Developing metal-salen complexes as redox mediators for lithium-air batteries

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The drive towards a greener, sustainable future is leading to increased research into next-generation energy storage devices. In due course, it is anticipated for renewable energy sources, such as solar and wind, to replace non renewable fuels as the major means of consumer energy production. However, their intermittency requires the use of electrical grid energy storage systems. As such, the development of efficient, sustainable energy storage devices is essential to further a greener future. One of the most promising is the lithium air battery, which offers ample competition to the current market leader, lithium ion batteries. The coupling of a lithium metal negative electrode and porous carbon, air-breathing positive electrode leads to a high theoretical gravimetric energy density of 3500 Wh kg 1 an order of magnitude greater than current lithium ion technology. The use of more abundant materials, such as lithium and carbon, is a monumental shift away from using less abundant, and more expensive transition metals in electrode structures. Furthermore, the cell chemistry relies on the reaction of lithium with oxygen, the latter of which is readily available from air. However, despite its high theoretical performance and sustainable design, there are significant issues to address before lithium-air batteries are commercialised. They suffer from sluggish reaction kinetics of the discharge/charge reactions; poor solubility and electrical conductivity of the discharge product, lithium peroxide (Li2O2); and passivation of the positive electrode by Li2O2. To tackle the electrochemical difficulties encountered in lithium air batteries, redox mediators can be added to the electrolyte. These are homogenous catalysts, which are capable of transferring electrons from the positive electrode to intermediate species in solution. In doing so, the rate performance of the cell during discharge/charge is significantly enhanced, and many of the issues outlined can be alleviated. This project will embark on developing a coherent mechanistic understanding of how redox mediators operate in lithium-air batteries. To do so, a class of molecule, known as metal salen complexes, will be used throughout the project. This class has been specifically chosen due to its high versatility in functionalisation. By modifying the structure of the molecule, its various properties can be fine-tuned, namely: the solvent reorganisation energy, binding site, and redox potential. It is expected by altering mediator structures and its properties outlined above, it will be forced to adopt an inner or outer-sphere electron transfer mechanism. Throughout the project, metal salen complex derivatives will be synthesised and initially screened for chemical and electrochemical stability in lithium-air battery electrolytes. Techniques such as cyclic voltammetry and nuclear magnetic resonance spectroscopy will be used to obtain mediator redox potentials, and mediator/electrolyte degradation. Chemical reaction of Li2O2 with mediators will be assessed, where techniques such as UV-Vis spectroscopy will be employed - promising mediators will be taken forward. Scanning electrochemical microscopy (SECM) will be employed to obtain kinetic information of Li2O2 in the presence of synthesised mediators. This technique will be coupled with others such as EPR and Raman spectroscopy to monitor formation of new intermediates or bonds. Finally, cell cycling will be employed to assess the stability of mediator in the presence of lithium metal; shuttling of mediator between electrodes; and performance of mediators in standard lithium-air cells.