Thermal runaway initialization temperature of fresh and aged large prismatic lithium-ion cells

Summary:

Driven by the field of electric vehicles, the energy density of lithium-ion cells is rapidly increasing. Therefore, safety aspects in lithium-ion battery design become even more important. In case of a single cell failure, a thermal runaway (TR) might be triggered which results in an uncontrolled heat release within the battery system. Heat might be transferred rapidly to neighbouring cells and result in cell-to-cell thermal propagation if the TR initialization temperature is reached. Efficient countermeasures have to be implemented during lithium-ion battery design in order to avoid thermal propagation. Therefore, a sufficient knowledge about TR initialization temperature is required.

In this work, the initial TR temperature of several fresh and aged large format prismatic NMC111 lithium-ion cells with 37 Ah in PHEV2 format has been analysed. Therefore, cells are placed individually in a heater. In a first step, heating starts until the ambient temperature reaches 80 °C. Secondly, this temperature is kept constant for two hours in order to preheat the cell. In the next steps, ambient temperature increases by 5 °C every 30 minutes until TR is triggered and the oven is deactivated. The TR initialization temperature is defined as the point where the heating rate reaches 1 °C/s.

Different measurements with fresh cells of the same cell chemistry showed that the TR initialization temperature is subject to statistical variations. By comparing measurement results of fresh and aged cells, a huge difference between the TR initialization temperatures is recognized. Although TR initialization temperature for aged cells seems to follow a distribution as well, the mean value is approx. 120 °C lower for aged cells than for fresh cells. This means that cell-to-cell propagation might start earlier in aged lithium-ion batteries as the TR initialization temperature is lower. Additionally, aged cells showed a massive amount of venting gas ignition and ejected material, which leads to lower surface temperatures compared to fresh cells.

Today, many safety devices for lithium-ion batteries are designed by only considering fresh lithium-ion cells. Future developments for lithium-ion battery technologies should take the presented variations of TR initialization temperature for aged and fresh lithium-ion cells into consideration.