Interfacial Behavior of Salt-in-Ionic Liquids: From Dry to Wet Regimes

IL-doped alkali-metal salts, commonly known as salt-in-ionic liquids (SiILs), have drawn attention over recent years as electrolytes in batteries. SiILs are a class of highly concentrated, strongly correlated, and asymmetric electrolytes, with low volatility, low flammability, and extraordinary thermal and chemical stability. It has been reported that the transference numbers of alkali metals are negative in Li-based SiILs when the mole fraction of Li+ is low. This behavior can be explained by the formation of negatively charged ionic clusters composed of alkali metal cations and anions. On the other hand, MD simulations have also suggested a different ionic arrangement under high concentrations of salts, where a percolated ionic network can form, leading to positive transference numbers of metal cations. In this work, we have focused on the interfacial structure and behavior of Na-based SiILs on charged surfaces. Sodium trifluoromethanesulfonimide (NaTFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) have been selected as the alkali-metal salt and the IL, respectively, and systematically mixed at different molar ratios. An extended Surface Forces Apparatus (eSFA) and Atomic Force Microscopy (AFM) were employed for probing structural features such as the arrangement and layering of ions at charged surfaces, while Wide Angle X-ray Scattering (WAXS) was used to explore the bulk structure of the SiILs. The effect of water as an additive to SiILs, so called water-in-salt-in-ionic liquids (WiSILs) has also been investigated, as an additional component to tune nanostructure ionic mobility, and interfacial interactions. By understanding the properties of electric double layer (EDL) and the formation of solid electrolyte interface (SEI), we aim to provide insights to design materials for the next-generation batteries beyond Li-based chemistries to improve energy resilience.