JPS62252927A - High capacitance electric double-layer capacitor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an electric double layer capacitor that uses a novel electrolyte and has high capacity and high withstand voltage.

(Prior Art) Electric double layer capacitors (capacitors), especially coin-shaped cells (European Patent No. 134706), which utilize an electric double layer formed at the interface between a polarizable electrode and an electrolyte, are used as small-sized, large-capacity capacitors in memory. As a backup power source, demand has been rapidly increasing in recent years.

Conventionally, alkali metal salts and tetraalkylammonium salts of perchloric acid, hexafluorophosphoric acid, fluoroboric acid, or trifluoromethanesulfonic acid have been proposed as solutes for electrolytes for electric/gas double-layer capacitors (especially 1977
-68254, 50-44463, 59-23
2409 publications).

However, when using these known solutes,
For example, when using a tetraalkylammonium salt, the oxidation resistance potential is low, and the withstand voltage of the resulting capacitor is low.
It was still not satisfactory in terms of capacity value, etc.

(Problems to be Solved by the Invention) The present invention attempts to solve the above-mentioned problems in the prior art, and aims to provide an electric double layer capacitor with excellent withstand voltage and capacity.

(Means for Solving the Problems) That is, the present invention provides an electrolytic solution used in an electric double layer capacitor formed at the interface between a polarizable electrode and an electrolytic solution, in which a compound of the general formula (I ) An electrolytic solution is obtained by dissolving a solute of a quaternary phosphonium salt represented by ,
or an alkyl group having 1 to 15 carbon atoms, or an alkyl group having 6 to 15 carbon atoms;
15 aryl groups, X is BF,
PF,, ClO4, ASF,, S b F b , A
I CI 4 or R, Sow CRr represents a fluoroalkyl group having 1 to 8 carbon atoms.

The electrolytic solution of the present invention itself has high conductivity and high chemical stability, so it not only has excellent durability, but also an electric double layer capacitor using the same has an increased oxidation-resistant potential. In particular, it is superior to conventional products in terms of capacity and withstand voltage, and in a particularly preferred embodiment, the capacity is improved by about 30% and the withstand voltage is improved by about 15%.

In the present invention, the general formula (1) is used as the solute of the electrolytic solution.
) It is important to use a quaternary phosphonium salt represented by:

In the general formula (1) of the above electrolyte, [] represents tetraalkylphosphonium or tetraarylphosphonium,
Here, the four alkyl groups or aryl groups bonded to the phosphorus atom may be a symmetrical phosphonium salt consisting of the same group, or an asymmetrical phosphonium salt consisting of at least two or more different groups. It may be.

The alkyl group is preferably a lower alkyl group having 1 to 4 carbon atoms. The aryl group preferably has 1 to 2 benzene nuclei, with phenyl groups being particularly preferred, and may also be a tetraalkyl or arylphosphonium salt in which an alkyl group and an aryl group are bonded to a phosphorus atom.

The symmetrical quaternary phosphonium is preferably tetramethylphosphonium, tetraethylphosphonium,
Tetrapropylphosphonium, tetrabutylphosphonium, dimethyldiethylphosphonium, and the like are preferred in terms of solubility and availability.

On the other hand, R, -R in the asymmetric quaternary phosphonium salt
Examples of combinations of , include, but are not limited to, methyltriethyl, dimethyldiethyl, ethyltrimethyl, diethyldiptyl, propyltributyl, triphenylethyl, etc. Among these, dimethyldiethyl , ethyltrimethyl, butyltriphenyl and the like are particularly preferred in terms of solubility in solvents and availability.

Furthermore, in the general formula (1), as X, 47-fluoroboric acid (BF4), 6-fluorinated phosphoric acid (PF6), perchlorine III (Cl 04 ), 677-arsenic acid (AsF6
), antimonic acid hexafluoride (S b Fh), aluminic acid tetrachloride (AICI, ), or Rr SOs (R
r is preferably a fluoroalkyl group having 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms.

The concentration of such solute in the electrolyte is 0.61 to 3.0.
M/l, particularly preferably 0.5 to 1.5 M/ffi. If the concentration is too low, loss increases due to an increase in internal resistance, while if it is too high, problems such as a decrease in stability due to solute precipitation in cold weather may occur.

In the present invention, the type of organic solvent is not particularly limited, and various conventionally known or well-known ones can be used, and electrochemically stable non-aqueous solvents such as propylene carbonate, γ-butyrolactone, and acetonitrile can be used. , dimethylformamide, 1°2-dimethoxyethane, sulfolane or nitromethane alone or in mixtures are preferred. Preferably, such solvents are used in a substantially anhydrous state.

Among these, propylene carbonate, γ-butyrolactone, or sulfolane alone or in mixtures are preferred.

A unit cell of the capacitor can be obtained by sandwiching a porous separator between electrodes processed and formed to match the shape of the electric double layer capacitor of the present invention, impregnating or filling it with the above electrolyte, and sealing it in a case. can.

The material of the polarizable electrode used in the present invention is not particularly limited, but it is preferable to use activated carbon powder or activated carbon fiber that is electrochemically inert to the electrolyte and has a large specific surface area.

In particular, activated carbon powder contains polytetrafluoroethylene (PT).
Electrodes that are made by adding a binder such as FE), roll-formed into a sheet, and preferably stretched in a uniaxial or biaxial direction have excellent capacity per unit volume, strength, and long-term reliability. Preferably used.

As the porous separator, for example, polypropylene fiber nonwoven fabric, glass fiber mixed fabric, etc. can be suitably used.

Further, the thickness of the separator is suitably 50 to 200 .mu.m, preferably 100 to 150 .mu.m.

The electrolytic solution according to the present invention can be used in any type of electric double layer capacitor, either a spiral structure or a coin structure. The spiral type (European patent 134706) Fig, 2) is
A metal net current collector and a polarizable electrode are rolled together with a roll or the like to obtain a polarizable electrode body, and the polarizable electrode body impregnated with the electrolyte of the present invention and the above separator are alternately stacked to form two sheets. It is constructed by storing polarizable electrode bodies wound in a spiral shape in a state of facing each other in a case.

Coin-shaped structure (European patent 134706 Fig,
4) houses a separator impregnated with the electrolytic solution, a sheet-shaped polarizable electrode opposed to both sides of the separator, and a combination thereof, and makes electrical contact with one of the polarizable electrodes at the bottom. a metal case, a metal lid fitted into the metal case and in electrical contact with the other polarizable electrode, interposed between a peripheral edge of the metal lid and an opening edge of the metal case to insulate each other; The metal case has a structure including a sealing body integrally caulked by the opening edge of the metal case. Electrical contact between the metal lid and the polarizable electrode, and between the metal case and the polarizable electrode, is made through a suitable current collector made of a metal net or a conductive resin.

The electrolytic solution of the present invention is particularly suitable for electric double layer capacitors having a coin-shaped structure.

The reason for this is that coin-type cells are required to be particularly compact, and electrical devices that are carried around (watches, televisions, VTRs, etc.)
This is because electrodes are required to have a large capacity per predetermined volume and have high mechanical properties against cracks and breakage.

Examples 1 to 14 In the following Examples and Comparative Examples, test devices were assembled as described below.

First, activated carbon fibers on the cathode side (
Specific surface area 2000 rrr/g, 3.14 cJ, 0.4
ml thickness), polypropylene fiber nonwoven fabric separator (4
, 9cJ, 0.4111 thickness), anode side activated carbon fiber (3,
14C1l! , 2 dragon thickness) are arranged one on top of the other. At this time, the activated carbon fibers are placed completely opposite each other with a separator in between.

Next, a polytetrafluoroethylene ring having threads on both the inside and outside of the container is screwed into the container to fix the positions of the activated carbon fibers and the separator.

Then, a threaded polytetrafluoroethylene rod with a platinum wire mesh current collector (200 mesh) attached to the tip was screwed into the opening of the ring, and the continuity between the platinum lead wire and the nickel container was measured using an LCR meter. Complete the following by checking with the two-terminal method. Note that the platinum lead wire is drawn out to the outside through a hole provided in the center of the rod.

Using the test apparatus assembled as described above, the characteristics of capacitors using various electrolytes consisting of solutes and solvents shown in Table 1 were evaluated.

In Table 1, TEP is tetraethylphosphonium,
TMP represents tetramethylphosphonium and TPP represents tetraphenylphosphonium.

The evaluation items were the decomposition voltage of the electrolytic solution, which is an index of capacity and withstand voltage, and each was measured using the following procedure.

To measure the capacity, first, a separate lake impregnated with a predetermined electrolytic solution and activated carbon fibers are set in a container, and then constant voltage charging is performed at 8V for 1 hour. After that, constant current discharge at 1 mA,
Measure the time it takes for the voltage between the terminals to reach 0■ during discharge,
The capacity was calculated from that value.

To determine the decomposition voltage, set the test capacitor in the same way as when measuring capacity, apply DC voltage, and calculate the leakage current (L) after 10 minutes.
C) was measured, and the sharp rise point of LC with respect to the time of application was taken as the decomposition voltage of the electrolytic solution.

Table 1 shows the results of tests using different types of electrolyte. The solute concentration was IM/2 in all cases, and number 14 shows a conventional example for comparison.

(Examples 15 to 31, Comparative Examples 1.2) Common to the Examples and Comparative Examples of the present invention is a unit cell of a coin-type electric double layer capacitor as shown in FIG. 1 (diameter: 20mm, thickness: 2mm). (hm) was prepared as follows. First, activated carbon powder (specific surface area: 200Or&/
10% by weight of polytetrafluoroethylene was added to g) and formed into a sheet by wet kneading. The sheet thus obtained was punched out into a disk shape to form polarizable electrodes 1 and 2 (diameter 15, thickness 0.7 mm), and these polarizable electrodes 1.2 were then interposed with a separator 3 made of a nonwoven polypropylene fiber fabric. They are housed in an outer container consisting of a stainless steel cap 4 and a stainless steel can 5, facing each other through the cap. Next, a predetermined electrolytic solution is injected into the unit cell to sufficiently impregnate the polarizable electrode 1.2 and the separator 3 with the electrolytic solution, and then the cap 4 and the can 5 are inserted through the polypropylene backing 6. The ends were caulked and sealed to integrate.

For each cell using the electric double layer capacitor unit cell prepared as described above and using various electrolytes with a phosphonium salt as a solute as shown in Table 2 at a concentration of 1.0 M#, 2. After measuring the initial capacity and internal resistance when a voltage of .8V was applied, the capacity after being stored at 70°C for 1000 hours while applying a voltage of 2.8V to this cell was measured. The capacity deterioration rate (%) was calculated from . The results of these measurements are shown in Table 2.

Note that the internal resistance was measured by an AC two-terminal method (frequency IKH2).

(Next summer, hereafter blank) Table 1 Table 2 (Effects of the invention) As explained above, according to the present invention, the internal resistance is low;
An electric double layer capacitor with excellent reliability and low capacity deterioration rate under high temperature conditions can be obtained.

[Brief explanation of drawings]

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

Claims (8)

[Claims] (1) In an electric double layer capacitor that utilizes an electric double layer formed at the interface between a polarizable electrode and an electrolyte, the electrolyte contains a quaternary phosphonium salt represented by general formula (I) in an organic solvent. An electric double layer capacitor characterized by being made of a molten material. ▲There are mathematical formulas, chemical formulas, tables, etc.▼(I) However, in the general formula (I) above, R_1, R_2,
R_3 and R_4 are each a hydrogen atom (all are H
), or an alkyl group having 1 to 15 carbon atoms or an aryl group having 6 to 15 carbon atoms, and X is BF_4, PF_6, ClO_4, AsF_
6, SbF_6, AlCI_4, or R_fSO_3
(R_f is a fluoroalkyl group having 1 to 8 carbon atoms). (2) The high capacity electric double layer capacitor according to claim 1, wherein the quaternary phosphonium salt is an asymmetric type in which at least two different types of alkyl groups or aryl groups are bonded to the phosphorus atom. . (3) The high capacity electric double layer capacitor according to claim 1, wherein the quaternary phosphonium salt is of a symmetrical type in which the same type of alkyl group or aryl group is bonded to the phosphorus atom. (4) The high capacity electric double layer capacitor according to any one of claims 1 to 3, wherein the alkyl group is a lower alkyl group having 1 to 4 carbon atoms. (5) The high capacity electric double layer capacitor according to any one of claims 1 to 3, wherein the aryl group has 1 to 2 benzene nuclei. (6) X is R_fSO_3, ClO_4 or BF_
4. The high capacity electric double layer capacitor according to claim 1, which is (7) The high capacity electric double layer capacitor according to any one of claims 1 to 6, wherein the organic solvent is propylene carbonate, γ-butyrolactone, sulfolane, or a mixture thereof. (8) The concentration of solute in the electrolyte is 0.1 to 3.0 M/l
A high capacity electric double layer capacitor according to any one of claims 1 to 7.