How to extinguish the fire of lithium battery?

Lithium batteries can be divided into two major parts, their own causes and external causes. Their own reasons mainly refers to a material, the thermal stability of the structure is good or bad, or not effect on the fire; external causes, refer to various methods of abuse, causing lithium battery fire.

Lithium battery positive electrode material, electrolyte composition and anode material, the thermal stability of these portions, directly affects the possibility of thermal runaway of battery cells occur.

Factors affecting the thermal stability of the anode material

Most of the anode materials currently in use are carbon materials. Under high temperature conditions, graphite easily reacts with the electrolyte, especially in the state where the battery charge is high, and LiC6 can increase the reaction intensity.

It has been found that the starting point of the temperature at which the negative electrode starts to react exotherm is related to the particle size of the carbon material. The larger the particle, the higher the temperature at which the reaction starts, and the safer it is. At the same time, carbon materials of different structures participate in the reaction of the electrolyte, and the heat release amount is not the same. Graphite has a larger heat release than amorphous carbon (mainly soft carbon and hard carbon).

Factors affecting the thermal stability of the cathode material

Lithium battery cathode materials, which are widely used at present, are all lithium compounds. The security level of lithium iron phosphate, lithium manganate and ternary lithium are in order. Some people have specifically studied the effects of cathode materials on the safety of these batteries.

It is considered that the higher the lithium content in the molecular formula of lithium, the worse the thermal stability and the lower the temperature at which the reaction with the electrolyte begins. There is a quantitative comparison, the proportional coefficient of each atom in the formula, when the coefficient of lithium is 0.25, the reaction temperature is 230 ° C; if this value becomes 1, the initial reaction temperature becomes 170 ° C. Further, if the positive electrode material contains a metal element other than lithium, the positive electrode material containing the manganese element is more thermally stable than the positive electrode material containing the nickel element.

Influencing factors of electrolyte thermal stability

Electrolyte can be said to be the core of thermal stability problems, and its stability directly affects the stability of the entire system. Some studies have been done on the thermal stability of the electrolyte, and the results show that:

The higher the content of dimethyl carbonate in the electrolyte, the worse the thermal stability is, and the easier it is to react with the positive and negative materials; the more compatible the electrolyte with the more types of materials, that is, at lower temperatures A variety of different salts react, indicating that the more active it is, the worse its thermal stability.

Thermal runaway caused by aging

Aging is a comprehensive process. The structure of the negative SEI film is aging, and damage occurs, causing a self-heating process; lithium dendrites accumulate in the negative electrode, causing internal short circuit or violent reaction with the electrolyte in a high temperature environment. The increase in internal resistance caused by aging increases the probability of heat accumulation. In general, there is a positive correlation between aging and thermal runaway risk.

The correct way to extinguish a lithium battery:

1. After the battery is on fire, the power supply should be cut off in time to evacuate the personnel in an orderly manner.

2. Open the windows near the sides of the battery box to prevent smoke from causing harm to the insiders.

3. Use water quickly to extinguish the fire and extinguish the fire according to the standard fire extinguishing method. (Dongguan Fenggang 2014 Mingyang Battery Factory fires, employees use dry powder fire extinguisher is invalid, Li Shujun, Ph.D., Ph.D., graduated from the Institute of High Energy Physics, Chinese Academy of Sciences, believes that due to the internal combustion of the battery, dry powder fire extinguishers will not work at this time. The best way is to use water to cool down.)

4. After the fire is extinguished, it is necessary to wait for the battery module to cool before processing and moving out of the battery compartment.

Lithium battery fire extinguishing is mainly caused by thermal runaway. If you need to extinguish the fire, you first need to understand the real cause of thermal runaway. The main causes of thermal runaway of lithium batteries are external short circuit, external high temperature and internal short circuit. ◎Internal short circuit: due to the abuse of the battery, such as the over-charge and over-discharge caused by the crystal, the magazine dust during the battery production process, will deteriorate the formation of the piercing diaphragm, resulting in a micro-short circuit, the release of electrical energy leads to temperature rise, temperature rise band The chemical reaction of the material further expands the short-circuit path and creates a larger short-circuit current. This mutual accumulation of mutually enhanced damage leads to thermal runaway. Taking a lithium cobalt oxide battery as an example, a typical thermal runaway process is briefly described. A: During the preparation phase, the battery is in a fully charged state; B: an internal short circuit occurs, a large current generates heat through the short-circuit point, and is thermally diffused by LiC6 to reach the decomposition temperature of the SEI film, and the SEI film begins to decompose, releasing a small amount of CO2 and C2H4, and the shell The body swells slightly. As the short-circuit position continues to discharge, the battery temperature rises continuously, and the chain solvent in the electro-hydraulic begins to disperse. LiC6 and electro-hydraulic also begin to react and exotherm, accompanied by C2H5F\C3H6\C3H8, but the reaction is slow. The heat release is small; C: As the discharge progresses, the temperature of the short-circuit position continues to rise, the partial shrinkage of the diaphragm melts, the short-circuit position expands, and the temperature further rises. When the internal temperature reaches the decomposition temperature of Li0.5Co02, the positive electrode decomposes instantaneously. And release O2, the latter reacts a large amount of heat in the electro-hydraulic reaction, and releases a large amount of CO2 gas, causing the internal pressure of the battery to increase. If the pressure is large enough, the battery casing is broken, causing the battery to explode; D: if the casing is blasted The pole piece is scattered, the temperature does not continue to rise, the reaction is terminated; but if the shell is only cracked, the pole piece is not scattered, then LiC6 continues to react with the electro-hydraulic, the temperature will continue High, but the heating rate decreased, since the reaction rate is slower, can be maintained for a long time; E: When the heat generation rate of the internal reaction of the battery is less than the heat dissipation rate, the battery begins to cool down until the internal reaction is completed; ◎ External short circuit: the probability of occurrence of danger in the actual vehicle operation is very low. First, the vehicle system is equipped with a fuse and a battery management system. BMS, the second is that the battery can withstand short-term high current impact. In the extreme case, the short-circuit point crosses the vehicle fuse and the BMS fails. The external short-circuit for a long time generally causes the weak connection point in the circuit to burn, which rarely causes the battery to run out of thermal events. Nowadays, more PACK companies adopt the practice of adding fuses in the loop, which can effectively avoid the hazards caused by external short circuits. ◎External high temperature: Due to the characteristics of the lithium battery structure, the SEI film, electrolyte solution, EC, etc. will decompose at high temperature, and the decomposition product of the electrolyte will react with the positive electrode and the negative electrode, and the cell diaphragm will melt and decompose, and various reactions Causes a lot of heat to be generated. The melting of the diaphragm causes an internal short circuit, which in turn increases the production of heat. The result of this cumulative mutual reinforcement damage is that the explosion-proof membrane of the battery core is broken, the electrolyte is ejected, and combustion and fire occur. For the above reasons, the lithium battery is fire extinguished. Let us look at the recommendations of Tesla and GM: 1. If there is a small fire, the flame does not spread to the high-voltage battery, you can use carbon dioxide or ABC dry powder fire extinguisher to extinguish the fire. 2. When thoroughly inspecting the fire, do not touch any high-voltage parts, always use insulation tools for inspection. 3. The gas cylinder, gas column and other components that store the gas can reach the extreme temperature of the boiling liquid expansion steam explosion. Before the "hot zone" of the accident is detected, dismantling with appropriate fine protection is required. 4. If the high-voltage battery is bent, twisted, or damaged in a fire, it may become unsatisfactory or suspect battery problems. That the amount of water used for fire-fighting should not be too small, and the amount of fire water should be sufficient. 5. It can take up to 24 hours for the battery to catch fire. Using a thermal imaging camera ensures that the high voltage battery is completely cooled before the end of the accident. If you don't have a thermal imaging camera, you must monitor if the battery will re-ignite. Smoke indicates that the battery is still hot, and monitoring is maintained until at least one hour after the battery is no longer smoking. In the emergency rescue manual of General Volanda, the fire extinguishing of electric vehicles is guided as follows: If the battery reaches a high enough temperature to leak and release electrolyte, the electrolyte must be flammable. This requires a large amount of water to cool the battery and extinguish the fire. Because the DC and AC systems are not grounded, firefighters can use water safely as the primary fire extinguishing agent and there is no risk of electric shock. ABC dry powder fire extinguishers do not extinguish the battery flame. Firefighters should avoid direct internal contact in any high-voltage components that are subject to fire or release operations, which can potentially result in electric shock.

The fire extinguishing agent that can really extinguish the battery fire is not present at present. An effective attempt is to use water to cool and suffocate control, but there is pressure inside the battery and electrolyte (chemical), so the combustion is complicated and cannot directly extinguish the fire. In addition, for electric vehicles, Shenzhen is said to use a small aerosol automatic fire extinguishing device. The size of the canned fish is magnetized on the side of the battery inside the vehicle. A small amount of blasting aerosol is used to spread the fire. The small space is very good.