Electrochemical insertion of lithium ions into disordered carbons derived from reduced graphite fluoride

Both covalent (obtained by direct fluorination at high temperature) and semi-ionic carbon fluorides (synthesized at room temperature) were reduced in order to obtain disordered carbons containing very small content of fluorine and different physical properties according to the reduction treatment (chemical, thermal or electrochemical). After a physical characterization (X-ray diffraction, electron spin resonance and FT-IR spectroscopies), the electrochemical behaviours of the pristine carbon fluorides and of the treated samples were investigated during the insertion of lithium using liquid carbonate-based electrolytes (LiClO₄-EC/PC, 50:50%, v/v). Both galvanostatic and voltammetric modes were performed and revealed that the voltage profiles and the capacities differed according to the starting material and the reduction treatment. Semi-ionic carbon fluoride treated in F₂ atmosphere for 2 h at 150 °C and then chemically reduced in KOH exhibits high reversible capacities (the reversible capacity is 530 mAh g⁻¹ in the second cycle); in this case, the voltage profiles show a large flat portion at potentials lower than 0.3 V which is attributed to the insertion/deinsertion of lithium ions between the small graphene sheets and/or the absorption of pseudo metallic lithium into the microporosity of the sample. Nevertheless, a part of the lithium ions are removed at potentials higher than 0.5 V versus Li⁺/Li limiting the useful capacity.     

Aqueous electrochemical intercalation of bromine into graphite fibers

For the first time, graphite fibers have been electrochemically intercalated with Br⁻ that have the same structure and properties as those intercalated from vapor phase Br₂. This was accomplished by intercalating pitch-based Thornel^® K-1100 graphite fibers at low temperature (near 0 °C) and high currents (2 A) for long times (6 h). The mechanism appears to be that Br⁻ is oxidized to aqueous Br₂ which, when sufficient local concentration builds up, intercalates the fiber. This was confirmed by intercalating K-1100 fiber in a saturated aqueous Br₂ solution without passing an electrical current. The applied voltage does apparently lower the activation energy of the reaction as evidenced by the observation that P-120 and P-100 fibers will not intercalate in aqueous Br₂ unless a voltage is applied.     

Theoretical study of stability of graphite intercalation compounds with Brønsted acids

The instability of acceptor graphite intercalation compounds (GICs) with Brønsted acids is a main obstacle to their extensive industrial use. Electron transfer from the solvated anion to the positively charged graphene layer is considered to be the first and rate-limiting step in the decomposition process, with the ionization potential of the intercalated anion being a measure of the stability of GICs. The effects of the hydrogen bonding and Coulomb interaction on the GIC stability are discussed.     

Flexible graphite as a heating element

Flexible graphite is an effective heating element. It provides temperatures up to 980 °C (though burn-off occurs in air at 980 °C), response half-time down to 4 s, and heat output at 60 s up to 5600 J. The electrical energy for heating by 1 °C is 1-2 J in the initial portion of rapid temperature rise. The temperature and heat output increase with decreasing thickness and with increasing power.     

Reversible intercalation of HF in fluorine-GICs

Reversible intercalation/deintercalation processes of HF were characterized for first stage fluorine-graphite intercalation compounds with semi-ionic C-F bonds (fluorine-GICs; C_xF) by X-ray diffraction and infrared absorption spectroscopy. C_xF(HF)_δ prepared by the intercalation of HF to C_xF possesses the interlayer distance of about 0.60 nm in the composition range of 2.1<x<2.6. Absorption peaks ascribed to the vibrational modes of carbon sheets in HF-intercalated C_xF are observed at higher wavenumbers with stronger intensities than those for HF-free C_xF. Attractive interaction is suggested to exist between the intercalated HF molecules and C_xF caused by the polar characters of both the H-F bonds in the former and the semi-ionic C-F bonds in the latter.     

Characterizations of expanded graphite/polymer composites prepared by in situ polymerization

Poly(styrene-co-acrylonitrile)/expanded graphite composite sheets with very low in-plane (8.5 × 10⁻³ Ω cm) and through-thickness (1.2 × 10⁻² Ω cm) electrical resistivities have been prepared. The expanded graphite was made by oxidation of natural graphite flakes, followed by thermal expansion at 600 °C. Microscopic results disclosed that the expanded graphite has a legume-like structure, and each “legume” has a honeycomb sub-structure with many diamond-shaped pores. After soaking the expanded graphite with styrene and acrylonitrile monomers, the polymer/expanded graphite composite granules were obtained by in situ polymerization of the monomers inside the pores at 80 °C. The functional groups and microstructures of the oxidized graphite, expanded graphite and composites in the forms of particles or sheets were carefully characterized using various techniques, including X-ray powder diffraction, thermogravimetry, optical and electron microscopy. It was found that the honeycomb sub-structure survived after hot-pressing, resulting in a graphite network penetrating through the entire composite body, which produces a composite with excellent electrical conductivity.     

Scanning tunneling microscopy study of a BiCl3–graphite intercalation compound

In the presence of chlorine gas, BiCl₃ was intercalated from the gas phase into highly oriented pyrolytic graphite; second-stage compounds were obtained. The surface structure in air was determined with a scanning tunneling microscope. A long-periodical rectangular structure was observed with an a-axis of 1429 pm and a b-axis of 1841 pm. This commensurate structure fits very well with X-ray diffraction data for a superlattice in BiCl₃ graphite intercalation compounds published by another group.     

Electrochemical insertion of sodium into graphite in molten sodium fluoride at 1025 °C

The electrochemical insertion of sodium into graphite was studied in molten sodium fluoride at 1025 °C. The results obtained evidenced two mechanisms for sodium insertion into graphite: sodium intercalation between the graphite layers and sodium sorption into the porosity of the material. Subsequent internal rearrangement of inserted sodium occurred, via transference from the pores towards the intercalation sites. In addition, the intercalation compound was found to undergo a fast decomposition process (k = 2.55 × 10⁻⁹ mol s⁻¹). X-ray diffraction analysis was used to confirm the formation of a high stage compound (Na_0.1C₈), the composition of which was consistent with compositions observed in the case of chemical vapor and electrochemical insertion of sodium, during experiments in the sodium perchlorate-ethylene cabonate electrolyte.     

Electrochemical, textural and microstructural effects of mechanical grinding on graphitized petroleum coke for lithium and sodium batteries

A graphitized coke material obtained from petroleum residua was mechanically ground at different milling times between 0 and 100 h. Electrochemical reactions with both lithium and sodium are significantly altered as a function of grinding time. Short-time ball milling of graphite (1 and 5 h) induces a limited decrease in particle size and an increase in microstrain content. Simultaneously, alkali metal intercalation and electrolyte decomposition are hindered, and thus the irreversible and reversible capacities decrease. For longer milling time (up to 100 h), average crystallite size decreases and particles adopt a lamellar shape. Simultaneously, the irreversible capacity increases and correlates with an increase of the resistance, as obtained by impedance spectroscopy. Ex-situ XRD shows that extensively ground graphite samples need a higher discharge specific capacity to reach the formation of n-stages as compared to non-ground graphite, this being indicative of lithium incorporation in energetically different sites to the interlayer space. Sodium storage capacity increases with prolonged grinding time. This effect is shown here for the first time for graphitized cokes.     

Electrochemical intercalation of potassium into graphite in KF melt

Electrochemical intercalation of potassium into graphite in molten potassium fluoride at 1163 K was investigated by means of cyclic voltammetry, galvanostatic electrolysis and open-circuit potential measurements. It was found that potassium intercalated into graphite solely between graphite layers. In addition, the intercalation compound formed in graphite bulk in molten KF was quite unstable and decomposed very fast. X-ray diffraction measurements indicate that a very dilute potassium-graphite intercalation compound was formed in graphite matrix in the fluoride melt. Analysis with scanning electron microscope and transmission electron microscope shows that graphite was exfoliated to sheets and tubes due to lattice expansion caused by intercalation of potassium in molten KF.     

On the role of dihydrogen in the co-intercalation reactions into graphite of potassium and chalcogen

The influence of dihydrogen gas on the reactions between graphite and liquid potassium containing a very small amount of a chalcogen (O, S, Se, Te) was studied. The reactions were carried out under a pure argon atmosphere in a stainless steel reactor, between 400 and 600°C. Controlled amounts of dihydrogen gas can be added in this reactor. When dihydrogen is strictly absent, the co-intercalation of potassium and chalcogen does not take place at 400°C: only potassium intercalates, leading to the KC₈ binary compound. The same experiments carried out with controlled amounts of dihydrogen at the same temperature lead to various ternary compounds with oxygen, sulphur, selenium and tellurium. However, at 600°C, and strictly without dihydrogen, co-intercalation occurs, but only for S, Se and Te, allowing the preparation of new well-defined ternary graphite intercalation compounds. The co-intercalation of potassium and oxygen is possible only in the presence of dihydrogen, at any temperature.     

Characterization of n-hexadecylalkylamine-intercalated graphite oxides as sorbents

Adsorption properties of graphite oxides hydrophobized by n-hexadecylamine, (C16)_xGO, were investigated using pyrene molecules as a model of nonionic organic contaminants. A large quantity of pyrene (28.5 mg/g) was adsorbed from a water-ethanol mixture (1:2) containing 2 mM of pyrene when (C16)_0.6GO was used. The isotherm of pyrene adsorption was better described by Freundlich equation rather than Langmuir equation, which indicated a single adsorption mechanism was involved. The change in the amount of adsorbed pyrene as a function of amine content in GO was very similar to that which occurs upon introduction of pyrene into (C16)_xGO films from chloroform solution, as determined by X-ray measurements. This suggests that pyrene molecules were adsorbed not only on the outer surface but also within the interlayer space of the intercalation compound. Swelling of the intercalation compound by ethanol can render the interlayers more organophilic and make access to hexadecylamine molecules bonded to the graphite oxide layer easier for pyrene molecules, especially in (C16)_xGOs with lower amine contents.     

Electrochemical performance of low temperature fluorinated graphites used as cathode in primary lithium batteries

The present study highlights the electrochemical performance of two series of fluorinated graphites used as the cathode in primary lithium batteries. These compounds were prepared under fluorine gas at room temperature using a catalytic atmosphere made of boron or chlorine fluoride, and then thermally treated between 100 and 600 °C. The electrochemical properties are correlated to a complete physico-chemical characterization, already performed by XRD, NMR, FT-IR and EPR. In particular, important parameters are taken into account: C-F bonding, carbon hybridization, fluorine content (i.e. F/C ratio) and amount of intercalated catalyst residues. It is shown that the average discharge potential of fluorinated graphite used in primary lithium batteries can be predicted owing to the chemical shift values (δ_C-F) obtained by solid ¹³C NMR. On the other hand, the higher capacity values are achieved for low temperature fluorinated graphite treated at the highest temperatures, i.e. for high fluorination level. The electrochemical performance study of these materials is completed by the study of the effect of simulated storage. The differences between the various samples during electrochemical tests and those observed using different electrolytes are discussed. Fluorinated graphites obtained with a chlorine catalyst or post-treated at temperatures higher than 450 °C are unaffected by ageing.