Universality in Nonaqueous Alkali Oxygen Reduction on Metal Surfaces: Implications for Li–O₂ and Na–O₂ Batteries

Nonaqueous metal–oxygen batteries, particularly lithium–oxygen and sodium–oxygen, have emerged as possible high energy density alternatives to Li-ion batteries that could address the limited driving range issues faced by electric vehicles. Many fundamental questions remain unanswered, including the origin of the differences in the discharge product formed, i.e., Li₂O₂ versus Li₂O in Li–O₂ batteries and NaO₂ versus Na₂O₂ in Na–O₂ batteries. In this Letter, we analyze the role of the electrode (electrocatalyst) in determining the selectivity of the discharge product through a tuning of the nucleation overpotential for a given electrolyte. On the basis of a thermodynamic analysis using density functional theory calculations, we demonstrate that the free energy of adsorbed LiO₂^* is a descriptor determining the nucleation overpotential for the formation of lithium peroxide, Li₂O₂, the primary discharge product in Li–O₂ batteries. Our analysis suggests that Au(100), Ag(111), and Au(111) are capable of nucleating Li₂O₂ with very low overpotentials. We also show that the free energy of adsorbed NaO₂^* is a descriptor determining the nucleation rate for sodium superoxide, NaO₂, the primary discharge product in Na–O₂ batteries. We explore trends in selectivity between 2e^– and 4e^– oxygen reduction for nucleating Li₂O₂ and Li₂O, respectively, and show that to a first approximation, the selectivity can be determined by a single descriptor, the free energy of adsorbed LiO₂^*. This is due to the existence of linear scaling between LiO₂^* and LiO* similar to that observed for OOH* and OH* for aqueous oxygen reduction. This analysis shows that for all materials that possess low nucleation overpotentials, the nucleation overpotential for 2e^– oxygen reduction is smaller than that for the 4e^– oxygen reduction. In the case of Na–O₂, we find that the trends in selectivity between nucleating NaO₂ and Na₂O₂ are determined by the free energy of adsorbed NaO₂^* and the reorganization energy associated with sodium-ion coupled electron transfer. This analysis provides a rational basis for the selection of the electrode (electrocatalyst) for tuning the nucleation and thereby potentially controlling the discharge product.