Enhancing the Initial Coulombic Efficiency of Sodium‐Ion Batteries via Highly Active Na₂S as Presodiation Additive

Abstract Recently, sodium‐ion batteries (SIBs) have received considerable attention for large‐scale energy storage applications. However, the low initial Coulombic efficiency of traditional SIBs severely impedes their further development. Here, a highly active Na₂S‐based composite is employed as a self‐sacrificial additive for sodium compensation in SIBs. The in situ synthesized Na₂S is wrapped in a carbon matrix with nanoscale particle size and good electrical conductivity, which helps it to achieve a significantly enhanced electrochemical activity as compare to commercial Na₂S. As a highly efficient presodiation additive, the proposed Na₂S/C composite can reach an initial charge capacity of 407 mAh g⁻¹. When 10 wt.% Na₂S/C additive is dispersed in the Na₃V₂(PO₄)₃ cathode, and combined with a hard carbon anode, the full cell achieves 24.3% higher first discharge capacity, which corresponds to a 18.3% increase in the energy density from 117.2 to 138.6 Wh kg⁻¹. Meanwhile, it is found that the Na₂S additive does not generate additional gas during the initial charging process, and under an appropriate content, its reaction product has no adverse impact on the cycling stability and rate performance of SIBs. Overall, this work establishes Na₂S as a highly effective additive for the construction of advanced high‐energy‐density SIBs.