Needle Penetration Studies on the Dynamics during Internal Short Circuits in Automotive Lithium-Ion Cells: Measurement of Internal Temperature and Short Circuit Current

Lithium-ion batteries (LIBs) have been widely applied in consumer electronic devices and electric vehicles for energy storage. Especially for the battery electric vehicles (BEVs), it is desirable to increase the volumetric energy density of LIB cells due to the limitation of a further volumetric expansion of the battery pack, which amounts to currently 220-400 L depending on the size of the BEV [1]. As an example, the capacity of the Li-ion battery pack in BMW i3s has been improved from 60 Ah over 94 Ah to 120 Ah over the course of a few years. Along with this trend of increasing energy density, however, the reactivity of the cells is increasing. Therefore, the potential safety issues are becoming more important. Between 2015 and 2018, a total of 15 fire incidents of BEVs were reported in Europe. In 2019, 113 fire incidents took place only within China [2]. Considering the decision by the European Union to ban the sale of new petrol and diesel vehicles from 2035, this concern with regards to the safety of LIBs would be growing progressively with the accelerated switch to BEVs. One of the main causes for a thermal runaway is an internal short circuit (ISC) [1,3]. Under many different abuse scenarios, ISC appears to trigger the thermal runaway (TR); (i) mechanical abuse (deformation of separator leading to ISC),(ii) electrical abuse (separator pierced by dendrite resulting to ISC) or iii) thermal abuse (shrinkage or melting of separator causing ISC) [1]. Nail penetration tests have been widely used to reproduce ISCs. Only the temperature on the cell surface and the cell voltage are measured in a conventional nail penetration test. However, there is also a challenge to understand the internal dynamics in a Li-ion cell in which an ISC takes place [4]. This study introduces an advanced needle penetration test that enables the measurement of both the internal temperature and the ISC current. As seen in Figure 1 (a), the amount of ISC current can be measured indirectly by replenishing the lost cell capacity from ISC with a power supply that is set to constant voltage mode (in this case, 4.2 V) and connected to the cell via a shunt. Additionally, the internal temperature can be measured using a thermocouple which is embedded in a specially crafted probe. The needle penetrates Li-ion cells with a slow penetration speed of 0.01 mm/s to understand the effect of piercing individual layers on the ISC with regard to electrical and thermal behaviour. Figure 1 (b) presents the ISC current and the internal temperature measured using this method. In the unstable ISC current signal, sharp periodical peaks are detectable. It is assumed that the ISC current is constantly increasing by several layer-to-layer shorts adding up and that its periodically decreasing profile originates from the increasing contact resistance by cell layers melting or being ruptured. Since thermal energy emerges from the ISC current, the internal temperature fluctuates dynamically corresponding to the ISC current profile, while surface temperature signals do not show any changes. This advanced needle penetration test offers the great research possibility to understand the internal dynamics of ISC that has been barely studied and to determine the internal onset cell temperature for TR. [1] X. Feng et al Energy Storage Mater. , 10 , 246 (2018). [2] Y.S. Duh et al J. Energy Storage , 41 , 102888 (2021). [3] D.P. Finegan et al J. Electrochem. Soc. , 164 , A3285 (2017). [4] S. Huang et al J. Electrochem. Soc. , 167 , 90526 (2020). Figure 1