Overcoming the Trade-Off between Initial Coulombic Efficiency and Rate Performance in Hard Carbon Anodes for Sodium-Ion Storage

Hard carbon (HC) has emerged as a promising anode for sodium-ion batteries owing to its low-voltage plateau and cost-effectiveness. However, HC anodes still suffer from a performance trade-off between the initial Coulombic efficiency (ICE) and rate capability. To address this issue, we propose a scalable synthesis method, the melt-spinning technique (kilogram scale) with a hexamethylenetetramine (HMTA) cross-linking-oxidation strategy, to multidimensionally regulate the structure of phenolic resin-derived hard carbon (CPF-1400) as high-performance anodes. Experimental studies demonstrate that the spatially cross-linked precursor with methylene bridge (-CH₂-) and rich carbonyl groups (C═O) effectively suppresses excessive graphitization (even at 1400 °C) and enlarges the spacing of carbon interlayers from 0.367 to 0.381 nm. Additionally, it enables the reduction of the specific surface area to merely 1.4 m² g^-1 and generates abundant and suitable-sized closed pores (0.315 cm³ g^-1, 1.26 nm) for CPF-1400. Therefore, CPF-1400 delivers an exceptional reversible sodium storage capacity of 431 mAh g^-1 with an unprecedentedly high ICE of 95%. Notably, it also retains a rate capability of 308 mAh g^-1 at 1 A g^-1, and it achieves a high energy density of 293 Wh kg^-1 assembled in full cells. Electrochemical analyses combined with in situ characterizations demonstrate a three-stage sodium storage mechanism in hard carbon and elucidate the correlation between the solid-electrolyte interphase (SEI) and battery performance.