Synthesis Of Graphene/Mesoporous Carbon/Sulfur Composite And Its Application In Lithium-Sulfur Batteries

As a novel secondary battery,lithium sulfur battery has the characteristics of high energy density(2600 Wh kg^-1),low cost,less pollution and so on.As a key component of lithium sulfur battery,the cathode material has an important influence on the electrochemical performance of lithium sulfur battery.However,the natural insulation of sulfur and lithium sulfide as well as the dissolution and shuttle ofpolysulfides in the electrolyte seriously affect the electrochemical performance of the Li-S batteries.The use of carbon materials for the modification of sulfur cathode will effectively solve the above problems.In this thesis,two kind of composite material were designed and synthesized,showing different morphologies and structures,which have a significant effect on the electrochemical properties of Li-S batteries.The main content was divided into the following two parts.1.Preparation of in-situ graphene/mesoporous carbon composites?MCG?MCG was fabricated via in situ growth of mesoporous carbon on the surface of graphene,while graphene was used as morphology-directed materials,F127 as pore forming agent,phenolic micelles as organic carbon source.The precursor was obtained by hydrothermal method,and then the mesoporous structure was obtained by frozen drying and annealing.The influences of the concentration of phenolic resin,the ratio of graphene to mesoporous carbon,pH were systematically studied.The composite exhibited a high initial discharge capacity of 1050 mAh g^-1,which indicated the composites can efficiently accelerate electrons and lithium-ions transfer.However,the pore structure of larger specific surface area increased the dissolution and diffusion of the intermediates into the electrolyte,leading to faster capacity fading.2.Preparation of graphene/mesoporous carbon spheres composites?MG?In order to solve the problems in the last work,the design idea of mesoporous carbon microspheres replaced the idea of in-situ growth of mesoporous carbon.On the surface of the controlled-growth mesoporous carbon spheres,graphene sheets were adsorbed by electrostatic interaction.After the high temperature treatment at 300?,the sulfur was impregnated into the mesoporous channel inside the mesoporous carbon spheres and the gap with the graphene.Mesoporous carbon spheres ensure high sulfur loading and uniform distribution of sulfur.Graphene can inhibit the dissolution and diffusion of polysulfide,and it can also increase the load of sulfur.The large-size graphene interlinked single mesoporous carbon spheres,facilitating faster electron and ions transport.The synergistic effect can improve the electrochemical performance of the composite,resulting in good rate performance and high coulombic efficiency.The specific discharge capacity was still about 510 mAh g^-1 after 300 cycles at a current density of 0.5 C,and the cycle efficiency is more than 98%with stable galvanostatic discharge profiles.