Molecular Engineering of High-Performance Lithium Sulfur Batteries

Abstract

The rechargeable Lithium-Sulfur (Li-S) battery is an attractive platform for high-energy, lowcost electrochemical energy storage due to the low cost of sulfur ($0.02/g) and the high theoretical energy density (2500 Wh/kg or 2800 Wh/L) of the sulfur cathode. Practical Li-S cells are limited by several fundamental issues, which derive from the complex solid-state and solution chemistry of the electrodes and electrolyte, such as the low conductivity of sulfur species, the dissolution and transport of long-chain lithium polysulfides (LiPS) into the electrolyte, and instability of the anode during recharge. This dissertation focuses on three critical aspect of the lithium sulfur battery aimed towards building high-performance lithium sulfur battery. Firstly, to sequester LiPS by creating species in the cathode that bind specifically with LiPS. Three studies were carried out under this topic. The first study was to find out the ideal polysulfide binding functional groups by both theoretical analysis and experimental tools, and amine group was targeted due to its high binding energy with LiPS, stability in the cell and wide availability. The second study then applied this idea in a more efficient and applicable way, by stabilizing LiPS on amine functionalized carbon nanotube. The third study found that the inorganic materials TiS2 also has high binding energy for LiPS, thus a hybrid cathode of TiS2 and sulfur was synthesized, where two species work synergistically to give higher capacity. The second method is to localize the dissolved LiPS by creating an ionic shielding for LiPS. A high-transference number membrane containing sulfonate groups was designed, in which the negatively charges on the membrane reject sulfur species (Sn 2-) due to the repulsive electrostatic interactions. Such unique characteristics are attractive in modifying ion transport within the cell and improving the battery performance. The last part of the dissertation will talk about the protection of lithium metal anode in lithium sulfur battery. We report on the chemistry and interfacial properties of artificial SEI films created by in-situ reaction of a strong Lewis acid AlI3, Li metal, and aprotic liquid electrolytes. We find that these SEI films impart exceptional interfacial stability to the Li metal anode.