Understanding thermal runaway lithium ion battery – causes, mechanism, and prevention
Thermal runaway lithium ion battery is a key safety problem in modern energy storage technology. To understand the mechanism of thermal runaway of lithium battery and its preventive measures is very important to ensure the safety and reliability of battery system.
This article will tell us the causes, mechanism, and prevention of thermal runaway lithium ion battery.
Table of Contents
What is thermal runaway in lithium ion batteries?
Lithium battery thermal runaway refers to the phenomenon that lithium battery current and internal temperature rise occur a cumulative mutually reinforcing use, resulting in lithium battery damage. The main body of thermal runaway in a narrow sense refers to a single cell. Generalized thermal runaway, the subject of which refers to PACK.
Simply put, thermal runaway is a positive energy feedback loop process: rising temperatures cause the system to heat up, which in turn causes the system to heat up, which in turn makes the system even hotter, and ultimately causes a fire or explosion.
What causes thermal runaway in lithium ion batteries?
The main factors that cause the thermal runaway of lithium-ion batteries are external short circuit, external high temperature and internal short circuit.
External short circuit
The probability of danger in actual vehicle operation is extremely low, one is that the vehicle system is equipped with fuse and battery management system BMS, and the other is that the battery can withstand a short time of large current impact.
External high temperature
Due to the characteristics of the lithium ion battery structure. At high temperatures, SEI film, electrolyte, EC and other decomposition reactions occur, the decomposition of lithium battery electrolyte will also react with positive and negative electrodes, the cell diaphragm will melt and decompose, and a variety of reactions will produce a lot of heat.
The melting of the diaphragm leads to an internal short circuit, and the release of electrical energy increases heat production.
The result of this cumulative and mutually reinforcing destructive use is that the explosion proof film of the battery cell is broken, the electrolyte is ejected, and the fire occurs.
Internal short circuit
Due to the wrong use of lithium-ion batteries or lithium battery quality defects, such as the branch crystals caused by overcharge and overdischarge, impurities in the battery production process dust, etc., will deteriorate the growth and puncture the diaphragm, there is a micro short circuit, the release of electrical energy leads to temperature rise, and the chemical reaction of the material brought by temperature rise expands the short circuit path, forming a larger short circuit current. A cumulative, mutually reinforcing destruction is formed, leading to thermal runaway.
In the cases of fire caused by thermal runaway of lithium batteries in recent years, most of them are first caused by internal short circuits, and its heat and temperature form an external high temperature environment for adjacent batteries, triggering thermal runaway of adjacent batteries, which leads to a chain reaction of the whole PACK.
Mechanism of Thermal runaway lithium ion battery
Lithium batteries are lithium ions embedded in carbon (petroleum coke and graphite) to form a negative electrode.
The cathode material is usually LixCoO2, LixNiO2 and LixMnO4, and the electrolyte is LiPF6+ diethylene carbonate (EC)+ dimethyl carbonate (DMC).
The main inducing factors of thermal runaway are mechanical damage, battery overcharge, internal short circuit , etc. Under the influence of various factors, the active material inside the lithium-ion battery has a violent exothermic reaction, and the internal temperature of the battery exceeds the controllable range, which ultimately leads to thermal runaway.
The exothermic chemical reaction in lithium ion batteries comprises the decomposition of SEI solid electrolyte interface film, the reaction of negative active material and electrolyte, the reaction of negative active material and binder, and the oxidation decomposition reaction of electrolyte.
How to prevent thermal runaway in lithium ion batteries?
The inducement of thermal runaway is multiple. For the situation of lithium battery thermal runaway, the current mainstream solution is to improve from the two aspects of external protection and internal improvement. External protection mainly refers to the upgrading and improvement of the system, and internal improvement is to improve the battery itself.
External protection
PTC (positive temperature coefficient) components
Install PTC components in lithium-ion batteries, considering the pressure and temperature inside the battery, when the battery is heated due to overcharge, the internal resistance of the battery is rapidly increased to limit the current, so that the voltage between the positive and negative terminals is reduced to a safe voltage, and the automatic protection function of the battery is realized.
Explosion-proof valve
When the internal pressure of the lithium battery is too large due to an anomaly, the explosion-proof valve is deformed, and the lead placed inside the battery for connection is cut off and the lithium battery charging stops.
Improved cooling system
The thermal management system is mainly responsible for controlling the temperature to ensure that the battery is always at a reasonable operating temperature. Usually, the thermal management system is controlled by the vehicle controller. When the temperature of the lithium battery is abnormal, heat dissipation or heating is carried out in time by the air conditioning system to ensure the safety and life of the battery.
Aerogel battery insulation sheet
The aerogel insulation pad can be assembled between the power battery cell and the module. When thermal control of the battery cell occurs, the aerogel with low thermal conductivity can play a heat insulation role, delaying or blocking the accident.
When the battery cell overheats and burns, the aerogel insulation sheet reaches class. A non-combustible performance can also effectively block or delay the spread of the fire, and can ensure that the battery pack does not burn or explode within 5 minutes, providing enough time for escape.
Internal improvement
Improve the electrolytic liquid system
As the blood of lithium-ion batteries, the lithium ion battery electrolyte directly determines the battery performance, and plays an important role in the battery’s capacity, operating temperature range, cycling performance and safety performance .
At present, the most widely used components in the electrolytic liquid system of commercial lithium-ion batteries are LiPF6, vinyl carbonate and linear carbonate. A large number of carbonate solvents with low boiling points and low flash points in the electrolyte will flash at a low temperature, which has great safety risks.
Therefore, many researchers try to improve the electrolytic liquid system to improve the safety performance of the electrolyte. If the main material of the battery does not undergo subversive changes in a short time, improving the stability of the electrolyte is an important way to enhance the safety of the battery.
Positive electrode material
Lithium-ion battery cathode material is unstable when the charging state lithium ion battery voltage is higher than 4V, and it is easy to thermal decomposition at high temperatures to release oxygen, and oxygen continues to react with organic solvents to produce a large amount of heat and other gases, reducing the safety of the battery. Therefore, the reaction between lithium ion battery anode and electrolyte is considered to be the main cause of thermal runaway.
For positive electrode materials, coating modification is a common method to improve their safety. Such as MgO, A12O3, SiO2, TiO2, ZnO, SnO2, ZrO2 and other substances on the surface of the positive electrode material, can reduce the reaction between the positive electrode and the electrolyte after Li+ removal, while reducing the oxygen release of the positive electrode, inhibit the phase change of the positive electrode material, improve its structural stability, reduce the disorder of the cation in the lattice, and reduce the positive electrode material. This reduces heat generation from side reactions during the cycle.
Separator
At present, the most widely used separator in commercial lithium-ion batteries is still polyolefin materials, whose main disadvantages are heat shrinkage at high temperatures and poor electrolyte wetting.
In order to overcome these defects, researchers have tried many methods, such as finding a heat-stable material instead, or adding a small amount of Al2O3 or SiO2 nanoparticle powder to the diaphragm, which not only uses ordinary diaphragm, but also improves the thermal stability of the positive electrode material.
Conclusion
The frequent occurrence of thermal runaway accidents of lithium-ion batteries is shocking. In order to avoid lithium-ion battery safety accidents, improve the safety of lithium-ion batteries, and avoid the occurrence of thermal runaway, it is necessary to take a multi-pronged approach from the battery formula design, structural design and thermal management design of battery packs to jointly improve the thermal stability of lithium-ion batteries and reduce the possibility of thermal runaway.
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Understanding thermal runaway lithium ion battery – causes, mechanism, and prevention
Thermal runaway lithium ion battery is a key safety problem in modern energy storage technology. To understand the mechanism of thermal runaway of lithium battery and its preventive measures is very important to ensure the safety and reliability of battery system.
This article will tell us the causes, mechanism, and prevention of thermal runaway lithium ion battery.
What is thermal runaway in lithium ion batteries?
Lithium battery thermal runaway refers to the phenomenon that lithium battery current and internal temperature rise occur a cumulative mutually reinforcing use, resulting in lithium battery damage. The main body of thermal runaway in a narrow sense refers to a single cell. Generalized thermal runaway, the subject of which refers to PACK.
Simply put, thermal runaway is a positive energy feedback loop process: rising temperatures cause the system to heat up, which in turn causes the system to heat up, which in turn makes the system even hotter, and ultimately causes a fire or explosion.
What causes thermal runaway in lithium ion batteries?
External short circuit
External high temperature
The melting of the diaphragm leads to an internal short circuit, and the release of electrical energy increases heat production.
The result of this cumulative and mutually reinforcing destructive use is that the explosion proof film of the battery cell is broken, the electrolyte is ejected, and the fire occurs.
Internal short circuit
Due to the wrong use of lithium-ion batteries or lithium battery quality defects, such as the branch crystals caused by overcharge and overdischarge, impurities in the battery production process dust, etc., will deteriorate the growth and puncture the diaphragm, there is a micro short circuit, the release of electrical energy leads to temperature rise, and the chemical reaction of the material brought by temperature rise expands the short circuit path, forming a larger short circuit current. A cumulative, mutually reinforcing destruction is formed, leading to thermal runaway.
In the cases of fire caused by thermal runaway of lithium batteries in recent years, most of them are first caused by internal short circuits, and its heat and temperature form an external high temperature environment for adjacent batteries, triggering thermal runaway of adjacent batteries, which leads to a chain reaction of the whole PACK.
Mechanism of Thermal runaway lithium ion battery
Lithium batteries are lithium ions embedded in carbon (petroleum coke and graphite) to form a negative electrode.
The cathode material is usually LixCoO2, LixNiO2 and LixMnO4, and the electrolyte is LiPF6+ diethylene carbonate (EC)+ dimethyl carbonate (DMC).
The main inducing factors of thermal runaway are mechanical damage, battery overcharge, internal short circuit , etc. Under the influence of various factors, the active material inside the lithium-ion battery has a violent exothermic reaction, and the internal temperature of the battery exceeds the controllable range, which ultimately leads to thermal runaway.
The exothermic chemical reaction in lithium ion batteries comprises the decomposition of SEI solid electrolyte interface film, the reaction of negative active material and electrolyte, the reaction of negative active material and binder, and the oxidation decomposition reaction of electrolyte.
How to prevent thermal runaway in lithium ion batteries?
External protection
PTC (positive temperature coefficient) components
Explosion-proof valve
Improved cooling system
The thermal management system is mainly responsible for controlling the temperature to ensure that the battery is always at a reasonable operating temperature. Usually, the thermal management system is controlled by the vehicle controller. When the temperature of the lithium battery is abnormal, heat dissipation or heating is carried out in time by the air conditioning system to ensure the safety and life of the battery.
Aerogel battery insulation sheet
The aerogel insulation pad can be assembled between the power battery cell and the module. When thermal control of the battery cell occurs, the aerogel with low thermal conductivity can play a heat insulation role, delaying or blocking the accident.
When the battery cell overheats and burns, the aerogel insulation sheet reaches class. A non-combustible performance can also effectively block or delay the spread of the fire, and can ensure that the battery pack does not burn or explode within 5 minutes, providing enough time for escape.
Internal improvement
Improve the electrolytic liquid system
As the blood of lithium-ion batteries, the lithium ion battery electrolyte directly determines the battery performance, and plays an important role in the battery’s capacity, operating temperature range, cycling performance and safety performance .
At present, the most widely used components in the electrolytic liquid system of commercial lithium-ion batteries are LiPF6, vinyl carbonate and linear carbonate. A large number of carbonate solvents with low boiling points and low flash points in the electrolyte will flash at a low temperature, which has great safety risks.
Therefore, many researchers try to improve the electrolytic liquid system to improve the safety performance of the electrolyte. If the main material of the battery does not undergo subversive changes in a short time, improving the stability of the electrolyte is an important way to enhance the safety of the battery.
Positive electrode material
Lithium-ion battery cathode material is unstable when the charging state lithium ion battery voltage is higher than 4V, and it is easy to thermal decomposition at high temperatures to release oxygen, and oxygen continues to react with organic solvents to produce a large amount of heat and other gases, reducing the safety of the battery. Therefore, the reaction between lithium ion battery anode and electrolyte is considered to be the main cause of thermal runaway.
For positive electrode materials, coating modification is a common method to improve their safety. Such as MgO, A12O3, SiO2, TiO2, ZnO, SnO2, ZrO2 and other substances on the surface of the positive electrode material, can reduce the reaction between the positive electrode and the electrolyte after Li+ removal, while reducing the oxygen release of the positive electrode, inhibit the phase change of the positive electrode material, improve its structural stability, reduce the disorder of the cation in the lattice, and reduce the positive electrode material. This reduces heat generation from side reactions during the cycle.
Separator
At present, the most widely used separator in commercial lithium-ion batteries is still polyolefin materials, whose main disadvantages are heat shrinkage at high temperatures and poor electrolyte wetting.
In order to overcome these defects, researchers have tried many methods, such as finding a heat-stable material instead, or adding a small amount of Al2O3 or SiO2 nanoparticle powder to the diaphragm, which not only uses ordinary diaphragm, but also improves the thermal stability of the positive electrode material.
Conclusion