Bio-based ether solvent and ionic liquid electrolyte for sustainable sodium–air batteries

Abstract

Sodium-air batteries (SABs) are receiving much attention in the last years due to their high theoretical high energy density (up to 1105 Wh kg-1). However, most of the studies on this technology are still based on organic solvents, in particular diglyme, a chemical considered highly flammable and toxic for the unborn child. Thus, the potential of the greener and low-toxic solvent 1,2,3 trimethoxypropane (TMP) as an alternative electrolyte to diglyme for SABs has been investigated for the first time in this publication. Through this work, it was found the reactivity of tertiary carbon present in TMP towards bare sodium metal. The addition of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyr][TFSI]) as co-solvent proved to be effective to limit the reactivity. Moreover, a Na-β-alumina disk was employed as anode protection, to separate TMP-based electrolyte and sodium metal. The new cell design resulted in improved cell performance: discharge capacities of up to 1.92 and 2.31 mAh cm-2, for 16.6 mol% NaTFSI in TMP and 16.6 mol% NaTFSI in TMP/[C4mpyr][TFSI], respectively. By means of SEM, Raman and 23NMR techniques NaO2 cubes as the major discharge product for both electrolyte compositions were identified. Moreover, it was observed that the hybrid electrolyte hindered the formation of side-products during discharge (ratio NaO2 to side-products in the hybrid electrolyte is 2.4 in comparison with the TMP based electrolyte 0.8) and a different charge mechanism for the dissolution of NaO2 cubes for each electrolyte was observed. The findings of this work show the high potential of TMP as a base solvent for SABs, and the importance of careful electrolyte composition design, in order to step towards greener and less toxic batteries.