Subsurface diffusion of oxide electrolyte decomposition products in metal fluoride nanocomposite electrodes

Conversion materials based on metal fluorides offer enhanced charge capacity as positive electrodes in Li-ion electrochemical cells. However, to date, little is understood about the interactions these structurally dynamic materials have with nonaqueous electrolytes during both lithiation and delithiation. Such interactions are explored using bismuth (III) fluoride. This work details the evolution of the electrolyte decomposition products at both highly oxidizing potentials (4.5 V vs. Li/Li+) and during delithiation of the positive electrode after having formed a decomposition layer via electrochemical reduction of the electrolyte solvents. For the first time, the impact of the electrolyte reactions on the surface and subsurface chemistry of the inorganic conversion material is explored. Observed results offer an explanation for the direct influence of electrolyte choice on cycling efficiency of metal fluoride conversion materials.

► XPS and NMR study of interactions of electrolytes with BiF3 during conversion.
► SEI decomposes during reconversion, resulting in oxyfluoride interfacial products.
► Mechanistic link established for SEI and capacity fade in BiF3 nanocomposites.