Sulfur-loaded monodisperse carbon nanocapsules anchored on graphene nanosheets as cathodes for high performance lithium-sulfur batteries

There is a growing demand to enhance the electrical conductivity of the cathode and to restrain the fast capacity decay during a charge-discharge process in lithium-sulfur (Li-S) batteries. This can be accomplished by developing novel methods for the synthesis of nanostructured materials that can act as effective cathode hosts. In this study, monodisperse carbon nanocapsules with a diameter of ∼20 nm anchored on a graphene nanosheet (MCNC/G) were prepared by a facile strategy, which involved mixing of iron-oleate and graphene, heat treatment, and finally, acid etching of iron oxide nanoparticles. This simple synthesis method could be suitable for mass production. We loaded MCNC/G with sulfur by a melting process, and tested the performance of the resulting MCNC/G-sulfur (MCNC/G-S) composites as the cathode material. As a result, the MCNC/G-S electrode infiltrated with 60 wt% sulfur delivers a high and stable reversible capacity of 525 mA h g^-1 after 100 cycles at a 0.5 C-rate with good capacity retention and excellent rate capability (630.5 mA h g^-1 at a high current density of 1C). The improved electrochemical performance could be attributed to the monodisperse carbon nanocapsules in the MCNC/G composite, which lead to small volume expansion and physical confinement of sulfur due to the void spaces inside the carbon nanocapsules during the charge-discharge process. Thus, these uniquely structured monodisperse carbon nanocapsules anchored on graphene nanosheets can be promising candidates for other energy storage applications.