Nitrogen-Doped Hard Carbon Nanosphere Anode Prepared by an Efficient Solvent-Free Strategy for High-Performance Sodium Storage

Developing facile and diverse polymer-derived porous carbon sphere synthesis strategies for sodium-ion battery (SIB) anodes is of great attraction, but the straightforward and scalable synthesis of carbon nanospheres remains a challenge. Different from the traditional sol–gel synthesis of metal–phenolic coordination spheres, here, a solvent-free regulating micelle transformation approach is involved to generate tannic acid–formaldehyde (TAF) polymer nanospheres by condensation reaction of tannic acid and formaldehyde decomposed from hexamethylenetetramine without adding external solvent and using F127 as the structure-directing agent. Subsequently, the TAF nanospheres were carbonized to obtain nitrogen-doped hard carbon nanospheres (NHCNs). The morphology and porous structure (especially the closed pores) of NHCNs were significantly affected by F127. The optimized NHCNs-3 exhibits uniform nanosphere morphology with obvious rough surfaces and abundant closed-pore structures, which significantly enhanced sodium-ion storage performance, including high reversible capacity of 377 mAh g^–1 at 0.05 A g^–1 and excellent rate performance (277 mAh g^–1 at 1 A g^–1), as well as good cycling stability (84.4% capacity retention after 1000 cycles at 1 A g^–1), outperforming most reported carbon nanosphere anodes. Series characterization confirmed that the sodium storage mechanism of NHCNs-3 includes defect adsorption, graphite intercalation, and pore-filling processes. Moreover, the SIB assembled with a Na₃V₂(PO₄)₃ cathode and NHCNs-3 anode exhibited high reversible capacity and retention at high current density, revealing its potential for practical application.