Superoxide stability for reversible Na-O-2 electrochemistry

Abstract

Stabilizing superoxide (O-2(-)) is one of the key issues of sodium-air batteries because the superoxide-based discharge product (NaO2) is more reversibly oxidized to oxygen when compared with peroxide (O-2(2-)) and oxide (O2-). Reversibly outstanding performances of sodium-oxygen batteries have been realized with the superoxide discharge product (NaO2) even if sodium peroxide (Na2O2) have been also known as the discharge products. Here we report that the Lewis basicity of anions of sodium salts as well as solvent molecules, both quantitatively represented by donor numbers (DNs), determines the superoxide stability and resultantly the reversibility of sodium-oxygen batteries. A DN map of superoxide stability was presented as a selection guide of salt/solvent pair. Based on sodium triflate (CF3SO3-)/dimethyl sulfoxide (DMSO) as a high-DN-pair electrolyte system, sodium ion oxygen batteries were constructed. Pre-sodiated antimony (Sb) was used as an anode during discharge instead of sodium metal because DMSO is reacted with the metal. The superoxide stability supported by the high DN anion/solvent pair (CF3SO3-/DMSO) allowed more reversible operation of the sodium ion oxygen batteries.