Outside-in directional sodium deposition through self-supporting gradient fluorinated magnesium alloy framework toward high-rate anode-free Na batteries

Constructing high-rate sodium anodes promotes the progress of high energy/power density Na metal batteries. However, it lacks effective strategies to regulate Na deposition behaviors under high current density and high capacity, greatly compromising their energy density, cycling life, and safety. Herein, we construct a selfsupporting framework host consisting of nitrogen-doped carbon hollow nanofibers with uniformly embedded MgF2 (MgF2@NCHNFs), facilitating outside-in directional Na deposition. During the first Na plating, the MgF2 in the tube wall of NCHNFs is in-situ converted to gradient fluorinated alloy architecture, where the outmost NaF homogenizes Na+ flux, the subsurface sodiophilic N sites and gradient distribution Mg sites facilitate Na deposition into the internal space of hollow nanofibers. Thus, the MgF2@NCHNFs exhibit dendrite-free and directional Na deposition behaviors even at high rate (10 mA cm-2 ) and high capacity (10 mAh cm-2 ). The superiorities of the Na-MgF2@NCHNFs are demonstrated in high-rate anode-less/anode-free sodium metal batteries using sodium vanadium phosphate and sulfur as cathodes. Furthermore, the pouch cells deliver a super- high capacity retention of 96.0 % over 400 cycles at 2 C. This work provides a new strategy for practical applications of multifunctional Na hosts in sodium metal batteries, and can extend to other metal batteries.