JP2006210817A - Nonaqueous electrolyte for electrochemical capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the matter of an electrochemical capacitor that the energy capacity is low. <P>SOLUTION: As the electrolyte of an electrochemical capacitor being mixed with an nonaqueous electrolyte, 1-50 mol% of electrolyte composed of an alkaline metal (potassium, rubidium, cesium) having atomic number of 19 or above is employed assuming the total quantity of electrolyte being contained is 100 mol%. Preferably, anion of that salt is organic anion such as bis-trifluoromethan sulfonic acid amide anion because it exhibits excellent breakdown voltage properties or viscosity of chain carbonate (e.g. ethyl methyl carbonate) and can be dissolved with high concentration into a solvent which cannot dissolve inorganic anion easily. As the remaining electrolyte being mixed with the electrolyte, a salt comprising an onium cation such as triethyl methyl ammonium ion and the organic anion is preferable. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention belongs to a non-aqueous electrolyte for electrochemical capacitors.

  Electrochemical capacitors are characterized by high charge / discharge efficiency, long life, and high output density. However, the energy density is lower than that of a secondary battery such as a Li battery or NiH battery, and an improvement in energy density is required. Among them, an electrochemical capacitor using a non-aqueous electrolyte solution is said to have a higher voltage capacity than a capacitor using an aqueous electrolyte solution, so that the energy capacity can be increased. At present, a nonaqueous electrolytic solution for an electrochemical capacitor in which a quaternary ammonium-boron tetrafluoride salt (for example, see Non-Patent Document 1) is dissolved in a propylene carbonate solvent has been put into practical use. In addition, an electrolytic solution for an electric double layer capacitor using a salt composed of a quaternary ammonium cation and a bistrifluoromethanesulfonic acid amide anion as an electrolyte has also been proposed (for example, Patent Document 1).

Tanahashi et al., Electrochemistry, 56, 892, 1988 JP-A-7-272882

  However, an electrochemical capacitor using such a non-aqueous electrolyte has a problem that its energy capacity is not sufficient, and therefore its use is limited. An object of the present invention is to provide a nonaqueous electrolytic solution having a higher capacity than an electrolytic solution obtained by dissolving a quaternary ammonium boron tetrafluoride salt in a conventionally used propylene carbonate solvent, and an electrochemical capacitor using the nonaqueous electrolytic solution. It is to be.

  The present inventors have intensively studied to solve the above problems. As a result, the capacitance is increased by adding an alkali metal salt whose cation is group 1A of the periodic table and whose atomic number is greater than or equal to potassium as a compound to be blended as an electrolyte in an electrolytic solution for electrochemical capacitors. I found out. As a result of further investigation based on the above findings, the present inventors have completed the present invention.

  That is, the present invention provides a nonaqueous electrolytic solution for an electrochemical capacitor containing an electrolyte and a nonaqueous solvent, and an alkali number of 19 or more when the total amount of the electrolyte contained in the electrolytic solution is 100 mol%. A nonaqueous electrolytic solution for an electrochemical capacitor, comprising 1 to 50 mol% of an electrolyte composed of a metal salt.

  Since the electrolytic solution of the present invention can increase the electrostatic capacity as compared with a conventionally known non-aqueous electrolytic solution, an electrochemical capacitor using the electrolytic solution as an electrolytic solution has an energy capacity without changing other members. Can be improved.

  The non-aqueous electrolyte solution of the present invention comprises an electrolyte and a non-aqueous solvent. The electrolyte is composed of two or more electrolytes, one of which must be an alkali metal salt having an atomic number of 19 or more, and the amount thereof is 100 mol% of the total amount of the electrolyte to be blended. When it does, it needs to exist in the range of 1-50 mol%.

  By blending an alkali metal salt having an atomic number of 19 or more as the electrolyte, the capacitance of the non-aqueous electrolyte increases. Since alkali ions having an atomic number of 19 or more, that is, ions of potassium (atomic number 19), rubidium (atomic number 37), and cesium (atomic number 55) have a smaller ionic radius than onium ions such as ammonium ions, onium It is considered that the capacitance increases by entering the pores that cannot be penetrated by ions. On the other hand, an alkali metal ion having an atomic number smaller than 19, that is, an ion of lithium (atomic number 3) or sodium (atomic number 11) is smaller, and once adsorbed, the cation is not eliminated. When a metal salt is blended, the capacitance is rather small. As the alkali metal, potassium or rubidium is preferable because potassium is inexpensive, and potassium is most preferable.

In the alkali metal salt having an atomic number of 19 or more as described above, the alkali metal ion is a cation. The anion which is a counter ion in the salt may be any known anion. Examples of such anions include boron tetrafluoride, phosphorus hexafluoride, arsenic hexafluoride, antimony hexafluoride, RfSO ₃ (where Rf is a fluoroalkyl group having 1 to 12 carbon atoms), RfCO ₂ ( However, Rf is a fluoroalkyl group having 1 to 12 carbon atoms), RfBF ₃ (where, Rf is a fluoroalkyl group having 1 to 12 carbon _{atoms), (Rf) a PF 6} -a ( where, Rf is 1 to carbon atoms a is an integer of 1 to ₆₎ at 12 fluoroalkyl ^{_{group; (Rf 1 SO 2) N}} - (SO 2 Rf 2) ( where Rf ₁ and Rf ₂ is a fluoroalkyl group having 1 to 12 carbon atoms, Rf ₁ and Rf ₂ may be the be the same group different ^{_{group), (Rf 1 SO 2)}} N -. (CORf 2) ( where Rf ₁ and Rf ₂ is a fluoroalkyl group having 1 to 12 carbon atoms, Rf ₁ and Rf ₂ are the same It may be also be a group different groups amides such _{as);. (RfSO 2) 3} C - ( where, Rf can be mentioned methide such fluoroalkyl group) having 1 to 12 carbon atoms.

  In general, organic anion salts have higher solubility in solvents than inorganic anion salts such as boron tetrafluoride and are excellent in voltage endurance, but inorganic anion salts are difficult to dissolve. On the other hand, it tends to dissolve and hardly precipitate even at low temperatures, but the capacitance tends to be small. However, by adding an alkali metal salt having an atomic number of 19 or more as in the present invention, the electrostatic capacity is increased, and the above disadvantages are eliminated. Therefore, the anion of the salt blended as an electrolyte in the nonaqueous electrolytic solution of the present invention is preferably an organic anion, such as bistrifluoromethanesulfonic acid amide, tris-trifluoromethanesulfonic acid methide, trifluoromethanesulfonic acid trifluoro. Acetic acid amide is more preferable, and bistrifluoromethanesulfonic acid amide is most preferable because it is relatively inexpensive.

  The combination of the above-mentioned cation and anion in the alkali metal salt having an atomic number of 19 or more blended as an electrolyte in the nonaqueous electrolytic solution of the present invention is not particularly limited, but a suitable salt is specifically exemplified. Bistrifluoromethanesulfonic acid amide-potassium salt, bistrifluoromethanesulfonic acid amide-rubidium salt, bistrifluoromethanesulfonic acid amide-cesium salt, tris-trifluoromethanesulfonic acid methide-potassium salt, trifluoromethanesulfonic acid trifluoroacetamide -Potassium salt etc. are mentioned. In addition, as the alkali metal salt having an atomic number of 19 or more to be blended in the non-aqueous electrolytic solution of the present invention, two or more kinds having different one or both of the cation and anion may be used.

  If the concentration of the alkali metal salt having an atomic number of 19 or more in the nonaqueous electrolytic solution of the present invention is too small, the effect is hardly obtained. On the other hand, when it is too high, salt tends to precipitate and conversely, the capacitance tends to decrease. Therefore, an appropriate blending amount for obtaining the effect of the present invention is 1 to 50 mol% when the total amount of the electrolyte blended in the electrolytic solution is 100 mol% (hereinafter, all based on this unless otherwise specified). Yes, preferably 2 to 30 mol%, particularly preferably 3 to 20 mol%.

  In the nonaqueous electrolytic solution of the present invention, an electrolyte other than the alkali metal salt having the atomic number of 19 or more (hereinafter, other electrolyte) is blended in a range of 99 to 50 mol%. As the electrolyte, any salt known as an electrolyte in a nonaqueous electrolytic solution for an electric double layer capacitor may be used. Specific examples of the anion in the electrolyte (salt) are the same as those exemplified as the alkali metal salt having the atomic number of 19 or more, and the preferred types are also the same. Examples of the cation include organic onium ions. Examples of organic onium ions that can be suitably used include symmetrical ammonium cations such as tetramethylammonium cation, tetraethylammonium cation, and tetrapropylammonium cation; ethyltrimethylammonium cation, triethylmethylammonium cation, triethylpropylammonium cation, diethyldimethylammonium Carbon number of the shortest substituent such as cation, tributylethylammonium cation, triethylisopropylammonium cation, N, N-dimethylpyrrolidinium cation, N-methyl-N-ethylpyrrolidinium cation, etc. Of ammonium cations; trimethylpropylammonium cation; Asymmetric ammonium cations such as limethylisopropylammonium cation, butyltrimethylammonium cation, hexyltrimethylammonium cation, octyltrimethylammonium cation, dodecyltrimethylammonium cation, dimethyldipropylammonium cation; divalent ammonium cations such as hexamethonium cation; Symmetrical imidazolium cations such as 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1,3-dipropylimidazolium cation, 1,3-dipropylimidazolium cation; 1-ethyl-3 -Methylimidazolium cation, 1-methyl-3-propylimidazolium cation, 1-isopropyl-3-propylimidazolium cation ON, asymmetric imidazolium cations such as 1-tert-butyl-3-isopropylimidazolium cation; pyridinium cations such as N-ethylpyridinium cation and N-butylpyridinium cation; trimethylsulfonium cation, triethylsulfonium cation tributylsulfonium cation, etc. Symmetric sulfonium cations such as diethylmethylsulfonium cation; asymmetric sulfonium cations such as dimethylpropylsulfonium and dimethylhexylsulfonium; tetramethylphosphonium cation, tetraethylphosphonium cation, tetrapropylphosphonium cation, tetrabutylphosphonium cation Tetraoctylphosphonium cation, tetraphenylphospho Symmetrical phosphonium cations such as a sulphonium cation; pseudosymmetrical phosphonium cations such as a trimethylethylphosphonium cation and triethylmethylphosphonium cation; and asymmetric phosphonium cations such as a hexyltrimethylphosphonium cation and a trimethyloctylphosphonium cation.

  As other electrolytes, the combination of the above-mentioned cation and anion is not particularly limited. Specific examples of suitable salts include bistrifluoromethanesulfonic acid amide-triethylmethylammonium salt, tris-trifluoromethanesulfone. Examples include acid methide-triethylmethylammonium salt, trifluoromethanesulfonic acid trifluoroacetamide-triethylmethylammonium salt, and the like. Moreover, as another electrolyte mix | blended with the non-aqueous electrolyte of this invention, you may use the 2 or more types from which either one or both of the said cation and anion differ.

  The amount of the electrolyte to be blended in the nonaqueous electrolytic solution of the present invention is not particularly limited and may be appropriately set as necessary. From the viewpoint of the electrical conductivity of the electrolytic solution and the internal resistance, 0.1 to It is desirable to be 2.0 mol / L.

  As a non-aqueous solvent blended in the non-aqueous electrolyte of the present invention, any known organic solvent as a solvent in the non-aqueous electrolyte may be used. The organic solvent to be used is appropriately selected from the electrochemical stability of the organic solvent and the solubility of the electrolyte. Moreover, the organic solvent may be used alone or in combination of two or more. Examples of suitable organic solvents include chain ethers (diethyl ether, dipropyl ether, methyl isobutyl ether, ethylene glycol dimethyl ether, etc.) and cyclic ethers (tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1, 3-dioxalane, etc.), amides (N, N-dimethylformamide, N-methylpyrrolidone, etc.), carboxylic acid esters (methyl acetate, ethyl acetate, etc.), lactones (γ-butyrolactone, etc.) and nitriles (acetonitrile, pro Pionitrile, 3-methoxypropionitrile, etc.) and carbonates (dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, etc.) chain carbonates, ethylene carbonate, propylene carbonate Cyclic carbonate) and sulfoxides (dimethyl sulfoxide, sulfolane, 3-methyl sulfonium run, etc.). Of these, carbonates, sulfoxides and ethers are preferred.

  Since the amount of water in the non-aqueous electrolyte affects the electrochemical capacitor performance, it is desirable that the non-aqueous electrolyte of the present invention contains as little water as possible. There may be a relatively large amount of water due to moisture absorption from the environment. The amount of water can be measured by an existing method such as the Karl Fischer method. As a method of reducing the amount of water, a method of dissolving in a sufficiently dehydrated chain carbonate in a sufficiently dehydrated electrolyte can be mentioned. Dehydration of the electrolyte can be achieved by heating under reduced pressure. The dehydration of the chain carbonate can be achieved by adding a dehydrating agent such as molecular sieve and distilling. By such a method, the water content is preferably 300 ppm or less, more preferably 100 ppm or less, and still more preferably 50 ppm or less.

  Further, the amount of tertiary amine in the electrolytic solution affects the electrochemical capacitor performance, and is preferably 50 ppm or less, and more preferably 20 ppm or less. The amount of tertiary amine can be measured by HPLC or the like. The method of reducing the amount of tertiary amine can be achieved by heating the electrolyte salt or the quaternary amine salt of the raw material under reduced pressure. However, since the quaternary ammonium decomposes and causes an increase in the amount of tertiary amine when the temperature during heating increases, the heating temperature is preferably 100 ° C. or less.

  The electrochemical capacitor using the electrolytic solution of the present invention is not particularly limited, and can be used as a capacitor by directly using the structure and material of a known electrochemical capacitor.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to these.

Method for Measuring Capacitance An electrochemical capacitor was prepared as follows and the capacitance was evaluated. Electrochemical capacitor electrodes (aluminum foil 30 μm, activated carbon layer 150 μm, electrostatic capacity 16 F / CC) purchased from Hosen Co., Ltd. were cut into 100 mm ² in a glove box with a humidity of −80 ° C. or less, and then 180 ° C. for 24 hours. It dried at the reduced pressure of 1 Pa or less. Next, the two electrodes were placed opposite to each other through a separator that had been dried at 180 ° C. for 24 hours under a reduced pressure of 1 Pa or less to produce an electrode element. Next, the electrode element was placed in an aluminum laminate cell and impregnated with an electrolyte under reduced pressure. The electrochemical capacitor produced by this method was charged at a constant current of up to 2.5 V at 0.5 mA in a 25 ° C. environment for 2 hours and then applied for 2 hours, and then discharged at a constant current of 0.25 mA to 0 V. The capacity was determined.

Abbreviations of electrolytes Abbreviations of electrolytes used in the examples and comparative examples are as follows.
TFSI-Li salt: bistrifluoromethanesulfonic acid amide-lithium salt TFSI-Na salt: bistrifluoromethanesulfonic acid amide-sodium salt TFSI-K salt: bistrifluoromethanesulfonic acid amide-potassium salt TFSI-Rb salt : Bistrifluoromethanesulfonic acid amide-rubidium salt / TSAC-K salt: 2,2,2-trifluoro-N- (trifluoromethanesulfonyl) acetamide-potassium salt / TEMA-TFSI: bistrifluoromethanesulfonic acid amide-triethylmethyl Ammonium salt A salt not commercially available was synthesized by the following method.

(1) TFSI-Na salt 101.7 g of commercially available TFSI-Li salt was added to 98% concentrated sulfuric acid and dissolved by heating. Thereafter, vacuum distillation was performed to obtain 92.6 g of TFSI-hydride. The obtained TFSI-hydride was dissolved in pure water, reacted with 18.0 g of sodium carbonate, excess sodium carbonate was filtered off, and the filtrate was concentrated to obtain 95.9 g of TFSI-Na salt.

(2) TFSI-Rb salt 43.1 g of commercially available TFSI-Li salt was added to 98% concentrated sulfuric acid and dissolved by heating. Thereafter, vacuum distillation was performed to obtain 38.7 g of TFSI-hydride. The obtained TFSI-hydride was dissolved in pure water and reacted with 14.6 g of rubidium carbonate, excess rubidium carbonate was filtered off, and the filtrate was concentrated to obtain 49.9 g of TFSI-Li salt.

(3) TSAC-K salt 2,2,2-trifluoro-N- (trifluoromethanesulfonyl) acetamide-potassium salt (hereinafter, K-TFSI) was produced by the following salt exchange method.

First, 69.42 g (0.4656 mol) of trifluoromethanesulfonamide was placed in a 1 L three-necked flask equipped with a thermometer, a dropping funnel, and a nitrogen balloon, 500 ml of dehydrated methanol was added and dissolved, and then tert-butoxy potassium 52 .25 g (0.4656 mol) was added and reacted at 60 ° C. for 3 hours. Thereafter, this solution was concentrated under reduced pressure to obtain a white powder. To this powder, 250 ml of dehydrated diethyl ether was added to form a slurry, cooled to 0 ° C., and then a mixed solution of 97.80 g (0.4656 mol) of trifluoroacetic anhydride and 250 ml of dehydrated diethyl ether was added dropwise. The resulting solution was stirred at 0 ° C. for 2 hours and then reacted at room temperature for 4 hours. Next, the obtained slurry was cooled and then filtered, and washed with diethyl ether to take out crystals. The crystals were dried under reduced pressure to obtain 116.75 g (yield 88.5%) of white powder. ^No peak was observed in ¹ H-NMR, and two singlet peaks of -91.19 ppm (3F) and -94,74 ppm (3F) were confirmed in ¹⁹ F-NMR. Further MS-ESI measurement (solvent: methanol) _in, CF ₃ CONSO ₂ CF 3 ^- anion having a molecular weight of 244.0, which is considered to ions was observed. The above measurement confirmed that the target metal salt was synthesized.

(4) TEMA-TFSI
Dissolve 75.8 g of commercially available special grade triethylmethyl chloride in 500 ml of ultrapure water, add 147.5 g of commercially available bistrifluoromethanesulfonic acid amide-lithium salt for electronic materials, and after stirring for 10 minutes, add 200 ml of commercially available special grade methylene chloride. In addition, it was allowed to stand for 1 hour for salt exchange. After standing, 300 ml of commercially available special grade methylene chloride was added, and the methylene chloride layer and the aqueous layer were separated. To the resulting methylene chloride layer, 250 ml of ultrapure water was added and stirred to separate the aqueous layer. Further, 250 ml of ultrapure water was added to the obtained methylene chloride layer and stirred to separate the aqueous layer. This was done a total of 5 times. The obtained methylene chloride layer was dried under reduced pressure at 60 mmHg and 35 ° C. to obtain 187.9 g of bistrifluoromethanesulfonic acid amide-triethylmethylammonium salt (hereinafter, TEMA-TFSI).

Example 1
56.5 g of TEMA-TFSI and 2.4 g of TFSI-K salt were dissolved in 62.4 g of ethyl methyl carbonate, and the electrolyte concentration was 1.5 mol / L (95 mol% of TEMA-TFSI, 5 mol% of TFSI-K salt). ) Was prepared. The capacitance of this electrolytic solution was measured. The results are shown in Table 1.

Examples 2 and 3
In the same manner as in Example 1, the ratio of TEMA-TFSI and TFSI-K salt was changed as shown in Table 1 to prepare an electrolyte solution composed of an ethyl methyl carbonate solution having an electrolyte concentration of 1.5 mol / L, The capacitance was measured. The results are shown in Table 1.

Comparative Example 1
An electrolytic solution composed of an ethylmethyl carbonate solution having an electrolyte concentration of 1.5 mol / L was prepared using only TEMA-TFSI without using a TFSI-K salt, and the capacitance was measured. The results are shown in Table 1.

Example 4
The electrolyte concentration was 1.5 mol / L (TEMA-TFSI was 95 mol%, TFSI-Rb salt) in the same manner as in Example 1 except that the TFSI-Rb salt whose cation was rubidium ion was used instead of the TFSI-K salt. Of 5 mol%) was prepared, and an electrostatic solution was measured. The results are shown in Table 1.

Comparative Examples 2 and 3
In place of the TFSI-K salt, an electrolyte concentration of 1.5 mol / L (TEMA) was used in the same manner as in Example 1 except that a TFSI-Li salt whose cation was lithium ion or a TFSI-Na salt whose sodium ion was sodium ion was used. -TFSI was 95 mol%) An electrolytic solution composed of an ethyl methyl carbonate solution was prepared, and the capacitance was measured. The results are shown in Table 1.

Example 5
The electrolyte concentration was 1.5 mol / L in the same manner as in Example 1 except that a TSA-K salt having a different anion was used instead of the TFSI-K salt (TEMA-TFSI was 95 mol%, TSA-K salt was 5 mol%). ) Was prepared, and the capacitance was measured. The results are shown in Table 1.

As understood from the results shown in Table 1 above, by adding an alkali metal salt having an atomic number of 19 or more, it is more electrostatic than when no such salt is added (Comparative Example 1). The capacity can be increased. In addition, even in the case of an alkali metal salt, the lithium or sodium salt having the atomic number smaller than 19 (Comparative Examples 2 and 3), on the contrary, the electrostatic capacity is lowered. It can be understood that it is necessary to have a large ion radius.

Claims (2)

  A non-aqueous electrolyte for an electrochemical capacitor containing an electrolyte and a non-aqueous solvent, wherein the electrolyte is an alkali metal salt having an atomic number of 19 or more when the total amount of the electrolyte contained in the electrolyte is 100 mol% Is contained in an amount of 1 to 50 mol%. An electrochemical capacitor using the nonaqueous electrolytic solution according to claim 1.