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).