General relationship between salt concentration and x-ray absorption for binary electrolytes

Hans-Georg Steinrück

doi:10.1063/5.0072947

Abstract

We derive a general relationship between salt concentration and x-ray absorption for binary electrolytes. Volume and mass conservation are considered. The relationship can be used to measure salt concentration profiles via x-ray absorption imaging during cell polarization and relaxation. In conjunction with concentrated solution theory, the derived relationship is as a powerful tool to accurately determine transport coefficients such as cation transference number in ion battery electrolytes and to test thermodynamic models. The relationship is electrolyte-agnostic, as long as the product between mass attenuation coefficient, inverse partial molar volume, and molar mass of the solvent and that of the salt are not equal.

Highlights

This profile can be straightforwardly experimentally produced in symmetric cation-active cells and calculated via concentrated solution theory, where the concentrationdependent transport coefficients serve as input parameters.[3]
It is shown that the salt concentration can be calculated for any binary electrolyte from the x-ray absorption, except in the special case, where the product between mass attenuation coefficient, inverse partial molar volume, and molar mass of the solvent and that of the salt are equal
We propose that laboratory x-ray absorption imaging can be a powerful technique toward measuring concentration profiles during polarization, from which the electrolyte transport coefficients can be extracted with high accuracy

Summary

Introduction

Measurements of the ion concentration profile within electrolytes between two planar electrodes are a powerful tool toward understanding ion transport.[1,2] This profile can be straightforwardly experimentally produced in symmetric cation-active cells and calculated via concentrated solution theory, where the concentrationdependent transport coefficients serve as input parameters.[3] It is of particular relevance to ion battery research, where the transport coefficients determine battery performance but are often challenging to determine.[4,5] Typically, assumption-based electrochemical methods are used.[6]. Direct measurements of the ion concentration profiles remain sparse These include x-ray phase contrast[7] and optical and Raman[8–13] and nuclear magnetic resonance (NMR)-based[1,14–17] imaging and can be used for inverse modeling or to independently verify thermodynamic models.[16]. These methods require complex, often impracticable, experimental setups and/or the presence of Raman- or NMR-active species

Objectives

Conclusion