Lithium Intercalation in LiFe2F6 and LiMgFeF6 Disordered Trirutile-Type Phases

LiFe2F6 and LiMgFeF6 disordered trirutile-type phases were prepared by high energy ballmilling, and their electrochemical activity toward lithium was investigated. Within the voltage range of 2.0-4.5 V, LiFe2F6 exhibited reversible reactivity with lithium, while LiMgFeF6 showed little to no reactivity. An in situ X-ray diffraction experiment was carried out to study the behavior of Li/LiFe2F6 cells. For Li1+xFe2F6, a reversible expansion of the a-axis occurred, indicating that Li atoms were intercalated in the rutile tunnels. For Li1-xFe2F6, there were minimal lattice constant shifts, but a broadening of most Bragg peaks and distinct charge-discharge hysteresis. We interpreted this as extraction of Li from the metal atom chains of the rutile structure. The theoretical specific capacity for Delta x=2 in LixFe2F6 is 224 mAh/g. Our experiments showed only about 126 mAh/g for this process from the stoichiometric LiFe2F6. However, up to 155 mAh/g specific capacity was obtained from nonstoichiometric Li1.2Fe2F6.2 prepared by adding an extra 0.2 mol of LiF per mole of LiFe2F6.