Lithium-ion batteries (LIBs) are a dominant state-of-the-art energy storage system and have importance in the automotive sector. Still, LIBs suffer from aging effects and serious hazards from failing batteries are possible. These failures can lead to exothermic chemical reactions inside the cell, ending up in thermal runaway (TR). TR has caused most electric vehicle (EV) fires. Since statistically most accidents with EVs happen after about one year of vehicle usage, in particular, the failing behavior of aged cells needs to be investigated. Little information was available in open literature about the influence of aging paths on the failing behavior and especially on the degassing behavior of large automotive LIBs.
Therefore, this study investigated the influence of three different aging paths (cyclic at -10°C and at 45°C and calendric at 60°C) on the thermal behavior, the vent gas emission, and the vent gas composition.
The results showed a clear effect of aging on the failing behavior. The expectation of a stronger reaction of the failing aged (cy-10°C) cell due to Li plating has not been confirmed. Because of the rest time after the Li plating, the plated Li might have intercalated or chemically reacted which had positive effects on the safety behavior of the cell during TR. In this study, using aged cells in overtemperature TR experiments ended up in reduced maximum temperatures, lower amount of produced gas, significantly lower amount of CO in the vent gas and lower mass loss than in the same experiments using fresh cells.
For each cell a first venting before the TR was observed. There were no significant changes for the first venting temperature between fresh cells and aged cells, but a significant higher average temperature for the second venting (start of TR) for cyclic aged cells at 45°C, which means an increase in thermal stability for cells cycled at 45°C. A decrease of thermal stability was observed for cells cycled at -10°C, because the second venting started at a lower temperature.
A linear correlation between the total amount of produced vent gas and the current capacity of each cell before the TR experiment was observed and reported as a new finding. 0.06 mol/Ah vent gas in overtemperature TR triggered cells could be a possible relation in general for NMC (622)-graphite chemistry cells with liquid electrolyte mixture (EC, EMC, DMC, DEC).
The main gas compounds produced during the TR (CO2, CO, H2) and higher hydrocarbons were independent of the aging path, but the exact vent gas composition differs between the different aging paths.
Aging of LIBs and the aging paths have a significant effect on the failing behavior of LIBs during TR, and consequently on the safety relevant parameters such as gas emission, maximum reached cell surface temperatures and vent gas composition. A general rule might be that a less violent TR behavior is observed for aged cells with power and capacity fade in comparison to fresh cells, if no fresh metallic Li plating is produced. If fresh metallic Li plating is produced inside the cell, a stronger TR reaction is expected. If we exclude the case of Li plating, these results show that experiments with fresh cells represent the worst case of TR reaction. Consequently, we recommend the use of fresh cells for the determination of safety-relevant parameters for the design of battery applications.
For more information, see the corresponding journal publication: Essl, C.; Golubkov, A.W.; Fuchs, A. Influence of Aging on the Failing Behavior of Automotive Lithium-Ion Batteries. Batteries 2021, 7, 23. https://doi.org/10.3390/batteries7020023.