Exploring the Redox Processes and the Associated Chemical Reactions in Next Generation Rechargeable Li-O2 Batteries

Abstract

The exceptionally high specific energy of Li-O2 rechargeable batteries makes them promising as a next-generation energy storage technology, beyond the conventional intercalation battery systems such as the lithium-ion batteries. However, practical realization remains elusive, primarily hindered by sluggish oxygen redox kinetics and an unclear understanding of the mechanisms related to the core redox processes. The use of electrode catalyst often leads to surface product formation, clogging the porosity of gas diffusion layers, and hence affecting device performance. A liquid-based redox mediator (RM) avoids surface related challenges by shifting the reaction to the solution phase. In this thesis, we have explored bioinspired porphyrin-based molecules, viz., first row transition metal phthalocyanines, as redox mediators to comprehend how the d-orbital occupancy affects battery performance and proposed redox mediation mechanisms for these complexes using DFT simulations. Using operando spectroscopy, complemented with DFT simulations, we have demonstrated that the orientation/interaction of (dis)charge/parasitic products on the M-N4 motif of the redox mediator is crucial for controlling redox efficiency. In addition to these fundamental studies, we have explored mass transport properties and interphase chemistry (SEI/CEI) to demonstrate the polarization and rate of the bottleneck reaction of Li-O2 batteries. We have also studied the solvation sheath of (dis)charge products in electrolytes for solution and surface reaction pathways in the presence of RM via spectroelectrochemistry and DFT calculations. In the context of sustainability and scalability for grid-scale energy storage systems, a portion of this thesis demonstrated aqueous zinc ion batteries. We have used a quinone-based redox-active cathode host and phthalocyanine-coated zinc anode to understand storage mechanisms and metalation kinetics. Overall, this thesis demonstrates a comprehensive investigation of various aspects of battery function and a fundamental understanding of the criteria for catalyst selection, electron transfer at the electrode/electrolyte interface, ion transport in electrolyte, and the solvation dynamics of counter ions with spectroscopy combined with electrochemistry.