Iron trifluoride synthesized via evaporation method and its application to rechargeable lithium batteries

Facile synthesis of rhombohedral type FeF3 introduced via two consecutive steps is introduced: i) acidic treatment of Fe2O3 followed by thermal evaporation at 80 °C resulting in hydrated β-FeF3·3H2O and ii) a simple thermal decomposition of the as-received β-FeF3·3H2O at 400 °C under an Ar atmosphere. A Rietveld refinement of x-ray diffraction data for the as-synthesized FeF3 indicates the formation of a highly crystalline FeF3 structure with a R3¯c space group. To overcome the high ionicity and improve the diffusivity, FeF3 is ball-milled with the aid of carbon (acetylene black). The electrochemical performance of nanosized FeF3 is not favored in voltage range of 1.5–4.5 V because the repetitive intercalation–conversion reaction accelerates the structural disruption within a few cycles, although a high capacity (518 mAh (g-fluoride)−1 at 20 mA g−1) is observed, assisted by the three-electron redox of Fe3+/0. Raising the lower cut-off voltage to 2 V, which allows only intercalation reaction, the FeF3 delivers a high capacity of 224 mAh g−1 with significantly improved capacity retention (71% at 100th cycle).

► FeF3 is readily synthesized via a direct reaction of Fe2O3 and HF.
► The resulting β-FeF3·3H2O transforms to FeF3 at 400 °C in an Ar atmosphere.
► The FeF3 delivers a high capacity of 224 mAh g−1 with good capacity retention.
► This results from the maintenance of the crystal structure by topotactic reaction.