Multi-Scale Architecture Regulation of Hard Carbons for High-Efficiency Sodium Storage Across Ambient and Subzero...

Hard carbons, despite their cost-efficient production and precursor availability, face critical electrochemical performance constraints from excessive defects, limited closed-pore structures, and poor interfacial stability. Herein, a multi-scale structural regulation strategy is proposed to tailor both micro- and nanoscale architectures of polymer-derived hard carbons for efficient sodium storage under both ambient and subzero conditions. The pitch-modulated carbonization directs the self-assembly of polyphosphazene (PZS) precursors into monodisperse microparticles while in situ forming nanoscale short-range-ordered graphitic domains. The resulting hard carbons integrate enhanced bulk conductivity, abundant closed pores, and defect-tailored low-surface-area microparticles, collectively enabling an inorganic-rich solid electrolyte interphase (SEI), fast Na⁺ transport, and suppressed side reactions. The optimized sample delivers a remarkable reversible capacity (413.7 mAh g^-1 at 0.05 A g^-1) with high initial Columbic efficiency (ICE) (87.1%) and excellent rate capability. More notably, it demonstrates high reversible capacity and exceptional cycling stability at -20°C, achieving a remarkable capacity retention of 98.8% after 3000 cycles and highlighting its practical viability under extreme conditions. The sodium storage mechanisms and accelerated kinetics are revealed through various in situ characterizations and computational techniques, providing deep insights into microstructure tailoring of hard carbons for high-performance sodium-ion batteries (SIBs).