Electrochemical studies of oxygen reduction for aprotic lithium-oxygen batteries

Abstract

Understanding oxygen electrochemistry lies at the heart of enabling many advanced energy storage and conversion technologies such as fuel cells, electrolyzers and metal-air batteries. Aprotic Li-02 electrochemistry is receiving much attention in this regard, as the Li-0 2 battery theoretically offers higher energy densities than conventional Li-ion systems at potentially lower cost. This thesis explores the relationship between the energetics of 02 redox processes, and nucleation, growth, and reactivity of Li-O products in Li-02 batteries. Using a combination of rotating disk techniques and first principles calculations, we first assess the influence of 02- and Li+ ion solvation on the energetics of 02/02- and Li+/Li redox processes. By combining these results with measurements of the redox potential of the Li+-02 reaction intermediate, we show that both the coupling strength and solubility of the Li-0 complex are rationalized by the combined solvation of Li+ and 02- ions, with greater combined solvation increasing solubility but decreasing coupling energy, respectively. We next extend these results to studying the influence of applied potential and Li'-0 solvation on the participation of soluble and solid species during Li202 growth, using the rotating ring disk electrode (RRDE) and electrochemical quartz crystal microbalance (EQCM) methods, respectively. As the applied potential increases, the reaction mechanism for Li20 2 formation switches from solution to surface-mediated, with the most likely pathways being Li+-02- disproportionation and 2e- transfer to 02, respectively. These insights are applied to understanding nucleation and growth of Li 20 2 in Li-02 batteries, using high surface area carbon-based electrodes as model systems. We first report, for the first time, the formation of large ~ 300 nm donut-shaped particles of Li20 2 at high applied potentials during Li-02 discharge, and smaller particles (< 50 nm) at lower potentials. The existence of these disparate potential-dependent growth morphologies of Li202 strongly supports the predominance of potential-dependent reaction mechanisms, as hypothesized based on RRDE and EQCM results. We also show, however, that while increasing Li+-02- solvation promotes higher discharge voltages, Li+-02- solvation does not scale with Li202 particle size, particularly at low applied potentials. We therefore proposed a classical growth model of Li202 particle size based on Li202 reactivity with the electrolyte and Li202 supersaturation. Lastly, the influence of aging and electrolyte pKa on discharge product chemistry was explored. Aging electrochemically formed Li202 in a dimethyl-sulfoxide-based electrolyte promoted its decomposition to LiOH, while LiOH was found to be more likely to form upon discharge with decreasing effective pKa of water in the electrolyte, indicating higher proton availability.