Redox Mediators and Electrolytes for High-Performance Li-O₂ Batteries

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The predicted theoretical energy density of a Li-O₂ battery is close to that of gasoline, which makes it one of the most promising forms of energy storage. To allow Li-O₂ batteries to achieve their full potential, multiple issues need to be overcome. One issue is the large charge and discharge overpotentials caused by sluggish kinetics of the reactions within the battery and the solid, insulating nature of the discharge product Li₂O₂. Applying soluble electrocatalysts (redox mediators or RMs) that aid the formation and decomposition of the Li₂O₂ can help to reduce the overpotentials. Other challenges associated with the Li-O₂ battery include instability, leakages, flammability and volatility of commonly used aprotic electrolytes. In this thesis, two series of compounds are investigated as solutions to these issues: (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) substituted imidazolium ionic liquids (TEMPOImILs) with different lengths of alkyl linkage between the TEMPO and imidazole moieties and with either H or CH₃ on the 2-position of the imidazole ring, and phenyl nitronyl nitroxides (RPTIOs) with varied substituents on the benzene ring. The compounds were characterized using NMR and FTIR together with elemental analysis, TGA, DSC and EIS for TEMPOImILs. Redox processes were studied using cyclic voltammetry and, for RPTIOs, UV-visible spectroelectrochemistry. The effect of the RMs on the battery performance was tested by assembling and cycling Li-O₂ batteries. For RPTIOs, cathodes were analysed after cycling of the batteries using SEM and XRD, and battery tests in argon were conducted to determine if there was a contribution to the capacity from redox shuttling of the RM. It was found that TEMPOImILs can serve both as charging RMs and safer electrolyte solvents for Li-O₂ batteries, while RPTIOs not only catalyse the charge process but also provide some improvement of the discharge performance. The length of alkyl chain in TEMPOImILs did not have any noticeable effect on the battery performance, whereas TEMPOImILs with 1-methylimidazolium cations provided substantially longer cycle life than those with 1,2-dimethylimidazolium cations. The charge potentials of the batteries with RPTIOs with the electron-donating groups were the lowest, showing that altering the structures of nitronyl-nitroxide-based RMs can directly affect battery performance. Overall, the use of the RMs in combination with other measures discussed in the thesis can lead to high-performance Li-O₂ batteries.
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