Physical and Theoretical Chemistry

Biography

Biography: Germain Vallverdu

Abstract

Electrochemical storage of energy through Li-ion devices is the commonly used solution to address the intermittent character of renewable energy and the increasing demand of nomad technologies. LiCoO₂ is the most widely used positive electrode material of today’s Li-ion batteries. In the last decade, much research has been performed to explore alternative materials as mixed transition metal oxides LiNi_xMn_xCo_1−2xO₂ (NMC). The surface reactivity of these electrode materials towards the electrolyte is a key feature that has a deep impact on the performance and lifetime of Li-ion cells and needs to be understood and controls. Within this framework, based on our previous experience on lithium layered oxides, we decided to study the surface reactivity of Li₂MnO₃ which can be view as a model compound for MnIV layered oxides such as NMC or even Li-rich materials. The strategy consists in coupling adsorption of gaseous probe molecule (SO₂), X-ray photoelectron spectroscopy (XPS) and DFT calculation in order to identify the influence of the oxidation state of the transition metal on the adsorption reaction type (basic/acidic or redox). We focus our study on strengthening the experimental/calculation coupling by studying the reactivity on a single crystal surface of Li₂MnO₃. Both approaches conclude to a redox adsorption mode with the formation of sulphate species. Chemical maps of the crystal surface after adsorption obtained by Auger spectroscopy provide information on the adsorption sites location. Stacking faults and spinel type default are usually encountered in the Li₂MnO₃ crystals. Thus, we completed this study with the investigation of the surface reactivity of Li₂MnO₃ polycrystals against the stacking faults rate. Moreover, the reactivity of Li_1+xMn_2-xO₄ spinel materials will be checked to determine the influence of the spinel type default on the surface reactivity.