The role of electrolyte anions in the Na-O2 battery: implications for NaO2 solvation and the stability of the sodium SEI formation in glyme-ethers.

Herein we investigate the influence of the sodium salt anion on the performance of Na-O2 batteries. To illustrate the solvent-solute interactions in various solvents, we use 23Na-NMR to probe the environment of Na+ in presence of different anions (ClO4-, PF6-, OTf- or TFSi-). Strong solvation of either the Na+ or the anion leads to solvent-separated ions where the anion has no measurable impact on the Na+ chemical shift. Contrarily, in weakly solvating solvents the increasing interaction of the anion (ClO4- < PF6- < TFSi- < OTf-) can indeed stabilize the Na+ due to formation of contact-ion-pairs. However, by employing these electrolytes in Na-O2 cells we demonstrate that changing from low DN anions (ClO4-) to high DN anions (OTF-) does not result in elevated battery performance. Nevertheless, a strong dependence of the solid electrolyte interphase (SEI) stability on the choice of sodium salt was found. By correlating physical properties with the chemical SEI composition, the crucial role of the anion in the SEI formation process is revealed. The remarkable differences and consequences for long-term stability are further established by cycling Na coin-cells, where electrolytes using NaTFSi are absolutely detrimental for metallic sodium, employing NaOTF and NaClO4 leads to short-term stability and only the combination of DME with NaPF6 allows for high efficiency and performance.