Dual anionic doping strategy towards synergistic optimization of Co₉S₈ for fast and durable sodium storage

Cobalt sulfides with high capacities and excellent redox activities have attracted comprehensive attention as promising anode materials for sodium-ion batteries (SIBs). However, the poor conductivity and large volume change during sodium storage render them an unprecedented challenge to rate capability and cycling stability. Herein, a dual anionic (N and Se) doping strategy is first proposed in the preparation of Co₉S₈ (N,Se-Co₉S₈) with structural and electronic optimization as anode material for SIB. It has been revealed that N doping can increase the electron density of Co₉S₈ at the Fermi level while Se doping could expand the lattice spacing and lower Co-S binding energy; the synergistic combination of N and Se enables Co₉S₈ to possess high conductivity, good Na⁺affinity, and effective Na⁺diffusion, leading to fast reaction kinetics and energetically stable performance upon charging/discharging. In consequence, the N,Se-Co₉S₈ delivers an exceptionally high capacity of 550 mAh g⁻¹ at a current density of 0.1 A g⁻¹, conspicuous rate performance of 411 mAh g⁻¹ at 5 A g⁻¹, and good cycling stability with a retained capacity of 406 mAh g⁻¹ over 3000 cycles at 5 A g⁻¹, among the best results of reported SIB anodes. The dual anionic doping strategy provides an efficient way to synergistically manipulate electrode materials for high-performance sodium ion storage.