Sulfur film-coated reduced graphene oxide composite for lithium-sulfur batteries

This paper reports a cathode material of a graphene-sulfur film nanostructure synthesized by a facile wet chemical oxidation technique. The hybrid material is synthesized by using ferric chloride as an oxidizing agent and polysulfide as the sulfur source. The oxidizing agent FeCl3 is introduced as a soft film template to form a sulfur film on graphene oxide (GO). This is ascribed to the formation of a ferric cations double layer (FCDL) on the graphene oxide surface with many negative functional groups. Material characterizations reveal that GO is reduced to reduced graphene oxide (RGO) and the sulfur materials are embedded in a three dimensional conducting composite network of RGO and polyethylene glycol (PEG), forming a continuous sulfur film coating on RGO. When fabricated as a cathode material for rechargeable lithium ion batteries, the sulfur film-coated graphene composite electrode exhibits high capacity, improved Coulombic efficiency and extraordinarily stable cycling performance. It is believed that the sulfur film is coated on RGO and protected by the PEG network, and therefore the high solubility of the polysulfide anions formed as the reaction intermediate in the discharge and charge processes is relieved to a large extent. The electronic conductivity of insulating sulfur is also improved substantially in the RGO-S-PEG network.