Rechargeable batteries with high energy density are desirable to solve the imminent global energy and environmental issues. Currently, lithium ion batteries dominate the portable electronics market, but are not satisfactory for the needs of electric vehicles due to their high cost and low energy density. Lithium-Sulfur (Li-S) battery is a promising candidate because of its high theoretical specific capacity, energy density and low cost. However, Li-S battery suffers from some problems, including the insulting nature of sulfur, volumetric change during cycling and shuttling phenomenon resulting from dissolving of polysulfides. How to increase the conductivity of cathode and effectively trap polysulfides are two important issues to be resolved.

In this thesis, to increase the co...[ Read more ]

Rechargeable batteries with high energy density are desirable to solve the imminent global energy and environmental issues. Currently, lithium ion batteries dominate the portable electronics market, but are not satisfactory for the needs of electric vehicles due to their high cost and low energy density. Lithium-Sulfur (Li-S) battery is a promising candidate because of its high theoretical specific capacity, energy density and low cost. However, Li-S battery suffers from some problems, including the insulting nature of sulfur, volumetric change during cycling and shuttling phenomenon resulting from dissolving of polysulfides. How to increase the conductivity of cathode and effectively trap polysulfides are two important issues to be resolved.

In this thesis, to increase the conductivity, I firstly prepared and tested planar graphene oxide-sulfur (G-S) composites with different sulfur contents as cathode materials for Li-S battery. The good performance of G-S composite is attributed to the proper sulfur content and uniformly dispersed sulfur nanoparticles on the GO sheets. Then, to effectively trap polysulfides, crumpled graphene balls were prepared via spray drying method and then deposited with sulfur nanoparticles. Compared with the previous G-S composites, the crumpled graphene balls-sulfur composites show superior cycling performance, which is attributed to the closed structure of the crumpled balls. At last, the crumpled graphene balls-sulfur composite was wrapped by reduced GO for further entrapping sulfur and polysulfides. The increased discharge capacity and coulombic efficiency result from the larger contact area of S, electrolyte and conductive agent, and trapping of polysulfides by crumpled balls and rGO.

In conclusion, three series of graphene-sulfur composites were prepared to increase the conductivity of cathode material and effectively trap sulfur. They were proved to be good cathode material candidates for Li-S battery.