March 14, 2024
Understanding lithium battery thermal runaway: principles, causes and prevention
Lithium batteries are widely used in electric vehicles, energy storage systems and portable electronic devices due to their high energy density and long cycle life. However, there is a risk of thermal runaway during the use of lithium batteries, which may cause fire or even explosion, seriously threatening personal and property safety. This article will explore the principles, stages, causes and preventive measures of lithium battery thermal runaway in depth, aiming to improve the understanding of lithium battery safety.
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What is lithium battery thermal runaway?
Thermal runaway refers to the chain reaction phenomenon inside the battery caused by various causes. This process releases a large amount of heat and harmful gases, which in turn causes the battery to catch fire and explode. The root cause of thermal runaway is the uncontrolled exothermic reaction inside the battery, which rapidly increases the battery temperature and forms a vicious cycle.
Stages of thermal runaway of lithium batteries
The lithium battery thermal runaway can be divided into three main stages:
- Self-heating stage (heat accumulation stage)
This stage usually ranges from 50°C to 140°C. It starts with the dissolution of the negative electrode SEI film. When the temperature reaches about 90°C, the dissolution of the SEI film becomes obvious, causing the negative electrode to be exposed to the electrolyte. The lithium-embedded carbon reacts exothermically with the electrolyte, further raising the temperature and promoting the decomposition of the SEI film.
- Thermal runaway stage
After the temperature exceeds 140°C, both the positive and negative electrode materials are added to the electrochemical reaction, and the increase in the mass of the reactants makes the temperature rise faster. The diaphragm begins to melt in large quantities, and the positive and negative electrodes are directly connected, causing a large-scale short circuit. At this time, the voltage will drop sharply. The violent reaction generates a large amount of gas and heat, and the expanding gas breaks through the battery cell shell, causing material spraying. If there are other batteries around, thermal runaway may spread.
- Thermal runaway termination stage
Once thermal runaway occurs, it can only be terminated after the reactants are exhausted. Firefighting means cannot temporarily terminate the ongoing thermal runaway. The fire extinguishing agent cannot really reach the ongoing reacting material.
Causes of lithium battery thermal runaway
The triggering causes of lithium battery thermal runaway can be divided into two categories: internal and external:
Internal causes (internal short circuit)
Lithium battery internal short circuit is one of the main causes of thermal runaway. The positive and negative electrodes are in direct contact, and the degree of contact is different, and the subsequent reactions triggered are also very different. Internal short circuits are usually caused by the following factors:
- Manufacturing defects: impurities (such as metal particles) mixed in the production process, uneven electrode coating, diaphragm damage or inconsistent thickness, etc., may lead to direct contact between the positive and negative electrodes or abnormal local resistance.
- Lithium dendrite growth: Under conditions such as overcharging, high current charging or low temperature charging, uneven deposition of lithium metal is prone to occur on the surface of the negative electrode, forming lithium dendrites. When lithium dendrites penetrate the diaphragm and contact the positive electrode, internal short circuits will be triggered.
External causes
- Mechanical abuse: such as extrusion, puncture, etc., may directly damage the battery structure and cause internal short circuits.
- Electrical abuse: including external short circuit, overcharge and over-discharge, etc., which may cause the internal temperature of the battery to rise and cause thermal runaway. Overcharging will cause the positive electrode material to decompose, release oxygen, and react violently with the electrolyte. Battery over-discharge may cause the negative electrode copper foil to dissolve and damage the battery structure.
- Thermal abuse: The external high temperature environment may accelerate the internal reaction of the battery, causing the temperature to rise and causing thermal runaway.
Detailed process analysis of thermal runaway of lithium batteries
In order to have a deeper understanding of the mechanism of lithium battery thermal runaway, it can be analyzed from the following three stages:
Thermal runaway start-up stage
In the initial stage, the internal temperature of the battery gradually rises, mainly due to the following factors:
- SEI film decomposition: SEI film will decompose at higher temperatures, generating gas and heat.
- Electrolyte decomposition: The electrolyte will also decompose at high temperatures, generating gas and heat.
- At the same time, as the temperature rises, the rate of some side reactions inside the battery gradually accelerates.
Thermal runaway development stage
As the temperature continues to rise (generally between 150 and 200°C), thermal runaway enters a rapid development stage. At this point, the cathode material begins to undergo significant decomposition reactions. Taking lithium cobalt oxide as an example, its decomposition reaction equation is roughly: LiCoO₂→Li₁₋ₓCoO₂ + xLi + 1/2O₂ (x is the lithium removal coefficient). The oxygen produced by decomposition will undergo a violent redox reaction with the organic solvent in the electrolyte, releasing a large amount of heat and gas (such as CO₂, H₂, etc.).
At the same time, lithium salts (such as LiPF₆) will also decompose at high temperatures to produce LiF and PF₅, a highly corrosive Lewis acid. PF₅ will further react with the organic solvent in the electrolyte, exacerbating gas and heat generation. In addition, the diaphragm may shrink, close pores or even melt in this temperature range, resulting in obstruction of ion conduction between the positive and negative electrodes, increased resistance, further aggravating heat generation inside the battery, forming a vicious cycle, and causing the battery temperature to rise sharply.
Severe thermal runaway stage
When the battery temperature rises to above 200°C, thermal runaway enters the most severe stage. At this time, the reaction between the positive and negative electrode materials and the electrolyte inside the battery breaks out completely, and the reaction rate is extremely fast, generating a large amount of heat and gas, causing the internal pressure of the battery to rise sharply. The battery shell may swell and rupture under the huge pressure, and the electrolyte may leak and come into contact with the air.
When encountering high temperature and fire source, it is very easy to cause combustion or even explosion. At the same time, the internal structure of the battery is severely damaged, the positive and negative active materials may decompose and fall off in large quantities, and the electrode collector may deform and melt, causing the battery to completely lose its charging and discharging function and fail completely.
How to prevent lithium battery thermal runaway?
Preventing thermal runaway of lithium batteries requires starting from multiple links such as design, manufacturing and use:
Optimize battery design
- Use safer electrode materials: For example, use lithium iron phosphate (LFP) (explore TYCORUN 76V 40Ah LiFePO4 battery) instead of ternary materials to improve the thermal stability of the battery.
- Use high temperature resistant diaphragms: Select diaphragm materials with higher melting points and better thermal stability to delay diaphragm melting and prevent internal short circuits.
- Optimize battery structure: Design a reasonable battery structure to enhance heat dissipation and reduce local temperature.
Strictly control the manufacturing process
- Improve the production process level: Avoid impurities, ensure uniform electrode coating, and reduce diaphragm defects.
- Strengthen quality inspection: Conduct strict performance and safety tests on batteries to screen out defective batteries.
Safely use batteries
- Avoid overcharge and over-discharge: Use chargers and battery management systems (BMS) with overcharge and over-discharge protection functions.
- Avoid external short circuits: Prevent metal objects from contacting the positive and negative poles of the battery.
- Avoid mechanical abuse: Prevent the battery from mechanical damage such as extrusion and puncture.
- Avoid high temperature environment: Store the battery in a cool and ventilated place, avoid direct sunlight and high temperature environment.
Battery Management System (BMS)
Real-time monitoring of battery status: BMS can monitor the battery voltage, current, temperature and other parameters in real time, and detect abnormal conditions in time.
- Provide protection function: BMS has protection functions such as overcharge, over-discharge, overcurrent, overtemperature, etc. to prevent battery safety accidents.
- Thermal management: Some advanced BMS also have thermal management functions (explore battery thermal management), which maintain the battery temperature within a safe range by controlling the cooling system.
Conclusion
Lithium battery thermal runaway is a complex process involving the interaction of multiple factors. By deeply understanding the principles, stages and causes of thermal runaway and taking effective preventive measures, the safety risks of lithium batteries can be significantly reduced and their application reliability in various fields can be improved. In the future, with the continuous development of battery technology, safer and more reliable lithium battery products will continue to emerge.
FAQ
What is lithium battery thermal runaway?
Thermal runaway refers to a chain chemical reaction that occurs inside a lithium battery when it is stimulated by overheating, short circuit or overcharging, causing the temperature to rise rapidly and uncontrollably, which may eventually cause a fire or explosion.
What are the main causes of thermal runaway of lithium batteries?
- Overcharge/overdischarge
- Internal short circuit
- External high temperature environment
- Physical damage to the battery (such as impact, puncture)
- Poor manufacturing or lack of protection circuit
Will a lithium battery explode if it has thermal runaway?
Yes, thermal runaway may cause a sudden increase in pressure inside the battery, eventually causing an explosion or violent combustion.
How to determine whether a lithium battery may have a risk of thermal runaway?
If the battery has abnormal heating (such as obvious heat when charging), bulging, leakage or odor (such as sour smell, burnt plastic smell), or a sudden drop in battery life or abnormal charging and discharging, it should be stopped immediately and professionally repaired.
What measures can users take to prevent lithium battery thermal runaway?
- Use smart batteries with BMS
- Avoid overcharge/over discharge
- Control ambient temperature (0~45℃)
- Use certified products
Will thermal runaway spread to other cells?
Yes, it is called "thermal runaway propagation". Without good thermal isolation, a cell with thermal runaway can quickly affect adjacent cells.
Which lithium battery is more prone to thermal runaway?
NCM and NCA have high energy density but are more prone to thermal runaway;
LiFePO₄ has better thermal stability and is relatively safe.
What should I do if thermal runaway occurs?
1. Immediately stay away from the battery
2. Do not use water to extinguish the fire, use a dry powder fire extinguisher
3. Cut off the power and seek professional help
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Willow
Willow is a materials engineer with a Master’s degree in Materials Science and Engineering, specializing in lithium-ion battery materials and energy storage technologies.
Her work focuses on EV battery swapping solutions, battery innovation, and new energy industry trends, aiming to translate research insights into practical applications for sustainable transportation.
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