Physico-chemical study of a corona discharge in gas mixtures containing humid air, carbon dioxide and nitrogen oxides

Gas mixtures containing humid air, various proportions of CO2 and traces of NO2 are submitted to a short gap point-to-plane DC corona discharge in a reactor at atmospheric pressure. These mixtures can be considered as the basic component of most industrial effluents. Coming after an electrical study previously published, this work is centered on the physico-chemical effects of the discharge. Concentric circular alterations appearing on a copper cathode have been analyzed by XPS, showing an oxidation and acidification spread over a great part of the surface by the electric wind. The NO2 removal and corresponding by-products have been studied by UV absorption spectroscopy on gas samples after treatment, in the 200–400 nm wavelength range where NO2 abatement and O3 concentrations can be evaluated. Being partly produced in liquid phase, HNO3 which is responsible of the acidification of cathode surface, can be studied only qualitatively. A few chemical mechanisms are propounded for both bulk and surface actions of the discharge

Electronic structure of thiogermanate and thioarseniate glasses: Experimental (XPS) and theoretical (ab initio) characterizations

cited By 9International audienceThe paper reports on structural investigation and electronic structure of thiogermanate and thioarseniate glasses by means of X-ray photoelectron spectroscopy (XPS) and ab initio calculation. Besides an increase of nonbridging sulfur (nbS) when increasing the amount of modifier (Ag2S), a limit in the breakdown between nbS and bridging sulfur (bS) atoms was observed for the intermediate Ag concentrations in the two families for the silver containing glasses. The influence of the modifier cation has been studied for the M2GeS3 glass family (M = Ag, Li, Na). Compared to alkali glasses, a more homogeneous electronic distribution on sulfur atoms was observed for Ag2GeS3. Furthermore, the electronic redistribution along Ge-S bonds is the most important in the case of Na2GeS3. In agreement with XPS results, Mulliken population analysis showed only a small difference (more important for Na than for Ag) between the charges on bS and nbS. According to XPS data (valence band), ab initio calculations on cluster models showed important changes - compared to GeS2 - in both Ge-S and Ge-Ge interactions, and support the concept of a nonlocalized effect of alkali-metal atoms

Li4NiTeO6 as a positive electrode for Li-ion batteries

Layered Li4NiTeO6 was shown to reversibly release/uptake B2 lithium ions per formula unit with fair capacity retention upon long cycling. The Li electrochemical reactivitymechanism differs from that of Li2MO3 and is rooted in the Ni4+/Ni2+ redox couple, that takes place at a higher potential than conventional LiNi1�xMnxO2 compounds.We explain this in terms of inductive effect due to Te6+ ions (or the TeO6 6� moiet

XPS valence band spectra and theoretical calculations for investigations on thiogermanate and thiosilicate glasses

International audienceThis paper reports on investigations of thiogermanate and thiosilicate crystals and glasses by means of XPS valence band spectra and theoretical calculations (FLAPW method). The calculations were achieved on three crystallized phases GeS2, Na2GeS3 and SiS2 and valence band spectra (visualization of the occupied electronic density of states) were precisely interpreted through modulated density of states and charge density maps. This information was used to go further in the structural investigations of some thiogermanate and thiosilicate glasses. In sodium thiogermanates, an increase in Ge–Ge bonds was revealed as the modifier content (Na2S) increases. In thiosilicates, the evolution of the valence spectra according to the nature of the alkaline atoms (Li, Na) has been interpreted as changes in the local connectivity of units (edge or corner sharing tetrahedra).This study exhibits the potentialities of valence band spectra to provide information on glassy systems

The structure of ionically conductive chalcogenide glasses: A combined NMR, XPS and ab initio calculation study

cited By 8International audienceThis paper reports on the structural investigation of lithium and sodium thiosilicate crystals and glasses by means of X-ray photoelectron spectroscopy and ab initio calculation. The results are analysed in conjunction with previously reported 29Si NMR data. While NMR proved to be an effective tool for the quantitative discrimination of edge- and corner-sharing tetrahedra existing in these materials, X-ray photoelectron spectroscopy (XPS) gives information on the nature of Si-S bonds, i.e. bridging and non-bridging bonds. The main result is the noticeable difference existing between the structures of lithium and sodium thiosilicate glasses, which, according to XPS data, is due to different electronic redistributions over the network when one or the other alkali is added, the sodium addition resulting in a change in the electronic distribution over the entire network

Study of intercalated Ti atom in tetrahedral or octahedral sites of titanium disulfide (001) surfaces: Theoretical scanning tunneling microscopy images

International audienceWe have performed ab initio linear combination of atomic orbitals-density functional theory calculations on biperiodic supercells to model the electronic and geometrical involvements of Ti intercalated atom in either octahedral or tetrahedral sites of the (001) Ti S2 surfaces. For each type of defect, both the relaxed atomic structure and the electronic properties of the defect states were carefully analyzed. For the titanium atom in the van der Waals gap, the partial filling of the conduction band is in agreement with the metallic behavior reported by experimental studies and the last filled states in the bottom of the conduction band-mainly developed on titanium 3d orbitals-permit us to explain the dark defects observed on the scanning tunneling microscopy image of the (001) Ti S2 surfaces. On the other hand, the intercalated titanium atom in the tetrahedral site which is just below the top sulfur atom plane governs the electronic density detected by the tip. It permits us to explain the triangular defect with a clear maximum of intensity in its center and dark sides

X-ray Photoemission Spectroscopy Study of Cationic and Anionic Redox Processes in High-Capacity Li-Ion Battery Layered-Oxide Electrodes

International audienceElectrode materials based on Li-rich layered oxides are of growing interest for high-energy Li-ion battery applications because of their staggering capacities associated with the emergence of a novel, reversible anionic process. However, the fundamental science at work behind this new process needs to be well understood for further optimization. Here we report on the redox mechanisms in high-capacity Li-rich materials Li2Ru1-xMxO3 and Li2Ir1-xMxO3, by combining X-ray photoemission spectroscopy (XPS) core peaks and valence intensity analyses. We fully confirm that these materials electrochemically react with Li via cumulative reversible cationic/anionic redox processes, but more importantly we reveal that, depending on the nature of the metal (Ru or Ir), there is a delicate balance between metal and oxygen contributions. For instance, we show a greater implication of oxide ions for Ir-based electrodes, consistent with the higher covalent character of Ir-O bonds compared to Ru-O bonds. We equally provide evidence that the oxygen redox process is responsible for the high capacity displayed by the Li-rich NMC Li1.2Ni0.13Co0.13Mn0.54O2 electrodes that are serious contenders for the next generation of Li-ion batteries. These combined results highlight the benefit of collecting both XPS core and valence spectra for a better understanding of anionic redox mechanisms in Li-rich layered oxides. © 2015 American Chemical Society

XPS valence characterization of lithium salts as a tool to study electrode/electrolyte interfaces of Li-ion batteries

International audienceX-ray photoelectron valence spectra of lithium salts LiBF4, LiPF6, LiTFSI, and LiBETI have been recorded and analyzed by means of density functional theory (DFT) calculations, with good agreement between experimental and calculated spectra. The results of this study are used to characterize electrode/electrolyte interfaces of graphite negative electrodes in Li-ion batteries using organic carbonate electrolytes containing LiTFSI or LiBETI salts. By a combined X-ray photoelectron spectroscopy (XPS) core peaks/valence analysis, we identify the main constituents of the interface. Differences in the surface layers' composition can be evidenced, depending on whether LiTFSI or LiBETI is used as the lithium salt. © 2006 American Chemical Society

XPS Investigation of Surface Reactivity of Electrode Materials: Effect of the Transition Metal

International audienceThe role of the transition metal nature and Al2O3 coating on the surface reactivity of LiCoO2 and LiNi1/3Mn1/3Co1/3O2 (NMC) materials were studied by coupling chemisorption of gaseous probes molecules and X-ray photoelectron (XPS) spectroscopy. The XPS analyses have put in evidence the low reactivity of the LiMO2 materials toward basic gaseous probe (NH3). The reactivity toward SO2 gaseous probe is much larger (roughly more than 10 times) and strongly influenced by the nature of metal. Only one adsorption mode (redox process producing adsorbed sulfate species) was observed at the LiCoO2 surface, while NMC materials exhibit sulfate and sulfite species at the surface. On the basis of XPS analysis of bare materials and previous theoretical work, we propose that the acidbase adsorption mode involving the Ni2+ cation is responsible for the sulfite species on the NMC surface. After Al2O3 coating, the surface reactivity was clearly decreasing for both LiCoO2 and NMC materials. In addition, for LiCoO2, the coating modifies the surface reactivity with the identification of both sulfate and sulfite species. This result is in line with a change in the adsorption mode from redox toward acidbase after Al/Co substitution. In the case of NMC materials, the coating induced a decrease of the sulfite species content at the surface. This phenomenon can be related to the cation mixing effect in the NMC

Role of the LiPF6 Salt for the Long Term Stability of Silicon Electrodes in Li Ion Batteries A Photoelectron Spectroscopy Study

cited By 83International audienceSilicon presents a very high theoretical capacity (3578 mAh/g) and appears as a promising candidate for the next generation of negative electrodes for Li-ion batteries. An important issue for the implementation of silicon is the understanding of the interfacial chemistry taking place during charge/discharge since it partly explains the capacity fading usually observed upon cycling. In this work, the mechanism for the evolution of the interfacial chemistry (reaction of surface oxide, Li-Si alloying process, and passivation layer formation) upon long-term cycling has been investigated by photoelectron spectroscopy (XPS or PES). A nondestructive depth resolved analysis was carried out by using both soft X-rays (100-800 eV) and hard X-rays (2000-7000 eV) from two different synchrotron facilities. The results are compared with those obtained with an in-house spectrometer (1486.6 eV). The important role played by the LiPF6 salt on the stability of the silicon electrode during cycling has been demonstrated in this study. A partially fluorinated species is formed upon cycling at the outermost surface of the silicon nanoparticles as a result of the reaction of the materials toward the electrolyte. We have shown that a similar species is also formed by simple contact between the electrolyte and the pristine electrode. The reactivity between the electrode and the electrolyte is investigated in this work. Finally, we also report in this work the evolution of the composition and covering of the SEI upon cycling as well as proof of the protective role of the SEI when the cell is at rest. © 2013 American Chemical Society