UCLA

High performance energy storage technologies are being developed to more effectively utilize renewable energies, as a result of the steadily expanding energy consumption for portable electronics and electric cars. Batteries with sulfur cathodes have attracted a lot of attention due to their inexpensive cost and great charge storage capacity. Polysulfide shuttling, the main issue in metal sulfur batteries, results in significant sulfur loss and quick deterioration. Despite significant efforts being devoted to improving their performance, the chemical pathway and shuttling mechanism in lithium/sodium sulfur batteries are still up for debate. In order to better mitigate the polysulfide shuttling issue, this work intends to examine the sulfur reaction mechanism and deposition behavior from a fundamental perspective. Additionally, effective catalysts can offer adequate active sites to improve the kinetics of sulfur reactions during charge/discharge, speeding up the conversion of polysulfides and reducing polysulfide accumulation that results in shuttling. To determine how catalysts affect the reaction network and battery performance, various catalysts are examined. This information could be employed to guide the rational design of the catalysts for sulfur cathodes in the future. In this work, Chapter 1 provides a background introduction for the tasks that follow. The behavior of sulfur deposition and basic reaction mechanisms such as the shuttling mechanism and molecular pathway in lithium sulfur batteries, were studied in Chapter 2 and Chapter 3 using heteroatom doped holey graphene frameworks as the modeling system. After building a systematic methodology to comprehend the reaction mechanism by combining electrochemistry (cyclic voltammetry), in situ Raman, and density functional theory, we further analyzed different catalysts in Chapter 4 and Chapter 5, including different heteroatom doped holey graphene frameworks and nitrogen doped mesoporous carbon materials. Finally, in Chapter 6, we took a step forward from lithium to sodium, and demonstrated a useful technique for separator modification to reduce polysulfide shuttling in sodium sulfur batteries. These findings attempt to address the shuttling issue by studying the fundamental reaction mechanism in metal sulfur batteries and would be very helpful as a blueprint for creating the next generation high energy density batteries.