Self-Template Construction of Hierarchical Bi@C Microspheres as Competitive Wide Temperature-Operating Anodes for Superior Sodium-Ion Batteries

Huge volume changes of bismuth (Bi) anode leading to rapid capacity hindered its practical application in sodium-ion batteries (SIBs). Herein, porous Bi@C (P-Bi@C) microspheres consisting of self-assembled Bi nanosheets and carbon shells were constructed via a hydrothermal method combined with a carbothermic reduction. The optimized P-Bi@C-700 (annealed at 700 °C) demonstrates 359.8 mAh g^-1 after 1500 cycles at 1 A g^-1. <i>In situ/ex situ</i> characterization and density functional theory calculations verified that this P-Bi@C-700 relieves the volume expansion, facilitates Na⁺/electron transport, and possesses an alloying-type storage mechanism. Notably, P-Bi@C-700 also achieved 360.8 and 370.3 mAh g^-1 at 0.05 A g^-1 under 0 and 60 °C conditions, respectively. Na₃V₂(PO₄)₃//P-Bi@C-700 exhibits a capacity of 359.7 mAh g^-1 after 260 cycles at 1 A g^-1. These hierarchical microspheres effectively moderate the volume fluctuation, preserving structural reversibility, thereby achieving superior Na⁺ storage performance. This self-template strategy provides insight into designing high-volumetric capacity alloy-based anodes for SIBs.