The power lithium-ion battery has the advantages of high energy density, long service life, high rated voltage, high power bearing capacity and low self discharge efficiency, and has become the ideal power battery for hybrid electric vehicles and pure electric vehicles. However, the safety problem of lithium-ion battery has become the main factor preventing it from being widely used in the power field.
With the increase of the number of electric vehicles, their safety problems are increasingly prominent, and the safety problems of electric vehicles mainly come from their power system – lithium ion batteries. The safety problem of lithium-ion battery is mainly caused by fire and explosion caused by thermal runaway, and one of the reasons for thermal runaway is short circuit inside the battery.
At present, the research on thermal runaway of lithium-ion battery mainly focuses on the modeling analysis and experimental analysis of one type of battery. This paper adopts different methods for 18650 battery and square battery to simulate the thermal runaway caused by short circuit in the cell of the battery module, and explore the method suitable for the verification of thermal runaway of power battery module.
1.Short circuit in the unit
In general, the risk of internal short circuit in abuse test is the greatest.
When the battery has an internal short circuit, the internal temperature rises. High temperature will induce a series of exothermic reactions of battery materials, and the heat generated by the reaction will further increase the internal temperature of the battery and intensify the exothermic reaction rate. Finally, the exothermic reaction and high temperature affect each other, presenting a runaway state, that is, thermal runaway, resulting in safety accidents such as combustion and explosion of lithium ion batteries. At present, the internal short circuit of the battery is mainly caused by the following two reasons: the defects of the first diaphragm, the pollution of raw materials or the residue of foreign objects will be continuously deteriorated and amplified during the transportation and use of the battery.
At this stage, even the battery manufacturer with the best quality control cannot completely avoid the metal impurities or burrs generated in the production process: the use of the second battery exceeds the applicable range of current, voltage and temperature specified by the manufacturer. A widely accepted method for testing internal short circuit in the industry should have the following characteristics:
(1) It can adapt to the change of battery structure and shape (cylindrical or square battery can be used
(2) The test results should be able to compare with those of other variables.
Various internal short circuits can be classified into the following four categories: negative pole to positive pole, negative pole to aluminum foil, copper foil to aluminum foil, and copper foil to positive pole
2.Introduction to short circuit method in single cell
In this paper, 18650 type battery and square battery with the same positive material are used for test simulation. The triggering method of short circuit in the test battery is resistance wire heating and nail penetration.
The resistance wire heating method was used to simulate the thermal runaway of lithium ion battery caused by the drastic change of ambient temperature. The battery diaphragm is composed of PP/PE/PP three-layer diaphragm, wherein the melting point of PP is 165 ℃ and that of PE is 135 ℃. By winding the resistance wire on the surface of the lithium-ion battery for heating, a large amount of heat is rapidly generated, which causes the diaphragm to deform and contract, and the positive and negative poles are connected, causing a short circuit in the battery, and eventually causing the battery to lose control of heat.
(2) Nail Penetration
The thermal runaway of the lithium-ion battery is caused by the penetration of the steel needle into the lithium-ion battery to simulate the metal foreign matters inside the battery. After the steel needle penetrates the battery, it is connected to the positive and negative electrodes as a metal conductor, which causes internal short circuit of the battery. The short circuit position will generate large current and quickly generate a lot of heat, which will eventually lead to thermal runaway of the battery.
3.Verification method for thermal runaway of power battery module
(1) Resistance wire heating
A 18650 battery was used in the test. The resistance wire with a certain internal resistance is wound on the surface of the battery through calculation, and a temperature sensor is arranged outside the battery housing, and then the battery is arranged in the center of the battery module. The battery adjacent to the battery is also arranged with the same temperature sensor. After the batteries are assembled into modules in parallel, use the charging method specified by the manufacturer to charge the batteries to full charge. Connect the resistance wire to the external power supply, and connect the temperature sensor to the temperature patrol detector; Record the initial state of the battery module (voltage, temperature, etc.), continue to heat the battery until it fails, disconnect the external power supply, and observe whether the module is out of control due to heat.
The test uses a square battery. Wrap the same resistance wire on the surface of the battery, and arrange temperature sensors on the surface of the battery to be heated and adjacent battery surfaces. After the battery forms a module, charge the module to full charge according to the charging method specified by the manufacturer. The heating method and temperature record are the same as the above tests. Keep heating the battery until it fails. Disconnect the external power supply and observe whether the square battery module is out of control.
(2) Nail penetration
A 18650 battery was used in the test. Select the location of the needle battery to arrange temperature sensors on the surface of this battery and adjacent batteries. After the batteries are assembled into modules in parallel, use the charging method specified by the manufacturer to charge the batteries to full charge. Fix the module on the needle stand and connect the temperature patrol instrument. Use a steel needle with a diameter of 1mm to puncture the battery at a speed of 1mm/s until it catches fire, and observe whether the module is out of control due to heat.
The test uses a square battery. Select the location of the needle punched battery, arrange temperature sensors on the adjacent battery surfaces, and fully charge the battery according to the charging method specified by the manufacturer after the battery forms a module. Use a steel needle with a diameter of 1mm to puncture the battery at a speed of 1mm/s until it catches fire, and observe whether the module is out of control due to heat.
By conducting resistance wire heating and pricking tests on 18650 battery and square battery, the battery in the module was short circuited, and the battery in the battery module was simulated to be out of control of heat. The appropriate verification method for thermal out of control was explored, and the following conclusions were drawn:
(1)By comparing the nail penetration and cation wire heating tests, we found that for the same type of battery, the internal short circuit triggered by the resistance wire heating causes the battery temperature to rise higher, which we believe is caused by the heat introduced by the resistance wire heating;
(2) During the heating process of the resistance wire, it will bring a lot of heat to the adjacent battery. If the module spacing is small, it is very likely that the heated battery and the adjacent battery will be out of control, which will lead to the failure of the test simulation. Therefore, we believe that the nail penetration method is more suitable for the safety verification of 18,650 battery and square battery module out of control heat;
(3) When the pressure relief safety valve is short circuited in the battery, it can reduce the internal pressure and temperature of the battery, prevent the battery from thermal runaway, and play a protective role;
(4) The fire caused by electrolyte spraying at high speed is not the main cause of temperature rise and failure of adjacent batteries.