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

Abstract

Extensive studies on the trirutile Li0.5FeF3 phase have been commissioned in the context of Li-Fe-F system for Li-ion batteries. However, progress in electrochemical and structural studies have 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 the ordered trirutile Li0.5FeF3 at elevated temperatures of 90 °C with the aid of a thermally stable ionic liquid electrolyte. The ordered trirutile Li0.5FeF3 is prepared by highenergy 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 twophase reaction with a minor volume change (1.09 % between Li0.5FeF3 and Li0.11FeF3) in the voltage range of 3.2−4.3 V. Extension of the lower cut-off 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 Li/Fe disordered trirutile structure at 4.3 V without showing the reconversion from LiF and rutile FeF2 to the ordered trirutile Li0.5FeF3 or FeF3.

High-Pressure Synthesis of Trigonal LiFe2F6: New Iron Fluoride with Li+ Tunnels as a Potential Cathode for Lithium-Ion Batteries

“…Regarding this result, a possible transition to an amorphous form of FeF 3 cannot be omitted. Such a result was encountered before in previous studies on fluorides in which pristine structures collapse upon cycling when too much lithium is inserted within the structure. ,, This issue can be avoided by using nanosized particles to minimize the ohmic resistance through cycling, and an advanced carbon coating was proven effective to maximize material protection. , Further experiments on the elaboration of the electrode composite material, use of ionic liquid electrolytes, or monitoring the measurement temperature will be explored for a possible positive impact on cyclability . To improve our result, we tried to study HP-LiFe 2 F 6 starting with a charge process, removing the lithium from the material first with the Fe 2+ oxidation reaction.…”

“…26 To improve the performance with a charge mechanism, LiFe 2 F 6 (called Li 0.5 FeF 3 ) was synthesized from the previous ball-mill synthesis followed by a thermal treatment at 400 °C under Ar. This material was studied electrochemically at 90 °C during cycling with an ionic liquid used as electrolyte (Li[FSA]-[C 2 C 1 im][FSA]); a reversible capacity at 90 mAh•g −1 was measured between 3.2 and 4.5 V. 27 The same conditions were used for a new disordered trirutile manganese−iron phase Li 1.2 MnFe 1.2 F 6.8 obtained by highenergy ball-milling, with a positive effect on the performance. 28 Offering a structure with lithium channels where enhanced lithium diffusions can occur, α-LiMnFeF 6 was synthesized by sol−gel synthesis and characterized electrochemically.…”

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries

“…However, such a figure of merit is negated by several roadblocks, namely enlisting among others a large irreversible capacity (>25%) and generally poor energy efficiency. Addressing these issues via innovative nanostructural designs is continuing to attract interest among the battery community not only for Li­(Na)-ion but also for F-ion shuttle batteries. , Fluorinated cathodes can possibly self-form a LiF enriched CEI (cathode electrolyte interface) with standard fluorine-rich electrolytes, therefore increasing the stability of fluorinated cathodes, and unusual electrolytes as ionic liquids could be more studied in terms of volume change or operating temperature. ,,, Lastly, although it was not the main scope of this manuscript, it is worth recalling that the treatment of electrode surface by fluorine or doping of electrodes with minute amounts of fluorine is current practice to minimize electrolyte degradation − and could be an interesting extension to the study of oxyfluorides material. We hope that this review could arouse interest from the battery community and provide a scientific sound ground for attracting young scientists eager to enter the field of F-based electrode materials and energy related problems in general.…”

“…At this temperature, a charge process was identified at 4 V followed by a structural change to a disordered form reaching a reversible capacity of 90 mAh g –1 in the range 3.2–4.5 V (Figure ). In the same conditions, the Mn 3+ /Mn 2+ redox reaction was unlocked in a Li-rich cation-disordered trirutile Li 1.06 Mn 0.88 Fe 1.06 F 6 …”

“…Galvanostatic charge–discharge voltage profiles (left), corresponding d Q /d V plots of the charge–discharge curves (right) of Li 0.5 FeF 3 during the initial three cycles from Zheng et al Reproduced from ref . Copyright 2021 American Chemical Society.…”