Oxygen-Crosslinker Effect on the Electrochemical Characteristics of Asphalt-Based Hard Carbon Anodes for Sodium-Ion Batteries

Because direct carbonation of asphalt usually yields ordered graphite structure with unfavorable storage of sodium. Here, the asphalt preoxidation at a specific temperature in the air introduces oxygen-containing groups to connect the unsaturated aliphatic hydrocarbon and aromatic side chains, forming a disordered carbon skeleton to inhibit melting and rearrangement during carbonization. The abundant oxygen-containing groups hinder the growth of the carbon layers during pyrolysis, which promotes the formation of disordered phases and abundant micropores in asphalt-based hard carbons (HCs). The simultaneous increase in initial coulombic efficiency, capacity, and transport behavior of sodium ions in HCs is achieved by adjusting the carbon layer and micropore evolution. The optimized HCs display excellent initial coulombic efficiency of 86.14% with remarkable reversible capacity of 313.83 mAh g⁻¹ at 0.1 C and high-rate capability with 140 mAh g⁻¹ at 5 C. Pairing with O3-NaNi_1/3Fe_1/3Mn_1/3O₂ cathode, the full cell delivers a higher reversible capacity of 255.7 mAh g⁻¹ with an initial coulombic efficiency of 83.7% and long cycle life. Based on the microstructure and electrochemical behaviors of asphalt-based HCs, the “adsorption-insertion-pores-filling” sodium storage mechanism is proposed, providing guidelines for designing high-energy-density sodium-ion batteries.