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 approximate to 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 mu m 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.