Thermal Runaway of EV Battery

Safety issues are the main obstacle to the large-scale application of lithium-ion batteries in electric vehicles.

With the continuous improvement of the energy density of lithium-ion batteries, improving their safety is increasingly urgent for the development of electric vehicles. Thermal runaway is a key issue in battery safety research. Therefore, this article provides a comprehensive review of the thermal runaway mechanism of commercial lithium-ion batteries used in electric vehicles. Summarized the abuse situations that may lead to thermal runaway. Abuse situations include mechanical abuse, electrical abuse, and thermal abuse. Internal short circuits are the most common feature of all abuse conditions.

Mechanical Abuse

Under external forces, the deformation of lithium battery cells and battery packs, as well as relative displacement of different parts of themselves, is the main external characteristic of mechanical abuse. The main forms for battery cells include collision, compression, and nail penetration. Considering the level of the battery pack, vibration issues also need to be considered.

In mechanical abuse, the most dangerous is nail penetration, where the conductor is inserted into the battery body, causing a direct short circuit between the positive and negative poles. Compared to collisions, squeezing, etc., it is only a probability that internal short circuits occur. The heat generation during the nail penetration process is more intense, and the probability of heating out of control is higher.

Previously, nail penetration was considered an alternative testing method for ISC. However, the repeatability of needle testing is being challenged by battery manufacturers. Some people believe that lithium-ion batteries with higher energy density will never pass the standard spike test. Improving the repeatability of nail penetration testing or finding alternative testing methods remains an open and challenging issue in the safety research of lithium-ion batteries.

Electrical Abuse

The electrical abuse of lithium batteries generally includes external short circuits, overcharging, and discharging, among which overcharging is the most likely to develop into thermal runaway.

External short circuit occurs when two conductors with a pressure difference are connected outside the cell. The external short circuit of the battery pack may be caused by deformation, immersion, conductor contamination, or electric shock during maintenance caused by car collisions.

Compared to nail penetration, the heat released by external short circuits usually does not heat the battery. The important link between external short circuit and thermal runaway is excessive temperature. When the heat generated by external short circuits cannot be dissipated well, the temperature of the battery will rise, and the high-temperature contact heat will lose control. Therefore, cutting off short-circuit current or dissipating excess heat are methods to suppress further harm caused by external short-circuit.

Overcharging

Due to its high energy content, it is the most hazardous type of electrical abuse. The generation of heat and gas are two common characteristics during overcharging. Heating comes from Ohmic heat and side reactions. Firstly, due to excessive lithium insertion, lithium dendrites grow on the anode surface. The time at which lithium dendrites begin to grow is determined by the stoichiometric ratio of the cathode and anode. Secondly, excessive detachment of lithium leads to the collapse of the cathode structure due to heating and oxygen release. The release of oxygen accelerates the decomposition of electrolytes, producing 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 substance in the battery cell comes into contact with air, it undergoes a violent reaction and releases a large amount of heat. Overcharging protection can be implemented from two aspects: voltage management and material adjustment.

Overdischarge

Inconsistent voltage between batteries within a battery pack is inevitable. Therefore, once the BMS fails to specifically monitor any individual battery cell with the lowest voltage, it will be over discharged. The mechanism of overdischarge abuse is different from other forms of abuse, and its potential danger may be underestimated.

During over discharge, the battery with the lowest voltage in the battery pack can be forcibly discharged by other batteries connected in series. During forced discharge, pole reversal causes the battery voltage to become negative, resulting in abnormal heating of the over discharged battery. The dissolved copper ions caused by overdischarge migrate through the membrane and form copper dendrites with lower potential on the cathode side. As the growth continues to increase, copper dendrites may penetrate the membrane, leading to severe ISC.

Thermal Abuse

Local overheating may be a typical heat abuse situation that occurs in battery packs. Thermal abuse rarely exists independently and often develops from mechanical and electrical abuse, and is ultimately a direct cause of runaway heating. In addition to overheating caused by mechanical/electrical abuse, overheating may be caused by loose connection contacts. The issue of loose battery connections has been confirmed. Thermal abuse is also the most commonly simulated situation, using controlled heating batteries to observe their reactions during the heating process.

Internal Short Circuit

Internal short circuit, direct contact between the positive and negative poles of the battery, of course, the degree of contact also varies greatly in the subsequent reactions triggered. Large scale ISC usually caused by mechanical and thermal abuse will directly trigger thermal runaway. On the contrary, the internal self-developed internal short circuit is relatively mild and generates very little heat, which will not immediately trigger thermal away.

The rate of energy release varies with the degree of diaphragm rupture and the length of time from ISC to thermal runaway. Spontaneous ISC is believed to originate from pollution or defects in the manufacturing process. Pollution/defects take days or even months to develop into spontaneous ISC, and the mechanisms involved in the long-term incubation process are quite complex.

The article introduced the current research results on the phenomenon, causes, and response strategies of thermal runaway. The most common characteristics of all abuse conditions are internal short circuits, including mechanical abuse, electrical abuse, and thermal abuse. Thermal runaway follows the mechanism of chain reaction, during which the decomposition reactions of battery component materials occur one after another.