Preparation Of Graphene-Based Cathode Materials For Lithium-Sulfur Batteries And Their Electrochemical Performances

Lithium-sulfur(Li-S)battery is an attractive and promising energy storage device due to its high theoretical energy density,low economic cost and good environmental safety.Carbonaceous materials are important skeletons widely used forimmobilizing sulfur to improve the performances of the cathodes of Li-S batteries.Graphene,a one-atomthick two-dimensional crystal made of conjugated carbon atoms,exhibits excellent electric conductivity,outstanding mechanical properties and chemical stability,huge specific surface area,and various supramolecular behaviors.Therefore,graphene materials have been extensively explored for developing high-performanceelectrodesof Li-S batteries.In this thesis,several freestanding and flexible graphene-based composite cathodes were prepared and their electrochemical performances were studied.The main contents are described as follows.The sulfur cathodes include the composite films of(1)mildly reduced less defective GO(mrLGO)/sulfur/carbon nanotubes(mrLGO/S/CNTs),(2)HI reduced LGO/S/CNTs(HI-rLGO/S/CNTs),(3)mildly reduced conventional GO/S/CNTs(mrCGO/S/CNTs),(4)HI-rCGO/S/CNTs,(5)mrLGO/S and(6)HI-rLGO/S.Their compositions,chemical structures,morphologies,and electrochemical performances werestudied in detail and the main results are summarized as follows.The mrLGO/S/CNTscathode showed the best performance,and Li-S battery based onit exhibiteda high initial specific capacity of 1,219 mAh g-1 at 0.2C and a stable specific capacity around 1,000 mAh g-1 after 200 cycles with a coulombic efficiency of 99%.Its excellent performance is mainly attributed to two factors:(1)mrLGO has a high conductivity(about 5 times that of mrCGO)and a large amount ofresidual oxygen containing groupsfor restraining the diffusion of polysufides to electrolyte;(2)the three-dimensional(3D)framework constructed by mrLGO sheets and CNTs provides more anchoring sites for polysufides and shortens the pathways of the ion/electron transportation,and thus improves the kinetics of the electrochemical reaction.