Flexible electrospun iron compounds/carbon fibers: Phase transformation and electrochemical properties

Flexible electrodes are currently widely studied for applications in energy storage, due to their elasticity and lightweight. Here we describe the fabrication of flexible electrodes obtained by embedding different iron-compound nanoparticles in carbon fibers. The composites are prepared by electrospinning followed by subsequent thermal annealing. We elucidate the phase evolution process of iron compounds in the carbon fibers, i.e., Fe3O4, Fe3O4/Fe3C and Fe3C being produced in the carbon fibers at 450 °C, 620 °C and 800 °C, respectively. In addition, we investigate the electrochemical performance of Fe3O4/carbon fibers (Fe3O4/CF), Fe3O4/Fe3C/carbon fibers (Fe3O4/Fe3C/CF) and Fe3C/carbon fibers (Fe3C/CF). When used as anode materials for lithium-ion batteries (LIBs), Fe3O4/CF, Fe3O4/Fe3C/CF and Fe3C/CF electrodes exhibit discharge capacities of 255.3, 478.1 and 169.2 mAh g−1 after 650 cycles at 1 A g−1, corresponding to 50.2%, 99.3% and 39.3% of the capacities after the second cycle, respectively. Compared with Fe3O4/CF and Fe3C/CF electrodes, the Fe3O4/Fe3C/CF electrode delivers higher capacity and better cycling stability. The excellent electrochemical performance of Fe3O4/Fe3C/CF is attributed to the high theoretical specific capacity of Fe3O4 and the excellent catalytic activity of Fe3C. Amorphous carbon fibers can buffer large volume changes during the lithium-ion insertion/extraction process. Finally, we evaluate the lithium storage properties of Fe3O4/CF and Fe3C/CF. Our work provides insights on the phase evolution and lithium storage mechanisms for iron-compound nanoparticles embedded in carbon fibers.