U.S. patent application number 12/489565 was filed with the patent office on 2009-10-15 for subfluorinated graphite fluorides as electrode materials.
This patent application is currently assigned to CALIFORNIA INSTITUTE OF TECHNOLOGY. Invention is credited to Andre HAMWI, Rachid YAZAMI.
Application Number | 20090258294 12/489565 |
Document ID | / |
Family ID | 46062847 |
Filed Date | 2009-10-15 |
United States Patent
Application |
20090258294 |
Kind Code |
A1 |
YAZAMI; Rachid ; et
al. |
October 15, 2009 |
Subfluorinated Graphite Fluorides as Electrode Materials
Abstract
Subfluorinated graphite fluorides of formula CF.sub.x wherein x
is in the range of 0.06 to 0.63, e.g., 0.10 to 0.46, are used as
electrode materials in electrochemical devices that convert
chemical energy to electrical current, e.g., batteries. The
invention additionally provides methods of manufacturing electrodes
with the subfluorinated graphite fluorides, as well as primary and
secondary batteries containing such electrodes.
Inventors: |
YAZAMI; Rachid; (Los
Angeles, CA) ; HAMWI; Andre; (Clermont-Ferrand,
FR) |
Correspondence
Address: |
GREENLEE WINNER AND SULLIVAN P C
4875 PEARL EAST CIRCLE, SUITE 200
BOULDER
CO
80301
US
|
Assignee: |
CALIFORNIA INSTITUTE OF
TECHNOLOGY
Pasadena
CA
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
Paris Cedex 16
|
Family ID: |
46062847 |
Appl. No.: |
12/489565 |
Filed: |
June 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11422564 |
Jun 6, 2006 |
7563542 |
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12489565 |
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11253360 |
Oct 18, 2005 |
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11422564 |
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60724084 |
Oct 5, 2005 |
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60724084 |
Oct 5, 2005 |
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Current U.S.
Class: |
429/217 ;
429/231.7 |
Current CPC
Class: |
H01M 4/382 20130101;
H01M 4/38 20130101; H01M 2004/021 20130101; Y02E 60/10 20130101;
H01M 6/16 20130101; H01M 4/133 20130101; H01M 4/587 20130101; H01M
4/621 20130101; H01M 4/5835 20130101; H01M 4/405 20130101; H01M
4/623 20130101; H01M 10/052 20130101; H01M 2004/028 20130101; H01M
4/625 20130101; H01M 2004/027 20130101; H01M 4/02 20130101; H01M
4/583 20130101; H01M 4/1393 20130101 |
Class at
Publication: |
429/217 ;
429/231.7 |
International
Class: |
H01M 4/58 20060101
H01M004/58; H01M 4/62 20060101 H01M004/62 |
Claims
1. An electrochemical device comprising an anode, a cathode, and an
ion-transporting material therebetween, wherein the cathode
comprises a subfluorinated graphite fluoride of formula CF.sub.x in
which x is in the range of 0.06 to 0.63.
2. The device of claim 1, wherein x is in the range of 0.06 to
0.52.
3. The device of claim 2, wherein x is in the range of 0.10 to
0.52.
4. The device of claim 3, wherein x is in the range of 0.10 to
0.46.
5. The device of claim 4, wherein x is in the range of 0.33 to
0.46.
6. The device of claim 1, wherein the subfluorinated graphite
fluoride comprises a particulate material.
7. The device of claim 6, wherein the subfluorinated graphite
fluoride has an average particle size in the range of about 1
micron to about 10 microns.
8. The device of claim 7, wherein the subfluorinated graphite
fluoride has an average particle size in the range of about 4
microns to about 7.5 microns.
9. The device of claim 8, wherein the subfluorinated graphite
fluoride has an average particle size of about 4 microns.
10. The device of claim 1, wherein the subfluorinated graphite
fluoride is in a composition further comprising a conductive
diluent and a binder.
11. The device of claim 10, wherein the conductive diluent is
selected from acetylene black, carbon black, powdered graphite,
cokes, carbon fibers, metallic powders, and combinations
thereof.
12. The device of claim 11, wherein the conductive diluent is
acetylene black.
13. The device of claim 10, wherein the binder is polymeric.
14. The device of claim 13, wherein the binder is a fluorinated
hydrocarbon polymer.
15. The device of claim 1, wherein the anode comprises a source of
ions of a metal selected from Groups 1, 2, and 3 of the Periodic
Table of the Elements.
16. The device of claim 15, wherein the ions are lithium ions.
17. The device of claim 16, wherein the source of lithium ions is
selected from lithium metal, a lithium alloy, and a carbon-lithium
material.
18. The device of claim 17, wherein the source of lithium ions is
lithium metal.
19. The device of claim 1, wherein the ion-transporting material
physically separates the anode and the cathode and prevents direct
electrical contact therebetween.
20. An electrochemical device comprising an anode, a cathode, and
an ion-transporting material therebetween, wherein the cathode
comprises a subfluorinated graphite fluoride of formula CF.sub.x in
which x is in the range of 0.06 to 0.63; wherein said
subfluorinated graphite fluoride of said cathode is made by
contacting a graphite powder having an average particle size in the
range of 1 micron to 10 microns with a gaseous source of elemental
fluorine at a temperature in the range of 375.degree. C. to
400.degree. C. for a time period of 5 to 80 hours.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/422,564, filed on Jun. 6, 2006 and
published as Publication No. US2007/0077495 on Apr. 5, 2007, which
is a Continuation-in-Part of U.S. patent application Ser. No.
11/253,360, filed Oct. 18, 2005, which claims the benefit under 35
U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/724,084
filed on Oct. 5, 2005, and U.S. patent application Ser. No.
11/422,564 also directly claims the benefit under 35 U.S.C. 119(e)
of U.S. Provisional Patent Application No. 60/724,084 filed on Oct.
5, 2005; all of which are hereby incorporated by reference in their
entireties to the extent not inconsistent with the disclosure
herein.
FIELD OF THE INVENTION
[0002] This invention relates generally to electrode materials, and
more particularly relates to the use of fluorinated carbon,
particularly subfluorinated graphite fluorides, as electrode
materials in electrochemical devices for generating electrical
current, e.g., lithium batteries.
BACKGROUND OF THE INVENTION
[0003] Since the pioneering work of Ruff et al. (1934) Z. Anorg.
Allg. Chem. 217:1, and of Rudorff et al. (1947) Z. Anorg. Allg.
Chem. 253:281, graphite has been known to react with elemental
fluorine at high temperatures to yield graphite fluoride compounds
of general formula (CF.sub.x).sub.n. Systematic studies on the
fluorination reaction later showed that the resulting F/C ratio is
largely dependent on the fluorination temperature, the partial
pressure of the fluorine in the fluorinating gas, and physical
characteristics of the graphite precursor, including the degree of
graphitization, particle size, and specific surface area. See
Kuriakos et al. (1965) J. Phys. Chem. 69:2272; Nanse et al. (1997)
Carbon 35:175; Morita et al. (1980) J. Power Sources 5:111;
Fujimoto (1997) Carbon 35:1061; Touhara et al. (1987) 2. Anorg.
All. Chem. 544:7; Watanabe et al. (1974) Nippon Kagaku Kaishi 1033;
and Kita et al. (1979) J. Am. Chem. Soc. 101:3832.
[0004] The crystal structure of highly fluorinated graphite
fluorides, i.e., (CF.sub.x).sub.n compounds with x>>0.5, has
been investigated by several groups (Nakajima et al., Graphites,
Fluorides and Carbon-Fluorine Compounds, CRC Press, Boca Raton,
Fla., p. 84; Charlier et al. (1994) Mol. Cryst. Liq. Cryst.
244:135; Charlier et al. (1993), Phys. Rev. B 47:162; Mitkin et al.
(2002) J. Struct. Chem. 43:843; Zajac et al. (2000) J. Sol. State
Chem. 150:286; Gupta et al. (2001) J. Fluorine Chem., 110-245;
Ebert et al. (1974) J. Am. Chem. Soc. 96:7841; Pelikan et al.
(2003) J. Solid State Chem. 174:233; and Bulusheva et al. (2002)
Phys. Low-Dim. Struct. 718:1). The Watanabe group first proposed
two phases: a first stage, (CF.sub.1).sub.n, and a second stage,
(CF.sub.0.5).sub.n the latter also commonly referred to as
(C.sub.2F).sub.n (Touhara et al., supra). In first stage materials,
the fluorine is intercalated between each carbon layer to yield
stacked CFCF layers, whereas in second stage materials, fluorine
occupies every other layer with a stacking sequence of CCFCCF.
Hexagonal symmetry was found to be preserved in both
(CF.sub.1).sub.n and (CF.sub.0.5).sub.n phases. Theoretical crystal
structure calculations were also carried out and different layer
stacking sequences were compared using their total energy (Charlier
et al. (1994), supra; Charlier et al. (1993) Phys. Rev. B 47:162;
and Zajac et al., Pelikan et al., and Bulusheva et al., all
supra).
[0005] (CF.sub.x).sub.n compounds are generally non-stoichiometric
with x varying between .about.0 and .about.1.3. For x<0.04,
fluorine is mainly present on the surface of the carbon particles
(Nakajima et al. (1999) Electrochemica Acta 44:2879). For
0.5.ltoreq.x.ltoreq.51, it has been suggested that the material
consists of a mixture of two phases, (CF.sub.0.5).sub.n and
(CF.sub.1).sub.n. "Overstoichiometric compounds," wherein
1.ltoreq.x.ltoreq..about.1.3, consist of (CF.sub.1).sub.n with
additional perfluorinated --CF.sub.2 surface groups (Mitkin et al.,
supra). Surprisingly, although they have been reported in the
literature (Kuriakos et al., supra; Nakajima et al. (1999)
Electrochemica Acta 44:2879; and Wood et al. (1973) Abs. Am. Chem.
Soc. 121), covalent type (CF.sub.x).sub.n materials with x<0.5
have not been investigated in view of their crystal structure
characterization. One possible reason of the focus on the
fluorine-rich materials comes from their potential application as
lubricants and as cathode materials for primary lithium batteries.
In fact, for the latter application, the energy density of the
battery, which is determined by its discharge time at a specific
rate and voltage, has been found to be an increasing function of
x.
[0006] The cell overall discharge reaction, first postulated by
Wittingham (1975) Electrochem. Soc. 122:526, can be schematized by
equation (1):
(CF.sub.x).sub.n+xnLinC+nxLiF (1)
[0007] Thus, the theoretical specific discharge capacity Q.sub.th,
expressed in mAhg-.sup.1, is given by equation (2):
Q th ( x ) = xF 3.6 ( 12 + 19 x ) ( 2 ) ##EQU00001##
where F is the Faraday constant and 3.6 is a unit conversion
constant.
[0008] The theoretical capacity of (CF.sub.x).sub.n materials with
different stoichiometry is therefore as follows: x=0.25,
Q.sub.th=400 mAhg-.sup.1; x=0.33, Q.sub.th=484 mAhg-.sup.1; x=0.50,
Q.sub.th=623 mAhg-.sup.1; x=0.66, Q.sub.th=721 mAhg-.sup.1; and
x=1.00, Q.sub.th=865 mAhg-.sup.1. It is interesting to note that
even a low fluorine-containing (CF.sub.0.25).sub.n material yields
a higher theoretical specific capacity than MnO.sub.2, i.e., 400
mAhg-.sup.1 versus 308 mAhg-.sup.1, respectively. Despite the
higher capacity, longer shelf life (on the order of 15 years), and
substantial thermal stability of (CF.sub.0.25).sub.n, MnO.sub.2 is
the most widely used solid state cathode in primary lithium
batteries, in part because of lower cost, and in part because of a
higher rate capability.
[0009] The lower rate performance of Li/(CF) batteries is
presumably due to the poor electrical conductivity of the
(CF).sub.n material. In fact, the fluorination of graphite at high
temperature (typically 350.degree. C..ltoreq.T.ltoreq.650.degree.
C.) induces a dramatic change in the stereochemical arrangement of
carbon atoms. The planar sp.sup.2 hybridization in the parent
graphite transforms into a three-dimensional sp.sup.3 hybridization
in (CF.sub.x).sub.n. In the latter, the carbon hexagons are
"puckered," mostly in the chair conformation (Rudorff et al.,
Touhara et al., Watanabe et al., Kita et al., Charlier et al.,
Charlier et al., Zajac et al., Ebert et al., Bulusheva et al., and
Lagow et al., all cited supra). Electron localization in the C--F
bond leads to a huge drop of the electrical conductivity from
.about.1.7 10.sup.4 Scm.sup.-1 in graphite to .about.10.sup.-14
Scm.sup.-1 in (CF).sub.n (Touhara et al., supra).
[0010] Accordingly, there is a need in the art for electrode
materials that would compensate for the low conductivity of
fluorinated carbon materials while preserving their high thermal
stability and high discharge capacity. Ideally, such electrodes
would enable, for example, the manufacture of lithium batteries
having increased battery performance when discharged, particularly
at high rates.
SUMMARY OF THE INVENTION
[0011] The invention is directed to the aforementioned need in the
art, and is premised on the discovery that electrodes fabricated
with "subfluorinated" carbon materials, e.g., graphite fluorides
CF.sub.x where x is in the range of 0.06 to 0.63, provide increased
battery performance upon discharge at a high rate.
[0012] In one aspect of the invention, then, an electrochemical
device is provided that comprises an anode, a cathode, and an
ion-transporting material therebetween, wherein the cathode
comprises a subfluorinated graphite fluoride of formula CF.sub.x in
which x is in the range of 0.06 to 0.63. The anode includes a
source of ions corresponding to a metal element of Groups 1, 2, or
3 of the Periodic Table of the Elements, e.g., lithium.
[0013] In another aspect of the invention, the aforementioned
electrochemical device is a primary lithium battery in which the
anode comprises a source of lithium ions, the cathode comprises a
subfluorinated graphite fluoride having an average particle size in
the range of about 4 microns to about 7.5 microns, and the
ion-transporting material is a separator saturated with a
nonaqueous electrolyte and physically separates the anode and
cathode and prevents direct electrical contact therebetween.
[0014] In a further aspect of the invention, an electrode is
provided for use in an electrochemical device that converts
chemical energy to electrode current, the electrode comprising a
subfluorinated graphite fluoride having an average particle size in
the range of about 4 microns to about 7.5 microns. Generally, the
subfluorinated graphite fluoride is present in a composition that
additionally includes a conductive diluent and a binder.
[0015] In still a further aspect of the invention, a method is
provided for preparing an electrode for use in an electrochemical
device, comprising the following steps:
[0016] contacting graphite powder having an average particle size
in the range of 1 micron to about 10 microns with a gaseous source
of elemental fluorine at a temperature in the range of about
375.degree. C. to about 400.degree. C. for a time period of about 5
to about 80 hours, producing a subfluorinated graphite fluoride
having the formula CF.sub.x in which x is in the range of 0.06 to
0.63;
[0017] admixing the subfluorinated graphite fluoride with a
conductive diluent and a binder to form a slurry; and
[0018] applying the slurry to a conductive substrate.
[0019] In still a further aspect of the invention, a rechargeable
battery is provided that includes:
[0020] a first electrode comprising a subfluorinated graphite
fluoride of formula CF.sub.x in which x is in the range of 0.06 to
0.63, the electrode capable of receiving and releasing cations of a
metal selected from Groups 1, 2, and 3 of the Periodic Table of the
Elements;
[0021] a second electrode comprising a source of the metal cations;
and
[0022] a solid polymer electrolyte that permits transport of the
metal cations and physically separates the first and second
electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 depicts the thermogravimetric analysis (TGA) curves
of graphite fluorides using a rate of 5.degree. C./minute as
evaluated in Example 2.
[0024] FIG. 2 provides the x-ray diffractometry measurements (XRD)
on the graphite fluorides as determined in Example 2.
[0025] FIG. 3 provides the results of X-ray photoelectron
spectroscopy (XPS) analysis of the graphite fluorides prepared as
described in Example 1 and characterized in Example 2, with the
C.sub.1s peaks in the primary spectrum having been
deconvoluted.
[0026] FIG. 4 is a graph showing a linear relationship between the
degree of fluorination and the C.sub.1s binding energies of the
graphite fluorides prepared as described in Example 1 and
characterized in Example 2.
[0027] FIG. 5 illustrates the discharge profile of the Li/graphite
fluoride cells prepared and evaluated as described in Example
3.
[0028] FIG. 6 illustrates the effect of discharge rate on the
discharge profile for sample CF.sub.0.52, as described in Example
3.
[0029] FIG. 7 is a Ragone plot indicating the performance of all
graphite fluoride cells prepared as described in Example 3.
DETAILED DESCRIPTION OF THE INVENTION
[0030] In one embodiment, the invention provides an electrochemical
device that converts chemical energy to electrochemical current,
such a device being exemplified by a lithium battery. The device
has a cathode, i.e., a positive electrode, comprising a
subfluorinated graphite fluoride; an anode, i.e., a negative
electrode, comprising a source of an ion corresponding to a metal
of Groups 1, 2, or 3 of the Periodic Table of the Elements; and an
ion-transporting material that physically separates the two
electrodes and prevents direct electrical contact therebetween.
[0031] The subfluorinated graphite fluoride is a carbon-fluorine
intercalation compound having an overall formula CF.sub.x wherein x
is in the range of 0.06 to 0.63, preferably in the range of 0.06 to
0.52, more preferably in the range of 0.10 to 0.52, still more
preferably in the range of 0.10 to 0.46, and optimally in the range
of 0.33 to 0.46. The subfluorinated graphite fluoride used in
connection with the present invention is generally a particulate
material, e.g., a powder, wherein the average particle size is
typically 1 micron to about 10 microns, preferably about 4 microns
to about 7.5 microns, and optimally about 4 microns.
[0032] In the electrochemical devices of the invention, the
subfluorinated graphite fluoride is normally present in a
composition that also includes a conductive diluent such as may be
selected from, for example, acetylene black, carbon black, powdered
graphite, cokes, carbon fibers, and metallic powders such as
powdered nickel, aluminum, titanium, and stainless steel. The
conductive diluent improves conductivity of the composition and is
typically present in an amount representing about 1 wt. % to about
10 wt. % of the composition, preferably about 1 wt. % to about 5
wt. % of the composition. The composition containing the
subfluorinated graphite fluoride and the conductive diluent also,
typically, contains a polymeric binder, with preferred polymeric
binders being at least partially fluorinated. Exemplary binders
thus include, without limitation, poly(ethylene oxide) (PEO),
poly(vinylidene fluoride) (PVDF), a poly(acrylonitrile) (PAN),
poly(tetrafluoroethylene) (PTFE), and
poly(ethylene-co-tetrafluoroethylene) (PETFE). The binders, if
present, represent about 1 wt. % to about 5 wt. % of the
composition, while the subfluorinated graphite fluorides represent
about 85 wt. % to about 98 wt. % of the composition, preferably
about 90 wt. % to 98 wt. % of the composition.
[0033] The subfluorinated graphite fluorides are prepared by
fluorination of a graphite material or a graphitizable material
(see U.S. Pat. No. 6,358,649 to Yazami et al.), with powdered
graphite having an average particle size in the range of 1 micron
to about 10 microns being preferred. A particle size of about 4
microns to about 7.5 microns is more preferred, with an
approximately 4 micron particle size being optimal.
[0034] An electrode provided with the aforementioned conductive
composition can be manufactured as follows:
[0035] Initially, the subfluorinated graphite fluoride is prepared
using a direct fluorination method, in which graphite powder
preferably having an average particle size in the range of 1 micron
to about 10 microns is contacted with a gaseous source of elemental
fluorine at a temperature in the range of about 375.degree. C. to
about 400.degree. C. for a time period of about 5 to about 80
hours, preferably about 15 to 35 hours. A subfluorinated graphite
fluoride as described above results. A suitable gaseous source of
elemental fluorine will be known to one of ordinary skill in the
art; an exemplary such source is a mixture of HF and F.sub.2 in a
molar ratio somewhat greater than 1:1, e.g., 1.1:1 to 1.5:1.
[0036] The resulting subfluorinated graphite fluoride is then
admixed with a conductive diluent and binder as described above,
with the preferred weight ratios being about 85 wt/% to about 98
wt. %, more preferably about 90 wt. % to about 98 wt. %,
subfluorinated graphite fluoride; about 1 wt. % to about 10 wt. %,
preferably about 1 wt. % to about 5 wt. %, conductive diluent; and
about 1 wt. % to about 5 wt. % binder.
[0037] Typically, the slurry formed upon admixture of the foregoing
components is then deposited or otherwise provided an a conductive
substrate to form the electrode. A particularly preferred
conductive substrate is aluminum, although a number of other
conductive substrates can also be used, e.g., stainless steel,
titanium, platinum, gold, and the like.
[0038] In a primary lithium battery, for example, the
aforementioned electrode serves as the cathode, with the anode
providing a source of lithium ions, wherein the ion-transporting
material is typically a microporous or nonwoven material saturated
with a nonaqueous electrolyte. The anode may comprise, for example,
a foil or film of lithium or of a metallic alloy of lithium (LiAl,
for example), or of carbon-lithium, with a foil of lithium metal
preferred. The ion-transporting material comprises a conventional
"separator" material having low electrical resistance and
exhibiting high strength, good chemical and physical stability, and
overall uniform properties. Preferred separators herein, as noted
above, are microporous and nonwoven materials, e.g., nonwoven
polyolefins such as nonwoven polyethylene and/or nonwoven
polypropylene, and microporous polyolefin films such as microporous
polyethylene. An exemplary microporous polyethylene material is
that obtained under the name Celgard.RTM. (e.g., Celgard.RTM. 2400,
2500, and 2502) from Hoechst Celanese. The electrolyte is
necessarily nonaqueous, as lithium is reactive in aqueous media.
Suitable nonaqueous electrolytes are composed of lithium salts
dissolved in an aprotic organic solvent such as propylene carbonate
(PC), ethylene carbonate (EC), ethyl methyl carbonate (EMC),
dimethyl ether (DME), and mixtures thereof. Mixtures of PC and DME
are common, typically in a weight ratio of about 1:3 to about 2:1.
Suitable lithium salts for this purpose include, without
limitation, LiBF.sub.4, LiPF.sub.6, LiCF.sub.3SO.sub.3,
LiClO.sub.4, LiAlCl.sub.4, and the like. It will be appreciated
that, in use, an applied voltage causes generation of lithium ions
at the anode and migration of the ions through the
electrolyte-soaked separator to the subfluorinated graphite
fluoride cathode, "discharging" the battery.
[0039] In another embodiment, the subfluorinated graphite fluoride
composition is utilized in a secondary battery, i.e., a
rechargeable battery such as a rechargeable lithium battery. In
such a case, the cations, e.g., lithium ions, are transported
through a solid polymer electrolyte--which also serves as a
physical separator--to the subfluorinated graphite fluoride
electrode, where they are intercalated and de-intercalated by the
subfluorinated graphite fluoride material. Examples of solid
polymer electrolytes include chemically inert polyethers, e.g.,
poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), and other
polyethers, wherein the polymeric material is impregnated or
otherwise associated with a salt, e.g., a lithium salt such as
those set forth in the preceding paragraph.
[0040] It is to be understood that while the invention has been
described in conjunction with the preferred specific embodiments
thereof, the foregoing description as well as the examples that
follow are intended to illustrate and not limit the scope of the
invention. Other aspects, advantages, and modifications within the
scope of the invention will be apparent to those skilled in the art
to which the invention pertains.
[0041] In the following examples, efforts have been made to ensure
accuracy with respect to numbers used (e.g., amounts, temperature,
etc.) but some experimental error and deviation should be accounted
for. Unless indicated otherwise, temperature is in degrees C. and
pressure is at or near atmospheric. All solvents were purchased as
HPLC grade, and all reagents were obtained commercially unless
otherwise indicated.
Example 1
Synthesis of (CF.sub.X).sub.N Materials
[0042] Four samples of (CF.sub.x).sub.n (A, B, C, D) were
synthesized by direct fluorination of a natural graphite powder
obtained from Centre National de la Recherche Scientifique (CNRS,
in Madagascar) and Clermont-Ferrand University Lab (France). The
average particle size for the precursor was 7.5 .mu.m for samples
A, B, and D whereas an average particle size of 4 .mu.m was used
for sample C. The fluorination temperature ranged from 375.degree.
C. to 400.degree. C., and was adjusted to obtain the desired F/C
ratios. A battery grade carbon monofluoride (E) derived from a
petroleum coke was obtained from Advance Research Chemicals Inc.
(ARC, Tulsa, Okla., USA). Table 1 summarizes the synthesis
conditions used for each sample:
TABLE-US-00001 TABLE 1 Synthesis conditions for the
(CF.sub.x).sub.n samples Sample A B C D E Precursor NG NG NG NG
Coke Particle Size ~7.5 .mu.m ~7.5 .mu.m ~4 .mu.m ~7.5 .mu.m ~15-30
.mu.m Temperature 375.degree. C. 380.degree. C. 390.degree. C.
400.degree. C. N/A Duration 17 hrs 32 hrs 20 hrs 80 hrs N/A NG =
natural graphite
Example 2
Physical Characterization of (CF.sub.X).sub.N Materials
Methods:
[0043] Scanning electron microscopy (SEM, JEOL instrument) was
performed to observe the particles' morphology and analyze their
composition via electron-dispersive x-ray (EDX) spectrometry.
Micrographs were taken at various magnifications ranging from
500.times. to 10,000.times..
[0044] The chemical composition of each sample was determined using
several methods. For samples A-D, the weight uptake during the
fluorination reaction was used to determine the F/C ratio. EDX
spectrometry provided semi-quantitative analyses of carbon and
fluorine for all samples. These measurements were acquired on the
SEM JEOL instrument with a Li-drifted Si crystal detector, at a
working distance of 10 mm, and analyzed using INCA software.
Additional elemental analysis was performed for sample E by a
carbonate fusion method at ARC.
[0045] The thermal stability of the material was investigated by
thermogravimetric analysis (TGA) performed on a Perkin Elmer Pyris
Diamond instrument. The weight loss of the material under argon
atmosphere was recorded while it was being heated at a rate of
5.degree. C.min.sup.-1 between 25.degree. C. and 900.degree. C.
[0046] X-ray diffractometry (XRD) measurements were performed on a
Rigaku instrument with CuK.sub..alpha. radiation. Silicon powder
(.about.5 wt. %) was mixed in all samples and used as an internal
reference. The spectra obtained were fitted on Xpert Highscore
software. The resulting profiles were used in combination with
CefRef software to determine the `a` and `c` crystal parameters of
the hexagonal cell (P.sub.-6m2) as proposed by Touhara et al.
(1987) Z. Anorg. All. Chem. 544:7.
Results:
[0047] The scanning electron micrographs showed particle sizes
ranging from about 2 to about 10 .mu.m while the observed particle
size of the commercially available (CF.sub.1).sub.n ranges from 10
to 35 .mu.m. In addition to the particle size, the morphology of
the two groups of samples seemed to differ. The sub-fluorinated
(CF.sub.x).sub.n samples consisted of very thin flakes while the
carbon monofluoride samples were bulkier. This difference
presumably derives from the use of a natural graphite precursor for
samples A, B, C, and D, and a larger petroleum coke precursor for
sample E.
[0048] The weight uptake during the fluorination of the graphite
materials was converted to an F/C ratio, with the measurements
averaged over a minimum of five different areas of the sample.
Table 2 summarizes the composition results obtained for each sample
and method. The composition of samples A, B, C, and D as determined
by weight uptake and EDX measurement correlated quite closely, as
illustrated by the results set forth in the table. The composition
of sample E as determined by a carbonate fusion method was
identical to that determined by EDX measurements.
TABLE-US-00002 TABLE 2 Chemical composition determined by weight
gain (A-D), EDX (A-E), and carbonate fusion method (E) Sample A B C
D E F/C Ratio Weight Gain 0.33 0.46 0.52 0.63 N/A EDX 0.36 0.47
0.60 0.67 1.08 ARC 1.08
[0049] Given the results summarized in Table 2, samples A, B, C, D,
and E will also be identified hereinafter as CF.sub.0.33,
CF.sub.0.46, CF.sub.0.52, CF.sub.0.63, and CF.sub.1.08,
respectively.
[0050] The TGA traces of all samples are shown in FIG. 1. Below a
temperature of 400.degree. C., materials A-D were found to be very
stable, with less than 1% observed loss of mass. Between
400.degree. C. and 600.degree. C., materials A-D underwent a
noticeable decrease in mass. While the profile was similar for A,
B, and C, material D exhibited a sudden drop in the temperature
range of 525.degree. C. to 580.degree. C. Above 600.degree. C., no
significant loss of mass was observed until about 900.degree. C.,
with the weight decreasing gradually, at a rate of less than 2% per
degree. Material E has the same thermogram profile as material D,
but exhibits somewhat higher thermal stability, beginning to
decompose at about 450.degree. C. and stopping at around
630.degree. C. Table 3 summarizes the TGA results, highlighting a
higher initial weight loss for CF.sub.0.52. While not wishing to be
bound by theory, it is presumed that this is due to the smaller
particle size, and thus larger surface area, of the precursor. More
surface adsorption effects cause greater initial weight loss at
lower temperatures.
TABLE-US-00003 TABLE 3 Summary of the TGA results on the
(CF.sub.x).sub.n powders Sample CF.sub.0.33 CF.sub.0.46 CF.sub.0.52
CF.sub.0.63 CF.sub.1.08 Tempe- 1% wt. Loss 380 374 328 393 426
rature 2% wt. Loss 423 427 403 459 467 After 3% wt. Loss 443 448
433 485 485 wt. % Remaining 55.6 49.7 40.9 35.7 18.4 at 800.degree.
C.
[0051] The XRD patterns, in FIG. 2, show a combination of broad and
sharp peaks, with intensity variations reflecting the difference in
the degree of fluorination. The sharper peaks originate from the
un-fluorinated precursor (graphite for CF.sub.0.33, CF.sub.0.46,
CF.sub.0.52, CF.sub.0.63, and coke for CF.sub.1.08) and are most
evident in samples CF.sub.0.33, CF.sub.0.46, CF.sub.0.52. The
strongest graphite peak (002) is observed at 26.5.degree. with
relative intensity decreasing with x. The broad peaks corresponding
to the fluorinated phase are found at about 10.degree., 25.degree.,
and 40-45.degree. for samples CF.sub.0.33 to CF.sub.0.63, and at
about 13.degree., 26.degree. and 41.degree. for sample CF.sub.1.08.
Table 4 shows the `a` and `c` parameters obtained for the
fluorinated phases assuming a hexagonal lattice structure.
TABLE-US-00004 TABLE 4 Summary of a and c parameters of the
hexagonal unit cell derived from XRD measurements Sample
CF.sub.0.33 CF.sub.0.46 CF.sub.0.52 CF.sub.0.63 CF.sub.1.08 a
({acute over (.ANG.)}) 2.54 2.54 2.54 2.54 2.54 c ({acute over
(.ANG.)}) 16.65 16.55 16.20 16.65 12.70
[0052] The C.sub.1s and F.sub.1s binding energy spectra were
collected and analyzed using X-ray photoelectron spectroscopy
(XPS). Deconvolution of the C.sub.1s, peaks (FIG. 3) revealed two
peaks other than the graphitic peak corresponding to x<1, and
three peaks in addition to the peak found at 285.5 eV
(corresponding to x=1). These peaks correspond to the
sp.sup.3-carbon from the C--F bonds, and the CF.sub.2 or CF.sub.3
bordering the graphene layers. Deconvolution of the F.sub.1s peaks
resulted in two peaks matching the C.sub.1s peaks. FIG. 4 shows a
linear relationship between the degree of fluorination and the
C.sub.1s binding energies.
Example 3
Electrochemical Performance of (CF.sub.X).sub.N Materials
[0053] Conventional 2032 coin cells were assembled to test the
electrochemical performance of the (CF.sub.x).sub.n materials. The
cathode was prepared by spreading a slurry of 5 g (CF.sub.x).sub.n,
0.62 g carbon black, and 0.56 g polytetrafluoroethylene
(PTFE)-based binder on an aluminum substrate. The anode was a
lithium metal disc, and the separator consisted of a microporous
polypropylene Celgard.RTM. 2500 membrane. The thicknesses of the
cathode, anode, and separator were 15 mm, 16 mm, and 17.5 mm
respectively. The electrolyte used was 1.2M LiBF.sub.4 in a 3:7 v/v
mixture of propylene carbonate (PC) and dimethyl ether (DME).
Stainless steel spacers and a wave washer were used to maintain
sufficient pressure inside the coin cell. The coin cells were
discharged on an Arbin instrument by applying a constant current
with a voltage cutoff of 1.5 V. The discharge rates ranged from
0.01 C to 2.5 C, at room temperature. The C-rate calculation was
based on a theoretical capacity Q.sub.th in mAh/g determined by
equation (2). A minimum of three cells were used for each test
condition.
Q th ( x ) = xF 3.6 ( 12 + 19 x ) ( 2 ) ##EQU00002##
[0054] The discharge profile of the Li/(CF.sub.x).sub.n cells is
shown in FIG. 5. While the battery grade carbon monofluoride
exhibited the characteristic plateau around 2.5 V, the discharge
profiles of samples CF.sub.0.33, CF.sub.0.46, CF.sub.0.52 differed
greatly in their voltage and shape. The discharge started at a
higher voltage of about 3 V, dropped to about 2.8 V, then slowly
decreased to about 2.5 V before a sharper drop to 1.5 V. The
discharge curve of sample CF.sub.0.63 falls in between the two
previous groups. In the latter sample the initial voltage is found
at around 2.7 V; the slope of the curve is flatter than that of
CF.sub.0.33, CF.sub.0.46, CF.sub.0.52, but steeper than
CF.sub.1.08. The discharge capacity differed depending on the
discharge rate as well as the F/C ratio. The variations in
potential are presumably due to the difference in the electrical
conductivity of the materials. The existence of an unfluorinated
graphitic phase may result in a higher conductivity between the
fluorinated grains of graphite fluoride, which reduce cathodic
overpotential. As a result, the lower the F/C the higher the
discharge voltage plateau.
[0055] For each material, the increase in the discharge current
caused a decrease in the average discharge voltage and a reduced
capacity. FIG. 6 illustrates the effect of the discharge rates on
the discharge profile for sample CF.sub.0.52. At the lowest
discharge rates (C/100 to C/5), the voltage drops gradually from an
open-circuit voltage of about 3.4 V to 3 V. The initial voltage
drop commonly observed in the fast discharges of
Li/(CF.sub.x).sub.n batteries was observed only for rates of 1 C or
higher. The discharge curves corresponding to 1.5 C, 2 C, and 2.5 C
are very similar in voltage and capacity, and exhibit a significant
voltage drop at the beginning of discharge. Similar effects were
observed for the other materials. Such a drop in the potential for
higher discharge rate is associated with a steep increase in the
overpotential at the higher discharge currents. Again, for the
sub-fluorinated samples, the conductivity of the materials should
be higher than that of the battery grade carbon monofluoride, and,
as a result, the cell over-potential at high discharge rates is
lower.
[0056] In order to compare the performance of the (CF.sub.x).sub.n
materials under different discharge rates, a Ragone plot is
presented in FIG. 7. It shows the achieved energy density E
(Whkg.sup.-1) versus the power density P (Wkg.sup.-1) traces. E and
P are determined from the discharge curves using equations (3) and
(4):
E = q ( i ) .times. < e i > m ( 3 ) P = i .times. < e i
> m ( 4 ) ##EQU00003##
[0057] In the equations for E and P, q(i) and <e.sub.i>
respectively represent the discharge capacity (Ah) and the average
discharge voltage (V) at current i (A), and m is the mass of active
(CF.sub.x).sub.n in the electrode (kg). Note that the P scale in
the Ragone plot is given as P.sup.1/2 for clarity. As expected,
carbon monofluoride exhibited a very high energy density (over 2000
Whkg.sup.-1) for low rates of discharge (<C/10) while the
sub-fluorinated graphites have significantly lower energy
densities. Below 1000 Wkg.sup.-1, the energy density was
approximately proportional to the F/C ratio of the materials.
Beyond that point, the operating voltage and discharge capacity of
carbon monofluoride are drastically reduced causing a large
decrease in the energy density. Similarly, the capacity of
materials A-D is also reduced; however, the operating voltage is
still greater than that of sample E, and the energy density is
greater than 500 Whkg.sup.-1 over 2.5 C.
[0058] Accordingly, the results show that partially fluorinated
graphite fluorides can outperform the traditional fluorinated
petroleum coke as electrodes in electrochemical devices such as
lithium batteries. Although lower fluorination content decreased
specific discharge capacity of the material somewhat, that decrease
was overshadowed by a very substantial increase in battery
performance at high discharge rates.
Example 4
Process for Making of (CF.sub.X).sub.N Materials
[0059] It is an objective of the present invention to provide
methods of making subflourinated carbon materials exhibiting useful
electronic and mechanical properties, particularly for use as
electrode materials for batteries. Methods of the present invention
are useful for making subfluorinated carbon materials having a
carbon to fluoride stoichiometry selected for a particularly
application, for example graphite fluorides, CF.sub.x, where x is
in the range of about 0.06 to about 0.63. The present invention
provides efficient methods for making significant quantities of
high quality graphite fluoride materials.
[0060] To demonstrate these capabilities of the present methods, we
carried out a systematic study of the influence of a number of
important process conditions on the yields and compositions of
graphite fluoride materials synthesized. Specifically, in the
synthesis conditions of CF.sub.x described herein, four main
parameters are considered: [0061] 1. Amounts of graphite in the
reactor (starting materials: Natural graphite from Madagascar of
7.5 .mu.m average grains size, and synthetic graphite from Timcal,
Co., Switzerland, average grains size 15-20 .mu.m) [0062] 2.
Reaction temperature [0063] 3. Time of Reaction [0064] 4. Fluorine
gas flow rate
[0065] In the methods of the present example, the graphite powder
is uniformly spread on a nickel boat with a density of
approximately 1 g/10 cm.sup.2, then it is introduced into the
reactor. The reactor is made of nickel, with a cylindrical shape
and horizontal setting. Its internal volume is about 5.5 liters.
The reactor is vacuum degassed for 2 hours, then fluorine gas is
flown. The fluorine pressure is 1 atmosphere. The reaction proceeds
under fluorine dynamic flow (open reactor). (Important note: if the
reactor is closed (static reactor), the fluorination reaction
becomes much slower.). The reactor is then heated at a rate of 1
degrees Celsius/minute. The reaction time is counted after the
reactor reached the target temperature until the reactor heating is
stopped. After the reactor cools down to the ambient temperature,
excess (unreacted) fluorine was evacuated under nitrogen flow until
no trace of free fluorine is in the reactor.
4.a. Effect of Temperature
[0066] Table 5 shows the yields and compositions of graphite
fluoride materials synthesized for reaction temperatures ranging
from 375 degrees Celsius to 490 degrees Celsius. In these
experiments, the graphite mass is 13 grams, the fluorine gas flow
rate is 1 g/hour and the reaction time is 14 hours.
TABLE-US-00005 TABLE 5 Yields and compositions of graphite fluoride
materials synthesized for temperatures ranging from 375 degrees
Celsius to 490 degrees Celsius Masse of Presence of Experiment
Temperature/ fluorinated graphite n.degree. .degree. C. graphite/g
Composition from XRD 1 375 20.00 CF.sub.0.34 Yes, a lot 2 390 22.67
CF.sub.0.47 Yes 3 400 23.28 CF.sub.0.50 Yes 4 490 27.62 CF.sub.0.71
No
4.b. Effect of the Graphite Mass
[0067] Table 6 shows the yields and compositions of graphite
fluoride materials synthesized for starting graphite masses ranging
from 11 grams to 17 grams. In these experiments, the reaction
temperature is 390 degrees Celsius, the fluorine gas flow rate is 1
g/hour and the reaction time is 17 hours.
TABLE-US-00006 TABLE 6 Yields and compositions of graphite fluoride
materials synthesized for starting graphite masses ranging from 11
grams to 17 grams Masse of Presence of Experiment Masse of
fluorinated graphite n.degree. graphite/g graphite/g Composition
from XRD 5 11 21.0 CF.sub.0.57 Yes 6 15 27.65 CF.sub.0.53 Yes 7 17
31.06 CF.sub.0.52 Yes 8 20 35.56 CF.sub.0.49 Yes
4.c. Effect of the Fluorine Flow Rate
[0068] Table 7 shows the yields and compositions of graphite
fluoride materials synthesized for fluorine gas flow rates ranging
from 0.5 g/hour to 2 g/hour. In these experiments, the reaction
temperature is 390 degrees Celsius, the starting graphite mass is
13 g and the reaction time is 17 hours.
TABLE-US-00007 TABLE 7 Yields and compositions of graphite fluoride
materials synthesized for fluorine gas flow rates ranging from 0.5
g/hour to 2 g/hour Fluorine Presence of Experiment flow rate MASSE
OF graphite n.degree. g/hour GRAPHITE/G Composition from XRD 9 0.5
20.15 CF.sub.0.35 Yes, a lot 10 0.7 23.00 CF.sub.0.48 Yes, a lot 11
0.8 24.57 CF.sub.0.56 Yes 12 1 26.05 CF.sub.0.63 Yes, a few 13 2
26.13 CF.sub.0.64 No, traces
4.d. Effect of the Reaction Time
[0069] Table 8 shows the yields and compositions of graphite
fluoride materials synthesized for reaction times ranging from 10
hours to 40 hours. In these experiments, the reaction temperature
is 390 degrees Celsius, the starting graphite mass is 13 g and the
fluorine gas flow rate is 1 g/hour.
TABLE-US-00008 TABLE 8 Yields and compositions of graphite fluoride
materials synthesized for reaction times ranging from 10 hours to
40 hours Masse of Presence of Experiment Reaction time/ fluorinated
graphite n.degree. hour graphite/g Composition from XRD 14 10 20.50
CF.sub.0.36 Yes, quite a lot 15 14 22.67 CF.sub.0.47 Yes, quite a
lot 16 16 23.10 CF.sub.0.49 Yes, quite a lot 17 18 24.95
CF.sub.0.58 Yes 18 20 26.15 CF.sub.0.64 Very few 19 24 27.00
CF.sub.0.68 No 20 40 27.72 CF.sub.0.71 No
4.E Synthesis of Larger Amounts
[0070] Table 9 shows the results of experiments wherein larger
amounts (e.g., about 55 grams to about 65 grams) of graphite
fluoride materials were synthesized. In these experiments, the
reaction temperature is 390 degrees Celsius, the reaction time is
17 hours and the fluorine gas flow rate is 2 g/hour.
TABLE-US-00009 TABLE 9 The results of experiments wherein larger
amounts (e.g., about 55 grams to about 65 grams) of graphite
fluoride materials were synthesized. Mass of Reaction MASSE OF
Presence of Experiment graphite Temperature/ FLUORINATED graphite
from no. (synthetic)/g .degree. C. GRAPHITE/G Composition XRD 21 30
375 54.77 CF.sub.0.52 Yes, a lot 22 30 390 60.10 CF.sub.0.63 Yes 23
30 490 65.38 CF.sub.0.74 No
* * * * *