Surface-dominated storage of heteroatoms-doping hard carbon for sodium-ion batteries

Though the intercalation mechanism of hard carbon anodes for Na-ion batteries (NIBs) gain decent electrochemical performances, the sluggish diffusion kinetics of Na-ions at low potential present huge challenge to high power applications. To improve the rate capability and cycling stability, especially the performance at high rates, herein, a novel N, S co-doping strategy based on the pseudocapacitive mechanism is developed via one-step synthesis, which endows the hard carbon with excellent rate performances and long-term cycling stability. Moreover, the electrochemical behaviors of N, S co-doping carbon materials outperform those of single heteroatom (N) doping materials, owing to the additional effective covalent S bonds and more defects. The optimized NSC2 (derived from 2:1 mass ratio of thiourea and sodium citric) delivers a high reversible capacity of 280 mAh g(-1) at 0.05 A g(-1) for 200 cycles and 223 mAh g(-1) at 1 A g(-1) for 2000 cycles, respectively, and preserves 102 mAh g(-1) at 10 A g(-1). This work provides a universal co-doping approach on modification of hard carbon materials for high power battery applications.