**5. Discussion**

The heat triggered TR experiments of a currently used high capacity cell—extracted from a modern mass-produced EV—enables studying hazards and quantify safety relevant parameters from this automotive cell. Since there are few papers available for failing high capacity NMC/LMO cells, the study of those hazards is even more important. Respective papers concentrate on single hazard categories. We concentrate on all five categorized hazards and the safety relevant parameters at different SOC. Table 5 sums up all safety relevant findings of the heat triggered battery failures of the fresh automotive pouch cell at 100%, 30% and 0% SOC.


**Table 5.** Summary of safety relevant parameters of overtemperature experiment of the fresh automotive pouch cell at 100%, 30% and 0% SOC.

The fully charged cell can be triggered thermally into TR. At 30% SOC and lower, it is not possible to trigger the cell into TR with the same heat setup (Table 5). If the cell is fully charged during thermal abuse the electrolyte reacts with the lithiated anode after the SEI breakdown [7,17]. Additionally, the stability of the delithiated cathode material is decreased [44]. If the cell is at 0% or 30% SOC the reaction of the lithiated anode with the electrolyte is reduced due to the lack of Li in the anode. No exothermal decomposition of those cells is observed. Increased safety with decreasing SOC is consistent with [12,17,43,44], although referenced literature describes different chemistries and cell

components: NCA and LFP [43]; NCA [44]; NMC/LTO [12]. The thermal interactions between several binder materials and anode carbon at 50% and 100% SOC is reported in [17].

Still one question is remaining: Which SOC is the minimum to trigger TR thermally? SOCcrit is defined as the lowest SOC to trigger TR. For this investigated cell it seems to be >30%, but there is no general answer for other cells, especially not for higher energy density cells. The SOC influences hazards, consequently safety and health risks from failing LIB. At failing cells with SOC < SOCcrit the vaporizing electrolyte and the electrolyte decomposition has the risk of flammable, toxic and corrosive gases. At cells with SOC > SOCcrit additional serious risks from heat generation, hot gas and particle emission due to the uncontrollable exothermal reaction need to be considered.

#### *5.1. Hazard Analysis of Failing Automotive Pouch Cells*

#### 5.1.1. Heat Generation/Temperature Increase

Temperature sensors on the cell surface show the TR propagation through the cell in 4.28 s. This rapid exothermal reaction and maximal cell temperatures above 700 ◦C can challenge prevention of TR propagation to neighboring cells and increase resulting risks and damage.

The comparison of the experiments at 100%, 30% and 0% SOC illustrates that the first venting of the investigated cell begins between *TV*<sup>1</sup> *cell* = 120 ◦C–130 ◦C cell surface temperature. The deviations between the measured *TV*<sup>1</sup> *cell* values may not be connected to the SOC and is explained as a measurement uncertainty. *TV*<sup>1</sup> *cell* is comparable with the measured temperature rate change (first venting) of overheated NMC pouch cells at about 120 ◦C plotted by Ren et al. [18]. Ren et al. shows in [17] (Figure 11) that the first venting appeared almost at the same temperature ~120 ◦C independent of the four different degradation paths and SOH. This would mean that aging effects, like SEI growth and electrolyte consumption, does not influence the first venting. For 50 Ah LMO prismatic metal can cells at our test stand the first venting was observed between *TV*<sup>1</sup> *cell* = 194 ◦C–220 ◦C [19]—far apart from our measured values for the pouch cell. This may indicate the influence of different cell design (metal can), vent design and chemistry (LMO) to *TV*<sup>1</sup> *cell*.

The next important temperature is the critical temperature *Tcrit cell* , where the temperature rate of the hottest sensor exceeds 10 ◦C/min, immediately before the full TR. At the fully charged cell *Tcrit cell* = 231 ◦C is comparable with the defined temperature T2 by Feng et al. [46]. Feng et al. correlated the influence of gravimetric energy density to the maximum reached temperature in [45] (Figure 6). Our result of *Tmax cell* = 715 ◦C fits the presented maximum temperature of NMC/LMO and NMC cells with similar energy density measured in [18] and [46]. At the TR, the cell temperature increases enormously due to chemical reactions inside the cell mainly produced by NMC degradation and reaction of the cathode and the solvent according to [12,17]. The maximum reached temperature can be significantly higher than 715 ◦C on the surface of the cell and even more inside the cell itself as demonstrated by [13]. The exothermic decomposition of the delithiated cathode material and the reaction between the released O2 with the solvent is speculated to be the reason for reaching the maximum cell surface temperature [17,22] at the fully charged cell.

Energy density, cathode material and cell design seem to be a main influencing factor for safety relevant and critical temperatures like the first venting as well as the maximum reached cell surface temperature.

#### 5.1.2. Gas Emission

Pressure increase at the first venting does not present any hazards. But the abrupt gas production at the TR and the venting rate of 18.7 L/s can lead to explosion of a battery pack.

The soft pouch packaging ruptured at *TV*<sup>1</sup> *cell* and the cell started to release gas continuously until the TR happens or the heating is stopped. The 100% SOC cell released 0.14 mol gas before the TR. During the TR, the cell released abrupt additional 2.17 mol of gas within 4 s. The 4 s reaction time is observed in the measured temperature and pressure data at the TR. The characteristic venting rate

is 0.8 mol/s (18.7 L/s) is comparable with the published results of Golubkov et al. for heated 50 Ah prismatic LMO cells (0.8 ± 0.3) mol/s [19]. This parameter is a relevant parameter for battery pack design and vent design. For higher energy densities and higher capacities increased maximum gas rates are expected. In addition, the reaction time of 4 s observed by the pouch cell may be di fferent for prismatic metal can cells.

The measured 1.3 L/Ah vent gas for this cell is barely within the literature review of Koch et al. of 1.3 L/Ah–2.5 L/Ah for current state-of-the-art batteries [34] and shows that the presented cell produced less gas compared with cells of similar capacity, energy density and chemistry, but the vent gas emission still needs to be considered as a serious safety risk. Compared to other state-of-the-art automotive pouch and metal can cells analyzed in our test setup, this investigated cell produces less gas per Ah at 100% SOC heat trigger, although no gas reducing electrolyte additives could be found. Roth et al. investigated the vent gas amount at di fferent cathode materials (LCO, NCA, NMC, LFP, LMO) and found that all cells produce about 1.2 L/Ah and that a main factor of predicting gas generation is the volume of the used electrolyte [27]. It needs to be mentioned that more vent gas is expected at the presence of O2 (as measured by Koch et al. as 1.96 L/Ah [34]) and at increasing SOC, like published at overcharge experiments of NCA and LFP cells in [43]. Additional published gas emission values are for NMC 1.2 L/Ah (0.9 Ah NMC) [27], 1.4 L/Ah (2 Ah NMC) [35] and 0.9 L/Ah (2.6 Ah NMC in air) [42]. Deviations from [42] may be explained due to di fferent vent gas amount calculation. The literature source reporting of 2.5 L/Ah is not experimentally determined.

Therefore, we assume that NMC/LMO cells produce between 1.2 L/Ah-2 L/Ah gas at thermal abuse. If the cell goes into TR (SOC ≥ SOCcrit) main influencing factors seem to be the capacity of the cell, the electrolyte amount, the SOC and present O2. According to Roth et al. cathode material has a minor influence on the gas amount.

#### 5.1.3. Gas Composition below SOCcrit—30% and 0% SOC

Vent gases measured at the 30% and 0% SOC cell and the first venting are dominated by CO2, H2O and electrolyte vapor. At this cell EC (irritant, PAC-1: 30 mg/m3) and DEC (flammable, PAC-1: 2 mg/m3) are the main electrolyte components. Lebedeva et al. state clear that most of the currently used LIB electrolytes are toxic, irritant or harmful in addition to being flammable and may even be carcinogenic [26]. Therefore, the opening of the cell and first venting below SOCcrit need to be handled as a serious risk due to irritant, toxic and flammable composites, especially at the early opening soft pouch packing and the vaporization of electrolyte inside a closed system (pack, garage, tunnel).

Beside significant electrolyte vapors the following gas components were measured at the heated 30% and 0% SOC cell in descending order: CO2, H2O, DEC, CO, H2, C2H4, CH4, C3H8, C2H6, C2H2. There are many studies reporting gas generation from electrolyte at cycling, formation and heating. The main gas components are similar to the measured gas components in this experiment (CO2, CO, C2H4, CH4, C3H8, H2, C2H6 [53–55]), although the exact gas concentration depends highly on the used electrolyte composition and the additives.

Gas generated at overheating of cells below SOCcrit are rarely published. Literature on high capacity NMC or NMC/LMO cells concerning the first venting or gassing at cells with SOC < SOCcrit is missing. Literature from small capacity cells: For a 3.35 Ah NCA cell Golubkov et al. presented on 25% SOC 18,650 cells at heating similar main gas compounds: CO2, H2, CH4, C2H4, CO [43] (electrolyte and higher hydrocarbons were not quantified). For a 1 Ah LCO cell with 50% PC, 20% EMC, 15% DEC and 10% DMC Kumai et al. measured before and after cycling tests significant di fferent gas compositions, but also the same main gas components: CH4, CO2, CO, C2H6, C3H8 and C3H6 [23] (H2 and electrolyte compounds were not quantified). The produced gases can also be compared with gases produced at the formation process and cycling of NMC cells: At a NMC(422)/graphite cell with 3:7 EC:EMC and LiPF6 at 100% SOC CO2, C2H4, C2H6, C2H5F, C3H8 and CH4 are measured in decreasing order [53]. Wu et al. investigated at LTO/NMC cells the gas generation at di fferent electrolyte compositions with and without cell formation (SEI) and found significant reduction in CO2 compared to cells with SEI [55].

Possible sources of the identified gases are therefore: for CO2: electrolyte [54] and SEI decomposition [5,55], for CO: EC [54], for C2H4: EC [54], SEI decomposition [5], for C2H6: DEC [54] and DMC [5], for H2: linear carbonates [55], C3H8 and CH4: DMC [55].

It seems that the cathode material plays a minor role for the gas composition at the first venting and at thermal abuse of cells below SOCcrit. The major influence appears to be the electrolyte composition.

#### 5.1.4. Gas Composition—100% SOC

Main components after TR are: 38% CO2, 23% H2, 17% CO, 8% H2O, 6% C2H4, 4% CH4 and electrolyte vapor 3% DEC. TR vent gas consists—apart from CO2 and H2O—of mainly toxic (CO) and flammable (H2, CH4, DEC) gases. Beside the risk of toxic and flammable atmosphere, fire and explosion are serious consequences.

CO2 is the most abundant gas component in the vent gas at the heat triggered TR at 100%, 30% and 0% SOC. At the 100% charged cell a 3.9 times higher CO2 amount was measured than at the 30% SOC cell. The ratio of CO2:CO = 9.3:1 for the 0% SOC and 30% SOC cell and CO2:CO = 2.3:1 for the 100% SOC cell. This observation can change at TR of LIBs with higher energy density, where CO2:CO ratios less than one are possible at TR [34] and more CO than CO2 is produced due to incomplete combustion reaction. Similar CO2:CO ratios of measured gases at heat triggered TR of NMC cells are observed in [40], although the investigated cell is a 1.5 Ah 18,650 cells with DMC:EMC:EC:PC (7:1:1:1) and an energy density of 133 Wh/kg (only CO2, H2, CO, CH4 and C2H4 were analyzed). In addition, perfect comparable main gas concentrations were measured for NMC cells with di fferent electrolyte compositions by Koch et al. The mean substance concentration values over 51 NMC LIBs fit perfectly for the presented results in this study: 37% CO2, 22% H2, 6% C2H4 and 5% CH4 [34] with the di fference in CO amount (28% CO by Koch et al.). The di fferent CO amount can be explained by the lower energy density at our NMC/LMO cell. Koch et al. did not quantify gaseous H2O and electrolyte [34]. For di fferent cathode materials similar gases, but di fferent gas concentrations, were observed [40]. If the same cell chemistry is analyzed, but di fferent triggers are used (like overcharge or nail penetration instead of overtemperature), di fferent preferred chemical reactions take place ending up in di fferent gas compositions [32].

As stated by Zhang et al. in literature no more than 10 gas species in the vent gas are quantified except for their own study [47]. Thus, in this study, 18 possible gas compounds during battery failures are presented. Additional gases identified by other authors, but not listed in this study, for instance C3H6 [34] and other higher hydrocarbons (less than 1.7% of the total gas emission according to [47]), were not identified. The deviations may be explained by di fferent cell chemistry, di fferent reaction probability, the test setup and the gas analysis methods. Commonly used electrolytes as EC, DEC, DMC and EMC absorb at similar wavenumber regions and can only be identified clearly at certain wavenumber regions with the FTIR.

Although for the presented experiments no hydrogen fluoride (HF) could be detected, HF is expected to be released by the cell in small amounts [32,36] and to undergo further reactions with the materials inside the reactor, the analysis region and the released particles. Beside the HF production, F may also remain in the cell itself and LiF can be formed. For another aged 18 Ah cell with NMC/LTO chemistry in our test setup, 66 ppm (0.396 mmol) HF were measured [37].

Adding up all quantified gas components at the presented results does not sum up to 100% in total. Possible reasons of the deviation are the sum of uncertainties of each gas component and gases which could not be identified/measured in this experiment.

In addition to the listed gases produced at the venting of cells with SOC < SOCcrit, at TR an increase of especially H2, CO2 and CO were observed. Though the total amount of measured electrolyte at the fully charged cell is reduced in comparison to the cell at 30% SOC (Figure 11), parts of the vent gas result from decomposing parts of 44 g EC, 59 g DEC, 3.7 g DMC according to [7,56,57] and result in mainly CO2 and H2O. Further sources for the gases are for H2: the reaction of binder material and Li

in the anode [42]; for CO2: oxidation of the electrolyte on the negative electrode surface and LiPF6 and further reaction with the released O2 of the decomposing cathode [5,21,27,54].

Concluding, the vent gas composition of a failing LIB may be highly sensitive to the SOC, the failure mode/trigger, the used electrolyte composition (especially for cells with SOC < SOCcrit), the chemistry and the energy density. This NMC/LMO cell produces similar gases and concentrations as published NMC cells.

#### 5.1.5. Particle Emission

The ejected particles contain elements that are potentially toxic and could act as an ignition source of the emitted burnable gasses, due to their high temperature [4,38]. Furthermore, most of the particles are smaller than 10 μm<sup>2</sup> and can therefore be inhaled deeply into the lungs [58].

Challenges to the particle analysis were the sampling method and the evaluation of the exact particle size and composition. Sampling is the bottleneck of any analytic method and may compromise the results, even when using a measurement method with high precision. During sampling, the material of interest should not be altered, and the sample should be representative. Several methods were tested and are described in [51]. However, the jet of air sampling method used in the end provides a uniform distribution of the particles on the carbon tape used in the SEM measurements, allowing the individual analysis of the particles regarding their size and composition. It has to be mentioned that the air sampling method is selective concerning the dimensions of the particles, but we assume that it is representative for these particles, which are relevant concerning hazards during inhalation.

The particles contain elements that are potentially toxic for humans including Al, Ni, F. Those elements were also reported in [38]. Thus, safety equipment for people handling cells after TR is important such as particle masks and protective clothing. However, the measured major particle size (<10 μm2) and the reported mass loss does not match with the observations of [38,47]. Zhang et al. show in [38] for a fully charged metal can cell particle matter account for 11.20% of the cell mass. Measured particle sizes were less than 0.85 mm at nearly 45% of particles. In [47] Zhang et al. report a mass loss of 28.53% at a 50 Ah cell due to gas and particle emission with a near 90% of the particles with a size of 0.5 mm in diameter. Zhang et al. measures lower maximum cell surface temperature (438 ◦C) [47]. The deviation in particle size may be explained due to di fferences in the cell design (metal can versus pouch), the chemical composition, the sample preparation techniques and the analysis methods.

In [38,47] four di fferent methods were used for the characterization of settleable particulate matter in the chamber, where the thermal runaway was investigated. In fact, very precise methods were applied, which have the drawback, that not one and the same sample can be used for each method. This is a grea<sup>t</sup> advantage of SEM combined with EDX, because after getting a specimen holder with disjunct fixated (carbon tape) particles the number, morphology, size and elemental composition (from the element boron (B) to uranium (U)) can be measured using only one methodical approach on the same sample. Hence a good statistic can be achieved, and even individual information of each particle is enabled. Additionally, it has to be highlighted that the only alteration of the sample is the application of a thin carbon layer on the particles, which is fundamental for imaging without charging, but is not compromising the elemental assessment. Thus, using SEM/EDX no heating of the material or dilution in a supporting liquid is needed as is prerequisite at several chemical or elemental analytical methods.

Beside elemental analysis using EDX even chemical analysis via Raman spectroscopy would help to identify particles. Especially organic materials (e.g., carbon rich particles) could be assessed. A new system called RISE (Raman Imaging and SEM) combines high resolution imaging using an SEM with chemical analysis by an integrated Raman microscope [59]. Thus, correlative microscopy combining morphologic, elemental and chemical investigation could be realized. In this special case the application of a carbon layer would be obstructive since it would mask the signal for Raman measurements. However, the used SEM enables a special vacuum mode (Variable Pressure), where imaging without charging and subsequent EDX and Raman analysis can be realized.

#### 5.1.6. Mass Reduction

At the TR, the investigated cell reduces the initial mass by 43% due to gas and particle emission. This result is comparable with pouch and hard case cells at 100% SOC overtemperature experiments by [34] reporting mass loss of 15–60% for NMC cells with 20–81 Ah. Zhang et al. measured significant lower mass loss (29%) for overheated prismatic NMC cell [47]. The mass loss of the 0% and 30% charged cells after the experiment after-treatment (15%) is comparable with the assumed amount of electrolyte (14%). Therefore, it is assumed that the mass loss of the 0% and 30% charged cell is mainly due electrolyte vaporization and decomposition of SEI, electrolyte and synthetic material.

The quantified mass reduction seems to depend on the SOC, the energy content of the cell and the cell design (metal can prismatic or cylindrical versus pouch cell).

#### *5.2. Forecast for Failing Behavior of Future Cells*

Cells with higher energy density than the investigated cell, which are currently planned for the next generation of EVs, may behave di fferently and it is possible that a TR even below 30% SOC can be triggered by heat. In TR experiments with di fferent cell generations and increasing gravimetric energy density at our test bench, the failing event results in more heat, higher mass loss, more gas and the gas composition changes towards increased toxic components (CO) compared to the presented results as indicated in [34]. New cell technology with increased Ni-content in NMCs are also supposed to have a reduced thermal stability and therefore failing behavior is supposed to change [47].

For comparability of experiments it is important to highlight influencing factors like the cell capacity, the SOC, the SOH, the energy density and the chosen TR trigger for each experiment. It is expected, that for instance the impact of overcharge triggered cells is higher than in heat triggered cells: gas amount and toxicity (CO) increase with SOC [33].

#### *5.3. Forecast for Failing Behavior of Aged Cells*

Aged cells (without Li plating) with increased SEI thickness and decreased electrolyte content are supposed to have a decreased heat generation and gas emission as observed by [18] and [35]. For pouch cells the first vent was observed at the same mean surface temperature for aged cells as for fresh cells [18], in contrast to a di fferent cell design in [35], where the first venting started at a lower temperature at the investigated cylindrical cell. Further investigations on the first venting at di fferent cell designs need to be done for early failure detection.

#### *5.4. Recommended Failure Detection*

As a result of the presented hazards and risks, special safety equipment and failure detection methods are recommended. For instance, temperature, pressure and gas monitoring is recommended at battery applications, especially inside the EV battery pack. This may enable failure detection at an early stage, as aimed by EVS–GTR. An unwanted opening of the cell could be detected with the proposed monitoring. Early failure detection is gaining more importance due to increasing cell energy density.
