Modulation of Surface/Interface States in Bi ₂ S ₃ /VS ₄ Heterostructure With CN Layer for High‐Performance Sodium‐Ion Batteries: Enhanced Built‐in Electric Field and Polysulfide Capture

Metal sulfides are promising materials for sodium-ion batteries (SIBs) owing to unique structures and high theoretical capacity. However, issues like poor conductivity, large volume changes, and polysulfide dissolution limit practical application. This study introduces a novel Christmas tree-like heterostructure composed of Bi₂S₃ and VS₄ encapsulated in nitrogen-doped carbon shell (Bi₂S₃/VS₄@CN), synthesized by sulfurizing dopamine-coated BiVO₄ precursor. The in situ synthesis ensures excellent lattice matching between Bi₂S₃ and VS₄, minimizing interface states and enhancing effective built-in electric field. This design accelerates electrochemical reaction kinetics; moreover, it promotes progressive reaction that mitigates structural fragmentation, suppresses degradation, and prevents polysulfide dissolution and shuttle. Additionally, the CN shell effectively passivates the surface states of Bi₂S₃ and VS₄ nanostructures, lowering surface barrier and improving overall conductivity. As a result, Bi₂S₃/VS₄@CN-based half-SIBs demonstrate remarkable long-cycle stability, maintaining 387.1 mAh g^-1 after 1600 cycles at 2 A g^-1, and excellent rate performance with 376.3 mAh g^-1 at 5 A g^-1. Full-SIBs using Na₃V₂(PO₄)₃//Bi₂S₃/VS₄@CN exhibit outstanding cycling stability, retaining 117.2 mAh g^-1 after 200 cycles at 1 A g^-1, along with 218 Wh kg^-1 high energy density at 145.3 W kg^-1. This work highlights the potential of heterostructures in advancing metal sulfide-based SIBs for high-performance energy storage.