Abnormal preferential oxygen functionalization on the surface of soft/hard carbon for sodium storage

Soft carbon (SC) and hard carbon (HC) have been widely used in the field of energy storage, and oxygen functional groups (OFGs) plays important roles on the electrochemical performances. Herein, we study the oxygen functionalization behaviors on the surface of classical SC and HC, by post-treatment of liquid phase oxidation. Surprisingly, CO contained OFGs were formed preferentially on the SC while CO contained OFGs on the HC, evidenced by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron (XPS) and absorption near-edge spectroscopies (XANES). The abundant sodium-ion storage sites contribute a capacity of 322 mAh/g at current density of 0.05 A/g after cycles for oxygen functionalized SC (OSC), higher than that of 256 mAh/g for oxygen functionalized HC (OHC). In addition, the mesoporous structure of SC and OSC also provides a good transport channel for sodium ions. The different oxygen functionalization behaviors on SC and HC in this work, provide some fundamental and universal guidance for surface modification of carbon materials.