(Invited) First-Principles Modeling of Degradation Mechanisms of the NaO2 Discharge Product in Na-O2 Battery Cells
Authors: Von Gunten Alex , Velinkar Kunal , Obi Onyema , Nikolla Eranda , Greeley Jeffrey 
Published: December 2024

The current climate crisis necessitates the development of improved electrochemical storage devices, such as aprotic alkali metal-O2 batteries (Li/Na-O2), which have greater than threefold higher thermodynamic gravimetric energy densities than Li-ion batteries. With a greater elemental abundance of sodium and low observed overpotential losses, Na-O2 batteries have become a focus of research interest. However, despite recent efforts, poor cell stability continues to plague Na-O2 cells, inhibiting commercial viability. A contributing factor to the poor cell stability is a degradation of the desired NaO2 discharge product at the cathode, resulting from undesirable interactions with the organic cell electrolyte under periods of cell idling where there is no charge transfer between the two electrodes. These undesirable interactions are hypothesized to degrade the NaO2 discharge product through two main pathways: (i) chemical transformation to hydrated sodium-oxygen species resulting from a chemical reaction with the electrolyte molecules and (ii) deleterious dissolution into the electrolyte. Thus far, practical solutions to improve stability have remained elusive, but elucidating molecular-level mechanistic details of these deleterious processes, using combined theory-driven and experimental approaches, could accelerate the discovery of practical design changes to overcome these stability bottlenecks.

The goal of the present work is to couple computational and experimental studies to elucidate key molecular-level mechanistic insights of the undesirable interactions between the cell electrolyte and the surface of the NaO2 discharge product. We utilize periodic Density Functional Theory (DFT) calculations and ab
initio molecular dynamics (AIMD) simulations to probe the two main NaO2 discharge product degradation mechanisms. To model the surface of the NaO2 discharge product, which in the case of high donor number electrolytes is deposited in a cubic confirmation, the cube faces and edges are represented as terrace and step facets, respectively, to capture the fact the undercoordinated cube edges could exhibit a different surface chemistry than the relatively pristine cube faces. At these two surface terminations, we elucidate the energetics that govern the degradation mechanisms of the NaO2 discharge product under cell idling conditions, as described below.

To probe the chemical transformation of the NaO2 discharge product towards hydrated sodium-oxygen products, we elucidate the energetics of hydrogen abstractions from the electrolyte molecules to the NaO2 surface. With regards to the hydrogen coverage on the NaO2 surface, we find that there exists a plateau in the abstraction thermodynamics for the uptake of hydrogen, and at high coverages, the terrace NaO2 surface undergoes a series of favorable surface reconstructions towards sodium hydroxide species. Interestingly, we find that the free energy of activation for hydrogen abstraction from the electrolyte is not strongly influenced by the surface termination (terrace vs. step), as one would typically expect based on coordination number arguments. For the study of the deleterious dissolution of the NaO2 discharge product, we elucidate the energetics of elementary material loss mechanisms from the surface. We find that it is thermodynamically favorable for the surface to dissolute as individual NaO2 units, maintaining the NaO2 stoichiometry of the surface, with the thermodynamic surface penalty to undergo dissolution for the step facet being approximately ½ of the penalty for the pristine terrace facet. Further, we find the free energy of activation for dissolution of the step facet to be approximately ½ of the energy barrier of the terrace facet, suggesting that, unlike in the hydrogen abstraction process, the NaO2 cube edges drive the undesirable dissolution during idling. 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.