Thermal runaway is a critical issue in battery technology, particularly in lithium-ion batteries commonly used in applications ranging from consumer electronics to electric vehicles. Understanding the risks associated with thermal runaway is essential to ensure safety and mitigate potential hazards. That’s why we’re taking a closer look at the risks of thermal runaway in batteries.

What is thermal runaway in lithium batteries?

Lithium-ion thermal runaway is a complex, chain-reaction phenomenon with potentially catastrophic consequences. Lithium thermal runaway typically begins with a breakdown of the solid electrolyte interface (SEI) membrane within the battery’s negative electrode. This failure can be caused by factors such as overcharging, physical damage, or manufacturing defects. Once the SEI membrane ruptures, the separator within the battery begins to decompose and melt. This breakdown compromises the structural integrity of the battery and promotes the diffusion of reactions between the electrodes and the electrolyte. When the negative electrode interacts with the electrolyte, it initiates further decomposition reactions, resulting in the release of heat and gas. This process can quickly escalate and spread to the positive electrode, exacerbating the thermal runaway phenomenon. Extensive decomposition and breakdown within the battery can lead to internal short circuits, causing localized heating and further accelerating the chain reaction. Eventually, the electrolyte catches fire, causing violent combustion and releasing heat and harmful gases.

People imagine lithium batteries as closed balls of energy. These small batteries exist in the form of reducing agents and oxidizing agents, which enable them to charge and discharge slowly or burn violently. Lithium-ion battery thermal runaway refers to a chain reaction triggered by multiple factors, which generates heat, raises the thermal runaway temperature of lithium-ion batteries to more than a thousand degrees Celsius, causes lithium batteries to ignite violently, and releases a large amount of heat and harmful gases inside the battery in a short period of time.

Therefore, when a lithium-ion battery has thermal runaway, the energy released by the entire battery pack is amazing. A battery pack consisting of 100 batteries with a charging capacity of 100Ah has a runaway energy of 240,000,000J, equivalent to 57 kilograms of TNT. Although scientists and engineers continue to improve the design and enhance the algorithm to effectively improve the safety of automotive lithium-ion battery packs, in real life we ​​still hear about some electric vehicles and mobile phones catching fire from time to time.

What causes battery thermal runaway?

1. Overcharge: The battery itself has overcharge protection, but if this overcharge protection fails and the battery continues to charge, it will cause overcharge and trigger thermal runaway. As the battery is used for a long time, aging becomes more serious and the consistency of the battery pack becomes worse. At this time, if the battery is overcharged, thermal safety problems are very likely to occur. Therefore, it is necessary to always follow the safe charging instructions.

2. Overheating: When the battery is discharged at a high speed or encounters extreme conditions, the internal temperature of the battery gradually increases. When a large amount of heat accumulates in the battery, if the discharge current is not limited in time, it may cause thermal runaway of the lithium battery.

3. Mechanical: Impact, internal short circuit and other behaviors that damage the battery pack may cause thermal runaway.

Thermal runaway of lithium batteries

Thermal runaway is divided into three stages: self-heating stage (50℃-140℃), runaway stage (140℃-850℃), and termination stage (850℃-room temperature). Some literature shows that the bulk melting temperature of the separator starts at around 140°C.

The self-heating stage, also known as the heat accumulation stage, starts with the dissolution of the SEI film. When the temperature reaches around 90°C, the dissolution of the SEI film becomes obvious. The dissolution of the SEI film exposes the carbon components of the negative electrode and the lithium embedded in the negative electrode to the electrolyte, triggering an exothermic reaction, which increases the temperature. Conversely, the increase in temperature accelerates the further decomposition of the SEI film. If there is no external cooling mechanism, this process will continue until the SEI film is completely decomposed.

In the runaway stage, once the temperature exceeds 140℃, the positive and negative electrode materials participate in the electrochemical reaction, resulting in a faster temperature increase due to the increase in the mass of the reactants. Observable changes include a sharp drop in voltage, which is described as follows: After reaching this temperature range, the separator begins to melt in large quantities, resulting in direct contact between the positive and negative electrodes, causing a large-scale short circuit.

In a short period of time, the intense reaction produces a large amount of gas and heat. The heat further heats the gas, causing the battery casing to expand and rupture, resulting in phenomena such as material ejection. The runaway reaches its most intense state, reaching the highest temperature at this stage. If there are other batteries nearby, thermal runaway may spread to them by transferring heat to the surrounding environment. Heat may be conducted to conductive parts or expand in volume. The batteries that were initially separated can now be in direct contact, thereby facilitating heat transfer between the battery casings.

In the termination stage, once thermal runaway occurs, it can only be terminated when all reactants are consumed. A report from the fire department indicates that for closed equipment containing high-energy substances such as lithium batteries, firefighting methods cannot immediately stop the ongoing thermal runaway. The fire extinguishing agent cannot effectively reach the reacting substances. In this case, firefighters face high risks and limited available measures. Generally speaking, the approach taken is to isolate the accident site. Thermal runaway can only terminate naturally after the reactants are consumed.

Conclusion

Once a lithium battery pack experiences thermal runaway, it is like an arrow that has been shot and cannot be retrieved. At present, the industry has a general understanding of the mechanism of thermal runaway. Future research will focus more on battery safety, thermal management, early prediction and warning of thermal runaway, and communication barriers in later notification. I believe that with the continuous exploration of industry experts, a comprehensive solution to battery thermal runaway will soon be realized. By then, people can drive electric vehicles more safely, use energy storage products on a large scale, and enjoy the new life brought by clean energy with peace of mind.