What is the Thermal Runaway of Lithium ion Battery?

Thermal runaway is the main object of research on improving the safety of lithium-ion batteries. Today, NPP will talk about  thermal runaway with everyone to get the reasons. At present, both power batteries and energy storage batteries are using lithium-ion batteries on a large scale, and people are still dissatisfied with the spontaneous combustion and explosion of batteries. The issue of battery safety is still a topic that cannot be avoided in the industry.

What is Thermal Runaway?

Overview

Thermal runaway refers to the chain reaction phenomenon caused by various incentives. The large amount of heat and harmful gases emitted by thermal runaways will cause the battery to catch fire and explode. Battery thermal runaway often begins with the decomposition of the negative electrode SEI film in the battery cell, and then the diaphragm decomposes and melts, resulting in a reaction between the negative electrode and the electrolyte, followed by decomposition of the positive electrode and the electrolyte. As a result, a large-scale internal short circuit was caused, causing the electrolyte to burn, and then spread to other cells, causing severe thermal runaway, and causing the entire battery pack to spontaneously ignite.

(*SEI film: SEI film is a layer of passivation film formed by the reaction of negative electrode material and electrolyte during the first charge formation of lithium battery. Its function is to cover the negative electrode material on the one hand and protect its structure from damage; On the one hand, it is able to allow lithium ions to pass through and be embedded in the negative electrode material.)

What are the stages of thermal runaway?

There are different opinions on how to divide the stages of thermal runaway. The core should be that the thermal trend will not be reversed beyond that point. It has been theorized that this point is a massive dissolution of the septum. Before that, the temperature drops, the activity of the substance decreases, and the reaction will slow down. Once this point is broken, the positive and negative electrodes are directly facing each other, the internal temperature of the battery cannot be lowered, and the continuation of the reaction cannot be terminated.

The theory divides thermal runaway into three stages,

Self-generating heat stage (50°C-140°C),

Thermal runaway stage (140°C-850°C),

Thermal runaway termination stage (850°C-room temperature),

Some documents provide a large-scale melting temperature of the separator starting at 140 °C.

Self-generating Heat Stage

The self-generating heat stage, also known as the heat accumulation stage, begins with the dissolution of the SEI film. When the temperature of the SEI film reaches about 90°C, its dissolution phenomenon will be clearly observed. The dissolution of the SEI film will make the negative electrode and the lithium-intercalated carbon components contained in the negative electrode directly exposed to the electrolyte.

The lithium-intercalated carbon reacts exothermically with the electrolyte, causing the temperature to rise. The increase in temperature in turn promotes the further decomposition of the SEI film. If there are no external cooling means, this process will roll forward until the SEI film is completely decomposed.

Thermal Runaway Stage

After the temperature exceeds 140°C, both the positive and negative electrode materials join the ranks of electrochemical reactions, and the increase in the number of reactants makes the temperature increase faster.

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 the heat heats the gas. The expanded gas breaks through the cell shell, and a phenomenon such as material ejection occurs, and the scattered material also takes away part of the heat. Thermal runaway reached its most intense state. The highest temperature is also reached at this stage.

The parameter change that can be observed outside is a sharp drop in voltage. The process is described as: after reaching this temperature range, the diaphragm begins to melt in large quantities, and the positive and negative electrodes are directly connected, resulting in a large-scale short circuit.

At this point, thermal runaway has begun and will not stop.

If there are other cells around, thermal runaway may spread to other cells at this stage by spreading the heat to the surroundings.

The heat may be conducted through the connected conductive parts, or due to volume expansion, the cells that originally kept the distance are already close to each other at this time, and the heat is directly conducted between the cell shells.

Thermal Runaway Termination

Once thermal runaway occurs, its termination can only be the complete combustion of reactants. A report from the fire department showed that for devices such as lithium batteries that contain high energy in a closed case, fire protection means are temporarily unable to stop the ongoing thermal runaway. An extinguishing agent that cannot actually touch the reacting substance in progress. Firefighters are at high risk at a fire scene, but the measures they can take are limited, generally isolating the scene of the accident.

Only when the reactants are exhausted can the thermal runaway process be terminated naturally.

What causes of thermal runaway?

The triggering causes of thermal runaways can be divided into two categories, internal and external.

Internal short circuit

Internal short circuit means that the positive and negative electrodes of the battery are in direct contact. The degree of contact is different, and the subsequent reactions triggered are also very different. Massive internal short circuits, usually caused by mechanical and thermal abuse, will directly trigger thermal runaway. On the contrary, the self-developed internal short circuit is relatively mild, and it generates very little heat, which will not immediately trigger thermal runaway. Common internal self-development includes manufacturing defects and various performance declines caused by battery aging, such as increased internal resistance, lithium metal deposition caused by long-term mild improper use, etc. Over time, this internal cause causes internal short circuit risk will gradually increase.

“Thermal runaway mechanism of lithium-ion battery for electric vehicles: A review” proposes three levels of internal short circuits, which is convenient for us to understand, as shown in the figure below:

Mechanical abuse

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. For example, foreign objects touched by the vehicle at high speed directly lead to the collapse of the internal diaphragm of the battery, which in turn causes a short circuit in the battery and triggers spontaneous combustion in a short period of time.

Electricity Abuse

The electrical abuse of lithium batteries generally includes external short circuits, overcharge, and over discharge. Among them, overcharging is the most likely to develop into thermal runaway.

External short circuit, when two conductors with a voltage difference are connected outside the cell, an external short circuit occurs. The external short circuit of the battery pack may be caused by deformation caused by car collision, water immersion, conductor pollution, or electric shock during maintenance, etc. Typically, the heat released by an external short circuit does not heat the battery compared to a puncture. From external short circuits to thermal runaways, the important link in the middle is that the temperature reaches a high point. When the heat generated by the external short circuit cannot be dissipated well, the battery temperature will rise, and the high temperature triggers thermal runaway. Therefore, cutting off the short-circuit current or dissipating excess heat are all methods 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. First, lithium dendrites grow on the anode surface due to excessive lithium intercalation. The point at which lithium dendrites start to grow is determined by the stoichiometric ratio of the cathode and anode. Second, the excessive deintercalation of Li leads to the collapse of the cathode structure due to heat generation and oxygen release (oxygen release from NCA cathodes [38]). The release of oxygen accelerates the decomposition of the electrolyte, producing a large amount of gas. Due to the increase in internal pressure, the vent valve opens and the battery begins to vent. After the active material in the battery comes into contact with air, it reacts violently and releases a lot of heat.

Over-discharge

Voltage inconsistencies between cells within the battery pack are inevitable. Therefore, once the BMS fails to specifically monitor the voltage of any individual cell, the cell with the lowest voltage will be over-discharged. The mechanism of over-discharge 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 forced to discharge by other batteries connected in series. During forced discharge, the poles are reversed and the battery voltage becomes negative, causing abnormal heating of the over-discharged battery. Dissolved copper ions induced by over-discharge migrate through the membrane and form copper dendrites with lower potential on the cathode side. As growth continues to rise, copper dendrites may penetrate the separator, causing severe internal short circuits.

Thermal Abuse

Localized overheating can be a typical thermal abuse situation that occurs in battery packs. Thermal abuse rarely exists in isolation, often develops from mechanical abuse and electrical abuse, and is the link that eventually directly triggers thermal runaway. In addition to overheating due to mechanical/electrical abuse, overheating can be caused by loose connection contacts. A loose battery connection issue has been confirmed. Thermal abuse is also currently the most simulated situation, using equipment to heat the battery in a controlled manner to observe its response during the heating process.

How to prevent and deal with thermal runaway?

We can see from the above that thermal runaway focuses on prevention and monitoring. Once thermal runaway occurs, there is no good way to stop it, just like a grenade that explodes, there is no way to extinguish it.

Prevention

Starting from the source, research on material improvement. The essence of thermal runaway mainly lies in the stability of positive and negative electrode materials and electrolytes. In the future, higher breakthroughs are needed in the coating and modification of positive electrode materials, the compatibility of the same body electrolyte and electrodes, and the improvement of the thermal conductivity of the battery core. Or choose a highly safe electrolyte to play a flame-retardant effect.

From the outside, use a safe and efficient thermal management system to suppress the temperature rise of the lithium-ion battery. As long as the temperature does not rise to the temperature where the SEI film starts to dissolve, terrible things will not happen. Passive cooling strategies such as adding fins, embedding metal foam, and coating phase change materials are the focus of improving the safety of lithium-ion battery applications in the future.

Monitoring

Late surveillance and control

When the cell diaphragm begins to dissolve in large quantities, a large-scale internal short circuit occurs inside the battery. The sudden voltage drop occurs at this stage, and the reason for the sudden drop is the large-scale short circuit at the positive and negative electrodes. By this stage, thermal runaway is completely impossible to contain. This process produces a detectable electrical parameter, the cell end voltage. In current BMS systems, only voltage acquisition can be accurate to each series module (with several cells in parallel). This phenomenon makes the management system know that the cell has failed. However, the moment when the voltage drop is detected is already an irreparable moment of thermal runaway. As the trigger signal of cooling measures, it has lost its meaning and can only be used as alarm information for personnel evacuation. What we can do is consider delaying the speed of thermal runaway transmission during product design and implementation. In reality, thermal runaway occurs very fast and can achieve devastating damage in a short time. Therefore, the harm of thermal runaway must be delayed or suppressed to win enough time for one to escape after an accident.

Middle and early monitoring

Thermal runaway early warning technology for BMS real-time monitoring

At present, it is easy to implement, through BMS monitoring temperature, voltage and other operating parameters, to detect early thermal runaway signs. In order to improve its fault monitoring ability, temperature sensor and voltage sensor with higher precision and reliability, and more accurate and effective state parameter estimation model can be constructed to detect abuse and abnormality as soon as possible, in which artificial intelligence can play a certain role. However, BMS scheme also has problems: it cannot be completely accurately simulated by external parameter monitoring, and cannot accurately reflect its internal electrochemical changes, thus preventing modern BMS to comprehensively assess the potential thermal runaway risk of battery cells.

Thermal runaway early warning technology based on internal state prediction.

Since it is difficult to fully control the light from the outside, let’s start from the inside. The current research direction includes: real-time detection of battery internal temperature, impedance, etc. through embedded foldable Bragg fiber optic sensors or electrochemical impedance meter frequency response analyzers, but still experimental In the laboratory stage, due to cost, technical and other issues, it cannot be applied to actual production.

Thermal runaway early warning technology based on gas detection

In the early stage of thermal runaway of lithium-ion batteries, due to the slow changes in characteristic identification parameters such as battery temperature, discharge voltage, and discharge current, battery failures cannot be detected early through modern BMS, and at this time, the electrochemical reaction inside the battery will produce a large amount of gaseous substances Therefore, it is theoretically feasible to use gas detection sensors to realize early warning of thermal runaway of lithium-ion batteries. At present, some companies have made related products in the direction of gas detection combined with fire protection.

Summary

A battery pack is like a match, and thermal runaway is like an overheated match head being ignited and triggering a cascade of reactions. At present, the industry has roughly figured out the mechanism of thermal runaway. Future research will focus more on battery body safety, thermal management, early prediction and early warning of thermal runaway, late notification and transmission obstruction, etc.