Characterizing mass transport and crossover reactions in Li-air batteries
Lithium air batteries (LABs) promise extremely high gravimetric energy densities. Similar to fuel cells, mass transport of redox active species, such as dissolved oxygen gas or redox mediators, is one of the challenges faced by LABs. While increasing mass transport increases LAB cell performance, it can also increase crossover of the redox active species from one electrode to the other, resulting in capacity loss and interfacial reactions. The interplay between mass transport and crossover of redox active species from one electrode has not been extensively discussed in literature and requires detailed characterization. In this work, mass transport effects and crossover rates were characterized using magnetic resonance methodologies, electrochemical techniques, and computational modelling. The crossover reactions that arise when redox active species migrate to the Li metal anode and how they affect the solid-electrolyte interphase (SEI) were then studied. Nuclear magnetic resonance (NMR) spectroscopy was used to measure oxygen mass transport in LABs in Chapter 3, capitalizing on the paramagnetic properties of O2. This method has not been previously used to measure O2 in LAB electrolytes and provides direct experimental evidence of the O2 coordination environment predicted by previous computational studies. The dissolved O2 were quantified during LAB operation using the NMR methodology. In addition, an increase in discharge capacity and decrease in overpotentials was observed when increasing O2 mass transport, both in LAB Swagelok and flow cells. Computational modelling supports these observations and suggests improved electrode utilization when O2 mass transport is improved. However, crossover of the dissolved oxygen was also observed in both computational and experimental studies, whereby O2 diffuses from the gas electrode over to the anode. Due to the reactivity of Li metal, the crossover of O2 changes both the SEI composition as well as the Li metal deposition morphologies as discussed in Chapter 4. Redox mediators are soluble catalysts employed to decrease overpotentials in LABs and promote solution-phase reactions. The mass transport and crossover of redox mediators in LABs were then studied in Chapter 5, focusing on reactions of TEMPO (2,2,6,6-tetramethylpiperidinyloxyl), which is a redox mediator used on charge. Due to its paramagnetic properties, operando magnetic resonance and computational methods were used, similar to the previous chapters studying dissolved O2. Again, an improvement in overpotentials when increasing the mass transport was observed while at the same time increasing the crossover and redox shuttling reactions. The reactions of TEMPO with the Li metal SEI were subsequently characterized in Chapter 6 with the aim of both understanding possible degradation reactions and using TEMPO as a probe to measure SEI properties. This thesis concludes with outlooks on methods to limit crossover reactions, engineering the Li metal SEI, and new methods to characterize the SEI.
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