An Air-Etching Strategy for Enlarging Plateau Capacity of Hard Carbon Anodes

Hard carbon (HC) anodes are regarded as the most promising anodes for sodium-ion batteries (SIBs), yet their energy density is severely limited by low-voltage plateau capacity, which remains difficult to enhance without sacrificing Initial Coulombic efficiency (ICE) or structural integrity. Here, we report a convenient air-etching strategy for the scalable synthesis of HC via a precursor-independent pore evolution pathway. By introducing a mild air oxidation process within a narrow kinetic window, the oxidation of pre-carbonized precursors preferentially occurs at edge and defect sites and suppresses over-etching, generating closable ultra-micropores (∼0.8 nm) without framework collapse. Upon graphitization, the ultra-micropores evolve into uniform distributed closed nanopores (∼2.8 nm) while maintaining a low specific surface area. The resulting HC exhibits a plateau-dominated sodium storage behavior and delivers a high ICE of 91.8% and a high reversible capacity of 460 mAh g −1 with a remarkably enlarged plateau capacity of 346.4 mAh g −1 , superior to most of HC reported previously. More importantly, the air-etching approach is applicable to diverse precursors, offering a low-cost and powerful route for achieving high-energy-density HC anodes and SIBs.