Recent Li-ion battery fire incidents in smartphones, hoverboards, e-cigarettes, electric vehicles, and aircrafts highlight the limited understanding of battery thermal runaway failure and inadequacy of current battery safety or protection mechanisms. Li-ion battery thermal runaway can occur due to the presence of material defects, operation under abusive/extreme conditions, or cumulative damage under normal operation. Electrical and thermal characterization of Li-ion batteries under thermal runaway conditions is necessary to improve the battery safety.

In the past various studies have been conducted to investigate thermal runaway failures under different operating conditions [1-3]. This work studies the effects of additional parameters and operating conditions on Li-ion battery thermal runaway induced by external heating of the battery. External heating is a common loading condition for batteries located in close proximity to an electronic circuit or inside a battery pack. A detailed electrical and thermal characterization of battery has been performed to understand the failure mechanisms and to provide guidance on battery monitoring and safety management.

Experiments were conducted on commercial 18650 Li-ion cells with 2600 mAh nominal capacity (at C/5) and nominal voltage of 3.6V. These cells have safety vents located below the positive terminal to allow release of gases. Cells in fully charged and fully discharged conditions were used for the experiments. A high temperature Nickel Chromium heating wire was wrapped around the cells and powered by a DC source to raise the cell temperature. The heating rate (current) of the wire was varied to study its effects on thermal runaway. Three different external heating cut-off conditions were investigated in this study: heating cut-off after the occurrence of thermal runaway, heating cut-off just after the trigger of cell venting, and heating cut-off at temperatures below cell venting trigger temperature. Two K-type thermocouples were mounted on cell near the two ends (terminals) to monitor the cell surface temperature. Cell voltage measurements were also recorded during the entire testing. All the experiments were conducted inside an in-house explosion chamber with a transparent glass window for the video recording.

The experimental results identify the operating conditions with high susceptibility to thermal runaway failure. The electrical and thermal observations indicate the possible cell failure mechanisms, reveal different stages of cell failure, and correlate the electrical and thermal failure behavior of the cell. The results show that the heating of fully charged cell up to a critical temperature level can lead to electrical failure (voltage drop below the cell cut-off voltage) even if the cell does not undergo thermal runaway. Parameters such as cell venting trigger temperature, cell maximum temperature, cell self-heating rate, voltage drop, and time duration of the thermal runaway have been estimated from the measurements. The learnings from this study form the basis of recommendations for improvement in battery safety management.

References

1. H. Maleki, G. Deng, A. Anani, J. Howard, Thermal stability studies of Li-ion cells and components, J. Electrochem. Soc. 146 (9) (1999) 3224-3229.
2. X. Liu, S.I. Stoliarov, M. Denlinger, A. Masias, K. Snyder, Comprehensive calorimetry of the thermally-induced failure of a lithium ion battery, J. Power Sources 280 (2015) 516-525.
3. A.W. Golubkov, S. Scheikl, R. Planteu, G. Voitic, H. Wiltsche, C. Stangl, G. Fauler, A. Thaler, V. Hacker, Thermal runaway of commercial 18650 Li-ion batteries with LFP and NCA cathodes – impact of state of charge and overcharge, RSC Adv. 5 (2015) 57171-57186.