Energy storage is a major challenge for modern society, with batteries being the prevalent technology of choice. Within this area, sodium oxygen (Na–O2) batteries have the capability to make a step change, thanks to their high theoretical energy density. In order to facilitate their use, the development of electrolytes is critical to overcome certain limitations that arise because of the technology’s unique chemistry, particularly relating to the stability of superoxide species. In this study, we have demonstrated the importance of selecting a suitable electrolyte to facilitate both a highly homogeneous distribution of the discharge products and to minimize the formation of undesirable reaction products. The combination of pyrrolidinium-based ionic liquid and diglyme can dramatically change the cell performance. The effect of sodium salt concentration as well as the amount of diglyme and N-butyl-N-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide, [C4mpyr]­[TFSI], in Na–O2 batteries has also been comprehensively studied by combination of experimental and simulation techniques.

The morphology and size of the discharge products in Na−O2 batteries are largely dominated by the interactions of the electrolyte with superoxide and sodium ions; therefore, an in‐depth understanding is essential for achieving high performance batteries. Herein, we report a framework designed to understand solvation and coordination in the recently discovered hybrid electrolytes, based on glyme and a pyrrolidinium ionic liquid. FTIR and NMR spectroscopic techniques, coupled with molecular dynamics simulations, have been used to characterize these systems. We demonstrate that the presence of ionic liquid in the hybrid electrolyte affects the superoxide coordination environment by weakening the glyme‐Na+ interactions, and generating solvent separated ion pairs. All these factors lead to different deposition mechanisms, which will determine the battery performance. The Na+ solvation shell compositions, anion conformers and relative free glyme content are also evaluated. The combinatorial approach used in this study‐experimental and computational – can be applied for further design of these hybrid electrolytes and other metal – O2 chemistries and electrochemical systems.

A series of hybrid electrolytes composed of diglyme and ionic liquids (ILs) have been investigated for Na–O2 batteries, as a strategy to control the growth and purity of the discharge products during battery operation. The dependence of chemical composition of the ILs on the size, purity, and distribution of the discharge products has been evaluated using a wide range of experimental and spectroscopic techniques. The morphology and composition of the discharge products found in the Na–O2 cells have a complex dependence on the physicochemical properties of the electrolyte as well as the speciation of the Na+ and superoxide radical anion. All of these factors control the nucleation and growth phenomena as well as electrolyte stability. Smaller discharge particle sizes and largely homogeneous (2.7 ± 0.5 μm) sodium superoxide (NaO2) crystals with only 9% of side products were found in the hybrid electrolyte containing the pyrrolidinium IL with a linear alkyl chain. The long-term cyclability of Na–O2 batteries with high Coulombic efficiency (>90%) was obtained for this electrolyte with fewer side products (20 cycles at 0.5 mA h cm–2). In contrast, rapid failure was observed with the use of the phosphonium-based electrolyte, which strongly stabilizes the superoxide anion. A high discharge capacity (4.46 mA h cm–2) was obtained for the hybrid electrolyte containing the pyrrolidinium-based IL bearing a linear alkyl chain with a slightly lower value (3.11 mA h cm–2) being obtained when the hybrid electrolyte contained similar pyrrolidinium-based IL bearing an alkoxy chain.