Investigation of Catastrophic Battery Failures

This Investigation of Catastrophic Battery Failures looks at field failures of lithium ion batteries in devices such as e-scooters and e-bikes. This is a summary of: Holloway, J.; Maharun, M.; Houmadi, I.; Remy, G.; Piper, L.; Williams, M.A.; Loveridge, M.J. Developing Preventative Strategies to Mitigate Thermal Runaway in NMC532-Graphite Cylindrical Cells Using Forensic SimulationsBatteries 202410, 104.

Author: Justin Holloway

Reports of e-bike and e-scooters catching fire and exploding while charging is prompting a rethink of safety protocols for lithium ion batteries (LIBs). One important tool for understanding these events is to carry out root cause analysis of the failure. This allows the failure mechanism to be determined and mitigation strategies hypothesised. Furthermore, these failures can be simulated within a laboratory with these mitigation strategies trialled. This process allows suitable safety controls to be in place during LIB use, as well as how additional controls can be applied. This in turn will reduce the impact these catastrophic failures have on people, equipment and the environment.

A recent study emulated a real life thermal event (shown by Figure 1a) where a module containing cylindrical LIBs went into thermal runaway. Analysis of the module showed a single cell had gone into thermal runaway beneath a battery management system (BMS). The failure mode was determined to be localised heating due to the metal–oxide–semiconductor field-effect transistors (MOSFETs) within the BMS device while the device was charging, similar to the e-bikes and e-scooters failures mentioned.

field failure investigation of escooter battery
Figure 1: Images of the a) failed module and b) setup for simulation of cell failure from the module.

Simulation Tests

Simulation testing was carried out within a safety testing chamber where a heating pad was attached to the outside of the cell (see Figure 1b). The heating pad applied localised temperatures of 100°C, 150°C, 200°C and 250°C for 3 hours. Temperatures were applied while the cell was at high SOC and low SOC. The low SOC test involved charging the cell while heating occurred. Damage to the cell was evaluated using x-ray tomography, mechanical testing and microscopy. Gases produced during testing were analysed using a mass spectrometer. The cells tested were the same as used in the module failure – commercial LG 18650 cells with nominal capacity 2.2Ah and chemistry NMC532/graphite.

Inducing Venting and Thermal Runaway in Cells

Cells were tested in a low SOC and a high SOC state. Testing the high SOC state induced venting and thermal runaway when the localised temperatures were 200°C and 250°C. Figure 2 shows an image of the cell after thermal runaway with the temperature at 250°C. The low SOC state cell with the same applied temperatures induced venting only. The likelihood of thermal runaway can thus be reduced by keeping cells at low SOC.

The study also analysed thermal runaway and venting as separate thermal events. Venting is known to occur in a cylindrical cell when the internal current interrupt device (CID) breaks relieving pressure buildup within the cell. Thermal runaway subsequently occurs with further heating. By removing the heat source after venting and allowing the cell to cool (in this case by convection cooling) the cell did not undergo thermal runaway. This has important implications, by detection of venting and the activation of a cooling mechanism can prevent thermal runaway.

field failure investigation of escooter battery
Figure 2: Image of the cell at high SOC after testing with 250°C. The cell had failed catastrophically, involving charring of the cell, ejection of componentry and deformation of the cell.

Mechanical Degradation

The cylindrical cell is analogous to a thin walled pressure vessel and is designed to fail at the top of the cell where the CID device is housed. The cell can must therefore be thick enough to withstand internal stresses and allow failure at the top of cell. Mechanical degradation of the cell can was analysed by hardness testing and microstructure evaluation after the tests. Results showed thermal runaway caused a significant decrease in mechanical properties, however the cell can was thick enough to withstand the stresses induced by venting and thermal runaway.

field failure investigation of escooter battery
Figure 3: Examples of mechanical degradation of cells after testing shown by x-ray computed tomography. a), b), c) are longitudinal scans and d), e), f) are axial scans. Cells that underwent thermal runaway (the 200°C and 250°C samples) show significant mechanical degradation – bulging shown by blue arrows and melting of the jelly roll shown by green arrows.

Exhumed Gas Analysis

Another important consideration for the safe use of LIBs is the gases that are evolved during venting and thermal runaway. In this study gases were monitored during testing. The gases detected include CH4, C2H4, C2H6, C3H8, H2, H2O, CO, O2, and CO2.

field failure investigation of escooter battery
Figure 4: A thermal image of the cell during the venting event. The hot venting gas is shown.

Summary

Commercial cylindrical cells, containing NMC532 cathode and graphite anode, were heated until gas generation inside the cell from breakdown of the SEI. This subsequently caused a cascade of reactions resulting in venting and catastrophic failure of the cell (thermal runaway). Mechanical degradation of the cell was observed by melting, recrystallisation, deformation and ejection of cell componentry. Catastrophic failure can be ameliorated by having the cell at low SOC or by removing the heat source during testing. A period of time was noted between venting and thermal runaway – an ideal time to activate a cooling mechanism – thus preventing thermal runaway.

Root cause analysis and failure simulation of these incidents is necessary to determine the failure mechanism and trial mitigation strategies. This process will enable a better understanding of battery safety and associated system design improvements. Additionally this approach will facilitate further regulatory approvals for consumer electronics.

For more details and depth please refer to the full paper:

Holloway, J.; Maharun, M.; Houmadi, I.; Remy, G.; Piper, L.; Williams, M.A.; Loveridge, M.J. Developing Preventative Strategies to Mitigate Thermal Runaway in NMC532-Graphite Cylindrical Cells Using Forensic SimulationsBatteries 202410, 104.

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