Briefly describe the fracture of copper foil in lithium ion battery during extrusion

For power batteries, safety and electrical performance are equally important. If a high-energy accident such as a collision occurs during the use of an electric vehicle, the lithium-ion battery may be severely deformed and causing serious safety problems such as internal short-circuit of the lithium-ion battery. When an internal short circuit occurs in the lithium-ion battery, 70% of the energy of the entire battery set will be released through the short circuit point within 60s, which will cause a rapid rise in local temperature, and then cause decomposition of positive and negative active substances and electrolytes, etc., resulting in thermal runaway of the lithium-ion battery.

In order to ensure the safety of lithium-ion batteries under the above conditions, a severe extrusion test was designed to investigate the safety performance of lithium-ion batteries in the event of a large deformation. Studies have shown that during the extrusion test, the deformation and uniform displacement of the electrode will occur first. As the degree of deformation increases, the current collector will slip along the 45-degree slip line. Finally, the diaphragm is too deformed. This causes the diaphragm to fail, causing a larger area short circuit to occur.

Once the internal short circuit occurs, the lithium ion battery may be out of control, and the high temperature will burn the battery. Even if there is no thermal runaway, the local high temperature will still melt the current collector, diaphragm, etc., so the lithium ion battery is squeezed. Structural changes in testing have always been a difficulty.

HsinWang et al. of Oak Ridge National Laboratory in the United States used 3DXCT technology to study the internal structure changes of lithium ion batteries during the extrusion test. It was found for the first time that copper foils produced microscopic fragments during extrusion testing, and these fragments were difficult. It has been discovered by conventional optical and electron microscopy that these hidden copper foil fragments may have a significant impact on the electrical performance and thermal runaway behavior of lithium-ion batteries, which deserves further study.

The commercial lithium cobalt oxide battery is used in the experiment. The battery core adopts a winding structure with a size of 30mm'40mm'4.5mm. In order to ensure that the lithium ion battery does not have thermal runaway during the extrusion test, HsinWang controls the battery's power consumption. Within 10% SoC, the following figure shows the structure of the battery after the battery is pressed and tested. The internal structure of the battery is not shown in Figure c. Figure d shows the internal structure of the battery after the extrusion test. After the extrusion test, the electrodes were symmetrically folded and wrinkled, and some of the electrode layers were bent to create a large gap between the electrode layers.

The figure below is a partially enlarged image. It can be noticed from the figure that the electrode has undergone severe folding and deformation in the incision direction. It can be seen from the comparison of Figure c and Figure d that the battery has a large amount of copper foil surface after extrusion. Crack.

The battery after the above experiment was disassembled, as shown in the following figure, it can be seen that there is a short-circuit point at the intermediate position where the extrusion is performed, but no obvious crack phenomenon occurs by optically observing the surface of the negative electrode.

However, by the X-ray radiography technique, a large number of cracks were observed in the copper foil as shown in the following figure b, but when the above electrode was observed by SEM, only a small amount of electrode breakage was observed. This indicates that although the copper foil generates a large number of cracks, since the graphite negative electrode coating is thick, the surface of the electrode does not have obvious cracks. Therefore, the fracture of the copper foil cannot be well observed by means of optics and SEM. However, the X-ray penetration is very good, graphite will hardly block X-rays, and the copper foil will effectively block X-rays. Therefore, X-ray imaging technology can well show the fracture of copper foil when it is squeezed.

The reason for the crack on the copper foil may be because the toughness of the copper foil is poor, and cracks are very likely to occur when pressed in the vertical direction. As for the cause of embrittlement of the copper foil, further research is needed, which may be related to the processing and electrode milling. It is related to the residual stress during the pressing process. Broken copper foil may have the following effects on lithium-ion batteries.

1) First, the broken copper foil cannot assume the function of the current collector, resulting in the loss of effective connection between the local active material and the conductive network.

2) The active material and the electrolyte are filled into the gaps of the broken copper foil, but they are not good electronic conductors and heat conductors, so when a short circuit occurs here, heat is hard to be conducted quickly.

3) The area to be squeezed, because the contact between the active material and the conductive network is deteriorated, resulting in a decrease in activity or a failure to participate in the charge and discharge reaction, resulting in a decrease in the capacity of the lithium ion battery.

4) Broken copper foil will reduce the mechanical properties of the negative electrode.

5) The final failure of the battery in the extrusion test is mainly caused by the failure of the positive electrode and the diaphragm.

6) At the initial stage of the occurrence of the short circuit, mainly the Al foil/positive electrode active material comes into contact with the copper foil chip/graphite active material of the negative electrode.

The above research is of great help for us to understand the heat runaway mechanism of lithium ion batteries in mechanical abuse, and also to judge whether lithium ion batteries can continue to be used after mechanical damage. Further research is needed on the crack generation mechanism of copper foil during extrusion.

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