Lithium-ion battery production site foreign body control

by:CTECHi     2021-07-07

The basic principle of the internal short circuit of the battery caused by the metal foreign body has two kinds of processes, as shown in Figure 1. In the first case, metal particles of larger size directly pierce the separator, causing a short circuit between the positive and negative electrodes, which is a physical short circuit.

In the second case, when metal foreign matter is mixed into the positive electrode, the potential of the positive electrode rises after charging, and the metal foreign matter dissolves at high potential, diffuses through the electrolyte, and then the negative electrode has a low potential The dissolved metal precipitates and accumulates on the surface of the negative electrode, and finally pierces the separator to form a short circuit, which is a chemical dissolution short circuit. The most common metal foreign objects on the battery factory site are Fe, Cu, Zn, Al, Sn, stainless steel, etc.

The process in which foreign matter is easily mixed into battery products at the battery manufacturing site includes the mixing of electrode slurry with metal impurities; the pole piece cutting process produces cutting burrs or metal chips; the winding process The chip is cut to produce burrs or metal foreign particles are mixed into the battery; the welding of the tab and the shell produces metal shavings, etc., as shown in Figures 3 and 4.

Regarding the control standards for metal foreign bodies and burrs, the burr size is generally less than half of the thickness of the diaphragm, but some manufacturers have stricter control requirements, and the burr does not exceed the coating, as shown in the following video:< /p>

Lithium wants to live

Honeycomb Energy's #Lithium-ion battery production foreign body and burr control standard

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In the process of inspection, the battery passes the voltage withstand test to detect the internal short-circuit unqualified product before the liquid injection; X-ray detects the battery cell Foreign matter inside; the aging process detects unqualified products through the battery voltage drop ΔV.

Metal foreign matter detected in the withstand voltage test

Insulation withstand voltage test generally adopts a safety gauge, and the electric core is hot pressed. During the shaping test, the instrument applies a voltage to the cell for a specified period of time, and then checks whether the current remains within the specified range, and judges whether there is a short circuit inside the positive and negative poles of the cell. Generally, the applied voltage is shown in Figure 5:

① Within a certain time t1, apply voltage to the cell from 0 to U.

②The voltage U is maintained for a period of time to t2.

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

In the test, because the positive and negative pole pieces are close to each other, only 15-30μm, a certain capacitance (stray capacitance) will be formed inside the bare cell. Exist, the test voltage must start from 'zero' and rise slowly to avoid excessive charging current. The larger the capacitance, the longer the ramp-up time t1, and the lower the voltage that can be increased at one time.

When the charging current is too large, it will definitely cause the tester to misjudge and make the test result incorrect. Once the stray capacitance of the tested cell is fully charged, only the actual leakage current will remain. Since the DC withstand voltage test will charge the battery under test, after the test, the battery under test must be discharged.

The diaphragm has a certain withstand voltage strength. When the applied voltage is too high, it will definitely break down the diaphragm and form a leakage current. Therefore, first of all, the cell 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 to be qualified.

If there is a certain size of foreign matter between the positive and negative electrodes, the separator is squeezed, the distance between the positive and negative electrodes is reduced, and the breakdown voltage between the positive and negative electrodes Will drop, if the same voltage is still applied, the leakage current may exceed the set alarm value. By setting the test voltage and other parameters, the size of the foreign body inside the battery can be statistically analyzed and judged, and then the test parameters can be set and the quality judgment standard can be set according to the actual production status and quality requirements of the product.

Example of foreign body size and withstand voltage test (values u200bu200bare assumed values)

In the test, the main parameters include voltage Slow rise time t1, voltage hold time t2, load voltage U, and alarm leakage current. As mentioned above, t1 and U are related to the stray capacitance of the cell. The larger the capacitance, the longer the ramp-up time t1 is required, and the lower the loading voltage U is. Moreover, U is also related to the withstand voltage strength of the diaphragm itself. If there is a foreign body inside the test cell, which causes an internal short circuit, the diaphragm is broken down. The specific situation is shown in Figure 7.

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

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