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 nanofiber 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 nanofiber. 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.