Structural engineering of hard carbon through spark plasma sintering for enhanced sodium‐ion storage

Abstract Hard carbon (HC) has emerged as one of the superior anode materials for sodium‐ion batteries (SIBs), with its electrochemical performance significantly influenced by the presence of oxygen functional groups and its closed pore structure. However, current research on the structural adjustment of these oxygen functional groups and the closed pore architecture within HC remains limited. Herein, energy‐efficient and contamination‐free spark plasma sintering technology was employed to tune the structure of coconut‐shell HC, resulting in significant adjustments to the content of carboxyl (decreasing from 5.71 at% to 2.12 at%) and hydroxyl groups (decreasing from 7.73 at% to 6.26 at%). Crucially, these modifications reduced the irreversible reaction of oxygen functional groups with Na + . Simultaneously, a substantial number of closed pores with an average diameter of 1.22 nm were generated within the HC, offering an ideal environment for efficient Na + accommodation. These structural changes resulted in a remarkable improvement in the electrochemical performance of the modified HC. The reversible specific capacity of the modified HC surged from 73.89 mAh·g −1 to an impressive 251.97 mAh·g −1 at a current density of 50 mA·g −1 . Even at 400 mA·g −1 , the reversible specific capacity increased significantly from 14.55 to 85.44 mAh·g −1 . Hence, this study provides a novel perspective for designing tailored HC materials with the potential to develop high‐performance SIBs.