Vailaya, Ganesh
(2025)
Developing an understanding of redox-shuttle mediators in
lithium-metal batteries.
PhD thesis, University of Nottingham.
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Abstract
The lithium-ion battery (LIB) is widely acknowledged as the state-of-the-art battery technology in use today and is used in consumer electronics, stationary storage, and electrified transportation. However, there is a drive towards developing technologies with higher energy densities. The lithium-sulfur (Li-S) and lithium-air (Li-air) batteries are two of the most promising next-generation technologies for high-energy applications, due to their high theoretical gravimetric energy densities, low raw material costs, and reduced environmental impact. Despite these potential advantages, both devices are still beset with difficulties, one of the more severe being the formation of insulating products during discharge of the battery. This leads to high overpotentials on charge, low active material utilisation, culminating in poor coulombic efficiencies and rapid cycle life decay. Redox mediators (RMs) have been touted in both systems as a solution to oxidise the insulating discharge products and overall improve the sluggish kinetics of the discharge/charge reactions.
The first half of this thesis, particularly Chapter 3, will focus on developing a structure-property relationship between redox mediators and their ability to target specific reactions in the Li-S battery. Their interaction with sulfur, lithium sulfide, lithium polysulfide and lithium metal will be better understood using a variety of techniques. Using liquid chromatography demonstrates there is a relationship between the redox potential of a mediator and its ability to drive certain sulfur redox processes. For example, high potential mediators are better able to drive Li2S oxidation to form S8, whereas mediators with lower redox potentials drive S8 reduction to lower chain polysulfides. This study also emphasises the redox potential alone is not a reliable indicator of the performance of the mediator, as lithium metal is shown to react with these molecules, leading to undesirable side effects in the battery. This has been proven using UME voltammetry, where some mediators undergo irreversible decomposition in contact with lithium.
In the second half of this thesis, in Chapter 4, a new redox mediator for the Li air charge process will be proposed. This molecule is capable of driving Li2O2 oxidation at potentials well before solvent degradation onset. In the literature, using Marcus theory, it is also suggested to have high kinetics in oxidising Li2O2 as its redox potential lies in the appropriate range. The importance of structural stability for RMs will be highlighted, as a simpler redox mediator structure is shown to undergo deprotonation in the cell, which leads to rapid capacity fade and premature cell death. Therefore, design principles will be established to create more stable, effective redox mediators in the desired voltage window. A variety of techniques, such as differential electrochemical mass spectrometry, cyclic voltammetry and galvanostatic cycling will be used to justify mediator performance.
In Chapter 5, a novel gas-handling demonstrator system will also be developed, where the effect of flow rate, gas composition and partial pressure of O2 will be explored on a closed Li-air flow cell. It is shown these factors lead to significant consequences for a practical Li-air cell, as capacities of the cell are severely impacted on moving to lower flow rates and partial pressures of O2.
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