Fluorination of multi-layered carbon nanomaterials

Fluoridation of multilayer carbon nanomaterials

Background of the invention

The present invention is in the field of fluorinated multilayer carbon nanomaterials, in particular multipared carbon nanotubes, multilayer carbon nanofibers, multilayer carbon nanoparticles, carbon nanowires and fluorinated carbon nanowires.

Fluorinated carbons are used commercially as a positive electrode material in primary lithium batteries. Graphite fluoridation enables fluorine intercalation between the carbon layers. Other industrial applications of fluorinated carbons include their use as solid lubricants or as reservoirs for highly active molecular oxidants, such as BrF3 and CIF3.

In a lithium / CFx cell, the global discharge reaction of the cell, first postulated by Wittingham (1975) Electrochem. Soc. 122: 526, can be schematized by equation (1):

Thus, the theoretical specific discharge capacity Qth, expressed in mAh • g-1, is given by equation (2):

where F is the Faraday constant and 3.6 is a unit conversion constant.

Therefore, the theoretical capacity of materials (CFx) n with different stoichiometry is as follows: x = 0.25, Qth = 400 mAh • g-1; x = 0.33, Qth = 484 mAh • g-1; x = 0.50, Qth = 623 mAh • g-1; x = 0.66, Qth = 721 mAh • g-1 and x = 1.00, Qth = 865 mAh • g-1.

The reactivity of the allotropic forms of carbon with fluorine gas differs greatly, due to the degree of graffiti or the type of carbon material (Hamwi A. et al .; J. Phys. Chem. Solids, 1996, 57 ( 6-8), 677-688). In general, the higher the degree of graffiti, the higher the reaction temperature. Carbon fluorides have been obtained by direct fluorination in the presence of fluorine or mixtures of fluorine and an inert gas. If graphite is used as a starting material, no significant fluorination is observed below 300 ° C. Between 350 and 640 ° C, two graphite fluorides are formed, which differ mainly in their crystalline structure and composition: poly (dicarbon monofluoride) (C2F) and poly (carbon monofluoride) (CF) n (Nakajima T. ; Watanabe

N. Graphite fluorides and Carbon-Fluorine compounds, 1991, CRC Press, Boston; Kita Y .; Watanabe N .; Fujii Y .; J. Am. Chem. Soc, 1979,101, 3832). In both compounds, carbon atoms adopt sp3 hybridization with associated distortion of carbon hexagons from a flat configuration to a "chair-shaped" or "boat-shaped" configuration. The poly (dicarbon monofluoride) is obtained at 350 ° C and has a characteristic structure, in which two adjacent fluorine layers are separated by two layers of carbon bonded by strongly covalent CC bonds along the c-axis of the reticular structure hexagonal (phase 2). On the other hand, the poly (carbon monofluoride), obtained at ̴ 600 ° C, has a structure with a single carbon layer between two adjacent fluorine layers (phase 1). Graphite fluorides obtained at 350 to 600 ° C have an intermediate composition between (C2F) n and (CF) n and consist of a combination of these two phases (Kita, 1979). Phase s denotes the number of carbon layers that separate two successive layers of fluorine. Thus, a compound of phase 1 has a sequence of stacking layers like FCF / FCF. , while a compound of phase 2 has the sequence FCCF / FCCF It is known that both poly (dicarbon monofluoride) and poly (carbon monofluoride) have a relatively poor electrical conductivity.

The use of fluorinated carbon nanotubes in batteries has been described in the patent literature. Japanese patent publication JP2005285440, Mashushita Electric Ind. Co. Ltd., discloses a non-aqueous electrolyte battery that includes a positive electrode made of a fluorocarbon that includes fluorinated carbon nanotubes and a negative electrode made of materials that can provide a source of lithium ions.

The reaction of multiple wall carbon nanotubes (NCPM) with fluorine has been described in the scientific literature. Hamwi et al. (1997) describe the fluoridation of carbon nanotubes with an outside diameter of between 20 and 40 nm, prepared by thermal decomposition of acetylene on cobalt catalysts supported by silica. Fluorination at an approximate temperature of 500 ° C for four hours in a pure fluorine atmosphere resulted in white compounds, indicative of complete fluorination. (A. Hamwi, H. Alvergnat, S. Bonnamy,

F. Beguin, 1997, Carbon, 35, 723). Touhara et al. (2002) describe the fluorination of template-synthesized carbon nanotubes with an outside diameter of 30 nm at temperatures between 50 ° C and 200 ° C for 5 days under 1 atm of fluorine gas (H. Touhara et al., 2002 , J. Fluorine Chem, 114,181-188).

The reaction of carbon fibers with fluorine has also been described. US Patent 6,841,610 on Yanagisawa et al. describes fluorinated carbon fibers in which the exposed edges of the carbon layers are fluorinated. The virgin carbon fiber starting material had a "spike" structure and an average diameter of approximately 100 nm. A fluorination temperature of 340 ° C, a partial fluorine pressure of 460 mm Hg, a partial nitrogen pressure of 310 mm Hg and a reaction time of 72 hours were described. Touhara et al. (1987) described the reaction of elemental fluorine and carbon fibers grown in steam phase under heat treatment with a diameter of approximately 10 microns at temperatures between 330 ° C and 614 ° C. No residual graphite was confirmed in any of the compounds. The F / C ratios observed were between 0.53 (at 345 ° C) and 0.99 at 614 ° C (Touhara et al 1987 Electrochemica Acta, Vol. 32, No. 2, 293-298).

Carbon-fluorine intercalation compounds have also been obtained by incorporating into the gas mixture other compounds capable of acting as a fluorination catalyst, such as HF or other fluorides. These methods can enable fluorination at lower temperatures. These methods have also enabled the preparation of intercalation compounds other than (C2F) n and (CF) n (N. Watanabe et al., "Graphite Fluorides", Elsevier, Amsterdam, 1988, pp. 240-246). These intercalation compounds prepared in the presence of HF or a metal fluoride have an ionic character when the fluorine content is very low (F / C <0.1), or an ionic-covalent character in case of higher fluorine contents ( 0.2 <F / C <0.5). In any case, the link energy measured by Electronic Spectroscopy for Chemical Analysis (ESCA) yields a value lower than 687 eV for the main peak of the line of F1s and a value lower than 285 eV for that of the line of C1s (T Nakajima, Fluorine-carbon and Fluoride-carbon, Chemistry, Physics and Applications, Marcel Dekker 1995 p. 13).

Hamwi et al. they have observed fluorination of multiple wall nanotubes (MWNT) at room temperature under a gaseous atmosphere of F2, HF and IF5 for approximately 10 hours. The F / C ratio, determined by mass absorption, was 0.4. It was reported that the Fourier Transform Infrared Spectroscopy spectra presented a wide range centered at approximately 1100 cm -1, indicating the presence of semi-ionic CF bonds (Hamwi 1997 ibid.).

U.S. Patent 5,106,606 to Endo et al. reveals fluorinated graphite fibers with a composition of C5F to C30F. The examples describe fluorination at room temperature in the presence of a silver fluoride catalyst.

Summary of the Invention

The invention discloses fluorinated multilayer carbon nanomaterials. Multi-layer carbon materials suitable for use with the invention include multi-wall carbon nanotubes (NCPM), multi-layer carbon nanofibers (NFCs), carbon onions, carbon nanowires and carbon nanowires. These fluorinated materials are indicated for use in electrochemical devices such as primary and secondary batteries. In particular, the use of partially fluorinated nanomaterials in lithium batteries can provide good battery performance with high discharge coefficients.

In one embodiment, the invention discloses a fluorinated multilayer carbon nanomaterial obtained by direct fluorination and having an average chemical composition CFx where x is the atomic ratio between fluorine and carbon. In one aspect of the invention, x is between 0.6 and 0.8. In one embodiment, the carbon nanomaterial has a substantially ordered multilayer structure before fluoridation.

In one embodiment, the invention discloses partially fluorinated multilayer carbon nanomaterials containing both non-fluorinated (unreacted) and fluorinated carbon. The unreacted carbon phase has a higher electrical conductivity than the fluorinated carbon product. When the partially fluorinated material is used in the cathode of a Li / CFX cell, the non-fluorinated component of the material guarantees electronic conductivity, while the fluorinated component is electrochemically active during discharge, according to Equation 1. For these nanomaterials partially fluorinated, the combination of these two phenomena allows obtaining high energy densities during discharge.

In another embodiment, the invention discloses fluorinated multilayer carbon nanomaterials containing at least two fluorinated carbon products: a less fluorinated and more electrically conductive product and a more fluorinated and less electrically conductive product. These fluorinated carbon materials may also contain non-fluorinated carbon. As previously described, the presence of a component with higher electrical conductivity is expected to increase the performance of the Li / CFX cell with high discharge coefficients.

The invention also discloses methods for fluoridation of multilayer carbon nanomaterials. In one embodiment, the methods of the invention involve placing the multilayer carbon nanomaterials with fluorine or with a mixture of fluorine gas at a temperature greater than about 375 ° C for a time greater than four hours.

The invention also discloses electrochemical devices that convert chemical energy into electrochemical current, such devices being exemplified by lithium batteries. Such a device incorporates a first electrode comprising at least one fluorinated multilayer carbon nanomaterial according to the invention, a second electrode; an ion transporting material called electrolyte and a separator material that physically separates the two electrodes and prevents direct electrical contact between them. In another embodiment, the electrolyte and the separator may be composed of a material, such as solid state polymer (POE, PPE), a gelled electrolyte or a solid state electrolyte (thin film of phosphorus lithium oxynitride (LiPON)) . In a lithium battery, the second electrode comprises a source of lithium ions. In one embodiment, the first electrode is a positive cathode or electrode and the second electrode is a negative anode or electrode. The anode may comprise a source of an ion corresponding to a metal of groups 1, 2

or 3 of the periodic table of the elements.

In another aspect of the invention, an electrode comprising a fluorinated multilayer carbon nanomaterial is disclosed. Generally, the fluorinated multilayer carbon nanomaterial is present in a composition that additionally includes a conductive diluent and a binder. Such electrodes can be used in electrochemical devices that convert chemical energy into electrical current and electronic devices.

Brief description of the drawings

Figure 1. Evolution of the molar ratio F: C as a function of temperature, estimated by mass absorption

(o) as through quantitative NMR data (•).

Figure 2. Clear-field MET images of virgin carbon nanofibers (a) and (b); of samples fluorinated at 420 ° C (c) and 480 ° C (d).

Figure 3. Power spectral density (DEP) functions calculated for the MET images of Fig. 2: virgin carbon nanofibers (1), fluorinated sample treated at 420 ° C (2 and 3). Curves 2 and 3 correspond to the entire image 2c and to the fiber core exclusively. The curves are displaced vertically for clarity. The dotted line indicates the periodicity of the graphite layers.

Figure 4. Flourished NFC X-ray diffraction patterns at temperatures between 380 and 480 ° C compared to NFC patterns.

Figure 5. Raman spectra of fluorinated NFCs at temperatures between 380 and 465 ° C compared to NFC standards.

Figure 6. NFC-FTF 19F NMR static spectra (380 <TF <480 ° C). The insert shows the spectra of the less fluorinated samples (TF = 380 and 390 ° C).

Figure 7. MAS 19F NMR spectra of NFC-FTF and graphite fluoride (C2F) n with a rotation speed of 10 kHz; * and markers identify lateral rotation bands related to isotropic peaks at -190 and 120 ppm, respectively.

Figure 8. Evolution of the spin-reticle relaxation time T1 as a function of the fluorination temperature (a) and the initial 19F magnetization curve of NFC-F420,

Figure 9. (a) NFC 13C NMR spectra fluoridated at temperatures between 380 and 480 ° C compared to the graphite fluoride spectrum (C2F) n, (b) MAS 13C NMR spectra of NFC-TF and fluoride Graphite (C2F) n obtained by cross polarization from 19F to 13C (the rotation speed is 10 kHz).

Figure 10. (a) Temporary evolution of 13C magnetization for carbon atoms with fluorine covalent bonding (o) and bound exclusively to carbons (sp3 C) (•) of NFC-F472 with a rotation speed of 14.5 kHz fulfilling the Hartmann-Hahn condition n = 1, (b) Fourier transform of the resulting oscillation as a function of the fluorination temperature. The graphite fluoride (C2F) n curve is added for comparison.

Figure 11. RPE spectra of fluorinated NFCs (a) (to facilitate comparison, intensities are divided by sample mass) and simulations of selected samples, NCF-F380 (b), NFC-F472 (c) and NCF-F480 (d).

Figure 12. Evolution with the fluorination temperature of the RPE parameters, line width (∆Hpp) and A / B ratio (a) and spin density Ds (b).

Figure 13. Cell discharge curve with a lithium anode and a fluorinated carbon nanofiber cathode, with a fluorine / carbon ratio of the fluorinated nanofibers of 0.21.

Figure 14. Cell discharge curve with a lithium anode and a fluorinated carbon nanofiber cathode, with a fluorine / carbon ratio of fluorinated nanofibers of 0.59.

Figure 15. Cell discharge curve with a lithium anode and a fluorinated carbon nanofiber cathode, with a fluorine / carbon ratio of fluorinated nanofibers of 0.76.

Figure 16. Cell discharge curve with a lithium anode and a fluorinated carbon nanofiber cathode, with a fluorine / carbon ratio of fluorinated nanofibers of 0.82.

Figure 17. Cell discharge curve with a lithium anode and a conventional carbon fluoride cathode, with a fluorine / carbon ratio of carbon fluoride of 1.0.

Figure 18. Open circuit voltage profiles of lithium / CFx cells at different temperatures.

Detailed description of the invention

The present invention is as described in the claims.

In a first aspect of the present invention, a fluorinated carbon nanomaterial obtained by direct fluorination of a carbon nanomaterial and having an average chemical composition CFX where x is the atomic ratio between fluorine and carbon and has a value of between 0.6 and 0.8, where the carbon nanomaterial has a substantially ordered multilayer structure before fluoridation and is selected from the group consisting of multi-walled carbon nanotubes, Multilayer carbon nanofibers, carbon onions, carbon nanowires and carbon nanowires.

In one embodiment, the invention discloses a multi-layer or multi-layer fluorinated carbon nanomaterial. As understood herein, a carbon nanomaterial has at least one dimension located between one nanometer and one micron. In one embodiment, at least one dimension of the nanomaterial is between 2 nm and 1000 nm. In nanotubes, nanofibers, nanowires or carbon nanowires, the diameter of the tube, fiber, nanowires or nanobars is within this range of sizes. In carbon nanoparticles, the diameter of the nanoparticle falls within this range of sizes. Carbon nanomaterials suitable for use with the invention include materials with total impurity levels below 10% and carbon materials doped with elements such as boron, nitrogen, silicone, tin and phosphorus.

Carbon nanomaterials suitable for use with the invention have multiple layers of carbon before fluoridation. In multi-walled nanotubes, the layers are formed by graphene layers that make up the nanotube's walls. In multilayer particles, the layers are formed by multilayer fullerenes.

As understood herein, the term "nanotube" refers to a discrete tube-shaped fibril, characterized by a diameter typically between about 1 nm and about 20 nm. In addition, the nanotube typically has a length greater than about 10 times its diameter, preferably greater than about 100 times its diameter. The term "multiple wall", as used to describe nanotubes, refers to nanotubes with a layered structure, so that nanotube comprises an outer region formed by multiple continuous layers of ordered atoms and a clearly differentiated inner central region or lumen . The layers are arranged in substantially concentric ways around the longitudinal axis of the fibril. In carbon nanotubes, the layers are graphene layers. Carbon nanotubes have been synthesized in different forms such as single, double and multiple wall carbon nanotubes, abbreviated as NCPS, NCPD and NCPM respectively. The diameter varies from about 2 nm in NCPS and NCPD to about 20 nm in NCPM. In one embodiment, the NCPMs used in the invention have a diameter greater than 5 nm, greater than 10 nm, between 10 and 20 nm or approximately 20 nm.

Multiple wall carbon nanotubes can be produced by vapor catalytic chemical deposition (DQV). In one embodiment, the carbon nanotubes produced by DQV are heat treated to improve their structural and microtextural characteristics before subjecting them to the fluorination process of the invention. Specifically, the carbon nanotubes are heated to a sufficiently high temperature, so that the graphene layers become substantially straight and well aligned with the axis of the tube. In one embodiment, the NCPMs are heated to obtain a substantially well-ordered structure. As understood herein, a carbon nanostructure is substantially well ordered when it has at least one peak in its X-ray diffraction pattern, peak that 1) appears in the angular area between 24.5 degrees and 26, 6 degrees at the 2 theta diffraction angle, using a monochromatic copper radiation, and 2) has a maximum mid-height width of less than 4 degrees at the 2 theta diffraction angle.

As understood herein, carbon nanofibers refer to carbon fibers with a diameter greater than 20 nm and less than 1000 nm. In different embodiments, the carbon nanofibers used in the invention are between 20 and 1000 nm, between 40 and 1000 nm or between 80 and 350 nm. Through carbon catalytic vapor deposition and term treatment, carbon nanofibers with concentric carbon layers similar to those of multiple wall nanotubes can be produced. In particular, carbon nanofibers produced by DQV are heated to a sufficiently high temperature, so that the carbon layers become substantially straight and well aligned with the fiber axis. In different embodiments, the carbon nanofibers are heated to a temperature greater than 1800 ° C or greater than 2500 ° C to obtain a substantially well-ordered structure.

As is known in the art, carbon vapor grown carbon fibers (FCCFV) with larger diameters (eg 10 microns) can also be produced by vapor catalytic deposition. These fibers can have a layer-like growth ring structure, concentrically arranged one above the other (Endo, M., 1988, Chemtech, 568-576). As used in the present invention, the term "carbon nanomaterials" is not intended to encompass FCCFVs with a diameter of one micron or greater.

Carbon nanoparticles can be conceived as structures related to large rather imperfect multilayer fullerenes (Harris, P., 1999, "Carbon Nanotubes and Related Structures", Cambridge University Press, Cambridge, p. 103). A form of carbon nanoparticle is known as "carbon onion." When fully formed, carbon onions are highly perfect in structure and have few obvious defects (Harris 1999). Carbon onions with diameters greater than 5 nm have been formed (Harris 1999). Nasibulin et al. describe the formation of carbon onions between 5 nm and 30 nm (Nasimbulin, AG, et al, 2005, Colloid J., 67 (1), 1-20), while Sano et al. describe the formation of carbon onions between 4 and 36 nm (Sano, N. et al, 2002, J. Appl. Phys., 92 (5), 2783). In different embodiments, the multi-wall carbon nanoparticles used in the invention have a diameter greater than 5 nm, greater than 10 nm, greater than 20 nm, between 5 and 35 nm or between 10 and 30 nm.

One form of carbon nanobars, grown by chemical vapor deposition activated by electron cyclotron resonance, was described by Woo et al. The filamentous carbon did not form a hollow tube. High resolution transmission electron microscopy has been used to reveal crystalline walls, with graphene layers somewhat messy and inclined around the axis of the bar. The average distance between graphene layers was determined to be greater than that given in NCPM (Woo, Y. et al., 2003 J. Appl. Phys. 94 (10, 6789).

Carbon threads, also called graphite threads, are known in the art. These materials appear to have a parchment-like structure, formed by an essentially continuous graphite structure (Harris 1999).

As understood herein, the fluoridation of a material implies the introduction of fluoride into the material. In the present invention, fluorination will typically involve the formation of bonds between carbon and fluorine. As is known in the art, fluorine is capable of forming both ionic and covalent bonds with carbon. In some cases, CF bonds have also been classified as intermediate in terms of strength between ionic and covalent bonds (eg partially ionic, semi-ionic, semi-valent). The fluorination method can influence the type of bond present in the fluorination product.

In the present invention, fluorinated multilayer carbon nanomaterials are produced by direct fluorination. In direct fluoridation, CF bonds tend to be of higher energy and have a more covalent character than CF bonds obtained by fluorine intercalation at low temperature. It is expected that fluorine-graphite intercalation compounds have a fluorine-carbon bond that varies between ionic and semi-valent, depending on fluorine content (Matsuo, Y. et al, 1995 Z. Anorg. There. Chemie, 621, 1943-1950). For example, Matsuo et al. (1995) classify spectral peaks of XPS F1s to 687 eV, 685 eV and 683 eV, respectively, as semi-valent, almost ionic and ionic. In contrast, the F1s peak in graphite fluoride with covalent bond is 689.3-689.6 eV (Watanabe 1988 ibid.).

In one aspect of the invention, at least part of the carbon in the fluorination product has covalent or almost covalent bonds with fluorine. In another aspect of the invention, at least part of the carbon in the fluorination product has covalent bonds with the fluorine. In one embodiment, the covalently or almost covalently bonded carbon to the fluorine is located under the surface of the fluorinated carbon nanomaterial.

As understood herein, a carbon-fluorine bond in the fluorination product is classified as almost covalent if the bond has an energy greater than the energy of the "semi-ionic" or "semi-valent" carbon-fluorine bonds in compounds of graphite intercalation fluorine obtained by fluorination of graphite at low temperature, but less than the typical energy of covalent carbon-fluorine bonds in the non-superficial region of polycarbonate monofluoride (C2F) not poly (carbon monofluoride) (CF) n.

The nature of the CF bond in the fluorination product can be determined by applying the appropriate analysis techniques. Such techniques are known to those skilled in the art, and include, but are not limited to, Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), light emitted photoelectron spectroscopy X (XPS) or electronic spectroscopy for chemical analysis (ESCA). The magnitude of the covalence in the CF bonds can be assessed by comparing the results of the fluoridation product analysis with those obtained for a "standard" generally accepted as having covalent CF bonds. A coincidence (within the experimental error) between the results of the analysis of the product of fluoridation and those of the "standard" could be considered indicative of a covalent bond. It is generally accepted that graphite fluorides poly (dicarbon monofluoride) (C2F) n and poly (carbon monofluoride) (CF) n have covalent CF bonds.

As discussed in Example 1, the solid-state 19F NMR spectra having a chemical shift peak centered at approximately -190 ppm / CFCI3 indicate the presence of fluorine atoms covalently bonded with carbon atoms. As a further example, 13C NMR spectra that have a chemical displacement resonance of 84-88 ppm / TMS indicate the presence of carbon atoms covalently bonded with fluorine atoms. Fourier transform infrared spectroscopy (FT-IR) spectra that show vibration bands centered at approximately 1215 cm -1 are also indicative of covalent CF bonds.

As understood herein, a partially fluorinated carbon material includes part of carbon material that has reacted with fluorine and part of carbon material that has not reacted with fluorine. Partially fluorinated carbon materials include materials in which mainly the outer portion has reacted with fluorine, while the inner region remains largely unreacted.

The average ratio between fluorine and carbon can be used as a measure of the magnitude of fluoridation. This average ratio can be determined by measurements by weight absorption or by NMR measurements, as described in Example 1. If the fluorine is not uniformly distributed over the entire wall thickness of the carbon material, this average proportion may differ from the proportions between fluorine and carbon on the surface, which can be determined by X-ray emitted photoelectron spectroscopy (XPS ) or ESCA.

In one aspect of the present disclosure, a fluorinated carbon nanomaterial obtained by direct fluorination and having an average chemical composition CFX where x is the atomic ratio between fluorine and carbon and has a value between 0.06 and 0 is disclosed. , 95, where the carbon nanomaterial has a substantially ordered multilayer structure before fluoridation. In other aspects of the present disclosure, x is between 0.06 and 0.68, between 0.3 and 0.66 and 0.95.

In one aspect of the present disclosure, x is between 0.006 and 0.68 and the fluorinated material has a) an X-ray diffraction peak in the angular range of 24.5-26.6 degrees using a monochromatic radiation source of Cu and b) a 19F NMR peak in the range between (-160) ppm and (-200) ppm / CFCI3. The DRX peak for non-fluorinated carbon nanomaterials is expected to be within the cited range.

In another aspect of the present disclosure, x is between 0.39 and 0.95 and the fluorinated material has a) an X-ray diffraction peak in the angular range of 9.8-15 degrees, b) an NMR peak 19F in the range between (-180) ppm and (200) ppm / CFCI3 and c) three 13C NMR peaks: a first peak in the range at 100-150 ppm / TMS, a second peak in the range 84-88 ppm / TMS and a third peak in the range 42-48 ppm / TMS.

In another aspect of the present disclosure, a fluorinated carbon nanomaterial comprising a non-fluorinated carbon phase and at least one fluorinated carbon product is disclosed in which at least part of the carbon is covalently or almost covalently bonded to the fluorine, and where the carbon nanomaterial has a substantially ordered multilayer structure before fluoridation. In different aspects of the present disclosure, the average ratio between fluorine and carbon is between 0.06 and 0.68, between 0.3 and 0.66 or between 0.3 and 0.6.

In another aspect of the present disclosure, a carbon nanomaterial that has been partially fluorinated and which comprises a non-fluorinated carbon phase and a fluorinated carbon product is disclosed, where the average ratio between fluorine and carbon is less than 0, 4 and the carbon nanomaterial has a substantially ordered multilayer structure before fluoridation.

In another aspect of the invention, the invention discloses a fluorinated carbon nanomaterial comprising at least one fluorinated carbon product in which at least part of the carbon is covalently or almost covalently bonded to the fluorine, and where the spacing between Layers is intermediate between that of poly (dicarbon monofluoride) graphite and that of poly (carbon monofluoride) graphite, where the carbon nanomaterial has a multilayer structure before fluoridation. In different aspects of the present disclosure, the average ratio between fluorine and carbon is less than 1.0, between 0.3 and 0.8, between 0.6 and 0.8 (in an embodiment of the present invention), between 0.39 and 0.95, between 0.39 and 0.86, between 0.39 and 0.68, between 0.68 and 0.86, or between 0.74 and 0.86.

In one embodiment, the fluorinated carbon product has some characteristics similar to those that would be produced by a mixture of graphite fluorides (C2F) n and (CF) n. X-ray diffraction analysis reveals that this product has 2Ɵ peaks centered at 12.0 degrees and 41.5 degrees. The spacing between layers is intermediate of this compound is approximately 0.72 nm. The 13C NMR spectra of this compound have a resonance present at 42 ppm, which indicates the presence of non-fluorinated sp3 carbon atoms. NMR analysis also indicates the existence of a covalent bond between carbon and fluorine. Groups CF2 and CF3 may also be present in small quantities.

Another fluorinated carbon product may have structural similarities with (CF) n. X-ray diffraction analysis reveals that this compound has 26 peaks centered at more than 12.0 degrees and less than 41.5 degrees. The spacing between layers of this compound is approximately 0.60 nm. NMR analysis also indicates the existence of a covalent bond between carbon and fluorine. Groups CF2 and CF3 may also be present in small quantities. Without adhering to any particular theory, it is believed that fluoridation of fluorinated multilayer carbon nanomaterials at relatively low temperatures (eg tempera tures below 420 ° C for NFC with an average diameter of approximately 150 nm) results primarily in the fluoridation of the surface of the carbon material. The rest of the carbon material remains a fluorinated. This surface fluoridation may include the formation of groups such as CF2, and CF3. In one embodiment, the ratio between fluorine and carbon associated with this regime is between 0 and 0.16.

At intermediate temperatures (eg temperatures between 420 ° C and 465 ° C for NFC with an average diameter of approximately 150 nm) it is believed that fluoridation penetrates beyond the surface of the material. In one embodiment, the fluorinated product has some crystallographic similarities with a mixture of graphite fluorides (C2F) n and (CF) n. In one embodiment, the multilayer carbon nanomaterials are partially fluorinated and contain a certain amount of unreacted carbon. In another embodiment, a second fluorination product in which part of the carbon is "slightly fluor inated" and has predominantly non-covalent bonds is present in addition to the first fluorination product, which has at least some of the carbon atoms covalently bonded. or almost covalent with fluoride. In one aspect of the present disclosure, the ratio between fluorine and carbon associated with this regime is between 0.31 and 0.79.

At higher temperatures (eg temperatures above 465 ° C for NFC with an average diameter of approximately 150 nm), the fluorination product begins to show greater crystallographic similarity with graphite fluoride (CF) n. Without limiting ourselves to any particular theory, it is believed that the similar phases (C2F) n present in the material initiate the conversion to a phase similar to (CF) n which also has covalent bonds. The conversion is accompanied by exfoliation. In one embodiment, the amount of non-fluorinated carbon is so small that it cannot be detected by DRX. In one aspect of the present disclosure, the ratio between fluorine and carbon is greater than 0.86.

The fluorinated multilayer carbon nanomaterials of the invention are prepared using a direct fluorination method, in which the multilayer carbon nanomaterials are contacted with a gaseous source of elemental fluorine. The fluorination conditions (including the temperature, time and fluorine pressure) are selected to obtain the desired degree of fluorination of the carbon materials. In one embodiment, suitable fluorination conditions for NFC fluorination with an average diameter of between 80 and 350 nm are selected.

In different embodiments, the fluorination temperature may be between 375 ° C and 480 ° C, between 400 ° C and 475 ° C, between 405 ° C and 465 ° C, or between 420 ° C and 465 ° C.

In different embodiments, the time period is longer than 4 hours, between 4 and 40 hours, between 4 and 20 hours, between 4 and 16 hours, between 4 and 12 hours, between 8 and 20 hours, of between 8 and 16 hours, between 8 and 12 hours, or about 16 hours.

In one embodiment, the fluorination is carried out at atmospheric pressure with a mixture of gas consisting essentially of F2 and inert gas. The percentage of fluoride in the mixture can be between 5% and 100%, between 10% and 90%, between 20% and 80%, between 20% and 60%, between 20% and 50% or approximately 20%.

In other embodiments, the fluorination can be carried out at a pressure lower than atmospheric pressure. In one embodiment, the fluorination can be carried out at pressures between 1 atm and 0.1 atm, or between 1 atm and 0.25 atm.

Those skilled in the art will know a suitable gaseous source of elemental fluorine, an example of such a source is a mixture of F2 and a sufficiently inert gas. Suitable inert gases include, but are not limited to, nitrogen and argon. Preferably, only traces of HF or other fluorides classified as intercalation catalysts are present in the gas mixture.

In one embodiment, the invention discloses a method for fluoridation of multilayer carbon nanomaterials, comprising the steps of exposing carbon nanomaterials to a gaseous source of elemental fluorine at a pressure between 1 atm and 0.1 atm at a temperature between 375 ° C and 480 ° C for a period of between 4 and 20 hours.

The fluorinated multilayer carbon nanomaterials of the invention can be subjected to heat treatment after fluoridation.

In electrochemical devices of the invention, fluorinated multilayer carbon nanomaterials are normally present in a composition that also includes a conductive diluent that can be selected from, for example, acetylene black, carbon black, graphite powder, cokes, fibers Carbon and metal powders such as nickel, aluminum, titanium and stainless steel powder. The conductive diluent improves the conductivity of the composition and is typically present in an amount that represents between about 1% by weight and about 10% by weight of the composition, preferably between about 1% by weight and about 5% by weight. composition weight. The composition containing the fluorinated multilayer carbon nanomaterial and the conductive diluent also typically contains a polymeric binder, with the preferred polymeric binders being at least partially fluorinated. Thus, examples of binders include, but are not limited to, 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 between about 1% by weight and about 5% by weight of the composition, while the fluorinated multilayer carbon nanomaterial represents between about 85% by weight and about 98% by weight of the composition, preferably between about 90% by weight and about 98% by weight of the composition.

Next, the resulting fluorinated multilayer carbon nanomaterial is added and mixed with a conductive diluent and a binder as described above, with the preferred proportions by weight located between approximately 85% by weight and approximately 98% by weight, more preferably between approximately 90% by weight and approximately 98% by weight of the fluorinated multilayer carbon nanomaterial; between about 1% by weight and about 10% by weight, preferably between about 1% by weight and about 5% by weight of conductive diluent and between about 1% by weight and about 5% by weight of binder .

Typically, then the suspension formed after the addition and mixing of the mentioned components is deposited or otherwise applied on a conductive substrate to form the electrode. A particularly preferred conductive substrate is aluminum, although various other conductive substrates, such as e.g. ex. stainless steel, titanium, platinum, gold and the like. The fluorinated multilayer carbon nanomaterial can be at least partially aligned during the deposition process. For example, cut alignment can be used to align the fluorinated multilayer carbon nanomaterial.

In another aspect of the present disclosure, a method for preparing an electrode for use in an electrochemical device is disclosed, comprising the following steps:

fluoride a multilayer carbon nanomaterial according to the methods of the invention;

add and mix the fluorinated multilayer carbon nanomaterial with a conductive diluent and a binder to form a suspension; and

apply the suspension to a conductive substrate.

In one embodiment, the invention discloses an electrochemical device comprising a first electrode and a second electrode, and an ion transport material disposed between both electrodes, wherein the first electrode comprises a fluorinated multilayer carbon nanomaterial according to the present invention.

In a primary lithium battery, for example, the aforementioned electrode acts as a cathode, while the anode provides a source of lithium ions, where the ion transporting material is typically a microporous or nonwoven material saturated with a non-aqueous electrolyte . The anode may comprise, for example, a lithium sheet or film or a lithium metal alloy (LiAl, for example), or lithium carbon, preferably a lithium metal sheet. The ion transporting material comprises a conventional "separator" material that has a low electrical resistance and has a high resistance, good chemical and physical stability and generally uniform properties. As noted above, preferred separators are microporous and nonwoven materials, e.g. ex. nonwoven polyolefins such as nonwoven polyethylene and / or nonwoven polypropylene, and microporous polyolefin films such as microporous polyethylene. An example of microporous polyethylene material is that obtained under the trade name Celgard® (eg, Celgard® 2400, 2500 and 2502) from Hoechst Celanese. The electrolyte is necessarily non-aqueous, since lithium is reactive in aqueous media. Suitable non-aqueous electrolytes are composed of lithium salts dissolved in an aprotic organic solvent such as propylene carbonate (PC), ethylene carbonate (EC), ethylmethyl carbonate (EMC), dimethyl ether (DME) and mixtures thereof . Mixtures of PC and DME are common, typically in a weight ratio of between about 1: 3 and about 2: 1. Suitable lithium salts for this purpose include, but are not limited to, LiBF4, LiPF6, LICF3SO3, LICIO4, LiAICI4, and the like. It will be appreciated that, during use, an applied voltage causes the generation of lithium ions in the anode and the migration of the ions through the electrolyte soaked separator towards the fluorinated multilayer carbon nanomaterial cathode, "discharging" the battery.

In one aspect of the present disclosure, an electrochemical device is disclosed which is a primary lithium battery in which the first electrode acts as the cathode, the second electrode acts as the anode and comprises a source of lithium ions, and the ion transport material physically separates the first electrode from the second and prevents direct electrical contact between them.

In another aspect of the present disclosure, the NCPM or NFC at least partially fluorinated in a secondary battery is used, that is, a rechargeable battery such as a rechargeable lithium battery. In this case, the cations, p. ex. Lithium ions are transported through a solid polymer electrolyte - which also acts as a physical separator - to the NCPM or NFC electrode at least partially fluorinated, where they are interleaved and uninterleaved by the NCPM or NFC material at least partially fluorinated. Examples of solid polymer electrolyte include chemically inert polyethers, e.g. ex. poly (ethylene oxide) (PEO), poly (propylene oxide) (PPO) and other polyethers, where the polymeric material is impregnated or otherwise associated with a salt, e.g. ex. a lithium salt like those mentioned in the previous paragraph.

In another aspect of the present disclosure, the invention discloses an electrochemical device that is a secondary lithium battery in which the second electrode comprises an ion source of a metal selected from groups 1, 2 and 3 of the Periodic Table of the elements and the ion transporting material comprises a solid polymer electrolyte that allows the transport of said metal cations and physically separates the first electrode from the second.

In another aspect of the present disclosure, a rechargeable battery is disclosed that includes:

a first electrode comprising a NCPM or NFC at least partially fluorinated, said electrode being capable of receiving and releasing cations of a metal selected from groups 1, 2 and 3 of the Periodic Table of the Elements, a second electrode comprising a source of the metal cations and a solid polymer electrolyte that allows the transport of the metal cations and physically separates the first electrode from the second.

A characteristic discharge profile of a NFC / fluorinated NFC cell is shown in Fig. 14; the ratio between fluorine and coal is approximately 0.59. These cells have a characteristic plateau corresponding to the formation of LiF according to Equation 1. The value of the plateau voltage depends on the discharge rate. For comparison, a characteristic discharge profile of a commercial Li / CF cell is shown in Figure 17.

The discharge profiles of fluorinated Li / NFC cells differ depending on the fluorine-carbon ratio of the NFC. In general, the higher the F / C ratio, the greater the discharge capacity, as indicated in Equation 2. Samples with proportions between fluorine and coal of 0.15 or less may not show a constant discharge potential .

Partially fluorinated NCPM and NFC may have an overpotential for low discharge times compared to fully fluorinated materials. This is related to the decrease in the electrical conductivity of the sample when the fluorinated parts advance, that is, the decrease in the amount of virgin carbon in the sample.

In addition, cells that include Li and NFC that are partially fluorinated may show better performance compared to commercial Li / CF cells at higher discharge rates (eg above 1C).

In one aspect of the present disclosure, once the electrochemical cell was assembled, it could be "pre-discharged" before use. The pre-discharge step involves the discharge of between 1% and 5% of the capacity of the cell. The pre-discharge of the cell can eliminate the initial voltage delay prior to the establishment of the tension plateau observed in the characteristic discharge profile of a fluorinated Li / NFC cell.

In the following examples, an attempt has been made to ensure accuracy in terms of the numbers used (eg quantities, temperature, etc.), but the possibility of some degree of experimental error and deviation should be considered. Unless otherwise indicated, the temperature is expressed in degrees Celsius and the pressure is atmospheric or near atmospheric. All solvents were purchased as HPLC grade, and all reagents were obtained commercially unless otherwise indicated.

Example 1: reactivity of carbon nanofibers with fluorine gas

Overview

The following study shows the reactivity of carbon nanofibers or nanofibers (NFC) with fluorine gas. Highly purified and graffiti CNFs were treated under a stream of fluorine gas for 16 hours at temperatures between 380 and 480 ° C. Different fluorination temperature zones have been revealed by direct physical-chemical analysis, such as DRX, Raman spectroscopy, RPE and solid-state NMR (13C and 19F). The comparison between several parameters, such as covalence of the CF bond, T1 time of nuclear relaxation spinreticle, density and environment of the free bonds, among others, makes it possible to determine the mechanism of fluorination, that is, the formation of graphite fluoride of the type (C2F) n as a precursor to the richest compound (CF) n. This is supported by the MET characterization as the fluorination advances from the outer parts of the carbon nanofibers and then propagates through the core without major structural changes of the fluorinated parts. Slight exfoliation of the sheets is necessary to achieve extended fluoridation and conversion into (CF) n; This takes place at fluorination temperatures above 472 ° C, with concomitant disappearance of the graphite structure.

1. Introduction

The reactivity of allotropic forms of carbon with fluorine gas differs greatly, due either to the degree of graffiti or the type of carbon material. The higher the degree of graffiti or the number of layers, the higher the temperature required for fluoridation. The fluorination temperature of the NCPMs is significantly higher compared to the NCPS, due to the presence of graphite multilayers surrounding the nanotube core. This temperature depends on the number of NCPM layers and is generally close to 400 ° C (Hamwi A .; Alvergnat H .; Bonnamy S .; Beguin F .; Carbon 1997, 35, 723; NakajimaT .; Kasamatsu S .; Matsuno Y. ; Eur. J. Solid. State Inorg. Chem. 1996, 33, 831).

Compared to the fluoridation of the NCPM, the structure of the NCPS favors fluoridation for temperatures of only 50 ° C (Mickelson ET; Huffman CB; Rinzler AG; Smalley RE; Hauge RH; Margrave JL; Chem. Phys. Lett. 1998, 296, 188; Kelly KF; Chiang IW; Mickelson ET; Hauge R .; Margrave JL; Wang X .; Scueria GE; Radloff C; Halas NJ; Chem. Phys. Lett. 1999, 313, 445. However, This structure is partially destroyed above 350 ° C.

In this work we have studied the reactivity of NFCs with pure fluorine gas in the temperature range of 380-480 ° C. The fluorinated compounds were characterized using various techniques, including: (1) DRX, MET and

19F 13C

Raman spectroscopy; (2) high resolution nuclear magnetic resonance and and, (3) electronic paramagnetic resonance. The cross-data analysis provides new information on the mechanism of fluoridation, the crystalline structure and the nature of the CF bond. The properties of the resulting materials will be discussed and compared with those of conventional graphite (CF) n (C2F) fluorides, also prepared for this study.

two. Experimental

fifteen High purity carbon nanofibers (> 90%), 2-20 microns in length, were provided courtesy of MER Corporation, Tucson, Arizona, USA. They were obtained by vapor catalytic chemical deposition (DQV) and subjected to heat treatment at 1800 ° C in an argon atmosphere to increase its crystallinity. Fluorinated carbon nanofibers (represented as NFC-FTF) were prepared with 200 mg of NFC at temperatures (TF) between 380 ° C and 480 ° C in a stream of F2. A reaction time of 16 h was used. The fluorination level 'x' (that is, the F: C molar ratio) was determined by gravimetry (weight absorption) and by quantitative 19F NMR measurements. X-ray diffraction dust patterns (DRX) were obtained using a Siemens D501 diffractometer with Cu (Kα) radiation (λ = 1.5406 Å).

The NMR experiments were performed with Tecmag Discovery and Bruker Advance spectrometers, with working frequencies for 1H, 13C and 19F of 300.1, 73.4 and 282.2 MHz, respectively. Two NMR Bruker samples were used: a static sample and a special cross polarization / magic angle rotation sample with fluoride decoupling on a 4 mm rotor. The 19F-13C match was optimized in polytetrafluoroethylene (PTFE); The pulse duration 19F π / 2 was 4 s. For MAS spectra, a simple sequence (µ acquisition) was used with a unique τ / 2 pulse duration of 3.5, 4 and 3.5 πs for 1H, 19F and 13C, respectively. The spin-reticulum T1 relaxation time was measured using a saturation recovery sequence and was calculated taking into account a magnetization curve with an evolution such as exp (-t / T1). The chemical shifts 1H and 13C were externally referenced to tetramethylsilane (TMS). The 19F chemical shifts were referenced with respect to CFCI3. In order to confirm the F: C molar ratio obtained by weight absorption and to determine the limit of this method, in particular for the higher fluorination temperature at which partial exfoliation and release of volatile fluorides can occur, volatile fluorides were carried out. 19F quantitative NMR measurements applying the same conditions for each sample, that is, similar receiver gain, Recycling time D1 (D1> 5T1 using the longest spin-reticulum relaxation time T1 corresponding to NFC-F480, therefore D1 = 3 s) and number of scans. The intensities are divided by the mass of the sample. For comparison purposes, two conventional samples (CF) n, obtained by direct reaction of natural graphite and petroleum coke with fluorine gas at 600 ° C, and a sample (C2F) n obtained by direct reaction were also analyzed by quantitative NMR of natural graphite with fluorine gas at 380 ° C, with compositions CF1.1, CF1.0 and CF0.6, respectively. Polyvinyldifluoride - (CF2-CH2) -n was used as a reference for fluorine content.

RPE spectra were obtained using a Bruker EMX digital X-band spectrometer (v = 9.653 GHz). Diphenylpicrilhydrazil (DPPH) was used as a calibration reference to determine both the resonance frequency and the densities of the spin carriers.

Raman spectra were recorded at room temperature using a JOBIN YVON T64000 with a charge device coupled as a multi-channel detector. The radiation source was an argon laser line of 514.5 nm. The laser power was adjusted to 10 mW.

The different samples were characterized by transmission electron microscopy (MET, FEI CM200 operating at 200 kV). The carbon nanofibers were dispersed in chloroform using ultrasonic treatment, and a few drops of suspension were deposited on copper observation gratings coated with ultrafine carbon / formvar films. The gratings were then dried under ambient conditions.

The quantitative analysis of the MET images was carried out in reciprocal space. Details of the method can be found in other sources (Basire C; Ivanov DA; Phys. Rev. Lett. 2000, 85, 5587). The two-dimensional power spectral density (P2 (s)) function was calculated from MET (u (r)) images up to the critical or Nyquist frequency, depending on the experimental sampling interval, such as:

where A expresses the image area, W (r) the window function (Press WH; Numerical Recipes in C, The Art of Scientific Computing, 1988, Plenum Press, New York) and s the 2D reciprocal space vector. The function P2 (s) was then transformed into the spectral density of monodimensional power (P1 (s)), where s represents the norm of s, according to the following equation:

3. Results 3.1 Fluoridation process

3.1 .ai Chemical composition in relation to fluorination temperature (TF)

The F: C ratio of the sample, obtained by weight absorption and NMR data, is plotted in Fig. 1 as a function of fluorination temperature; The values ​​are also summarized in Table 1. The two methods yield similar results for TF up to 450 ° C. However, at higher temperatures a pronounced discrepancy appears between the two methods, since gravimetry underestimates the actual amount of fluoride, whose determination is more accurate by NMR. In fact, the NMR method was tested with two standard graphite fluorides based on compounds of (CF) n and (C2F) n, and yields the correct F: C ratio. In all likelihood, the origin of the discrepancy lies in the thermal decomposition of fluorinated NFCs at T> 450 ° C, which generates volatile alkyl fluorides such as CF4, C2F6 and others and, as a consequence, causes weight loss. In conclusion, using the quantitative NMR data, the drastic increase in fluorine content for TF greater than 420 ° C is clearly demonstrated. However, a progressive increase in fluorine content is then registered in parallel to the treatment temperature, contrary to what is determined by weight absorption.

In Figure 1 and Table 4, four fluorination temperature zones can be distinguished:

<dl><dt>-</dt><dd>i) for TF below 420 ° C, the fluorination level is low: 0 &lt;x &lt;0.2; </dd></dl>

In particular, for TF = 380 ° C the composition is CF0.04 while for graphite at the same reaction temperature it reaches CF0.60 (Dubois M .; Giraudet J .; Guerin K .; Hamwi A .; Fawal Z. ; Pirotte P .; Masin F .; J. Phys. Chem. B2006, 110, 11800).

<dl><dt>-</dt><dd>ii) when TF is in the 420-435 ° C range, the F: C ratio is drastically increased: 0.31 &lt;x &lt;0.7. </dd></dl>

<dl><dt>-</dt><dd>iii) when TF is between 435 and 450 ° C, the F: C ratio is practically constant: x ~ 0.7-0.8. </dd></dl>

<dl><dt>-</dt><dd>iv) for TF greater than 450 ° C, a jump in the composition is observed up to 465 ° C, and then the proportion stabilizes around ax ~ 1.0. </dd></dl>

Unlike NCPS, NCPM and larger-diameter carbon nanotubes react with fluoride at higher temperatures due to their graphite structure. Specifically, NCPS react with fluoride at temperatures of only 50 ° C to form compound CF0,114 (Mickelson 1998; Kelly 1999 ibid.). In the case of NCPM, the higher the number of layers, the higher the reaction temperature with fluoride (Hamwi 1997 ibid.). In our case, the number of graphene layers (similar to the NCPM layers) of virgin NFCs is approximately 35. This value demonstrates a high degree of NFC graffiti.

Probably a fluorination of the surface of the NFCs takes place during the first temperature range, while the fluorination of the less accessible graphene layers takes place during the second temperature range. In order to confirm this interpretation, various physicochemical characterizations with increasing TF values ​​were investigated.

3.1.a.ii Chemical composition in relation to fluoridation time

For a fluorination temperature of 430 ° C, the fluorination time was varied between 4 and 16 hours (keeping the other fluorination conditions unchanged). The F / C ratios obtained by measuring weight absorption were as follows: 0.22 for a fluoridation time of 4 hours, 0.38 for a fluorination time of 8 hours, 0.55 for a fluorination time of 12 hours and 0.60 for a fluoridation time of 16 hours.

3.1.a.iii Chemical composition in relation to fluorination pressure

For a fluorination temperature of 430 ° C and a fluoridation time of 16 hours, the fluorination pressure was reduced to 0.3 atm (keeping the other fluorination conditions unchanged). For this pressure, the F / C ratio obtained by measuring weight absorption was 0.53, compared to an F / C ratio of 0.60 at 1 atm.

3.1 .b. Layer stacking structure and morphology

The bright-field MET images of carbon nanofibres subjected to further treatment confirm this structural order by revealing the presence of graphite layers (Fig. 2a). The well-defined periodicity of the layers is reflected in a Bragg peak in the corresponding PED curve (Fig. 3), positioned at approximately 0.34 nm. The distribution of diameters is quite narrow, covering between 80 and 350 nm (Fig. 2b). The average diameter (&lt;Φ&gt;) is estimated at around 150 nm from observations of several parts of the gross sample. Due to the accommodation of fluorine atoms within graphene layers, fluoridation results in a moderate increase in the average diameter after the reaction at 420 ° C (&lt;Φ&gt; = 160 nm), and to a greater extent at 480 ° C ( &lt;Φ&gt; = 180 nm). The graphite structure is maintained with fluoridation at 420 ° C (Fig. 2c), contrary to what happens in fluorination at 480 ° C (Fig. 2d). For NFC-F420, fiber morphology reveals two distinct structures present in the periphery and in the fiber core. The PED function (Fig. 3) corresponding to the image in Fig. 1c shows a broad peak with a maximum at approximately 1.5-2.0 nm-1 in addition to the usual periodicity of the graphene layer determined for the sample untreated (Fig. 2a). This additional feature in the PED curve indicates the presence of layers, which are less ordered and more separated in space due to the accommodation of fluorine atoms. Note that the fiber core of the same sample does not manifest this increase in periodicity (Fig. 3).

When considering the crystalline order and the orientation of the graphene layers resulting from the post-1800 ° C treatment, it is found that virgin nanofibers and NCPMs have many things in common. Thus, in the following sections, calculations similar to those applied for NCPMs will be presented.

3.2 Structural evolution

The DRX patterns of virgin and fluorinated carbon nanofibers are compared in Fig. 4. The pattern of virgin carbon nanofibers is similar to that of NCPM synthesized by arc discharge (Okotrub AV; Yudanov NF; Chuvilin AL; Asanov IP ; Shubin YV; Bulusheva LG; Gusel'nikov AV; Fyodorov IS; Chem. Phys. Lett. 2000, 323, 231) or by DQV (Nakajima 1996, ibid.). The main peaks correspond to the graphite diffraction lines (002), (100), (101), (004) and (110) for the 2θ values ​​of 26 ° 3 (distance between layers d = 0.338 nm), 43 ° 5 (0.207 nm), 45 ° (0.201 nm), 54 ° 4 (0.169 nm) and 77 ° 9 (0.123 nm), respectively. The most pronounced reflection (002) is associated with an average spacing between layers of 0.338 nm, in accordance with the results of the bright-field MET. The peak width (002) (Δ2Ɵ = 0.72 °) characterizes both the average number of carbon layers (approximately 35) and the coherence length Lc along the c axis (11.8 nm). With regard to the NCPM, the symmetry of the lines (hk0) and the weak intensity of the peaks (hkl) reflect the absence of a positional correlation between carbon atoms in different layers of a nanoparticle. The fluorination of carbon nanofibers progressively changes the structure as a function of TF, as the DRX shows. With 405 &lt;TF &lt;420 ° C, a new phase appears with corresponding peaks in 2Ɵ values ​​centered at 12.0 ° and 41.5 ° attributed to the peaks (001) and (100) of a fluorograph matrix in a hexagonal system . This phase coexists with that of virgin carbon nanofibers. However, the widening of the peak observed in this new phase could be caused by a lower stacking order of the fluorinated layers. At 435 &lt;TF &lt;450 ° C, the virgin carbon nanofiber phase disappears and only the fluorinated nanofiber phase is present. Fluorinated carbon nanofibers have a 0.72 nm layer spacing, which varies between that of (C2F) n (d spacing 0.81 nm) and that of (CF) n (d spacing equal to 0.60 nm ) (Nakajima et al., 1991, ibid.). With TF of approximately 480 ° C, a structure similar to (CF) n is obtained with a spacing between layers of 0.60 nm. In Figure 5, the Raman spectra of the fluorinated carbon nanofibers are compared with those of the starting carbon nanofibers. The latter has two bands: one at 1345 cm-1 attributed to mode D and a second at 1570 cm-1 corresponding to mode G and assigned to a double resonance Raman effect on sp2 carbon atoms. In NCPMs, the D band probably originates mainly from defects in the tube walls (Osswald S .; Flahaut E .; Ye H .; Gogotsi Y .; Chem. Phys. Lett. 2005, 402, 422). However, both in the walls of the tube and in other forms of carbon, the contribution of defects to the band is still not fully understood

D. The G mode is due to a reticulum vibration in the optical plane with Eg symmetry of the active graphical mode in Raman. As Rao et al. Revealed, there are no active vibration modes in Raman either for fluorine or for carbon-fluoride bonds between 1250 and 1700 cm -1 (Rao AM; Fung AWP; di Vittorio SL; Dresselhaus MS; Dresselhaus G .; Endo M .; Oshida K .; Nakajima T .; Phys. Rev. B 1992, 45, 6883).

As the F: C ratio increases, so does the intensity of the D band. For temperatures below 420 ° C, the NFC-FTF spectra are similar to those of the NFC. More particularly, the ID / IG ratio of the integrated intensity of the D and G bands is practically constant (ID / IG = 0.30, Table 1) within the experimental error ranges. This proportion is related to the structural disorder in the NFC. Therefore, as fluoridation progresses, the ID / IG ratio increases as does the disorder. With 420 &lt;TF &lt;435 ° C the disorder is drastically increased (see Table 1) and a new mode appears at approximately 1620 cm-1. This characteristic of weak intensity was verified using a high density of phonon states for zone center phonons (Chien TC; Dresselhaus MS; Endo M. Phys. Rev. B 1982, 26, 5867). With 435 &lt;TF &lt;465 ° C, the D-band induced by the disorder increases but does so moderately, and the ID / IG ratio reaches a maximum of 1.12. Finally, for TF greater than 465 ° C, Raman spectra of the resulting materials cannot be registered due to fluorescence phenomena.

As a complement to DRX measurements, Raman spectroscopy underlines the decrease in structural coherence caused by fluoride bonding. This evolution is further demonstrated by investigating the average crystal coherent domain size (La) obtained using the inverse relationship between domain size and the ID / IG intensity ratio formulated by Knight and White (Knight DD; White WS; J Mater. Res. 1989, 4, 385). The La value decreases substantially from 14.7 nm for NFCs to 3.9 nm for NFC-F480.

3.3 Solid State NMR

Except in the case of NFC-F380, for which the static 19F NMR spectrum reveals two asymmetric contributions at -170 and -190 ppm / CFCI3, all spectra have a similar shape regardless of the fluorination temperature (Fig. 6) . The maximum mid-height width of the 19F NMR, with a similar value (5.104 Hz) for all samples, is explained by a strong dipolar homonuclear coupling between the fluorine nuclei such as that found in other fluorinated carbons, such as covalent gratifo fluorides: (C2F) n and (CF) n (Dubois, 2006 ibid .; Panich AM; Synth. Metals 1999, 100, 169; Touhara H .; Okino F .; Carbon 2000, 38, 241; Panich AM; Shames AI; Nakajima T .; J. Phys. Chem. Solids 2001, 62, 959; KrawietzT.R .; Haw JF; Chem. Commun. 1998, 19, 2151; Dubois M .; Guerin K .; Pinheiro JP; Fawal Z .; Masin F .; Hamwi A .; Carbon 2004, 42, 1931; Giraudet J .; Dubois M .; Guerin K .; Pinheiro JP; Hamwi A .; Stone WEE; Pirotte P .; Masin F .; J. Solid State Chem. 2005, 118,1262. hereinafter Giraudet 2005a), semiionic compounds (Guerin 2004 and Giraudet 2005a ibid.) and fluorinated charcoal (Touhara 2000 ibid .; Hagaman EW; Murray DK; Cul GDD; Energy &amp; Ft / e / 1998, 12, 399). The center of this symmetric resonance peak is -190 ppm and is attributed to fluorine atoms covalently bonded to carbon atoms (Dubois 2006; Dubois 2004; Giraudet 2005a ibid.). The increase in the content of fluorine nuclei occurs without significant changes in the shape of the peak, which indicates a similar environment for fluorine atoms although the F: C ratios are different. The case of CNF-F380 is different, since two groups of fluorine nuclei are detected (δ = -170 and -190 ppm) (inserted in Fig. 6); they are the result of different interactions between fluorine and carbon atoms or from different environments. Fmóvil interleaving resulting from traces of HF molecules cannot be ruled out, as is the case in type (C4F) n (Panich 1999 ibid.). The low fluoride content (F: C = 0.04) can then lead to an inhomogeneous fluorination on the surfaces of the NFC.

The NMF MAS 19F NMR spectra of NFC-FTF at ambient temperature recorded at a rotation speed of 10.0 kHz are shown in Fig. 7. The spectrum for (C2F) has also been added in order to show its great similarity to the NFC fluoridated The maximum half-height width of NFC-F428 (6800 Hz) is greater than that of the other fluorinated samples (4100 Hz). This can be explained by the efficiency of the MAS experiment, which depends on the structural order of the sample studied; This particular compound is less ordered, as revealed by the DRX. (C2F) n and all types of FCN studied above 428 ° C have more similar spectra, and therefore are not affected by the degree of fluoridation. This confirms that both the CF bond and the fluorine environment are similar in these samples. A pronounced isotropic peak is present at -190 ppm / CFCI3 along with its lateral rotation bands. This peak corresponds to fluorine atoms involved in covalent CF bonds. This covalent character was confirmed by FT-IR spectroscopy (not shown here), which reveals a vibration band at 1215 cm-1, attributed to the covalent CF bond in (C2F) n (Kita 1979 ibid.).

A second less intense resonance at -120 ppm (visible as a shoulder on one of the lateral rotation bands of the CF line) indicates the presence of CF2 groups. However, the content of the CF2 groups is small but sufficient to be detected by NMR MAS 19F. The lateral bands of the resonance&gt; CF2 are also present, in particular the band superimposed on the left side of the isotropic peak of the CF groups and resulting in a shoulder. These groups could be attributed to fluorine atoms located on the edge of the graphite layers or structural defects. These 19F MAS experiments allow other groups to be detected (-CF3) despite their very low quantity. Several narrow lines are present in the -60 / -90 ppm range, superimposed with side rotation bands of the CF and&gt; CF2 peaks. These groups may be located at the edges of the fluorocarbon sheet, and probably have a rotational movement around the CC bonds, which would explain the narrowness of the resonance.

The evolution of the spin-reticulum relaxation time (T1) with the fluorination temperature shown in Fig. 8a provides complementary information on fluoride accommodation if the values ​​are compared with those obtained for conventional high temperature graphite fluoride types ( CF) ny (C2F) n, which yield 450 and 210 ms, respectively (Dubois 2006 ibid .; Giraudet J .; Dubois M .; Guerin K .; Hamwi A .; Masin F .; J. Phys. Chem. Solids 2006, 67 (5-6), 1100.). For a wide range of fluorination temperatures (405 &lt;TF &lt;450 ° C), that is, in correspondence with a wide range of F: C ratios ranging from 0.16 to 0.74, the spin-reticule relaxation times they approach the value determined for (C2F) n (Fig. 8a). This fact, together with the other characterizations, suggests a fluoridation mechanism that involves the formation of (C2F) n and its spread to the nucleus without major structural changes. This assumption will be confirmed below by NMR experiments MAS 13C. Therefore, when the reaction temperature is increased to 480 ° C, T1 progressively approaches a value of 450 ms, similar to that measured in (CF) n prepared using petroleum coke. The conversion from the structure of the type (C2F) to the structure of the type (CF) n goes through a partial exfoliation of fluorinated layers to allow the additional absorption of fluorine.

The case of NFC-F390 is special, since the fluorine content is very low (F: C = 0.09) and the fluorinated parts are essentially located on the surface of the NFC. We believe that the external surface is strongly fluorinated and this could explain the high T1 value (492 ms) recorded for this compound.

Contrary to what happens in carbon fluoride (CF) n (fluorinated coke), the presence of paramagnetic centers constitutes an important relaxation factor in the case of (C2F) n

2001ibid .; Giruadet 2006 ibid.). This process is underlined by the linearity of the 1vs t1 / 2 magnetization curve for short recovery times (Fig. 8b). In fact, under certain conditions, when the spin diffusion constant has the appropriate value, the magnetization for short recovery periods evolves as t1 / 2 (Blumberg WE; Phys. Rev. 1960, 119, 79). This curve is linear for all fluorinated NFCs, as exemplified by the sample obtained at 420 ° C, which provides an additional demonstration of the structural similarities between NFC-FTF and (C2F) n.

13C NMR provides additional information about the nature of the interaction between carbon and fluorine atoms, that is, the CF bond, and about the presence of non-fluorinated carbon atoms. Again in this case, the less fluorinated sample (CNF-F380) differs from the other NFC-FTFs since it has a single wide resonance centered near 120 ppm / TMS (Fig. 9a). This shape is similar to that of pure graphite. Due to the low fluorine content, only a small part of the carbon atoms are linked to fluorine atoms. When the fluorine content reaches 0.16, the line becomes asymmetric and two well-defined resonances are shown at 84-88 and 42 ppm for F: C ≥ 0.31, both related to carbon atoms that manifest sp3 hybridization. The first line (with area represented as SCF) is attributed to carbon atoms covalently bonded with fluorine atoms, as expected in view of the fluorine content found in this temperature range (Panich 1999; Dubois 2004 .; Giraudet 2005a ibid) The other peak is related to non-fluorinated sp3 carbon atoms (Csp3) as is the case of (C2F) n (Dubois 2006 ibid.). A chemical displacement of 42 ppm corresponds to sp3 carbon atoms, as Wilkie et al. for (CxF) n (x&gt; 1) (Wilkie CA; Yu G .; Haworth DT; J. Solid Sate Chem. 1979, 30,197). 1979, 30,197). According to the proposed structural model of (C2F) n consisting of pairs of fluorographs connected by pairs through inter-layer CC covalent bonds (Watanabe N .; Physica B1981, 105, 17; Sato Y .; Itoh K .; Hagiwara R .; Fukunaga T .; Ito Y .; Carbon 2004, 42, 3243), this line could be attributable to sp3 carbon atoms. Since only half of the carbon atoms are fluorinated, the hybridized sp3 carbon atoms are bound exclusively to other carbon atoms. The peak resonance of the pure diamond is expected at 35 ppm (Duijvestjn MJ; Van der Lugt C; Smidt J .; Wind RA; Zilm KW; Staplin DC; Chem. Phys. Lett. 1983, 102, 25), therefore a weak interaction between fluorine and carbon can result in a chemical shift value similar to that observed in the case of (C2F) n (δ = 42 ppm) (Hamwi 1996 ibid; Dubois 2004 ibid.).

The third large resonance peak centered near 120 ppm is mainly attributed to non-fluorinated sp2 carbon atoms, but also to carbon atoms in weak interaction with fluorine (≈ 140 ppm) (Hamwi 1996 ibid; Dubois 2004 ibid.).

The area ratio of the two sp3 carbon resonances is practically constant regardless of the fluorine content; we have SC-F / SCSP 3 = 2.43, 2.37 and 2.34 for fluorinated NFCs at 428, 465 and 472 ° C, respectively. For comparison purposes, the graphite fluoride (C2F) n spectrum is also shown (this sample was obtained by fluorination of graphite at 380 ° C, has a F: C ratio of 0.60 and a small amount of sp2 C (Dubois 2006 ibid.). In this case, the SC-F / SCSP 3 ratio is close to 1.5.

On the other hand, the content of sp2 carbon atoms decreases continuously as the fluorination temperature increases, that is, the content of F: C increases. It should be noted that this process also takes place with (C2F) n when the temperature is increased from 350 to 380 ° C (CF0.51 and CF0.60, respectively) (Dubois 2006 ibid.). Then, FNC fluoridation results in an increase in the F: C ratio without significant structural modification. The formed CF bonds are mostly covalent.

NMR measurements made using MAS and cross polarization 19F → 13C could differentiate between the various carbon atoms. A comparison of the spectra obtained by MAS and CP-MAS for NFC-FTF and (C2F) n (Figs. 9a and 9b) confirms our attributions for the three types of carbon atoms. Since CF groups are favored with CP-MAS compared to their second carbon neighbors, that is, sp3 carbon atoms, only the peak corresponding to CF bonds from the fluorocarbon matrix is ​​increased, unlike the two hybridized sp3 carbon atoms bound exclusively to other carbon atoms (SC-c) and sp2 graphite carbon atoms (SG), the latter disappearing completely.

Considering the peak at 145 ppm, revealed by applying these conditions, the measurements also reveal the presence of sp2 carbon atoms in weak interaction with fluorine, as is the case of graphite fluorides at room temperature (Dubois 2004; 2005a ibid.) . However, such atoms are only present in a very low concentration.

In addition, the resonance of groups&gt; CF2 is also favored by CP-MAS and a small line at 110 ppm is observed as shoulder of the SC-F peak (Fig. 9b). Such groups have already been observed in other studies in various (CF) n (Kita 1979; Touhara 2000; Panich 2001; Krawietz 1998; Wilkie 1979 all ibid.).

While MAS spectra are similar for samples treated between 428 and 472 ° C, the peak intensity related to sp3 carbon atoms decreases significantly for NFC-F480, confirming that the nature of this sample has changed. The spectrum of NFC-F480 is very close to that observed for (CF) n, unlike the other spectra, which have similarities to the spectrum of (C2F) n.

The length of the CF link can be determined by NMR since these data are included in the expression of the dipole coupling. The latter information is lost when using the Hartmann-Hahn cross polarization (CP) associated with MAS, but the dipole coupling can be reintroduced into the spectrum thanks to the reverse cross polarization sequence (PCI). This method and experimental conditions are fully explained in a previous publication about (CF) n (Giraudet J .; Dubois M .; Hamwi A .; Stone WEE; Pirotte P .; Masin F .; J. Phys. Chem. B 2005, 109,175, hereinafter Giraudet 2005b).

For short contact times, it is observed that the amplitude of the CP signal is oscillating, with a frequency

ɸ related to the length of the CF link (Bertani P .; Raya J .; Reinheimer P .; Gougeon R .; Delmotte L; Hirschinger J .; Solid State Magn. Res. 1999, 13, 219). This behavior is observed only in carbon covalently bonded with fluorine, and not in sp3 carbon atoms. Using the PCI sequence, the integrated peak intensity of the carbon spectra was calculated as a function of the contact time, thus revealing the CP dynamics

was ϕ = 3976.0 ± 18.6 Hz. From

(Dubois 2006; Giruadet 2005b; Bertani 1999, all ibid.).

With regard to (C2F) n (Dubois 2006 ibid.) And (CF) n (Giruadet 2005b ibid.), The Fourier transform of the 13C magnetization evolution for CF groups yields the Pake-like structure for all grades of fluoridation of the NFCs (Fig. 10b). The length of the CF link (rCF) can be deduced from the Pake structure using the wings, which are related to dipole fluctuation. S1 is the separation between said wings. Link length

(nm) (Dubois 2006; Giruadet 2005b; Bertani 1999, all ibid.).

It can be estimated from the equation:

The S1 value is 7700 Hz for all fluorinated NFC samples. In all the cases studied, a CF link distance of 0.138 ± 0.002 nm is determined. It should be noted that these values ​​estimated by NMR could be overestimated due to possible molecular movements that reduce the value of the second moment, inversely proportional to the distance CF. The CF link lengths in NFC-FTF are close to those obtained by the same NMR procedure for (C2F) n (Dubois 2006 ibid.) And (CF) n (0.138 nm) (Giruadet 2005b ibid.) Indicating that the nature of the CF bond is similar in these three types of compounds.

3.4 RPE Study

The virgin sample has no RPE signal within the spectrometer detection limits. Figure 11a shows the fluorinated NFC RPE spectra. The origin of the main broad line was attributed to carbon-free bonds that have a localized spin. Such spin carriers have been postulated for other fluorinated carbons obtained in an F2 atmosphere at 600 ° C, starting with graphite (Panich 2001 ibid.) Or graphite fluoride at room temperature (Dubois 2004; Giraudet 2006 ibid), but also for amorphous carbon thin film (Yokomichi H .; Morigaki K .; J. Non-Cryst. Solids 2000, 266, 797; Yokomichi H .; Hayashi T .; Amano T .; Masuda A .; J. Non-Cryst. Solids 1998, 227, 641) or nano-sized graphite fluorides (Takai K .; Sato H .; Enoki T .; Yoshida N .; Okino F .; Touhara H .; Endo M .; Mol. Cryst. Liq. Cryst 2000, 340, 289).

The RPE parameters are summarized in Figs. 12a and 12b and in Table 2. Again, fluorinated NFCs at the lowest and highest temperature (TF &lt;405 ° C and TF = 480 ° C) differ from the other samples. The spectrum of NFC-F380 is asymmetric (the ratio of A / B intensity of the positive and negative parts of the derived curve is close to 0.6 (Fig. 12a)). This asymmetry, which reveals different contributions to the spectra, gradually disappears as fluoridation increases. Thus, A / B leads to 1. The simulation of the spectra reveals three contributions for NFC-F380, NFC-F472 and NFC-F480 (Figs. 11b, 11c and 11d, respectively, as well as Table 2). These simulations were carried out using WinSimfonia (Bruker software).

First, by analogy with (C2F) n and (CF) n, the broad line 4 observed could be due to the combined contributions of free bonds in interaction with oxygen (Dubois 2006 ibid.) And, in the cases of (CF) and CNF-F480, to the unsolved super-hyperfine structure that has practically the same line width for both samples (Giraudet 2005b ibid). Since the line widths are very similar, line 2 could be related to free links located in the neighborhood, similar to (C2F) n (Tab. 2). For the same reason, we can attribute line 3 to structural defects similar to those present in (CF) n (Giraudet 2006 and 2005b ibid). The content of these spin carriers increases with the fluorination temperature and becomes predominant for NFC-F480 (the free links of line 2 disappeared completely). Line 1, responsible for asymmetry (Fig. 11b), appears for the reaction at the lowest temperature (380 and 390 ° C) when inhomogeneous fluorination occurs near the surface. The narrowness of this signal is the result of various interactions with the fluorocarbon matrix and / or the presence of F-intercalated. On the one hand, the density of spin (Ds that is, the number of free bonds per mass of the sample) increases continuously with the fluorination temperature, as shown in Fig. 12b, due to the propagation of the fluorinated parts , which contain paramagnetic defects; It should be noted that the NFCs do not have RPE lines. For TF located between 380 and 472 ° C, Ds approximates that of (C2F) n (17 1019 spins.g-1, see table 2). On the other hand, Ds increases drastically for NFC-F480 and becomes increasingly similar to that of type (CF) n (15.6 1020 spins.g-1, this sample was obtained with natural graphite) according to structural conversion of (C2F) n in (CF) n, which has been previously postulated in this paper.

Four. General debate

The particular structure of the NFCs seems to favor the formation of materials of the type (C2F) as the fluorination progresses. This structure (C2F) n, consisting of pairs of fluorograph layers, is formed and preserved independently of the fluorination temperature (405 &lt;TF &lt;450 ° C), over a wide range of composition (from F : C from 0.16 to 0.74). This characteristic can be explained by the existence of a fluoridation process near the external walls when low temperatures are applied, and which then advances towards the internal walls as the temperature increases. This mechanism differs from that of graphite fluoridation, since there is a progressive conversion from (C2F) n to (CF) n as the temperature increases within the range 350/600 ° C (Nakajima 1991; Kupta V .; Nakajima T. ; Ohzawa Y .; Zemva B .; J. Fluorine Chem. 2003,120,143). While (C2F) n and (CF) n are formed by a fluorine intercalated phase with graphene layers (Kupta 2003 ibid.), This intermediate phase does not appear to be involved during the formation of NFCs. An increase in the fluorination temperature up to 480 ° C causes a partial decomposition of the NFCs. The mechanism could be similar to a partial peel. The 13C NMR spectrum (Fig. 11) of this resulting material clearly shows the low contents of both virgin sp2 carbon atoms and sp3 carbon atoms related to type (C2F) n. The partial decomposition of fluorinated parts leads to the conversion of (C2F) n to (CF) n.

Although the evolution of the F: C ratio for NFCs in the fluoridation conditions applied here could be expected, this study reveals some interesting new features. In contrast to previous works dedicated to NCPM (Hamwi 1997 ibid.), The high degree of purity of our gross NFC allows the chemical composition to be accurately deduced from the F: C ratio. In previous studies, this circumstance was underestimated due to the presence of reactive carbon in the virgin sample, which formed volatile carbon and fluorine derivatives. Furthermore, the increase in the F: C ratio within the narrow temperature range [420-435 ° C] had never been described before; only a process of intense fluoridation between 400 and 500 ° had been previously suggested (Nakajima 1996 ibid.). In addition, another important finding in this study is the determination of both a temperature limit before the decomposition of the NFCs and a limit of the level of fluoridation for the NFCs. Based on the complementary characterization using NMR, Raman and DRX, it is possible to classify fluorinated materials into three different types, depending on the treatment temperature:

for lower fluorination temperatures (TF &lt;420 ° C), the fluorinated samples have a low fluorine content and their structure is similar to that of virgin NFCs, as revealed by Raman diffusion and DRX. The fluorine atoms are located on the surface of the NFC, that is, on the outer walls.

In the 420-465 ° C temperature range, drastic changes take place as a result of the increase in the F: C ratio, which is rapid for T &lt;435 ° C. Subsequently the fluoridation process slows down; First, the samples become biphasic (NFC and NFC-F) and the new NFC-F phase has crystallographic similarities with graphite fluoride type (C2F) n, which involve paired fluorograph layers connected by covalent CC bonds interlayer This has been demonstrated by the presence of hybridized sp3 carbon atoms bound exclusively to carbon atoms in the fluorocarbon interlayer. The curvature of the graphene layers and / or their stacking seems to limit both fluoridation compared to graphite and to favor the formation of this phase of type (C2F) n. Raman dispersion reveals that the concentration of structural defects increases with fluorine content. In addition, the incorporation of fluorine atoms takes place through the formation of covalent CF bonds. The type of interaction between carbon and fluorine atoms does not vary with fluorine content. Therefore, the fluoridation process must start from the external walls, forming a configuration (C2F) n, and then move towards the core of the NFC.

5 When the fluorination temperature is increased above 465 ° C, a slight exfoliation occurs that decomposes the NFC-FTF as clearly shown in the case of NFC-F480. This mechanism was revealed by both 13C NMR and DRX, which clearly show the conversion of (C2F) n into (CF) n made possible by partial exfoliation. However, the conversion of (C2F) n to (CF) n is low for treatment temperatures of 472 ° C and takes place mainly at TF≥480 ° C. The occurrence of fluorescence in the dispersion spectra

10 Raman could be indicative of the formation of higher (CF) na TF.

5. conclusion

The reaction of fluorine gas with carbon nanofibers in a temperature range of 380 to 480 ° C has been studied. The fluorine content increases from CF0.31 to CF0.70 in a narrow temperature range [420-435 ° C]. 472 ° C appears as the maximum temperature before the partial decomposition of the NFC. At lower temperatures, only superficial fluorination occurs. In the temperature range 420-435 ° C, the samples become biphasic and the new crystallographic phase, in addition to the gross NFCs, shows some similarities with graphite fluoride type (C2F) n. This phase is formed, regardless of the fluorine content, above a reaction temperature of 420 ° C, suggesting that the fluorination takes place from the outer walls inwards as the fluorination temperature increases. In addition, the incorporation of fluorine atoms takes place through the

twenty formation of covalent CF bonds, regardless of fluorine content. When comparing graphite fluorides of types (CF) n and (C2F) n, the structural parameters such as distance between layers, spin-reticle relaxation time T1, density and environment of paramagnetic free bonds, are sufficiently different to allow us to determine which phase is formed during the fluoridation of carbon nanofibers as a function of temperature.

25 Table 1. F: C ratio obtained by weight absorption and by quantitative NMR, together with Raman ratio ID / IG of the various fluorinated NFCs.

<dl><dt>TF (° C) </dt><dd>Gravimetry F: C NMR F: C b / lG </dd></dl>

<dl><dt>CNF-F380 </dt><dd> 380 0,04 0,06 0,30 </dd></dl>

<dl><dt>CNF-F390 </dt><dd> 390 0,09 0,09 0,29 </dd></dl>

<dl><dt>CNF-F405 </dt><dd> 405 0,16 0,15 0,26 </dd></dl>

<dl><dt>CNF-F420 </dt><dd> 420 0,31 0,39 0,78 </dd></dl>

<dl><dt>CNF-F428 </dt><dd> 428 0,59 0,59 0,89 </dd></dl>

<dl><dt>CNF-F435 </dt><dd> 435 0,70 0,68 1,02 </dd></dl>

<dl><dt>CNF-F450 </dt><dd> 450 0,74 0,74 1,12 </dd></dl>

<dl><dt>CNF-F465 </dt><dd> 465 0,78 0,86 0,90 </dd></dl>

<dl><dt>CNF-F472 </dt><dd>472 0.73 0.90 Fluorescence </dd></dl>

<dl><dt>CNF-F480 </dt><dd>480 0.70 1.04 Fluorescence </dd></dl>

<dl><dt>CNF-F490 </dt><dd>490 Partial decomposition </dd></dl>

Table 2. Data from the RPE spectra

<dl><dt>ΔHPP (G) </dt><dd>A / B Line 3 Ds (spins.g-1) </dd></dl>

<dl><dt>Line 1 </dt><dd>Line two Line 3 Line 1 Line two </dd></dl>

<dl><dt>± 0.2 G </dt><dd /><dt>± 0.2 G </dt><dd /><dt>± 0.2 G </dt><dd /><dt>± 0.2 G </dt><dd /><dt>± 0.2 G </dt><dd /><dt>± 0.2 G </dt><dd /></dl>

<dl><dt>NFC-F380 </dt><dd> 6,2 15,0 - 60 0,6 0,5 1020 </dd></dl>

<dl><dt>NFC-F472 </dt><dd> - 11,0 19,5 60 1 2,4 1020 </dd></dl>

<dl><dt>NFC-F480 </dt><dd>- - 17.4 80 ESHF * 1 7.7 1020 </dd></dl>

<dl><dt>(C2F) n </dt><dd> - 13,5 80 1 1,7 1020 </dd></dl>

<dl><dt>(CF) n (from petroleum coke) </dt><dd>20.9 80 ESHF * 1 15.6 1020 </dd></dl>

* super-hyperfine structure (ESHF) with (2nl + 1) = 7 lines, where = 6 is the number of neighboring fluorine nuclei (nuclear spin number I = 1/2) (coupling constant A = 45 ± 2 G, width line ∆HPP = 36 G ± 2G) (Dubois 2006; Giraudet 2006)

** S1 = SESHF + ESHP not resolved

Data for (C2F) n taken from Dubois 2006; data for (CF) n taken from Giraudet 2005b

Example 2: Electrochemistry of fluorinated carbon nanofibers

For electrochemical tests, the electrodes were composed of at least partially fluorinated carbon nanofiber sample, a conductive material and a binder. For the samples tested at a constant discharge rate of 10 Akg-1 whose results are shown in Table 3, the electrode composition was approximately 80% by weight of fluorinated nanofibers, 10% by weight graphite and 10% by weight polyvinylidene difluoride (DFPV) as a binder. The electrode was then mounted in a two electrode cell in which the electrolyte was composed of a solution of 1 mol.L-1 of LiCIO4 dissolved in propylene carbonate. A microporous DFPV film containing the electrolyte was inserted between the graphite fluoride electrode and a lithium metal sheet.

For the samples whose results are shown in Figures 13-16 and Table 4, the electrode composition was approximately 75% by weight fluorinated nanofibers, 10% by weight graphite of acetylene black and 15% by weight difluoride of polyvinylidene (DFPV) as a binder. These three materials were mixed together in acetone solution with 20% bis (n-butyl) phthalate (DBF). The solution was then evaporated until a thin film of CFX was obtained. The film was cut to the desired diameter and dried overnight in vacuo. The electrolyte was LiBF4 dissolved in propylene carbonate (PC) and dimethyl ether (DME). The separator was Celgard®, thickness of 25 microns, porosity of 55%. The separator containing the electrolyte was inserted between the graphite fluoride electrode and a sheet of lithium metal.

For the samples whose results are shown in Figure 18, the electrolyte was 1 M LiBF4 dissolved in propylene carbonate (PC) and dimethyl ether (DME) (3: 7) (5% discharge at C / 20 RT) .

The discharge profile of fluorinated carbon Li / nanofiber cells is shown in Figs. 13-16. For reference, Fig. 17 shows a discharge profile for a conventional Li / CF cell. These cells have a characteristic plateau corresponding to the formation of LiF according to equation 1.

The main electrochemical characteristics of fluorinated NFC are summarized in Tables 3 and 4. In Table 3, the discharge rate yielded a constant value of 10 Akg-1. The energy density reached, Spec (Whkg-1 units), and the power density, Pspec (Wkg-1 units), are determined from the discharge curves by applying equations (5) and (6):

In the equations for Spec and Pspec, q (i) and &lt;ei&gt; respectively represent the discharge capacity (Ah) and the average discharge voltage (V) with a current i (A), and m is the mass of (CFx) n active at the electrode (kg).

At a constant discharge rate of 10 Akg-1, the average potential of all the samples studied is approximately the same and is equivalent to 2.5 V. This result is consistent with a covalent CF bond that does not change with fluoridation, as is has established through 13C NMR. In addition, since the fluorination temperature range is narrow, the non-dependence of the nature of the CF bond with respect to the reaction temperature is easily understandable. Since the power density P (W kg-1) is directly proportional to the average discharge voltage and the current density applied to the cell (which was a constant of 10 A kg- '), the power density is also constant at 25 W kg-1 regardless of the fluorinated compound.

On the other hand, discharge profiles differ greatly in their tension and shape. This result is mainly due to the increase in F / C with the fluorination temperature. The higher the F / C, the greater the discharge capacity. Only the 405 ° C fluoridated sample should not present a constant discharge potential like the other fluorinated MWCNTF. That may be related to the fluoride fixation site. In fact, since F is expected to be located on the surface of the, the concentric shape of the sheets results in a slight dispersion of the CF bond energy and the different electrochemical potentials of the formation of LiF lead to a gradual discharge plateau . For the other samples a potential constant discharge is present and corresponds to the formation of LiF from fluorine intercalated between the fluorocarbon matrix. While the discharge voltage and F / C fluorine content of F are close to the compound (C2F), the capacity of the fluorinated MWCNTF between 428 and 450 ° C is 30% higher than that of the commercial (C2F).

As regards the energy density, the maximum value obtained is high and the evolution of the energy density takes place as that of the capacity at fluorination temperature since the discharge voltage is constant. The maximum of the electrochemical performances of MWCNTF465 is due to the unique evolution of the faradaic performance with the fluorination temperature. The faradaic performance, which is defined as the proportion of the discharge capacity in the theoretical specific discharge is low for the less fluorinated and is approximately 100% for a fluorination temperature greater than 450 ° C. As would be expected, the presence of (even in small quantities) of high electrical conductivity, together with the insulating carbon fluoride, should favor the columbic efficiency.

For the samples in Table 3, it was observed that the low potential of discharge time increased with the fluorination temperature. This is related to the decrease in the electrical conductivity of the sample when the fluorinated parts advance, that is, the decrease in the amount of virgin NCPM in the sample. However, the diffusion of lithium seems to limit electrochemical processes and determine electrode yields. This could be explained by the structure of the MWCNTFTT: a biphasic domain cannot constitute a preferred route for the diffusion of lithium, while with a fluorination temperature greater than 450 ° C, a graphite fluoride structure with a low defect level allows to lithium diffuse more easily by particles in general and more particularly when the current density is low as in our case.

A maximum of the electrochemical yields (Spec, Pspec) was obtained for the fluorination temperature at which F / C is maximum and the graphite fluoride phase of type (C2F) is mainly present in the compound, that is, for a treatment temperature equal to 465 ° C.

The stability of open circuit voltage over time (self-discharge) was investigated for several fluorinated NFC compositions. Figure 18 shows the excellent temperature stability of these materials.

Table 3

<dl><dt>TF (° C) </dt><dd>F / C ID / IG C (mA / g) &lt;E&gt; (V) Spec (Wh / kg) Pspec (W / kg) Yield %</dd></dl>

<dl><dt>NFC380 </dt><dd> 380 0,044 0,30 </dd></dl>

<dl><dt>NFC390 </dt><dd> 390 0,090 0,29 </dd></dl>

<dl><dt>NFC405 </dt><dd> 405 0,160 0,26 153 2,49 381 24,9 54 </dd></dl>

<dl><dt>NFC420 </dt><dd> 420 0,312 0,78 325 2,50 813 25,0 61 </dd></dl>

<dl><dt>NFC428 </dt><dd> 428 0,587 0,89 530 2,53 1341 25,3 78 </dd></dl>

<dl><dt>NFC435 </dt><dd> 435 0,700 1,02 </dd></dl>

<dl><dt>NFC450 </dt><dd> 450 0,737 1,12 </dd></dl>

<dl><dt>NFC465 </dt><dd> 465 0,775 0,90 760 2,40 1824 24,0 98 </dd></dl>

<dl><dt>NFC472 </dt><dd>472 0.730 F 725 2.50 1813 25.0 96 </dd></dl>

<dl><dt>NFC480 </dt><dd>480 0.700 F </dd></dl>

* F: fluorescence

Table 4

<dl><dt>CFx </dt><dd>Theoretical capacity (mAh / g) Max. vel. available load Max. energy density (Wh / kg) Max. power density (W / kg)</dd></dl>

<dl><dt>Commercial CFx (x = 1.0) </dt><dd>865 1 C 2012 1370 </dd></dl>

<dl><dt>NFC, x = 0.21 </dt><dd>352 6C 620 3434 </dd></dl>

<dl><dt>NFC, x = 0.59 </dt><dd>681 6C 1587 7866 </dd></dl>

<dl><dt>NFC, x = 0.76 </dt><dd>771 6C 1749 8057 </dd></dl>

As they should be interpreted herein, the terms "comprise", "understand", "included" specify the presence of the characteristics, elements, steps or components referred to, but do not exclude the presence or addition of one or more other characteristics, elements, steps, components or group of these Where the terms "comprise", "understand", "included" are optionally substituted by grammatically analogous terms, e.g. ex .; "Consistent / consists (n)" or "consisting essentially of / consists (n) essentially in" to thus describe other embodiments that are not necessarily coextensive, it is also intended to encompass embodiments other than the invention.

REFERENCES