Sulfur‐Enhanced n‐π Conjugation Unlocks High‐Voltage and 75C Rate in Sodium Dual‐Ion Batteries

ABSTRACT Sodium dual‐ion batteries (SDIBs) based on organic cathodes hold immense promise for sustainable energy storage. However, their practical deployment is severely constrained by the trade‐off between high operating voltage and rapid charge kinetics, particularly under high‐rate conditions where conventional nitrogen‐rich cathodes suffer from charge localization and structural polarization. Herein, a rational molecular design strategy leveraging sulfur‐enhanced n‐π conjugation to simultaneously boost redox potential and accelerate anion storage kinetics is reported. By strategically incorporating sulfur atoms into the phenothiazine backbone and coupling them with phenazine moieties, a conjugated polymer, poly[10‐methyl‐3‐(phenazin‐5(10H)‐yl)‐10H‐phenothiazine] (P(PhTz)) is constructed. Theoretical calculations reveal that the lone‐pair electrons of sulfur engage in effective n‐π conjugation, which not only extends the π‐delocalization range but also lowers the highest occupied molecular orbital energy level, thereby stabilizing the radical cation intermediates and elevating the oxidation potential. In half cells, P(PhTz) delivers an average discharge potential of 3.34 V (vs. Na + /Na) and a capacity of 185 mAh g −1 , couple with a 75C rate capability that outperforms most organic cathodes. When paired with a hard carbon anode, the full cell achieves a high energy density of 536 Wh kg −1 cathode with stable cycling over 500 cycles, while simultaneously maintaining ultra‐high rate capability up to 75C.