Investigations of the Thermal Runaway Process of a ...

IN DEGREE PROJECT CHEMICAL SCIENCE AND ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2021

Investigations of the Thermal Runaway Process of a Fluorine-Free Electrolyte Li-Ion Battery Cell

TAMARA PATRANIKA

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY, BIOTECHNOLOGY AND HEALTH

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AuthorTamara PatranikaMaster in Chemical Engineering for Energy and EnvironmentKTH Royal Institute of Technology

ExaminerGöran LindberghSchool of Engineering Sciences in Chemistry, Biotechnology and HealthKTH Royal Institute of Technology

SupervisorsHanna EllisSwedish Defence Research Agency (FOI)

Guiomar HernándezDepartment of Chemistry - Ångström Laboratory, Structural ChemistryUppsala University

Jonas MindemarkDepartment of Chemistry - Ångström Laboratory, Structural ChemistryUppsala University

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AbstractThis project aims to investigate the thermal runaway process of fluorine-free lithium-ionbattery cells and to compare this with a commercially used fluorinated electrolyte.The cells consisted of a silicon-graphite composite anode and a LiNi0.6Mn0.2Co0.2O2

(NMC622) cathode. The non-fluorinated electrolyte used was based on lithiumbis(oxalato)borate (LiBOB) in organic solvents with the additive vinylene carbonate(VC). Moreover, the fluorinated electrolyte consisted of LiPF6 in the same organicsolvents together with VC and fluoroethylene carbonate (FEC).

The thermal stability measurements have included Accelerating Rate Calorimetry(ARC) and Differential Scanning Calorimetry (DSC). Moreover, both coin cells andpouch cells have been examined by ARC. However, thermal runaway could not bedetected for either types of cells, concluding that a greater amount of active materialwas needed. In order to measure the thermal reactions of the battery components,DSC was used. These results concluded that the anode was more thermally stablewith a non-fluorinated electrolyte. However, the thermal stability appeared to be lowerfor the cathode, therefore, further investigation is needed for confirmation of the cathode.

KeywordsLithium-ion batteries, Electrolyte, Non-fluorinated, Thermal stability

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SammanfattningDetta projekt syftar till att undersöka den termiska rusningsprocessen hos ettlitiumjonbatteri med en fluorfri elektrolyt och jämföra den med en kommersiellt användfluor-innehållande elektrolyt. Battericellerna innehöll silikon-grafit som anod ochLiNi0.6Mn0.2Co0.2O2 (NMC622) som katod. Den fluorfria elektrolyten var baserad pålitium bis(oxalato)borat (LiBOB) i organisk lösning med additivet vinylen karbonat(VC). Det jämfördes med en fluor-innehållande elektrolyt med LiPF6 i samma organiskalösning tillsammans med VC och fluoroetylene karbonat (FEC).

De termiska stabilitetstesterna utfördes med Accelerating Rate Calorimetry (ARC) ochDifferentiell svepkalorimetri (DSC). Både knappceller och pouchceller har undersöktsmed hjälp av ARC. Trots flera försök med olika uppställning kunde den termiskarusningen inte bli detekterad för någon av celltyperna, med slutsatsen att en störremängd aktivt material behövs. Istället användes DSC för att undersöka de termiskareaktionerna hos batteri-komponenterna. Resultaten visade att anoden var mer termiskstabil med den fluorfria elektolyten, medan samma elektrolyt visade mindre termiskstabilitet på katoden. Vidare undersökningar behövs dock för bekräftelse av katoden.

NyckelordLitiumjonbatteri, Elektrolyt, Fluorfri, Termisk stabilitet

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AcknowledgementsI would like to thank my supervisor, Hanna Ellis at Totalförsvarets forskningsinstitut(FOI) for her valuable knowledge in thermal testing of batteries, and great supportthroughout the project. Also, I want to thank my supervisor Guiomar Hernández atthe Department of Chemistry at Uppsala University for sharing her endless knowledge inbatteries and tirelessly answering my questions. Further my supervisor Jonas Mindemarkat the Department of Chemistry at Uppsala University, I want to thank for his valuableinput when writing this report. Finally, I would like to thank my mother and grandmotherfor all the support they have given me throughout my studies, and for being the bestinspiration a young female engineer could ask for.

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AbbreviationsARC Accelerating Rate CalorimetryBMS Battery management systemDMC Dimethyl carbonateDSC Differential Scanning CalorimetryEC Ethylene carbonateEIS Electrochemical Impedance SpectroscopyEMC Ethyl methyl carbonateEV Electric vehiclesEV-ARC Extended volume ARCFEC Fluoroethylene carbonateHF Hydrogen fluorideHWS Heat-Wait-SeekLIB Lithium-ion batteryLiBOB Lithium bis(oxalato)borateLiPAA Lithium polyacrylate binderLiPF6 Lithium hexafluorophosphateNMC LiNixMnyCozO2

NMC622 LiNi0.6Mn0.2Co0.2O2

LiBOB Lithium bis(oxalato)boratePE PolyethylenePF5 Phosphorus pentafluoridePP PolypropylenePVDF Polyvinylidene fluorideSEI Solid electrolyte interfaceSOC State of chargeSOH State of healthTR Thermal RunawayVC Vinylene carbonate

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Contents

1 Introduction 21.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Background 52.1 Structure of Lithium-Ion Batteries . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Cathode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.2 Anode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.3 Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.4 Separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Mechanisms of Thermal Runaway . . . . . . . . . . . . . . . . . . . . . . 92.3 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3.1 Electrochemical Impedance Spectroscopy . . . . . . . . . . . . . . 112.3.2 Accelerating Rate Calorimetry . . . . . . . . . . . . . . . . . . . . 122.3.3 Differential Scanning Calorimetry . . . . . . . . . . . . . . . . . . 14

3 Method 163.1 Coin cell preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.2 Pouch cell preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.3 Electrochemical Impedance Spectroscopy . . . . . . . . . . . . . . . . . . 173.4 Accelerating Rate Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . 183.5 Differential Scanning Calorimetry . . . . . . . . . . . . . . . . . . . . . . 19

3.5.1 Separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.5.2 Coin cell o-ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.5.3 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.5.4 Electrolyte and salt . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4 Results and Discussion 224.1 Cycling and cell properties (EIS) . . . . . . . . . . . . . . . . . . . . . . 224.2 Thermal reactions in full cells . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2.1 Coin cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.2.2 Pouch cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.3 Thermal reactions on cell components . . . . . . . . . . . . . . . . . . . . 284.3.1 Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.3.2 Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.3.3 Salt and electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . 32

5 Conclusion 34

6 References 35

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1 IntroductionThe increased demand for energy in the world is a great challenge, as well as ensuring thisenergy to have no or limited carbon dioxide emissions. Even though renewable energysources such as solar and wind power are available, these are intermittent, which furtherincreases the need for energy storage [1]. Lithium-ion batteries (LIBs) are used in a largenumber of applications, such as mobile phones, computers, and cameras [2]. One of theadvantages of these types of batteries is the high energy density (150 Wh/kg), whichmakes them economically beneficial. A comparison of the energy density of differenttypes of energy storage is shown in Figure 1, which shows that LIBs have an outstandingperformance [3]. Moreover, the batteries are available in various sizes, both for smaller(0.1 Ah) and larger (160 Ah) applications. The operating voltage of a single cell variesbetween 2.5-4.2 V, which is approximately three times the voltage of a nickel-cadmiumbattery. This makes it possible to use fewer cells per battery [4]. However, the ambitionis to use LIBs for both electric vehicles (EV) and grid storage on a larger scale. Theseapplications might generate heat at long term usage, which makes it important that thebattery functions are electrochemically intact at elevated temperatures. Therefore, thematerials used in the LIBs must be optimized and improved [5].

Figure 1: Comparison of the energy density between different types of energy storage.Reprinted from [6], Copyright 2021, with permission from Elsevier.

Although the production of LIBs is increasing rapidly, the aspects of safety and theenvironmental interests of the products are still a challenge, since reactive compoundsare used. Starting with the environmental concerns and requirements, there are variousalternatives for electrode materials that meet the current demands [7]. A commoncommercially used material for the cathode is LiCoO2 coated on a sheet of aluminum.Using cobalt increases both the cost and the environmental concerns of the material, sincecobalt is highly toxic [8]. Also, mining cobalt is a process that have large negative socialimpact on the workers due to child labour and unsafe working conditions, especially inthe world’s leading producer of cobalt, the Democratic Republic of Congo [9]. Therefore,a substitute for this element is investigated, and LiNiO2 and LiNixMnyCozO2 (NMC) arebeing considered [8]. However, the electrolyte of LIBs still needs further development tomeet the current environmental demands. Most electrolytes contain salts with fluorine,which can undergo defluorination reactions producing toxic gases, as hydrogen fluoride(HF). In order to make the electrolyte more environmentally suitable the composition ofthe salt must be changed so that fluorinated compounds are not used [7].

Furthermore, LIBs are expected to be a suitable option for automotive applications.In 2016, the fuels that powered the world’s vehicle application depleted 49 % of the

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oil resources. Using EVs would not only enable independence from fossil fuels but alsolower greenhouse gas emissions by 20 %. If EVs were powered by renewable energy, theemissions would be reduced by 40 %. Moreover, the issues of long time and high powerstorage need to be resolved together with the thermal stability before LIBs are fullyimplemented in automotive applications [6]. However, with the rapid development ofLIBs, these will likely be a suitable option for EVs, thanks to their high energy density [1].

Due to this increasing demand for LIBs within the automotive industry, additionalpressure is put on the recycling of the used material. It is predicted that the waste fromEV batteries will be 340 000 metric tonnes yearly, by 2040 [10]. Considering this, it isof importance to ensure a workable recycling process, but also to develop batteries withsustainable materials. The US Advanced Battery Consortium considers batteries in theautomotive industry to be at the end of life when the cell capacity is less than 80 % ofthe starting capacity. Thereafter, the batteries are to be recycled, or reused with a newpurpose, for example, grid energy storage. However, the market of LIBs has acceleratedfaster than for the recycling processes to develop, which is partly due to the complextechnology and structure of the LIBs. The focus on the materials to be recycled hasbeen on the compounds that influence the production cost. Therefore, valuable metalssuch as nickel and cobalt are a target for recycling. However, while recycling the cells torecover the previously mentioned metals the electrolyte is exposed to the atmosphere,creating a severe safety hazard due to the toxic gases generated. The electrolyte musttherefore undergo various separation steps during the recycling process [10].

Regarding the safety aspects of LIBs, they show unstable behavior at highly increasedtemperatures. When the cell gets heated, exothermic reactions start to occur, thefirst being the decomposition of the solid electrolyte interface (SEI) layer, which is apassivating layer on the anode surface. While the SEI layer protects the anode fromfurther electrolyte decomposition, it can still react with the electrolyte in the case of anexothermic reaction. As a result, the temperature increases further, making it possiblefor more exothermic reactions to develop. These reactions cause the battery to self-heatin an uncontrolled manner, a phenomenon called thermal runaway (TR). At TR, theelevated temperature together with the organic solvents can start a fire [11].

The most commonly used salt for electrolytes is lithium hexafluorophosphate (LiPF6)dissolved in organic solvents. In the TR process, the battery emits gases, such as HFand phosphorus pentafluoride (PF5), which can be even more harmful in the applicationof aviation or submarines since the gases are highly toxic and corrosive. Two possibleevents to trigger the TR is short-circuiting and overheating. In the case of overheating,the gases evolved might evaporate and emit from the battery. The gases can in turnignite directly or later, which creates a dangerous environment [12]. Due to the issuesof the fluorinated electrolyte, this bounds the applications of the LIBs below 50 �,which shows the importance of ensuring a more stable electrolyte [13], [14]. Xu et al.[15] have suggested lithium bis(oxalato)borate (LiBOB) as an effective alternative tothe fluorine-based electrolyte. Moreover, Hernández et al. [7] have used the salt inhigh-density batteries employing silicon-based electrodes with high mass loading. LiBOBhas shown a higher thermal stability than the fluorinated electrolyte and it has beencategorized as a biodegradable material. Furthermore, LiBOB has a lower reactivitythan LiPF6 [7].

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Since LIBs degrade at high temperatures, cooling systems are used in order to keepthe battery from overheating, and thereby prevent the degradation. If this systemwere to fail, the cell is instead disconnected by a so-called battery management system(BMS). This can then regulate the degrading of the LIB [4]. Furthermore, Figure 2shows a projection of long-term storage testing on LIBs used for EV applications atdifferent temperatures (20 �, 40 �). It shows that the capacity for the cells is decreasedwhen used at elevated temperatures [16]. Furthermore, state of health (SOH) is ameasurement used to estimate the capacity of a battery, since a reduction of the SOHindicates that the internal resistance is increased, and thereby the capacity is decreased.Moreover, it has been shown that the SOH is reduced when the cell operates at ahigher temperature, in line with the projection in Figure 2 [17]. Therefore, the capacityat elevated temperature is an important issue to resolve to enable further applications [16].

Figure 2: Projection of the lifetime of LIBs for EV applications at different temperatures,assuming one fully charged and discharged cycle per day (100 % depth of discharge(DOD)) and a 350 km usage per cycle. Reprinted from [16] under CC-BY 4.0(http://creativecommons.org/licenses/by/4.0/).

1.1 PurposeThe purpose of this project was to develop a deeper understanding of the thermalstability of a fluorinated and non-fluorinated electrolyte, in order to achieve a saferbattery. In the case of TR a fluorinated electrolyte emits toxic gases, such as HF, whichcan be avoided by removing fluorine from the electrolyte. Hence, providing a safer andmore environmentally friendly electrolyte. Moreover, such batteries would be a suitableoption for marine applications, where evacuation in the case of a fire is not possible.

Thereafter, the aim was to compare the temperature at which TR occurs for respectiveelectrolyte. Thereafter the different components of the cells were investigated, todetermine the thermal reactions.

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2 BackgroundThis section aims on explaining the structure of LIBs as well as common materials usedfor the different components. The process and reactions of TR are also put forward.

2.1 Structure of Lithium-Ion BatteriesLIBs consist of two electrodes, a positive (cathode) and a negative (anode). The lithiumions move between the two electrodes when the battery is being charged and discharged.The electrode consists of an active material coated on a metal foil, called a currentcollector [4]. For the cathode, the active material is often a metal oxide, such as lithiumcobalt oxide or NMC, which is then placed on a current collector of aluminum. Whilelithium cobalt oxide is more commonly used on the market, the usage of NMC isincreasing [18]. The active material of the anode is commonly made of graphite, witha current collector of copper. The current collector and the active material are puttogether by a binder and polyvinylidene fluoride (PVDF) is commonly used. Since nometallic lithium is used in the batteries, they are less reactive and a safer option to use.Furthermore, to enable the movement of ions between the electrodes, an electrolyte isused. Liquid electrolytes consist of a salt, containing the ions, that is dissolved in one ormultiple solvents [4].

Because the electronic conductivity of the active material is poor, it is common to usean additive to improve the performance. For commercial cells, conductive-nanocarbonis often used [19]. Furthermore, the two electrodes are separated by a microporousmaterial, called a separator. [4].

When charging the LIB, the lithium ions move from the metal oxide on the cathodethrough the electrolyte and are inserted into the anode. Simultaneously, the electronstravel through an external circuit, before arriving at the anode. On discharge, the reversereactions occur, hence the lithium ions are extracted from the anode and move throughthe electrolyte and intercalate in the cathode, while the electrons move through the outercircuit and produce the electrical energy of the battery [20]. Consequently, at charging,the cathode is oxidized while the anode is reduced [4]. The mechanism of these reactionsare shown in Equations 1 and 2, with LiCoO2 as an example of the metal oxide. However,Co can be replaced by other metals.

Cathode : LiCoO2

charge��*)��discharge

Li1�xCoO2 + xLi++ xe

� (1)

Anode : C + xLi++ xe

� charge��*)��discharge

LixC (2)

2.1.1 Cathode Materials

When designing and developing a cathode, there are several requirements.The material must contain lithium, preferably a large quantity, and allow theintercalation/deintercalation of it without changing the structure, to ensure a long cyclelife of the battery. Moreover, the material must not be soluble in the liquid electrolyteand have a good electronic conductivity for the ions to move at a high rate [4].

There are two kinds of structures for the cathode, a layered structure and athree-dimensional spinel structure. The first mentioned has lithium ions between

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the layers of oxygen, while the latter consists of an octahedral structure with lithiumin an eighth of these positions [4]. One example of a layered structure electrode isNMC, which is often used in automotive applications, due to the high specific capacityof layered structures. On the other hand, spinel structure, like LiMn2O4, achieves fastkinetics, which allows for higher power applications, with the drawback that the specificcapacity is lower [21].

2.1.2 Anode Materials

One of the most common active materials for the anode is graphite, which is anintercalation material [22]. This is used because of the stability and low voltage plateauit provides [20]. Since the electrolyte is not stable at the low operating voltage of thiselectrode, it will start to degrade on the surface of the anode forming a passivating layercalled SEI. Despite that this layer contributes to an irreversible capacity loss, a stableSEI layer is beneficial since it prevents further degradation of the electrolyte [23]. Theideal anode would have a high specific capacity and stable cycling whilst having low orno irreversible capacity loss. However, these characteristics often contradict one another[4].

Another option for the active material of the anode is silicon, which forms an alloy withlithium [24]. Comparing the gravimetric capacities of these, the silicon (3579 mAh/g)trumps the graphite (372 mAh/g) [20]. However, the volume of silicon is highly increasedwhen being lithiated, and decreased during delithiation. Thereby, the silicon electrodeshows great volume changes during each cycle [25]. This in turn leads to a decreasein conductivity, due to cracks in the electrode particles that create voids and leads toimpaired contact, a phenomenon called pulverization. The capacity difference betweenthe two materials can be explained by looking at the half-reactions. As shown in Equation3 and 4, it takes six carbon atoms to intercalate to one lithium atom, whilst one siliconcan bind up to 3.75 lithium atoms if it is fully lithiated forming Li15Si4 [20].

LixSi ⌦ xLi++ Si + xe

� (3)

LiC6 ⌦ Li++ 6C + e

� (4)

2.1.3 Electrolyte

There are four types of electrolytes for LIBs: gel, polymer, ceramic, and liquid, whereasthe latter is the most frequently used due to its high ionic conductivity which providesbetter performance. The electrolyte regularly contains a lithium salt dissolved in anorganic solvent [4]. Moreover, it is important that the electrolyte has a low viscosity andcan entirely wet the separator and the electrode to ensure a good contact with all thecomponents [26]. To enable the electrochemical reactions in a LIB at a high rate, theelectrolyte must have a high ionic conductivity [5].

One lithium salt that meets this conductivity demand is LiPF6, which also is the mostcommercially used salt for LIBs [4]. However, LiPF6 has shown problems with thermalstability, as well as with reactivity at elevated temperatures. Moreover, the salt isalso sensitive to moisture, which in turn makes the preparation of the electrolyte morecomplicated. When reaching TR, moisture in contact with the cell can react with the

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electrolyte, forming the highly corrosive and toxic compound HF [27]. To enable thecapacity increase for LIBs to be suitable for the automotive industry, the electrolyte andthe chosen lithium salt play a big role [15].

The requirements of the solvent in the electrolyte have proven difficult to fulfill with asingle solvent since it needs to both dissolve the lithium salt and have a low viscosity toensure the movement of ions, while still being stable at the interface of the electrodes [5].Therefore, it is common to use more than one solvent to increase cell performance, suchas ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate(EMC) [4], [26].

An important feature of the electrolyte is the formation of the SEI layer, which iscreated when the electrolyte degrades and lithium ions are consumed into the layer, asan irreversible reaction [4]. To ensure a stable LIB cell, it is crucial to have a stable SEIlayer since this will prevent any further degradation of the electrolyte. Moreover, thelayer also works as an electric insulating block between the electrode and the electrolyte,which hinders the electrons to pass, while allowing the lithium ions to move insidethe cell. However, the thickness of the layer will affect the diffusion of the ions [28].Furthermore, the stability of the electrode also depends substantially on the formation ofthe SEI layer. Within the build-up of the layer at each cycle, irreversible reactions occurand the coulombic efficiency is lowered. This phenomenon is especially important whenusing a silicon electrode since the surface of the electrode correlates to the amount of SEIformed. The silicon electrode is pulverized over several cycles, which creates a highersurface area and thereby allowing additional silicon in contact with the electrolyte andan unwanted increased formation of the SEI layer. Therefore, the capacity of the siliconelectrode decreases at each cycle [15], [20]. To ensure the formation of the SEI layer,it is common to use additives [7], [28]. As the additives are included in the electrolyteformulation to form an SEI, they will affect its composition, hence affecting the stabilityof the cell. Two commonly used additives, especially when using a silicon electrode, arefluoroethylene carbonate (FEC) and vinylene carbonate (VC). The degradation productsof these compounds are similar, since the dehydrofluorination of FEC results in VC, andthereafter the reduction mechanisms are similar. An essential difference between the twoadditives is the main reduction product of FEC, which is LiF [28].

In order to receive less toxic and corrosive products, LiPF6 could be replaced withfluorine-free alternatives, such as LiBOB. The salt also forms a stable SEI layer on thegraphite anode [5], [27] and is more thermally stable [15] than LiPF6. However, LiBOBshows issues of interaction with the cathode at voltages higher than 4.2 V [5].

To analyze the effects of fluorine in the electrolyte in high-energy-density full cellswith silicon-based anodes, Hernández et al. [7] compared LiPF6 and LiBOB. With thenon-fluorinated electrolyte, VC was added as an additive. This was also added for thefluorinated electrolyte, together with FEC [7]. The cells were tested with galvanostaticcharge and discharge cycling, where all electrolytes showed a high resistance at thebeginning of the first cycle and thereafter the resistance decreased. This change inresistance was due to the first formation of the SEI layer and degradation of theelectrolyte, which in turn increased the viscosity of the electrolyte in the cell. However,the non-fluorinated electrolyte showed higher cell resistance compared to the LiPF6

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based electrolyte. Moreover, without the additives, both types of electrolyte presented adecrease in capacity. This was explained by the cells being unable to form a stable SEIlayer on the graphite-silicon electrode. However, the stability was improved with theaddition of the additives [7].

At low currents, the non-fluorinated cells showed a slower increase in cell resistance,which gives better capacity retention (84.4 % after 200 cycles) and in turn indicates thatthe cycle life is extended since the anode is stabilized. However, at higher currents, theoverpotential increased and the fluorinated electrolyte showed a better capacity retention(78.5 % after 300 cycles) compared to the non-fluorinated (59.4 % after 300 cycles).Still, a partially fluorine-free electrolyte was also tested, together with both additivities,providing a capacity retention of 79.6 % after 300 cycles. This shows that decreasing theamount of fluorine in the electrolyte will provide effective batteries, whilst improving theenvironmental factor [7].

Furthermore, the cells were also tested at 40 �, which is a temperature the battery couldreach at in warmer climates or at fast charging. At this temperature, the fluorinatedelectrolyte proved to be non-efficient with a capacity retention at 49 % after 300 cycles.However, the non-fluorinated electrolyte showed a capacity retention of 77 % after 300cycles at the same temperature, which indicates that these types of batteries are bettersuited at elevated temperatures. Furthermore, the morphology of the anode surface wasalso investigated, which can be correlated to the electrolyte and in turn influence theelectrochemical performance. The cells with fluorinated electrolyte showed an unevensurface on the anode, and a correlation between fluorine and silicon, which indicatesthe degradation of the electrolyte and formation of the SEI layer on the silicon. Thenon-fluorinated electrolyte cells were not able to form a stable SEI layer to stabilize thesilicon at high currents. Therefore, it had varying cycling stability at different currents [7].

2.1.4 Separators

The separator is a microporous film that is used to electrically isolate the anode andcathode from one another to avoid a short circuit within the cell. The membrane isoften a polymer with a thickness of around 10-30 µm that allows for the transport oflithium ions through the electrolyte [4], [29]. It is of importance for the performancethat the material of the separator does not change in thickness, and that the materialis wetted completely by the electrolyte. A common pore size for commercial separatorsis 0.03–0.1 µm with a porosity of 30–50 %. Both polyethylene (PE) and polypropylene(PP) are often used as separators. However, if the temperature of the battery were toreach the melting point of the polymer (135 � and 165 � respectively), the polymerwould lose its porosity. Still, this can be used as a safety mechanism if the cell was tobe overheated [4]. It is common to use two or more layers of polymers with differentmelting points. When the temperature is increased, the polymer with the lower meltingpoint will melt and in turn fill up the pores of the remaining layer, which hinders boththe ion transport and the current [30]. However, the separator tends to break downalmost immediately after the pore collapsing, resulting in short-circuit and initiating theTR process [31].

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2.2 Mechanisms of Thermal RunawayDespite that LIBs are used in various applications and are suitable for electric andhybrid vehicles, the safety features are to be further developed before their productionis exponentially increased. The organic solvents in the electrolyte are highly flammable,which under some circumstances can cause fire and lead to explosions. However,non-flammable electrolytes have been investigated as an alternative but have shownto be unsuited with current electrodes used in industry or too expensive [32]. Whendeveloping batteries for automotive applications, safety concerns must be addressed foreach stage of the manufacturing process, starting from the material selection and celldesign to the position in the vehicle. Depending on the type of battery, different typesof failures and safety issues are of concern, but the most common is the generation ofheat and gas when the batteries are exposed to abuse [6]. The state of charge (SOC) hasproven to be an important factor when determining the effects of an eventual TR process[27]. Looking at the composition of the battery, the electrolyte and the anode can beconsidered as a fuel, with the cathode as an oxidizer, together creating an explosiveenvironment. Therefore, the thermal stability of LIBs is of great importance [33].

Figure 3: Causes and consequences of the thermal runaway process, reconstructed from[6] and [12].

When the electrolyte is heated, this can trigger exothermic reactions, which in turnincreases the temperature further and thereby makes it possible for additional exothermicreactions to occur, initiating the TR process. Figure 3 illustrates the TR process, itscauses and its outcomes. The main reason for TR is the instability of the electrolyte [32].In Table 1 the main mechanisms occurring at TR are summarized, provided by Wang etal. [6]. Here, most batteries are stable at 80 �, while at approximately 120 °C the SEIlayer dissolves in the electrolyte, which creates an unprotected surface on the graphiteelectrode that can react exothermally with the electrolyte [6].

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Table 1: The reactions at different temperatures for the thermal runaway for LIBs,reconstructed from [6] and [34]

Temperature [°C] Associated reactionsand targets

Comment

80-120 SEI layer Exothermic, SEI layerbreaks

> 120 Intercalated lithium withelectrolyte

Exothermic

130-140 PE separator melts Endothermic

160-170 PP separator melts Endothermic

200 Solvents - LiPF6 Exothermic, slow kinetics

200-230 Positive active materialdecomposition

Exothermic, emission ofoxygen that reacts withelectrolyte

240-250 LiC6 + binder Exothermic

240-250 LiC6 + electrolyte Exothermic

This first stage of the SEI layer decomposing is well studied in literature and occurs inthe temperature range of approximately 80 � to 120 �. These exothermic reactions arecaused by the degradation of the SEI layer, along with the de-intercalation of lithium onthe graphite anode surface. When the SEI layer is decomposed, intercalated lithium atthe anode is exposed to the electrolyte [11]. Consequently, the surface of the anode is leftunprotected, which makes it possible for the intercalated lithium to react exothermicallywith the electrolyte, which occurs above 120 °C [34]. Moreover, at 200 °C the electrolytestarts to decompose, also an exothermic reaction [34]. Furthermore, the collapsingtemperature of the separator is dependent on the melting point of the polymer used,which is an endothermic reaction [6]. The melting of the separator increases the internalresistance and blocks the pores. Thereafter, the separator collapses and causes shortcircuit in the cell [34].

When the temperature reaches 200 °C, the active material on the cathode decomposes,which releases oxygen in an exothermic reaction. The oxygen can react further with theelectrolyte, which creates additional exothermic reactions [34]. This stage is categorizedto continue until the end of the TR process and is the dominating reaction [11].

2.3 Measurement TechniquesThis section aims on explaining the measurement techniques that have been used withinthe project.

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2.3.1 Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS) can be used to observe the chemicalproperties of LIBs, among other applications. The internal degradation of the cell canbe observed, which enables the possibility to develop safety features of the cell. By usingEIS, the features of the different components of the cell can be investigated and provideinformation regarding the electrochemical properties, such as the exchange currentdensities and charge transfer resistances [35]. In practice, EIS quantifies the impedance,by applying a sinusoidal wave to the system, and then measuring the response. This inturn makes it possible to quantify the resistive, capacitive, and inductive properties ofthe LIBs, which can then be used to interpret the charge transport and kinetics reactionsin the cell [36].

Impedance can be defined as the power that works against the electrical current in acircuit. It is measured in Ohms, and the correlation between impedance (Z ), voltage(V ) and current (I ), together with the frequency (!) is described in Equation 5 [35].

Z(j!) =V (!)

I(!)(5)

EIS is measured over a variety of frequencies and provides the impedance characteristicsof this range. The measurement itself is done by applying an oscillating voltage orcurrent to a circuit and then observing the response of the respective current or voltage.The responding curve will be phase-shifted due to the different parameters of impedance,resistance, capacitance, and inductance [37]. The significant feature of a resistor is thatthe current is proportional to the voltage and that there is no dependence on frequency.However, for a capacitor, constant current cannot flow trough but charge insteadaccumulates [38]. Moreover, the impedance in capacitors is inversely proportional to thefrequency. This means that a decrease in impedance will occur with an increase of thefrequency [39].

Although impedance is measured in the same unit as resistance, it is not equivalent toit. In a DC circuit, the impedance can be approximated to consist of only resistance,since the current is linear. However, this can only be assumed when the voltage andcurrent are in phase, which is a rare condition since the capacitive and inductive effectsare present at around most frequencies. Therefore, impedance gives a more generalexplanation of the oppositional power in the circuit [35].

To interpret the results, imaginary values of the impedance need to be processed, usuallyby an adapted software. The results can be presented in different ways and a commonmethod is to plot the imaginary values of the impedance against the real values, whichis referred to as the Nyquist plot (see Figure 4). The plot provides an indication ofthe solution resistance (Rs) in the cell, which does not depend on the frequency andis shown by the intersection with the real axis at the range of higher frequencies. Thecharge transfer resistance (Rct) is shown by the intersection of the real axis at lowerfrequencies. Moreover, at lower frequencies, the diffusion of ions can be observed, sincethis process is slower than the migration. However, the Nyquist plot itself does not

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show any frequency dependency but is helpful to characterize LIBs [35]. In Figure 4, fis the frequency that is commonly referred to as the relaxation frequency and relatesto the time constant of the circuit. The time constant is inversely proportional to thefrequency [39]. In impedance measurements, the real values are often positive, while theimaginary values are negative, hence the negative imaginary values are used for bettervisualization in the Nyquist plot [37].

Figure 4: Reconstruction of Nyquist plot from [35].

Another way of presenting the EIS data is by using a Bode plot, which plots thephase shift and impedance magnitude, as a function of the frequency, which providesinformation regarding how these factors depend on the frequency. This plot canin turn be used to calculate the time constants of the electrochemical reactionssince these are proportional to the inverse of the frequency. The Bode plot can beused as a complement to the Nyquist plot, to conclude from the experimental results [40].

2.3.2 Accelerating Rate Calorimetry

To analyze the thermal behavior of LIB cells, Accelerating Rate Calorimetry (ARC)is a useful tool. The instrument provides information regarding the thermal behaviorof the individual components in the cell, as well as thermal hazards of the full cell[41]. Moreover, the analysis can also collect information regarding activation energyand thermal kinetic parameters. ARC approximates an adiabatic environment, whichsimulates that the heat produced by the sample, is used to heat the sample itself. Theseoccurring temperature changes are due to exothermic reactions in the cell, which causesthe cell to self-heat [42]. ARC provides information regarding the temperature andpressure changes of the sample and can contain various types of samples, both highexplosives, liquid, and solids [43].

The set-up is constructed by a blast-proof chamber, with a calorimeter assemblyinside. The calorimeter has a top, side, and bottom sensor that contains heaters andthermocouples (see Figure 5) [43]. Inside the calorimeter, a container referred to as

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the canister is put. The sample is placed inside the canister, and the thermocouplesare attached to the sample. To enable the adiabatic conditions, the temperature of thecalorimeter and the canister is held the same. However, the thermocouples in the systemmight drift during the test and thereby not provide the required adiabatic environment.Therefore, a drift correction is being made every 50 °C, by adapting the temperaturedifference required [44].

Figure 5: Set up of coin cells in ARC, reconstructed and modified from [45].

During the operation, ARC uses a mode referred to as Heat-Wait-Seek (HWS). Firstly,a start and an end temperature of the experiment are chosen, along with the heatincrement. From the chosen start temperature, the temperature is increased andthereafter the instrument enters a waiting mode, to reach isothermal equilibrium [46].At this stage, the temperature difference between the canister and the calorimeteris minimized, so that the calorimeter adjusts the temperature after the increase [35].Finally, the seeking mode is initiated, which monitors the temperature change of the cellfor 10 minutes, to detect if the cell has entered an exothermic mode. The temperaturechange is compared to a temperature rate sensitivity value to detect any exothermicreactions. If the temperature change is greater than the threshold, this will be consideredas an exothermic reaction and the initiation of the TR process. As previously mentioned,the instrument will simulate an adiabatic environment to ensure that there is no heatexchange between the cell and the chamber. This ensures that the heat producedby the cell during the exothermic reaction is used to heat the cell, thereby the cellis considered self-heating. The ARC will then track the temperature until either thetemperature change is below the sensitivity limit, or at the end of the TR process. If thetemperature change is lower than the threshold value, the instrument will again initiatethe heating mode, increasing the temperature by 5 °C [46]. The HWS will continue untilan exothermic reaction is determined, or until the temperature exceeds the chosen endtemperature, at which the instrument will initiate cooling by letting in compressed airinto the system. The end temperature is often put between 250-300 °C depending onthe type of cell [35]. The HWS mode is illustrated in Figures 6 and 7.

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Figure 6: Illustration of the HWS mode in ARC, reconstructed from [47].

Figure 7: Schematic image of the HWS mode in ARC, reconstructed from [47].

The output data provided by ARC is the surface temperature of the cell, thetemperature and pressure of the canister, and the self-heating rate of the cell, which canhelp investigating the reactions of TR. Other than that, the data provided can be usedto receive information regarding activation energy, the kinetics, and the thermodynamicheat of the reaction [35]. However, when performing the analysis, the instrumentassumes total adiabatic conditions meaning that there is no heat absorbed or lost fromthe canister. Moreover, it is also assumed that the temperature of the cell is uniform andthat there is no temperature gradient between the surface and insides of the cell [48].

2.3.3 Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) is used to identify the physical properties ofmaterials, often polymers, such as melting point and transition temperatures [49]. Twopans are put in the instrument, one containing the sample and the other used as a

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reference. Both pans are isolated from the ambient environment inside the instrument[50].

The calorimeter measures the difference in heat flow between the inserted sample andthe reference. Both objects are undergoing the same temperature differences at thesame pressure [51]. The containers of the sample and the reference are in contact witha thermometer, which is in turn connected to an oven which regulates the temperatureof the instrument. Both the sample and the reference are assumed to be isothermal,hence there is no temperature difference within the cell. The sample and the referenceare connected to the surroundings of the instrument by thermal links. Moreover, thetemperature of the instrument adapts to the changing temperature of the sample [51].This set-up provides the same heating rate of the sample and the reference, while theheat flow differs. The heat flow of the sample is thereby the difference in heat flowbetween the two pans. By dividing the heat flow of the sample by the chosen heatingrate, the heat capacity is provided. Using the mass of the sample the specific heatcapacity can be calculated, which then can be used to provide sufficient information ofthe sample [50]. DSC can be used as a complement to ARC since it is mostly used tomonitor the exothermic reactions, while with DSC both endo- and exothermic reactionscan be investigated [52].

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3 MethodThis section describes the methods used to compare the thermal stability of LIB cellsusing a fluorinated and a non-fluorinated electrolyte, in addition to the cell preparation.Both coin cells and pouch cells were prepared. In order to investigate the electrochemicalproperties of the assembled cells, EIS was used. Furthermore, ARC was used todetermine the thermal behavior in full cells. Different set ups were used, initially singlecoin cells and thereafter four and eight coin cells. Finally, DSC was used to characterizethe thermal behavior of the cell components.

3.1 Coin cell preparationThe two electrodes used for this project were kindly provided by Varta. The anodeconsisted of a silicon-graphite composite with a lithium polyacrylate binder (LiPAA)and a copper current collector, with a theoretical capacity of 2.4 mAh/cm2. The cathodeconsisted of LiNi0.6Mn0.2Co0.2O2 (NMC622) with a PVDF binder and aluminum asthe current collector, with a theoretical capacity of 2.2 mAh/cm2. The separator usedwas Celgard 2325 with a 17 mm diameter. Both electrodes were punched out with adiameter of 16 mm, and thereafter dried under vacuum at 120 °C for 12 hours, whilethe separator was dried under vacuum at 70 °C for 5 hours. This was done in order toremove any moist from the cell components as it is detrimental for the cell performance.Water and oxygen residues can react with the electrodes and electrolytes degradingthem. Thereafter, the cells were assembled in an argon glovebox according to the setupin Figure 8, and thereafter crimped.

Figure 8: Coin cell assembly set up.

The fluorinated electrolyte used was 1 M LiPF6 dissolved in a mixture of EC and EMC, ina ratio of 3:7 v/v with the additives FEC 10 vol % and VC 2 vol %. The non-fluorinatedelectrolyte used was 0.7 M LiBOB dissolved in a mixture of EC and EMC, in a ratioof 3:7 v/v with the additive VC 2 vol %. All cells were prepared with 75 µL of electrolyte.

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After assembling, all cells were rested for 6 hours and then precycled twice at the currentneeded to cycle the cell in 20 hours (C/20) between 3.0 and 4.2 V followed by a constantvoltage step at 4.2 V until the current decreased to C/50. The current was calculatedbased on the capacity of the NMC. Thereafter, the cells were cycled three times at C/2in the same voltage range and a constant voltage step at 4.2 V until the current droppedto C/20. Finally, one more charge step was carried out in order to have the cells in thefully charged state where they are more reactive and susceptible to undergo TR process.

At a later stage it was discovered that the assembled cells leaked during the ARCmeasurements. It was then decided to glue the cells prior to the ARC measurement.This was done with JB Weld Orginal Cold Weld Epoxy glue, to ensure that the sealingwould be intact throughout the process. The cells were then left rested for the glueto harden for 24 hours. Furthermore, four coin cells were run simultaneously in theARC in order to increase the mass loading. Thereafter, eight glued coin cells were runsimultaneously to combine the improved sealing and increased mass loading.

3.2 Pouch cell preparationThe same preparation was used for the pouch cell electrodes as the coin cell electrodes,but with a diameter of 26 mm. Other than that, the same separator but with a diameterof 30 mm was used, with respective electrolyte. The cells were assembled according tothe set-up in Figure 9 in an argon glovebox with 60 µL of electrolyte put on the cathode,and then additional 60 µL after the separator was put in place. Thereafter the cells wereput through the same cycling as the coin cells, and in the charged state.

Figure 9: Pouch cell assembly set-up, with the pouch material as casings in grey.

3.3 Electrochemical Impedance SpectroscopyTo investigate the electrochemical properties of the cells, potentiostatic EIS wasperformed after the cycling, using a Bio-Logic MPG2 potentiostat. The settings areshown in Table 2, which were chosen to asses a complete spectrum for the Nyquist plot.

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Table 2: Settings used for EIS after cycling.

Settings Value

Frequency range 10 kHz – 10 mHz

Amplitude 5 mV

Measurement points 10 per decade

3.4 Accelerating Rate CalorimetryAn extended volume ARC (EV-ARC) from Thermal Hazard Technology was used tocompare the thermal stability of the constructed cells. A drift test and a calibrationof the instrument and the thermocouples were conducted to ensure the isothermal andadiabatic features of ARC. This was conducted with a metal object of aluminum, thatwas placed inside of the canister. The calibration also ensures that the calorimeter isadiabatically and isothermally stable at all temperatures. This makes it possible toseparate exothermal reactions from instabilities in the system.

The cells were charged to 4.2 V at C/2, and then connected to the instrument perFigure 10. As seen in the figure, eight coin cells were coupled in series, with thethermocouple placed on the end of the pack at the anode in order to increase the massof the active material put in the ARC. Furthermore, Kapton tape was used both to holdthe cells together and to attach the thermocouple. A steel wire was also placed aroundthe cells for further stability, as shown in Figure 10. The entire set-up was then placed inthe center of the canister, held by the thermocouple for it not to be in contact with thewalls of the container. The settings used in ARC are shown in Table 3. The temperaturerange was adopted to allow for the TR to appear. The chosen waiting time and detectionsensitivity was recommended by the manufacturer.

(a) One cell (b) Multiple cells

Figure 10: Set-up of coin-cells for ARC measurements.

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Table 3: Setting used in EV-ARC.

Settings Value

Start temperature 50 °C

End temperature 300 °C

Heat increment 5 °C

Wait time 15 min

Detection sensitivity 0.02 °C/min

The settings shown in Table 3 were also applied for the pouch cells. Two cells werecoupled together with the thermocouple in between, with the anodes facing each other,see Figure 11. Kapton tape was used to put the thermocouple in place, and to make surethe cells were kept together.

(a) One cell (b) Full set up with two cells

Figure 11: Set-up of pouch cells in ARC measurements

3.5 Differential Scanning CalorimetryDSC was performed with Mettler Toledo DSC 3+ STAR and various runs were conductedat different scanning rates. These runs were done to determine the properties of thedifferent components of the assembled cells and investigate the thermal reactions on asmaller scale. The mass of the samples varied between 2 mg – 10 mg.

3.5.1 Separator

For the investigation of the melting point of the polymeric separator used in the cells,Celgard 2325, the heating rate of 10 K/min was chosen to match the rate found inliterature [53]. The settings are shown in Table 4.

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Table 4: Settings used for DSC for separator.

Settings Value

Heating rate 10 K/min

Temperature range 35 – 180 °C

Inert gas N2, 80 ml/min

Sample pan Al 40 µL

3.5.2 Coin cell o-ring

The o-ring of the coin cell set up was tested in the DSC in order to examine the meltingpoint, to draw conclusions regarding the leakage of the coin cells. The settings for thesemeasurements are shown in Table 5.

Table 5: Settings used for DSC for coin-cell o-ring.

Settings Value





3.5.3 Electrodes

For the investigation of the thermal behavior of the electrodes, two different measurementswere conducted. Firstly, cycled cells of both types of electrolyte were disassembled atthe charged state to extract the electrodes, which were punched out and then sealed inan argon glovebox. The charged state was chosen since it is more reactive. Secondly, thecycled electrodes were also prepared for the DSC with 3 µL of electrolyte to simulate anenvironment more similar to the fully assembled coin cell, simultaneously as it provides amore reactive scenario than the cycled electrodes. The settings for the electrode testingare shown in Table 6. In order to compare the results with the full cells brought inthe ARC, the same temperature range was adopted. However, the heating rate wasincreased to 10 K/min, which has been common in literature [53].

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Table 6: Settings used for DSC for the electrodes, electrolytes and salts.

Settings Value





3.5.4 Electrolyte and salt

In order to investigate the impact of the electrolytes and the salts, they were alsomeasured individually in the DSC. This was also conducted as a complement to analyzethe previous measurements. The settings used are shown in Table 6.

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4 Results and DiscussionIn this section, the results of the cycling and thermal testing are presented. The thermaltesting is divided in two sections, the thermal reactions in full cells, and on the cellcomponents. The results from the full cell testing presents the different set ups, startingwith one coin cell, and thereafter the set up with four, and eight coin cells. Finally, theset up of multiple pouch cells are presented. Further, the results of the thermal reactionson cell components are presented.

4.1 Cycling and cell properties (EIS)Prior to the thermal measurements, the cycling, which is shown in Figure 12, wasanalyzed. The cells used for the thermal measurements all showed stable voltage profiles.Further, the results of the EIS measurements are shown in Figure 13. What can beobserved is that the non-fluorinated electrolyte shows a higher cell resistance than thefluorinated, which corresponds with previous literature [7]. Furthermore, it can beobserved that the cells were reproducible based on the EIS measurments in Figure 13.

(a) Non-fluorinated (b) Fluorinated

Figure 12: Voltage profiles of (a) a non-fluorinated and (b) a fluorinated cell.


Figure 13: EIS measurements of cells with (a) non-fluorinated and (b) electrolytefluorinated electrolyte.

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4.2 Thermal reactions in full cells4.2.1 Coin cells

The first experiments made in the ARC were conducted on one fluorinated and then onone non-fluorinated coin cell. The resulting graphs are shown in Figure 14, where noindication of TR can be detected. Following, a decrease in temperature can be seen forthe non-fluorinated cell around 140 °C, which signifies the separator melting since this isan endothermic reaction in contrary to the degradation reactions of the electrode materialthat is exothermic. During the experiment of the fluorinated cell, the site sufferedfrom a power outage, resulting in the full graph not shown in the figure. However,TR is expected to occur within the temperature range that is included. Thereby itcan be concluded that neither of the single cells reached TR, despite the separator melting.


Figure 14: ARC measurement of one (a) non-fluorinated and (b) fluorinated coin cell.

The DSC measurement of the separator is shown in Figure 15. The two endothermic peaksmatch the melting points of the two polymers that the separator consists of. Moreover, thepreviously mentioned suggested melting point for the separator in the ARC measurementcorresponds to these values. The exothermic peak at 40 °C is a defect of the instrument.

Figure 15: DSC measurment of Celgard 2325.

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What can be further deduced from the experiments of the single coin cells is that thesealing of the cells was not intact. Pictures of the cells after the ARC measurements areshown in Figure 16. The bottom part of the cell is tilted upwards, and what is suggestedto be electrolyte has leaked through the sealing of the cell.


Figure 16: Non-fluorinated (uncleaned) and fluorinated cell (cleaned) after ARCmeasurement.

Combining the results that no TR was detected with the possible leakage, it was assumedthat the set-up for one coin cell in the ARC was not optimal. Two possible outcomeswere considered, the first one being that one coin cell did not provide enough activematerial for the ARC to detect the required temperature changes. Secondly, that theleakage of electrolyte destroys the cell before TR occurs. To investigate the leaking, aDSC measurement of the o-ring in the coin-cell setup was conducted, which is shown inFigure 17. In the graph, the melting point of the o-ring can be seen around 160 °C. Sincethe TR reactions are expected to occur around that temperature region for the cells, itcannot be excluded that the coin cell o-ring affects the method set-up, contributing tothe leakage in the coin cells.

Figure 17: DSC measurement of coin cell o-ring.

To confront the first mentioned concern that a single coin cell did not provide enoughmass, it was decided to run multiple cells in the ARC in order to increase the activemass used in the experiment. Thereby, four fluorinated cells were coupled in series, withthe thermocouple placed on the anode with Kapton tape (see Figure 18).

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Figure 18: Four fluorinated cells coupled in series put in the ARC.

The resulting graph of the four fluorinated cells is shown in Figure 19. No significanttemperature change is seen, which suggests that no TR reaction is detected. Moreover,the graph enters a flat plateau at approximately 250 °C, which is an indication of thethermocouple detaching from the set-up.

Figure 19: ARC measurement of four fluorinated cells coupled in series.

In order to investigate the impact of the active mass and the sealing, four commercialprimary lithium cells (CR2450) were measured in the ARC, coupled in the same way asthe previous measurement. Since the material of the commercial cells are different, thethermal reactions are expected to be different. However, the measurement was conductedto compare the sealing and mass of the active material, which is still comparable. Thetotal capacity for the coupled commercial cells added up to 8 mAh, comparing with thefour fluorinated cells with a total capacity of almost 11 mAh.

The rapid temperature increase shown in Figure 20 indicates the initiation of TR reactionsat 110 °C. However, curve is thereafter flattened, so that the instrument does not detectthe exothermic data further and instead continues the heat-wait-seek mode. Still, thethermal reactions of coin cells differ from those seen in previous literature of cylindricalcommercial cells but correspond to measurements done on coin cells by the supplier[54]. Thereby it can be concluded that the thermal reactions were detected for the fourcommercial coin-cells, hence making it possible with the given mass loading. Moreover,the commercial cells after the measurement are seen in Figure 20. What can be observedis that the cells have reacted, which confirms the theory that TR has occurred.

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(a) (b)

Figure 20: ARC measurement of four commercial cells (CR2450). Resulting graph (a)and the cells after the measurement (b).

To investigate the impact of the possible leakage, the cells were glued with epoxy glue,which is shown in Figure 21. Combining the two issues of the mass loading of the cellswith the possible leakage, eight glued cells were coupled in series and tested in ARC. Thisprovided a capacity of approximately 16 mAh. The thermocouple was still placed on theanode with Kapton tape. Moreover, the eight cells were held together by Kapton tapeand an steel wire.

Figure 21: One coin cell glued with epoxy-glue.

The results for the eight glued cells of respective electrolyte are shown in Figure 22.What can be seen is that none of the cells shows an exothermic temperature increase.For the fluorinated cells, the thermocouple seems to have de-attached from the set-upsince the measurement did not finish but shows the flat plateau. However, the lack ofexothermic reactions shown in the graphs does not exclude a TR process occurring inthe cells, only that the instrument does not detect this. Thereby it is concluded that theassembled coin cells do not have enough mass to be measured in the EV-ARC.

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Figure 22: ARC measurements of eight glued coin cells with non-fluorinated electrolyteand fluorinated electrolyte.

4.2.2 Pouch cells

In order to increase the amount of active material and improve the sealing, it was decidedto run pouch cells in the ARC. This provided the possibility to increase the size of theelectrodes, from 16 mm to 26 mm which provided a discharge capacity of approximately10 mAh. Moreover, the pouch material used for these experiments was the same onethat industry uses and was expected to handle the high temperatures in the ARC. Theresulting graphs for the two fluorinated and non-fluorinated cells are shown in Figure 23.


Figure 23: ARC measurement of two pouch cells, (a) non-fluorinated and (b) fluorinated.

Despite the increased mass of active material, no indication of TR are shown inFigure 23. The pouch cells after the ARC measurements are shown in Figure 24. Oneconcern of the pouch cells was if the plastic that covers the outer layer of the materialwere to melt and cover the thermocouple. However, even though that the material wasweakened by the ARC measurement, the material and sealing was intact. The cell wasslightly inflated, but had not opened at the edges. Thereby it is suggested that TRwas reached, but not detectable in EV-ARC. Furthermore, it can be observed for the

27

non-fluorinated cells to not have as stable plateaus for the wait and seek phases above250 °C. This can be partly due to the instability of the instrument at high temperatures,but similar behavior in ARC has been explained by the degradation of the LiBOB salt [55].

Figure 24: Two fluorinated pouch cells after ARC measurment.

Moreover, what can be observed for all the ARC measurements containing thefluorinated electrolyte, is that the thermocouple detaches from the set-up, causing theflat temperature plateau. The same behaviour is not detected for the samples with thenon-fluorinated electrolyte. This is a further indication that the cells are reacting inthe ARC, but the reactions are not detected. However, more measurements need to beconducted to confirm if this implies that the fluorinated electrolyte causes more violentreactions, hence causing the thermocouple to detach from the set up.

4.3 Thermal reactions on cell componentsSince the measurements of the full cells in the ARC did not detect any indication of TR,it was decided to run DSC on the cell components. This was done in order to investigatethe thermal reactions of the cycled anode and cathode, also with additional electrolyte.The salt and electrolyte were also measured individually to determine their impact.

4.3.1 Cathode

Low reproducibility was observed for DSC measurements for the cathode, which wasconsidered to depended on the time the electrodes were stored in the argon gloveboxafter the disassembling. In order to avoid this effect, the results presented in this studyare done on electrodes that were not stored in the glovebox, but directly punched outand sealed in DSC pans after disassembling. However, more studies investigating theimpact of sample storage are needed in order to get full reproducibility and understanding.

The results from the cycled cathode without additional electrolyte are shown in Figure 25.As previously mentioned, the results depended on the sample storage, and therefore it isnot possible to report an onset temperature for the degradation of the cathode. However,the results did not differ much between the fluorinated and non-fluorinated electrolyteswithin the same sample preparation, as shown in Figure 25. Therefore, it can be presentedthat the two electrolytes that were compared, do not affect the thermal reactions of thecycled cathode because the exothermic reactions start at the same temperature for bothcathodes with different electrolytes.

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Figure 25: DSC measurement of the cycled cathode.

Moreover, the results of the cycled cathode with additional electrolyte can be seen inFigure 26. Firstly, looking at the fluorinated sample, it can be stated that no exothermicreactions occur before 250 °C. However, it can be seen that the peak at 300 °C ofthe sample including the electrolyte increase in magnitude. This has previously beenreported to be a result of the reaction between the organic solvents in the electrolyteand the cathode, rather than a reaction with the salt [56].

Secondly, the non-fluorinated electrolyte shows an exothermic peak at 200 °C whichcorresponds to the degradation of the active material on the cathode, NMC622 [34].Furthermore, endothermic peaks are observed at 145 °C for the non-fluorinated electrolyteand 160 °C for the fluorinated. This implies that the non-fluorinated electrolyte is lessstable for the cathode. Moreover, these endothermic peaks were not seen for the cycledcathode without the additional electrolyte, which implies that the peak is an effect ofthe electrolyte. A suggested explanation for the non-fluorinated electrolyte being lessthermally stable is the oxygen evolution that occurs from the decomposition of theNMC. The oxygen can react further with the organic solvents, and might create a moreexothermic reaction with LiPF6 than with LiBOB.

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Figure 26: DSC measurement of cycled cathode with additional electrolyte.

4.3.2 Anode

The results of the cycled anode, without additional electrolyte are shown in Figure 27.In the higher temperature region, similar exothermic peaks can be observed for bothelectrolytes. However, at lower temperatures, a broad exothermic peak can also beobserved for the fluorinated electrolyte, with an onset temperature of 70 °C which issuggested to be the degradation of the SEI layer [56]. Moreover, this peak has also beenreported to occur for full graphite anodes at 120 °C [57]. However, given this study thatthe anode also consists of silicon and not fully graphite, it is suggested that this peak stillis due to the degradation of the SEI layer. In turn, this implies that the non-fluorinatedelectrolyte which does not show this exothermic peak has a more thermally stable SEIlayer.

Figure 27: DSC measurements of the cycled anode without additional electrolyte.

The results of the cycled anode, with additional electrolyte are shown in Figure 28. The

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first exothermic peak shown is for the electrode with fluorinated electrolyte at 80 °Cwhich correlates with the previously seen peak in Figure 27 [56]. Interestingly, the samepeak is not present for the electrode with non-fluorinated electrolyte, as for the cycledanode without additional electrolyte. Thereby this suggests that the non-fluorinatedelectrolyte provides a more thermally stable SEI layer than for the fluorinated electrolyte.It has been reported that the reaction between lithiated graphite and LiBOB shows lessexothermic behaviour than for the same material reacting with LiPF6 [58]. This furtherexplains the suggestion that the non-fluorinated electrolyte is more thermally stable thanthe fluorinated electrolyte. Furthermore, the endothermic peaks shown for both typesof electrolytes at 150 °C are due to the solvents evaporating. These peaks have beenreported for similar solvents [57].

Figure 28: DSC measurments of cycled anode with additional electrolyte.

The exothermic peaks in Figure 28 at 200-300 °C are seen for both electrolytes. Thesereactions have previously been reported as a reaction of the lithium-intercalated carbonwith electrolyte since they have been proven to become more exothermic with anincreased level of lithium [57]. However, the peaks of the electrodes with fluorinatedelectrolyte shows greater amplitude than for the non-fluorinated electrolyte, whichsuggests that the latter is less reactive. Furthermore, the exothermic peaks for the anodeat elevated temperatures have also been reported to be due to lithium reacting withthe PVDF binder material [56]. Even though that binder is not used for the anode inthis study, the peak could still be a reaction with the LiPAA binder. The two types ofreactions can however not be distinguished from one another in the DSC curve.

The endothermic peak for the non-fluorinated electrolyte has previously been reportedas the decomposition of LiBOB [55]. To confirm this, further analyses were conductedby running DSC measurements of LiBOB. The fluorinated electrolyte does not show asimilar endothermic peak for the LiPF6 decomposition, which could depend on the highlyexothermic reactions occurring simultaneously, blocking it from being visible. However,given the previously mentioned reactions of the SEI layer degradation and the differencein magnitude of the exothermic peaks, it can be concluded that the anode shows higherthermal stability for the non-fluorinated electrolyte than for the fluorinated one.

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4.3.3 Salt and electrolytes

The DSC measurements of the non-fluorinated and fluorinated electrolyte are shownin Figure 29. What can be observed for the fluorinated electrolyte is the previouslyseen endothermic peak of solvent evaporation at 130 °C. However, the peak for thenon-fluorinated electrolyte occurs at 176 °C. This in turn can be interpreted as thecomposition of the non-fluorinated electrolyte being more thermally stable. The integralof the non-fluorinated endothermic peak is smaller then for the fluorinated, which meansthat the reaction or the phase transition requires less heat. This further confirms thesuggestion that the non-fluorinated electrolyte is more thermally stable.

Figure 29: DSC measurement of non-fluorinated and fluorinated electrolyte

In order to confirm the endothermic peak at 280 °C in Figure 28, suggested to be thedecomposition of LiBOB, the salts used in both electrolytes were measured in the DSC.Figure 30 shows the resulting curve for both salts, with a magnification of the endothermicpeak at 298 °C. Firstly, LiBOB has been reported to degrade at 290 °C [55], whichcorrelates o the peaks seen at 280 °C and 298 °C in this study. This further confirmsthe endothermic peak in Figure 28 to be the decomposition of LiBOB. Secondly, it canbe observed in Figure 30 that LiPF6 melts at close to 150 °C and degrades at 225 °C.However, the previously reported melting point for LiPF6 is closer to 200 °C, while thedecomposition temperature has been just below 300 °C [59]. These differences might bedue to the different heating rates used for the DSC analyses. To confirm this data, furtheranalyses need to be done with multiple heating rates.

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Figure 30: DSC measurement of LiBOB and LiPF6, with a magnification of theendothermic peak at 298 °C

33

5 ConclusionThis study aimed to investigate and compare the thermal stability of a fluorinated andnon-fluorinated electrolyte in a LIB. Firstly, ARC measurements were conducted on bothcoin cells in different set ups and then on pouch cells with a higher active material loading.However, the instrument was not able to detect TR in either of the cells. For futurestudies, cells with higher capacity, or coin cells with better sealing should be investigated.

In order to investigate the thermal behavior of the components of the cell, DSCmeasurements were conducted. These measurements concluded that the silicon-graphiteanode was more stable with the non-fluorinated electrolyte used in this study.Furthermore, the NMC622 cathode indicates to be less stable with the non-fluorinatedelectrolyte, but further DSC measurements are needed to make this conclusion.

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6 References[1] D. Larcher and J.-M. Tarascon, “Towards greener and more sustainable batteries

for electrical energy storage,” Nature Chemistry, vol. 7, no. 1, pp. 19–29, Jan. 2015.[2] J. Ordoñez, E. J. Gago, and A. Girard, “Processes and technologies for the recycling

and recovery of spent lithium-ion batteries,” Renewable and Sustainable EnergyReviews, vol. 60, pp. 195–205, Jul. 2016.

[3] B. Diouf and R. Pode, “Potential of lithium-ion batteries in renewable energy,”Renewable Energy, vol. 76, pp. 375–380, Apr. 2015.

[4] D. Linden and T. Reddy, Handbook of Batteries. New York, USA, UNITEDSTATES: McGraw-Hill Professional Publishing, 2001, isbn: 978-0-07-141475-3.

[5] K. Xu, “Electrolytes and Interphases in Li-Ion Batteries and Beyond,” ChemicalReviews, vol. 114, no. 23, pp. 11 503–11 618, Dec. 2014.

[6] Q. Wang, B. Jiang, B. Li, and Y. Yan, “A critical review of thermal managementmodels and solutions of lithium-ion batteries for the development of pure electricvehicles,” Renewable and Sustainable Energy Reviews, vol. 64, pp. 106–128, Oct.2016.

[7] G. Hernández, A. J. Naylor, Y.-C. Chien, D. Brandell, J. Mindemark, and K.Edström, “Elimination of Fluorination : The Influence of Fluorine-Free Electrolyteson the Performance of LiNi1/3Mn1/3Co1/3O2/Silicon-Graphite Li-Ion BatteryCells,” vol. 8, no. 27, pp. 10 041–10 052, 2020.

[8] D. V. Medić, S. M. Milić, S. Č. Alagić, I. N. Ðorđević, and S. B. Dimitrijević,“Classification of spent Li-ion batteries based on ICP-OES/X-ray characterizationof the cathode materials,” Hemijska industrija, vol. 74, no. 3, pp. 221–230, 2020.

[9] L. Mancini, N. A. Eslava, M. Traverso, and F. Mathieux, “Assessing impacts ofresponsible sourcing initiatives for cobalt: Insights from a case study,” ResourcesPolicy, vol. 71, p. 102 015, Jun. 2021.

[10] Y. Bai, N. Muralidharan, Y.-K. Sun, S. Passerini, M. Stanley Whittingham,and I. Belharouak, “Energy and environmental aspects in recycling lithium-ionbatteries: Concept of Battery Identity Global Passport,” Materials Today, vol. 41,pp. 304–315, Dec. 2020.

[11] N. E. Galushkin, N. N. Yazvinskaya, and D. N. Galushkin, “Mechanism of ThermalRunaway in Lithium-Ion Cells,” Journal of the Electrochemical Society, vol. 165,no. 7, A1303, May 2018.

[12] F. Larsson, P. Andersson, P. Blomqvist, and B.-E. Mellander, “Toxic fluoride gasemissions from lithium-ion battery fires,” Scientific Reports, vol. 7, no. 1, p. 10 018,Aug. 2017.

[13] S. Wilken, M. Treskow, J. Scheers, P. Johansson, and P. Jacobsson, “Initial stages ofthermal decomposition of LiPF6-based lithium ion battery electrolytes by detailedRaman and NMR spectroscopy,” RSC Advances, vol. 3, no. 37, pp. 16 359–16 364,Aug. 2013.

[14] R. Wagner, M. Korth, B. Streipert, J. Kasnatscheew, D. R. Gallus, S. Brox, M.Amereller, I. Cekic-Laskovic, and M. Winter, “Impact of Selected LiPF6 HydrolysisProducts on the High Voltage Stability of Lithium-Ion Battery Cells,” ACS AppliedMaterials & Interfaces, vol. 8, no. 45, pp. 30 871–30 878, Nov. 2016.

35

[15] C. Täubert, M. Fleischhammer, M. Wohlfahrt-Mehrens, U. Wietelmann, and T.Buhrmester, “LiBOB as Electrolyte Salt or Additive for Lithium-Ion BatteriesBased on LiNi0.8Co0.15Al0.05O2 / Graphite,” Journal of the ElectrochemicalSociety, vol. 157, no. 6, A721, Apr. 2010.

[16] J. E. Harlow, X. Ma, J. Li, E. Logan, Y. Liu, N. Zhang, L. Ma, S. L. Glazier,M. M. E. Cormier, M. Genovese, S. Buteau, A. Cameron, J. E. Stark, andJ. R. Dahn, “A Wide Range of Testing Results on an Excellent Lithium-Ion CellChemistry to be used as Benchmarks for New Battery Technologies,” Journal ofThe Electrochemical Society, vol. 166, no. 13, A3031, Sep. 2019.

[17] A. E. Mejdoubi, A. Oukaour, H. Chaoui, H. Gualous, J. Sabor, and Y. Slamani,“State-of-Charge and State-of-Health Lithium-Ion Batteries’ Diagnosis Accordingto Surface Temperature Variation,” IEEE Transactions on Industrial Electronics,vol. 63, no. 4, pp. 2391–2402, Apr. 2016.

[18] E. Billy, M. Joulié, R. Laucournet, A. Boulineau, E. De Vito, and D. Meyer,“Dissolution Mechanisms of LiNi1/3Mn1/3Co1/3O2 Positive Electrode Materialfrom Lithium-Ion Batteries in Acid Solution,” ACS Applied Materials & Interfaces,vol. 10, no. 19, pp. 16 424–16 435, May 2018.

[19] W. Ren, K. Wang, J. Yang, R. Tan, J. Hu, H. Guo, Y. Duan, J. Zheng, Y. Lin,and F. Pan, “Soft-contact conductive carbon enabling depolarization of LiFePO4cathodes to enhance both capacity and rate performances of lithium ion batteries,”Journal of Power Sources, vol. 331, pp. 232–239, Nov. 2016.

[20] A. Casimir, H. Zhang, O. Ogoke, J. C. Amine, J. Lu, and G. Wu, “Silicon-basedanodes for lithium-ion batteries: Effectiveness of materials synthesis and electrodepreparation,” Nano Energy, vol. 27, pp. 359–376, Sep. 2016.

[21] J. Belt, V. Utgikar, and I. Bloom, “Calendar and PHEV cycle life aging ofhigh-energy, lithium-ion cells containing blended spinel and layered-oxide cathodes,”Journal of Power Sources, vol. 196, no. 23, pp. 10 213–10 221, Dec. 2011.

[22] J. Xu, Y. Dou, Z. Wei, J. Ma, Y. Deng, Y. Li, H. Liu, and S. Dou, “Recent Progressin Graphite Intercalation Compounds for Rechargeable Metal (Li, Na, K, Al)-IonBatteries,” Advanced Science, vol. 4, no. 10, p. 1 700 146, 2017.

[23] V. A. Agubra and J. W. Fergus, “The formation and stability of the solid electrolyteinterface on the graphite anode,” Journal of Power Sources, vol. 268, pp. 153–162,Dec. 2014.

[24] V. G. Khomenko, V. Z. Barsukov, J. E. Doninger, and I. V. Barsukov, “Lithium-ionbatteries based on carbon–silicon–graphite composite anodes,” Journal of PowerSources, IBA – HBC 2006, vol. 165, no. 2, pp. 598–608, Mar. 2007.

[25] K. Higa and V. Srinivasan, “Stress and Strain in Silicon Electrode Models,” Journalof The Electrochemical Society, vol. 162, no. 6, A1111, Mar. 2015.

[26] B. Flamme, G. R. Garcia, M. Weil, M. Haddad, P. Phansavath, V.Ratovelomanana-Vidal, and A. Chagnes, “Guidelines to design organicelectrolytes for lithium-ion batteries: Environmental impact, physicochemicaland electrochemical properties,” Green Chemistry, vol. 19, no. 8, pp. 1828–1849,2017.

[27] K. Xu, “Nonaqueous Liquid Electrolytes for Lithium-Based RechargeableBatteries,” Chemical Reviews, vol. 104, no. 10, pp. 4303–4418, Oct. 2004.

36

[28] A. L. Michan, B. S. Parimalam, M. Leskes, R. N. Kerber, T. Yoon, C. P. Grey,and B. L. Lucht, “Fluoroethylene Carbonate and Vinylene Carbonate Reduction:Understanding Lithium-Ion Battery Electrolyte Additives and Solid ElectrolyteInterphase Formation,” Chemistry of Materials, vol. 28, no. 22, pp. 8149–8159,Nov. 2016.

[29] Z. J. Zhang and P. Ramadass, “Lithium-Ion Battery Separators1,” in Lithium-IonBatteries: Science and Technologies, M. Yoshio, R. J. Brodd, and A. Kozawa, Eds.,New York, NY: Springer, 2009, pp. 1–46.

[30] C. J. Orendorff, “The Role of Separators in Lithium-Ion Cell Safety,” TheElectrochemical Society Interface, vol. 21, no. 2, p. 61, Jan. 2012.

[31] M. F. Lagadec, R. Zahn, and V. Wood, “Characterization and performanceevaluation of lithium-ion battery separators,” Nature Energy, vol. 4, no. 1,pp. 16–25, Jan. 2019.

[32] H. F. Xiang, H. Wang, C. H. Chen, X. W. Ge, S. Guo, J. H. Sun, and W. Q. Hu,“Thermal stability of LiPF6-based electrolyte and effect of contact with variousdelithiated cathodes of Li-ion batteries,” Journal of Power Sources, vol. 191, no. 2,pp. 575–581, Jun. 2009.

[33] T. H. Dubaniewicz Jr and J. P. DuCarme, “Internal short circuit and acceleratedrate calorimetry tests of lithium-ion cells: Considerations for methane-air intrinsicsafety and explosion proof/flameproof protection methods,” Journal of LossPrevention in the Process Industries, vol. 43, pp. 575–584, Sep. 2016.

[34] R. Spotnitz and J. Franklin, “Abuse behavior of high-power, lithium-ion cells,”Journal of Power Sources, vol. 113, no. 1, pp. 81–100, Jan. 2003.

[35] E. P. Randviir and C. E. Banks, “Electrochemical impedance spectroscopy:An overview of bioanalytical applications,” Analytical Methods, vol. 5, no. 5,pp. 1098–1115, Feb. 2013.

[36] “Electrochemical Impedance Spectroscopy,” in Electrochemical ImpedanceSpectroscopy, Section: 5, John Wiley & Sons, Ltd, 2017, pp. 89–127, isbn:978-1-119-36368-2.

[37] M. Lacey, The principles of electrochemical impedance spectroscopy, Mar. 2021.[Online]. Available: http://lacey.se/science/eis/eis-principles/ (visitedon 03/29/2021).

[38] A. Lasia, “Definition of Impedance and Impedance of Electrical Circuits,” inElectrochemical Impedance Spectroscopy and its Applications, A. Lasia, Ed., NewYork, NY: Springer, 2014, pp. 7–66, isbn: 978-1-4614-8933-7.

[39] M. Lacey, Simple circuits with resistors and capacitors. [Online]. Available: http://lacey.se/science/eis/simple-circuits/ (visited on 06/07/2021).

[40] S. M. Rezaei Niya and M. Hoorfar, “Study of proton exchange membrane fuel cellsusing electrochemical impedance spectroscopy technique – A review,” Journal ofPower Sources, vol. 240, pp. 281–293, Oct. 2013.

[41] X. Feng, M. Fang, X. He, M. Ouyang, L. Lu, H. Wang, and M. Zhang,“Thermal runaway features of large format prismatic lithium ion battery usingextended volume accelerating rate calorimetry,” Journal of Power Sources, vol. 255,pp. 294–301, Jun. 2014.

37

http://lacey.se/science/eis/eis-principles/

http://lacey.se/science/eis/simple-circuits/

http://lacey.se/science/eis/simple-circuits/

[42] B. Oh, Y.-E. Hyung, D. R. Vissers, and K. Amine, “Accelerating ratecalorimetry study on the thermal stability of interpenetrating network typepoly(siloxane-g-ethylene oxide) polymer electrolyte,” Electrochimica Acta, EighthInternational Symposium on Polymer Electrolytes, vol. 48, no. 14, pp. 2215–2220,Jun. 2003.

[43] “An Introduction to Accelerating Rate Calorimetry,” Thermal Hazard Technology,Manual Supplier. [Online]. Available: www.thermalhazardtechnology.com.

[44] D. Townsend and J. TOU, “Thermal Hazard Evaluation by an Accelerating RateCalorimeter,” Elsevier Scientific Publishing Company, Amsterdam, Tech. Rep.,1980.

[45] H. Ishikawa, O. Mendoza, Y. Sone, and M. Umeda, “Study of thermal deteriorationof lithium-ion secondary cell using an accelerated rate calorimeter (ARC) and ACimpedance method,” Journal of Power Sources, vol. 198, pp. 236–242, Jan. 2012.

[46] B. Lei, W. Zhao, C. Ziebert, N. Uhlmann, M. Rohde, and H. J. Seifert,“Experimental Analysis of Thermal Runaway in 18650 Cylindrical Li-Ion CellsUsing an Accelerating Rate Calorimeter,” Batteries, vol. 3, no. 2, p. 14, Jun. 2017.

[47] T. T. D. Nguyen, S. Abada, A. Lecocq, J. Bernard, M. Petit, G. Marlair, S. Grugeon,and S. Laruelle, “Understanding the Thermal Runaway of Ni-Rich Lithium-IonBatteries,” World Electric Vehicle Journal, vol. 10, no. 4, p. 79, Dec. 2019.

[48] “Analysis of Accelerating Rate Calorimetry Data: Theory,” ThermalHazard Technology, Manual Supplier. [Online]. Available: www .thermalhazardtechnology.com.

[49] C. Schick, “Differential scanning calorimetry (DSC) of semicrystalline polymers,”Analytical and Bioanalytical Chemistry, vol. 395, no. 6, p. 1589, Oct. 2009.

[50] K. Lukas and P. K. LeMaire, “Differential scanning calorimetry: Fundamentaloverview,” Resonance, vol. 14, no. 8, pp. 807–817, Aug. 2009.

[51] C. Demetzos, “Differential Scanning Calorimetry (DSC): A Tool to Study theThermal Behavior of Lipid Bilayers and Liposomal Stability,” Journal of LiposomeResearch, vol. 18, no. 3, pp. 159–173, Jan. 2008.

[52] J. S. Gnanaraj, E. Zinigrad, L. Asraf, H. E. Gottlieb, M. Sprecher, M. Schmidt,W. Geissler, and D. Aurbach, “A Detailed Investigation of the Thermal Reactionsof LiPF6 Solution in Organic Carbonates Using ARC and DSC,” Journal of TheElectrochemical Society, vol. 150, no. 11, A1533, Sep. 2003.

[53] Z. Zhang, D. Fouchard, and J. Rea, “Differential scanning calorimetry materialstudies: Implications for the safety of lithium-ion cells,” Journal of Power Sources,vol. 70, no. 1, pp. 16–20, Jan. 1998, Publisher: Elsevier.

[54] I. Hutchins, “Accelerating Rate Calorimetry,” Thermal Hazard Technology,Experimental report THT20210, Mar. 2021.

[55] E. Zinigrad, L. Larush-Asraf, G. Salitra, M. Sprecher, and D. Aurbach, “On thethermal behavior of Li bis(oxalato)borate LiBOB,” Thermochimica Acta, vol. 457,no. 1, pp. 64–69, Jun. 2007.

[56] E. P. Roth and G. Nagasubramanian, “Thermal stability of electrodes inLithium-ion cells,” Journal of the Electrochemical Society, Feb. 2000.

38

www.thermalhazardtechnology.com



[57] A. D. Pasquier, F. Disma, T. Bowmer, A. S. Gozdz, G. Amatucci, and J.-M.Tarascon, “Differential Scanning Calorimetry Study of the Reactivity of CarbonAnodes in Plastic Li-Ion Batteries,” Journal of The Electrochemical Society,vol. 145, no. 2, p. 472, Feb. 1998.

[58] J. Jiang and J. R. Dahn, “Comparison of the Thermal Stability of LithiatedGraphite in LiBOB EC/DEC and in LiPF6 EC/DEC,” Electrochemical andSolid-State Letters, vol. 6, no. 9, A180, Jun. 2003.

[59] H. Yang, G. V. Zhuang, and P. N. Ross, “Thermal stability of LiPF6 salt and Li-ionbattery electrolytes containing LiPF6,” Journal of Power Sources, vol. 161, no. 1,pp. 573–579, Oct. 2006.

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