As a source of energy for pure electric vehicles, the important reason for lithium-ion battery fires is the thermal runaway caused by battery overheating, which is most likely to occur during battery charging and discharging processes. Due to the inherent internal resistance of lithium-ion batteries, they generate a certain amount of heat while outputting electrical energy to power pure electric vehicles, causing their temperature to increase. When their temperature exceeds their normal operating temperature range, it will damage the entire battery's lifespan and safety. In pure electric vehicles, the power lithium battery system is composed of multiple individual power lithium battery cells. During operation, a large amount of heat accumulates in the narrow battery box. If the heat cannot be quickly dissipated in time, high temperatures can affect the lifespan of the power lithium battery and even cause thermal runaway, leading to accidents such as fires and explosions. In principle, there are four important reasons for thermal runaway:
1. Mechanical abuse
During a car collision, due to the use of external forces, the lithium-ion battery cells and battery packs deform, causing relative displacement in different parts of themselves, resulting in the battery separator being torn and internal short circuits occurring; Flammable electrolyte leakage ultimately triggers a fire. In mechanical abuse, puncture injury is the most serious, as it may lead to the insertion of conductors into the battery body, causing a direct short circuit between the positive and negative poles. In contrast, collisions, squeezing, and other factors only have a probability of internal short circuits occurring, and the generation of heat during the puncture process is more intense, resulting in a higher probability of heating out of control.
2. Electricity abuse
Electricity abuse is caused by improper use of batteries, including external short circuits, overcharging, and over discharging. Among them, the harm caused by excessive discharge is the smallest, but the increase in copper dendrites caused by excessive discharge will reduce the safety of the battery and increase the probability of thermal runaway. External short circuit is the result of two conductors with a pressure difference being connected outside the battery cell. When an external short circuit occurs, the heat generated by the battery cannot be dissipated well, and the temperature of the battery will also rise, causing high temperature contact heat to lose control.
Overcharging is the most harmful form of electrical abuse. Due to excessive lithium insertion, lithium dendrites grow on the anode surface. Secondly, excessive deintercalation of lithium leads to the collapse of the cathode structure due to heating and oxygen release (NCA cathode oxygen release). The release of oxygen accelerates the decomposition of electrolytes, resulting in a large amount of gas. Due to the increase in internal pressure, the exhaust valve opens and the battery begins to exhaust. After the active substances in the battery cells come into contact with air, they undergo a violent reaction, releasing a large amount of heat, which can cause the battery pack to burn and catch fire.
3. Heat abuse
Thermal abuse refers to the localized overheating in the battery, which rarely exists independently and often develops through mechanical and electrical abuse, and is ultimately a direct result of accidents such as thermal runaway. Heat abuse is generally caused by high external environment or short circuits caused by excessive battery heat when the temperature control system is not functioning, leading to uncontrolled heating. In terms of reasons, the causes of thermal abuse are the most complex. Collisions, damage to battery packs, internal structure, performance of batteries, or failure of other thermal management systems and air conditioning systems can all lead to the occurrence of thermal abuse.