Physicochemical properties of N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide for sodium metal battery applications

The physicochemical properties of a range of NaNTf2 (or NaTFSI) salt concentrations in N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (or C(3)mpyrFSI) ionic liquid were investigated by DSC, conductivity, cyclic voltammetry and diffusivity studies. Cyclic voltammetry indicated a stable sodium plating behavior with a current of 5 mA cm(-2) at 25 degrees C to 20 mA cm(-2) at 100 degrees C, along with high reversibility identifying this electrolyte as a possible candidate for sodium-ion or sodium metal battery applications. Na-23 NMR chemical shifts and spectral linewidths (FWHM) indicate a complex coordination of the Na+ ion which is dependent on both temperature and salt concentration with an apparently stronger coordination to the NTf2 anion upon increasing the NaNTf2 concentration. Temperature dependent PFG-NMR diffusion measurements show that both FSI and NTf2 have a comparable behaviour although the smaller FSI anion is more diffusive.