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.
