Role of Disorder in NaO₂ and Its Implications for Na–O₂ Batteries

There is a need for more energy dense batteries, and Na–O₂ batteries have emerged as an attractive option. In this work, we present a density functional theory (DFT) study utilizing the Hubbard U correction to probe structural and magnetic disorder in NaO₂, the primary discharge product of Na–O₂ batteries. We show that NaO₂ exhibits a large degree of rotational and magnetic disorder. A three-body Ising model is necessary to capture the subtle interplay of this disorder. Our Monte Carlo (MC) simulations demonstrate for the first time that energetically favorable, ferromagnetic (FM) phases near room temperature consist of alternating bands of O₂ dimers oriented along two of four cubic cell body diagonals. Using hybrid density functional theory calculations, we find that bulk structures are insulating, with a subset of FM structures showing a moderate gap (<2 eV) in one spin channel. The insulating nature of NaO₂ implies that growth of the discharge product is most likely occurring due to the solution mechanism pathway involving a chemical dissolution of NaO₂ into Na⁺ and O₂^–, similar to what is seen in Li–O₂ batteries.