Solvation Behaviour in Electrochemical Double Layers: Modelling Solvation Energy

Abstract

Electrolyte modelling promises to speed up processes surrounding design and selection of electro-lytes for batteries [1]. Models which can accurately describe dilute electrolytes are known for some time. Highly concentrated electrolytes, however, are more difficult to model. Solvation mechanics can play a very important role in the battery performance, for example when comparing the intercalation capacity of graphite electrodes regarding Li and Na-Ions [2]. Therefore, the desolvation inside the double layer plays a central role in the performance of graphite electrodes [3]. For this reason, including solvation behaviour in the model makes it applicable to a much wider range of batteries.

Dreyer et al. [4] presented a model for electrolytes which includes solvation effects. The model as-sumes a dilute solution and fixes the number of solvent molecules bound to each ion. We have recently presented a theory for highly concentrated electrolytes based on a free energy which includes electro-static energy, the contributions of the molar volumes of the species and the entropic energy [5,6]. In this contribution, we propose to add an interaction energy for ion solvation in concentrated electrolytes. Our model is able to describe the local solvation state of the electrolyte, especially in highly concentrated electrochemical double layers.

To characterise the ion-solvent interaction, two parameters per ion species are chosen: The maxi-mum number of solvent molecules binding to a single ion and the binding energy per bound solvent molecule. This form of the free energy yields plausible results in equilibrium condition, though we have not yet tested this free energy on a full battery model.