Non-Uniform Li Plating Behavior of Li-Ion Cells in a Battery Pack with Bottom, Symmetric Top-Bottom, or Side Cooling Thermal Management Systems during Fast Charging
Authors: Forouzan Mehdi M , Khaleghi Rahimian Saeed , Han Sangwoo , Liu Ying , Tang Yifan 
Published: February 2020

For optimized operation and safety consideration, a Thermal Management System (TMS) is consequential for a battery pack. For example, during fast charging in hot climates the cooling system is needed to remove the extra heat generated by the cells to avoid thermal run away and explosion. On the other hand, during fast charging in a cold weather, the TMS should add heat to the cells to make it possible to charge the battery to 80% SOC without serious damage and degradation (such as Li plating at high rates and low temperatures). It is recommended by battery manufacturers that the batteries should not be charged beyond the range of 20 to 40 , especially at higher rates mainly because of cell degradation and safety. However, pack designers often do not consider the effects of TMS design on the cell degradation and lifetime and primarily focus on safety issues.
Different commonly used TMS designs for automotive industry are basically heat exchanger systems located on the bottom or top of the battery pack, or around the individual cells inside the pack (see Fig. 1), so heat can be released or added when necessary. The temperature of the cells in the pack can be controlled during charge and discharge for better operation, minimized degradation, and safety issues. The main problem with these systems is that sometimes the TMS introduces non-uniform temperature through the cell (normal to the TMS plate) for bottom and top cooling and in radial direction for side cooling that in turn induces other types of non-uniformities such as non-uniform current density, local state of charge, and Li plating, resulting in a non-uniform degradation.
The main objective of this work is to understand the effects of different TMS designs on the amount of Li plating and non-uniform behavior by a combination of an electrochemical (EC) and a thermal (T) model or an ECT model. In order to capture non-uniform behavior, we consider three regions in the z direction for bottom cooling design, based on our previous work [1], as shown in Fig. 2. For side cooling design, three regions are considered in the r direction in a way that the regions’ volumes are equivalent. Due to the shared current collectors of all regions, the regions are treated as parallel connected cells and have similar voltage. According to the Ohm’s law, local current densities are controlled by varied internal resistance of each region (V=IR). The resistance of each region may change by SOC, temperature, transport properties, and microstructure of each regions [2], however in this work the focus is non-uniform temperature due to different TMS designs rather than a non-uniform microstructure[1-3]. The non-uniform temperature resulting from a particular TMS design effects non-uniform resistance and in-turn non-uniform current density. Due to the fact that temperature, current density, and SOC are all important factors that affect the amount of Li plating in each region, it is not easy to predict the amount and non-uniformity degree of Li plating in a cell with bottom, top, or side cooling system during fast charging. The developed ECT model in COMSOL Multiphysics can provide a real insight into this important concept of non-uniform degradation that should be taken into consideration by pack and TMS designers.
[1] M.M. Forouzan, B.A. Mazzeo, D.R. Wheeler, J. Electrochem. Soc., 165 (2018) A2127-A2144.
[2] J.E. Vogel, M.M. Forouzan, E.E. Hardy, S.T. Crawford, D.R. Wheeler, B.A. Mazzeo, Electrochimica Acta, (2018).
[3] M.M. Forouzan, M. Wray, L. Robertson, D.R. Wheeler, J. Electrochem. Soc., 164 (2017) A3117-A3130.

Figure 1