Scotch-tape-like exfoliation of graphite assisted with elemental sulfur and graphene–sulfur composites for high-performance lithium-sulfur batteries

Abstract

A new composite structure of graphene–sulfur with a high electrochemical performance is proposed. Scotch-tape-like sulfur-assisted exfoliation of graphite is developed to produce the graphene–sulfur composites and freestanding low-defect graphene sheets. The intimate interaction between sulfur and graphene, attributed to the similar electronegativities of the two elements, is stronger than the van der Waals forces between adjacent π–π stacked graphene layers. This causes cleavage of the graphene layers when the sulfur molecules stick to the surface and edges of the graphite, similar to Scotch tape in micromechanical exfoliation processes. This approach enables us to obtain graphene with an electrical conductivity as high as 1820 S cm⁻¹ and a Hall mobility as high as 200 cm² V⁻¹ s⁻¹, superior to most reported graphene. Furthermore, the graphene sheets which uniformly anchor sulfur molecules provide a superior confinement ability for polysulfides, sufficient space to accommodate sulfur volumetric expansion, a large contact area with the sulfur and a short transport pathway for both electrons and Li⁺. The unique structure containing 73 wt.% sulfur exhibits excellent overall electrochemical properties of 615 mA h g⁻¹ at the 1 C (1 C = 1675 mA g⁻¹) rate after 100 cycles (corresponding average Coulombic efficiency of over 96%) and 570 mA h g⁻¹ at 2 C. These encouraging results represent that sulfur molecules bound onto graphene sheets could be a promising cathode material for lithium batteries with a high energy density.