Sulfur Modification for Battery and Optical Applications

Abstract

Understanding and developing a new sulfur chemistry give great opportunities for the synthesis of advanced functional materials, because of the strong relationship between sulfur and the environment on the Earth, as well as the economic demands to address surplus problem of sulfur from oil refinery industry. Although there are a number of superior functionalities with sulfur, such as high refractive index and electrochemical activity, its utilization and processing are challenging due to the poor physico-chemical properties resulted from its orthorhombic molecular crystalline nature. There have been several efforts to incorporate sulfur into amorphous polymeric materials, but simple preparation methods for high sulfur content polymers are still lacking. In this thesis, extremely simple chemistry to prepare high sulfur content copolymers is introduced, and various kinds of their applications, including IR optical devices and cathodes for Li-S batteries, are demonstrated with the significantly improved processibility and performances of the sulfur copolymers. Moreover, based on these sulfur copolymers, novel hybridization strategies are proposed for the enhanced functionalities, and finally, a couple of conformal coating methods are reported to stabilize the surface of sulfur copolymers when they are utilized in electrochemical electrodes. After a brief introduction on unique characteristics and challenging issues of sulfur, in chapter 1, the novel synthetic chemistry of sulfur copolymers, and their properties are discussed in chapter 2, followed by facile processing the polymers for the various applications. By the inverse vulcanization, high sulfur contents copolymers were synthesized, and their completely amorphous and viscous nature was analyzed. A melt process was conducted with these amorphous polymers using PDMS imprint nano pattering technique, and solution process was also possible due to the solubility of the copolymers at increased temperature. By using these processing methods, the sulfur copolymers were utilized in various applications, such as IR lens, transistors, photonic crystals, and Li-S batteries. Superior functionalities of the copolymers were attributed to the intrinsic properties of elemental sulfur. Hybridization of sulfur copolymers with functional nanomaterials were introduced in chapter 3, with a novel bi-functionality of oleylamine which is a comonomer for the sulfur copolymeric matrices. Because the double bond in the middle of oleylamine links to linear polysulfide, and amine functional group is attached the surface of inorganic nanoparticles, the nanocomposites with sulfur copolymers were prepared by adding inorganic salts into the copolymer mixture. The reaction mechanism facilitating simple one-pot synthesis of the sulfur copolymeric nanocomposites were revealed by various characterization tools. In chapter 4, the preparation of graphene sulfur copolymeric nanocomposites are exhibited based on similar dual reactivity of oleylamine. Especially, significant enhancement in battery performance was demonstrated with these nanocomposites due to the intense electrical contact facilitated by graphene within the copolymers. The surface modification of sulfur copolymers is discussed in chapter 5, more focused on the potential application in Li-S batteries. Because the interface interaction between sulfur cathode and electrolyte is a serious problem in Li-S batteries, direct deposition of protecting layers on sulfur cathode was demonstrated with drastically enhanced cycle performance of the polymer coated sulfur cathodes. Polymer conformal coating methods such as layer-by-layer (LbL) deposition on top of the sulfur cathode are introduced to further increase the capacity retention of Li-S batteries.