Study on the mechanism of 18650 battery failure caused by axial pressure

by:CTECHi     2021-07-21

In the event of a safety accident such as an electric vehicle collision, it is necessary for the power battery to not fire or explode to ensure the safety of the drivers and passengers. Therefore, the power battery safety test includes extrusion, acupuncture, etc. Whether a lithium-ion battery safety performance test under extreme abuse conditions can pass these rigorous safety tests is the ultimate standard for evaluating the safety of a lithium-ion battery.

In the extrusion test, the outer shell of the lithium-ion battery is first deformed, and then it begins to squeeze the battery core. Because of the strength of the diaphragm prepared by the current dry stretching process in the transverse and diagonal directions Therefore, when the deformation of the battery cell reaches a certain level, the transverse direction of the separator will be broken first, causing the positive and negative electrodes of the lithium-ion battery to directly contact, causing a short circuit, and instantly releasing a large amount of heat, resulting in the negative electrode SEI film and the positive electrode active material. It decomposes and reacts with the electrolyte, causing thermal runaway of the lithium-ion battery, and finally causing the lithium-ion battery to catch fire and explode.

In order to avoid thermal runaway of lithium-ion batteries in the extrusion test and improve the safety of lithium-ion batteries, it is necessary to conduct in-depth research on the mechanism of thermal runaway of lithium-ion batteries in the extrusion test. , So as to carry out targeted safety design for lithium-ion batteries, thereby improving the safety of lithium-ion batteries in the extrusion test. Let's take a look at the latest research results of the Massachusetts Institute of Technology in the United States.

Juner Zhu of Massachusetts Institute of Technology and others used 18650 battery to study the mechanism of thermal runaway of lithium ion battery during axial compression, and used The finite element analysis model was simulated and analyzed. The model restored the impact of different axial pressures on the lithium-ion battery. The analysis results were verified by CT scanning. The simulation analysis results found two types that can be explained in the extrusion test. The cause of the short circuit of the lithium-ion battery.

Because the 18650 battery in the power battery pack is generally assembled vertically, in the case of the battery pack falling, the axial compression is the main reason for the deformation of the lithium-ion battery, so Juner Zhu's main research The mechanism of lithium-ion battery short-circuit caused by battery deformation under axial pressure is described.

Some traditional models assume that the interior of the lithium-ion battery is a uniform whole, so when predicting the axial compression test of the 18650 battery, the test result cannot be accurately predicted. This is mainly due to the lithium-ion battery cell The special structure of the lithium ion battery causes the upper and lower parts of the battery cell to be inconsistent. At the same time, due to the unique structure of the upper cover of the lithium ion battery (that is, the positive electrode), the lithium ion battery may be internally short-circuited when it is subjected to axial pressure. Previously, it caused a short circuit in the lithium-ion battery.

18650 battery is mainly composed of three parts: safety valve, coil core and low carbon steel shell. The safety valve is usually composed of positive temperature coefficient material, aluminum safety valve, stainless steel positive terminal, gas gasket, etc. The battery core is composed of positive electrode, negative electrode and diaphragm. In this experiment, the active material of the positive electrode is composed of LiCoO2. The loading speed of the axial load is 5mm/min, and all the test batteries have been completely discharged before the test (SOCu003d0).

The test results show that the pressure of the 18650 battery in the axial pressure test shows a slow rise-rapid rise-slight decline-rapid rise trend, and the voltage test shows that the 18650 battery is deformed to 4mm Failure to occur under the circumstance of, 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, rather than the open circuit of the internal structure.

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

First of all, the shell of the upper cover area of u200bu200bthe battery begins to undergo plastic deformation. After the deformation exceeds 1mm, the deformed shell begins to squeeze the upper part of the battery core. As the degree of deformation increases, the battery begins to deform. Deformation occurs, and there is a slight drop in the pressure curve, and then as the contact area between the battery case and the battery cell increases, 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 there was almost no deformation under the battery.

The disassembly of the 18650 battery after the test shows that although the battery core has been severely deformed, the positive and negative electrodes have not broken, but the separator is at a distance from the upper edge. A crack appeared at the 1.3mm position, which directly caused a short circuit in the battery and a sudden drop in voltage. This crack may be caused by the intrusion of the sharp edge of the metal foil. In addition, the thickness of the diaphragm at some locations has greatly decreased, which is mainly caused by the recessed shell squeezing the battery core.

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

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 with the broken diaphragm

3. The positive and negative electrodes pass through the diaphragm The thinned area is in contact

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

From the test results, when the axial deformation of the 18650 battery reaches 4mm, it will cause the internal Short circuit, so special consideration should be given to 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, special attention should be paid to the safety design of the upper part of the 18650 battery.

Article source: New energy leader

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