High-Capacity Interstitial Mn-Incorporated Mn_xFe_3-xO₄/Graphene Nanocomposite for Sodium-Ion Battery Anodes

Sodium-ion batteries (SIBs) have attracted wide attention because of their prospects for grid-scale electrical regulation and cost effectiveness of sodium. In this regard, iron oxides (FeO_x) are considered as one of the most promising anode candidates due to their high theoretical capacity and low cost. Unfortunately, the utilization of FeO_x anodes suffers from sluggish reaction kinetics and significant lattice variation, causing insufficient rate performance and fast capacity degradation during the sodiation/desodiation process. In this study, Mn ions are incorporated through interstitial sites into a Fe₃O₄ lattice to form the Mn-incorporated Fe₃O₄/graphene (M-Fe₃O₄/G) composites through a facile hydrothermal method. Confirmed by XRD Rietveld refinement and the first-principles calculation, Mn occupation into the body structure can effectively condense the electron density around the Fermi level and thus contributes to the increased electrical conductivity and improved electrochemical properties. Accordingly, the M_0.1Fe_2.9O₄/G composite demonstrates a high reversible capacity of 439.8 mA h g^-1 at a current density of 100 mA g^-1 over 200 cycles. Even at a high current density of 1 A g^-1, the M-Fe₃O₄/G composites remain stable for over 1200 cycles, delivering a capacity of 210 mA h g^-1. Coupled with a Na₃V₂(PO₄)₃-type cathode, the Mn-incorporated Fe₃O₄/G composites demonstrate good suitability in full SIBs (161.2 mA h g^-1 at the current density of 1 A g^-1 after 100 cycles). The regulation of Mn ions in the Fe₃O₄ lattice provides insights into the optimization of metal oxide anode candidates for their application in SIBs.