**1. Introduction**

The market of battery electric vehicles (BEV) and hybrid electric vehicles (HEV) increases, especially in China, the U.S. and the EU [1,2]. LIBs are significantly used in the automotive sector. However, there are still challenging requirements for LIBs in the automotive sector such as costs, fast charging, lifetime, increasing energy density and safety.

It is known that battery failures can lead to critical situations inside the vehicle. The worst case is the uncontrollable exothermic chemical reaction—the TR. TR caused most of EV fires according to Sun and Huang et al., who published a review about EV fire incidents in [3]. TR is a self-accelerating exothermic reaction inside the cell which can be started by a hot spot produced inside the cell (hot spot, particle short circuit) or by a heat source outside the cell (electrical failure) [4–7]. Current methods to characterize possible battery failures are battery abuse tests like overcharge, overtemperature, over-discharge, nail penetration and fire tests. These abuse tests show the influence of cell chemistry on the failing behavior and the thermal stability of the cell [4].

Thus, the cell chemistry is an important parameter for battery safety. State-of-the-art battery chemistries used in BEVs and HEVs are based on Li-ion technology. Currently used materials are: LiNiMnCoO2 (NMC), LiNiCoAlO2 (NCA), LiMn2O4 (LMO), LiFePO4 (LFP) and LiCoO2 (LCO) as cathode; graphite and carbonaceous materials as anode; regular electrolyte mixtures of ethylene carbonate (EC), diethylene carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC); a Li-salt such as LiPF6 and a separator between the electrodes [8]. The cells are encased with sealed laminated foils (pouch cells) or metallic casings. During the first charge of the LIB an organic passivation layer—the so-called solid electrolyte interface (SEI)—develops on the anode.

Several decomposition stages of those cell materials in overheated LIBs have been published [9–12]. Main reactions according to literature include for the listed cell chemistries in general:


During battery failures, like the TR, violent reactions inside the cell produce significant amounts of hot, toxic and flammable gas and the cell ejects hot particles. The released gas and particles may cause serious safety and health risks, like fire, explosion and toxic atmosphere.

These critical situations need to be analyzed in order to minimize the risks from failing LIBs and to increase safety. To reach an acceptable level of safety in EVs and to enable early failure detection, the Electrical Vehicle Safety—Global Technical Regulation (EVS-GTR) aims to harmonize vehicle regulations worldwide. These regulations discuss suitable tests to characterize safety risks [24].

It is essential to identify comparable hazards and safety parameters to evaluate the failing behavior of different cell types reliably and in order to set necessary safety measures. But which hazards need to be addressed, which safety relevant parameters need to be quantified and which methods are suitable for a comprehensive hazard analysis of a cell?

#### *1.1. Categorized Hazards from LIBs*

In literature several important hazards from failing state-of-the-art batteries are reported resulting in main five hazards, which may lead to safety and health risks (Figure 1): electrolyte vaporization, heat generation, gas emission, gas concentration and particle emission. Hazards based on high voltage and current are not considered in this study. The first venting and the TR of the cell can cause the following hazards:

#### *Batteries* **2020**, *6*, 30

**Figure 1.** A failing battery can lead to hazards at the first venting and at the TR. Five categorized hazards (orange) and their consequences on safety and health (red) are presented. The battery failures are influenced by several factors.

## 1.1.1. Electrolyte Vaporization

Electrolyte vaporizes starting at the first venting of the cell. Contemporary electrolytes for LIBs are known to be flammable, irritant, toxic, and/or corrosive depending on the exact composition of the electrolyte mixture [4,25,26] and need to be considered as a safety and health risk. Electrolytes are assumed to be a major source of poor safety with high volume gas decomposition, large combustion enthalpy and flammability of solvent vapor [27].

#### 1.1.2. Heat Generation

Heat generation [4,19,28–30] and significant temperature increase is one safety hazard of the TR, which may lead to TR propagation to neighboring cells or battery fire [31]. Safety relevant parameters are the cell temperature at the first venting of the cell, the TR onset temperature, the maximum reached cell surface temperature and the vent gas temperature. The temperature of the produced vent gas and the ejected particles out of the cell can reach critical high temperatures up to 1000 ◦C [19] and may damage the cell surrounding materials irreversibly.

#### 1.1.3. Gas Emission

Gas emission [4,23,32,33] is another hazard with the possible consequence of explosion and rapid destruction of the pack. At the TR significant amount of gas [34,35] is produced within seconds. Safety relevant parameters are the amount of produced gas (in mol or liter) and the venting rate (in mol/s or L/s). The gas emission at TR for current state-of-the-art batteries with regular electrolytes is expected in the range of 1.3 L/Ah up to 2.5 L/Ah (at STP: 298.15 K, 100 kPa) [34]. Characteristic venting rates are (0.8 ± 0.3) mol/s at heat ramp TR experiments of 50 Ah prismatic LMO cells [19].

#### 1.1.4. Gas Composition

Main gas compounds at TR are carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2) and hydrocarbons [31,32]. The produced gas is toxic and flammable [25,36]. Except for CO2 and H2O all produced gases are flammable, explosive and deflagration of the produced vent gas in contact with O2 is possible. In addition, small amounts of toxic gases like hydrogen fluoride (HF) can be produced by decomposition of fluorine compounds as LiPF6 [31,37].

#### 1.1.5. Particle Emission

At TR solid hot particles of active materials and aerosols can be released by the failing cell, which are critical to ignite the combustible vent gas [4,38]. Particles should be considered as additional

toxic hazard [4] and health risk. The ejected material is a mixture of solid particles, aerosols of active material, parts of current collector foil and electrolyte from the cell.

Figure 1 presents these five hazards assigned to the first venting and the second venting, the TR. The battery failing behavior on cell level and the resulting hazards are influenced by: the energy content of the cell (capacity and the energy density) [34,35,39], the chemistry/active material and separator [4,40], the electrolyte composition and additives [27,41], the failure case/trigger [4,42], the design of the cell housing (pouch versus metal can) [28], the SOC [17,23,43,44] and the state-of-health (SOH)/aging history [18,45]. Additionally, the presence of surrounding gases like O2 changes the resulting hazards [42] due to additional chemical reactions.

Many researchers have studied single hazard categories from failing LIBs for di fferent cell types and di fferent chemistries [19,28,32,33], but mainly for small capacity cells with <5 Ah [32,33,42,43]. Since NMC/graphite composites are currently one of the preferred LIB chemistries in EVs and higher cell capacities and higher energy densities lead to more severe TR reaction [34,46], this study focuses on the failing behavior of modern high capacity NMC and NMC/LMO cells.

Single hazard categories from NMC and NMC/LMO cells with >20 Ah are published in [13,25,34,38,46–48]: Fang and Gao et al. concentrate on the heat generation during heat triggered TR for 25 Ah NMC [13], 1–50 Ah NMC and NMC/LMO [46] and TR propagation of 42 Ah prismatic BEV [48] cells. Ren et al. evaluate heat generation at di fferent SOH [18]. Koch et al. focus on gas emission (amount), gas composition and mass loss at overtemperature experiments in an atmosphere of air (present O2) [34]. Nedjalkov et al. analyze the gas composition in air (present O2) with a nail trigger to force TR [25]. Zhang et al. focus on particle emission [38] and gas composition [47] after heating the cell.

Beside valuable information on single hazard categories, to the best of the authors knowledge, only little information is available in literature on the following hazards and safety relevant parameters of high capacity NMC or NMC/LMO cells. Nevertheless, this information is of relevance for various R&D activities towards significant safety improvements of batteries:


Additionally, a contribution of the following parameters at failing high capacity NMC or NMC/LMO cells in N2 atmosphere would be relevant for the scientific community in this field:

• A study of the five mentioned hazards including quantification of the safety relevant parameters for the same specific cell.

• Comprehensive gas composition analysis at the first venting or at abuse experiments of cells with low SOC, where no self-heating into TR can be triggered.

Therefore, for a comprehensive hazard analysis a study on relevant parameters and measurement principles need to be addressed for all five mentioned hazards. In this study, these five hazards are characterized, safety relevant parameters are quantified, and measurement principles are provided from a large capacity NMC/LMO cell currently used in modern EV. Overtemperature experiments are conducted on three cells with di fferent SOCs (100%, 30% and 0%). The investigated hazards (and quantified safety relevant parameters) are:


Hazards from this automotive NMC/LMO pouch cells have, to the authors' knowledge, not been the subject of any scientific publication, but, as will be shown, are important to investigate.

#### *1.2. Structure of the Study*

This study describes a comprehensive hazard analysis, safety parameter quantification and TR measurement principles of a fresh 41 Ah automotive Li-ion pouch cell. It starts with a brief investigation of initial cell material in Section 2, an introduction of the TR test bed and the applied methods in Section 3, presenting the failing behavior and hazards from the heat triggered cell in Section 4 and ending with comparing the results with existing literature in Section 5. The TR experiments of the same cell at di fferent SOC (100%, 30% and 0%), but same TR trigger are compared to evaluate the influence of SOC to the failing behavior.

#### **2. Investigated Cell**

The investigated cell is a fresh high energy density 41 Ah Li-ion pouch cell designed for EV applications and used in a currently available EV. We extracted the cells from an EV.

The total mass of the fresh pouch cell is 865 g (Table 1). The cell consists of an electrode stack which is sealed in laminated foil. This electrode stack has 22 anode layers, 21 cathode layers and 42 separator layers. The anode layers consist of Copper (Cu) foils (current collector of the anode), which are coated with graphite on both sides. Likewise, the cathode layers consist of aluminum (Al) foils, which were coated on both sides with a mixture of NMC and LMO (spinel). The graphite particles have an average size of 25 μm and the NMC/LMO particles 12–15 μm [49]. The separator has an Al2O3 coating facing the cathode side. Fluorine (F) was detectable in the anode and cathode material [49].


**Table 1.** Specification of the automotive Li-ion pouch cell.

The electrolyte consists of a mixture of EC, DEC and DMC solvents with 1 mol LiPF6 per liter. The following molar ratios, namely 48% EC, 48% DEC and 4% DMC were determined by 1H NMR and 13C NMR analysis. No FEC and VC electrolyte additives were found by this investigation (Table 1).

The mass split of the discharged cell presented in Figure 2 is estimated based on the investigations of the cell material and considers the cell design and data from literature for NMC cells [32,43]. The mass of SEI, binder and carbon black are omitted. It is assumed that 14% of the initial mass of the cell is electrolyte and conducting salt. This corresponds to 121.5 g of electrolyte, consisting of 44 g of EC, 59 g of DEC, 3.7 g of DMC and 14.8 g of LiPF6.

**Figure 2.** Estimated mass split of the investigated fresh automotive pouch cell in discharged condition.

#### **3. Experimental Setup and Test Methods**

Three experiments with fresh automotive pouch cells are 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%. Each single cell is triggered into the failing behavior separately by heat. During the heating phase, temperatures at several positions on the cell surface and inside the test reactor, the voltage of the cell and the pressure inside the reactor are measured.

## *3.1. Reactor Setup*

TR experiments are carried out inside a gastight 40 bar pressure resistant stainless-steel reactor. The test-rig is published in [19,37,50] and is shown in Figure 3. The stainless-steel reactor with the implemented sample holder has a free volume of 121.5 L. The experiments can be done in N2 atmosphere or in air. For safety reasons most experiments are done in N2 atmosphere, as are the presented ones.

**Figure 3.** Test rig for thermal runaway experiments on automotive cells [19,50] designed for different cell geometries and different sample holders.

## *3.2. Experimental Method:*

In the experiments the response of each cell (mounted inside a sample holder) to heat is measured and safety relevant parameters are quantified. The sample holder presented in Figure 4 is heated by two heater stripes (max. 500 W each) on the top stainless-steel plate and two heater stripes (max. 500 W each) on the bottom stainless-steel plate. To minimize the thermal coupling between the stainless-steel plates and the cell, insulating mica sheets (thermal conductivity of 0.23 W/mK) with 2 mm are placed between the cell and the stainless-steel plates. The mica sheets also provide channels for the thermocouple wires. Each mica sheet has positions for thermocouples. The tips of the thermocouples protrude through the mica sheets and are squeezed between the mica sheet and the cell surface. Because the mica sheets are thermal insulators, the thermocouple tips measure the cell surface temperature.

The heater increases the temperature of the cell (also compare Figure 6 heater output, black line). Though with the presented setup it is not possible to define the exact heating rate before the experiment, the average heating rate is calculated after the experiment. The heating rate is defined as the increase of the average cell surface temperature per minute between 30 ◦C and 200 ◦C.

**Figure 4.** Cell sample holder (**a**) open and (**b**) closed; two heater stripes (red) on the top and two on the bottom side of the stainless-steel plates (dark gray), thermal insolating mica sheets (beige) between the cell (symbolic geometry and design of a pouch cell (blue)) and the stainless-steel plates, thermocouples attached on the mica sheets facing the cell surface.

The experiment method consists of several subsequent steps:

Sample and experiment preparation:


Experimental steps:

7. The data acquisition system is started: measurement of cell surface temperature, cell voltage, temperature and pressure inside the reactor. The cell is pulsed with a battery cycler (±1 A pulses) in order to ge<sup>t</sup> information on the cell resistance.


Experiment after-treatment:


#### *3.3. Heat Generation Analysis*/*Temperature Measurement*

Up to 30 thermocouples type k inside the reactor are used in each TR experiment. The temperature of the pouch cell surface is measured with twelve thermocouples on the cell top and twelve on the cell bottom positioned in defined regular distances (50 mm, arrangemen<sup>t</sup> 4 × 3, see Figure 5). *TV*<sup>1</sup> *cell* describes the average measured cell surface temperature of all thermocouples at the first venting. *TV*<sup>2</sup> *cell* describes the average measured cell surface temperature of all thermocouples at the second venting. The onset temperature *Tonse<sup>t</sup> cell* is the temperature when the temperature of the cell heating rate is faster than the heating rate of the heat ramp. The critical temperature *Tcrit cell* describes when the temperature rate of the selected sensor exceeds 10 ◦C/min (detailed description in [19]). The maximal cell surface temperature *Tmax cell* is the maximum recorded temperature of one of the thermocouples (depends on the position of the origin of the TR). The gas temperature is measured inside the reactor at four different positions. The average reactor temperature is used to calculate the vent gas amount produced at the battery failure.

**Figure 5.** Scheme of the thermocouples positions on the surface of the pouch cell (red) and at different positions inside the TR reactor (green).

#### *3.4. Gas Emission Analysis*

The pressure inside the reactor is measured with a GEMS 3300B06B0A05E000 sensor. The pressure and the average gas temperature measured at equilibrium state, 5 min after the TR, are used to calculate the amount of released vent gas. The amount of released gas nv (mol) is calculated with the ideal gas equation and is presented in liter at standard temperature and pressure (STP: 298.15 K, 100 kPa, Vmol = 24.465 L/mol). The amount of gas produced starting at *TV*<sup>1</sup> *cell* and ending at the *TV*<sup>2</sup> *cell* is defined as nv1. nv2 is the gas produced after *TV*<sup>2</sup> *cell* and during the TR. The characteristic venting rate .*n*ch (mol/s) is calculated with the minimal duration Δt50% (s) to produce 50% of the venting gas nch50% (mol). For the calculation of the safety relevant parameters (amount of released gas and characteristic venting rate) the same calculation is used as described in [19]).

#### *3.5. Gas Composition Analysis*

The gas composition is quantified with two complementary methods in parallel: A Fourier transform infrared spectrometer (FTIR) and a gas chromatograph (GC). In contrast to [40,43] the

gas analysis is enhanced with FTIR spectroscopy. The results of the two methods are combined for each measurement and—depending on expected gas components and their concentration range—the measured results of a method, either FTIR or GC, can be chosen.

The downstream connection from the reactor to the gas analysis is heated to ~130 ◦C. Thus, all gases with a condensation temperature below 130 ◦C will stay gaseous and will be detected. One converse example is the commonly used electrolyte component EC with a boiling point of 246 ◦C. Hence, it is very unlikely to measure EC absorbance peaks in the used test setup. The reactor gas consists of N2 and the vent gas, which is added by the cell. Since the produced vent gas does not contain N2, the amount of N2 can be subtracted to calculate the concentration of each component of the vent gas only. The concentration of any gas component (cv/%) in the vent gas is calculated with the measured concentration of this gas component in the reactor gas (cm) and the measured N2 concentration (cN2) in the reactor gas:

$$\mathbf{c}\_{\mathbf{v}} = ((\mathbf{c}\_{\mathbf{m}} \times 100)(100 - \mathbf{c}\_{\mathbf{N}2})) \tag{1}$$

#### 3.5.1. FTIR Spectrometer (FTIR)

A Bruker MATRIX-MG01 FTIR is used with 0.5 cm<sup>−</sup><sup>1</sup> wavenumber resolution. The MCT detector is N2 (l) cooled. The FTIR measurement chamber itself is heated to 190 ◦C. The interior space of the FTIR spectrometer is purged with N2 (g) for at least 2 h to reduce the influence of surrounding gases to the measurement. For the background measurement 100 scans are averaged. A number of 40 scans are used for each data point. To avoid contamination a cold trap and a particle filter are added in front of the FTIR gas measurement chamber. The quantification of the gas compounds is done with the software OPUS GA by Bruker. For each gas analyzed with FTIR a certain absorbance wavenumber region is chosen and compared with a reference spectrum. The setting of the software OPUS GA is optimized for the expected gases and concentrations and validated with the test gas. The FTIR spectrometer is currently optimized for: CO, CO2, CH4, C2H6, C2H4, C2H2, DEC, DMC, EC, EMC, H2O, C6H14, HF, C4H10 and C3H8.

#### 3.5.2. Gas Chromatograph (GC)

For gas analysis with GC the 3000 Micro GC (G2802A) is used with three columns and TCD detectors. The three-channel system includes Molsieve (10 m × 320 μm × 12 μm), Plot U (8 m × 320 μm × 30 μm) and OV1 (8 m × 150 μm × 2.0 μm). The injector temperature and the sample inlet temperature are set to 100 ◦C for all three channels. The column temperature of the Molsieve channel is 80 ◦C (at 30 psi) and 60 ◦C for the Plot U and OV1 channel (40 psi each). Injection time for Molsieve and Plot U is 15 ms and 10 ms for the OV1 channel.

Since the GC uses corrosion sensitive columns, the gas is washed in water washing bottles at room temperature before entering the GC. These washing bottles are directly applied after passing the FTIR gas measurement chamber. Gases that do not dissolve or condensate in the water can be measured. The GC is calibrated for: H2, O2, N2, CH4, CO, CO2, C2H6, C2H4, C2H2.

#### 3.5.3. Accuracy of the Gas Quantification

The accuracy of the gas analysis for the presented experiments is validated with test gas of different concentrations and the systematic and statistic uncertainties for FTIR and GC analyzed gas components are added up (Table 2). The FTIR measures spectra continuously over time with a low standard deviation of the measured value (dependent on gas compound <0.2% of the measured value). The GC is calibrated with test gas at a specific uncertainty of each component Δtest gas = ±1%.

The gas quantification method of the FTIR measured spectra is optimized for the expected gas concentrations produced at first venting and during TR. FTIR measurements have advantages at low gas concentrations like for gaseous and toxic HF, but disadvantages in symmetric molecules without change of dipole moment like H2 and if the absorption peaks of gases are at similar wavelengths. The GC has its benefits at high concentrations of permanent gases, especially H2, N2 and O2 which cannot be measured with FTIR spectrometer.


**Table 2.** Accuracy of the FTIR and GC gas quantification optimized for expected gas concentrations.

LOD: limit of detection at the specific setting in parts per million (ppm). -: not calibrated for quantitative analysis or not possible to measure.

From the gas compounds quantified with both methods the result of one method, either FTIR or GC, is chosen depending on expected gas components and their concentration range. For small concentrations of CO, CO2, CH4, C2H6, C2H4, C2H2 the measured FTIR concentration values are chosen because of the lower LOD. If the measured concentration of C2H4 is significantly higher than the LOD, the GC measured value is chosen because of the higher accuracy compared to the FTIR.

#### *3.6. Particle Collection and Particle Analysis*

The ejected particles are sampled after the TR and investigated using scanning electron microscopy (SEM) at the Institute of Electron Microscopy and Nanoanalysis (FELMI) at Graz University of Technology. The analysis is focused on particle size distribution (PSD) and particle composition. A ZEISS Sigma 300 VP (Variable Pressure) and a FEI Quanta 200 ESEM (Environmental SEM) are used for the investigation of the released particles after TR. The following SEM detection modes are used:


For the SEM investigations the particles have to be fixed on a sample holder. The fixation must enable a homogeneous distribution without agglomeration of the particles. Gasser showed that the most reliable sampling method is to collect particles from inside the reactor with a spatula and spraying them by a jet of air on a double-sided adhesive carbon tape [51]. This method is used for the sample preparation and subsequently the particles are analyzed with SEM/EDX to measure particle size and particle elemental composition.

Prior to the investigation, EDX simulations are performed with the public access program NIST DTSA-II [52]. Therewith the electron beam interaction was simulated, to be able to assess the best beam energy for SEM-EDX measurements of particles with the measured particle sizes [51].

#### *3.7. Mass Reduction Analysis*

The weight of the test sample is measured before and after the experiment using a scale (KERN K8) with a measurement uncertainty of ±0.01 g. After the experiment after-treatment including the heating of the reactor, the vacuum and the N2 flushing the weight of the remaining cell and large parts (>30 mm length) of the cell outside the cell housing are measured.
