A Comparative Study of alpha- and beta-Li3FeF6: Structure and Electrochemical Behavior

Two materials in the Li-Fe-F system, monoclinic (alpha)- and orthorhombic (beta)-Li3FeF6, have been investigated as hosts for lithium insertion reactions. Both of them are related to the cryolite-type structure. Although the as-prepared materials do no insert lithium reversibly, decreasing the particle size by mechanical milling in the presence of carbon activates them. In alpha-Li3FeF6, the synthesis procedure yields particles with a smaller size and, hence, processing time can be decreased. After milling alpha-Li3FeF6 for 5 h, a fully reversible behavior involving ca. 0.45 Li/formula is observed at ca. 3.0 V, whereas for alpha-Li3FeF6 the same processing time produces a material that exhibits a capacity corresponding to 0.12 Li/formula. Capacity increases notably after 12 h of milling, reaching 100 mAh g(-1) (0.7 Li/formula) in the alpha-polymorph. For the beta-polymorph, capacity also increases, getting closer to the one obtained for the alpha-polymorph in as much as particle sizes are becoming more similar after prolonged milling. The characteristics of the insertion reaction make these two cryolites interesting as cathode materials for batteries. Because the alpha-polymorph can be prepared at room temperature from an aqueous solution, this polymorph seems to be more adequate to develop a cathode material. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3454238] All rights reserved.