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

High Resolution Image Download MS PowerPoint Slide 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 Li 2 O 2 as the discharge product at the cathode, whereas Na–O 2 batteries usually form NaO 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 NaO 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 NaO 2 discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na + ion solvent shell structure on the NaO 2 growth mechanism. Strong solvent–solute interactions in long-chain ethers shift the formation of NaO 2 toward a surface process resulting in submicrometric crystallites and very low capacities (ca. 0.2 mAh/cm 2 (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/cm 2 (geom) ). This work provides a new way to look at the key role that solvents play in the metal–air system.