Determination of the Transport Properties of Liquid Electrolytes Using a Multi-Electrode Cell
Authors: Rabette Clément , Lombard Tristan , Chehab Jean-Paul , Farkhondeh Mohammad , Fleutot Benoit , Delacourt Charles 
Published: May 2020

Reliable electrochemical models are necessary for performing numerical battery-related studies. These models must be comprehended with a number of input parameters.
Many literature papers deal with the determination of electrolyte transport properties [1- 4] but only a minority of them proposes an exhaustive characterization (namely the determination of the transference number, the conductivity, the diffusion coefficient and the thermodynamic factor, for the simplest case of a binary electrolyte). Furthermore, when comparison can be made[5], it is often the case that data gathered from different sources for a same electrolyte are not in good quantitative agreement with each other [1,6].
The most commonly used experimental methods for a complete determination of the set of properties are: The Hittorf method [2,3] (for transference number), the restricted diffusion method [7] (for diffusion coefficient), the electrochemical impedance spectroscopy (for conductivity) and the concentration cell (for the activity coefficient).
Back in 2016 [6], Farkhondeh and coworkers proposed to alleviate the tedious multi-experimental protocol by introducing a novel multi-electrode cell design, in which the potential between inner Li reference electrodes is measured during and after a current pulse applied between two outer Li working electrodes. The measured potential is free of any electrode polarization and can be fitted with a mathematical model of the electrolyte so that conductivity, salt diffusion coefficient, and lithium transference number are simultaneously determined in single experiment.
Our current work builds on the prior art by Farkhondeh and coworkers. It deals with the use of a “multi-electrode electrochemical cell”. Different routes are being explored and will be discussed in the presentation, namely:

Using more reference electrodes for the method to gain accuracy, and possibly to evaluate the composition dependence of parameters in a limited number of experiments (fig.1).

Bypassing metallic-lithium-related problems by finding alternatives to be used as active electrodes.

Theoretical and practical aspects will be discussed and LiPF6 dissolved in 1:1 weight proportions EC:DEC will be investigated as a benchmark electrolyte and compared with already-published data [1].
Our work aims at (i) showing how the parameters evaluation process could be simplified, and (ii) reduce drastically the number of experiments needed for fully characterizing a liquid electrolyte. This is an essential step towards the elaboration of an electrolyte-property database, supplying valuable information for modeling, as well as formulation-engineering, purposes.

Fig. 1 : Representation of voltage measured between reference electrodes (blue red and orange crosses), during a current-pulse (black dashed line) of 1.9 A/m² and following relaxation. These experimental data were used to fit the electrolyte transport parameters using a mathematical model (in purple, green and cyan). Determined parameters are the following : . D = 2.85×10-10 m²/s , t+
0
= 0.217 and
κ

eff
= 0.602 S/m. The gradient-colored strip at the bottom highlights the parts of the signal where the different transport parameters have the largest sensivity. The multi-reference cell is represented in inset: (dark-grey: Li metal reference electrodes, dark grey: polyethylene spacers).
References :
[1] H. Lundgren, M. Behm, and G. Lindbergh, “Electrochemical Characterization and Temperature Dependency of Mass-Transport Properties of LiPF6 in EC:DEC,” J. Electrochem. Soc., vol. 162, no. 3, pp. A413–A420, 2014.
[2] L. O. Valo̸en and J. N. Reimers, “Transport Properties of LiPF[sub 6]-Based Li-Ion Battery Electrolytes,” J. Electrochem. Soc., vol. 152, no. 5, p. A882, 2005.
[3] T. Hou and C. W. Monroe, “Composition-Dependent Thermodynamic and Mass-Transport Characterisation of Lithium Hexafluorophosphate in Propylene Carbonate,” Manuscr. Submitt. Publ., p. 135085, 2019.
[4] Yanping Ma, Marc Doyle, Thomas F Fuller, Marca M. Doeff, Lutgard C. De Jonghe, and John Newman. The measurement of a complete set of transport properties of a concentrated solid polymer electrolyte solution. Journal of The Electrochemical Society, 142(6):1859–1868, 1995
[5] A. Ehrl, J. Landesfeind, W. A. Wall, and H. A. Gasteiger, “Determination of Transport Parameters in Liquid Binary Electrolytes: Part II. Transference Number,” J. Electrochem. Soc., vol. 164, no. 12, pp. A2716–A2731, 2017.
[6] FARKHONDEH, Mohammad, PRITZKER, Mark, FOWLER, Michael, et al. Transport property measurement of binary electrolytes using a four-electrode electrochemical cell. Electrochemistry Communications, 2016, vol. 67, p. 11-15..
[7] Herbert S. Harned and Douglas M. French. A conductance method for the determination of the diffusion coefficients of electrolytes. Annals of the New York Academy of Sciences, 46(1):267–284, 1945

Figure 1