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The causes of lithium-ion battery fire can be divided into two main parts: self caused and external caused. Self reasons mainly refer to the quality of the thermal stability of one's own materials and structure, which has an impact on the occurrence of a fire; External factors refer to the fires caused by various abusive means of lithium-ion batteries.

Lithium batteries are composed of positive electrode materials, negative electrode materials, and electrolyte, and the thermal stability of these parts directly affects the possibility of thermal runaway of the battery cell.

Factors affecting the thermal stability of negative electrode materials

The vast majority of negative electrode materials currently used are carbon materials. Under high temperature conditions, graphite is prone to react with the electrolyte, especially in high battery states, where LiC6 can enhance the intensity of the reaction.

A study has found that the temperature starting point at which the negative electrode begins to react and release heat is related to the particle size of the carbon material. The larger the particle size, the higher the temperature at which the reaction begins, making it safer. Meanwhile, carbon materials with different structures participate in the reaction of electrolytes, and their heat release is not the same. Graphite releases more heat than amorphous carbon (mainly referring to soft and hard carbon).

Factors affecting the thermal stability of positive electrode materials

The widely used positive electrode materials for lithium batteries are all lithium compounds. Lithium iron phosphate, lithium manganese oxide, and ternary lithium, if generalized, their safety is arranged from high to low. And someone has specifically studied the impact of positive electrode materials on the safety of these batteries.

Research suggests that the higher the lithium content in the molecular formula of lithium compounds, the poorer their thermal stability, and the lower the temperature at which they begin to react with the electrolyte. There is a quantitative comparison of the proportion coefficients of each atom in the molecular formula. When the coefficient of lithium is 0.25, the reaction temperature is 230 ℃; If this value becomes 1, the initial reaction temperature will become 170 ℃. In addition, if the positive electrode material contains metal elements other than lithium, the positive electrode material containing manganese has better thermal stability than the positive electrode material containing nickel.

Factors affecting the thermal stability of electrolytes

Electrolyte can be said to be the core of thermal stability issues, and its stability directly affects the stability of the entire system. A series of studies have been conducted on the thermal stability of electrolytes, and the results show that:

The higher the content of dimethyl carbonate in the electrolyte, the poorer its thermal stability, and the easier it is to react with positive and negative electrode materials; The electrolyte has poor compatibility with more types of materials, which means it can react with various different salts at lower temperatures, indicating that the more active it is, the poorer its thermal stability.

Thermal runaway caused by aging

Aging is a comprehensive process, where the negative electrode SEI membrane structure ages, causing damage and triggering self heating; The accumulation of negative lithium dendrites can cause internal short circuits or intense reactions with the electrolyte in high temperature environments. The increase in internal resistance caused by aging increases the probability of heat accumulation. Overall, there is a positive correlation between aging and the risk of thermal runaway.

The correct method for extinguishing lithium-ion battery fires:

1. After the battery catches fire, the power supply should be cut off in a timely manner and personnel should be evacuated calmly and orderly.

2. Open the windows on both sides of the battery compartment to prevent smoke from causing harm to personnel inside the vehicle.

3. Quickly use water to extinguish the fire and follow the standard extinguishing method. (In 2014, a fire broke out in Mingyang Battery Factory in Fenggang, Dongguan, and the dry powder extinguisher used by employees was invalid. Li Shujun, a doctor of physics who graduated from the Institute of High Energy Physics of the Chinese Academy of Sciences, believed that because of the spontaneous combustion and explosion inside the battery, the dry powder extinguisher could not work at this time, and the best way was to use water to cool it.)

4. After the fire is extinguished, it needs to wait for the battery module to cool down before handling and removing it from the battery compartment.

Lithium ion battery fire suppression is mainly caused by thermal runaway. If fire suppression is needed, the real cause of white heat runaway needs to be identified first. The main factors causing thermal runaway in lithium batteries are external short circuits, external high temperatures, and internal short circuits. Internal short circuit: Due to the abuse of batteries, such as dendrites caused by overcharging and over discharging, and magazine dust during battery production, it will deteriorate and form piercing membranes, resulting in micro short circuits. The release of electrical energy leads to temperature rise, and the material chemical reaction caused by temperature rise expands the short circuit path, forming a larger short circuit current. This cumulative and mutually reinforcing damage leads to thermal runaway. Taking lithium cobalt oxide battery cells as an example, a typical thermal runaway process is briefly described below. A: In the preparation stage, the battery is in a fully charged state; B: When an internal short circuit occurs, a large current passes through the short circuit point and generates heat, which diffuses through LiC6 and reaches the SEI film decomposition temperature. The SEI film begins to decompose, releasing a small amount of CO2 and C2H4. The shell slightly swells. As the short circuit position continues to discharge, the battery temperature continues to rise, and chain solvents in the electrolyte begin to disperse. LiC6 and the electrolyte also begin to react and release heat, accompanied by the production of C2H5F \ C3H6 \ C3H8, but the reaction is slow and the heat release is relatively small; C: As the discharge proceeds, the temperature at the short-circuit location continues to rise, the diaphragm contracts and melts locally, the short-circuit location expands, and the temperature further rises. When the internal temperature reaches the decomposition temperature of Li0.5Co02, the positive electrode instantly decomposes and releases O2, which reacts with the electrolyte to release a large amount of heat and CO2 gas, causing an increase in the internal pressure of the battery. If the pressure is large enough, it can break through the battery shell and cause the battery to explode; D: If the shell explodes and the polarizers scatter, the temperature will not continue to rise and the reaction will terminate; But if the shell only cracks and the polarizers are not scattered, LiC6 will continue to react with the electrolyte, and the temperature will continue to rise, but the heating rate will decrease. Due to the slow reaction rate, it can be maintained for a longer time; E: When the heat generation rate of the internal reaction in the battery is less than the heat dissipation rate, the battery begins to cool down until the internal reaction is complete; External short circuit: The probability of danger occurring during actual vehicle operation is very low. Firstly, the entire vehicle system is equipped with fuse and battery management system BMS, and secondly, the battery can withstand short-term high current shocks. In extreme cases, when the short circuit point crosses the vehicle fuse and the BMS fails, a prolonged external short circuit generally leads to the burning of weak connection points in the circuit, rarely causing thermal runaway events in the battery. Nowadays, many PACK companies have adopted the method of adding a fuse in the circuit, which can more effectively avoid the harm caused by external short circuits. External high temperature: Due to the characteristics of the structure of lithium batteries, the SEI film, electrolyte, EC, etc. will undergo decomposition reactions at high temperatures. The decomposition products of the electrolyte will also react with the positive and negative electrodes, and the battery cell separator will melt and decompose, resulting in a large amount of heat generation due to various reactions. The melting of the diaphragm caused an internal short circuit, and the release of electrical energy increased the production of heat. The cumulative and mutually reinforcing destructive effect leads to the rupture of the explosion-proof film of the battery cell, the spraying of electrolyte, and the occurrence of combustion and fire. Based on the above reasons, for extinguishing lithium batteries, let's take a look at Tesla and General Motors recommendations: 1. If a small fire occurs and the flame does not spread to the high-voltage battery section, carbon dioxide or ABC dry powder fire extinguishers can be used to extinguish the fire. When thoroughly inspecting the fire, do not come into contact with any high-voltage components and always use insulated tools for inspection. 3. Inflatable cylinders, gas pillars, and other components for storing gas can reach the extreme temperature of boiling liquid expansion vapor explosion. Before detecting the "hot zone" of the accident, it is necessary to dismantle it with appropriate and meticulous protection. 4. If the high-voltage battery is bent, twisted, or damaged in a fire, it will become unsightly or suspect that the battery has a problem. So the amount of water used for firefighting should not be too small, and there should be sufficient water for firefighting. 5. It may take 24 hours for the battery to catch fire and be completely extinguished. By using a thermal imaging camera, it is possible to ensure that the high-voltage battery is completely cooled before the end of the accident. If there is no thermal imaging camera, it is necessary to monitor whether the battery will reignite. Smoking indicates that the battery is still very hot, and monitoring should be maintained until at least one hour after the battery no longer smokes. The emergency rescue manual of General Motors Volanda provides guidance on the firefighting of electric vehicles: if the battery reaches a sufficiently high temperature, leaks and releases electrolytes, the electrolyte must be flammable. This requires a large amount of water to cool the battery and extinguish the fire, as the DC and AC systems are not grounded, allowing firefighters to safely use water as the main extinguishing agent without the risk of electric shock. ABC dry powder fire extinguishers will not extinguish battery flames. Firefighters should avoid direct internal contact with any high-voltage components during firefighting or relief operations, as this may potentially cause electric shock.

There is currently no fire extinguishing agent that can truly extinguish battery fires. An effective attempt is to cool with water and control suffocation. However, there is pressure and electrolyte (chemicals) inside the battery, so the combustion is more complex and cannot be directly extinguished. In addition, for electric vehicles, Shenzhen is said to use a small aerosol automatic fire extinguishing device, which is the size of a fish can. The magnet is attached to the battery inside the vehicle body, and a small amount of explosive aerosol is used to diffuse the fire, making it very useful in small spaces.

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