Reasons and countermeasures for lithium battery explosion and fire

1. Analysis of factors causing lithium battery explosion and fire Lithium-ion batteries are mainly composed of positive electrode materials, negative electrode materials, electrolytes and separators, and mainly rely on the intercalation and deintercalation of Li+ between the two electrodes during charge and discharge. Batteries generally use materials containing lithium as positive electrode materials, but some materials have poor chemical stability and thermal stability, and are prone to fire and explosion accidents during overcharging, impact and short circuit. In addition to positive electrode materials, the quality of negative electrode materials directly affects the performance of lithium-ion batteries. Traditional carbon negative electrode materials are prone to form solid electrolyte interface films in electrolytes, causing irreversible loss of initial capacity and reducing the efficiency of the first charge and discharge. Secondly, since the potential of the carbon negative electrode is close to that of metallic lithium, when the battery is overcharged, metallic lithium is easily precipitated on the surface of the carbon negative electrode, which may form lithium dendrites and cause short circuits.

2. The impact of overcharging on the internal structure of lithium-ion batteries A large number of experimental studies on overcharging of the internal composition principle and thermal reaction mechanism of lithium-ion batteries have shown that the higher the ambient temperature, the greater the risk of overcharging, the higher the temperature reached by the battery, and the shorter the time to reach the highest temperature. Moreover, the more cycles, the earlier the battery explodes. This is because after multiple charges and discharges, the irreversible charge and discharge process will cause tiny defects in the internal structure of the battery. This defect will be prominent in the case of high-rate overcharge, which will have an adverse effect on the safety performance of the battery. In daily life, overcharging is a very common safety hazard. During the overcharging process, the voltage of the lithium battery will lose control and continue to rise. The positive electrode material in the lithium-poor state will decompose and release heat. When the positive electrode potential rises to the electrolyte oxidation decomposition potential, the electrolyte will oxidize and decompose on the positive electrode surface, releasing a large amount of gas and heat. Experiments show that the temperature inside the battery before the fire was 140°C higher than the temperature outside the battery. It is proposed that the rapid accumulation of heat inside the battery is the result of the reaction between the positive electrode material and the electrolyte. This series of irreversible reactions may cause the internal pressure and temperature of the battery to rise sharply, causing the battery to swell and rupture, causing fire and explosion.

3. The impact of short circuit on lithium ions The electrode materials and electrolytes of lithium batteries are flammable and easily damaged, resulting in short circuits in the battery. The huge energy stored in the short-circuited lithium battery will be quickly released in the form of heat. This rapid heat release behavior will cause the internal temperature of the battery to rise sharply. The electrolyte is easy to ignite at high temperatures, causing the battery to burn as a whole. If there are flammable materials nearby, it will cause an explosion, threatening people's safety and health.

4. Causes of short circuits in lithium batteries

Lithium dendrites

When lithium batteries are overcharged and charged and discharged during the cycle, the lithium deposition on the surface of the negative electrode will be uneven, and dendrites are easily formed. At this time, the lithium dendrites grow significantly. When the dendrites fall off or break, "dead lithium" is produced, causing irreversible lithium. When sharp dendrites penetrate the diaphragm, the positive and negative electrodes are short-circuited and in a self-discharge state, accompanied by a large amount of heat generation, causing the lithium battery to catch fire or even explode. Overcharging is the main cause of lithium-ion battery overheating. The needle-shaped lithium metal crystals on the pole piece will cause micro short circuits. The gradually rising battery temperature will vaporize the electrolyte and increase the internal pressure of the battery pack, causing a violent oxidation reaction inside the battery, causing the battery to burn. Further explanation is that the combustion caused by overcharging does not necessarily occur when the battery is charged. It may occur when the material is not burned and the battery shell is not broken. As the heat inside the battery accumulates to the natural temperature of the battery, it will spontaneously combust or even explode. When the lithium battery is over-discharged, the negative carbon sheet structure will collapse. When it is charged and used again, it is very easy to have an internal short circuit and cause a fire.

External impact

Lithium ions are highly active and have a low initial explosion temperature. When they are hit by falling objects from high altitude or by a free fall object with a speed of about 3.5m/s, 25J of energy is instantly generated inside them, causing the battery temperature to rise by 120℃. The heat quickly accumulates to the initial explosion temperature of the battery, causing thermal runaway and chain reaction explosions, which can easily cause serious casualties. For example, in the early morning of May 26, 2012, a BYD E6 motor vehicle in Shenzhen was hit violently from the left rear by a luxury sports car and caught fire, killing three people on board.

Diaphragm defects

The AHP method was used to calculate that the main factor affecting the fire hazard of lithium-ion batteries is the diaphragm. The reason is that the heat release rate of the diaphragm and the smoke parameter values (CO, CO2) generated are relatively large. The incomplete combustion of the diaphragm leads to more smoke, which is easy to irritate human skin and respiratory tract, hindering the fire fighting and rescue work. At the same time, during the cutting process of lithium batteries, cutting burrs are prone to appear on the wide edges of the pole pieces. Once these burrs pierce the diaphragm, a short circuit will occur inside the battery. By coating inorganic/organic coatings, the performance of the diaphragm can be greatly improved, and the fire (short circuit) safety hazards caused by cutting burrs can be avoided, which greatly improves the safety of the battery and improves the performance of the battery.

5、 Countermeasures for lithium battery explosion and fire

The relative density of metallic lithium is only half that of water. When burned at high temperatures, lithium quickly melts into a liquid state. When the fire extinguishing agent hits the surface of lithium, it is easy to cause the "sinking" phenomenon of the fire extinguishing agent. In the literature on lithium fire extinguishing experiments, it was observed that liquid lithium flows on the surface and cannot extinguish the fire. Therefore, in order to extinguish lithium battery fires, it is necessary to first analyze the special characteristics of lithium fires, correctly select fire extinguishing agents, and take appropriate firefighting methods and disposal measures.

1. Reasonable selection of fire extinguishing agents and scientific disposal of lithium fires

Lithium metal has extremely high reactivity at high temperature. If chemical suppression, isolation and cooling extinguishing agents (such as aqueous film forming foam, carbon dioxide, ABC dry powder extinguishing agent) are simply used, the extinguishing agent can not form an effective air barrier coating on the surface of burning lithium metal. Once the extinguishing agent is stopped, the metal may re ignite. At present, Lith-X or copper powder fire extinguishing agent (rare gas argon is used as the driving gas) is used. On the one hand, copper powder reacts to generate a low activity copper lithium alloy, passivating the surface of liquid lithium to extinguish the fire. On the other hand, it can quickly dissipate the heat generated by combustion, causing the burning lithium to rapidly cool down and reduce the combustion intensity, ultimately extinguishing lithium battery fires.

2. Pay attention to maintaining a safe distance and handle emergency situations correctly

For lithium battery production and storage fires, on-site commanders should contact engineering and technical personnel as soon as possible, inquire in detail about the type and quantity of lithium batteries, and correctly select fire extinguishing agents. At the same time, set up observation posts, plan the driving route of fire trucks reasonably, and issue instructions in advance if there is a possibility of explosion at any time, quickly withdraw from the battle, and ensure the safety of firefighters themselves.

3. On site monitoring of thermal runaway temperature to prevent reignition

For lithium battery fire, it is necessary to increase the water consumption on the fire site to reduce the temperature inside the battery, use spray or spray water gun for cooling, reduce the temperature on the surface and inside of the battery, to a certain extent, reduce the exhaust of electrolyte, prevent the internal combustion of the battery, and effectively protect the safety of the battery.