Formation of graphene-encapsulated CoS2 hybrid composites with hierarchical structures for high-performance lithium-ion batteries

Transition metal sulfides (TMSs) are considered as the most promising alternative anode materials for advanced lithium-ion batteries (LIBs). Here, we report a hierarchically structured CoS2 nanosphere/graphene (CoS2/G) composite, fabricated by a simple hydrothermal method. This composite, assembled with CoS2 nanoparticles uniformly distributed on the graphene, exhibits excellent electrochemical performance. In particular, the CoS2/G electrode material delivers a high rate capability of around 398 mA h g−1 at a current density of 3500 mA g−1. Moreover, a discharge capacity of about 400 mA h g−1 can be obtained after 1000 cycles at a current density of 500 mA g−1. X-ray absorption spectroscopy is used to characterize the sample for the first time, and the results demonstrate that CoS2/G is reduced to metallic Co and Li2S when discharged to 0.01 V. In subsequent charge–discharge processes, the metallic Co cannot be fully oxidized to CoS2, which is the main cause of capacity loss for the CoS2 electrode.