High-efficiently doping nitrogen in kapok fiber-derived hard carbon used as anode materials for boosting rate performance of sodium-ion batteries

The engineering of plant-based precursor for nitrogen doping has become one of the most promising strategies to enhance rate capability of hard carbon materials for sodium-ion batteries; however, the poor rate performance is mainly caused by lack of pyridine nitrogen, which often tends to escape because of high temperature in preparation process of hard carbon. In this paper, a high-rate kapok fiber-derived hard carbon is fabricated by cross-linking carboxyl group in 2,6-pyridinedicarboxylic acid with the exposed hydroxyl group on alkalized kapok with assistance of zinc chloride. Specially, a high nitrogen doping content of 4.24% is achieved, most of which are pyridine nitrogen; this is crucial for improving the defect sites and electronic conductivity of hard carbon. The optimized carbon with feature of high nitrogen content, abundant functional groups, degree of disorder, and large layer spacing exhibits high capacity of 401.7 mAh g−1 at a current density of 0.05 A g−1, and more importantly, good rate performance, for example, even at the current density of 2 A g−1, a specific capacity of 159.5 mAh g−1 can be obtained. These findings make plant-based hard carbon a promising candidate for commercial application of sodium-ion batteries, achieving high-rate performance with the enhanced pre-cross-linking interaction between plant precursors and dopants to optimize aromatization process by auxiliary pyrolysis.