Ion-conductive gradient sodiophilic 3D scaffold induced homogeneous sodium deposition for highly stable sodium metal batteries

Sodium (Na) metal is considered as an appealing anode material for high-energy density Na batteries owing to its low cost, low redox potential and high theoretical capacity. However, several issues triggered by the uncontrolled Na dendrites hinder the practical applications of Na metal anodes, such as the poor cycling reversibility, short lifespan and even the unexpected safety hazard. Herein, the gradient SnO2-modified 3D carbon nanofibers (SnO2-CNFs) scaffold is fabricated, in which, the in-situ formed sodiophilic Na-Sn alloys and Na2O during initial cycle can guide Na homogeneous deposition in a "bottom-up" mode. Moreover, Na2O with excellent ionconductivity can decrease diffusion barrier to promote fast Na+ ion diffusion and uniform deposition into the scaffold. These features have positive effect on not only inhibiting dendrite growth but also improving space utilization of the 3D scaffold. As a result, the designed SnO2-CNFs electrode can maintain a high Coulombic efficiency (CE) of 99.88% after 1500 cycles (3000 h) at 3 mA cm-2 and 3 mAh cm-2. Besides, it can attain a CE of 99.68% after 400 cycles (800 h) when the current density and areal capacity increase to 5 mA cm-2 and 5 mAh cm-2. For the symmetric cell assembled by SnO2-CNFs@Na electrodes, excellent cycling performance for 1500 h at 5 mA cm-2 and 5 mAh cm-2 is displayed. Moreover, the assembled full cell using Na3V2(PO4)3@C@CNTs as cathode and SnO2-CNFs@Na as anode exhibits an outstanding capacity retention of 95.1% after 400 cycles at a large current density of 5 C, evidencing the feasible application of the designed SnO2-CNFs scaffold to realize stable Na metal anodes for advanced Na metal batteries.