Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-01-22
2002-03-19
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C423S449400
Reexamination Certificate
active
06358649
ABSTRACT:
The present invention concerns a fluorinated carbon, a method for its preparation, and its utilization as active electrode material.
Fluorinated carbons, which comprise the fluorides of carbon and the carbon-fluorine intercalation compounds, are known for their applications as lubricants on the one hand, and as cathode materials in lithium electrochemical generators on the other hand. The first application is based on the low surface energies stemming from the presence of hydrophobic C—F groups. The second application is permitted by the relative facility of intercalation of lithium ions during the process of discharge of the generator, as well as by the marked reducing character of lithium as regards the C—F bond.
Various methods are known for the preparation of fluorinated carbons, and give different products.
Carbon fluorides have been obtained by direct fluorination. A process of direct fluorination of carbon was described in 1934 [O. Ruff et al., Z. Anorg. Allgem. Chem., 217, 1(1934)]. It enabled a gray compound to be obtained, of composition CF
0.92
. A process of direct fluorination of graphite at temperatures of 410° C. to 550° C. enables a series of fluorinated carbons to be obtained, the composition of which was from CF
0.676
to CF
0.988
[W. Rüdorff et al., Z. Anorg. Allgem. Chem., 253, 281(1947)]. The carbon fluorides thus obtained correspond to a structure (CF)
n
in which the layers of carbon are constituted by an infinite network of hexagonal “chair form” cycles bonded together by an sp
3
type of bond. The C—F bond is purely covalent and the compounds are electrical insulators. Another method by direct fluorination, permitting a carbon fluoride having the formula (C
2
F)
n
to be obtained, and in which the C—F bond is a covalent bond, was described by Y. Kita et al., J. Am. Chem. Soc., 101,3832 (1979).
The compounds thus obtained by direct fluorination, (CF)
n
in particular, are utilized at present as cathode material in commercial lithium batteries. These batteries discharge around 2.4 V to 2.5 V at a current density of about 1 mA/cm
2
.
The essential characteristic of graphitic fluorides is the high energy of the C—F bond, which can be measured by, for example, ESCA (XPS) spectrometry and which gives values of the most intense peaks of the line F
1s
≧688.5 eV and the line C
1s
≧290 eV. Furthermore, the sp
3
hybridization of carbon brings about an increase of the length of the C—C bond within the hexagon. The parameter a of the structure, which is 2.46 Å in graphite, exceeds 2.50 Å in (C
2
F)
n
and (CF)
n
[N. Watanabe et al., “Graphite Fluorides”, Elsevier (1988), p. 50]. Moreover, in all the cases of direct fluorination of a natural or synthetic graphite or of a coke, the temperature of fluorination is necessarily greater than 400° C. if it is desired to obtain a compound which is rich in fluorine (F/C>0.6) which can be used in a battery.
Carbon-fluorine intercalation compounds, in which F/C<0.5, have been obtained by various methods of fluorination of carbon at ambient temperature. A first method consists of reacting graphite with a gaseous mixture of F
2
+HF, and permits stage 1 intercalation compounds to be obtained with a composition going from C
4
F to C
3.6
F [W. Rüdorff et al., Chem. Ber., 80, 417 (1947)]. The stage s denotes the number of layers of carbon separating two successive layers of fluorine. Thus a compound of stage 1 has a sequence of lamination of the layers as C/F/C/F . . . , and a compound of stage 2 has the sequence F/C/C/F/C/C . . . .
Another known method consists of causing graphite to react with fluorine in the presence of HF or of a metallic fluoride such as LiF, SbF
5
, WF
6
, CuF
2
or AgF, and enables intercalation compounds C
x
F of stages 1 to 4 to be obtained, with 2≦x≦16 [T. Nakajima, et al., Z. Naturforsch. 36b, 1419 (1981)]. Likewise, the synthesis is known of similar products of stage 1 to stage 4 having a composition of C
2
F to C
10
F, by reaction at ambient temperature of graphite with an atmosphere of F
2
containing a small quantity of a fluoride HF, AsF
5
, IF
5
or OsF
6
. In all the intercalation compounds thus obtained, the F/C ratio is at most equal to 0.5. Now it is known that the capacity of a generator containing a fluorinated carbon as active material of an electrode increases with the proportion of fluorine. A proportion of fluorine lower than 0.5 is thus insufficient. Moreover, the fluorine contained in these compounds is less strongly bound to carbon, permitting it to have a mobility between the planes. Because of this, the fluorine can disintercalate, become dissolved in the electrolyte, and react with the lithium electrode, bringing about a phenomenon of self-discharge. These intercalation compounds prepared in the presence of HF or of a metal fluoride have an ionic character when the fluorine content is very low (F/C<0.1), or an iono-covalent character for higher fluorine contents (0.2<F/C<0.5). In any case, the bonding energy measured by ESCA gives a value less than 687 eV for the most important peak of the F
1s
line and a value less than 285 eV for that of the C
1S
line [T. Nakajima, Fluorine-carbon and Fluoride-carbon, Chemistry, Physics and Applications, Marcel Dekker (1995) p.13]. Moreover, the carbon remains hybridized in sp
2
and the crystallographic parameter a in the plane remains in the neighborhood of 2.46 Å as in the case of graphite.
Carbon-fluorine intercalation compounds in which F/C>0.5 have been obtained by a method of fluorination by means of a gaseous mixture of HF, F
2
, and a metallic or non-metallic fluoride MF
n
[A. Hamwi et al., Synt. Metals, 26, 89 (1988]. This method enables compounds of formula CF
x
M
y
to be obtained, of stage 1, having a F/C ratio comprised between 0.52 and 0.8 and a M/C ratio comprised between 0.02 and 0.06. This method has a disadvantage, however, for the preparation of fluorinated carbons intended to be used as active electrode material. The compounds obtained have relatively little stability when used as active material of an electrode in an electrochemical generator having a lithium negative electrode. This instability becomes evident as a loss of capacity of the battery by self-discharge, particularly at higher temperatures. This loss is principally due to the high content of the impurity M. Moreover, the compounds in which the value of y is very low, near to the lower limit y=0.02, are difficult to obtain by this method.
The present invention has as its object to provide a new fluorinated carbon having a lower proportion of impurities and thus a good stability when used in an electrochemical generator having a lithium negative electrode, as well as a sufficient fluorine content for its use in an electrochemical generator.
The invention thus has as its object a fluorinated carbon, a method for its preparation, and its utilization as active material of the positive electrode in a battery whose negative electrode is a lithium electrode.
The fluorinated carbon of the present invention is characterized in that:
it corresponds to the formula CF
x
M
y
in which x>0.6, y<0.018, and M represents an element chosen from among I, Cl, Br, Re, W, Mo, Nb, Ta, B, Ti, P, As, Sb, S, Se, Te, Pt, Ir and Os;
its crystallographic parameter a, corresponding to the line (100) of the lattice, is such that 2.46 Å≦a≦2.49 Å;
the C—F bond energy is characterized by the strongest lines F
1s
, and C
1s
at positions such that 687.5 eV≦F
1s
≦688.5 eV and 287 eV≦C
1s
≦290 eV, in the ESCA spectra.
Such a bond energy corresponds to an iono-covalent type of bond.
The method for preparing a fluorinated carbon of the invention is characterized in that:
during a first step, a carbon compound chosen from natural or synthetic graphites and graphitizable carbons with a mosaic texture by a thermal treatment are reacted with a gaseous mixture (HF+F
2
), in the presence of a fluoride MF
n
at a temperature between 15°
Hamwi Andre
Hany Pascal
Yazami Rachid
Burns Doane Swecker & Mathis L.L.P.
Centre National de la Recherche Scientifique
Chaney Carol
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