Note: Descriptions are shown in the official language in which they were submitted.
WO 01/56100 CA 02365630 2001-09-21 PCT/USO1/02778
BACKGROUND OF THE INVENTION
s 1. FIELD OF THE INVENTION
The present invention relates to carbon materials for use in energy
storage cells and batteries. More specifically, the present invention relates
to
carbon materials for use in dual graphite energy storage cells and batteries.
to
2. DESCRIPTION OF RELATED ART
Carbon is used as an active material for battery electrodes for many
different structures ranging from soft, amorphous carbon to hard, crystal and
15 graphite and in many different forms such as powders and fibers.
Traditionally, these carbons have been bound or pasted to a metal substrate
to provide an electrical path from the active material to the battery
terminals.
The materials used to bind the carbon to the metal typically interfere with
the
electrochemistry, add resistance to the electrode, and increase the weight of
2o the electrode. Metal current collectors can contribute as much as half of
the
weight of a battery electrode.
Previously, carbonaceous materials were used as an active material in
negative electrode for absorbing and discharging lithium ions. In these uses,
25 the carbonaceous material has a layered structure more disordered than
graphite and has hexagonal net phases with selected orientation. These
materials, therefore, included both a graphite-like layered structured part
and
a turbulence layered structured part. In U.S. Patent 5,244,757 to Takami, et
al. there are specific parameters relating to the graphite-like layered
structure
3 0 of the carbonaceous material. This patent, along with all of the prior
art, is
limited because the carbon material is formed into a fiber having fine
-1
WO 01/56100 CA 02365630 2001-09-21 PCT/USO1/02778
structures in either a lamellar type or Brooks-Taylor orientation.
U.S. Patent 5,677,084 to Tsukamoto, et al. discloses carbon fibers that
are used in carbonaceous material in the form of a unidirectionally arranged
s body in combination with electrically conductive fibers or foil. A problem
disclosed in the prior art is the concern that proper conductivity occurs. In
order to overcome this problem the carbon particles or fibers were attached to
a matting or adhered to an electro conducting foil, such as a metal foil to
the
entire carbonaceous area. This provided sufficient conductivity, thereby
1 o enabling proper conductivity of the carbon material.
U.S. Patents 5,862,035 and 6,094,788 both to Farahmandi, et al.
disclose a double layer capacitor which utilizes aluminum which is
impregnated into a commercially available carbon cloth made up of bundles
15 of active carbon fibers. As with the above prior art, the Farahmandi, et
al.
patents require the aluminum to ensure proper conductivity of the carbon
material and cover the entire carbonaceous area.
Numerous patents and technical literature describe electrical energy
2o storage devices utilizing carbonaceous material such as carbon or graphite
as
an electrode material. The function of the carbon or graphite in the prior art
has been primarily that of a current collector, or as a reactive material to
form
new compounds, or as an additive to a metal or ceramic material for the
storage (doping/de-doping and/or intercalation/ deintercalation) of lithium
ions
2s in a lithium ion secondary battery. Lithium ion secondary batteries concern
the shuttling of the lithium ions from one electrode to the other (cathodes
and
anodes) with no direct use of anions that may be present in the system for
energy storage. The carbon materials used in dual graphite systems are
chosen with different requirements than seen in much of the prior art. U.S.
3 o Patent 5,993,997 to Fujimoto, et al. for instance, describes the use of a
carbon compound material capable of occluding and discharging lithium (or
-2
WO ~1/561~~ CA 02365630 2001-09-21 PCT/US~l/02778
doping/de-doping) whicf~ is then shuttled to the negative electrode composed
mainly of a carbon material that intercalates and deintercalates the lithium
in
opposition to the reaction occurring at the opposite electrode. This patent is
typical of the prior art. The dual graphite energy storage system is very
s different. The graphites chosen for dual graphite systems are used strictly
to
intercalate and deintercalate both cations and anions at two different
electrodes. The ions are strictly drawn out of the electrolyte solution for
intercalation, and never from one electrode to the other. The cations migrate
and intercalate into one electrode at the same time the anions migrate and
to intercalate into the other electrode. The reverse process also occurs
simultaneously. This explains why many skilled in the art refer to this
technology as dual intercalating. The carbonaceous materials used in dual
graphite systems require a unique selection process that is inherent with the
technology since the requirements are interdependent. Additionally, the
15 results of individual half-cells do not predict the final result of a
completed
dual graphite cell. All components in the dual graphite technology depend on
the other components, including the cation intercalating carbon fiber, the
anion intercalating carbon fiber, the ionizable salt and its concentration,
and
the solvent.
The prior art describing dual graphite energy storage devices is limited.
U.S. Patent 4,865,931 and U.S. Patent 4,830,938 to McCullough, et al.
describe a dual graphite energy storage system in which the carbonaceous
material has a Young's modulus of greater than 1 MSI but less than 75MS1.
The crystal structures of charged and discharged carbon anodes have
been extensively studied in the prior art for typical secondary batteries
unlike
the dual graphite technology. The prior art discusses the ability of carbon
fiber to dope/de-dope or intercalate/deintercalate lithium for use in
3 o electrochemical cells, however the references do not include the specifics
required for use with anion use. Of special interest is the work reviewed by
-3
WO 01/56100 CA 02365630 2001-09-21 PCT/USO1/02778
Matsumura, et al., in which the group investigates the explanations for high
discharge capacities for lithium ion cells beyond 372 mAh/g, that do not fit
the
typical models as theorized for carbons by those familiar with the art. They
use a model to describe how carbons allow for several types of interactions
s with lithium to be possible, where lithium is intercalated between the
graphitic
layers and doped at the edges of the layers. Where Lc is the crystalline
thickness and the interlayer spacing is d(002), the surface of the crystallite
is
proportional to 1/((Lc/d002)+1), and relates in a linear fashion to lithium
discharge capacity in mAh/g, allowing for C>sLi where capacity increases with
to small crystallite size Lc and large interlayer distance when La crystallite
size is
smaller than 100angstroms. However, the relationship of anion discharge
capacity in dual graphite cells is not like that of lithium cells.
With regard to the use of a carbon foam material or a carbon mat in
15 which the fibers are thermally fused to each other, U.S. Patent 5,145,732
to
Kyutoku, et al. discloses the use of a carbon felt material however the
material is referred to as a thermal insulator, expressing that the material
is
not principally conductive nor principally one of continuous carbon structure,
and in addition the material is impregnated with a resin. Other prior art
2 o references disclose the use of a carbon aerogel for use in a battery.
Additionally, U.S. Patent 5,932,185, refers to the use of carbon foams as
electrodes where the thickness of the electrode is less than 40 mils.
It would therefore be useful to develop a new carbon material that has
2s proper conductivity without requiring the addition of a metal in order to
function properly.
-4-
WO 01/56100 CA 02365630 2001-09-21 PCT/USO1/U2778
SUMMARY OF THE INVENTION
The present invention provides a carbon material for use in a dual
graphite battery. The carbon material includes a carbonaceous material
having a Young's modulus of greater than 75MS1. Also provided by the
present invention is a conductive carbon material for use in an energy storage
system, wherein the carbon material includes a carbonaceous material
selected from the group consisting essentially of a single conductive fiber, a
multiplicity of conductive fibers, conductive fibers formed into a cloth, a
to carbon foam and a carbon mat in which the fibers are thermally fused
together. Included in the invention is a carbon material or fiber having a
crystallite surface calculated by 1/[(Lc/d002)+1] of less than or equal to
0.025
for anion intercalation, and a method for making stabilized unidirectional
cloth
by affixing a webbing to a carbonaceous material.
DESCRIPTION OF THE DRAWINGS
Other advantages of the present invention are readily appreciated as
the same becomes better understood by reference to the following detailed
2o description when considered in connection with the accompanying drawings
wherein:
Figure 1 is a graph comparing the degree of graphitization of the
carbon fiber tested as the as the anion intercalation/deintercalation fiber
versus the anion fiber discharge capacity measured in mAh/g;
Figure 2 is a graph depicting the crystallite surface (as calculated by
1/((Lc/d002)+1)) versus the anion discharge capacity for various types of
carbon fibers; and
Figure 3 is a schematic representation of various types of continuous
-5-
WO 01/56100 CA 02365630 2001-09-21 PCT/USOl/02778
carbon fibers that can be used in dual graphite cells and batteries from a top
view; (3)(a) shows a woven material; (3)(b) shows a unidirectional material;
(3)(c) shows a biaxial braid material; (3)(d) shows a triaxial braid material;
and
(3)(e) shows an end result of a carbon foam or a carbon mat in which fibers
s are thermally fused to each other.
DETAILED DESCRIPTION OF THE INVENTION
Generally, the present invention provides a carbon material for use in a
to dual graphite battery. The carbon material is made of a carbonaceous
material having a Young's modulus greater than 75MS1.
The terminology used to describe a dual graphite cell during a
galvanostatic cycle is different than that of a typical battery. Since a dual
15 graphite cell incorporates the intercalation and deintercalation of both an
anion and a cation, the terminology generally employed by those in the
battery art of "anode" and "cathode" do not translate well in the dual
graphite
technology, as the electrode polarities are clearly switched between charge
and discharge of the cell. The terminology employed by those familiar in the
2o art of the dual graphite technology, more clearly delineates the
electrodes,
and is very simplistic. Electrodes are simply referred to as "cation
electrode",
which denotes the electrode that intercalates and deintercalates the cation
(i.e. Li); and "anion electrode", which denotes the electrode that
intercalates
and deintercalates the anions (i.e. BF4). This terminology eliminates the
2s confusion that can occur during dual graphite battery discussions when the
state of charge or discharge is not being specified.
The present invention uses a conductive carbon/graphite material
consisting of one or more of the following:
(1) a single conductive fiber;
-6-
CA 02365630 2001-09-21
WO 01/56100 PCT/USO1/02778
(2) a multiplicity of conductive fibers;
(3) a multiplicity of conductive fibers formed into a cloth form such
as a woven fabric, unidirectional mat, biaxial braid, triaxial braid;
(4) a carbon foam; and,
(5) a carbon mat in which the fibers are thermally fused to each
other.
All of these carbon fiber forms are equally effective in that they are all
conductive, light in weight, and deliver good cell capacity. These forms can
to be used in any combination, but the preferred embodiment uses the same
form for the anion and cation intercalating fiber with in the same cell in
order
to provide the cell with the best cell compression, etc. The difference
between the forms is the number of terminal connections required to ensure
full utilization of the material.
Energy storage, especially in dual graphite systems, is greatly
enhanced by the proper selection of carbon materials. Increasing the energy
capacity of the anion intercalating electrode means that less volume and
weight of carbonaceous material is required to achieve more energy storage.
2 o This in turn increases the entire energy storage device's energy density
giving
the device more energy per weight and volume. In addition, less total
carbonaceous material in the device reduces the total cost of the device.
Through the understanding of the relationship between the carbon's degree
of graphitization, crystallite surface and intercalation capacity, greatly
improved energy storage is achieved.
In addition, it is shown in the current invention that the degree of
graphitization and/or carbonization is an important factor in the performance
of the material as an electrode element. Greatly improved energy storage,
3 o greater than 100mAh/g anion discharge capacity, is achieved by optimizing
the carbon for the anion intercalating electrode (or what would traditionally
be
_7_
WO 01/56100 CA 02365630 2001-09-21 PCT/USO1/02778
referred to as the cathode in electrochemical cells) through optimizing its
degree of graphitization. Previous efforts to increase energy storage, which
was no greater than 100mAh/g, were focused on aspect ratio and surface
area. Figure (1 ) shows a clear representation of this. Carbonaceous
s materials having about 90 percent carbonization, are referred to in the
literature as partially carbonized. Carbonaceous materials having from 91 to
98 percent carbonization are referred to in the literature as a carbonized
material, while materials having a carbonization of greater than 98 percent
are referred to as graphitized. The present invention uses the term carbon
to fiber or material to describe all levels of carbonization described above
for
simplicity.
Figure (2) clearly shows the relationship of anion discharge capacity is
opposite that of lithium as described by Matsumura, et al. patent.
15 Accordingly, the anion preferentially stores/intercalates in the d002
spacing of
the graphite, versus the surfaces or edges of the crystal structure of the
carbon fiber. Therefore, capacity is linked more closely to La than it is to
Lc.
Carbon fibers most preferred for anion intercalation/deintercalation
electrodes
have a crystallite surface calculated by 1/((Lc/d002)+1) of less than or equal
2 o to 0.025.
The present invention provides a method for the use of continuous
carbon fiber, not formed electrodes with binders as seen in much of the prior
art (such as the reference by Steel, J.A. and Dahn, J.R.). The present
2s invention also provides for the use of a non-aqueous electrolyte, unlike
some
of the prior art references such as the reference by Noel, M. and Santhanam,
R. Additionally, all tests performed on the fibers of the present invention
were
done in a dual graphite cell where the anode and cathode were both carbon
fibers unlike the majority of the prior art (such as the reference by
3 o Santhanam, R. and Noel, M.). A noble metal is only used occasionally as a
reference electrode, but not as a counter or working electrode in the present
_g_
CA 02365630 2001-09-21
WO 01/56100 PCT/USO1/02778
invention. Through extensive testing, it has been proven that the results of
individual half-cells do not predict the final result of a completed dual
graphite
cell. All components in the dual graphite technology depend on the other
components, including the cation intercalating carbon fiber, the anion
s intercalating carbon fiber, the ionizable salt and its concentration, and
the
solvent.
Of extreme importance, in any application using carbon/graphite
materials, is the contact between every individual fiber piece being carried
1 o completely to the exterior of the device, or to the central area of
thermal or
electrical collection. Electrically and thermally conductive bonds must,
therefore, occur between every individual fiber and the metal, then in turn
every individual fiber to the other fibers in the bundle (tow), every fiber
bundle
to every other fiber bundle in the cloth formation used, and finally to the
entire
15 metal substrate in order to obtain 100% utilization of all carbon/graphite
in the
system where a cloth is used. This fact translates to required penetration and
uniform bond formation with all portions of the carbon/graphite.
The present invention is applicable to a wide variety of conductive
2 o materials. For example, the present invention is applicable to various
forms
and grades of carbon and graphite particularly graphite fibers, formed from
coal tar or petroleum pitches which are heat treated to graphitize to some
degree the carbonaceous matter. In addition, the present application is
applicable to the various polymers which on heating to above about 800C
2s lose their non-carbon or substantially lose their non-carbon elements
yielding
a graphite like material (a material having substantial polyaromatic
configurations or conjugated double bond structures) which results in the
structure becoming conductive and are in part at least graphitic in form.
3o Complete connection to all carbon/graphite is essential to obtain full
utilization of the material in any use. This keeps the amount of the
relatively
_g_
CA 02365630 2001-09-21
WO 01/56100 PCT/USO1/02778
expensive materials to a minimum, which translates to lower product costs
and waste. For a battery, this also translates to the ability to obtain higher
energy densities by using only the stoichiometric amount of materials required
for the system to function. In the case of a dual graphite energy storage
s device, poor utilization of the carbon material leads to overall loss in
cell
capacity.
Carbon/graphite fibers, and their various forms, have the least amount
of resistance in the axial direction, or along the length of the fiber.
Electrical
1 o and thermal energy is carried more efficiently along the length of a fiber
than
it is between fibers that are only in direct physical contact with each other,
even when these fibers are held under pressure or with binders. Binders
themselves, though often called conductive, are not as conductive as the fiber
itself. Fibers that have only surface contacts with each other, have a large
15 increased resistance between them due to these factors. For these reasons,
it is preferential to utilize all fibers in a manner that takes advantage of
the low
resistance axial direction. For this reason, continuous fibers are often
preferential to any form of carbon powder, chopped fibers, felt type mats,
mesocarbon microbeads, etc.
An advantage to the use of the different carbon/graphite forms can be
seen. Those forms that require the lowest number of connection sites have
the least amount of collector area that must be accommodated in an end use.
In applications where weight and or space is a critical factor, the least
2s amount of collector weight and area used by the collector is typically an
important consideration, since it is this collector that generally contributes
the
most in terms of weight, space, and often cost. Specifically in a battery, the
reduced amount of collector translates to improved energy density of the end
products, which in turn provides more possible end uses, and tower costs.
Carbon/graphite powders, chopped fibers, felt type mats, mesocarbon
-10-
CA 02365630 2001-09-21
WO 01/56100 PCT/USO1/02778
microbeads, etc., requ re an individual contact point for every piece of
carbon/graphite to the collector. This often means that the collector has a
large surface area, and thus uses a great deal of space, adds extra weight
and costs to the end product. Continuous fibers of various forms are often
s preferred. For example, a woven cloth contains continuous fibers that run in
two directions that are perpendicular to each other. This means that the
woven cloth has fiber ends exposed on four sides. The woven cloth then
requires at least two edges of collection to utilize all carbon/graphite
materials
in the cloth, and thus takes more space and weight for current collection than
to for instance a unidirectional cloth, but less than is required for a
graphite
powder which would have one entire side of the material coated as a collector
and would require a binder. Unidirectional cloths, or braids such as biaxial
or
triaxial, contain continuous fibers that run in essentially one direction. The
fibers start and then end with only two edges of exposed fiber ends; these
15 then require only one edge of collection. A carbon/graphite foam, or mat of
thermally bonded fibers, requires only one point of collection to attach all
carbon/graphite together, since the material is fused together creating
essentially one continuous fiber. Figure (3) depicts these various carbon
forms schematically.
The present invention includes the method and use of a unidirectional
carbon fiber material. U.S. Patent 5,677,084 refers to the use of a sheet of
unidirectionally arranged carbon fibers; however, this prior art reference
requires the fibers to be placed on a metal foil sheet, or the fibers are
pasted
and coated with a resin, that is in contact with the entire fiber surface.
Figure
(3)(b) shows carbon fibers laying in a parallel arrangement. This fabric can
be held in place by using a cross-stitch, or outer stitch, or with a web mat.
The present invention reduces weight, maintains cloth shape and improves
handling of these unidirectional carbon fibers. The unidirectional carbon
3 o fibers may be sprayed or covered with either polypropylene, polyethylene,
or
Teflon or glass, or combinations thereof that are stable in the battery
-11
WD 01/56100 CA 02365630 2001-09-21 PCT/USOl/02778
environment, in a webbing/mat which covers the fiber surface on top and/or
bottom, and extends beyond the carbon fiber in the perpendicular direction so
that in the case of the fiber sandwich, the top and bottom covers are melted
together and to the carbon fabric.
In the case of the carbon fiber being covered with the webbing/mat the
melting provides a stabilized fabric by melting the webbing/mat onto the
carbon fiber. The mat can also be woven, but due to cost is more effectively
a random or nonwoven. The mat can be applied directly to the fibers as they
to are oriented off a spool and then the fiber and polymer and/or glass mat
can
be run through a hot roller or other heat source such as IR tamps. Several
other processing options also exist with the same end product formed. The
end result is a stable and easily handled cloth.
The unidirectional carbon fiber cloths can be of any size or shape, as
determined by the end use. The material can be used as a battery electrode
and separator pair where two sandwiches are placed adjacent to one another
where the carbon acts as electrodes and the polymer as a separator.
Optionally, a thinner mat can be applied to the carbon and an additional layer
of separator material is placed between the carbon/polymer sandwiches.
The nature of the dual graphite system is not as restricted by electrode
thickness as found in similar technologies. As such, thicker electrodes can be
used than those described in the prior art, and the electrode thickness is
2s limited only by the cell design and intended use. Figure (3)(e) shows a
material in which the end result is a mat or foam of carbon that is
essentially
all one fiber. Carbon foams and thermally fused felts are being produced
using various different methods and in various degrees of graphitization. The
foam/fused felt has a structure similar to a sponge with a portion of the
3 o volume consisting of solid carbon while the remainder is void. Since the
individual strands of carbon are inherently connected to each other, an entire
-12-
WO 01/$6100 CA 02365630 2001-09-21 PCT/[JSOl/0277g
piece of carbon foam/fused felt acts as a single piece of carbon. Therefore,
by connecting to a single atom of the carbon, you connect to all of the atoms
in the structure. This is particularly useful with electrical current in
batteries
where the carbon material can act as both the active material for energy
s storage and as the current collector for electric current. The amount of
metal
needed in a typical electrode can constitute as much as half of the weight,
whereas carbon foam/fused mat reduces it to virtually nothing.
Although particular embodiments of the present invention have been
to described in the foregoing description, it will be understood by those
skilled in
the art that the invention is capable of numerous modifications, substitutions
and rearrangements without departing from the spirit or essential attributes
of
the invention.
15 The invention presented specifies the various types, and forms, of
carbonaceous materials that are optimal for the use in dual graphite cells,
which are incorporated to form dual graphite batteries. The dual graphite
energy storage device is different from all other batteries. The dual graphite
cell functions strictly on the intercalation and deintercalation of anions and
2 o cations, where no electrochemical reactions are required for energy
storage
and use.
The carbonaceous materials used in dual graphite cells, and/or
batteries, have requirements specific to this technology. Improved anion
2s intercalation and deintercalation capacity is seen as the degree of
graphitization of the carbon fiber increases. Exact electrode capacities
differ
depending upon the supporting electrolyte used, and depending upon the
cation intercalation/deintercalation fiber used.
3 o Various forms of the carbon fiber electrodes can be used in a dual
graphite cell or battery, however those requiring the least amount of current
-13
CA 02365630 2001-09-21
WO 01/56100 PCT/USO1/02778
collector are preferable. The dual graphite cell or battery obtains increasing
levels of energy density as the continuous carbon fiber to current collector
area ratio increases. This means that the order of preferred materials are: a
carbon mat (in which the fibers are thermally fused to each other) and a
s carbon foam, a multiplicity of conductive fibers formed into a cloth form
(such
as a woven fabric, unidirectional mat, biaxial braid, triaxial braid), a
multiplicity
of conductive fibers, and a single conductive fiber.
to
A dual graphite cell was built through the following steps.
Unidirectional carbon cloth, of the design previously described, was cut to
the
15 desired size. Current collectors were placed upon one edge of each
electrode. A thin layer of a typical battery separator was placed between the
two electrodes. The electrodes were placed in an air and watertight package.
The package void space was filled with a typical battery electrolyte. Upon
charge and discharge the dual graphite cell repeatably achieved 180mAh/g of
2 o both anion and cation capacity.
Experimentally, graphite foam was used from various sources to make
25 electrodes for cation intercalation and for anion intercalation in a dual
graphite
battery. Electrically conductive carbon ink was used to join the foam to a
metal strip, and the metal was then coated to protect it from corrosion in the
electrolyte. The foam materials for the two electrodes were in a one to one
weight ratio. A thin layer of a typical battery separator was placed between
3 o the two electrodes. The electrodes were placed in an airtight and
watertight
package. The package void space was filled with a typical battery electrolyte.
-14
WO 01/56100 CA 02365630 2001-09-21 PCT/USOl/02778
In at least one of the carbon foams tested the following data was achieved:
>228 perchlorate capaci~y mAh/g and >228 lithium capacity mAh/g.
Throughout this application various publications are referenced by
s author and year. Full citations for the publications are listed below. The
disclosures of these publications in their entireties are hereby incorporated
by
reference into this application in order to more fully describe the state of
the
art to which this invention pertains.
to The invention has been described in an illustrative manner, and it is to
be understood that the terminology used is intended to be in the nature of
words of description rather than of limitation.
Obviously, many modifications and variations of the present invention
15 are possible in light of the above teachings. It is, therefore, to be
understood
that within the scope of the appended claims, the invention may be practiced
otherwise than as specifically described.
-15-
CA 02365630 2001-09-21
WO 01/56100 PCT/USO1/02778
s 4,725,422
4,830,938
4,865,931
5,145,732
5, 244, 757
l0 5,527,643
5,741,472
5,626,977
5,677,084
5,773,167
15 5,862,035
5,898,564
5,932,185
5, 993,997
6,094,788
Matsumura, Y., et al., Interactions between disordered carbon and lithium in
2s lithium ion rechargeable batteries, Carbon, Vol. 33 (No. 10), 1995, p1457
1462.
Noel, M. and Santhanam, R., Electrochemistry of graphite intercalation
compounds, Journal of Power Sources, Vol. 72, 1998, p53-65.
Santhanam, R. and Noel, M., Influence of polymeric binder on the stability
and intercalation/de-intercalation behaviour of graphite electrodes in non-
aqueous solvents, Journal of Power Sources, Vol. 63, 1996, p1-6.
Steel, J.A. and Dahn, J.R., Electrochemical intercalation of PF6 into
graphite,
Journal of The Electrochemical Society, Vol. 147(No. 3), 2000, p892-898.
-16-
