Source : “Thermal Runaway Mechanism of Lithium Ion Battery for Electric Vehicles: A Review” X. Feng, et al Tsinghua University – Beijing
Internal short circuit and thermal runaway
As explained in the WATTALPS TechLetter on lithium-ion battery safety, the thermal runaway of a battery cell is the event to avoid to ensure safety in a battery system. Thermal runaway is a self-heating phenomenom, which can potentially lead to fire or explosion. When the battery reaches a temperature threshold, parasitic exothermic reactions occur and heat the battery even more. The reaction becomes unstable and can lead to a battery fire or an explosion in certain circumstances.
Source : “Thermal Runaway Mechanism of Lithium Ion Battery for Electric Vehicles: A Review” X. Feng, et al Tsinghua University – Beijing
The electronic battery management system ensures active protection against thermal runaway by maintaining cell in a safe operation zone (“electrical abuse” / “thermal abuse” of the figure here above). Additional protection measures in the battery design and its integration in the vehicle will strongly limit the probability of an internal short circuit linked to an accident (battery crush or penetration of conductive parts into the battery casing). These protection measures can be validated during qualification tests (ECE R100 V2, SAEJ2464, IEC62619…).
Nevertheless, it is not possible to prevent a cell internal short-circuit due to the presence of a metallic part inside the cell (error in the manufacturing process of the lithium-ion cell).
Probability of occurrence of an internal short-circuit due to the cell manufacturing process
According to the literature, the probability of occurrence of an internal short-circuit for a 18650 cell is of 0,1 ppm (source : “Thermal Runaway Mechanism of Lithium Ion Battery for Electric Vehicles: A Review” X. Feng, et al Tsinghua University – Beijing). This event will therefore occur more often for big batteries and for products made in big quantities. As example, it is estimated that one Tesla battery every 10,000 will undergo an internal short circuit linked to an error in the manufacturing process.
Thermal runaway propagation
It is therefore prominent to understand the behavior of a lithium-ion cell during a thermal runaway and put in place appropriate mitigation measures. Indeed, the heat generated during the thermal runaway of a lithium-ion cell can be enough to trigger thermal runaway of nearby cells and thus creating a chain reaction that can lead to explosion in worst cases. This is why more and more sectors make it compulsory to test a battery for thermal runaway propagation (DNV-GL for marine application, SAE for electric vehicles, IEC 62619 for stationary and industrial batteries…).
You will find in the following link a video of a thermal runaway propagation of a small battery made of five 18650 cells (LCO chemistry is the most reactive one).
Two different strategies
To prevent thermal runaway propagation, two different strategies are deployed:
- Use lithium-ion cells which present no risk in case of internal short circuit. This strategy strongly limits the use of lithium-ion cells. Indeed, high energy density cells evacuate more energy during the thermal runaway. Some LFP (Iron Phosphate) cells enables this type of design but their energy density is greatly reduced: 260 Wh/L for the best LFP cells against 700 Wh/L for the best lithium-ion cells.
Battery modules and packs manufactured from LFP cells provide energy densities ranging from 90 to 166 Wh/L (Excluding heating/cooling system).
- Develop a mitigation system enabling to manage cell thermal runaway and prevent cell to cell propagation. This strategy enables to use a wide range of lithium-ion cells but needs a much greater design and validation work. The design indeed has to evacuate safely the vent gas from the thermal runaway but also integrate passive heat dissipating systems to prevent adjacent cells from reaching their temperature limit triggering thermal runaway. WATTALPS modules have passed the non-propagation tests according to IEC 62619 with NCA lithium-ion cells, having an energy density higher than 700Wh/L.
WATTALPS safe modules and battery packs provide energy densities ranging from 220 to 300 Wh/L and integrate a coolant circulation network to easily cool down or warm up the battery.
NB : if you are not familiar with the terms LFP or NCA, do not hesitate to read our TechLetters on positive and negative electrodes of lithium-ion batteries, as well as the one on electrical energy storage.