Chemical Oxidation Synergistically Regulates Surface Chemistry and Pore Structure of Cotton Stalk-Based Hard Carbon for Enhanced Sodium Storage Performance

Biomass-derived hard carbon (HC) represents a promising anode candidate for sodium-ion batteries, owing to its disordered structure and abundant micropores. This study systematically investigates three chemical oxidation strategies (NaClO, H 2 SO 4 and H 2 O 2 +NaOH) applied to cotton stalk-derived HC carbonized at 1300 °C. The NaClO-treated sample delivers the optimal overall electrochemical performance, achieving a high discharge capacity of 330.2 mAh g −1 at 0.1 C, a high initial Coulombic efficiency (ICE) of 81.7%, and a capacity retention of 81.9% after 1000 cycles at 2 C (from 226.0 to 185.1 mAh g −1 ). This superiority is attributed to the formation of a three-dimensional hierarchical pore network and optimal oxygen functional groups. The H 2 SO 4 treatment yields a discharge capacity of 323.2 h g −1 , an ICE of 75.3%, and a capacity retention of 77.1% after 1000 cycles (from 183.5 to 141.5 mAh g −1 ), benefiting from structural densification. The H 2 O 2 +NaOH treatment delivers a capacity of 269.8 mAh g −1 and an ICE of 74.2%, exhibiting a distinct activation behavior likely due to its thin pore walls and abundant open mesopores. Overall, all treated samples significantly outperformed the pristine HC, which exhibits a discharge capacity of 320.3 mAh g −1 , an ICE of 68.0%, and a retained capacity of 90.4 mAh g −1 after 1000 cycles at 2 C.