Targeted Etching Strategy to Expand Low-Voltage Plateau of Pitch-Based Hard Carbon for Sodium Storage

To address the lack of voltage plateau, low plateau capacity, and poor cycling stability in pitch-derived carbon anodes, a hard carbon material was synthesized via CO₂-assisted etching combined with high-temperature carbonization of preoxidized pitch. Preoxidation introduces C-O/C═O groups into pitch, suppressing its pyrolysis melting and enabling uniform nanopore formation. Subsequent CO₂ etching at 800 °C selectively expands micropores while reducing oxygen content (from 14.94% to 8.05%), and final carbonization further converts open pores into closed pores. These integrated strategies endow the material with outstanding sodium storage performance, achieving an initial Coulombic efficiency of 90%, a reversible capacity of 347.6 mAh g^-1 (including a plateau capacity of 249.4 mAh g^-1), and 87.3% capacity retention after 200 cycles at 100 mA g^-1. The enhanced stability originates from the formation of a thin solid electrolyte interphase with organic-inorganic gradient structure and a triphasic mechanism involving adsorption (>0.10 V), intercalation (0.01-0.10 V), and pore filling (<0.01 V), highlighting the critical role of closed-pore engineering of pitch-derived hard carbon for high-performance sodium-ion batteries (SIBs).