Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF3 using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, and by revisiting prior experimental studies of FeF2 and CuF2. Metal fluoride lithiation is instead dominated by diffusion-controlled displacement mechanisms, and a clear topological relationship between the metal fluoride F− sublattices and that of LiF is established. Initial lithiation of FeF3 forms FeF2 on the particle’s surface, along with a cation-ordered and stacking-disordered phase, A-LixFeyF3, which is structurally related to α-/β-LiMn2+Fe3+F6 and which topotactically transforms to B- and then C-LixFeyF3, before forming LiF and Fe. Lithiation of FeF2 and CuF2 results in a buffer phase between FeF2/CuF2 and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.
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 authors upon reasonable request.
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X.H. is supported by funding from the Engineering and Physical Sciences Research Council (EPSRC) Doctoral Prize, Adolphe Merkle and the Swiss National Science Foundation (Program NRP70 no. 153990) and European Commission via Marie Skłodowska-Curie actions (MSCA) (grant 798169). A.S.E. acknowledges financial support from the Royal Society. E.C.-M. acknowledges funding from the European Commission via MSCA (grant 747449) and RTI2018-094550-A-l00 from the Ministerio de Ciencia e Innovación. H.S.G. is supported by EPSRC via Industrial Cooperative Awards in Science and Technology studentship. Z.L. is supported by the Faraday Institution (grant number FIRG017). C.J.P. is supported by the Royal Society through a Royal Society Wolfson Research Merit award and by EPSRC grant EP/P022596/1. A.L.G. acknowledges funding from the European Research Council (grant 788144). This research was supported as part of the North Eastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, and Office of Basic Energy Sciences under award number DE-SC0001294. Work done at Argonne and use of the Advanced Photon Source, an Office of Science User Facility operated for the US Department of Energy Office of Science by Argonne National Laboratory, was supported by the US Department of Energy under contract no. DE-AC02-06CH11357. Work done at Diamond Light Source was under proposal EE17315-1. We thank G. Ceder and other North Eastern Center for Chemical Energy Storage members for many stimulating discussions concerning fluoride-based conversion reactions and on the origins of structural hysteresis. We also acknowledge help from S. Dutton, T. Dean, A. Docker, M. Leskes and D. Keeble.
The authors declare no competing interests.
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a) Galvanostatic profile of n-FeF3’s first cycle with distinct reaction processes indicated by different colour backgrounds. The 6Li MAS NMR chemical shifts (red) from our previous study10 are labelled at their respective state of charge with the main phase identification indicated. Black squares mark the states of charge where ex situ X-ray total scattering experiments were performed to acquire b) XRD and c) PDF patterns. Green, blue and pink dotted lines indicate unique Bragg or PDF features from FeF3, LixFeyF3 and FeF2, respectively. The red arrow highlights a drastic decrease in the samples’ particle sizes upon charge. The evolution of the phase mole fractions obtained from the PDF refinement is shown in d) with the deduced step-by-step mechanism diagram indicated on the left, which echoes with the simplified scheme shown in Fig. 5d.
The reference phases in the phase diagram are labelled and indicated by light blue circles so as to show the positions of A- and B-LixFeyF3, whose Fe concentration is off-stoichiometric. The reaction pathways associated with the FeF3 and FeF2 systems are respectively marked by using green and pink dashed arrows. Each reaction process is also labelled with its respective roman numeral used to label the equations in the manuscript.
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Hua, X., Eggeman, A.S., Castillo-Martínez, E. et al. Revisiting metal fluorides as lithium-ion battery cathodes. Nat. Mater. (2021). https://doi.org/10.1038/s41563-020-00893-1