(Invited) Experimental Characterization and Modeling of Heat Generation Rates of Lithium-Ion Batteries with NMC/C Chemistries
Authors: Choe Song-Yul 
Published: December 2020

The operating temperature of a battery strongly affects overall chemical reactions, ion transport, intercalation and deintercalation process and consequently efficiency, cycle life, and safety of battery systems. Therefore, battery thermal management system (BTMS) should be optimally designed to ensure a highly efficient and reliable operation of the battery system, which requires characterization and analysis of heat generated during operations rather than temperature.
Characterization of heat generation rates of a battery requires a highly accurate and dynamic calorimeter that is not commercially available. Therefore, we designed a new multifunctional calorimeter for lithium-ion batteries using thermoelectric assemblies (TEA), which enables the measurement of heat generation rate as a function of C rates, state-of-charge (SOC) and temperatures. In addition, the calorimeter also works as a thermostat that is capable of tracking an alternating reference temperature, so that flexibility and accuracy of experiments are ensured. This capability enables to decouple the temperature fluctuation during charging and discharging operations and the measurement of the entropy coefficient.
In this paper, the cells used as an example for the experiments are large format pouch type lithium ion batteries with LMO/NMC-Graphite chemistry. The lumped heat generation rate of the cell is measured at different C rates, state-of-charge (SOC) and temperatures. In addition, analysis of the data has shown that both the heat generation rate and the ratio of the total heat generation to the total energy stored or retrieved of the tested cells increase as the current increases or the temperature decreases.
Characterization of the heat source terms includes measurement of reversible and irreversible heat sources that can be determined by calorimetric methods. Particularly, the entropy coefficient (dUOC/dT) in the reversible source is the key parameter that determines the amount of the reversible heat generated during operations. Currently, either potentiometric or calorimetric method is used to measure the value of the entropy coefficient, which drawbacks are long measurement time and inaccuracy. Thus, a new method is proposed based on the calorimeter that works as a thermostat in conjunction with an estimation technique. In order to reduce the measurement time, a time-domain background voltage offset correction and a frequency-domain entropy coefficient determination have been newly developed and applied. Comparison between the developed one and the classical methods has shown a drastic reduction of the measurement time and a similar accuracy to the potentiometric method. The proposed method is also used to investigate effects of temperatures and aging on the entropy coefficient. As a result, the reversible heat source term dependent upon the operating conditions such as SOC, temperature and cycle life can be characterized, which are the important factors for design of battery thermal management systems.
Moreover, a calorimetric method for the characterization of the reversible and irreversible heat sources is also developed by applying a sinusoidal current excitation to the cell and analyzing the heat generation response in frequency domain. The determined parameters of both heat sources are compared with those measured by hybridized time frequency domain analysis method and EIS analysis, respectively, which shows a good match. Moreover, the calculated reversible and irreversible heat sources terms are compared with the experimental measurement. Results have shown improved performances in the characterization of irreversible and reversible heat with respect to measurement speed and accuracy.
Finally, a thermal model that includes irreversible and reversible heat source terms is developed and then incorporated into a reduced-order electrochemical model (ROM). The electrochemical thermal model is validated against the heat generation rate of the cells measured by the developed calorimeter aforementioned. The model is capable of predicting heat generation rates up to 3C from -30°C to 35°C. Based on the validated thermal model, effects of C rates and temperatures on heat sources can be analyzed. The results have shown that the reversible heat is dominant at low C rates, while the irreversible one is dominant at high C rates because the Joule heating, which is the main irreversible heat source, is proportional to the square of the current. Besides, the reversible heat source is dominant at high temperatures, while the irreversible one is dominant at low C rates because battery internal resistance sources such as contact resistance and ionic resistivity rapidly increase at low temperatures.
In addition, the lumped model is used to develop a two-dimensional electrochemical thermal model that is validated with temperature images captured by an Infrared thermal camera.
Presented experimental characterization of heat generation and the associated theoretical analysis by electrochemical thermal models can provide basics and fundamentals for an optimal design of future battery thermal management systems.