In Situ Formation of Co 9 S 8 /N-C Hollow Nanospheres by Pyrolysis and Sulfurization of ZIF-67 for High-Performance Lithium-Ion Batteries.

Co9 S8 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 Co9 S8 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, Co9 S8 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. Co9 S8 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 Co9 S8 /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 Cu2 S/C.