Multifunctional Calorimeter for Thermal Characterization of Pouch and Cylindrical Cell
Authors: Du Xiaoniu , Choi Chihwan , Garrick Taylor R. , Choe Song-Yul 
Published: December 2024

Thermal management system (TMS) is crucial for the batteries to reject the heat generated during charging and discharging and maintain the proper working temperature. The design of cooling system requires accurate quantification of heat generation rate (HGR) of the cells. During actual operations, the HGR is further affected by various conditions such as temperature gradient and mechanical pressure. The inhomogeneous distribution of current density within the cell can cause a temperature gradient across the cell body. The volume changes taking place during charging and discharging can cause mechanical pressure to the adjacent cells or busbar in the battery pack. These conditions can affect the electrochemical-thermal behavior of the cells [1], [2]. In this work, a multifunctional calorimeter has been developed that enables accurate measurement of HGR under temperature gradient and mechanical pressure as testing conditions. Most of commercially available calorimeters generally have a large thermal time constant between the measuring points and the heat source of the cell while the developed one provides direct contact between the cell and the cooling plate and, a flexible fit to the dimensions, shape and format of the measured cells. In addition, it facilitates an accurate regulation of a constant reference temperature to create isothermal conditions (± 0.1 °C at continuous charging, ± 0.5 °C at pulse profile) and a tracking control that allows for determination of the entropy coefficient (EC, dUoc/dT) [3] for not only different formats of pouch cells but also cylindrical cells as shown in Figure (a).
To study the temperature gradient effect, the cell is divided into three equal segments in the longitudinal direction, whose temperatures are separately controlled, creating uniform temperature (MMM, 25 °C) and high-low-high (HLH 28-18-28 °C) temperature conditions across the cell. The results At BOL have shown that the integrated total heat generation over the full discharge range (THG) is suppressed by the HLH when the high C-rates are applied. Further, two aging profiles are applied with the conditions of MMM and HLH. At MOL (400 cycles), the capacity fades of the MMM and HLH conditions are around 4% and 7%, respectively, as shown in Figure (b,1). Also, the HGR exhibits that HLH condition generates more heat than the MMM case after the cell is aged as shown in Figure (b,2).
To study the pressure effect, the pressure conditions were set to be 30 kPa, 100 kPa, 170 kPa, 240 kPa and 310 kPa. The results have shown that while the discharge capacity of the cell is not significantly affected by the increase of compressive mechanical pressure, the HGR reduces as the pressure increases, especially at high C-rates, as shown in Figure (c). We further separately probe the reversible and irreversible heat generation heat source to root the cause of the decrease in HGR. The EC is measured at the five selected pressure conditions, and the trend and magnitude are found to be very similar at all SOC, which indicates that the reversible HGR is minimally affected by the increasing pressure. Then, the impedance measured by Electrochemical Impedance Spectroscopy (EIS) have shown a decreasing trend of Ohmic resistance and Charge Transfer resistance as the pressure increases, but not the SEI resistance. Consequently, the decrease in HGR as the compressive pressure increases is attributed to the decrease in Joule heating relating to the Ohmic resistance and Charge Transfer resistance, which is caused by the reduced contact resistance between the current collector and electrode and the decrease of ion transport distance in electrolyte.
Furthermore, the accurate temperature regulation allows us to control a sinusoidal temperature profile and determine the EC using the Hybridized time-frequency method. The results show that temperature does not have a significant impact on the EC within 0-45 °C, as shown in Figure (d,1). At around 40% SOC, there is a transition point where the reversible HGR has been changed from exothermic to endothermic process if during continuous discharging. Moreover, the EC at different capacity fades (CF) has shown a gradual shift of the peak from B to B’ in the SOC direction, which might be related to the mechanism change during the aging, as shown in Figure (d,2).
[1] Du, X., Hu, Y., Song, M., Choi, C., Choe, S. Y., Labaza, C., Gao, J., Koch, B. J., & Garrick, T. R. (2023). Journal of Power Sources, 587, 233688.
[2] Du, X., Hu, Y., Choe, S. Y., Garrick, T. R., & Fernandez, M. A. (2023). Journal of Power Sources, 573, 233117.
[3] Hu, Y., Choe, S. Y., & Garrick, T. R. (2020). Electrochimica Acta, 362.

Figure 1