Abstract
The kinetic of LiFePO4 chemical delithiation was studied on sintered powders synthesized by precipitation method and partially oxidized using Br2 in acetonitrile. Thorough X-ray diffraction analyses using Rietveld refinements revealed a two-phase reaction with a slight deviation of the lattice parameters. Coherency domains of the two end members were determined using the Thomson-Cox-Hasting function and they linearly increase for the lithium-poor phase at the detriment of the lithium-rich phase. The HRSTEM-EELS measurements on partially delithiated samples validate the Shrinking Core mechanism with a core of LiFePO4 surrounded by a shell of FePO4. Thus, each particle undergoes a two-phased transformation. Furthermore, the HRSTEM-EELS reveals that the grain boundaries (resulting from the heat treatment in order to improve phase crystallinity) strongly hinder the delithiation kinetics. Chemical delithiation triggers agglomerates breaking and particle splitting at grain boundaries. This new aspect may provide information on the impact of microstructure on the electrochemical performances of LiFePO4 materials.