Electrolyte solution for capacitors, electric double layer capacitor, and lithium ion capacitor

Abstract

The present invention provides a highly conductive, highly voltage-resistant, and stable liquid electrolyte solution for capacitors which does not coagulate and is free from precipitation of salts in a wide temperature range, particularly at low temperatures, shows excellent electrical characteristics, and has excellent long-term reliability. The present invention also provides an electric double-layer capacitor and a lithium ion capacitor produced using the electrolyte solution for capacitors. The present invention relates to an electrolyte solution for capacitors including: an organic solvent; and a quaternary ammonium salt or lithium salt dissolved in the organic solvent, the organic solvent containing acetonitrile and a chain alkyl sulfonic compound represented by the formula (1): ##STR00001##
wherein R.sup.1 and R.sup.2, which may be the same as or different from each other, each independently represent a straight or branched chain C1-C4 alkyl group.

Claims

1. A lithium ion capacitor produced using an electrolyte solution comprising: an organic solvent; and a quaternary ammonium salt or lithium salt dissolved in the organic solvent, the organic solvent containing acetonitrile, a chain alkyl sulfonic compound represented by the formula (1), and sulfolane represented by the formula (2): ##STR00010## wherein R.sup.1 and R.sup.2, which may be the same as or different from each other, each independently represents a straight or branched chain C1-C4 alkyl group.

2. A lithium ion capacitor produced using an electrolyte solution comprising: an organic solvent; and a quaternary ammonium salt or lithium salt dissolved in the organic solvent, the organic solvent containing acetonitrile and a chain alkyl sulfonic compound represented by the formula (1): ##STR00011## wherein R.sup.1 and R.sup.2, which may be the same as or different from each other, each independently represents a straight or branched chain C1-C4 alkyl group.

Description

DESCRIPTION OF EMBODIMENTS

(1) The present invention is explained in more detail below based on examples, but is not limited to these examples.

(2) Table 1 shows formulas of chain alkyl sulfone compounds (ethyl methyl sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone), a cyclic sulfone compound (sulfolane), a chain nitrile compound (acetonitrile), and a carbonate ester compound (propylene carbonate), used as a solvent in examples and comparative examples.

(3) TABLE-US-00001 TABLE 1 Compound name Formula Chain alkyl sulfone compound represented by formula (1) Ethyl methyl sulfone (EMS) Ethyl isopropyl sulfone (EIPS) Ethyl isobutyl sulfone (EIBS) Cyclic sulfone compound Sulfolane Chain nitrile compound Acetonitrile Carbonate ester compound Propylene carbonate

EXAMPLE 1

(4) Ethyl methyl sulfone (EMS, boiling point 239 C., melting point 34 C., viscosity 6 cP (35 C.)) (80 parts by mass) and acetonitrile (20 parts by mass) were placed in a glass screw vial and mixed to prepare a solvent in a dry box under an argon atmosphere in which the dew point was controlled to not more than 50 C. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L with a stirring bar. The screw vial was then sealed. The content was stirred for about 1 hour to completely dissolve the electrolyte using a magnetic stirrer. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was measured using a Karl Fischer moisture meter (produced by Hiranuma Sangyo Corporation, AQ-2200), and was determined as less than 100 ppm.

EXAMPLE 2

(5) Ethyl isopropyl sulfone (EIPS, boiling point 265 C., melting point 11 C., viscosity 6 cP (25 C.)) (16 parts by mass), sulfolane (64 parts by mass), and acetonitrile (20 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 3

(6) Ethyl methyl sulfone (18 parts by mass), sulfolane (72 parts by mass), and acetonitrile (10 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 4

(7) Ethyl methyl sulfone (20 parts by mass), sulfolane (60 parts by mass), and acetonitrile (20 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 5

(8) Ethyl methyl sulfone (60 parts by mass), ethyl isopropyl sulfone (20 parts by mass), and acetonitrile (20 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 6

(9) Ethyl methyl sulfone (70 parts by mass) and acetonitrile (30 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 7

(10) Ethyl isopropyl sulfone (70 parts by mass) and acetonitrile (30 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 8

(11) Ethyl isobutyl sulfone (EIBS, boiling point 261 C., melting point 16 C., viscosity 4 cP (25 C.)) (70 parts by mass) and acetonitrile (30 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 9

(12) Ethyl isopropyl sulfone (95 parts by mass) and acetonitrile (5 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

EXAMPLE 10

(13) Ethyl isopropyl sulfone (20 parts by mass) and acetonitrile (80 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

COMPARATIVE EXAMPLE 1

(14) Triethylmethylammonium tetrafluoroborate was added to acetonitrile to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

(15) The electrolyte solution for capacitors obtained in Comparative Example 1 was highly volatile, and therefore needed to be used and stored at a temperature of 25 C. or lower for suppression of volatilization.

COMPARATIVE EXAMPLE 2

(16) Triethylmethylammonium tetrafluoroborate was added to ethyl methyl sulfone to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

COMPARATIVE EXAMPLE 3

(17) Triethylmethylammonium tetrafluoroborate was added to propylene carbonate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

COMPARATIVE EXAMPLE 4

(18) Triethylmethylammonium tetrafluoroborate was added to sulfolane to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

COMPARATIVE EXAMPLE 5

(19) Ethyl isopropyl sulfone (80 parts by mass) and sulfolane (20 parts by mass) were mixed to prepare a solvent. To the solvent was added triethylmethylammonium tetrafluoroborate to a concentration of 1.0 mol/L. Thus, an electrolyte solution for capacitors was prepared. The moisture value of the prepared electrolyte solution for capacitors was determined as less than 100 ppm.

(20) <Evaluation>

(21) (1) Measurement of Properties of Electrolyte Solution for Electric Double-Layer Capacitors

(22) The conductivities (mS/cm) at 20 C. and 30 C. and the potential windows (V) of the electrolyte solutions for capacitors prepared in examples and comparative examples were measured. The conductivities were measured using LCR HiTESTER 3532-50 produced by HIOKI E.E. CORPORATION, and the potential windows were determined by measuring oxidation decomposition potential and reduction decomposition potential from a voltage range in which a current density of not more than 0.2 mA/cm.sup.2 was measured at a sweep rate of 5 mV/s using electrochemical analyzer Mode1660C produced by ALS. In the measurement, a glass carbon working electrode (diameter 1.7 mm), a platinum wire counter electrode, and a Ag/Ag.sup.+ reference electrode were used. Table 2 shows the results. In Table 2, EMS represents ethyl methyl sulfone, EIPS represents ethyl isopropyl sulfone, and EIBS represents ethyl isobutyl sulfone.

(23) TABLE-US-00002 TABLE 2 Electrolyte solution composition Chain alkyl sulfone compound represented Conductivity Potential by formula (1) Propylene (mS/cm) window EMS EIPS EIBS Acetonitrile Sulfolane carbonate 20 C. 30 C. (V) Example 1 80 20 9.3 1.7 +2.5 to 3.2 Example 2 16 20 64 10.1 2.0 +2.3 to 3.2 Example 3 18 10 72 9.3 0.8 +2.3 to 3.2 Example 4 20 20 60 11.4 2.2 +2.3 to 3.2 Example 5 60 20 20 10.5 2.1 +2.6 to 3.3 Example 6 70 30 18.2 4.6 +2.5 to 3.2 Example 7 70 30 15.7 3.7 +2.5 to 3.2 Example 8 70 30 15.1 3.4 +2.5 to 3.2 Example 9 95 5 5.6 0.6 +2.5 to 3.2 Example 10 20 80 33.8 19.5 +2.6 to 3.2 Comparative 100 53.1 28.0 +2.6 to 3.2 Example 1 Comparative 100 4.1 Coagulated +2.6 to 3.3 Example 2 Comparative 100 11.7 1.1 +1.8 to 3.2 Example 3 Comparative 100 2.7 Coagulated +2.3 to 3.2 Example 4 Comparative 80 20 4.5 Coagulated +2.3 to 3.2 Example 5 The electrolyte solution for capacitors obtained in Comparative Example 1 is highly volatile, and therefore needs to be used and stored at a temperature of 25 C. or lower for suppression of volatilization.
(2) Measurement of Properties of Electric Double-Layer Capacitor

(24) Electric double-layer capacitors were prepared using the electrolyte solutions for capacitors prepared in examples and comparative examples in the following way.

(25) Two 0.1 mm-thick aluminum sheets having a 14 mm-diameter circular shape coated with activated carbon were used as polarizing electrodes disposed to face each other, and a 0.05 mm-thick cellulose separator having a 17 mm-diameter circular shape was disposed between the aluminum sheets. These were housed in an stainless steel outer container (diameter 20 mm, height 3.2 mm, thickness of stainless steel 0.25 mm) equipped with a polypropylene gasket, and impregnated with an electrolyte solution for capacitors. The container was engaged with a crimping machine to prepare a CR2032 size coin-shaped electric double-layer capacitor.

(26) The characteristics of the prepared electric double-layer capacitors were determined through the charge-discharge test at 20 C. and 30 C. Specifically, each capacitor was allowed to stand for not less than 30 min under the given measurement temperature to adjust the temperature of the capacitor to the given temperature, a rated voltage of 2.5 V was applied to the capacitor for 30 min, and the capacitor was discharged at a constant current of 2 mA. The time until the voltage between the capacitor terminals reaches from 2 V to 1 V was measured to determine an electrostatic capacity. The lower limit of the discharge voltage was 0.0 V. The internal resistance was determined similarly to the measurement of the electrostatic capacity in such a way that a rated voltage of 2.5 V was applied to the capacitor for 30 min and the capacitor was discharged at a constant current of 100 mA, and the internal resistance was calculated by IR drop. Table 3 shows the results.

(27) TABLE-US-00003 TABLE 3 Electrolyte solution composition Chain alkyl sulfone Electrostatic Internal compound represented capacity resistance by formula (1) Propylene (F) () EMS EIPS EIBS Acetonitrile Sulfolane carbonate 20 C. 30 C. 20 C. 30 C. Example 1 80 20 0.41 0.34 11.0 38 Example 2 16 20 64 0.42 0.33 12.9 41 Example 3 18 10 72 0.43 0.33 11.1 101 Example 4 20 20 60 0.44 0.35 9.0 45 Example 5 60 20 20 0.42 0.33 11.8 43 Example 6 70 30 0.48 0.37 5.8 21 Example 7 70 30 0.46 0.35 6.9 25 Example 8 70 30 0.45 0.35 7.1 29 Example 9 95 5 0.44 0.34 15.8 93 Example 10 20 80 0.48 0.42 3.5 4.9 Comparative 100 0.50 0.45 2.0 3.4 Example 1 Comparative 100 0.42 Coagulated 22.0 Coagulated Example 2 Comparative 100 0.48 0.33 9.1 88 Example 3 Comparative 100 0.41 Coagulated 28.0 Coagulated Example 4 Comparative 80 20 0.42 Coagulated 21.5 Coagulated Example 5 The electrolyte solution for capacitors obtained in Comparative Example 1 is highly volatile, and therefore needs to be used and stored at a temperature of 25 C. or lower for suppression of volatilization.

(28) Tables 2 and 3 show that the capacitors using the electrolyte solutions for capacitors of the examples in which a solvent mixture containing acetonitrile is used function even at low temperatures, and the electric double-layer capacitors using such electrolyte solutions have sufficient electrostatic capacity even at low temperatures.

INDUSTRIAL APPLICABILITY

(29) The present invention can provide a highly conductive, highly voltage-resistant, and stable liquid electrolyte solution for capacitors which does not coagulate and is free from precipitation of salts in a wide temperature range, particularly at low temperatures, shows excellent electrical characteristics, and has excellent long-term reliability. The present invention can also provide an electric double-layer capacitor and a lithium ion capacitor produced using the electrolyte solution for capacitors.