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Lessons learned from FeSb2O4 on stereoactive lone pairs as a design principle for anion insertion



Wasif Zaheer, George Agbeworvi, Saul Perez-Beltran, Justin Andrews, Yierpan Aierken, Conan Weiland, Cherno Jaye, Young-Sang Yu, David Shapiro, Sirine Fakra, Daniel A. Fischer, Jinghua Guo, David Prendergrast, Sarbajit Banerjee


Fluoride-ion batteries are an attractive energy storage concept analogous to lithium-ion batteries but feature an inverted paradigm where anions (fluoride-ion), and not cations, are the principal charge carriers. Insertion hosts that can reversibly insert fluoride ions at room temperature are exceedingly sparse and fundamental design principles are almost entirely unexplored. Using FeSb2O4 as a model system, we reveal using a combination of spectroscopic probes of valence and conduction band states that the topochemical solution-phase insertion of F-ions at room temperature involves Fe2+/Fe3+ redox but is mediated by critical multi-center synergies between iron and antimony centers across the 1D tunnel structure. The separation of the primary transition metal redox center from the p-block coordination site in FeSb2O4 alleviates structural strain by enabling compensatory contraction and expansion of FeO6 and SbO3 polyhedra, respectively, allowing for reversible accommodation of the inserted fluoride-ions in the 1D tunnels with minimal change in volume. The Sb 5s2 stereoactive electron lone pair lobes are destabilized and distorted by fluoride-ion insertion. As such, p-block electron lone pairs play a critical role in weakening anion—lattice interactions, which is critical to enabling reversible fluoride-ion diffusion across micron-sized length-scales (demonstrated here with nanometer-scale resolution using X-ray ptychography). The results illuminate a key mechanistic principle that anti-bonding interactions stemming from stereoactive lone pairs can be used to mediate anion—lattice interactions, and suggest that anion insertion hosts can be designed by pairing redox-active transition metals with p-block cations bearing stereochemically active electron lone pairs.
Cell Reports Physical Science


F-ion intercalation, battery materials, HAXPES, XANES, STXM, X-ray ptychography


Zaheer, W. , Agbeworvi, G. , Perez-Beltran, S. , Andrews, J. , Aierken, Y. , Weiland, C. , Jaye, C. , Yu, Y. , Shapiro, D. , Fakra, S. , Fischer, D. , Guo, J. , Prendergrast, D. and Banerjee, S. (2021), Lessons learned from FeSb2O4 on stereoactive lone pairs as a design principle for anion insertion, Cell Reports Physical Science, [online],, (Accessed December 7, 2023)
Created October 20, 2021, Updated December 31, 2022