In Situ Formation of Co9S8/N-C Hollow Nanospheres by Pyrolysis and Sulfurization of ZIF-67 for High-Performance Lithium-Ion Batteries

Graphical Better batteries : Co 9 S 8 /N-C hollow spheres were successfully synthesized through synchronous pyrolysis and sulfurization of ZIF-67 (see figure). These structures deliver a distinguished performance when applied as the anode in lithium-ion batteries. Co 9 S 8 is considered a promising candidate as the anode material in lithium-ion batteries (LIBs) because of its remarkable electrical conductivity, high theoretical capacity, and low cost. However, the practical application of Co 9 S 8 is greatly restricted because of its poor cycling stability and rate performance, which result mainly from the large volume expansion and dissolution of the polysulfide intermediates during the charge/discharge process. In this report, Co 9 S 8 embedded in N-rich carbon hollow spheres are successfully designed and synthesized through an in situ pyrolysis and sulfurization process, employing the well-known ZIF-67 as the precursor and ethanethiol as the sulfur source. Co 9 S 8 nanoparticles embedded in the N-rich hollow carbon shell exhibit excellent lithium storage properties at a high charge/discharge rate. A discharge capacity of 784 mAh g −1 is obtained upon battery testing at a current density of 1 C (544 mA g −1 ). Even upon cycling at a current density of 4 C, the as-prepared Co 9 S 8 /N-C can still deliver a discharge capacity of 518 mAh g −1 . The excellent battery performance can be attributed to the hollow structure as well as the N-rich carbon encapsulation. Moreover, this metal–organic framework sulfurization route also shows good generality for the synthesis of other metal sulfide–carbon composites such as ZnS/N-C and Cu 2 S/C.