(255a) Long-Range Interactions in Highly Concentrated Electrolytes

Ionic liquids (ILs) are a highly promising class of electrolytes for myriad applications, from energy storage to solvents for reactions and lubricants. These electrolytes, in their pure form, only comprise of ions, which gives rise to a unique combination of properties, such as being non-flammable, non-volatile, and excellent solvents, but less prone to thermal and electrochemical decomposition compared to traditional organic solvents. ILs can thus operate at higher voltages and offer reduced safety concerns. In addition to ILs, other highly concentrated electrolytes have been also found to offer advantages in energy applications, an example of which is aqueous LiTFSI. Advancing the fundamental knowledge of the electrical double layer of highly concentrated electrolytes can help establish design principles for targeted applications. In the first part of this talk, I will summarize our recent progress in understanding the electrical double layer of highly concentrated electrolytes based on measurements with a Surface Forces Apparatus, and Atomic Force Microscopy, among others.

In their pure form, ILs exhibit high viscosity and moderate ionic conductivity. Doping the IL with alkali metal salts bearing the same anion as the host IL, a concept known as salt-in-ionic liquid (SiIL), has the potential to facilitate ion dissociation, reduce viscosity, and augment the conductivity of the metal cations. This behavior opens up opportunities for harnessing the advantageous properties of IL electrolytes in battery applications. It is well known that short-range oscillatory forces in ILs originate from the overscreening provided by ion layers that accumulate close to the charged surface. In the second part of this talk, I will show how salts modify such overscreening and present new results of long-range surface forces mediated by SiILs. I will also compare experiments and theory to propose an alternative explanation of the long-range force found in highly concentrated electrolytes.