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.

Carbonaceous material with favorable K⁺ intercalation feature is considered as a compelling anode for potassium-ion batteries (PIBs). However, the inferior rate performance and cycling stability impede their large-scale application. Here, a facile template method is utilized to synthesize boron doping carbon nanobubbles (BCNBs). The incorporation of boron into the carbon structure introduces abundant defective sites and improves conductivity, facilitating both the intercalation-controlled and capacitive-controlled capacities. Moreover, theoretical calculation proves that boron doping can effectively improve the conductivity and facilitate electrochemical reversibility in PIBs. Correspondingly, the designed BCNBs anode delivers a high specific capacity (464 mAh g⁻¹ at 0.05 A g⁻¹) with an extraordinary rate performance (85.7 mAh g⁻¹ at 50 A g⁻¹), and retains a considerable capacity retention (95.2% relative to the 100th charge after 2000 cycles). Besides, the strategy of pre-forming stable artificial inorganic solid electrolyte interface effectively realizes high initial coulombic efficiency of 79.0% for BCNBs. Impressively, a dual-carbon potassium-ion capacitor coupling BCNBs anode displays a high energy density (177.8 Wh kg⁻¹). This work not only shows great potential for utilizing heteroatom-doping strategy to boost the potassium ion storage but also paves the way for designing high-energy/power storage devices.