With the rapid development of the electric vehicle industry, lithium-ion batteries are widely used in the field of power batteries due to their advantages of high energy density, no memory effect and high safety. Due to the special nature of electric vehicles, higher requirements are also placed on the safety of power batteries, such as in the event of a collision or other safety accidents involving an electric vehicle.

　　With the rapid development of the electric vehicle industry, lithium batteries are widely used in the field of power lithium batteries due to their advantages of high energy density, no memory effect and high safety. Due to the particularity of electric vehicles, higher requirements are placed on the safety of power lithium batteries. For example, in the event of a collision or other safety accidents in electric vehicles, the power lithium batteries must not catch fire or explode to ensure the safety of drivers and passengers. , therefore, the power lithium battery safety test experiments include extrusion, acupuncture and other tests related to the safety performance of lithium batteries under extreme abuse conditions. Whether it can pass these stringent safety tests is an important factor in evaluating the safety of a lithium battery. The ultimate standard of sex.

　　In the extrusion test, the lithium battery shell first deforms, and then begins to squeeze the battery core. Since the separator prepared by the current dry stretching process has low strength in the transverse and diagonal directions, the deformation of the battery core occurs. When it reaches a certain level, the transverse direction of the separator will break first, causing the positive and negative electrodes of the lithium battery to be in direct contact, causing a short circuit, instantly releasing a large amount of heat, causing the negative SEI film, positive electrode active material and electrolyte to decompose, resulting in lithium Thermal runaway occurs in the battery, eventually causing the lithium battery to catch fire and explode.

　　In order to prevent thermal runaway of lithium batteries during the extrusion test and improve the safety of lithium batteries, it is necessary to conduct in-depth research on the mechanism of thermal runaway of lithium batteries during the extrusion test, so as to carry out targeted safety measures for lithium batteries. Designed to improve the safety of lithium batteries in extrusion tests. Let’s take a look at the relevant research results of the Massachusetts Institute of Technology in the United States.

　　Juner Zhu and others from the Massachusetts Institute of Technology in the United States used 18650 batteries to study the mechanism of thermal runaway of lithium batteries during axial extrusion, and conducted simulation analysis using a finite element analysis model. This model restored the different axial The analysis results of the impact of axial pressure on lithium batteries were verified by CT scans. The simulation analysis results found two reasons that can explain the short circuit of lithium batteries during the extrusion test.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%

　　Since the 18650 batteries in power lithium battery packs are generally assembled vertically, axial extrusion is an important cause of lithium battery deformation when the battery pack is dropped, so JunerZhu has focused on the battery deformation under axial pressure. The mechanism that causes lithium battery short circuit. Some traditional models assume that the interior of the lithium battery is a uniform whole, so they cannot accurately predict the test results when predicting the 18650 battery axial compression test. This is mainly due to the special structure of the lithium battery cell, resulting in the The upper and lower parts are not exactly the same. At the same time, due to the unique structure of the lithium battery upper cover (that is, the positive electrode), when the lithium battery is subjected to axial pressure, it may cause a short circuit in the lithium battery before an internal short circuit occurs.

　　The 18650 battery mainly consists of three parts: safety valve, core and mild steel casing. Safety valves usually consist of positive temperature coefficient materials, aluminum safety valves, stainless steel positive terminals, gas seals, etc. The battery core consists of positive electrodes, negative electrodes and separators. In this test, the active material of the positive electrode is LiCoO2. The loading speed of axial load is 5mm/min, and all test batteries have been fully discharged (SOC=0) before the test. The test results show that in the axial pressure test, the pressure of the 18650 battery shows a trend of slow rise - rapid rise - slight drop - rapid rise. The voltage test shows that the 18650 battery will fail only when the deformation reaches 4mm. , and through experiments, it was found that the voltage drop of the 18650 battery was mainly caused by the internal short circuit of the battery, rather than the open circuit of the internal structure. In order to study the failure mechanism of 18650 under axial pressure, JunerZhu also used finite element software to analyze it. The materials in the model mainly used an elastic-plastic model, and the anisotropic characteristics of various materials were taken into account. In the model Containing millions of calculation units, the loading speed of the axial load is set to 1m/s.

　　The simulation results reproduce the deformation process of the 18650 battery under axial load. First, the casing in the upper cover area of the battery begins to undergo plastic deformation. After the deformation exceeds 1 mm, the deformed casing begins to squeeze the upper part of the battery core. As the degree of deformation increases, the battery core begins to deform, thereby causing the pressure to increase. There is a slight decrease in the curve, and then with the increase in the contact area between the battery case and the cell, the pressure curve shows a rapid upward trend. The CT scan results also well verified the above analysis. The deformation of the test battery mainly occurred in the upper structure, and almost no deformation occurred when the battery was lowered.

　　Disassembly of the 18650 battery after the test showed that although the battery core was severely deformed, the positive and negative electrodes did not break. Instead, a crack appeared in the separator 1.3mm from the upper edge, which directly caused the battery to fail. A short circuit occurs, the voltage drops, and this crack may be caused by the intrusion of a sharp edge of the metal foil. In addition, the thickness of the diaphragm has dropped significantly in some locations, which is mainly caused by the dented shell squeezing the battery core.

　　Judging from the above analysis results, the possible reasons for the short circuit of the 18650 battery under axial pressure include the following.

　　Low temperature and high energy density 18650 3350mAh-40℃ 0.5C discharge capacity ≥60%

　　Charging temperature: 0~45℃ Discharge temperature: -40~+55℃ Specific energy: 240Wh/kg -40℃ Discharge capacity retention rate: 0.5C Discharge capacity ≥ 60%

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　　1. The shell is in contact with the positive and negative electrodes through the broken diaphragm.

　　2. The positive and negative electrodes are in contact through the ruptured separator

　　3. The positive and negative electrodes are in contact through the thinned area of the separator.

　　4. The safety valve is squeezed and comes into contact with the battery core

　　Judging from the test results, when the axial deformation of the 18650 battery reaches 4mm, it will cause an internal short circuit, so special consideration must be given to the safety design of the battery pack. In addition, since deformation mainly occurs in the upper part of the 18650 battery during axial pressure, special attention must be paid to the safety design of the upper part of the 18650 battery.

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