Bismuth (Bi) has attracted significant attention as a highly promising anode material for sodium-ion batteries (SIBs) owing to its high theoretical specific capacity and safe sodium storage potential. However, its practical application is severely hindered by poor intrinsic electrical conductivity, sluggish Na⁺ insertion/extraction reaction kinetics, and severe volume expansion during the charge/discharge process. To address these issues, this work proposes a nitrogen/sulfur co-doped carbon coating strategy to effectively enhance the high-rate sodium storage performance of Bi-based anodes. Specifically, a series of bismuth/nitrogen-sulfur co-doped carbon composites (NS-C@Bi-X, where X represents the carbonization temperature) was synthesized through a facile liquid-phase coprecipitation combined with a high-temperature carbonization strategy. In a typical procedure, ammonium bismuth citrate, urea and trithiocyanuric acid were dissolved in anhydrous ethanol under continuous stirring for 12 h. The obtained precursors were subsequently carbonized at various temperatures (700, 800, and 900 ℃) for 3 h under an Ar atmosphere with a heating rate of 5 ℃•min^-1. Structural and morphological characterizations, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), confirm that the Bi nanoparticles are uniformly encapsulated within the N, S co-doped carbon matrix. In this material system, the N, S co-doped carbon layer not only improves the electrical conductivity of the material but also provides abundant active sites for sodium storage, while the interconnected open carbon coating network effectively buffers the volume expansion of Bi nanoparticles. The electrochemical performance was evaluated in coin cells using 1.0 mol•L^-1 NaPF₆ in diglyme as the electrolyte, operating within a voltage window of 0.01~1.8 V. Notably, NS-C@Bi-800 exhibits outstanding sodium storage rate capability and excellent cycling stability, delivering a high specific capacity of 257 mAh•g^-1 even at an ultrahigh current density of 20 A•g^-1. After 500 cycles at 1 A•g^-1, it can still retain a specific capacity of 304 mAh•g^-1 with a high capacity retention of 87%. Furthermore, by analyzing the relationship between its compositional structure and sodium storage kinetics, the working mechanism underlying its high-rate response and tolerance to large-current charge/discharge is elucidated.

Key words: sodium-ion battery, Bi-based anode, N/S co-doping, carbon coating, high-rate performance