Mechanistic origin of low polarization in aprotic Na-O₂ batteries

Research interest in aprotic sodium-air (Na-O₂) batteries is growing because of their considerably high theoretical specific energy and potentially better reversibility than lithium-air (Li-O₂) batteries. While Li₂O₂ has been unequivocally identified as the major discharge product in Li-O₂ batteries containing relatively stable electrolytes, a multitude of discharge products, including NaO₂, Na₂O₂ and Na₂O₂·2H₂O, have been reported for Na-O₂ batteries and the corresponding cathodic electrochemistry remains incompletely understood. Herein, we provide molecular-level insights into the key mechanistic differences between Na-O₂ and Li-O₂ batteries based on gold electrodes in strictly dry, aprotic dimethyl sulfoxide electrolytes through a combination of in situ spectroelectrochemistry and density functional theory based modeling. While like Li-O₂ batteries, the formation of oxygen reduction products (i.e., O₂^-, NaO₂ and Na₂O₂) in Na-O₂ batteries depends critically on the electrode potential, two factors lead to a better reversibility of Na-O₂ electrochemistry, and are therefore highly beneficial to a viable rechargeable metal-air battery design: (i) only O₂^- and NaO₂, and no Na₂O₂, form down to as low as ∼1.5 V vs. Na/Na⁺ during discharge; (ii) solid NaO₂ is quite soluble and its formation and oxidation can proceed through micro-reversible EC (a chemical reaction of the product after the electron transfer) and CE (a chemical reaction preceding the electron transfer) processes, respectively, with O₂^- as the key intermediate.