Co9S8@carbon nanospheres as high-performance anodes for sodium ion battery

Co 9 S 8 @C nanospheres with a 5 nm carbon shell and 50 nm Co 9 S 8 core have been synthesized by a facile hydrothermal method followed by an annealing process. The electrochemical results indicate that the Co 9 S 8 @C nanospheres are an excellent anode material for sodium ion batteries. They deliver a specific capacity of 305 mA h g −1 after 1000 cycles at current density of 5 A g −1 and a high specific capacity of 405 mA h g −1 at current density of 500 mA g −1 . The good cycling performance and rate capability are attributed to the synergistic effects of the carbon shell and the ultra-small size of Co 9 S 8 , which can enhance the electrical conductivity of the composite, as well as resisting the strain caused by repeated charge/discharge processes.