Phase Evolution of Trirutile Li_0.5FeF₃ for Lithium-Ion Batteries

Extensive studies on trirutile Li0.5FeF3 phase have been commissioned in the context of the Li–Fe–F system for Li-ion batteries. However, progress in electrochemical and structural studies has been greatly encumbered by the low electrochemical reactivity of this material. In order to advance this class of materials, a comprehensive study into the mechanisms of this phase is necessary. Therefore, herein, we report for the first time overall reaction mechanisms of ordered trirutile Li0.5FeF3 at elevated temperatures of 90 °C with the aid of a thermally stable ionic liquid electrolyte. Ordered trirutile Li0.5FeF3 is prepared by high-energy ball milling combined with heat treatment followed by electrochemical tests, X-ray diffraction, and X-ray absorption spectroscopic analyses. Our results reveal that a reversible topotactic Li+ extraction/insertion from/into the trirutile structure occurs in a two-phase reaction with a minor volume change (1.09% between Li0.5FeF3 and Li0.11FeF3) in the voltage range of 3.2–4.3 V. The extension of the lower cutoff voltage to 2.5 V results in a conversion reaction to LiF and rutile FeF2 during discharging. The subsequent charge triggers the formation of the disordered trirutile structure at 4.3 V without showing the reconversion from LiF and rutile FeF2 to ordered trirutile Li0.5FeF3 or FeF3.