U.S. patent application number 11/971188 was filed with the patent office on 2008-05-08 for electrochemical double layer capacitor.
This patent application is currently assigned to Maxwell Technologies, Inc.. Invention is credited to Xiaomei Xi, Linda Zhong, Bin Zou.
Application Number | 20080106850 11/971188 |
Document ID | / |
Family ID | 37497210 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080106850 |
Kind Code |
A1 |
Zhong; Linda ; et
al. |
May 8, 2008 |
ELECTROCHEMICAL DOUBLE LAYER CAPACITOR
Abstract
Active electrode material, such as fibrillized blend of
activated carbon, polymer, and conductive carbon, is pretreated by
immersion in a sealing coating. After the active electrode material
is dried, the coating seals micropores of the activated carbon or
another porous material, thus preventing exposure of water
molecules or other impurities trapped in the micropores to outside
agents. At the same time, the sealing coating does not seal most
mesapores of the porous material, allowing exposure of the
mesapores' surface area to the outside agents. The pretreated
active electrode material is used for making electrodes or
electrode assemblies of electrical energy storage devices. For
example, the electrodes may be immersed in an electrolyte to
construct electrochemical double layer capacitors. Pretreatment
with the sealing coating reduces the number of water molecules
interacting with the electrolyte, enhancing the breakdown voltage
of the capacitors.
Inventors: |
Zhong; Linda; (San Diego,
CA) ; Xi; Xiaomei; (Carlsbad, CA) ; Zou;
Bin; (San Diego, CA) |
Correspondence
Address: |
MAXWELL TECHNOLOGIES, INC.
9244 BALBOA AVENUE
SAN DIEGO
CA
92123
US
|
Assignee: |
Maxwell Technologies, Inc.
San Diego
CA
92123
|
Family ID: |
37497210 |
Appl. No.: |
11/971188 |
Filed: |
January 8, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11609291 |
Dec 11, 2006 |
7317609 |
|
|
11971188 |
Jan 8, 2008 |
|
|
|
11087409 |
Mar 23, 2005 |
7147674 |
|
|
11609291 |
Dec 11, 2006 |
|
|
|
Current U.S.
Class: |
361/502 |
Current CPC
Class: |
H01G 11/28 20130101;
H01M 4/0404 20130101; H01M 4/621 20130101; H01M 4/96 20130101; Y02E
60/50 20130101; H01G 11/26 20130101; H01G 9/02 20130101; Y02E 60/10
20130101; Y02E 60/13 20130101; H01G 11/32 20130101; H01G 11/38
20130101 |
Class at
Publication: |
361/502 |
International
Class: |
H01G 9/155 20060101
H01G009/155 |
Claims
1. An electrochemical double layer capacitor comprising: a first
and a second electrode, wherein each electrode comprises an active
electrode material comprising: a porous material comprising a
plurality of unsealed mesapores and a plurality of sealed
micropores interspersed among the plurality of unsealed mesapores,
at least a portion of the plurality of micropores having water
molecules sealed therein; a porous separator disposed between the
first electrode material and the second electrode material; and a
first current collector and a second current collector, wherein the
first electrode is disposed between the porous separator and the
first current collector, and the second electrode is disposed
between the porous separator and the second current collector.
2. An electrochemical double layer capacitor according to claim 1
wherein the plurality of sealed micropores comprises a coating
sealing the plurality of sealed micropores.
3. An electrochemical double layer capacitor according to claim 2
wherein the plurality of unsealed mesapores is not sealed by the
coating.
4. An electrochemical double layer capacitor according to claim 3
wherein the plurality of unsealed mesapores comprises at least a
majority of mesapores of the porous material as measured by surface
area.
5. An electrochemical double layer capacitor according to claim 1
wherein the porous material comprises a porous film material.
6. An electrochemical double layer capacitor according to claim 1
wherein the porous material comprises activated carbon, conductive
carbon, and a binder.
7. An electrochemical double layer capacitor according to claim 10
further comprising an electrolyte in which at least a portion of
the first electrode, at least a portion of the second electrode,
and at least a portion of the separator are immersed.
8. An electrochemical double layer capacitor according to claim 13
further comprises a container holding the first electrode, the
second electrode, the separator, and the electrolyte.
9. An electrochemical double layer capacitor according to claim 1
wherein the porous material comprises a calendared porous film
material.
10. An electrochemical double layer capacitor according to claim 1,
wherein the active electrode material of the first electrode, the
active electrode material of the second electrode, and the porous
separator are immersed in an electrolyte.
11. An electrochemical double layer capacitor according to claim 1,
wherein the porous material comprises activated carbon.
14. An electrochemical double layer capacitor according to claim 1,
wherein the porous material comprises dry carbon particles and
binder particles.
15. An electrochemical double layer capacitor comprising: a first
and a second electrode material, wherein the electrode material
comprises a porous activated carbon comprising a plurality of
unsealed mesapores and a plurality of sealed micropores
interspersed among the plurality of unsealed mesapores, at least a
portion of the plurality of micropores having water molecules
sealed therein; a porous separator disposed between the first
electrode material and the second electrode material; a first
current collector and a second current collector, wherein the first
electrode material is disposed between the porous separator and the
first current collector, and the second electrode material is
disposed between the porous separator and the second current
collector; an electrolyte, wherein the first electrode material,
the second film electrode material and the porous separator are
immersed in the electrolyte; and a container, the container holding
the electrolyte, the separator, and the first and second electrode
material.
16. An electrochemical double layer capacitor according to claim 15
wherein the plurality of sealed micropores comprises a coating
sealing the plurality of sealed micropores.
17. An electrochemical double layer capacitor according to 16
wherein the plurality of unsealed mesapores is not sealed by the
coating.
18. An electrochemical double layer capacitor according to claim 17
wherein the plurality of unsealed mesapores comprises at least a
majority of mesapores of the porous material as measured by surface
area.
19. An electrochemical double layer capacitor according to claim 15
wherein the porous material comprises a porous film material.
20. An electrochemical double layer capacitor according to claim
15, wherein the electrode material comprises dry activated carbon
particles and binder particles.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional application and
claims the benefit of U.S. patent application Ser. No. 11/609,291,
filed Dec. 11, 2006 and entitled "Pretreated Porous Electrode,"
which is currently pending and which is a continuation application
of U.S. patent application Ser. No. 11/087,409, filed Mar. 23,
2005, entitled "Pretreated Porous Electrode and Method for
Manufacturing Same," which issued as U.S. Pat. No. 7,147,674 on
Dec. 12, 2006. Each of these applications are incorporated by
reference in their entirety as though fully set forth herein.
FIELD OF THE INVENTION
[0002] The present invention generally relates to processing of
porous materials. More specifically, the present invention relates
to porous electrodes and to energy storage devices, such as
electrochemical double layer capacitors, fabricated using porous
electrodes.
BACKGROUND
[0003] Electrodes are widely used in many devices that store
electrical energy, including primary (non-rechargeable) battery
cells, secondary (rechargeable) battery cells, fuel cells, and
capacitors. Important characteristics of electrical energy storage
devices include energy density, power density, maximum charging
rate, internal leakage current, equivalent series resistance (ESR),
and durability, i.e., the ability to withstand multiple
charge-discharge cycles. For a number of reasons, double layer
capacitors, also known as supercapacitors and ultracapacitors, are
gaining popularity in many energy storage applications. The reasons
include availability of double layer capacitors with high power
densities (in both charge and discharge modes), and with energy
densities approaching those of conventional rechargeable cells.
[0004] Double layer capacitors use electrodes immersed in an
electrolyte (an electrolytic solution) as their energy storage
element. Typically, a porous separator immersed in and impregnated
with the electrolyte ensures that the electrodes do not come in
contact with each other, preventing electronic current flow
directly between the electrodes. At the same time, the porous
separator allows ionic currents to flow between the electrodes in
both directions. As discussed below, double layers of charges are
formed at the interfaces between the solid electrodes and the
electrolyte. Double layer capacitors owe their descriptive name to
these layers.
[0005] When electric potential is applied between a pair of
electrodes of a double layer capacitor, ions that exist within the
electrolyte are attracted to the surfaces of the oppositely-charged
electrodes, and migrate towards the electrodes. A layer of
oppositely-charged ions is thus created and maintained near each
electrode surface. Electrical energy is stored in the charge
separation layers between these ionic layers and the charge layers
of the corresponding electrode surfaces. In fact, the charge
separation layers behave essentially as electrostatic capacitors.
Electrostatic energy can also be stored in the double layer
capacitors through orientation and alignment of molecules of the
electrolytic solution under influence of the electric field induced
by the potential.
[0006] In comparison to conventional capacitors, double layer
capacitors have high capacitance in relation to their volume and
weight. There are two main reasons for these volumetric and weight
efficiencies. First, the charge separation layers are very narrow.
Their widths are typically on the order of nanometers. Second, the
electrodes can be made from a porous material, having very large
effective surface area per unit volume. Because capacitance is
directly proportional to the electrode area and inversely
proportional to the widths of the charge separation layers, the
combined effects of the large effective surface area and narrow
charge separation layers result in capacitance that is very high in
comparison to that of conventional capacitors of similar size and
weight. High capacitance of double layer capacitors allows the
capacitors to receive, store, and release large amounts of
electrical energy.
[0007] Electrical energy storage capability of a capacitor is
determined using a well-known formula, to wit: E = C * V 2 2 . ( 1
) ##EQU1## In this formula, E represents the stored energy, C
stands for the capacitance, and V is the voltage of the charged
capacitor. Thus, the maximum energy (E.sub.m) that can be stored in
a capacitor is given by the following expression: E m = C * V r 2 2
, ( 2 ) ##EQU2## where V.sub.r stands for the rated voltage of the
capacitor. It follows that a capacitor's energy storage capability
depends on both (1) its capacitance, and (2) its rated voltage.
Increasing these two parameters is therefore important to capacitor
performance. Indeed, because the total energy storage capacity
varies linearly with capacitance and as a second order of the
voltage rating, increasing the voltage rating is the more important
of the two objectives for improving capacitors.
[0008] Voltage ratings of double layer capacitors are generally
limited by chemical reactions (reduction, oxidation) and breakdown
that take place within the electrolytic solutions in presence of
electric field induced between capacitor electrodes. Electrolytic
solutions currently used in double layer capacitors are of two
kinds. The first kind of electrolytic solutions includes organic
solutions, such as propylene carbonate. Long lifetime prior art
double layer capacitors made with organic electrolytes can boast
voltage ratings approaching 2.5 volts.
[0009] Double layer capacitors may also be made with aqueous
electrolytic solutions, for example, potassium hydroxide and
sulfuric acid solutions. Double layer capacitor cells manufactured
using aqueous electrolytes and activated carbon are typically rated
at or below 1.2 volts in order to achieve a commercially acceptable
number of charge-discharge cycles. Even small increases above 1.2
volts tend to reduce substantially the number of charge-discharge
cycles that the capacitors can withstand without significant
deterioration in performance.
[0010] The 2.5 volt rating is considerably below voltage rating
theoretically achievable in organic electrolyte-based double layer
capacitors. According to some calculations, double layer capacitors
made with an organic electrolyte and activated carbon should
perform reliably at voltages ranging to about 3.2-3.5 volts.
Achieving this range, however, has been an elusive goal because of
early decomposition and breakdown of the electrolyte. The problem
results, at least in part, from presence of impurities within the
activated carbon and within the electrolyte. Water is one of these
impurities.
[0011] Trace amounts of water and other impurities can be found in
most electrolytes, and they may affect capacitor reliability,
durability, and voltage rating. Highly purified electrolytes,
however, are commercially available at reasonable cost.
[0012] The active material of the electrode--activated carbon or
another porous material, for example--almost invariably has some
impurities, including water. Water may be present in the raw
carbon, and it may be introduced or added during the electrode
manufacturing process. In practice, purifying activated carbon has
proven to be a much more difficult task than purifying electrolyte.
Water molecules can attach to the carbon in several ways, including
by means of VanderWaal's forces responsible for physical bonding,
and chemical (covalent and hydrogen) bonding forces.
[0013] Whatever the nature of the bond between a water molecule and
activated carbon, a high energy "excited site" is formed around it.
Electrolyte interacts with the water molecules and decomposes more
readily near such sites than elsewhere in the capacitor. The
trapped water functions deleteriously at the capacitor's working
potential, so that the maximum application voltage is affected by
the water devolution voltage. This is believed to be a major
contributing cause to the lower actual-versus-theoretical breakdown
voltage of double layer capacitors.
[0014] It would be desirable to increase actual breakdown voltage
of double layer capacitors. It would also be desirable to increase
reliability and durability of double layer capacitors, as measured
by the number of charge-discharge cycles that a capacitor can
withstand without significant deterioration in its operating
characteristics. Because capacitor breakdown voltage and durability
are both compromised by interaction between electrolyte and water
molecules trapped in the activated carbon, it would be desirable to
reduce the interactions or eliminate the interactions altogether.
More generally, it would be desirable to provide a method for
preventing impurities attached to porous materials from interacting
with surrounding gas or liquid in which the porous material is
immersed.
SUMMARY
[0015] A need thus exists for methods for preventing or reducing
exposure of high energy excited sites within porous materials to
outside agents. Another need exists for porous materials with
reduced exposure of water and other impurities trapped in the
materials to outside agents. A further need exists for electrodes
made from porous materials having reduced content of water
molecules that can interact with surrounding gas or liquid in which
the electrodes are immersed. Still another need exists for double
layer capacitors and other electrical energy storage devices that
employ electrodes made from these materials.
[0016] Various embodiments of the present invention are directed to
methods, electrodes, electrode assemblies, and energy storage
devices that satisfy one or more of these needs. An exemplary
embodiment of the invention herein disclosed is a method for
processing porous material. According to this method, the porous
material is treated with a sealing coating capable of sealing
impurities in micropores of the porous material. The porous
material is then dried, so that the coating seals water molecules
in the micropores. Treatment may involve, for example, immersing
the porous material in the sealing coating, and then draining the
sealing coating from the porous material before the material is
dried. The coating may be such that it does not seal at least a
majority of mesapores of the porous material as measured by surface
area, while sealing at least a predetermined percentage of water
molecules in the micropores of the material.
[0017] In other aspects of the invention, the porous material
includes activated carbon in particulate form, fibril-forming
binder, and conductive carbon. These ingredients may be blended,
for example, dry-blended, and subjected to high-shear forces in
order to fibrillize the material. The high-shear forces may be
applied using non-lubricated techniques.
[0018] To make an electrode, the porous material processed as
described above may be coated onto one or both sides of a current
collector so that film or films of the material are formed on the
current collector when the material is dried. To densify the films,
the electrode may be calendered. The electrode may then be used in
a double layer capacitor, for example, by providing a second
electrode, interposing a porous separator between the two
electrodes, and immersing the separator and the two electrodes in
an electrolyte.
[0019] In another aspect, an electrode assembly is made by coating
a porous separator with the porous material processed as described
above, so that films of the material are formed on the separator.
Current collectors may then be attached to the surfaces of the
films that are not in contact with the separator. The resulting
electrode assembly may be calendered to densify the films. A double
layer capacitor is obtained when the assembly is immersed in an
electrolyte.
[0020] In one embodiment, a method for processing porous material
comprises steps of: providing a porous material, at least some of
the porous material comprising micropores, at least some of the
micropores having impurities disposed therein; treating the porous
material with a sealing coating to seal the impurities in
micropores of the porous material; and drying the porous material.
The treating step may comprise immersing the porous material in the
sealing coating, the method further comprising: draining the
sealing coating from the porous material before the drying step.
The sealing coating may be such that it does not seal at least
majority of mesapores of the porous material as measured by surface
area. The sealing coating may be capable of sealing water molecules
in micropores of the porous material. The step of providing the
porous material may comprise providing activated carbon. The step
of providing the porous material may comprise: providing a
fibril-forming binder; providing conductive carbon; blending the
activated carbon, the fibril-forming binder, and the conductive
carbon, thereby obtaining blended active electrode material; and
applying high-shear forces to the blended active electrode material
to fibrillize the blended active electrode material. The step of
providing the porous material further may comprise: providing a
fibril-forming binder; providing conductive carbon; dry-blending
the activated carbon, the fibril-forming binder, and the conductive
carbon, thereby obtaining blended active electrode material; and
applying a non-lubricated high-shear force technique to the blended
active electrode material to dry fibrillize the blended active
electrode material. The method may comprise processing porous
material, wherein the step of providing activated carbon comprises
providing the activated carbon in particulate form; providing a
current collector; and coating the current collector with the
fibrillized active electrode material before the step of drying,
thereby obtaining the electrode. The step of coating the current
collector may comprise coating both sides of the current collector
with the fibrillized active electrode material so that first and
second films of active electrode material are formed on both sides
of the current collector. The method may comprise calendering the
current collector with the films after the step of drying. The
method may comprise: making first and second electrodes by
providing a porous separator; disposing the separator between the
first and second electrodes so that active electrode material is
interposed between the separator and respective current collector
of the electrodes; and immersing the electrodes and the separator
in an electrolyte. The method may comprise providing processed
porous material wherein the step of providing activated carbon
comprises providing the activated carbon in particulate form;
providing a porous separator; and coating the porous separator with
the porous material before the step of drying; whereby the
electrode assembly is obtained. The step of coating the porous
separator may comprise coating both sides of the porous separator
with the active electrode material so that a first film of active
electrode material is formed on a first side of the porous
separator and a second film of active electrode material is formed
on a second side of the porous separator. The method may comprise
attaching a first current collector to the first film so that the
first film is disposed between the first current collector and the
porous separator; and attaching a second current collector to the
second film so that the second film is disposed between the second
current collector and the porous separator. The method may further
comprise calendering the electrode assembly. The method may
comprise comprising: making the electrode assembly of claim; and
immersing the electrode assembly in an electrolyte. The method for
providing film of active electrode material may comprise providing
processed porous material and calendering the processed porous
material to obtain the film of active electrode material. The
method may comprise providing processed porous material and
calendering the processed porous material to obtain a first film of
active electrode material and a second film of active electrode
material; providing a porous separator; providing a first current
collector and a second current collector; attaching the first film
to the porous separator and to the first current collector so that
the first film is disposed between the porous separator and the
first current collector; attaching the second film to the porous
separator and to the second current collector so that the second
film is disposed between the porous separator and the second
current collector, and the porous separator is disposed between the
first and second films; and immersing the porous separator and the
first and second films in an electrolyte.
[0021] In one embodiment, an electrochemical double layer capacitor
comprises: a first and a second electrode material, wherein the
active electrode material comprises a porous material and a sealing
coating that seals water molecules in micropores of at least some
of the porous material; a porous separator disposed between the
first electrode material and the second electrode material; a first
current collector and a second current collector, wherein the first
electrode material is disposed between the porous separator and the
first current collector, and the second electrode material is
disposed between the porous separator and the second current
collector; and an electrolyte, wherein the first electrode
material, the second film electrode material and the porous
separator are immersed in the electrolyte. The porous material may
comprise activated carbon. The porous material may comprise dry
fibrillized carbon particles and fibril-forming binder
particles.
[0022] In one embodiment, an electrochemical double layer capacitor
comprises: a first and a second fibrillized electrode material,
wherein the fibrillized electrode material comprises a porous
activated carbon and a sealing coating that seals water molecules
in micropores of at least some of the activated carbon; a porous
separator disposed between the first electrode material and the
second electrode material; a first current collector and a second
current collector, wherein the first electrode material is disposed
between the porous separator and the first current collector, and
the second electrode material is disposed between the porous
separator and the second current collector; an electrolyte, wherein
the first electrode material, the second film electrode material
and the porous separator are immersed in the electrolyte; and a
container, the container holding the electrolyte, the separator,
and the first and second electrode material.
[0023] Active electrode material, such as fibrillized blend of
activated carbon, polymer, and conductive carbon, can thus be
pretreated by immersion in a sealing coating. After the active
electrode material is dried, the coating seals micropores of the
activated carbon or another porous material, thus preventing
exposure of water molecules or other impurities trapped in the
micropores to outside agents. At the same time, the sealing coating
does not seal most mesapores of the porous material, allowing
exposure of the mesapores' surface area to the outside agents. The
pretreated active electrode material is used for making electrodes
or electrode assemblies of electrical energy storage devices. For
example, the electrodes may be immersed in an electrolyte to
construct electrochemical double layer capacitors. Pretreatment
with the sealing coating reduces the number of water molecules
interacting with the electrolyte, enhancing the breakdown voltage
of the capacitors.
[0024] These and other features and aspects of the present
invention will be better understood with reference to the following
description, drawings, and appended claims.
BRIEF DESCRIPTION OF THE FIGURES
[0025] FIG. 1 illustrates selected steps of a process for making an
electrode wherein active electrode material is pretreated with a
sealing coating;
[0026] FIG. 2 illustrates selected steps of a process for making an
electrode assembly wherein paste of pretreated electrode material
particles is deposited on a separator of a double layer
capacitor;
[0027] FIG. 3 illustrates selected steps of another process for
making an electrode with pretreated active electrode material;
[0028] FIG. 4 illustrates selected steps of a process for making an
electrode film with pretreated active electrode material; and
[0029] FIG. 5 illustrates, in a high-level manner, cross-section of
an electrode assembly of a double layer capacitor.
DETAILED DESCRIPTION
[0030] Reference will now be made in detail to several embodiments
of the invention that are illustrated in the accompanying drawings.
Same reference numerals may be used in the drawings and the
description to refer to the same or like parts or steps. The
drawings are in simplified form and not to precise scale. For
purposes of convenience and clarity only, directional terms, such
as top, bottom, left, right, up, down, over, above, below, beneath,
rear, and front may be used with respect to the accompanying
drawings. These and similar directional terms should not be
construed to limit the scope of the invention in any manner.
[0031] The words "embodiment" and "variant" refer to particular
apparatus or process, and not necessarily to the same apparatus or
process. Thus, "one embodiment" (or a similar expression) used in
one place or context can refer to a particular apparatus or
process; the same or a similar expression in a different place can
refer to a different apparatus or process. The expression
"alternative embodiment" and similar phrases are used to indicate
one of a number of different possible embodiments. The number of
potential embodiments is not necessarily limited to two or any
other quantity.
[0032] The expression "active electrode material" and similar
phrases signify material that enhances the function of the
electrode beyond simply providing a contact or reactive area
approximately the size of the visible external surface of the
electrode. In a double layer capacitor electrode, for example, a
film of active electrode material includes particles with high
porosity, so that the surface area of the electrode exposed to an
electrolyte (in which the electrode is immersed) is increased well
beyond the area of the visible external surface; in effect, the
surface area exposed to the electrolyte becomes a function of the
volume of the film made from the active electrode material. The
meaning of the word "film" is similar to the meaning of the words
"layer" and "sheet"; "film" does not necessarily imply a particular
thickness of the material. The references to "fibrillizable binder"
and "fibril-forming binder" within this document are intended to
convey the meaning of polymers, co-polymers, and similar ultra-high
molecular weight substances capable of fibrillation. Such
substances are often employed as binder for promoting cohesion in
loosely-assembled particulate materials, i.e., active filler
materials that perform some useful function in a particular
application. When used to describe processing of porous materials,
the words "pretreat," "treat" and their inflectional morphemes
refer to subjecting the porous material to contact with a sealing
coating to seal impurities within micropores of the material. For
example, the material may be immersed in the coating, mixed with
the coating, sprayed with the coating, exposed to condensation of
coating vapors, or otherwise brought in contact with the coating.
Note that "treat" and "pretreat" have a different meaning when
these words are used to describe processing of current collectors,
as is explained in context. "Calender" and "nip" as used in this
document mean a device adapted for pressing and compressing.
Pressing may be, but is not necessarily, performed using rollers.
When used as a verb, "calender" means processing in a press, which
may, but need not, include rollers.
[0033] Other and further definitions and clarifications of
definitions may be found throughout this document.
[0034] Referring more particularly to the drawings, FIG. 1
illustrates selected steps of a process 100 for fabricating an
electrode of a double layer capacitor. Although the process steps
are described serially, certain steps may also be performed in
conjunction or in parallel, in a pipelined manner, or otherwise.
There is no particular requirement that the steps be performed in
the same order in which this description lists them, except where
explicitly so indicated, otherwise made clear from the context, or
inherently required. Not all illustrated steps are strictly
necessary, while other optional steps may be added to the process
100. A high level overview of the process 100 is provided
immediately below; more detailed explanations of the steps of the
process 100 and variants of the steps are provided following the
overview.
[0035] At step 105, fibrillized particles of active electrode
material are provided. At step 110, the fibrillized particles are
dried to evaporate water molecules that may be present within the
active electrode material. At step 115, the particles are mixed
with or immersed in a sealing coating. The sealing coating is
capable of sealing water molecules (and possibly also other
impurities) in micropores of the active electrode material. The
coating may also perform as an adhesive promoting cohesion of the
particles of the active electrode material and adhesion of the
particles to a surface, for example, current collector or separator
surface. In some embodiments, two coatings are used: one for
sealing the micropores, the other for acting as an adhesive. The
sealing coating is such that it does not seal at least a majority
(as measured by surface area) of mesapores of the active electrode
material. At step 120, a current collector is provided. At step
125, the treated active electrode material is mixed with one or
more processing material or liquid as known to those skilled in the
art to form a slurry like paste, which is then coated onto the
current collector. As will be discussed below, the current
collector can be coated on two sides. At step 130, the paste is
dried, resulting in an electrode sheet that includes (1) the
current collector, and (2) one or two active electrode material
layers. At step 135, the electrode sheet is calendered to densify
the active electrode material layers. At step 140, the calendered
sheet is formed into one or more electrodes/electrode assemblies
for use in double layer capacitors.
[0036] We now turn to a more detailed description of the individual
steps of the process 100, beginning with the step 105 in which
fibrillized active electrode material is provided.
[0037] According to one technique for obtaining the fibrillized
active electrode material, particles of active electrode material
are dry-blended or otherwise mixed together with a fibrillizable
binder (e.g., a polymer) and a conduction promoter to form a dry
powder material. Dry-blending may be carried out, for example, for
1 to 10 minutes in a V-blender equipped with a high intensity
mixing bar, until a uniform dry mixture is formed. Those skilled in
the art will identify, after perusal of this document, that
blending time can vary based on batch size, materials, particle
size, densities, as well as other properties, and yet remain within
the scope of the present invention. The resulting dry powder
material is dry fibrillized (fibrillated) using non-lubricated
high-shear force techniques, such as jet milling, pin milling,
hammer milling, or similar techniques known to a person skilled in
the art. The shear forces that arise during the dry fibrillation
process physically stretch the polymer particles, causing the
polymer to form a network of fibers that bind the polymer to the
conduction promoter and to the active electrode particles. The
polymer acts as a matrix for holding the active electrode particles
and the conduction promoter particles within the fibrillized
material.
[0038] In some embodiments, the active electrode material and the
conduction promoter used in this process are, respectively,
activated carbon and conductive carbon or graphite. Suitable
activated carbon materials are available from a variety of sources,
including Nuchar.RTM. powders sold by Westvaco Corporation of
Stamford, Conn.; and YP-17 activated carbon particles sold by
Kuraray Chemical Co., LTD, Shin-hankyu Bldg. 9F Blvd. C-237,
1-12-39 Umeda, Kiata-ku, Osaka 530-8611, Japan.
[0039] The polymers used in electrode embodiments in accordance
with the present invention include, without limitation,
polytetraflouroethylene (PTFE or Teflon.RTM.), polypropylene,
polyethylene, co-polymers, and various polymer blends.
[0040] The specific proportions of the activated carbon, conductive
carbon, and polymer used in selected exemplary embodiments are as
follows: 85-90 percent by weight of activated carbon, 5-8 percent
by weight of PTFE, and 2-10 percent by weight of conductive carbon.
Other exemplary embodiments contain 85-93 percent of activated
carbon, 3-8 percent of PTFE, and 2-10 percent of conductive carbon.
Yet other exemplary embodiments contain activated carbon and PTFE,
and do not use conductive carbon.
[0041] It should be noted that the references to dry-blending, dry
powders, other dry processes, and dry materials used in the
manufacture of the active electrode material films do not exclude
the use of electrolyte in the double layer capacitors. As has
already been mentioned, the electrodes and the separator are
typically immersed in and impregnated with an electrolytic solution
in order to make a double layer capacitor. Furthermore, even though
additives, such as solvents, liquids, and the like, are not
necessarily used in the manufacture of certain embodiments
disclosed herein, a certain amount of impurity, for example,
moisture, may be absorbed by the active electrode material from the
surrounding environment. Those skilled in the art will understand,
after perusal of this document that the dry particles used with
embodiments and processes disclosed herein may also, prior to being
provided by particle manufacturers as dry particles, have
themselves been preprocessed with additives and, thus, contain one
or more pre-process residues. For these reasons, one or more of the
embodiments and processes disclosed herein may utilize a drying
step prior to a final electrolyte impregnation step so as to remove
or reduce the aforementioned pre-process residues and impurities.
It is identified that even after one or more drying steps, trace
amounts of the aforementioned pre-process residues and impurities
may be present in the active electrode material and the electrode
film made from the material.
[0042] The drying step 110 may involve air-drying the fibrillized
particles. Alternatively, the particles are force-dried at an
elevated temperature. For example, the particles may be subjected
to a temperature between about 100 and 150 degrees Celsius. It has
been identified that subjecting the active electrode material to
the elevated temperature substantially reduces the presence of
water molecules held by physical bonding forces (VanderWaal's
forces) in mesapores and macropores of the material. At the same
time, water molecules held by the physical binding forces in
micropores may remain trapped because of the small size of the
micropores and capillary effects. Note that for the purposes of
this document, we roughly divide the pores according to their
dimensions (diameters or longest dimensions) along the following
lines:
[0043] Micropores--under about 2 nanometers;
[0044] Mesapores--between about 2 and about 25 nanometers; and
[0045] Macropores--over about 25 nanometers.
[0046] It has been identified that drying has a less pronounced
effect on the water molecules held by chemical bonding forces
within the active electrode material than on water molecules held
by physical bonding forces. The probable reason for the diminished
effect is that chemical bonding forces are generally stronger than
VanderWaal's forces.
[0047] In some embodiments, the drying step 110 is performed prior
to fibrillizing the active electrode particles.
[0048] At the step 115, the fibrillized particles are mixed with a
sealing coating. The coating is "sealing" in the sense that it
penetrates the micropores of the active electrode material and
surrounds the water molecules (and possibly other impurities)
within the micropores. The water molecules become sealed within the
micropores. In some embodiments, more than 30 percent of water
molecules in the micropores are sealed. In more specific
embodiments, at least 50 percent of water molecules in the
micropores are sealed. In yet more specific embodiments, at least
80 percent of water molecules in the micropores are sealed. When
the active electrode material is subsequently immersed in an
electrolyte, the sealed water molecules are not able to interact
with the electrolyte, or the effect of such interaction is
diminished. Consequently, the number of high energy excited sites
is reduced. The other desirable (but not strictly necessary)
property of the coating is that it can act as an adhesive.
[0049] One sealing coating adapted for use in methods described
throughout this document is known by the trade name Electrodag.RTM.
EB-012. It is available from Acheson Colloids Company, 1600
Washington Avenue, Port Huron, Mich. 48060; telephone number (810)
984-5581; www.achesonindustries.com. The Electrodag.RTM. EB-012 is
a water-based dispersion of graphite in a thermoplastic binder.
[0050] Other coatings adapted for use in the described methods can
be selected from the Adcote.RTM. line of solvent-based adhesives,
available from Rohm and Haas Company, 100 Independence Mall West,
Philadelphia, Pa. 19106-2399; telephone number 215-592-3000;
facsimile number 215-592-3377; www.rohmhaas.com.
[0051] In one embodiment, the active electrode material particles
are first immersed in a sealing coating. The sealing coating is
then drained through a filter, and the particles are dried,
allowing the micropores to be sealed. After treatment with the
sealing coating, the active electrode particles are mixed with an
adhesive coating. The resulting material may then as be mixed with
one or more processing material or liquid to obtain a slurry-like
paste.
[0052] The current collector provided in the step 120 may be made
of a sheet of conductive material, such as metal sheet, foil,
screen, or mesh. In one electrode embodiment, the current collector
is a sheet of aluminum foil approximately 40 microns thick. In
alternative embodiments, the thickness of the foil is between about
20 and about 100 microns. In other, more specific embodiments, the
thickness of the aluminum foil is between about 30 and about 50
microns. In still other alternative embodiments, the current
collector is relatively thick and is better described as a
plate.
[0053] Conductive materials other than aluminum can also be used in
the current collector. These materials include, for example,
silver, copper, gold, platinum, palladium, steel, and tantalum, as
well as various alloys of these metals. Non-metal materials are
also potential candidates for use in the current collector.
[0054] In some embodiments, the current collector may be pretreated
to enhance its adhesion properties. Treatment of the current
collector may include mechanical roughing, chemical pitting, and/or
use of a surface activation treatment, such as corona discharge,
active plasma, ultraviolet, laser, or high frequency treatment
methods known to a person skilled in the art.
[0055] In the step 125, the paste made with the treated active
electrode particles is applied uniformly to one or both sides of
the current collector, so that one or two films of active electrode
material are formed after the paste is dried in the following step.
The advantage of applying the paste to both sides of the current
collector is that the two films or layers of the active electrode
material may be made at the same time, resulting in an electrode
assembly that includes two electrodes sharing the current
collector.
[0056] At the step 130, the paste applied to the current collector
may be allowed to air-dry, or it may be force-dried at an elevated
temperature. Drying at elevated temperature has the advantage of
shortening the drying time and, therefore, shortening the overall
time for manufacturing electrodes. After the paste is dried, film
(or films) of active electrode material is (are) formed on the
current collector.
[0057] At the step 135, the current collector and the film(s) are
processed in a calender or another high-pressure nip. As a result
of this step, the active electrode material of the films is
compacted and densified under the pressure applied by the nip.
Compaction in a nip generally does not significantly reduce
porosity on a small scale level. Because compacting reduces the
film's volume while keeping pore surface area relatively unchanged,
the normalized effective surface area of the material is increased.
The volumetric efficiency of the active electrode films is
therefore also increased. Moreover, compacting tends to decrease
the equivalent series resistance of the capacitors built with
electrodes made from the resulting current collector-film product.
Structural integrity of the films and the films' adhesion to the
current collector may also be improved as a result of
calendering.
[0058] At the step 140, the combination of the current collector
and the one or two films is shaped for use as electrodes, for
example, trimmed to predetermined dimensions.
[0059] FIG. 2 illustrates selected steps of a process 200 for
fabricating an electrode assembly wherein the paste of pretreated
electrode material particles is deposited on a separator of a
double layer capacitor. Although the process steps are described
serially, certain steps may also be performed in conjunction or in
parallel, in a pipelined manner, or otherwise. There is no
particular requirement that the steps be performed in the same
order in which this description lists them, except where explicitly
so indicated, otherwise made clear from the context, or inherently
required. Not all illustrated steps are strictly necessary, while
other optional steps can be added to the process 200. A high level
overview of the process 200 is provided immediately below; more
detailed explanations of the steps of the process 200 and variants
of the steps are provided following the overview.
[0060] At step 205, fibrillized particles of active electrode
material are provided. At step 210, the fibrillized particles are
dried to evaporate the water molecules within the active electrode
material. At step 215, the particles are mixed with a sealing
coating, after which a slurry-like paste composition is formed. The
sealing coating is capable of sealing micropores in the active
electrode material. The coating may also perform as an adhesive
promoting cohesion of the particles of the active electrode
material and adhesion of the particles to a surface, for example,
current collector or separator surface. In some embodiments, two
coatings are used: one for sealing micropores, the other for acting
as an adhesive. At step 220, a porous separator sheet is provided.
At step 225, the paste obtained in the step 215 is applied to both
sides of the porous separator. Note that in alternative embodiments
the paste is applied to only one side of the separator. At step
230, the paste is dried, resulting in electrode films being formed
on the separator. At step 235, two current collectors are provided.
At step 240, the current collectors are attached to the surfaces of
the active electrode material films that are opposite the surfaces
of the films adjacent to the separator sheet. At step 245, the
combination of the separator sheet, active electrode material
films, and current collectors is calendered. The resulting
calendered product is formed into a shape appropriate for use in a
double layer capacitor, at step 250.
[0061] The steps 205 through 215 of the process 200 are similar or
identical to the steps 105 through 115 of the process 100 of FIG.
1.
[0062] The separator provided in the step 220 is made from a porous
material that allows an electrolyte to pass through its pores or
holes. At the same time, the separator material is capable of
preventing direct electrical contact between the films of active
electrode material disposed on each side of the separator. In
various embodiments, the separator materials used include glass,
polyethylene, polyphenylene sulfide, rayon, polypropylene,
polyetheretherketone, other polymers, as well as compositions,
laminates, and overlays of these materials. Furthermore, sheets
formed using woven and unwoven fibers of these and other substances
can also be used in making the separators. Separators of various
embodiments further include cellulose, paper, and cotton linter. In
one particular embodiment, the separator is made from TF3045 paper
available from Nippon Kodoshi Corporation of Japan.
[0063] In the step 225, the paste made with the treated active
electrode particles is applied uniformly to the sides of the
separator, so that films of active electrode material are formed on
the separator after the paste is dried in the following step.
[0064] At the step 230, the paste applied to the separator may be
allowed to air-dry, or it may be force-dried at an elevated
temperature. After the paste is dried, the films of active
electrode material are formed on the separator.
[0065] Each of the current collectors provided in the step 235 may
be similar or identical to the current collector provided in the
step 120 of the process 100. For example, each current collector
may be made of thin aluminum foil.
[0066] Turning next to the step 240, the current collectors may be
attached to the active electrode films using an adhesive. In
certain alternative process embodiments, the current collectors are
deposited on the films using high-energy metallization techniques,
such as flame spraying, arc spraying, plasma spraying, and high
velocity oxygen fuel (HVOF) thermal spraying. In other embodiments,
the current collectors are applied onto the films by vapor
deposition, for example, low-pressure or sub-atmospheric chemical
vapor deposition (LPCVD or SACVD). In still other embodiments, the
current collectors are simply brought into contact with the films
before calendering performed in the step 245, which laminates the
current collectors to their respective films under high pressure,
in addition to densifying the active electrode films. The step 245
is similar to the step 135 of the process 100.
[0067] At the step 250, the combination of the porous separator,
films, and current collectors is shaped for use as electrodes, for
example, trimmed to predetermined dimensions.
[0068] Films of active electrode material pretreated with a sealing
coating may be made before they are attached to the current
collectors (as in the process 100) or to the porous separator (as
in the process 200). FIG. 3 illustrates selected steps of one such
process 300. Although the process steps are described serially,
certain steps may also be performed in conjunction or in parallel,
in a pipelined manner, or otherwise. There is no particular
requirement that the steps be performed in the same order in which
this description lists them, except where explicitly so indicated,
otherwise made clear from the context, or inherently required. Not
all illustrated steps are strictly necessary, while other optional
steps can be added to the process 300.
[0069] At step 305, two films of active electrode material are
provided. In some process embodiments, polymer powder, active
electrode material powder (e.g., activated carbon), conduction
promoter powder (e.g., conductive carbon or graphite), and possibly
other powder materials are blended, for example, using a
dry-blending process. The proportions of the materials, the
specific materials used, and the blending operation may be similar
or identical to those described in relation to the process 100 of
FIG. 1.
[0070] The dry powder material (that results from mixing and
blending) is fibrillized (fibrillated) using non-lubricated
high-shear techniques, such as jet milling, pin milling, hammer
milling, or similar techniques known to a person skilled in the
art. The fibrillized material is then fed into one or more
high-pressure nips, such as roll mills, calenders, belt-presses, or
flat plate presses, to press the material into films.
[0071] After the active electrode films are made, they are immersed
in a sealing coating, for example, the Electrodag.RTM. EB-012 or
Adcote.RTM. coating, at step 310. At step 315, the films treated
with the sealing coating are dried, for example, air-dried or force
dried at an elevated temperature. At step 320, a current collector
is provided. This step is similar to the step 120 of the process
100. The active electrode films are attached to the current
collector in step 325. Attachment may be performed using a number
of different techniques, including these:
1. Using an adhesive layer between each film and the current
collector, optionally followed by calendering.
2. Using high-energy metallization techniques, such as flame
spraying, arc spraying, plasma spraying, and HVOF thermal
spraying.
3. Using vapor deposition, for example, LPCVD and SACVD
techniques.
[0072] At step 330, the current collector and the films are
processed in a calender or another high-pressure nip. At step 335,
the combination of the current collector and the one or two films
is shaped for use as electrodes, for example, trimmed to
predetermined dimensions.
[0073] Treatment of porous material with a sealing coating may be
performed before the material is fibrillized. FIG. 4 illustrates
selected steps of a process 400 for making film of fibrillized
active electrode material wherein the porous material is treated
with a sealant before the material is fibrillized. Although the
process steps are described serially, certain steps may also be
performed in conjunction or in parallel, in a pipelined manner, or
otherwise. There is no particular requirement that the steps be
performed in the same order in which this description lists them,
except where explicitly so indicated, otherwise made clear from the
context, or inherently required. Not all illustrated steps are
strictly necessary, while other optional steps can be added to the
process 400.
[0074] At step 405, activated carbon is provided. For example,
carbon particles may be activated using thermal or chemical
activation techniques that increase carbon porosity. At step 410,
the activated carbon particles are washed to remove solid
impurities.
[0075] At step 415, the carbon particles are force-dried at
elevated temperature, or simply allowed to dry at room temperature.
The step 415 may be similar to the step 110 of the process 100
described above.
[0076] At step 420, the carbon particles are treated with a sealing
coating. This step is similar to the step 115 of the process 100.
As has been described in relation to the process 100, the coating
is "sealing" in the sense that it penetrates the micropores of the
active electrode material and surrounds the water molecules (and
possibly other impurities) within the micropores. The water
molecules become sealed within the micropores. At step 425, the
sealing coating with which the carbon particles have been treated
is dried, for example, force-dried or allowed to dry at ambient
temperature.
[0077] At step 430, the activated carbon particles are mixed with
fibrillizable binder and, optionally, with particles of a
conduction promoting material, such as conductive carbon. The
mixture is then blended. After blending, the resulting dry powder
material is fibrillized using, for example, non-lubricated
high-shear force techniques, such as jet milling, pin milling,
hammer milling, or similar techniques known to a person skilled in
the art. This is done at step 435. Mixing, blending, fibrillation,
and specific materials and proportions used in the steps 430 and
435 may be similar or identical to those that have been described
in relation to the step 105 of the process 100.
[0078] At step 440, film or films are formed from the fibrillized
material. In some exemplary embodiments, the fibrillized material
is fed into one or more high-pressure nips, such as roll mills,
calenders, belt-presses, or flat plate presses, to press the
material into films. In other exemplary embodiments, particles of
the fibrillized material are mixed with an adhesive to form a
slurry-like paste composition, which may be deposited on a current
collector or porous separator, and allowed to dry, as has been
described in relation to step 125/130 and 225/230 of the processes
100 and 200, respectively. In yet other embodiments, treatment with
sealant as described herein is performed on particles that are used
form extruded type electrode films, as are known to those skilled
in the extruded electrode arts.
[0079] The electrodes, electrode assemblies, and electrode films
obtained through the processes 100, 200, 300, and 400 may be used
in double layer capacitors and other electrical energy storage
devices. FIG. 5 illustrates, in a high level manner, cross-section
of an electrode assembly 500 of a double layer capacitor. In the
Figure, the components of the assembly 500 are arranged in the
following order: (1) first current collector layer 505, (2) first
active electrode film 510, (3) porous separator 520, (4) second
active electrode film 530, and (5) second current collector 535. A
double layer capacitor using the electrode assembly 500 further
includes an electrolyte and a container, for example, a sealed can,
that holds the electrolyte. The assembly 500 is disposed within the
container (can) and immersed in the electrolyte.
[0080] To understand better various steps of the processes 100,
200, 300, and 400, a person skilled in the art may also benefit
from reading U.S. patent application Ser. No. 10/817,701, filed 2
Apr. 2004 and one or more provisional referenced therein. These
commonly assigned patent documents are hereby incorporated by
reference as if fully set forth herein, including all figures,
tables, claims, and additional subject matter incorporated by
reference therein.
[0081] Additional details for manufacturing double layer capacitors
are described in various sources, including Farahmandi et al., U.S.
Pat. No. 6,585,152, entitled METHOD OF MAKING A MULTI-ELECTRODE
DOUBLE LAYER CAPACITOR HAVING SINGLE ELECTROLYTE SEAL AND
ALUMINUM-IMPREGNATED CARBON CLOTH ELECTRODES; and in Bendale et
al., U.S. Pat. No. 6,631,074, entitled ELECTROCHEMICAL DOUBLE LAYER
CAPACITOR HAVING CARBON POWDER ELECTRODES. These commonly-assigned
patents are hereby incorporated by reference as if fully set forth
herein, including all figures, tables, claims, and additional
subject matter incorporated by reference therein.
[0082] The inventive active electrode films, electrodes, electrode
assemblies, energy storage devices, and processes used in the
course of their fabrication are described above in considerable
detail for illustration purposes. Neither the specific embodiments
of the invention as a whole, nor those of its features, limit the
general principles underlying the invention. In particular, the
invention is not limited to the specific materials and proportions
of constituent materials used for fabricating the electrodes. The
invention is also not limited to electrodes used in double layer
capacitors, but extends to other electrode applications. The
specific features described herein may be used in some embodiments,
but not in others, without departure from the spirit and scope of
the invention as set forth. Many additional modifications are
intended in the foregoing disclosure, and it will be appreciated by
those of ordinary skill in the art that, in some instances, some
features of the invention will be employed in the absence of a
corresponding use of other features. The illustrative examples
therefore do not define the metes and bounds of the invention and
the legal protection afforded the invention, which function is
served by the claims and their equivalents.
* * * * *
References