li ion 18650 battery pack sales

　　Mechanism of failure of 18650 cylindrical lithium battery caused by axial pressure

　　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 such as 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 batteries, such as safety accidents such as collisions of electric vehicles

　　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 safety accident such as a collision of an electric vehicle, the power lithium battery must not catch fire or explode to ensure the safety of drivers and passengers. Therefore, the power lithium battery safety test includes extrusion, acupuncture and other tests related to the safety performance of the lithium battery under extreme abuse. Whether it can pass these rigorous safety tests is the key to evaluating the safety of a lithium battery. ultimate standard of sex.

　　In the extrusion test, the lithium battery first deforms the shell, and then begins to squeeze the cell. Since the separator prepared by the current dry stretching process has low strength in the transverse and diagonal directions, it is difficult to deform the cell when the cell deforms. When it reaches a certain level, the transverse direction of the diaphragm will break first, causing direct contact between the positive and negative electrodes of the lithium battery, a short circuit occurs, and a large amount of heat is released instantly, resulting in the decomposition reaction of the negative electrode SEI film, positive electrode active material and electrolyte, resulting in lithium Thermal runaway occurs in the battery, which eventually causes the lithium battery to catch fire and explode.

　　In order to prevent thermal runaway of lithium batteries in 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 in extrusion tests, so as to carry out targeted safety measures for lithium batteries. Design 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.

　　JunerZhu et al. from the Massachusetts Institute of Technology used 18650 batteries to study the mechanism of thermal runaway of lithium batteries in the process of axial extrusion, and used the finite element analysis model to perform simulation analysis. The impact of the vertical pressure on the lithium battery, the analysis results were verified by CT scans, the simulation analysis results found two reasons that can explain the short circuit of the lithium battery in the extrusion test.

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

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

　　Since the 18650 battery in the power lithium battery pack is generally assembled vertically, the axial extrusion is an important reason for the deformation of the lithium battery when the battery pack is dropped, so JunerZhu mainly studied the deformation of the battery under axial pressure. Mechanisms leading to short circuits in lithium batteries. 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 axial compression test of the 18650 battery. This is mainly due to the special structure of the lithium battery cell, which leads to The upper part and the lower part are not completely consistent. At the same time, due to the unique structure of the lithium battery 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 the internal short circuit occurs.

　　The 18650 battery is mainly composed of three parts: a safety valve, a winding core and a mild steel case. The safety valve is usually composed of positive temperature coefficient material, aluminum safety valve, stainless steel positive terminal, gas sealing gasket, etc. The battery cell is composed of positive electrode, negative electrode and diaphragm. In this test, the active material of the positive electrode is LiCoO2. The loading speed of the axial load was 5mm/min, and all the test cells had been fully discharged (SOC=0) before the test. The test results show that the pressure of the 18650 battery shows a trend of slow rise-rapid rise-slight drop-rapid rise in the axial pressure test, and the voltage test shows that the 18650 battery will not fail until the deformation reaches 4mm , And through experiments, it is found that the voltage drop of the 18650 battery is mainly caused by the internal short circuit of the battery, not the internal structure of the battery. In order to study the failure mechanism of 18650 under axial pressure, JunerZhu also used finite element software to analyze it. The material in the model mainly adopts the elastoplastic model, and considers the anisotropy characteristics of various materials. In the model Contains millions of computing units, and the loading speed of the axial load is set to 1m/s.

　　The simulation results reproduce the deformation process of the 18650 battery under the axial load. First of all, the casing in the upper cover area of the battery begins to undergo plastic deformation. After the deformation exceeds 1mm, the deformed casing begins to squeeze the upper part of the battery core. With the increase in the deformation, the battery core begins to deform. There was a slight drop on the curve, and then as the contact area between the battery case and the battery cell increased, the pressure curve showed a rapid upward trend. The CT scanning results also verified the above analysis well. The deformation of the test battery mainly occurred in the upper structure, and the lower battery hardly deformed.

　　The dismantling of the 18650 battery after the test showed that although the battery core was severely deformed, the positive and negative electrodes did not break, but a crack appeared in the diaphragm at a distance of 1.3 mm from the upper edge, which directly caused the battery to break. There was a short circuit, a sudden drop in voltage, and this crack was probably caused by the intrusion of the sharp edge of the metal foil. In addition, the thickness of the diaphragm has dropped greatly in some places, which is mainly due to the extrusion of the battery core by the concave shell.

　　From the above analysis results, the possible reasons for the short circuit of the 18650 battery under axial pressure are as follows.

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

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

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

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

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

　　4. The safety valve is squeezed and is in contact with the cell

　　According to the test results, when the axial deformation of the 18650 battery reaches 4mm, an internal short circuit will occur, so special consideration should be given to the safety design of the battery pack. In addition, since the deformation mainly occurs on the upper part of the 18650 battery during axial pressure, special attention should be paid to the safety design of the upper part of the 18650 battery.

　　Deformation and failure mechanisms of 18650 battery cells under axial compression, Journal of Power Source, 336 (2016), JunerZhu, et.al