**4. Results**

Three experiments with fresh automotive pouch cells were conducted. In the first experiment the cell is charged to 100%. In the second experiment the cell is charged to 30% and in the third to 0%. The first venting of the cell could be observed at all three test samples. The TR could only be triggered at the fully charged cell.

#### *4.1. Heat Generation*/*Temperature Response*

One critical hazard of a failing cell is heat generation, which can be detected by measuring the temperature response of the cell to the trigger (Figure 6). The experiment of the fresh automotive pouch cell at 100% SOC is compared to the 30% SOC cell in Figure 6a,c during the whole heat ramp experiment and Figure 6b,d at the main exothermic event.

**Figure 6.** Overtemperature experiments of a fresh automotive pouch cell at (**<sup>a</sup>**,**b**) 100% SOC and (**<sup>c</sup>**,**d**) 30% SOC: (**<sup>a</sup>**,**<sup>c</sup>**) show the temperatures at up to 30 different positions during the heat ramp experiment measured on the cell surface (red) and inside the reactor (green). The heater output of the sample holder in% is plotted (black line). The cell voltage times 100 is plotted in blue. (**b**,**d**) show the temperature measured at the main exothermic event. In (**b**) ±1 A pulses are visible (blue).

#### 4.1.1. Experiment with the 100% SOC Cell

As the fully charged cell is heated it shows a minor temperature excursion in the range of *TV*<sup>1</sup> *cell* = 130 ◦C—the first venting of the cell—10,300 s after activating the heat ramp (Figure 6a). The pouch cell opens. If the cell gets heated up further, the cell reaches the onset temperature. The onset of the main exothermic reaction is detected at *Tonse<sup>t</sup> cell* = 170 ◦C. The voltage of the cell started decreasing during the heating phase at 70 ◦C and dropped completely to 0 V at 203 ◦C cell surface temperature. The second venting starts at *TV*<sup>2</sup> *cell* = 212 ◦C. The main exothermic reaction developed to a rapid TR at *Tcrit cell* = 231 ◦C (self-heating beyond 10 ◦C/min). At 100% SOC the cell exhibited an exothermic reaction after 19,397 s and reached a maximum temperature of *Tmax cell* = 715 ◦C on the cell surface. The main exothermic reaction begun at a location between the center of the cell and the positive tab of the cell. Within 4.28 s the exothermic reaction propagated through the cell (time between the rapid increase of the first thermocouple and the increase of the last thermocouple in Figure 6b).

#### 4.1.2. Experiment with the 30% and 0% SOC Cell

Compared to the fully charged fresh cell, the cell with 30% SOC behaves di fferently using the same overtemperature setup (Figure 6c,d). After reaching the first venting at about *TV*<sup>1</sup> *cell* = 127 ◦C, no exothermic reaction can be detected even by heating beyond 231 ◦C. The 30% SOC cell is heated with a constant rate of 0.36 ◦C/min until 38,000 s and afterwards with an increased rate up to 309 ◦C (Figure 6c). After reaching the 309 ◦C maximum cell surface temperature, the heat ramp is stopped.

The 0% SOC cell also could not be triggered into TR by heat. At *TV*<sup>1</sup> *cell* = 120 ◦C cell surface temperature, the first venting is detected. The experiment is stopped heating up to 240 ◦C.

#### *4.2. Gas Emission*

#### 4.2.1. Experiment with the 100% SOC Cell

The pressure inside the reactor increases slowly at the first venting of the pouch cell and abruptly at the TR (Figure 7a). Figure 7b shows that the gas emission of the cell at the TR takes in total about 4 s. About 50% of the gas is produced in Δt50% = 1.44 s and 90% in Δt90% = 3.22 s.

**Figure 7.** Absolute pressure (green) versus time of the fully charged cell (**a**) during the whole experiment and (**b**) at the TR only. The maximum pressure is reached 4 s after the TR starts. 50% of the gas is produced in 1.44 s (red line). 90% of the gas is produced in 3.22 s (blue).

The fully charged cell released during the first venting nv1 = 0.14 mol of gas (Figure 8a). During the main TR reaction, the cell released additional nv2 = 2.17 mol of gas with a characteristic venting rate of . *n*ch = 0.8 mol/s (18.7 L/s). The calculated produced vent gas amount is shown in Figure 8a. At 100% SOC in total nv = 2.31 mol gas, which is equivalent to 52 norm liters (at 0 ◦C, 1013.25 hPa) and 57 L at STP, are produced. The fully charged cell produced 0.06 mol/Ah (equivalent to 15 mol/kWh, 1.3 L/Ah) during the overtemperature TR experiment.

**Figure 8.** Produced vent gas amount nv in mol and liter at STP during the experiments of the (**a**) 100% and (**b**) 30% SOC cell. At the 100% SOC cell two venting stages are measured: A first venting starting at *TV*<sup>1</sup> *cell* and a second venting starting at *TV*<sup>2</sup> *cell*. The 30% SOC cell released gas starting at the first venting at *TV*<sup>1</sup> *cell*until the heating was stopped.

4.2.2. Experiment with the 30% and 0% SOC Cell

The 30% SOC cell released nv = 0.53 mol (13 L) gas during the first venting and constant evaporation of electrolyte until the heating is stopped at 309 ◦C (Figure 8b). Compared with nv1 of the fully charged cell, the 30% cell released nv = 0.11 mol until Tcell = 212 ◦C. The discharged cell shows a similar behavior and produces nv = 0.41 mol (10 L) gas until the heating is stopped at 240 ◦C. In these cases, after the first venting, additional gas is produced during the heating phase.

**Figure 9.** Produced vent gas amount in mol for 0% (green), 30% (red) and 100% SOC (blue) pouch cell at overtemperature experiments at first venting and second venting in comparison.

Figure 9 shows the produced gas amount in mol of the 0%, 30% and 100% charged cell for the first venting, the second venting and the total gas emission. In case of the 0% and 30% cell no second venting could be triggered, therefore, the gases produced until the heating is stopped are added up to the first venting. Hence, the amount of produced gas at the first venting is higher at the 0% and the 30% SOC cells than at the 100% SOC cell.

#### *4.3. Vent Gas Composition*

The main gas components at the heat triggered cell at 0% and 30% SOC are CO2, DEC, H2O with minor components like CO, H2, C2H4, CH4, C3H8, C2H6, C2H2 (Figure 10). The main gas components of the fully charged cell are in descending order at the first venting DEC, H2O, CO2, CO, C2H6, H2, C2H4 and at the TR CO2, H2, CO, H2O, C2H4, CH4, DEC, C4H10, C2H6, C2H2 (Table 3, Figure 10). In Table 3 the measured gas concentration values of the experiment at 100% and 30% SOC are listed as well as the vent gas composition in% and mol according to Equation (1).

**Table 3.** Measured gas concentration values at heat triggered fresh automotive pouch cell at 100% SOC versus 30% SOC in N2.


cm: measured gas concentration including N2 atmosphere; cv/% Vol: vent gas in volume%, according to Equation (1); cv/mol: vent gas in mol.

**Figure 10.** Measured gas composition in mol: immediately after the first venting at *TV*<sup>1</sup> *cell* of the 100% SOC cell (yellow); after the heat ramp was stopped at the 0% (green) and 30% SOC cell (red); and after the TR of the 100% SOC cell (blue); experimental setup in N2.

The measured gas components at the 30% SOC and 0% SOC cell match with the gas compounds measured at the beginning of the first venting of the 100% SOC cell at about 120-130 ◦C cell surface temperature. Additionally, it is assumed that the quantified gases at the 30% and 0% SOC cell are dominated by SEI decomposition, electrolyte vapor and decomposition reaction of the electrolyte above 200 ◦C [5]. At the experiments of the 100%, 30% and 0% SOC cell no HF is detected.

The FTIR spectra of vent gases produced at the 100% (blue) and the 30% (red) charged cell are compared directly in Figure 11. The absorbance spectrum shows for the 30% SOC cell significant higher absorption peaks of the used electrolyte DEC between 1000–1850 cm<sup>−</sup><sup>1</sup> than at the venting of the fully charged cell. In the spectrum of the gas produced at the 100% SOC cell the electrolyte absorption peaks decreased (decomposition of the electrolyte, TR reaction and less long heating time at the 100% SOC cell) and CO, CO2, CH4 and C2H4 increased.

**Figure 11.** FTIR spectrum of the gas composition measured after the TR of the 100% SOC cell (blue) in comparison to the spectrum measured after stopping the heat ramp at the 30% SOC cell (red).

#### *4.4. Particle Emission*

Imaging of particles collected after the TR is performed using SEM. SE images deliver topographic contrast (Figure 12a). Although BSE imaging enables material contrast (Figure 12b), where particles with higher mean atomic number appear comparatively brighter and particles of different composition could be discerned by different gray levels, SE imaging is used to enhance the visibility of carbonated particles on the carbon substrate. To determine the PSD, SE images are binarized by gray value thresholding. Results of the measured average particle areas are presented in Table 4. Due to different reasons, like image noise or image resolution, particles segmented with the threshold method which are beneath 2 μm<sup>2</sup> in area have a big relative uncertainty. The investigation of the particle size shows that most of the particles have an area smaller than 10 μm<sup>2</sup> and about half of the particles are smaller than 5 μm2.

**Table 4.** Average measured area (a) of particles and average number of particles produced from an automotive pouch cell (at 100% SOC) at overtemperature.


**Figure 12.** SEM images of particles assembled after the TR. (**a**) SE image shows the topographic contrast; (**b**) BSE measurement shows the material contrast of the same area of the sample. Particles were positioned on a carbon adhesive tape.

To obtain a precise particle composition EDX analysis is used. Therefore, the combination of the SEM with an Oxford XMax 80 EDX detector is applied using the software AZtec for EDX control an evaluation. Therewith it is possible to simultaneously obtain the PSD and the elemental composition of every individual particle. With this setup five di fferent categories of particles are identified and assigned the following classes:


The identified particles were parts of the cell active material and were ejected by the cell due to the exothermic reaction. The Mn rich particles (class 2 and 3) result from the cathode. The C rich particles originate from the anode. F and P may result from the salt LiPF6. A small amount of C measured at almost every particle can result from the used carbon tape, the conducting carbon in the cathode or the carbon coating which was performed prior to the investigation in order to ge<sup>t</sup> an electrically conductive surface of the specimen.

In the Supplementary Materials SEM images of particles of the listed classes and the correlated spectra are explained. Exemplarily Figure 13 shows (a) the SE image, (b) the BSE image and (c) the EDX spectrum of a particle of class 1. The main elements in this particle are O and Al, as shown in the EDX spectrum. For the most particles of this class the chemical formula Al2O3 can be assumed.

**Figure 13.** Analysis of a particle of class 1: (**a**) SE image, (**b**) BSE image, (**c**) EDX spectrum. The presented scale in (**a**) and (**b**) is 10 μm.

#### *4.5. Mass Reduction*

Since no TR could be triggered at the 0% and 30% SOC cell, the initial cell mass of 865 g is reduced by 15% during the whole experimental test including the aftertreatment. Considering the amount of vent gas and the molar mass of the measured main gas components produced until the heat ramp was stopped, the 30% SOC cell released in total 27 g uncondensed gas during the heat ramp experiment. We assume that the mass reduction of 15% is due to the measured gas, condensed gas and additional gases produced at the experiment after-treatment.

At the 100% SOC overtemperature experiment the initial cell mass of 868 g reduced to 491 g after the TR. This means a cell mass reduction by 43%. This mass reduction can be explained as the sum of released gas, liquids and ejected particles at the TR. Considering the amount of vent gas and the molar mass of the measured main gas components H2, CO and CO2 and the side products CH4, C2H4, DEC, H2O, C2H6, C4H10 in total 74 g not condensed gas is released during the TR experiment. The measured gas components are about 20% of the lost cell mass during TR and about 9% of the initial cell mass. The result of the total mass of produced gas is used to assume the mass of the produced particles at the TR. The total mass loss (377 g) minus the gas amount (74 g) results in ~300 g particles. We assume that EC, one of the main electrolyte components, condensed after the TR. Gas with high boiling temperature will condensate on the colder reactor walls, but the amount of condensed gas is not the focus of this study.

#### *4.6. Optical Observation of the Cell after TR*

The pouch foil of the fully charged cell is heavily damaged on the top and bottom side after the TR and the Cu foil is visible on the top. The foil opened on all three welded sides except for the side with the terminals. In Figure 14 the cell stack including metallically glossy droplets are visible. We assume that these are Al droplets from the Al current collector. At the 30% and the 0% SOC cell no visible

openings of the pouch foil surface are observed. The pouch is still closed on the sides of the terminals. An opening is observed opposite the terminals.

**Figure 14.** The pouch cell after TR was opened on the welded sides. Droplets were visible between the stacked cell layers.
