First-Principles Investigation of the Li-Fe-F Phase Diagram and Equilibrium and Nonequilibrium Conversion Reactions of Iron Fluorides with Lithium

We have used density functional theory (DFT) to investigate the ternary phase diagram of the Li−Fe−F system and the reactions of Li with iron fluorides. Several novel compounds, not previously identified in the Li−Fe−F system, are predicted to be stable. Electrochemical voltage profiles, derived from the evolution of the Li chemical potential in the calculated phase diagram, are in reasonable agreement with experimental trends. The effect of particle size on the Fe that precipitates when Li_xFeF₃ reacts with Li is also investigated. We find that when 1 nm Fe particles form, the potential for this reaction is considerably reduced from its bulk value and relate this to the experimental observations. Furthermore, we formulate a model for the significant hysteresis that is observed in the lithiation and delithiation of FeF₃. Nonequilibrium paths derived by assuming much faster diffusion of Li than Fe are in reasonable agreement with experimental profiles. Our kinetic model predicts that the iron fluoride reaction follows a different path through the phase diagram during conversion (discharge) and reconversion (charge), which results in the voltage profile hysteresis observed during experiment. The proposed kinetic model also explains why upon extraction of Li from a 3/1 mixture of LiF and Fe a rutile FeF₂-like structure can form, even when iron should be oxidized to Fe³⁺ by extraction of three Li⁺ per Fe.