Guided and supported by policies, electric vehicles have entered a period of rapid development. The government and major engine manufacturers have formulated corresponding development plans for electric vehicles. In the future, electric vehicles will gradually replace traditional fuel vehicles. However, after a series of reported electric vehicle collisions, fires and even explosions occurred again, making people focus on the safety of electric vehicles. The electric vehicle mainly uses the power battery as the power source, which leads to a series of problems such as power battery pack, high and low voltage wire harness and load end protection. However, due to the characteristics of integrated high voltage and high electric energy, the layout of power battery pack on the vehicle and collision protection need to be focused.
The existing laws and regulations put forward corresponding detection standards for the mechanical abuse of battery packs. However, under real road conditions, the forms of automobile accidents are different, and there is a risk that the battery packs will be squeezed after the collision of vehicles, leading to fire and explosion. Therefore, it is necessary to study the collision safety of power batteries.
Some scholars have studied the safety of batteries from the components used in batteries, such as electrode materials, separators and electrolyte components There are also a series of experimental studies on the power battery, such as indentation, three-point bending, acupuncture and drop weight tests, to explore the mechanical response of the power battery under load. The simulation analysis of the battery pack structure and the crash performance of new energy vehicles is also carried out, so as to provide guidance for the structural safety design of vehicles with power batteries.
In this paper, a certain type of lithium ion power battery is taken as the research object, and the extrusion test is carried out in different directions of the cell. Through the collection of extrusion pressure, temperature and voltage data, the direction of weak resistance to extrusion of the cell is found out. Then, the extrusion damage tolerance of the cell is further determined through the analysis of different loading conditions. Secondly, the anti extrusion ability of modules in different directions is analyzed.
1.Power Battery Thermal Runaway
Taking lithium ion power battery as an example, its main working mechanism involves the delithiation and lithium insertion of cathode and anode materials, as well as the transmission and diffusion of lithium ions in electrolyte (mass) and diaphragm materials. Most of the safety problems of power batteries are directly related to the material composition of the battery. Due to mechanical abuse, such as battery overheating, overcharging, discharging, impact, extrusion, etc., chemical reactions will occur between the internal battery materials, which will lead to runaway heating, and eventually lead to fire and explosion accidents.
After a car collision accident, the battery structure will deform after being impacted and squeezed, resulting in the failure of internal component materials, including the rupture of the diaphragm or the rupture of the cathode and anode materials, which will lead to internal short circuit; The internal short circuit generates a large amount of Joule heat, which leads to the decomposition of materials in the single battery. When the temperature is between 80 ℃ and 120 ℃, the solid electrolyte interface (SEI) first decomposes.
When the SEI membrane decomposition reaction proceeds to a certain extent, a series of subsequent chemical reactions will occur. The thermal runaway of lithium ion battery can be divided into the following five reaction stages according to the temperature rise: SEI membrane decomposition reaction, negative electrode and electrolyte reaction, positive active substance decomposition reaction, adhesive reaction and electrolyte decomposition reaction. The heat generating reaction inside the battery will also produce a large amount of gas, and the pressure inside the structure will rapidly increase and gather; When the pressure reaches the bearing limit of the structure, fire and explosion may occur. When a single battery fails, it will also quickly spread to adjacent batteries, leading to the failure of the battery system, resulting in more serious consequences.
When the power battery is short circuited, the voltage starts to drop and the temperature starts to rise, so the temperature and voltage can be used as the judgment basis for the failure of the power battery. Considering that the temperature after internal short circuit can only be transferred to the battery surface for a certain time, and is greatly affected by the ambient temperature, compared with temperature, voltage is a parameter that can relatively quickly and accurately judge the short circuit failure of power battery. The critical value of extrusion failure of this type of battery can be determined by the rise of temperature and the start of drop of voltage.
Figure 1 shows the temperature and voltage data collected during the process of thermal runaway of the cell being squeezed in the test. Within 0~36s, the cell is squeezed to a certain extent, the voltage is still stable at 4.14V, and the battery temperature is 26.2 ℃; During 37-38s, the voltage drops to 2mV, and the battery temperature increases to 30.5 ℃; When the load reaches 50 s, the measured voltage is always 2 mV, and the temperature rises to 131 ℃. At this time, it is observed that a large amount of smoke is emitted from the cell; During 50~51s, the battery temperature rose from 131 ℃ to 614 ℃, and then fire and explosion occurred.
Figure 2 shows the phenomenon of smoke, fire and explosion when the extrusion heat of single battery is out of control in the test.
2.Power Battery Crush Test
The power battery pack is generally arranged under the floor on the vehicle. In case of front collision or rear end collision, there is enough energy absorption space in front of and behind the vehicle, the structural deformation of the floor is small, and the power battery pack will not be damaged due to extrusion; However, when the vehicle is subject to side impact, the lateral energy absorption space is small, and the impact force is transmitted to the floor beam, the central channel and other areas through the threshold. The power battery pack is easy to be squeezed and damaged, thus causing damage to the internal modules and monomer. Therefore, it can provide a basis for the safe layout design of power battery packs and failure judgment in simulation to determine the extrusion damage tolerance of individual cells by testing and studying the damage and failure of battery cells and modules under extrusion conditions.