With the rapid development of electric vehicle industry, lithium-ion batteries are widely used in the field of power batteries due to their high energy density, no memory effect and high safety, etc. Due to the particularity of electric vehicles, the safety of battery is also proposed Higher requirements, such as electric vehicles in the event of a collision and other safety incidents, the need for battery power is not fire, explosion, to ensure the safety of occupants, so in the power battery safety experiments include extrusion, acupuncture and so on The testing of lithium-ion battery safety in the most extreme cases of abuse, the passing of these harsh safety tests, is the ultimate standard for evaluating the safety of a lithium-ion battery.
In the extrusion test, the lithium ion battery is first deformed shell, and then began to form the cell extrusion, due to the current dry-stretch process to prepare the diaphragm in the lateral and diagonal low intensity, so the core Deformation to a certain extent, the diaphragm will be the first fracture occurred in the lateral, resulting in direct contact with the positive and negative lithium-ion battery, a short circuit, the instant release of large amounts of heat, resulting in negative SEI film, positive electrode active material and electrolyte decomposition reaction, Lead to thermal runaway lithium-ion battery, eventually leading to lithium-ion battery fire and explosion.
In order to avoid the thermal runaway of lithium-ion batteries in the extrusion test and to improve the safety of lithium-ion batteries, the mechanism of thermal runaway of lithium-ion batteries in the extrusion test needs to be further studied to make lithium-ion batteries Targeted safety design, thereby enhancing the safety of lithium-ion batteries in the extrusion test. Let's take a look at the latest MIT research results.
Juner Zhu and others at the Massachusetts Institute of Technology (MIT) studied the mechanism of thermal runaway of lithium-ion batteries in the axial extrusion process using a 18650 battery and conducted a simulation analysis using the finite element analysis model, which reduced the differences Axial pressure on the lithium-ion battery, the results of the analysis have been verified by CT scanning. The simulation results show that the two can explain the squeeze test causes lithium-ion battery short circuit causes.
As the 18650 battery is generally assembled vertically in the power battery pack, axial compression is the main reason for the deformation of the lithium ion battery in the event of a drop in the battery pack or the like. Therefore, Juner Zhu mainly studied the battery under axial pressure Deformation of the lithium-ion battery short circuit caused by the mechanism of Some traditional models due to the assumption that the internal lithium-ion battery is a homogeneous whole, it is predicted in the axial compression test of the 18650 battery can not accurately predict the test results, mainly due to the lithium-ion battery The special structure of the cell, resulting in the upper and lower batteries are not exactly the same, at the same time as the lithium-ion battery cover (that is, the positive) unique structure allows the lithium-ion battery under axial pressure may be included Before short circuit occurs, it causes short circuit of lithium ion battery.
18650 battery is mainly composed of three parts: safety valve, core and low carbon steel casing. Safety valve is usually positive temperature coefficient of material, aluminum safety valve and stainless steel positive terminal, gas seals and other components, batteries from the positive, negative and diaphragm , The composition of the active material in the positive electrode of the test is LiCoO2. The loading speed of the axial load is 5 mm / min, and all the test cells have been fully discharged before the test (SOC = 0) .Test results show that the 18650 battery Axial pressure test showed a slow rise in pressure - rapid rise - a slight decline - the trend of rapid rise, and voltage tests show that the deformation of the 18650 battery will fail in the case of 4mm, and through the test found that 18650 The voltage drop of the battery was mainly caused by the internal short circuit of the battery, not the disconnection of the internal structure.In order to study the mechanism of 18650 failure under axial pressure, Juner Zhu also analyzed it by using the finite element software. The material in the model The main use of the elastic-plastic model, and considering the anisotropic characteristics of various materials, the model contains millions of computing units, The loading speed to the load is set to 1 m / s. The simulation results reproduce the deformation of the 18650 battery under axial load. First, the housing in the top cover of the battery begins to deform plastically. After the deformation exceeds 1 mm , The deformation of the shell began to squeeze the upper part of the battery core, with the degree of deformation increases, the core began to appear deformed, resulting in a slight pressure drop in the curve, and then with the battery shell and the contact area of the cell The pressure curve shows a trend of rapid increase.CT scan results also verify the above analysis, the experimental battery deformation occurs mainly in the superstructure, the battery is almost no deformation under the change.
Disassembly of the 18650 battery after the test shows that although the cell has undergone severe deformation, but the positive and negative did not break, but instead of the diaphragm at a distance of 1.3mm from the upper edge of a crack, which led directly to the battery A short circuit occurs, and the voltage drops suddenly, and the crack may be caused by the intrusion of the sharp edge of the metal foil In addition, the thickness of the separator is greatly decreased at some positions, mainly due to the depressed shell pressing the cell .
From the above analysis results, the axial pressure caused 18650 battery short circuit may be the following main reasons.
1. The shell contacts the positive and negative electrodes through the ruptured diaphragm
2. Positive and negative contact through the rupture of the diaphragm
3. Positive and negative contact through the thinner area of the diaphragm
4. Safety valve is squeezed, contact with batteries
From the test results, when the axial deformation of the 18650 battery reaches 4mm will trigger an internal short circuit, it needs to be particularly considered in the safety design of the battery pack. In addition, deformation occurs mainly in the upper part of the 18650 battery under axial pressure, The top of the 18650 battery safety design also have special care.
