Unlocking the Ultrafast Deposition Kinetics within Bi‐Tailored Core‐Shell Structured Carbon Nanofibers for Highly Efficient and Ultrastable Sodium Metal Batteries

Sodium metal anodes (SMA), featuring high energy content, low electrochemical potential and easy availability, are a compelling option for sustainable energy storage. However, notorious sodium dendrite and unstable solid‐electrolyte interface (SEI) have largely retarded their widespread implantation. Herein, porous amorphous carbon fiber embedded with Bi nanoparticles in nanopores (Bi@NC) was rationally designed as a 3D host for SMA. In‐situ and ex‐situ characterizations, along with theoretical simulations unlock that the in‐situ formed Na‐Bi alloy significantly accelerates sodium metal nucleation and sodium ion diffusion kinetics, enabling uniform sodium plating within the void spaces and a stable SEI outside the carbon fiber. Particularly, the Bi@NC electrode achieved a high Coulombic efficiency of 99.99% at 3 mA cm‐2 and 3 mAh cm‐2 in half‐cell tests, a cycle life of 1000 hours at 5 mA cm‐2 and 10 mAh cm‐2, and sustained performance over 600 cycles under harsh conditions under 30 mA cm‐2 and 3 mAh cm‐2 within symmetrical cells. The full battery assembled with a Na3V2(PO4)3@C cathode and Bi@NC anode delivered long‐term cyclability over 800 cycles, demonstrating its potential for flexible application of sodium‐based energy storage systems. This work highlights the Bi@NC electrode as a promising candidate for high‐performance and flexible sodium metal batteries.