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

Abstract

MnFe₂O₄ nanodots (∼3.3 nm) homogeneously dispersed in porous nitrogen-doped carbon nanofibers (denoted as MFO@C) were prepared by a feasible electrospinning 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 + 4Na₂O) and then to MnO and Fe₂O₃ during the following charge (Mn + 2Fe + 4Na₂O → MnO + Fe₂O₃ + 8Na⁺ + 8e^–). The subsequent cycles occur through reversible redox reactions of MnO + Fe₂O₃ + 8Na⁺ + 8e^– ↔ Mn + 2Fe + 4Na₂O, of which the reduction/oxidation of MnO/Mn takes place at a lower potential than that of Fe₂O₃/Fe. Furthermore, a soft package sodium-ion full battery with MFO@C anode and Na₃V₂(PO₄)₂F₃/C cathode was assembled, delivering a stable capacity of ∼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.