Effect of coating asphalt softening point on the pre-oxidation pathway and sodium storage performance of derived hard carbon

This study clarifies the nonlinear relationship between the softening point of coating asphalt and its oxidative cross-linking behavior, as well as the sodium storage performance of the derived hard carbon. Comparative analysis of asphaltes with low (80 ℃), medium (160 ℃) and high (260 ℃) softening points revealed that both the 80 and 260 ℃ asphaltes incorporated a higher oxygen content (20%−25%) during oxidation, leading to the formation of a deeply cross-linked structure dominated by anhydride and ester groups. This effectively suppressed graphitization during carbonization, yielding hard carbon with large interlayer spacing, high disorder, and abundant closed pores. The derived hard carbon exhibited superior sodium storage performance, the initial charge capacities of EPOC-80 and EPOC-260 reached 314.7 and 306.6 mA·h/g, with first-cycle coulombic efficiencies of 81.3% and 79.3%, respectively, along with excellent cycling stability and rate capability. In contrast, the medium softening point asphalt (160 ℃) showed limited oxygen incorporation (~5%) and insufficient cross-linking after oxidation, resulting in a densely packed hard carbon with smaller interlayer spacing (3.46 Å) and restricted sodium storage sites, which led to a significantly reduced capacity of 173.2 mA·h/g. This work provides new design principles and theoretical support for optimizing hard carbon anode structures through precise control of the precursor softening point.