Modeling the Influence of Solvation on the Electrochemical Double Layer of Salt / Solvent Mixtures

TL;DR

This paper enhances electrolyte modeling by incorporating solvation interactions, enabling detailed analysis of the electrochemical double layer in various salt/solvent mixtures, especially at high concentrations, with implications for battery chemistry optimization.

Contribution

It introduces a combined local solvation and electrolyte model to study EDL structures, accounting for ion-solvent interactions and concentration effects, which was not previously possible.

Findings

Salt concentration influences solvent shell stability.

Ion-solvent interaction strength affects potential at solvation shell removal.

Model can qualitatively predict EDL structures based on key parameters.

Abstract

Modelling electrolytes accurately on both a nanoscale and cell level can contribute to improving battery chemistries.[Armand and Tarascon, Nature, 2008, 451, 652-657] We previously presented a thermodynamic continuum model for electrolytes.[arXiv:2010.14915] In this paper we include solvation interactions between the ions and solvent, which alter the structure of the electochemical double layer (EDL). We are able to combine a local solvation model -- permitting examination of the interplay between electric forces and the ion-solvent binding -- with a full electrolyte model. Using this, we can investigate double layer structures for a wide range of electrolytes, especially including highly concentrated solutions. We find that some of the parameters of our model significantly affect the solvent concentration at the electrode surface, and thereby the rate of solvent decomposition. Firstly, an increased salt concentration weakens the solvation shells, making it possible to strip the solvent in the EDL before the ions reach the surface. The strength of the ion-solvent interaction also affects at which potential difference the solvation shells removed. We are therefore able to qualitatively predict EDL structures for different electrolytes based on parameters like molecule size, solvent binding energy and salt concentration.