Katia Guérin

A 7Li NMR study of a hard carbon for lithium-ion rechargeable batteries

Negative electrodes for lithium–ion batteries containing a hard carbon with different binders were characterised by 7Li NMR experiments. The spectra of unwashed lithiated electrodes, although different in profile and position at room temperature for the different binders used, exhibited a common feature at temperatures lower than 200 K, namely the presence of three signals, in addition to that of the electrolyte salt, similar whatever the electrode technology. They were assigned to covalently bonded, intercalated and pseudo-metallic lithium species, respectively. At room temperature, an exchange was observed between at least two of these signals; its characterisation is complicated by the paramagnetic nature of some of the components which induces a change in their position and width upon heating simultaneously with the exchange process.

Effect of Graphite Crystal Structure on Lithium Electrochemical Intercalation

The electrochemical intercalation of lithium into various graphite materials has been examined in LiPF{sub 6} solutions of ethylene carbonate (EC)/dimethyl carbonate (DMC) or EC/DMC/propylene carbonate (PC). When the graphite powder samples contained at least 30% of the rhombohedral form, no exfoliation was observed, even with electrolytes containing a large amount (80%) of PC. However, when the graphites were heat-treated at temperatures above 1,000 C, the faradaic losses due to the exfoliation reappeared, even though the rhombohedral phase content was unchanged. From Raman spectroscopy measurements, a correlation was found between the irreversible capacity due to the exfoliation and the ratio, R, of the integrated intensity of the disorder-induced line at 1,350 cm{sup {minus}1} to the Raman-allowed line at 1,580 cm{sup {minus}1}. This suggests that structure defects, probably localized in grain boundaries between rhombohedral and hexagonal domains, hinder the layer opening necessary for the intercalation of solvated lithium species at the beginning of the first electrochemical cycle.

On the irreversible capacities of disordered carbons in lithium-ion rechargeable batteries

The irreversible capacity of disordered carbons, used as anodes of lithium-ion batteries, is known to result from their generally large surface area (leading to large volume of passivating layer) and from the presence of surface functional groups and of heteroatoms left from the precursor, which can react irreversibly with lithium. In this paper, we show that the irreversible capacity of disordered carbons can result from an additional mechanism: lithium trapping in the internal porosity. A series of carbon samples with low specific surface area and low heteroatom content has been investigated by X-ray diffraction, helium pycnometry, complex impedance spectroscopy and galvanostatic cycling. A clear correlation has been found between the irreversible capacity and the internal pore volume.

On the choice of graphite for lithium ion batteries

Graphites as active materials for negative electrode in lithium batteries are particularly attractive because of their large capacity of lithium intercalation and their low average voltage. In some conditions, they are known to suffer from low reversibility of the initial intercalation process. This phenomenon is shown to be unambiguously related to an exfoliation of graphene layers, that can occur even in EC based electrolytes. Occurrence of a clear correlation between the extent of irreversible behaviour and rhombohedral phase content of graphites is discussed. Milling or thermal treatment of pristine graphites are also shown to influence electrochemical properties.

Fluorination of anatase TiO2 towards titanium oxyfluoride TiOF2: a novel synthesis approach and proof of the Li-insertion mechanism

The reactivity of pure molecular fluorine F2 allows the creation of new materials with unique electrochemical properties. We demonstrate that titanium oxyfluoride TiOF2 can be obtained under molecular fluorine from anatase titanium oxide TiO2, while the fluorination of rutile TiO2 leads only to pure fluoride form TiF4. Contrary to most fluorides, TiOF2 is air-stable and hydrolyses poorly under humid conditions. Such a stability makes it possible for TiOF2 to be studied as an electrode material in Li-ion secondary battery systems. It shows the capacity as high as 220 mA h g−1 and good cyclability at high current rates at an average potential of 2.3 V vs. Li+/Li. At such a potential, only Li+ insertion occurs, as proven by in operando XRD/electrochemistry experiments.

Solid State NMR study of nanodiamond surface chemistry

Solid state NMR measurements using 13C, 1H and 19F nuclei (MAS, CP-MAS) underline the surface chemistry of nanodiamonds from different synthesis (detonation, high pressure high temperature and shock compression). The comparison of the spin-lattice relaxation times T1 and physicochemical characterization (spin densities of dangling bonds, specific surface area and Raman and infrared spectroscopies) for the various samples, as synthesized, chemically purified and fluorinated allows the nature and the location of the various groups, mainly C-OH, C-H and C-F to be investigated. C-OH groups are located only on the surface whereas C-H and dangling bonds seem to be distributed in the whole volume. Fluorination was studied as a chemical treatment for purification and change of the hydrophobicity through the conversion of the C-OH groups into covalent C-F bonds.

Chemical and electrochemical intercalation of lithium into boronated carbons

Abstract Boron-substituted carbons have been produced by chemical vapour deposition from acetylene and boron trichloride precursors at 1140°C. Li intercalation was investigated, chemically from the Li vapour and electrochemically in Li–carbon cells. In both cases, the amount of intercalated Li increases with the boron content of the carbon, up to a value of 13 at.% boron. Above this value, the intercalation of lithium is not so efficient. This behaviour is understood by assuming that boron, which acts as an electron acceptor and hence favours the intercalation of electron donors like lithium, can enter substitutionally into the carbon lattice up to a certain limit, which is close to 13 at.% for the materials deposited at 1140°C.

On the Electrochemical Intercalation of Lithium into Graphitizable Carbons

Abstract Coke samples heat-treated between 1200 and 2800°C have been characterized by X-ray diffraction and their performance as electrodes for lithium batteries has been determined on coin cells with LiTFSI dissolved in EC/DMC as electrolyte. The data show that the electrochemical behaviour is strongly dependent on the structural characteristics of the carbon materials. It is shown that Merings graphitization model can account for the values of reversible capacities measured for the different materials, as well as for the appearance of voltage plateaus for carbon heat-treated above 2000°C.

One-shot versus stepwise gas–solid synthesis of iron trifluoride: investigation of pure molecular F2 fluorination of chloride precursors

Iron trifluoride has been known for a long time and here we provide new insights into its detailed synthesis. Anhydrous iron trifluorides with different structures have been synthesized via one-shot and stepwise gas–solid fluorination under a gaseous flow of molecular fluorine from 150 °C. The degree of hydration of the precursors has a direct influence on the crystalline phase obtained: one-shot fluorination of FeCl2·4H2O leads to the hexagonal tungsten bronze-type FeF3 while fluorination of dehydrated iron chlorides gives the rhombohedral FeF3 with traces of oxy(hydroxy)fluorides. A stepwise procedure at 150, 250 and 350 °C tends to lead to pure phases and avoid the crystallization of oxy(hydroxy)fluorides in the final powder.

First Insight into Fluorinated Pt/Carbon Aerogels as More Corrosion-Resistant Electrocatalysts for Proton Exchange Membrane Fuel Cell Cathodes

This study evaluates the fluorination of a carbon aerogel and gives first insights into its durability when used as platinum electrocatalyst substrate for proton exchange membrane fuel cell (PEMFC) cathodes. Fluorine has been introduced before or after platinum deposition. The different electrocatalysts are physico-chemically and electrochemically characterized, and the results discussed by comparison with commercial Pt/XC72 from E-Tek. The results demonstrate that the level of fluorination of the carbon aerogel can be controlled. The fluorination modifies the texture of the carbons by increasing the pore size and decreasing the specific surface area, but the textures remain appropriate for PEMFC applications. Two fluorination sites are observed, leading to both high covalent C-F bonds and weakened ones, the quantity of which depends on whether the treatment is done before or after platinum deposition. The order of the different treatments is very important. Indeed, the presence of platinum contributes to the fluorination mechanism, but leads to amorphous platinum, which is demonstrated rather inactive towards the oxygen reduction reaction. On the contrary, a better durability was demonstrated for the fluorinated and then platinized catalyst compared both to the same but not fluorinated catalyst and to the reference commercial material (based on the loss of the electrochemical real surface area after accelerated stress tests).