Precise control of the resin-based hard carbon pseudo graphite and closed pores structure to enhance sodium storage...

Hard carbon is recognized as one of the most promising anode materials for sodium-ion batteries due to its high specific capacity and low cost. Enhancing the closed-pore structure, pseudo-graphite structure, and defects in hard carbon represents an effective strategy for improving the performance of hard carbon anodes. Thermosetting phenolic resin is considered one of the most promising precursors for hard carbon materials, owing to its multifunctional and tunable microstructure. In this study, hard carbon is designed to have a high proportion of pseudo-graphite structure and closed pores at the molecular level. It is found that urea promotes the formation of sp³-hybridized carbon and defects. The microcrystalline structure of hard carbon can be precisely tuned by controlling the sp³C/sp²C ratio. This evolution involves the formation of long-range ordered graphite-like structures, short-range pseudo-graphite structures, and ultimately an amorphous structure with a cross-linked graphite. A successful relationship is established between the evolution of the hard carbon microstructure and the formation of closed pores. The optimized HCUP-40 hard carbon material exhibits a high initial coulombic efficiency of 94.44 % at a current density of 0.1 A g^-1, a reversible specific capacity of 418.90 mAh g^-1, and excellent cycle stability, maintaining 184.80 mAh g^-1 after 1000 cycles at a current density of 5 A g^-1. This study provides valuable insights into the regulation of hard carbon microstructure and the design of high-capacity anode materials.