With the increasing popularity of electric vehicles, the safety and electrical performance of electric vehicles need more and more attention. If high-energy accidents such as collisions occur during the use of electric vehicles, the lithium-ion battery may undergo severe deformation, which may cause serious safety problems such as internal short-circuits in the lithium-ion battery.
When an internal short circuit occurs in a lithium-ion battery, 70% of the energy of the entire battery pack will be released through the short-circuit point within 60s, causing a local temperature to rise rapidly, which in turn causes the decomposition of positive and negative active materials, electrolyte, etc., which lead to thermal runaway of lithium ion battery.
In order to ensure the safety of lithium-ion batteries under the above circumstances, people have designed rigorous squeeze tests to examine the safety performance of lithium-ion batteries in the event of huge deformation. The research shows that during the crush test, the deformation and uniform displacement of the electrode will occur first. With the increase of the deformation, the current collector will slip along the 45-degree slip line. Finally, the diaphragm is deformed too much, causing the diaphragm to fail, causing a larger area short circuit.
Once an internal short circuit occurs, it may lead to thermal runaway of the lithium ion battery, and the resulting high temperature will burn the battery. Even if there is no thermal runaway, the local high temperature will still melt parts of the current collector, the separator, etc., so we need to study the structural changes of lithium ion batteries in the crush test.
Later, it was discovered that copper foil will produce microscopic fragments during the extrusion test, and these fragments are difficult to find through traditional optical and electron microscopes. These hidden copper foil fragments may have a significant impact on the electrical performance and thermal runaway behavior of lithium-ion batteries. Broken copper foil may have the following effects on lithium-ion batteries.
- Broken copper foil cannot assume the role of a current collector, causing the local active material to lose its effective connection with the conductive network.
- Active materials and electrolyte will fill the gaps of broken copper foil, but they are not good electronic conductors and thermal conductors, so when a short circuit occurs here, it is difficult to conduct heat quickly.
- In the squeezed area, due to the poor contact between the active material and the conductive network, the activity is reduced, or it cannot participate in the charge and discharge reaction, causing the capacity of the lithium ion battery to decrease.
- Broken copper foil will reduce the mechanical properties of the negative electrode
- The final failure of the battery in the crush test is mainly caused by the failure of the positive electrode and the separator.
- At the beginning of the short circuit, the Al foil / positive electrode active material is in contact with the copper foil fragments / graphite active material of the negative electrode.
The best choice for testing this type of lithium battery is to use the battery crush nail penetration tester, so that you can not only observe the reaction of the battery during the crush, but also see what happens to the battery during the nail penetration.