Solvation effects in Electrochemical Double Layers and static simulations

Abstract

Batteries play a key role in future sustainable energy networks. Modelling these electro-chemical systems accurately provides a path to improved battery chemistries. In recent years, research has identified highly concentrated electrolytes as promising constituents for battery cells with high energy density. These are, however, complex systems, where the bulk electrolyte concentration and external electric potential significantly influences the electrochemical double layer (EDL) structure. It is challenging to model the behaviour of highly concentrated electrolytes near a charged surface.

Here, we present a continuum transport theory for these materials, which incorporates solvation effects. The model is built up from the second law of thermodynamics by enforcing positive entropy production. By analysing contributions to entropy production, we can establish the free energy of the electrolyte system, which in turn constitutes the basic equations of the transport theory. Our continuum model can describe both static and dynamic systems of highly concentrated electrolytes. To analyse the EDL, static simulations have proven effective due to their very low computation cost. Using this method, we are able to achieve concentration profiles at EDL length scales, when adding nonlocal contributions even oscillatory behaviour can be recreated.

Solvation is a key effect to consider when modelling reactions at the electrode surface or intercalation. The desolvation at the electrode surface constitutes an energy barrier, which must be overcome. Previous solvation models use a static ion-solvent coordination number, therefore the stripping of the solvation shell in the double layer cannot be described. We address this concept by incorporating local solvation in our model, enabling a description of the electric forces desolvating ions in the EDL.

Solvation is added to the free energy of the system by modifying the statistics which determine the entropic contribution. Additionally, we include a term to represent the ion-solvent interaction energy. Thus, two novel parameters are used to define the interaction – the maximum number of solvent molecules binding to a single ion, and the binding energy.

We supplement our analytic discussion by numerical double layer simulations of an electrolyte consisting of an IL with a solvent, neutral or charged. Our results capture the relationship of ion-solvent binding energy and the desolvation potential. We can extract qualitative results down to a molecular scale from our model, allowing us to predict coarse grained behaviour of MD-simulations. By estimating the solvent coordination number at the electrode surface, it is possible to obtain a desolvation energy barrier.