Electric double layer capacitor

An electric double layer capacitor utilizing an electric double layer formed by the interface of an electrolyte solution and polarizable electrodes, wherein the electrolyte solution comprises a solute dissolved in at least one solvent selected from the group consisting of sulfolane and a derivative thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electric double layer capacitor, and 
more particularly to an electric double layer capacitor having excellent 
high temperature load characteristics and low temperature characteristics. 
2. Discussion of Background 
As an electrolyte solution to be used for such an electric double layer 
capacitor, there have been proposed electrolyte solutions obtained by 
dissolving an electrolyte such as a tetraalkylammonium salt, ammonium salt 
or alkali metal salt of perchloric acid, hexafluorophosphoric acid, 
tetrafluoroboric acid or trifluoromethane sulfonic acid, in an organic 
solvent such as propylene carbonate, .gamma.-butyrolactone, acetonitlile 
or dimethylformamide (Japanese Unexamined Patent Publications Nos. 
50255/1973, 68254/1974 and 232409/1984). 
However, with such conventional electric double layer capacitors, the 
working voltage per unit cell was about 1.8 V. A working voltage at a 
level of 5.5 V is required for their main use as a memory back-up electric 
power source. Therefore, it has been common to laminate three cells in 
series to obtain a commercial product. If the working voltage per unit 
cell can be improved to a level of 2.75 V or higher, the desired level of 
working voltage can be obtained by laminating only two cells, whereby the 
costs can be reduced. On the other hand, with conventional electric double 
layer capacitors, if a working voltage of 2.75 or higher is applied, 
decomposition of the solvent of the electrolyte solution occurs, thus 
leading to problems such that the capacity decreases, the internal 
resistance increases since the outer casing expands due to the generation 
of gas, and further the electrolyte solution is likely to leak from the 
cells. Such phenomena for degradation are remarkable particularly when the 
capacitors are used at high temperatures. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to solve the above-mentioned 
problems. 
The present invention provides an electric double layer capacitor utilizing 
an electric double layer formed by the interface of an electrolyte 
solution and polarizable electrodes, wherein the electrolyte solution 
comprises a solute dissolved in at least one solvent selected from the 
group consisting of sulfolane and a derivative thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the present invention, the derivative of sulfolane to be used as a 
solvent for electrolyte is preferably 3-methylsulfolane or 
2,4-dimethylsulfolane. Such sulfolane or its derivatives may be used alone 
independently. However, in some cases, such sulfolane and its derivative 
may be used in combination as a solvent mixture in the present invention. 
Sulfolane by itself has a solidifying point as high as 28.5.degree. C. and 
a high dielectric constant as its characteristic. Whereas, 
3-methylsulfolane or 2,4-dimethylsulfolane has a low solidifying point and 
has good low temperature characteristics. Therefore, such a solvent 
mixture may have the characteristics of both materials i.e. good low 
temperature characteristics and high dielectric constant. When sulfolane 
is mixed with its derivative, the amount of the derivative is preferably 
from 20 to 70% by weight, more preferably from 30 to 60% by weight, to 
obtain adequate low temperature characteristics and low internal 
resistance. 
Sulfolane and sulfolane derivatives such as 3-methylsulfolane and 
2,4-dimethylsulfolane, to be used in the present invention, have high 
electrochemical stability, and they are hardly susceptible to electrolytic 
oxidation or reduction. They have a wide useful range of electric 
potential. Thus, they are suitable as a solvent for electrolyte to be used 
for an electric double layer capacitor, and they are capable of providing 
an electric double layer capacitor having a highly dependable excellent 
working voltage characteristic, whereby no decomposition of the solvent 
results even when a high voltage of 3 V or higher is applied. 
However, a sulfolane solvent has a high viscosity and a relatively high 
solidifying point. Therefore, when it is used for an electrolyte solution, 
the electric conductivity tends to be low, and the internal resistance of 
the capacitor tends to increase particularly in a low temperature region, 
whereby the capacity is likely to decrease. It has been found that in such 
a case, the problems can be solved by mixing to the sulfolane solvent 
propylene carbonate or butylene carbonate as a solvent having a low 
solidifying point and excellent low temperature characteristics and 
electrochemical stability. 
The content of propylene carbonate in the solvent mixture used in the 
present invention is preferably from 10 to 80% by volume, more preferably 
from 15 to 60% by volume, most preferably from 20 to 50% by volume, in 
order to obtain excellent electrochemical stability and low temperature 
characteristics at the same time even when a high voltage is applied. 
Likewise, the content of butylene carbonate in the solvent mixture, is 
preferably from 10 to 80% by volume, more preferably from 15 to 60% by 
volume, most preferably from 20 to 50% by volume. 
Further, it has been found very effective to incorporate to the sulfolane 
solvent chlorobenzene 
##STR1## 
as a solvent having a low solidifying point and excellent low temperature 
characteristics and electrochemical stability. The content of 
chlorobenzene in the solvent mixture of the present invention, is 
preferably from 10 to 70% by volume, more preferably from 20 to 60% by 
volume, most preferably from 30 to 50% by volume, to satisfy the 
properties such as the electrochemical stability and low temperature 
characteristics when a high voltage is applied, and the solubility for 
electrolyte. 
With respect to the electrolyte for the electrolyte solution of the present 
invention, there is no particular restriction. For instance, an 
electrolyte formed by the combination of a cation such as an alkali metal, 
an alkaline earth metal, tetraalkylammonium or tetraalkylphosphonium, with 
an anion such as tetrachloroaluminic acid, tetrafluoroboric acid, 
hexafluorophosphoric acid, hexafluoroarsenic acid, perchloric acid or 
trifluoromethanesulfonic acid, may suitably be used. Among these salts, a 
tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate, perchlorate or 
trifluoromethanesulfonate of tetraalkylphosphonium or tetraalkylammonium 
is particularly suitable as the electrolyte for the present invention in 
view of the solubility to the solvent, the electric conductivity of the 
solution and the electrochemical stability. 
There is no particular restriction as to the material for the polarizable 
electrodes to be used in the present invention. However, it is preferred 
to employ activated carbon powder or activated carbon fiber which is 
electrochemically inert to the electrolyte solution and which has a large 
specific surface area. Particularly preferred is an electrode obtained by 
adding a binder such as polytetrafluoroethylene (PTFE) to activated carbon 
powder, rolling the mixture to form a sheet, and, if necessary, subjecting 
the sheet to stretching treatment, since it is superior in the capacity 
per unit volume, in the strength and in the dependability for a long 
period of time. 
Now, the present invention will be described in further detail with 
reference to Examples and Comparative Examples. However, it should be 
understood that the present invention is by no means restricted to such 
specific Examples. 
EXAMPLES 1 to 11 and COMATIVE EXAMPLES 1 to 3 
In the following Examples and Comparative Examples, the test apparatus was 
assembled as follows: 
Firstly, in an internally threaded cylindrical nickel container having a 
bottom, an activated carbon fiber cloth (specific surface area: 2000 
m.sup.2 /g, 3.14 cm.sup.2, 0.4 mm in thickness) as a cathode side 
polarizable electrode, a separator made of a non-woven fabric of 
polypropylene (4.9 cm.sup.2, 0.4 mm in thickness) and an activated carbon 
fiber cloth (3.14 cm.sup.2, 2 mm in thickness) as an anode side 
polarizable electrode each impregnated with an electrolyte solution to be 
tested, were overlaid one after another. In this case, the activated 
carbon fiber cloths were arranged to face each other with the separator 
interposed therebetween. 
Then, an externally and internally threaded ring of polytetrafluoroethylene 
was screwed in this container to fix the positions of the activated carbon 
fiber cloths and the separator. 
Then, a threaded rod of polytetrafluoroethylene having provided at the 
forward end with a platinum net current collector (200 mesh) having a 
platinum lead wire, was screwed in the opening of the above-mentioned 
ring. The assembling was completed by confirming the electric connection 
of the platinum lead wire and the nickel container by an AC two-terminal 
method using a LCR meter. The platinum lead wire was led out through a 
hole provided at the center of the above-mentioned rod. 
By using the test apparatus assembled as described above, the properties of 
capacitors in which various electrolyte solutions comprising the solutes 
and solvents as identified in Table 1, were used so that they were 
adequately impregnated to the anode and cathode composed of activated 
carbon fibers, were evaluated. 
The evaluation was made with respect to the decomposition voltage of the 
electrolyte solution as an index for the working voltage, and the capacity 
retention after the storage at a high temperature. The measurements were 
conducted, respectively, as follows. 
For the measurement of the decomposition voltage, the test capacitor was 
set, and then a direct current voltage was applied. Ten minutes later, the 
leakage current was measured, and the voltage at which the leakage current 
increased abruptly when the applied voltage was gradually increased, was 
taken as the decomposition voltage. 
The measurement of the capacity retention (Io) after the storage at a high 
temperature, was conducted as follows. Firstly, a test capacitor was set, 
and then charging was conducted at a constant voltage of 2.8 V for 1 hour. 
Then, discharging was conducted at a constant current of 1 mA, whereby the 
time until the terminal voltage during the discharging became 1.0 V was 
measured, and the initial capacity (Fo) was calculated from the measured 
value. 
Then, the same test cell was stored in a constant temperature tank of 
85.degree. C. for 1,000 hours while applying a voltage of 2.8 V, and then 
the capacity (F) after the storage was measured in the same manner as 
above, whereupon the capacity retention after the storage at a high 
temperature was calculated by Io=F/Fo.times.100. 
The results of the tests in which the type of the electrolyte solution was 
varied, are shown in Table 1. Comparative Examples 1 to 3 represent 
conventional electrolyte solutions. In the Table, TEA represents 
tetraethylammonium, and TBA represents tetrabutylammonium. 
TABLE 1 
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Capacitor properties 
Capacity 
Electrolyte Decomposition 
retention 
No. Solvent Solute voltage (V) 
Io (%) 
__________________________________________________________________________ 
Example 
1 Sulfolane 0.7 M TEA.BF.sub.4 
5.4 91 
2 Sulfolane 0.7 M TBE.BF.sub.4 
5.3 89 
3 3-Methylsulfolane 
0.5 M TEA.BF.sub.4 
5.4 88 
4 2,4-Dimethylsulfolane 
0.5 M TEA.BF.sub.4 
5.3 85 
5 Sulfolane 70% 
0.6 M TEA.BF.sub.4 
5.4 90 
3-Methylsulfolane 
30% 
6 Sulfolane 50% 
0.6 M TEA.BF.sub.4 
5.4 89 
3-Methylsulfolane 
50% 
7 Sulfolane 30% 
0.5 M TEA.BF.sub.4 
5.4 88 
3-Methylsulfolane 
70% 
8 Sulfolane 50% 
0.5 M TEA.BF.sub.4 
5.3 87 
2,4-Dimethylsulfolane 
50% 
9 Sulfolane 60% 
0.6 M TEA.BF.sub.4 
5.3 88 
3-Methylsulfolane 
20% 
2,4-Dimethylsulfolane 
20% 
10 Sulfolane 70% 
0.8 M TEA.BF.sub.4 
5.2 89 
Propylene carbonate 
30% 
11 3-Methylsulfolane 
70% 
0.8 M TEA.BF.sub.4 
5.1 86 
.gamma.-Butyrolactone 
30% 
Comparative 
Example 
1 Propylene carbonate 
0.7 M TEA.BF.sub.4 
4.9 64 
2 .gamma.-Butyrolactone 
0.7 M TEA.BF.sub.4 
4.9 60 
3 Dimethylformamide 
0.7 M TEA.BF.sub.4 
4.7 53 
__________________________________________________________________________ 
EXAMPLES 12 to 21 and COMATIVE EXAMPLE 4 
In each of the Examples of the present invention and the Compartive 
Example, a unit cell (diameter: 20 mm, thickness: 2.0 mm) of a coin-shaped 
electric double layer capacitor as shown in FIG. 1 was prepared as 
follows. 
Firstly, 10% by weight of polytetrafluoroethylene was added to activated 
carbon powder (specific surface area: 2000 m.sup.2 /g), and the mixture 
was formed into a sheet by a wet-type kneading. The sheet thus obtained, 
was punched out to obtain disc-shaped polarizable electrodes 1 and 2 
(diameter: 15 mm, thickness: 0.7 mm). These polarizable electrodes 1 and 2 
facing to each other with a separator 3 of a non-woven fabric of 
polypropylene fiber interposed therebetween, were placed in a container 
comprising a stainless steel cap 4 and a stainless steel can 5. Then, the 
predetermined electrolyte solution was injected in the unit cell so that 
the polarizable electrodes 1 and 2 and the separator 3 were adequately 
impregnated with this electrolyte solution. Then, the edges of the cap 4 
and the can 5 were caulked with a polypropylene packing 6 interposed 
therebetween, for sealing. 
By using the unit cell of an electric double layer capacitor prepared as 
described above, the initial capacity (Fo) and the internal resistance 
upon application of a voltage of 2.8 V were measured with respect to each 
of cells containing various electrolyte solutions as shown in Table 2, as 
solutes. Then, each cell was stored at 70.degree. C. for 1000 hours while 
continuously applying a voltage of 2.8 V thereto, whereupon the capacity 
(F) and the internal resistance were measured. The measured values are 
presented in Table 2 so that they can readily be compared with the initial 
values. The internal resistance was measured by an alternate current 
two-terminal method (frequency: 1 KHz) at 20.degree. C. and -10.degree. C. 
The results of Examples 12 to 21 and Comparative Example 4 are shown in 
Table 2. 
TABLE 2 
__________________________________________________________________________ 
Solvents Capacitor properties 
Propylene After application of 2.8 
V 
carbonate 
Electrolyte 
Initial values at 70.degree. C. for 
1000 hrs. 
Sulfonate solvent 
(% by and Internal resistance .OMEGA. 
Capacity 
Internal 
(% by volume) 
volume) 
concentration 
-10.degree. C. 
20.degree. C. 
Fo resistance 
Capacity 
__________________________________________________________________________ 
F 
Example 
12 Sulfolane 70 
30 0.8 M 30.2 11.2 2.42 18.5 2.18 
(Et).sub.4 NBF.sub.4 
13 Sulfolane 70 
30 1.0 M 34.7 13.2 2.60 17.3 2.37 
(Bu).sub.4 NPF.sub.6 
14 Sulfolane 70 
30 1.2 M 35.8 12.8 2.38 16.6 2.29 
(Bu).sub.4 PBF.sub.4 
15 3-Methylsulfolane 
60 
40 1.0 M 41.3 14.7 2.36 20.9 2.22 
(Bu).sub.4 PBF.sub.4 
16 2,4-Dimethylsulfolane 
60 
40 1.0 M 38.6 13.9 2.36 19.5 2.16 
(Et).sub.4 NBF.sub.4 
17 Sulfolane 90 
10 1.0 M .infin. 
15.7 2.30 20.8 2.14 
(Bu).sub.4 PBF.sub.4 
18 Sulfolane 20 
20 1.0 M 43.7 14.5 2.34 21.8 2.15 
(Bu).sub.4 PBF.sub.4 
19 Sulfolane 50 
50 1.0 M 30.0 11.6 2.45 22.3 2.17 
(Bu).sub.4 PBF.sub.4 
20 Sulfolane 20 
80 1.0 M 24.5 9.8 2.52 24.6 2.01 
(Bu).sub.4 PBF.sub.4 
21 Sulfolane 70 
30 1.0 M 26.3 10.3 2.47 27.7 1.89 
LiBF.sub.4 
Comparative 
Example 
4 -- 100 1.0 M 17.2 7.5 2.61 38.5 1.69 
(Et).sub.4 NBF.sub.4 
__________________________________________________________________________ 
Note: Et: C.sub.2 H.sub.5 (i.e. ethyl group), Bu: C.sub.4 H.sub.9 (i.e. 
nbutyl group) 
As is evident from Table 2, according to the present invention, it is 
possible to provide a highly reliable electric double layer capacitor 
having a small temperature dependency of the internal resistance and a 
small deterioration in the capacity as compared with the conventional 
capacitors even when a high voltage of a level of 2.75 V or higher is 
applied at a high temperature. Thus, when it is used at a constant level 
of 5.5 V as a memory back-up electric power source, it is possible to 
reduce the costs by changing the conventional three-cell structure to a 
two-cell structure. Further, with the two-cell structure, the capacity can 
h=increased by 1.5 times the capacity with the three-cell structure, which 
is practically extremely advantageous. 
EXAMPLES 22 to 26 and COMATIVE EXAMPLE 5 
In the same manner as in Examples 12 to 21, the initial capacity (Fo) and 
the internal resistance upon application of a voltage of 2.8 V were 
measured with respect to each of cells containing various electrolyte 
solutions wherein a solvent mixture of a sulfolane solvent and 1-butylene 
carbonate was used, as shown in Table 3. Then, each cell was stored at 
70.degree. C. for 1,000 hours while continuously applying a voltage of 2.8 
V thereto, whereupon the capacity was measured, and the capacity 
deterioration rate (%) from the initial capacity (Fo) was calculated. The 
internal resistance was measured by an alternate current two-terminal 
method (frequency: 1 KHz) at 20.degree. C. and -25.degree. C. The results 
of Examples 22 to 26 and Comparative Example 5 are shown in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Solvents Capacitor properties 
Butylene 
Sulfolane Initial 
Capacity 
Internal 
Internal 
carbonate 
solvent capacity 
deterioration 
resistance 
resistance 
.OMEGA. 
(% by volume) 
(% by volume) Electrolyte 
(Fo) (%) (20.degree. C.) 
(-25.degree. 
__________________________________________________________________________ 
C.) 
Example 
22 50 Sulfolane 
50 (C.sub.4 H.sub.9).sub.4 PBF.sub.4 
2.20 2.4 15.4 65.3 
23 50 Sulfolane 
50 (C.sub.2 H.sub.5).sub.4 NBF.sub.4 
2.28 2.8 13.3 64.7 
24 30 3-Methylsulfolane 
70 (C.sub.4 H.sub.9).sub.4 PBF.sub.4 
2.32 2.9 18.2 73.8 
25 33.3 Sulfolane 
33.3 (C.sub.4 H.sub.9).sub.4 PBF.sub.4 
2.31 3.0 21.2 88.2 
26 33.3 3-Methylsulfolane 
33.3 (C.sub.2 H.sub.5).sub.4 NBF.sub.4 
2.35 3.5 22.3 91.2 
Comparative 
Example 
5 Propylene carbonate 
100 (C.sub.2 H.sub.5).sub.4 NBF.sub.4 
2.61 35.2 7.5 30 
__________________________________________________________________________ 
Note 1: The concentration of the electrolyte was 1.0 M in each of Example 
and Comparative Example. 
Note 2: The capacity deterioration rate (%) was measured after applicatio 
of 2.8 V at 70.degree. C. for 1,000 hours. 
It is evident from Table 3 that by using an electrolyte solution in which a 
solvent mixture of a sulfolane solvent and butylene carbonate is used 
according to the present invention, it is possible to provide a highly 
reliable electric double layer capacitor having a small temperature 
dependency of the internal resistance, and small deterioration of the 
capacity as compared with the conventional electrolyte solutions even when 
a voltage as high as 2.75 V or higher is applied at a high temperature. 
EXAMPLES 27 to 33 and COMATIVE EXAMPLES 6 to 7 
In each of the Examples of the present invention and the Compartive 
Example, a unit cell (diameter: 20 mm, thickness: 2.0 mm) of a coin-shaped 
electric double layer capacitor as shown in FIG. 2 was prepared as 
follows. 
Firstly, 10% by weight of polytetrafluoroethylene was added to activated 
carbon powder (specific surface area: 2000 m.sup.2 /g), and the mixture 
was formed into a sheet by a wet-type kneading. The sheet thus obtained, 
was punched out to obtain disc-shaped polarizable electrodes 11 and 12 
(diameter: 15 mm, thickness: 0.7 mm). These polarizable electrodes 11 and 
12 facing to each other with a separator 13 of a non-woven fabric of 
polypropylene fiber interposed therebetween, were placed in a container 
comprising a stainless steel cap 14 and a stainless steel can 15 so that 
they were bonded a graphite type conductive adhesive 17. Then, the 
predetermined electrolyte solution was injected in the unit cell so that 
the polarizable electrodes 11 and 12 and the separator 13 were adequately 
impregnated with this electrolyte solution. Then, the edges of the cap 14 
and the can 15 were caulked with a polypropylene packing 16 interposed 
therebetween, for sealing. The polarizable electrodes 11 and 12 were 
bonded to the inside of the cap 14 and the inside of the can 15, 
respectively, by the conductive resin layers 17. 
By using the unit cell of an electric double layer capacitor prepared as 
described above, the initial capacity (Fo) at 20.degree. C. and the 
internal resistance upon application of a voltage of 2.8 V were measured 
at 20.degree. C. and -10.degree. C. with respect to each of cells 
containing various electrolyte solutions as shown in Table 4, as solutes. 
Then, each cell was stored at 70.degree. C. for 1000 hours while 
continuously applying a voltage of 2.8 V thereto, whereupon the capacity 
(F) and the internal resistance were measured at 20.degree. C. The 
measured values are presented in Table 4 so that they can readily be 
compared with the initial values. The internal resistance was measured by 
an alternate current two-terminal method (frequency: 1 KHz). The results 
of Examples 27 to 33 and Comparative Example 6 and 7 are shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Capacitor properties 
After application of 28 
V 
Electrolyte 
Initial values at 70.degree. C. for 
1000 hrs. 
Solvents and Internal Internal 
Sulfolane solvent 
Chlorobenzene 
concentra- 
resistance .OMEGA. 
Capacity 
resistance 
Capacity F 
(% by volume) (% by volume) 
tion -10.degree. C. 
20.degree. C. 
Fo (F) 
20.degree. C. 
(F)EGA. 
__________________________________________________________________________ 
Example 
27 Sulfolane 60 40 0.8 M 31.5 12.1 2.37 19.3 2.09 
(Bu).sub.4 PBF.sub.4 
28 Sulfolane 60 40 0.4 M 28.8 11.0 2.42 17.4 2.15 
(Bu).sub.4 PBF.sub.4 
0.3 M 
(Et).sub.4 PBF.sub.4 
29 Sulfolane 60 40 0.3 M 27.4 10.8 2.44 16.5 2.09 
(Bu).sub.4 NBF.sub.4 
0.3 M 
(Et).sub.4 NBF.sub.4 
30 3-Methylsulfolane 
70 30 0.8 M 34.2 12.2 2.35 19.7 2.01 
(Bu).sub.4 PBF.sub.4 
31 2,4-Dimethylsulfolane 
70 30 0.8 M 33.1 12.4 2.36 20.8 2.00 
(Bu).sub.4 PBF.sub.4 
32 Sulfolane 40 60 0.7 M 28.4 11.3 2.39 20.5 1.98 
(Bu).sub.4 PBF.sub.4 
33 Sulfolane 80 20 0.8 M 37.9 13.9 2.29 24.6 2.05 
(Bu).sub.4 PBF.sub.4 
Comparative 
Example 
6 Propylene carbonate 
100 
-- 0.8 M 22.1 8.7 2.56 49.1 1.79 
(Bu).sub.4 PBF.sub.4 
7 Propylene carbonate 
100 
-- 0.8 M 19.3 7.9 2.62 37.3 1.70 
(Et).sub.4 NBF.sub.4 
__________________________________________________________________________ 
Note: Et: C.sub.2 H.sub.5 (i.e. ethyl group), Bu: C.sub.4 H.sub.9 (i.e. 
nbutyl group)