Revisiting conversion reaction mechanisms in lithium batteries: lithiation-driven topotactic transformation in FeF2

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Karki, Khim
Wu, Lijun
Ma, Ying
Armstrong, Mark J.
Holmes, Justin D.
Garofalini, Stephen H.
Zhu, Yimei
Stach, Eric A.
Wang, Feng
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American Chemical Society
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Intercalation-type electrodes have now been commonly employed in todayâ s batteries due to their capability of storing and releasing lithium reversibly via topotactic transformation, conducive to small structural change, but they have limited interstitial sites to hold Li. In contrast, conversion electrodes feature high Li-storage capacity, but often undergo large structural change during (de)lithiation, resulting in cycling instability. One exception is iron fluoride (FeF2), a conversion-type cathode that exhibits both high capacity and high cycling stability. Herein, we report a lithiation-driven topotactic transformation in a single crystal of FeF2, unveiled by in situ visualization of the spatial and crystallographic correlation between the parent and converted phas-es. Specifically, conversion in FeF2 resembles the intercalation process but involves transport of both Li+ and Fe2+ ions within the F-anion array, leading to formation of Fe preferentially along specific crystallographic ori-entations of FeF2. Throughout the process, the F-anion framework is retained, creating a checkerboard-like structure, within which the volume change is largely compensated, thereby enabling the high cyclability in FeF2. Findings from this study, with unique insights into conversion reaction mechanisms, may help to pave the way for designing conversion-type electrodes for the next-generation lithium batteries.
Lithium batteries , Conversion reaction mechanisms , Topotactic transformation , Iron fluorides , FeF2
Karki, K., Wu, L., Ma, Y., Armstrong, M. J., Holmes, J. D., Garofalini, S. H., Zhu, Y., Stach, E. A. and Wang, F. (2018) 'Revisiting conversion reaction mechanisms in lithium batteries: lithiation-driven topotactic transformation in FeF2', Journal of the American Chemical Society. doi:10.1021/jacs.8b07740
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© 2018, American Chemical Society. This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society after technical editing by the publisher. To access the final edited and published work see