Iron–carbon composites as electrode materials in lithium batteries

Iron–carbon composites have been prepared by adding Fe2O3 hematite to a petroleum vacuum residue as a carbon precursor. Iron contents and other synthesis parameters were varied to evaluate the evolution of the observed iron phases. XRD patterns and 57Fe Mössbauer spectroscopy was used to monitor the reduction of the initial iron oxide to pyrrhotite (Fe7S8) intermediate, austenite (FexC) and metallic iron as final reaction product when annealing temperature and time were high. Pyrrhotite has demonstrated to be electrochemically active and beneficial for the capacity retention in lithium test cells. Those samples with the higher pyrrhotite contribution have a good capacity retention achieving values close to 350 mAh/g after 30 cycles. AC measurements evidenced that iron species inhibit the increase of both the charge transfer resistance and the surface film resistance upon cycling.