Physicochemical properties of Pyr13TFSI-NaTFSI electrolyte for sodium batteries

Ionic liquids (ILs) are an increasingly important component of electrolytes for lithium and sodium batteries. Here, the physicochemical properties of the system N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr13TFSI) ionic liquid and NaTFSI are investigated vs. the concentration of the sodium salt and the temperature. The explored concentration range was (1-x) Pyr13TFSI: x NaTFSI with x (mole fraction) = 0, 0.02, 0.05, 0.1, 0.2. 23Na solid-state NMR reveals that the Na+ ions exist in two distinct environments: mobile Na+ ions (1), and Na+ ions involved in clusters or even bigger interacting networks (2). The ratio between mobile and bonded Na+ populations increases with temperature and decreases with increasing salt concentration, reaching 100% at 60 °C for the most diluted compositions. Raman spectroscopy allows to identify the quantity of free and bonded anions depending on the concentration, and to measure the number of Na+ ions solvating the TFSI− anion (SN = 4). The combined NMR and Raman results allow us to estimate the salt solubility range, x = 0.12 ± 0.02. The composition x = 0.1 showed satisfying stability when cycled versus high-potential cathodic material Na0.44MnO2 (NMO) in a cell Na/IL/NMO.