Lithium-ion battery thermal runaway process, is the thermal runaway reaction of different lithium batteries the same?
Lithium-ion battery thermal runaway process, is the thermal runaway reaction of different lithium batteries the same? At present, the safety problem of the automotive power battery system is the safety of the battery group. The automotive power battery accidents in recent years are all caused by the thermal runaway of a certain battery cell in the lithium battery pack, which generates a lot of heat, which causes the surrounding battery cells to be heated and generate thermal runaway. Thermal runaway spread within a battery pack is a major concern for battery pack safety.
Thermal runaway is the most serious safety incident for lithium-ion batteries. The thermal runaway of lithium-ion batteries is due to the fact that the rate of heat production is much higher than the rate of heat dissipation, and a large amount of heat accumulates inside the lithium-ion battery, causing a rapid rise in the temperature of the lithium-ion battery, resulting in spontaneous shrinkage and melting of the diaphragm, and decomposition of positive and negative active materials. The exothermic reaction caused the lithium-ion battery to catch fire and explode.
Lithium-ion battery thermal runaway process:
Battery thermal runaway is caused by the fact that the heat generation rate of the battery is much higher than the heat dissipation rate, and a large amount of heat is accumulated and not dissipated in time. Essentially, "thermal runaway" is an energy-positive feedback loop process: an elevated temperature causes the system to heat up, the system heats up and the temperature rises, which in turn makes the system hotter, loosely divided , the battery thermal runaway can be divided into three stages:
1. The internal thermal runaway stage of the battery
Due to internal short circuit, external heating, or self-heating of the battery itself during high current charging and discharging, the internal temperature of the battery rises to about 90°C to 100°C, and the lithium salt LiPF6 begins to decompose; the chemical activity of the carbon negative electrode in the charged state is very high, Close to metallic lithium, the SEI film on the surface decomposes at high temperature, and the lithium ions embedded in graphite react with the electrolyte and the binder, further pushing the battery temperature to 150 °C, and a new violent exothermic reaction occurs at this temperature. .
2. Battery bulge stage:
When the temperature of the lithium battery reaches above 200°C, the positive electrode material decomposes, releasing a large amount of heat and gas, and the temperature continues to rise. At 250-350°C, the lithium intercalated negative electrode begins to react with the electrolyte.
3. Battery thermal runaway, explosion failure stage:
During the reaction process, the charged cathode material begins to undergo a violent decomposition reaction, and the electrolyte undergoes a violent oxidation reaction, releasing a large amount of heat, generating high temperature and a large amount of gas, and the lithium battery will burn and explode.
Do different lithium batteries have the same thermal runaway reaction? :
When heated externally, all lithium batteries undergo a thermal runaway that releases fumes and gases. For about half of the working cells, within about 15 seconds after thermal runaway, the gas accumulated in the oven was ignited causing a gas explosion with a major fume release process. Cells that were cycled or not, did not affect the occurrence of gas explosions, which occurred at all cycle aging levels from 0-300 full deep cycles.
Compared with other types of batteries, lithium-ion batteries generate more heat, and their gas emissions are at a higher risk of explosion and fire. These risks are far from being fully understood, and it is possible to improve system safety through research and accident analysis. The type and severity of risk depends on the application and the size of the battery system. Due to the transmissibility of battery and module failures, failure consequences can increase significantly as battery system size increases.
The lithium cobalt oxide battery has the highest thermal runaway temperature, which can reach 850 degrees Celsius; the nickel-cobalt-manganese ternary lithium battery is the second, which can reach 670 degrees Celsius; the lithium iron phosphate battery has no obvious thermal runaway, and the maximum temperature is about 400 degrees Celsius. The power of the thermal runaway process obviously depends on the energy of the lithium-ion battery. The lithium cobalt oxide battery has the highest specific energy, and the temperature change and gas discharge are also the most severe.
There are many factors of thermal runaway, generally divided into two categories, internal factors and external factors. The main internal factors are: internal short circuit caused by battery production defects; improper use of the battery, resulting in internal lithium dendrites and short circuit between positive and negative electrodes. External factors are mainly: external factors such as extrusion and acupuncture lead to short circuit of lithium ion battery; external short circuit of battery causes excessive heat accumulation inside the battery; excessive external temperature leads to decomposition of SEI film and positive electrode material.
The above is the thermal runaway process of lithium-ion batteries and the thermal runaway reactions of different lithium batteries. Generally, after thermal runaway occurs, it will spread downward. For example, after the first section of thermal runaway, heat transfer will begin to spread, and then the whole group will follow one by one like firecrackers. In short, in terms of thermal runaway expansion and suppression, R&D personnel should start from two aspects: safety protection design and lithium battery management system.
