Flourinated Electrolytes for High Performance Rechargeable Lithium-Sulfur Batteries

The high demand for clean, efficient, and renewable energy and, the necessity for solving the CO2 issue and global warming are only a few of the major motivations for exploring renewable energy technologies. Since the energy must be stored in order for renewable energy to become part of a practical energy solution, there have been many studies of secondary batteries for energy storage applications that benefit from high specific energy, high rate capability, high safety, and low cost. Lithium-sulfur (Li-S) batteries have received a great amount of attention in recent years, as sulfur exhibits an order of magnitude higher theoretical specific capacity than that achievable with intercalation-type cathodes in lithium-ion batteries. In addition, sulfur is abundant in nature and non-toxic, which leads to low cell cost and significant environmental benefits. However, low active material utilization and poor cycle life hinder the commercial application of the Li-S chemistry. In this research project, different methods and solutions have been applied to achieve high capacity and efficiency with a long cycle life of the Li-S battery. In the first part of this thesis, the effects of different electrolyte solvents and fluorinated additives on the Li-S battery were investigated, as electrolyte is one of the key components in determining the performance of this battery. It is observed that fluorinated solvents have an outstanding effect in improving the cycle life and efficiency and will also assist in improving the self discharge. In the second part of this study, we report on a modification to the traditional Li-S battery configuration. The performance of Li-S batteries using Teflon® coated carbon paper (TCCP) was investigated in this study for the first time. The TCCP is composed of carbon microfibers that act as an excellent substrate while the hydrophobic Teflon (PTFE) coating facilitates the absorption of soluble polysulfides to the cathode. This new configuration indicates great improvement of the Li-S performance in comparison to conventional cells. Even though the current state of the Li-S battery is still far away from the requirements for practical applications, based on our studies utilizing fluorinated solvents and additive can open a new window for engineering Li-S cells with much improved performance.