In the face of a growing climate crisis, the need to transition away from fossil fuels is evident. Over the past decade, we have seen electrification of the transportation sector and investment in renewable energy projects. In order to fully electrify our society, energy storage installments for the power grid will become a critical part of our infrastructure. Among many options, batteries represent the most promising technology for energy storage. Currently, Li-ion batteries dominate electric vehicles, consumer electronics, and now utility-scale batteries. However, there are numerous drawbacks to Li-ion chemistry that have prompted research in alternative batteries. The energy density of current Li-ion cells are limited by the added weight of intercalation host materials. Thus, in large-scale energy storage projects for the grid, the cost of Li-ion cells for long-duration storage is prohibitive compared to fossil-fuel-derived energy. Moreover, material mining for the rare-earth metals in these cells raise environmental and ethical concerns, and accelerated demand presents problems for the supply chain. Thus, we need a better, cheaper, and safer alternative.

Metal-air batteries are a broad class of electrochemical systems that address these considerations. These batteries transfer high-energy electrons from earth-abundant metals like Na, Zn, Li, Fe, or Mg to molecular oxygen typically on a carbon substrate. In these cells, a range of electrochemical reactions is possible. For example, in nonaqueous ether-based electrolytes, oxygen reduction results in the formation of lithium peroxide and sodium superoxide in Li-air and Na-air batteries, respectively. In alkaline Zn batteries, the discharge product is zinc oxide. Because each of these systems is unique and product selectivity is not necessarily evident for a given battery composition (i.e., peroxides, superoxides, oxides, and hydroxides have all been observed as discharge products in metal-air batteries), it is difficult to rely on electrochemical or spectroscopic methods alone to characterize metal-air battery electrochemistry. Therefore, throughout this work, we employ rigorous quantitative gas analysis and titration techniques developed to study metal-air batteries and gain insights into the fundamental processes that occur in them.

After the introductory section, Chapter 2 delves into work in nonaqueous Na-air batteries, comparing two different charging schemes. This study shows that a gentler, constant current/ constant voltage charging method prevents oxidation of electrode and electrolyte degradation products. Ex-situ chemical titrations of the cathode also indicate the buildup of sodium carbonate and other degradation species occurs during discharge, so ultimately the charging method does not improve the overall cell reversibility. We do, however, note that CC/CV charging prevents severe capacity fade when the cathode is made from a lower surface area graphitic carbon material. Thus, charging at lower overpotentials limits electrode passivation.

In Chapters 3 and 4, we discuss aqueous Zn electrochemistry, first in the absence of O2 and subsequently in a Zn-air battery. Because of its relative stability in aqueous electrolytes, Zn metal is an attractive next-generation rechargeable battery material. However, corrosion limits its cycling efficiency. This work discusses a new titration method based on quantitative mass spectrometry and ICP-OES to determine the stability of Zn in a promising aqueous electrolyte. When we test this aqueous electrolyte in a Zn-O2 cell, we observe zinc peroxide formation and quantify its reversibility. In addition, experiments with isotopically labeled O2 allow us to make mechanistic insights on the oxygen reduction reaction.

Finally, in Chapter 5, a novel Mg-O2 cell in an ethanol solvent demonstrates magnesium peroxide formation. Although the oxygen consumption rate indicates the electrochemical formation of this product is nearly ideal (i.e., occurs via a net 2 e- per O2 process), once it is formed, MgO2 is reactive with the electrolyte and electrode materials. Interestingly the magnesium peroxide that does not decompose is able to electrochemically oxidize during charge. This work demonstrates the first observation of a 2 electron oxygen reduction to form magnesium peroxide in a Mg-O2 battery, as well as the first demonstration of oxygen evolution from the oxidation of magnesium peroxide, albeit at less than 100% efficiency.

The goal of this thesis is to investigate three unique metal-air systems and generate insights using quantitative methods. We realize all of these metal-air chemistries will benefit from further research and development. We hope that with these rigorous techniques, we can better understand where to focus our efforts.