Explosions caused by electric vehicle batteries: Xinhua News Agency
Why do lithium battery explosions seem to occur so frequently in recent years?
Whether it's an electric vehicle or an energy storage power station, they all rely on a crucial component—the battery. Almost all electric vehicles and over 70% of chemical energy storage power stations use lithium batteries, the same type of batteries used in our mobile phones and laptops.
Lithium-ion batteries have made electrical energy portable, propelling the development of our information age. As a result, three scientists who made the greatest contributions to the development of lithium-ion battery technology were awarded the Nobel Prize in Chemistry.
Thanks to the development of lithium batteries, their applications are very close to our lives; our mobile phones, cameras, and Bluetooth headsets all use them. But why are there so many accidents involving lithium batteries when they are used in electric vehicles?
This is essentially a matter of probability. For example, a certain imported battery used in an imported electric vehicle is claimed to have an accident probability of only one in ten million. However, each vehicle needs to install 8,000 of these batteries, meaning that ten million batteries could power 1,250 electric vehicles. Theoretically, in one of these 1,250 electric vehicles, one battery might experience an accident. If this accident is a fire or explosion, it could trigger a chain reaction among the surrounding batteries, potentially leading to a major fire in the electric vehicle.
The same applies to accidents involving energy storage power stations. While an electric vehicle can store approximately 50-100 kWh of electricity, a typical energy storage battery container can store 1000 kWh, and a medium-to-large-scale energy storage power station often consists of dozens of such containers. It's conceivable that with such a large-scale battery usage, occasional accidents are not uncommon. Furthermore, the consequences of fires and explosions in electric vehicles and energy storage power stations are far more severe than those involving mobile phone batteries, and current fire prevention measures are almost ineffective against them. Of course, we cannot ignore the fact that in this era of rapid and widespread information dissemination, serious incidents that occasionally result in casualties are more likely to have a significant social impact.
Why do lithium batteries burn or even explode?
A lithium battery is an energy-containing device, mainly composed of a positive electrode, a negative electrode, an electrolyte, and a separator. After charging, the positive electrode is generally a transition metal oxide, which has strong oxidizing properties; the negative electrode is graphite with a large number of lithium embedded inside, which has extremely strong reducing properties. The electrolyte is generally an organic ester, characterized by low melting point and flammability.
It is particularly important to note that firecrackers in our daily lives are also a type of energy-containing device. Many people know that the gunpowder they contain consists of sulfur (chemical formula S), potassium nitrate (chemical formula KNO3), and charcoal. Among them, potassium nitrate is a strong oxidizing agent, while sulfur and charcoal are reducing agents. When an external stimulus exceeding 120 degrees Celsius is applied, a violent oxidation-reduction reaction occurs inside the firecracker, releasing a large amount of gas and heat, causing the gunpowder to burn and the firecracker to explode.
Therefore, it is theoretically possible for lithium batteries to undergo highly exothermic redox reactions, and the flammable electrolyte within them can further promote these reactions, leading to combustion or even explosion. How powerful is a lithium battery fire or explosion? From the perspective of stored energy alone, the electrical energy of a typical lithium battery with an energy density of 150 Wh/kg is approximately 1/10 of the heat energy density produced by a TNT explosion.
Recent studies have definitively demonstrated that, under certain conditions, the positive and negative electrodes in lithium-ion battery accidents can directly undergo violent redox reactions. Even aluminum and copper current collectors can directly participate in the reaction as reducing agents, generating heat significantly higher than the energy stored in the battery. Generally, in a confined space, the highest temperature of a lithium-ion battery in a safety accident can reach over 800℃. The explosive heat generated by the explosion of a 43.4g lithium-ion battery is equivalent to 5.45g of TNT, reaching 1/8 of the TNT equivalent.
The reason why lithium batteries convert their internal chemical energy into electrical energy in a controllable and continuous manner through electrochemical reactions rather than violent redox reactions is because the separator effectively physically isolates the positive and negative electrodes and provides electronic insulation (along with the presence of an ion-conducting electrolyte). However, when various internal or external factors cause the separator to fail, leading to direct contact between the positive and negative electrodes, this internal short circuit results in the instantaneous release of electrical energy, generating a large amount of heat and high temperatures. This instantly disrupts the stability of the battery's internal chemical system, causing redox reactions involving the negative and positive electrolytes, the negative and positive electrodes, and even the current collector. This instantaneous release of heat causes the electrolyte to vaporize instantly, spraying out of the battery casing along with powdered positive and negative electrode materials, leading to combustion or even explosion. This process is called thermal runaway (TR).
According to statistics on electric vehicle accidents in recent years, most accidents are caused by "spontaneous combustion," including when the battery is stationary (not charging or discharging), while driving (when the battery is discharging), and while charging. A small number of accidents occur due to external heat sources, collisions, and control circuit failures.
Spontaneous combustion falls under the category of spontaneous thermal runaway, which is collectively referred to as thermal runaway under various abuse conditions (thermal abuse, mechanical abuse, and electrical abuse). Although the mechanisms leading to temperature rise and combustion are similar in both scenarios, the ease of studying them differs significantly. Currently, research on thermal runaway under abuse conditions has made great progress in recent years due to the controllable initiation conditions, and it is now possible to quantitatively describe the mechanisms of thermal runaway triggered by various abuse conditions and the subsequent hazards. However, spontaneous thermal runaway, due to its complex and unpredictable causes, and the fact that the battery is completely destroyed after thermal runaway, making it difficult to restore its pre-runaway microscopic state, remains a research challenge.
Why is it so difficult to predict thermal runaway in lithium batteries?
Spontaneous thermal runaway is currently the biggest safety concern for electric vehicles. Why is it so difficult to prevent? This all stems from battery manufacturing.
If every single battery were 100.000000000% identical, from the microscopic electrode material particles and separator to the macroscopic electrode sheets and casing, then a battery pack made from thousands or hundreds of thousands of such batteries would certainly have better safety characteristics. You might notice that the way "100%" is expressed here is a little different, with about ten zeros after it. This represents an ideal expectation—high consistency across the entire battery scale.
As is well known, the consequence of battery inconsistency is that degraded batteries will deteriorate faster. Some become passivated and deactivated, and fail directly; while others go down a completely different path - internal short circuit, leading to thermal runaway, combustion, and explosion.
Why can't we predict this most dangerous type of spontaneous internal short circuit?
The main reasons are twofold: first, the decay process to internal short circuit is very slow and the external voltage signal is not obvious; second, the batteries that have malfunctioned all directly enter destructive thermal runaway within minutes, completely destroying the batteries and making it impossible to trace back the evidence, which also slows down the progress of research in this field.
Accurately simulating the process of spontaneous internal short circuits remains a challenge. Furthermore, batteries are like black boxes. While we can use electrochemical spectroscopy and in-situ CT techniques to monitor the electrochemical reactions and internal microstructural changes of individual batteries from the outside, we cannot predict which of tens of millions of batteries will "suddenly die" after several months or years, nor can we conduct detailed studies on its entire life cycle evolution. Each battery is virtually free of spontaneous thermal runaway risk when it leaves the factory, but which one will "suddenly die" on a summer night or winter morning six months or three years later, causing a large-scale combustion accident? This is currently very difficult to predict.
Doesn't this look like the human body?
Battery material parameters and manufacturing processes are similar to our genes; battery charging and discharging regimes are like our dietary habits; and temperature changes in the battery's operating environment are like our growth environment. As we grow, some people will develop chronic inflammation or more serious vascular lesions, which may develop into cancer or stroke in a short period of time. This is similar to a short circuit in a battery and the subsequent thermal runaway.
If we had the ability to monitor the health status of everyone on Earth 24/7 in real time, we could detect abnormalities early and take action, reducing the risk of cancer and stroke. However, this is clearly impractical. Similarly, it is difficult for us to provide comprehensive real-time monitoring of every single battery. Currently, we can equip a module consisting of dozens of batteries with devices to monitor voltage and overall temperature, but this is far from meeting the requirements for researching and preventing spontaneous thermal runaway of individual battery cells.
One thing is certain: improving battery consistency enhances the safety and reliability of battery packs. However, perfect consistency is impossible. Even considering just the particles of the positive and negative electrode active materials, the shape, surface condition, and defects of each particle are all visible under sufficiently high-resolution equipment. Besides raw materials, battery manufacturing involves dozens of complex processes, making it extremely difficult to maintain consistency. Although the power lithium battery industry invests hundreds of millions to achieve higher processing precision, the numerous raw materials and complex manufacturing processes make improving consistency a never-ending task.
Electric vehicles will undoubtedly continue to develop, and my country will continue to promote the application of large-scale energy storage technology in its energy system. Given my country's current energy structure, electric vehicles hold a crucial position in my country's medium- and long-term energy strategy and future sustainable development. It is believed that with the continued rapid development of battery technology, its reliability and safety will significantly improve in the next 5-10 years.
However, it is virtually impossible to completely eliminate lithium battery combustion accidents.
Of course, while respecting objective realities, there is still much work to be done to improve safety. Firstly, there are innovative early warning technologies. For example, a recent report from Stanford University showed that sensitive detection of hydrogen signals can advance the warning time for lithium-ion battery thermal runaway by 5 minutes, enough time for occupants of electric vehicles to escape. Secondly, battery "self-poisoning" technology is also quite effective. Its mechanism involves releasing specific chemical substances in the early stages of thermal runaway, causing passivation and "paralysis" of the battery's internal structure, thus breaking the chain of thermal runaway.
We must acknowledge the safety risks of lithium batteries, vigorously develop innovative and efficient safety enhancement technologies, and continuously improve battery manufacturing consistency. One day, such sensational news will no longer be a part of our lives, and we can use electric vehicles with peace of mind.
