Thermal characterization of high-performance battery cells during charging and discharging using optical temperature measurement methods

"Lithium-ion batteries currently used in electric vehicles have strong thermal limitations. Thus, both excessively high and low temperatures lead to an accelerated aging process in form of capacity losses through SEI-layer buildup on the one hand and lithium-plating on the other hand. Additionally, temperature gradients within a cell, as well as temperature differences between individual cells have to be minimized in order to increase the system’s durability. With a properly designed battery-thermal-management (BTM), not only the aging process can be minimized but also extreme cases like thermal runaways can be prevented, which otherwise can cause fatal ecological and economical damage or in the worst-case-scenario exposes and harms vehicle occupants to fire, explosion or intoxication due to a damaged battery cell system. For the application of an efficient BTM, a detailed characterization of the battery’s thermal behavior during the charge and discharge cycle with different loads is essential. This paper deals with high-performance pouch cells, as they have a favorable ratio of heat transfer surface area to battery volume for BTM. However, they also present a particular challenge for the acquisition of the surface temperature distribution, as they do not have a solid shell. An external force must be applied to this cell type to prevent it from inflating, when it is under higher loading conditions, because this volume change is expected to change the cell’s thermal behavior. Therefore, the determination of the surface temperature in the present study was carried out while the cell was fixated and forced to stay in its geometric shape. The mechanical fixation has an influence in the thermal behavior and therefore the temperature distribution should be probed in this configuration but this also makes the optical measurement of temperature more difficult as pressure transmission plates must be made transparent. The measured surface temperature distribution can then be used as an input for a FEM-simulation to provide information on the whole cell’s internal temperature field. To the authors´ knowledge, no thermal investigations in this configuration have been reported. Two different optical temperature measurement methods are applied. The first technique is IR-thermography, which is based on the measurement of heat radiation from the surface, and which is restricted to regions on the surface using small IR-transmitting sapphire windows. The second technique, phosphor thermometry, exploits the temperature dependent luminescence properties of thermographic phosphor materials. The phosphors are applied as a thin coating on the surface of the battery and illuminated by a light source. The resulting luminescence emission in the visible range is detected to infer the temperature of the whole surface, since normal glass can be used. The surface temperature distribution obtained by these two techniques are provided and the advantages or disadvantages of each technique are described in terms of precision, accuracy, sensitivity, temporal resolution, spatial resolution and also implementation expenses. The internal temperature field provided by the heat transfer model for a known surface distribution is also presented and the impact of this integral thermal characterization on the BTM is discussed."