Synthesis by Thermal Decomposition of Two Iron Hydroxyfluorides: Structural Effects of Li Insertion

The synthesis and the electrochemical properties of iron hydroxyfluorides with an open structure, prepared from the thermal decomposition of Fe2+Fe3+F5(H2O)2 and Fe2+Fe23+F8(H2O)2 under ambient air, are reported. The combination of thermogravimetric analyses, X-ray thermodiffraction (XRD), and infrared spectroscopy has evidenced two unstable lacunar oxyfluorides that transform rapidly into hydroxyfluorides under ambient air. Their formulations and structural features have been determined from powder XRD, Mössbauer spectrometry, and pair distribution function, leading to Fe3+F2.5(OH)0.5 with the pyrochlore network and to Fe3+F2.66(OH)0.34 with the hexagonal tungsten bronze (HTB) structure. The electrochemistry of these hydroxyfluorides indicates excellent and similar capacities with a significant decrease after the first cycle to reach sustained values of ∼110 mAh g–1 upon cycling. In situ XRD measurements reveal that the pyrochlore network undergoes amorphization (xLi+ > 0.5) while the HTB phase transforms into the ReO3-type phase (xLi+ > 0.33).