Analysis and Investigation of Thermal Runaway Propagation for a Mechanically Constrained Lithium-Ion Pouch Cell Module

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Article Analysis and Investigation of Thermal Runaway Propagation for a Mechanically Constrained Lithium-Ion Pouch Cell Module

Luigi Aiello 1,* , Ilie Hanzu 2,3 , Gregor Gstrein 1 , Eduard Ewert 4, Christian Ellersdorfer 1 and Wolfgang Sinz 1

1 Vehicle Safety Institute, Graz University of Technology, Inffeldgasse 23/I, 8010 Graz, Austria; [email protected] (G.G.); [email protected] (C.E.); [email protected] (W.S.) 2 Institute for Chemistry and Technology of Materials, Graz University of Technology, (NAWI Graz), Stremayrgasse 9, 8010 Graz, Austria; [email protected] 3 Alistore—ERI European Research Institute, CNRS FR3104, Hub de l’Energie, Rue Baudelocque, 80039 Amiens, France 4 Dr. Ing. h.c. F. Porsche Aktiengesellschaft, Porschestr. 911, 71287 Weissach, Germany; [email protected] * Correspondence: [email protected]; Tel.: +43-(316)-873-30367

Abstract: In this paper, tests and analysis of thermal runaway propagation for commercial modules consisting of four 41 Ah Li-ion pouch cells are presented. Module samples were tested at 100% state-of-charge and mechanically constrained between two steel plates to provide thermal and mechanical contact between the parts. Voltage and temperature of each cell were monitored during the whole experiment. The triggering of the exothermal reactions was obtained by overheating one cell of the stack with a flat steel heater. In preliminary studies, the melting temperature of the separator was measured (from an extracted sample) with differential scanning calorimetry Citation: Aiello, L.; Hanzu, I.; and thermogravimetric analysis techniques, revealing a tri-layers separator with two melting points Gstrein, G.; Ewert, E.; Ellersdorfer, C.; (≈135 ◦C and ≈170 ◦C). The tests on module level revealed 8 distinct phases observed and analyzed in Sinz, W. Analysis and Investigation of the respective temperature ranges, including smoking, venting, sparkling, and massive, short circuit Thermal Runaway Propagation for a condition. The triggering temperature of the cells resulted to be close to the melting temperature Mechanically Constrained of the separator obtained in preliminary tests, confirming that the violent exothermal reactions of Lithium-Ion Pouch Cell Module. thermal runaway are caused by the internal separator failure. Postmortem inspections of the modules Batteries 2021, 7, 49. https://doi.org/ 10.3390/batteries7030049 revealed the internal electrical failure path in one cell and the propagation of the internal short circuit in its active material volume, suggesting that the expansion of the electrolyte plays a role in the short Academic Editor: Carlos Ziebert circuit propagation at the single cell level. The complete thermal runaway propagation process was repeated on 5 modules and ended on average 60 s after the first thermal runaway triggered cell Received: 31 May 2021 reached a top temperature of 1100 ◦C. Accepted: 13 July 2021 Published: 19 July 2021 Keywords: thermal runaway; propagation analysis; thermal trigger; overheating; lithium ions batteries; internal short circuit; gas venting; differential scanning calorimetry Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Propagation speed of thermal runaway (TR) in a Li-ion battery module is an important phenomenon to consider as part of the battery pack design process. In fact, the timing between a first detectable TR event and a battery pack hazardous condition needs to Copyright: © 2021 by the authors. be conformal to homologation requirements [1]. Testing a single module opportunely Licensee MDPI, Basel, Switzerland. triggered in TR condition can be useful to understand the propagation process before This article is an open access article scaling it on battery pack level. Since TR exothermal behavior can differ based on the distributed under the terms and triggering methods, a research about the initiation of TR was conducted. As found in conditions of the Creative Commons literature, a Li-ion cell failure can result from a variety of abuse conditions: mechanical, Attribution (CC BY) license (https:// electrical, thermal, or manufacturing defects [2–8]. In general, the exothermal behavior creativecommons.org/licenses/by/ 4.0/). of the TR can change if the cell is triggered under different abuse conditions [9–13]; as a

Batteries 2021, 7, 49. https://doi.org/10.3390/batteries7030049 https://www.mdpi.com/journal/batteries Batteries 2021, 7, 49 2 of 18

consequence, the initiation of TR is a fundamental aspect to consider to obtain test results in the direction of the desired target. On the module level, if the TR is triggered in a first cell, this will start exothermal reactions, causing thermal and mechanical stress to the adjacent cells and likely propagating with a cascade process involving multiple cells, or, in worst case, the whole module [11,14–19]. Considering the aforementioned, if the TR propagation speed needs to be analyzed in a Li-ion cells module, since the triggering method affects the exothermal behavior of the first cell, it must be carefully chosen since it could affect the propagation speed. In fact, if the initiation is induced externally, for example, by a mechanical actuator (i.e., nail penetration), its exothermal behavior could be very different with respect to the next triggered cells (induced by cascade effect), likely also causing a difference in propagation velocity. Since the aim of this study is the TR propagation speed, the triggering method must reproduce, as close as possible, the stress and abuse loads that a cell in TR applies to the adjacent ones during the cascade effect. Furthermore, the triggering method must be robust and exhibit good repeatability. In the literature, several methods were found to initiate TR in a system with multiple Li- ion cells, however, all the investigated procedures can be reclassified into three main cases: object penetration [14], external heating [9,11,15,17–19], or electrical overcharge [20,21]. In general, the overheating of the Li-ion cell can be obtained by an external thermal source or by an abusive electrical operation, i.e., forcing an electric current and/or voltage above the nominal values. For example, in Cheng et al. [9], the thermal runaway is triggered using a suitable heater placed at the center of the prismatic cells stack. This initiates the TR by overheating the central cells. In this case, the heater must be chosen of the same size of the cell surface and power must be enough to reach a TR state. Instead, Feng et al. [14] utilizes a sharp object to penetrate the first cell of the stack, thus initiating the TR not in the center but on the side of the stack. Electrical overcharging can also push a Li-ion cell to TR, as indicated in Sun et al. [20]; however, since the state of charge (SOC) is a parameter that affects the TR behavior [22–27], this triggering method would not be preferred if the desired outcome is the exothermal behavior of the module. The overheating can also be obtained by the electrical induction phenomenon. Since the battery housing is generally made of steel/aluminum, an excitation electrical current can be induced on it with an external electromagnetic field [13]. Based on the conducted literature research, a test design was developed for the evaluation of TR propagation in Li-ion pouch cell module. The proposed test concept allows mechanical constraint to be applied on the cell stack and to precisely measure the amount of heat introduced in-to the system, thus evaluating the amount of energy for TR activation. The samples of pouch Li-ion battery modules were obtained from a commercialized battery pack of an all-electric vehicle. Module description is reported in Section 2.1. Further details of the battery pack, modules, chemistry and internal materials are reported in publication Kovachev et al. [28]. The experiment presented in this manuscript was conducted to evaluate the speed of thermal runaway propagation between pouch cells in a commercial Li-ion battery module. The produced data on module level can be utilized for validation of simulation models and then further extended for battery pack simulations.

2. Experimental The aim of the experiment was to obtain more insights into TR of a single cell and TR propagation, as well as suitable input data for simulation purposes. The TR of the first cell was triggered by overheating the cell at the bottom of the cell stack. The novelty of the test setup is the triggering method: a custom flat steel heater—connected to a high power electrical supply—positioned at the bottom of the cell stack allowed to provide controlled heat and, concurrently, to constrain the stack in between two steel plates. The heater was produced with laser cut technology from 1 mm steel foil; it was then possible to fulfill minimum power requirements and to fit the battery cell surface for a homogeneous heat distribution. Voltage and current provided to the heater were constantly monitored during the experiment allowing to monitor the amount of heat introduced into the system. The Batteries 2021, 7, x FOR PEER REVIEW 3 of 19

duced with laser cut technology from 1 mm steel foil; it was then possible to fulfill mini- Batteries 2021, 7, 49 mum power requirements and to fit the battery cell surface for a homogeneous heat3 dis- of 18 tribution. Voltage and current provided to the heater were constantly monitored during the experiment allowing to monitor the amount of heat introduced into the system. The module,module, composed composed by by four four Li-ion Li-ion pouch pouch stack stackeded cells cells was was placed placed in in between between the the two two steel steel platesplates to to constrain constrain the the cells cells during during the the whole whole test. test. A sketch A sketch of the of the assembly assembly is reported is reported in Figurein Figure 1. 1.

Figure 1. Parts concept description of test assembly for thermal runaway propagation test. Flat heater Figure 1. Parts concept description of test assembly for thermal runaway propagation test. Flat heaterat bottom at bottom of stack of isstack meant is meant to trigger to trigger first cell firs withoutt cell without affecting affecting mechanical mechanical compression compression in stack. in stack. The steel plates were bolted to each other at minimum pressure to guarantee thermal contact between the parts and a displacement constraint. Thermocouples were placed in The steel plates were bolted to each other at minimum pressure to guarantee thermal strategic points—at the very top and bottom of the stack, and in between every cell—and contact between the parts and a displacement constraint. Thermocouples were placed in applied with aluminum thermal paste to make a better thermal contact with the part. As strategic points—at the very top and bottom of the stack, and in between every cell—and indicated in Figure1, two plates of phenolic paper were sandwiched between the steel applied with aluminum thermal paste to make a better thermal contact with the part. As plates of the stack. There are two reasons for this: the first is to separate the cell stack indicated in Figure 1, two plates of phenolic paper were sandwiched between the steel from the steel plates to avoid strong thermal losses, the second is to guarantee electrical plates of the stack. There are two reasons for this: the first is to separate the cell stack from insulation of the steel heater from the steel plate. the steel plates to avoid strong thermal losses, the second is to guarantee electrical insula- tion2.1. of Samples the steel Description heater from the steel plate. The Li-ion cell module samples were obtained from an all-electric vehicle which 2.1. Samples Description was commercialized in 2016. Each module contained a stack of four Li-ion pouch cells gluedThe together Li-ion cell on themodule surface samples of the were central obtained contact from area an and all-electric kept in place vehicle by which an external was commercializedplastic/silicon frame.in 2016. The Each four module holes incontained the plastic a stack frame of were four madeLi-ion to pouch receive cells four glued steel togethercylinders on for the reasons surface ofof mechanicalthe central contact constraint. area Furthermore,and kept in place the cellsby an were external connected plastic/siliconwith a 2p2s frame. electrical The four configuration: holes in the theplastic tabs frame were were welded made on to three receive copper four step-shaped steel cylin- dersbars for ending reasons with of threemechanical output constraint. terminals. Fu Therthermore, single cells the cells were were laminated connected pouch-type with a 2p2swith electrical a rated configuration: capacity of 41 the Ah tabs and were a nominal welded voltage on three of copper 3.7 V. step-shaped The chemistry bars of end- the ingelectrodes with three was output determined terminals. by an The analytic single cell cells dissection, were laminated as described pouch-type in the publication with a rated of capacityKovachev of et41al. Ah [28 and]. The a nominal cathode voltage chemistry of is3.7 a V. spinel The ofchemistry NMC and of LMO, the electrodes and the anode was determinedelectrode is by made an analytic of Graphite. cell Eachdissection, cell had as adescribed weight of in approximately the publication 800 of g Kovachev with a size et of al.290 [28]. mm The× 216cathode mm ×chemistry8 mm. Pictures is a spinel of the of moduleNMC and and LMO, its tabs and connections the anode areelectrode reported is madein Figure of Graphite.2. Each cell had a weight of approximately 800 g with a size of 290 mm x 216 mmFour x 8 resin mm. spotsPictures were of found the module at the fourand its corners tabs connections of each cell withare reported the purpose in Figure of fixing 2. the plastic frames to the cells themselves, and in correspondence of the resin spots, the external pouch of the cells were presenting holes of 3 mm diameters. While dismantling, the silicon edges showed some resistance when pulled apart; this was probably due to a thin layer of glue placed all around the frame. Batteries 2021, 7, 49 4 of 18 Batteries 2021, 7, x FOR PEER REVIEW 4 of 19

FigureFigure 2. (a 2.) Module(a) Module sample sample used used for for test test is is composed composed ofof four cells cells stacked stacked and and glued glued to toeach each other; other; each each cell cellis surrounded is surrounded by a plastic-silicon frame; (b) Top view of copper busbars: cells are electrically connected in the 2s2p configuration; (c) by a plastic-silicon frame; (b) Top view of copper busbars: cells are electrically connected in the 2s2p configuration; (c) Detail Detail of the connection busbars of module. of the connection busbars of module. Four resin spots were found at the four corners of each cell with the purpose of fixing 2.2.the Samples plastic Preparationframes to the cells themselves, and in correspondence of the resin spots, the externalThe modulespouch of werethe cells prepared were presenting preserving holes the of manufacturer’s3 mm diameters. glueWhile between dismantling, the cells tothe guarantee silicon edges that theshowed thermal some contact resistance cell-to-cell when pulled was optimal.apart; this At was first, probably the modules due to a were setthin to 100%layer of SOC glue with placed astandard all around procedure: the frame. a charging current was applied at 1-C rate (82 A), and once the charging device switched to Voltage Control mode, the procedure was ended2.2. Samples when Preparation electrical current was below the C/20 C-rate. The SOC setting was carried out withThe a modules programmable were prepared DC power preserving supply the (Model: manufacturer’s EA-PSI 9000glue 3U),between an electronicthe cells to load (Model:guarantee EA-EL that 9000the thermal B), and contact a coulomb cell-to-cell counter was implemented optimal. At first, with the a precisionmodules were DC Current set Transducerto 100% SOC (Model: with a CT-1000). standard procedure: The modules a charging included current frames was made applied ofplastic at 1-C rate and (82 silicon materialsA), and gluedonce the directly charging to thedevice pouch switched bag edges to Voltage of each Control cell. The mode, frames the wereprocedure clearly was added byended the manufacturer when electrical to current ensure was the below cellsalignment the C/20 C-rate. and avoidThe SOC undesired setting was contacts carried between out thewith busbars a programmable and/or current DC collectors.power supply Since (Model: those plasticEA-PSI frames 9000 3U), were an external electronic to the load active material(Model: area, EA-EL they 9000 were B), and considered a coulomb not counter relevant implemented for the cell-to-cell with a precision heat conduction; DC Current hence, theTransducer frames were (Model: removed CT-1000). with The a specialmodules disassembling included frames procedure. made of plastic The steps and silicon and tools materials glued directly to the pouch bag edges of each cell. The frames were clearly utilized for the module preparation are shown in Figure3. added by the manufacturer to ensure the cells alignment and avoid undesired contacts To separately monitor the electric voltage of each cell, the electric busbars connections between the busbars and/or current collectors. Since those plastic frames were external to needed to be removed before proceeding with the test setup. the active material area, they were considered not relevant for the cell-to-cell heat conduction;At hence, first, the frames accessible were busbars removed connections with a special were disassembling cut with a ceramicprocedure. knife; The then,steps the fourand resin tools spotsutilized (as for illustrated the module in pr Figureeparation3b) were are shown removed in Figure with 3. a 3 mm drill bit. At this point, the first plastic frame could be detached with a minimum pullout force, and the next tabs became accessible to advance with the preparation procedure.

2.3. Equipment and Sensors The heater was built from a 1 mm steel foil suitably shaped by laser cutting and it was electrically supplied with the same programmable DC power supply used for the SOC setting (power supply: EA-PSI 9000 3U). Voltage and current provided to the heater were measured during the whole experiment to evaluate the heat power introduced into the system. The heater was designed to obtain the required heating power and to ﬁt the sizes of the battery. Voltages and temperatures (captured with type K thermocouples) were measured at 2 kHz with NI expandable data logger (cDAQ-NI 9178). The electric current was measured with a precision electric current transducer (model: CT-1000) from FLUCS company. Batteries 2021, 7, 49 5 of 18 Batteries 2021, 7, x FOR PEER REVIEW 5 of 19

FigureFigure 3. Procedure 3. Procedure for for partial partial disassembling disassembling of of Li-ionLi-ion cellcell module. (a (a) )First First two two levels levels of ofaccessible accessible busbars busbars are removed are removed with a ceramic cutter; (b) Resin spots of first two cells are removed with a 3 mm drill bit; (c) Plastic frames are removed with a ceramic cutter; (b) Resin spots of ﬁrst two cells are removed with a 3 mm drill bit; (c) Plastic frames are removed uncovering next busbars layers; (d) Procedure is repeated for further busbars layers. uncovering next busbars layers; (d) Procedure is repeated for further busbars layers. To separately monitor the electric voltage of each cell, the electric busbars connec- 2.4.tions Test needed Setup andto be Procedure removed before proceeding with the test setup. ThermocouplesAt first, the accessible were positionedbusbars connections to obtain were meaningful cut with a informationceramic knife; regarding then, the the propagationfour resin spots of TR, (as henceillustrated thetemperature in Figure 3b) sensorswere removed were placed with a between3 mm drill each bit. cellAt this as well aspoint, at the the top first and plastic bottom frame of thecould stack, be deta as describedched with a in minimum Figure4. pullout As illustrated, force, and since the the Batteries 2021, 7, x FOR PEER REVIEW 6 of 19 originalnext tabs glue became between accessible the cells to advance was preserved, with the the preparation thermocouple procedure. numbers 2, 3, and 4 could not be placed in the center of the stack but just at the edge of the internal glued surface. 2.3. Equipment and Sensors The heater was built from a 1 mm steel foil suitably shaped by laser cutting and it was electrically supplied with the same programmable DC power supply used for the SOC setting (power supply: EA-PSI 9000 3U). Voltage and current provided to the heater were measured during the whole experiment to evaluate the heat power introduced into the system. The heater was designed to obtain the required heating power and to fit the sizes of the battery. Voltages and temperatures (captured with type K thermocouples) were measured at 2 kHz with NI expandable data logger (cDAQ-NI 9178). The electric current was measured with a precision electric current transducer (model: CT-1000) from FLUCS company.

FigureFigure2.4. 4.Test 4. DescriptionDescription Setup and of Procedure sensors’ of sensors’ posi positioningtioning for TR for propagation TR propagation experiment. experiment. Thermocouples Thermocouples 2,3 2,3 and and4 could 4 couldThermocouples not not be be placed placed on onwere same same positioned verticalvertical axis toof of thermocouplesobtain thermocouples meaningful 1 and 1 and 5 sinceinformation 5 since manufacturer’s manufacturer’s regarding gluethe was gluepreserved.propagation was preserved. Therefore, of TR,Therefore, middlehence middle the area temperature betweenarea between cells sens cells wasors was not were not accessible. accessible. placed between each cell as well as at the top and bottom of the stack, as described in Figure 4. As illustrated, since the TheThe triggering triggering of TR of TRwas was performed performed by providing by providing an electric an electriccurrent currentto the heater to the heater original glue between the cells was preserved, the thermocouple numbers 2, 3, and 4 could described in Section 2.3 with the target power of 600 W, provoking a temperature increase describednot be placed in Section in the 2.3center with of thethe targetstack but power just at of the 600 edge W, provokingof the internal a temperature glued surface. increase ofof about about 2 °C/min. 2 ◦C/min. When When threshold threshold temperature temperature was reached was a reached visible increase a visible of temper- increase of tem- atureperature occurred, occurred, then the then electric the electriccurrent was current manually was manuallyinterrupted interruptedby the operator. by theThe operator. experimentsThe experiments were conducted were conducted outdoors outdoorsand recorded and with recorded a digital with camera a digital that was camera acti- that was vated just a few minutes before the TR critical temperature was reached. Furthermore, an activated just a few minutes before the TR critical temperature was reached. Furthermore, additional surveillance camera (remotely controlled) was put in place to allow the opera- an additional surveillance camera (remotely controlled) was put in place to allow the tors to detect smoke and fire in real time and manually deactivate the heater when TR started.operators A blower to detect was smokepositioned and at ﬁre approxim in realately time two and meters manually from deactivatethe module theto flush heater when away smoke during the TR, this device was active during the whole test duration. A total of five repetitions were conducted with the aim to increase the chances of observing a common behavior of TR and its propagation. Since the TR behavior is usually violent— due to extreme gas venting and high temperatures—loss of contact of the temperature sensors or voltage cables in some tests was considered possible.

2.5. Analysis of the Separator with Differential Scanning Calorimeter Since thermal runaway threshold temperature mainly depends on the separator softening/melting temperature [3,4,9,10], the separator material of the Li-ion cells under analysis was investigated. The exothermal/endothermal behavior of the separator was investigated by simultaneous thermal analysis (STA) that provided information on both mass loss and heat flow. The instrument used was STA 449C Jupiter from “NETZSCH”. The temperature program consisted of a temperature ramp from 20 °C to 550 °C at a heating rate of 10°/min. Al2O3 (alumina) crucibles were used, and the atmosphere was helium. The preparation of the samples for the DSC analysis was done disassembling a battery in the glovebox and cutting 5 mm disks out of the separator layer. During the extraction, most of the electrolyte vaporized and got lost; however, the samples were sealed immediately in a small aluminum container. In the results, the effect of the residual electrolyte is well-visible in the expected temperature range.

3. Results 3.1. DSC Analysis of Separator

The aim of this measurement is the determination of materials phase change temperatures; in particular, the melting temperature of the separator. In Figure 5 are shown the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of the separator as taken from the opened cell, with no solvent rinsing and after several days of storage under ambient atmosphere. Batteries 2021, 7, 49 6 of 18

TR started. A blower was positioned at approximately two meters from the module to ﬂush away smoke during the TR, this device was active during the whole test duration. A total of ﬁve repetitions were conducted with the aim to increase the chances of observing a common behavior of TR and its propagation. Since the TR behavior is usually violent—due to extreme gas venting and high temperatures—loss of contact of the temperature sensors or voltage cables in some tests was considered possible.

2.5. Analysis of the Separator with Differential Scanning Calorimeter Since thermal runaway threshold temperature mainly depends on the separator softening/melting temperature [3,4,9,10], the separator material of the Li-ion cells under analysis was investigated. The exothermal/endothermal behavior of the separator was investigated by simultaneous thermal analysis (STA) that provided information on both mass loss and heat ﬂow. The instrument used was STA 449C Jupiter from “NETZSCH”. The temperature program consisted of a temperature ramp from 20 ◦C to 550 ◦C at a heating rate of 10◦/min. Al2O3 (alumina) crucibles were used, and the atmosphere was helium. The preparation of the samples for the DSC analysis was done disassembling a battery in the glovebox and cutting 5 mm disks out of the separator layer. During the extraction, most of the electrolyte vaporized and got lost; however, the samples were sealed immediately in a small aluminum container. In the results, the effect of the residual electrolyte is well-visible in the expected temperature range.

3. Results 3.1. DSC Analysis of Separator The aim of this measurement is the determination of materials phase change temperatures; in particular, the melting temperature of the separator. In Figure5 are shown the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of the separator as taken from the opened cell, with no solvent rinsing and after several days of storage under ambient atmosphere. At temperatures between 50 and 200 ◦C, three endothermic peaks related to the separator can be seen. The process occurring at lower temperatures around 100 ◦C likely corresponds to the shrinking of the polyolefin part of the separator; water evaporation may also contribute in this temperature region. The peaks centered at 134 ◦C and 170 ◦C very likely correspond to the melting point of different separator components such as polyethylene and polypropylene. The other endothermal and exothermal peaks can be explained taking into consideration that the separator was not rinsed with any solvent before analysis. Samples were simply taken from cell after disassembly with no further treatment. Consequently, the investigated separator contained a significant quantity of solvents, very likely those having a lower vapor pressure such as ethylene carbonate (EC), as well as some lithium hexaflourophosphate (LiPF6) salt. The other two components of the solvent mixture, namely diethyl carbonate (DEC) and dimethyl carbonate (DMC), have high vapor pressures and are thus volatile. It is plausible to consider that DEC and DMC completely evaporated during the preparation for analysis and storage of the separator under ambient atmosphere for several days before measurements. Indeed, in Figure5 we see an endothermic peak centered about 34–35 ◦C that very nicely fits to the melting temperature of crystalline EC embedded in the separator. The endothermic peak centered at 134 ◦C can very likely be ascribed to the melting point of the polyethylene (PE) layer(s) in the separator, a phenomenon that is known to occur around 130 ◦C. Further on, the endothermic peak centered at 170 ◦C is characteristic to the melting point of polypropylene (PP) layer of the separator. This confirms that the separator is a multilayer separator, made of PE and PP each with slightly different mechanical and thermal properties and porous so that a liquid electrolyte can be retained in the separator and ensure ion conduction. At temperatures above 250 ◦C, the LiPF6 salt is known to decompose exothermally into LiF and PF5, a reaction that matches very well the major endothermic peak around 270 ◦C. Batteries 2021, 7, 49 7 of 18 Batteries 2021, 7, x FOR PEER REVIEW 7 of 19

Figure 5. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of separator as taken Figure 5. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of separator as taken from from opened cell, with no solvent rinsing and after several days of storage under ambient atmosphere. opened cell, with no solvent rinsing and after several days of storage under ambient atmosphere. At temperatures between 50 and 200 °C, three endothermic peaks related to the sep- 3.2. Observable TR Phases arator can be seen. The process occurring at lower temperatures around 100 °C likely cor- respondsDuring tothe the tests,shrinking we observed of the polyolefin a sequence part of of phases the separator; in common water for evaporation all the experiments may and—foralso contribute each step—identified in this temperature the region. respective (externally measured) temperature ranges. The identifiedThe peaks eight centered phases—associated at 134 °C and 170 to°C observable very likely correspond phenomena—were to the melting correlated point to theof expecteddifferent separator physical components processes occurring such as po insidelyethylene the cells and andpolypropylene. listed in Table The1 other. The en- eight phasesdothermal described and exothermal in Table1 peaks cannot can be be identified explained fortaking each into of consideration the four cells that of the sep- stack; inarator fact, oncewas not the rinsed first cell with was any triggered, solvent before gas venting, analysis. andSamples fire did were not simply allow taken us to from have a clearcell after sight disassembly on the module. with Furthermore,no further treatm theent. speed Consequently, of heat released the investigated from a triggered separator cell is certainlycontained higher a significant than the quantity heating of power solvents, of thevery steel likely heater, those provokinghaving a lower a faster vapor transition pres- betweensure such phases as ethylene for the successivecarbonate cells,(EC), complicatingas well as some the phaseslithium identification. hexaflourophosphate Supplying the(LiPF6) heater, salt. the The first other cell two of the components stack was of warmed-up the solvent mixture, at a rate namely of approximately diethyl carbonate 0.2 ◦C/s (measured(DEC) and on dimethyl thermocouple carbonate 1) from(DMC), ambient have high temperature vapor pressures until the and TR are trigger thus volatile. temperature It wasis plausible reached. to At consider this temperature that DEC and threshold, DMC completely the temperature evaporated increase during can the be preparation sustained by thefor self-heat analysis generationand storage of of the the cellseparator but it doesunder not ambient correspond atmosphere to the for very several first self-heating days be- temperature;fore measurements. in fact, Indeed, that would in Figure be related 5 we see to thean endothermic decomposition peak of centered the solid about electrolyte 34– interface35 °C that (SEI) very layer, nicely occurring fits to the already melting from temperature about 80–90 of crystalline◦C[4,10]. EC Adiabatic embedded experiment in the mustseparator. be performed to precisely observe this temperature threshold. TheThe self-heatingendothermic ratepeak due centered to this at phenomenon134 °C can very (SEI likely decomposition) be ascribed to wouldthe melting be not enoughpoint of to the provide polyethylene a visible (PE) temperature layer(s) in th increasee separator, of the a systemphenomenon and a that further is known deeper to TR state.occur The around reason 130 is °C. that Further this test on, is the not endothermic performed inpeak an centered adiabatic at condition: 170 °C is characteristic a cooling effect fromto the the melting external point environment of polypropylene is constantly (PP) laye present,r of the requiring separator. the This external confirms heating that devicethe separator is a multilayer separator, made of PE and PP each with slightly different me- to be active until higher temperature is reached. chanical and thermal properties and porous so that a liquid electrolyte can be retained in Considering the above-mentioned, no visible effects were registered during the ex- the separator and ensure ion conduction. ternal warm-up phase until 170 ◦C(phase 1). In the range 170–190 ◦C(phase 2), a slight At temperatures above 250 °C, the LiPF6 salt is known to decompose exothermally white smoke was detected, probably due to the electrolyte evaporation through the sealing; into LiF and PF5, a reaction that matches very well the major endothermic peak around during this phase, impulsive and temporary increase of internal pressure were detected 270 °C. by observing slight movements and bending of the steel plates. As soon as phase 3 started, the3.2. heater Observable was turnedTR Phases off. A fast increase in temperature was detected as well as a strong movement of the steel plates, which indicates a strong increase in internal pressure that was During the tests, we observed a sequence of phases in common for all the experi- certainly related to local internal short circuits due to mechanical failure of the separator, ments and—for each step—identified the respective (externally measured) temperature and in turn leads to the rupture of the pouch bag (phase 4). The breakdown of the pouch bag led to a consequent gas venting and drop of the internal pressure, and once hot gases are released to the surroundings with available oxygen, their combustion can be initiated Batteries 2021, 7, 49 8 of 18

(phase 5). Local short circuits melt down small pieces of the current collectors (presumably the aluminum of the cathode current collector), thus generating flying burning embers that can potentially initiate the combustion of the vented gases. In every repetition test, combustion of the vented gases always occurred, so that a phase 6 was specially introduced to define this event. Since all the events were recorded by a high definition camera, in Figure6 are reported the frames relative to the beginning of each phase described in Table1 . The violent exothermal reaction phase is followed by a slower heat generation condition; this is indicated by phase 7, where uncombusted active materials are still present, decomposing gradually and fading to a passive cooling condition. In this phase, small regions of the active material could be still functioning, thus presenting a fading exothermal behavior. Once all the parts are combusted, the system enters phase 8: during this last phase, the system cools slowly down to ambient temperature.

Table 1. Description of TR phases and their observable phenomena and respective external temperature ranges.

Temperature Observable Phenomena Range (External) Phase 1 No visible effect <170 ◦C Slight smoke release, possible electrolyte leakage, impulsive and Phase 2 170–190 ◦C discontinuous increase of internal pressure Phase 3 Fast swelling, fast increase of internal pressure and temperature 190–220 ◦C Pouch bag opens, gas venting, decrease of internal pressure, Phase 4 >220 ◦C decompression could cause instantaneous temperature drop Phase 5 Gas venting, sparks initiate the combustion of vented gas >220 ◦C Phase 6 Deep short circuit and gas combustion, triggering of next cells - Rise of temperature and pressure is over, system starts to Phase 7 - decrease its temperature, uncombusted materials are still present Phase 8 All active materials are combusted, system cools down slowly -

The described TR phases were observed for all the ﬁve experiments performed; temperatures and voltages were monitored and reported in the next sections.

3.3. Temperature Measurements The position of the sensors were described in Figure4, ﬁve thermocouples were positioned at every layer of the stack (in between the cells) to obtain the temperature gradients in TR propagation direction. Thermocouples are numbered in such a way that the higher the number, the higher the distance from the heater (as illustrated in Figure4 and consequently—during TR events—the temperatures of the thermocouples increased accord- ing to the numbering order. In some cases—due to violent gas venting—thermocouples were losing contact with the parts (temporary or for the rest of the experiment), with consequent loss of temperature data. Regarding the experiments described in this study, two of ﬁve tests showed partial loss of data, although the lost data could be reconstructed by interpolation. For results demonstration just one experiment will be considered, and the temperature results of the other tests can be found in the AppendixA. Plots of the recorded temperatures—overview and zoom on TR propagation—are illustrated in Figure7. Analysis of TR propagation speed can be carried out from the temperature data of Figure7b: it is possible to observe the trigger temperature from Thrmcpl 1 (the one closest to the heater) of about 200 ◦C. Furthermore, referring to the same temperature curve (Thrmcpl 1), a small negative peak is present just before the temperature rise, corresponding to the main gas venting event. This peak is not present on the next 4 thermocouples when TR is triggered. The two thermocouples Thrmcpl 3 and Thrmcpl 4 reach a higher temperature with respect to the others. The reason can be addressed to the thermocouples position: since they are placed internally to the stack, there is less heat exchange with the surroundings during the experiment. Batteries 2021, 7, x FOR PEER REVIEW 9 of 19

generation condition; this is indicated by phase 7, where uncombusted active materials are still present, decomposing gradually and fading to a passive cooling condition. In this Batteries 2021, 7, 49 9 of 18 phase, small regions of the active material could be still functioning, thus presenting a fading exothermal behavior. Once all the parts are combusted, the system enters phase 8: during this last phase, the system cools slowly down to ambient temperature.

FigureFigure 6. Description 6. Description of the of the TR TR phases phases observed observed and and recordedrecorded with with a ahigh-definition high-deﬁnition camera: camera: (a) initial (a) initial warm warm up phase, up phase, no observable effects; (b) slight smoke detected, probably due to electrolyte evaporation through sealing; (c) gas venting, no observablesparks and effects; embers (b are) slight not yet smoke present, detected, smoke is probably mainly of due white to electrolytecolor; (d) gas evaporation venting with through increased sealing; speed, (gasc) gas hasventing, a sparksdarker and emberscolor, sparkles are not and yet embers present, occur; smoke (e) due is mainlyto sparkles of whiteand embers, color; vented (d) gas gas venting initiate combustion; with increased (f) deep speed, TR and gas has a darkercombustion color, sparkles of vented and gases; embers (g) occur; system ( estarts) due to to cool sparkles down, anduncombusted embers, ventedactive materials gas initiate are combustion;still present and (f) burning deep TR and combustionslowly; of(h) vented active materials gases; ( gare) system all combusted, starts to system cool down,cools down. uncombusted active materials are still present and burning slowly; (h) active materials are all combusted, system cools down. The described TR phases were observed for all the five experiments performed; temperatures and voltages were monitored and reported in the next sections. 3.4. Voltage Measurements Since the violent exothermal behavior of TR is mainly triggered by the internal electrical short circuit [4,10], voltage measurements of the cells provide a more precise information regarding the TR triggering time and propagation. The external temperature rise—monitored by temperature sensors—is an effect occurring with a certain time delay with respect to the internal electrical short circuit. Plotting the temperature and voltage data, this phenomenon and the related delay, which in this case was approximately 3 s (as shown in Figure8), are observable. Figure9 shows the voltages of the 4 cells in the thermal runaway time window for the test carried out on “Module 2”. It is possible to observe several small peaks on the voltage Batteries 2021, 7, x FOR PEER REVIEW 10 of 19

3.3. Temperature Measurements Batteries 2021, 7, 49 10 of 18 The position of the sensors were described in Figure 4, five thermocouples were positioned at every layer of the stack (in between the cells) to obtain the temperature gradients in TR propagation direction. Thermocouples are numbered in such a way that the curveshigher (in the particular number, the cell higher numbers the distance 3 and 4), from those the peaksheater are(as illustrated due to the ininstability Figure 4 and of the internalconsequently—during short circuit brought TR events—the about during temperatures the warmup of the phase. thermocouples In Figure9 increased at time ≈ac-900 s, cording to the numbering order. In some cases—due to violent gas venting—thermocou- a massive short circuit occurs in cell number 1. After 12 s, the cell number 2 is triggered, ples were losing contact with the parts (temporary or for the rest of the experiment), with also exhibiting a faster transition to massive short circuit compared to cell 1. In the same consequent loss of temperature data. Regarding the experiments described in this study, plot (i.e. Figure9) a difference in terms of propagation time and severity of the internal two of five tests showed partial loss of data, although the lost data could be reconstructed shortby interpolation. circuit—by aFor more results gentle demonstration discharging just curve—can one experiment be observed will be considered, for cells 3and and 4. Thisthe difference temperature can results be attributed of the other to tests thedifferent can be found stack in pressure the Appendix status A. when Plots cellof the 3 and 4 arerecorded triggered. temperatures—overview Once cell 1 and cell and 2zoom initiated on TR TR propagation—are and vented gas, illustrated the internal in Fig- stack pressureure 7. drops.

FigureFigure 7. Plots 7. Plots of temperatures of temperatures recorded recorded by by the the 5 5 thermocouples thermocouples for TR propagation propagation test; test; (a) ( aOverview) Overview of 5 of temperature 5 temperature sensors;sensors; time time window window is set is set from from beginning beginning to to end end of of experiment; experiment; ( b)) Zoom Zoom of of time time window window where where temperatures temperatures rise risein in correspondence to TR. correspondence to TR. Analysis of TR propagation speed can be carried out from the temperature data of A comparison of the triggering time of the 5 tested modules—regarding the voltage Figure 7b: it is possible to observe the trigger temperature from Thrmcpl 1 (the one closest drop behavior—is reported in Table2. In some cases, it was not possible to derive the TR to the heater) of about 200 °C. Furthermore, referring to the same temperature curve initiation(Thrmcpl time; 1), thisa small is indicated negative in thepeak table is aspresent “Loss ofjust Contact” before (LoC).the temperature Appendix Arise, shows the temperature and voltage measurement curves for each conducted test. The heating power was manually regulated by the operator and turned off when TR was visibly triggered by temperature observation (as illustrated in phase three from Table1). For this reason, the triggering time is different in each test.

3.5. Postmortem Inspection After the completion of the experiments, the modules were inspected to ﬁnd possible evidence to conﬁrm the steps and phases observed and recorded during the experiment. Evidence of internal short circuit and triggered active material were found by removing Batteries 2021, 7, x FOR PEER REVIEW 11 of 19

corresponding to the main gas venting event. This peak is not present on the next 4 thermocouples when TR is triggered. The two thermocouples Thrmcpl 3 and Thrmcpl 4 reach a higher temperature with respect to the others. The reason can be addressed to the thermocouples position: since they are placed internally to the stack, there is less heat exchange with the surroundings during the experiment.

Batteries 2021, 7, 49 3.4. Voltage Measurements 11 of 18 Since the violent exothermal behavior of TR is mainly triggered by the internal electrical short circuit [4,10], voltage measurements of the cells provide a more precise information regarding the TR triggering time and propagation. The external temperature therise—monitored pouch bag from by thetemperature top cell ofsensors—is the stack, an thus effect clearly occurring revealing with a thatcertain the time propagation delay of thewith short respect circuit to wasthe internal due to localelectrical pressure short circuit. increase as consequence of short circuit initiation. ReferringPlotting to cell the number temperature 4 (the and cell voltage at the data, top of this the phenomenon stack) of the and tested the related module delay, 2, the sign ofwhich initial in short this circuitcase was and approximately consequent 3 propagation s (as shown in was Figure signiﬁcant, 8), are observable. as reported in Figure 10.

FigureBatteriesFigure 8. 2021Comparison ,8. 7 , Comparisonx FOR PEER of REVIEW temperatureof temperature and and voltage voltage for cell 1 1 trigge triggeredred to to TR TR condition. condition. Time Time between between short shortcircuitcircuit and12 of and 19 temperature rise is approximately 3 s. temperature rise is approximately 3 s. Figure 9 shows the voltages of the 4 cells in the thermal runaway time window for the test carried out on “Module 2”. It is possible to observe several small peaks on the voltage curves (in particular cell numbers 3 and 4), those peaks are due to the instability of the internal short circuit brought about during the warmup phase. In Figure 9 at time ≈ 900 s, a massive short circuit occurs in cell number 1. After 12 s, the cell number 2 is triggered, also exhibiting a faster transition to massive short circuit compared to cell 1. In the same plot (i.e. Figure 9) a difference in terms of propagation time and severity of the internal short circuit—by a more gentle discharging curve—can be observed for cells 3 and 4. This difference can be attributed to the different stack pressure status when cell 3 and 4 are triggered. Once cell 1 and cell 2 initiated TR and vented gas, the internal stack pressure drops.

FigureFigure 9. Voltage 9. Voltage behavior behavior of cells of cells after after trigger trigger of TR. of TR. First First cell cell exhibits exhibits a fastera faster transition transition in in deep deep thermal thermal runawayrunaway indi- indicating cating a more severe electrical short circuit condition. After venting of the first cell, stack pressure decreases reducing a more severe electrical short circuit condition. After venting of the ﬁrst cell, stack pressure decreases reducing electrical electrical short circuit severity for successive cells. Discontinuities of voltages for cells 3 and 4 are due to venting of adjacent short circuitcells. severity for successive cells. Discontinuities of voltages for cells 3 and 4 are due to venting of adjacent cells.

Table 2. ATR comparison propagation of times the triggering based on time the voltage of the 5 drop tested measurements. modules—regarding Some values the voltage are missing— indicateddrop behavior—is as: Loss of reported Contact (LoC)—sincein Table 2. In some cables cases, contacts it was were not lostpossible during to derive test due the toTR violent gasinitiation venting. time; this is indicated in the table as “Loss of Contact” (LoC). Appendix A shows the temperature and voltage measurement curves for each conducted test. TR Triggering Propagation Time—Based on Voltage Drops Table 2. TR propagation times based on the voltage drop measurements. Some values are miss- Cell 1 Cell 2 Cell 3 Cell 4 ing—indicated as: Loss of Contact (LoC)—since cables contacts were lost during test due to violent gasModule venting. 1 1947.4 s 1958.8 s (+11.4) LoC LoC Module 2 897.9 s 909.9 s (+12.0) 924.4 s (+14.5) 941.9 s (+17.5) TR Triggering Propagation Time—Based on Voltage Drops Module 3 660.5 s LoC LoC LoC Cell 1 Cell 2 Cell 3 Cell 4 Module 4 1157.4 s 1168.3 s (+10.9) 1184.4 s (+16.1) 1203.1 s (+18.7) Module 1 1947.4 s 1958.8 s (+11.4) LoC LoC Module 5 1045.3 s 1051.1 s (+5.8) 1062.3 s (+11.2) 1075.0 s (+12.7) Module 2 897.9 s 909.9 s (+12.0) 924.4 s (+14.5) 941.9 s (+17.5) Module 3 660.5 s LoC LoC LoC Module 4 1157.4 s 1168.3 s (+10.9) 1184.4 s (+16.1) 1203.1 s (+18.7) Module 5 1045.3 s 1051.1 s (+5.8) 1062.3 s (+11.2) 1075.0 s (+12.7)

The heating power was manually regulated by the operator and turned off when TR was visibly triggered by temperature observation (as illustrated in phase three from Table 1). For this reason, the triggering time is different in each test.

3.5. Postmortem Inspection After the completion of the experiments, the modules were inspected to find possible evidence to confirm the steps and phases observed and recorded during the experiment. Evidence of internal short circuit and triggered active material were found by removing the pouch bag from the top cell of the stack, thus clearly revealing that the propagation of the short circuit was due to local pressure increase as consequence of short circuit initiation. Referring to cell number 4 (the cell at the top of the stack) of the tested module 2, the sign of initial short circuit and consequent propagation was significant, as reported in Fig- ure 10. Batteries 2021, 7, 49 12 of 18 Batteries 2021, 7, x FOR PEER REVIEW 13 of 19

FigureFigure 10. 10.Inspection Inspection of of batterybattery modulemodule afterafter test completion ( (a)) Perspective Perspective view view ( (bb)) Top Top view. view. Traces Traces of of burned burned material material on the path of internal short circuit and high temperature electrolyte are well visible on top layer. on the path of internal short circuit and high temperature electrolyte are well visible on top layer.

LookingLooking atat Figure 1010,, it is possible to to make make assumptions assumptions about about the the process process of of internal internal shortshort circuit and and its its propagation propagation during during TR. TR. As Asthe thefirst ﬁrst short short circuit circuit is triggered, is triggered, the elec- the electrolytetrolyte warms warms up upquickly quickly and and expands expands in inits its surrounding, surrounding, thus thus triggering triggering mechanical mechanical stressstress onon thethe adjacentadjacent layerslayers leading to an avalanche effect. effect. The The short short circuit circuit propagates propagates withwith branchingbranching shape until it reaches the cell edges, as as shown shown in in Figure Figure 10.10 .A A sketch sketch to to illustrateillustrate howhow thisthis phenomenonphenomenon can take place place on on single single layer layer level level is is shown shown in in the the results results discussiondiscussion section.section. Batteries 2021, 7, 49 13 of 18 Batteries 2021, 7, x FOR PEER REVIEW 14 of 19

4. Results Discussion 4. Results Discussion The presented data and postmortem inspection can be analyzed to make assumptions The presented data and postmortem inspection can be analyzed to make assump- regarding the electrothermal behavior of the cells when internal failure occurs. When tions regarding the electrothermal behavior of the cells when internal failure occurs. When thermal stress is applied (by the external heater) at a certain temperature, the separator thermal stress is applied (by the external heater) at a certain temperature, the separator loses its mechanical stability. This temperature threshold can be estimated from the melting loses its mechanical stability. This temperature threshold can be estimated from the melt- point obtained by DSC analysis (as illustrated in Figure5). In Figure 11, it is graphically ing point obtained by DSC analysis (as illustrated in Figure 5). In Figure 11, it is graph- explained the process of local separator failure and its consequence involving the electrolyte ically explained the process of local separator failure and its consequence involving the and the adjacent layers. electrolyte and the adjacent layers.

Figure 11. Description of expected internal phenomena occurring during initial warming up phase and TR triggering. Figure 11. Description of expected internal phenomena occurring during initial warming up phase and TR triggering. Temperature ranges refer to the DSC analysis and not to measured ones during TR propagation test. In fact, during ex- Temperature ranges refer to the DSC analysis and not to measured ones during TR propagation test. In fact, during periment external temperature is measured while described processes are triggered by internal temperatures. experiment external temperature is measured while described processes are triggered by internal temperatures. Batteries 2021, 7, 49 14 of 18

Once the separator fails in a precise location, this failure propagates, as explained in Figure 11. From the plot of Figure8, it is possible to understand the speed of this internal short circuit propagation: within 15 s the voltage drops to zero and the temperature rises to about 700 ◦C. Furthermore, it is possible to deduct that the thermal runaway severity shows a dependency on the external force applied to the stack. Looking at the voltage drop of each cell (as illustrated in Figure9), when TR occurs in the first cell the stack is loaded with the maximum pressure, as soon as the first cell is triggered and in deep TR, the thickness of the cell stack will be reduced lowering the stack pressure. Consequently, the successive cells will sustain different pressure when TR will be triggered. The effect of this phenomenon can be evaluated observing the time between the first voltage variation and the massive internal short circuit in Figure9. In fact, the cell number two shows a longer initial discharge before to get into the massive short circuit condition, this variation gets higher and higher for next cells as it is visible for cells three and four. From the postmortem inspection (as illustrated in Figure 10), the path of the electrical short circuit propagation on single cell level is visible, and the propagation is provoked by the overpressure of the electrolyte in the area of the first short circuit. On module level, the propagation cell-to-cell can be evaluated observing the temperatures or the voltages. Since the internal separator failure of a cell is the precursor of massive internal short circuit and TR, the voltage drop reveals the very first cell failure. Since thermocouples cannot be placed on the inner material of the cell, the measured temperature will not be a precise indicator of the TR event. This time discrepancy is explained in Figure8 , where voltage and temperature relative to one cell are compared. For this reason, the time intervals of TR propagation, reported in Table2, are based on cells’ voltage drop.

5. Conclusions The presented work gives insights about thermal runaway (TR) process in terms of electrical and thermal behavior in a Li-ion batteries module. A stack of four commercial Li-ion cells were triggered to TR—by overheating the first cell of the stack—and the evolu- tion observed by voltage and temperature sensors. The information about TR propagation timing is of great interest since the Global Transport Regulation [1] defines a minimum time window of 5 min in between the first TR event and a hazardous situation inside the passenger compartment. In this manuscript, more insights about the timing and TR process were provided reporting temperatures, voltages, visible effects, and postmortem inspection. Eight distinct phases were identified by observing five repetition tests conducted on equiv- alent samples at 100% SOC and in same testing conditions. Furthermore, physical meaning of the observed effects were hypothesized for each phase in the respective temperature range, and an ideal illustration was provided of what is expected to take place on single cell level. The provided assumptions could be particularly useful for the implementation of finite element models and computational fluid dynamics simulations of the TR process as well as from electric and thermodynamic perspectives. From the results and postmortem inspection, the electrolyte seems to play an important role on the propagation speed of the electrical short circuit on single cell level due to the increase of pressure as consequence of the increase of temperature. The provided data could be useful to predict, for example, the gas venting point on the pouch bag in case of TR condition. Furthermore, voltage and temperatures can be useful for the electrothermal simulation validation of TR process. For future test improvements, it could be of great interest to measure the pressure of the stack during the whole experimental procedure.

Author Contributions: Conceptualization, L.A.; Data curation, L.A.; Investigation, L.A.; Method- ology, L.A.; Project administration, C.E. and W.S.; Supervision, W.S.; Visualization, L.A.; Writing— original draft, L.A.; Writing—review & editing, L.A., I.H., G.G., E.E. and W.S. All authors have read and agreed to the published version of the manuscript. Batteries 2021, 7, 49 15 of 18

Batteries 2021, 7, x FOR PEER REVIEW 16 of 19 Funding: This work origins from the research project SafeBattery (grant no. 856234). The K-project SafeBattery is funded by the federal ministry for transport, innovation, and technology (BMVIT), federal ministry of digital and economic affairs (BMDW), Austria and Land Styria within the program COMET—Competencefederal ministry of digital Centers and economic for Excellent affairs Technologies. (BMDW), Austria The program and Land COMET Styria iswithin administered the pro- by thegram FFG. COMET—Competence This work was also Centers based on for the Excellent research Technologies. project SafeLIB The (grant program no. COMET 882506). is The adminis- K-project SafeLIBtered by isthe funded FFG. This by the work Austrian was also federal based ministry on the research for climate project action, SafeLIB environment, (grant no. energy,882506). mobility The innovationK-project SafeLIB and technology is funded by (BMK), the Austrian federal federal ministry ministry of digital for andclimate economic action, affairsenvironment, (BMDW), en- the ergy, mobility innovation and technology (BMK), federal ministry of digital and economic affairs provinces Styria and Upper Austria within the program COMET—Competence Centers for Excellent (BMDW), the provinces Styria and Upper Austria within the program COMET—Competence Cen- Technologies. The program COMET is administered by the FFG. ters for Excellent Technologies. The program COMET is administered by the FFG. Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Data Availability Statement: Statement: TheThe whole whole data data of of interest interest was was incl includeduded thanks thanks to the to the Appendix Appendix A. A. Acknowledgments:Acknowledgments: WeWe would would like like to tothank thank all allour ourcolleagues colleagues from from VSI and VSI ICTM and ICTMinstitutes institutes for the for theteamwork teamwork and andeffort effort put into put reaching into reaching the objectiv the objectives.es. The targets The targetswere achieved were achieved thanks to thanks the valu- to the valuableable input input of the of consortium the consortium members members of the Safe of theBattery SafeBattery project. project. “Open Access “Open Funding Access Funding by the Graz by the GrazUniversity University of Technology”. of Technology”. ConﬂictsConflicts of Interest: TheThe authors authors declare declare no no conflict conﬂict of of interest. interest.

Appendix A The following Figures Figures A1 A1 and and A2 A2 shows shows the the temperature temperature measurements measurements and and voltage voltage measurements of of modules modules 2, 2, 3, 3, 4, 4, 5. 5.

Figure A1. Cont. Batteries 2021, 7, x FOR PEER REVIEW 17 of 19

BatteriesBatteries2021 2021, 7,, 497, x FOR PEER REVIEW 17 of16 19 of 18

FigureFigureFigure A1. A1.A1. Temperature TemperatureTemperature measurements measurements ofof of modulesmodules modules 2, 2, 3, 3, 4,4, 4, 5.5. 5.

Figure A2. Cont. Batteries 20212021,, 7,, x 49 FOR PEER REVIEW 1817 of of 19 18

FigureFigure A2. A2. VoltageVoltage measurements of modules 1,3,4,5.

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