As the energy source for pure electric vehicles, a major cause of lithium-ion battery fires 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 the electric vehicle, causing their temperature to rise. When this temperature exceeds their normal operating temperature range, it will damage the battery's lifespan and safety. In pure electric vehicles, the power lithium battery system consists of multiple individual lithium battery cells. During operation, a large amount of heat accumulates within the confined battery pack. If this heat cannot be dissipated quickly and effectively, high temperatures will affect the battery's lifespan and may even lead to thermal runaway, resulting in fires and explosions. In principle, the causes of thermal runaway mainly fall into four categories:
Causes of thermal runaway:
The first category is external factors affecting the battery. If the ambient temperature is too high, the battery's heat dissipation is poor, or its internal windings are not properly designed, heat dissipation will be obstructed. In addition, companies should pay special attention to another situation: the battery's spindle can sometimes conduct heat into the battery. The battery's metal spindle, made of copper or aluminum, has good thermal conductivity. Components in the circuit that are high-heat sources must be kept away from the battery's spindle to allow heat to dissipate effectively. Qiu Xinping added.
The second category is internal battery factors. One is micro-short circuits, which can occur in two ways: one is short circuits during battery manufacturing, including burrs; the other is short circuits during use. Both of these can cause micro-short circuits, leading to localized battery temperatures exceeding the thermal runaway temperature, thus resulting in thermal runaway.
Third, overcharging the battery can lower the thermal runaway temperature of the materials. For example, in the later stages of battery use, the battery capacity has already decreased, and at this point, the battery itself is already in an overcharged state, so the thermal runaway temperature will naturally decrease, Qiu Xinping pointed out.
Fourthly, the amount of electrolyte used is crucial. If too much electrolyte is used, the explosion will be very dangerous, and the flames will shoot very high and far. Therefore, the amount of electrolyte used must be strictly controlled.
The most serious occurrence during a car collision is when the external force deforms the individual lithium-ion battery cells and battery pack, causing relative displacement of different parts, leading to tearing of the battery separator and internal short circuit; leakage of flammable electrolyte ultimately causes a fire. Among mechanical abuse, puncture damage is the most severe, as it can cause a conductor to penetrate the battery body, resulting in a direct short circuit between the positive and negative electrodes. In contrast, collisions and crushing only have a probabilistic chance of causing an internal short circuit; the heat generated during puncture is more intense, increasing the probability of thermal runaway.
Battery abuse is primarily caused by improper battery use, and includes several types: external short circuits, overcharging, and over-discharging. Of these, over-discharging causes the least harm, but the rise of copper dendrites due to over-discharging reduces battery safety and increases the likelihood of thermal runaway. An external short circuit occurs when two conductors with a voltage difference are connected outside the battery cell. When an external short circuit occurs, the heat generated by the battery cannot dissipate effectively, causing the battery temperature to rise and triggering thermal runaway.
Overcharging is one of the most dangerous forms of battery abuse. Excessive lithium insertion leads to the growth of lithium dendrites on the anode surface. Secondly, excessive lithium deintercalation causes the cathode structure to collapse due to heat generation and oxygen release (oxygen release at the NCA cathode). This oxygen release accelerates electrolyte decomposition, resulting in a large amount of gas. Due to the increased internal pressure, the vent valve opens, and the battery begins to release gas. When the active materials in the cell come into contact with air, a violent reaction occurs, releasing a large amount of heat, which can ignite the battery pack and cause a fire.
Thermal abuse primarily refers to localized overheating within a battery. It rarely occurs independently and often develops from mechanical and electrical abuse, ultimately triggering accidents such as thermal runaway. Thermal abuse typically results from excessive battery heat caused by high external environmental temperatures or malfunctioning temperature control systems, leading to short circuits and ultimately thermal runaway. The causes of thermal abuse are the most complex; collisions or damage to the battery pack, malfunctions in the battery's internal structure and performance, or failures in other thermal management or air conditioning systems can all contribute to its occurrence.
Internal short circuits occur when the positive and negative electrodes of a battery come into direct contact. The degree of contact significantly affects the subsequent reactions. Large-scale internal short circuits caused by mechanical or thermal abuse typically lead directly to thermal runaway. The causes of internal short circuits are complex. For example, overcharging a lithium-ion battery can cause dendrites to accumulate and puncture the battery separator, resulting in an internal short circuit. Alternatively, collisions or punctures can directly cause the positive and negative electrodes to come into contact, leading to thermal runaway. Compared to internal short circuits caused by external factors, internal short circuits resulting from defects inherent in the battery manufacturing process are generally milder. The heat generated by an inherent internal short circuit is minimal and does not immediately trigger thermal runaway. Furthermore, these inherent defects may take some time to evolve into a milder internal short circuit.
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).
