U.S. patent application number 10/639913 was filed with the patent office on 2004-03-25 for non-symmetric capacitor.
This patent application is currently assigned to Aerovox Incorporated. Invention is credited to Arora, Mulk, Hudis, Martin, Hutchinson, Robert, Marincic, Nikola.
Application Number | 20040057194 10/639913 |
Document ID | / |
Family ID | 22331877 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040057194 |
Kind Code |
A1 |
Hudis, Martin ; et
al. |
March 25, 2004 |
Non-symmetric capacitor
Abstract
A capacitor having an-electrochemical cathode electrode, an
electrolytic anode electrode, and a substantially non-aqueous
electrolyte disposed between the cathode and anode electrodes. The
cathode includes a metal and a finely divided material, i.e. a
conducting material with a very high ratio of surface area to
volume. The anode includes an oxide forming metal and a
corresponding metal oxide. The substantially non-aqueous
electrolyte is in contact with the finely divided material forming
a double layer electrochemical cathode capacitor. The cathode
provides the foundation for interleaving a plurality of anodes with
a plurality of cathodes. Insulating layers separate the interleaved
anodes from the cathodes.
Inventors: |
Hudis, Martin;
(Mattapoisett, MA) ; Hutchinson, Robert; (Near
Dorchester, GB) ; Marincic, Nikola; (Winchester,
MA) ; Arora, Mulk; (Huntsville, AL) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Assignee: |
Aerovox Incorporated
|
Family ID: |
22331877 |
Appl. No.: |
10/639913 |
Filed: |
August 12, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10639913 |
Aug 12, 2003 |
|
|
|
10273013 |
Oct 17, 2002 |
|
|
|
10273013 |
Oct 17, 2002 |
|
|
|
09358577 |
Jul 21, 1999 |
|
|
|
09358577 |
Jul 21, 1999 |
|
|
|
09110223 |
Jul 6, 1998 |
|
|
|
6208502 |
|
|
|
|
Current U.S.
Class: |
361/516 |
Current CPC
Class: |
H01G 11/22 20130101;
H01G 11/46 20130101; Y02E 60/13 20130101; H01G 9/0425 20130101;
H01G 9/04 20130101; H01G 9/035 20130101; H01G 9/145 20130101; H01G
11/42 20130101; H01G 9/02 20130101 |
Class at
Publication: |
361/516 |
International
Class: |
H01G 009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 1999 |
WO |
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 an electrochemical cathode comprising a metal
coated with a finely divided material, an electrolytic anode
comprising an oxide forming metal and a corresponding metal oxide,
and a substantially non-aqueous electrolyte in contact with the
finely divided material on the cathode and the metal oxide on the
anode.
2. The capacitor of claim 1 wherein a double layer of charge is
formed at the interface between the finely divided material and the
substantially non-aqueous electrolyte.
3. The capacitor of claim 1 wherein the cathode comprises a metal
selected from the group consisting of nickel, titanium, platinum,
other noble metals, other non-oxidizing metals and metals forming a
conducting or semiconducting oxide.
4. The capacitor of claim 1 wherein the anode comprises aluminum
coated with aluminum oxide.
5. The capacitor of claim 1 wherein the anode comprises a metal
selected from the group consisting of tantalum, niobium, zirconium,
titanium, and alloys thereof.
6. The capacitor of claim 1 wherein the finely divided material is
selected from the group consisting of activated carbon powder,
carbon fibers, and graphite.
7. The capacitor of claim 1 wherein the substantially non-aqueous
electrolyte comprises an ethylene glycol solvent mixture with
additives.
8. The capacitor of claim 1 wherein the substantially non-aqueous
electrolyte comprises a butyrolactone solvent mixtures with
additives.
9. The capacitor of claim 1 wherein the anode has a larger surface
area than the cathode.
10. The capacitor of claim 1 wherein the cathode comprises an
expanded nickel mesh.
11. The capacitor of claim 1 further comprising a separator
separating the electrolytic anode from the electrochemical
cathode.
12. The capacitor of claim 11 wherein the separator comprises
kraft, manila, hemp papers, and composites thereof.
13. The capacitor of claim 11 wherein the separator comprises a
composite of paper and polypropylene fibers.
14. The capacitor of claim 1 wherein the anode comprises a metal
foil wound into a roll.
15. The capacitor of claim 1 wherein the anode comprises a
flattened aluminum wire wound into a roll.
16. The capacitor of claim 1 comprising a plurality of parallel
connected capacitors, each capacitor comprising an electrochemical
cathode, an electrolytic anode, and a substantially non-aqueous
electrolyte.
17. The capacitor of claim 16 further comprising separators
electrically separating the anodes from the cathodes.
18. 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 plurality of electrochemical cathodes, a
plurality of electrolytic anodes interleaved with the
electrochemical cathodes, a plurality of separators separating the
cathodes from the anodes, an electrolyte disposed between the
cathodes and anodes, and wherein the plurality of anodes and
cathodes form a plurality of parallel connected capacitors.
19. The capacitor of claim 18 wherein the electrolyte is
substantially non-aqueous.
20. The capacitor of claim 18 wherein each electrochemical cathode
comprises a metal having top and bottom surfaces coated with a
finely divided material.
21. The capacitor of claim 18 wherein each electrolytic anode
comprises a metal and a corresponding metal oxide.
22. The capacitor of claim 18 further comprising a cathode lead
electrically connecting the cathodes to each other aid to a cathode
terminal.
23. The capacitor of claim 18 further comprising an anode lead
electrically connecting the anodes to each other and to an anode
terminal.
24. The capacitor of claim 18 wherein the anode is formed as a
stack of individual metal sheets.
25. The capacitor of claim 24 further comprising a collector strip
connecting the individual metal sheets to each other.
26. An electrolytic capacitor comprising an anode, said anode
comprising a stack of individual metal sheets electrically
connected to each other.
27. An AC start capacitor comprising a first electrolytic anode, a
second electrolytic anode, a floating electrochemical cathode
interleaved between the first and second electrolytic anodes, an
electrolyte, and wherein said first and second electrolytic anodes
comprise a metal and a corresponding metal oxide, said
electrochemical cathode comprises a metal having top and bottom
surfaces coated with a finely divided material and an AC voltage is
connected to said first and second electrolytic anodes.
28. The capacitor of claim 27 wherein the electrolyte is
substantially non-aqueous.
29. The capacitor of claim 27 further comprising separators
separating the first and second anode from the floating
cathode.
30. A method of forming a capacitor electrode comprising
transporting a continuous sheet of metal, cutting pieces of the
metal sheet at spaced intervals, stacking the pieces of the metal,
and welding the stacked pieces of the metal.
31. A method of forming a capacitor comprising fabricating a
plurality of anodes by transporting a continuous sheet of a first
metal, cutting pieces of the first metal sheet at spaced intervals,
stacking the pieces of the first metal, and welding the stacked
pieces of the first metal, fabricating a plurality of cathodes by
transporting a continuous sheet of a second metal, and cutting
pieces of the second metal, and interleaving the anodes and
cathodes, while separating them with insulating separators.
32. The capacitor of claim 1 wherein the cathode comprises a
titanium sheet.
33. The capacitor of claim 32 wherein the cathode comprises a
hydrated amorphous ruthenium oxide coating applied to the titanium
sheet.
34. The capacitor of claim 18 wherein a plurality of anodes are
arranged in a first layer, and a plurality of cathode areas are
arranged in a second layer with the cathode areas approximately
aligned with the anodes.
35. The capacitor of claim 34 wherein the plurality of anodes
comprises wound aluminum anodes in the shape of disks.
36. The capacitor of claim 35 wherein the plurality of cathode
areas comprise hydrated amorphous ruthenium oxide areas applied to
a titanium sheet.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of copending U.S.
application Ser. No. 09/110,223, filed Jul. 6, 1998. This
application also claims priority from copending PCT application
serial no. PCT/US99/15207, filed 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 C.
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 double-layer. Double-layer capacitors rely solely on
interfacial charge separation across a boundary between an
electrolyte and a conducting surface or an insulating surface such
as a metal 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 one aspect, the invention features a capacitor having an
electrochemical cathode, an electrolytic anode and a substantially
non-aqueous electrolyte disposed between the cathode and anode. The
cathode includes a metal and a finely divided material, the anode
includes an oxide forming metal and a corresponding metal oxide,
and the substantially non-aqueous electrolyte is in contact with
the finely divided material and the metal oxide. The cathode
structure results in high capacitance, permitting much higher
energy density.
[0009] In preferred implementations of the invention, one or more
of the following features may be incorporated. The cathode may be a
metal selected from the group consisting of nickel, titanium,
platinum, other noble metals, other non-oxidizing metals and metals
forming a conducting or semiconducting oxide. The cathode may also
be an expanded nickel mesh. The anode may be aluminum coated with
aluminum oxide. The anode may also be a metal selected from the
group consisting of tantalum, niobium, zirconium, titanium, and
alloys thereof. The finely divided material may be selected from
the group consisting of activated carbon powder, carbon fibers, and
graphite. The cathode may be a hydrated amorphous ruthenium oxide
coating applied to a titanium sheet. The substantially non-aqueous
electrolyte may be an ethylene glycol solvent mixture with
additives, or a butyrolactone solvent mixtures with additives.
[0010] In another aspect the invention features a capacitor having
a plurality of electrochemical cathodes, a plurality of
electrolytic anodes interleaved with the electrochemical cathodes,
and the plurality of anodes and cathodes form a plurality of
parallel connected capacitors. A plurality of separators separate
the cathodes from the anodes, and an electrolyte may be disposed
between the cathodes and anodes. A cathode lead electrically
connects the cathodes to each other and to a cathode terminal. An
anode lead electrically connects the anodes to each other and to an
anode terminal.
[0011] In preferred implementations of the invention, the anode may
be formed as a stack of individual metal sheets, and the individual
metal sheets may be connected to each other by a collector strip.
There may be a plurality of anodes arranged in a first layer, and a
plurality of cathode areas arranged in a second layer with the
cathode areas approximately aligned with the anodes. The plurality
of anodes may be wound aluminum anodes in the shape of disks. The
plurality of cathode areas may be hydrated amorphous ruthenium
oxide areas applied to a titanium sheet.
[0012] In another aspect the invention features an AC start
capacitor having a first electrolytic anode, a second electrolytic
anode, a floating electrochemical cathode interleaved between the
first and second electrolytic anodes, and a non aqueous
electrolyte. The first and second electrolytic anodes include a
metal and a corresponding metal oxide, and the electrochemical
cathode includes a metal having top and bottom surfaces coated with
a finely divided material. An AC voltage is connected to the first
and second electrolytic anodes.
[0013] In another aspect the invention features forming a capacitor
by fabricating a plurality of anodes, fabricating a plurality of
cathodes, and then interleaving the anodes and cathodes while
separating them with insulating separators. The anodes and cathodes
are then connected in parallel to each other. The anodes are
fabricated by transporting a continuous sheet of a first metal,
cutting pieces of the first metal sheet at spaced intervals,
stacking the pieces of the first metal, and welding the stacked
pieces of the first metal. The cathodes are fabricated by
transporting a continuous sheet of a second metal, and cutting
pieces of the second metal.
[0014] Among the advantages of the invention are that the
non-symmetric capacitors can be used in high voltage applications
without a series construction. They have increased energy density
over conventional electrolytic capacitors, improved service life,
reduced time constant, and increased power density over serially
connected chemical capacitors.
[0015] 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
[0016] FIG. 1 is a cross-sectional view of a prior art symmetric
aluminum electrolytic capacitor.
[0017] FIG. 1A is an equivalent circuit diagram of a prior art
symmetric electrolytic capacitor.
[0018] FIG. 2 is a cross-sectional view of a non-symmetric
electrolytic/electrochemical capacitor embodiment of the
invention.
[0019] FIG. 2A is an exploded perspective view of another
non-symmetric electrolyte/electrochemical capacitor embodiment of
the invention.
[0020] FIG. 3 is a perspective view of an anode roll useful in some
embodiments of the invention.
[0021] FIG. 4 is a cross-sectional view of a thin etched aluminum
foil.
[0022] FIG. 5 is a perspective view of an expanded nickel mesh,
used in the cathode of an embodiment of the invention.
[0023] FIG. 6 is a perspective view of the expanded nickel mesh
coated with carbon powder, used in the cathode of an embodiment of
the invention.
[0024] FIG. 7 is a diagrammatic view of a multi-cell non-symmetric
electrolytic/electrochemical capacitor, according to another
embodiment of the invention.
[0025] FIG. 8 is a flow chart of the process for fabricating a
capacitor according to the invention.
[0026] FIGS. 9A to 9D show impedance spectroscopy data for
embodiments of the invention with a carbon coated nickel cathode,
etched and formed aluminum anode and non-aqueous electrolyte.
[0027] FIGS. 10A to 10D show impedance spectroscopy data for
embodiments of the invention with a ruthenium oxide coated titanium
cathode, etched and formed aluminum anode and non-aqueous
electrolyte.
[0028] FIGS. 11A to 11D show impedance spectroscopy data for
embodiments of the invention with carbon coated nickel
electrodes(anode and cathode)and with butyrolactone based
electrolyte.
[0029] FIGS. 12A to 12D show impedance spectroscopy data for
embodiments of the invention with carbon coated nickel
electrodes(anode and cathode) and with ethylene glycol based
electrolyte.
[0030] FIG. 13A is a diagrammatic view of a double anode structure
for an AC start capacitor.
[0031] FIG. 13B is a plot of the applied AC voltage across the
double anode structure of FIG. 13A.
[0032] FIG. 14A is a diagrammatic view of a double anode with a
floating capacitor structure for an AC start capacitor, according
to an embodiment of this invention.
[0033] FIG. 14B is a plot of the applied AC voltage across the
double anode with the floating capacitor structure of FIG. 14A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] Referring to FIG. 2, a multi-section non-symmetric capacitor
50 includes capacitor sections 51a to 51c. Although only three
sections 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.
[0035] The anodes 52 are formed by winding etched and formed
aluminum foil 82 into a wide roll (in one example the width may be
in the range between 50 to 100 cm) and then slitting the wide roll
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.
The anode may also be fabricated by winding a flattened small
diameter etched aluminum wire into a roll. The aluminum oxide layer
is subsequently formed.
[0036] 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.
[0037] The cathode 54 is formed by coating a nickel mesh 70 (FIG.
5) with a finely divided material, such as carbon powder 72 ( FIG.
6). A finely divided material is a conducting material or any
material which supports Faradaic charge transfer at an interface
with an electrolyte and has a very high ratio of surface area to
volume. The surface of the nickel mesh is coated with a layer of a
non-insulating nickel oxide 74. The carbon powder 72 provides the
interface with the electrolyte which functions as a double layer
electrochemical capacitor. The nickel and the non-insulating nickel
oxide layer function as a current collecting conductor. In one
example, the thickness of the cathode is in the range between 0.003
to 0.004 inch, and the nickel mesh has 9000 holes per square
inch.
[0038] Other 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 which form a
conducting or a semiconducting oxide, such as nickel and
titanium.
[0039] Another cathode current collector, and the one presently
preferred, 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 collector will work with an
ethylene glycol electrolyte.
[0040] A non-insulating oxide forming current collector is desired
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 double
layer cathode 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 double layer capacitor with the
large capacitance diminishes.
[0041] The finely divided material, which together with the
electrolyte forms the double layer capacitor, may be carbon powder,
carbon fibers, graphite, platinum powder, ceramic oxide powder,
such as ruthenium oxide, or mixtures of these powder materials.
[0042] 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.
[0043] 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.
[0044] 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 provide very high cathode gain for carbon coated nickel or
titanium current collectors, even though they are non-aqueous and
have a near neutral ph.
[0045] 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 and have metal tabs 53, which are
ultrasonically welded to an anode lead 60. The anode lead 60 may be
aluminum or other inert high conductivity metal, such as nickel or
platinum, to prevent chloride contamination within the capacitor.
Similarly, the cathodes 54 have ultrasonically welded metal tabs
57. The metal tabs 57 are ultrasonically welded to a cathode lead
62. The cathode lead 62 may be aluminum for a nickel based cathode
or nickel for a titanium based cathode. The anode and cathode leads
60 and 62 are connected to the voltage terminals 60a and 62a,
respectively, located on the top cover 59.
[0046] 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.
[0047] 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.
[0048] 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. Other geometric
configurations for the anode sheets are possible, including oval,
and circular.
[0049] The anode material is not limited to pre-etched and formed
aluminum. The block can be formed from thin very pure, soft or hard
aluminum foil which is etched before welding and formed after it
has been welded into a cubic geometry.
[0050] The cathodes 54 are shown as sheets of material that are
interleaved between separators 56 and anodes 52.
[0051] The collector plates 61 are non-etched aluminum strips which
are spot welded to the ends of the individual aluminum sheets
52a.
[0052] 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.
[0053] 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 a soft nickel expanded mesh (or
titanium mesh) coated on both sides with carbon particles. The
second spool provides the anode material which may be a spool of
very narrow slit etched and formed aluminum.
[0054] 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 hydrated 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.
[0055] 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.
[0056] Referring to FIG. 8, the process of making capacitors
according to the invention 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.degree. C.). Finally, the vent
is sealed, and the capacitor performance is evaluated by bridge
measurements and DC leakage current measurements at rated
voltage.
[0057] 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.
[0058] FIGS. 9A to 9D and 10A to 10D show plots of capacitance (9A,
20A), 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.
[0059] 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.
[0060] 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).
[0061] Since the invention uses an aluminum electrolytic anode,
voltages as high as 600 V DC can be applied to the anode. Most of
the applied voltage can be 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.
[0062] 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.
[0063] Another advantage of the invention 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.
[0064] 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. With the new construction, a small
diameter very pure, soft or hard aluminum wire (e.g., 150
micrometers) can be flattened and wound into the interleaving anode
structures before it is etched and formed. With the etching and
forming being done after the anode is wound, etching is not limited
by the mechanical properties of the aluminum material and the anode
capacitance can be increased by 50%.
[0065] Another advantage of the invention 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 carbon coated nickel cathode, hydrogen gassing
is reduced resulting in reduced chemical activity and considerably
extended life for the capacitor.
[0066] Another advantage of the invention 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 carbon 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.
[0067] Another key design variable with the interleaving
construction is the thickness of the anode; the smaller the anode
thickness the shorter the time constant. By using a narrow anode
foil width (or a small diameter flattened wire), 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 and half the cost of an
electrolytic capacitor.
[0068] The interleaving construction provides also 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.
[0069] 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.
[0070] Referring to FIG. 14A, an AC start electrolytic capacitor,
according to this invention, includes in addition to the two anodes
102 and 104 a floating cathode 108 interleaved between the opposite
polarized anodes. The floating cathode may be an electrolytic
aluminum with aluminum oxide cathode. 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.
[0071] In one embodiment of this invention the floating cathode is
an electrochemical cathode including a carbon coated expanded
nickel mesh in contact with an electrolyte. The electrolyte may be
aqueous or non-aqueous.
[0072] 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.
* * * * *