Beyond lithium ion [electronic resource] : developing high energy density lithium sulfur batteries
Keywords: Chemical Engineering.

The emerging applications of electric vehicles (EV) and grid scale energy storage are pushing the limit of energy storage technologies. To meet the US Department of Energy (DOE)'s targets for EV batteries and grid storage, battery chemistries beyond the current lithium ion systems are required. Among the many new chemistries studied, lithium sulfur battery is one of the most promising technologies that could have high specific energy and low cost. In this thesis, I will examine the main challenges in lithium sulfur batteries and present my study on using nanoscale engineering approaches to address the problems of both the sulfur cathode and the lithium metal anode. Lithium sulfur battery has a theoretical specific energy of around 2600 Wh/kg, around 10 times that of the current lithium ion battery technology. The large abundance of sulfur also means that battery cost can be significantly reduced by replacing the expensive transition metals used in conventional lithium ion batteries. However, sulfur is a highly insulating material and the intermediate discharge products lithium polysulfides can easily dissolve into the electrolyte. In the first part of my study, I will describe my work on using nanostructure materials to improve the sulfur cathode performance. By using nanostructure design, sulfur can be embedded into nanoscale conductive matrix, which significantly improve the sulfur utilization and reduce the polysulfide dissolution. We demonstrated that high specific capacity of around 1400 mAh/g could be achieved using the hollow carbon nanofiber encapsulated sulfur cathode structure. I will also present my study on the interfacial properties in the sulfur cathode, their potential effect on the initial capacity decay and our solutions to address the problem. The change in binding strength between the sulfur cathode and the conductive carbon matrix was observed using ex-situ¬ TEM study. We tackle this problem by functionalizing the carbon surface with amphiphilic polymers that allow anchoring of the polar lithium sulfides species to the non-polar carbon surface. We also used a patterned surface to confirm this phenomenon, by demonstrating controlled spatial deposition of lithium sulfide. Based on the study, we fabricated a hybrid electrode consisting of metal oxide particles decorated carbon nanofiber current collectors, which show marked improvement in stabilizing the sulfur cathode performance. For the anode side, I will present my research on using nanoscale engineering approach to improve the lithium metal anode. Lithium metal has long been considered the "holy grail" in lithium battery research, due to its high specific capacity and the lowest potential among all lithium anode materials. However, the problems of lithium dendrite formation and low cycling Coulombic efficiency have prevented lithium metal anode from successful application. By introducing a nanoscale interfacial layer of interconnected hollow carbon spheres onto the lithium surface, we demonstrate that lithium dendrite formation can be largely suppressed at a practical current density and the cycling Coulombic efficiency significantly improved. Our work provides a new direction in addressing the long-standing lithium metal problems. I will also talk about the semi-liquid flow battery design for grid storage, by paring lithium polysulfide catholyte with lithium metal. The energy density and power density can be potentially decoupled in the semi-liquid flow batteries. The catholyte (lithium polysulfide solution) can be stored in an external tank and pumped into the battery chamber on demand. The system has a very high energy density of around 170 Wh/kg (190 Wh/L), with an impressive cycle life of more than 2400 cycles at constant capacity charging of 200 mAh/g.