DEMO - Portrait

Publications récentes

• HAL : Dernières publications

  • [hal-01670660] Surface reactivity of Li2MnO3: first-principles and experimental study

    28 mars 2018 03:15

    The present paper deals with the surface reactivity of (001) oriented Li2MnO3 crystals investigated from a multi-techniques approach combining, material synthesis, X-ray photoemission spectroscopy (XPS), Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES) and first-principle calculations. Li2MnO3 is considered as a model compound suitable to go further in the understanding of the role of tetravalent manganese atoms in the surface reactivity of layered lithium oxides. The knowledge of the surface properties of such materials is essential in order to understand the mechanisms involved in parasitic phenomena responsible for early aging or poor storage performances of lithium-ion batteries. The surface reactivity was probed through the adsorption of SO2 gas molecules on large Li2MnO3 crystals in order to be able to focus the XPS beam on the top of the (001) surface. A chemical mapping and XPS characterization of the material before and after the SO2 adsorption show in particular that the adsorption is homogeneous at the micro and nanoscale and involves Mn reduction, while first-principle calculations on a slab model of the surface allow us to conclude that the most energetically favorable species formed is a sulfate with charge transfer implying reduction of Mn.

  • [hal-01680977] Morphology and surface reactivity relationship in the Li1+xMn2–xO4 spinel with x = 0.05 and 0.10: a combined first-principle and experimental study

    21 mars 2018 19:57

    This article focuses on the surface reactivity of two spinel samples with different stoichiometries and crystal morphologies, namely Li1+xMn2–xO4 with x = 0.05 and 0.10. LiMn2O4 compounds are good candidates as positive electrode of high-power lithium-ion batteries for portable devices. The samples were investigated using both experimental and theoretical approaches. On the experimental point of view, they were characterized in depth from X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS) analyses. Then, the reactivity was investigated through the adsorption of (SO2) gaseous probes, in controlled conditions, followed by XPS characterization. First-principle calculations were conducted simultaneously to investigate the electronic properties and the reactivity of relevant surfaces of an ideal LiMn2O4 material. The results allow us to conclude that the reactivity of the samples is dominated by an acido-basic reactivity and the formation of sulfite species. Nonetheless, on the x = 0.05 sample, both sulfite and sulfate species are obtained, the later, in lesser extent, corresponding to a redox reactivity. Combining experimental and theoretical results, this redox reactivity could be associated with the presence of a larger quantity of Mn4+ cations on the last surface layers of the material linked to a specific surface orientation.

  • [hal-00713263] The IR spectrum of supercritical water: Combined molecular dynamics/quantum mechanics strategy and force field for cluster sampling

    13 mars 2018 19:39

    Supercritical water was analyzed recently as a gas of small clusters of waters linked to each other by intermolecular hydrogen-bonds, but unexpected "linear" conformations of clusters are required to reproduce the infra-red (IR) spectra of the supercritical state. Aiming at a better understanding of clusters in supercritical water, this work presents a strategy combining classical molecular dynamics to explore the potential energy landscape of water clusters with quantum mechanical calculation of their IR spectra. For this purpose, we have developed an accurate and flexible force field of water based on the TIP5P 5-site model. Water dimers and trimers obtained with this improved force field compare well with the quantum mechanically optimized clusters. Exploration by simulated annealing of the potential energy surface of the classical force field reveals a new trimer conformation whose IR response determined from quantum calculations could play a role in the IR spectra of supercritical water.

  • [hal-00764350] Assessing ocean-model sensitivity to wind forcing uncertainties

    11 janv. 2018 07:28

    In this paper, we assess the short-term forecast error of a mesoscale primitive-equation open-ocean model, induced by uncertainties in wind forcing. Statistics calculated from an ensemble of ocean states show that temperature forecast error is strongest at the top of the ensemble-mean thermocline, as a consequence of vertical displacement of the mixed-layer base around its ensemble mean. Horizontal pattern of the temperature error in the mixed-layer is mainly explained by horizontal advection and surface heat flux fluctuations. These two mechanisms and entrainment through the mixed-layer bottom are presented as the three processes responsible for thermal forecast error growth in the modeled upper ocean.

  • [hal-01485877] A-VCI: a flexible method to efficiently compute vibrational spectra

    11 janv. 2018 07:28

    The Adaptive Vibrational Configuration Interaction (A-VCI) algorithm has been introduced as a new method to efficiently reduce the dimension of the set of basis functions used in a Vibrational Configuration Interaction (VCI) process. It is based on the construction of nested basis for the discretization of the Hamiltonian operator according to a theoretical criterion that ensures the convergence of the method. The purpose of this paper is to study the properties and outline the performance details of the main steps of the algorithm. New parameters have been incorporated to increase flexibility, and their influence have been thoroughly investigated. The robustness and reliability of the method are demonstrated for the computation of the vibrational spectrum up to $3000$ cm$^{-1}$ of a widely studied $6$-atom molecule (acetonitrile). Our results are compared to the most accurate up to date computation, and we also give a new reference calculation for future work on this system. The algorithm has also been applied to a more challenging $7$-atom molecule (ethylene oxide). The computed spectrum up to $3200$ cm$^{-1}$ is the most accurate computation that exists today on such a system.