Ion Clusters and Networks in "Water-in-Salt Electrolytes"

TL;DR

This paper develops a simplified thermodynamic model for water-in-salt electrolytes, accurately predicting ionic cluster populations, gelation points, and ion association behaviors crucial for battery performance.

Contribution

It introduces a minimal-parameter model tailored for WiSEs that matches simulation data on ionic clustering and gelation, advancing understanding of their complex ionic structures.

Findings

Model accurately predicts ionic cluster distributions

Identifies critical salt concentration for gelation

Quantifies ion incorporation into ionic gels

Abstract

Water-in-salt electrolytes (WiSEs) are a class of super-concentrated electrolytes that have shown much promise in replacing organic electrolytes in lithium-ion batteries. At the extremely high salt concentrations of WiSEs, ionic association is more complicated than the simple ion pair description. In fact, large branched clusters can be present in WiSEs, and past a critical salt concentration, an infinite percolating ionic network can form spontaneously. In this work, we simplify our recently developed thermodynamic model of reversible ionic aggregation and gelation, tailoring it specifically for WiSEs. Our simplified theory only has a handful of parameters, all of which can be readily determined from simulations. Our model is able to quantitatively reproduce the populations of ionic clusters of different sizes as a function of salt concentration, the critical salt concentration for ionic gelation, and the fraction of ions incorporated into the ionic gel, as observed from molecular simulations of three different lithium-based WiSEs. The extent of ionic association and gelation greatly affects the effective ionic strength of solution, the coordination environment of active cations that is known to govern the chemistry of the solid-electrolyte interface, and the thermodynamic activity of all species in the electrolyte.