Preparation And Electrochemical Performance Of Carbon/sulfur Composite Cathode Materials For Li-S Batteries

With the rapidly increasing demand for electrical vehicles and portable electronic devices,rechargeable lithium-sulfur(Li-S) batteries have gained increased attention because of their high theoretical specific capacity(1675 mAh g-1) and energy density(approximately 2600 Wh kg-1).Sulfur, as one of the most abundant elements on earth, is non-poisonous, inexpensive and environment friendly. As such, sulfur is a potential electrode material for the next generation of high-performance energy storage devices. Nevertheless, commercial application of Li-S batteries is restricted by low electrical conductivity of sulfur/polysulfides, large volume expansion of sulfur upon lithiation, dissolution of polysulfides and the resulting shuttling effect. Among the cathode materials for lithium-sulfur batteries, carbon/sulfur materials are the most attractive. In this thesis, two carbon/sulfur(C/S) composites have been successfully synthesized and explored as cathode materials for Li-S batteries to pursue improved electrochemical performance.The features of the carbon substrate are crucial for the electrochemical performance of lithium-sulfur(Li-S) batteries. Nitrogen doping of carbon materials is assumed to play an important role in sulfur immobilisation. In this study, natural silk fibroin protein is used as a precursor of nitrogen-rich carbon to fabricate a novel, porous, nitrogen-doped carbon material through facile carbonisation and activation. Porous carbon, with a reversible capacity of 815 m Ah g-1at 0.2 C after 60 cycles, serves as the cathode material in Li-S batteries. Porous carbon retains a reversible capacity of 567 mAh g-1, which corresponds to a capacity retention of 98% at1 C after 200 cycles. The promising electrochemical performance of porous carbon is attributed to the mesoporous structure, high specific surface area and nitrogen doping into the carbon skeleton. This study provides a general strategy to synthesise nitrogen-doped carbons with a high specific surface area, which is crucial to improve the energy density and electrochemical performance of Li-S batteries.Graphene oxide(GO), has high surface area, chemical stability, mechanical strength and flexibility, is a useful material for electrochemical energy storage applications, especially for coating and protecting sulfur. In this paper, a hollow sulfur@graphene oxide core-shell composite is synthesized for high-performance lithium-sulfur batteries through a facile two-step solution method with the assistance of polyvinylpyrrolidone. Scanning electron microscope observation shows that the hollow sulfur sphere is coated by the homogeneous graphene oxide nanolayer, and thermogravimetric analysis demonstrates the composite holds sulfur loading as high as 92 wt%. The composite served as cathode in lithium-sulfur batteries shows its capacity as high as 745 mAh g-1at 0.2 C after 20 cycles. It also has an excellent cycling stability even at ahigh current density of 1 C(73% retention and 96% coulombic efficiency after 150 cycles),implying that the diffusion of the polysulfides into electrolyte can be greatly inhibited. It is believed that the structure design of graphene oxide coated hollow sulfur sphere can be developed for the practical application of high-performance lithium-sulfur batteries.