Electric double layer capacitor

The invention relates to an electric double layer capacitor in which a metallic piece for a case at the side of an anode is formed with an aluminum layer. The aluminum layer has an electrochemically stable anodized film according to an applied voltage and has so low a resistance as not to present any practical problem. This property is effectively utilized to impart a breakdown voltage not less than 3 V to the capacitor.

TECHNICAL FIELD 
This invention relates to an electric double layer capacitor utilizing an 
electric double layer formed at the interface between polarizable 
electrodes and an electrolyte. 
BACKGROUND TECHNOLOGY 
Known electric double layer capacitors of the type mentioned above are 
comprised of, as shown in FIG. 1, polarizable electrodes 1 which are 
obtained by press molding of active carbon particles, by applying a 
mixture of active carbon particles and an appropriate binder onto a 
collector metal, or by forming a spray coating layer of aluminium on 
active carbon fibers. The polarizable electrodes 1 are accomodated in a 
stainless steel case 2 and are facing each other through an electrolytic 
solution and a separator 3. The metallic case 2 is sealed through a gasket 
4 at a peripheral opening thereof. 
Another type of known capacitor now in use is shown in FIG. 2 in which a 
non-polarizable electrode 5 is used as one electrode. 
In these known arrangements, solvents used for the electrolytic solution 
are propylene carbonate, r-butyrolactone, N,N-dimethylformaldehyde, and 
acetonitrile. In the case of polarization at the anode, the stainless 
steel in the electrolytic solution cannot make a complete passive state 
but dissolves in the solution. The potential at which the current starts 
to run owing to the dissolution is 2.3-2.4 volts which is determined 
depending on the decomposition potential of the solvent at the cathode. 
This is lower than the potential of oxidation of active carbon or the 
potential of decomposition of the electrolyte in the electrolytic solution 
using the organic solvents. Accordingly, when the stainless steel case 2 
is used as a current collector, the potential at the anode is restricted 
by the potential of dissolution of the stainless steel, so that a 
potential of 3 V which is in an electrochemically stable potential region 
determined by the polarizable electrode 1 and the electrolytic solution 
could not be effectively utilized. 
For instance, when an excess voltage over a voltage at which a leakage 
current starts to increase is applied, large amounts of iron, nickel and 
the like are detected in the electrode at the side of the anode, from 
which it has been confirmed that the stainless steel is dissolved and iron 
ions are moved from the anode toward the cathode. 
As will be seen from the above, when stainless steel is used as the 
metallic case 2, it is difficult to effectively utilize a potential of 3 V 
which is in an electrochemically stable region determined by the active 
carbon polarizable electrode 1 and the electrolytic solution. In order to 
obtain an electric double layer capacitor of a breakdown voltage which 
enables one to use 3 V, it is necessary to use a material which allows 
passage of a reactive current at a potential equal to or larger than the 
active carbon of the polarizable electrode 1 and which has sufficient 
strength when the anode is subjected to polarization in a solvent used. 
One such a material may be titanium which forms a passive state in 
electrolytic solutions. As shown in FIG. 3, however, the breakdown voltage 
becomes higher than in the case using stainless steel. With electrolytic 
solutions using propylene carbonate and tetraethylammonium perchlorate, 
the region where the reactive current flows increases by about 0.8 V. 
However, the internal resistance increases, so that when the voltage drop 
becomes large in the case where the electric double layer capacitor is 
used, there is an attendant problem that such a capacitor cannot be in 
use. The present invention is contemplated to solve the above problem and 
has for its object the provision of an electric double layer capacitor 
which has a high breakdown voltage over 3 V. 
DISCLOSURE OF THE INVENTION 
The present invention contemplates to solve the above problem and has for 
its object the provision of an electric double layer capacitor having a 
high breakdown voltage over 3 V. 
More particularly, the present invention has such an arrangement that a 
metallic case piece at the side of an anode which is in contact with a 
conductive electrode and electrolyte is provided with an aluminium layer 
on the inner surface thereof. When aluminium is formed on the surface in 
contact with the electrolyte, an oxide film is formed on the aluminium 
layer according to an been applied voltage. In a region to which a voltage 
has once applied, no reactive current passes, so that the reaction of 
dissolution can be inhibited. Thus, there can be obtained an electric 
double layer capacitor which is electrochemically stable even when a 
potential of 3 V is applied. 
Because the aluminium oxide film thickness is small and thus the resistance 
is low, an increase of the internal resistance as in the case using 
titanium does not occur at a low voltage of about 3 volts. In this case, 
the collector metal serves also as a material for the case and should have 
high strength sufficient for the case. If the case is constituted of 
aluminium alone, the strength is not satisfactory. In addition, because a 
limitation is placed on the thickness of the case from the standpoint of a 
product size, too large a thickness is not practical. Under these 
restricting conditions, it is necessary to use a material, such as 
stainless steel, which has little problem in electric connection on use as 
an external terminal and high strength, in combination with aluminium.

BEST MODE FOR CARRYING OUT THE INVENTION 
Embodiments of the invention are described with reference to FIGS. 4 
through 7. 
An embodiment shown in FIG. 4 includes polarizable electrodes 6 each made 
of an active carbon fiber cloth or a molding of a mixture of an active 
carbon powder and a binder. An aluminium conductive electrode 7 is formed 
on one side of the electrodes 6 by plasma spray coating. The polarizable 
electrodes 6 are encased in metallic case pieces 9, 10 of stainless steel 
which have an aluminium layer 8 on the inner surfaces thereof in such a 
way that the conductive electrodes 7 contact the inner surfaces of the 
case pieces 9,10. The conductive electrodes 7 are connected to the 
metallic case pieces 9, 10 by spot welding. One of the polarizable 
electrodes 6 which is a counter electrode at the side of the cathode is 
impregnated with an electrolytic solution which has 10 wt% of 
tetraethylammonium tetrafluoroborate added to propylene carbonate. An 
ionpermeable separator 11 is provided between the polarizable electrodes 6 
and a gasket 12 is arranged around the opened periphery of the metallic 
case pieces 9, 10 and the metallic piece 10 is subjected to curling to 
complete a sealed case. 
A capacitor shown in FIG. 5 is an embodiment where no aluminium layer is 
formed on the inner surface of the metallic case piece 10 at the side of 
the anode. 
In FIG. 6, there is shown a capacitor which is similar to FIG. 4 but a 
non-polarizable counter electrode 13 of, for example, lithium is used as a 
cathode and the metallic case pieces 9, 10 have, respectively, aluminium 
layers 8 on the inner surfaces thereof. 
FIG. 7 shows another embodiment in which a non-polarizable electrode is 
used as the counter electrode 13 and the metallic case piece 10 has no 
inner aluminium layer at the side of the counter electrode 13. 
The metallic case pieces 9, 10 may be made of, aside from stainless steel, 
iron, nickel, titanium and copper alloys. 
The present invention is described by way of examples. 
Example 1 
In the embodiments shown in FIGS. 4 and 5, a 250 micrometer thick aluminium 
conductive electrode 7 is formed by plasma spray coating on one surface of 
a polarizable electrode 6 consisting of a phenolic active carbon fiber 
cloth (thickness 0.5 mm, specific surface area 2000 m.sup.2 /g). This 
double-layered construction is punched in the form of a disk having a 
diameter of 2 cm to obtain electrodes. The electrodes are impregnated with 
an electrolytic solution having 10 wt% of tetraethylammonium 
tetrafluoroborate in propylene carbonate, after which they are superposed 
through separator 11. This unit is encased in a stainless steel case made 
of pieces 9, 10 which is covered with an aluminium layer 8 (such as a 
layer having a purity of 99.86% and a thickness of 70 micrometers or a 
layer having a purity of 99.99% and a thickness of 60 micrometers) only on 
the inner surface contacting the anode and on inner surfaces contacting 
both electrodes. A gasket 12 is provided in an opening end between the 
metallic case pieces 9, 10 and caulked to close the opening. 
Several characteristics of the electric double layer capacitors according 
to the invention are indicated in Table 1 as Nos. 1-3. In Table 1, 
characteristics of an electric double layer capacitor for comparison in 
which the stainless steel case is not covered with aluminium on the inner 
surfaces thereof are indicated as No. 6. 
EXAMPLE 2 
Coconut shell active carbon particles are mixed with a polyflon binder and 
molded (thickness 0.5 mm, specific surface area 800 m.sup.2 /g) to obtain 
a polarizable electrode 6. The electrode is formed with a 250 micrometer 
thick aluminium conductive electrode 7 by plasma spray coating. This 
double-layered construction is punched into disks having a diameter of 2 
cm to obtain electrodes. The electrodes are impregnated with an 
electrolytic solution of 10 wt% of tetraethylammonium tetrafluoroborate in 
propylene carbonate and superposed through a separator 11 and sandwiched 
between metallic case pieces 9, 10 in which the stainless steel case piece 
9 alone or pieces 9, 10 are covered with an aluminium layer 8 (purity 
99.86%, thickness 70 micrometers) on the inner surface or surfaces thereof 
The metallic case pieces 9, 10 are closed with a gasket 12 at the opening 
therebetween. 
In Table 1, the characteristics of the electric double layer capacitors of 
the invention are shown as Nos. 4 and 5. 
TABLE 1 
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Life under high temperature & 
Internal 
Break- 
loading conditions 
Active Capaci- 
Resist- 
down 3.0 V, 70.degree. C. 1000 hours 
No. 
Case Construction 
Carbon tance 
ance Voltage 
(Variation in capacitance) 
__________________________________________________________________________ 
Capacitors of Invention: 
1 anode: Al-covered 
phenolic 
3 F 0.16 
ohms 
3 V -5% 
stainless active carbon 
steel fibers 
(Al purity: 
99.86%) 
cathode: stainless 
steel 
2 anode: Al-covered 
phenolic 
3 F 0.16 
ohms 
3 V -5% 
stainless active carbon 
steel fibers 
(Al purity: 
99.99%) 
cathode: stainless 
steel 
3 anode: Al-covered 
phenolic 
3 F 0.16 
ohms 
3 V -5% 
stainless active carbon 
steel fibers 
(Al purity: 
99.99%) 
cathode: Al-covered 
stainless 
steel 
(Al purity: 
99.86%) 
4 anode: Al-covered 
coconut 0.8 
F 0.1 
ohm 
3 V -5% 
stainless shell active 
steel carbon particles 
(Al purity: 
99.86%) 
cathode: stainless 
steel 
5 anode: Al-covered 
coconut 0.8 
F 0.1 
ohm 
3 V -5% 
stainless shell active 
steel carbon particles 
(Al purity: 
99.86%) 
cathode: Al-covered 
stainless 
steel 
(Al purity: 
99.99%) 
Prior Art Capacitor: 
6 stainless steel 
phenolic 
3 F 0.16 
ohms 
2.3 
V -95% 
for both anode 
active carbon 
and cathode 
fibers 
__________________________________________________________________________ 
EXAMPLE 3 
As shown in FIGS. 6 and 7, a polarizable electrode 6 of an acrylic active 
carbon fiber cloth (thickness 0.5 mm, specific surface area 800 m.sup.2 
/g) is formed with a 250 micrometer thick aluminium conductive electrode 7 
by plasma spray coating. This double-layered construction is punched into 
a disk having a diameter of 2 cm, thereby obtaining an anode electrode. 
This electrode is superposed with a lithium non-polarizable electrode 13 
having a diameter of 2 cm through a separator 11 to obtain an electrode 
pair. This pair is impregnated with an electrolytic solution of 10 wt% of 
lithium tetrafluoroborate in propylene carbonate and sandwiched with 
stainless steel case pieces 9, 10 in which the stainless steel piece 9 
alone or the pieces 9, 10 are covered with an aluminum layer (purity 
99.99%, thickness 60 micrometers) on the inner surfaces thereof. The 
opening end between the case pieces 9, 10 is provided with a gasket 12 and 
caulked to close the pieces. 
In Table 2, there are shown characteristics of the electric double layer 
capacitors of the invention as Nos. 1 and 2. For comparison, there are 
also shown characteristics of an electric double layer capacitor in which 
no aluminium layer is formed on the inner surfaces of a metallic case as 
No. 3. 
TABLE 2 
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Life under High Temperature 
Internal 
Break- 
and Loading Conditions 
Combinations 
Capaci- 
Resist- 
down 3.0 V, 70.degree. C. 1000 hours 
No. 
Case Construction 
of Electrodes 
tance 
ance Voltage 
(Variation in capacitance) 
__________________________________________________________________________ 
Capacitors of Invention: 
1 anode: covered 
facing type 
6 F 0.15 ohms 
3 V -5% 
stainless of a pola- 
steel rizable 
(Al purity: 
electrode & 
99.99%) a non-pola- 
cathode: stainless 
rizable 
steel electrode 
2 anode: Al-covered 
6 F 0.15 ohms 
3 V -5% 
stainless 
steel 
(Al purity: 
99.99%) 
cathode: Al-covered 
stainless 
steel 
Prior-art Capacitor: 
3 stainless steel 6 F 0.15 ohms 
2.8 V 
-505% 
for both an anode 
and a cathode 
__________________________________________________________________________ 
INDUSTRIAL UTILITY 
As will be understood from the foregoing, according to the present 
invention, an electric double layer capacitor of a high breakdown voltage 
of 3 V or higher can be readily fabricated in which an aluminium layer is 
formed at least on the inner surface of a metallic case at the side of the 
anode. The aluminium layer is formed with an electrochemically stable 
anodized film according to an applied voltage and the resistance of the 
film is so low as not to present practical problems. This property of the 
aluminium film is effectively utilized for the fabrication.