MnFe2O4@C Nanofibers as High-Performance Anode for Sodium-Ion Batteries

MnFe2O4 nanodots (similar to 3.3 nm) homogeneously dispersed in porous nitrogen-doped carbon nanofibers (denoted as MFO@C) were prepared by a feasible electro-spinning technique. Meanwhile, MFO@C with the character of flexible free-standing membrane was directly used as binder and current collector-free anode for sodium-ion batteries, exhibiting high electrochemical performance with high-rate capability (305 mA h g(-1) at 10000 mA g(-1) in comparison of 504 mA h g(-1) at 100 mA g(-1)) and ultralong cycling life (ca. 90% capacity retention after 4200 cycles). The Na-storage mechanism was systematically studied, revealing that MnFe2O4 is converted into metallic Mn and Fe after the first discharge (MnFe2O4 + 8Na(+) + 8e(-) -> Mn + 2Fe + 4Na(2)O) and then to MnO and Fe2O3 during the following charge (Mn + 2Fe + 4Na(2)O -> MnO + Fe2O3 + 8Na(+) + 8e(-)). The subsequent cycles occur through reversible redox reactions of MnO + Fe2O3 + 8Na(+) + 8e(-) <-> Mn + 2Fe + 4Na(2)O, of which the reduction/oxidation of MnO/Mn takes place at a lower potential thyan that of Fe2O3/Fe. Furthermore, a soft package sodium-ion full battery with MFO@C anode and Na3V2(PO4)(2)F-3/C cathode was assembled, delivering a stable capacity of similar to 400 mA h g(-1) for MFO@C (with 100 cycles at 500 mA g(-1)) and a promising energy density of 77.8 Wh kg(-1) for the whole battery. This is owing to the distinctive structure of very-fine MnFe2O4 nanodots embedded in porous N-doped carbon nanofibers, which effectively improves the utilization rate of active materials, facilitates the transportation of electrons and Na+ ions, and prevents the particle pulverization/agglomeration upon prolonged cycling.