Design And Preparation Of Sulfur-based Composites And Their Application In Lithium-sulfur Batteries

Lithium sulfur batteries (LSBs) are the most promising candidate for the next generation energy storage system due to the ultrahigh theoretical energy density (2500 Wh kg-1 or 2800 Wh L-1). However, the application of LSBs is hindered by the poor cycle performance and low Coulomb efficiency. It is important for preparating high-performance cathode material of LSBs. In this paper, the hollow porous silica nanobelts, the three-dimensional silica aerogels, and the three-dimensional graphene aerogels were used as skeleton of sulfur-based composite cathode materials which exhibit good electrochemical lithium storage properties. The main works of this paper are as follows:(1) By using the CuO nanobelts as templates, the hollow porous SiO2 nanobelts were prepared via the hydrolysis and polycondensation process of tetraethyl orthosilicate (TEOS)and the subsequent template etching process. And then the hollow porous SiO2/S composite was synthesized by melt diffusion method. Hollow porous silica nanobelt is both of a carrier of sulfur and a container for in situ desorption of polysulfide, showing good cycle stability. For example, the cell delivers an initial discharge capacity of 679 mAh g-1 and retained to 534 mA h g-1 even after cycling for 70 cycles at 0.1 C,corresponding to an average capacity loss of around 0.31% per cycle, and the Coulomb efficiency is close to 100%.(2) By using tetraethyl orthosilicate (TEOS) as the silicon source, sublimed sulfur as sulfur source, the three-dimensional silica aerogels/S hybrids (SASHs) were in situ synthesized through the hydrolysis and polycondensation of TEOS, without the need for high temperature treatment. Three-dimensional porous silica aerogels (SAs) matrix has dual function of both physical adsorption and chemical adsorption of soluble polysulfide.When evaluated as cathde amterials for LSBs, the SASHs show good electrochemical performance. For example, the SASHs deliver an initial discharge capacity of 762 mAh g-1 and retained to 600 mA h g-1even after cycling at 0.1 C for 110 cycles, corresponding to an average capacity loss of around 0.20% per cycle, and the Coulomb efficiency is close to 100%.(3) By using the sublimed sulfur as sulfur source, the graphene oxide /S hydrogel was in situ prepared by the self-assembly of chitosan chains with graphene oxide (GO)nanosheets, where GO works as the two-dimensional cross-linker dues to its multifunctional groups on both sides. Finally, the three-dimensional reduced graphene oxide (rGO) aerogel/S composites were prepared by the reduction of GO with hydrazine hydrate. The reduced graphene netork has two functions, i.e. sulfur carrier and conductive skeleton. The rGO aerogel/S composites exihibit superior cycle stability and rate capability. For example, the cell delivered an initial discharge capacity of 711 mAh g-1 and retained to 465 mA h g-1 even after cycling for 247 cycles at 0.1 C, corresponding to an average capacity loss of around 0.14% per cycle. Moreover, reversible capacities of 420 and 320 mA h g-1 are obtained at 0.5 and 1C, respectively.