A Functionalized Carbon Surface for High-Performance Sodium-Ion Storage

Sodium-ion batteries (SIBs) are promising for large-scale energy storage systems and carbon materials are the most likely candidates for their electrodes. The existence of defects in carbon materials is crucial for increasing the sodium storage ability. However, both the reversible capacity and efficiency need to be further improved. Functionalization is a direct and feasible approach to address this issue. Based on the structural changes in carbon materials produced by surface functionalization, three basic categories are defined: heteroatom doping, grafting of functional groups, and the shielding of defects. Heteroatom doping can improve the electrochemical reactivity, and the grafting of functional groups can promote both the diffusion-controlled bulk process and surface-confined capacitive process. The shielding of defects can further increase the efficiency and cyclic stability without sacrificing reversible capacity. In this Review, recent progresses in the ways to produce surface functionalization are presented and the related impact on the physical and chemical properties of carbon materials is discussed. Moreover, the critical issues, challenges, and possibilities for future research are summarized.