Lithium-ion batteries are widely used in new energy vehicles. In recent years, the explosion accidents of new energy vehicles have caused widespread concern and attention to their safety. Lithium-ion batteries may burn and explode under extreme conditions such as overcharge, collision and high temperature, and it is very difficult to put out.

1.Analysis of buring process and characteristics of lithium-ion battery

Lithium-ion battery is mainly composed of positive electrode, negative electrode, diaphragm, electrolyte, shell and other components. The cathode material is usually lithium intercalated transition metal oxide or polyanion compound; The anode material is mostly graphite. The electrolyte is mainly composed of organic mixed solution and lithium salt.

The diaphragm is mainly used to isolate the positive and negative materials, prevent the short circuit caused by the passage of electrons, and let the ions in the electrolyte pass through. When the battery is used, carbon negative electrode, positive active material and electrolyte will undergo electrochemical or chemical reaction to release heat, which will cause the temperature rise of the battery and promote the reaction. In the case of abuse such as short circuit, high temperature, collision, etc., it is easy to cause thermal runaway inside the battery.

The thermal runaway that rapidly releases energy causes the electrolyte to burn, and the battery temperature rises rapidly. Because other components of the lithium battery, such as Shi Zhao negative electrode, diaphragm and positive electrode, are also combustible substances, which leads to the burning and even explosion of the battery composition materials.

After the battery shell is broken, the air and lithium undergo a violent oxidation reaction, which also causes the battery to burn or even explode. When the lithium ion battery burns, the positive material will decompose and release oxygen, and the battery combustion will also release combustible gases such as CO; The large amount of heat released by the internal reaction provides energy for the combustion of lithium-ion batteries. These factors also make the lithium battery burn even in a closed environment. Once it burns, it is very difficult to put it out.

2.Test methods and equipment

2.1 Test layout

The front of the DGBELL Test Chamber is equipped with an observation window with a size of 150 mm x 150 mm. Thermocouple installation interface is reserved at the back of the chamber, and pressure sensor installation interface is reserved at the top and both sides. During the test, three thermocouples are installed on the back of the chamber, and the installation position is 10 cm above the battery. The thermocouples are connected to the paperless recorder.

On the upper part of the test chamber, one pressure sensor is installed on the left and right sides respectively, which is connected with the data acquisition system to monitor the explosion pressure in the test chamber. During the test, the infrared thermometer is used to monitor the surface temperature of the battery. The test mainly monitors the combustion temperature of lithium battery and the explosion pressure in the tank. The combustion temperature is monitored by armored K-type thermocouple, and the monitoring temperature range is 0~1000 ℃; Use a paperless recorder to record the thermocouple monitoring value, with a recording interval of 1s.

The explosion pressure in the test chamber is collected by the pressure sensor and data acquisition system, and the measuring range of the pressure sensor is 0~30 kPa; The surface temperature of lithium battery is monitored by infrared thermometer, which can monitor the range of – 20~550 ℃.

2.2 Test method

The battery used for the test is a three-element lithium battery for electric vehicles with the shell removed. The three-element material refers to lithium nickel-cobalt manganate, which uses nickel salt, cobalt salt and manganese salt as raw materials. The proportion of nickel, cobalt and manganese can be adjusted, and the anode material is graphite. The rated voltage of the battery is 48 V, the rated current is 10 A, the state of charge is the factory power, and the battery size is 285 mm x 92 mm x 72 mm. By heating the bottom plate of the test chamber, the battery is continuously heated and ignited. After the battery catches fire, stop heating the bottom plate of the chamber.

3.Buring and Explosion test of lithium battery

When the first open fire occurs, the surface temperature of the battery measured by the infrared thermometer is about 205 ℃. After the open fire occurs, the battery burns violently, the combustion flame fills the whole test chamber, and sprays out from the observation window. The maximum temperature in the test chamber measured by the thermocouple is 705 ℃.

The combustion temperature monitored in the test chamber changes with time. The combustion temperature of lithium ion battery can be roughly divided into the initial growth stage, the sharp rise stage and the decay stage. In the initial growth stage, the temperature in the test chamber slowly rises due to the combined effect of external heating and battery heat release. At this stage, the battery shell is damaged and smoke is generated inside.

Before and after the battery produces open fire, the combustion temperature enters a sharp rise stage, the battery burns violently, and the explosion occurs from time to time during the combustion process, with the maximum distance of up to 4 m. After that, the temperature began to decline and entered the decay stage. The heat generated by the internal reaction of the battery accumulates in a large amount, and the instantaneous release causes the explosion phenomenon in the combustion process.

Before and after the battery has an open fire, the temperature in the test chamber begins to rise sharply. The chemical reaction rate inside the battery is very fast and the heat is released rapidly. At this time, it is difficult to take fire extinguishing measures to achieve a good cooling effect. In the test, the peak temperature is still high, so fire extinguishing and cooling measures should be taken at the initial temperature growth stage.