synthesis of iron - based fluoride materials : fef2.2(oh)0.8 was prepared by using a precipitation method assisted by microwave heating as detailed in reference  14 . fef2.2(oh)0.8 was subsequently annealed at 300 and 350  c for 1  h under ar , yielding fef2.2(oh)0.8xox/2x/2 with varied content of anionic vacancies (  ) . electrochemical characterizations : electrochemical reactions with lithium were performed using coin cells comprising iron - based fluoride and metallic lithium as electrodes . the various components were hand milled with nmp solvent prior to deposit on aluminum foil . the area of the electrode was 1.0  cm with a typical mass loading of 23  mg . the electrolyte consists of the commercially available lp30 ( 1.0  m lipf6 dissolved in a mixture of ethylene carbonate and ethyl methyl carbonate ( 3:7 v / v ) . cells were cycled between 2 and 4.2  v at 50  ma  g current density . pair distribution function ( pdf ) analysis : lithiated and delithiated electrodes were recovered in a glove box , washed with dmc and packed in kapton capillaries . high - energy x - ray data were collected at the 11-id - b station at the advanced photon source ( argonne national laboratory ) using a large amorphous - silicon - based area detector . the data were corrected for background scattering , compton scattering , and detector effects within pdfgetx2 software . data were subsequently fourier transformed to obtain the pdf , g(r).22 refinement of the pdf data was performed with pdfgui.23
the mechanism of lithium insertion that occurs in an iron oxyfluoride sample with a hexagonal  tungsten  bronze ( htb)-type structure was investigated by the pair distribution function . 
 this study reveals that upon lithiation , the htb framework collapses to yield disordered rutile and rock salt phases followed by a conversion reaction of the fluoride phase toward lithium fluoride and nanometer - sized metallic iron . 
 the occurrence of anionic vacancies in the pristine framework was shown to strongly impact the electrochemical activity , that is , the reversible capacity scales with the content of anionic vacancies . 
 similar to feof - type electrodes , upon de - lithiation , a disordered rutile phase forms , showing that the anionic chemistry dictates the atomic arrangement of the re - oxidized phase . 
 finally , it was shown that the nanoscaling and structural rearrangement induced by the conversion reaction allow the in situ formation of new electrode materials with enhanced electrochemical properties .