Proceedings of 7th Transport Research Arena TRA 2018, April 16-19, 2018, Vienna, Austria Transport of Li-Ion Batteries: Early Failure Detection by Gas Composition Measurements Christiane Essl*, Andrey W. Golubkov, Rene Planteu, Bernhard Rasch, Alexander Thaler and Anton Fuchs

VIRTUAL VEHICLE Research Center, Inffeldgasse 21a, 8010 Graz, Austria

Abstract

Improved volumetric energy density of the battery system is requested to increase the range of upcoming electric cars. To guarantee safety of enhanced Li-ion batteries in electric vehicles operation as well as for transport and storage is of crucial importance for acceptability of this technology. Failure cases of Li-ion batteries causing high temperatures and undesirable chemical reactions which can result in exothermic reactions and furthermore thermal runaway have to be avoided reliably. This paper presents potential failure cases of large batteries and approaches how to detect these failures. Important parameters as well as the composition of gas release with the focus on hydrogen fluoride (HF) are provided. The aim is to improve gas composition measurement, to learn more about the Li-ion failure cases and to use early failure detection for preventing the release of harmful toxic and inflammable gases. In this paper the gas composition of the failure case “second degassing” is measured and analyzed with an FTIR spectrometer.

Keywords: Li-ion batteries; thermal runaway; FTIR spectroscopy; Hydrogen fluoride; battery safety; electric vehicles Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

Nomenclature

SOC State of charge FTIR Fourier-transform infrared (FTIR-spectroscopy) GC Gas Chromatography ATL Amperex Technology Limited (Cell producer) TLV Threshold limit value HF Hydrogen Fluoride CO Carbon Monoxide CH4 Methane CO2 Carbon Dioxide C2H4 Ethylene C2H6 Ethane LiPF6 Lithium Hexafluorophosphate PF5 Phosphorus Pentafluoride LiF Lithium Fluoride H2 Hydrogen DMC Dimethylcarbonate NMC Nickel Manganese Cobalt Oxide Li(NiMnCo)O2 LTO Lithium Titanate Oxide Li4Ti5O12 ppm part per million

1. Introduction

Technical improvements, industrialization towards large-scale production and governmental support lower the costs of electric cars. The range of upcoming middle class electric cars (from 2017 onwards) will increase to at least 300 km (Shanhan 2017). This increase in range will be enabled – among others - by improved volumetric energy density of the battery system. State of the art Li-ion batteries used in EVs/HEVs vent large amount of toxic and inflammable gases in case of certain failures Beside the use of the Li-ion batteries in electric vehicles, safety should be guaranteed at the transport of these batteries. The motivation of our project is to improve battery safety by examining concepts for early failure detection in Li-ion battery systems and gain insight in gas composition at error cases.

This paper will show potential failure cases such as vaporizing electrolyte of a leaky battery, the first degassing of a Li-ion cell at a temperature rise above 140°C and the second degassing of a Li-ion cell in thermal runaway according to Golubkov et al. (2014, 2015). Each failure case will lead to a significant composition of released gases. Some of these gases are very toxic and inflammable. Research on the failure case of Li-Ion battery fire has been intensively published e.g. Wang et al. (2012) while other failure cases are less documented in literature. This paper therefore concentrates on failure cases other than battery fire.

To guarantee safety and examine concepts for early failure detection, it is important to gain insights in gas composition and the amount of released gas at the battery failure cases described in Figure 1. Therefore, a battery test bed for the investigation of the battery behavior under misuse tests is enhanced with optical online gas analysis (FTIR spectrometer) to quantize emitted gases of different Li-ion cells. With this setup, the evolving gases are analyzed and quantified with the main focus on toxic (CO, HF) and inflammable gases (CO, CH4, C2H4…) of Li-ion cells for a state of the art Li-ion battery. With the gained information of the gas composition, it is possible to derive concepts for early failure detection with economic and commercially available gas sensors.

The venting behavior of large Li-ion cells is not yet sufficiently described in the literature. The paper will show real world gas composition measurements of one selected Li-ion battery failure case, the second degassing of a Li-ion cell in thermal runaway, and venting of automotive Li-ion cells. Especially the detection and quantification of HF are in the focus of the work. Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

Figure 1: Scheme of potential failure cases (from left to right): electrolysis of refrigerant in case of a leaky battery-cooling system, vaporizing electrolyte of a leaky battery, the first degassing of a Li-ion cell at a temperature rise above 140°C and the second degassing of a Li-ion cell in thermal runaway

2. Theory

Thermal runaway experiments of Li-ion batteries are published e.g. by Golubkov (2014, 2015) and for 18650 cells by Lammer, M. (2017) with GC-measured gas compositions. Gas compositions measured at battery fire are published by F. Larsson et al. (2017) and P. Ribière et al. (2012). Because of the special interest in HF measurement, the HF related reactions are discussed in this section.

The most commonly used conducting salt in the electrolyte of Li-ion cells is fluorine containing LiPF6. It offers a good compromise of ion-mobility, solubility and thermal stability. H. Yang et al. (2006) investigated that pure LiPf6 is stable up to 380 K in an inert atmosphere and dissociates into lithium fluoride (LiF (s)) and phosphorus pentafluoride (PF5 (g)):

퐿푖푃퐹6 → 퐿푖퐹 + 푃퐹5 (1)

Degradation of the salt corresponds to the dissociation of the salt in dry condition. If the conducing salt is in contact with water (300 ppm) the decomposition onset temperature reduces and a chemical reaction between LiPF6 and water vapour produces POF3 and HF. POF3 reacts again with water to build HF.

퐿푖푃퐹6 + 퐻2푂 → 퐿푖퐹 + 푃푂퐹3 + 2 퐻퐹 (2)

3. Method

3.1. Update of the battery abuse test rig

In order to qualify and quantify vent gases for a failure case of Li-ion batteries, an existing battery abuse test rig is used and extended with a new FTIR spectrometer. The battery lab test rig was developed at Virtual Vehicle to carry out thermal runaway experiments on automotive cells (see Figure 2). The environment inside the reactor with a volume of 121 l can be flushed with N2 or Ar. Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

Figure 2: Test rig at Virtual Vehicle Research Center for thermal runaway experiments on automotive cells (Golubkov, 2016)

For experiments, the temperature inside the reactor is measured at different positions on the cell surface, of the vent gas and at different positions inside the test rig. There is a specific sample holder shown in Figure 3. The cell can be overheated, overcharged or forced by other methods into thermal runaway. After the produced failure case of an automotive Li-ion cell, the gas is analyzed with a FTIR spectrometer. Until 2017, the produced gas at this test rig was analyzed by a gas chromatograph and therefore the main released gases such as CO, CO2, CH4, C2H4 and H2 of the thermal runaway event were already known.

Figure 3: Sample holder as part of the existing test bed for battery misuse tests at Virtual Vehicle (Golubkov, 2017)

3.2. Introducing FTIR spectroscopy

The existing GC alone is not able to measure HF, POF3 and PF5, therefore the FTIR spectrometer is introduced as new gas composition measurement technology. Due to the specific rotational and vibrational modes of each molecule, molecules have their own characteristic infrared absorption spectrum, which can be used to analyze the whole vent gas spectrum over a broad absorbance wavenumber region qualitatively with the FTIR spectrometer. Symmetric two-atomic gases such as H2 and O2 cannot be analyzed with an FTIR spectrometer.

The FTIR spectrometer used for the experimental investigations has a path length of 10 cm and a resolution better than 0.5 cm-1. It is able to measure the absorption bands of already known and expected gases as well as Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

unknown infrared absorbing gases. The resulting measured spectra are used for qualitative and quantitative analysis of the released gas composition during Li-ion battery failures. For each potential failure case such as vaporizing electrolyte of a leaky battery, the first degassing of a Li-ion cell at a temperature rise above 140°C and the second degassing of a Li-ion cell in thermal runaway specific gases are expected and can be measured.

3.3. Optimization of the spectroscopy measurement

The FTIR spectrometer requires well-defined pressure and temperature of the gas. Additionally it is very important to guarantee defined particle separation in front of the FTIR gas cell in order to keep the gas cell clean. Therefore, the integration of the FTIR spectrometer is complex and needs some time and modification. The calibration has to be done for each gas separately and is highly sensitive to temperature and pressure influences.

After each experiment, the SOC, the failure case, the maximum cell- and vent gas temperature, the amount and composition of the vent gases are available.

4. Experiment

With the testbed and the online FTIR spectrometer potential failure cases such as vaporizing electrolyte of a leaky battery, the first degassing of a Li-ion cell at a temperature rise above 140°C and the second degassing of a Li-ion cell in thermal runaway can be investigated.

This paper shows the gas composition of a second degassing of a Li-ion cell in thermal runaway analyzed by a FTIR Spectrometer. The experiment was done with an already aged 18 Ah ATL cell inside the test rig. The published cell has a NMC cathode and a LTO anode. For this experiment, the cell is charged to 100% SOC and overheated afterwards with a constant heating rate. The environment in the reactor was during the experiment filled with N2.

The temperature was measured during the experiment at different positions inside the reactor, on the cell surface and near the cell rupture-plate. The temperature measurements are shown in Figure 4. The maximum reached temperature of the vent gas at this experiment is 427°C.

Figure 4: Temperature measurements of a failure case of second degassing of a Li-ion cell in thermal runaway at different positions on the cell surface, vent gas positions and reactor positions (Golubkov, 2017) Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

For the thermal runaway, the self-heating rate of the cell is larger than the cooling rate because of the exothermic reaction. As the cell temperature increases, the vent pipe of the cell opens and releases a large amount of inflammable (and partially toxic) gases into the test rig. After the TR of the Li-ion cell in the closed reactor, the reactor is opened and the gas composition (e.g. CO, CO2, CH4, C2H4, C2H6, H2O, HF) is analyzed by the FTIR spectrometer.

4.1. Qualitative gas composition analysis with FTIR spectrometer

In case of the ATL 18 Ah Li-ion cell the spectrum of a failure case of a thermal runaway is shown in Figure 5: the spectra of CO, CO2, DMC, C2H4, and H2O can be separated, because these molecules absorb infrared light of a different frequency. In the region between 2750 – 3200 cm-1 a lot of higher hydrocarbons absorb infrared light. That is why it is difficult to separate the gases exactly.

Figure 5: FTIR spectrometer transmission spectrum of vent gas of the second degassing of a Li-ion cell in thermal runaway

The FTIR spectrometer has the significant advantage to detect a device-specific wavelength region spectrum at once. The detector, the light source and the used optical components define the possible wavelength region and therefore also unknown gases, which absorb light in this wavelength region, can be analyzed. From the obtained transmission spectra the absorbance spectra can be calculated (Figure 6). Transmission and absorbance spectra are often used in spectroscopy.

In comparison to GC, even HF can be analyzed with the FTIR spectrometer. To the best of our knowledge the quantification of HF at a thermal runaway experiment without battery fire has not been published; therefore the HF measurement is focused in this paper.

Figure 6: FTIR absorbance spectrum of vent gas of the second degassing of a Li-ion cell in thermal runaway

HF absorption peaks appear between 3650 and 4200 cm-1 as well as between 50 and 350 cm-1. The spectral range of the spectrometer and gases absorbing at the same wavelength as HF have to be considered to characterize HF. The spectrometer used for the experimental investigations has a spectral range between 700 and 7000 cm-1. -1 CO2 and H2O also absorb light in the wavelength region of HF (CO2 absorbs between 3600 and 3750 cm and -1 H2O absorbs dependent on the gas concentration up to 4050 cm ). The absorbance spectra of realistic vent gas concentrations of HF, CO2 and H2O are plotted in Figure 7 for 10 cm path length, at 1 atm and 463 K. Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

Figure 7: Absorbance spectra for realistic vent gas concentrations between 3650 and 4200 cm-1 for 10 cm path length, at 1 atm and 463 K by Spectraplot (2017)

Figure 7 shows that the relevant wavelength region to characterize HF is in the case of vent gas analysis and with our equipment between ~4000 and 4200 cm-1. Figure 8 shows the FTIR measurement of a second degassing of a Li-ion cell in thermal runaway in the region between 3900 and 4200 cm-1 with highlighted HF peaks.

In Figure 8 the absorption peaks of HF are shown. In this experiment the measured HF concentration is weak and therefore the absorption peaks are also low, but are definitely caused by HF according to the HITRAN database.

Figure 8: FTIR absorbance spectrum of HF and other vent gas of a Li-ion cell in thermal runaway

4.2. Quantitative composition analysis with FTIR spectrometer

For quantitative gas composition analysis the used FTIR spectrometer has to be calibrated for each gas with defined temperature, pressure and gas concentration. Certain corrections such as the baseline-correction have to be distinguished. The calibration of the FTIR is quite time consuming. All factors influencing the absorbance should be considered in the concentrations calculation. The test method has been validated for the measured gas components with certified test gas. The HF test gas was connected directly after the test rig to the analysis path and got analyzed with the FTIR spectrometer.

The measured vent gas concentrations of the introduced 18 Ah cell experiment with the FTIR spectrometer are shown in Figure 9. Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

Figure 9: FTIR measured concentrations in ppm of a failure case of second degassing of a Li-ion cell in thermal runaway in ppm measured at 1068.86 hPa and 420 K

The concentrations measured after the measured gas concentration converged to a stable concentration value are shown in Figure 10 for the identified gases in detail. In this example the 18 Ah ATL cell charged to 100% SOC releases 66 ppm HF. The overall released amount of vent-gas was calculated with the ideal gas law, using the measured reactor pressure and gas temperature inside the reactor. The ATL cell released 1.0039 mol of vent-gas. Therefore the amount of released HF was 396 µmol.

FTIR measured and identified vent gas concentrations are given in ppm in Table 1. H2 is also expected at a thermal runaway, but cannot be analyzed with an FTIR spectrometer.

Table 1: FTIR spectrometer measured concentrations of second degassing of a ATL 18 Ah Li-ion cell in thermal runaway in ppm at 980.1 hPa and 420 K Gas Concentration Component / ppm

C2H4 6.221 C2H6 994 CH4 2.202 CO 7.621

CO2 74.657 DMC 23.142

H2O 12.260 HF 66

The gas composition depends on the maximal reached temperature and on the chemical reactions at this temperature. Therefore, the cell temperature maximum, the maximal vent gas temperature as well as the amount of not condensed gas are relevant to compare the measurements. The concentrations shown in Table 1 are measured with the reactor volume of 121 l at 1.4 bar.

Figure 10: Plot of FTIR spectrometer measured concentrations of a failure case of second degassing of a Li-ion cell in thermal runaway in ppm at 980.1 hPa and 420 K Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

5. Conclusion

Failure cases of Li-ion cells like a second degassing of a Li-ion cell in thermal runaway have been analysed at a test rig with thermal and mechanical sensors. This paper presents results of gas composition analysis with an FTIR spectrometer where an 18 Ah ATL cell with NMC cathode and LTO Anode was charged to 100% SOC and forced into thermal runaway. The used Li-ion cell reached a maximal vent gas temperature of 427°C and produced 1.0039 mol gas. The gas composition of the vent gas or in general produced gas during the failure case is analysed with a calibrated FTIR spectrometer qualitatively and quantitatively. Especially the concentration measurement of toxic HF was focussed and turned out to be possible with the FTIR spectrometer. During the exothermic reaction of the 18 Ah ATL cell under exclusion of air in inert N2 test rig, mainly carbonaceous gases, especially CO2, CO and hydrocarbons, were released. For the thermal runaway 7.621 ppm CO, 74.657 ppm CO2, 6.221 ppm C2H4, 2.202 ppm CH4 and 66 ppm HF in a 121 l reactor were measured during the failure case of second degassing of a Li-ion cell with the FTIR spectrometer. Thereby, HF was the only measured fluoride gas at the thermal runaway. H2 is also expected at a thermal runaway, but cannot be analyzed with an FTIR spectrometer and has to be completed with a GC measurement. Possible sources of the gaseous HF at Li-ion failure cases are the fluorine electrolyte salt LiPF6 and the binder material of the electrodes.

For the transport of Li-ion batteries it is important to guarantee gas concentrations of all gases below the TLV of each gas to guarantee safe working conditions. To give an impression of the toxicity of HF: the short time TLV- value of HF in Austria is defined for 15 minutes with 3 ppm. One important question concerning transport of Li-ion batteries is which volume per automotive (failure) venting cell is needed at minimum in order to get HF concentration lower than the TLV of HF. For this example it means that 66 ppm in a reactor with a 121 l volume and a pressure of 1.4 bar inside the reactor at constant temperature have to be diluted to guarantee safe working conditions. In this case at least additional 4 m³ per second degassing cell is needed to get a HF concentration value beneath TLV. For the sake of completeness, the CO short time TLV-value in Austria is defined for 15 minutes with 600 ppm.

For a better interpretation of the measured HF concentration, the results were compared to the gas composition measurements, especially the ratio of HF to the main produced gas component CO2, of electric vehicle fire. At our induced thermal runaway experiment under exclusion of air, the ratio of HF:CO2 was 1:1130 (in ppm respectively mol). According to Lecocq et al. (2012) HF was emitted in significant quantities during fire tests of a full electric vehicle (EV) and analogous internal combustion engine (ICE) vehicle. In case of the EV the ratio HF:CO2 was 1:136 (in mol) and at ICE HF:CO2 was 1:371 (in mol). That means our result of HF:CO2 ratio at a single thermal runaway cell is comparable with the results of Lecocq et al. (2012) considering that the measured HF concentration of the ICE without Li-ion cells was about half of the HF of the full electric vehicle fire. For complete vehicle fires, the source of HF may be fluorinated materials in the vehicle (e.g. from a fluorinated refrigerant in the air conditioning system). This example shows that safety issues concerning toxic gases, like HF, at Li-ion cell failure cases need to be understood and clarified.

Furthermore, toxic gases such as CO and inflammable gases such as CO, CH4 and C2H4 have to be considered. Detection of Li-ion battery failures is very important for electric vehicles, transport and storage of Li-ion batteries. Early failure detection, especially for transport, is possible with gas sensors (for instance a sensor for CO2) and temperature sensors. Each failure case has its own gas composition and corresponding to this the gas sensors can be chosen. Further high energy cells are designated for research test series in 2018 with common NMC cathode and graphite anode as well as commonly used electrolyte and additives.

Acknowledgements

This work was accomplished at the VIRTUAL VEHICLE Research Center in Graz, Austria. The work reported in this paper was partially funded by Austrian Research Promotion Agency FFG funding program “Industrienahe Dissertation” (Project Gabsi - 857934) and by the COMET K2 - Competence Centers for Excellent Technologies Programme of the Austrian Federal Ministry for Transport, Innovation and Technology (bmvit), the Austrian Federal Ministry of Science, Research and Economy (bmwfw), the Province of Styria and the Styrian Business Promotion Agency (SFG). Christiane Essl / TRA2018, Vienna, Austria, April 16-19, 2018

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