**6. Conclusions**

A comprehensive hazard analysis of modern automotive high capacity NMC/LMO—graphite pouch cells was performed at three overtemperature TR experiments. The investigated cells are currently used in commercially available mass-produced EVs.

In the first experiment the cell is charged to 100%, in the second to 30% and in the third to 0% SOC. The results confirm the influence of the SOC on the failing behavior of the LIB. The fully charged cell could be triggered into TR, but the cells with SOC ≤ 30% could not. The experiments show that there

are serious risks (safety and health) at failing state-of-the-art Li-ion cells resulting from electrolyte vapor, generated heat, gas and particles at TR as toxic and flammable gas, explosion and fire. Safety relevant hazards are electrolyte vaporization, heat generation, gas emission including gas rate, gas composition including electrolyte and particle emission including size and content of the particles.

Main findings of the investigated automotive cells are:

	- - The main gas components are after the first venting and constant gas production until the heating is stopped in descending order CO2, DEC, H2O, CO, H2, C2H4, CH4, C3H8.
	- - One presented hazard is electrolyte vaporization. Commonly used electrolyte components such as EC, DEC, DMC, EMC in an unsealed cell are critical due to the consequential irritant, toxic, cancerogenic and flammable atmosphere. At this cell EC (irritant, PAC-1: 30 mg/m3) and DEC (flammable, PAC-1: 2 mg/m3) are the main electrolyte components. It is important to address this hazard especially in large traction battery EV applications, where significant amounts of electrolyte may vaporize inside a closed system (pack, garage, tunnel).
	- - Enormous heat is generated by the cell, the cell surface temperatures increased above 700 ◦C. The main exothermic reaction developed to a rapid TR when the hottest measured part of the cell reached 231 ◦C. Within 4.28 s the TR propagated through the cell. This high surface temperature can lead to TR propagation to neighboring cells and irreversible damage of the battery pack.
	- - Overall, 2.31 mol (57 L, 1.3 L/Ah) of *gas* is produced. The cell released 0.14 mol before the TR. During the TR, the cell released in 4 s additional 2.17 mol with a characteristic rate of 0.8 mol/s (18.7 L/s). 50% of the gas is produced in 1.4 s. The abrupt pressure increase at the TR is a serious risk inside a closed volume.
	- - The cell mass reduces by 43% of the initial mass. This mass reduction can be explained as the sum of released gas and ejected particles at TR.
	- - The main gas components are: 38% CO2, 23% H2, 17% CO, 8% H2O, 6% C2H4, 4% CH4 and 3% electrolyte vapor (DEC). The measured gas components are about 20% of the lost cell mass during TR and 9% of the initial cell mass. Toxic (CO) and flammable (H2, CH4, DEC, etc.) gas components are dangerous when entering the passenger compartment.
	- - A large number of ejected particles are smaller than 10 μm2. Novel nondestructive sampling and analysis methods were used to evaluate the particle parameters: The smallest analyzed particles have an area of 0.1 μm2, thus a circle equivalent diameter of roughly 6 nm. A total of twelve elements were detected in the particles, including elements like Al, Ni or F. These ejected hot particles (~35% of the initial cell mass) may ignite the vent gas, are carcinogenic and respirable for humans.

To reach an acceptable level of safety in EVs a comprehensive analysis of hazards is very important. In order to define testing standards, the battery hazard influencing factors (such as energy content of the cell, chemistry, the failure case/trigger, cell design, SOC and SOH) must be characterized clearly. The five presented hazards addressed in this study should also be considered in future work for di fferent cell types. We recommend to include in the quantification of safety relevant parameters

such as the maximum reached cell surface temperature, the amount if produced vent gas, the venting rate, the composition of the produced gases at the first venting and the TR including electrolyte vapor and the size and composition of the produced particles to cover the most significant hazards at battery failures.

Our future work is aimed to evaluate the influence of di fferent triggers, cell design (pouch versus prismatic metal can) and aging on the failing behavior of large automotive Li-ion cells with higher capacity than the presented sample. To guarantee safety at LIB applications it is important to be aware of potential safety and health risks originated from failing cells.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2313-0105/6/2/30/s1, Document S1: SEM images of particle classes.

**Author Contributions:** Conceptualization, C.E.; methodology for gas analysis, C.E.; methodology for particle analysis, E.G., A.Z., M.N., C.E.; software, C.E.; validation, C.E., A.W.G.; formal analysis, C.E., A.W.G., E.G.; investigation, C.E.; data curation, C.E., E.G.; writing—original draft preparation, C.E.; writing—review and editing, A.W.G., A.F., M.N., A.Z., E.G., E.E.; visualization, C.E., A.W.G., E.G.; supervision, A.F.; All authors have read and agreed to the published version of the manuscript.

**Funding:** The publication was written at VIRTUAL VEHICLE Research GmbH in Graz and is partially funded by the COMET K2—Competence Centers for Excellent Technologies Program of the Federal Ministry for Transport, Innovation and Technology (BMVIT), the Federal Ministry for Digital and Economic Affairs (BMDW), the Austrian Research Promotion Agency (FFG), the Province of Styria and the Styrian Business Promotion Agency (SFG). The study shows the results of the FFG project SafeBattery. The K-project SafeBattery is funded by the BMVIT, BMDW, Austria and Land Steiermark within the framework of the COMET—Competence Centers for Excellent Technologies program. The COMET program is administered by the FFG.

**Acknowledgments:** Special thanks to the institution ICTM for the cooperation in the project SafeBattery, particularly Ilie Hanzu and Petra Kaschnitz for the analysis of the electrolyte of the investigated cell. Special thanks to the Institute of Electron Microscopy and Nanoanalysis (FELMI) of Graz University of Technology for the cooperation at the particle analysis in the course of Eva Gassers Master's thesis, especially to Werner Grogger. The setup of the electron microscope Sigma 300 VP was enabled by the project "HRSM-Projekt ELMINet Graz — Korrelative Elektronenmikroskopie in den Biowissenschaften" (i.e., a cooperation within "BioTechMed-Graz", a research alliance of the University of Graz, the Medical University of Graz and Graz University of Technology), which was financed by the Austrian Federal Ministry of Education, Science and Research. The combination with the EDX detector could be realized by the project "Innovative Materialcharakterisierung" (SP2016-002-006), which is part of "ACR Strategisches Projektprogramm 2016" of the Austrian Cooperative Research (ACR), where a support by the Austrian Federal Ministry for Digital and Economic A ffairs is to be mentioned. The thermal runaway test stand was developed with technical and financial support by AVL List GmbH.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results.
