Local Oxygen Reconstruction Enables Dual‐Ion Active Sites in Carbon Cathode for High Energy Density Sodium‐Ion Capacitors

Abstract Carbon materials are the promising cathode material for sodium‐ion capacitors (SICs) with high energy/power density, however, clarifying the evolution processes of functional groups in carbon materials and revealing their energy storage mechanisms are full of challenges. Inspired by the ancient practice of alchemy, which sought to purify Dan medicine and remove impurities through precise control of the refining temperature, the local oxygen reconstruction strategy, to alter the functional groups species in SP 3 ‐C, is pioneeringly utilized, achieving targeted regulation of carbonyl groups with increase from 27.9 to 43.3 at%, which efficiently change the electronic structure of the carbon framework and realize the dual‐ion adsorption of Na + and ClO 4 − , according well with the theoretical calculations. As expected, the obtained carbon cathode delivers a specific capacity of 145 mAh g −1 , higher than that of the parent carbon material (95 mAh g −1 ). Impressively, the ex situ X‐ray Photoelectron Spectroscopy and in situ Raman reveals that carbonyl can act as dual‐ion active sites for Na + and ClO 4 − through pseudocapacitive behavior under different voltage states. Notably, the assembled SIC using the carbonyl‐rich carbon cathode exhibits an ultrahigh energy density of 162 Wh kg −1 . This work opens a novel avenue for regulating the carbonyl content of carbon materials.