Mechanistic Insights into Oxygen Reduction Reactions in Non-Aqueous Metal-Air Batteries
Authors: Rawal Saurin Hiren , McKee William C. , Xu Ye 
Published: February 2020

Non-aqueous metal-air batteries have generated strong research interest because they possess much greater theoretical gravimetric energy storage density than today’s rechargeable battery technologies.1-4 This has re-kindled research into the fundamentals of oxygen reduction reaction (ORR) in non-aqueous electrolytes because they are the central electrochemical processes at metal-air cathodes. Understanding the reaction mechanisms is critical to improving cycling efficiency and making rechargeable metal-air batteries a reality. ORR in these metal air batteries typically lead to discharge products that are either metal superoxides or metal peroxides. The discharge products can grow either through a surface mediated mechanism5 that produces the discharge product directly on the electrode or a solution mediated mechanism6-7 which leads to the precipitation of nanoparticles. In this talk, we will present the results of extensive DFT-based theoretical studies of ORR by different cations (M, where M=H, Li, Na, and K) on Au surfaces. The computational electrode approach allows us to shed light on the mechanism of surface-mediated ORR, which in a stable organic solvent such as DMSO always involves the superoxide anion (O2
-) as the first reduction product within one volt of the relevant open cell potential. This is followed by the direct reduction of O2 by cation/electron transfer to form MO2 and M2O2 species at lower potentials. We will show that in modeling the electrochemical stability of the adsorbed molecular superoxide species, the effects of the interfacial electric field and solvent molecules need to be included to avoid errors on the order of a few tenths of a volt. The field effect can be readily approximated by the first-order Stark effect. At the same time, the formation and solvation of O2
- open up pathways to thermochemical ORR in the solution phase, which leads preferentially to the formation of solid Li2O2 vs. solid NaO2 and KO2 at small overpotentials. We propose, based on additional theoretical calculations, the reaction pathways for the formation of alkali superoxide and peroxide in several solvents that explain the observed product selectivity.

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