Oxygen Functional Group Modification of Cellulose-Derived Hard Carbon for Enhanced Sodium Ion Storage

Oxygen-containing groups in carbon materials have been demonstrated to be effective in the anodic sodium-ion storage process; however, the effect of specific oxygen-containing groups on the sodium-ion storage in the carbon framework remains to be explored. Based on a mechanochemical process (exemplified by ball milling in the presence of dry ice), a selectively modified cellulose-derived hard carbon (BHC-CO2) with a high oxygen content of 19.33 at. % and carboxyl-dominant groups was prepared in this work. The fabricated BHC-CO2 anode exhibits excellent electrochemical performance with a high reversible capacity of 293.5 mA h g–1 at a current density of 0.05 A g–1, two times as high as that of the oxygen-deficient BHC-CO2-H2 anode, demonstrating the significant role of oxygen-containing groups in enhancing the Na+ storage. Moreover, the BHC-CO2 anode has an excellent high-rate cycling stability with a specific capacity of 80.0 mA h g–1 even after 2000 cycles at 1 A g–1. Qualitative analyses of capacitive effect combined with density functional theory calculations further reveal that carboxyl groups introduced by the mechanochemical process facilitate Na+ adsorption on the carbon surface, enhancing the capacitive Na+ storage process and thus greatly improving the capacity. This work demonstrates the role of carboxyl on Na+ storage by carbonaceous materials and provides theoretical guidance for the oxygen functional group modification of carbon materials to enhance the sodium-ion storage.