Phase Evolution of Trirutile Li<sub>0.5</sub>FeF<sub>3</sub> for Lithium-Ion Batteries
Yayun Zheng, Shinya Tawa, Jinkwang Hwang, Yuki Orikasa, Kazuhiko Matsumoto, Rika Hagiwara
Abstract
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.