Fluorination Of Graphite Research Articles

Tuning fluorine and oxygen distribution in graphite oxifluorides for enhanced performances in primary lithium battery

Fluorine Graphite Intercalation Compounds are one of the most common cathode used in primary lithium batteries, mainly for their high delivering potential. Nevertheless, to increase their capacities, fluorine can be partially substituted by oxygen. Oxifluorides offer then versatility to design cathode materials delivering the upmost performances. Combining the choice of the precursor (either graphite oxide or sub-fluorinated graphite fluoride) and two-step synthesis processes, which include Hummers' oxidation and direct fluorination, allow the tuning of the fluorine and oxygen atoms distribution. According to the sequence, i.e. fluorination/oxidation or oxidation/fluorination, several oxifluorides were then prepared. On one hand, direct fluorination using F2 gas resulted in homogeneous fluorine dispersion when precursor was graphite oxide. On the other hand, Hummers’ oxidation was modulated according to the temperature parameter. COH, COC, COOH, CF, CF2 and CF3 were identified through multinuclear solid-state MAS-NMR. Covalence and environment, especially in-plane organization of those chemical groups, were assessed. When oxifluorides are then used as cathode in primary lithium battery, the better performances are obtained for 3 phases compounds where fluorinated part insured high potential whereas oxygenated part allowed durability to be reached; carbonaceous region providing conductivity. A maximum of about 2400 Wh/kg of energy density is attempted.

Fluorination of nuclear graphite IG-110 in molten 2LiF-BeF2 (FLiBe) salt at 700 ºC

It is important for the development of advanced nuclear reactors to understand if the exposure to molten fluoride salts changes physical properties of graphite and its ability to absorb tritium. This paper demonstrated that the interaction between high purity nuclear grade graphite IG-110 and molten salt mixture 2LiF-BeF2 (FLiBe) at 700 °C for twelve hours in argon envirnoment resulted in graphite fluorination and changes in graphite microstructure. XPS C1s spectra analysis shows an increase in F functionalization (from 3.5% to 9.0–19.3%), an increase in the sp3/sp2 ratio (from 0.1496 to 0.1798), and F1s spectrum analysis confirms appearance of CF bonds. Raman spectra show an increase in ID/IG ratio from 0.35 to 0.69 and an appearance of peak shoulders characteristic of CF bond formation. GDMS depth analysis from the surface to 5 μm depth shows a higher depth penetration of F compared to that of Li and Be, indicative of surface salt reduction and bulk graphite fluorination. FLiBe reduction by graphite fluorination is not thermodynamically favorable, but certain carbon sites in the graphite are more reactive than the majority of carbon atoms in graphite. We argue that graphite fluorination occurred at these reactive sites, leading to the reduction of salt at the graphite surface. XRD and Raman confirmed microstructural changes. This indicates that the fluorination process may introduce new active sites as existing active sites are consumed, which could be advantageous for tritium chemisorption from FLiBe.

Large-scale synthesis of fluorinated graphene by rapid thermal exfoliation of highly fluorinated graphite.

Weakly fluorinated graphene nanosheets were efficiently prepared via fast thermal exfoliation of highly fluorinated graphite. This scalable method consists of a fast temperature increase (around 10 °C s-1) of fluorinated HOPG performed under an argon atmosphere, without any post-treatment. The mechanism of exfoliation induced by the defluorination step, evolving volatile fluorocarbons, has been highlighted thanks to thermogravimetric analysis (TGA), high temperature X-ray diffraction (HTK-XRD), X-ray Photoelectron Spectrometry (XPS) studies on temperature, and Raman and infrared spectroscopy. The advantages of the described process are its high-yield for the preparation of fluorinated graphene nanosheets, its ease of implementation and scalability. Moreover, the fluorine content drastically decreases during such a process and graphene layers are slightly functionalized, i.e. conductive contrary to the precursor.

Fluorine-graphite intercalation compound (C4F)n at high pressure: Experimental and theoretical study

In this paper the structural and vibrational properties of intercalated fluorine graphite (C4F)n under compression up to 40 GPa are investigated by means of Raman spectroscopy, X-ray diffraction and ab initio calculations. The theoretical study was performed within the framework of the density functional theory, taking into account van der Waals corrections to the total energy. We studied the effect of anisotropic and isotropic deformation on the crystal properties, in relation to the layered structure of the sample and in particular to the CF bonding. The evolution with pressure of the phonon modes at the center of the Brillouin zone was calculated and compared with experimental data. Both our experimental data and theoretical models indicate that a structural transition occurs around 10 GPa. Above this pressure the (C4F)n structure becomes dynamically unstable, characterized by a “soft mode” and a change in symmetry due to anisotropic strain components.

Basic Phytotoxicity of Carbon Foils Prepared by Oxidization of Graphite Combined with Fullerene, Fluorine Graphite and Biochar

This article describes the preparation of hybrid-based compounds of graphene oxide, which are obtained by shared oxidization of graphite with fullerene C60, fluorine graphite and biochar. Suspensions of these compounds were prepared into the form of the foils. Subsequently its morphology, internal structure, internal identification groups, thermal stability is described but this work is primarily focused on its phytotoxicity by germination seed test on white mustard, which was performed directly onto the foils.

Synergistic effects of AlB2 and fluorinated graphite on the mechanical and tribological properties of hybrid fabric composites

In present work, the dry sliding wear properties of hybrid PTFE/Nomex fabric/phenolic composite (hybrid fabric composite), filled with aluminum diboride (AlB2), fluorinated graphite (FGr), and simultaneously filled with above fillers were investigated using a pin-on-disc tribo-meter. The wear rate and friction coefficient of hybrid fabric composite reinforced by different fillers were determined. Besides, the effects of fillers on hardness, thermal stability, thermal conductivity of hybrid fabric composites were systemically studied. The results showed that 3 wt% AlB2 filled hybrid fabric composite exhibited excellent anti-wear properties. The optimal filler percentage was determined by changing the content of FGr from 2 to 8 wt% whereas AlB2 was kept constant at 3 wt%. Those results mainly attributed to the improved hardness, thermal stability and thermal conductivity of hybrid fabric composite. Furthermore, the transfer film formed on the counterpart pin surfaces was demonstrated to be efficient in enhancing the tribological properties of filler reinforced hybrid fabric composites. The worn surfaces of the hybrid fabric composites were studied by scanning electron microscopy (SEM) and the corresponding wear mechanisms were discussed based on the characterizations.

Photoexfoliation of two-dimensional materials through continuous UV irradiation

This paper describes the preparation of fluorinated graphene nanosheets (FGNs) through photoexfoliation of fluorinated graphite (FG) in the liquid phase. We discovered that UV irradiation of FG dispersions in N-methyl-2-pyrolidone facilitated exfoliation to give FGNs. Transmission electron microscopy and atomic force microscopy revealed that the average thickness of the FGNs was approximately 3 nm; they were considerably thinner than the nanosheets prepared using a conventional sonication approach. Furthermore, when the FGNs were deposited uniformly onto substrates (through spin coating), they formed effective cathode interlayers for polymer solar cells (PSCs), the efficiency of which was 60% greater than that of PSCs containing FGNs prepared through ultrasonication.

Facile synthesis and spectroscopic characterization of fluorinated graphene with tunable C/F ratio via Zn reduction

A facile approach has been developed to synthesize fluorinated graphene (FG) nanosheets with tunable C/F ratio through liquid-phase exfoliation of fluorinated graphite (FGi) in N-methyl-2-pyrrolidone (NMP) and subsequent reduction with Zn powder. The evolution of surface composition and electronic structure of FG is investigated by X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES) spectroscopy and Raman spectroscopy. It is shown that the C/F atomic ratio of FG can be easily varied by Zn reduction with different reaction time and addition of diluted HCl solution. Both XPS and XANES results indicate the effective removal of covalent CF bonds and progressive restoration of sp2-hybridized structure upon the treatment of FG with Zn. Raman measurements further reveal that Zn reduction can effectively restore the π-electron conjugated electronic structure and reduce the optical gap of FG. This study provides a simple and effective route to tailor the surface composition and electronic structure of FG for potential applications.

Experimental and DFT high pressure study of fluorinated graphite (C2F)n

We studied the high pressure behavior of poly(dicarbon monofluoride) (C2F)n up to 37 GPa both theoretically, within the first-principle framework of the density functional theory, taking into account van der Waals corrections to the total energy, and experimentally, combining in situ Raman spectroscopy, XRD and X-ray photoelectron spectroscopy (XPS) measurements. The (C2F)n structure has a complex mixed sp2 sp3 character where graphene layers are intercalated among sp3 carbon hybridized structural domains. We observed three phase transitions in the high pressure range explored. The first transition is observed around 12 GPa and is dominated by strain effects. Above 16 GPa the intercalated graphene layers corrugate and change from the sp2 to sp3 hybridization state. Above 25–27 GPa the new pressure-induced phase becomes unstable. We find that, by varying applied pressure or by changing the hybridization due to the fluorination of graphite, the band structure of (C2F)n can be modified, resulting in valence and conduction bands touching at the K point, as in graphene, thus turning (C2F)n into a semimetal. This could provide a new pathway to reversibly engineer the band structure and conductivity for applications in high energy density batteries.

Design on CFx Cathode for Sodium Secondary Batteries

After investigations on many kinds of fluorides, carbon fluoride (or graphite fluoride, CFx) is the best material as cathode of primary lithium battery with ultrahigh energy density [1-2], while F2 gas is difficult to operate as cathode. And many attempts for extending the application concepts of CFx in lithium batteries were made to conclude no any evidence on its reversibility[3]. Since sodium batteries have been attracting much attention as one of the post-lithium ion battery[4]. CFx/Na cells were explored and the reversibility of these cells was also proved[5-6]. Here, we summarize these designed CFx materials for sodium secondary batteries, as shown in Figure 1. Commercial CFx and F-MWCNTs were synthesized by direct fluorination of graphite and mutil-wall carbon nanotubes (MWCNTs) using F2 gas. While CGF powders were prepared by simply ball-milling of the commercial CFx, FeF3 and GNS, with higher surface area, electronic conductivity, more active sites and specific channels for the transportation and storage of fluorine ions. Furthermore, disordered CFx exhibits excellent electrochemical performance due to its intrinsic amorphous character. Figure 1 Three kinds of CFx materials for sodium secondary batteries:(a) commercialCFx, (b) CFx/GNS/FeF3(CGF) composite, (c) F-MWCNTs Commercial CFx was first used in sodium secondary battery[5]; however, much larger overpotential(2000mV) between charge and discharge voltage and terrible cyclic performance (5 cycles) hindered its practice adoption. These drawbacks can be overcome by the introduce of catalysts and the change of the intrinsic characteristic of CFx materials[5-6], which results in a gap of only 0.8 volt and enhanced cycle life. For practical uses, some popular approaches in nanotechnology and electrolyte chemistry such as the core-shell structure or encapsulation or coating composites, and using specially designed solid electrolytes will be expected to prevent the dissolution of fluorine for improving cycle life for Na-CFxbattery in the future. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC21373137) and Shanghai Science and Technology Development Fund (15DZ2282000). Reference [1] N. Watanabe, M. Fukuda, U.S. Patent 3 536 532, 1970 and 3 700502, 1972. [2] M. Fukuda, Iijima, T. In Collins, D.H., Ed.; Power Sources 5;Academic Press: New York, 1975; p 713. [3] G. G. Amatucci, N. J. Pereira, Fluorine Chem. 2007, 128, 243. [4] D. S. Michael, K. Donghan,L. Eungje, and S. J. Christopher, Sodium-Ion Batteries, Adv. Funct. Mater. 2013, 23, 947. [5] W. Liu, H. Li, J. Y.Xie, and Z. -W. Fu, ACS Appl. Mater. Interfaces, 2014, 6, 2209. [6] W. Liu, Z. Shadike, Z. C. Liu, W. Y. Liu, J. Y.Xieand Z. -W. Fu, Carbon, 2015, 523. [7] Y. Shao, H. Yue, Z. Gong and Y. Yang, The 18th National Conference on Electrochemistry. August, 2015, Haerbin, China. Figure 1

Long-term stability of fluorine–graphite intercalation compound prepared under high pressure of fluorine

Fluorine–graphite intercalation compounds were prepared from highly oriented pyrolytic graphite (HOPG) and natural graphite under high pressure of fluorine (16MPa) at room temperature and were analyzed by means of X-ray diffraction. The rate of intercalation under high pressure of fluorine was higher than that under ordinary pressure. The compositions of the compounds obtained from HOPG and natural graphite were estimated by the weight change before and after fluorination to be C3.41F and C5.62F, respectively. They were blue–black compounds composed of two different phases. One phase is a conventional stage II CxF with the repeat distance of ca. 0.93–0.96nm which was the same compound as that obtained under ordinary pressure of fluorine, and the other was also a stage II with a different repeat distance of ca. 1.03–1.18nm. The fluorine species intercalated between the graphene layers can easily move between both phases and the phase transition occurs periodically. The decomposition rate of fluorine-intercalated HOPG was extremely low and it shows a long-term stability in air for 26years.

ChemInform Abstract: Fluorine Compounds as Energy Conversion Materials

AbstractReview: 35 refs.

Fluorine Patterning in Room-Temperature Fluorinated Graphite Determined by Solid-State NMR and DFT

Fluorination of graphite at room temperature allows producing graphite fluoride compounds with a controlled content of fluorine. Here we combine solid-state NMR spectroscopy and DFT calculations to study the structure and reveal the fluorine patterning in graphite fluorides C2Fx intercalated with acetonitrile. Two major chemical states of carbon, namely the atoms covalently bound to fluorine and the bare atoms, are detected by 13C MAS NMR irrespective of the degree of fluorination. The data indicate that although all graphene sheets were subjected to fluorination, the near-planar configuration is preserved. The interaction between host C2Fx matrix and acetonitrile molecules is of van der Waals character. Decomposition of the 19F MAS NMR spectra reveals occurrence of at least six fluorine environments in each sample. By DFT calculations distinct 19F chemical shifts are attributed to isolated, end chain, “linked” (which include midchain, cyclic, and branched) CF groups and infinite CF arrays. The assignment is confirmed by 19F RFDR, which is sensitive to dipolar coupling. Analyzing the data for C2Fx samples with different degrees of fluorination x, an evolution of the fluorine pattern is proposed. The reported calculated 19F NMR shielding parameters provide classification criteria for assignment of 19F NMR chemical shifts in fluorinated carbon materials.

Structural characteristics of dicarbon polyfluoride

In chemical analysis and XRPA studies, it has been found that the products of fluorination of graphite, graphite bifluoride, and tetracarbon polyfluoride with gaseous and liquid fluorooxidants (ClF3, ClF5, XeF2, and FNO3) and their solutions in anhydrous hydrogen fluoride and freon-113 at 20–100°C, having a limiting composition, are second stage intercalated compounds based on dicarbon polyfluoride with the corresponding fluorooxidant, or the reduced form of fluorooxidant, or a fluorooxidant and a solvent. The degree of C-F bond ionicity in C2F was higher than in CF, but lower than in C4F. However, the bond is closer in character to the covalent C-F bond in monocarbon polyfluoride.

Reversible Intercalation of Fluoride-Anion Receptor Complexes in Graphite

We have demonstrated a route to reversibly intercalate fluoride-anion receptor complexes in graphite via a nonaqueous electrochemical process. This approach may find application for a rechargeable lithium–fluoride dual-ion intercalating battery with high specific energy. The cell chemistry presented here uses graphite cathodes with dissolved in a nonaqueous solvent through the aid of anion receptors. Cells have been demonstrated with reversible cathode specific capacity of approximately at discharge plateaus of upward of , with graphite staging of the intercalant observed via in situ synchrotron X-ray diffraction during charging. Electrochemical impedance spectroscopy and nuclear magnetic resonance studies suggest that co-intercalation of the anion receptor with the fluoride occurs during charging, which likely limits the cathode specific capacity. The anion receptor type dictates the extent of graphite fluorination, and must be further optimized to realize high theoretical fluorination levels. To find these optimal anion receptors, we have designed an ab initio calculations-based scheme aimed at identifying receptors with favorable fluoride binding and release properties.

Magnesium batteries: Towards a first use of graphite fluorides

Graphite fluorides obtained by fluorination of graphite at room temperature and after a subsequent re-fluorination treatment were characterised by X-ray diffraction and 19F MAS NMR. Their electrochemical performances as cathode materials in magnesium batteries have been investigated. Four different electrolytes (0.5 or 1 M Mg(ClO 4) 2 in ACN, DMSO, PC and THF) were used for these tests. The specific energy and power densities were estimated for all media. A comparison of the performances between lithium and magnesium batteries was realised. The effect, on the electrochemical performances, of a re-fluorination treatment at 250 °C was also studied.

Synthesis of perfluoroalkanes by fluorination of graphite with elementary fluorine in a vertical ascending gas-dust flow

Synthesis of lower pefluoroalkanes, tetrafluoromethane, hexafluoroethane, octafluoropropane, and decafluorobutane, in a high-temperature reaction of graphite with fluorine in a reactor with a vertical ascending gas-dust flow was studied.

Synthesis of perfluoroalkanes in high-temperature fluorination of graphite with fluorine in a reactor with a free-falling graphite bed

Reaction between fluorine and graphite in a reactor with a free-falling bed of graphite was studied in relation to the temperature in the reaction zone, ratio of the feeding rates of fluorine and graphite, and dilution of fluorine with an inert gas.

Room temperature graphite fluorination process using chlorine as catalyst

The present study highlights the physicochemical characteristics of two fluorinated graphites. These compounds were prepared at room temperature in an atmosphere of F2, HF, and chlorine, obtained using either Cl2 gas or heated potassium chloride, and have the same fluorination level CF0.4; treatments under fluorine at temperatures higher than 400 °C allow an increase in fluorination level up to CF1. NMR and EPR measurements show the influence of two preparation modes on catalytic residues and structural defects.

Study on Electrochemical Performance of Fluorinated Graphite and Its Application in Alkaline Batteries

The electrochemical characteristics and performance of fluorinated graphites (FGs) and its application in conventional alkaline batteries were investigated. FG as a cathode additive in alkaline batteries can improve the discharge performance significantly. The discharge performances of alkaline batteries in which the cathode contained a different weight percentage of FG and those with a fixed weight percent of FG with different fluorine contents were measured. The storage stability of cells with various test cathodes was determined. XRD studies were carried out on different graphites, on FG, and on various types of cathode samples before and after discharge.