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Why do electric bicycles catch fire when charging?

Writer: admin Time:2022-03-30 11:19 Browse:

Why do electric bicycles catch fire when charging? 5 questions about lithium-ion batteries
Since the beginning of the 21st century, there has been a shortage of traditional non-renewable energy sources such as coal and oil, and the serious environmental pollution caused by traditional non-renewable energy sources has prompted the development of a renewable, clean and efficient new energy system as soon as possible.
Renewable new energy mainly includes solar energy, wind energy, tidal energy, hydro energy and geothermal energy, etc. Although these renewable energy sources are rich in resources and low in development cost, they are difficult to integrate into the grid due to their own intermittent and fluctuating characteristics. use. In order to store and reuse these renewable energy sources when there is a surplus, large-scale development of energy storage technology is essential.
Are lithium-ion batteries safe?
The current energy storage technologies are mainly divided into three categories: physical energy storage, chemical energy storage and electromagnetic energy storage. Among them, chemical energy storage mainly includes flow batteries, batteries, supercapacitors, etc. Due to the advantages of stable voltage, safety and reliability, low price, wide application range, abundant raw materials and high recycling rate, batteries have become the most mature and reliable energy storage technology at present.
According to the different materials used, batteries can be divided into lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, lithium-ion batteries, etc. Among them, lithium-ion batteries have developed rapidly due to their high energy density, high output voltage, long service life, and low self-discharge rate. They have been widely used in renewable energy, smart grids, distributed power generation, microgrid systems, and new energy vehicles. , industrial energy saving, emergency power supply, household energy storage, warehousing, logistics and other fields.
With the mass production and application of lithium-ion batteries, fires and even explosions have occurred to varying degrees in the fields of energy storage, warehousing, and logistics at home and abroad.
In April 2019, an explosion occurred at an energy storage station in Arizona, the United States, injuring 4 firefighters, including 2 seriously; on April 16, 2021, an explosion at a lithium battery energy storage station in Beijing's South Fourth Ring Road, injuring 2 firefighters Sacrifice, 1 firefighter was injured; in February 2019, a fire broke out in the warehouse of Ocado, the largest fresh food e-commerce company in the UK, in Hampshire. After investigation, the fire was caused by the collision of robots in the warehouse. The overall loss is expected to exceed $400 million.
In addition, reports on pure electric vehicle fire accidents have also attracted widespread attention, and the proportion of fires caused by electric bicycles is also high. New energy fire accidents are highly sudden, the burning fire spreads rapidly, lasts for a long time, and the potential danger of the accident is large, accompanied by the release of toxic gas and the danger of explosion, and the risk of re-ignition is high. Once the lithium-ion battery pack is burned, it is difficult to fight and it is easy to cause secondary disasters.
What are the classifications of lithium-ion batteries?
Lithium-ion batteries mainly include lithium cobalt oxide batteries, lithium manganate batteries, nickel-cobalt-manganese ternary lithium batteries, lithium iron phosphate batteries, etc. At present, lithium iron phosphate batteries are most commonly used in energy storage stations. A few countries use nickel-cobalt-manganese ternary lithium batteries. The commonly used power batteries are mainly nickel-cobalt-manganese ternary lithium batteries and lithium iron phosphate batteries.
The structure of a lithium-ion battery is generally composed of a positive electrode, a negative electrode, a separator and an electrolyte. In addition, the battery also includes a binder, a conductive agent, a current collector, and a packaging material. Lithium-ion battery diaphragms, electrolytes and other materials are flammable or combustible materials that are easily decomposed by heat, and the generated gas products are flammable, resulting in thermal runaway.
With the development of electric vehicles, people have higher and higher requirements for the cruising range of electric vehicles, and require faster and faster battery charging, which increases the content and activity of active substances in the battery, and also increases the lithium-ion battery in disguise. Fire risk; increasing battery energy density also increases the potential of lithium-ion batteries to release energy.
Why do lithium-ion batteries catch fire?
The main cause of lithium-ion battery fire is the thermal runaway of the battery. When the battery is thermally out of control, the temperature of the battery can quickly rise to 400 degrees Celsius to 1000 degrees Celsius, and then fire, explosion and other accidents occur. The main reasons for thermal runaway of lithium-ion batteries can be roughly divided into mechanical abuse, electrical abuse, self-defect and high temperature environment.
Mechanical abuse includes types such as bumping, crushing, and puncturing. During use, storage and transportation of lithium-ion batteries, due to external forces such as extrusion, collision, and puncture, the battery cells or battery packs may be deformed, resulting in damage to the battery diaphragm and internal short circuit, which eventually leads to fire. In mechanical abuse, the puncture injury is the most serious, which means that the sharp conductor pierces the battery body, causing a direct short circuit between the positive and negative electrodes. The puncture causes the battery to short-circuit at the puncture point, and the short-circuit area generates a large amount of heat due to the short-circuit to form a local hot zone. When the temperature of the hot zone exceeds the critical point, thermal runaway will occur, causing smoke, fire or even explosion. Collisions, squeezing, and punctures are similar in that they cause localized internal shorts that may cause thermal runaway. The difference is that collision and extrusion are only probabilistic internal short circuits. In contrast, the heat generation during the puncture process is more intense, resulting in a higher probability of thermal runaway.
Electrical abuse mainly includes overcharge, overdischarge, short circuit and other types, mainly caused by improper use of batteries. When the battery is overcharged, the voltage of the positive electrode gradually increases, and the delithiation process becomes difficult, which leads to a sharp increase in the internal resistance of the battery, so a large amount of Joule heat is generated, and at the same time, the positive electrode oxygen releases a large amount of heat, and the negative electrode after the temperature rises There is also an exothermic reaction with the electrolyte. Thermal runaway occurs when a series of exothermic reactions cause the battery's internal temperature to rise above a certain level. When the battery is charged, lithium ions are deposited on the surface of the negative electrode to form lithium dendrites. When the battery is over-discharged, the voltage of the battery decreases, which may cause the copper foil of the negative electrode to dissolve and accelerate the failure of the battery. There are two types of short circuits: external short circuits and internal short circuits; external short circuits refer to the direct conduction between the positive and negative electrodes of lithium-ion batteries without the load. As the temperature rises, the high temperature triggers thermal runaway.
Self-defects mainly include burrs, impurities, process defects, inconsistencies and other types. During the production process of the battery, there are metal burrs around the battery pole pieces or metal particles are mixed in the pole pieces. Under certain conditions, the burrs or metal particles pierce the separator and cause an internal short circuit. Battery modules or battery packs require lithium-ion monomers to maintain good consistency during production and use. When there are production process defects or battery management system management defects, it will cause the problem of inconsistency of lithium ion cells, resulting in overcharge of battery cells and self-discharge between cells during the overall use of battery modules or battery packs. , accelerate the aging of the battery cells, the battery performance declines, and eventually lead to thermal runaway.
High temperature environment includes thermal shock, poor heat dissipation and other types. A high temperature environment will be formed when the battery is subjected to external heat and heat generated inside the battery and the battery cooling system fails. The high temperature environment will accelerate the aging of the lithium-ion battery, and the battery performance will be attenuated, resulting in an increase in the battery heat during use, a vicious cycle, and ultimately triggering thermal runaway. Excessive temperature may also directly decompose the battery separator material, forming an internal short circuit, which in turn leads to thermal runaway. In order to solve the problem of thermal runaway of lithium-ion batteries, it can be carried out from two aspects: improving the intrinsic safety of lithium-ion batteries and strengthening external protection. From the current research focus, improving the intrinsic safety of lithium-ion batteries mainly focuses on the development of positive and negative electrode materials with high thermal stability, which can reduce or eliminate the formation of needle-like metal crystals, and the development of new types of separators that are resistant to high temperature and mechanical force. , Develop flame retardant electrolytes, develop solid-state batteries with low heat production, etc. Strengthening external protection mainly includes optimizing the battery manufacturing process and strictly checking the battery's testing before leaving the factory; setting up battery monitoring and protection devices to continuously improve the management level of the battery management system; setting up a battery cooling system to enhance its cooling capacity.
How to choose a lithium-ion battery?
For consumers, in order to avoid lithium-ion battery safety accidents, the following points should be done:
First, when purchasing lithium-ion batteries and related products, you should choose products from regular manufacturers and choose a charger that matches the voltage and current of the battery;
Second, when charging lithium-ion batteries and related products, they should be properly charged according to the instructions for use to avoid excessive charging time;
Third, when storing, moving, and using lithium-ion batteries and related products, try to avoid the batteries being subjected to strong external forces such as bumps and extrusions;
Fourth, the storage and use of lithium-ion batteries and related products in high temperature and other environments should be avoided;
Fifth, do not disassemble or dissect the battery at will;
Sixth, do not change the electrical circuit of the electric bicycle without permission, and replace the large-capacity battery without permission, which will easily lead to overloading and short circuit of the wire.
How to put out a lithium-ion battery fire?
Regarding the fire extinguishing of lithium-ion battery fires, the most commonly used fire extinguishing media in energy storage stations are heptafluoropropane, perfluorohexanone, and water mist, and the most commonly used fire extinguishing media in vehicle-mounted fire extinguishing devices are dry powder and aerosol. The above fire extinguishing medium is only effective for the initial fire of lithium ion battery, and except for water, it cannot prevent the re-ignition of lithium ion battery. Lithium-ion battery fires require a large amount of water and take a long time to fight, which will cause water pollution. It is generally used for the final stage of disposal that cannot be extinguished.
When encountering a lithium-ion battery fire, you should first wear personal protective equipment and respiratory protective equipment, and then start to put out the fire.
For pure electric vehicle fires, it should be noted that in addition to close-range operations when treating or rescuing trapped persons, fire fighting and disposal should be kept at a distance of 5 meters. When putting out a fire, first cut off the power supply, and then put out the fire at the upper wind to avoid inhaling toxic fumes. It is strictly forbidden to cut, pry and disassemble vehicle structural and electrical components during firefighting and rescue to avoid electric shock and expansion of battery thermal runaway. The main focus is to control the fire, and continuously cool the battery pack to reduce the risk of explosion. Monitor battery temperature in real time to prevent thermal diffusion and re-ignition. Before the battery is fully discharged, the wreckage of the vehicle should be placed in the open air, and a reasonable safety distance should be maintained from the surrounding area. Vehicle wreckage should be kept under 24-hour monitoring, and professionals should be arranged for it.
For storage lithium-ion battery fires such as energy storage stations, the fire situation should be studied and judged before the fire is extinguished. When the fire accident does not explode in the early stage, it is strictly forbidden to directly spray water on the battery pack. When a large-scale combustion or explosion of a lithium-ion battery has occurred, a large amount of water should be used to cool the fire to prevent the accident from expanding. During the disposal process, keep a safe distance from the lithium-ion battery and protect against electric shock. Do not blindly open or demolish the closed lithium-ion battery storage room that is on fire to prevent explosion or flashover. It can carry out necessary on-site inspection, smoke exhaust, and explosion suppression operations, and use drones and other equipment to carry out fire fighting to reduce personnel safety risks.
With the advancement of technology, the research and development of new lithium-ion batteries with high energy density and good safety, the improvement of the management level of battery management systems, the research and development and application of accurate, efficient, applicable and safe lithium-ion battery fire prevention technology and equipment will be It is the future development trend of lithium-ion battery safety.