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Why do lithium-ion batteries catch fire?
The main cause of fire in lithium-ion batteries is thermal runaway of the battery. When the battery thermal runaway occurs, the temperature of the battery can quickly rise to 400 degrees Celsius to 1000 degrees Celsius, leading to fires, explosions and other accidents. The main causes of thermal runaway in lithium-ion batteries can be roughly divided into mechanical abuse, electrical abuse, own defects and high temperature environment.
Mechanical abuse includes types such as collision, crushing and puncture. During the use, storage and transportation of lithium-ion batteries, due to external forces such as extrusion, collision, and puncture, the battery cells or battery packs may be deformed, resulting in damage to the battery separator and internal short circuit, eventually causing a fire. Among mechanical abuse, puncture injury is the most serious. It refers to a sharp conductor penetrating into the battery body, causing a direct short circuit between the positive and negative poles. The puncture causes the battery to short-circuit at the puncture point. The short-circuit area generates a large amount of heat due to the short circuit and forms a local hot zone. When the temperature of the hot zone exceeds the critical point, thermal runaway will occur, causing smoke, fire or even explosion. Collision, extrusion, and puncture are similar to each other in that they cause local internal short circuits, which may cause thermal runaway. The difference is that collision and extrusion are only probabilistic internal short circuits. In contrast, the heat generated during the puncture process is more intense, and the probability of thermal runaway is higher.
Electrical abuse mainly includes overcharge, over-discharge, short circuit and other types, which are mainly caused by improper use of batteries. When the battery is overcharged, the positive electrode voltage gradually increases, and the delithiation process becomes difficult. This causes the internal resistance of the battery to increase sharply, thus generating a large amount of Joule heat. At the same time, the oxygen in the positive electrode releases a large amount of heat, and the temperature of the negative electrode increases. It will also react exothermically with the electrolyte. Thermal runaway occurs when a series of exothermic reactions cause the internal temperature of the battery to rise to a certain level. When the battery is charged, lithium ions are deposited on the surface of the negative electrode to form lithium dendrites. The lithium dendrites can easily pierce the separator and cause a short circuit between the positive and negative electrodes. When the battery is over-discharged, the voltage of the battery decreases, which may cause the copper foil of the negative electrode to dissolve and accelerate battery failure. Short circuit includes two types: external short circuit and internal short circuit; external short circuit refers to the direct conduction between the positive and negative electrodes of the lithium-ion battery without passing through the load. When an external short circuit occurs, the heat generated by the battery cannot be effectively dissipated, and the battery temperature will also change accordingly. As it rises, the high temperature triggers thermal runaway.
Self-defects mainly include burrs, impurities, process defects, inconsistencies and other types. During the manufacturing process of the battery, there are metal burrs around the battery pole pieces or metal particles mixed in the pole pieces. Under certain conditions, the burrs or metal particles penetrate the isolation film and cause an internal short circuit. Battery modules or battery packs require lithium-ion cells to maintain good consistency during production and use. When there are production process defects or battery management system management defects, it will cause inconsistency in lithium-ion cells, resulting in overcharging of battery cells and self-discharge between cells during the overall use of the battery module or battery pack. , accelerating the aging of battery cells, battery performance degradation, and ultimately triggering thermal runaway.
High-temperature environments include thermal shock, poor heat dissipation, etc. High-temperature environments will accelerate the aging of lithium-ion batteries and the degradation of battery performance, resulting in an increase in battery heat during use, creating a vicious cycle and eventually triggering thermal runaway. Excessively high temperatures may also directly decompose the battery separator material, causing an internal short circuit, which may lead to thermal runaway. In order to solve the thermal runaway problem of lithium-ion batteries, we can improve the intrinsic safety of lithium-ion batteries and strengthen external protection. Judging from current research hotspots, improving the intrinsic safety of lithium-ion batteries mainly focuses on the development of positive and negative electrode materials with high thermal stability that can reduce or eliminate the formation of needle-like metal crystals, and the development of new separators that are resistant to high temperatures and mechanical forces. , develop flame-retardant electrolytes, develop solid-state batteries with low heat production, etc. Strengthening external protection mainly includes optimizing the battery manufacturing process and strictly controlling the testing of batteries before leaving the factory; setting up battery monitoring and protection devices to continuously improve the management level of battery management systems; setting up battery cooling systems to enhance their heat dissipation capabilities.