Kinetics of phase transformations in lithium-sulfur batteries

Abstract

Sulfur is a promising positive electrode for lithium batteries with the potential to create the step-change improvement in energy density and cost needed for the widespread adoption of electric vehicles and renewable energy. However, lithium-sulfur batteries suffer from a number of challenges, among them poor rate capability resulting in part from a complex dissolution-precipitation mechanism which produces electronically insulating end members S₈ and Li₂S. Few studies have heretofore been performed on rate-limiting mechanisms in Li-S batteries, which must be elucidated in order to inform rational design of electrodes with high capacity and rate capability. Polysulfide solutions, intermediates in the electrochemical reduction of sulfur, are used for the first time to make an efficient, high energy density flow battery, enabled by a novel flow battery architecture using a percolating network of nanoscale conductive carbon. An extensive experimental study of exchange current density for redox of higher order polysulfide solutions and their ionic conductivity has been conducted. The type and amount of electrolyte solvent has been found to influence both of these. The second portion of this thesis characterizes the kinetics of Li₂S electrodeposition, which is responsible for three-quarters of the theoretical capacity of the sulfur cathode. Kinetics are found to be highly dependent on solvent choice in a manner similar to exchange current density. Furthermore, electrodeposition kinetics are found to slow considerably at the low electrolyte/sulfur ratios which are needed for high energy density and low cost. Materials such as conductive oxides can serve as nucleation promoters and help solve this challenge. The morphology of precipitates is found to be dependent on discharge rate, with large, discrete particles forming at low rates. A model was for describing 3-D electrodeposition of Li₂S under the influence of a soluble redox mediator which enables efficient utilization of conductive surface area and prevents passivation of conductive carbon with insulating Li₂S.