Lithium-ion energy storage battery fire: Causes, characteristics and fire extinguishing strategies!

1. Causes of battery fire
Battery fire is the result of the comprehensive action of oxidizer, combustibles and ignition source. Therefore, the thermal runaway mechanism of the battery can be simplified as a "thermal runaway triangle". The combustion process can be terminated with the destruction of any element of the "thermal runaway triangle". Therefore, a deep understanding of the "thermal runaway triangle" can guide us in effectively extinguishing battery fires.

In the case of thermal abuse, electrical abuse or mechanical abuse, there may be an abnormal temperature rise inside the sodium-ion battery, and the materials inside the battery undergo a series of exothermic reactions, further increasing the temperature and reaction rate of the battery, and eventually triggering thermal runaway. The types of combustible materials in the thermal runaway process of batteries are complex and diverse, including graphite negative electrode (flammable solid material), electrolyte solvent (flammable liquid), hydrogen and hydrocarbon gases (flammable gas) and metals (combustible metal). Moreover, the composition of combustible materials changes continuously in the process of thermal runaway. In addition, the thermal runaway evolution of the battery does not all depend on the environmental oxygen supply, and the high temperature decomposition of the positive electrode material can also release the required oxygen. Therefore, battery fire is a complex and changeable fire situation.

2. Battery fire characteristics
Compared with traditional fire, battery thermal runaway fire has its own uniqueness, summarized as follows:

(1) High temperature rise rate. The battery will spontaneously exothermic reaction under the conditions of abuse. After triggering thermal runaway, the internal exothermic reaction of the battery is severe, and a large amount of heat is generated in a short time, causing the battery temperature to rise sharply. In the process of thermal runaway, the maximum temperature rise rate of the battery even exceeds 100 ℃/s, and the surface temperature exceeds 1000℃. Such a high temperature far exceeds the ignition point of other combustible materials inside and outside the battery module, which easily leads to greater fire accidents;

(2) accompanied by intense jet fire, and high heat release rate. As the temperature rises, the chemical reaction inside the battery intensifies and releases a large amount of gas, which accumulates inside the battery, resulting in a gradual increase in internal pressure. When a certain threshold is reached, the battery safety valve breaks and ejects a large number of flammable gases, electrolytes and material particles, resulting in a fierce fire, and the flame height can even reach 1~2 m.

(3) The fire spreads fast. When one of the battery modules fails and triggers thermal runaway, the close-packed arrangement can quickly transfer heat to the adjacent battery, resulting in thermal runaway propagation within the battery module, leading to catastrophic consequences.

(4) It is difficult to extinguish the fire, and the fire is easy to reignite. In the process of thermal runaway, most of the exothermic reactions occur inside the battery. Due to the obstruction of the housing, it is difficult for the extinguishing agent to enter the inside of the battery to block the thermal runaway chain reaction. Therefore, during the fire extinguishing process, the heat inside the battery continues to be generated, resulting in a fire that is difficult to extinguish. Even if the fire is successfully extinguished, if the battery temperature cannot be completely reduced to eliminate the heat source, it is still difficult to inhibit the internal chemical reaction. At this time, the three elements of ignition source, fuel and oxidizer have not been eliminated, and the battery fire is easy to reignite after the release of fire extinguishing agent.

(5) Fire has the risk of explosion. Thermal runaway processes produce a large number of gases, including CO, CO2, CH4, C2H4, H2, and electrolyte vapors.

(6) Thermal runaway gas is toxic. In addition to being combustible and explosive, the gas produced in the battery fire also has certain toxicity, which is one of the main causes of casualties. CO is one of the most toxic gases in the fire, which can compete with oxygen for hemoglobin in the blood, reduce the oxygen supply capacity of hemoglobin, and cause hypoxia damage to human tissues.

3. Fire Fighting Strategies
The main function of fire extinguishing agent is to eliminate one or two factors in the "fire triangle", so as to achieve the purpose of suppressing the occurrence of fire and controlling the spread of fire. The mechanism of fire suppression can be divided into isolation, asphyxiation, cooling and chemical suppression. At present, the commonly used fire extinguishing agents used to suppress battery fires can be divided into gas fire extinguishing agents (carbon dioxide, heptafluoropropane, perfluorohexanone, liquid nitrogen, etc.), solid fire extinguishing agents (dry powder and aerosol fire extinguishing agents, etc.) and liquid fire extinguishing agents (water spray, water mist, foam, etc.).
Gas extinguishing agents are widely used in precision instruments and electrical fires because of their advantages of non-conductivity, no corrosion, no residue and fast flow speed. At the same time, the gas extinguishing agent can play a better fire-fighting effect in the confined space. Among them, the fire extinguishing mechanism of carbon dioxide (CO2) and other inert gases is mainly asphyxiation. Because CO2 is difficult to reduce the temperature of the battery, the fire-fighting cooling effect of CO2 is limited, and it is difficult to completely extinguish the battery fire. Liquid nitrogen has a good cooling and fire extinguishing effect on battery fire, showing a good application prospect. As a substitute for halon with excellent fire extinguishing performance, sevofluoropropane and perfluorohexanone are widely used to extinguish battery fires, and their fire extinguishing mechanisms mainly include asphyxiation, cooling and chemical suppression. Both gasification and decomposition can absorb the surrounding heat, reduce the temperature of the fire zone, and dilute the oxygen concentration. At the same time, they decompose at high temperatures to produce fluorides such as CF3 and CF2 that trap free radicals and interrupt the combustion chain reaction. However, HF will be produced during the process, and when the concentration is high, it will cause damage to human health and equipment.

Solid fire extinguishing agents such as dry powder and aerosol have the advantages of high fire extinguishing efficiency, convenient storage and low cost. Dry powder is a solid powder formed by drying, crushing and mixing inorganic salts with fire extinguishing efficiency. The main ingredient of ABC dry powder is ammonium phosphate, which can isolate, asphyxiate, cool, and chemically suppress the flame. After the dry powder is released into the flame zone, the decomposition products at high temperatures can capture H· and HO· free radicals, interrupting the combustion chain reaction. At the same time, the decomposition of the dry powder will absorb heat and produce ammonia and water vapor, diluting the oxygen concentration in the flame zone. In addition, the dry powder falls on the surface of the high-temperature fuel, melting to form a glass covering that isolates oxygen and suffocates the fuel. However, ABC dry powder has limited cooling capacity, and after the release, the battery temperature is still high, which may lead to reignition.
For liquid fire extinguishing agents, the direct use of a large amount of water spray can effectively take away the heat and prevent the heat from spreading out of control to adjacent battery cells, but it may cause secondary explosion risk or electrical short circuit problems due to contact with the electrolyte to produce hydrogen after the battery is damaged. Water mist increases the efficiency of water evaporation with a smaller drop size, allowing for more efficient cooling and less water consumption, while reducing these risks. Foam extinguishing agent can not only provide good thermal insulation effect, but also cover the surface of the burning material, isolate oxygen, inhibit flame reignition, especially suitable for dealing with the flow of fire or the ground overflow of flammable electrolyte liquid. However, for different types of battery fires, it is crucial to choose the right extinguishing method and extinguishing agent to ensure the safe and effective extinguishing of the fire.

Energy storage battery fires not only threaten the safety of facilities, but also pose serious challenges to the environment and personnel health. To understand the cause of fire, master the characteristics of fire, and choose the appropriate extinguishing agent and strategy is the key to prevent and deal with fire. Through the use of advanced fire extinguishing technology combined with perfect daily maintenance and management, the fire risk can be effectively reduced to ensure the stable operation of the energy storage system.