Electric Field Induced Associations in the Double Layer of Salt-in-Ionic-Liquid Electrolytes

TL;DR

This paper develops a theory for the electric double layer in salt-in-ionic-liquid electrolytes, revealing how ion associations change under different electric fields, with predictions to be validated by simulations and experiments.

Contribution

It introduces a novel theoretical model accounting for thermoreversible ion aggregation in the EDL of SiILs, highlighting field-dependent association behaviors.

Findings

IL cations populate the EDL at negative voltages

Large negative voltages break cation-anion associations, favoring alkali metal cations

SiILs become more aggregated at positive voltages, screening charge more effectively

Abstract

Ionic liquids (ILs) are an extremely exciting class of electrolytes for energy storage applications because of their unique combination of properties. Upon dissolving alkali metal salts, such as Li or Na based salts, with the same anion as the IL, an intrinsically asymmetric electrolyte can be created for use in batteries, known as a salt-in-ionic liquid (SiIL). These SiILs have been well studied in the bulk, where negative transference numbers of the alkali metal cation have been observed from the formation of small, negatively charged clusters. The properties of these SiILs at electrified interfaces, however, have received little to no attention. Here, we develop a theory for the electrical double layer (EDL) of SiILs where we consistently account for the thermoreversible association of ions into Cayley tree aggregates. The theory predicts that the IL cations first populate the EDL at negative voltages, as they are not strongly bound to the anions. However at large negative voltages which are strong enough to break the alkali metal cation-anion associations, these IL cations are exchanged for the alkali metal cation because of their higher charge density. At positive voltages, we find that the SiIL actually becomes $\textit{more aggregated while screening the electrode charge}$ from the formation of large, negatively charged aggregates. Therefore, in contrast to conventional intuition of associations in the EDL, SiILs appear to become more associated in certain electric fields. We present these theoretical predictions to be verified by molecular dynamics simulations and experimental measurements.