Characterizing Thermodynamic Properties in Concentrated Binary Electrolytes: Combined Differential Reference Concentration Cell and Hittorf Methods

Abstract

Accurate models of ion transport in isothermal binary solutions require measurements of various properties, including ionic conductivity, salt diffusivity, cation transference number (t+ 0), and the thermodynamic Darken factor (χ). In this work a novel method is proposed to quantify composition-dependent values of χ(1-t+ 0 ). Recent studies of highly concentrated “solvent-in-salt” electrolytes suggest a strong variation of transference number with respect to composition. In fact t+ 0 for electrolyte systems may double in magnitude in the highly concentrated regimes.1 Previous studies have quantified the Darken factor by assuming that t+ 0 is relatively constant across the concentration range of interest. Furthermore, concentration-cell experiments typically fix arbitrary reference concentrations that may be far from the test concentration across the liquid junction. Such methods employ integral values of t+ 0; the accuracy of thermodynamic properties, and thereby numerical models, may be compromised if there are substantial concentration differences and concomitant local changes in t+ 0.2 An alternative approach to concentration-cell experiments would apply differential forms of the equations relating potential difference, composition, thermodynamic factor, and cation transference number.3 We propose a novel approach to concentration-cell experiments in which a matrix of liquid-junction potentials banded about a variable reference concentration is employed. When coupled with independent Hittorf-cell measurements of t+ 0, this 3-dimensional surface for potential difference can be numerically fitted to reliably and accurately determine differential values of the thermodynamic factor χ across the solubility range of an electrolyte. We demonstrate this method with the electrolyte lithium hexafluorophosphate in propylene carbonate (LiPF6:PC) to determine concentration-cell liquid-junction potential (U) across a wide the solubility range. Transference-number measurements from Hittorf experiments are then used to isolate χ from differential measurements of χ(1-t+ 0 ) as a function of electrolyte particle fraction (y). Suo, L., Hu, Y. S., Li, H., Armand, M. & Chen, L. A new class of Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries. Nat. Commun. 4, 1–9 (2013).Lundgren, H., Scheers, J., Behm, M. & Lindbergh, G. Characterization of the Mass-Transport Phenomena in a Superconcentrated LiTFSI:Acetonitrile Electrolyte. J. Electrochem. Soc. 162, A1334–A1340 (2015).Newman, J. & Thomas-Alyea, K. E. Electrochemical Systems. (John Wiley & Sons, 2004). Figure 1