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

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journal contribution
posted on 2018-07-16, 00:00 authored by Oleg Sapunkov, Vikram Pande, Abhishek Khetan, Venkatasubramanian Viswanathan
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.