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.

Owing to their high theoretical capacity, metal fluorides have attracted significant interest as materials for fabricating the cathode of lithium secondary batteries. In the present study, a nanocomposite of LiF and FeF2 is prepared by a fluorolytic solgel method in an ethanol solution, for use as the cathode material of a lithium secondary battery. The produced LiF/FeF2 composite is characterized by broad X-ray diffraction peaks attributed to the nanosized (~10 nm) LiF and FeF2 crystals, a large Brunauer-Emmett-Teller surface area of 119 m 2 g −1 , and adsorption-desorption hysteresis, attributed to the presence of mesopores. The results of charge-discharge tests indicates an initial discharge capacity of 225 mAh (g-LiF/FeF2) −1 through reversal conversion at a current rate of 10 mA (g-LiF/FeF2) −1. Based on a combination of galvanostatic intermittent titration, X-ray absorption, and X-ray diffraction investigations, a new reaction mechanism is developed, namely, the conversion of the local environment of an Fe atom from a rutile-type FeF2 structure to a rhenium trioxide-type FeF3 structure during charging, with the subsequent discharge resulting in the insertion of Li + into the rhenium trioxide-type FeF3 structure, followed by the conversion reaction to LiF and FeF2.

The Li[FSA]-[C 2 C 1 im][FSA] (FSA − : bis(fluorosulfonyl)amide and C 2 C 1 im + : 1-ethyl-3methylimidazolium) ionic liquids have been studied as electrolytes for Li secondary batteries, though their thermal, physical, and electrochemical properties have not been systematically characterized. In this study, the thermal and transport properties of Li[FSA]-[C 2 C 1 im][FSA] ionic liquids as a function of the Li[FSA] molar fraction and temperature, in view of their operation at both room and intermediate temperatures. Differential scanning calorimetric analysis revealed that this system has a wide liquidphase temperature range from Li[FSA] fractions of 0.0 to 0.4 and indicated the existence of the Li[C 2 C 1 im][FSA] 2 line compound. Single-crystal X-ray diffraction analysis was used to determine the crystal structure of Li[C 2 C 1 im][FSA] 2 , which consists of Li + octahedrally coordinated by six O atoms originating from four FSA − anions. The temperature dependences of the viscosity and ionic conductivity were fitted by the Vogel-Tammann-Fulcher equation, and the viscosity and molar ionic conductivity were connected by the fractional Walden rule. Lithium-metal deposition/dissolution efficiency decreased with increasing measurement temperature and decreasing Li[FSA] fraction. Aluminium corrosion at positive potentials was investigated by a potential step method, which revealed that the stability of an aluminium electrode was improved at high Li[FSA] fractions at 298 K and the corrosionlimit potential decreased at elevated temperatures. .

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FeF 3 possesses a high theoretical capacity of 712 mAh g −1 owing to the three-electron reaction. However, various drawbacks, such as the large voltage hysteresis of the conversion reaction, prevent its practical use in lithium secondary batteries. In this study, the charge-discharge behavior of FeF 3 in an ionic liquid electrolyte at 363 K was investigated to elucidate the mechanisms and cause of the reduced overpotentials of the charge-discharge reactions. An evident plateau with an equilibrium potential of 3.42 V vs. Li + /Li during the initial discharge, indicating the two-phase reaction of FeF 3 to form another phase nominally composed of non-trirutiletype LiFe 2 F 6 , was confirmed. Lithium cation was inserted into LiFe 2 F 6 , resulting in a gradual decrease in the rest potential. The lithium-inserted phase was finally converted to LiF and FeF 2 at the end of the one-electron discharge. The conversion of FeF 2 to LiF and Fe in the ionic liquid electrolyte at 363 K was completed at >2.0 V and 71.2 mA g −1 , even though the reaction did not occur at 298 K unless the electrode was discharged below 2.0 V. This difference in the operating voltage of the conversion reaction was mainly due to the suppression of the Li + diffusion overpotential at 363 K.

Iron(III) fluoride (FeF 3 ) with low crystallinity and high surface area has been synthesized by a fluorolysis method, and applied as a positive electrode material for lithium secondary batteries. The FeF 3 prepared with a high molar ratio of hydrogen fluoride exhibits a high surface area and crystallinity. The FeF 3 sample treated at 300 °C under a F 2 /Ar atmosphere exhibits the highest surface area and was selected for further electrochemical test in view of improvement of performance in the potential region of conversion reactions. The initial discharge (lithiation) capacity was 676 mAh g −1 that was close to the theoretical capacity of FeF 3 (712 mAh g −1 ).Compared to the highly crystalline FeF 3 commercially available, the synthesized FeF 3 in the present study exhibited a low discharge capacity attributed to the insertion reaction because of the low crystallinity, and showed a low overpotential and larger capacity corresponding to the conversion reaction owing to the higher surface area of the fluorides.

A composite of lithium fluoride (LiF) and iron difluoride (FeF2) has been prepared by the reaction of the corresponding metal acetates and anhydrous hydrogen fluoride in ethanol in view of its application as a positive electrode material for lithium secondary batteries. The obtained LiF/FeF2 composite shows weak XRD peaks assigned to LiF and FeF2, and crystallite size is estimated to be 12 nm by the Scherrer’s equation. The composite powder exhibits a high N­2-adsorption capacity with adsorption-desorption loop due to the existence of meso-pores, and BET surface area is calculated to be 119 m2 g−1. In the charge–discharge test, this composite shows an initial charge capacity of 235 mAh (g-LiF/FeF2)−1 and discharge capacity of 225 mAh (g-LiF/FeF2)−1 at 10 mA (g-LiF/FeF2)− 1. Although the discharge reaction from the initial cycle occurs with small overpotential, the charge reaction exhibits large overpotential over 300 mV even at a slow rate below 10 mA (g-LiF/FeF2)− 1.