Molecular Descriptor-Directed Microstructural Regulation of Asphalt-Derived Hard Carbons for Advanced Sodium-Ion...

Asphalt is a promising precursor for hard carbon anode in sodium-ion batteries, owing to its low cost and high carbon yield. However, its complex molecular composition leads to uncontrolled carbonization and unpredictable electrochemical performance. Here, we introduce two quantitative molecular descriptors: the aromatic substitution index (f_a ^s) and short-chain substitution index (R) that enable precise correlation between precursor molecular topology and hard carbon microstructure. Through systematic solvent fractionation and multi-scale characterization, we discover that short-chain pendant alkyl groups with high R values enhance oxidative crosslinking to suppress graphitization, while high-degree aromatic substitution with high f_a ^s values promotes the formation of uniform closed pores and optimizes structural disorder. The optimized hard carbon achieves superior sodium storage performance (326 mAh g^-1, 93% capacity retention after 500 cycles, 204 mAh g^-1 at 5 A g^-1) with exceptional rate capability. This descriptor-based approach provides both fundamental insights into microstructure formation and a practical strategy for precursor selection, paving the way for rationally designed high-performance hard carbon anodes.