High-Pressure Synthesis of Trigonal LiFe2F6: New Iron Fluoride with Li+ Tunnels as a Potential Cathode for Lithium-Ion Batteries

To overcome the challenge of synthesizing new and promising iron fluoride cathode materials, high-pressure synthesis was employed to obtain a new form of LiFe2F6 crystallized in the trigonal P321 α-LiMnFeF6 structure. Starting from solid fluorine sources, the trigonal structure was obtained for the first time for LiFe2F6 at 2.7 GPa, 500 °C, and characterized by synchrotron X-ray diffraction. Rietveld refinements revealed that a new form of LiFe2F6 possesses the Na2SiF6-type structure, which would provide enhanced Li+ diffusions. A Mössbauer spectrometry analysis was done with a paramagnetic structure obtained at 300 K and a magnetic sextet at 77 K with a 53 T hyperfine field for Fe3+ similarly to α-LiMnFeF6. The first electrochemistry tests indicate a good first cycle capacity of ∼150 mAh·g–1 with a charge process corresponding to Li+ disinsertion, a value doubled when compared to α-LiMnFeF6. These first experimental results show a new synthetic idea to discover materials designed for next generation batteries.