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What is claimed is:
1. A metal collector foil for an electric double layer capacitor,
comprising:
an etched metal collector foil having an oxide film in an amount not
greater than 300 mg/m.sup.2, and a capacitance per unit surface area not
less than 150 .mu.F/cm.sup.2.
2. The metal collector foil according to claim 1, wherein the etched metal
collector foil is free from a dielectric layer formed by an anodic
formation process on a surface of the etched metal collector foil.
3. A method of producing a metal collector foil for use in an electric
double layer capacitor, comprising the steps of:
preparing a plain metal foil;
etching the metal foil in a chloride solution to dissolve a surface of the
metal foil; and
controlling the growth of an oxide film on the surface of the etched metal
foil and the capacitance per unit surface area of the etched metal foil
concurrently and separately such that the amount of the oxide film is not
greater than 300 mg/m.sup.2, and the capacitance per unit surface area is
not less than 150 .mu.F/cm.sup.2.
4. The method according to claim 3, wherein the etched metal collector foil
is free from a dielectric layer formed by an anodic formation process on
the surface of the etched metal collector foil.
5. An electric double layer capacitor comprising:
a positive electrode and a negative electrode each having a metal collector
foil and an electrode material formed mainly from activated carbon and
bonded to both opposite surfaces of the metal foil;
a dielectric separator disposed between the positive and negative
electrodes; and
a liquid electrolyte impregnated in the electrode material to enable
charging and discharging of the electric double layer capacitor,
wherein the metal collector foil is an etched metal foil having an oxide
film on the opposite surfaces thereof, the amount of the oxide film,
immediately before the bonding of the etched metal foil relative to the
electrode material, is greater than 300 mg/m.sup.2, and a capacitance per
unit surface area of the etched metal foil, immediately before the bonding
of the etched metal foil relative to the electrode material, is not less
than 150 .mu.F/cm.sup.2.
6. The electric double layer capacitor according to claim 5, wherein the
etched metal collector foil is free from a dielectric layer formed by an
anodic formation process on the surface of the etched metal collector
foil.
7. A metal collector foil for an electric double layer capacitor,
comprising:
an etched metal collector foil having been subjected to an etching process
in an etching solution having a chlorine iron such that a capacitance per
unit area of the etched metal collector foil obtained when the etched
metal collector foil is subjected to an anodic formation process with
application of a withstanding voltage of 65.5 volts is in a range of 1.7
to 2.3 .mu.F/cm.sup.2, the etched metal collector foil having a tensile
strength not less than 9,000 N/cm.sup.2 and a residual chlorine
concentration not greater than 1.0 mg/m.sup.2.
8. A method of producing a metal collector foil for use in an electric
double layer capacitor, comprising the steps of:
preparing a plain metal foil;
etching the metal foil in an etching solution having a chlorine iron such
that a capacitance per unit area of the etched metal collector foil
obtained when the etched metal collector foil is subjected to an anodic
formation process with application of a withstanding voltage of 65.5 volts
is in a range of 1.7 to 2.3 .mu.F/cm.sup.2, and the etched metal collector
foil has a tensile strength not less than 9,000 N/cm.sup.2 ; and
washing the etched metal foil to the extent that a residual chlorine
concentration of the etched metal foil is not greater than 1.0 mg/m.sup.2.
9. The method according to claim 8, wherein the metal foil is a plain
aluminum foil, the etching is carried out at a temperature of 40 to
50.degree. C. in a 5% hydrochloric acid solution with an AC current
applied at 50 Hz with an electrolytic current density of 0.25 A/cm.sup.2
and the quantity of electricity 35 to 40 A.multidot.min/dm.sup.2, and the
washing is carried out at a temperature of 50.degree. C. in a pH1 acid
solution for 60 seconds.
10. An electric double layer capacitor comprising:
a positive electrode and a negative electrode each having a metal collector
foil and an electrode material formed mainly from activated carbon and
bonded to both opposite surfaces of the metal foil;
a dielectric separator disposed between the positive and negative
electrodes; and
a liquid electrolyte impregnated in the electrode material to enable
charging and discharging of the electric double layer capacitor,
wherein the metal collector foil is an etched metal collector foil having
been subjected to an etching process in an etching solution having a
chlorine iron such that a capacitance per unit area of the etched metal
collector foil obtained when the etched metal collector foil is subjected
to an anodic formation process with application of a withstanding voltage
of 65.5 volts is in a range of 1.7 to 2.3 .mu.F/cm.sup.2, and the etched
metal collector foil has a tensile strength not less than 9,000 N/cm.sup.2
and a residual chlorine concentration not greater than 1.0 mg/m.sup.2.
11. A metal collector foil for use in an electric double layer capacitor,
comprising:
an etched aluminum foil formed from a plain aluminum foil of an ordinary
degree of purity not greater than 99.8%, the etched aluminum foil
containing at least one of Cu, Ni, Zn, Sn and Fe with a content of Cu, Ni,
Zn or Sn not greater than 10 ppm and a content of Fe not greater than 300
ppm.
12. The metal collector foil according to claim 11, wherein the purity of
the aluminum foil prior to etching is about 99.63-99.66%.
13. A method of producing a metal collector foil for use in an electric
double layer capacitor, comprising the steps of:
preparing a plain aluminum foil having an ordinary degree of purity not
greater than 99.8% and containing at least one of Cu, Ni, Zn, Sn and Fe;
and
etching the plain aluminum foil in a hot solution of 5% hydrochloric acid
to thereby obtain an etched aluminum foil of an aluminum content of not
greater than 99.8% and containing at least one of Cu, Ni, Zn, Sn and Fe
with a content of Cu, Ni, Zn or Sn not greater than 10 ppm and a content
of Fe not greater than 300 ppm.
14. The method according to claim 13, wherein the plain aluminum foil
contains about 99.63-99.66% aluminum and at least one of Cu, Ni, Zn, Sn
and Fe with a Cu content not greater than 23 ppm, an Ni content not
greater than 17, a Zn content not greater than 18 ppm, an Sn content of
not greater than 19 ppm and an Fe content not greater than 348 ppm.
15. An electric double layer capacitor comprising:
a positive electrode and a negative electrode each having a metal collector
foil and an electrode material formed mainly from activated carbon and
bonded to both opposite surfaces of the metal foil;
a dielectric separator disposed between the positive and negative
electrodes; and
a liquid electrolyte impregnated in the electrode material to enable
charging and discharging of the electric double layer capacitor,
wherein the metal collector foil is an etched aluminum foil formed from a
plain aluminum foil of an ordinary degree of purity not greater than
99.8%, and the etched aluminum foil contains at least one of Cu, Ni, Zn,
Sn and Fe with a content of Cu, Ni, Zn or Sn not greater than 10 ppm and a
content of Fe not greater than 300 ppm.
16. The electric double layer capacitor according to claim 15, wherein the
purity of the aluminum foil prior to etching is about 99.63-99.66%. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to a metal collector foil for use in an
electric double layer capacitor, a method of producing the metal collector
foil, and an electric double layer capacitor using the metal collector
foil.
BACKGROUND OF THE INVENTION
Various electric double layer capacitors are known heretofore. Japanese
Patent Laid-open Publication (JP-A1) No. 11-283871 discloses an invention
that focuses in particular on the strength of a metal collector foil for
such electric double layer capacitors. It is stated in this publication
that certain improvements in an electrode assembly have been proposed but
the proposed electrode assembly is relatively weak in strength and hence
is likely to be damaged during its manufacture or when it is laminated
with a separator to form a capacitor. Taking this prior problem into
consideration, the invention disclose in the aforesaid Japanese
publication seeks to provide a metal collector foil for use in the
electrode assembly and having a strength which is sufficient to withstand
a rupture energy of at least 3.0 kg mm.
Based on the disclosure of JP-A1-11-283871, many sample electric double
layer capacitors were produced by the present inventors for evaluation.
The produced samples indicated that the metal collector foil was
satisfactory in terms of strength, but due to the resistance value
increasing beyond an allowable limit as the times goes on, the operation
performance as a rechargeable battery deteriorated significantly. Through
an investigation made on various factors, the present inventors have found
that an oxide film produced on the surface of the metal collector foil
affects the operation performance of the capacitor. This is because the
oxide film is an insulator, and so the performance of the rechargeable
battery deteriorates as the amount of oxide film increases.
A plain collector foil (i.e., a collector foil before being subjected to an
etching process) has a flat and smooth surface and hence is likely to
allow the occurrence of electrode separation when it is used for adhesive
bonding with an electrode material. To deal with this problem, it has been
an ordinary practice to etch the plain collector foil to form a dense
network of microscopic channels or pits in surface area thereby to
increase an increased bonding strength relative to the electrode material.
For convenience of manufacture, many etched collector foils are stored for
a period of from several hours to several days rather than advanced to a
subsequent processing operation. After etching, the foil surface is
activated and hence actively reacts with oxygen in surrounding air. Thus,
an oxide film produce on the surface area of the etched collector foil
unavoidably grows up during the storage of the etched collector foil. An
attempt to remove the oxide film just before the etched foil is subjected
to a subsequent process (i.e., a bonding process in which an electrode
material is attached by adhesion bonding to the surface the etched
collector foil) may induce an additional cost, which increases the
manufacturing cost of the capacitor.
Another finding through the afore-mentioned investigation is that chlorine
remaining on the surface of the etched collector foil has a great
influence on the age-related deterioration of the capacitor. The residual
chlorine is caused by chlorine ions contained in an etching solution.
Accordingly, the etched collector foil necessarily involves residual
chlorine.
The residual chlorine is generally removed by washing. To improve the
quality, the washing operation is repeated several times. Such repeated
washing operation is, however, objectionable from the viewpoint of
manufacturing cost.
Japanese Patent Laid-open Publication (JP-A1) No. 2001-176757 discloses the
use of 99.99% pure aluminum foil as a collector foil in an electric double
layer capacitor. The disclosed aluminum collector foil has a copper
content blow 150 ppm. As is well known, in the manufacture of aluminum,
bauxite ore is used as a starting material to produce alumina, which is
then placed in an electrolytic furnace for purification to produce an
aluminum ingot having an ordinary degree of purity in the range of from
90.0 to 99.85%. When needed, the aluminum ingot with ordinary purity is
subjected to a secondary purification process in which the three-phase
electrolyzing method or the segregation method is used to increase the
purity of the aluminum ingot to 99.99% or higher. Such high purity
aluminum is expensive, as it requires purification to be done repeatedly.
The 99.99% pure aluminum foil disclosed in JP-A1-2001-176757 is also a
high purity aluminum foil and hence expensive to manufacture. The use of
the high purity aluminum foil in the manufacture of an electric double
layer capacitor necessarily increases the manufacturing cost of the
capacitor.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a metal
collector foil and its production method that can effectively suppress the
age-related deterioration of an electric double layer capacitor in which
the collector foil is used, to thereby ensure high operation performance
of the capacitor over a long period of use.
Another object of the present invention is to provide a metal collector
foil and its production method that are highly cost-effective and can
lower the manufacturing cost of an electric double layer capacitor in
which the collector foil is used.
A further object of the present invention is to provide an electric double
layer capacitor using the metal collector foil.
According to a first aspect of the present invention, there is provided a
metal collector foil for an electric double layer capacitor, which
comprises an etched metal collector foil having an oxide film in an amount
of not greater than 300 mg/m.sup.2, and a capacitance per unit surface
area not less than 150 .mu.F/cm.sup.2.
Since the amount of oxide film produced on the etched collector foil does
not exceed 300 mg/m.sup.2, it is possible to suppress an objectionable
increase in the internal resistance. An additional suppressing effect
against the rise of internal resistance is achieved by the etched
collector foil having a capacitance not less than 150 .mu.F/cm.sup.2. By a
combination of the amount of oxide film and the capacitance that are
controlled within given ranges, the capacitor can perform its prescribed
functions reliably and stably over a long period of use without causing
age-related deteriorations.
According to a second aspect of the present invention, there is provided a
method of producing a metal collector foil for use in an electric double
layer capacitor, comprising the steps of: preparing a plain metal foil;
etching the metal foil in a chloride solution to dissolve a surface of the
metal foil; and controlling the growth of an oxide film on the surface of
the etched metal foil and the capacitance per unit surface area of the
etched metal foil concurrently and separately such that the amount of the
oxide film is not greater than 300 mg/m.sup.2, and the capacitance per
unit surface area is not less than 150 .mu.F/cm.sup.2.
According to a third aspect of the present invention, there is provided an
electric double layer capacitor comprising a positive electrode and a
negative electrode each having a metal collector foil and an electrode
material formed mainly from activated carbon and bonded to both opposite
surfaces of the metal foil, a dielectric separator disposed between the
positive and negative electrodes, and a liquid electrolyte impregnated in
the electrode material to enable charging and discharging of the electric
double layer capacitor. The metal collector foil is an etched metal foil
having an oxide film on the opposite surfaces thereof. The amount of the
oxide film, immediately before the bonding of the etched metal foil
relative to the electrode material, is greater than 300 mg/m.sup.2, and a
capacitance per unit surface area of the etched metal foil, immediately
before the bonding of the etched metal foil relative to the electrode
material, is not less than 150 .mu.F/cm.sup.2.
The etched metal collector foil is preferably free from a dielectric layer
formed by an anodic formation process on a surface of the etched metal
collector foil.
According to a fourth aspect of the present invention, there is provided a
metal collector foil for an electric double layer capacitor, comprising an
etched metal collector foil having been subjected to an etching process in
an etching solution having a chlorine iron such that a capacitance per
unit area of the etched metal collector foil obtained when the etched
metal collector foil is subjected to an anodic formation process with
application of a withstanding voltage of 65.5 volts is in a range of 1.7
to 2.3 .mu.F/cm.sup.2. The etched metal collector foil has a tensile
strength not less than 9,000 N/cm.sup.2 and a residual chlorine
concentration not greater than 1.0 mg/m.sup.2.
By virtue of the capacitance per unit area in a range of 1.7 to 2.3
.mu.F/cm.sup.2 and the residual chlorine concentration not greater than
1.0 mg/m.sup.2, the etched metal foil when used in an electric double
layer capacitor is able to suppress age-related deteriorations of the
capacitor, thus enabling the capacitor to operate with high performance
qualities over a long period of use. Furthermore, the etched metal foil
having a tensile strength not less than 9,000 N/cm.sup.2 is able to
withstand a tensile force tending to damage or break the metal foil during
its manufacture or during winding into a cylindrical electrode element.
This may lead to a reduction of the manufacturing cost.
According to a fifth aspect of the present invention, there is provided a
method of producing a metal collector foil for use in an electric double
layer capacitor, comprising the steps of: preparing a plain metal foil;
etching the metal foil in an etching solution having a chlorine iron such
that a capacitance per unit area of the etched metal collector foil
obtained when the etched metal collector foil is subjected to an anodic
formation process with application of a withstanding voltage of 65.5 volts
is in a range of 1.7 to 2.3 .mu.F/cm.sup.2, and the etched metal collector
foil has a tensile strength not less than 9,000 N/cm.sup.2 ; and washing
the etched metal foil to the extent that a residual chlorine concentration
of the etched metal foil is not greater than 1.0 mg/m.sup.2.
It is preferable that the metal foil is a plain aluminum foil, the etching
is carried out at a temperature of 40 to 50.degree. C. in a 5%
hydrochloric acid solution with an AC current applied at 50 Hz with an
electrolytic current density of 0.25 A/cm.sup.2 and the quantity of
electricity 35 to 40 A.multidot.min/dm.sup.2, and the washing is carried
out at a temperature of 50.degree. C. in a pH 1 acid solution for 60
seconds.
According to a sixth aspect of the present invention, there is provided an
electric double layer capacitor comprising a positive electrode and a
negative electrode each having a metal collector foil and an electrode
material formed mainly from activated carbon and bonded to both opposite
surfaces of the metal foil, a dielectric separator disposed between the
positive and negative electrodes, and a liquid electrolyte impregnated in
the electrode material to enable charging and discharging of the electric
double layer capacitor. The metal collector foil is an etched metal
collector foil having been subjected to an etching process in an etching
solution having a chlorine iron such that a capacitance per unit area of
the etched metal collector foil obtained when the etched metal collector
foil is subjected to an anodic formation process with application of a
withstanding voltage of 65.5 volts is in a range of 1.7 to 2.3
.mu.F/cm.sup.2. The etched metal collector foil has a tensile strength not
less than 9,000 N/cm.sup.2 and a residual chlorine concentration not
greater than 1.0 mg/m.sup.2.
According to a seventh aspect of the present invention, there is provided a
metal collector foil for use in an electric double layer capacitor,
comprising an etched aluminum foil formed from a plain aluminum foil of an
ordinary degree of purity not greater than 99.8%, the etched aluminum foil
containing at least one of Cu, Ni, Zn, Sn and Fe with a content of Cu, Ni,
Zn or Sn not greater than 10 ppm and a content of Fe not greater than 300
ppm. The purity of the aluminum foil prior to etching may be about
99.63-99.66%.
The use of an ordinarily-purified aluminum foil achieves a considerable
reduction in the material cost of the electric double layer capacitor and
contributes to the saving of a variable energy resource, which may
otherwise be consumed during a three-phase electrolytic purification
process when a highly-purified aluminum foil is needed.
According to an eighth aspect of the present invention, there is provided a
method of producing a metal collector foil for use in an electric double
layer capacitor, comprising the steps of: preparing a plain aluminum foil
having an ordinary degree of purity not greater than 99.8% and containing
at least one of Cu, Ni, Zn, Sn and Fe; and etching the plain aluminum foil
in a hot solution of 5% hydrochloric acid to thereby obtain an etched
aluminum foil of an aluminum content of not greater than 99.8% and
containing at least one of Cu, Ni, Zn, Sn and Fe with a content of Cu, Ni,
Zn or Sn not greater than 10 ppm and a content of Fe not greater than 300
ppm.
Preferably, the plain aluminum foil contains about 99.63-99.66% aluminum
and at least one of Cu, Ni, Zn, Sn and Fe with a Cu content not greater
than 23 ppm, an Ni content not greater than 17, a Zn content not greater
than 18 ppm, an Sn content of not greater than 19 ppm and an Fe content
not greater than 348 ppm.
According to a ninth aspect of the present invention, there is provided an
electric double layer capacitor comprising a positive electrode and a
negative electrode each having a metal collector foil and an electrode
material formed mainly from activated carbon and bonded to both opposite
surfaces of the metal foil, a dielectric separator disposed between the
positive and negative electrodes, and a liquid electrolyte impregnated in
the electrode material to enable charging and discharging of the electric
double layer capacitor. The metal collector foil is an etched aluminum
foil formed from a plain aluminum foil of an ordinary degree of purity not
greater than 99.8%, and the etched aluminum foil contains at least one of
Cu, Ni, Zn, Sn and Fe with a content of Cu, Ni, Zn or Sn not greater than
10 ppm and a content of Fe not greater than 300 ppm.
BRIEF DESCRIPTION OF THE DRAWINGS
Certain preferred structural embodiments of the present invention will be
described in detail herein below, by way of example only, with the
reference to the accompanying drawings, in which:
FIG. 1 is a perspective view, with parts cut away and with parts extended
for clarity, of an electric double layer capacitor according to an
embodiment of the present invention;
FIG. 2 is an enlarged cross-sectional view of a main portion of the
electric double layer capacitor;
FIG. 3 is a flowchart showing a sequence of processing steps that are
achieved to carry out a method according to a first embodiment of the
present invention to produce the electric double layer capacitor shown in
FIG. 1;
FIG. 4 is a diagrammatical view showing the principle of capacitance
measurement of an etched metal collector foil that is used in the
production of the electric double layer capacitor;
FIG. 5 is a graph showing the relationship between the amount of oxide film
W and the resistivity increase rate after 2,000 hours aging;
FIG. 6 is a graph showing the relationship between the capacitance C0 and
the resistivity increase rate after 2,000 hours aging;
FIG. 7 is a flowchart showing a sequence of processing steps that are
achieved to carry out a method according to a second embodiment of the
present invention to produce an electric double layer capacitor of the
same structure as shown in FIG. 1;
FIG. 8 is a graph showing the relationship between the capacitance at 65.5V
and the resistivity increase rate after 2,000 hours;
FIG. 9 is a graph showing the relationship between the capacitance at 65.5V
and the cell resistivity;
FIG. 10 is a graph showing the relationship between the capacitance at
65.5V and the tensile strength;
FIG. 11 is a graph showing the relationship between the residual chlorine
concentration and the resistivity increase rate after 2,000 hours aging;
and
FIG. 12 is a flowchart showing a sequence of processing steps that are
achieved to carry out a method according to a third embodiment of the
present invention to produce an electric double layer capacitor of the
same structure as shown in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings and FIG. 1 in particular, there is shown an
electric double layer capacitor 10 in which a metal collector foil
according to a first embodiment of the present invention is used. The
electric double layer capacitor 10 includes an elongated strip of positive
electrode or anode 11 and an elongated strip of negative electrode or
cathode 12 that are laminated together with a separator 13 disposed
therein. The laminated anode and cathode 11 and 12 are tightly wound into
a roll and received in a cylindrical container 14. Numeral 15 denotes an
end seal plate for sealing an open end of the container 14, 16 a positive
tab or terminal connected to the anode 11, 17 a negative tab or terminal
connected to the cathode 12, and 18 an inlet for filling an electrolyte
into the container 14.
As diagrammatically shown on enlarged scale in FIG. 2, the anode 11 and the
cathode 12 are each formed by a collector foil 21 made of metal such as
aluminum, and an electrode material 22 formed mainly from activated carbon
and bonded in the form of a sheet on both opposite surfaces of the metal
collector foil 21. In FIG. 2, only one electrode material layer 22 for
each of the anode 11 and the cathode 12 is shown for the purpose of
illustration.
The electrode materials 22, 22 have an adequate amount of liquid
electrolyte impregnated therein. When a DC voltage is applied across the
positive and negative terminals 16 and 17, positive and negative ions
electrostatically absorb to the interior of the electrode materials 22, 22
and to the surfaces of the metal collector foils 21, 21 so that they
create a positive pole and a negative pole, respectively. Upon
discharging, the absorbed ions shift to create motion or transfer of an
electron so that an electric current can be taken out through the positive
and negative terminals 16, 17.
FIG. 3 is a flowchart showing a sequence of processing steps achieved to
produce the electric double layer capacitor 10 according to the first
embodiment of the present invention. As shown in this figure, a step ST01
prepares a metal collector foil, which is formed from, for example, a
plain aluminum foil having a purity of at least 99.8%. The plain aluminum
foil has a flat and smooth surface because it has not been surface-treated
by an etching process or an anodic formation process.
At a step ST02, the aluminum foil is then etched in an etching solution
containing hydrochloric acid to preferentially dissolve the surface of the
aluminum foil. With this etching process, a roughed surface layer having
fine pits is formed on the aluminum foil. The fine pits in the roughened
surface serve to anchor an electrode material when the electrode material
is later bonded to the aluminum foil. The etched aluminum foil is washed
or otherwise cleaned for neutralization to the extent that the residual
chlorine concentration of the cleaned collector foil meets a control
criterion (not greater than 1.0 mg/m.sup.2). Excessive cleaning of the
etched foil can thus be avoided.
Then, a step ST03 undertakes storage of the etched collector foil. The
etched collector foil is stored in an exposed state at room temperature in
the atmosphere for several days. This storage step ST03 is done for the
purpose of producing a test sample. In the ordinary manufacturing process,
the etched collector foil is placed in a bag of resin film and the bag is
then tightly sealed with a deoxidant received in the sealed bag. As an
alternative, the etched collector foil is placed in the bag and after that
the bag may be vented or deaerated, then tightly sealed with a nitrogen
gas filled therein. By thus limiting or isolating the etched collector
foil from contact with oxygen, it is possible to slow the growth of the
oxide film.
A step ST04 performs measurement of the amount of oxide film. More
concretely, upon the lapse of the storage period, the weight w1 of the
sample foil (with an oxide film formed thereon) is measured. The sample
foil is then immersed in a chromic-phosphoric solution to remove the oxide
film, and after that by washing and drying the sample foil, we can obtain
a sample foil, which is free from oxide film. The oxide-film-free sample
foil is weighed on a scale to determine a weight w2 thereof. New, we can
obtain the amount of oxide layer W in accordance with the following
equation: W=(w1-w2)/area of the sample foil.
Then, a step ST05 determines whether or not the amount of oxide film W is
smaller than or equal to 300 mg/m.sup.2. The basis for using the criterion
will be described later on. When the result of determination is
affirmative (i.e., W.ltoreq.300 mg/m.sup.2), a step ST06 is then performed
to measure the capacitance C0 of the etched collector foil. Alternatively
when the determination result is negative (i.e., W>300 mg/m.sup.2), the
etched aluminum foil is judged as a defective foil.
FIG. 4 diagrammatically shows a principle of the capacitance measurement
used in ST04, where the etched foil 21 and an opposite electrode 32
disposed to surround the foil 21 are immersed in an aqueous solution 31
including ammonium adipic acid in a test container 30 and applied with a
DC voltage with a capacitance meter 33 connected across the foil 21 and
the electrode 32. By using the arrangement shown in FIG. 4, a capacitance
value C0 of the etched foil 21 itself can be measured by the capacitance
meter 33.
Then, a step ST07 shown in FIG. 3 determines whether or not the measured
capacitance C0 of the etched foil 21 is greater than or equal to 150
.mu.F/cm.sup.2. The basis for using this criterion will be described
later. When the result of determination is affirmative (i.e.,
C0.gtoreq.150 .mu.F/cm.sup.2), a step ST08 is then performed wherein an
electrode material provided in the form of a sheet is adhesively bonded to
the surface of the etched foil. Alternatively when the determination
result is negative (i.e., C0<150 .mu.F/cm.sup.2), the etched aluminum
foil is judged as a defective foil.
The step ST08 is followed by a step ST09 where two foils with electrode
materials carried thereon are wound into a cylindrical element together
with a separator. The cylindrical element is then placed in a cylindrical
container at a step ST10 and after that an open end of the cylindrical
container is hermetically closed by a sealing end plate at a step ST11.
Subsequently, at a step ST12, a liquid electrolyte is filled in the
container to impregnate the electrode materials. An electric double layer
capacitor, such as one shown in FIG. 1, is thus obtained. The order or
sequence of ST04-ST07 may be altered.
For a more complete understanding, the first embodiment of the present
invention will now be described in greater detail with reference to the
following examples.
EXAMPLES
For comparative purposes, eight samples were prepared under the conditions
given below.
1. Materials
1-1. Metal Collector Foil
1-1-1. Pretreatment (Common to All Samples)
A plain aluminum foil was etched at 50.degree. C. in a 5% hydrochloric acid
solution with a 50 Hz AC current applied with an electrolytic current
density of 0.25 A/cm.sup.2 and a quantity of electricity of 35
A.multidot.min/dm.sup.2.
The etched aluminum foil was washed at 50.degree. C. in a pH1 aqueous acid
solution for one minute and the washed foil was dried with hot air heated
at 180.degree. C.
1-1-2. Posttreatment and Measurement
In Example 1, upon completion of the drying process, an amount of oxide
film W on the etched foil and a capacitance C0 of the etched foil were
determined through measurements taken in the manner as described above.
In Example 2, after drying, the etched foil was stored at 25.degree. C. in
the atmosphere for 200 hours, and after the lapse of the storage time, the
same measurements were taken to determine an amount of oxide film W on the
etched foil and a capacitance C0 of the etched foil.
In Example 3, after drying, the etched foil was stored at 25.degree. C. in
the atmosphere for 500 hours, and after the lapse of the storage time, the
same measurements were taken to determine an amount of oxide film W on the
etched foil and a capacitance C0 of the etched foil.
In Example 4, after drying, the etched foil was stored at 25.degree. C. in
the atmosphere for 700 hours, and after the lapse of the storage time, the
same measurements were taken to determine an amount of oxide film W on the
etched foil and a capacitance C0 of the etched foil.
In Comparative Example 1, after drying, the etched foil was stored at
25.degree. C. in the atmosphere for 1,000 hours, and after the lapse of
the storage time, the same measurements were taken to determine an amount
of oxide film W on the etched foil and a capacitance C0 of the etched
foil.
In Comparative Example 2, after drying, the etched foil was stored at
25.degree. C. in the atmosphere for 1,500 hours, and after the lapse of
the storage time, the same measurements were taken to determine an amount
of oxide film W on the etched foil and a capacitance C0 of the etched
foil.
In Comparative Example 3, after drying, the etched foil was stored at
25.degree. C. in the atmosphere for 2,000 hours, and after the lapse of
the storage time, the same measurements were taken to determine an amount
of oxide film W on the etched foil and a capacitance C0 of the etched
foil.
In Comparative Example 4, after drying, the etched foil was stored at
25.degree. C. in the atmosphere for 3,500 hours, and after the lapse of
the storage time, the same measurements were taken to determine an amount
of oxide film W on the etched foil and a capacitance C0 of the etched
foil.
After the measurements, the etched foil was immediately subjected to a
subsequent process. The following conditions were common to all of
Examples 1-4 and Comparative Examples 1-4.
1-2. Electrode Material
90 parts by weight of activated carbon, 5 parts by weight of graphite
powder and 5 parts by weight of polytetrafluoroethylene (PTFE) were mixed
together, kneaded and formed by rolling into a sheet-like electrode
material having a thickness of 145 .mu.m, a width of 100 mm and a length
of 1,200 mm.
1-3. Adhesive
A conductive adhesive composed of polyvinyl alcohol (PVA), graphite and
amorphous carbon.
1-4. Separator
A porous film of artificial silk having a thickness of 75 .mu.m and a width
of 105 mm.
1-5. Container
A cylindrical container having a diameter of 40 mm and a height of 130 mm.
1-6. Electrolyte
A liquid organic electrolyte consisting of toriethylmonomethylammonium
polytetrafluoroborate/propylene carbonate (TEMA.BF4/PC).
2. Preparation of Samples
For each of the seven samples, the sheet-like electrode material was
bonding with the adhesion to both opposite surfaces of the etched aluminum
foil. Two such aluminum foils were wound into a cylindrical element
together with the separator. The cylindrical element was placed in the
container and an open end of the container was tightly sealed.
Subsequently, the liquid electrolyte was filled in the container. An
electric double layer capacitor was thus produced.
3. Additional Measurements
3-1. Cell Resistivity Measurement
Using new or unused samples, a resistance value (.OMEGA.) was measured by
means of an ohm meter connected across the positive and negative terminals
16, 17 (see FIG. 1) of each sample capacitor. The measured resistance
value A (.OMEGA.) was multiplied by an area B (cm.sup.2) of the anode and
cathode collector foils thereby to obtain a cell resistivity
(.OMEGA.cm.sup.2). Thus, the resistivity (.OMEGA.cm.sup.2)=the resistance
value A (.OMEGA.).times.the area B (cm.sup.2) of anode and cathode
collector foils. It is noted, however, that the cell resistivity thus
obtained is a value when the electrode materials of 145 .mu.m thick (see
3-1 above) are used.
3-2. Measurement of Resistivity Increase Rate after 2000 Hours Aging
After the measurement of cell resistivity, each sample capacitor was
subjected to an aging process during which a DC voltage of 2.5V was
continuously applied in a high temperature (45.degree. C.) atmosphere for
2,000 hours. After the elapse of 2,000 hours, application of the DC
voltage was terminated. Then, constant voltage discharge was started at
room temperature with a current value kept at 30 A. The discharge
continued until the voltage showed a drop from 2.5 V to 1.0 V. After the
end of the discharge, a cell resistivity after 2,000 hours aging was
obtained for each sample capacitor, in the same manner as described above
in the preceding paragraph 3-1. The thus obtained cell resistivity after
2,000 hours aging was compared with the cell resistivity previously
obtained so as to determine to what extent the cell resistivity increases
after the 2,000 hours aging. The thus determined increase in the cell
resistivity is indicated by percent and referred to as "cell resistivity
increase rate after 2,000 hours aging".
Table 1 given below shows the storage time, amount of oxide film W,
capacitance C0, cell resistivity and cell resistivity increase rate after
2,000 hours aging that are taken with respect to each of the eight sample
capacitors. The data shown | | |
