Synthesis of planar-sheet graphite fluorides using fluorometallate salts of transition metals in anhydrous hydrogen fluoride

doi:10.1016/0008-6223(93)90131-S

Carbon

Volume 31, Issue 3, 1993, Pages 437-443

https://doi.org/10.1016/0008-6223(93)90131-S Get rights and content

Abstract

Planar-sheet graphite fluorides, C_xF, are prepared at ambient temperature by the reaction of graphite with the transition fluorometallates K₂MnF₆ and K₂NiF₆ in anhydrous hydrogen fluoride (AHF). In the case of K₂MnF₆, this represents the first chemical route to graphite fluorides without use of elemental fluorine. For each reagent, the net reactions are identified by stoichiometry and X-ray diffraction of the solid products. The graphite fluorides generated at the oxidation limit have stoichiometries close to C₆F and C₂F for the Mn⁴⁺ and Ni⁴⁺ fluorometallates, respectively. Na₃FeF₆ will also oxidize graphite to C_xF in AHF, although homogeneous products were not obtained due to rapid solvolysis of the fluorometallate. CoF₃ and KF react in AHF to release F₂. C_xF is produced when graphite is reacted with this oxidizing combination. The reactions and oxidation limits are discussed in terms of a thermodynamic model for graphite intercalation. A synthetic strategy for the preparation of planar-sheet graphite fluorides in organic solvents is proposed.

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Electrochemical oxidation of graphite in aqueous hydrofluoric acid solution at high current densities
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Citation Excerpt :
However, the capacity of it still lower than that of graphite fluoride, therefore, preparation of CxF with higher fluorine content has been extensively studied. In order to obtain CxF with high fluorine content, solid-gas reaction using fluorine gas in the presence of catalysts such as solid metal fluorides, gaseous halogen fluorides, etc. has been usually employed [8–20]. Among them by using KAgF4 under high F2 gas pressure in anhydrous HF solution, CxF with the minimum x value (highest fluorine content) of 1.2 has been prepared [18].
Electrochemical oxidation of graphite was performed at high current densities in 47% HF aqueous solution. Cyclic voltammetry indicated that covalent CF bonding formed above 2.4 V vs Pb/PbF₂. The sample obtained at higher current densities than 200 mA cm⁻² was stage 1 type material with an interlayer spacing of 0.55 nm and it contained a considerable amount of oxygen, together with the covalently bonded fluorine. The discharge profile of this sample as a cathode of lithium primary battery was similar to that of C_2.5F prepared under F₂ gas atmosphere and the capacity reached 550 mAh g⁻¹. This strongly indicated that not only fluorine but also oxygen in this sample was utilized.
Carbothermal treatment for the improved discharge performance of primary Li/CF<inf>x</inf> battery
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Carbothermal treatment was used to improve the discharge rate performance of primary lithium/carbon monofluoride (Li/CF_x with x = 1) batteries. The treatment was carried out by heating a mixture of CF_x and carbon black (CB) just below the decomposition temperature of CF_x under nitrogen for 2 h. In the treatment, poly(vinylidene fluoride-co-hexafluoropropylene) (Kynar) was used as a fluorinated polymer binder to press the CF_x/CB mixture into pellets. It was shown that the content of Kynar significantly affected the discharge performance of the resulting treated-CF_x (T-CF_x). This can be attributed to the catalytic effect of HF formed by the pyrolysis of Kynar on the decomposition of CF_x and on the reaction of CB with the volatile fluorocarbons formed by the decomposition of CF_x. The discharge performance of T-CF_x cathode was also affected by the temperature of carbothermal treatment and by the ratio of CF_x to CB. In this work the best result was obtained from a treatment conducted at 470 °C on a 87CF_x/10CB/3Kynar (by weight) mixture. In the discharge condition of C/5 and 20 °C, the Li/CF_x cell with such-obtained T-CF_x cathode showed about 95 mV higher voltage than the control cell while retaining nearly the same specific capacity. Impedance analyses indicate that the improved discharge performance is mainly attributed to a reduction in the cell reaction resistance (R_cr) that includes an ohmic resistance related to the ionic conductivity of the discharge product shell and a Faradic resistance related to the processes of charge-transfer and Li⁺ ion diffusion in the CF_x reaction zone.
Effect of reaction time on the composition of graphite bis(trifluoromethanesulfonyl)imide
2007, Carbon
Graphite intercalation compounds (GICs) of composition C_xN(SO₂CF₃)₂ · δF are prepared under ambient conditions in 48% hydrofluoric acid, using K₂MnF₆ as an oxidizing reagent. The stage 2 GIC product structures are determined using powder XRD and modeled by fitting one dimensional electron density profiles. A new digestion method followed by selective fluoride electrode elemental analyses allows the determination of free fluoride within products, and the compositional x and δ parameters are determined for reaction times from 0.25 to 500 h.
Influence of cointercalated HF on the electrochemical behavior of highly fluorinated graphite
2004, Journal of Power Sources
Highly fluorinated graphite was prepared at room temperature using high oxidation state transition metal complex fluoride (K₂PdF₆, K₂MnF₆, K₂NiF₆ or KAgF₄) and elemental fluorine under pressure ((5.9–11.8) ×10⁵ Pa) in anhydrous liquid HF (aHF). The composition of the fluorinated graphite samples ranged from C_1.1F to C_1.9F containing small amounts of HF. IR absorption spectra revealed that stage 1 phase of C_xF contained several different phases with planar (sp²) and puckered (sp³) graphene layers. Electrochemical discharge of the fluorinated graphite showed that profile of discharge potential and discharge capacity varied depending on the amount of cointercalated HF. The C_xF samples with less amounts of HF and HF₂^δ− had relatively flat discharge potentials and large discharge capacities. The discharge capacity reached 500–600 mAh/g in 1 mol/dm³ LiClO₄–propylene carbonate solution at 25 °C. Chemical diffusion coefficients of Li⁺ ion in the intermediate discharge products were (4.4–13) ×10⁻¹² cm²/s from impedance measurement.
Electrochemical behavior of graphite highly fluorinated by high oxidation state complex fluorides and elemental fluorine
2000, Electrochimica Acta
Highly fluorinated graphites have been synthesized by using elemental fluorine under pressure and high oxidation state complex fluoride, K₂NiF₆ or KAgF₄ in anhydrous hydrogen fluoride. Their electrochemical discharge as a cathode of primary lithium battery has been investigated in organic electrolyte solution. Main products were stage 1 compounds or the stage 1 involving stage 2 and 3 phases as minor components with approximate compositions of C_1.6F. X-ray photoelectron spectra indicated the existence of nearly semi-ionic C–F bonds. Electrochemical studies demonstrated that the C_xF samples prepared using KAgF₄ and elemental fluorine exhibited high and flat discharge potentials of 3.2 V versus Li/Li⁺ and discharge capacities of ∼500 mA h g⁻¹ at cut-off potential 1.5 V.
Fluorine reactivity with graphite and fullerenes. Fluoride derivatives and some practical electrochemical applications
1996, Journal of Physics and Chemistry of Solids
The reactivity of fluorine with graphite depends strongly on reaction temperature and purity of the fluorine atmosphere. The presence of fluoride impurities accelerates the reaction even at room temperature. The CF bond character ranges from covalent to ionic or semi-ionic. The electrochemical performance of fluorinated graphite products as cathode material in lithium cells is related to lamellar structure, ionicity and fluorine content. Unlike graphite, fullerenes react readily with pure fluorine even at room temperature. Highly fluorinated C₆₀ compounds of high crystallinity are obtained at 300 °C. A fully fluorinated product C₆₀F₆₀ can also be isolated.

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Synthesis of planar-sheet graphite fluorides using fluorometallate salts of transition metals in anhydrous hydrogen fluoride

Abstract

Solid State Ionics

Synth. Met.

Carbon

Carbon

Synth. Met.

Z. Anorg. Allg. Chem.

J. Am. Chem. Soc.

J. Chem. Soc. Chem. Comm.

Phil. Trans. R. Soc. Lond.

J. Electrochem. Soc.

J. Chem. Soc. Chem. Comm.