| Summary: | <p>It is becoming increasingly clear that current lithium-ion battery technology does not possess a gravimetric energy density great enough to cater to the demands of our growing energy needs. By dispensing with the heavy cathode materials, metal-oxygen battery systems have far greater gravimetric energy densities, potentially enabling them to supersede lithium-ion technology and make the complete electrification of transport a viable endeavour. </p>
<p>Metal-oxygen batteries, however, are not without their limitations. Lithium-oxygen cells suffer from very large charging overpotentials, high levels of decomposition of cell components at all points of the cell cycle (particularly so on charge, due to the presence of singlet oxygen), and insoluble and insulating discharge products, which passivate the cathode leading to cell death. Potassium-oxygen cells, whilst displaying superior stability and smaller charging overpotentials, also form passivating discharge products. This thesis addresses a number of the issues and questions surrounding lithium- and potassium-oxygen cells.</p>
<p>Chapter 3 investigates the relationship between charging potential and the quantity of lithium carbonate arising from both ethereal electrolyte and carbon cathode decomposition. Use of operando electrochemical mass spectrometry and fluorescence spectroscopy reveals that formation of singlet oxygen only occurs above 3.31 V, with minimal quantities formed between 3.31 – 3.45 V. Low potential redox mediators are demonstrated to minimise cell decomposition by suppressing the charge potential below this level. </p>
<p>Chapter 4 details the synthesis and characterisation of novel redox mediators, identified through the use of DFT. The reaction kinetics of redox mediators with lithium peroxide are investigated through the use of scanning electrochemical microscopy, revealing that said reaction kinetics are governed by Marcus theory.</p>
<p>Chapter 5 demonstrates the use of phenol as a protic phase transfer catalyst to increase the discharge capacity of potassium-oxygen cells, with operando electrochemical mass spectrometry revealing an electron to oxygen ratio of 1.02 on discharge, indicative of potassium superoxide formation.</p>
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