New energy lithium battery laser welding precautions

Electric vehicles have been favored since their inception. Electric vehicles rely on power lithium batteries for power supply. At present, the battery shell made of aluminum alloy material accounts for more than 90% of the entire power battery. Aluminum batteries are welded together to protect the internal electrode material and prevent electrolyte leakage. However, traditional welding and packaging methods have been unable to meet the requirements of the rapid development of power lithium batteries, and laser welding technology has good adaptability to the aluminum casing of power batteries. The difficulty of its welding lies in the extremely high reflectivity of the aluminum alloy to the laser, the high sensitivity of pores during the welding process, and some problems and defects will inevitably occur during welding.

The difficulties of power lithium battery welding mainly include the following:

1. There are two main types of pores in the laser welding process of aluminum alloys: hydrogen pores and pores generated by bubble bursting. Because the cooling rate of laser welding is too fast, the hydrogen hole problem is more serious, and there is another type of hole caused by the collapse of the small hole in the laser welding.

2. Since aluminum alloys are typical eutectic alloys, hot cracks are prone to occur during welding, including weld crystallization cracks and HAZ liquefaction cracks. Due to the segregation of components in the weld area, eutectic segregation will occur and grain boundary melting will occur. Under the action of stress Liquefaction cracks will form at the grain boundaries and reduce the performance of the welded joint.

Other difficulties:

1. Welding of soft-package tabs requires higher welding tooling. The tabs must be pressed firmly to ensure the welding gap.

2. The welding of the cylindrical cell is mainly used for the welding of the positive electrode. Due to the thin shell of the negative electrode, it is very easy to be welded through.

3. When the square battery combination is welded, the poles or connecting pieces are polluted and thick. When the connecting pieces are welded, the pollutants are decomposed, and it is easy to form welding explosion points and cause holes; the battery with thin poles and plastic or ceramic structural parts under it, Easy to weld through. When the pole is small, it is easy to be welded to the point of burning of the plastic, forming an explosion point.

Precautions for laser welding of power lithium batteries

1. The thickness of the power battery is generally less than 1.0 mm, and the thickness of the mainstream battery is mainly 0.6 mm and 0.8 mm according to the different capacity. At present, laser welding methods are mainly divided into side welding and top welding. The main advantage of side welding is that the impact on the inside of the cell is small, and the spatter will not easily enter the inner side of the shell cover.

2. Since it may cause bulges after welding, which will have a slight impact on the assembly of the subsequent process, the side welding process has higher requirements on the stability of the laser, the cleanliness of the material and the matching gap between the top cover and the shell . For top welding, since it is welded on one surface, a more efficient galvanometer scanning welding method can be used, but it has high requirements for shell entry and positioning of the previous process, and high requirements for equipment automation.

3. Because the positive pole of the lithium-ion battery cell is an aluminum strip, it cannot be soldered directly, so a nickel strip needs to be added. Usually, the ultrasonic metal spot welding method is used: the high-frequency oscillation of the ultrasonic wave is used to make the friction between the two metal sheets partially Hyperthermia occurs, and fusion joins. Frequency: 20K~35KHZ Time: 0.3/S

4. The appropriate welding waveform should be selected during laser welding. The commonly used pulse waveforms include square wave, peak wave, double peak wave, etc. The reflectivity of the aluminum alloy surface to light is too high. When the high-intensity laser beam hits the material surface, the metal surface 60%-98% of the laser energy will be lost due to reflection, and reflectivity varies with surface temperature. Generally, sharp wave and double peak wave are the best choices when welding aluminum alloys. The pulse width of the slow-down part behind this welding waveform is longer, which can effectively reduce the generation of pores and cracks.