrolytic solution for an electrochemical capacitor. Is also included.

  Since the electrolytic solution for an electrochemical capacitor of the present invention was able to increase the capacitance of the positive electrode, the capacitance of the entire capacitor could be increased.

  Since the electrolytic solution for an electrochemical capacitor of the present invention is for an electrochemical capacitor having a specific negative electrode and a positive electrode, these electrodes will be described first.

[electrode]
The electrode of the electrochemical capacitor of the present invention includes two types of electrodes, and the ion species to be occluded or adsorbed is limited. An electrode mainly composed of a carbon material that can occlude and release alkali metal ions and / or alkaline earth metal ions can occlude only alkali metal ions and / or alkaline earth metal ions. Become. In addition, an electrode mainly composed of activated carbon can adsorb anions, which becomes a positive electrode.

  As a material capable of occluding and releasing alkali metal ions and / or alkaline earth metal ions, which are main constituent materials of the negative electrode, graphitic carbon materials are preferable. For example, natural graphite, artificial graphite, graphitized mesophase carbon micro beads (MCMB), graphitized mesophase carbon fiber (MCMB), graphite whisker, graphitized carbon fiber, and the like. One or more of these may be molded by adding a binder resin or the like as necessary. Can be used. Among these, graphitized mesophase carbon microbeads (MCMB) are preferable because they have many ion storage sites and are easy to handle. Activated carbon is not preferred in the present invention because of its low specific gravity and bulkiness, and a small number of ion storage sites.

  As binder resins, polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene- Tetrafluoroethylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether- Fluororesin such as tetrafluoroethylene copolymer; Acrylic resin such as ethylene- (meth) acrylic acid copolymer, ethylene- (meth) acrylic acid methyl copolymer; polyethylene, polypropylene Rubbers such as SBR; polyolefins such emissions polyamide; polyimides; polyurethanes; polyvinyl pyrrolidone, and the like. These may be used alone or in combination. From the viewpoint of solvent resistance to non-aqueous solvents, polyvinylidene fluoride, ethylene- (meth) acrylic acid copolymer, polyamide, polyimide, polyurethane, polyvinylpyrrolidone and the like are suitable. The amount of the binder resin is preferably about 1 to 30% by mass in the negative electrode in order to balance properties such as strength and capacity.

The positive electrode, BF ₄ ^- is not particularly limited as long anion as it can adsorb the like, large activated carbon the specific surface area is preferable. For example, there are coconut shell activated carbon, petroleum coke activated carbon and the like, but it is preferable to use petroleum coke activated carbon to obtain a large capacity capacitor.

[Nonaqueous solvent for electrolyte]
What can be used as the non-aqueous solvent of the electrolytic solution of the present invention is a carbonate system such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; a lactone system such as γ-butyrolactone; Oxygen-containing cyclic compounds such as dioxolane, dioxane, 3-methyl-2-oxazolidinone, tetrahydrofuran, 2-methyltetrahydrofuran, maleic anhydride, succinic anhydride, phthalic anhydride, propane sultone, dihydropyran, 3-methyl sydnone; formamide, dimethyl Formamides such as formamide and dimethylacetamide; dimethyl sulfoxide; sulfolane; phosphate triesters; nitrogen-containing compounds such as nitromethane, nitrobenzene, and acetonitrile ; Alkoxy ethane such as 1,2-dimethoxyethane: toluene, and the like. These solvents may be used alone or in combination of two or more.

[Alkali metal salts, alkaline earth metal salts]
The electrolytic solution of the present invention contains an alkali metal salt and / or an alkaline earth metal salt, and these are present in the electrolytic solution as alkali metal ions and alkaline earth metal ions. Examples of the alkali metal include Li, Na, and K, and examples of the alkaline earth metal include Mg and Ca. Li is most preferred.

Examples of the lithium salt include lithium dicyanotriazolate (LiDCTA), LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiB [C ₆ H ₄ (CF ₃ )] and LiB [C ₆ represented by the following formula (1). H ₃ (CF ₃ ) ₂ ], LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiClO ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiC ₂ F ₅ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC ₈ F ₁₇ SO ₃ , LiCF ₃ CO ₂ , LiBOB (lithium bis (oxalate) borate) represented by the following formula (2), LiTFDT represented by the following formula (3) Lithium 4,4,5,5-tetrafluoro- [1,3,2] dithiozolidinium 1,1,3,3-tetraoxide) and the like. Among them, those in which an anion has a cyclic structure are likely to cause intercalation to the positive electrode, are effective in improving the capacitance, and are a preferred embodiment of the present invention. Examples of the lithium salt in which the anion has a cyclic structure include LiDCTA, LiBOB, LiTFDT, and the like. In particular, LiDCTA showing a high capacitance when used in combination with an onium salt is preferable.

  LiDCTA is disclosed in Japanese Patent Application No. 2006-271962, in which diaminomaleonitrile is dispersed in an acidic aqueous solution, sodium nitrite is added to synthesize dicyanotriazole, and then a lithiation reagent is reacted. LiDCTA can be obtained.

[Organic onium salt]
The electrolytic solution of the present invention contains an organic onium salt. This is because when this organic onium salt is used in combination with the above lithium salt or the like, an increase in the capacitance of the positive electrode is particularly recognized (combination effect). Examples of organic onium salts include organic onium salts, quaternary ammonium salts, quaternary phosphonium salts, tertiary sulfonium salts, secondary iodonium salts, etc., but solubility in nonaqueous solvents, electrolytes From the standpoint of dissociation properties and the like, a quaternary ammonium salt or a quaternary phosphonium salt is preferable, and a quaternary ammonium salt is particularly preferable.

The quaternary ammonium salt is an imidazolium cation, pyridinium cation, pyrrolidinium cation, piperidinium cation, R ¹ R ² R ³ R ⁴ N ⁺ (R ^{1 to} R ⁴ may be the same or different and have 1 to 6 carbon atoms. And a salt such as a cation represented by formula (1). In particular, a salt of R ¹ R ² R ³ R ⁴ N ⁺ is preferable, and a tetraethylammonium (TEA) salt is particularly preferable because it does not cause irreversible decomposition on the negative electrode surface. Examples of the anionic species include dicyanotriazolate ions (DCTA), PF ₆ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ , C (CN) ₃ ⁻ , N (CN) ⁻ , N (SO ₂ CF) represented by the following formula: ₃ ) ₂ ⁻ , CF ₃ SO ₃ ⁻ , C (CF ₃ SO ₂ ) ₃ ⁻ , AsF ₆ ⁻ , SbF ₆ ^{− and the} like. In particular, cyanotriazolate, bis (oxalate) borate, PF ₆ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ , C (CN) ₃ ⁻ , N (CN) ⁻ , N (SO ₂ CF ₃ ) ₂ ⁻ , B ( CN) ₄ ^- salt to anion is preferably a like. The most suitable examples of the salt are TEADCTA, TEABF _{4 and the} like.

[Concentration of alkali metal salt, alkaline earth metal salt and organic onium salt in electrolyte]
The total concentration of the alkali metal salt, alkaline earth metal salt and organic onium salt in the electrolytic solution is preferably 3.0 mol / dm ³ or less in the electrolytic solution. If the above salts are present in excess of 3.0 mol / dm ³ , the viscosity of the electrolyte will be too high, and there will be undissociated salts that do not substantially contribute to the increase in capacitance, resulting in high costs. This is not preferable. The total concentration of salts, more preferably 2.5 mol / dm ³ or less, more preferably 2.0 mol / dm ³ or less. The lower limit of the total salt concentration is preferably 0.1 mol / dm ³ . When the amount is less than 0.1 mol / dm ^3, the absolute amount of ions contributing to occlusion and adsorption decreases, and the capacitance may be lowered. 0.2 mol / dm ³ or more, and even more preferably 0.3 mol / dm ³ or more.

  The ratio of the alkali metal salt and alkaline earth metal salt to the organic onium salt is 0.1 / 99.9 to 99.9 / 0.1 in terms of molar ratio. That is, the effect of increasing the capacitance of the positive electrode does not appear. More preferably, it is 0.5 / 99.5 to 99.5 / 0.5, and further preferably 1.0 / 99.0 to 99.0 / 1.0.

  The electrolytic solution of the present invention can be obtained by dissolving the alkali metal salt and / or alkaline earth metal salt and the organic onium salt in the above preferred concentration range in a non-aqueous solvent.

  The electrochemical capacitor of the present invention includes the above-described negative electrode and positive electrode, has the electrolytic solution of the present invention, and can be manufactured by a known method.

  The present invention will be described in further detail with reference to the following examples. However, the following examples are not intended to limit the present invention, and all modes modified and implemented without departing from the spirit of the present invention are included in the present invention.

Synthesis example 1
400 g of water is put in a reaction vessel, 98 g (1 mol) of sulfuric acid and 216 g (2 mol) of diaminomaleonitrile are sequentially added, and the mixture is immersed in an ice bath while stirring with a paddle blade, while maintaining the internal temperature at 20 ° C. An aqueous solution obtained by dissolving 0.45 g (2.05 mol) in 410 g of water was dropped over 2 hours. After completion of the dropping, stirring was continued for 30 minutes, and then the reaction solution was suction filtered through a cellulose filter having a pore size of 0.2 μm to remove insoluble matters. Water was distilled off from the filtrate using a rotary evaporator until the reaction product was dried. To the obtained solid, 400 g of diisopropyl ether was added to extract dicyanotriazole. Furthermore, after adding 400 g of water to the diisopropyl ether phase and washing, dicyanotriazole was obtained by distilling the diisopropyl triazole diisopropyl ether solution with a rotary evaporator. The dicyanotriazole was purified by sublimation under reduced pressure at 125 ° C. and 30 Pa for 50 hours.

  100 g (0.84 mol) of dicyanotriazole was dissolved in 1000 g of water, and lithium carbonate was added little by little while stirring at room temperature until the pH of the reaction solution reached 7. The reaction solution was suction filtered through a cellulose filter to remove insoluble matters. Water was distilled off from the filtrate using a rotary evaporator until the reaction product was dried. 100 g of the obtained LiDCTA and 1000 g of toluene were put in a reaction vessel, and azeotropic dehydration was performed at 50 ° C. for 20 minutes and at 130 ° C. for 100 minutes while refluxing toluene. This was suction filtered through a polytetrafluoroethylene filter having a pore size of 0.2 μm, and the solid was dried at 120 ° C. under reduced pressure. Purified LiDCTA was obtained.

Experiment No. 1
A system using tetraethylammonium tetrafluoroborate (TEABF ₄ ; manufactured by Toyama Pharmaceutical Co., Ltd.) as the organic onium salt, LiBF ₄ (manufactured by Kishida Chemical Co., Ltd.) as the lithium salt, and LiDCTA obtained in Synthesis Example 1 above For the system used, the capacitance was measured by changing these mixing ratios. As the non-aqueous solvent, a mixture of ethylene carbonate (EC; manufactured by Kishida Chemical Co., Ltd.) and dimethyl carbonate (DMC; manufactured by Kishida Chemical Co., Ltd.) at 1: 1 (volume ratio) was used. The concentration of the electrolyte (total of TEABF ₄ and lithium salt) in the electrolytic solution was 1 mol / dm ³ .

In this experiment, activated carbon was used as an electrode of a capacitor in order to verify whether the electrolytic solution of the present invention increases the capacitance of the positive electrode. The activated carbon fiber cloth (manufactured by Kuraray Chemical Co., Ltd.) has a specific surface area of 2000 m ² / g, a negative electrode cut into a circle with a diameter of 1 cm, and a positive electrode with a circular electrode with a diameter of 1.2 cm. Pasted with a conductive adhesive and impregnating each electrode while reducing the above electrolyte, and then facing in a polytetrafluoroethylene sealed cell, placing a silver wire as a reference electrode between them, The capacitor was separated by a separator immersed in an electrolyte (“Celgard (registered trademark) 2500” manufactured by Celgard). The capacitor was produced in an argon-substituted glove box.

  Cyclic voltammetry (CV) was measured using an electrochemical measurement system (Hokuto Denko; “HZ-3000”). The voltage was changed between −1 and 1 V (vs. Ag), and the scanning speed was changed between 2 and 100 mV / s. The capacitance was calculated by dividing the amount of electricity passed at CV by the scanning potential range.

FIG. 1 shows the electrostatic capacity at room temperature (25 ° C.) and a scanning speed of 2 mV / s for the LiBF ₄ -TEABF ₄ system and the LiDCTA-TEABF ₄ system. In either system, compared with the lithium salt alone (TEABF ₄ is 0 mol%) and TEABF ₄ alone (100 mol%), it can be seen that the electrostatic capacitance of the positive electrode is increased. In particular, the capacitance of LiDCTA-TEABF ₄ system greatly increased.

Further, in FIG. 2 shows LiBF ₄ and 0.5mol / dm ^3, TEABF ₄ and 0.5 mol / dm ³ including the CV capacitors using the electrolytic solution results (scanning speed 2 mV / s). FIG. 3 shows the CV results (scanning speed 2 mV / s) of the capacitor using the electrolytic solution containing LiDCTA 0.8 mol / dm ³ and TEABF ₄ 0.2 mol / dm ³ .

Experiment No. 2
A mesophase carbon particle (made by Osaka Gas Chemical Co., Ltd .; mesomicrocarbon beads: MCMB25-28), which is a graphite material, is fired at 2800 ° C., and polyvinylidene fluoride (Kureha Co., Ltd .: KF1100) is 95: 5 ( (Mass ratio) and mixed onto a copper foil to form a negative electrode. A bipolar coin cell was prepared using lithium foil as the positive electrode and reference electrode. This coin cell was subjected to a constant current charge / discharge test at a current density of 0.5 mA / cm ² using a charge / discharge test apparatus manufactured by Nagano. The electrolyte was a mixture of EC and DMC at 1: 1 (volume ratio), LiBF ₄ alone dissolved at 1.0 mol / dm ³ (FIG. 4), and LiBF ₄ at 0.8 mol / dm ^3. TEABF ₄ dissolved in 0.2 mol / dm ³ (FIG. 5), LiDCTA dissolved in 0.8 mol / dm ³ and TEABF ₄ dissolved in 0.2 mol / dm ³ (FIG. 6) were used.

  Comparing FIG. 4 (Comparative Example) with FIGS. 5 and 6 (Examples), the charge / discharge capacity per gram of MCMB is almost the same, and there is a particularly noticeable decrease in capacity in the charge / discharge characteristics of the graphite negative electrode. There wasn't.

Experiment No. 3
Experiment No. The same TEABF ₄ as used in No. 1 was 0.8 mol / dm ³ and LiDCTA 0.8 mol / dm ^3, and A constant current charge / discharge test was performed using the same non-aqueous solvent and capacitor as used in 1. The measurement device uses a secondary battery charge / discharge test device “BTS-2400” manufactured by Nagano Co., Ltd., and changes the current density to 0.5 mA / cm ² and the cut-off voltage (end voltage) from 1.5 to 3.0 V. I let you. The charge / discharge profile is shown in FIG. FIG. 8 shows the results of a constant current charge / discharge test using an electrolyte solution that does not contain LiDCTA. 9 shows a profile with a cutoff voltage of 2V in the profile of FIG. 7, and FIG. 10 shows a profile with a cutoff voltage of 2.5V in the profile of FIG.

  Comparing the charge / discharge profiles of FIGS. 7 and 9 with the charge / discharge profiles of FIGS. 8 and 10, the profiles of FIGS. 7 and 9, which are examples of the present invention, are convex upward during charging and convex downward during discharging. If no intercalation occurs in the positive electrode, the profile becomes a straight line during charging and discharging (FIGS. 8 and 10). However, as shown in FIGS. Represents that. Further, from FIG. 7, it was found that this charge / discharge reproducibility was high, and electrode deterioration and irreversible adsorption / intercalation did not occur.

  From each experimental result, since anions such as LiDCTA have a cyclic structure, it is considered that the intercalation phenomenon to the positive electrode occurred and the capacitance increased.

Experiment No. 2 is a graph of the composition of the electrolytic solution and the capacitance in FIG. Experiment No. 1 is a measurement result of cyclic voltammetry of a capacitor using the electrolytic solution of the present invention in FIG. Experiment No. 2 is a measurement result of cyclic voltammetry of a capacitor using another electrolytic solution of the present invention in FIG. Experiment No. 2 is a graph showing a constant current charge / discharge test result of a comparative example in FIG. Experiment No. 2 is a graph showing a constant current charge / discharge test result of a capacitor using the electrolytic solution of the present invention in FIG. Experiment No. 2 is a graph showing a constant current charge / discharge test result of a capacitor using another electrolytic solution of the present invention in FIG. Experiment No. 3 is a charge / discharge profile of the example in FIG. Experiment No. 3 is a charge / discharge profile of a comparative example in FIG. Experiment No. 3 is a charge / discharge profile of the example in FIG. Experiment No. 3 is a charge / discharge profile of a comparative example in FIG.

Claims (4)

An electrolytic solution for an electrochemical capacitor comprising a negative electrode capable of occluding and releasing alkali metal ions and / or alkaline earth metal ions, and a positive electrode capable of adsorbing anions, comprising a non-aqueous solvent, an alkali Metal salt and / or alkaline earth metal salt and organic onium salt are included, and the total concentration of alkali metal salt, alkaline earth metal salt and organic onium salt is 3.0 mol / dm ³ or less with respect to the entire electrolyte. Electrolyte for electrochemical capacitors characterized by the above-mentioned.   The electrolytic solution for an electrochemical capacitor according to claim 1, wherein the alkali metal ion is a lithium ion, and the alkali metal salt is a lithium salt.   The electrolytic solution for an electrochemical capacitor according to claim 1 or 2, wherein the organic onium salt is a quaternary ammonium salt.   The negative electrode which can occlude and discharge | release alkali metal ion and / or alkaline-earth metal ion, the positive electrode which can adsorb | suck an anion, and the electrolysis for electrochemical capacitors of any one of Claims 1-3 An electrochemical capacitor comprising a liquid.