Lithium-Ion Cell Safty Experiments Under Adiabatic Conditions: Nail Penetration

Abstract

Listen

Translate article

We are introducing a new measurement method for nail penetration tests for lithium-ion cells. Nail penetration is a useful abuse test for lithium-ion cells to simulate a catastrophic external caused (e.g. car crash with extensive deformations) internal short circuit. In principle a nail is forced into the cell, leading not only to an immediate short circuit of the electrode layers which is resulting in a fast self-heating of the entire cell but also to a contamination of the electrode material and the electrolyte with oxygen and water.An important safety test guideline for the use of batteries in motor vehicle is the SAND 99-0497 from the US research institute Sandi National Laboratories. Besides a nail penetration test, there is also a thermal abuse test with an accelerating rate calorimeter (ARC) prescribed. The heat generation and pressure development is measured in an ARC under adiabatic conditions. An environment without the possibility of heat dissipation is the worst-case scenario for an abuse test and is simulating the abuse condition of a single cell packed in a larger cell stack of a lithium-ion battery with a defect or not sufficient battery cooling. Measurement Setup– All experiments were carried out in a modified ARC® 254 from Netzsch Gerätebau GmbH (Germany). A schematic view of the ARC measurement setup is provided in Fig. 1. The modifications consist of a pneumatic press and a pneumatic valve to control the speed of the steel nail. The sample holder for an 18650 type lithium-ion cell is keeping the cell in place and providing a possibility to measure the sample temperature on the cell surface.Several research groups investigated the thermal stability of cathode materials for lithium-ion cells in contact with electrolyte. [1-3] Therefore, a 1.1 Ah graphite / lithium-iron-phosphate (LFP) high power cell and a 2.6 Ah graphite / lithium-nickel-manganese-cobalt-dioxide (NMC) and lithium-cobalt-dioxide (LCO) blend, high energy cell at different states of charge (SoC) and states of health (SoH) were used to investigate the influence of different cell types and cathode materials. Results– To the best of our knowledge, there was no literature or experience for this kind of experiments available. To evaluate and classify the results from the here presented tests, an adiabatic nail penetration test was performed on five identical LFP cells at 50 % SoC. In Fig. 2 the temperature and the pressure inside the ARC tube are plotted against the elapsed time for all five experiments. The nail impact into the cell leads to an immediately short circuit of multiple electrode layers and significant damage to the cell body, causing a contamination of the electrodes and electrolytes. This incident is reflected by the fast increase of the temperature (Fig. 2). The peak temperature and pressure (standard deviation of all five tests: ±1.46 K and ±0.01°bar) are a good characteristic of the adiabatic nail penetration experiment.The peak temperature is rising with increasing SoC for the LFP cells (Fig. 3). Furthermore, the measurement parameters e.g. peak temperature, peak pressure, heat rates and press rates are for several experiments with different cell chemistries, SoCs and SoHs compared to each other. The temperature and pressure development for all experiments are carefully analysed and new endothermic reactions are identified. Conclusion– The here presented nail penetration experiment under adiabatic conditions is an advancement in the field of mechanical abuse tests through the selected measurement conditions. This method enables a very detailed and accurate analysis of the nail penetration experiment as tool to analyze worst-case scenarios.