Based on the experiment of thermal runaway fire characteristics of lithium batteries, the thermal runaway characteristics of 18650 lithium ions with charge amounts of 20%, 30%, 50%, 70%, and 100% were summarized, including thermal runaway propagation, heat release rate, temperature, mass loss, and released gas. Analyze the risk characteristics of thermal runaway of lithium batteries in the cockpit, cabin, and cargo hold, as well as the ability of internal fire extinguishing and ventilation systems and other facilities to withstand lithium battery fires. Introduce the thermal runaway experiment under simulated flight changing environment, providing reference for the development of large-scale lithium battery related experiments.
Batteries experience thermal runaway due to heating, short circuits, or external force collisions, which not only violently releases a large amount of gas and heat, but also easily spreads from one battery to adjacent batteries. Although the phenomenon of thermal runaway varies depending on the type of battery, it can be summarized as four stages: first, gas is alternately ejected from the small holes (the outer packaging is burned through to form small holes), then internal substances emerge from the existing small holes at the positive end, then internal substances are ejected, and finally all internal substances are ejected. When standing upright, explosions are prone to occur in the horizontal direction, and gases and electrolytes are released in the vertical direction. The ejected substances are mainly copper, graphite, and aluminum.
1 The Influence of Charge Quantity on Thermal runaway Characteristics
Regarding the study of the effect of State of Charge (SOC) on the thermal runaway characteristics of lithium-ion batteries, researchers have found that the self ignition temperature of the nickel cobalt 18650 battery is about 10 ℃. The thermal stability of lithium-ion batteries decreases with the increase of SOC, and the domino chain effect of thermal runaway propagation is verified.
As the SOC of the battery increases, the starting temperature of thermal runaway gradually decreases, while the ending temperature of thermal runaway first increases and then decreases, and the mass loss gradually increases. When the SOC is 0%, the CO content in the generated gas is the highest, and the smoke toxicity is the strongest; At 50%, the maximum amount of flue gas is generated, but the CO content is relatively low, and the characteristics of lithium-ion battery jet combustion are the most obvious
1.1 Transmission of thermal runaway
18650 lithium-ion batteries with SOC of 20%, 30%, 50%, 70%, and 100% are divided into 5 groups, each consisting of 4 batteries and 1 100W cylindrical heater. It was found that when the SOC was 50%, all four batteries experienced thermal runaway, with temperatures reaching over 700 ℃; When SOC is 40%, two batteries experience thermal runaway; When SOC is 30%, 70%, and 100%, only one battery loses thermal control.
1.2 Maximum Heat Release Rate
In the experiment on the influence of thermal runaway, the system was divided into 5 groups based on SOC values of 20%, 30%, 50%, 70%, and 100%. The system was heated with alcohol, and the changes in system heat were recorded using a cone calorimeter. The peak heat release rate (PHRR) was determined by the relationship between the changes in SOC and the peak heat release rate (PHRR). The results show that PHRR increases with the increase of SOC, with an average PHRR of 50% and a minimum PHRR of 20%. When thermal runaway occurs, the maximum value of PHRR appears around 50% SOC, which is also the most prone to the spread of thermal runaway between batteries and poses the greatest danger
Exploring the relationship between temperature and SOC changes in 18650 lithium-ion batteries during different stages of thermal runaway through temperature measurement experiments. The experiment found that the temperature T during the first release of gas due to thermal runaway was independent of SOC and remained around 200 ℃; The temperature T2 during the second release of gas is also independent of SOC, approximately 260 ℃. The highest temperature T MAX during the entire thermal runaway process increases with the increase of SOC. At 0%, the maximum temperature is approximately 600 ℃, and at 100%, the maximum temperature exceeds 1000 ℃.
1.4 Quality loss
Exploring the mass loss during the thermal runaway process in the thermal runaway mass loss experiment. After the first release of gas, the mass loss is about 2-3g, and after the second release of gas, the mass loss is about 17g. The total duration of the two releases is about 2s. Exploring the relationship between the variation law of thermal runaway mass loss and SOC, it was found that the mass m after the first release of gas from thermal runaway does not change with the variation of SOC, and the mass loss during this stage is a constant value; The mass m2 after the second release of gas decreases with the increase of SOC. The mass loss of the battery during the entire thermal runaway process shows an increasing trend with the increase of SOC. Under the same SOC, the thermal runaway mass loss is a linear function of the storage capacity. The larger the storage capacity, the more thermal runaway mass loss.
1.5 Release of gas
In the gas release hazard experiment, 400 18650 lithium-ion batteries with a SOC of 50% were placed in a 10 square meter pressure chamber to experience thermal runaway. The measured pressure showed that the maximum pressure they could generate was 193.1kPa. When a single battery loses its thermal control, approximately 6L of flammable gas can be released within 3s, and the total amount of released gas and the law of each component increasing with the increase of SOC are obtained.
The gas composition may vary depending on the type of battery. Conduct pressure testing experiments in cargo hold 737. Measure the pressure generated by the ignition of the gas released by the lithium battery thermal runaway in the 737 cargo hold at a true environmental state of 70% cargo capacity. The results indicate that when the gas released from thermal runaway of 18650 lithium-ion batteries with 8 SOC of 50% or 3 SOC of 100% is ignited, the flame smoke and pressure generated will cause the fire extinguishing system in the cargo hold to fail.