High Capacity Na–O₂ Batteries: Key Parameters for Solution-Mediated Discharge

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