Designing Zwitterionic Gel Polymer Electrolytes with Dual‐Ion Solvation Regulation Enabling Stable Sodium Ion Capacitor

Abstract Sodium ion capacitors (SICs) show high energy/power densities owing to the special dual‐ion energy storage mechanism with cation intercalation and anion adsorption. However, the strong ion‐solvent interactions make it difficult for interfacial ion desolvation, which not only limits the ion transport kinetics, but also results in the solvent co‐intercalation into electrode materials. Here, an advanced zwitterionic gel polymer electrolyte (GPE) is developed to weaken the ion‐solvent interactions. The 3‐(1‐vinyl‐3‐imidazolio) propanesulfonate (VIPS) zwitterions help to lower the desolvation barriers, enabling fast ion transfer kinetics for constructing stable quasi‐solid‐state SICs. Furthermore, the decomposition of VIPS contributes to the formation of S‐ and N‐based inorganic interphase on the surface of hard carbon anode, which reduces the Na + ion diffusion barriers and improves electrochemical compatibility. The designed Zwitterionic GPE can stabilize 4.0 V hard carbon//activated carbon SICs with 95.3% capacity retention after 9000 cycles, showing a high energy density of 140.2 Wh kg −1 . This study highlights the regulation of ion‐solvent chemistry and provides a guiding principle in electrolyte design for advanced hybrid ion capacitors.