With the large-scale application of lithium-ion batteries in new energy vehicles, people have begun paying more and more attention to the safety of lithium-ion batteries, especially Tesla's recent explosion of electric vehicles. Let people worry about the safety of power batteries. In order to ensure the safety of lithium-ion batteries, a series of safety tests must be carried out before leaving the factory, including extrusion, acupuncture and short-circuit tests. This is a simulation of the safety of a power battery after an accident occurs in an electric vehicle. It has important reference significance for the safety of a power battery. There are many factors that affect the results of a crushing experiment, such as the extrusion speed and the position of the extrusion. Influencing the final test results, Brandy Dixon et al. of the Massachusetts Institute of Technology (MIT) recently evaluated and tested the factors that affect the test results of the crushing test. The test results indicate certain factors, such as whether there is electrolyte, the speed of the crush test, etc. It will have a significant impact on the test results, but some factors, such as the location of the squeeze test, have little impact on the test results. .
In the experiment, a soft-packaged lithium-ion battery was used with a capacity of 52 Ah. The positive electrode material was NCA system. The battery was tested under two conditions. One was dry state without electrolyte, the other was with electrolyte, and discharged to zero. The status of %SoC.
1. Effect of electrolyte on extrusion test results
In order to facilitate the analysis of the influence of the extrusion test on the internal structure of the lithium-ion battery, we will use a dry cell that has not been injected with the electrolyte, or evaporate the electrolyte and perform the extrusion test. However, few studies have analyzed the electrolyte for the extrusion test. Effect of the results. Brandy Dixon performed round plane extrusion test and hemispherical extrusion test using dry cells without electrolyte and ordinary cells after venting. Circular plane extrusion test showed no dry cells with electrolyte in the test. A larger force feedback (shown in the blue curve below) will be generated. The force feedback of the dry battery when the deformation reaches 4mm is 167kN, and the battery with electrolyte is only 150kN.
The same rule was also found in the hemispherical extrusion test. The maximum force generated by the dry cell reached 7.7kN (after the deformation reached 4mm and then failed), and the maximum force of the ordinary battery with electrolyte was only 5.2kN (the deformation was After 3mm failure, this test shows that the electrolyte in the battery will actually have a significant effect on the results of the lithium ion battery compression test. The polymer separator will soften after soaking in the electrolyte, and therefore will be more than in the dry state. Early failure. From the test results of Brandy Dixon, the squeeze test results obtained with dry cells will be larger than the actual state of the cells. Therefore, the results obtained from the squeeze cell test of dry cells do not fully represent the lithium ion battery. Actual usage.
2. Effect of squeeze position on test results
Lithium-ion batteries may be located in different positions in the lithium-ion battery during the accidental squeezing deformation. When we perform the squeezing test, we often choose to use the central position of the battery as the test position. Then the squeezing test results can characterize the lithium ion battery. In this case, Brandy Dixon analyzed the impact of the position of the squeeze on the test results by performing a squeeze test at different locations on a dry lithium-ion battery. Brandy Dixon chose four positions for testing. The first position is the center of the battery, about 65mm from the long edge of the lithium-ion battery, the second position is about 30mm from the long edge, the third position is about 10mm from the long edge of the lithium-ion battery, the fourth and fifth The dots are located at the edge of the lithium-ion battery, and the center of the fifth dot is located on the edge of the lithium-ion battery. Figure a below shows the test result. From the results, unless the extrusion position is on the edge of the lithium-ion battery, The position of the squeeze test basically has no effect on the test result, that is, we can choose any position except the edge during the squeeze test.
3. Effect of extrusion speed on test results
In the actual use of lithium-ion batteries, they often face different extrusion rate tests, then whether the extrusion speed will have a significant impact on the test results? For this reason, Brandy Dixon uses a speed of 0.2mm/min to 20mm/min. The crush test, the test results are shown in Figure c below, from which it can be seen that the extrusion speed will have a significant impact on the maximum force of the crush test. For example, at 20mm/min, the maximum force is 8.4 kN, while the maximum force at a speed of 0.2 mm/mm is only 7.3 kN, but if the extrusion speed is an order of magnitude, such as 10 mm/min and 20 mm/min, 1 mm/min and 2 mm/min will be the same Test Results.
4. Repeatability of the squeeze test
As a destructive test, the squeeze test can only be tested by sampling. Therefore, whether the test results obtained by the test battery are reproducible and how important it is to represent other identical batteries becomes an important issue. In order to verify whether the test results of the squeezing test are reproducible, Brandy Dixon used a hemispherical squeezing head with a diameter of 12.6mm, 28mm and 44mm, respectively, to perform a squeezing test on a lithium ion battery. The results are shown in Figure c below. Judging from the test results, the curves obtained by the extrusion test heads of different diameters are highly coincident in the multiple extrusion test, and there is only a slight difference in the maximum force. This shows that the repeatability of the extrusion test is very good. Measure the safety of the same batch of batteries only by measuring a small number of lithium-ion batteries.
Squeeze testing is a very important safety test for power batteries. Brandy Dixon's research indicates for us which factors will have a significant impact on the results of the squeeze test and requires us to exercise strict controls, and those factors have an impact on the final test results. Not too big, the restrictions can be relaxed. At the same time, Brandy Dixon's research also shows that the crush test has a very good repeatability, so we can perform the crush test on a small number of batteries to characterize the safety of the same batch of batteries. .
