1. Research background
Energy shortage and environmental pollution are the main problems facing humanity, and the development of new energy has become a global research focus. Lithium ion batteries, especially lithium iron phosphate (LFP) batteries, have become the preferred battery for energy storage due to their performance advantages. Electrochemical energy storage (EES) power plants are widely used, but the safety issues of lithium-ion batteries have attracted much attention. At present, there is insufficient understanding of the hazards of thermal runaway (TR) behavior in gas generation and flames for high-capacity lithium iron phosphate batteries (280Ah). This study investigated the thermal runaway characteristics (heat release rate, combustion heat, battery surface temperature) and gas generation patterns (gas type and composition ratio) of 280AhLFP batteries using external heating method. The gas generation characteristics and flame macroscopic behavior of thermal runaway were analyzed, and the evolution laws of battery thermal runaway and fire risk under different states of charge (SOC) were elucidated. The influence of SOC on the characteristic parameters of battery thermal runaway was also explored. This study reveals the TR behavior of LFP batteries in EES at 50% and 100% SOC, providing reference data for EES fire prevention and emergency response design.
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2. Experimental setup
2.1 Battery Example
This study used a 280Ah lithium-ion battery with lithium iron phosphate (LiFePO4) as the positive electrode material and graphite (C) as the negative electrode material. The detailed physical parameters are shown in Table 1. Use the NEWARECT-4004-5V20A NFT device to charge and discharge the battery. Discharge the battery with a current of 20A until the cut-off voltage reaches 2.5V. The battery is charged using a constant current and constant voltage mode, with a charging current of 20A and cut-off currents and voltages of 2.8A and 3.65V. Before testing, fully charge the battery (100% SOC), and then discharge the battery to the desired state of charge according to experimental requirements.
| Parameter | Unit | Value |
| Dimension(length x height x thickness) | mm³ | 173.9 x 71.7 x 207.3 |
| Nominal capacity | Ah | 280 |
| Nominal energy | Wh | 896 |
| Mass | kg | 5.55 ± 0.30 |
| Nominal voltage | V | 3.2 |
| Charge and discharge voltage | V | 2.5 - 3.65 |
| Operating temperature (charging) | ℃ | 0 - 60 |
| State of charge | % | 50,100 |
| Specific heat Capacity | J/(kg·K) | 1030 |
| Density | kg/m³ | 2147.2 |
| Thermal conductivity | W/(m·K) | X/Y/Z : 20.5/20.5/4.92 |
2.2 Experimental apparatus and methods
2.2.1 Experimental setup
Figure 1 shows the experimental platform used in the work, including a combustion chamber manufactured according to the ISO9705 standard with dimensions of 1.8m × 1.8m × 2m and other experimental equipment. There is a smoke exhaust duct on the upper side of the combustion chamber. All experiments were conducted in the combustion chamber.
2.2.2 Experimental Methods
Use a heating plate to cause thermal runaway of a 280Ah lithium iron phosphate (LiFePO4) battery. Measure the surface temperature of the battery using a K-type thermocouple, measure the heat release rate (HRR) during the TR process using a heat release rate measuring device, and obtain the total heat generation of thermal runaway through integration. Use Fourier transform infrared spectrometer (FTIR spectrometer) to detect gas composition, and use Mettler balance to collect real-time mass changes. When a large amount of smoke is emitted, use an electronic ignition device to ignite the sprayed electrolyte and combustible gas. Thermocouples are distributed on the heating surface and back surface of the battery (as shown in Figure 2, Tf and Tb respectively), and the measured temperature on the side of the battery and the temperature at the opening position of the safety valve are denoted as Ts and Tup, respectively. Place five thermocouples to measure the temperature above the safety valve at different heights, which are 5cm, 10cm, 20cm, 30cm, and 40cm away from the safety valve.
3. Results and Discussion
3.1 Gas production and flame behavior during TR process
At 100% SOC, the battery exhibits significant gas production and flame behavior during the TR process, as shown in Figure 3. After the safety valve opens at 0 seconds, a large amount of electrolyte sprays out at 1 second, causing a change in flame color due to the presence of flammable substances. At 60 seconds and 175 seconds, the two cores inside the battery experienced thermal runaway, causing two intense gas production and flame spraying phenomena. This indicates that although gas ignition has little effect on the thermal runaway process, the entire process of battery thermal runaway lasts about 240 seconds, and its risks mainly manifest in severe gas production and jet flames. In a confined space, ignition of combustible gases can lead to explosions, while intense flame sprays can cause serious thermal radiation effects on surrounding batteries and the environment.
3.2 Thermal runaway analysis of battery surface temperature
The surface temperature of the battery is a key parameter in evaluating the TR process of the battery. Figure 4 shows the surface temperature changes of the battery under 50% SOC and 100% SOC conditions. Figures 4 (a) and (b) depict the temperature changes under gas production conditions, while (c) and (d) show the temperature changes under ignition conditions. The observation results indicate that under the same SOC, the surface temperature changes of the battery under the two conditions have similar trends. Although flames appear above the battery and have a certain jet velocity, their radiated heat has limited direct impact on the surface of the battery, so the effect of gas combustion on the surface temperature of the battery is relatively small. For batteries with 50% SOC, the thermal runaway process is relatively slow, as shown in Figure 4 (a) and (c). Under gas production conditions, the temperature on the side of the battery rapidly increases and triggers thermal runaway at 3200 seconds, with the highest temperatures reaching 434.9 ° C (front) and 307.3 ° C (back), respectively. Under ignition conditions, the temperature on the side of the battery sharply increases at 3169 seconds, with the highest temperature slightly higher than the gas production condition. The highest temperatures on the front and back surfaces are 475.9 ° C and 319.6 ° C, respectively. Meanwhile, the study also analyzed the changes in battery voltage. Under gas and flame conditions, when a battery with 50% SOC experiences thermal runaway, its voltage will decrease slowly, with a duration of about 400 seconds. This indicates that during thermal runaway, the internal reaction rate of 50% SOC batteries is slower and the duration of the thermal runaway process is longer.
In order to further analyze the regularity characteristics of the thermal runaway process, Figure 5 depicts the curves of temperature rise rate and time, as well as temperature and temperature rise rate. DT/dt represents the rate of temperature rise. Based on the temperature rise rate on the back of the battery, when the temperature rise rate exceeds 0.5 ° C/s, the reaction inside the battery is defined as irreversible. For a battery with 50% SOC, the duration of temperature rise rate exceeding 0.5 ° C/s is 80 seconds, while for a battery with 100% SOC, this duration is 200 seconds. Meanwhile, the peak temperature rise rate of thermal runaway in 100% SOC batteries is also higher than that in 50% SOC batteries. According to the temperature change curve and dT/dt, the thermal runaway process of the battery can be divided into four stages: the first stage is the heating state, with a temperature rise rate maintained at 0.03-0.04 ° C/s. The internal temperature of the battery is low, and the heat source is transferred to the battery through the heating plate. The second stage is the initial stage of thermal runaway, where the temperature rise rate gradually increases to 1 ° C/s. The SEI film inside the battery begins to decompose, and the electrolyte evaporates into electrolyte vapor, causing an increase in internal pressure and accelerating internal reactions. The third stage is the thermal runaway stage, where the rapid reaction of internal materials produces a large amount of gas, which is manifested as the diffusion of a large amount of combustible smoke in the absence of an external ignition source, and in the presence of flames, it is manifested as intense jet flames. The fourth stage is the cooling stage. After the battery loses thermal control, the surface temperature of the battery can reach 500 ° C. Due to the battery still being in a high temperature state, there is still a certain degree of danger.
3.3 Gas Generation and Flame Temperature Analysis
Figure 6 shows the gas temperature changes of 50% SOC and 100% SOC batteries at different heights under gas generation conditions. By analyzing the surface temperature of the battery, it can be concluded that the thermal runaway duration of 50% SOC batteries is longer than that of 100% SOC batteries, and this conclusion can also be verified in the gas temperature curve. The time when the temperature of a 50% SOC battery is above 50 ° C lasts for about 500 seconds, and the highest gas temperature at 5cm is relatively low, at 173.2 ° C; The high temperature duration of 100% SOC batteries is shorter, but the highest gas temperature at 5cm is higher, reaching 325.7 ° C, which is about twice that of 50% SOC batteries (as shown in Figure 6 (b)). The reason is that batteries with higher SOC have more intense internal reactions, faster gas generation rates, and shorter convective heat transfer time between high-temperature gas and the surrounding environment. Under the action of convective heat transfer, the temperature at the measurement point along the height of the battery gradually decreases, and the gas temperature near the battery safety valve is relatively high. When the measurement point is 50cm away from the battery safety valve, the gas temperature generated by the 100% SOC battery does not reach 40 ° C.
During the experiment, four main gases, CO, CH4, C2H4, and CO2, were measured during the thermal runaway process using a Fourier transform infrared spectrometer. It was found that carbon dioxide was produced the most during thermal runaway, with a much higher proportion than other gases, followed by carbon monoxide, methane, ethylene, and other hydrocarbon gases. Due to the inability of the instrument to measure hydrogen gas, its concentration was not analyzed. In addition, according to the analysis of the proportion of these four gases in Figure 6 (d), carbon dioxide accounts for 51.2% and carbon monoxide accounts for 22.9%. However, considering the large amount of hydrogen gas generated during the thermal runaway process, the proportion of carbon dioxide shown in Figure 6 (d) is not the proportion of all gas components. Due to the high flammability of the generated gas, the risk of TR is greater. Therefore, under pure gas conditions, thermal runaway behavior mainly brings toxicity, suffocation, and combustion risks.
In the actual scenario of energy storage batteries, fires often occur after the battery touches the heat TR, so ignition operation should be carried out after the battery safety valve is opened, and the gas temperature after ignition should be analyzed. As shown in Figure 7, five temperature measurement points are arranged vertically above the battery to measure the flame temperature at different heights. After the safety valve is opened, the ignition is immediately started, and the temperature at each measuring point rises sharply. Due to thermal runaway inside the battery, a large amount of gas is produced, and a violent jet fire appears above the safety valve. From the temperature curve, it can be seen that the highest temperature initially occurs at a height of 10cm, and the temperatures at heights of 5cm and 20cm are almost the same. In the later stage of thermal runaway, the flame gradually decreases, and the highest temperature occurs at a height of 5cm, with stable combustion of gas until the flame is extinguished. Compared with the temperature under gas production conditions, the temperature above the battery significantly increases after the flame appears, as shown in Figure 7 (b). The highest temperature of the flame above the battery at 50% SOC can reach about 750 ° C, and the temperature of the battery at 100% SOC is even higher, with a peak temperature of over 900 ° C (see Figure 7 (b)).
3.4 Quality Loss Analysis
Figure 8 shows the quality loss and quality loss rate of 50% SOC and 100% SOC batteries during thermal runaway under gas production conditions. Before the rapid decline in quality, both types of SOC batteries experienced a slow phase of quality decline, with a loss of approximately 100-200g. This slow descent process is related to the safety valve design of the battery. When the internal pressure of the battery reaches a certain level, the safety valve will slightly release the pressure. Due to the safety valve being fully opened, the rate of quality loss during this process is relatively slow. As the gas inside the battery increases, the internal pressure gradually rises. When the internal pressure reaches the pressure limit of the safety valve, the safety valve ruptures, causing a large amount of gas and electrolyte to spray out, resulting in a linear decrease in mass, as shown in Figure 8. During this process, the quality loss rate is approximately 110g/s.
Multiple cores inside the battery caused multiple peaks in the quality loss rate during thermal runaway. The internal reaction of 50% SOC batteries is slow, corresponding to two smaller peaks of 2.3g/s and 1.25g/s, respectively. Due to its relatively high capacity, 100% SOC batteries experience more severe thermal runaway processes, with two peak mass loss rates of 12.9g/s and 15.25g/s, respectively, as shown in Figure 8 (b). In addition, for 100% SOC batteries, there were two smaller peaks in the mass loss rate during the thermal runaway gas generation process.
Figure 9 shows the mass change and mass loss rate during the thermal runaway process under flame conditions. The process of thermal runaway is generally the same as that under gas generation conditions, but when the safety valve is opened, the mass loss rate is relatively low. The mass loss rates corresponding to 50% SOC and 100% SOC are 69.9g/s and 92.9g/s, respectively. The reason is that the ignition operation is carried out when the safety valve is opened, and some electrolyte and gas are not completely sprayed out, but burned completely at this time. Although the mass loss rate is low, it still far exceeds the two peak values of thermal runaway (the two peak values of 50% SOC flame are 2.05g/s and 1.2g/s, and the two peak values of 100% SOC are 8.05g/s and 9.95g/s, both lower than the mass loss rate under gas production conditions). By comparing the total mass loss under two conditions, it can be concluded that the mass loss under flame conditions is greater than that under gas production conditions.
3.5. Analysis of Heat Release Rate
After the battery safety valve is opened, ignition is carried out. According to the oxygen consumption theory, the heat release rate of the battery under thermal runaway combustion is measured as shown in Figure 10. For a 50% SOC battery, the first peak of the heat release rate after ignition is 57.107 kW. Integrating the heat release rate during the experiment yields a total heat generated by combustion of 20.79 MJ. The first peak heat release rate of the 100% SOC battery after ignition is 62.485 kW. Due to its high gas production rate, the peak heat release rate at the strongest moment of thermal runaway reaches 85.667 kW, which is much higher than the heat release rate of the 50% SOC battery as shown in Figure 10 (b). After integrating the entire experimental heat release rate, the total heat generated by combustion is 25.97 MJ. Although the thermal runaway duration and flame duration of 50% SOC batteries are longer, their total combustion heat is only 5.18MJ less than that of 100% SOC batteries.
4. Conclusion
(1) The impact of SOC on the surface temperature of batteries is greater than that of flames. Under gas and flame conditions, the highest surface temperature of a 100% SOC battery during thermal runaway is greater than that of a 50% SOC battery, while at the same SOC, the surface temperature of the battery under gas and flame conditions is almost the same.
(2) The flame temperature is much higher than the gas production temperature. The gas temperature generated by thermal runaway of 100% SOC batteries can reach 325.7 ° C, while the peak flame temperature can exceed 900 ° C. After gas ignition, it has a significant impact on the environment above and around the battery, mainly reflected in the radiation effect of high-temperature flames on the environment. In the absence of an external source of fire, the accumulation of a large amount of gas can pose a risk of poisoning, suffocation, and explosion.
(3) For 50% SOC and 100% SOC batteries, the peak mass loss rate under gas production conditions is greater than that under flame conditions, and the internal structure and thermal runaway process of the battery are determined based on the peak mass loss rate. The peak heat release rate of 100% SOC batteries after thermal runaway combustion is relatively high, but the thermal runaway duration of 50% SOC batteries is longer and the flame exists for a longer time. The total heat released by the combustion of 50% SOC and 100% SOC batteries is only 5.18 MJ different.