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 dif-ferences. 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 con-cerns 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 performances. In contrast to the Li-O2 system, high solubilities of the NaO2 discharge product do not necessarily lead to in-creased 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 to-wards 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 for-mation 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.