Co9S8@carbon yolk-shell nanocages as a high performance direct conversion anode material for sodium ion batteries

Cobalt sulfides based on conversion mechanisms are considered as promising anode materials for sodium-ion batteries due to their appropriate working voltage and high practical capacities. But the severe volume change and structure transformation make their cycle stability and rate capability unsatisfactory. In this study, metalorganic framework derived Co9S8@carbon yolk-shell nanocages (Co9S8@CYSNs) was prepared and its direct conversion mechanism was carefully demonstrated for the first time by various spectroscopic techniques and first-principles calculations. The unique hierarchical structure of Co9S8@CYSNs composed of Co9S8 nanoparticles dispersed in amorphous carbon matrix inside a rigid carbon shell was capable of accelerating the conversion reaction, shortening the Na+ diffusion distance and providing a fast electron transport channel. Benefiting from the accelerated electrochemical reactions and high activities of nanosized particles, the Co9S8@CYSNs exhibited a large discharge capacity of 549.4 mA h g(-1) at 0.1 A g(-1). In addition, a superior rate performance of 100 mA h g(-1) at 10 A g(-1) and excellent cycle stability with a very low capacity decay of 0.019% per cycle over 800 cycles at 10.0 A g(-1) were achieved because of the confine effect of the carbon shell and improved charge transfer reactions of the electrode.