The Na–O2 battery offers an interesting alternative to the Li–O2 battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal–O2 batteries, they display significant differences. For instance, Li–O2 batteries form Li2O2 as the discharge product at the cathode, whereas Na–O2 batteries usually form NaO2. A very important question that affects the performance of the Na–O2 cell concerns the key parameters governing the growth mechanism of the large NaO2 cubes formed upon reduction, which are a requirement of viable capacities and high performance. By comparing glyme-ethers of various chain lengths, we show that the choice of solvent has a tremendous effect on the battery performance. In contrast to the Li–O2 system, high solubilities of the NaO2 discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na+ ion solvent shell structure on the NaO2 growth mechanism. Strong solvent–solute interactions in long-chain ethers shift the formation of NaO2 toward a surface process resulting in submicrometric crystallites and very low capacities (ca. 0.2 mAh/cm2 (geom)). In contrast, short chains, which facilitate desolvation and solution-precipitation, promote the formation of large cubic crystals (ca. 10 um), enabling high capacities (ca. 7.5 mAh/cm2 (geom)). This work provides a new way to look at the key role that solvents play in the metal–air system.

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