A Molecular-Sieving Pitch Shell Decouples Surface Passivation from Sodium Storage in Hard-Carbon Anodes for Sodium-Ion Batteries

Pitch carbon coating is a practical route to reconcile the capacity–kinetics trade-off of biomass-derived hard-carbon anodes for sodium-ion batteries, yet the role of the coating sequence has not been isolated. Here, an identical, industrially realistic 2 wt % pitch loading was applied to a lychee-wood hard carbon either before carbonization (co-carbonization, HC-PB) or after it (thin-shell carbonization at 800 °C, HC-PA25), and both were benchmarked head-to-head against the pitch-free baseline. Neither route alters the turbostratic bulk, the 324–328 mAh g–1 first-sodiation capacity, or its plateau/slope partition. Dual-probe (N2/CO2) adsorption shows that the two sequences engineer the pores oppositely. The post-coated shell seals the open surface porosity (N2-accessible area 115 → 3.6 m2 g–1) while retaining the internal ultramicropores, including the whole sub-0.45 nm population (0.068 vs 0.064 cm3 g–1), a molecular-sieving gate, whereas co-carbonization erases that population entirely (its pore-size distribution starts at 0.48 nm) and kinetically gates the rest (40% lower CO2 uptake, pronounced trapping hysteresis). The sieving shell raises the initial Coulombic efficiency to 91.3%, stabilizes an organic-rich solid electrolyte interphase (SEI), and delivers the best long-term cycling stability (237.7 mAh g–1 after 1000 cycles at 5C/2C) while maintaining a high rate capability (259.0 mAh g–1 at 10C desodiation, 88.3% retention). Although co-carbonization gives HC-PB the highest apparent Na+ diffusivity, the decisive advantage of the pitch-after route lies at the interface, and in Na4Fe3(PO4)2P2O7 full cells it translates into 89.6% capacity retention after 200 cycles vs 81.2% for the baseline and only 65.1% for the co-carbonized HC-PB.