Electrochemical Performance Studies of Cathode Materials Based on Sulfur/Carbon Composites and Transition Metal Chalcogenides for Rechargeable Lithium- and Magnesium-Ion Batteries

Abstract

Rechargeable batteries are poised to make a major contribution to the emerging electric vehicle (EV) and large-scale energy storage markets. To that end, sulfur has emerged as one of the most promising cathode materials because of its low cost, non-toxicity, and high specific capacity and energy density of 1675 mAh/g and 2600 Wh/kg, respectively. In spite of these promises, the utilization of elemental S has proven difficult due to its reactivity in conventional electrolytes and the shuttle phenomena, in which sulfide ions are shuttled between the electrodes during the charge-discharge process, thereby significantly diminishing the performance of the cell. In order to mitigate some of the intrinsic problems associated with elemental S, this thesis attempted to utilize facile means to 1) confine S in a conductive matrix and 2) synthesize transition metal polychalcogenides (TMPCs) using either vanadium, manganese, or copper as the metal center for both Li and Mg cells. All the synthesize TMPCs were proven to be monosulfides using Energy Dispersive X-ray Spectroscopy (EDS) and powder X-ray Diffraction (XRD), with the exception of CuSx, which does form a TMPC but decomposes upon annealing to form CuS. Electectrochemically, S-activated carbon powder/fiber, VSx and MnSx cathodes exhibited poor voltammetric and charge-discharge performance. However, CuSx cathodes showed improved reversibility upon crystallization and voltage optimization with initial charge-discharge capacities ranging from 220-480 mAh/g in Li cells at a current density of 100 mA/g. The potential for further optimizing and possibly stabilizing CuSx at the TMPC phase offers a new means of utilizing S and its sulfide derivatives in rechargeable batteries.