Synergistic CO₂ etching and carbonization induces closed-pore structures for plateau-dominant sodium storage

Hard carbon is widely regarded as one of the most promising anode materials for sodium-ion batteries (SIBs), yet achieving high energy density requires a significant enhancement of the low-voltage plateau capacity near ~ 0.1 V (vs. Na⁺/Na). Although closed-pore structures dominate plateau storage, their formation mechanisms remain elusive. We present a synergistic strategy combining CO₂ etching with high-temperature carbonization to systematically elucidate the evolution of closed pores and their influence on sodium storage behavior. CO₂ etching generates open pores that reorganize into closed pores during secondary treatment. Crucially, precursor selection dictates closed-pore density, with N-rich chitosan-derived hard carbon developing denser closed-pore architecture than exclusively O-doped precursors. The optimized hard carbon anode delivers a high reversible capacity of 388.8 mAh·g⁻¹ at 0.05 A·g⁻¹, with excellent cycling stability (83.8% capacity retention after 800 cycles at 0.5 A·g⁻¹). In-situ and ex-situ analyses demonstrate that Na⁺ ions reversibly fill the engineered closed pores, accounting for over 200 mAh·g⁻¹ (approximately 57% of the total reversible capacity) via a plateau-dominated storage. Consequently, full cells assembled with this optimized hard carbon anode achieve an energy density of 165.2 Wh·kg⁻¹. This work offers new mechanistic insights into pore evolution and provides a practical route for tailoring high-performance hard carbon anodes for next-generation SIBs.