title: Non-aqueous Spectroelectrochemistry of Dioxygen for Alkali Metal-Oxygen Batteries
Creators: name: Aldous, IM
Publisher: name: University of Liverpool Repository
Year: 2017

description: The rechargeable non-aqueous alkali metal-oxygen batteries (also referred to as ‘metal-air’ batteries), such as lithium and sodium-air, offer improved energy storage opportunities over other existing battery systems. Some severe technological problems need to be addressed for the successful development of this system into a commercially viable device. The development of efficient cathode materials and stable electrolytes that allow the reversible electrochemical reduction of oxygen is an area where significant research is necessary. For example, lithium oxygen (Li-O2) cells generate electrical current when lithium ions react with reduced oxygen at the negatively polarised electrode during discharge, the as formed LixO2 reaction products can be reversed during the charging cycle. The problem is that the discharge products (superoxides and peroxides of lithium and sodium) and the metastable intermediates formed are highly reactive, and conventional battery electrolytes easily degrade in their presence. For a greater understanding of the oxygen reduction mechanism and for the direct detection of the intermediates and reaction products formed during the oxygen reduction process, surface sensitive spectroscopy techniques can be utilised. The development of in situ spectroelectrochemical environments for such a purpose is a technical challenge that is unique to each spectroscopy. The information herein has been obtained through in situ surface enhanced Raman spectroscopy (SERS) and UV/Vis spectroscopy in conjunction with common electrochemical techniques. The Raman effect is weak and careful preparation of the surface is necessary to provide enhancement of Raman signals from surface present species during O2 reduction. UV/Vis provides different challenges in the preparation of a sealed electrochemical environment, which the cell and working electrode are transparent to UV/Vis light. Chapter 4 describes the how the size of the supporting salt (tetra alkyl ammonium) cation influences both the electrokinetic and voltage hysteresis between oxygen reduction and evolution reactions (ORR and OER). This was found to be due from the rearrangement of the cation at the electrode interface, under potential control. The use of in situ SERS was used in Chapter 5 to study the effect of solvent on the ORR in the presence of Na+. The findings conclude a solvent dependent mechanism whereby highly solvating solvents like amides and sulfoxides detect only NaO2, and nitriles and glymes detect Na2O2 as respective discharge products. A comparative study of the effect of alkali metal cation size is discussed in Chapter 6 showing the changes in reversibility of O2 with cations of increased ionic radius. Larger cations, such as Na+, K+ and Cs+ display quasi-reversibility, as opposed to the irreversibility of Li+. Finally in Chapter 7 the detection and characterisation of the electronic state of oxygen released from electrooxidised Li2O2, NaO2 and KO2 was obtained through in situ UV/Vis spectroscopy. Li2O2 producing a higher percentage of reactive singlet oxygen per mass of oxidised discharge product compared with NaO2 and KO2. Within this chapter initial findings of a known singlet oxygen sink, 1,4-diazabicyclo[2.2.2]octane (DABCO), is discussed as a possible electrolyte additive.