Developing sodium-oxygen (Na-O₂) batteries with high energy efficiency depends on identifying stable solvents resistant to decomposition. Here, we focus on the solvent stability against Hydrogen-abstraction by superoxide radicals during discharge. Using a detailed quantitative analysis, we show that the solvent’s resistance to H-abstraction is determined by its acid dissociation constant, pKa. The ether (TEGDME)-based electrolyte with a larger pK_a value (i.e. 46-52) leads to form a much larger percentage of NaO₂ among the discharge products than the carbonate (EC/PC with pK_a of 20.9) and the ionic liquid (PP13TFSI with pK_a of 30.0)-based ones. It is indicated that the larger value of pK_a corresponds to the better electrolyte stability, beneficial to high round-trip efficiency of the Na-O₂ battery.

Based on these results, the cyclability and rate-capability of Na-O₂ batteries with TEGDME-based electrolyte have been optimized under various operation conditions. Under static O₂/Ar atmosphere, they exhibit high energy storage (i.e. capacity of 4200 mAh g^-1 at 0.1 mA cm^-2), excellent cyclability (i.e. the batteries cycle 130 times with 750 mAh g^-1 depth of discharge at 0.1 mA cm^-2) and good rate performance (i.e. ~1500 mAh g^-1 at 1.0 mA cm^-2 with pre-deposition of a NaO₂ nucleus layer). Such high performance indicates the importance of stable electrolyte and operating condition for high performance Na-O₂ batteries. Further, the fundamental issues associated with the phase transformation from NaO₂ to Na₂O₂·2H₂O will be discussed in this presentation.