Reaction Pathways of Iron Trifluoride Investigated by Operation at 363 K Using an Ionic Liquid Electrolyte

Abstract

FeF3 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 FeF3 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 FeF3 to form another phase nominally composed of non-trirutile-type LiFe2F6, was confirmed. Lithium cation was inserted into LiFe2F6, resulting in a gradual decrease in the rest potential. The lithium-inserted phase was finally converted to LiF and FeF2 at the end of the one-electron discharge. The conversion of FeF2 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.