Analysis of the causes of fires in power lithium-ion battery packs
The most important reason for lithium-ion battery pack fires is that the heat inside the battery is not released as required by the design, and the fire occurs when the heat reaches the ignition point of the internal and external combustibles. The important reasons for this include external short circuit, external high temperature and internal short circuit.
As the energy source for pure electric vehicles, a major cause of fires in 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 a certain internal resistance, they generate heat while supplying power to electric vehicles, causing their own temperature to rise. When this temperature exceeds their normal operating temperature range, it will damage the lifespan and safety of the entire lithium-ion battery pack.
A power lithium battery system is composed of multiple power lithium battery cells. During operation, a large amount of heat accumulates in the small battery box. If the heat cannot be dissipated quickly and in time, the high temperature will affect the life of the power lithium-ion battery pack or even cause thermal runaway, which may lead to accidents such as fire and explosion.
To address thermal runaway in lithium-ion battery packs, the mainstream solutions in China currently focus on two main aspects: external protection and internal improvements. External protection primarily refers to system upgrades and improvements, while internal improvements refer to enhancements made to the battery itself.
1. External short circuit
External short circuits may be caused by improper operation or misuse. Due to external short circuits, the discharge current of the lithium-ion battery pack is very large, which will cause the iron core to heat up. The high temperature will cause the diaphragm inside the iron core to shrink or completely fail, resulting in an internal short circuit and causing a fire.
2. Internal short circuit
Due to an internal short circuit, the high-current discharge of the battery cell generates a large amount of heat, burning the diaphragm and causing a significant short circuit. This results in high temperatures, causing the electrolyte to decompose into gas and leading to excessive internal pressure. When the core's outer casing cannot withstand this pressure, the core will catch fire.
3 Overcharge
When the iron core is overcharged, the excessive release of lithium from the positive electrode alters its structure. Excess lithium can easily migrate to the negative electrode and deposit on its surface. When the voltage exceeds 4.5V, the electrolyte decomposes, releasing a large amount of gas. All of these factors can cause a fire.
4. Excessive moisture content
Water can react with the electrolyte in the core to produce gas. During charging, it can react with lithium to form lithium oxide, causing core capacity loss and easily leading to overcharging and gas formation. Water has a low decomposition voltage, making it prone to decomposition into gas during charging. When these gases appear, the internal pressure of the core increases. If the core's outer shell cannot withstand these gases, the core will explode.
5. Insufficient negative electrode capacity
When the negative electrode has insufficient capacity relative to the positive electrode, or no capacity at all, some or all of the lithium generated during charging cannot be inserted into the interlayer structure of the graphite in the negative electrode and will deposit on the surface of the negative electrode. The protruding dendrites are more likely to cause lithium precipitation during the next charge cycle. After tens to hundreds of charge-discharge cycles, the dendrites will grow and eventually pierce the separator paper, causing an internal short circuit.
