Tailoring the Micronanostructure of Hard Carbon via Ball-Milling for Sodium-Ion Storage

Engineering microcrystalline and pore structures of hard carbons is crucial for optimizing their sodium storage performance. This work presents a solid-state mechanochemical approach for tailoring the structure of hard carbons using phenolic resin-based carbon as an exemplification. Mechanical ball-milling can crush carbon particles and break the C-C/C═C bonds, leading to submicrometer-sized particles enriched with carbon defects and oxygen-bearing functional groups. Small-sized particles enable their uniform assembly during the subsequent milling process with pitch; the abundant defects lead to the formation of more small-sized (∼2 nm) closed pores as the microcrystalline form develops during the subsequent carbonization process. Additionally, due to the presence of pitch-derived soft carbon, the optimal sample (BPHC) obtained at 1500 °C possesses both an abundance of closed pores and a high degree of crystallinity. As a result, BPHC shows a high reversible capacity of 304 mAh g^-1 with an initial Coulombic efficiency of 82.2% at 0.03 A g^-1, as well as high rate performance (50.6 mAh g^-1 at 2 A g^-1). When coupled with the Na₃V₂(PO₄)₃ cathode, BPHC as an anode in a full cell exhibits a high reversible capacity of 280.8 mAh g^-1 at 0.03 A g^-1 with excellent cycling performance. This work offers theoretical guidance for tailoring the micronanostructure and enhancing the electrochemical performance of hard carbons.