Activating graphite with defects and oxygenic functional groups to boost sodium-ion storage

Sodium-ion batteries have been one of the most promising alternatives for lithium-ion batteries (LIBs) for large-scale energy storage systems due to cost-efficiency and rich resources of sodium. However, graphite, a commercial anode material of LIBs, shows a very low reversible capacity for sodium-ion storage because of the weak binding between sodium and graphite. Herein, an activated graphite (AG) material with abundant defects including edges and vacancies with oxygenic functional groups is well-designed and fabricated by a facile and eco-friendly ball-milling method. The structural evolutions during the ball-milling process and their effects on electrochemical sodium-ion storage performance are investigated. A stable reversible capacity of 139.1 mA h g^-1 can be achieved at 1.0 A g^-1 even after 4500 cycles for the AG-50 electrode with the 50-hour ball-milling treatment, amounting to a very low decay ratio of 0.0034% per cycle. Based on physical characterizations and density functional theory calculations, the greatly improved specific capacity and cycling stability of the AG anode for sodium-ion storage can be attributed to the enlarged interlayer space, increased specific surface area, and introduced defects caused by ball-milling treatment, which provide vast active sites for reversible sodium-ion storage based on a adsorption/desorption mechanism, thus leading to great improvement in the specific capacity of the AG electrode. These results can provide a meaningful reference for the application of modified graphite for high-performance sodium storage.