Reversible Ionic Aggregation Kinetics in Concentrated Electrolytes

Abstract

Here we develop and test a formalism for reversible ionic aggregation kinetics in an example concentrated electrolyte. Specifically, building on previous equilibrium work of McEldrew and co-workers in the context of concentrated electrolytes, and non-equilibrium properties of thermoreversible polymers and patchy particle systems, we develop the formalism for how ionic associations in electrolytes change subject to a step-change in conditions. This is achieved through solving a macroscopic rate equation of open/occupied association sites, which is a solution of the reversible Smoluchowski aggregation equation. We compare the derived equations against atomistic molecular dynamics simulations of a salt-in-ionic liquid. Good qualitative agreement is obtained, but quantitative differences are found, which highlights the multiple time scales of the associations that exist in concentrated electrolytes. We hope this formalism acts as the starting point for investigating these properties in other electrolytes, and developing it further to investigate the non-Newtonian behaviour of concentrated electrolytes, double layer charging, and the slow dynamics of these electrolytes in confinement.

Figures (7)

• Figure 1: Schematic of non-equilibrium kinetics of ionic aggregation in the studied salt-in-ionic liquid (SIIL). The alkali metal cation and IL anion bind together to form aggregates. From step-changing a property of the system, we investigate how they evolve in time. The alkali metal cation and IL anions are shown with 3 dangling bonds, to represent they can form associations, while the IL cations are shown without, since they do not associate strongly with the anions.
• Figure 2: Coordination numbers between Na$^+$ and TFSI$^-$ in NaTFSI$_{0.75}$EMIMTFSI$_{0.25}$ as a function of time, for the charge rescaling case, at the indicated temperatures, from MD simulations and theory.
• Figure 3: Gelation criteria in NaTFSI$_{0.75}$EMIMTFSI$_{0.25}$ as a function of time, for the charge rescaling case, at 300 K, for MD and theory. The percolation point is given by $p_{+-}p_{-+} = [(f_+ - 1)(f_- - 1)]^{-1}$.
• Figure 4: Cluster distributions $c_{lm}$, with $m$ TFSI$^-$ anions and $l$ Na$^+$ cations in an aggregate, at various times in the MD simulation and theory, as indicated, at 300 K.
• Figure 5: Coordination numbers between Na$^+$ and TFSI$^-$ as a function of time, for the charge rescaling case, for the indicated mole fractions, in NaTFSI$_{x}$EMIMTFSI$_{1-x}$, at 300 K, for MD simulations and theory.