Authors gracefully thank Marc Steen and Natalia Lebedeva for the excellent support reviewing and discussing this manuscript.

The authors have nothing to disclose.

The need of decarbonizing the economy combined with increasing energy demands&#160;&#9472;&#160;resulting from socio-economic developments and from climate change &#9472;&#160;requires a major shift in the energy system to address challenges posed by global warming and fuel shortage1,2. Clean energy technologies such as wind energy and solar energy are regarded as best alternatives to a fossil-fuel dominated energy system3; however, they are intermittent and the storage of energy will help to ensure continuity of energy supply. Properties such as high specific energy density, stable cycling performance and efficiency make lithium-ion batteries (LIBs) promising candidates as electrochemical energy storage system. The cost and the lack of reliable operation of LIBs may hamper a wider application in the power grid, in the form of large stationary battery system4,5. An additional aspect to consider is that the combination of high energetic materials with flammable organic solvent-based electrolytes can lead to hazardous conditions such as fire, release of toxic gases and explosion6,7. Therefore, one must address safety issues in LIBs.
Since early commercialization, a number of accidents in current applications (portable electronic devices, electric cars, and auxiliary power unit in aircraft) were reported in the news8,9. For instance, despite high quality production, the Sony laptop batteries incident10, the two Boeing 787 incidents11,12, Samsung Galaxy Notes 7 incidents13 are assumed to happen by internal short circuits in a cell. Tests have been developed to assess safety hazards14,15,16,17. Overcharge, over-discharge, external heating, mechanical abuse and internal/external short-circuit are failure mechanisms that are known to trigger thermal runaway (TR)18 and have been included in some standards and regulations. During this process, a series of exothermic reactions occur causing a drastic and rapid increase in temperature. When the heat generated cannot be dissipated fast enough, this condition develops into a TR19,20. Additionally, a single cell can then generate sufficient heat to trigger the neighboring cells, within a module or within a pack assembly, into TR; creating a thermal propagation (TP) event. Mitigation strategies such as increasing cell spacing in a module, the use of insulation materials and a specific style of cell interconnecting tab have all proven to curb the propagation phenomenon21. Also, the electrolyte stability and the structure stability of various cathode materials in the presence of electrolyte, at elevated temperature, have been investigated in order to reduce the likelihood of TR22.
Juarez-Robles et al. showed the combined effects of the degradation mechanisms from long-term cycling and over-discharge on LIB cells23. Depending on the severity of the discharge, phenomena like Li plating, cathode particle cracking, dissolution of Cu current collector, cathode particle disintegration, formation of Cu and Li bridges were reported as the main degradation mechanisms observed in these tests. Furthermore, they studied the combined effect resulting from aging and overcharge on a LIB cell to shed light on the degradation mechanisms24. Due to the extent of the overcharge regime, degradation behaviors observed for the cell were capacity fade, electrolyte decomposition, Li plating, delamination of active material, particle cracking and production of gases. These combined abuse conditions may cause the active materials to undergo exothermic reactions that can generate sufficient heat to initiate thermal runaway.
To avoid safety-related problems, lithium-ion batteries have to pass several tests defined in various standards and regulations14. However, variety in cell designs (pouch, prismatic, cylindrical), applicability of tests limited to a certain level (cell, module, pack), different evaluation and acceptance criteria defined, highlight the need to unify guidelines and safety requirements in standards and regulations25,26,27. A reliable, reproducible and controllable methodology, with uniform test conditions to trigger an internal short circuit (ISC) with subsequent TR, along with uniform evaluation criteria, is still under development28. Furthermore, there is not a single agreed protocol to assess the risks associated with the occurrence of TP in a battery during normal operation20,25.
In order to establish a testing protocol that simulates a realistic field failure scenario, a massive number of input parameter combinations (e.g., design parameters of the cell such as capacity, surface to volume ratio, thickness of the electrodes, ISC triggering method, location, etc.) need to be investigated experimentally to determine the best way to trigger TR induced by internal short circuit. This requires prohibitive lab efforts and costs. An alternative approach consists of the use of modeling and simulation to design a suitable triggering method. Nonetheless, 3D-thermal modeling of batteries can be prohibitively computationally expensive, considering the number of assessments needed to cover the effect of all possible combinations of parameters potentially governing TR induced by an internal short circuit.
In the literature, thermal decomposition models have been developed to simulate electrochemical reactions and thermal response of different types of lithium-ion batteries under various abuse conditions, such as nail penetration29, overcharge30, or conventional oven test31. In an effort to understand the cathode material stability, Parmananda et al. compiled experimental data of accelerating rate calorimeter (ARC) from the literature32. They have extracted kinetic parameters from these data, developed a model to simulate the calorimetric experiment and use these kinetic parameters for thermal stability&#160;prediction of a range of cathode materials32.
In references29,30,31 and numerous other studies, a combination of the same models33,34,35,36&#160;&#9472;&#160;describing&#160;respectively; heat release from decomposition of anode and solid electrolyte interface (SEI) layer; decomposition of cathode and decomposition of electrolyte&#160;&#9472;&#160;has been used repeatedly during several years as the basis for modeling thermal runaway. The latter has also been improved over time, for instance, by adding venting conditions37. However, this series of model was initially developed to capture the onset temperature of a TR and not for modeling thermal runaway severity.
Since thermal runaway is an uncontrolled thermal decomposition of battery components, it is of utmost importance to identify the decomposition reactions in anode and cathode to be able to design safer Li-ion batteries cells and more accurate testing methodologies. To this purpose, the goal of this study is the investigation of thermal decomposition mechanisms in NMC (111) cathode and graphite anode for the development of a simplified yet sufficiently accurate reaction kinetic model, which can be used in simulation of TR.
Here, we propose the use of coupled analytical equipment: Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) in a single simultaneous thermal analysis (STA) instrument. This equipment is coupled to the gas analysis system, which consists of Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS). The hyphenated STA/FTIR/GC-MS techniques will allow us to acquire a better understanding of the causes and processes of thermal runaway in a single cell. Moreover, this will help to identify thermal decomposition processes. Hyphenation refers to the online combination of different analytical techniques.
The set-up of this custom-made integrated system is shown in Figure 1. The STA equipment, used in the present study, is located inside a glovebox, which guarantees the handling of components in a protective atmosphere. The latter is coupled with the FTIR and GC-MS via heated transfer lines (150 &#176;C) to avoid the condensation of evaporated materials along the lines. The hyphenation of these analytical techniques allows simultaneous study of thermal properties and identification of released gases, providing information of the mechanisms of the thermally induced decomposition reactions. In order to further reduce the impact of unwanted chemical reactions in the electrodes during sample preparation, sample handling and sample loading are performed inside an argon-filled glove box. The disassembled electrodes are not rinsed nor any additional electrolytes are added to the crucible.
STA allows for the identification of phase transitions during the heating process, along with accurate determination of temperatures and enthalpies associated with these phase transitions, including those without mass change. The combination of on-line FTIR and GC-MS methods with the STA provides a qualitative assessment of gases evolved from the sample during its thermal decomposition. This is the key in identifying thermally induced reaction mechanisms. Indeed, STA/FTIR/GC-MS coupled system allows correlating the mass changes, heat flow, and detected gases.
FTIR and GC-MS each have their advantages and limitations. The high sensitivity of GC-MS allows rapid and easy detection of molecules from peaks of low intensity. Furthermore, FTIR data well complement the information provided by MS spectrum patterns to achieve the structural identification of organic volatile species. However, FTIR is less sensitive. In addition, diatomic molecules, such as H2, N2, O2, do not possess a permanent dipole moment and are not infrared active. Therefore, they cannot be detected using infrared absorption. On the contrary, small molecules such as CO2, CO, NH3, and H2O can be identified to a high degree of certainty38. Altogether, the information provided by these complementary methods makes it possible to gain insight of the gases emitted during thermal characterization.
In order to check the state of the art in terms of thermal decomposition reactions identified for Lithium Nickel Manganese Cobalt oxide (NMC (111)) cathode, graphite (Gr) anode and 1M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) = 50/50 (v/v) electrolyte, a literature review was performed. Table 1, Table 2, and Table 3 summarize the main findings.
In the field of thermal characterization of battery components, the sample preparation method has a significant effect on DSC experimental results since this has an influence on the DSC signal. Many studies have reported different approaches in terms of electrode handling. Some variations include 1) scratching active material from electrode without prior rinsing nor adding extra amount of electrolyte, e.g.,54,55; 2) rinsing/drying/scratching active material and adding a given amount of electrolyte with it in the crucible e.g.,55,56; 3) rinsing/drying/scratching active material without adding electrolyte at a later stage (e.g., see reference57). However, in the literature, there is no general agreement on the sample preparation techniques. Washing the electrode affects the integrity and the reactants in the SEI58, which in turn, modify the amount of heat generated from its decomposition34,59. On the other hand, there is no clear indication or sufficient details on the amount of added electrolyte to the harvested materials prior to thermal analysis.
In this work, the SEI modification is minimized by not washing the electrode and excluding electrolyte addition, in an attempt to characterize the electrode material in its original state, while keeping its residual electrolyte content. Realizing that SEI thermal decomposition is a potential trigger to thermal runaway, this preparation method is expected to allow for a better understanding of the thermal properties of the electrode, under test conditions, without the dissolution of some SEI products. Indeed, the breakdown of SEI layer on the anode is generally the first stage of battery failure that initiates a self-heating process39,41,60.
Another important issue in thermal analysis is the measurement conditions (type of crucible, open/closed crucible, atmosphere) that are affecting the DSC signal to be measured. In this case, the use of a hermetically closed crucible is clearly not suitable for the hyphenated STA/GC-MS/FTIR techniques, which implies the identification of evolved gases. In a semi-closed system, the size of the opening in the perforated crucible lid can have a strong influence on the measurement results. If the size is small, the thermal data is comparable to a sealed crucible61. On the contrary, a large hole in the lid is expected to decrease the measured thermal signal because of the early release of low temperature decomposition products. As a result, these species would not be involved in higher temperature processes61. Indeed, a closed or semi-closed system allows longer residence time of the species, transformed from condensed to vapor phase inside the crucible. A laser-cut vent hole of 5 &#181;m in the crucible lid has been selected for the investigation of thermal behavior and evolved gases of graphite anode and NMC (111) cathode. Considering the size of the laser-cut hole, we assume the system inside the crucible may most probably depict, a simple but reasonable approximation of the dynamic inside both, a closed battery cell and a battery cell venting.
This present work is built upon an earlier publication by the same authors48. However, this paper focuses in more detail on the experimental part, highlighting the benefits of the used techniques and testing conditions to reach the goal of this work.
To the best of the authors&#39; knowledge, there is limited research published on the thermal behavior of electrode material, using of the exact combination of these analytical instruments STA/FTIR/GC-MS, analytical parameters and sample preparation/ handling to elucidate chemical reaction mechanisms at material level during thermal decomposition. At the cell level, Fernandes et al. investigated the evolved gases in a continuous way, using FTIR and GC-MS, in a battery cylindrical cell undergoing an overcharged abuse test, in a closed chamber62. They have identified and quantified the gases during this test, but the understanding of reaction mechanisms still remains unclear. Furthermore, to develop a TR runaway model, Ren et al. have also conducted DSC experiments at material level to calculate kinetic triplet parameters of exothermic reactions55. They have identified six exothermic processes, but the reaction mechanisms were not determined, and they did not use coupled gas analysis techniques.
On the other hand, Feng et al. have proposed a three-stage TR mechanism in LIB cell with three characteristic temperatures that can be used as indexes to assess thermal safety of battery63. For this purpose, they have used a thermal database with data from ARC. Nevertheless, details of the chemical reactions underlying these three mechanisms are not provided.
In this study, the data obtained through these thermal analysis methods are essential for the development of the kinetic model where the main thermal decomposition processes should be determined and properly described. The kinetic thermal triplet, namely the activation energy, frequency factor, and heat of reaction, are calculated for the different sub-processes taking place in both electrodes during thermally induced decomposition, using three different heating rates: 5, 10 and 15 &#176;C /min. When applicable, the Kissinger method64,65 was used for the determination of activation energy and frequency factor, following the Arrhenius equation. The Kissinger method is applicable when DSC peak shifts to higher temperature with increasing heating rate. The reaction enthalpy is obtained by integrating the area of the reaction peak, as measured by DSC. From these thermal data and the measurement uncertainties, a reaction kinetic model is proposed to simulate the dynamics of a thermal runaway. In the second part of this work66, this newly developed model will be used to determine the probability of a TR event as a function of the parameters of an ISC triggering method.
The scheme depicted in Figure 2 summarizes the sequence of steps needed to undertake the protocol. The first step consists of assembling the electrochemical cell with the battery materials under investigation, namely, NMC (111)/Gr.
In order to be able to harvest the battery materials after electrochemical cycling and state of charge (SOC) adjustment to 100%, a re-sealable electrochemical cell supplied by EL-CELL (ECC-PAT-Core) was used. This allowed a smooth cell opening process without damage to the electrodes. Once the battery materials are harvested, thermal characterization is carried out.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Glove box MB 200B</td>
			<td>MBraun</td>
			<td></td>
			<td>Glove box filled with argon to ensure inert atmosphere.&#160; H2O &#60; 0.1 ppm; O2 &#60; 0.1 ppm</td>
		</tr>
		<tr>
			<td>&#160;STA 449 F3 Jupiter&#174;</td>
			<td>Netzsch</td>
			<td></td>
			<td>Simultaneous Thermal Analysis for thermal characterization. The STA equipment is located inside the glove box. Type of furnace used: silver</td>
		</tr>
		<tr>
			<td>Agilent 7820A GC- Agilent 5977E MS</td>
			<td>Agilent</td>
			<td></td>
			<td>Gas Chromatograph equipped with an quadrupole Mass Spectrometer. GC-MS is coupled to STA via heated lines in order to identify the released gases during thermal characterization</td>
		</tr>
		<tr>
			<td>Bruker Vertex 70 V FTIR</td>
			<td>Bruker</td>
			<td></td>
			<td>&#160;Fourier Transform Infrared spectrometer equipped with a TG-IR box. FTIR is coupled to STA via heated lines in order to identify the released gases during thermal characterization</td>
		</tr>
		<tr>
			<td>18mm graphite electrode disc</td>
			<td>Customcell</td>
			<td>363586011</td>
			<td>graphite electrode disc with an area capacity of 2.24 mAh.cm-2</td>
		</tr>
		<tr>
			<td>18mm NMC (111) electrode disc</td>
			<td>Customcell</td>
			<td>363662011</td>
			<td>LiNiMnCoO2 electrode disc with stochiometry ratio 111 and an area capacity of 2.0 mAh.cm-2</td>
		</tr>
		<tr>
			<td>1.0 M LiPF6 in EC/DMC=50/50 (v/v)</td>
			<td>Sigma-Aldrich</td>
			<td>746711-100ML</td>
			<td>Electrolyte</td>
		</tr>
		<tr>
			<td>&#160;2325 trilayer film separator</td>
			<td>Celgard&#174;</td>
			<td></td>
			<td>Film separator. 22 mm diameter discs should be cut from the film</td>
		</tr>
		<tr>
			<td>High precision cutting pliers</td>
			<td>EL Cell</td>
			<td></td>
			<td>1) Used in paragraph 2 for cutting with a fixed 18 mm diameter the bare copper and aluminum foils (non-coated). These current collectors were bought from the same supplier as for the elecrode discs 2) Used for cutting 22 mm diameter Cegard separator discs</td>
		</tr>
		<tr>
			<td>ECC-PAT-Core (2014 version) kit using: a) reusable stainless steel (SS) upper plunger, b) reusable SS lower plunger of type 50 (size in &#181;m), c) single-use PP insulation sleeves</td>
			<td>&#160;EL Cell</td>
			<td></td>
			<td>Electrochemical cell assembly</td>
		</tr>
		<tr>
			<td>&#160;Maccor Series 4000</td>
			<td>Maccor</td>
			<td></td>
			<td>Battery cycler. It was used to cycle 2 times the electrochemical cell and to adjust afterwards the SOC to 100%</td>
		</tr>
		<tr>
			<td>aluminum crucibles with 5 &#181;m laser-pierced lid</td>
			<td>Netzsch</td>
			<td>6.239.2-64.81.00</td>
			<td>Crucible used for the thermal analysis and identification of released gases from cycled electrode material (graphite anode or NMC (111) cathode)</td>
		</tr>
		<tr>
			<td>Precision balance AE240&#160; type AE240-S inside Glove box</td>
			<td>Mettler Toledo</td>
			<td></td>
			<td>Battery materials are measured at &#181;g precision</td>
		</tr>
		<tr>
			<td>Analogue digital multimeter ABB model MetraWatt M2005</td>
			<td>Gossen MetraWatt</td>
			<td></td>
			<td>Used to measure of the freshly assembled electrochemical cell in paragraph 1.2.6</td>
		</tr>
	</tbody>
</table>

identification and quantification of decomposition mechanisms in lithium-ion batteries; input to heat flow simulation for modeling thermal runaway

The risks and possible accidents related to the normal use of lithium-ion batteries remain a serious concern. In order to get a better understanding of thermal runaway (TR), the exothermic decomposition reactions in anode and cathode were studied, using a Simultaneous Thermal Analysis (STA)/Gas Chromatography-Mass Spectrometry (GC-MS)/Fourier Transform Infrared (FTIR) spectrometer system. These techniques allowed the identification of the reaction mechanisms in each electrode, owing to the analysis of evolved gaseous species, the amount of heat released and mass loss. These results provided insight into the thermal events happening within a broader temperature range than covered in previously published models. This allowed the formulation of an improved thermal model to depict TR. The heat of reaction, activation energy, and frequency factor (thermal triplets) for each major exothermic process at material level were investigated in a Lithium Nickel-Manganese-Cobalt-Oxide (NMC (111))-Graphite battery cell. The results were analyzed, and their kinetics were derived. These data can be used to successfully simulate the experimental heat flow.

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Identification and Quantification of Decomposition Mechanisms in Lithium-Ion Batteries; Input to Heat Flow Simulation for Modeling Thermal Runaway

This work aims at determining the reaction kinetics of Li-ion battery cathode and anode materials undergoing thermal runaway (TR). Simultaneous Thermal Analysis (STA)/Fourier Transform Infrared (FTIR) spectrometer/Gas Chromatography Mass Spectrometry (GC-MS) were used to reveal thermal events and to detect evolved gases.

The risks and possible accidents related to the normal use of lithium-ion batteries remain a serious concern. In order to get a better understanding of ...