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

Transitionmetal sulfides (TMSs) are considered as themost promising alternative anodematerials 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.