Dual-regulation of pore confinement and mouth size for enhanced sodium storage in hard carbon

Hard carbon (HC) remains a leading anode candidate for sodium-ion storage, yet its application is hindered by low initial Coulombic efficiency (ICE) and limited plateau capacity due to uncontrolled defect density and open porosity. Here, we propose a scalable dual-regulation strategy that simultaneously tunes pore mouth size and defect chemistry to enhance sodium storage performance. Using phenol-formaldehyde resin as the carbon precursor and phosphorus pentoxide (P₂O₅) as a bifunctional sacrificial template and dopant source, we synthesize phosphorus-functionalized hard carbon (PF-PHC) featuring a high density of closed pores with well-confined sub-nanometer pore entrances. The in-situ sublimation of P₂O₅ during pyrolysis promotes the formation of closed-pore architectures, while residual phosphorus atoms effectively passivate vacancy-type defects, thereby reducing irreversible Na⁺ adsorption and mitigating excessive solid electrolyte interphase (SEI) formation. As a result, PF-PHC achieves an ICE of 89.3% and a plateau capacity of 289 mAh g⁻¹. In-situ characterizations reveal that regulating pore mouth dimensions decouples Na⁺ and solvent access, enabling highly selective ion transport and stable interfacial chemistry. Sodium-ion hybrid capacitors (SIHCs) assembled based on PF-PHC deliver exceptional rate performance and outstanding long-term cycling stability, retaining 98.2% after 10,000 cycles at 2 A g⁻¹. This study establishes pore mouth engineering as a robust and scalable design principle for advancing next-generation HC-based sodium storage materials.