Rationally designed yolk–shell Co9S8–Co1−xS hollow spheres for advanced sodium-ion storage

Abstract

As one of the most promising sodium-ion battery (SIB) anodes, transition metal sulfides (TMSs) have been extensively studied for their high capacity and abundant resources. However, the performance of TMS electrodes has been plagued by the poor rate capability and short cycle life, resulting from the huge volume change and the shuttling of polysulfides during cycling. Here, rationally-designed nitrogen-doped carbon-coated Co₉S₈–Co_1−xS yolk–shell hollow spheres (Co₉S₈–Co_1−xS@NC) are synthesized as the SIB anode. The hollow nanostructure shortens the diffusion length of sodium ions/electrons and affords abundant space to accommodate the volume expansion during sodiation. The Co₉S₈–Co_1−xS@NC anode delivers a large sodium-ion storage capacity of 578 mA h g⁻¹ even after 500 cycles at 0.5 A g⁻¹ and superior rate capability and cycling stability (323 mA h g⁻¹ at 5 A g⁻¹ over 1200 cycles) in a half cell. In addition, when combined with the advanced Na₃V₂(PO₄)₂O₂F electrode, the full SIB shows a highly stable specific capacity and reversibility. The reaction mechanism of Co₉S₈–Co_1−xS@NC was studied using operando X-ray diffraction (operando XRD), revealing the reversible conversion between cobalt sulfides and Co/Na₂S.