U.S. patent application number 10/193366 was filed with the patent office on 2002-11-28 for anode constructions for nonsymmetric capacitors.
This patent application is currently assigned to Aerovox Incorporated, a Delaware corporation. Invention is credited to Hudis, Martin.
Application Number | 20020176221 10/193366 |
Document ID | / |
Family ID | 22331877 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020176221 |
Kind Code |
A1 |
Hudis, Martin |
November 28, 2002 |
Anode constructions for nonsymmetric capacitors
Abstract
A capacitor of the type having a cathode and an anode and an
electrolyte disposed between the cathode and the anode, the
capacitor comprising a cathode, an electrolytic anode comprising a
stack of metal sheets, the sheets being coated with a metal oxide,
wherein the metal sheets are electrically connected in parallel by
one or more electrically conductive elements in electrical contact
with the sheets, and an electrolyte in contact with the cathode and
the metal oxide on the sheets.
Inventors: |
Hudis, Martin;
(Mattapoisett, MA) |
Correspondence
Address: |
G. ROGER LEE
Fish & Richardson P.C.
225 Franklin Street
Boston
MA
02110-2804
US
|
Assignee: |
Aerovox Incorporated, a Delaware
corporation
|
Family ID: |
22331877 |
Appl. No.: |
10/193366 |
Filed: |
July 11, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10193366 |
Jul 11, 2002 |
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09761966 |
Jan 17, 2001 |
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09761966 |
Jan 17, 2001 |
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09358577 |
Jul 21, 1999 |
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09358577 |
Jul 21, 1999 |
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09110223 |
Jul 6, 1998 |
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6208502 |
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Current U.S.
Class: |
361/511 |
Current CPC
Class: |
Y02E 60/13 20130101;
H01G 9/02 20130101; H01G 9/0425 20130101; H01G 11/46 20130101; H01G
9/04 20130101; H01G 9/145 20130101; H01G 11/22 20130101; H01G 11/42
20130101; H01G 9/035 20130101 |
Class at
Publication: |
361/511 |
International
Class: |
H01G 004/32; H01G
009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 1999 |
PCT/US99/15207 |
Claims
What is claimed is:
1. A capacitor of the type having a cathode and an anode and an
electrolyte disposed between the cathode and the anode, the
capacitor comprising a cathode, an electrolytic anode comprising a
stack of metal sheets, the sheets being coated with a metal oxide,
wherein the metal sheets are electrically connected in parallel by
one or more electrically conductive elements in electrical contact
with the sheets, and an electrolyte in contact with the cathode and
the metal oxide on the sheets.
2. The capacitor of claim 1 wherein the sheets are rectangular, and
the stack is a parallelepiped.
3. The capacitor of claim 1 wherein the cathode is an
electrochemical cathode comprising a metal current collector coated
with a finely divided material.
4. The capacitor of claim 1 or 3 wherein openings are formed
through the stack of sheets to facilitate communication between the
electrolyte and the metal oxide at locations within the interior of
the stack of sheets.
5. The capacitor of claim 1 or 3 wherein the sheets are separate
elements electrically connected at their edges.
6. The capacitor of claim 1 or 3 wherein the sheets are a
continuous, fan-folded sheet, with each fold forming one sheet in
the stack.
7. The capacitor of claim 3 wherein the finely divided material
comprises carbon particles and the capacitance of the
electrochemical cathode is provided by a double layer effect.
8. The capacitor of claim 3 wherein the finely divided material
comprises a conducting metal oxide and the capacitance of the
electrochemical cathode is provided by an oxidation reduction
reaction.
9. The capacitor of claim 3 wherein the electrolyte is
substantially nonaqueous.
10. A capacitor of the type having a cathode and an anode and an
electrolyte disposed between the cathode and the anode, the
capacitor comprising an electrochemical cathode comprising a metal
layer coated with a finely divided material, an electrolytic anode
winding comprising at least one oxide-coated metal layer wound into
a winding, and an electrolyte in contact with the cathode and the
metal oxide in the anode winding.
11. The capacitor of claim 10 wherein the cathode is adjacent the
anode winding, so that the cathode extends generally in a plane
normal to the axis about which the anode is wound.
12. The capacitor of claim 11 wherein there are a plurality of
cathodes and a plurality of anode windings and the cathodes are
interleaved between the anode windings.
13. The capacitor of claim 11 wherein there are a plurality of
anode windings arranged in an anode layer and wherein the cathode
comprises at least one layer forming a cathode layer positioned
generally parallel to the anode layer and normal to the axis about
which the anode windings are wound.
14. The capacitor of claim 13 wherein there are a plurality of
interleaved anode and cathode layers.
15. The capacitor of claim 10 wherein the anode windings are
circular or oval in cross section.
16. The capacitor of claim 10 wherein the finely divided material
comprises carbon particles and the capacitance of the
electrochemical cathode is provided by a double layer effect.
17. The capacitor of claim 10 wherein the finely divided material
comprises a conducting metal oxide and the capacitance of the
electrochemical cathode is provided by an oxidation reduction
reaction.
18. The capacitor of claim 10 wherein the electrolyte is
substantially nonaqueous.
19. A capacitor of the type having a cathode and an anode and an
electrolyte disposed between the cathode and the anode, the
capacitor comprising a cathode, an electrolytic anode winding
comprising a plurality of oxide-coated metal layers wound about an
axis, wherein the metal layers are electrically connected in
parallel by one or more electrically conductive elements in
electrical contact with the layers, and an electrolyte in contact
with the cathode and the metal oxide on the layers.
20. The capacitor of claim 20 wherein there are at least three
oxide-coated layers wound to form the anode winding.
21. A capacitor of the type having a cathode and an anode and an
electrolyte, the capacitor comprising an electrolytic anode
comprising a plurality of oxide-coated metal layers arranged one
layer over another layer, and an electrochemical cathode comprising
at least one metal layer coated with a finely divided material,
wherein the coated layer of the cathode is arranged substantially
parallel to the plurality of anode layers, wherein the layers of
the anode each include a multiplicity of tiny holes extending
substantially all the way through the layer to provide paths for
conductive ions to flow between the cathode layer and the anode
layers.
22. The capacitor of claim 21 wherein the layers of the anode and
cathode are wound into a winding.
23. The capacitor of claim 21 wherein the layers of the anode and
cathode are stacked in a stack.
24. The capacitor of claim 21 further comprising spacer layers
separating the anode layers from the cathode layers.
25. The capacitor of claims 19 or 21 wherein the electrolyte is
substantially nonaqueous.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part application of
and claims priority to U.S. application Ser. No. 09/358,577, filed
on Jul. 21, 1999, which is a continuation-in-part of copending U.S.
application Ser. No. 09/110,223, filed on Jul. 6, 1998, and which
also claims priority from copending PCT Application Serial No.
PCT/US99/15207, filed on Jul. 6, 1999.
BACKGROUND OF THE INVENTION
[0002] This invention relates to non-symmetric
electrolytic/electrochemica- l capacitors.
[0003] A typical symmetric aluminum electrolytic capacitor (FIG. 1)
includes an aluminum anode foil, an aluminum cathode foil, and a
conductive liquid electrolyte, such as ethylene glycol. Ethylene
glycol is a substantially non-aqueous electrolyte, i.e. it contains
less than 3% of water. The liquid electrolyte is retained by a
porous paper separator which acts as a spacer between the anode and
cathode foils. The anode and cathode foils are connected to
external terminals via aluminum tabs.
[0004] The surfaces of the aluminum anode and cathode foils are
coated with a layer of an insulating aluminum oxide, which is
formed by an electro-chemical oxidation process called forming. For
the forming process, a constant voltage is applied to the aluminum
foils. The formation voltage is higher than a typical rated working
voltage of the capacitor. The aluminum oxide thickness is
proportional to the applied voltage. In one example, an aluminum
electrolytic capacitor may have rated working voltages up to 600 V
and forming voltages in the range of 850 to 900 V.
[0005] The insulating aluminum oxide is in contact with the
conductive electrolyte. The aluminum anode and cathode foils, the
corresponding aluminum oxides, and the electrolyte with the
separator form two capacitors connected in series (FIG. 1A). The
thickness of the insulating aluminum oxide layer determines the
breakdown voltage of the capacitor. By varying the aluminum oxide
layer thickness, the specific capacitance (i.e., capacitance per
surface area) of each capacitor is varied. Increasing the aluminum
oxide layer thickness reduces the specific capacitance and
increases the breakdown voltage of the capacitor. The specific
capacitance may be increased by increasing the active surface area
of the aluminum foil. The active surface area of the aluminum foil
is increased by etching.
[0006] Another class of capacitors are the electrochemical
capacitors. Electrochemical capacitors fall into two categories:
Faradaic and non-Faradaic (double-layer). Non-Faradaic capacitors
rely solely on interfacial charge separation across a boundary
between an electrolyte and a conducting surface or an insulating
surface such as some metal oxides like aluminum oxide and tantalum
oxide. The Faradaic capacitors are often referred to as
pseudo-capacitors. Pseudo-capacitors have enhanced charge storage
derived from charge transfer through a chemical reaction that takes
place across the interface between an electrolyte and a conducting
surface. The charge transfer can occur, for example by: (1) surface
charge attachment to a metal hydride like ruthenium hydride, (2)
volume charge diffusion into a metal like silver coated palladium,
or (3) an oxidation/reduction reaction at the surface of an oxide
like ruthenium oxide.
[0007] Non-symmetric electrolytic/electrochemical capacitors use a
conventional electrolytic capacitor at the anode and an
electrochemical capacitor at the cathode. Evans U.S. Pat. No.
5,737,181 describes a non-symmetric capacitor that has a
pseudo-capacitor ruthenium oxide ceramic cathode, a tantalum anode
and an aqueous electrolyte. Non-symmetric capacitors with modified
metal cathode surfaces are disclosed in Libby U.S. Pat. No.
4,780,797 and Rogers U.S. Pat. No. 4,523,255, which describe very
aggressive aqueous electrolytes (e.g., sulfuric acid) that have
high conductivity and are compatible with tantalum and tantalum
oxide anodes.
SUMMARY OF THE INVENTION
[0008] In general, the invention features a capacitor of the type
having a cathode and an anode and an electrolyte disposed between
the cathode and the anode, the capacitor comprising a cathode, an
electrolytic anode comprising a stack of metal sheets, the sheets
being coated with a metal oxide, wherein the metal sheets are
electrically connected in parallel by one or more electrically
conductive elements in electrical contact with the sheets, and an
electrolyte in contact with the cathode and the metal oxide on the
sheets.
[0009] In preferred implementations, one or more of the following
features may be incorporated. The sheets may be rectangular, and
the stack may be a parallelepiped. The cathode may be an
electrochemical cathode comprising a metal current collector coated
with a finely divided material. Openings may be formed through the
stack of sheets to facilitate communication between the electrolyte
and the metal oxide at locations within the-interior of the stack
of sheets. The sheets may be separate elements electrically at
their edges. The sheets may be a continuous, fan-folded sheet, with
each fold forming one sheet in the stack. The finely divided
material may comprise carbon particles and the capacitance of the
electrochemical cathode is provided by a double layer effect. The
finely divided material may comprises a conducting metal oxide and
the capacitance of the electrochemical cathode is provided by an
oxidation reduction reaction. The electrolyte may be substantially
nonaqueous.
[0010] In a second aspect, the invention features a capacitor of
the type having a cathode and an anode and an electrolyte disposed
between the cathode and the anode, the capacitor comprising an
electrochemical cathode comprising a metal layer coated with a
finely divided material, an electrolytic anode winding comprising
at least one oxide-coated metal layer wound into a winding, and an
electrolyte in contact with the cathode and the metal oxide in the
anode winding.
[0011] In preferred implementations one or more of the following
features may be incorporated. The cathode may be adjacent the anode
winding, so that the cathode extends generally in a plane normal to
the axis about which the anode is wound. There may be a plurality
of cathodes and a plurality of anode windings and the cathodes are
interleaved between the anode windings. There may be a plurality of
anode windings arranged in an anode layer and wherein the cathode
comprises at least one sheet forming a cathode layer positioned
generally parallel to the anode layer and normal to the axis about
which the anode windings are wound. There may be a plurality of
interleaved anode and cathode layers. The anode windings may be
circular or oval in cross section. The finely divided material may
comprise carbon particles and the capacitance of the
electrochemical cathode is provided by a double layer effect. The
finely divided material may comprise a conducting metal oxide and
the capacitance of the electrochemical cathode is provided by an
oxidation reduction reaction. The electrolyte may be substantially
nonaqueous.
[0012] In a third aspect, the invention features a capacitor of the
type having a cathode and an anode and an electrolyte disposed
between the cathode and the anode, the capacitor comprising a
cathode, an electrolytic anode winding comprising a plurality of
oxide-coated metal layers wound about an axis, wherein the metal
layers are electrically connected in parallel by one or-more
electrically conductive elements in electrical contact with the
layers, and an electrolyte in contact with the cathode and the
metal oxide on the layers.
[0013] In preferred implementations, one or more of the following
features may be incorporated. There may be at least three
oxide-coated layers wound to form the anode winding. The
electrolyte may be substantially nonaqueous.
[0014] In a fourth aspect, the invention features a capacitor of
the type having a cathode and an anode and an electrolyte, the
capacitor comprising an electrolytic anode comprising a plurality
of oxide-coated metal layers arranged one layer over another layer,
and an electrochemical cathode comprising at least one metal layer
coated with a finely divided material, wherein the coated layer of
the cathode is arranged substantially parallel to the plurality of
anode layers, wherein the layers of the anode each include a
multiplicity of tiny holes extending substantially all the way
through the layer to provide paths for conductive ions to flow
between the cathode layer and the anode layers.
[0015] In preferred implementations, one or more of the following
features may be incorporated. The layers of the anode and cathode
may be wound into a winding. The layers of the anode and cathode
may be stacked in a stack. Spacer layers may separate the anode
layers from the cathode layers. The electrolyte may be
substantially nonaqueous.
[0016] The improved anode constructions provide a nonsymmetric
capacitor that performs well and that may be manufactured at lower
cost.
[0017] Other features and advantages of the invention will be
apparent from the following description of preferred embodiments,
and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a cross-sectional view of a prior art symmetric
aluminum electrolytic capacitor.
[0019] FIG. 1A is an equivalent circuit diagram of a prior art
symmetric electrolytic capacitor.
[0020] FIG. 2 is a cross-sectional view of a non-symmetric
electrolytic/electrochemical capacitor embodiment of the
invention.
[0021] FIG. 2A is an exploded perspective view of another
non-symmetric electrolyte/electrochemical capacitor embodiment of
the invention.
[0022] FIG. 3 is a perspective view of an anode roll useful in some
embodiments.
[0023] FIG. 4 is a cross-sectional view of a thin etched aluminum
foil.
[0024] FIG. 5 is a perspective view of an expanded nickel mesh that
may be used in the cathode.
[0025] FIG. 6 is a perspective view of the cathode collector coated
with a finely-divided powder.
[0026] FIG. 7 is a diagrammatic view of a multi-cell non-symmetric
electrolytic/electrochemical capacitor.
[0027] FIG. 8 is a flow chart of the process for fabricating a
capacitor.
[0028] FIGS. 9A to 9D show impedance spectroscopy data for
embodiments with a carbon coated nickel cathode, etched and formed
aluminum anode and non-aqueous electrolyte.
[0029] FIGS. 10A to 10D show impedance spectroscopy data for
embodiments with a ruthenium oxide coated titanium cathode, etched
and formed aluminum anode and nonaqueous electrolyte.
[0030] FIGS. 11A to 11D show impedance spectroscopy data for
embodiments with carbon coated nickel electrodes (anode and
cathode) and with butyrolactone based electrolyte.
[0031] FIGS. 12A to 12D show impedance spectroscopy data for
embodiments with carbon coated nickel electrodes (anode and
cathode) and with ethylene glycol based electrolyte.
[0032] FIG. 13A is a diagrammatic view of a double anode structure
for an AC start capacitor.
[0033] FIG. 13B is a plot of the applied AC voltage across the
double anode structure of FIG. 13A.
[0034] FIG. 14A is a diagrammatic view of a double anode with a
floating capacitor structure for an AC start capacitor.
[0035] FIG. 14B is a plot of the applied AC voltage across the
double anode with the floating capacitor structure of FIG. 14A.
[0036] FIG. 15 shows the process for winding an anode with multiple
current collector sheets.
[0037] FIG. 16 is an end view of another non-symmetric
electrolytic/electrochemical capacitor embodiment of the invention,
in which both the anode and cathode layers are wound in the same
winding. A portion of the winding is shown enlarged to provide a
diagrammatic view of the cathode, anode, and separator layers.
[0038] FIG. 17 is a diagrammatic cross sectional view of a tiny
portion of the embodiment of FIG. 16, showing the cathode, anode,
and separator layers in cross section.
[0039] FIG. 18 is a diagrammatic view of a multi-cell non-symmetric
electrolytic/electrochemical capacitor wherein the anode layers are
parallel to the cathode layers.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] FIG. 2 shows a first embodiment of a multi-section
non-symmetric capacitor 50. Although only three capacitor sections
51a to 51c are shown for purposes of illustration, a typical
embodiment may have more sections (e.g., 5 to 50, or many more).
Each capacitor section 51a includes an anode 52, a cathode 54, an
electrolyte 58 (liquid that impregnates the capacitor or partially
fills the interior of container 55), and separator disks 56. In one
example, the anodes 52, cathodes 54, and the separator disks 56
have annular shapes with central openings 63, 65, and 67,
respectively. The anodes 52 are interleaved with the cathodes 54,
and the individual capacitor sections are stacked so that a
sequence of cathode/anode/cathode/anode is formed. The separator
disks 56 separate the cathodes 54 from the anodes 52 within a
capacitor section, and from the anodes in the adjacent capacitor
section. The stacked sections 51a, 51b, and 51c are supported by a
plastic support member 49, which is fed through the central
openings 63, 65 and 67. The section assembly is placed in a plastic
container or other suitable container, such as an aluminum case 55
which is closed on the top with a cover 59. The cover has a vent 61
and anode and cathode voltage terminals 60a, 62a. The vent is
configured to open when the pressure inside the capacitor exceeds a
certain value, e.g., 75 psi.
[0041] The anodes 52 may be formed by splitting and winding etched
and formed aluminum foil 82. For example, starting with a wide roll
(in one example the width may be in the range between 50 to 100
cm), the material may be split (in one or more splitting steps)
into 3-23 mm wide sheets, which are then wound to form a plurality
of narrow rolls 80 (FIG. 3). Conventional processes may be used to
etch the aluminum foil and form the aluminum oxide layer 84 (shown
diagrammatically in FIG. 4). The etching process produces micron
size holes 86 in the foil. The holes increase the effective surface
area of the anode. In one example, the aluminum foil is 6 mm wide,
and has a thickness of 100 micrometers. The etched aluminum holes
86 with the aluminum oxide layer 84 may be approximately 40
micrometers deep on both surfaces.
[0042] Alternatively, the anode may be fabricated by winding a
flattened small diameter etched aluminum wire into a roll. The
aluminum oxide layer is subsequently formed.
[0043] The insulating aluminum oxide surface of the anode contacts
the electrolyte 58, to form an anode capacitor consisting of
conductive aluminum/insulating aluminum oxide/conductive
electrolyte.
[0044] The cathode 54 may be formed by coating a metal current
collector with a finely divided material. A finely divided material
is a conducting material or any material which supports an
electrochemical capacitance at an interface with an electrolyte and
has a very high ratio of surface area to volume. In some instances,
the surface of the current collector is coated with a layer of a
non-insulating oxide. The finely divided material provides the
interface with the electrolyte which functions as an
electrochemical capacitor. In one example, the thickness of the
cathode is in the range between about 75 to 100 micrometers.
[0045] A variety of materials may be used for the cathode current
collector. They include materials that do not oxidize, such as
noble metals, platinum or palladium, or materials that form a
conducting or a semiconducting oxide, such as nickel and titanium.
If nickel is used, e.g., a nickel mesh as shown in FIG. 5, care
should be taken to avoid an electrolyte in which ammonia is used,
as this may produce corrosion in the nickel.
[0046] A non-insulating oxide forming current collector is required
because if a thin insulating oxide layer develops between the high
gain cathode capacitor and the current collector, it will
contribute to the formation of an additional capacitor. The
additional capacitor will be formed between the conductive current
collector/insulating oxide/conductive carbon powder. This
additional capacitor will be connected in series with the
electrochemical capacitor. When two capacitors are connected in
series the capacitor with the smallest capacitance (in this case
the additional capacitor) dominates and the benefit of the
increased energy density of the electrochemical capacitor with the
large capacitance diminishes.
[0047] The finely divided material, which together with the
electrolyte forms the electrochemical capacitor, may be carbon
powder, carbon fibers, graphite, platinum powder, oxide powders,
such as ruthenium oxide, or mixtures of these powder materials. The
material must provide a capacitor-like voltage/current relationship
for the cathode.
[0048] The electrolyte 58 is a conducting liquid using either
gamma-butyrolactone or ethylene glycol as the major solvent. The
gamma-butyrolactone is a substantially non-aqueous solvent, using
an aromatic dicarboxylic acid/organic base as the major ionogen for
conductivity. A cosolvent, such as methoscypropiomitrile, is added
in concentrations up to 15% to modify low temperature
characteristics. Specific depolarizers, such as benzyl, are added
to reduce gassing, and organic phosphates to improve the oxide
stability. The ethylene glycol system is also a substantially
non-aqueous electrolyte, and uses aliphatic dicarboxylic
acids/ammonia as ionogens. A water content of 1.5 to 2.5% improves
the oxide formation capability during aging and low temperature
characteristics. Specific depolarizers, such as p-nitrobenzoic acid
are added to reduce gassing, and organic phosphates to improve the
oxide stability.
[0049] These electrolyte mixtures have very attractive properties
for the aluminum anode. These include ability to form an insulating
oxide layer through aging on the cut anode surfaces, long shelf
life, ability to withstand high surge voltages, both low and high
temperature performance with a small temperature coefficient,
strong gas absorption properties to provide long service life, and
ability to work with low cost separators. The electrolyte mixtures
also work well with many of the possible cathodes (e.g., titanium
with a ruthenium oxide or carbon powder coating), even though they
are non-aqueous and have a near neutral ph.
[0050] A presently preferred cathode is titanium printed with
hydrated amorphous ruthenium oxide as disclosed in U.S. Pat. Nos.
5,600,535 and 5,621,609, granted to the United States of America on
Feb. 4, 1997 and Apr. 15, 1997, respectively. Such a cathode will
work with the above described ethylene glycol electrolyte. The
cathode may also be constructed by printing ruthenium oxide on
another current collector material (e.g., aluminum). The titanium
substrate is preferably a foil about 12.5 to 50 micrometers thick
(preferably 25 micrometers), which may be in the form of individual
sheets or a roll.
[0051] Printing is accomplished by first coating both sides of the
titanium with a conductive adhesion layer, e.g., a Rexam (TM)
carbon-rubber material produced by the COER-X process (material
available from Rexam Graphics, South Hadley, Mass.). The adhesion
layer should provide a highly conductive, but noncapacitive,
surface onto which the ruthenium oxide may be deposited. The
surface of the carbon-rubber coating has a roughness that allows
the ruthenium oxide to remain adhered. The coating should be kept
thin to minimize its resistance. A thickness of about 5 micrometers
has been found to function well, but other thicknesses (e.g., 2.5
to 12.5 micrometers) can be used.
[0052] After the titanium foil has been coated with the
carbon-rubber layer, hydrous amorphous ruthenium oxide powder is
deposited onto the coated surfaces as a thin film (e.g., by screen
printing) about 5 micrometers thick (other thicknesses, e.g., 2.5
to 12.5 micrometers may also be used). The hydrous amorphous
ruthenium oxide has been mixed with a proton conducting binder,
which serves to provide mobile protons to the hydrous amorphous
ruthenium oxide and to bind the ruthenium oxide particles so as to
maintain close interparticle contact to minimize internal
resistance. Suitable proton conducting binders are organic polymers
having a fluorinated backbone and terminal sulfonic acid carboxylic
acid groups on a fluorinated chain pendant to said fluorinated
backbone. A preferred proton conducting binder is a fluorinated
material arising from copolymerization of tetrafluoroethylene and
FS02CF2CF2OC(CF3)FCF2OCF=CF2 (Dupont Nafion.TM. in an aqueous
solution). More detail on depositing the hydrous amorphous
ruthenium oxide as a thin film can be found in Chen et al. U.S.
application Ser. No. 09/137,227, filed by T. B. Kim Technologies
International, Inc., of Los Angeles, Calif. The hydrous amorphous
ruthenium oxide thin film is preferably applied by screen printing,
using a printing solvent to suspend the ruthenium oxide particles
and proton conducting binder. The ruthenium oxide is printed in a
specific pattern matching the geometry of the stacked individual
anode rolls, thereby reducing the amount of expensive ruthenium
oxide that is used.
[0053] The cathode may also be constructed by adhering carbon
powder to an aluminum current collector. Aluminum, which is already
in use in the anode, has the advantage of being fully compatible
with the preferred nonaqueous electrolyte. The difficulty with
using aluminum for the current collector of the cathode has been
that aluminum tends to form an insulating passive oxide when
exposed to air. Such an oxide layer is unacceptable, for it
provides, in effect, a small further capacitor in series with the
principal capacitance provided by the cathode and anode, thereby
unacceptably lowering the overall capacitance. By using aluminum
foil on which carbon powder has been adhered without an intervening
oxide layer, these problems may be overcome. One such carbon coated
aluminum foil product is available from W. L. Gore, Microfiltration
Technologies Group, Dallas, Tex.
[0054] A further alternative is to bond Rexam (TM) adhesion layer
to an aluminum cathode collector without an intervening oxide
layer. Then, either ruthenium oxide or carbon powder may be
applied.
[0055] Referring again to FIG. 2, the capacitor sections 51a to 51c
are connected in parallel to each other. The aluminum anodes 52 are
spirally wound disks of standard etched and formed aluminum, about
100 Tm thick, 50 mm in diameter, and 6.3 mm wide. The material can
be obtained from a variety of sources (e.g., Aerovox, Beckermel,
Satma, or JCC). A plurality (e.g., 3 to 10) of layers of aluminum
are wound together and electrically connected in parallel so as to
reduce the overall resistance of the anode for the same overall
capacitance.
[0056] Electrical connections to the anodes are made using metal
tabs 53 (small pieces of soft aluminum) mechanically attached
between the brittle anodes and anode leads 60. The anode leads 60
are lightly etched and formed aluminum. Cathodes 54 also have metal
tabs 57, which are ultrasonically welded to a cathode lead 62. The
cathode lead 62 is preferably soft aluminum. The anode and cathode
leads 60 and 62 are connected to the voltage terminals 60a and 62a,
respectively, located on the top cover 59.
[0057] The separator disks 56 may be constructed from standard
duplex or plain kraft paper or similar materials like Manila or
Hemp (e.g., 50-200 micrometers thick). Other separator materials
which can be used with the butyrolactone electrolyte mixtures and
the ethylene glycol electrolyte mixtures include Kraft, manila or
hemp fibers, or composites made from other paper separators and
polypropylene fibers.
[0058] The much higher capacitance of the cathode compared to the
anode allows the cathode to have a much smaller surface area, e.g.,
0.2% to 2% of the area of the anode.
[0059] FIG. 7 shows an alternative multi-section capacitor 50,
which includes cathodes 54, separators 56, collector plates 61, and
cubic anodes 52, which are formed as stacks of etched and formed
aluminum sheets 52a. The aluminum sheets 52a have a rectangular
shape. The cubic anodes 52 can be built in a progressive
cut-to-length line with two degrees of freedom. The stacked,
etched, and formed aluminum sheets 52a are spot welded using a
laser dot matrix pattern or ultrasonic welding. The cathodes 54 are
shown as sheets of material that are interleaved between separators
56 and anodes 52. The collector plates 61 are conductive material
conductively adhered to the ends of the individual aluminum sheets
52a.
[0060] This construction further reduces the series resistance and
provides an attachment point for the anode lead 60, which can be
spot-welded to each of the individual anode collector plates 61.
This construction puts all the individual anode sheets 52a in
parallel and leads to a further reduction in the series
resistance.
[0061] This multi-section capacitor 50 with anodes constructed from
stacked aluminum sheets may be assembled using automated assembly
equipment fed from two spools of material. One spool provides the
cathode material which may be titanium printed hydrous amorphous
ruthenium oxide. The second spool provides the anode material which
may be a spool of very narrow etched and formed aluminum.
[0062] Other geometric configurations for the anode are possible.
The stacked sheets may be oval or circular. Multiple layers of
aluminum may be rolled simultaneously to form anode rolls (as shown
in FIG. 15), with each layer electrically connected in parallel.
This has the same effect as connecting layers of a stack in
parallel. Overall resistance of the anode is reduced for the same
overall capacitance.
[0063] The butyrolactone and ethylene glycol electrolyte systems
are compatible with various polymeric materials (e.g., nylon and
polypropylene), thus adding to the low cost packaging options that
can be obtained through this approach. This total package has the
advantages of providing a very economical capacitor, using a set of
materials that have a long field history under electrochemical
conditions within a broad temperature range, and providing the
basis for a new capacitor with the same long service life under
electrochemical conditions.
[0064] Referring to FIG. 8, one process of making the capacitors
includes the following steps: First, the anodes and cathodes are
fabricated, and the separator disks are cut. The anodes,
separators, and cathodes are then interleaved to form a stack.
Anode and cathode tabs are ultrasonically welded to the anode and
cathode leads, respectively, and the stack assembly is placed into
a container. The anode and cathode leads are then connected to
anode and cathode voltage terminals, respectively, located on the
container cover. The stack assembly is vacuum impregnated with a
substantially non-aqueous electrolyte. The container is closed with
the cover, additional electrolyte may be added, and the capacitor
is aged at a rated voltage (e.g., 50 V DC) and elevated temperature
(e.g., 85 EC). Finally, the vent is sealed, and the capacitor
performance is evaluated by bridge measurements and DC leakage
current measurements at rated voltage.
[0065] Another multi-section capacitor construction is shown in
FIG. 2A. The aluminum anodes 52 are arranged in groups (e.g., eight
are shown) in the same layer, rather than having only a single
anode in each layer, as in FIG. 2. Leads from each anode are
connected to anode lead 60, which leads to terminal 60a on the
cover 59. The cathodes are titanium with hydrous amorphous
ruthenium oxide printed in circular areas aligned with the aluminum
anodes; cathode leads 62 connect the cathode sheets to terminal 62a
on the cover 59. The stack of aluminum anode layers and cathode
layers fits within a rectangular shaped housing 55. Insulating
separator sheets (not shown) formed from the same materials as
discussed above for separator disks 56 are positioned between the
anode and cathode layers. Although circular anodes and matching
circular cathode areas are shown in the figure, it may be
preferable to use other shapes to increase the density of the
capacitor; e.g., oval shaped anodes and matching oval shaped
cathode areas could be substituted.
[0066] The assembly is formed by stacking the various components,
i.e., separator material (one to three sheets typically), one layer
of titanium coated foil, separator material (one to three sheets
typically), an array of 8 anodes (two columns and 4 rows) with the
tab material leads adhered together to form one lead per layer,
separator material (one to three sheets typically), one layer of
titanium coated foil, etc. The assembly may contain a number of
layers (e.g., 10 to 20). The geometry of the capacitor can be
varied and there is no standard number of anodes in an array and
there is no standard or preferred number of stack layers in a
complete assembly.
[0067] The individual anode layers and cathode layers are connected
in parallel by adhering the individual aluminum leads to an
aluminum bar across the top of the assembly. The assembly resembles
a block, which is approximately 9 cm by 11 cm by 24 cm.
Polypropylene insulation (18 Tm) is wrapped around the rectangular
assembly and held in place with electrical tape. The two aluminum
bars (one for the cathode connections and one for the anode
connections) contain holes, which are bolted to the terminals in
the cover (one for the anode and one for the cathode). The cover
assembly is fabricated from a punched sheet of 40 mil food grade
aluminum stock, folded and welded with holes for aluminum ceramic
terminals. The cover assembly also contains a small vent hole which
is sealed with a rubber plug after impregnation. The cover assembly
with the capacitor is purshed down inside of an aluminum can, which
has also been fabricated from food grade aluminum sheets. The cover
assembly is welded to the top of the aluminum case. The complete
capacitor (vent plug open) is impregnated in electrolyte using a
standard vacuum-pressure impregnation oven. The impregnated
capacitor is then aged using a standard oven aging power supply.
The capacitor after aging is cooled to room temperature and the
vent plug is sealed into the capacitor.
[0068] Referring to FIGS. 16 and 17, another construction for the
non-symmetric capacitor has anode layers 52, cathode layers 54, and
separator layers 56 wound in the same roll. A single such roll
could form the capacitor, or multiple rolls could be connected
together internally. The enlargement of the end of the winding in
FIG. 16 shows the various layers, as does the cross sectional view
in FIG. 17, which shows an enlargement of a tiny area of a cross
section through the winding of FIG. 16. The anode and cathode
layers are constructed as taught for the other disclosed
embodiments, with one important exception. The anode layers are
tunnel etched, so that tiny etching holes extend substantially
fully through each anode layer. The tunnel etched holes are coated
with aluminum oxide in a forming process. MacFarlane U.S. Pat. No.
5,584,890 discloses a symmetric electrolytic capacitor in which
both anode and cathode are oxide coated aluminum, and the tunnel
etching is provided in one of the anode and cathode.
[0069] In this alternative embodiment, the tunnel etching provides
a path for conductive ions to flow between the cathode layer and
anode layers. Three anode layers 52 separated by spacer layers 56
from a single cathode layer 54 are shown. But varying numbers of
anode layers (preferably between 1 and 7 layers) could be
interspersed between each cathode layer. And the cathode layer
could be formed by providing a plurality of adjacent cathode
layers.
[0070] The arrangement of FIGS. 16 and 17, in which cathode and
anode layers are wound together, is preferred over the arrangement
of FIGS. 1 and 2A, in which anode windings are interspersed between
cathode sheets, as the width of the winding becomes greater. It
appears that it may be preferable to use the arrangement of FIGS.
16 and 17 when the width of the winding (e.g., about 6 mm in FIGS.
2 and 2A) is greater than about seven times the aggregrate
thickness of the anode layers (e.g., each anode layer is about 100
micrometers in FIGS. 2 and 2A). Otherwise, the arrangement of FIGS.
2 and 2A may be preferable.
[0071] FIG. 18 shows an alternative to the arrangement of FIGS. 16
and 17. The anode layers 52, spacer layers 54, and cathode layers
56 are stacked instead of wound. This permits a construction
comparable to that of FIG. 7, but with the cathode layers built
into the stacks rather than oriented perpendicularly to the edges
of the stacked anode layers. Tiny etching holes extend
substantially fully through each anode layer 52 just as in the
embodiment of FIGS. 16 and 17.
[0072] FIGS. 9A to 9D show the impedance spectroscopy of a
non-symmetrical capacitor constructed with a cathode made of carbon
coated nickel expanded mesh. FIGS. 10A to 10D show the impedance
spectroscopy of a non-symmetrical capacitor constructed with a
cathode made of ruthenium oxide coated titanium expanded mesh. In
both cases, the anode is constructed from etched aluminum foil. The
electrolyte is a butyrolactone system.
[0073] FIGS. 9A to 9D and 10A to 10D show plots of capacitance (9A,
10A), resistance(9B, 10B), impedance (9C, 10C), and phase angle
(9D, 10D) versus frequency. These data demonstrate capacitor
performance limited by the capacitance of the anode for frequencies
that are less than 1 Hz for this specific design. The frequency
limit may be changed by adjusting the design parameters of the
capacitor, for example the carbon coating thickness and anode
geometry.
[0074] The measurements in FIGS. 11A to 11D and 12A to 12D, show
the performance of symmetrical capacitors having carbon coated
nickel screen electrodes. In FIGS. 11A to 11D, the electrolyte is
butyrolactone based and in FIGS. 12A to 12D, the electrolyte is
ethylene glycol based. The curves demonstrate capacitive
performance with both electrolyte systems.
[0075] One of the advantages of this non-symmetric capacitor is
that it can be used for high voltage application without series
construction. Conventional electrochemical capacitors are limited
to low voltages (e.g., less than or equal to 4.5 V). Higher voltage
capacitor ratings using electrochemical capacitor technology
require series construction. Series construction of electrochemical
capacitors leads to voltage sharing problems, high series
resistance, and sealing problems (the individual series section
must be isolated electrically).
[0076] Since the capacitor uses an aluminum electrolytic anode,
voltages as high as 600 V DC can be applied to the anode. Most of
the applied voltage is placed on the anode, by choosing the anode
capacitance to be a small fraction of the cathode capacitance. The
interleaving construction leads to anode sections, that are
operated in parallel, not in series. This eliminates the voltage
sharing problems, leads to a much lower series resistance, and
eliminates the sealing problems.
[0077] The lower series resistance is important for high current
pulsing applications and continuous ripple current applications.
Low series resistance directly relates to an increase in the peak
current, that can be pulsed through the capacitor. Low series
resistance also relates to a lower power factor for continuous
ripple current applications.
[0078] Another advantage of the capacitor is increased energy
density for the non-symmetric capacitor. The ability to operate
with a very limited amount of cathode material leaves more room
within the same size case for anode material. This leads to an
increase in the energy stored in the capacitor by a factor of 2 to
3 over conventional electrolytic capacitors.
[0079] A further advantage comes from possible construction
differences. Conventional aluminum electrolytic capacitors require
etched foils, which must have some mechanical strength and
flexibility so they can be formed and wound on a high-speed
automatic forming and winding machine. This limits the etching and
therefore the specific capacitance of the anode foil and, in turn,
the amount of stored energy.
[0080] Another advantage of the capacitor is improved service life.
In DC applications of conventional aluminum electrolytic
capacitors, the leakage current and gassing of the aluminum foils
(primarily at the cathode) can limit the life of the capacitor by
forming internal gas pressure which vents and sets the life of the
capacitor. With the capacitors disclosed, hydrogen gassing is
typically reduced resulting in considerably extended life for the
capacitor.
[0081] Another advantage is improved time constant. Electrochemical
capacitors are inherently slow devices (long time constant) because
of the high contact resistance across the large porous active
surface area of the electrodes. The thick porous oxide layer
provides a long conducting path and many conducting interfaces,
which lead to a very high resistance. The thickness of the cathode
layer is designed so that the cathode capacitance provides the
correct voltage across the anode. By substituting the thick porous
layer with a finely divided powder coating the thickness of the
cathode is reduced. This causes a reduction in the time constant.
Shorter time constant can also be obtained by selecting the aspect
ratio (surface area to volume ratio) of the specific carbon
particles used in the coating on the nickel (or titanium) wire.
[0082] Another important design variable with the interleaving
construction is the width of the anode roll; the smaller the width
the shorter the time constant. By using a narrow anode foil width,
the time constant can be decreased to 5 milliseconds. This is still
too slow for high ripple current computer grade applications, but
it is suitable for high CV (capacitance and voltage) low ripple
current computer grade applications. In addition to the reduced
time constant, the capacitor has less than half the size of an
electrolytic capacitor.
[0083] The interleaving construction also provides the basis for a
new approach to start (AC) electrolytic capacitors. Start (AC)
electrolytic capacitors operate for a relatively short period of
time and have a power factor between 2.5 to 6.5%. Conventional
electrochemical capacitors have a power factor higher than 10% at
line frequency and therefore are not suitable for AC start
capacitor applications.
[0084] Referring to FIG. 13A, a typical double layer structure AC
start electrolytic capacitor 100 has two anodes 102 and 104 wound
back to back and separated by a separator 106. The formed aluminum
oxide on the anodes functions as a diode, i.e. acts as an insulator
in the forward bias direction and as a resistor in the reverse bias
direction (bias is the direction of the applied electric field
compared to the direction of the formed polarization electric
field). When an AC voltage V is applied across the structure 100,
at any point of the voltage waveform, one anode is a capacitor and
the other a resistor (FIG. 13B). Therefore, the structure 100
behaves like a capacitor with a high power factor due to the
resistance of the second anode.
[0085] Referring to FIG. 14A, an AC start electrolytic capacitor,
includes in addition to the two anodes 102 and 104 a floating
cathode 108 interleaved between the opposite polarized anodes.
Separators 106 separate the anodes from the cathode. The behavior
of the double anode with the floating cathode structure is the same
as the double anode structure (FIG. 14B) except that the floating
cathode reduces the anode formation voltage and thus the aluminum
oxide thickness on the anodes. The reduced aluminum oxide thickness
results in reduction of the time constant, size and cost of the AC
start capacitor.
[0086] Replacing the floating aluminum cathode with a floating
electrochemical cathode provides the same reduction in the anode
formation voltage and has the additional benefit of reduced weight
and size.
* * * * *