Achieving a High Plateau Capacity in Pitch-Based Carbon Anodes via Dual-Step Pre-Oxidation for Sodium-Ion Batteries

Preoxidation is a widely adopted strategy to prepare pitch-based hard carbon (HC) for sodium-ion batteries, as the introduced oxygen-containing functional groups can restrain the graphitization of pitch during carbonization. However, low-softening-point pitch remains largely unexplored for HC production, and the unclear oxidative cross-linking mechanism limits structural control and performance optimization of HC. To address these challenges, this study employs a dual-step preoxidation method to fabricate HC from low-softening-point pitch, accompanied by an in-depth investigation of the molecular structure evolution of pitch during oxidation. The results demonstrate that liquid-phase oxidation increases the aromaticity of pitch and introduces C═O groups by oxidizing the alkyl side chains; subsequent solid-phase oxidation further increases the oxidation depth, generating abundant cross-linked structures dominated by O-C═O and C-O bonds. These cross-linked structures hinder the oriented growth of carbon layers during carbonization, ultimately forming disordered HC structures with large interlayer spacing, small microcrystalline size, and numerous closed pores. Therefore, the optimized HC exhibits an impressive reversible capacity of 357.6 mAh g^-1 along with a high plateau capacity of 274.2 mAh g^-1 at 20 mA g^-1. This work provides a feasible approach for transforming low-softening-point pitch into HC.