Nitrogen-Doped Yolk–Shell-Structured CoSe/C Dodecahedra for High-Performance Sodium Ion Batteries

In this work, nitrogen-doped, yolk–shell-structured CoSe/C mesoporous dodecahedra are successfully prepared by using cobalt-based metal–organic frameworks (ZIF-67) as sacrificial templates. The CoSe nanoparticles are in situ produced by reacting the cobalt species in the metal–organic frameworks with selenium (Se) powder, and the organic species are simultaneously converted into nitrogen-doped carbon material in an inert atmosphere at temperatures between 700 and 900 °C for 4 h. For the composite synthesized at 800 °C, the carbon framework has a relatively higher extent of graphitization, with high nitrogen content (17.65%). Furthermore, the CoSe nanoparticles, with a size of around 15 nm, are coherently confined in the mesoporous carbon framework. When evaluated as novel anode materials for sodium ion batteries, the CoSe/C composites exhibit high capacity and superior rate capability. The composite electrode delivers the specific capacities of 597.2 and 361.9 mA h g^–1 at 0.2 and 16 A g^–1, respectively