As the driving range of electric vehicles continues to increase, ternary materials with higher energy density are gradually replacing lithium iron phosphate materials with better stability. Although ternary materials bring higher energy density to power lithium batteries, their thermal stability is relatively low. In particular, high-nickel materials begin to decompose at around 200°C, releasing O2. This also makes ternary lithium batteries less safe than lithium iron phosphate batteries. Therefore, they are more prone to thermal runaway in the event of electrical abuse, thermal abuse, or mechanical abuse.
Recently, Xian Xuelei (first author) and Dong Haibin (corresponding author) of the Tianjin Fire Research Institute of the Ministry of Emergency Management studied the thermal runaway behavior of ternary soft-pack lithium batteries under two conditions: thermally triggered and overcharge-triggered thermal runaway. The study showed that under thermally triggered conditions, the main cause is the ejection of a mixture of O2 produced by the high-temperature decomposition of the positive electrode material and the vaporization of the electrolyte, resulting in the battery directly ejecting flames. In contrast, overcharge-triggered thermal runaway mainly originates from the large amount of alkane gas produced by the reaction between the negative electrode and the electrolyte, causing the battery to eject flames first and then ignite.
The basic information of the soft-pack ternary lithium battery used by the authors in the experiment is shown in the figure below. The positive electrode material of the battery is NCM811, the negative electrode is graphite, and the battery capacity is 107Ah.
To observe the behavior of the aforementioned pouch lithium battery during thermal runaway, the authors designed the observation system shown in the figure below, using a high-speed camera (4000 frames/s), a thermal imaging camera, and a regular camera to record the entire thermal runaway process. In the experiment, the authors used two methods to trigger thermal runaway. The first method involved heating the battery by adding a hot plate to the bottom of the battery to heat the fully charged battery and trigger thermal runaway. The second method involved continuously charging the battery using a charging and discharging device, triggering thermal runaway through overcharging.
The key steps involved in thermal triggering are:
Fully charged
Heating until thermal runaway
Stop heating
Overcharge triggering includes the following steps:
Fully charged
The battery was continuously charged at a current of 50A until thermal runaway occurred.
Stop charging
Heating triggers thermal runaway
The thermal triggering process can be divided into five steps: 1. Heating, battery bulging; 2. One end bursting; 3. The other end bursting; 4. Quiet combustion; 5. End. Test data showed that when the battery was heated to 215.7℃, it began to bulge significantly and the electrolyte burst, with one end of the battery flaming. Approximately 7 seconds later, the other end of the battery also began to flame, and the battery's perimeter began to crack and flame, a process lasting 19 seconds with a maximum temperature of 720.1℃. The battery then entered a quiet combustion process, lasting 28 seconds. Throughout the heating process, the battery voltage remained essentially unchanged until the bursting occurred, after which the voltage suddenly dropped to 0V. This indicates that we cannot predict battery thermal runaway by detecting abnormal voltage.
To verify the trigger temperature of battery thermal runaway, the authors heated the battery to 100°C, 150°C and 200°C respectively. The test results showed that the battery did not experience thermal runaway at 100°C and 150°C, and thermal runaway was only triggered when the heating temperature exceeded 200°C.
The image below shows the moment of thermal runaway of the battery, captured by the author using a high-speed camera. It can be seen that the battery ruptured near the tabs, and the battery exploded very quickly, with a large amount of flames shooting out from the hole in the aluminum-plastic film in less than 0.5 seconds.
Overcharge triggers thermal runaway
Overcharging-triggered thermal runaway can also be divided into 5 stages: 1. Battery swelling; 2. One end bursting; 3. The other end bursting; 4. Quiet combustion; 5. End.
Tests revealed that when the battery was overcharged to 126% SoC, the highest surface temperature reached 98.7℃, with a maximum temperature rise rate of 1.7℃/min. Subsequently, the battery's expansion and temperature rise rates increased rapidly, with the surface and tabs experiencing temperature rise rates of no less than 5℃/min. Then, one end of the battery experienced a burst, followed by another burst approximately 2 seconds later. Simultaneously, the other two long sides of the battery showed signs of tearing, and flames began to shoot out in multiple directions. This entire process lasted approximately 5 seconds. The battery then began to burn quietly, a process that lasted approximately 43 seconds. Throughout this process, the battery voltage continuously increased until the burst occurred, at which point the voltage suddenly dropped to 0V.
The image below shows a battery explosion caused by overcharging, recorded by a high-speed camera. As can be seen from the image, the tabs at one end of the battery cracked first, spraying out mist and solid particles in a high-temperature molten state. During the explosion, an electric spark appeared at the opening of the battery, igniting the electrolyte and causing a violent explosive combustion.
In a battery explosion caused by overcharging, the electrolyte ejected from the battery does not burn initially. However, when the flammable gases and liquids reach a certain concentration with the oxygen in the air, the molten particles ejected from the battery will suddenly ignite them, causing an explosive combustion (as shown in the figure below).
Xian Xuelei's research shows that there are certain differences in the behavior of thermal runaway triggered by heating and thermal runaway triggered by overcharge. When the battery casing is ruptured during heating-triggered thermal runaway, the gas ejected contains a considerable amount of O2, so the battery directly ejects flames. However, when thermal runaway is triggered by overcharge, the mixture of liquid and gas ejected by the battery, as well as molten particles, is ignited only after mixing with oxygen in the air to reach a certain concentration, resulting in an explosive combustion that is even more intense than thermal runaway triggered by heating.
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Research on Thermal Runaway and Fire Characteristics of Ternary Lithium-ion Power Lithium Batteries, *Energy Storage Science and Technology*, 2020, 19(01):239-249, Xian Xuelei, Dong Haibin, Zhang Shaoyu, Li Yi, Liu Lianxi, Yu Dongxing, Sheng Yanfeng, Yi Chengyi, Han Guang
