Polymer‐Immobilized Organic Salt Anions Enable Enhanced Anode Kinetics and Interfacial Stability in Sodium‐Ion Capacitors

ABSTRACT Sodium‐ion capacitors (SICs) are energy storage devices capable of simultaneously achieving high power density and high energy density. However, due to differences in the kinetics of the anode and cathode, conventional liquid electrolytes struggle to maintain cycling stability and are prone to leakage. To address these issues, herein, we designed a gel polymer electrolyte (GPE) by in situ polymerizing the functional organic salt sodium 2‐acrylamido‐2‐methylpropanesulfonate (AMPSNa) with the crosslinking agent Pentaerythritol triacrylate (PETA). This approach achieves dual transport regulation of anode kinetics by anchoring anions to optimize both Na + desolvation kinetics and SEI stability. Upon dissociation, AMPSNa anchors the AMPS − ion to the polymer backbone, weakening Na + solvent interactions. This modifies the Na + coordination environment and facilitates interfacial transport. Concurrently, the reduction of AMPS − generates inorganic solid electrolyte interphase (SEI) components forming a stable, highly ion‐conductive interfacial layer that accelerates Na + transport. The AC||HC SIC assembled using the GPE‐AMPSNa electrolyte achieved a maximum energy density of 171.2 Wh kg −1 , while significantly reducing the difference in the migration numbers of Na + and ClO 4 − . Additionally, it demonstrates outstanding cycling stability (96.1% retention after 10 000 cycles). These findings provide a viable new approach for the preparation of high‐performance SICs.