Oxygen-mediated molecular crosslinking-engineered hard carbon anodes for advanced sodium-ion batteries

In the realm of commercial sodium-ion batteries (SIBs), hard carbon (HC) stands out as the most promising anode material; however, precisely modulating its microstructure to unlock high reversible capacity remains a formidable challenge. This study proposes a simple and efficient molecular cross-linking reaction to engineer the precursor's molecular configuration and functional groups. By introducing oxygen to trigger esterification between hydroxyl and carboxyl groups, a robust cross-linked carbon network is constructed. This molecular-level structural engineering ultimately yields an optimized HC architecture characterized by a highly distorted graphite lattice, an expanded closed-pore volume (0.1123 cm³ g⁻¹), and abundant Na⁺ adsorption sites. Theoretical and experimental analyses reveal that this unique structure not only enhances the Na⁺ adsorption energy and lowers the migration barrier, but also effectively facilitates the formation of quasi-metallic sodium clusters within the closed pores. Consequently, the optimized HC anode delivers a superior reversible capacity of 364.7 mAh g⁻¹. Furthermore, it exhibits exceptional cycling durability, retaining 205.4 mAh g⁻¹ after 2500 cycles at a high rate of 2 C, alongside robust electrochemical performance at −25 °C. This simple and effective molecular cross-linking strategy provides valuable guidance for the advanced application of HC in SIBs.