CO₂-Etching Creates Abundant Closed Pores in Hard Carbon for High-Plateau-Capacity Sodium Storage

Hard carbon (HC) has become the most promising anode material for sodium-ion batteries (SIBs), but its plateau capacity at ≈0.1 V (Na⁺/Na) is still much lower than that of graphite (372 mAh g⁻¹) in lithium-ion batteries (LIBs). Herein, a CO₂-etching strategy is applied to generate abundant closed pores in starch-derived hard carbon that effectively enhances Na⁺ plateau storage. During CO₂ etching, open pores are first formed on the carbon matrix, which are in situ reorganized to closed pores through high-temperature carbonization. This CO₂-assisted pore-regulation strategy increases the diameter and the capacity of closed pores in HC, and simultaneously maintains the microsphere morphology (10–30 µm in diameter). The optimal HC anode exhibits a Na-storage capacity of 487.6 mAh g⁻¹ with a high initial Coulomb efficiency of 90.56%. A record-high plateau capacity of 351 mAh g⁻¹ is achieved, owing to the abundant closed micropores generated by CO₂-etching. Comprehensive in situ and ex situ tests unravel that the high Na⁺ storage performance originates from the pore-filling mechanism in the closed micropores.