Structure and Defect Engineering of V₃S_4−xSe_x Quantum Dots Confined in a Nitrogen‐Doped Carbon Framework for High‐Performance Sodium‐Ion Storage

Abstract Constructing quantum dot‐scale metal sulfides with defects and strongly coupled with carbon is significant for advanced sodium‐ion batteries (SIBs). Herein, Se substituted V₃S₄ quantum dots with anionic defects confined in nitrogen‐doped carbon matrix (V₃S_4−xSe_x/NC) are fabricated. Introducing element Se into V₃S₄ crystal expands the interlayer distance of V₃S₄, and triggers anionic defects, which can facilitate Na⁺ diffusions and act as active sites for Na⁺ storage. Meanwhile, the quantum dots tightly encapsulated by conductive carbon framework improve the stability and conductivity of the electrode. Theoretical calculations also unveil that the presence of Se enhances the conductivity and Na⁺ adsorption ability of V₃S_4−xSe_x. These properties contribute to the V₃S_4−xSe_x/NC with high specific capacity of 447 mAh g⁻¹ at 0.2 A g⁻¹, and prominent rate and cyclic performance with 504 mAh g⁻¹ after 1000 cycles at 10 A g⁻¹. The sodium‐ion hybrid capacitors (SIHCs) with V₃S_4−xSe_x/NC anode and activated carbon cathode can achieve high energy/power density (maximum 144 Wh kg⁻¹/5960 W kg⁻¹), capacity retention ratio of 71% after 4000 cycles at 2 A g⁻¹. This work not only synthesizes V₃S_4−xSe_x/NC, but also provides a promising opportunity for designing quantum dots and utilizing defects to improve the electrochemical properties.