A Comparative Study of α- and β-Li₃FeF₆: Structure and Electrochemical Behavior

Abstract

Two new cathode materials in the Li-Fe-F system are presented: α- and β-Li3FeF6. Both of them are related to the cryolite-type structure. Though the as prepared materials do no intercalate lithium reversibly, decreasing of particle size by mechanical milling in the presence of carbon activate them. Performances of α- Li3FeF6 are better since the synthesis procedure yields particles with smaller size and processing time can be reduced. For example, a fully reversible behavior involving ca. 0.45 Li/formula can be obtained using 5 h of milling for α-Li3FeF6 while for β-Li3FeF6 the same processing produces a material that exhibits a capacity corresponding to 0.12 Li/formula. For 12 h of milling capacity increases notably reaching 100 mAh g-1 (0.7 Li/formula) in the case of the α polymorph. Interestingly, the α-polymorph can be prepared at near room temperature. The easy and cheap synthesis of α-Li3FeF6 makes this material a very promising electrode material.