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

The Na-O-2 battery offers an interesting alternative to the Li-o(2) battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal-O-2 batteries, they display significant differences. For instance, Li-O-2 batteries form Li2O2 as the discharge product at the cathode, whereas Na-O-2 batteries usually form Na-O-2. A very important question that affects the performance of the Na-O-2 cell concerns the key parameters governing the growth mechanism of the large Na-O-2 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-O-2 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/cm((geom))(2)). 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/cm((geom))(2)). This work provides a new way to look at the key role that solvents play in the metal air-system.