How should lithium battery fire be put out

Lithium battery fire

Lithium battery fire can be divided into two major causes, internal and external causes. Its own cause mainly refers to its own material, structure thermal stability, the impact of fire or not; External cause, refers to various abuse means, caused by lithium battery fire.

1.1 own causes

Lithium battery is composed of positive electrode material, negative electrode material, and electrolyte. The thermal stability of these parts directly affects the possibility of thermal runaway of the cell.

Factors affecting the thermal stability of anode materials

At present, most of the anode materials are carbon materials. Under high-temperature conditions, graphite is prone to react with electrolyte, especially in the state of high battery charge. LiC6 can enhance the intensity of the reaction.

Some studies have found that the starting point of the temperature at which the negative electrode begins to react with heat is related to the granularity of carbon material. The larger the particle, the higher the temperature at which it begins to react, and the safer it is. At the same time, carbon materials with different structures participate in the reaction of electrolyte, the heat release is not the same, graphite is more heat release than amorphous carbon (mainly refers to soft carbon and hard carbon).

Factors affecting the thermal stability of anode materials

Lithium battery anode materials, which are widely used at present, are all lithium compounds. Lithium iron phosphate, lithium management and lithium ternary are, broadly speaking, ranked from high to low in safety. The effect of anode materials on the safety of these batteries has been studied.

According to the study, the higher the content of lithium in the compound molecular formula, the worse the thermal stability and the lower the starting temperature of the reaction with the electrolyte. A quantitative comparison, formula of the ratio coefficient of individual atoms, when lithium coefficient is 0.25, the reaction temperature is 230 ℃; If this value to 1, the initial reaction becomes a temperature of 170 ℃. In addition, if the anode materials contain other metal elements besides lithium, the anode materials containing manganese have better thermal stability than those containing nickel.

Factors affecting thermal stability of electrolyte

Electrolyte can be said to be the core of thermal stability, its stability directly affects the stability of the whole system. Some people have done a series of studies on the thermal stability of electrolyte, and the results show that:

The higher the content of dimethyl carbonate in the electrolyte is, the worse the thermal stability is and the easier it will react with the anode and cathode materials. The more types of materials the electrolyte is incompatible with, i.e. it can react with a variety of different salts at a lower temperature, indicating that the more reactive it is, the worse its thermal stability will be.

The heat of aging is out of control

Aging is a comprehensive process. The negative SEI membrane structure ages and breaks, which leads to spontaneous heating process. Negative lithium dendrites accumulate, causing internal short circuit or intense reaction with electrolyte in high temperature environment. The internal resistance of aging increases, increasing the probability of heat accumulation. In general, aging is positively correlated with the risk of thermal runaway.

Lithium battery fire fighting is mainly due to the cause of heat runaway, if you need to extinguish the fire, the first need to understand the real cause of heat runaway. The main causes of runaway heat in lithium batteries are external short circuit, external high temperature and internal short circuit. Have internal short circuit: because of the abuse of the battery as the overcharge before put in crystal, the magazines such as dust in the process of producing batteries, exacerbating generated pierced diaphragm, produce micro short circuit, the electric energy that cause temperature rise of temperature of material chemistry and enlarged the short-circuit path, formed the bigger short-circuit current, the accumulation of each other and enhance each other's damage, leading to thermal runaway. A typical runaway heat process is described in the following example of lithium cobalt oxide cells. A: in the preparation stage, the battery is fully charged; B: internal short circuit occurs, large current through the short circuit point and generate heat, and through LiC6 thermal diffusion, SEI decomposition temperature, the SEI film starts to break down, let out a small amount of CO2 and C2H4, shell slight bulge, with the continuous discharge of short circuit position, battery temperature rising, the electro-hydraulic chain in solvent began to disperse, LiC6 with electro-hydraulic began exothermic reaction, accompanied by C2H5F, C3H6, C3H8, but slower reaction, heat quantity is small; C: as you progress through the discharge, short circuit position temperatures continue to rise, the diaphragm local contraction to melt, short circuit position, temperature rise further, when the internal temperature reaches Li0.5 Co02 decomposition temperature, decomposition of the positive moments, and O2 release, the latter in electrohydraulic moment reaction gives off a lot of calories, and release a large amount of CO2 gas, causing the battery internal pressure increases, if the pressure is enough big, break through the battery shell, cause the battery explosion; D: if the shell explodes and the polar plates scatter, the temperature will not continue to rise and the reaction will be terminated; However, if only the shell cracked and the polar plates were not scattered, then LiC6 continued to react with the electro-hydraulic liquid, and the temperature would continue to rise, but the rate of temperature rise would decrease. Due to the slow reaction rate, the LiC6 could last for a long time. E: when the heat generation rate of the internal reaction of the battery is lower than the heat dissipation rate, the battery begins to cool down until the internal reaction is completed; 2, external short circuit: the actual vehicle operation risk probability is very low, one is the whole car system is equipped with fuse wire and battery management system BMS, two is the battery can withstand a short time of large current impact. In the extreme case, the short circuit point crosses the fuses of the whole vehicle, and BMS fails at the same time. The external short circuit for a long time will generally cause the weak point of connection in the circuit to burn out, and rarely cause the battery to lose control of heat. Now, more PACK enterprises have adopted the practice of adding fuse wire in the circuit, which can effectively avoid the harm caused by external short circuit. 3. External high temperature: due to the characteristics of lithium battery structure, SEI film, electrolyte and EC will decompose at high temperature. The decomposed substance of electrolyte will also react with the positive electrode and the negative electrode. The melting of the diaphragm causes an internal short circuit, and the release of electrical energy increases heat production. This cumulative mutual enhancement of damage, the result is that the cell explosion - proof film rupture, electrolyte ejected, fire occurred. For the above reasons, the fire-fighting treatment for lithium battery is recommended by Tesla and general motors. 1. In case of small fire, if the flame does not spread to the high-pressure battery, carbon dioxide or ABC dry powder fire extinguisher can be used. 2. When thoroughly inspecting the fire, do not contact any high-voltage components and always use insulation tools for inspection. 3. The gas bottle, gas pillar and other components used to store the gas can reach the extreme temperature of boiling liquid, expanding steam and explosion. Disassembly with appropriate fine protection shall be carried out before the "hot zone" of the accident is detected. 4. If the high-voltage battery is bent, twisted or damaged in a fire, it will simply become disfigured, or the battery is suspected to be faulty. So fire fighting water consumption should not be too little, fire water should be enough. Battery fires can take up to 24 hours to completely extinguish. The use of thermal imaging cameras ensures that the high-voltage batteries are fully cooled before the accident is over. Without thermal-imaging cameras, the battery must be monitored for restarting. The smoke indicates that the battery is still hot, and the monitoring should be maintained until at least an hour after the battery is no longer smoking. The general motors volt's emergency response manual for electric vehicles tells you that if the battery reaches a high enough temperature to leak and release electrolytes, 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 and firefighters can safely use water as the primary extinguishing agent without the risk of electric shock. ABC dry powder extinguishers do not extinguish battery flames. Fire fighters should avoid direct contact within any high pressure assemblies during fire fighting or relief operations, which can potentially lead to electric shocks.

In view of the lithium battery fire extinguishing research, has aroused the attention in our country. At an auto industry exhibition at the end of October, more than one report discussed the research results of lithium battery fire, but it only summarized the characteristics of lithium battery fire for the time being, and the research on specific fire fighting methods was not very in-depth.

Germany, the United States and the United Kingdom led the way in the study of fire fighting methods.

Germany, for electric vehicle fire fighting, conducted a comparative experiment. It turns out that electric car fires can be put out with water, but consume a lot of water. With the addition of f-500 and Firesorb additives, the fire fighting effect was greatly improved.

Research in the United States has found that lithium battery fires are essentially caused by runaway heat and that cooling is a key aspect of fire-fighting methods. For portable lithium battery fire, screening test results show that water-based extinguishing agent has the best cooling effect, while gas and dry powder extinguishing agent has the poor effect.

The UK mainly conducted experiments on lithium battery fire of portable equipment in the process ofspecial. Halon and fe-36 were selected as the fire extinguishing agents, and they were used to dispose of lithium battery fire on flights.

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