Surprising Enhancement in the Ion Transport Properties and Sodium Metal Cycling in a Water Tolerant Ionic Liquid Electrolyte

Present energy demand is hard to sustain due to increasing energy consumption. Thus new energy storage and supply technologies are required which are reliable, safe, inexpensive and show high energy and power densities. Among various battery chemistries, Na based batteries have been identified as potential efficient stationary energy storage technologies. Electrolytes are an integral component of a sodium battery spatially ionic liquid based electrolytes have attracted great interest1. The use of additives during electrolyte preparation is often carried out to achieve better transport performance, tune the composition of the solid electrolyte interphase, SEI, and alter other characteristics1–3. Recently, electrolyte additives for Na batteries is an emerging area for researchers3 The effect of water on the properties of sodium salt solutions of ionic liquids (ILs) was investigated in order to design electrolytes for sodium battery applications using water as an additive. Water amounts ranging from 100 to 10000 ppm were added to a superconcentrated NaFSI salt (50 mol%) solution in the ionic liquid N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI)4. While the thermal properties (glass transition temperature) are little dependent on the water content, the viscosity and, in particular the ionic conductivity (fig1a), are strongly affected. Whereas addition of the NaFSI salt to the IL significantly increases viscosity with a concommitant decrease in conductivity, adding water up to 10000 ppm (0.99 wt%) remarkably restores the values close to those of the neat ionic liquid (i.e., reversing the detrimental effect of Na salt addition on conductivity). Na|Na symmetrical cell cycling performance is strongly dependent on the applied current density as well as on the water content (fig1b). At higher current densities (1.0 mAcm-2) the polarisation profiles show a water dependence suggesting that water is being actively involved in the formation of a solid electrolyte interphase (SEI) for high water content samples (1000 – 5000 ppm). The work shown here suggests that water may be a convenient and inexpensive additive in high concentrated ionic liquid electrolytes for sodium which can improve device performance. Reference: Armand, M., Endres, F., MacFarlane, D. R., Ohno, H. & Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 8, 621–9 (2009). Yan, Y. et al. Roles of Additives in the Trihexyl(tetradecyl) Phosphonium Chloride Ionic Liquid Electrolyte for Primary Mg-Air Cells. J. Electrochem. Soc. 161, A974–A980 (2014). Komaba, S. et al. Fluorinated ethylene carbonate as electrolyte additive for rechargeable Na batteries. ACS Appl. Mater. Interfaces 3, 4165–4168 (2011). Forsyth, M. et al. Novel Na+ Ion Diffusion Mechanism in Mixed Organic-Inorganic Ionic Liquid Electrolyte Leading to High Na+ Transference Number and Stable, High Rate Electrochemical Cycling of Sodium Cells. J. Phys. Chem. C 120, 4276–4286 (2016). Figure 1