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

Abstract

Extensive\nstudies on trirutile Li_0.5FeF₃ phase\nhave been commissioned in the context of the Li–Fe–F\nsystem for Li-ion batteries. However, progress in electrochemical\nand structural studies has been greatly encumbered by the low electrochemical\nreactivity of this material. In order to advance this class of materials,\na comprehensive study into the mechanisms of this phase is necessary.\nTherefore, herein, we report for the first time overall reaction mechanisms\nof ordered trirutile Li_0.5FeF₃ at elevated temperatures\nof 90 °C with the aid of a thermally stable ionic liquid electrolyte.\nOrdered trirutile Li_0.5FeF₃ is prepared by high-energy\nball milling combined with heat treatment followed by electrochemical\ntests, X-ray diffraction, and X-ray absorption spectroscopic analyses.\nOur results reveal that a reversible topotactic Li⁺ extraction/insertion\nfrom/into the trirutile structure occurs in a two-phase reaction with\na minor volume change (1.09% between Li_0.5FeF₃ and Li_0.11FeF₃) in the voltage range of 3.2–4.3\nV. The extension of the lower cutoff voltage to 2.5 V results in a\nconversion reaction to LiF and rutile FeF₂ during discharging.\nThe subsequent charge triggers the formation of the disordered trirutile\nstructure at 4.3 V without showing the reconversion from LiF and rutile\nFeF₂ to ordered trirutile Li_0.5FeF₃ or FeF₃.