Investigation of the lithiation and delithiation conversion mechanisms of bismuth fluoride nanocomposites

Combined in situ X-ray diffraction, in situ X-ray absorption spectroscopy, and selected area electron diffraction analyses have confirmed the occurrence of a reversible conversion reaction in the BiF3/C nanocomposite upon cycling, which leads to the formation of Bi-0 and LiF during lithiation and the reformation of BiF3 during delithiation. It has been shown that only the high-pressure tysonite phase of BiF3 reforms during the oxidation sweep and that no bismuth fluoride compound with an oxidation state of the bismuth lower than 3 is formed as intermediate during the lithiation or delithiation reactions. Finally, it has been demonstrated that the different plateaus or pseudo plateaus observed on the lithiation and delithiation voltage profiles stem from polarization changes brought about by the dramatic structural changes occurring in the nanocomposite upon cycling. A model, based on the variation of the electronic and ionic transport mechanisms as a function of the state of completion of the conversion and reconversion reactions, is proposed to explain those polarization changes.