Microstructural Evolution Of Iron Oxyfluoride/Carbon Nanocomposites Upon Electrochemical Cycling

High electrochemical performance iron oxyfluoride conversion electrode undergoes complex electrochemical reaction mechanisms upon cycling. In this work, a combination of selected area electron diffraction (SAED) and scanning transmission electron microscopy/electron energy loss spectroscopy (STEM/EELS) analysis techniques have been used to understand the conversion-reconversion mechanisms of FeO_0.7F_1.3/C upon cycling. Considerable changes have been observed with cycling. For the fully delithiated electrodes, the nanometer scale intermixing of amorphous rutile and nanocrystalline rocksalt phases is stable up to 20 cycles; however, upon further cycling the amount of amorphous rutile phase decreased and amount of rocksalt phase increased gradually, implying incomplete reconversion reactions with increasing cycle number. In addition, a progressive growth of solid electrolyte interphase (SEI) layer was observed with cycling, which is mainly composed of LiF. Interestingly, Fe²⁺ and Fe nanoparticles were found trapped in the SEI layer with increasing cycle number. Upon cycling, the combined progressive increase in Fe²⁺ content and insulating LiF (from SEI and conversion product) give rise to the observed capacity loss