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What is lithium battery thermal runaway?

With the vigorous development of new energy vehicles and electrochemical energy storage, the installed capacity of lithium batteries is increasing day by day. However, the frequent accidents of new energy vehicles and energy storage power stations have drawn much attention to the safety of lithium batteries. Among them, the most dangerous factor is thermal runaway. Let’s learn more about the lithium battery thermal runaway.

What is lithium battery thermal runaway?

In GB 38031-2020 “Safety Requirements for Traction Batteries for Electric Vehicles” and GB 38032-2020 “Safety Requirements for Electric Buses”, specific explanations and related response requirements are given for thermal runaway of electric vehicles. Thermal runaway refers to the phenomenon that the battery temperature rises uncontrollably due to the exothermic chain reaction of battery cells.

The large amount of heat and harmful gases emitted by lithium battery thermal runaway can cause the battery to catch fire and explode. It often begins with the decomposition of the SEI diaphragm of the negative electrode in the battery cell, and then the decomposition and melting of the diaphragm, resulting in a reaction between the negative electrode and the electrolyte, followed by decomposition of the positive electrode and the electrolyte, causing a large-scale internal short circuit, and then spreading to other cells. causing serious thermal runaway, causing the entire battery pack to spontaneously ignite.

Lithium battery thermal runaway stage division

The large-scale melting temperature of the diaphragm provided in the literature starts at 140°C, so there are theories that divide the thermal runaway into three stages: the self-generating heat stage (50°C-140°C), the thermal runaway stage (140°C-850°C), and the thermal runaway stage (140°C-850°C). Runaway termination stage (850°C-normal temperature).

Self-generating heat stage

The self-generating heat stage, also known as the heat accumulation stage, begins with the dissolution of the SEI film, so that the negative electrode and the lithium-intercalated carbon components contained in the negative electrode are directly exposed to the electrolyte, and the lithium-intercalated carbon reacts exothermicly with the electrolyte, resulting in temperature raised.

Thermal runaway stage

In the stage of thermal runaway, after the temperature exceeds 140°C, the positive and negative electrode materials undergo electrochemical reactions, which accelerates the temperature rise. A sharp drop in voltage can be observed externally. In a short period of time, the violent reaction generates a large amount of gas and generates a large amount of heat at the same time, and expansion occurs. The heat spreads to the surroundings, and the thermal runaway spreads to other cells.

Termination of thermal runaway

Termination of thermal runaway. Once lithium battery thermal runaway occurs, the process of thermal runaway can be terminated naturally only when the reactants are exhausted.

What causes lithium battery thermal runaway

Relevant experts have proposed that there are three main thermal runaway mechanisms for high specific energy power batteries: one is that the internal short circuit caused by the puncture of the diaphragm causes thermal runaway. The second is that the positive stage of the high specific energy battery releases active oxygen, and the oxygen evolution density decreases with the increase of the specific energy. The third is that the negative electrode decomposes active lithium, which is caused by fast charging or overcharging.

We understand that the triggering causes of thermal runaway can start from internal and external factors. Refer to the diagram below.

what cause thermal runaway
What cause thermal runaway

An internal short circuit, it means that the positive and negative terminals of the battery are in direct contact. Massive internal short circuits, usually caused by mechanical and thermal abuse, will directly trigger thermal runaway. The self-developed internal short circuit will not trigger thermal runaway immediately, but the risk will gradually increase as time accumulates. “Thermal runaway mechanism of lithium ion battery for electric vehicles: A review” proposes three levels of internal short circuit, as shown in the figure below:

Three levels of short circuit
Three levels of short circuit

Mechanical abuse refers to the deformation of lithium battery cells and battery packs under the action of external forces, and the relative displacement of different parts of themselves. The main forms for the battery include impact, extrusion and puncture.

Electrical abuse generally includes external short circuit, overcharge, and overdischarge. Among them, overcharge is the most likely to develop into thermal runaway.

External short circuit, that is, two conductors with a voltage difference are connected outside the cell. The reason may be deformation due to car collision, water immersion, conductor contamination or electric shock during maintenance, etc. When the heat generated by the external short circuit cannot be dissipated, the high temperature triggers thermal runaway. Therefore, cutting off the short-circuit current or dissipating excess heat are all ways to suppress further damage caused by the external short circuit.

Overcharging, due to its energies, is one of the most harmful types of electrical abuse. Heat and gas generation are two common features in the overcharging process. The heat comes from ohmic heat and side reactions. After the active material in the battery comes into contact with air, it reacts violently and releases a lot of heat.

Over-discharge, due to the existence of voltage difference, during the over-discharge period, the battery with the lowest voltage in the battery pack is forced to discharge by other batteries connected in series, the poles are reversed, and the battery voltage becomes negative, resulting in abnormal heating. Dissolved copper ions induced by overdischarge migrate through the membrane and form copper dendrites with a lower potential on the cathode side, which may penetrate the separator and cause a severe internal short circuit.

Thermal abuse, localized overheating may be a typical thermal abuse situation that occurs in a battery pack. Thermal abuse rarely exists in isolation and often develops from mechanical and electrical abuse and can also be caused by loose connector contacts.

We can see that most of the causes of Lithium battery thermal runaway are related, as shown in the figure:

Thermal runaway cause relationship diagram

How to stop Lithium battery thermal runaway?

Once Lithium battery thermal runaway occurs, it is currently difficult to terminate it. We can only avoid Lithium battery thermal runaway from the source.

We can only start from the source and study the improvement of materials. Improve the stability of positive and negative electrode materials and electrolytes; from the outside, select a safe and efficient thermal management system to suppress the temperature rise of lithium-ion batteries. Use the thermal runaway early warning technology of BMS real-time monitoring to monitor and prevent the occurrence of thermal runaway.

We can also install the aerosol fire suppression system in the battery box/compartment in advance to extinguish the early fire caused by thermal runaway and minimize the loss of property and personnel.

What is aerosol fire suppression system?

Lithium battery thermal runaway video

Following videos shows that when the lithium battery thermal runaway , without the protection of the aerosol fire suppression system, the out-of-control battery caught fire instantly, and all the 12 fast ternary lithium batteries in the battery box exploded due to thermal runaway. And the three batteries that were overcharged under the protection of the aerosol fire extinguisher device thermally ran out of control and did not start open fire, the remaining nine batteries are intact.

It has been verified that the aerosol fire suppression system has the effect of suppressing and extinguishing the ternary lithium battery thermal runaway.

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