Mechanism study on axial failure of 18650 battery

With the rapid development of the 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. Due to the special nature of electric vehicles, higher requirements are also placed on the safety of power batteries.

For example, in the event of a safety accident such as a collision of an electric vehicle, the power battery does not need to be fired or exploded, and the safety of the driver and passenger is ensured. Therefore, in the power battery safety test, the extrusion, acupuncture, etc. are involved in extreme abuse. The safety performance test of lithium-ion batteries in these cases can pass these stringent safety tests and is the ultimate standard for evaluating the safety of a lithium-ion battery.

In the extrusion test, the lithium ion battery first deformed the outer casing and then began to form a squeeze on the electric core. Since the diaphragm prepared by the current dry stretching process has lower strength in the lateral and diagonal directions, the battery is in the battery core. When the deformation reaches a certain level, the transverse direction of the diaphragm will first break, resulting in direct contact between the positive and negative electrodes of the lithium ion battery, short circuit occurs, and a large amount of heat is instantaneously released, resulting in decomposition reaction of the negative electrode SEI film, the positive electrode active material and the electrolyte. The lithium ion battery is thermally out of control, which eventually causes the lithium ion battery to catch fire and explode.

In order to avoid the thermal runaway of the lithium ion battery in the extrusion test and improve the safety of the lithium ion battery, it is necessary to carry out in-depth research on the mechanism of the thermal runaway of the lithium ion battery in the extrusion test, thereby conducting the lithium ion battery. Targeted safety design to enhance the safety of lithium-ion batteries in extrusion testing. Let's take a look at the latest research results of the Massachusetts Institute of Technology.

Juner Zhu et al. of the Massachusetts Institute of Technology in the United States used 18650 batteries to study the mechanism of thermal runaway of lithium-ion batteries during axial extrusion, and performed simulation analysis using a finite element analysis model. The influence of the axial pressure on the lithium ion battery was verified by CT scan. The simulation analysis found two reasons for the short circuit of the lithium ion battery in the extrusion test.

Since the 18650 battery is generally vertically assembled in the power battery pack, axial extrusion is the main cause of deformation of the lithium ion battery in the case of battery pack drop, etc., therefore Juner Zhu mainly studied the battery under axial pressure. The mechanism of deformation causing a short circuit in a lithium ion battery.

Some traditional models assume that the interior of a lithium-ion battery is a uniform whole, so the prediction results cannot be accurately predicted when predicting the axial compression test of the 18650 battery. This is mainly due to the special structure of the lithium-ion battery cells, resulting in electricity. The upper and lower parts of the core are not completely identical. At the same time, due to the unique structure of the upper cover of the lithium ion battery (ie, the positive electrode), the lithium ion battery may cause lithium ion battery before the internal short circuit occurs when the axial pressure is applied. A short circuit has occurred.

The 18650 battery consists of three main components: a safety valve, a core and a low carbon steel housing. The safety valve is usually composed of positive temperature coefficient material, aluminum safety valve and stainless steel positive terminal, gas gasket, etc. The battery core is composed of a positive electrode, a negative electrode and a separator. In this test, the active material of the positive electrode is LiCoO2. The axial load was loaded at 5 mm/min and all of the test cells were fully discharged (SOC = 0) before the test.

The test results show that the pressure of the 18650 battery in the axial pressure test shows a slow rise - rapid rise - a slight drop - a rapid rise, while the voltage test shows that the 18650 battery will fail when the deformation reaches 4mm And through experiments, it was found that the voltage drop of the 18650 battery is mainly caused by the internal short circuit of the battery, not the internal structure.

In order to study the mechanism of failure of 18650 under axial pressure, Juner Zhu also analyzed it with finite element software. The materials in the model mainly used elastoplastic model, and considered the anisotropy characteristics of various materials. It contains millions of calculation units, and the loading speed of the axial load is set to 1 m/s. The simulation results reproduce the deformation of the 18650 battery in the case of axial load.

Firstly, the shell of the upper cover area of ​​the battery begins to plastically deform. After the deformation degree exceeds 1 mm, the deformed outer shell begins to squeeze the upper part of the battery core. As the degree of deformation increases, the battery core begins to deform, so that the pressure curve There was a slight drop in the upper part, and then the pressure curve showed a rapid upward trend as the contact area between the battery case and the cell increased. The CT scan results also well verified the above analysis. The deformation of the test cell mainly occurred in the upper structure, and the battery was almost never deformed.

The disassembly of the tested 18650 battery showed that although the battery core was severely deformed, the positive and negative electrodes did not break. Instead, the diaphragm showed a crack at a position 1.3 mm from the upper edge, which directly led to the battery. A short circuit occurs and the voltage drops, and this crack may be caused by the sharp edge intrusion of the foil. In addition, the thickness of the diaphragm is greatly reduced at some locations, mainly due to the extruded housing of the recessed casing.

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

1. The outer casing is in contact with the positive and negative electrodes through a broken diaphragm

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

3. Contact between the positive and negative electrodes through the thinned area of ​​the diaphragm

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

From the test results, when the axial deformation of the 18650 battery reaches 4mm, an internal short circuit is caused, so it is necessary to specially consider the safety design of the battery pack. In addition, since the deformation mainly occurs in the upper part of the 18650 battery under axial pressure, the safety design of the upper portion of the 18650 battery is also particularly concerned.

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