Controlled formation of closed-pore structures in modified phenolic resin-derived carbons for enhanced sodium storage

Hard carbon anodes face the critical challenge of low initial coulombic efficiency (ICE) and capacity fading arising from insufficient control of the closed-pore structure, which must be addressed to advance sodium-ion batteries (SIBs). In this work, we present a carbon structure engineering strategy to enhance anode performance by treating cyano-functionalized phenolic resin-derived carbon microspheres with NH₃. Cyano side groups increase crosslinking density during resin curing, facilitating the formation of closed-pore structures with low specific surface areas. Stepwise carbonization in ammonia and nitrogen atmospheres tailors the closed-pore structure and surface morphology. Small-angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HR-TEM) reveal that the closed-pore walls undergo mild etching, inducing pore surface defects. Compared with nitrogen treatment, NH₃ significantly enhances nitrogen incorporation and electrochemical activity. The optimized sample carbonized at 1200 °C exhibits a specific surface area of 1.87 m² g^-1, an ICE of 81.81%, and a first-cycle discharge capacity of 373.3 mAh g^-1 at a current density of 30 mA g^-1. <i>In situ</i> X-ray Diffraction (XRD) and the galvanostatic intermittent titration technique (GITT) reveal a three-step sodium storage mechanism: adsorption, intercalation, and pore-filling, with rough pore surfaces accelerating filling kinetics. This strategy provides a scalable approach for designing high-performance carbon-based anodes.