Rational design of mangosteen-like bismuth nanospheres coated by N-doped carbon shell as superb composite anode for high-performance sodium-ion batteries

Metallic bismuth (Bi) anode materials are promising candidates for alkali-ion batteries due to its high theoretical gravimetric/volumetric capacity. However, the pronounced volume change from Bi to full sodiation phase Na3Bi, inducing the structural collapse, and ionic conduction interruption upon cycling, hinders their implementation in sodium-ion batteries (SIBs). Herein, a mangosteen-like bismuth nanosphere coated by N-doped carbon shell (Bi@NC) was designed and synthesized to address these limitations. Due to the nanosize of Bi core and in situ, generated N-doped carbon shell, this Bi@NC anode can not only effectively accommodate the volume change during the alloying/dealloying process but also provide high-speed channels for electron/ion transport to the highly active bismuth nanospheres. The Bi@NC electrode exhibits outstanding cycling stability (1000 cycles at 5 A g−1 with an impressive capacity retention of 96.5 %) and rapid sodium storage performance (392.8 mAh g−1 at 20 A g−1). Importantly, ex-situ techniques and kinetic analysis have revealed the distinctively sharp multiphase transitions and the "battery-capacitor dual-mode" sodium-storage mechanism. This research introduces a new approach to enhance the performance of bismuth-based anodes in advanced SIBs and demonstrates potential for practical applications.