Optimizing solvated anion accessibility and pseudocapacitive charge storage in carbon cathode toward high-energy sodium-ion capacitor

Carbon based sodium-ion capacitors (SICs) are becoming promising energy storage devices owing to the high energy/power densities and the advantages in price and environmental friendliness. However, the lack of methods to precisely tune the intrinsic texture of carbon cathode and understand its capacitive behaviors have limited the development of high-energy SICs. Herein, a novel carbon framework derived from magnolol-based epoxy resin that shows ultrahigh surface area and uniform distributed pore is developed for cathode of SICs. Significantly, it is proved that the diameter of solvated ClO4− ion is 1.48 nm in multicomponent organic electrolytes; and the maximum adsorptive capacitance is achieved by optimizing solvated ClO4− ion accessibility. Furthermore, the surface-induced pseudocapacitance between C = O/pyridinic-N groups and ClO4− ions is revealed. On this basis, the well-constructed SICs deliver an ultrahigh energy density of 195.4 Wh kg−1 and an ultra-stable cyclability of 91.2% capacitance retention after 16,000 cycles. The rational design principles of advanced carbon materials and the in‑depth investigation based on the understanding of capacitive behavior in NaClO4 electrolyte would push the development of high-energy and ultra-stable SIC systems.