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Hazardous characteristics of charge and discharge of lithium-ion batteries under adiabatic environment and hot environment
International Journal of Heat and Mass Transfer 141 (2019) 419–431
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Hazardous characteristics of charge and discharge of lithium-ion batteries under adiabatic environment and hot environment Tianfeng Gao, Zhirong Wang ⇑, Shichen Chen, Linsheng Guo Nanjing Tech University, Nanjing 211816, China
a r t i c l e
i n f o
Article history: Received 5 March 2019 Received in revised form 23 May 2019 Accepted 21 June 2019
Keywords: Thermal runaway Charge and discharge Jet fire Electric heating Thermal insulation Gas leakage
a b s t r a c t The 18,650 lithium-ion battery was charged and discharged at different rates, first in an environment with heat source, and then in one with heat-insulation, to study the effects of different thermal environments on the thermal behavior of lithium batteries. In the case of charging and discharging in adiabatic environments, the battery did not have thermal runaway, and the maximum temperature and temperature rise rate increased with the increase of the charging and discharging rates. During the 4 C discharge process, the resistance increased due to the action of the Positive Temperature Coefficient (PTC) element, and the loop current decreased, which made the highest temperature lower than that in the 3 C discharge process. In the presence of an external heat source, when charged and discharged at different rates, the battery was thermally out of control. The thermal runaway behavior of the battery during charging was a large amount of gas leakage. At the time of discharge, due to the high initial State of Charge (SOC), at the 0.5 C, 1 C, 2 C rates, there was thermal runaway following a large amount of gas leakage. The same phenomenon also occurred at the 3 C, 4 C rates, but without continuous combustion. In the environment of external heat source, results showed that the thermal runaway heat release was always greater in the charge process than in the discharge process. With the increase of the charge and discharge currents, the battery power before and after the thermal runaway remained almost the same. Therefore, the ratio of the thermal runaway heat release in the charge process to that in the discharge process kept increasing and finally approached 1. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction During charging and discharging of lithium-ion batteries, electrochemical reaction heat, polarization heat and Joule heat are continuously generated due to internal electrochemical reactions [1–3]. With the charging and discharging, the heat accumulates and the battery temperature rises steadily, which eventually leads to the occurrence of thermal runaway, which may cause a fire or an explosion. In recent years, many scholars have studied the thermal behavior of lithium-ion batteries during charging and discharging. Guo et al. [4] studied the thermal behavior of lithium-ion batteries under different ambient temperatures and heat dissipation conditions. It was found that maintaining good heat dissipation conditions during the charge and discharge process could reduce the heat generation of the battery and provide an important basis for its safe design. Few studies, however, have been reported on the changes in battery voltage and current during charging and discharging, and on the characteristics of thermal runaway behavior. ⇑ Corresponding author. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.ijheatmasstransfer.2019.06.075 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
Song et al. [5,6] established a micro-calorimetric platform to study the thermal behavior of LiFePO4 batteries at different ambient temperatures and discharge rates. Their research showed that the total thermal behavior of lithium-ion battery during charging and discharging was exothermic, and the heat generation increased with the increase of charging and discharging current. Srinivasan et al. [7] used a two-dimensional model of electrochemical and thermal behavior to study the thermal behavior of lithium-ion batteries. It was found that entropy changes had a greater impact on the heat of the battery at different magnifications. Jhu et al. [8] used the VSP2 adiabatic calorimeter to study the thermal runaway of the 18,650 lithium-ion battery, and determined the thermodynamic parameters of the LiCoO2 battery reaction using the Arrhenius model. The 18,650 lithium-ion battery is a cylindrical lithium-ion battery with a diameter of 18 mm and a length of 65 mm. Jeon et al. [9] studied the thermal behavior of lithium-ion batteries during cyclic charge and discharge, and found that at low discharge rates, the heat generated by entropy occupied the main part, while at high discharge rates, Joule heat occupied the main part. The above scholars mainly studied the energy changes during the charge and discharge of lithium batteries. These studies provide a theoretical
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Nomenclature Q C T m n Rate
generation of heat [J] specific heat capacity [J/(g°C)] temperature [°C or K] quality [g] ratio of heat release rate of charge or discharge [C]
Subscripts in in r runaway cu copper b battery loss loss c charge
basis for the calculation of calorific value of battery during thermal runaway. But they didn’t make studies on energy release during thermal runaway due to charging and discharging of battery. Liu et al. [10] studied the variation of battery temperature during battery charging with different currents. They found that the thermal runaway initial temperature decreased with the increase of charging current. Wang et al. [11] studied the variation of lithium-ion battery temperature during constant current, constant voltage charging and constant current discharge in adiabatic environment. The cycle magnifications of the experiments were 0.5 C, 1 C and 1.5 C. C-rate is the measurement of the charge and discharge current with respect to its nominal capacity. However, the selected charge and discharge rates are relatively small, so the research has certain limitations. Kim et al. [12] simulated the temperature distribution and heat generation behavior of lithium-ion battery during Direct Current (DC) constant voltage charging by finite element method. Gerver et al. [13] used two-dimensional electrochemical model and three-dimensional thermal model to simulate the changes of voltage, current and temperature during charging of the battery, and obtained the distribution function of current density, potential and temperature of lithium-ion battery. Wu et al. [14] used a two-dimensional transient model with different heat dissipation methods to simulate the temperature distribution of a lithium-ion battery. It was found that forced convection could effectively slow down the temperature rises of the battery compared with natural convection. Kim et al. [15] studied the effects of electrode structure on the thermal behavior of lithium polymer battery by simulation method, and calculated the temperature distribution of lithium battery based on potential and current density distribution. Cheng et al. [16] built a finite element thermal model of 18,650 Li-ion batteries for its thermal characteristics simulation. They found that cell elements such as the steel case, the electrolyte, and the relief valve have larger impacts on the internal temperature distribution than the surface temperature during charge. Panchal et al. [17,18] examined the heat and mass transfer (temperature and mass flow rate of water) field as well as voltage profiles for a 20 Ah Graphite/LiFePO4 LIB pack at low currents of 20 A (1 C) and 40 A (2 C) with the selected ambient conditions using unique water-cooling methods with 35 °C, 25 °C, 15 °C, and 5 °C. They found that increasing discharge currents and ambient conditions result in an increased surface temperature at 3 spots; close to the ve electrode, close to the +ve electrode, and near the middle part of the LIB cell. Feng et al. [19] established a thermal analysis database. Three characteristic temperatures are summarized based on the common features of the cells in the database. They found that internal short circuits are responsible for very little of the total heat generated during thermal runaway. Yang
d
discharge
Superscripts °C degree centigrade Acronyms AC alternating current DC direct current LiCoO2 lithium cobalt oxide LiFePO4 lithium iron phosphate PTC positive temperature coefficient SEI solid electrolyte interphase SOC state of charge VSP2 vegetative storage protein 2
et al. [20] studied the unbalanced discharge and aging caused by the temperature difference of the parallel battery. It was found that the temperature difference between the parallel batteries greatly aggravated the discharge imbalance between the batteries, and accelerated the loss of battery capacity. The capacity loss rate of the parallel battery pack increased linearly with the increase of the temperature difference, and this trend increased with the rise of temperature. Lee et al. [21] established a thermal runaway model for lithium-ion batteries and proposed a method for delaying thermal runaway of lithium-ion batteries. Panchal et al. [22] used mathematical models to predict the transient temperature and voltage distribution of lithium-ion batteries at different discharge rates, and conducted air-cooling tests. It was found that the surface temperature of the battery increased with the increase of discharge rate. Li et al. [23] used infrared imaging technology to study the temperature distribution of lithium batteries under natural convection conditions. It was found that the heat release rate of the positive electrode was higher than that of the negative electrode, and the temperature distribution became increasingly uneven with the increase of discharge rate and discharge amount. The above scholars studied the temperature change law in the process of battery runaway. However, they didn’t make studies on the hazardous characteristics of charge and discharge of lithium-ion batteries under adiabatic environment and hot environment. Previous studies were focused only on the battery itself. The experimental conditions were relatively simple, and the effects of hot environment on heat production were not compared. In this paper, the thermal behavior of 18,650 lithium-ion battery was studied during charging and discharging in adiabatic environment and that with external heat source. The heat generation law of lithium-ion battery during charging and discharging in different thermal environments was compared. Wherein, the hot environment was used to simulate the thermal behavior of other batteries when the single battery in the battery pack was thermally out of control. The adiabatic condition where the battery was selfheated during charging and discharging was used to simulate the thermal behavior of the battery under heat-insulated conditions (such as in an electric vehicle).
2. Materials and methods 2.1. Experimental device The experimental device mainly includes a LAND chargedischarge testing device, a heat preservation device and an electric heating device.
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(1) The LAND charge-discharge testing device The LAND (Wuhan LAND Electronic Co. Ltd.) charge-discharge testing device (LANHE CT2001B) can, by detecting the voltage and current changes of the battery during charging and discharging, generate a voltage and current curve. The device has 8 channels, each of which can be independently charged and discharged in any charge and discharge mode. The physical map is shown in Fig. 1.
temperature measured by the thermocouple is considered to be temperature of the battery. During the battery cycle charging and discharging process, the thermocouple is directly attached to the outer surface of the battery with high temperature insulating tape. In the electric heating experiment, the electric resistance wire is evenly wound around the outer surface of the copper tube, and the two ends of the electric resistance wire are connected with a DC stabilized power supply, and the electric heating power can be controlled by controlling the voltage and current of the DC stabilized power supply.
(2) The heat preservation device (3) The electric heating device The heat preservation device is to charge and discharge the battery in an adiabatic environment, to prevent heat exchange between the inside and outside of the battery. The heat preservation device comprises a copper tube and a heat insulating cotton. The copper tube is wrapped with a ceramic fiber blanket with a thickness of 100 mm and placed in an iron container. The schematic diagram of the heat preservation device is shown in Fig. 2 (a) [24]. In the experiment, the thermocouple actually measures the temperature of the copper tube. Since the thermal conductivity of copper is very high, the temperature of the battery and the temperature of the copper tube can be taken to equal. Therefore, the
The electric heating device creates a high temperature environment that continuously heats up. It is composed of a DC regulated power supply and a resistance wire, and the electric heating power can be controlled by adjusting the voltage and current of the power supply, and the electric resistance wire is evenly wound around the outside of the copper pipe, thereby creating a high temperature environment. The technical parameters of the DC regulated power supply are shown in Table 1. The schematic diagram of the electric heating device is shown in Fig. 2 (b) [24]. (4) Battery used in the experiment The Samsung 18,650 lithium-ion battery was used in the experiment. Its technical specifications are shown in the Table 2. (5) Thermocouple used in the experiment OMEGA armored K-type thermocouple was used in the experiment. Its technical specifications are shown in the Table 3.
Table 1 Parameters of DC regulated power supply.
Fig. 1. Picture of LAND charge-discharge testing device.
Technical indicators
Parameter
Model Voltage adjustable range (V) Current adjustable range (A) Accuracy Input voltage
WYJ-5A30V 0–30 V 0–5 A ±1.5% Alternating Current (AC) 220 V ± 10%
Fig. 2. The heat preservation device and the electric heating device schematic.
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Table 2 The technical specifications of Samsung 18,650 lithium-ion battery. Technical indicators
Parameter
Model Nominal capacity Nominal voltage Anode material Cathode material Electrolyte material Quality
Samsung 18,650 lithium-ion battery 2600 mA h 4.2 ± 0.05 V LiCoO2 Graphite LiPF6 45 g
Table 3 The technical specifications of OMEGA armored K-type thermocouple. Technical indicators
Parameter
Model Specification Range Response time
TJ36-CAXL-116U-2 1.6 * 300 mm 50 to 1200 300 ms
where: Cb is the specific heat capacity of lithium-ion battery, the value of this paper is 0.85 J/(g°C); mb is expressed as the mass of the lithium-ion battery, g, calculated value 45 g; Ti+1-Ti represents the temperature difference of the lithium-ion battery at a certain time, °C. Calculation of heat loss Qloss of the system was performed by using 15 W electric heating for blank experiment. The battery that failed to be reacted was placed in the copper tube. When the blank test was performed, it can be seen from the 15 W heating curve that the temperature rose very slowly after the temperature exceeded 350 °C. The heat dissipation and heat generation reached equilibrium, that is, it can be considered that the electrical heating process is similar to the thermal behavior of a lithium-ion battery during the thermal insulation process. When the battery was out of control, the temperature was high and the temperature changed fast. So, the approximate calculation of Qin = Qloss is considered here. Take the electric heating process as an example. Since the thermal runaway process of the lithium-ion battery is complicated, the following assumptions are made before the thermal runaway calculation.
2.2. Experimental method (1) Environmental test method for external heat source Lithium-ion batteries of different SOC were electrically heated at 15 W, and charging and discharging experiments were performed to obtain data on battery temperature changes, to study the temperature variation characteristics of thermal runaway of different lithium-ion batteries in high temperature environments. (2) Experimental method for adiabatic environment The lithium-ion battery was placed in a copper tube, and the battery was charged and discharged with different charging and discharging power in an adiabatic environment to obtain voltage and current changes during charging and discharging of the lithium-ion battery.
(1) The occurrence of thermal runaway is shorter and the temperature rises faster. It is considered that the heat generated by 15 W electric heating is equal to the loss of heat. (2) The specific heat capacity of the lithium-ion battery does not change before and after the thermal runaway, and the specific heat capacity of the copper rod does not change with temperature variations. (3) A small amount of gas is released before the battery is out of control. Since the amount of gas released is small, it is assumed that the quality of the battery remains unchanged before the battery is out of control. Therefore, the heat calculation formula generated by the battery thermal runaway process is as shown in (4).
Q r ¼ C cu mcu ðT iþ1 T i Þ þ C b mb ðT iþ1 T i Þ
ð4Þ
2.3. Calculation model and method The heat generated by the thermal runaway process refers to the heat generated by the battery during the thermal runaway from the start to the end of the whole thermal runaway process. The main calculation is the thermal change of the thermal runaway process of the battery. The heat generated by the thermal runaway process system includes the heat Qr generated by the thermal runaway of the battery and the heat input to the system by the electric heating Qin. These two parts of energy mainly pass through the heat absorbed by the copper tube Qcu, the heat absorbed by the battery Qb, and the heat loss Qloss transmitted to the hot environment through heat radiation and heat conduction. The heat balance equation is shown in Eq. (1) below.
Q in þ Q r ¼ Q cu þ Q b þ Q loss
ð1Þ
The heat absorbed by the copper tube Qcu is calculated as shown in (2).
Q cu ¼ C cu mcu ðT iþ1 T i Þ
ð2Þ
where: Ccu is the specific heat capacity of copper. The value of this paper is 0.39 J/(g°C); mcu is the mass of copper tube, g, the calculated value is 133.5 g; Ti + 1-Ti is the temperature of the copper tube at a certain moment, °C. The heat absorbed by the battery Qb is calculated as shown in (3)
Q b ¼ C b mb ðT iþ1 T i Þ
ð3Þ
3. Results and analysis 3.1. Temperature change characteristics of lithium-ion battery charge and discharge in adiabatic environment 3.1.1. Temperature change characteristics of lithium-ion battery under different charging rates The calculation formula of C-rate is shown in (5).
C rate ¼ Charge or discharge current=Nominal capacity
ð5Þ
The LAND charge-discharge testing device was used to first charge the lithium-ion battery to 4.2 V at a constant current, and then charge it to 100 mA at a constant voltage of 4.2 V. Different charging rates at 0.5 C, 1 C, 2 C, 3 C, and 4 C were set for the constant current charging step. The surface temperature changes of the battery during charging are shown in Fig. 3. The maximum temperature and temperature rise rate of the battery during charging are shown in Fig. 4. The rate of temperature rise of the battery increases with the increase of the charge rate. During charging, the heat generated by the battery mainly includes reversible heat and irreversible heat. The reversible heat is the h eat generated by the electrochemical reaction, and the irreversible heat mainly includes the polarization heat and the Joule heat [1–3]. When the current passes through the impedance composed of the positive and negative electrodes of the battery, the electrolyte, and the separator, Joule
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Fig. 3. Temperature curve of battery charging at 2 C, 3 C and 4 C. Fig. 6. The highest temperature and temperature rise rate at different discharge rate.
Fig. 4. The highest temperature and temperature rise rate at different charge rate.
heat is continuously generated. As the charging rate increases, the rate of heat generation also increases, so the rate of heat generation of the battery increases [25]. 3.1.2. Temperature change characteristics of lithium-ion battery under different discharge rates Constant current discharge on a 100% SOC battery was performed with 0.5 C, 1 C, 2 C, 3 C, and 4 C in an adiabatic environment. The temperature change curves during discharge are shown in Fig. 5. The maximum temperature and temperature rise rate of the battery during discharge are shown in Fig. 6. Figs. 7– 11 show voltage and current changes during discharge at each rate. During the discharge process, the temperature rise rate of the lithium-ion battery increases as the discharge rate increases. The heat production during battery discharge mainly includes electrochemical reaction heat generation, polarization heat generation
and ohmic heat [1–3]. When the lithium-ion battery is discharged at a slow rate, the heat of reaction will occupy the main part. When the battery is discharged at a high rate, Joule heat occupies the main part [9]. From the voltage and current curve of the 0.5 C discharge process, the voltage of the battery dropped rapidly from 2.75 V to 0.50 V or lower, and the battery did not discharge with the set 0.5 C current. The discharge current rapidly decreased and the battery temperature began to decline. The similar phenomenon occurred in the 1 C and 2 C discharge processes. When it started to discharge at 3 C, the battery voltage suddenly decreased, and then there was a small increase. At 1020 s, the voltage dropped rapidly from 2.55 V to 0.5 V or lower. When discharged at 4 C, the battery voltage dropped to 1.69 V at the beginning of discharge, the discharge current dropped to 10 A, and then the voltage and current began to slowly decrease. At 1980 s, the voltage and current of the battery dropped dramatically. From the above phenomenon, when the battery voltage drops to about 2.7 V, the voltage will drop rapidly. The reason is that when the voltage drops to 2.7 V, the battery is in an overdischarge state. Combined with the temperature profile, it can be found that the battery has reached the maximum temperature. After that, the battery temperature begins to drop. When the battery is discharged at 4 C, since the current exceeds the critical value, the action of the PTC element inside the battery is triggered. The resistance of the PTC element is rapidly increased, resulting in a decrease in the current of the entire discharge circuit, and the Joule heat and polarization heat of the battery are also reduced [26]. Therefore, the maximum temperature of the 4 C discharge process is less than the maximum temperature of the 3 C discharge. It can be concluded from the above experiments that although the lithium-ion battery does not thermally lose control in an adiabatic environment, the battery temperature rises during charging or discharging. Therefore, when a lithium-ion battery is charged and discharged in a sealed or semi-closed space where heat dissipation is poor, the lithium-ion battery should be cooled in time by other means such as liquid cooling to dissipate the heat generated by the lithium-ion battery and to prevent the damage to the battery. 3.2. Temperature change characteristics of lithium-ion battery charge and discharge in hot environment
Fig. 5. Temperature curve of battery discharging at 2 C, 3 C and 4 C.
3.2.1. Effect of charging rate on thermal runaway of lithium-ion battery The LAND charge-discharge testing device was used to first charge the lithium-ion battery to 4.2 V at a constant current in
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Fig. 7. Voltage and current curve of battery discharging at 0.5 C.
Fig. 8. Voltage and current curve of battery discharging at 1 C.
Fig. 9. Voltage and current curve of battery discharging at 2 C.
the presence of an external heat source, and then charge it to 100 mA at a constant voltage of 4.2 V. Different charging rates at 0.5 C, 1 C, 2 C, 3 C, and 4 C were set for the constant current charging step. In the presence of an external heat source, when the lithium-ion battery was charged at different rates, the battery thermal runaway process was similar. At all charging rates, during the thermal runaway process, the battery only showed a large amount of deflation, and there was no jet fire or stable combustion. When charging at 0.5 C, the thermal runaway reaction of the battery was slower than the thermal runaway reaction of other magnifications, which is not so severe, and the deflation time and the amount of deflation
Fig. 10. Voltage and current curve of battery discharging at 3 C.
Fig. 11. Voltage and current curve of battery discharging at 4 C.
were relatively small. Therefore, when 0.5 C is charged, the maximum temperature because of thermal runaway of the battery is lower than the highest temperature of several other magnifications. Fig. 12 is a typical characteristic picture of the thermal runaway process of a lithium-ion battery during charging at different rates. In the early stage of thermal runaway, a small amount of white flue gas was slowly discharged from the positive electrode of the battery. After the heat was out of control, the battery instantaneously released a large amount of white smoke from the positive electrode, and the process of a large amount of deflation lasted for about 20 s. The leaking white smoke had an irritating odor. When the battery was heavily deflated, a small amount of gas was slowly released from the positive electrode. As the temperature of the battery further dropped, the venting gas slowly decreased and disappeared. The battery did not form a jet fire or stable combustion during charging, which was related to the thermal runaway reaction inside the battery. Due to the small capacity of the battery during the charging process, the internal reaction is relatively gentle, the temperature rise rate is not fast, and the generated pressure is not high. So, the gas and electrolyte substances discharged from the gas do not reach the ignition point and so are not ignited. As shown in Fig. 12 (a)–(e), when charging, the thermal runaway shows a large amount of deflation mode. This model can be summarized into the following three typical phases: (1) The stage of slow release of gas at the beginning of thermal runaway; (2) The stage of large-scale venting of gas in the event of thermal runaway; (3) The stage of a small amount of gas releasing phase at the end of thermal runaway.
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Fig. 12. Typical characteristic of battery thermal runaway at different charging rate.
The battery temperature change and temperature rise rate curves during charging are shown in Figs. 13 and 14. It can be seen from the figures that the thermal runaway onset temperature of the lithium-ion battery is closer. The maximum temperature rise rate increases with the increase of the charge rate. Between 1500 s and 2000 s (Fig. 14), the battery temperature rise rate had a negative value at which point the battery temperature dropped a little. The main reason is that when the temperature is between 160 °C and 180 °C, the battery safety valve is opened, and the gas is released and the heat of the battery is removed. The battery temperature then continued to rise. Before the thermal runaway occurred, the rise rate of the battery temperature increased with the increase of the charging rate. Due to the
action of the PTC element, the 4 C charging process also resulted in a maximum temperature which was less than that during 3 C charging process. The thermal runaway temperatures of the lithium-ion batteries were 337.3 °C, 404.9 °C, 433.1 °C, 436.1 °C, and 427.0 °C, respectively. Figs. 15–19 record the relationship between the voltage, current and SOC of the battery during charging of the lithium-ion battery. When charging at 1 C, 2 C, 3 C, 4 C rates, the battery directly enters the constant voltage charging step. As charging and heating continued, the battery failed and the current dropped rapidly. From the temperature curve, it can be seen that no matter what ratio the battery is charged, when the battery temperature reaches 140 °C to 150 °C, the battery will be ineffective and can no longer be charged.
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Fig. 13. Temperature variation curve of battery charging at different current under electrical heating condition. Fig. 16. Voltage and current curve of battery charging at 1 C under electrical heating condition.
Fig. 14. Temperature rise rate of battery charging at different current under electrical heating condition. Fig. 17. Voltage and current curve of battery charging at 2 C under electrical heating condition.
Fig. 15. Voltage and current curve of battery charging at 0.5 C under electrical heating condition. Fig. 18. Voltage and current curve of battery charging at 3 C under electrical heating condition.
This phenomenon may be due to the fact that the diaphragm material of the 18,650 lithium-ion battery used in the experiment is mainly composed of polyethylene and polypropylene. When the battery temperature reaches 130–165 °C, the melting point of the diaphragm is reached, and the battery fails [27]. Analysis from the final power of the battery shows that, when the 1 C, 2 C, 3 C, and 4 C are charged, the battery finally retains similar amounts of power. Therefore, the thermal runaway onset temperature is also similar to the thermal runaway end temperature of the battery. However, due to the increase of the charging rate, the heat generation rate of the battery increases, so the time for the battery
to reach the thermal runaway onset temperature is relatively short.
3.2.2. Effect of discharge rate on thermal runaway of lithium-ion battery The battery with full charge was discharged at 0.5 C, 1 C, 2 C, 3 C, 4 C rates with external heat source to study the law and danger of thermal runaway of lithium-ion battery under different discharge rates.
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Fig. 19. Voltage and current curve of battery charging at 4 C under electrical heating condition.
Under the electric heating condition, during the discharge process at different magnifications, the battery thermal runaway process also differs. When discharging at 0.5 C, the battery continued to deflate for a total of 9 s when it was out of control, and a small combustion flame appeared after the air was deflated, and the small flame burned for 10 s. When discharging at 1 C, the battery continued to be deflated for 4 s when it was out of control. After the deflation, a large combustion flame appeared and kept burning for 5 s before dying out. When discharging at 2 C, the battery was instantly deflated, and then a jet flame formed, which was violent due to intense reaction. When discharging at 3 C, the battery continued to deflate for a maximum of 15 s, and there was no burning flame after a large amount of deflation. When discharging at 4 C, the battery continued to deflate for 20 s. After the deflation was completed, the battery did not show a burning flame. Viewed from the thermal runaway process, when the battery is discharged at 0.5 C, 1 C, and 2 C, the burning flame of the battery appeared after the battery was deflated. When the battery was discharged at 3 C and 4 C, there was no combustion during the thermal runaway process. Therefore, the maximum temperature of the thermal runaway process at 0.5 C, 1 C, and 2 C discharges is higher than that at 3 C and 4 C discharges. Fig. 20 is a typical characteristic diagram of thermal runaway of a lithium-ion battery during discharge at various rates. Based on the analysis of the thermal runaway process at five different rates during discharge, the thermal runaway of the battery can be divided into two modes. One is a large amount of deflation mode and the other is a large amount of deflation and combustion mode. Both modes have two identical phases, (1) a small amount of slow deflation in the early stage of thermal runaway; (2) A large amount of deflation in an instant. At low rate discharge, after a large amount of gas venting, a short burning phase occurs. The temperature change curve, runaway temperature and mass change before and after the control of the battery during discharge are shown in Figs. 21–23. The battery showed runaway at all the five rates. The runaway onset temperature increased with the increase of the discharge rate, while the end temperature decreased. When the battery was out of control at 0.5 C, 1 C, 2 C discharges, there was jet fires, and the reaction was more intense. Thus, the battery lost more mass and the temperature was higher. Figs. 24–28 show the voltage and current curves of the battery during discharge. Fig. 29 shows the relationship between
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battery voltage and battery discharge when discharging at different rates. It is found from the voltage and current curves that the discharge was performed at a constant current, and the battery temperature kept rising due to the simultaneous electric heating. Certain temperatures would cause a sudden decrease of the discharge current and the battery voltage, and the battery was damaged and could not be discharged normally. Through the voltage and current curves and the temperature curve, it can be found that the battery failed at 80–130 °C. This is because at 80–120 °C, the SEI film undergoes a decomposition reaction, resulting in failure of the internal material of the battery. Therefore, when a single battery in the battery pack is out of control, the charging or discharging process should be immediately cut off by the interlocking device to prevent the thermal runaway accident from further expanding. 3.3. Heat release from the thermal runaway process of lithium-ion batteries In this section, the heat release from the out-of-control process is calculated based on the temperature change of this process to analyze the relationship between the heat release and the charge-discharge current. The thermal models and calculation methods associated with the calculations in this section have been mentioned in Section 2.3. Fig. 30 is a graph showing the thermal runaway heat generation of the battery at different charge and discharge rates. It can be seen from the figure that the heat released by the thermal runaway during the discharge process is higher than the heat released during the charging process. The heat released by the thermal runaway during the charging process tends to increase with the increase of the charging rate, but the heat release of the 4 C charging is slightly reduced. When the discharge is at 0.5 C, 1 C, 2 C, the heat released by thermal runaway is higher, and the heat released by thermal runaway tends to decrease as the discharge rate increases. The above results are also consistent with those of the experiments in Sections 3.1 and 3.2. A dimensionless parameter n is defined to compare the amount of heat released by thermal runaway during charging and discharging at the same rate. Its calculation formula is shown in (6).
n ¼ Q c =Q d
ð6Þ
Where, Qc represents the thermal runaway heat release during the charging process, and Qd represents the thermal runaway heat release during the discharge process. The calculation results of thermal runaway heat release during charge and discharge at the same rate are shown in Table 4. In the process of low magnification (0.5 C), the uncontrolled heat release in the charging process is small, and the uncontrolled heat release in the discharge process is large, and the dimensionless value n is small. With the increase of the magnification, the thermal runaway heat release in both the charging and discharging processes, and the value n tends to be 1. The reason may be that the initial battery charge is 0 before charging, so the energy contained in the battery is very small. On the other hand, the initial charge of the battery before discharging is 100%, so the battery contains more energy. During low-rate charge and discharge, the amount of change in electricity is small. Before the runaway, the electricity in the battery during charging is still low, while it is high when the battery is discharging. Therefore, there is less heat loss in the charging process, and the heat is
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Fig. 20. Typical characteristic of battery thermal runaway at different discharging rate.
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Fig. 21. Temperature variation curve of battery discharging at different current under electrical heating condition.
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Fig. 25. Voltage and current curve of battery discharging at 1 C under electrical heating condition.
Fig. 22. Initial and finish temperature of Thermal runaway at different discharging current.
Fig. 26. Voltage and current curve of battery discharging at 2 C under electrical heating condition.
Fig. 23. Quality loss of battery at different discharging current.
Fig. 27. Voltage and current curve of battery discharging at 3 C under electrical heating condition.
Fig. 24. Voltage and current curve of battery discharging at 0.5 C under electrical heating condition.
out of control during the discharge process. With the increase of the charge and discharge rates, there are greater changes in electricity. Before the loss of control, the electricity in the battery is similar during the charge process and the discharge process, so the heat release is also similar in the out-of-control process, and thus the value of n is close to 1.
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4. Conclusions
Fig. 28. Voltage and current curve of battery discharging at 4 C under electrical heating condition.
The 18,650 lithium-ion battery is difficult to generate thermal runaway when it is charged and discharged in an adiabatic environment. In this condition, the maximum temperature during charging and discharging increases with the increase of charge and discharge rate. At the same rate, the maximum temperature in the charging process is greater than that in the discharging process. When there is a external heat source, the battery will be thermal runaway during the charging and discharging process. During the 3 C and 4 C discharge processes, since a large amount of electricity has been consumed, only some gas is leaked during the thermal runaway process and there is no continuous combustion. At the same rate, the thermal runaway heat release during the charging process is always greater than that during discharging process. Therefore, when a single battery in the battery pack is thermally out of control due to excessive heat accumulation, the power supply should be cut off in time by the interlocking device, and the battery pack should be cooled. This will be an effective way to prevent thermal runaway of the entire battery pack, and hence accidents. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted Acknowledgements
Fig. 29. Relationship between voltage and capacity of battery discharging at different current.
The authors are grateful for the support given by National Natural Science Foundation of China under Grant No. 51874184, Key R & D programs (Social Development) in Jiangsu Province under Grant No. BE2016771, Key Natural Science Foundation in Jiangsu Province under Grant No. 18KJA620003, and Jiangsu Project Plan for Outstanding Talents Team in Six Research Fields (TD-XNYQC002). The authors also thank Associate Prof. Yang Hongqi with Nanjing Tech University for revising and proofreading this manuscript. References
Fig. 30. Heat production of battery thermal runaway process at different chargedischarge rate.
Table 4 Thermal runaway heat release of the battery at the same rate. Rate
Qc
Qd
n
0.5 C 1C 2C 3C 4C
3.19 kJ 9.86 kJ 12.65 kJ 12.78 kJ 11.75 kJ
26.17 kJ 27.96 kJ 27.29 kJ 22.26 kJ 12.39 kJ
0.122 0.353 0.464 0.574 0.948
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Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Hazardous characteristics of charge and discharge of lithium-ion batteries under adiabatic environment and hot environment Tianfeng Gao, Zhirong Wang ⇑, Shichen Chen, Linsheng Guo Nanjing Tech University, Nanjing 211816, China
a r t i c l e
i n f o
Article history: Received 5 March 2019 Received in revised form 23 May 2019 Accepted 21 June 2019
Keywords: Thermal runaway Charge and discharge Jet fire Electric heating Thermal insulation Gas leakage
a b s t r a c t The 18,650 lithium-ion battery was charged and discharged at different rates, first in an environment with heat source, and then in one with heat-insulation, to study the effects of different thermal environments on the thermal behavior of lithium batteries. In the case of charging and discharging in adiabatic environments, the battery did not have thermal runaway, and the maximum temperature and temperature rise rate increased with the increase of the charging and discharging rates. During the 4 C discharge process, the resistance increased due to the action of the Positive Temperature Coefficient (PTC) element, and the loop current decreased, which made the highest temperature lower than that in the 3 C discharge process. In the presence of an external heat source, when charged and discharged at different rates, the battery was thermally out of control. The thermal runaway behavior of the battery during charging was a large amount of gas leakage. At the time of discharge, due to the high initial State of Charge (SOC), at the 0.5 C, 1 C, 2 C rates, there was thermal runaway following a large amount of gas leakage. The same phenomenon also occurred at the 3 C, 4 C rates, but without continuous combustion. In the environment of external heat source, results showed that the thermal runaway heat release was always greater in the charge process than in the discharge process. With the increase of the charge and discharge currents, the battery power before and after the thermal runaway remained almost the same. Therefore, the ratio of the thermal runaway heat release in the charge process to that in the discharge process kept increasing and finally approached 1. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction During charging and discharging of lithium-ion batteries, electrochemical reaction heat, polarization heat and Joule heat are continuously generated due to internal electrochemical reactions [1–3]. With the charging and discharging, the heat accumulates and the battery temperature rises steadily, which eventually leads to the occurrence of thermal runaway, which may cause a fire or an explosion. In recent years, many scholars have studied the thermal behavior of lithium-ion batteries during charging and discharging. Guo et al. [4] studied the thermal behavior of lithium-ion batteries under different ambient temperatures and heat dissipation conditions. It was found that maintaining good heat dissipation conditions during the charge and discharge process could reduce the heat generation of the battery and provide an important basis for its safe design. Few studies, however, have been reported on the changes in battery voltage and current during charging and discharging, and on the characteristics of thermal runaway behavior. ⇑ Corresponding author. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.ijheatmasstransfer.2019.06.075 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
Song et al. [5,6] established a micro-calorimetric platform to study the thermal behavior of LiFePO4 batteries at different ambient temperatures and discharge rates. Their research showed that the total thermal behavior of lithium-ion battery during charging and discharging was exothermic, and the heat generation increased with the increase of charging and discharging current. Srinivasan et al. [7] used a two-dimensional model of electrochemical and thermal behavior to study the thermal behavior of lithium-ion batteries. It was found that entropy changes had a greater impact on the heat of the battery at different magnifications. Jhu et al. [8] used the VSP2 adiabatic calorimeter to study the thermal runaway of the 18,650 lithium-ion battery, and determined the thermodynamic parameters of the LiCoO2 battery reaction using the Arrhenius model. The 18,650 lithium-ion battery is a cylindrical lithium-ion battery with a diameter of 18 mm and a length of 65 mm. Jeon et al. [9] studied the thermal behavior of lithium-ion batteries during cyclic charge and discharge, and found that at low discharge rates, the heat generated by entropy occupied the main part, while at high discharge rates, Joule heat occupied the main part. The above scholars mainly studied the energy changes during the charge and discharge of lithium batteries. These studies provide a theoretical
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Nomenclature Q C T m n Rate
generation of heat [J] specific heat capacity [J/(g°C)] temperature [°C or K] quality [g] ratio of heat release rate of charge or discharge [C]
Subscripts in in r runaway cu copper b battery loss loss c charge
basis for the calculation of calorific value of battery during thermal runaway. But they didn’t make studies on energy release during thermal runaway due to charging and discharging of battery. Liu et al. [10] studied the variation of battery temperature during battery charging with different currents. They found that the thermal runaway initial temperature decreased with the increase of charging current. Wang et al. [11] studied the variation of lithium-ion battery temperature during constant current, constant voltage charging and constant current discharge in adiabatic environment. The cycle magnifications of the experiments were 0.5 C, 1 C and 1.5 C. C-rate is the measurement of the charge and discharge current with respect to its nominal capacity. However, the selected charge and discharge rates are relatively small, so the research has certain limitations. Kim et al. [12] simulated the temperature distribution and heat generation behavior of lithium-ion battery during Direct Current (DC) constant voltage charging by finite element method. Gerver et al. [13] used two-dimensional electrochemical model and three-dimensional thermal model to simulate the changes of voltage, current and temperature during charging of the battery, and obtained the distribution function of current density, potential and temperature of lithium-ion battery. Wu et al. [14] used a two-dimensional transient model with different heat dissipation methods to simulate the temperature distribution of a lithium-ion battery. It was found that forced convection could effectively slow down the temperature rises of the battery compared with natural convection. Kim et al. [15] studied the effects of electrode structure on the thermal behavior of lithium polymer battery by simulation method, and calculated the temperature distribution of lithium battery based on potential and current density distribution. Cheng et al. [16] built a finite element thermal model of 18,650 Li-ion batteries for its thermal characteristics simulation. They found that cell elements such as the steel case, the electrolyte, and the relief valve have larger impacts on the internal temperature distribution than the surface temperature during charge. Panchal et al. [17,18] examined the heat and mass transfer (temperature and mass flow rate of water) field as well as voltage profiles for a 20 Ah Graphite/LiFePO4 LIB pack at low currents of 20 A (1 C) and 40 A (2 C) with the selected ambient conditions using unique water-cooling methods with 35 °C, 25 °C, 15 °C, and 5 °C. They found that increasing discharge currents and ambient conditions result in an increased surface temperature at 3 spots; close to the ve electrode, close to the +ve electrode, and near the middle part of the LIB cell. Feng et al. [19] established a thermal analysis database. Three characteristic temperatures are summarized based on the common features of the cells in the database. They found that internal short circuits are responsible for very little of the total heat generated during thermal runaway. Yang
d
discharge
Superscripts °C degree centigrade Acronyms AC alternating current DC direct current LiCoO2 lithium cobalt oxide LiFePO4 lithium iron phosphate PTC positive temperature coefficient SEI solid electrolyte interphase SOC state of charge VSP2 vegetative storage protein 2
et al. [20] studied the unbalanced discharge and aging caused by the temperature difference of the parallel battery. It was found that the temperature difference between the parallel batteries greatly aggravated the discharge imbalance between the batteries, and accelerated the loss of battery capacity. The capacity loss rate of the parallel battery pack increased linearly with the increase of the temperature difference, and this trend increased with the rise of temperature. Lee et al. [21] established a thermal runaway model for lithium-ion batteries and proposed a method for delaying thermal runaway of lithium-ion batteries. Panchal et al. [22] used mathematical models to predict the transient temperature and voltage distribution of lithium-ion batteries at different discharge rates, and conducted air-cooling tests. It was found that the surface temperature of the battery increased with the increase of discharge rate. Li et al. [23] used infrared imaging technology to study the temperature distribution of lithium batteries under natural convection conditions. It was found that the heat release rate of the positive electrode was higher than that of the negative electrode, and the temperature distribution became increasingly uneven with the increase of discharge rate and discharge amount. The above scholars studied the temperature change law in the process of battery runaway. However, they didn’t make studies on the hazardous characteristics of charge and discharge of lithium-ion batteries under adiabatic environment and hot environment. Previous studies were focused only on the battery itself. The experimental conditions were relatively simple, and the effects of hot environment on heat production were not compared. In this paper, the thermal behavior of 18,650 lithium-ion battery was studied during charging and discharging in adiabatic environment and that with external heat source. The heat generation law of lithium-ion battery during charging and discharging in different thermal environments was compared. Wherein, the hot environment was used to simulate the thermal behavior of other batteries when the single battery in the battery pack was thermally out of control. The adiabatic condition where the battery was selfheated during charging and discharging was used to simulate the thermal behavior of the battery under heat-insulated conditions (such as in an electric vehicle).
2. Materials and methods 2.1. Experimental device The experimental device mainly includes a LAND chargedischarge testing device, a heat preservation device and an electric heating device.
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(1) The LAND charge-discharge testing device The LAND (Wuhan LAND Electronic Co. Ltd.) charge-discharge testing device (LANHE CT2001B) can, by detecting the voltage and current changes of the battery during charging and discharging, generate a voltage and current curve. The device has 8 channels, each of which can be independently charged and discharged in any charge and discharge mode. The physical map is shown in Fig. 1.
temperature measured by the thermocouple is considered to be temperature of the battery. During the battery cycle charging and discharging process, the thermocouple is directly attached to the outer surface of the battery with high temperature insulating tape. In the electric heating experiment, the electric resistance wire is evenly wound around the outer surface of the copper tube, and the two ends of the electric resistance wire are connected with a DC stabilized power supply, and the electric heating power can be controlled by controlling the voltage and current of the DC stabilized power supply.
(2) The heat preservation device (3) The electric heating device The heat preservation device is to charge and discharge the battery in an adiabatic environment, to prevent heat exchange between the inside and outside of the battery. The heat preservation device comprises a copper tube and a heat insulating cotton. The copper tube is wrapped with a ceramic fiber blanket with a thickness of 100 mm and placed in an iron container. The schematic diagram of the heat preservation device is shown in Fig. 2 (a) [24]. In the experiment, the thermocouple actually measures the temperature of the copper tube. Since the thermal conductivity of copper is very high, the temperature of the battery and the temperature of the copper tube can be taken to equal. Therefore, the
The electric heating device creates a high temperature environment that continuously heats up. It is composed of a DC regulated power supply and a resistance wire, and the electric heating power can be controlled by adjusting the voltage and current of the power supply, and the electric resistance wire is evenly wound around the outside of the copper pipe, thereby creating a high temperature environment. The technical parameters of the DC regulated power supply are shown in Table 1. The schematic diagram of the electric heating device is shown in Fig. 2 (b) [24]. (4) Battery used in the experiment The Samsung 18,650 lithium-ion battery was used in the experiment. Its technical specifications are shown in the Table 2. (5) Thermocouple used in the experiment OMEGA armored K-type thermocouple was used in the experiment. Its technical specifications are shown in the Table 3.
Table 1 Parameters of DC regulated power supply.
Fig. 1. Picture of LAND charge-discharge testing device.
Technical indicators
Parameter
Model Voltage adjustable range (V) Current adjustable range (A) Accuracy Input voltage
WYJ-5A30V 0–30 V 0–5 A ±1.5% Alternating Current (AC) 220 V ± 10%
Fig. 2. The heat preservation device and the electric heating device schematic.
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Table 2 The technical specifications of Samsung 18,650 lithium-ion battery. Technical indicators
Parameter
Model Nominal capacity Nominal voltage Anode material Cathode material Electrolyte material Quality
Samsung 18,650 lithium-ion battery 2600 mA h 4.2 ± 0.05 V LiCoO2 Graphite LiPF6 45 g
Table 3 The technical specifications of OMEGA armored K-type thermocouple. Technical indicators
Parameter
Model Specification Range Response time
TJ36-CAXL-116U-2 1.6 * 300 mm 50 to 1200 300 ms
where: Cb is the specific heat capacity of lithium-ion battery, the value of this paper is 0.85 J/(g°C); mb is expressed as the mass of the lithium-ion battery, g, calculated value 45 g; Ti+1-Ti represents the temperature difference of the lithium-ion battery at a certain time, °C. Calculation of heat loss Qloss of the system was performed by using 15 W electric heating for blank experiment. The battery that failed to be reacted was placed in the copper tube. When the blank test was performed, it can be seen from the 15 W heating curve that the temperature rose very slowly after the temperature exceeded 350 °C. The heat dissipation and heat generation reached equilibrium, that is, it can be considered that the electrical heating process is similar to the thermal behavior of a lithium-ion battery during the thermal insulation process. When the battery was out of control, the temperature was high and the temperature changed fast. So, the approximate calculation of Qin = Qloss is considered here. Take the electric heating process as an example. Since the thermal runaway process of the lithium-ion battery is complicated, the following assumptions are made before the thermal runaway calculation.
2.2. Experimental method (1) Environmental test method for external heat source Lithium-ion batteries of different SOC were electrically heated at 15 W, and charging and discharging experiments were performed to obtain data on battery temperature changes, to study the temperature variation characteristics of thermal runaway of different lithium-ion batteries in high temperature environments. (2) Experimental method for adiabatic environment The lithium-ion battery was placed in a copper tube, and the battery was charged and discharged with different charging and discharging power in an adiabatic environment to obtain voltage and current changes during charging and discharging of the lithium-ion battery.
(1) The occurrence of thermal runaway is shorter and the temperature rises faster. It is considered that the heat generated by 15 W electric heating is equal to the loss of heat. (2) The specific heat capacity of the lithium-ion battery does not change before and after the thermal runaway, and the specific heat capacity of the copper rod does not change with temperature variations. (3) A small amount of gas is released before the battery is out of control. Since the amount of gas released is small, it is assumed that the quality of the battery remains unchanged before the battery is out of control. Therefore, the heat calculation formula generated by the battery thermal runaway process is as shown in (4).
Q r ¼ C cu mcu ðT iþ1 T i Þ þ C b mb ðT iþ1 T i Þ
ð4Þ
2.3. Calculation model and method The heat generated by the thermal runaway process refers to the heat generated by the battery during the thermal runaway from the start to the end of the whole thermal runaway process. The main calculation is the thermal change of the thermal runaway process of the battery. The heat generated by the thermal runaway process system includes the heat Qr generated by the thermal runaway of the battery and the heat input to the system by the electric heating Qin. These two parts of energy mainly pass through the heat absorbed by the copper tube Qcu, the heat absorbed by the battery Qb, and the heat loss Qloss transmitted to the hot environment through heat radiation and heat conduction. The heat balance equation is shown in Eq. (1) below.
Q in þ Q r ¼ Q cu þ Q b þ Q loss
ð1Þ
The heat absorbed by the copper tube Qcu is calculated as shown in (2).
Q cu ¼ C cu mcu ðT iþ1 T i Þ
ð2Þ
where: Ccu is the specific heat capacity of copper. The value of this paper is 0.39 J/(g°C); mcu is the mass of copper tube, g, the calculated value is 133.5 g; Ti + 1-Ti is the temperature of the copper tube at a certain moment, °C. The heat absorbed by the battery Qb is calculated as shown in (3)
Q b ¼ C b mb ðT iþ1 T i Þ
ð3Þ
3. Results and analysis 3.1. Temperature change characteristics of lithium-ion battery charge and discharge in adiabatic environment 3.1.1. Temperature change characteristics of lithium-ion battery under different charging rates The calculation formula of C-rate is shown in (5).
C rate ¼ Charge or discharge current=Nominal capacity
ð5Þ
The LAND charge-discharge testing device was used to first charge the lithium-ion battery to 4.2 V at a constant current, and then charge it to 100 mA at a constant voltage of 4.2 V. Different charging rates at 0.5 C, 1 C, 2 C, 3 C, and 4 C were set for the constant current charging step. The surface temperature changes of the battery during charging are shown in Fig. 3. The maximum temperature and temperature rise rate of the battery during charging are shown in Fig. 4. The rate of temperature rise of the battery increases with the increase of the charge rate. During charging, the heat generated by the battery mainly includes reversible heat and irreversible heat. The reversible heat is the h eat generated by the electrochemical reaction, and the irreversible heat mainly includes the polarization heat and the Joule heat [1–3]. When the current passes through the impedance composed of the positive and negative electrodes of the battery, the electrolyte, and the separator, Joule
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Fig. 3. Temperature curve of battery charging at 2 C, 3 C and 4 C. Fig. 6. The highest temperature and temperature rise rate at different discharge rate.
Fig. 4. The highest temperature and temperature rise rate at different charge rate.
heat is continuously generated. As the charging rate increases, the rate of heat generation also increases, so the rate of heat generation of the battery increases [25]. 3.1.2. Temperature change characteristics of lithium-ion battery under different discharge rates Constant current discharge on a 100% SOC battery was performed with 0.5 C, 1 C, 2 C, 3 C, and 4 C in an adiabatic environment. The temperature change curves during discharge are shown in Fig. 5. The maximum temperature and temperature rise rate of the battery during discharge are shown in Fig. 6. Figs. 7– 11 show voltage and current changes during discharge at each rate. During the discharge process, the temperature rise rate of the lithium-ion battery increases as the discharge rate increases. The heat production during battery discharge mainly includes electrochemical reaction heat generation, polarization heat generation
and ohmic heat [1–3]. When the lithium-ion battery is discharged at a slow rate, the heat of reaction will occupy the main part. When the battery is discharged at a high rate, Joule heat occupies the main part [9]. From the voltage and current curve of the 0.5 C discharge process, the voltage of the battery dropped rapidly from 2.75 V to 0.50 V or lower, and the battery did not discharge with the set 0.5 C current. The discharge current rapidly decreased and the battery temperature began to decline. The similar phenomenon occurred in the 1 C and 2 C discharge processes. When it started to discharge at 3 C, the battery voltage suddenly decreased, and then there was a small increase. At 1020 s, the voltage dropped rapidly from 2.55 V to 0.5 V or lower. When discharged at 4 C, the battery voltage dropped to 1.69 V at the beginning of discharge, the discharge current dropped to 10 A, and then the voltage and current began to slowly decrease. At 1980 s, the voltage and current of the battery dropped dramatically. From the above phenomenon, when the battery voltage drops to about 2.7 V, the voltage will drop rapidly. The reason is that when the voltage drops to 2.7 V, the battery is in an overdischarge state. Combined with the temperature profile, it can be found that the battery has reached the maximum temperature. After that, the battery temperature begins to drop. When the battery is discharged at 4 C, since the current exceeds the critical value, the action of the PTC element inside the battery is triggered. The resistance of the PTC element is rapidly increased, resulting in a decrease in the current of the entire discharge circuit, and the Joule heat and polarization heat of the battery are also reduced [26]. Therefore, the maximum temperature of the 4 C discharge process is less than the maximum temperature of the 3 C discharge. It can be concluded from the above experiments that although the lithium-ion battery does not thermally lose control in an adiabatic environment, the battery temperature rises during charging or discharging. Therefore, when a lithium-ion battery is charged and discharged in a sealed or semi-closed space where heat dissipation is poor, the lithium-ion battery should be cooled in time by other means such as liquid cooling to dissipate the heat generated by the lithium-ion battery and to prevent the damage to the battery. 3.2. Temperature change characteristics of lithium-ion battery charge and discharge in hot environment
Fig. 5. Temperature curve of battery discharging at 2 C, 3 C and 4 C.
3.2.1. Effect of charging rate on thermal runaway of lithium-ion battery The LAND charge-discharge testing device was used to first charge the lithium-ion battery to 4.2 V at a constant current in
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Fig. 7. Voltage and current curve of battery discharging at 0.5 C.
Fig. 8. Voltage and current curve of battery discharging at 1 C.
Fig. 9. Voltage and current curve of battery discharging at 2 C.
the presence of an external heat source, and then charge it to 100 mA at a constant voltage of 4.2 V. Different charging rates at 0.5 C, 1 C, 2 C, 3 C, and 4 C were set for the constant current charging step. In the presence of an external heat source, when the lithium-ion battery was charged at different rates, the battery thermal runaway process was similar. At all charging rates, during the thermal runaway process, the battery only showed a large amount of deflation, and there was no jet fire or stable combustion. When charging at 0.5 C, the thermal runaway reaction of the battery was slower than the thermal runaway reaction of other magnifications, which is not so severe, and the deflation time and the amount of deflation
Fig. 10. Voltage and current curve of battery discharging at 3 C.
Fig. 11. Voltage and current curve of battery discharging at 4 C.
were relatively small. Therefore, when 0.5 C is charged, the maximum temperature because of thermal runaway of the battery is lower than the highest temperature of several other magnifications. Fig. 12 is a typical characteristic picture of the thermal runaway process of a lithium-ion battery during charging at different rates. In the early stage of thermal runaway, a small amount of white flue gas was slowly discharged from the positive electrode of the battery. After the heat was out of control, the battery instantaneously released a large amount of white smoke from the positive electrode, and the process of a large amount of deflation lasted for about 20 s. The leaking white smoke had an irritating odor. When the battery was heavily deflated, a small amount of gas was slowly released from the positive electrode. As the temperature of the battery further dropped, the venting gas slowly decreased and disappeared. The battery did not form a jet fire or stable combustion during charging, which was related to the thermal runaway reaction inside the battery. Due to the small capacity of the battery during the charging process, the internal reaction is relatively gentle, the temperature rise rate is not fast, and the generated pressure is not high. So, the gas and electrolyte substances discharged from the gas do not reach the ignition point and so are not ignited. As shown in Fig. 12 (a)–(e), when charging, the thermal runaway shows a large amount of deflation mode. This model can be summarized into the following three typical phases: (1) The stage of slow release of gas at the beginning of thermal runaway; (2) The stage of large-scale venting of gas in the event of thermal runaway; (3) The stage of a small amount of gas releasing phase at the end of thermal runaway.
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Fig. 12. Typical characteristic of battery thermal runaway at different charging rate.
The battery temperature change and temperature rise rate curves during charging are shown in Figs. 13 and 14. It can be seen from the figures that the thermal runaway onset temperature of the lithium-ion battery is closer. The maximum temperature rise rate increases with the increase of the charge rate. Between 1500 s and 2000 s (Fig. 14), the battery temperature rise rate had a negative value at which point the battery temperature dropped a little. The main reason is that when the temperature is between 160 °C and 180 °C, the battery safety valve is opened, and the gas is released and the heat of the battery is removed. The battery temperature then continued to rise. Before the thermal runaway occurred, the rise rate of the battery temperature increased with the increase of the charging rate. Due to the
action of the PTC element, the 4 C charging process also resulted in a maximum temperature which was less than that during 3 C charging process. The thermal runaway temperatures of the lithium-ion batteries were 337.3 °C, 404.9 °C, 433.1 °C, 436.1 °C, and 427.0 °C, respectively. Figs. 15–19 record the relationship between the voltage, current and SOC of the battery during charging of the lithium-ion battery. When charging at 1 C, 2 C, 3 C, 4 C rates, the battery directly enters the constant voltage charging step. As charging and heating continued, the battery failed and the current dropped rapidly. From the temperature curve, it can be seen that no matter what ratio the battery is charged, when the battery temperature reaches 140 °C to 150 °C, the battery will be ineffective and can no longer be charged.
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Fig. 13. Temperature variation curve of battery charging at different current under electrical heating condition. Fig. 16. Voltage and current curve of battery charging at 1 C under electrical heating condition.
Fig. 14. Temperature rise rate of battery charging at different current under electrical heating condition. Fig. 17. Voltage and current curve of battery charging at 2 C under electrical heating condition.
Fig. 15. Voltage and current curve of battery charging at 0.5 C under electrical heating condition. Fig. 18. Voltage and current curve of battery charging at 3 C under electrical heating condition.
This phenomenon may be due to the fact that the diaphragm material of the 18,650 lithium-ion battery used in the experiment is mainly composed of polyethylene and polypropylene. When the battery temperature reaches 130–165 °C, the melting point of the diaphragm is reached, and the battery fails [27]. Analysis from the final power of the battery shows that, when the 1 C, 2 C, 3 C, and 4 C are charged, the battery finally retains similar amounts of power. Therefore, the thermal runaway onset temperature is also similar to the thermal runaway end temperature of the battery. However, due to the increase of the charging rate, the heat generation rate of the battery increases, so the time for the battery
to reach the thermal runaway onset temperature is relatively short.
3.2.2. Effect of discharge rate on thermal runaway of lithium-ion battery The battery with full charge was discharged at 0.5 C, 1 C, 2 C, 3 C, 4 C rates with external heat source to study the law and danger of thermal runaway of lithium-ion battery under different discharge rates.
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Fig. 19. Voltage and current curve of battery charging at 4 C under electrical heating condition.
Under the electric heating condition, during the discharge process at different magnifications, the battery thermal runaway process also differs. When discharging at 0.5 C, the battery continued to deflate for a total of 9 s when it was out of control, and a small combustion flame appeared after the air was deflated, and the small flame burned for 10 s. When discharging at 1 C, the battery continued to be deflated for 4 s when it was out of control. After the deflation, a large combustion flame appeared and kept burning for 5 s before dying out. When discharging at 2 C, the battery was instantly deflated, and then a jet flame formed, which was violent due to intense reaction. When discharging at 3 C, the battery continued to deflate for a maximum of 15 s, and there was no burning flame after a large amount of deflation. When discharging at 4 C, the battery continued to deflate for 20 s. After the deflation was completed, the battery did not show a burning flame. Viewed from the thermal runaway process, when the battery is discharged at 0.5 C, 1 C, and 2 C, the burning flame of the battery appeared after the battery was deflated. When the battery was discharged at 3 C and 4 C, there was no combustion during the thermal runaway process. Therefore, the maximum temperature of the thermal runaway process at 0.5 C, 1 C, and 2 C discharges is higher than that at 3 C and 4 C discharges. Fig. 20 is a typical characteristic diagram of thermal runaway of a lithium-ion battery during discharge at various rates. Based on the analysis of the thermal runaway process at five different rates during discharge, the thermal runaway of the battery can be divided into two modes. One is a large amount of deflation mode and the other is a large amount of deflation and combustion mode. Both modes have two identical phases, (1) a small amount of slow deflation in the early stage of thermal runaway; (2) A large amount of deflation in an instant. At low rate discharge, after a large amount of gas venting, a short burning phase occurs. The temperature change curve, runaway temperature and mass change before and after the control of the battery during discharge are shown in Figs. 21–23. The battery showed runaway at all the five rates. The runaway onset temperature increased with the increase of the discharge rate, while the end temperature decreased. When the battery was out of control at 0.5 C, 1 C, 2 C discharges, there was jet fires, and the reaction was more intense. Thus, the battery lost more mass and the temperature was higher. Figs. 24–28 show the voltage and current curves of the battery during discharge. Fig. 29 shows the relationship between
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battery voltage and battery discharge when discharging at different rates. It is found from the voltage and current curves that the discharge was performed at a constant current, and the battery temperature kept rising due to the simultaneous electric heating. Certain temperatures would cause a sudden decrease of the discharge current and the battery voltage, and the battery was damaged and could not be discharged normally. Through the voltage and current curves and the temperature curve, it can be found that the battery failed at 80–130 °C. This is because at 80–120 °C, the SEI film undergoes a decomposition reaction, resulting in failure of the internal material of the battery. Therefore, when a single battery in the battery pack is out of control, the charging or discharging process should be immediately cut off by the interlocking device to prevent the thermal runaway accident from further expanding. 3.3. Heat release from the thermal runaway process of lithium-ion batteries In this section, the heat release from the out-of-control process is calculated based on the temperature change of this process to analyze the relationship between the heat release and the charge-discharge current. The thermal models and calculation methods associated with the calculations in this section have been mentioned in Section 2.3. Fig. 30 is a graph showing the thermal runaway heat generation of the battery at different charge and discharge rates. It can be seen from the figure that the heat released by the thermal runaway during the discharge process is higher than the heat released during the charging process. The heat released by the thermal runaway during the charging process tends to increase with the increase of the charging rate, but the heat release of the 4 C charging is slightly reduced. When the discharge is at 0.5 C, 1 C, 2 C, the heat released by thermal runaway is higher, and the heat released by thermal runaway tends to decrease as the discharge rate increases. The above results are also consistent with those of the experiments in Sections 3.1 and 3.2. A dimensionless parameter n is defined to compare the amount of heat released by thermal runaway during charging and discharging at the same rate. Its calculation formula is shown in (6).
n ¼ Q c =Q d
ð6Þ
Where, Qc represents the thermal runaway heat release during the charging process, and Qd represents the thermal runaway heat release during the discharge process. The calculation results of thermal runaway heat release during charge and discharge at the same rate are shown in Table 4. In the process of low magnification (0.5 C), the uncontrolled heat release in the charging process is small, and the uncontrolled heat release in the discharge process is large, and the dimensionless value n is small. With the increase of the magnification, the thermal runaway heat release in both the charging and discharging processes, and the value n tends to be 1. The reason may be that the initial battery charge is 0 before charging, so the energy contained in the battery is very small. On the other hand, the initial charge of the battery before discharging is 100%, so the battery contains more energy. During low-rate charge and discharge, the amount of change in electricity is small. Before the runaway, the electricity in the battery during charging is still low, while it is high when the battery is discharging. Therefore, there is less heat loss in the charging process, and the heat is
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Fig. 20. Typical characteristic of battery thermal runaway at different discharging rate.
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Fig. 21. Temperature variation curve of battery discharging at different current under electrical heating condition.
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Fig. 25. Voltage and current curve of battery discharging at 1 C under electrical heating condition.
Fig. 22. Initial and finish temperature of Thermal runaway at different discharging current.
Fig. 26. Voltage and current curve of battery discharging at 2 C under electrical heating condition.
Fig. 23. Quality loss of battery at different discharging current.
Fig. 27. Voltage and current curve of battery discharging at 3 C under electrical heating condition.
Fig. 24. Voltage and current curve of battery discharging at 0.5 C under electrical heating condition.
out of control during the discharge process. With the increase of the charge and discharge rates, there are greater changes in electricity. Before the loss of control, the electricity in the battery is similar during the charge process and the discharge process, so the heat release is also similar in the out-of-control process, and thus the value of n is close to 1.
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4. Conclusions
Fig. 28. Voltage and current curve of battery discharging at 4 C under electrical heating condition.
The 18,650 lithium-ion battery is difficult to generate thermal runaway when it is charged and discharged in an adiabatic environment. In this condition, the maximum temperature during charging and discharging increases with the increase of charge and discharge rate. At the same rate, the maximum temperature in the charging process is greater than that in the discharging process. When there is a external heat source, the battery will be thermal runaway during the charging and discharging process. During the 3 C and 4 C discharge processes, since a large amount of electricity has been consumed, only some gas is leaked during the thermal runaway process and there is no continuous combustion. At the same rate, the thermal runaway heat release during the charging process is always greater than that during discharging process. Therefore, when a single battery in the battery pack is thermally out of control due to excessive heat accumulation, the power supply should be cut off in time by the interlocking device, and the battery pack should be cooled. This will be an effective way to prevent thermal runaway of the entire battery pack, and hence accidents. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted Acknowledgements
Fig. 29. Relationship between voltage and capacity of battery discharging at different current.
The authors are grateful for the support given by National Natural Science Foundation of China under Grant No. 51874184, Key R & D programs (Social Development) in Jiangsu Province under Grant No. BE2016771, Key Natural Science Foundation in Jiangsu Province under Grant No. 18KJA620003, and Jiangsu Project Plan for Outstanding Talents Team in Six Research Fields (TD-XNYQC002). The authors also thank Associate Prof. Yang Hongqi with Nanjing Tech University for revising and proofreading this manuscript. References
Fig. 30. Heat production of battery thermal runaway process at different chargedischarge rate.
Table 4 Thermal runaway heat release of the battery at the same rate. Rate
Qc
Qd
n
0.5 C 1C 2C 3C 4C
3.19 kJ 9.86 kJ 12.65 kJ 12.78 kJ 11.75 kJ
26.17 kJ 27.96 kJ 27.29 kJ 22.26 kJ 12.39 kJ
0.122 0.353 0.464 0.574 0.948
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