Causes of fire in power lithium-ion battery packs
The essence of lithium-ion battery pack fires is that the heat inside the battery fails to be released as designed, causing the internal and external combustibles to ignite and catch fire. The main causes include external short circuits, external high temperatures, and internal short circuits.
1. Internal short circuit: Due to the misuse of the battery, such as overcharging and over-discharging leading to dendrite formation, impurities and dust in the battery production process, etc., the battery will deteriorate and puncture the separator, resulting in a micro short circuit. The release of electrical energy leads to a temperature rise, and the chemical reaction of the material caused by the temperature rise further expands the short circuit path, forming a larger short circuit current. This mutually cumulative and mutually reinforcing damage leads to thermal runaway.
2. External Short Circuit: Taking electric vehicles as an example. The probability of a dangerous situation occurring during actual vehicle operation is very low. Firstly, the vehicle system is equipped with fuses and a Battery Management System (BMS). Secondly, the battery can withstand short-term high-current surges. In extreme cases, the short circuit point may extend beyond the vehicle's fuses, and the BMS may fail. A prolonged external short circuit generally only causes weak points in the circuit to burn out, rarely leading to thermal runaway of the battery. Currently, many battery pack companies use fuses in the circuit, which is more effective in preventing hazards caused by external short circuits.
3. External High Temperatures: Due to the structural characteristics of lithium-ion batteries, the SEI film, electrolyte, and EC will decompose under high temperatures. The decomposition products of the electrolyte will also react with the positive and negative electrodes, causing the cell separator to melt and decompose. These multiple reactions generate a large amount of heat. The melting of the separator leads to an internal short circuit, and the release of electrical energy further increases heat generation. This cumulative, mutually reinforcing destructive effect results in the rupture of the cell's explosion-proof membrane, electrolyte leakage, and subsequent combustion and fire.
Analysis of the causes of lithium-ion battery fires
As the energy source for pure electric vehicles, a major cause of fires involving lithium-ion battery packs is thermal runaway due to overheating, which is most likely to occur during battery charging and discharging. Because lithium-ion batteries have inherent internal resistance, they generate heat while supplying power to the electric vehicle, causing their temperature to rise. When this temperature exceeds their normal operating range, it will damage the battery's lifespan and safety. In pure electric vehicles, the power lithium battery system consists of multiple individual lithium-ion battery cells. During operation, a significant amount of heat accumulates within the confined battery pack. If this heat cannot be dissipated quickly and effectively, high temperatures can affect the battery's lifespan and even lead to thermal runaway, potentially causing fires and explosions.
Solutions to thermal runaway
▲ Improved cooling method
The thermal management system is primarily responsible for controlling temperature, ensuring the battery remains at a reasonable operating temperature. Typically, the thermal management system is controlled by the vehicle's controller. When the battery pack temperature becomes abnormal, the air conditioning system provides timely cooling or heating to ensure battery safety and lifespan. Battery cooling methods are categorized into four types based on the heat conduction method and medium: air cooling, liquid cooling, phase change materials (solid), and combined cooling (air/water cooling + solid cooling).
Internal improvements involve modifying the material structure within the battery cell to enhance its heat resistance and heat dissipation performance. Current research focuses on developing solid-state electrolytes, modifying the structure of the positive and negative electrodes, and introducing safer separator materials – all mainstream methods for improving battery thermal performance from within.
What should be done if a lithium-ion battery catches fire?
Having analyzed the causes of battery fires, let's look at Tesla's recommended methods for extinguishing lithium-ion battery fires:
1. In case of a small fire that has not spread to the high-voltage battery area, carbon dioxide or ABC dry powder fire extinguishers can be used to extinguish the fire.
2. When thoroughly inspecting for fire, do not come into contact with any high-voltage components, and always use insulated tools for inspection.
3. If a high-voltage battery is bent, twisted, or damaged during a fire—in short, becomes unrecognizable—or if a battery problem is suspected, then the amount of water used for firefighting must be sufficient.
4. A battery fire may take up to 24 hours to be completely extinguished. Using a thermal imaging camera can ensure that the high-voltage battery is completely cooled before the incident ends. If a thermal imaging camera is not available, the battery must be monitored for reignition. Smoke indicates that the battery is still hot; monitoring should continue for at least one hour after the battery stops smoking.
