Thermal-Electrochemical Lithium-Ion Battery Simulations on Microstructure and Porous Electrode Scale

Heat production within a battery cell is governed by local fluxes of ions and electrical currents in the microstructure of the battery electrodes. These fluxes on the other hand are strongly coupled with thermal transport, leading to a fully coupled reaction transport model for batteries [1]. It describes ion, charge and heat transport on the length scale of the microscopic active particles that build up an electrode. The implementation of the model in our simulation software BEST [2] allows simulations of transport and reactions in three-dimensionally fully resolved electrode structures. Ion concentration, potential and temperature are obtained in each spatial point of the simulation domain. Thus the dependence of battery performance, heat production and heat distribution in the electrode microstructure can be studied.

Due to its geometric complexity this approach is not viable for a simulation on cell scale. To this end effective or homogenized models are commonly employed [3]. By applying the volume-averaging technique to our thermal-electrochemical microscopic theory the corresponding thermal-electrochemical porous electrode theory can be derived [4]. Since volume-averaging neglects the details of the electrode structure it is obvious that ion fluxes and electrical currents cannot be recovered in detail. However, many heat sources of our model depend on the magnitude and distribution of these fluxes (e.g. Joule heating) and are strongly localized on the interfaces between electrolyte and active particles. It is therefore a priory not clear whether upscaled models of this type are at all useful for predictive cell scale simulations if heat production and temperature distribution is of interest.

In our contribution we will present the fully coupled thermal battery models for both micro and cell scale. We focus on a simulation study where we compare micro- with equivalent cell scale simulations based on the derived porous electrode theory. Whereas the numerically averaged results agree very well with the results of porous electrode theory, strong fluctuations around this average (e.g. of the overpotential) are observed in microstructures. Especially for the prediction of degradation phenomena, these fluctuations are crucial and cannot be recovered in porous electrode theory. Our results raise the question to which extent porous electrode theory can be used to predict degradation phenomena in batteries.

[1]   A. Latz and J. Zausch, Journal of Power Sources 196 (2011) 3296.

[2]   www.itwm.fraunhofer.de/best

[3]   J. Newman and W. Tiedemann, AIChE 21, (1975) 25.

[4] A. Latz, J. Zausch, Multiscale modeling of lithium ion batteries : thermal aspects, Beilstein J. Nanotechnol. 6 (2015) 987–1007. doi:10.3762/bjnano.6.102.