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.