What caused the 18650 battery to fail?

With the rapid development of electric vehicle industry, lithium-ion batteries are widely used in the power battery field due to their high energy density, no memory effect and high safety. Due to the particularity of electric vehicles, the safety of power cells is also required to be higher.

For example, in the event of a safety accident such as a collision with an electric car, the power battery must not catch fire or explode to ensure the safety of the driver. Therefore, in the power battery safety test, it includes tests such as extrusion and acupuncture that involve the safety performance of lithium ion batteries under extreme abuse. Whether or not these rigorous safety tests can be passed is the ultimate standard for evaluating the safety of a lithium ion battery.

In the extrusion test, the lithium ion battery first deforms the shell and then begins to squeeze the core. Since the diaphragm prepared by the dry tensile process is currently less strong in the transverse and diagonal directions, the deformation of the core reaches a certain degree. When, The transverse layer of the diaphragm will first break, resulting in direct contact between the positive and negative poles of the lithium-ion battery, short-circuit, and instantaneous release of a large amount of heat, resulting in the decomposition of the negative SEI membrane, the positive polar active material, and the electrolyte, resulting in the thermal runaway of the lithium-ion battery., It eventually led to a lithium-ion battery fire and explosion.

In order to avoid thermal runaway in the extrusion test of lithium ion batteries and improve the safety of lithium ion batteries, it is necessary to conduct in-depth research on the mechanism of thermal runaway in the extrusion test of lithium ion batteries, so as to carry out targeted safety design of lithium ion batteries. In order to improve the safety of lithium-ion battery in extrusion test. Here's a look at the latest MIT research.

JunerZhu et al. of the Massachusetts Institute of Technology used the 18650 battery to study the mechanism of thermal runaway of lithium-ion batteries during axial extrusion, and used the finite element analysis model to perform simulation analysis. The model restores the effects of different axial pressures on lithium-ion batteries, and the results of the analysis are verified by CT scanning. The simulation results show that two kinds of reasons can be explained to cause short-circuit of lithium-ion batteries in extrusion tests.

Since 18650 cells are generally vertically assembled in power batteries, axial compression is the main cause of deformation of lithium ion cells in the event of a drop in the battery pack. Therefore, JunerZhu mainly studied the mechanism of lithium ion battery short-circuit caused by battery deformation under axial pressure.

Some traditional models assume that the interior of a lithium ion battery is a homogeneous whole, so it is impossible to accurately predict the test results when predicting the 18650 battery axial compression test. This is mainly due to the special structure of the lithium ion battery core. The upper and lower parts of the core are not exactly the same, and due to the unique structure of the lithium-ion battery cover(ie, the positive electrode), the lithium-ion battery may be exposed to axial pressure before the internal short circuit occurs. Causes a short circuit to the lithium-ion battery.

The 18650 battery consists of three main parts: a safety valve, a roll core, and a low-carbon steel shell. The safety valve is usually composed of a positive temperature coefficient material, an aluminum safety valve, a stainless steel positive extremon, a gas sealing pad, etc.. The core consists of a positive electrode, a negative electrode, and a diaphragm. In this experiment, the active substance of the positive electrode is LiCoO2. The axial load load speed is 5mm/min, and all test cells have been fully discharged before the test(SOC = 0).

The test results showed that the pressure of the 18650 battery in the axial pressure test showed a slow upward trend-rapid upward trend-slight downward trend-and the voltage test showed that the 18650 battery would not fail until the deformation reached 4mm. Moreover, it was found through tests that the voltage sudden drop of the 18650 battery was mainly caused by the internal short circuit of the battery rather than the internal structure.

In order to study the mechanism of 18650 failure under axial pressure, JunerZhu also used finite element software to analyze it. The materials in the model mainly used ElastoPlastic models and considered the Anisotropy of various materials. The model includes millions of computing units, and the load speed of the axial load is set to 1m/s. The simulation results reproduce the deformation of the 18650 battery under axial load.

First, the shell in the upper cover area of the battery begins to undergo plastic deformation. After the deformation exceeds 1 mm, the deformed shell begins to squeeze the upper part of the battery core. With the increase of the deformation, the core begins to deform. As a result, there is a slight drop in the pressure curve, and then with the increase of the contact area between the battery shell and the core, the pressure curve shows a rapidly rising trend. The results of CT scan also verify the above analysis. The deformation of the test battery mainly occurs in the superstructure, and the subbattery modification has almost no deformation.

The dismantling of the 18650 battery after the test showed that although the core had undergone serious deformation, the positive and negative poles did not break. Instead, a crack occurred in the diaphragm at a distance of 1.3 mm from the upper edge, which directly caused the battery. Short circuit, Voltage suddenly dropped, and this crack may be caused by the sharp edge of the metal foil invasion. In addition, the thickness of the diaphragm has dropped greatly at some locations, which is mainly due to the compression of the core of the sunken shell.

From the above analysis results, the possible reasons for the 18650 battery short-circuit under axial pressure are mainly the following.

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

2. Positive and negative poles contact through a ruptured diaphragm

3. Positive and negative poles contact through the area where the diaphragm is thin

4. The relief valve is squeezed and contacted with the core

From the test results, an internal short circuit will be caused when the axial deformation of the 18650 cell reaches 4mm, so special consideration needs to be given to the safety design of the battery pack. In addition, since deformation occurs mainly in the upper part of the 18650 battery at axial pressure, the safety design of the upper part of the 18650 battery must also be particularly concerned.

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