Sol-gel processing and electrochemical characterization of monoclinic Li 3 FeF 6

To find new cathode materials for future applications in lithium-ion batteries, lithium transition metal fluorides represent an interesting class of materials. In principle the Li intercalation voltage can be increased by replacing oxygen in the cathode host structure with the more electronegative fluorine. A facile pyrolytic sol–gel process with trifluoroacetic acid as fluorine source was established to synthesize monoclinic Li3FeF6 using nontoxic chemicals. The acicular Li3FeF6 powder was characterized with X-ray diffraction and a detailed structure model was calculated by Rietveld analysis. For the preparation of cathode films to cycle versus lithium monoclinic Li3FeF6 was ball milled with carbon and binder down to nanoscale. After 100 cycles galvanostatic cycling (C/20) 47 % fully reversible capacity of the initial capacity (129 mAh/g) could be retained. To the best of our knowledge the results presented in this work include the first rate performance test for monoclinic Li3FeF6 up to 1 C maintaining a capacity of 71 mAh/g. The redox reaction involving Fe3+/Fe2+ during Li insertion/extraction was confirmed by post-mortem XPS and cyclic voltammetry.