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Description  |
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FIELD OF THE INVENTION
The present invention relates to an electrode metal material for electrical
components such as capacitors and batteries which are used in contact with
electrolyte, to a capacitor and a battery formed of the electrode metal
material, and to a method of producing the electrode metal material and
the capacitor and battery thereof.
PRIOR ART
At present, there are, for example, electric double-layer capacitors and
electrolytic capacitors available as electrical components which are used
in contact with electrolyte. Such electric double-layer capacitors have
been applied to large-capacitance capacitors chargeable at up to about 3
V, and used for backup power sources of microcomputers, memory devices,
timers, and the like.
Typically, an electric double-layer capacitor comprises a pair of
polarizable electrodes or double-layer electrodes disposed face-to-face
via an insulating separator therebetween and immersed in electrolyte. The
electrode is produced by applying an activated carbon layer on the surface
of an electrode metal material made of a valve metal and used as a
mechanical supporter and, at the same time, electric collector.
Some types of electric double-layer capacitors use an organic-solvent based
electrolytic solution as electrolyte, such as a tetraethyl ammonium salt
which is added to an organic solvent, such as propylene carbonate. The
examples of conventional electric double-layer capacitors using
organic-solvent based electrolyte include a type in which a pair of
electric double-layer electrodes is wound and enclosed in a container, and
another type in which a pair of double-layer electrodes is laminated or
stacked, both types having been disclosed in U.S. Pat. No. 5,150,283.
In the case of the winding type of capacitors, as shown in FIG. 7, an
electrode metal material 1 is formed of etched aluminum foil having a
thickness of 20 to 50 .mu.m, and a paste obtained from a powder mixture of
activated carbon particles, a desired binder and a desired conductive
agent is applied to the above-mentioned metal foil to form a film. This
film, that is, an activated carbon layer 30 (a polarizable electrode)
mainly consisting of activated carbon particles, is used to form an
electric double-layer electrode 3.
A lead 6 is connected to each of the electrode metal materials 1 of the
pair of electric double-layer electrodes 3 and 3, respectively. These
electrodes 3 and 3 are disposed face-to-face with a separator 5
therebetween and wound like a coil. The electric double-layer electrodes
is immersed in non-aqueous electrolyte under vacuum to impregnate the
activated carbon layers 30 and the separators 5 with the electrolyte, then
placed in an aluminum case 70, the opening 7 of the aluminum case 70 being
sealed with a watertight packing 8. The electrolyte in the electric
double-layer capacitor has used polypropylene carbonate as an organic
solvent, and a tetraethyl ammonium salt as an electrolyte, for example.
Furthermore, in a button-type electric double-layer capacitor,
schematically shown in FIGS. 9 and 10, activated carbon layers 30 are
joined to disc-like sheets 1 made of a valve metal material, respectively,
to form a pair of double-layer electrodes 3. The pair of double-layer
electrodes 3 and 3 are disposed face-to-face via an insulating separator 5
therebetween, and accommodated in a metal container comprising two mating
members. The valve metal material sheets of the two double-layer
electrodes are joined to the inner surface sides of the bottom member 60
and the lid member 61 of the metal container. Both the bottom and lid
members are joined to each other so as to be watertight by using an
insulating ring packing 69 at the peripheral portion thereof. The interior
of the capacitor is filled with non-aqueous electrolyte so that the
double-layer electrodes and the activated carbon layers are immersed
therein sufficiently. The non-aqueous electrolyte is a solution of
tetraethyl ammonium perchlorate added in propylene carbonate in the same
way as described above.
An electrolytic capacitor is known as a capacitor in which non-aqueous
electrolyte is used. In the anode of the capacitor, a dielectric film is
formed by chemically treating the valve metal foil. In the cathode, the
valve metal foil is used as it is. Usually, both the electrodes are
disposed face-to-face, wound into a coil, and hermetically enclosed in a
container while being immersed in electrolyte.
In the case of the conventional electric double-layer capacitor, the valve
metal sheet or foil, on which a polarizable electrode is formed as a film,
has a naturally oxidized film specific to the valve metal constituting an
electrode structure while the foil is handled. When this foil is used to
form an electrode structure, a thin, insulating oxidized film 4 is
frequently formed at the interface between the aluminum foil 1 used as a
valve metal material and the polarizable electrode 3, as schematically
shown in FIG. 6.
Furthermore, the above-mentioned non-aqueous electrolyte typically includes
slight amounts of water and oxygen. For this reason, the valve metal
material constituting the electrode structure reacts with the water
content in the electrolyte during use of the capacitor, and the surface of
the metal is oxidized. Therefore, when the electric double-layer capacitor
formed of this kind of metal is used for extended periods of time, its
equivalent series resistance (ESR), i.e., the internal resistance of the
capacitor used as a power source, increases gradually, and, in some cases,
its capacitance decreases.
This problem due to the oxidation of the metal portion of the electrode has
also occurred in the case of the button-type electric double-layer
capacitor in the same way.
Furthermore, the anode of the electrolytic capacitor using non-aqueous
electrolyte is provided with a dielectric insulating layer formed by
anodizing a valve metal such as aluminum. In addition, its cathode in
direct contact with the electrolyte is also formed of the valve metal such
as aluminum. In this case, an oxide film is formed on the surface of the
metal used for the cathode because of oxidation with the water content in
the electrolyte. This causes a problem of the capacitor increasing in
internal resistance, just like the problem described above.
With respect to batteries using electrodes in contact to non-aqueous
electrolyte, a lithium ion secondary battery is known which has high
charge-discharge cycle performance with high energy density in a compact
shape.
A lithium ion secondary battery, as shown in FIG. 11, comprises a positive
electrode 35, a negative electrode 37, facing to the positive electrode, a
film separator 5 for separating both electrodes 35 and 37, and a
non-aqueous electrolytic solution in which both the electrodes are placed
and contained in a casing 71. The positive electrode 35 is, as an example,
formed of a mixture of positive active substance such as LiCoO.sub.2,
conductive material such as acetylene black, and a binder including
carboxylmethylcellulose and polyflorovinylidene which mixture is applied
on both sides of aluminum foil as an electrode metal material 1 for an
electric collector. On the other hand, the negative electrode 37 is formed
of a mixture of negative active substance such as graphite and a binder
such as carboxylmethylcellulose and styrene-butadiene rubber which mixture
is applied on both sides of copper foil as an electric collector. The
electrolytic solution is a non-aqueous solvent of a mixture of
propylenecarbonate and 1,2-dimethoxyethane containing LiPF.sub.6 as
electrolyte. A porous polypropylene film is used as a separator.
In conventional lithium ion secondary batteries, aluminum foil is formed
with natural oxide film on its surface during dealing with the foil so
that thin isolating film have often been formed in the interface between
the aluminum foil and the positive electrode on the aluminum foil.
Further, since the above non-aqueous electrolytic solution also contains
slight amount of water and oxygen, the aluminum foil in the battery have
been oxidized on its surface, in use, gradually over long time due to
reaction of aluminum surface with water in the electrolytic solution,
causing a lithium ion secondary battery to increase in equivalent series
resistance, i.e., internal resistance and resulting in low capacity at
high discharge rate.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is provide a valve metal
material capable of being formed into electrodes used in contact with
non-aqueous electrolyte to reduce internal resistance of a capacitor or
battery.
Another object of the present invention is to provide a method of producing
a valve metal material capable of being formed into electrodes used in
contact with non-aqueous electrolyte to reduce internal resistance of such
a capacitor or battery.
A still another object of the present invention is to provide a capacitor
capable of having a low internal resistance by restricting the change in
the resistance of the electrode metal material constituting the electrodes
used in contact with non-aqueous electrolyte.
A still another object of the present invention is to provide a non-aqueous
secondary battery having low internal resistance by restricting the change
in the resistance of the electrode metal material constituting the
electrodes used in contact with non-aqueous electrolyte.
A yet still another object of the present invention is to provide a method
of producing a capacitor capable of having a low internal resistance by
restricting the change in the resistance of the electrode metal material
constituting the electrodes used in contact with non-aqueous electrolyte.
A yet still another object of the present invention is to provide a method
of producing a non-aqueous secondary battery having low internal
resistance by restricting the change in the resistance of the electrode
metal material constituting the electrodes used in contact with
non-aqueous electrolyte.
An electrode metal material in accordance with the present invention is
formed of a valve metal material containing carbon particles on the
surface, and is used to form electrodes. The carbon particles in the
carbon-containing metal material ensure direct contact with a conductor
(including electrolyte) to electrically connect the electrode metal
material to the conductor.
In particular, the carbon-containing metal material comprises a valve metal
material and numerous carbon particles fixed in the surface of the valve
metal material and exposed to the surface. In the present invention, the
carbon particles may be projected slightly so as to be exposed to the
surface of the valve metal material in order to enhance the conductivity
and joining characteristic to a conductor to become contact therewith.
The electrode metal material in accordance with the present invention may
be used to obtain electrode structures used in contact with non-aqueous
electrolyte. This kind of carbon-containing metal material itself may be
an electrode making contact with electrolyte. Alternatively, the electrode
metal material may have an activated carbon layer coated on the surface,
i.e., a polarizable electrode. The former corresponds to the cathode of an
electrolytic capacitor, and the latter corresponds to the double-layer
electrode of an electric double-layer capacitor.
Further, the electrode metal material may be used to support a positive
electrode including a positive active substance on the surface of the
electrode metal material, the positive electrode being used for a
non-aqueous electrolytic secondary battery, e.g., a lithium ion secondary
battery.
In the electrolytic capacitor, the carbon particles of the
carbon-containing metal material, exposed to the surface thereof, can make
direct contact with the electrolyte to ensure conductivity between the
metal material and the electrolyte. In addition, inside the electric
double-layer capacitor, the carbon particles of the carbon-containing
metal material, exposed to the surface thereof, can make direct contact
with the activated carbon layer to ensure conductivity between the metal
material and the activated carbon layer. Further, in the lithium ion
secondary battery, the carbon particles of the carbon-containing metal
material are exposed to the surface thereof, to make direct contact with
the active substances in the positive electrode, ensuring conductivity
between the electrode metal material and the positive electrode.
In any of the cases, even if the carbon-containing metal material makes
contact with electrolyte solution, and the metallic surface thereof is
oxidized by water contained in non-aqueous electrolyte, the conductivity
noted above remains almost unchanged over long time periods.
More particularly, numerous carbon particles may project on the surface of
the valve metal material. Therefore, it is preferable that only the metal
surface of the valve metal material may be removed such that the carbon
particles are left projected on the removed surface. This projection
configuration of the surface of the valve metal material ensures
conductivity to the activated carbon layer in the capacitor or active
substance in the battery, and also enhances strength of joining to the
activated carbon layer or positive electrode.
More particularly, the metallic surface of the valve metal material may be
coated with a passive film. In this case although the metallic surface of
the valve metal material itself may lose conductivity, the metallic
surface is prevented stably from oxidation because of no contact with the
electrolyte, and the valve material has stable conductivity via the carbon
particles for extended periods of time.
The valve metal material in accordance with the present invention can be
formed into sheet. The term "sheet" herein refers to plate, sheet, film
and foil. The valve metal material may be formed of products other than
sheet, having a small thickness with a desired shape. The electrode metal
material may have a shape of net or punched plate. this may be is adequate
to apply, for example, the positive electrode thereon to produce
non-aqueous secondary battery.
The sheet and other formed products may include carbon particles at least
on one side thereof and also may include carbon particles on both sides
thereof.
A method of producing a valve metal material for electrodes in accordance
with the present invention contains driving or squeezing numerous carbon
particles into the surface thereof. Pressing using dies or rolling using
rollers may be employed to drive powder of carbon particles into a valve
metal sheet, then, carbon particles being fixed in the surface of the
valve metal sheet with the particle exposed on the surface.
Another method may be adopted where a slurry of carbon particles is applied
and dispersed on a surface of a valve metal material and pressed to
squeeze the carbon particles into the surface, then, obtaining
carbon-containing electrode metal material. The carbon slurry may comprise
carbon particles and a solvent, particularly, volatile dispersing liquid
without any binder used. The dispersing liquid may be water, alcohol or
other volatile liquid because after drying, it is preferable that only
carbon particles remain dispersed on the surface without containing any
impurity such as binder solid.
Prior to pressing in the above methods, the valve metal material may
preferably be roughened on the surface, particularly be made porous in a
thin layer of the surface, facilitating carbon particles to engage and
embed in the porous surface layer effectively.
Also, a method of producing the valve metal material for electrodes in
accordance with the present invention may include a step wherein the
powder material for the valve metal and carbon particles are semi-melted
in a mixture condition and subjected to pressure so as to be formed into a
dense metal ingot. The metal ingot, including carbon particles dispersed
inside, is forged or rolled into a product having a desired shape, and
then the carbon particles are exposed to the surface of the product.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described below in detail referring to the
accompanying drawings, in which:
FIG. 1 is a schematic sectional view showing an example of a
carbon-containing valve metal material in accordance with the present
invention, wherein carbon particles are fixed in the surface of the valve
metal sheet;
FIG. 2 is a schematic sectional view showing another example of a
carbon-containing valve metal material in accordance with the present
invention;
FIG. 3 is a schematic sectional view showing still another example of a
carbon-containing valve metal material in accordance with the present
invention;
FIG. 4 is a schematic sectional view showing yet still another example of a
carbon-containing valve metal material in accordance with the present
invention, wherein carbon particles are fixed on both sides of the valve
metal sheet;
FIG. 5 is a schematic partially-sectional view showing a double-layer
electrode used for an electric double-layer capacitor formed of the
carbon-containing valve metal material in accordance with the present
invention;
FIG. 6 is a schematic partially-sectional view showing a double-layer
electrode used for a conventional electric double-layer capacitor;
FIG. 7 is a schematic partially-cutaway perspective view showing a winding
type electric double-layer capacitor;
FIG. 8 is a schematic partially-sectional view showing a double-layer
electrode used for a button-type electric double-layer capacitor formed of
the carbon-containing valve metal material in accordance with the present
invention;
FIG. 9 is a schematic sectional view showing the button-type electric
double-layer capacitor;
FIG. 10 is a schematic partially-cutaway perspective view showing the
button-type electric double-layer capacitor.
FIG. 11 shows a schematic partial cross sectional view of a non-aqueous
secondary battery; and
FIG. 12 shows a cross sectional structure of a positive electrode for a
non-aqueous secondary battery which is formed on both sides of an carbon
containing electrode metal material according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A valve metal material for electrodes in accordance with the present
invention includes carbon particles fixed on the surface thereof as
described above. The valve metals can be selected from among metals
capable of forming a passivated layer on the surface thereof. For example,
such valve metals may include tantalum, aluminum, titanium, niobium,
zirconium, bismuth, silicon and hafnium. Alternatively, the valve metal
can be selected from among alloys including these elements capable of
generating a valve action, such as for example, a titanium-based alloy
including boron and tin, a titanium-based alloy including chromium and
vanadium, a titanium-based alloy including vanadium and antimony, and an
aluminum-based alloy including titanium. The most desirable material is
aluminum, in particular, high-purity aluminum.
The electrode metal material of the present invention is formed into the
form having a desired thickness, for example, into sheet. The thickness of
the sheet may be in a range of 10 .mu.m to 5 mm, although the thickness
depends on the kind of capacitor or battery, or the kind of electrode.
Generally, for winding type electric double-layer capacitors and
electrolytic capacitors, metal foil having a thickness of 50 to 500 .mu.m
is preferably used to provide flexibility and sufficient winding turns. On
the other hand, for button-type electric double-layer capacitors, the
valve metal material, when also used as a part of the wall or bottom
portion of the container, should preferably have a larger thickness of
about 0.50 to 3.0 mm, for example, to provide strength to the wall or
bottom portion.
A base metal plate for providing strength may be cladded with the
above-mentioned thin valve metal material, and carbon particles may be
included in the clad valve metal material used. A highly
corrosion-resistant metal or alloy, such as nickel or stainless steel, may
be used as this kind of base metal. Such base metal plate cladded with
valve metal material may used for a casing which also supports a positive
electrode of button- or coin-type secondary batteries.
On the other hand, carbon particles are formed of conductive carbon
particles, such as graphite or carbon black. Carbon black as an example
may be acetylene black. Furthermore, carbon particles may be particles of
activated carbon.
The diameter of carbon particles should preferably be in the range of 0.01
to 50 .mu.m, more preferably, in the range of 0.1 to 10 .mu.m. In
addition, the carbon particles can have one of particulate, granular and
fibrous forms. In the case of fibrous carbon particles, the
above-mentioned particle diameter in the range of 0.1 to 50 .mu.m refers
to the fiber length thereof.
The content of carbon particles should preferably be in a range of 1 to 90%
of the area percentage of the carbon with respect to the whole surface
area of the valve metal material. If the area percentage of the carbon is
less than 1%, it may be difficult to sufficiently reduce contact
resistance at the surface. The area percentage of the carbon should
preferably be higher. However, if the area percentage of the carbon is
more than 90%, it becomes difficult to stably hold carbon particles
pressed into the surface of the valve metal by a press method.
Accordingly, the area percentage of the carbon particles on the surface
should preferably be in a range of 5 to 60%, more preferably, in a range
of 10 to 40%.
The valve metal material should preferably have a rough surface. In
particular, carbon particles should preferably project slightly from the
surface of the metal material. The projection of the carbon particles can
be performed by subjecting the surface to electrolytic etching in an
acidic aqueous solution. The exposure of numerous carbon particles can
increase contact frequency to an activated carbon layer for an electric
double-layer electrode structure. Furthermore, the activated carbon layer
can be firmly fixed by an anchor effect. Also, the exposed carbon
particles from the electrode can increase contact frequency to active
substance contained in the positive electrode of non-aqueous secondary
batteries.
FIG. 1 shows a carbon-driven metal material 1 wherein nearly particulate
carbon particles 2 are driven on one side of a sheet of valve metal
material 10. This figure shows a schematic view of an example of the valve
metal material in which the carbon particles 2 are partially embedded in
the surface of the metal material and the upper portions thereof project
from the surface.
FIG. 2, similar to FIG. 1, is a conceptual view showing a condition wherein
the carbon particles 2 are crushed and wholly embedded in the surface of
the metal material. However, in the carbon-driven metal material 1, the
surfaces of the carbon particles are still exposed to the surface of the
metal material, and the carbon particles can be used to ensure
conductivity. This condition may be obtained when relatively soft carbon
particles are pressed strongly.
FIG. 3 is a view showing a condition wherein the carbon-driven metal
material 1 shown in FIG. 2 is subjected to electrolytic etching to remove
its metallic surface 11, thereby allowing the carbon particles to project
from a etched surface. FIG. 4 is a view showing a condition wherein the
carbon particles driven on both sides of the sheet of the valve metal
material which sides are subjected to etching, thereby allowing the carbon
particles to project from both etched surfaces.
Furthermore, the whole surface of the carbon-containing metal material may
be roughened by blasting. Blasting makes the surface of the valve metal
material rough directly, and the carbon particles expose. In addition, for
electric double layer capacitors, the activated carbon layer can be fixed
firmly to the roughened surface, and the contact resistance of the surface
can be reduced.
A passive film my preferably be formed on the surface of the
carbon-containing metal material (for example, the metallic surface 11
shown in FIGS. 3 and 4). Even if water is present in electrolyte while the
valve metal material is used as an electrode, the passive film protects
the surface of the valve metal material against oxidation and corrosion.
Therefore, the electrodes can be stabilized further, without adversely
affecting the conductivity thereof due to the existence of the carbon
particles.
The passive film may have only a thickness capable of withstanding working
voltage of a capacitor comprising the film. For example, in the case of an
electric double-layer capacitor rated at 2.5V to 3.5V, the film should
only have a thickness capable of withstanding a voltage in the range of 4
to 5 V. In this case, the valve metal material is provided with a passive
film having a thickness of 40 .ANG. to 60 .ANG. or more. Also, for the
positive active electrode of non-aqueous secondary batteries, the passive
film formed on the carbon containing electrode metal material may have
higher withstand voltage than 3V, preferably of 4 to 5V.
With respect to lithium ion secondary batteries, FIG. 11 shows a
winding-type lithium ion secondary battery, in which a pair of electrodes
i.e., a positive electrode 35 and a negative electrode 37, between which a
separator 5 is inserted, are wound, penetrated with an non-aqueous
electrolyte in a casing 71 for sealing.
An electrode metal material of the present invention is used to form the
positive electrode 35 which comprise an mixture of positive active
substance, conductive material and a binder which is formed on both sides
of the electrode metal material. The positive active substance may be a
compound capable of absorbing and emitting any ions of H.sup.+, Li.sup.+,
Na.sup.+ and K.sup.+, preferably, oxides or chalcogenides of transition
metals, or carbon, particularly, lithium-containing transition metal
oxides. The transition metal may be one or more selected from a group of
Co, Ni, Fe, V and Mn. A conductive material may be an electron-conductive
material not to chemically react in the battery, such as natural graphite,
synthetic graphite, carbon black, acetylene black or carbon fibers. As a
binder, polysaccharides, thermo-plastic resins or rubber-like elastic
polymers may be used. A binder may include starch, polyvinylalcohol,
carboxylmethylcellulose, hydroxy-propylcellulose, polyflorovinylidene,
etc.
An electrolyte comprises an organic solvent and a salt soluble to the
solvent to dissociate. A solvent may be one or a mixture of propylene
carbonate, ethylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran,
.gamma.-butyrolactone, and 1,2-dimethoxyethane. The salt as an
electrolytic substance may be selected from LiPF.sub.6, LiClO.sub.4,
LiBF.sub.4 and LiCF.sub.3 SO.sub.3.
The separator 5 may be thin insulating material capable of penetrating an
electrolyte therethrough such as polypropylene porous film, woven or
unwoven glass cloth, paper made of manila paper and rayon.
In this battery structure, FIG. 12 shows a cross sectional structure of a
positive electrode 35 for a non-aqueous secondary battery which is formed
on both sides of an carbon containing electrode metal material 1 according
to the present invention. Since the electrode metal material 1 of the
present invention which supports the positive electrodes 35 is provided
with carbon particles on its surface, the carbon particles 2 can
effectively connect the electrode metal material 1 of a valve metal 10
directly with a positive electrode 35 effectively even though a thin
insulating film 4 has been formed in interfaces between the metallic
surface of the electrode metal material 1 and the positive electrode 35 by
oxidation of the metallic surface of the electrode material metal. This
results in reduction in equivalent series resistance and capacity loss of
batteries.
Some methods may be adopted to produce an electrode metal material formed
of a valve metal material containing numerous carbon particles at least in
the surface thereof.
In a first method, a mixture of valve metal powder and carbon powder is
heated near its melting point and pressurized in a container to make an
ingot so that the carbon powder may be contained in the valve metal ingot.
This method further includes a step of subjecting the ingot of
carbon-containing valve metal produced by the above step to plastic
working in order to obtain a desired shape of the valve metal material. In
the plastic working step, hot or cold forging or rolling can be utilized,
whereby forming sheet or foil having a desired thickness or a formed
product having any other shapes.
A second method includes a carbon-powder driving step wherein carbon
particles are driven into the surface of a valve metal material by
pressurizing carbon particles dispersed on the surface of the valve metal
material.
The carbon particle driving step in the second method can also be
accomplished by a press technique where dies are used to press and fit
carbon particles into the surface of the valve metal material. The dies
may have a flat, hardened plate or the like.
Furthermore, the carbon particle driving step can also be accomplished by a
rolling method wherein rollers are used to drive carbon particles into the
surface of the valve metal material. In either of the two methods, carbon
particles can be pressed and fitted in the surface of the valve metal
material and fixed.
By using the second method, carbon particles are driven in the surface of
the valve metal material having a desired thickness. The above-mentioned
carbon particle driving step can be performed by applying a surface
pressure of 0.5 to 10000 kg/cm.sup.2 in a direction perpendicular to the
surface of the metal material. This pressure is determined depending on
the hardness of the surface of the valve metal material and the hardness
and particle size of the carbon particles.
Furthermore, the carbon particle driving step may also be used as a step of
pressing or forging a valve metal blank into a formed product having a
desired shape. In other words, in this case, the carbon particle driving
step is carried out when the valve metal ingot is hot or cold worked. In
this step, just when the valve metal material is pressurized by hot or
cold forging or rolling, the carbon particles are driven into the forged
or rolled surface.
Prior to the carbon particle driving step, numerous carbon particles are
dispersed on the surface of a valve metal material, for example, in the
form foil, sheet or plate. Usually, carbon powder may be powdered on the
surface.
Preferably, a slurry containing carbon particles and liquid may be applied
on a surface of a valve metal material, facilitate the dispersing Of
carbon particles. The liquid in the slurry may be a volatile dispersing
liquid to facilitate dehydration of the slurry. The slurry may not contain
a binder which remain in a powder after drying. The dispersing liquid may
be water, alcohol or other volatile liquid because after drying, it is
preferable that only carbon particles remain dispersed on the surface
without containing any impurity such as binder solid. Preferably, the
carbon slurry may be dried on the surface of the valve metal material, and
in the carbon particle driving step, is pressed to squeeze the carbon
particles into the surface, obtaining carbon-containing electrode metal
material.
Prior to pressing in the above methods, the valve metal material may
preferably be roughened on the surface, allowing carbon particles to
engage and embed easily on the rough surface.
A technique for roughening a surface includes blasting sand or other harden
powder onto the surface. Other roughening method includes chemical or
electrolytic etching which makes the surface porous in a thin layer under
the valve metal surface. By this etching, a great number of pores are
produced opening to the surface of the valve metal material and extending
in some depth under the surface and dispersed uniformly in an area of the
surface.
Such a roughened or porous surface may easily receive and hold the applied
carbon powder or dried carbon slurry on the surface, preventing the carbon
particles to be sent flying. Roughening of a valve metal surface can
eliminate the need for use of any binder mixing in the carbon slurry,
avoiding impurity inclusion on the carbon embedded surface.
Particularly, the porous surface layer formed by etching is easy to be
deformed by pressing or rolling on the surface, allowing carbon particles
to be embedded easily in high carbon density into the surface-deformed
layer from the porous surface layer during pressing or rolling.
The roller used in rolling may be an roller embossed on its surface. The
embossed roller can make an embossed pattern on the carbon embedded
surface at the same time during driving carbon particle driving.
Furthermore, in the method in accordance with the present invention, the
surface of the carbon containing metal material may preferably be
coarsened after the carbon particle driving step.
For this purpose, it is desired that, after the carbon particle driving
step, the method includes a step of exposing the carbon particles to the
surface by electrolytically etching in an acidic aqueous solution. By this
treatment, the carbon particles exposed on the surface project from the
surface and carbon particles slightly embedded below the surface also are
exposed to the surface. The exposure of numerous carbon particles can
increase the contact frequency of an activated carbon layer for an
electric double-layer electrode structure. Furthermore, the activated
carbon layer can be firmly fixed by an anchor effect.
After the carbon particle driving step, the method may include blasting as
a step of exposing carbon particles to the surface. This blasting
technique can also accomplish direct roughing of the surface and exposing
of the carbon particles by blasting.
The production method may preferably include a step of forming a passive
film on the metallic surface of the carbon containing metal material after
the carbon particle exposing step. Formation of the passive film may use a
technique of heating the carbon-containing metal material oxidized in an
oxidative atmosphere, such as in air. The heat treating is performed at
300-620.degree. C.
Alternatively, another method is used, wherein the carbon-containing metal
material is anodized using anodic oxidation of a metallic surface of the
valve metal.
The passive film should have a thickness capable of withstanding an applied
voltage of 4 to 5 V in the case of an electric double-layer capacitor
rated at 2.5 to 3.5 V, for example. In this case, the valve metal material
is provided with a passive film having a thickness of 40 .ANG. to 60 .ANG.
or more.
Capacitors in accordance with the present invention include electric
double-layer capacitors and electrolytic capacitors. In both types of the
capacitors, non-aqueous electrolyte is used, and the valve metal material
thereof makes contact with the electrolyte.
FIG. 7 is a schematic view showing a winding type capacitor used as a kind
of electric double-layer capacitor. The winding type capacitor is provided
with flexible electric double-layer electrodes. The electrode comprises
thin valve metal foil used as a valve metal material, and activated carbon
layers bonded to both sides of the foil. Numerous carbon particles are
fixed on the surface of the foil so as to expose, thereby making contact
with the activated carbon layers.
A pair of electric double-layer electrodes, holding a separator
therebetween, is wound to a coil and enclosed in a container while being
immersed in non-aqueous electrolyte, thereby forming an electric
double-layer capacitor. The electrolyte is formed of an organic solvent
not including water and a salt dissolved in such a solution so as to be
dissociated. A solution wherein tetraethyl ammonium perchlorate used as an
electrolyte is added to propylene carbonate used as a solvent is taken as
an example.
The activated carbon layer is formed in a thin film by forming activated
carbon powder into a paste form and by applying the paste to the surface
of the valve metal foil.
The paste is obtained, for example, by kneading a mixture of activated
carbon powder, conductive carbon powder and an appropriate binder, such as
cellulose or fluororesin, as necessary, together with water or other
solvent. The coated paste film is appropriately dried and heated together
with the valve metal foil to cure the binder, whereby the film is fixed to
obtain an electric double-layer electrode.
Leads are connected to the pair of electric double-layer electrodes, one
lead to each electrode. Furthermore, the electrodes were wound while
holding a separator therebetween so as to be formed into a coil. The
separator is formed of an appropriate thin material that is insulating and
water-permeable, such as glass-fiber woven or non-woven cloth.
The coil comprising the electric double-layer electrodes and the separator
is immersed in electrolyte and accommodated in a metal container having a
bottom. The opening of the container is sealed with a sealing material.
The leads pass through the sealing material and are extended outside.
With the above-mentioned structure of the electrode, even when a thin
insulating film 4 is present at the interface between the foil-like metal
material 10 of the electrode metal material 1 and the polarizable
electrode 30 of the electric double-layer capacitor as shown in FIG. 5, no
oxidized film is formed on the surfaces of carbon particles 2 exposed from
the electrode foil 10. For this reason, electric conduction can be
maintained by the carbon particles 2 at portions wherein the carbon
particles 2 are present. As a result, the equivalent series resistance
(ESR) of the electric double-layer capacitor decreases, and the conduction
portions increase in number, whereby the capacitance thereof increases.
FIGS. 9 and 10 show a button-type electric double-layer capacitor. An
activated carbon layer 30 is bonded to a disc-like sheet 10 formed of the
valve metal material of the present invention via an adhesive layer 9,
thereby forming a pair of double-layer electrodes 3. The two double-layer
electrodes 3 are disposed face-to-face with an insulating separator 5
therebetween, and accommodated inside a metal container comprising two
mating portions 60 and 61.
The valve metal material sheets 10 and 10 of the two double-layer
electrodes 3 and 3 are joined to the inner surface sides of the bottom
portion 60 and the lid portion 61 of the metal container, respectively.
The bottom portion and the lid portion are then joined to each other at
their peripheral portions so as to be watertight via an insulating ring
packing 69. The container is filled with non-aqueous electrolyte so that
the double-layer electrodes and the activated carbon layers are
sufficiently immersed in the electrolyte. The non-aqueous electrolyte may
be, for example, a solution of tetraethyl ammonium perchlorate used as an
electrolyte in propylene carbonate used as a solvent, as described above.
Double-layer electrodes 3 for the button-type electric double-layer
capacitor are shown in FIG. 8. The activated carbon layer 30, i.e., the
polarizable electrode 30, is formed of a sheet made of activated carbon
particles or fibers.
For example, the activated carbon layer 30 is obtained as described below.
A paste is prepared by mixing activated carbon powder, a solvent and an
appropriate binder, and this paste is formed into thin film, which are
dried and cured to obtain sheets including activated carbon particles. The
sheets are used as the activated carbon layer 30.
The sheet of activated carbon fiber is formed of fiber activated at the
carbonization step of phenol-based resin fiber, for example. The activated
carbon fiber is woven into a sheet.
The above-mentioned activated carbon particle sheet or the activated carbon
fiber sheet is stamped into sheet pieces having a desired shape, and the
sheet pieces are bonded to the carbon-containing side of the valve metal
material sheet so as to be assembled into the double-layer electrode 3. An
organic adhesive 9, being conductive, is usually used for the bonding.
The conductive adhesive may be used to firmly bond a sheet of chemically
active carbon fiber or the like to the valve metal material sheet.
Furthermore, the adhesive 9 is used to electrically connect the carbon
particles on the valve metal material side to some parts of the fiber and
particles on the activated carbon side | | |
