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

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.