From Solvate to Cell: A Molecular Engineering Approach to the Lithium-Sulfur Battery

Abstract

The lithium-sulfur (Li-S) battery system has been widely lauded as a candidate to replace lithium-ion (Li-ion) technology in weight-critical applications such as electric vehicles, aerospace missions, and personal electronics. The attractiveness of this design comes from its titular active materials, which can theoretically store >2300 Wh/kg in comparison to ~400 Wh/kg for Li-ion. Additionally, sulfur is cheap and earth-abundant, reducing the potential cost and environmental impact of the system. However, despite decades of research, a commercially-competitive Li-S battery remains elusive. This is largely due to functional challenges such as poor conductivity, electrolyte-soluble reaction intermediates, and anode surface passivation, which reduce the capacity, efficiency and cycle life of practical cells. Although many of these issues are specific to Li-S chemistry, contemporary research often borrows heavily from Li-ion conventions in attempting to address them (to varying degrees of success). Alternately, Li-S battery design may be approached from a “bottom-up” or “rational molecular design” perspective, in which the materials, fabrication techniques, and analytical methods are designed de novo based on the unique functional demands of the system. This doctoral dissertation broadly details my efforts to develop and study free-standing gel electrolytes for the Li-S system, successfully integrate them into working devices, and demonstrate their effect on cell performance. Chapter 1 introduces the motivating factors behind this research, basic Li-S operating principles, and major functional challenges. Chapter 2 reviews the existing literature on Li-S chemistry and cell designs, including common strategies to improve cell performance. Chapter 3 presents the design, fabrication, and electrochemical properties of solvate ionogel (SIG) electrolytes based on solvate ionic liquid Li(G4)TFSI and functional poly(ethylene glycol) methacrylates. Chapter 4 explores the structure-property relationships of SIGs with regards to solvent additives and polymer molecular structures. Chapter 5 details the development of quasi-solid-state (QSS) Li-S battery designs through integration of SIGs into sulfur/carbon composite cathodes and porous polypropylene separators, the electrochemical performance of QSS cells, and the origin of their cycling characteristics. Finally, Chapter 6 summarizes these results and their impact, concluding with suggestions for future research that may build upon the work herein.