Adsorption Preference of HF over Ethylene Carbonate Leads to Dominant Presence of Fluoride Products in LiFePO4 Cathode- Electrolyte Interface in Li-Ion Batteries

The electrolyte in Li-ion batteries is a Li-containing salt, such as LiPF6, dissolved in an organic solvent, such as ethylene carbonate (EC). LiPF6 reacts with trace amount of water present in the solvent to produce HF. The adsorption of HF and EC molecules on the cathode surface is the initial step in the series of reactions that leads to the formation of carbonates, polycarbonates, fluorides, and oxyfluorides which comprise the cathode-electrolyte interface (CEI). The composition of CEI has implications on the transfer of Li ions across the interface and the cycle life of the cell. Experiments show that the CEI on LiFePO4 contains only fluoride-containing species while the carbonate decomposition products are absent. Here, we attempt to explain this adsorption behavior. We compute the adsorption energy of EC and HF on different LiFePO4 cathode surfaces as a function of surface coverage using density functional theory (DFT). We check the adsorption preference for five low-index surfaces found in the Wulff shape of LiFePO4. We find that the reconstructed Wulff shape of LiFePO4 in an electrolyte environment contains facets adsorbed entirely by fluoride (F-) and no carbonate-adsorbed facets appear in it. We further discuss the implications of such adsorption preference of fluoride on the kinetics of Li-ion transfer across the CEI and on the cycle life of cell.