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

Iron compounds have been extensively used as anode materials of lithium-ion batteries because of their low-cost and excellent electrochemical performance. However, iron compounds have a poor cyclic stability as a result of their drastic volume change in the charge/discharge processes. Herein, we propose an effective strategy to improve the cyclic performance of iron compounds by carbon coating and mixing with manganese oxide. To implement this, flexible carbon fibers embedded with Fe/Mn-based nanoparticles are produced by electrospinning with subsequent annealing process. By adjusting the annealing temperatures, the phase transformation of Fe/Mn-based compounds within carbon fibers is investigated in details. As the annealing temperature increases, Fe/Mn precursors (iron acetylacetonate and manganese acetylacetonate) in the fibers decompose gradually and then Fe3O4 and MnO nanoparticles form successively. When the temperature rises to 800 °C, the formed Fe3O4 is gradually reduced by carbon to produce Fe3C while MnO remains unchanged. The carbon fibers loaded with different Fe/Mn-based compounds (Fe3O4, Fe3O4/MnO, Fe3O4/Fe3C/MnO and Fe3C/MnO) are obtained under different annealing temperatures. Among these composites, carbon fibers loaded with Fe3O4/Fe3C/MnO nanoparticles exhibit the best specific capacity (750.23 mAh g−1 after 250 cycles) and cyclic stability, ascribing to the synergistic effect of iron and manganese compounds. Furthermore, it is found that charge transfer impedance of the anodes is reduced and Li+ diffusion ability is improved as the annealing temperature increases, which is ascribed to the increasing content of Fe3C and the short Li+ diffusion distance resulted from the reduced diameter of carbon fibers. The excellent electrochemical performance indicates that the hybrid electrospun nanofibers have a great potential application for flexible lithium-ion batteries.