The Nature and Impact of Side Reactions in Glyme-based Sodium-Oxygen Batteries

Although Na-O-2 batteries have a low overpotential and good capacity retention, degradation reactions of glyme-based electrolytes are the primary reason for inefficiency in cell performance. The discharge capacity is accounted for through analysis of the side-products. Although sodium superoxide is the primary product (90% theoretical), quantitative and qualitative evaluation of the side-products (using H-1 NMR, iodometric titration, and on-line mass spectrometry) shows the presence of sodium acetate (similar to 3.5 %), and three-fold less sodium formate, methoxy (oxo) acetic anhydride, and sodium carbon-ate. Our reaction mechanism proposes two paths for their formation. Because the side-products are not fully removed during oxidation, they accumulate on the cathode upon cycling. Resting the cell at open circuit potential during discharge results in consumption of the superoxide through the reaction with diglyme, which greatly increases the fraction of side products, as also confirmed by ex situ reaction studies. These findings have implications in the search for more stable electrolytes.