Résumé
The last decades LiFePO4 (triphylite) has been extensively studied due to interesting electrochemical properties that make it an attractive positive electrode candidate for Li-ion batteries. LiFePO4 used as a cathode material exhibits a high retention in cycling, a structural stability of the delithiated phase FePO4 and a low cost of its components. Recent developments show the possibility of significantly increasing power densities. However, the LiFePO4 nanoparticles show significant amounts of structural defects according to the synthetic route used. For instance, the presence of Fe and vacancies in Li crystallographic sites (Pnma space group) has a significant effect on electrochemical behavior by hindering cation diffusion. LiFePO4 also has a high reactivity to O2 at moderate temperatures (300-500°C depending on the size) leading to the gradual diffusion of Fe from the core to the surface of the material accompanied by the formation of Fe2O3 nanoparticles. Therefore, the composition material LixFeyPO4 exhibits very high degree of crystalline defects. These transformations were evidenced by X-ray diffraction and electron diffraction at different temperatures. These olivine compounds outside stoichiometry have an order of defects leading to the formation of a superstructure. However, in order to get a better insight into the mechanisms related to these transformations, in situ investigation in a real-time is necessary. Environmental ETEM, in which a pression of O2 can be injected, coupled with a heating holder is a perfect characterization platform to monitor oxidation reactivity of LiFePO4 up to 700°C. This project focuses on the study of the structural mechanisms associated with the temperature reactivity of LiFePO4 under an oxidizing atmosphere using environmental TEM (TITAN). The aim is to quantify the kinetics of FeyOx nanoparticle formation, and the appearance of superstructures induced by Fe diffusion from the core to the surface of the material.