A Volume-based Description of Transport in Incompressible Liquid Electrolytes and its Application to Ionic Liquids

TL;DR

This paper introduces a volume-based theoretical framework for understanding ion transport in incompressible liquid electrolytes, especially ionic liquids, revealing limitations of traditional center-of-mass based models and improving interpretation of experimental data.

Contribution

It presents a novel thermodynamically consistent volume-based approach to analyze transport in highly correlated electrolytes, challenging existing assumptions of local momentum conservation.

Findings

Volume-based description aligns well with electrophoretic NMR data for ionic liquids.

Traditional center-of-mass models are inadequate for highly concentrated electrolytes.

Local momentum conservation does not hold at the macroscopic scale in these systems.

Abstract

Transference numbers play an important role for understanding the dynamics of electrolytes and assessing their performance in batteries. Unfortunately, these transport parameters are difficult to measure in highly concentrated, liquid electrolytes such as ionic liquids. Also, the interpretation of their sign and magnitude has provoked an ongoing debate in the literature further complicated by the use of different language. In this work, we highlight the role of the reference frame for the interpretation of transport parameters using our novel thermodynamically consistent theory for highly correlated electrolytes. We argue that local volume conservation is a key principle in incompressible liquid electrolytes and use the volume-based drift velocity as reference. We apply our general framework to electrophoretic NMR experiments. For ionic liquid based electrolytes, we find that the results of the eNMR measurements can be best described using this volume-based description. This highlights the limitations of the widely used center-of-mass reference frame which for example forms the basis for molecular dynamics simulations - a standard tool for the theoretical calculation of transport parameters. It shows that the assumption of local momentum conservation is incorrect in those systems on the macroscopic scale.