Ionic liquids (ILs) are organic salts that consist solely of ions. They are potential alternatives to conventional organic solvents in energy applications due to their low volatility, low flammability, and extraordinary thermal and chemical stability. In particular, IL-doped Na salts have been explored as electrolytes in batteries resulting from the natural abundance and low production cost of Na. However, there still exist knowledge gaps in understanding the molecular structure and interactions of salt-in-ionic-liquid electrolytes (SiILs) under confinement. To unveil the behavior of SiILs, sodium trifluoromethanesulfonimide (Na[TFSI]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) have been selected and mixed at different molar ratios. Surface forces apparatus (SFA) and atomic force microscopy (AFM) were employed to measure the interfacial structure and interactions of the electrolytes. Unlike neat ILs, SiILs have displayed forced separations of surfaces at certain applied pressures, and long-range force interactions, which could be possibly explained by the ion aggregation theory. By investigating the behavior of SiILs, we hope to better explore the potential of utilizing concentrated SiILs for energy storage devices.
