Recently a number of electric car fires and explosions once again the safety of the power battery has been pushed to the forefront, lithium ternary materials instead of lithium iron phosphate material battery in the electric car range increased at the same time, but also led to a reduction in battery safety, in the event of a collision accident is prone to power battery combustion and other safety incidents. In order to increase the escape time of the passengers in the car when the power battery combustion accident, generally need to increase the fire extinguishing system inside the battery box to inhibit the spread of thermal runaway in the battery box interior. Unlike traditional organic combustion, the battery system that contains reducing agents, also contains oxidizing agents, so the simple isolation of air can not stop the combustion of lithium batteries, that is, the traditional fire extinguishing agents are not necessarily suitable for extinguishing power battery fires.
Power lithium-ion battery fire mainly has the following characteristics.
1. fast fire speed, long duration
According to the NFPA (National Fire Protection Association) experiments, the fire is only a few seconds in the power battery module rapid spread, lasting about 27min.
2. High combustion temperature
According to the experiments of NFPA, the maximum temperature of the external battery can reach 283-1090 degrees Celsius, and the internal temperature of the battery can reach 572-1121 degrees Celsius, which is high enough to ignite the internal battery module and other combustible materials inside the car and produce a bigger fire.
3. Difficult to extinguish
For power batteries, most of the combustion reaction occurs inside the battery shell, the battery shell prevents the fire extinguishing agent from taking effect. In practice, the extinguishing time of power battery fires often exceeds the oxygen supply time of the oxygen cylinders carried by firefighters, posing a certain threat to the personal safety of firefighters.
In order to verify the effectiveness of different extinguishing agents on power battery fires, the FAA tested aqueous solution-based extinguishing systems such as water, AF-31, AF-21, etc., gaseous extinguishing agents such as FM-200, FE-36, and dry powder extinguishing agents, and the study showed that the cooling effect of the extinguishing agent is the key factor affecting the extinguishing effect, thus water-based extinguishing agents are the most effective in extinguishing lithium-ion batteries, while dry powder and gaseous extinguishing agents are basically ineffective on lithium-ion battery fires.
Some Chinese research institutes, such as the University of Science and Technology of China, have shown that carbon dioxide, dry powder and other extinguishing agents are not very effective in extinguishing lithium-ion battery fires, which is actually not difficult to understand, mainly because the main working principle of these extinguishing agents is to isolate the oxygen in the air, so that the combustion reaction can not be carried out, but in the lithium-ion battery internal cathode material, especially in the charging state of The cathode material itself is a strong oxidizer, while the cathode active material, electrolyte, etc. are reducing agents, already have the basic conditions of combustion, so the ordinary air-isolating fire extinguishing agent does not also have an effect on lithium-ion battery fires, only the rapid cooling effect of the extinguishing agent, quickly take away the heat generated by combustion, so that the combustion reaction does not have the conditions to continue, in order to put out lithium-ion battery fires. .
Although water as an extinguishing agent to extinguish power battery fires is very effective, however, in practice there are still many problems, such as long extinguishing time, water consumption, so water is not a very efficient extinguishing agent. 2009 Bosch Germany tested the effectiveness of water, foam, dry powder and F-500 (a micro-cellular extinguishing agent, developed by Americandangerous goods The Bosch test showed that F-500 was the best choice for extinguishing power battery fires, and the DEKRA test in 2013 also showed that F-500 was a highly effective extinguishing agent for power battery fires, significantly reducing the time to extinguish power battery fires and reducing the amount of extinguishing agent consumed. consumption.
The working principle of F-500 extinguishing agent is to effectively reduce the surface tension of water through additives, which can form tiny water droplets when sprayed, and these droplets can spread to the interior of lithium-ion batteries and quickly absorb heat to achieve the effect of fire extinguishing.
In the International Symposium on Fire Science and Fire Engineering held in 2017, Weitao Luo (first author) and Shunbing Zhu (corresponding author) from Nanjing University of Technology analyzed the effect of different extinguishing agents on extinguishing power battery fires. In the experiment, the authors used 18650 battery as the research object and tested the effect of pure water, 5% F-500 solution and their own configuration of 5% solution (anionic/non-ionic surfactant) on extinguishing lithium-ion battery fires, and also tested the effect of water mist on extinguishing lithium-ion battery fires. From the experimental results (shown below), the effect of using F-500 extinguishing agent and the anionic and nonionic solution extinguishing agent prepared by the authors was significantly better than that of pure water extinguishing agent. The battery temperature using both extinguishing agents dropped rapidly, and the lithium-ion battery fire was extinguished quickly, and the extinguishing time was significantly reduced compared to that of pure water. The authors' experiments show that even with spray fire extinguishing, the effect of F-500 extinguishing agent and the anionic and non-ionic solution prepared by the authors is significantly better than that of pure water, and it takes only 4 seconds to effectively suppress the burning of lithium-ion batteries.
From the above analysis, it is easy to see that although pure water can extinguish lithium-ion battery fires, the efficiency is very low, the extinguishing time is long and the water consumption is large, so it is not suitable for such occasions as electric vehicles where space is limited and escape time is very precious, while F-500 extinguishing agent makes water form tiny droplets by reducing the surface tension of water, thus significantly improving the efficiency of extinguishing fires and making the extinguishing time significantly shorter. The amount of extinguishing agent is greatly reduced, making it ideal for use on electric vehicles. Even in some special occasions such as aircraft, the use of this type of extinguishing agent can also be used to quickly extinguish the fire when the battery of electronic equipment burns, reducing the risk than water.
