U.S. patent application number 09/866492 was filed with the patent office on 2002-12-05 for fullerene-based secondary cell electrodes.
Invention is credited to Cagle, Dawson W..
Application Number | 20020182506 09/866492 |
Document ID | / |
Family ID | 25347728 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020182506 |
Kind Code |
A1 |
Cagle, Dawson W. |
December 5, 2002 |
Fullerene-based secondary cell electrodes
Abstract
This invention provides methods and materials for the
manufacture of lithium ion rechargeable battery electrodes
comprising fullerene compounds which have not been exposed to
oxygen. Fullerene compounds are intercalated into carbonaceous
materials to form electrodes having superior
intercalation/deintercalation properties. Fullerene monomers, such
as C.sub.60, C.sub.70, C.sub.74, C.sub.76, C.sub.78, C.sub.80,
C.sub.84 and C.sub.80-100 and fluorinated derivatives thereof, are
useful in the methods and materials of this invention. Materials
incorporating C.sub.74 are particularly preferred. The invention
also provides fullerene polymer materials useful for lithium ion
electrodes with greatly improved electrochemical properties.
Inventors: |
Cagle, Dawson W.;
(Arlington, VA) |
Correspondence
Address: |
ROTHWELL, FIGG, ERNST & MANBECK, P.C.
1425 K STREET, N.W.
SUITE 800
WASHINGTON
DC
20005
US
|
Family ID: |
25347728 |
Appl. No.: |
09/866492 |
Filed: |
May 29, 2001 |
Current U.S.
Class: |
429/231.8 ;
252/182.1; 252/502; 252/511; 429/217 |
Current CPC
Class: |
H01M 4/622 20130101;
B82Y 30/00 20130101; H01M 4/5835 20130101; H01M 4/583 20130101;
Y02E 60/10 20130101; H01M 10/0525 20130101; H01M 4/625 20130101;
H01M 4/587 20130101 |
Class at
Publication: |
429/231.8 ;
429/217; 252/182.1; 252/502; 252/511 |
International
Class: |
H01M 004/58; H01M
004/62; H01B 001/04; C01B 031/02 |
Claims
1. A composition comprising a carbonaceous material selected from
the group consisting of graphite, pitch, coal pitch, coke,
synthetic graphite, carbon black, lamellar graphite, carbon-arc
generated soot and mixtures thereof, intercalated with C.sub.74,
wherein said C.sub.74 has not been exposed to oxygen.
2. A composition of claim 1 further comprising one or more
additional fullerene wherein said additional fullerene has not been
exposed to oxygen.
3. A composition of claim 2 wherein said additional fullerene is
selected from the group consisting of C.sub.60, C.sub.70, C.sub.76,
C.sub.78, C.sub.84, and mixtures thereof.
4. A composition of claim 1 comprising about 1% to about 99%
fullerene by weight.
5. A composition of claim 1 comprising about 1% to about 50%
fullerene by weight.
6. A composition of claim 1 comprising about 10% to about 20%
fullerene by weight.
7. A composition of claim 1 further comprising a polymeric
binder.
8. A composition of claim 7 wherein said polymeric binder is
selected from the group consisting of polyvinylidene fluoride,
polyethylene oxide, polyethylene oxide, polyethylene,
polypropylene, polytetrafluoroethylene, polyacrylates, substituted
derivatives thereof, copolymers thereof, and mixtures thereof.
9. An anode for a lithium ion secondary battery comprising a
composition of claim 7.
10. A lithium ion secondary battery comprising an anode of claim
9.
11. A method of making a carbonaceous anode, comprising: (a)
providing a carbonaceous material selected from the group
consisting of graphite, pitch, coal pitch, coke, synthetic
graphite, carbon black, lamellar graphite, carbon-arc generated
soot and mixtures thereof; (b) providing at least one fullerene
selected from C.sub.60, C.sub.70, C.sub.74, C.sub.76, C.sub.78,
C.sub.80, C.sub.84, C.sub.80-.sub.100 and mixtures thereof, wherein
said at least one fullerene has not been exposed to oxygen; (c)
mixing said carbonaceous material and said at least one fullerene
to form a mixture which contains about 1% to about 50% fullerene;
(d) heating said mixture to a temperature of about 400.degree. C.
to about 900.degree. C. and holding said mixture at a temperature
of about 400.degree. C. to about 900.degree. C. for about 1 to
about 8 hours to form a carbonaceous fullerene intercalate
composition; (e) annealing said composition at a rate of about
1.degree. C./min to about 10.degree. C./min until the composition
reaches room temperature; (f) mixing with said composition a
polymeric binding material to form a fullerene
intercalate-polymeric binder mixture which is about 3% to about 15%
polymeric binder; and (g) pressing said fullerene
intercalate-polymeric binder mixture at a pressure of about 2 atm
to about 100 atm for about 1 hour to about 8 hours to form a
carbonaceous anode, wherein all of the steps (a)-(g) are conducted
in an atmosphere lacking oxygen.
12. A method of claim 11 wherein said carbonaceous material is
graphite.
13. A method of claim 11 wherein said at least one fullerene is
C.sub.74.
14. A method of claim 11 wherein said mixture of step (c) contains
about 1% to about 50% fullerene.
15. A method of claim 11 wherein said mixture of step (c) contains
about 5% to about 20% fullerene.
16. A method of claim 11 wherein said mixture of step (c) contains
about 10% to about 20% fullerene.
17. A method of claim 11 wherein said heating of step (d) is to a
temperature of about 500.degree. C. to about 600.degree. C. for
about 8 hours.
18. A method of claim 11 wherein said annealing is performed at a
rate of about 1.degree. C./min to about 5.degree. C./min.
19. An anode made according to a method of claim 11.
20. A lithium ion secondary battery anode comprising a C.sub.74
star polymer wherein said C.sub.74 star polymer has been
synthesized from C.sub.74 that has not been exposed to oxygen prior
to polymer formation.
21. An anode of claim 20 further comprising a polymeric binder
selected from the group consisting of polyvinylidene fluoride,
polyethylene oxide, polyethylene, polypropylene,
polytetrafluoroethylene, polyacrylates, substituted derivatives
thereof, copolymers thereof and mixtures thereof.
22. An anode of claim 20 wherein said C.sub.74 star polymer is a
C.sub.74 bis-EDOT-arylene copolymer.
23. A lithium ion secondary battery comprising an anode of claim
20.
24. A composition comprising fluorinated graphite intercalated with
fluorinated C.sub.74, wherein said fluorinated C.sub.74 has not
been exposed to oxygen prior to fluorination.
25. A composition of claim 24 wherein said fluorinated C.sub.74
consists essentially of C.sub.74F.sub.48.
26. A composition of claim 24 comprising about 10% to about 20%
fullerene by weight.
27. A composition of claim 24 further comprising a polymeric
binder.
28. A cathode for a lithium ion secondary battery comprising a
composition of claim 27.
29. A lithium ion secondary battery comprising a cathode of claim
28.
30. A method of making a carbonaceous cathode, comprising: (a)
providing fluorinated graphite; (b) providing fluorinated C.sub.74;
(c) mixing said fluorinated graphite and said fluorinated C.sub.74
to form a mixture which contains about 1% to about 25% fluorinated
C.sub.74; (d) heating said mixture to a temperature of about
100.degree. C. to about 900.degree. C. and holding said mixture at
a temperature of about 100.degree. C. to about 900.degree. C. for
about 1 hour to about 8 hours to form a fluorinated fullerene
intercalate composition; (e) annealing said composition at a rate
of about 1.degree. C./min to about 10.degree. C./min until the
composition reaches room temperature; (f) mixing with said
composition a polymeric binder to form a fluorinated C.sub.74
intercalate-polymeric binder mixture which is about 3% to about 15%
polymeric binder; and (g) pressing said fluorinated C.sub.74
intercalate-polymeric binder mixture at a pressure of about 2 atm
to about 100 atm for about 1 hour to about 8 hours to form a
carbonaceous cathode, wherein said fluorinated C.sub.74 has not
been exposed to oxygen prior to fluorination.
31. A method of claim 30 wherein said mixture of step (c) contains
about 1% to about 20% fluorinated C.sub.74.
32. A method of claim 30 wherein said mixture of step (c) contains
about 5% to about 20% fluorinated C.sub.74.
33. A method of claim 30 wherein said mixture of step (c) contains
about 10% to about 20% fluorinated C.sub.74.
34. A method of claim 30 wherein said annealing is performed at a
rate of about 1.degree. C./min to about 5.degree. C./min.
35. A method of claim 30 wherein said fluorinated C.sub.74 is
C.sub.74F.sub.48.
36. A cathode made according to a method of claim 30.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] This invention relates to methods and compositions for the
construction of anodes, cathodes and batteries of the lithium ion
secondary (rechargeable) type. In particular, the invention relates
to fullerene-based secondary cell components and methods and
compositions for their construction.
[0003] 2. Description of the Background Art
[0004] Lithium ion secondary (rechargeable) cells are commonly used
as power sources in portable electronic devices. Such rechargeable
cells generally use a lithium transition metal oxide (very often
Li.sub.2CoO.sub.2 (lithium colbaltate)) cathode and an anode
composed of a highly porous carbonaceous material, usually graphite
or a pyrolyzed organic material. A lithium ion-soluble electrolyte
is placed between the two electrodes, and the cell is charged.
During the electrochemical process of charging, some of the lithium
ions in the cathode material migrate from the cathode to the
carbonaceous anode layer and completely intercalate into it. During
discharge, the negative charge held by the anode is conducted out
of the battery through its negative terminal, and the Li.sup.+ ions
migrate through the electrolyte to their original location in the
cathode. When this migration is complete, the cell has been
completely discharged, and the lithium ions in the battery are at
an electronic "ground state."
[0005] The porous carbonaceous anode material can reversibly
incorporate ions within its crystal lattice with only small
structural changes. Structurally, graphite is a planar sheet of
carbon atoms arranged in a honeycomb. The carbon layers are stacked
to form what is commonly known as hexagonal (2H) or rhombohedral
(3R) graphite. A certain amount of random stacking or disorder in
the structure is common in graphite and in other carbon forms such
as cokes, petroleum cokes, synthetic graphites, carbon blacks and
the like.
[0006] Lithium ion batteries possess four main advantages over
other rechargeable cells such as nickel metal hydride,
nickel-cadmium, and lead-acid cells. First of all, lithium ions,
due to their small size, can intercalate between carbon layers more
easily and completely than larger battery ions such as nickel and
lead. Because of this property, lithium ion cells do not form
battery "memory" ion channels. Secondly, the electrical potential
of lithium ions is the most similar to graphite (carbon) of any
metal ion. This allows the easiest possible charge transfer between
carbon battery anodes and migrating lithium ions. Better charge
transfer gives these cells more efficient, complete, and
long-lasting discharge rates. Thirdly, lithium cells are much less
toxic than comparable secondary cells which use lead, cadmium or
nickel metal ions. Fourthly, lithium, the lightest metal, has a
high charge-to-mass ratio and thus produces a battery of lighter
weight.
[0007] Although current lithium battery technology constitutes an
enormous advance over that of previous commercial secondary cells,
there are a number of shortcomings involving the current materials
used as battery electrodes. Commercial lithium secondary cell
cathodes are composed of lithium salts, such as lithium colbaltate,
which allow less than 50% lithium ion migration to the anode.
Largely due to the presence of two distinct ionization energies in
the active cathode material (i.e.,
[Li.sub.2CoO.sub.4].rarw..fwdarw.[Li.sup.+]
[LiCoO.sub.4].rarw..fwdarw.[2- Li.sup.+] [CoO.sub.4.sup.2-]) the
battery never achieves its full theoretical charge potential. To
overcome the activation energy barrier for lithium ion formation in
the cathode, more energy must be put into the battery during
charging than is returned during discharge. Some of the energy used
to overcome this barrier is regained when the original compound is
reformed, but some is lost as heat. Cathodes which do not have this
high energy barrier would require less energy to charge and would
achieve a more complete charge. Therefore, materials with more
efficient charge transfer chemistries would be highly
desirable.
[0008] The anodes used in lithium ion batteries also have certain
characteristics which prevent optimal performance. Due to the
nature of graphitic and pyrolyzed carbon anode materials,
electrodes made from them are inevitably irregularly sized,
non-directionally specific, and possess non-predictable "pockets"
of charging where lithium ions can intercalate into the anode. The
extended structure of the carbon compound chosen for the anode
therefore influences both the total amount of lithium which can be
intercalated within it and at what voltage. The electrical charge
stored in the anode must be able to freely migrate between all
points of the anode and the negative cell terminal for optimal
performance. Poor orientational control increases resistance in the
anode and reduces cell charge transfer efficiency as well. The
carbon electrodes currently in use commercially employ various
forms of amorphous or graphene layered carbon. By their very
nature, the structure of these materials cannot be controlled at a
molecular level to maximize their affinity for lithium ions.
[0009] Heat-treating these prior art carbonaceous materials
increases their crystallinity and affects both their structure and
their ability to intercalate lithium. Current methods of hydraulic
compression and heat treatment significantly help this problem by
creating a more regular structure, but the problem cannot be
completely solved until the anode is manufactured in a manner such
that the order and porosity is more controlled. Therefore, new
carbon materials which can be manufactured with more controlled
structure would be highly desirable and produce more efficient
batteries.
[0010] Fullerenes are spherical or partially spherical aromatic
compounds composed solely of triconjugate (Sp.sup.2-hybridized)
carbon atoms. As such, they resemble an ideal graphite sheet, but
for the strain which their spherical shape imposes on the normally
planar aromatic structure. This strain causes fullerenes to be more
reactive than a continuous aromatic sheet. Fullerene molecules are
highly electronegative as well, and possess unusual magnetic and
electrical properties.
[0011] Improvements in the art of lithium ion batteries and battery
electrodes therefore would be highly desirable. Such electrodes
would enable the manufacture of smaller, lighter rechargeable
batteries with longer life and more efficient charging for use in
portable electronic devices such as telephones, CD players, hearing
aids, computers, and the like, or any device where a high
efficiency light weight rechargeable battery is desirable.
SUMMARY OF THE INVENTION
[0012] Accordingly, this invention provides a composition
comprising a carbonaceous material such as graphite, pitch, coal
pitch, coke, synthetic graphite, carbon black, lamellar graphite,
carbon-arc generated soot or mixtures thereof, intercalated with
C.sub.74 which has not been exposed to oxygen. The compositions may
also include additional fullerene compounds which have not been
exposed to oxygen, such as C.sub.60, C.sub.70, C.sub.76, C.sub.78,
C.sub.4 or mixtures thereof. Preferred compositions comprise about
1% to about 25% fullerene by weight and about 75% to about 99%
graphite by weight. Compositions also may further include one or
more polymeric binder, such as polyvinylidene fluoride,
polyethylene oxide, polyethylene, polypropylene,
polytetrafluorethylene, polyacrylates, substituted derivatives
thereof, copolymers thereof and mixtures thereof. Further
embodiments of the invention include anodes for lithium ion
secondary batteries comprising the compositions described above,
and lithium ion secondary batteries comprising such anodes.
[0013] The invention further provides a method of making a
carbonaceous anode comprising (a) providing a carbonaceous material
such as graphite, pitch, coal pitch, coke, synthetic graphite,
carbon black, lamellar graphite, carbon-arc generated soot and
mixtures thereof; (b) providing at least one fullerene such as
C.sub.60, C.sub.70, C.sub.76, C.sub.78, C.sub.84 or mixtures
thereof wherein the fullerenes have not been exposed to oxygen; (c)
mixing the carbonaceous material and the fullerene(s) to form a
mixture which contains about 1% to about 25% fullerene; (d) heating
the mixture to a temperature of about 400.degree. C. to about
900.degree. C. and holding the mixture at a temperature of about
400.degree. C. to about 900.degree. C. for about 1 hour to about 8
hours to form a carbonaceous fullerene intercalate composition; (e)
annealing the composition at a rate of about 1.degree. C./min to
about 10.degree. C./min until the composition reaches room
temperature; (f) mixing with the composition a polymeric binding
material to form a fullerene intercalate-polymeric binder mixture
which is about 3% to about 15% polymeric binder; and (g) pressing
the fullerene intercalate-polymeric binder mixture at a pressure of
about 2 atm to about 100 atm for about 1 hour to about 8 hours to
form a carbonaceous anode, wherein all of the steps (a)-(g) are
conducted in an atmosphere lacking oxygen. Preferred methods
involve graphite and C.sub.74. The invention also provides anodes
made by the method described above.
[0014] In yet a further embodiment, the invention provides a
lithium ion secondary battery carbonaceous anode comprising a
C.sub.74 star polymer wherein the C.sub.74 star polymer has been
synthesized from C.sub.74 which has not been exposed to oxygen
prior to polymer formation. Anodes also may include a polymeric
binder such as polyvinylidene fluoride, polyethylene oxide,
polyethylene, polypropylene, polytetrafluorethylene, polyacrylates,
substituted derivatives thereof, copolymers thereof or mixtures
thereof. The invention also provides lithium ion secondary
batteries comprising such anodes.
[0015] In yet a further embodiment, the invention provides a
composition comprising fluorinated graphite intercalated with
fluorinated C.sub.74 has not been exposed to oxygen prior to
fluorination. Compositions also may include a polymeric binder such
as polyvinylidene fluoride, polyethylene oxide, polyethylene,
polypropylene, polytetrafluorethylene, polyacrylates, substituted
derivatives thereof, copolymers thereof or mixtures thereof. The
invention also provides cathodes comprising the above compositions
and batteries comprising such cathodes.
[0016] In yet a further embodiment, the invention provides a method
of making a carbonaceous cathode comprising (a) providing
fluorinated graphite; (b) providing fluorinated C.sub.74; (c)
mixing the fluorinated graphite and fluorinated C.sub.74 to form a
mixture which contains about 1% to about 25% fullerene; (d) heating
the mixture to a temperature of about 100.degree. C. to about
900.degree. C and holding the mixture at a temperature of about
100.degree. C. to about 900.degree. C. for about 1 hour to about 8
hours to form a fluorinated fullerene intercalate composition; (e)
annealing the composition at a rate of about 1.degree. C./min to
about 10.degree. C./min until the composition reaches room
temperature; (f) mixing with the composition a polymeric binder to
form a fluorinated fullerene intercalate-polymeric binder mixture
which is about 3% to about 15% polymeric binder; and (g) pressing
the fluorinated fullerene intercalate-polymeric binder mixture at a
pressure of about 2 atm to about 100 atm for about 1 hour to about
8 hours to form a carbonaceous cathode, wherein the fluorinated
C.sub.74 has not been exposed to oxygen prior to fluorination. The
invention also provides cathodes made according to the above
method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a conceptual illustration of individual fullerene
molecules and graphite structures before and after heating at
800-900.degree. C. under an argon atmosphere.
[0018] FIG. 2 is a conceptual illustration showing the benefit of
heated versus unheated fullerene-graphite mixtures in terms of the
ability of lithium to freely intercalate and bind to the carbon
structures.
[0019] FIG. 3 is a schematic diagram of a sublimation tube furnace
suitable for separating fullerene species from a mixture of
fullerenes and carbonaceous material or for subliming crude or
purified materials onto a cooled target for direct use as a
fullerene anode.
[0020] FIG. 4 is a diagram outlining steps for electrochemical
purification of oxygen-sensitive C.sub.74.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The macromolecular structure of fullerene-based electrodes
can be controlled by selecting molecules for favorable electronic
structure, for their reactivities with other carbonaceous binder
materials and/or for tendency to acquire a favorable crystalline
form. The techniques described below are methods which can be used
to vary the chemico-physical properties of carbon compounds to
create fullerene-based carbon electrodes with improved performance
suited to many different applications.
[0022] Unlike the graphite or other carbon materials traditionally
used in rechargeable lithium batteries, the structure of
fullerene-based compositions can be better controlled during
manufacture. Fullerene compositions, having a more ordered
structure than ordinary graphite, result in electrodes in which
lithium ions can intercalate more easily and completely, thereby
increasing the possible energy density and reducing the capacity
fade of the cell due to irreversible lithium ion intercalation.
More complete lithium elimination during anode discharging also is
possible with this technique.
[0023] Different ratios of fullerene or fullerene-like compounds
may be added to traditional carbonaceous materials to improve their
electrochemical properties. Additionally, different fullerene bulk
materials may be used to produce fullerene electrodes with
different characteristics. The fullerene-based carbon electrode
materials may be heat-treated to increase their uniformity of
structure and lithium ion capacity. Use of fullerene compounds
allows for greater control of structure during manufacture,
producing a unique, reproducible electrode material which is both
more uniform and more electrically conductive than standard
graphite electrodes. Both anodes and cathodes may be produced using
fullerene or fullerene-based compounds to take advantage of these
improved structural properties.
[0024] By the term "fullerene electrode," it is meant any
electrode, anode or cathode, containing fullerene compounds,
including those consisting essentially of one or more fullerene
types and those containing fullerene mixed with one or more
additional compound. The term "fullerene compounds" includes
C.sub.60, C.sub.70, C.sub.74, C.sub.76, C.sub.78 or any of the
materials sometimes referred to as "small band gap fullerenes, "
larger C.sub.80-C.sub.100 compounds such as C.sub.84 and C.sub.100
or any of the so-called "giant fullerenes" (>C.sub.100 spheroid
or partially formed spheroid molecules) and the like. Chemically or
physically modified fullerenes compounds such as fluorinated
fullerenes or adducts and derivatives (such as, for example, those
described in Cardulla et al., Helv. Chim. Acta 80:343-371, 1997;
Zhou et al., J. Chem. Soc., Perkin Trans. 2:1-5, 1997; Okino et
al., Synth. Metals 70:1447-1448, 1995; Okino et al., Recent
Advances in the Chemistry and Physics of Fullerenes and Related
Materials, vol. 3, 1996, pp. 191-199; Haddon et al., Nature
350:320-322, 1991; Chabre et al., J. Am. Chem. Soc. 114:764-766,
1992; Gromov et al., Chem. Commun. 2003-2004, 1997; Strasser et
al., J. Phys. Chem. B 102:4131-4134, 1998; Cristofolini et al.,
Phys. Rev. B: Cond. Matter Mater. Phys. 59:8343-8346, 1999; Wang et
al., Synthetic Metals 103(1):2350-2353, 1999; Wang et al., Mater.
Res. Soc. Symp. Proc. 413:571, 1996; Kallinger et al., Synthetic
Metals 101:285-286, 1999; Kajii et al., Synthetic Metals
86:2351-2352, 1997; and Araki et al., Synthetic Metals 77:291-298,
1996, the disclosures of which are hereby incorporated by
reference, also are encompassed within this term, as well as
polymeric, copolymeric and crosslinked fullerene compounds and
compositions. These compounds per se have been described in the
prior art. The term includes all physical forms of the materials,
including, for example, solids, gases, vapors, solutions,
emulsions, powders, thin films and the like.
[0025] Specifically, the fullerenes and fullerene materials which
may be used in this invention are C.sub.60, C.sub.70, C.sub.74,
C.sub.76, C.sub.78 and C.sub.80-100, C.sub.74 is greatly preferred
for use in electrodes for lithium ion secondary batteries because
of its highly favorable electrical properties, which previously
have been unknown. C.sub.74 is highly electronegative and allows
lithium ions to move through the structure of the carbonaceous
electrodes extremely easily and quickly, leading to a light weight
battery with greatly improved efficiency, capacity and long life.
In addition, C.sub.74 is easier to synthesize and purify since it
is one of the most abundant fullerenes. Electrodes consisting
essentially of C.sub.74 as the fullerene component are contemplated
for use in the electrode materials of this invention, as well as
fullerene mixtures in which the majority or even only a minority of
the fullerene compounds are C.sub.74. Therefore, highly purified
C.sub.74, semi-purified C.sub.74 or fullerene mixtures containing
C.sub.74 may be used to make the compositions, electrodes and
batteries of this invention, and are encompassed in the term
"C.sub.74." Preferred fullerene materials are at least 50% up to
99.9% or nearly 100% C.sub.74.
[0026] Recently, fullerenes have been discovered to be highly
reactive with oxygen, including atmospheric oxygen. It is very
important, therefore, that all the fullerene compounds used in
these inventive materials and electrodes be kept away from oxygen
during all stages of production, including fullerene synthesis,
purification, electrode synthesis, cell manufacture and packaging.
Therefore, an inert atmosphere, such as argon or helium, must be
used during all stages of manufacture of batteries containing these
materials. Molecular oxygen from the atmosphere or other sources
can be absorbed on the surface of fullerene molecules, resulting in
the partial oxidation of the fullerene and dimerization or
polymerization. The prior art has described the use of C.sub.60 or
C.sub.70 to make some types of fullerene-based electrodes, and has
described how to synthesize and purify them. However, no mention is
made in the prior art that the materials are to be synthesized and
manipulated in the absence of air at all stages. C.sub.60 and
C.sub.70 samples will partially react with each other to form
dimers (i.e., C.sub.60--O--C.sub.60) and polymers when exposed to
air for just a few seconds. Brief exposure results in a reaction
which is only partial, leaving most of the material undisturbed.
However, the quantity that does react may create a passivating
layer, and poison the electronic activity of the bulk fullerene
material, making them far less useful for battery electrodes. Prior
art fullerene materials, which have been exposed to the air for any
length of time, however brief, therefore are not fullerenes but in
fact fullerene dimers with quite different structures and bulk
properties which make them unsuitable for use in lithium ion
electrodes. C.sub.74 is particularly reactive in this way and loses
its highly beneficial and advantageous electrochemical properties
if exposed to oxygen, even for a very short time. The methods and
materials of the invention disclosed here eliminate this problem by
ensuring that the fullerenes are not oxidized.
[0027] Generally, anodes for lithium batteries with improved
electrical conductivity, greater electron density storage capacity
(mAh/g) and better lithium intercalation-deintercalation properties
over standard carbon anodes may be produced using any of the
fullerenes. In one embodiment, fullerene compounds, preferably
C.sub.74, are mixed with graphite, pitch, coal pitch, coke,
synthetic graphite, carbon black, carbon-arc generated soot
electrode binder material or any other carbon-containing material
known in the art for carbon anode production. Mixtures of these
compounds also are suitable. Electrodes may be made having about 1%
fullerene by weight to about 99% fullerene by weight. Generally,
fullerene is desirably present at least about 10% by weight of the
electrode material, desirably about 10% to about 50% and preferably
about 10% to about 20% or 25% fullerene. It is contemplated that
these ranges may be expanded or varied as called for by the types
of materials used and the desired application of the electrode or
battery. Those of skill in the art consider it routine to conduct
experimentation to optimize ratios of components to achieve the
desired result.
[0028] The mixture of the fullerene compound(s) and other
carbonaceous material(s) preferably is pyrolyzed according to
methods known in the art. Generally, the mixtures are heated to
near the sublimation point of the added carbonaceous material
(e.g., graphite, coke, and the like) and well past the sublimation
point of the fullerenes, in closed, pressurized containers, held at
this elevated temperature, and then slowly annealed over a period
of about 1 hour to about 8 hours. Hydraulic pressure at about 50
atm to about 200 atm may be used. Annealing involves a slow
lowering of the temperature from the reaction temperature to about
room temperature, usually at a rate of about 1.degree. C./min to
about 10.degree. C./min, however slower or faster rates may be used
as is convenient. Generally a rate of about 1.degree. C./min to
about 5.degree. C./min is preferred for the compounds of this
invention. Suitable temperatures for any monomeric fullerene
compound are usually about 400.degree. C to about 900.degree. C. or
from about 500.degree. C. to about 900.degree. C., although
fluorinated fullerene compositions may be treated at temperatures
from about 100.degree. C. to about 900.degree. C. The preferred
temperature range is about 500.degree. C. to about 600.degree. C.
As a result of the heating, the distance between graphene sheets
increases and the fullerene molecules sublime and are able to move
between the sheets, further separating the graphite layers. This
new structural configuration allows for a higher degree of
reversible lithium ion intercalation. See FIGS. 1 and 2. The
diagrams depict C.sub.60 molecules, but the concept is equally
applicable to any fullerene, including C.sub.74.
[0029] In the electrodes of the embodiments described above,
unoxidized fullerenes intercalate between and around the carbon
layers of graphite or other organic material. The
fullerene-containing materials thus form more easily accessible
intercalation sites for lithium ions when the cell is charged.
Furthermore, this invention takes advantage of the ability of each
fullerene molecule to attract and reversibly absorb up to six
electrons. Because of the fullerene's ability to bind these
electrons, it is beneficial to put as much fullerene in the
electrode as reasonably possible (generally 1%-50% fullerene by
weight of the carbonaceous electrode composition is attainable and
results in an electrode with greatly improved properties over the
prior art) to achieve the most improvement in electron density
storage. However, a certain amount of binder is required to form
the electrode from loose powder (generally 5-10% by weight).
Therefore, the total composition of the electrode is about 1% to
about 25% fullerene, about 3% to about 15% binder and the remainder
graphite or other carbonaceous material. Electrodes comprising 1%,
2%, 3%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 25% fullerene, or up to
99% fullerene are contemplated. Binder material may be present as
3%, 5%, 8%, 10%, 12% or 15% of the total electrode composition
weight. Preferred compositions generally are about 5% to about 20%
fullerene and about 5% to about 10% binder. Most preferred
compositions generally are about 10% to about 20% fullerene and
about 5% to about 10% binder by weight. An example of a preferred
electrode composition is 10% fullerene, 10% binder and 80%
graphitic carbon by weight. These materials are more highly
controllable in structure and concomitant electrochemical
properties than ordinary graphite or other standard carbonaceous
electrode materials since they can be synthesized and purified to
form a reproducible and uniform product. Electrodes made according
to this method have improved electrical conductivity, electron
density storage, and intercalation/deintercalation properties over
both standard carbon anodes and prior art electrodes containing
fullerenes.
[0030] Generally, fullerenes may be synthesized according to any
convenient method, including any prior art method such as the
carbon arc method, (also referred to as the Kratschmer-Huffman
method) and purified by any convenient means such as slow
concentration of solutions, diffusion methods, cooling of saturated
solutions, precipitation with a solvent, sublimation or
electrochemically, and by liquid chromatographic separation, but in
an inert atmosphere at all stages. An exemplary method for C.sub.74
synthesis, adapted from Deiner and Alford, Nature 393: 668-671,
1998, is provided in Example 1.
[0031] For purification, after synthesis in an anaerobic
environment, the fullerene compounds (e.g., C.sub.60, C.sub.70,
C.sub.74, C.sub.80-100, and mixtures thereof) may be placed in an
inert atmosphere furnace, vaporized and condensed at a specific
temperature for each fullerene species onto a cool metal or
graphite substrate. Crude or purified materials also may be
condensed in the same manner onto a substrate of suitable size and
proportion to form a battery electrode. See FIG. 3 for a diagram of
a suitable sublimation tube furnace. In the furnace (100), inert
gas, preferably argon, (1, 2) is continually flushed across a
heated tube furnace chamber (20) containing fullerene material held
in an inert boat (10). The temperature of the furnace is gradually
increased. At about 500-800.degree. C., the fullerene material
becomes vaporized and moves from the graphite, ceramic or inert
metal boat (10) in the center of the furnace (100) towards the
cooler end of the furnace tube chamber (20). When the material
reaches the outer region of the furnace, the vaporized fullerene
condenses onto a cooled substrate (50). The target substrate should
be made of metal or graphite for the fullerene to adhere to the
substrate and so that the substrate will withstand the high
temperatures of the furnace. The substrate, coated with deposited
fullerene is eventually removed (under an inert atmosphere).
[0032] Fullerene deposition may be performed at varying rates to
produce fullerene compositions having different degrees of
crystallinity, as desired, forming an electrode which then may be
assembled into lithium ion secondary cells. All of the
fullerene-containing materials are kept away from atmospheric air
or sources of oxygen during all stages of manufacture, including
synthesis, purification, sublimation and assembly. This specific
technique allows for better electrode thickness and fullerene
crystallinity control than any prior method and permits better
control of the electronic properties of the electrodes.
[0033] Film electrodes may be synthesized from anaerobically
produced fullerenes, e.g., C.sub.74. The C.sub.74 may be placed in
an electrically conductive organic solvent or solution and
electrolytically reduced at a voltage of -1.0V versus an AgNO.sub.3
reference electrode. Suitable organic solvents include CH.sub.3CN,
toluene or benzene with an electrolyte such as TBAPF.sub.6, or
TBAPF.sub.6-doped CH.sub.3CN. The electrolytic reduction causes
C.sub.74.sup.n- and some other fullerene anions to dissolve into
solution. Particulate material is removed from the solution and the
dissolved fullerene compounds may be redeposited onto a high
surface area cathode. A voltage of +0.4V (versus AgNO.sub.3
reference electrode) will cause C.sub.74 .sup.n- anions to
redeposit, leaving the remaining fullerides in solution. Purified
fullerene compounds on the high surface area cathode may be
redissolved into any desired electrolytic solution and then
redeposited onto the surface to be used in the electrode. See
Example 4.
[0034] Varying thicknesses of the C.sub.74 material can be achieved
by replenishing the C.sub.74 material-containing electrolyte
solution or by slowly ramping the current to the system. The
resulting electrode may then be placed in a (Li.sub.2CoO.sub.2 (or
other lithium ion source) .parallel. polymer electrolyte .parallel.
C.sub.74 material) cell for testing. Completely uniform films of
fullerene-based compounds may be prepared by gas phase deposition,
or chemical vapor deposition (CVD) onto the surface of graphite
anodes. Multiple layers of cathode and anode then may be stacked in
an orderly manner, occupying very little space.
[0035] High carbon content anodes with improved performance
according to the invention may be constructed using fullerene
copolymers as well as fullerene monomers. These fullerenes are
polymerized in either straight-chain (linear) or crosslinked (star)
form. A conductive, highly reduced, porous, rubber-like fullerene
material which easily accommodates intercalation and
deintercalation of lithium cations results from star polymers.
Electrically conductive linear fullerene polymer compounds,
composed of C.sub.60, C.sub.70 or C.sub.74 monomers or a mixture
thereof, including derivatives of C.sub.60, C.sub.70 or C.sub.74,
also may be used. Linear copolymers of fullerenes may be
synthesized according to methods available in the prior art, for
example in Loy and Assink, J. Am. Chem. Soc. 114:3977-3978, 1992
(C.sub.60-p-xylylene copolymer), the disclosures of which are
incorporated by reference.
[0036] Fullerene polymers having electrically conductive linking
structures which crosslink between fullerene monomers in the
polymeric structure are especially preferred. Most crosslinking
species having this property have a conjugated (alternating double
and single bonds) backbone. Preferably, there are about 3 to about
20 atoms linking the fullerene monomers. Any number of atoms which
allows for complete pi-bond conjugation from fullerene to fullerene
(alternating single and double bonds) is suitable. Any fullerene
type may be used, however fluorinated fullerenes generally are not
used for this application. Specific conductive polymer types which
are useful include polyaniline (PANI), poly(p-phenylene) (PPP),
polyacetylene (PA), polythiophene (PT), polypyrrole,
polyisothionaphthene, polyethylenedioxythiophene (PEDOT),
poly(phenylenevinylene), substituted derivations thereof and
copolymers thereof. An exemplary alternating co-polymer is
bis-EDOT-arylene. (Sotzing et al., Chem. Mater. 8:882, 1996).
[0037] A conductive star polymer also may be synthesized in which
two or more different conjugated arms radiate from the central
core. The repeating units of these polymers include not only
polymers with hundreds or thousands of repeating units, but also
oligomers with 4-10 repeating units. Thus, the fullerene polymers
suitable for use with this invention include those with four to
about 10,000 repeating units.
[0038] Without wishing to be bound by theory, it is believed that
these fullerene polymer compounds are particularly advantageous for
use as anodes because the linking structures hold the fullerene
monomers apart so that lithium ions may move freely through the
structure. Therefore preferred polymers have a structure open
enough for the ions to move freely, but not so open that space is
wasted and charge density within the compound is sacrificed.
[0039] Fullerene polymers are superior electrode materials for
several reasons. First, fullerene polymers can hold a large
electrical potential on each fullerene without chemically degrading
and the polymers provide a low resistance, evenly ordered conduit
for this high potential to reach the battery terminal. Second, the
higher degree of order brought about by the polymer can distribute
charge more evenly throughout the electrode than randomly
structured, pyrolyzed carbon materials. Third, the HOMO-LUMO gap
for fullerenes is similar to graphite and can be tailored to the
engineering needs of a target cell. Fourth, crosslinked fullerene
polymers can be made sufficiently porous and regular to allow
improved lithium ion intercalation.
[0040] Most desirable polymers are those which exhibit long range
ordering of the polymers in the solid state. Polymer structures
which promote such long range ordering and also which favor
interchain charge hopping with favorable intermolecular spacings
and pi overlap are thus advantageous and, generally preferred among
the fullerene polymer compounds. Examples of such compounds are the
poly(3-alkylthiophenes), and other compounds which may be
regioregular, such as 3-substituted polypyrroles and
1,4-polythiophenes. In general, any conductive star polymer may be
used in the compositions, electrodes and batteries of this
invention, including those described in U.S. Pat. No. 6,025,462 to
Wang and Rauh, which is hereby incorporated by reference. C.sub.74
or other fullene copolymers may be synthesized with polyaniline,
poly(p-phenylene), polyacetylene, polythiophene, polypyrrole,
polyisothionaphthene, polyethylenedioxythiophene,
poly(phenylenevinylene), substituted derivatives thereof, or
copolymers thereof. Preferably, regular intervals are maintained
between fullerenes in the polymer structure.
[0041] Polymeric fullerenes may be composed of any fullerene
monomer or combination of monomers, however, C.sub.74 has extremely
favorable electrical conductivity properties such as its degenerate
valency orbitals and therefore is preferred. Due to the multiple
orbital degeneracies of C.sub.74, and the electrically conductive
links between each C.sub.74 molecule, this electrode material
provides greatly improved, more regular
intercalation/deintercalation sites for lithium ions over prior
carbon electrode art. These electrodes provide almost no electrical
resistance whatsoever, and as a result, can produce the most
favorable intercalation-deintercalation sites for lithium ions of
any known fullerene. Lithium ion intercalation capability is
dramatically improved compared with either conventional porous
graphitic battery anodes or anodes made with partially oxidized (or
air-exposed) fullerene-containing material.
[0042] Therefore, it is important to note that the fullerene
monomers must be handled exclusively in an oxygen-free environment
to retain their unique electrochemical activity. Forming these
compounds in an inert atmosphere using fullerenes which have never
been exposed to oxygen, results in a product which is surprisingly
improved. Once the polymer is synthesized, exposure to oxygen does
not harm the material.
[0043] Fluorinated C.sub.74 has unexpectedly advantageous
electrical properties which make it useful for the manufacture of
cathodes. Fluorinated graphite intercalated with fluorinated
C.sub.74 has greatly improved properties, providing easier and more
regular lithium ion intercalation and deintercalation into and from
the cathode. Similar efforts using fluorinated graphite alone (also
known as fluorinated carbon, CF.sub.8) have met with some success,
but the structure of the material was inherently difficult to
control, for the reasons discussed. Fluorinated C.sub.74, however,
possesses unique electrical and redox properties which allow
C.sub.74-based cathodes to provide a lithium ion source in which
more lithium ions are present in the material and more lithium ions
are able to exit the cathode during charging than most inorganic
salts or oxides (the standard in this industry). In addition, the
lithium ions are far more easily dissociated from the cathode for
ion migration.
[0044] C.sub.74 may be directly fluorinated by exposure to F.sub.2
gas according to methods known in the prior art. Once fluorinated,
the fullerene material is white in color and is air inert.
Therefore, once fluorinated, further manipulation of this material
may be carried out in air atmosphere, although special attention
should be paid to keep conditions free of water to prevent water
contamination of the final battery. Direct fluorination yields a
product which may contain from about 2 up to 74 atoms of fluorine
per C.sub.74 molecule. The term "fluorinated C.sub.74" therefore
encompasses any degree of fluorination, including
C.sub.74F.sub.74-2, wherein x=0-16. On average, the degree of
fluorination is about 48 fluorine atoms per C.sub.74 molecule,
resulting in C.sub.74F.sub.48. Intercalates made up of fluorinated
graphite and fluorinated C.sub.74 can be made in the same manner
described above for non-fluorinated compounds.
[0045] The following examples are provided to illustrate the
invention described above and are not intended to be limiting.
EXAMPLES
Example 1
C.sub.74 Synthesis
[0046] Fullerenes are synthesized by the carbon-arc
(Kraetschmer-Huffman) method, and sublimed directly from the soot
onto a water-cooled target under vacuum at 750.degree. C. The
resulting sublimate is collected and washed 3 times with degassed
o-xylene under a helium atmosphere using 1 ml or more o-xylene for
every milligram of material washed or until the wash is colorless.
This wash removes fullerenes which are soluble in organic solvents
in their neutral state, leaving C.sub.74 and some other compounds
which are insoluble under these conditions. Following washing, the
solvent is removed by heating the resulting insoluble material
under vacuum for 2 hours. The solid then is suspended in a degassed
0.1 M solution of tetrabutylammonium hexafluorophosphate
(TBA.sup.+PF.sub.6) electrolyte in acetonitrile (CH.sub.3CN). The
resulting solution then is electrochemically reduced at -1.0V
versus a Ag/AgNO.sub.3 reference electrode. Both the working
(cathode) and the auxiliary (anode) electrodes are platinum. The
working electrode is a high surface area platinum mesh electrode.
After the cell is equilibrated at -1.0V, the electrodes are removed
from the solution containing numerous different fullerene anions in
solution, and the electrolyte solution is filtered with a 0.45
.mu.m membrane filter. The solution then is reoxidized at +0.4V
using the same electrochemical system. Reoxidation precipitates
near-pure C.sub.74 onto the surface of the working electrode,
although some minor impurities of C.sub.76through C.sub.100 are
present. See FIG. 4 for a schematic diagram of this method.
Example 2
Fullerene-Graphite Anodes for Lithium Secondary Batteries
[0047] Lithium ion battery carbon anodes with improved electrical
conductivity, greater electron density storage (mAh/g), and
intercalation/deintercalation properties over standard carbon or
air-exposed fullerene anodes are synthesized using fullerenes as
follows. Purified C.sub.74, which has been synthesized with no
exposure to oxygen is mechanically ground together with graphite
(10% C.sub.74, 90% graphite by weight) under an inert atmosphere.
The mixture then is slowly heated to 500.degree. C. at a rate of
1.degree. C./min under vacuum in a sealed container. The mixture is
held at 500.degree. C. for 8 hours, and then cooled at a rate of
5.degree. C./min to room temperature. This mixture is then
mechanically combined with polyethylene binder material (10%
binder, 90% fullerene graphite mixture by weight) and pressed onto
a metal electrode terminal at a pressure of 50 atm for 2 hours
under vacuum using a hydraulic press heated to 50.degree. C., and
then allowed to come to room temperature. The resulting electrode
may be used as an anode in whatever batter configuration is
desired. The fullerene materials are kept away from atmospheric air
during all stages of manufacture, including synthesis,
purification, mixing, pyrolysis, pressing, storage and
assembly.
Example 3
C.sub.74 Polymer Anodes
[0048] Purified C.sub.74 is mechanically mixed with
bis-EDOT-arylene in a ratio of 10% C.sub.74 to 90% bis-EDOT-arylene
by weight in an inert atmosphere and reacted according to prior art
methods. The resulting C.sub.74 copolymer ("star polymer") is then
mixed with a binder material as described in Example 1 and pressed
with a heated hydraulic press at 50.degree. C. and a pressure of 50
atm onto a metal battery terminal for 2 hours. Although the
C.sub.74 monomer is not exposed to oxygen prior to polymerization,
the polymer may be exposed to air without harm. After cooling to
room temperature, the anode may be assembled into a lithium ion
battery.
Example 4
Fluorinated Fullerene Cathode Materials for Lithium Ion Secondary
Batteries
[0049] Materials for fullerene-based lithium ion battery cathodes
are synthesized by direct fluorination of C.sub.74. C.sub.74 is
dried under dynamic vacuum (<1 torr) for 8 hours with heating,
and placed in a pressure vessel. The vessel is filled with 1 atm
F.sub.2 gas and then heated to 100.degree. C. for 48 hours with
mechanical agitation of the solid. The solid, off-white material
which results has been chemically altered (fluorinated) to produce
an air-inert material which can then be exposed to oxygen. This
material is mechanically mixed with fluorinated carbon (CF.sub.8)
and a polyethylene binder in a mixture containing 10% fluorinated
C.sub.74, 7% polyethylene binder material and 83% CF.sub.8. This
mixture then is pressed with a heated hydraulic press at 50.degree.
C. and a pressure of 50 atm onto a metal battery terminal for 2
hours. The resulting electrode is removed and placed in a cell as
the anode with a lithium metal cathode in an electrolyte solution
composed of tetrabutylammonium hexafluorophosphate electrolyte in
acetonitrile. A +0.1V potential (with respect to the Li/Li.sup.+
couple) is placed on the system, and lithium ions are allowed to
migrate to the fluorinated fullerene electrode until the system has
come to equilibrium. The fullerene-based electrode then is removed,
washed with clean acetonitrile and dried. The electrode is now
ready to be placed in a battery as the cathode (lithium ion
source).
* * * * *