Thermal analysis of battery components is crucial for developing effective battery thermal management strategies that improve the performance of batteries, extend their operational life, and prevent thermal runaway, which is essential to build inherently safer batteries.
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The Importance of Thermal Analysis of Batteries
Safer batteries, specifically Li-ion batteries, have gained significant attention in the last few years owing to their extensive use as energy storage systems in many applications, including smartphones and electric vehicles.
Li-ion cells are lightweight and possess a high energy density, making them suitable as portable energy storage systems. However, these features have also made them more susceptible to thermal runaways.
Thermal runaway occurs when a lower amount of heat is dissipated from the cell compared to the heat generated within the cell due to self-heating, leading to higher storage of heat in the cell, which increases the cell temperature that results in even higher self-heating.
Cell temperatures continue to increase rapidly due to thermal runaway, eventually leading to explosions, production of toxic gases, and combustion, which can be fatal for users of devices powered by Li-ion batteries.
Internal or external short circuits, environmental conditions, excessive heat, improper discharging or charging of the cell, mechanical damage, hot spot creation in the cell due to latent defects within it, heat generated by adjacent electronics, and parasitic reactions are the major causes of thermal runaway.
Battery safety and management systems can be designed to decrease the impact and frequency of external sources. However, addressing an internal defect that is influenced by the charge, age, and health of a cell remains challenging.
Thus, the thermal runaway pathways must be characterized using dedicated testing methodologies and equipment. Additionally, the incorporation of compatibility, safety, and other advanced testing methods in the initial stages of the battery development process can significantly reduce the time-to-market and facilitate the creation of intrinsically safer systems and cells.
Major Thermal Analysis Methods
The cell design’s primary heat transfer properties must be understood comprehensively by measuring the specific heat capacity, thermal conductivity, and thermal diffusivity to determine the amount of heat that can be dissipated by the cells.
Accelerating rate calorimetry (ARC), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and laser flash analysis (LFA) are primarily utilized for the thermal analysis of batteries.
Temperature changes and delithiation/lithiation can change the physical properties of materials, including durability, flexibility, strength, volume, and dimensions. In a cell, these changes are not uniform and can increase mechanical stress, which adversely affects material performance, and eventually the battery performance.
For instance, polymer separators shrink substantially at higher temperatures, which impacts the battery performance. Thus, measuring the shrinkage/expansion behavior is critical for predicting the stresses and deformations in the separator in batteries.
TMA can determine the dimensional changes of liquids, pasty materials, and solids as a function of time and/or temperature under a defined mechanical force. Moreover, TMA is also related to dilatometry, which determines the changes in the length of samples under negligible load to understand their thermal behavior.
DSC and Thermogravimetric Analysis (TGA)
DSC and TGA can be utilized to measure the thermal stability of the batteries. DSC is the most extensively used thermal analysis method and can determine the specific heat capacity of a cell.
DSC can also accurately measure other thermal properties, including enthalpies and reaction temperatures, glass transition temperatures, and crystallization/melting temperatures.
The DSC method is suitable for investigating slurries, separators, electrolytes, and electrodes. For instance, DSC can effectively investigate the energy released during a reaction between electrode and electrolyte.
The LFA technique is an effective, versatile, and fast method to measure the thermal diffusivity of electrode coating, electrodes, and separators with high accuracy. Obtaining accurate component-level thermophysical property data is crucial for detailed thermal modeling of the battery.
Adiabatic calorimetry is used most extensively to measure self-heating, which typically leads to thermal runaway in a Li-ion cell. ARC can effectively measure the pressure and thermal properties of exothermic chemical reactions. Moreover, ARC measurement of cell components can assist in understanding the stability of various material combinations.
Studies on Thermal Analysis of Li-ion Batteries
In a study published in the Journal of Power Sources, researchers investigated the heat sources in the thermal runaway processes of Li-ion batteries composed of different chemistries using DSC and ARC.
The adiabatic thermal runaway features for four types of commercial Li-ion batteries were investigated using ARC, while the reaction characteristics of component materials within these batteries, including the separator, anode, and cathode, were determined using DSC.
The critical component reaction valleys and peaks measured by DSC matched the temperature rise rate fluctuations measured by ARC. Thus, the relevance between the ARC curves and DSC curves was utilized to investigate the heat source in the thermal runaway process and identify the thermal runaway mechanisms.
The analysis and results indicated that the severity of thermal runaway was highly influenced by the internal short circuit. The internal short circuit generated an extra electrical heat that was comparable to the heat released by the chemical reactions, which was responsible for the self-heating. However, the occurrence of the internal short circuit was not the only cause of thermal runaway, as in specific cells, thermal runaway occurred without any internal short circuit.
In another study published in the IOP Conference Series: Materials Science and Engineering, researchers displayed an approach to monitor battery temperature to prevent Li-ion battery failures.
Initially, they established a simulation model based on the second-order resistor-capacitor (RC) circuit model of the heat dissipation and generation of the Li-ion battery pack using SIMULINK. The battery pack temperature was tested under The New European Driving Cycle conditions.
Additionally, the heat dissipation coefficient in the simulation model was optimized by comparing it with the experimental battery temperature data based on the genetic algorithm using MATLAB.
The optimization result demonstrated that the difference between the actual temperature and the simulated temperature was within one degree, which indicated that the optimization result-based model can reflect the actual temperature change accurately.
To summarize, thermal analysis is playing a critical role in preventing thermal runaway and will continue to be relevant during the development of future battery designs. The analytical approach of the thermal runaway mechanisms can effectively guide the safety design of commercial Li-ion batteries.
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References and Further Readings
Zhang, H., Hao, K., Liu, W., Deng, Y., Wang, Y. (2020). Thermal Analysis for Lithium-Ion Battery Pack based on Parameter Estimation based on Genetic Algorithm. IOP Conference Series: Materials Science and Engineering, 793. https://www.researchgate.net/publication/340561326_Thermal_Analysis_for_Lithium-Ion_Battery_Pack_based_on_Parameter_Estimation_based_on_Genetic_Algorithm
He, X., Feng, X., Wang, L., Zheng, S. (2018). Probing the heat sources during thermal runaway process by thermal analysis of different battery chemistries. Journal of Power Sources, 378, 527-536. https://doi.org/10.1016/j.jpowsour.2017.12.050
Thermal Analysis and Rheology of Batteries [Online] Available at https://analyzing-testing.netzsch.com/_Resources/Persistent/7/5/f/7/75f703be7ff0de38746bfce0ee775dc3daa595a6/Thermal%20Analysis%20and%20Rheology%20of%20Batteries.pdf (Accessed on 07 February 2023)
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