Foreign matter control on lithium ion battery production site

There are two basic processes of internal short circuit of battery caused by metal foreign matters, as shown in Figure 1. In the first case, large metal particles directly pierce the diaphragm, causing a short circuit between the positive and negative electrodes, which is a physical short circuit.

In the second case, when the metal foreign matter is mixed with the positive electrode, the positive electrode potential rises after charging, the metal foreign matter dissolves at high potential, diffuses through the electrolyte, and then the metal with low potential dissolved in the negative electrode is deposited on the negative electrode surface, finally pierces the diaphragm to form a short circuit, that is, a short circuit of chemical solution. The most common metal impurities in battery plants include iron, copper, zinc, aluminum, tin, stainless steel, etc.

At the battery production site, the battery products are easy to mix with foreign matters, including electrode slurry mixed with metal impurities; Cutting burrs or metal chips generated during pole cutting; When the electrode piece is cut off in the winding process, burrs or metal foreign matter particles are mixed into the iron core. Welding of lug and shell will produce metal chips, etc., as shown in the figure. 3 and 4.

For the control standard of metal foreign matters and burrs, generally speaking, the burr size is less than half of the diaphragm thickness, but some manufacturers have stricter control requirements, and the burr does not exceed the coating.

During the test, the battery is tested for internal short circuit nonconforming products through voltage test before injection; X-ray detected foreign bodies in cells. Aging process through battery voltage drop δ V Inspect the unqualified products.

Detection of metal foreign matters by withstand voltage test

The insulation withstand voltage test generally uses a safety meter. During the battery hot pressing test, the instrument applies a voltage to the battery for a specified period of time, and then checks whether the current is kept within the specified range to determine whether there is a short circuit inside the positive and negative electrodes of the battery. In general, the applied voltage is shown in Figure 5:

① Increase the voltage on the battery from 0 to U within a certain time T1.

② The voltage U remains at T2 for a period of time.

③ After the test, cut off the test voltage and discharge the stray capacitance of the battery.

During the test, the anode plates are close to each other, only 15 to 30 microns. A certain capacitance (stray capacitance) can be formed inside the bare battery. Because of the capacitance, the test voltage must start from "zero" and rise slowly. In order to avoid excessive charging current, the larger the required capacitance is, the slower it rises. The longer the t1 time is, the lower the voltage can be increased.

When the charging current is too large, it will inevitably lead to the misjudgment of the tester, resulting in incorrect test results. Once the stray capacitance of the tested battery is fully charged, only the actual leakage current remains. Since the DC voltage test will charge the tested battery, please ensure that the battery is discharged after the test.

The diaphragm has a certain voltage strength. When the load voltage is too high, the diaphragm will definitely break down and form a leakage current. Therefore, first of all, the core insulation test voltage should be lower than the breakdown voltage. As shown in Figure 6, when there is no foreign matter between the positive and negative electrodes, the leakage current under the test voltage is less than the specified value, and the battery is judged as qualified.

If there is a certain size of foreign matter between the positive and negative electrodes, the diaphragm will be squeezed, the distance between the positive and negative electrodes will decrease, and the breakdown voltage between the positive and negative electrodes will drop. If the same voltage is applied at the same time, the leakage current may exceed the set alarm value. By setting parameters such as test voltage, you can statistically analyze and judge the size of foreign matters in the battery. Then, according to the actual production situation and quality requirements, you can set test parameters and formulate quality judgment standards.

Sample foreign matter size and withstand voltage test (assumed value)

In the test, the main parameters include the slow voltage rise time T1, voltage holding time T2, load voltage U and alarm leakage current. As mentioned above, T1 and U are related to the stray capacitance of the battery. The larger the capacitance is, the longer the slow rise time T1 is required, and the lower the load voltage U is. In addition, U is also related to the compressive strength of the diaphragm itself. If there is foreign matter in the test unit, it will cause internal short circuit and the diaphragm will be damaged, as shown in Figure 7.

Therefore, the insulation withstand voltage test of lithium battery is an important part of product process inspection, which can detect unqualified products and improve the safety factor of final battery products. The actual test needs to consider many factors, such as parameter settings and judgment criteria.