Nano-CoF3 prepared by direct fluorination with F2 gas: Application as electrode material in Li-ion battery
Résumé
Nano-CoF3 powders were synthesized by direct fluorination at various temperatures (up to 300 °C) of cobalt nanoparticles and their electrochemical features in lithium battery were investigated. The structure and composition of the raw and fluorinated materials are explored by SEM/TEM, HRTEM, XRD and XPS experiments. The influence of the fluorination temperature on the crystallite size has been evidenced by XRD. It is shown that such fluorination process allows the direct synthesis of nano-CoF3, at TF2 = 100 °C. The XPS investigation of mixtures of CoF3 and acetylene black has shown that, when handled in very dry atmospheres, CoF3-based samples can be used to prepare electrodes for Li-ion batteries in safe conditions. The electrochemical reaction of nano-CoF3 powders vs. lithium metal studied in organic medium involves a conversion process as described in the case of cobalt oxide. Successive impedance measurements were carried out at different discharge states to follow the modifications occurring at the electrode/electrolyte interface during the discharge step. The evolution of the reversible capacity retention of Li+ vs. time capacity of the powdered materials as well as the cycle life of the battery were investigated as a function of the fluorination temperature used for the preparation of the cobalt fluoride nanopowders. The best electrochemical performances were obtained with nano-CoF3 powders prepared at fluorination temperature of 100 °C, for which a reversible capacity of about 390 mAh/g is obtained after subsequent cycles.