In Situ Mossbauer Effect Study of Lithium Intercalation in LiFe2F6

A sample of Li1.2Fe2F6.2 with a disordered trirutile structure was prepared by high energy ball milling, and reversible lithium intercalation in this material was investigated using in situ Mossbauer spectroscopy. During the first discharge to 2.0 V (adding lithium), the percentage of Fe3+ decreased linearly with lithium content from 46 to 11%, while the percentage of Fe2+ increased linearly from 54 to 89%. No evidence of Fe-0 was observed in this potential range indicating that displacement reactions do not occur above 2.0 V. During the first charge (removing lithium) to 4.5 V, the percentage of Fe3+ increased from 11 to 68%, while the percentage of Fe2+ decreased from 89 to 32%. The change in the Fe oxidation state showed a 1:1 correlation with z in Li1.2 +/- zFe2F6.2 between 2.0 and 4.5 V. When Li/Li1.2Fe2F6.2 cells were charged to voltages higher than 4.5 V, the percentage of Fe3+ did not increase linearly with the cell capacity, indicating that side reactions occur above 4.5 V. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3473809] All rights reserved.