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

Abstract 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.