Determination of Transport Parameters in Liquid Binary Electrolytes: Part II. Transference Number

Abstract

In the literature, various numerical methods for the simulation of ion-transport in concentrated binary electrolytes for lithium ion batteries can be found, whereas the corresponding transport parameters are rarely discussed. In this contribution, a novel method for the determination of the transference number in non-aqueous electrolytes is proposed. The method is based on data from a concentration cell and on the value of the thermodynamic factor obtained from independent measurements based on quantifying the redox potential of ferrocene. The concentration dependent transference numbers obtained by this new method are compared to values obtained by the classical approach, which is based on experiments in a polarization cell and a concentration cell. For the latter, a set of commonly used and some newly proposed analysis methods as well as their theoretical justification are discussed. Using an exemplary electrolyte (lithium perchlorate in a mixture of ethylene carbonate and diethyl carbonate), we will demonstrate that our newly proposed method based on concentration cell experiments and a thermodynamic factor derived from independent measurements is a more accurate approach for obtaining concentration dependent transference numbers. At the end, the experimentally determined concentration dependent transference numbers are compared to data available in the literature.

Interactions and Transport in Highly Concentrated LiTFSI‐based Electrolytes

“…The electrolytes with higher transport numbers do not perform significantly better than the more conductive electrolytes with lower transport numbers. However, as recently been highlighted, experiments to better measure the transport numbers using applied electric fields, both with electrophoretic NMR (which excludes contributions from ion‐pairs) and electrochemical polarisation (which includes diffusion in concentration gradients), are highly desired.…”

Ion Transport in Organic Electrolyte Solutions for Lithium-ion Batteries and Beyond

“…In particular, Newman and Chapman used the refractive index in order to measure the concentration differences between two points of the vertical cell [62]. The binary diffusion coefficients calculated based on short or long time relaxation can be significantly different [63,64]. Other techniques that are used to monitor the relaxation of the concentration gradient or for measuring the transference number include the following:…”

“…A constant current is obtained after the steady state (Iss) salt concentration profile is achieved, and the net anion flux is zero. The transference number is calculated as t+ = Iss/I0 [63], assuming that convection and instabilities at the Li electrodes can be neglected, the ions of the electrolyte are completely dissociated, and the applied polarization voltage is small [65]. However, in real cells, charge transfer and conduction through the dynamic passivation layer occur on the electrode.…”

Comparing Cycling Characteristics of Symmetric Lithium-Polymer-Lithium Cells with Theoretical Predictions

“…7 In addition, modeling these systems requires knowledge of the thermodynamic factor, (1+dln ± /dlnm), which quantifies the change in the mean molal activity coefficient of the salt,  ± , with the molality, m, of the solution. We note in passing that measuring these four parameters in conventional liquid electrolytes is complicated due to convection; [18][19][20] convection is suppressed in polymers due to high viscosity. Although both cells in Figure 1 were cycled at the same current density of i ss = ± 0.02 mA/cm 2 , the cell containing an electrolyte with a lower salt concentration (r av = 0.02) reaches a much lower potential at steady-state compared to the cell with the higher concentration electrolyte (r av = 0.14).…”

“…These values are used in our analysis to normalize the potential of each cell according to thickness.Transient Model -Comsol ParametersThe transient model, based on a macro-homogeneous model by Newman and coworkers,10,26,27 is used to calculate the time-dependence of the potential across a lithium-PEO/LiTFSI-lithium symmetric cell during dc polarization. The governing equations for this model (eq [15][16][17][18][19][20],. are solved numerically using Comsol 5.3.…”