The ion-transport model for concentrated electrolyte solutions introduced by Newman and Thomas-Alyea¹is frequently used for numerical simulations of battery systems and depends on three ion transport parameters: the conductivity, the transference number and the binary diffusion coefficient. In addition, the thermodynamic factor, which is derived from the mean molar activity coefficient, is required for the correct description of the thermodynamic behavior of a binary electrolyte solution.

The resistive ion transport of the binary electrolyte in a lithium ion battery causes the formation of temperature gradients, especially in large format cells. To understand the influence of local temperature inhomogeneity on, e.g., the aging of the cell, it is necessary to have a profound understanding of the temperature dependence of the ion transport parameters.

While a vast spectrum of physico-chemical parameters can be found in the literature only few publications study their temperature dependence, e.g., Valoen and Reimers². In addition, some of the parameters are often fitted to match experimental performance data and may thus be fitting parameters rather than intrinsic physico-chemical parameters with predictive capability.

We present the results of our temperature dependent study of the ionic transport parameters from commonly used lithium electrolytes by using a new method to determine the thermodynamic factor³and combining it with concentration cell experiments to determine transference numbers. Using common polarization cell experiments, we additionally determine diffusion coefficients while conductivities are measured using turn-key equipment.

We chose common battery electrolytes, e.g., LiPF₆ in EC:EMC (3:7 w:w) and LiPF6 in EC:DMC (1:1 w:w) for our analysis. Exemplarily shown in Figure 1. is the relative temperature dependence of the transference number, thermodynamic factor and the diffusion coefficient for a 0.1 M, 1 M and a 2 M LiClO₄in EC:DEC (1:1 w:w) electrolyte. Our results allow us to use numerical experiments to study the temperature dependent transport limitations in binary electrolyte and their influence on the cell performance.

Adaption of our transport parameters in Newman type battery models will enhance numerical predictions. The study of temperature variations in large format cells might help to understand the observed, spatially dependent aging in, e.g., cylindrical cells and thereby enables the design and improvement of better lithium ion batteries in the future.

Figure 1. Relative change of transference number, binary diffusion coefficient and thermodynamic factor versus their 20°C value a LiClO₄in EC:DEC (1:1 w:w) electrolyte at 0.1, 1 and 2 M concentrations.

References

[1] J. Newman and K. Thomas-Alyea, Electro-chemical Systems, 3rd ed., Wiley Interscience, Hoboken, (2004).

[2] L. O. Valøen and J. N. Reimers, J. Electrochem. Soc., 152, A882 (2005).

[3] J. Landesfeind, A. Ehrl, M. Graf, W. A. Wall, and H. A. Gasteiger, J. Electrochem. Soc., 163, A1254–A1264 (2016).

Acknowledgements

We gratefully acknowledge the funding by the Bavarian Ministry of Economic Affairs and Media, Energy, and Technology for its financial support under the auspices of the EEBatt project.