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In situ stress analysis of lithium-ion battery cell

Periodic Reporting for period 1 - LIB STRESS (In situ stress analysis of lithium-ion battery cell)

Reporting period: 2018-07-30 to 2020-07-29

Thermal runaway may occur for a lithium ion battery (LIB) cell when its temperature exceeds a certain value. The temperature of the LIB cell depends on the heat balance between its heat production and heat dissipation. The temperature inside the cell is difficult to measure because it is sealed. However, surface temperature, which can give a warning sign before failure of the cell, can be easily measured and used to provide an indication of cell internal temperature. Work is also ongoing correlating such warning signs with battery failure.

Lithium-ion batteries (LIB) play a pivotal role in the electrification of transport and energy storage systems (ESS) as well as in consumer market. However, the recurrent re and explosion incidents involving LIBs in mobile phones, laptops, electric vehicles (EVs), hybrid EVs, airplanes and ESS have raised increasing concern regarding their safety. When LIBs are subjected to abuse conditions, thermal runaway (TR) may be induced, leading to fire and even explosions. In such scenarios, multiple electrical, electrochemical, physical, and chemical processes take place simultaneously across different scales.

The Fellowship aims to analyse the thermal behaviour of lithium ion cells during abuse conditions and thermal runaway, giving particular focus to the evolution of thermal runaway and the resulting gases emissions and fires. The specific objectives of the research include:
- Investigate LIB cell temperature changes under thermal abuse.
- Improve both OpenFOAM and COMSOL to study the TR evolution.
- Validate the above predictions with published data and new data to be generated in collaboration with others.
- Investigate the combined effects of electrochemical, mechanical and thermal on LIB behaviour and performance.
- Seek collaboration with industrial partners to give the research an industrial dimension.
A comprehensive analysis has been conducted for the thermal runaway (TR) characteristics of type 21700 cylindrical LIBs with a specific energy of 266 W∙h/kg. The batteries with both 30% state of charge (SOC) and 100% SOC were triggered to TR by uniform heating using a flexible heater in a laboratory environment. High definition cameras were used to capture TR behaviour and flame evolution from different viewpoints. Correlation between the heat release rate (HRR) and the mean flame height of turbulent jet diffusion flame were used to estimate the HRRs of LIBs. Additional characteristics of cell failure (for cells with 100% and 30% SOC) were also noted for comparison, including: number of objects ejected from the cell; sparks and subsequent jet fires. An approach has been developed to estimate the HRRs from TR triggered fires and results compared with previous HRR measurements for type 18650 cylindrical cells with a similar cathode composition.

A simplified mathematical model has been developed for the evolution of heating-induced TR of LIBs. This model only requires a minimum number of input parameters, and some of these unknown parameters can be obtained from accelerating rate calorimeter (ARC) tests and previous studies, removing the need for detailed measurements of heat flow of cell components by differential scanning calorimetry. The model was firstly verified by ARC tests for a commercial cylindrical 21700 cell for the prediction of the cell surface temperature evolution with time. It was further validated by uniform heating tests of 21700 cells conducted with flexible and nichrome wire heaters, respectively. The validated model was finally used to investigate the critical ambient temperature that triggers battery TR. The predicted critical ambient temperature is between 127 °C and 128 °C. The model has been formulated as lumped 0D, axisymmetric 2D and full 3D to suit different heating and geometric arrangements and can be easily extended to predict the TR evolution of other LIBs with different types and chemistry. It can also be easily implemented into other computational fluid dynamics (CFD) code.
The model was then validated with heating tests by both flexible heater and nichrome-wire heaters. The variation of the normalised amount of reactant and degree of conversion with time and temperature was used to further explain the change of temperature rising rate of the cell during TR. The model has achieved reasonably good agreement with the measurements for the time to reach the maximum temperature.

In order to address the abnormal conditions under mechanical abuse, e.g. impact by a projectile. Analysis has been conducted for nail penetration induced TR for both pouch and cylindrical cells. Comparison has been made with published experimental data.

Presentations and events:
1. Oxford Battery Modelling Symposium, 16-17 March 2020, Virtual meeting, University of Oxford, UK.
2. Aerospace Battery Safety and Standards workshop, 18 Nov. 2019, WMG, University of Warwick, UK
3. Cenex Events, 4-5 Sep. 2019, Millbrook Proving Ground, Bedfordshire, UK.
4. The 1st Int. Symp. on Lithium Battery Fire Safety (1st ISLBFS), 18-20 July 2019, Hefei, China.
Provided experimental insight for high specific energy type 21700 LIBs with different state of charge (SOCs) under uniform electrical heating. The whole process from venting to TR, and the resulting fire were recorded by HD and high speed cameras. The characteristics of spark and jet fire events during the combustion of cells were analysed. The transient flame heights were calculated by image processing. The effects of SOC on fire behaviour were presented and explained with flammability limit analysis and vent gas components of type 18650 LIBs. Published correlations between HRR and the mean flame height of turbulent jet diffusion flames were extended to estimate the HRRs in the cell fire. HRR predictions were compared with the reported values for similar LIB cells.
Developed an empirical approach to predict the transient heat release rates (HRRs) of the 21700 LIB. This simplified approach can be used as an alternative means to estimate the HRRs in addition to the commonly used oxygen consumption method which also has its limitations.

Developed and validated a simplified mathematical model for predicting the evolution of heating induced TR of 21700 cells has been developed. This model assumes that the exothermic reactions during TR follow two Arrhenius expression to describe the decomposition reaction and autocatalytic reaction. These assumptions have reduced the input parameters required to calculate heat generate rates generated by exothermic reactions. The model has been formulated as lumped 0D, axisymmetric 2D and full 3D. The lumped model (0D) can be used for predicting ARC tests and the critical ambient temperature when the Biot number is small, the 2D axisymmetric model is used when the heating conditions are axisymmetric and the 3D model can be used with neither of the above conditions can be met, such as the flexible heater tests in the present study. The model has been implemented in COMSOL Multiphysics 5.4® in the present study, but it can be easily implemented into other CFD codes as well.

Progress has also been achieved for predicting nail penetration induced thermal runaway in pouch and cylindrical cells. For the later, analysis has also been extended to cell clusters to examin the propagation of thermal runaway with and without mitigation measures.