JPH0351284B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an electric double layer capacitor, and more particularly to an electric double layer capacitor having excellent high-temperature load resistance and low-temperature characteristics. (Prior Art) Electrolytes used in such electric double layer capacitors have traditionally been perchloric acid, hexafluorophosphoric acid, tetrafluoroboric acid, or trifluoromethanesulfonic acid tetraalkylammonium salts, ammonium salts, or alkali. Electrolytes obtained by dissolving electrolytes such as metal salts in organic solvents such as propylene carbonate, γ-butyrolactone, acetonyl, and dimethylformamide are known (Japanese Patent Laid-Open No.
No. 48-50255, JP-A-49-68254, JP-A-59-
Publications such as No. 232409). (Problems to be Solved by the Invention) However, in such conventional electric double layer capacitors, the withstand voltage of the unit cell is generally around 1.8V, and when used as a memory backup power supply, which is the main application, Since it required a withstand voltage of 5.5V, it was commercialized with three cells stacked in series. The withstand voltage of the unit cell
If you increase it to 2.75V or more, you can stack 2 cells,
However, with conventional electric double layer capacitors, when a voltage of 2.75V or more is applied, the solvent in the electrolyte decomposes, reducing the capacity of the capacitor and causing gas generation. Internal resistance may increase due to expansion of the outer case, and electrolyte may leak from the cell.
There was a problem in that such deterioration phenomenon was particularly noticeable when used at high temperatures. (Means for Solving the Problems) In order to solve the above problems, the present invention provides an electric double layer capacitor that utilizes an electric double layer formed at the interface between a polarizable electrode and an electrode solution, in which sulfolane is used as an electrolyte. The present invention provides an electric double layer capacitor characterized by using a solution in which an electrolyte is dissolved in a mixed solvent of propylene carbonate and at least one type of solvent selected from among Capacitors manufactured by the company have excellent high-temperature load resistance and low-temperature characteristics. Sulfolane and sulfolane derivatives used in the present invention, such as 3-methylsulfolane or 2,4-dimethylsulfolane, have high electrochemical stability, are resistant to electrolytic oxidation or electrolytic reduction, and have the highest usable potential range. It is one of the widest range of solvents. For this reason, it is suitable as a solvent for the electrolyte used in electric double layer capacitors, and the solvent does not decompose even when a high voltage of 3V or more is applied.
It is possible to provide an electric double layer capacitor that is highly reliable and has excellent withstand voltage characteristics. However, on the other hand, sulfolane solvents have a high viscosity and a relatively high freezing point, so when used as an electrolyte solution, the electrical conductivity is low, and the internal resistance of the capacitor increases, particularly in low temperature regions, resulting in a decrease in capacitance. There was a drawback that it was difficult to use. The present invention solves the above-mentioned drawbacks by mixing propylene carbonate, which is a solvent with a low freezing point and excellent low-temperature properties and electrochemical stability, with a sulfolane solvent. The content of propylene carbonate in the mixed solvent used in the present invention is preferably 10 to 80% by volume in order to achieve both electrochemical stability and low-temperature properties when a high voltage is applied. Preferably it is 15-60% by volume, most preferably 20-50% by volume. The electrolytes of the electrolyte in the present invention include those commonly used, such as alkali metals, alkaline earth metals, tetraalkylammonium,
Cations such as tetraalkylphosphonium,
Salts formed in combination with anions such as tetrachloroaluminic acid, tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroarsenic acid, perchloric acid, and perfluoromethanesulfonic acid are preferably used. Among these salts, tetrafluoroborates, hexafluorophosphates, and hexafluorophosphates of tetraalkylphosphonium or tetraalkylammonium are preferred in terms of solubility in solvents, electrical conductivity of solutions, and electrochemical stability. arsenate,
Perchlorate salts or trifluoromethanesulfonate salts are particularly suitable as electrolytes in the present invention. The material of the polarized electrode used in the present invention is not particularly limited, but it is preferable to use activated carbon or activated carbon fibers that are electrochemically inert to the electrolyte and have a large specific surface area. In particular, electrodes made by adding a binder such as polytetrafluoroethylene (PTFE) to activated carbon, roll-forming it into a sheet, and subjecting it to stretching treatment as necessary, have excellent capacity per unit volume, strength, and long-term performance. It is preferable because it has excellent reliability. (Example) Next, Examples and Comparative Examples will be specifically described based on the drawings. As an example of the present invention and a relatively common unit cell of a coin-shaped electric double layer capacitor (diameter 20 mm, thickness 2.0 mm) as shown in FIG.
was produced as follows. First, 10% by weight of polytetrafluoroethylene was added to activated carbon powder (specific surface area: approximately 2000 m ² /g) and formed into a sheet by wet kneading. The sheet thus obtained was punched out into a disk shape to obtain polarizable electrode 1 (diameter 15 mm, thickness 0.7 mm), and polarizable electrode 1 and polarizable electrode 2 having the same composition and shape were made of polypropylene. A stainless steel cap 4 and a stainless steel can 5 are inserted through a separator 3 made of fiber nonwoven fabric.
Stored in an outer container consisting of Next, a predetermined electrolyte is injected into the unit cell, and the polarizable electrode 1,
After the electrolytic solution was sufficiently impregnated into the cap 4 and the separator 3, the ends of the cap 4 and the can 5 were caulked and sealed with a polypropylene packing 6 interposed therebetween. Using the electric double layer capacitor unit cell manufactured as described above, the initial capacitance (Fo) and After measuring the internal resistance, subsequently apply 2.8V to this cell.
The capacitance (F) and internal resistance after storage at 70° C. for 1000 hours while applying a voltage of 100° C. were measured, and these measured values are shown in Table 1 so that they can be compared with the initial values.
Internal resistance was measured at 20°C and -10°C by the AC two terminal method (frequency 1KHz). The measurement results for Examples 1 to 10 and Comparative Examples 1 and 2 are shown in Table 1.

[Table] As is clear from Table 1, according to the present invention,
Even when high voltages of 2.75V or higher are applied at high temperatures, there is less deterioration in capacity compared to conventional products.
Moreover, it is possible to provide a highly reliable electric double layer capacitor whose internal resistance has small temperature dependence. Therefore, it can be used as a memory backup power source.
When used with a 5.5V rating, costs can be reduced by changing from the conventional 3-cell stack to a 2-cell stack. Furthermore, if two cells are stacked, the capacity can be increased by 1.5 times compared to the case where three cells are stacked, which is extremely advantageous in practical terms. (Effects of the Invention) As explained above, according to the present invention, a highly reliable electric double layer capacitor with little capacity deterioration under high temperature conditions and low temperature dependence of internal resistance can be obtained.

[Brief explanation of drawings]

FIG. 1 is a partial sectional view showing an embodiment of an electric double layer capacitor according to the present invention. 1, 2...Polarizable electrode, 3...Separator, 4
... Cap, 5... Can, 6... Packing.

Claims (1)

[Claims] 1. An electric double layer capacitor that utilizes an electric double layer formed at the interface between a polarizable electrode and an electrolytic solution, wherein the electrolytic solution is at least one selected from sulfolane and sulfolane derivatives.
An electric double layer capacitor characterized by using a solution in which an electrolyte is dissolved in a mixed solvent of various types of solvents and propylene carbonate. 2. The electric double layer capacitor according to claim 1, wherein the content of propylene carbonate in the mixed solvent is 10 to 80% by volume. 3. The electrolyte is a combination of a cation made of tetraalkylphosphonium or tetraalkylammonium and an anion made of tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroarsenic acid, perchloric acid or trifluoromethanesulfonic acid. The electric double layer capacitor according to claim 2, which is a salt.