Carboxyl‐Dominant Oxygen Rich Carbon for Improved Sodium Ion Storage: Synergistic Enhancement of Adsorption and Intercalation Mechanisms

Oxygen‐containing groups in carbon materials have been shown to affect the carbon anode performance of sodium ion batteries; however, precise identification of the correlation between specific oxygen specie and Na + storage behavior still remains challenging as various oxygen groups coexist in the carbon framework. Herein, a postengineering method via a mechanochemistry process is developed to achieve accurate doping of (20.12 at%) carboxyl groups in a carbon framework. The constructed carbon anode delivers all‐round improvements in Na + storage properties in terms of a large reversible capacity (382 mAg −1 at 30 mA g −1 ), an excellent rate capability (153 mAg −1 at 2 A g −1 ) as well as good cycling stability (141 mAg −1 after 2000 cycles at 1.5 A g −1 ). Control experiments, kinetic analysis, density functional theory calculations, and operando measurements collectively demonstrate that carboxyl groups not only act as active sites for Na + capacitive adsorption through suitable electrostatic interactions, but also gradually expand d ‐spacing by inducing a repulsive force between carbon layers with Na + preadsorbed, and hence facilitate diffusion‐controlled Na + insertion process. This work provides a new insight in the rational tunning of oxygen‐containing groups in carbon for boosting reversible Na + storage through a synergy of adsorption and intercalation processes.