Scalable synthesis of electrode materials with long cyclic life, high energy and power densities is a prerequisite for next generation Li ion batteries. Freestanding composite films are prepared by one-pot electrospinning, in which ultrafine Fe3O4 nanoparticles are uniformly dispersed in a continuous carbon nanofiber (CNF) matrix. The Fe3O4/CNF electrodes deliver remarkable electrochemical performance, e.g. a reversible capacity of 881mAhg-1 at 0.2Ag-1, excellent cyclic stability of 687 mAhg-1 after 350 Cycles at 0.5Ag-1 and a high-rate capability of 422mAhg-1 after1000cyclesat5.0Ag-1 with 84% capacity retention. These values are among the highest ever reported for various nanostructured iron oxide-based electrodes. Even after prolonged cycles, the CNF matrix containing ultrafine nanocrystals remains structurally sound without damage. In contrast, the Fe3O4/CNF-750 electrode containing larger Fe3O4 particles obtained at a higher carbonization temperature of 750 °C presents faster capacity decay and cracking of CNF matrix due to larger volume expansion. The in-situ TEM analysis is used to provide an insight into real-time structural evolution and conversion reactions. It is revealed that (i) upon initial lithiation, the Fe3O4 nanoparticles embedded in the CNF are gradually reduced to Fe nano grains along the Li ion diffusion direction; and(ii)after delithiation, a new oxide at ion product, FeO, emerges, instead of Fe3O4. Their reversible phase conversion from Fe3O4 to Fe is the first of its kind reported for Fe3O4 electrodes although ha similar phenomenon has been proposed for other electrode materials, like Fe2O3and Co3O4.

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