The application of transition metal fluorides as energy-dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities and limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single-crystalline, monodisperse iron(ii) fluoride nanorods. Near theoretical capacity (570 mA h g−1) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr1,3FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 °C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface-specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semicoherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF2.
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The authors declare that all data supporting the findings of this study are included within the paper and its Supplementary Information files. Source data are available from the corresponding author upon reasonable request.
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This publication arises from research funded by the John Fell Oxford University Press Research Fund. We acknowledge financial support from the Henry Royce Institute (through UK Engineering and Physical Science Research Council grant no. EP/R010145/1) for capital equipment. I.C. acknowledges support from a Modentech studentship. T.-U.W and H.-W.L. acknowledge the 2019 Research Fund (1.190031.01) of Ulsan National Institute of Science and Technology and Individual Basic Science & Engineering Research Program (NRF-2019R1C1C1009324) through the National Research Foundation of Korea funded by the Ministry of Science and ICT. N.G. and A.R. acknowledge support from the Royal Society. We are grateful to the David Cockayne Center for Electron Microscopy for the use of their electron microscopes, and we thank P. Holdway for assistance in the collection of XPS data. A.W.X. thanks P. Quijano Velasco and K. Hurlbutt for helpful scientific discussion. We also thank S. Yin and E. Für DeBier.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
a,b, Dark field STEM and corresponding EELS spectra for an FeF2 nanorod from Stage 2 of discharge. Yellow arrows denote direction of EELS line scan and black dashed boxes indicate areas over which the displayed spectra are averaged. EELS spectra from the core of the nanrod exhibit a splitting of the Fe L3 peak, specifically broadening toward higher energy loss values indicative of Fe3+. The greater width of this peak is evident when compared to the EELS spectra acquired from the surface. This increase in Fe oxidation state indicates that a disproportionation to form the trirutile phase has occurred. This oxidation is not a result of air exposure as this would oxidize material at the surface first. Instead, the EELS spectra near the nanorod surface are still indicative of Fe0 or Fe2+. Scale bar, 40 nm (a).
a,b, Dark field STEM and corresponding EELS spectra for an FeF2 nanorod from Stage 3 of discharge. Line scans taken at different positions along the length of the nanorod indicate the presence of two distinct regions. There is a clear distinction between a region containing a strong Fe3+ signal related to the trirutile phase and a region containing only Fe0 and Fe2+ species. This data suggests that the onset of conversion coincides with the reduction of the Fe3+ rutile formed during the initial disproportionation as well as the formation of metallic iron in the nanorod interior. Scale bar, 50 nm (a).
a,b, Dark field STEM and corresponding EELS spectra for an FeF2 nanorod from Stage 3 of discharge.Upon charging to the full capacity at 4.0 V vs. Li+/Li, the widespread presence of Fe3+ is observed at the interior of the nanorod. This potential is higher than the standard reduction potential of Fe3+ + e− → Fe2+ (3.8 V vs. Li+/Li), and this transition is facilitated by the kinetically limited reinsertion of Fe from the double layer shell which causes the core of the nanorod to reconvert with a stoichiometry closer to FeF3.40 This delayed reconversion of the double layer shell is evidenced by EELS spectra from the nanorod surface, which show the distinct presence of Fe2+. Fe3+ is generated at the beginning of discharge and reformed at the end of charge. Scale bar, 30 nm (a).
a,b, Bright field TEM images comparing single nanorods after 15 cycles in the IL and LP30 electrolytes. c,d, The corresponding FFTs colored to differentiate between multiple phases present. Greater shape preservation is observed in the IL electrolyte. Strong rocksalt reflections in (c) indicate largely incomplete reconversion. Scale bars, 20 nm.
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Xiao, A.W., Lee, H.J., Capone, I. et al. Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes. Nat. Mater. 19, 644–654 (2020). https://doi.org/10.1038/s41563-020-0621-z
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