Preoxidation Achieves Bond Energy Conversion to Regulate the Microstructure of Phenolic Resin-Derived Hard Carbon for Efficient Sodium Storage

Phenolic resin (PF) is a very promising precursor material for hard carbon (HC) relying on high residual carbon yield and structural tunability, while, the highly ordered structure of PF-derived HC restricts the formation of closed pores, accordingly attenuating the sodium-ion storage capacity and kinetic performance. In this work, we develop a PF-derived HC material with an optimized cross-linked structure, increased disorder, and an enriched closed pore network with a microstructure-regulation strategy through hydrothermal treatment combined with preoxidation processes. The preoxidation process converts lower bond energy C–C and C–O bonds in PF into higher bond energy C═O and C(O)–O bonds, thereby improving the cross-linking degree of PF. Upon carbonization, this bond energy transformation further enhances the disordering of the HC structure, thus enhancing the structural stability of the HC structure during sodiation/desodiation cycles. When used as an anode material, the optimized sample, which has been preoxidized at 220 °C, indicates a higher reversible capacity of 341.9 mAh g–1. Compared with the nonpre-oxidized sample, the optimal sample exhibited a 122.7 mAh g–1 increase in plateau capacity, a 17.6% improvement in Coulombic efficiency, and enhanced rate capability. This study contributes to a theoretical understanding and a practical approach to designing the enriched closed pore structure for a high-performance HC anode.