Quantitative identification of emissions from abused prismatic Ni-rich lithium-ion batteries

Journal Pre-proof Quantitative identification of emissions from abused prismatic Ni-rich lithium-ion batteries Yajun Zhang, Hewu Wang, Weifeng Li, Cheng Li PII:

S2590-1168(19)30031-1

DOI:

https://doi.org/10.1016/j.etran.2019.100031

Reference:

ETRAN 100031

To appear in:

eTransportation

Received Date: 5 November 2019 Revised Date:

8 November 2019

Accepted Date: 11 November 2019

Please cite this article as: Zhang, Y., Wang, H., Li, W., Li, C., Quantitative identification of emissions from abused prismatic Ni-rich lithium-ion batteries, eTransportation, https://doi.org/10.1016/ j.etran.2019.100031. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Quantitative identification of emissions from abused prismatic Ni-rich lithium-ion batteries Yajun Zhang, Hewu Wang, Weifeng Li *1, Cheng Li State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, PR China Abstract The emissions from lithium-ion batteries in the process of thermal runaway are one of the important sources of electric vehicle fire hazards. The purpose of this study is to reveal the gaseous and solid emission characteristics of lithium-ion batteries after thermal runaway. A 50 Ah commercial prismatic cell with Li(Ni0.6Mn0.2Co0.2)O2 cathode was triggered to thermal runaway by external heating in a sealed chamber with nitrogen atmosphere. Elements of settleable particles were detected, and size distributions were analyzed. The gaseous emissions were detected and their compositions were classified by boiling temperatures. The results show that the total emissions accounted for 28.53% of the cell mass. The settleable particles accounted for 17.00% of the emission mass, and 30 elements were found in them. Near 90% of the particle mass were those with a size less than 0.5 mm in diameter, and the median size of these particles was approximately 397 um. A total of 31 compositions were found in the gaseous emissions, of which 17 were compositions with boiling temperatures below 25°C, 8 compositions between 25°C and 90°C, and 6 compositions between 90°C and 185°C. The gases with boiling temperatures below 90°C accounted for 32.75% of the emission mass, while the remaining gases and undetected particles accounted for 50.25%. These results further clarified the sources of lithium-ion battery fire hazards. Key words: Lithium-ion batteries; thermal runaway; emissions; vent gas; particulate matter. Nomenclature Al Ar As Ba Br C Ca CH4 C2H2 C2H4 C2H6 C3H4 C3H6 C3H8

aluminum argon arsenic barium bromine carbon calcium methane ethyne ethylene ethane propyne propylene propane

i I K Li LIB Mg Mn n n0 ni nstable_ideal N N2

a certain composition in the vent gases iodine potassium lithium lithium-ion battery magnesium manganese molar amount initial amount of gas in the chamber molar amount of substance i the stable amount of VG calculated by ideal gas equation nitrogen nitrogen

*   E-ml : lwf@c (W L) 1

C4H6 C4H8 C4H8 C4H8 C4H8 C4H10 C5H10 C5H10 C5H10 C5H10 C5H10 C5H10 C5H12 C H C6H12 C9H18 C3H6O3 C4H8O3 C5H10O3 Cl Co CO CO2 Cr Cu Dx

DMC DEC EMC F H H2 H O HCl

1,3-butadiene 1-butylene 2-methyl propene trans-2-butene cis-2-butene n-butane 1-pentene cis-2-pentene trans-2-pentene 2-methyl-1-butene 2-methyl-2-butene 3-methyl-1-butene n-pentane benzene 2-methyl-1-pentene 2,4-dimethyl-1-heptene dimethyl carbonate methyl ethyl carbonate diethyl carbonate chlorine cobalt carbon monoxide carbon dioxide chromium copper particle size corresponding to a cumulative particle size distribution percentage of x% dimethyl carbonate diethyl carbonate methyl ethyl carbonate fluorine hydrogen element hydrogen water vapor hydrogen chloride

Na Ni NMC O P Pchamber PM QR QD R S Sb SOC Sr T1 T2 T3 Tboiling Tboiling, max Tenv Tdetection Tsampling Tstable Ti Sn V VG VGx VGx+ VL Vchamber VS x0 xi Zr Zn

sodium nickel LixNiyMnzCo1-y-zO2 oxygen phosphorus internal pressure of the sealed chamber particle matters Quantitative ratio Quantitative degree gas constant sulfur antimony state of charge strontium cell surface temperature cell jet zone temperature temperature near the sealed chamber wall boiling point the maximum boiling point environment temperature in the chamber gas detection temperature the gas sampling temperature the stable value of the VG temperature titanium tin vanadium vent gas vent gas with boiling point below x vent gas with boiling point above x vent liquid the chamber volume vent solid molar concentration of N2 molar concentration of substance i zirconium zinc

1. Introduction In recent years, regulatory restrictions on emissions and environmental awareness have boosted the promotion of electric vehicles or plug-in hybrid electric vehicles, and led to reductions in the usage of traditional internal combustion engine vehicles [1-5]. The global stock of electric and plug-in hybrid electric vehicles surpassed the 3 million mark in 2017, accompanied by an average annual increase of 56% in 2015 and 2016 (1.2 million and 2 million, respectively) according to the International Energy Agency [6]. After many years of development,

2

lithium-ion batteries (LIBs) have become increasingly acceptable as the main power source in vehicles, given their higher energy density, longer calendar or cycle life, and increased reliability [7]. However, safety aspects concerning these vehicles have received increasing attention due to the hazards of battery vent gas (VG) or smoke, and difficulty in firefighting, which usually caused by failures of on-board large-capacity power batteries [8]. One major battery failure mode is called thermal runaway [9], wherein an increase in temperature changes conditions, which drives a further increase in temperature. With the occurrence of LIB thermal runaway, more and more gases are generated inside the battery. When the pressure inside the battery reach a certain value, the battery safety valve for the hard-shell battery is released, or the area in the aluminum plastic film with a lower allowable pressure for the pouch battery develops a crack. Then, the battery vents out emissions [10-12]. In addition to the jet stream of gaseous emissions, liquids and solids inside the battery are also vented out [10-12], forming liquid emissions and solid emissions. The vent gas emissions released by LIBs in the process of thermal runaway are one of the important sources of fire hazards [13]. Therefore, many researchers have studied the formation mechanisms of the VG emissions, and effects of state of charge (SOC), battery type, and battery aging on the compositions and amounts of the VG emissions [13-27]. The VG emissions form mainly from the decompositions of the negative solid-electrolyte interface film [28, 29], reactions of the negative electrodes with the electrolytes [28], decompositions of positive electrode materials [28, 30-32], decompositions of electrolytes [33], decompositions of binders [34], and reactions between various material decomposition products. Somandepalli et al. [22] studied the variations in the compositions, concentrations, and volumes of the VG with SOC. They placed a battery in a sealed 6 L chamber filled with argon (Ar) and heated it to vent. The results showed that the VG mainly included carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), and hydrocarbons with different numbers of carbon atoms such as methane (CH4), ethylene (C2H4), and ethane (C2H6). As the SOC increased, the CO2 concentration decreased, the CO and CH4 concentrations increased, H2 remained relatively unchanged, and total hydrocarbons varied between 20% and 30%. The normalized volume of the VG increased with increasing SOC. However, existing research on LIBs has the following problems: Firstly, there were few open literatures [27] reporting the VG emissions released by large-capacity power batteries with Ni-rich cathodes. The battery capacities were 2.5 Ah, 2.1 Ah, 1.1~2.6 Ah, 1.1/3.35 Ah and 20~80 Ah in Ref. [14], [22], [25], [26] and [27]. However, Ref. [27] did not provide details of the battery, the test process, and the 3

results of the research. Many open literatures reported the VG emissions released by LIBs using LixFePO4 (LFP) [14], LiCoO2 (LCO) [22], and Lix(Ni0.80Co0.15Al0.05)O2 (NCA) [26] as the cathodes, but the battery using NMC as the cathode received more attention due to its higher energy density, especially the Ni-rich cathodes. The VG emissions were studied by AW Golubkov et al. [25] using Li(Ni0.45Mn0.45Co0.10)O2 as the cathode; however, the Ni content was relatively low. Secondly, only H2, CO, CO2 and hydrocarbons with lower carbon numbers (the number of carbon atoms was usually not higher than 4) were researched in open literatures [25-27]. For example, AW Golubkov et al. [25, 26] only gave the testing results of H2, CO, CO2, CH4, C2H2, C2H4 and C2H6. Similarly, only H2, CO, CO2, CH4, C2H4, C2H6, propylene (C3H6), and propane (C3H8) were tested in the work of S. Kocha, et al [27]. Hydrocarbons in the VG emissions with higher carbon numbers were not detected. These substances are often flammable and explosive, and are extremely harmful to humans. The lack of hydrocarbons with higher carbon number in the test results leads to an underestimation of the potential hazards of VG emissions. Thirdly, due to the low sampling temperature (Tsampling), hydrocarbons having a higher carbon number probably were not be quantitatively detected due to liquefaction. For example, Somandepalli et al. [22] sampled the VG at a temperature of 60°C (the heating temperature of the sample before detection was not given), while the maximum temperature of the VG emissions detected in the chamber during venting was approximately 150°C. Although the results contained a variation in hydrocarbons with high carbon numbers and their mole fractions, these results could not be used for quantitative studies. One of the carbon compounds was ethylbenzene (C6H5C2H5). Its liquefaction temperature, i.e. the boiling point (Tboiling) of the substance, is approximately 136°C. It is highly volatile and explodes when exposed to flames, heat, or oxidizing agents. C6H5C2H5 is also listed as a Class 2B carcinogen. Due to its lower Tsampling, the detection of ethylbenzene concentration was often erroneous, leading to inaccurate evaluation of the ignitability, explosiveness, and toxicity of VG emissions. Finally, the lack of an indicator to evaluate the quantitative identification of VG emissions, made the test results misleading. For example, some studies detected only a few gas species, such as CO, H2, CO2 and some hydrocarbons with a carbon number less than 4, and the results were easy to make the readers think that there was no other gas in the VG emissions [ 25-27]. Somandepalli et al. [22] derived the mole percentages of gases with Tsampling
cathode, and derive quantitative parameters to measure the extent to which the identification is quantitative. The existence of vent liquid (VL) emissions from LIBs were confirmed by Wang et al. [10]. They used a high-speed camera to photograph the battery vent process and found that the jet stream ejecting from the battery exhibited liquid characteristics. Since there were liquid electrolytes inside the battery, it was supposed that the VL emissions contained electrolytes or high-molecular-weight substances with a high Tboiling. In addition, Wang et al. [10] also found that the temperature of the jet zone decreased with time at the initial vent stage, and supposed that this was due to the evaporation of VL emissions. It should be noted that if a certain gas is at a temperature lower than its Tboiling, it will be converted from gaseous to liquid state. Therefore, VG emissions with high Tboiling can also be classified as VL emissions.

In addition to VG and VL emissions, the battery jet is often accompanied by vent solid (VS) emissions, e.g. high-temperature particulate matter (PM) during eruption and combustion [35,36]. Sparks may be included in the high-temperature PM of VS emissions, whose temperature could be as high as 1,200°C. Once the combustible mixture and high-temperature PM are simultaneously ejected into the ambient environment and react with oxygen, they ignite to cause a fire or even explosion. In addition, as high-temperature PM have large kinetic energy, they may be scattered around and meet combustible materials, making them highly inflammable. The particle size distribution characteristics of settleable PM in the process of LIB thermal runaway can provide guidance on the suppressing of sparks and reducing the risk of ignition, as well as establish a fire safety active control strategy for LIBs. In addition, the elemental composition of settleable PM can provide guidance on proper handling of the LIB fire residue; that is, whether it can be released into the natural environment or needs to be recycled first, etc. The existing studies on high-temperature PM venting during the LIB thermal runaway process, are, to the best of our knowledge, inconclusive. A preliminary study on the particle size distribution, and elemental composition of settleable PM at air atmosphere has been conducted in our previous research [37]. The purpose of this research is to reveal the characteristics of emissions released by lithium-ion batteries during the vent and thermal runaway process. The test was performed on a 50-Ah commercial prismatic cell with (Ni0.6Mn0.2Co0.2)O2 as the cathode. The battery vent and thermal runaway were triggered by external heating in a sealed chamber with nitrogen atmosphere at 100% state of charge (SOC). VG and settleable VS emissions were

5

collected and detected after the experiment. The compositions of LIB VG emissions were revealed at a larger scale, and quantitative parameters were derived to measure the extent to which the identification of VG emissions was quantitative. The mass distribution, size distribution, and elemental composition of the VS emissions collected were also revealed. In addition, the percentages of emissions with different states were calculated. 2. Experimental procedure 2.1. Experimental setup We used a commercial cell based on nickel-manganese-cobalt (NMC) cathode. According to the manufacturer’s data, the cell nominal capacity and voltage were 50 Ah and 3.65 V, respectively. Additional information is listed in Table 1. The cell was composed of a lid, a shell, and a core. The safety valve, tab, and terminal of the cell were installed on the lid. The cathode, the separator, and the anode were curled to form the core. The cathode was composed of the cathode active material and the cathode current collector. The cathode active material of the cell in this research was mainly Li(Ni0.6Mn0.2Co0.2)O2, and the current collector was an aluminum foil. Similarly, the anode was composed of the anode active material and the anode current collector. The anode active material of the cell was mainly graphite, and the current collector was a copper foil. Table 1 Detailed technical specifications of the test cell. Parameters Cell mass (g) Nominal capacity (Ah) Nominal voltage (V) Minimum voltage (V) Maximum voltage (V) Cathode active material Cathode coating thickness (µm) Anode active material Anode coating thickness (µm) Cathode current collector Cathode current collector thickness (µm) Anode current collector Anode current collector thickness (µm) Shell Material

Specifications 870 50 3.65 2.75 4.25 Li(Ni0.6Mn0.2Co0.2)O2 61 Graphite 73 Aluminum foil 16 Copper foil 10 Aluminium alloy

In order to collect the emissions released by the cell, a high-pressure sealed chamber was designed (for structure diagram, see Fig. 1). The chamber had a volume of 230 L and could withstand a maximum pressure of 2 MPa. In order to trigger the vent and thermal runaway of the cell, an electric heater with adjustable power was placed inside the chamber, heating the cell evenly by in-chamber gas-convection heat exchange. The maximum power of the heater was 5 kW. The functions of the insulation board in the chamber were to a) support and fix the test cell and b) 6

prevent direct heating by the electric heater. The cell was vertically fixed on the insulation board with iron wires to ensure a certain spray direction. Three temperature sensors and a pressure sensor were installed in the chamber to measure the temperature and pressure parameters of the experimental process. The temperatures of the cell surface, jet zone and the wall of the sealed chamber were represented by T1, T2, T3, respectively. The internal pressure of the sealed chamber was indicated by the symbol P.

Fig. 1. Structure diagram of the sealed chamber. 2.2. Experimental method The cell SOC was 100% in this research, which was the most dangerous operating condition. The cell was fully charged before the test under standard conditions. The experiment consisted of the following steps: a) The cell underwent an open circuit voltage check, charged up to the selected SOC, placed on top of the thermal insulation board, and fixed. b) The heater was placed under the thermal insulation board. c) The sealed reactor was evacuated and flushed with N2. d) The thermal runaway was initiated by turning on the heater. The heating power of the heater was 5 kW. The heater heated the cell by heating the gas inside the reactor, to make the cell wall as evenly heated as possible. The cell surface temperature rate of increase was approximately 3.7

·min-1 before the vent. The cell

transited into thermal runaway and emissions were released as the safety valve opened. The cell VG emissions were sampled at 185 , which was the average temperature of T2 and T3, since the pressure and temperature in the reactor had been stable for a while after thermal runaway. The VG emissions sampled were reheated to 90

before detection. A gas chromatography system (Agilent Technologies Inc, America;

7

Agilent 7890A) was used to detected CO, CO2, H2, and hydrocarbon in the VG sample. A gas chromatography-mass spectrometer (Agilent Technologies Inc, America; Agilent 7890B-5977A) was used to detected dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC) in the VG sample. An ion chromatograph (Metrohm, Switzerland; 930 Compact) was used to detect hydrogen chloride (HCl) in the VG sample. The cell settleable VS emissions in the chamber were collected when the temperature in the chamber dropped to room temperature after the thermal runaway. A million-point electronic balance (Sartorius, Germany; BSA224S) and several sample sieves with different hole diameters were used to detect mass. A Malvern particle size analyzer (Malvern Panalytical, United Kingdom; Hydro2000MU) was used to analyze the particle size. An element analyzer (Elementar Analysensysteme, Germany; Vario EL cube) was used to detected carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S). An ion chromatograph (Dionex, America; ICS-2000) was used to detected halogen elements including fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). An inductively coupled plasma mass spectrometer (Agilent Technologies Inc., America; Agilent-7900) was used to detected phosphorus (P), silicon (Si), boron (B), arsenic (As), mercury (Hg), and all the non-radioactive metallic elements in the periodic table. 3. Calculation parameter The molar amount of VG emissions released inside the chamber was calculated by applying the ideal gas law: n=

−

(1)

where n is the molar amount of VG emissions, p is the recorded pressure in the chamber, V is the chamber volume, R is the gas constant, Tenv is the environment temperature in the chamber, and n0 is the initial amount of gas, i.e. N2, in the chamber at the start of the experiment. Tenv was calculated by the following equation: T

=

(2)

where T2 is the recorded temperature in the injection zone of a cell, and T3 is the recorded temperature near the chamber wall. The quantitative ratio (QR) of VG emissions was defined by the ratio of quantitative molar amount nq to the total amount of the VG emissions, as shown as follows: =

× 100%

(3)

where nq is the quantitative molar amount of the VG emissions, and ntotal is the total amount of the VG emissions. 8

The total amount of VG emissions was replaced by its stable amount as follows: × 100%

≈ "

(4)

#

where nstable is the stable amount of VG emissions in the chamber. The molar amount of VG composition was determined by the ratio of its molar concentration to N2 molar concentration detected by GC as follows: $

=

% &'

&%

(5)

where ni is the molar amount of a certain composition i of VG emissions, xi is the concentration of composition i detected by GC, and x0 is the N2 molar concentration detected by GC. 4. Results and discussion 4.1. Thermal runaway characteristics Fig. 2 shows the variation in the cell surface temperature and its rate of increase during thermal runaway. The time at which the safety valve is opened is taken as 0 min in the horizontal ordinate. As time progresses, the cell surface temperature increases gradually, at an average rate of increase of approximately 3.7°C·min-1, as shown in Fig. 2(a), with a maximum rate of increase of approximately 18.0°C·min-1, as shown in Fig. 2(b). It then begins to rise rapidly at a cell surface temperature of 167.2°C as the safety valve is released after being heated for 44.60 minutes, and finally decreases. During this process, the cell surface temperature reaches its maximum rate of increase of temperature of 930°C·min-1 at 204.6°C within 0.13 minutes (8 seconds) since the safety valve is released. It reaches its peak of 437.6°C within 2.67 minutes (163 seconds) since the safety valve is released.

9

a

550

Cell surface temperature (℃ ℃)

500 450 400 350 300 250 200 150 100 50 0 -45 -40 -35 -30 -25 -20 -15 -10 -5 Time (min)

0

5

10 15 20 25

Cell surface temperature rise rate (℃ ℃·min-1)

b 1000

100

10

1 0

100

200 300 Temperature (℃ ℃)

400

500

Fig. 2. Variation in cell surface temperature vs. time (a) and variation in rate of increase of cell surface temperature vs. cell surface temperature (b). 4.2 Gas emissions In order to obtain the amount of VG emissions, jet zone temperature T2, temperature T3 near the sealed-reactor wall, and the internal pressure of the sealed reactor were detected and the ideal gas state equation was used, as 10

shown in Eq. 1. The pressure inside the sealed reactor was relatively uniform, but the temperature was stratified. Hence, the average value of jet zone temperature T2, and temperature T3 near the sealed-reactor wall was taken as the average temperature, i.e. environment temperature Tenv inside the reactor during the calculation. The calculation results show that, with the cell vent and thermal runaway, the molar amount curve of the VG emissions attain two peaks before finally stabilizing at 3.83 mol, as shown in Fig. 3. Since the internal temperature of the sealed reactor is severely stratified as the cell is venting, the error of the molar amount calculated before the internal temperature of the sealed reactor is relatively large. Therefore, the two peak values of the VG molar amount around the time of venting are incredible. In contrast, the calculated VG molar amount becomes accurate after the temperature and pressure data in the sealed reactor tends to stabilize. Therefore, in this research, the VG molar amount that does not change with time after the end of the vent and thermal runaway is defined as a stable amount, i.e. nstable_ideal. The corresponding environment temperature in the sealed reactor is defined as the stable value of VG emissions, i.e. Tstable, as shown in Fig. 3. It should be noted that, in the VG emissions, during the period from the start of the cell vent to the time at which the molar amount tends to stabilize, some physical and chemical reactions such as liquefaction and decomposition will occur inevitably. The specific reactions are unclear and should be studied further.

7 T₁

450

T₂

T₃

Tenv

P

n 6

400 5

350 nstable_ideal=3.83 mol

300

4

250 3

200 150

2

Tstable=Tsampling=185℃ ℃

VG molar amount (mol)

℃) and pressure (kPa) Temperature (℃

500

100 1

50 0

0 -4

-2

0

2

4 6 Time (min)

8

10

12

14

Fig. 3. Variation in temperature, pressure and VG molar amount vs. time. Table 2 shows the identification results of the collected VG emissions. The Tsampling, i.e. the average temperature 11

inside the sealed chamber at the time of gas sampling, is 185°C, as shown in Fig. 3. The gas detection temperature (Tdetection), i.e. the temperature to be increased when the gas flowed from the sampling device to the detection device, is 90°C. The results reveal that there are 31 substances found in the VG emissions collected, mainly including non-hydrocarbons, alkanes, alkenes, alkynes, aromatic hydrocarbons, etc. There are 3 components in the non-hydrocarbons, including CO2, CO, H2. There are 5 components in the alkanes, including CH4, C2H6, C3H8, n-butane (C4H10), and n-pentane (C5H12). There are 13 components in the alkenes, including C2H4, C3H6, 1-butylene (C4H8), 2-methyl propene (C4H8), trans-2-butene (C4H8), cis-2-butene (C4H8), 1-pentene (C5H10), cis-2-pentene (C5H10), trans-2-pentene (C5H10), 2-methyl-1-butene (C5H10), 2-methyl-2-butene (C5H10), 3-methyl-1-butene (C5H10), 2-methyl-1-pentene (C6H12). These substances found also include dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), water vapor (H2O), hydrogen chloride (HCl), etc. It should be noted that DMC, DEC and EMC are the main components of the electrolytes. Table 2 Compositions of vent gases and their thermal characteristics. Category

Num.

Name

Formula

-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Carbon dioxide Carbon monoxide Hydrogen Methane Ethane Propane n-Butane n-Pentane Ethylene Propylene 1-Butylene 2-Methyl propene Trans-2-Butene Cis-2-Butene 1-Pentene Cis-2-Pentene Trans-2-Pentene 2-Methyl-1-Butene 2-Methyl-2-Butene 3-Methyl-1-Butene 2-Methyl-1-pentene Ethyne Propyne 1,3-Butadiene Benzene

CO2 CO H2 CH4 C2H6 C3H8 C4H10 C5H12 C2H4 C3H6

-

Sum.

Non-HC

Alkane

Alkene

Alkyne Aromatic HC

Tboiling [38, 39]

nGC

Qualitative -

C6H12 C2H2 C3H4 C4H6 C6H6

-78 -192 -253 -161 -88 -42 -1 36 -104 -48 -6 -7 1 4 30 37 37 31 37 20 62 -28 -23 -5 80

mmol 875.66 1030.58 1147.91 147.17 16.53 2.96 0.81 1.08 173.38 39.78 4.97 5.91 0.67 0.40 1.48 0.27 0.54 0.94 0.13 0.27 1.34 0.94 8.47 6.72 2.02

-

-

3470.92

-

C4H8

C5H10

Yes

12

26 Electrolyte 27 28 29 Others 30 31

DMC EMC DEC 2,4-Dimethyl-1-Heptene Water vapor Hydrogen chloride

C3H6O3 C4H8O3 C5H10O3 C9H18 H O HCl

91 107 126 134 100 -

1.08 11.16 5.38 0.40 0.02 <0.13

No

In order to achieve quantitative identification of a certain substance, its Tboiling should not be greater than Tdetection, and the Tdetection should not be greater than Tsampling. If the Tdetection is lower than the Tboiling of a certain substance, the identification result cannot be quantitative because the substance is in the gas-liquid phase, but can be qualitative due to the presence of gas vapor. The Tsampling is available in many open literatures [14, 22, 25-27], but the Tdetection is not explicitly given, as shown in Table 4. This easily leads to the ambiguity in the VG identification results. Therefore, in order to accurately describe the VG identification results, the value of the maximum boiling point (Tboiling, max) of the VG compositions that can be quantitatively identified should be stated clearly. Combination of VG and its Tboiling, max is used to classify the VG emissions. To evaluate the VG identification degree, a quantitative degree (QD) is defined by the Tboiling, max of quantitative VG compositions according to substance type. In the present research, the Tboiling, max of the first 25 gas species is lower than the Tdetection of 90°C, so they can be quantified. That is, the QD is 90°C. This indicates that the total amount, the types of compositions, and their proportions of VG90 can be determined, as shown in Table 4. Fig. 4 shows the molar percentage of each composition in the VG90. The compositions with a percentage greater than 1% include H2, CO, CO2, C2H4, CH4, and C3H6, and the remaining compositions account for 1.63%. Gas generation and the individual compositions can be explained by thermal decomposition and reactions of electrolyte, binder, and electrode materials, as summarized in the open literatures [25-27]. Although the content of macromolecular substances in VG emissions are lower, they are more prone to self-ignition than small molecular substances due to their lower self-ignition points. Therefore, they may play important roles in causing battery fire. This requires special research, and is out of the scope of this study. Table 3 Sampling and detection temperatures in open literatures and the current research. Literature

Tsampling

Tdetection

V. Somandepalli, et al, 2014 [22] AW. Golubkov, et al, 2013 [25] AW. Golubkov, et al, 2015 [26] Y. Fernandesa, et al, 2018 [14] S. Kocha, et al, 2018 [27]

60 130 250 unavailable unavailable

Unavailable Unavailable Unavailable Unavailable Unavailable 13

Current research

185

90

Table 4 The quantitative identifications of VG90 and VG185. VG

VG90GC VG185ideal

Molar percentage in VG185

Composition type

Total molar amount

Molar percentage

% 90 100

Yes Yes

Yes Yes

Yes No

100%SOC, NCM, VG90

CO2, 25.23%

CH4, 4.24% C3H6, 1.15% C2H6, 0.48%

C2H4, 5.00% C4H8, 0.34% Others, 1.63% C4H4, 0.24%

CO, 29.69% H2, 33.07%

C4H6, 0.19% C5H10, 0.10%

C3H8, 0.09% C6H6, 0.06% C6H12, 0.04% C5H12, 0.03% C2H2, 0.03% C4H10, 0.02%

Fig. 4. Molar percentage of each composition in VG90. According to the above VG classification method, the molar percentages in the total amount and their cumulative mole percentage of VG compositions with Tboiling can be obtained, as shown in Fig. 5. The molar amount of a certain composition of VG emissions can be determined by its concentration ratio relative to N2 detected by GC, as shown in Eq. (5). However, the liquefaction of some substances, and the instability and stratification of pressures and temperatures in the sealed reactor make it difficult to determine the total molar amount. Therefore, the stable amount of VG emissions can be used to represent the total VG amount, as shown in Eq. 4. When the vent and thermal runaway processes complete, the average temperature of the gas inside the sealed reactor stabilizes at 185°C, i.e. Tstable = 185°C, as shown in Fig. 3. Therefore, the VG after being stable can be expressed as VG185. The stable molar amount of VG emissions, i.e. nVG185, can be obtained using Eq. 1. Seventeen, 8 and 6 types of substances are detected in VG25 (i.e. the vent gas, which exists in a gaseous state at room temperatures), VG25 to VG90, and 14

VG90 to VG185, respectively. This means that the boiling points of the VG compositions are widely distributed. The difference in the cumulative molar amount between VG25 and VG90 is only approximately 0.2%. It can be concluded that, among the VG emissions that could be quantified, the amount of substances that exist as liquids at room temperature is small. In addition, the difference in the cumulative molar amount between VG-30 and VG40 is lower than 1%. This means that the amount of VG emissions in the gaseous state does not change much from winter to summer. Since the Tsampling may be lower than the vent temperature or the Tstable, some substances cannot be quantitatively identified due to liquefactions, e.g. VG90 to VG185, as shown in Fig. 5. Therefore, a second indicator called quantitative ratio (QR) has been proposed in this paper, to evaluate the extent to which the vent gases can be identified quantitatively. QR is defined by the ratio of the quantitative amount to the total VG amount according to the amount of substance, as shown in Eq. 3. However, the real total VG amount is hard to be determined. So, it is replaced by the stable amount, as shown in Eq. 4. The results show that the QR, i.e. the ratio of nVG90 to nVG185, is 90.60%. In addition, the VG is sampled at Tstable. Therefore, the total molar amount and species of VG185 compositions can be determined, but the percentage of each composition cannot be quantified, as shown in Table 4. In summary, the VG emissions are quantitatively identified over a large range, and the number of components that can be quantitatively identified reaches 25. In addition, according to the boiling point, the VG compositions can be effectively classified, and the QD and QR can be used to effectively evaluate the extent to which LIB VG emissions can be identified quantitatively. The QD and QR are 90°C and 90.60%, respectively.

15

110

VG-30=88.9% Cumulative molar percentage in VG185

100

Molar percentage (%)

90

Molar percentage in VG185

80

VG90=89.9% VG185=100%

VG25=89.7% VG40=89.8%

70 60 50 40 30

H2

CO

20 CH4C2H4

10 0

C3H6/C3H8/C3H4 /C4H8/C4H8/C4H6 /C H /C H /C H CO2 /C4H10/C4H8 /C4 H8 5 10 5 10 5 10 /C5H12/C5H10/C5H10 /C5H10/C6H12/C6H6 C2H2

VG90 to VG85： DMC/EMC /DEC/C9H18 /H2O

C2H6

-10 -270

-210

-150

-90 -30 30 Boiling point (℃ ℃)

90

150

210

Fig. 5. Variation in mole percentage and cumulative mole percentage of VG compositions in VG185 vs. boiling points. 4.3 Solid emissions Fig. 6 shows the variation in mass of the collected settleable solid emissions, i.e. PM as a function of the equivalent particle size. The equivalent particle size is defined as the mean value of the largest size and the smallest size in a certain particle size interval. The particle size interval selected in Fig. 7 is 0.5 mm. The results show that the settleable PM emissions with particle size below 0.5 mm account for 90% of the PM mass.

16

105

90 Cumulative mass percentage

Mass percentage (%)

70

100 60 50 40

95

30 20 90 10

Cumulative mass percentage (%)

Mass percentage

80

0 -10

85 0

1

2

3

4 5 6 7 8 9 10 11 12 13 14 15 Equivalent particle size (mm)

Fig. 6. Variation in mass percentage of PM vs. equivalent particle size. Fig. 7 shows the particle size distribution of PM with the equivalent particle size below 0.5 mm. In particles with the particle size of less than 0.5 mm, D90, D50 and D10 are 390 µm, 195 µm and 19 µm, respectively. This means that 10%, 50% and 90% of the PM volume can be flited by pore sizes of 390 µm, 195 µm and 19 µm, so that they cannot be released to the outside world. This will reduce the probability of the high temperature particles and flammable VG from contacting the outside air at the same time, thereby improving the LIB fire safety. In addition, particle size analysis indicate that the collected PM has a minimum particle size of 2.9 µm. This indicates that there exists dust, e.g. PM10, in the PM emissions released by the LIBs. Therefore, it is necessary to take safety precautions when handling particulate matters to prevent them from entering the human respiratory system. Fig. 8 shows the elemental composition of the collected PM. A total of 30 elements are detected in the collected PM. They are arranged from big to small according to their mass percentage: carbon (C), nickel (Ni), oxygen (O), copper (Cu), aluminum (Al), cobalt (Co), manganese (Mn), lithium (Li), sulfur (S), chlorine (Cl), hydrogen (H), fluorine (F), potassium (K), phosphorus (P), iron (Fe), zirconium (Zr), strontium (Sr), sodium (Na), calcium (Ca), iodine (I), bromine (Br), titanium (Ti), chromium (Cr), barium (Ba), arsenic (As), vanadium (V), tin (Sn), zinc (Zn), magnesium (Mg) and antimony (Sb), as shown in Fig. 8. The mass percentages of the first nine elements greater than 0.5% are 28.0%, 20.8%, 12.3%, 9.5%, 9.4%, 7.0%, 6.3%, 3.6%, and 0.9%, respectively. The molar ratio of Ni: Co: Mn was 6: 2: 2, which is consistent with their relative molar ratio in the positive electrode materials. From the 17

point of view of the elemental composition of the PM, they contain a large amount of metal elements, which can be recycled. It should be noted that particles contain elements [40, 41] such as Al, Li, F, As, and Sn, which are potentially toxic to the human body, with mass percentages of 9.4%, 3.6%, 0.3%, 0.002% and 0.001%, respectively. They can be enriched thousands of times by biomagnification of the food chain before they finally enter the human body [40]. Therefore, the particles released by the LIB after the thermal runaway must be collected and treated to avoid entering the ecological environments such as water and soil.

100 Volume percentage

90

Cumulative volume percentage Volume percentage (%)

16

80

D50: 195 μm

70

D90: 390 μm

12

60 50

8

40

D10: 19 μm

30

4

20 10

0

Cumulative volume percentage (%)

20

0 0

100

200 300 Equivalent particle size (µm)

400

500

Fig. 7. Variation in volume percentage of PM with particle size below 0.5 mm vs. equivalent particle size.

18

a

100%SOC, NMC, Settleable PM 9.4675%, Cu 9.3793%, Al 6.9576%, Co 6.3429%, Mn 3.6400%, Li 0.9110%, S

12.3395%, O

0.4104%, Cl

0.0833%, Fe 0.0621%, Zr

0.4029%, H

0.0580%, Sr

0.3384%, F 2.1616%, Others 0.3263%, K

0.0576%, Na 0.0442%, Ca 0.0284%, I 0.0140%, Br 0.0106%, Ti 0.0035%, Cr 0.0015%, Ba

20.7536%, Ni

0.3145%, P 28.0484%, C

0.0014%, V 0.0013%, Sn 0.0008%, Zn 0.0007%, Mg 0.0001%, Sb

0.0015%, As

b

100% SOC, NMC, Settleable PM

57.2%, metallic element

42.8%, nonmetallic element

Fig. 8. Elemental composition of the collected PM 4.4 Composition of emissions The total mass of the cell emissions, i.e. the cell mass loss during the vent and thermal runaway process is 248.2 g, accounting for 28.53% of the cell mass. Fig. 9 shows mass percentages of the compositions of the LIB emissions VG25, VG25 to VG90, and settleable PM account for 32.52%, 0.23% and 17.00% of the emission mass, respectively. Seventeen compositions are identified in VG25, CO2, CO, H2, CH4, C2H6, C3H8, C4H10, C2H4, C3H6, 19

C4H8 (4 types), C5H10, C2H2, C3H4 and C4H6, as shown in Table 2 and Fig. 5. Eight compositions are identified in VG25 to VG90, C5H10 (6 types), C6H12 and C H , as shown in Table 2 and Fig. 5. Thirty elements are found in the settleable PM, C, Ni, O, Cu, Al, Co, Mn, Li, S, Cl, H, F, K, P, Fe, Zr, Sr, Na, Ca, I, Br, Ti, Cr, Ba, As, V, Sn, Zn, Mg and Sb, as shown in Fig. 8. The remaining 50.25% are mainly VG90+ (i.e. the gas with Tboiling above 90°C) and dust. Six substances are identified in VG90 to VG185, DMC, EMC, DEC, C9H18, H2O, and HCl. The dust should be composed of PM2.9 because the minimum size of the settleable PM collected is 2.9 µm.

100% SOC, NMC, battery vent emissions 17.00%, settleable PM, 30 elements

50.25%, VG90++dust, more than 6 compositons in VG90+, PM2.9, ect.

32.52%, VG25, 17 compositons 0.23%, VG25 to VG90, 8 compositions

Fig. 9. Mass percentage of the compositions of the LIB emissions 4.5 Significance of this research The academic and engineering significance of the research are as follows: 1)

Firstly, there was a lack of open literatures [27] reporting the VG emissions released by large capacity power batteries with Ni-rich cathodes. The battery capacities were usually below 4 Ah [14, 22, 25-27], and the Ni content was relatively low (no more than 50% [25]). A 50Ah commercial prismatic battery with Li(Ni0.6Mn0.2Co0.2)O2 cathode was used to research the VG emissions. In this paper a power battery specifically used in new energy vehicles is selected. Its capacity and nickel content reach 50 Ah and 60%, respectively.

2)

No more than 10 species of VG compositions were identified quantitatively in open literatures [14, 22, 25-27]. In the current research, the VG emissions are identified in a larger range. The number of quantitative compositions of VG emissions reaches 25, the quantitative degree is 90°C, and the quantitative ratio is 90.60%. 20

Although the amount of macromolecular substances is small, these substances tend to catch fire more easily and are more toxic. Therefore, this is of great significance for further comprehensive identification of LIB fire sources and assessment of vent gas hazards from the perspective of power battery safety and pollution performances. 3)

This research proposes a classification method and two evaluating parameters, including quantitative degree and quantitative ratio, for the vent gases of LIBs. These two evaluating parameters will give a more objective and accurate description of the gas identification results. This lays the foundation for further standardization of LIB gas emission research from the perspective of power battery safety performance.

4)

There are currently very few open literatures [37] reporting the characteristics of battery PM emissions. This study gives the mass distribution, particle-size distribution, and elemental composition of the settleable PM emissions. This gives an important reference value for suppressing the ignition sources of LIBs, such as high-temperature particles, from the perspective of power battery safety performance. In addition, the result can lay the foundation for proper handling of the LIB fire residue from the perspective of power battery pollution performance, and answer the question of “whether the LIB particulate emissions need to be recycled” from the perspective of power battery recyclability performance.

5)

The current research gives the composition percentage of battery emissions and the possible compositions. This reveals the characteristics of emissions released by LIBs to a satisfactory degree, and further enhances the understanding of LIB emissions.

5 Conclusions A 50-Ah commercial prismatic cell with (Ni0.6Mn0.2Co0.2)O2 as the cathode was used to reveal the characteristics of emissions released by lithium-ion batteries during the vent and thermal runaway process. The main conclusions are summarized as follows: (1) When the heater was turned on, the cell surface temperature increased slowly, and then began to rise rapidly at 167.2°C as the safety valve was opened after being heated for 44.60 minutes, and finally decreased. During this process, the peak value of the cell surface temperature and the rates of increase of the maximum temperature were 437.6°C and 930°C·min-1, respectively. (2) The boiling point could be used to effectively classify the vent gas compositions of the LIBs. There were 31 substances found in VG185, and 25 of them could be wholly quantified. In VG25, VG25 to VG90, and VG90 21

to VG185, 17, 8, and 6 substances were detected, respectively. In VG90, compositions with a percentage greater than 1% included H2, CO, CO2, C2H4, CH4, and C3H6, and the remaining compositions accounted for 1.63%. The quantitative degree and quantitative ratio could be used to evaluate the extent to which the VG could be quantitatively identified, and their values were 90

and 90.60% in this research, respectively.

(3) The settleable PM emissions with particle size below 0.5 mm accounted for 90% of the PM mass. In settleable PM with the particle size of less than 0.5 mm, D90, D50 and D10 were 390 µm, 195 µm and 19 µm, respectively. There were 30 elements found in those PM. The other solid emissions, i.e. dust, were supposed to be composed of PM2.9 due to that the minimum size of the settleable PM collected was 2.9 µm. (4) The total emission accounted for 28.53% of the cell mass. VG25, VG25 to VG90, and the settleable PM accounted for 32.52%, 0.23% and 17.00% of the emission mass, respectively. The remaining 50.25% were VG90+ and dust. Acknowledgements This research was supported by the Ministry of Science and Technology of the People's Republic of China under the Grant No. 2019YFE010186, the China Postdoctoral Science Foundation under the Grant No. 2018M631464, and the National Natural Science Foundation of China under the Grant No. U1564205. References:

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[18] Y. Chen, Investigation on LiCo1/3Ni1/3Mn1/3O2 cathode material and safety of lithium-ion battery, Tianjin: Tianjin University, 2006. [19] H. Chen, Z. Tang, X. Lu, Research of explosion mechanism of lithium-ion battery, Prog. Chem. 18 (2006) 823-831. [20] X. Feng, M. Ouyang, X. Liu, L. Lu, Y. Xia, X. He, Thermal runaway mechanism of lithium ion battery for electric vehicles: A review, Energy Storage Mater. 10 (2017) 246-267. [21] B. Long, R. Xu, Y. Liu, Gas-flammability testing for Li-ion cells during abusing, Battery 44 (2014) 121-123. [22] V. Somandepalli, K. Marr, Q. Horn, Quantification of combustion hazards of thermal runaway failures in lithium-ion batteries, SAE Int. J. Alt. Power. 3 (2014) 98-104. [23] Q.Q. Liu, D.J. Xiong, R. Petibon, C.Y. Du, J.R. Dahn, Gas evolution during unwanted lithium plating in Li-ion cells with EC-based or EC-free electrolytes, J. Electrochem. Soc. 163 (2016) A3010-A3015. [24] F. Larsson, S. Bertilsson, M. Furlani, I. Albinsson, B.E. Mellander, Gas explosions and thermal runaways during external heating abuse of commercial lithium-ion graphite-LiCoO2 cells at different levels of ageing, J. Power Sources 373 (2018) 220-231. [25] A.W. Golubkov, D, Fuchs, J. Wagner, H. Wiltsche, C. Stangl, G. Fauler, G. Voitic, A. Thaler, V. Hacker, Thermal-runaway experiments on consumer Li-ion batteries with metal-oxide and olivin-type cathodes, RSC Adv. 4 (2013) 3633-3642. [26] A.W. Golubkov, S. Scheikl, R. Planteu, G. Voitic, H. Wiltsche, C. Stangl, G. Fauler d, A. Thaler, V. Hacker, Thermal runaway of commercial 18650 Li-ion batteries with LFP and NCA cathodes - Impact of state of charge and overcharge, RSC Adv. 5 (2015) 57171-57186. [27] S. Kocha, A. Fill, K. P. Birke, Comprehensive gas analysis on large scale automotive lithium-ion cells in thermal runaway, J. Power Sources 398 (2018) 106-112. [28] R. Spotnitz, J. Franklin, Abuse behavior of high-power, lithium-ion cells, J. Power Sources 113 (2003) 81-100. [29] M.N. Richard, J.R. Dahn, Accelerating rate calorimetry research on the thermal stability of lithium intercalated graphite in electrolyte. I. Experimental, Fuel Energ. Abs. 41 (1999) 2068-2077. 24

[30] H. Wang, A. Dang, K. Huang, Oxygen evolution in overcharged LixNi1/3Co1/3Mn1/3O2 electrode and its thermal analysis kinetics, Hubei Elec. Power 29 (2011) 1583-1588. [31] D.D. MacNeil, J.R. Dahn, The reaction of charged cathodes with nonaqueous solvents and electrolytes: I. Li0.5CoO2. J. Electrochem. Soc. 148 (2001) A1205-A1210. [32] Q. Wang, J. Sun, C. Chen, Thermal stability of delithiated with electrolyte for lithium-ion batteries, J. Electrochem. Soc. 154 (2007) A263-A267. [33] S.E. Sloop, J.K. Pugh, S. Wang, J.B. Kerr, K. Kinoshita, Chemical reactivity of PF5 and LiPF6 in ethylene carbonate/dimethyl carbonate solutions, Electrochem. Solid St. 4 (2001) 357-364. [34] A.D. Pasquier, F. Disma, T. Bowmer, A.S. Gozdz, G. Amatucci, J.M. Tarascon, Differential scanning calorimetry research of the reactivity of carbon anodes in plastic Li-ion batteries, J. Electrochem. Soc. 145 (1998) 472-477. [35] Chen M, Yuen R, Wang J. An experimental research about the effect of arrangement on the fire behaviors of lithium-ion batteries. J. Therm. Anal. Calorim. 129 (2017) 181-188. [36] Zhang Q, Guo C, Qin S. Research on lithium-ion batteries explosive characteristics and aviation transportation safety. J. China Safety Sci. 26 (2016) 50-55. [37] Y. Zhang, H. Wang, W. Li, C. Li, Y. Ouyang, Size distribution and elemental composition of vent particles from abused prismatic Ni-rich automotive lithium-ion batteries. J. Energy Storage 26 (2019) 100991. [38] L. He, Combustion in Fire Fighting, Beijing: Chain Machine Press, 2018. [39] F. Cardarelli, Materials Handbook, Harbin: Harbin institute of technology press, 2014. [40] J.Y. Hu, J.Y, Dai, Advance in studies on human distribution and toxic effects of perfluoroalkyl and polyfluoroalkyl substances, Asian J. Ecotoxicol. 8 (2013) 650-657. [41] X. Yang, Trace elements and health, New observations on nutritional health (47th issue): Trace Elements and Health Album 1 (2017). Table captions Table 1 Detailed technical specifications of the test cell. Table 2 Compositions of vent gases and their thermal characteristics. Table 3 Sampling and detection temperatures in open literatures and the current research. 25

Table 4 The quantitative identifications of VG90 and VG185. Figure captions Fig. 1. Structure diagram of the sealed chamber. Fig. 2. Variation in cell surface temperature vs. time (a) and variation in rate of increase of cell surface temperature vs. cell surface temperature (b). Fig. 3. Variation in temperature, pressure and VG molar amount vs. time. Fig. 4. Molar percentage of each composition in VG90. Fig. 5. Variation in mole percentage and cumulative mole percentage of VG compositions in VG185 vs. boiling points. Fig. 6. Variation in mass percentage of PM vs. equivalent particle size. Fig. 7. Variation in volume percentage of PM with particle size below 0.5 mm vs. equivalent particle size. Fig. 8. Elemental composition of the collected PM. Fig. 9. Mass percentages of the compositions of the LIB emissions.

26

Highlights: The battery used herein has high capacity and Ni-rich cathode. Twenty-five gases were quantitatively identified. Quantitative degree and quantitative ratios were 90

and 90.60%, respectively.

Thirty elements were found in the settleable PM emissions. The battery mass loss’s 49.75% was quantitatively identified.

AUTHOR DECLARATION We wish to draw the attention of the Editor to the following facts which may be considered as potential conflicts of interest and to significant financial contributions to this work. Declarations of interest: none We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from [email protected]. Signed by all authors as follows: Yajun Zhang Hewu Wang Weifeng Li Cheng Li