From sodium to lithium: unlocking high-performance fluorophosphate cathode materials through topotactic ion exchange

Ion-exchange synthesis offers a powerful route to access Li-based fluorophosphates that are otherwise unattainable through direct methods. In this study, Na₀.₉₂Li₁.₀₅FePO₄F/C (NLFPF/C) was successfully prepared via a chemical ion exchange reaction using Na₂FePO₄F/C (NFPF/C) as a precursor. XRD and Rietveld refinement confirmed that the orthorhombic Pbcn framework was preserved after exchange, while Raman and TGA demonstrated that the conductive carbon coating remained intact. ICP-OES and solid-state NMR showed a nearly one-to-one Na$^+$/Li$^+$ substitution, with Li$^+$ occupying both Na sites in the Na₂FePO₄F framework. Variable-temperature XRD and TGA indicated structural decomposition above 350 °C, leading to the formation of NaFeP₂O₇. NFPF/C electrochemical profile exhibited two distinct voltage plateaus characteristic of biphasic Na$^+$ migration, whereas NLFPF/C showed significantly different electrochemical behavior consisting of sloping profiles that indicate a solid-solution mechanism. NLFPF/C delivered a high discharge capacity of 139 mAh g$^{−1}$ at 0.1C with excellent capacity retention and improved coulombic efficiency over 100 cycles. ... mehrEx-situ XRD measurements confirmed reversible structural evolution without formation of secondary phases, suggesting a highly stable framework under cycling. These findings demonstrate the high stability and energy density of NLFPF/C and highlight ion exchange as an excellent strategy for designing advanced Li-based polyanionic cathodes.