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.