| Graphene is a new two dimensional carbon material, well known for its superior electrical conductivity, high surface area of over 2600 m 2/g, ultrathin thickness, structure flexibility and good mechanical properties. Nowadays, graphene has been used in fabrication of anode and cathode together with other materials for the lithium batteries. Elemental sulfur is one of the most promising cathode materials for the next generation lithium batteries, especially for the high-energy, high-power batteries that will be applied in powering hybrid electric vehicles and in storing electricity from renewable sources, due to its highest specific capacity, abundant resources, and low toxicity. In this thesis, we have synthesized graphene based composite materials through the development of an aqueous one-pot wet chemical method, extensively characterized the synthesized composites and explored them as cathode materials of lithium-sulfur battery. The main results of this thesis research are summarized as follows:;1. Sulfur-reduced graphene oxide composite (SGC) materials with uniformly dispersed sulfur on the reduced graphene oxide sheets are prepared by developing a simple aqueous one-pot synthesis method, in which the formation of the composite is achieved through simultaneous oxidation of sulfide and reduction of graphene oxide. The sulfur contents in the SGC, determined by thermogravimetry and elementary analysis, have been adjusted in the range from 20.9% to 72.5%. (Chapter 3).;2. We have tested the electrochemical performances of the as-synthesized SGC materials as cathode of Li-S battery after optimizing electrolyte and voltage window. Under the electrochemical condition, we have found that the SGC-63.6%S can deliver a reversible capacity as high as 804 mAh/g after 80 cycles of charge-discharge at a current density of 312 mA/g (ca. 0.185 C), and 440 mAh/g after 500 cycles at 1250 mA/g (ca. 0.75 C). We have further shown that the SGC cathodes with the sulfur content around 60% in the SGC can achieve a maximal sulfur utilization. (Chapter 4).;3. It was shown that the commonly used heat treatment cannot improve the electrochemical performance of the SGC cathode because after heat treatment some sulfur initially dispersed uniformly on the reduced graphene oxide sheets aggregates into large particles, resulting in a severe polysulfides shuttle effect. On the other hand, the heat treatment study has confirmed once again the uniform dispersion of sulfur on the reduced graphene oxide in the SGC materials during the one pot synthesis. (Chapter 5).;4. In order to improve further the electric transportation property of the composite materials, we have synthesized multiwall carbon nanotubes-sulfur-graphene composite by developing an appropriate synthesis method. In this method, the surface of MWCNTs is modified at first with --OH groups, and then the --OH is replace by --SH via adding the Na 2S to grow sulfur on the MWCNTs' surface. The result demonstrated that the sulfur-MWCNTs composite can improve the coulombic efficiency of Li-S battery. In the case of MWCNTs-S-rGO with 30% MWCNTs and 10% reduced graphene oxide, the cathode can deliver a reversible capacity of 500 mAh/g after 200 full cycles with a low coulombic efficiency of 108%. (Chapter 6). |
