Synthesis, crystal structure and electrochemical properties of LiFePO₄F cathode material for Li-ion batteries

Highlights: • Tavorite-structured LiFePO₄F with high purity is successfully synthesized by a two-step solid-state route. • Rietveld refinement shows that open pathways for 3D ion transport exist in LiFePO₄F and its discharged state (Li₂FePO₄F). • The voltage plateaus on 2.71 / 2.86 V, in galvanostatic discharge-charge cycling, indicate a low electrode polarization of 0.15 V. • This work attained the largest initial discharge capacity at the highest rate (1 C) reported to date. • This work attained almost the same capacity retention at a tenfold higher rate (1 C) than that (0.1 C) within the maximum cycles of 100 reported to date. - Abstract: Tavorite-structured lithium-metal-fluorophosphates for Li⁺ transition have been recognized as a good alternative to olivine-type cathodes for lithium-ion batteries. They show an exceptional ionic conductivity, excellent thermal stability and capacity retention. In this work, LiFePO₄F with high purity is successfully synthesized by a two-step solid-state route. Rietveld refinement shows that open pathways for 3D ion transport exist in LiFePO₄F and its discharged state (Li₂FePO₄F). Cyclic voltammetry data exhibit a clear indication of the Fe^3+/2+ redox couple that involves a two-phase transition. Galvanostatic discharge-charge cycling was examined at the rates 0.1 - 5 C up to 1000 cycles. The voltage plateaus on 2.71 / 2.86 V indicate a low electrode polarization of 0.15 V. This work attained the largest initial discharge capacity at the highest rate (1 C) reported to date, and almost the same capacity retention at a tenfold higher rate (1 C) than that (0.1 C) within the maximum cycles of 100 reported to date