(418c) First-Principles Modeling of Discharge Product Surface Thermodynamics in Na-O2 Batteries

With the current climate crisis, the growing need for utilization of renewable energy resources necessitates the development of improved electrochemical storage devices. One technology that shows promise in this regard is aprotic alkali metal-O₂ batteries (Li/Na-O₂), which have greater than threefold higher theoretical gravimetric energy densities than state-of-the-art Li-ion batteries¹. With a greater elemental abundance of sodium and low observed overpotential losses, Na-O₂ batteries have become a focus of research interest². Despite recent efforts, however, poor cell lifespans continue to plague Na-O₂ cells (≈10s of cycles), remaining well below commercial viability³. A contributing factor to these poor cell lifespans is an inherent instability (parasitic reactivity) at the cathode between the desired discharge product (NaO₂) and the organic electrolyte, leading to undesirable side product formation and a rapid decline in cell capacity. The formation mechanism for these side products is unknown, and one hypothesis is that the surface of the discharge product first undergoes an undesirable dissolution into the electrolyte before subsequently forming the side products⁴. Thus far, practical solutions to improve stability have remained elusive. By elucidating the molecular-level mechanisms of these deleterious processes, using both theory-driven and experimental approaches, it may be possible to overcome these bottlenecks.

The goal of the present work is to couple computational and experimental studies to elucidate key molecular-level mechanistic insights of the parasitic reactivity occurring at the surface of the NaO₂ discharge product. We utilize Density Functional Theory (DFT) calculations to study the surface thermodynamics of the NaO₂ discharge product, of which the knowledge significantly trails that of the closely related Li-O₂ battery chemistry. We elucidate thermodynamically relevant surface facets and surface layer stoichiometries by constructing an ab-initio surface phase diagram of the NaO₂ discharge product. We discover an inverse relationship between coordination and stability of surface terminations under vacuum. We further observe a competing energetic effect between smoothness and packing density at the surface layer. In addition, we identify a representative set of surface features (including terraces, steps, and defects) for use in subsequent studies of parasitic reactivity mechanisms at the NaO₂ discharge product surface.

As an extension of the surface phase diagram, an ab-initio mechanistic surface dissolution analysis is applied to the identified surface terminations, probing the effect of coordination on the thermodynamics governing surface dissolution. In this analysis, the thermodynamic energy barriers are compared between surface terminations, and as with the surface stability, we discover that there is an inverse relationship between coordination and the thermodynamic barrier for surface dissolution. In considering both stoichiometric and off-stoichiometric pathways, we find dissolution will follow a concerted pathway, such that stoichiometric NaO₂ units will dissolute from the surface, rather than as individual Na⁺ and O₂^- ions. In closing, we comment how the current work lays the groundwork for future combined computational-experimental studies and how they may be used to propose practical design changes.

References:

(1) Samira, S.; Deshpande, S.; Greeley, J.; Nikolla, E. Aprotic Alkali Metal–O ₂ Batteries: Role of Cathode Surface-Mediated Processes and Heterogeneous Electrocatalysis. ACS Energy Letters 2021, 6 (2). https://doi.org/10.1021/acsenergylett.0c02506.

(2) Kwak, W.-J.; Rosy; Sharon, D.; Xia, C.; Kim, H.; Johnson, L. R.; Bruce, P. G.; Nazar, L. F.; Sun, Y.-K.; Frimer, A. A.; Noked, M.; Freunberger, S. A.; Aurbach, D. Lithium–Oxygen Batteries and Related Systems: Potential, Status, and Future. Chemical Reviews 2020, 120 (14). https://doi.org/10.1021/acs.chemrev.9b00609.

(3) Sun, Q.; Liu, J.; Xiao, B.; Wang, B.; Banis, M.; Yadegari, H.; Adair, K. R.; Li, R.; Sun, X. Visualizing the Oxidation Mechanism and Morphological Evolution of the Cubic‐Shaped Superoxide Discharge Product in Na–Air Batteries. Advanced Functional Materials 2019, 29 (13). https://doi.org/10.1002/adfm.201808332.

(4) Kim, J.; Park, H.; Lee, B.; Seong, W. M.; Lim, H.-D.; Bae, Y.; Kim, H.; Kim, W. K.; Ryu, K. H.; Kang, K. Dissolution and Ionization of Sodium Superoxide in Sodium–Oxygen Batteries. Nature Communications 2016, 7 (1). https://doi.org/10.1038/ncomms10670.