Water-Dispersed Sulfur Nanoparticles With High Activity: Controlled Fabrication And Use In Lithium-Sulfur Batteries

Owing to the advantages of high-energy density,environmental friendliness and financial feasibility,lithium sulfur batteries have emerged as one of the most promising next-generation energy-storage systems.Despite these great advantages,many challenges have to be overcome before the practical application,such as the poor conductivity of sulfur,severe shuttle effects caused by the soluble long-chain lithium polysulfide and large volume expansion during the charge/discharge process.Functional sulfur host materials with physical confinement and chemical affinity to lithium polysulfide have been proved efficient on improving the performance of lithium sulfur battery.However,it still suffers with the problems of low sulfur utilization efficiency and energy density.Here,sulfur nanoparticles have been selected as a research target,and a kind of water-dispersed sulfur（WDS）nanoparticle with unique aqueous dispersion property and high electrochemical activity has been synthesized.Its reaction mechanism and potential applications in lithium sulfur batteries are comprehensively investigated and several high performance cathode materials for lithium sulfur battery have been successfully fabricated through the structure designation and surface modification.Three-dimensional graphene/sulfur composition（G/S）is prepared by a one-pot hydrothermal method using graphene oxide and sulfur-based compound as reactants.The uniform distribution of sulfur on graphene is realized by this simple method.The graphene in the composition is interconnected with each other to form a three dimensional porous structure,which can greatly improve the conductivity of the cathode materials and restrict the dissolution of lithium polysulfides.By optimizing the drying method of the 3D graphene-based materials,we obtain a high density graphene/sulfur composition（DGS）with dense pore structure,and the electrochemical tests illustrate that DGS can greatly accommodate the volume expansion and improve the performance of lithium sulfur battery.Through confining the gaseous comproportionation reaction between sulfur containing air pollutants,H₂S and SO₂,in water,we develop a completely clean method to prepare WDS with controllable size,and this method is free of surfactants and organic solvents.WDS has shown its great potential in lithium sulfur battery application and makes it possible for achieving solution hybridization with other conductive materials in any fraction of sulfur.An aqueously fabricated WDS/carbon nanotube cathode delivers the theoretical capacity of 1673 m Ah g^-1 at 0.5 A g^-1,and a capacity of～750 m Ah g^-1 even at a high current density of 5.0 A g^-1.And a new-concept sulfur-based interlayer structure with low sulfur content can effectively inhibit the shuttling of polysulfides,greatly improving the cycling performance of the Li-S battery.Based on the unique properties of WDS,a three-dimensional porous structure constructed by carbon nanotube is used as the conductive substrate to prepare free-standing flexible lithium sulfur battery cathode materials.By introducing functionalized interlayer structures,the flexible cathode materials with hierarchical structures are prepared and greatly restrict lithium polysulfide by physical confinement and chemical adsorption.The as-fabricated flexible lithium sulfur battery exhibits surprisingly cycling stability and excellent mechanical properties.To further improve the practical energy density of lithium sulfur battery,graphene-based porous carbon/sulfur composition with 88%sulfur content is constructed by high conductive three dimensional graphene porous structure and sulfur nanoparticles with high electrochemical activity.Furthermore,nitrogen doping method is also employed to modify the surface chemistry property of non-polar graphene,which will enhance the chemical interaction with polysulfides and improve the electrochemical performance of lithium sulfur battery.