With the increase in sales of new energy vehicles, power battery installed capacity is also growing rapidly. Data show that in 2021 China's new energy vehicle sales of 3.521 million units, an increase of 157.5%; power battery installed capacity reached 154.5GWh, an increase of 142.8%. Battery manufacturing enterprises have to expand production capacity, academician Ouyang Minggao in the electric vehicle 100 experts said that by 2025 China's power battery production capacity will reach 3,000 GWh. In the power battery manufacturing process, low cost, high quality and high efficiency is the pursuit of the three major goals of the manufacturing enterprises, therefore, to achieve the three goals of the technical process and intelligent equipment is highly favored by the battery manufacturers.
Power battery is also a complex system, from the cell, battery module, battery pack, through a manufacturing process, and finally assembled into a whole power battery system. Among them, the connection of material to material, module to module, and battery pack structure involves the highly demanding welding process - laser welding.
Power battery manufacturing process welding method and process of reasonable choice, will directly affect the cost of the battery, quality, safety and consistency of the battery. The next step is to organize the content of the power battery welding.
Power battery common welding applications
Power batteries are divided into square, cylindrical and soft pack batteries. Nowadays, in the production of power batteries, the use of laser welding links mainly include
Middle process: welding of pole lugs (including pre-welding), spot welding of pole bands, pre-welding of the core into the shell, sealing welding of the top cover of the shell, sealing welding of the liquid injection port, and so on;
Back-channel process: including welding of connecting tabs when battery PACK module, and welding of explosion-proof valve on the cover plate after the module.
Welding of battery shell and cover plate
The shell and cover of power battery play the roles of encapsulating electrolyte and supporting electrode materials, providing a stable and closed environment for the storage and release of electric energy, and its welding quality directly determines the sealing and pressure strength of the battery, thus affecting the life and safety performance of the battery. The battery shell is mainly made of Al3003 aluminum alloy, the thickness of which is generally between 0.6 and 0.8 mm, and is generally welded by low-power pulse laser. The connection between the case and the cover is shown in the figure. The main quality problems of the laser weld at this location are imperfections, porosity, and couching, which will reduce the sealing performance of the battery.
Battery Positive and Negative Terminal Welding
Battery pole is the battery positive and negative contact piece, generally speaking, the positive electrode uses aluminum, negative electrode uses copper, its function is to let the battery pole through the connecting piece welding, the formation of series and parallel circuit, forming a battery module.
Battery explosion-proof valve seal welding
Explosion-proof valve is a thin-walled valve body on the battery sealing plate, when the internal pressure of the battery exceeds the specified value, the explosion-proof valve valve body takes the lead in rupturing and venting, releasing the pressure and avoiding the battery from bursting. Explosion-proof valve structure is ingenious, mostly with laser welding firmly in a certain shape of the two aluminum metal sheet. When the pressure inside the battery rises to a certain value, the aluminum piece ruptures from the designed groove position, preventing the battery from further expanding and causing an explosion. Therefore, this process on the laser welding process requirements are extremely stringent, the requirements of the weld sealing, strict control of heat input to ensure that the weld destructive pressure value stabilized within a certain range (generally in the range of 0.4 ~ 0.7MPa), too large or too small will have a great impact on the safety of the battery.
Battery adapter welding
The adapter piece and flexible connection are the key components connecting the battery cover with the battery cell. It must take into account the requirements of overcurrent, strength and low spattering of the battery at the same time, so in the welding process with the cover plate needs to have enough weld width, and need to make sure that no particles fall on the electric cell to avoid a short circuit of the battery. As for the negative electrode material, copper belongs to the low absorption rate of high-reflective material, which requires higher energy density to weld, Lianyin laser's latest blue light composite laser can well solve the traditional process problems such as high reflective and spattering.
Battery pole welding
The poles on the battery cover are divided into internal and external connections. The internal connection of battery is the welding between the pole lugs of the battery cell and the pole post of the cover plate; the external connection of battery is the welding of the pole post of the battery through the connecting piece to form a series and parallel circuit to form a battery module.
The main problem with laser welding of battery poles is also the blow hole defect, the cause of which is similar to that of explosion-proof valves. The pole weld is essentially the mating surface between the aluminum adapter and the pole, and the hole in the aluminum block is only about 6 mm in diameter, which makes it very easy for impurities such as stamping oil and cleaning agents to remain in the area. The high energy density of the laser causes a temperature surge in the weldment, which leads to the rapid vaporization of the impurities left at the pole, which causes bubbles to escape and leave the molten pool overcoming the surface tension of the molten pool, resulting in a blown hole defect. During this process, the rapid change in pulsed laser power further increases the tendency to form blowholes. Therefore, in addition to intensive pre-weld cleaning, blowholes can also be reduced by optimizing the laser power variation.
Power Battery Module and PACK Welding
Battery module can be understood as a combination of lithium-ion cells connected in series and parallel, with a single battery monitoring and management device. The structural design of the battery module often determines the performance and safety of a battery pack. Its structure must support, fix and protect the electric cell. At the same time, how to meet the overcurrent requirements, current uniformity, how to meet the temperature control of the battery cell, and whether there is a serious abnormality can be cut off, to avoid chain reaction, etc., will be the standard to judge the merits of the battery module.
At the same time, due to the copper and aluminum heat transfer are very fast and very high reflectivity to the laser, the thickness of the connecting piece is relatively large, so it is necessary to use a higher power laser to be able to achieve the welding.
Conclusion
At present, the main problems of laser welding in power batteries are porosity, cracks, poor molding, blowholes and other welding defects. These defects lead to reduced strength, sealing and conductivity of the battery pack, leading to a series of safety problems such as battery explosion, liquid leakage and heat generation. In response to these problems, a large number of studies focus on process optimization, by adjusting the laser welding power, pulse width, welding speed, defocusing amount and other parameters can effectively reduce defects.
It is not difficult to see that the power battery welding process is a delicate work, any small problem will affect the performance and safety of the subsequent finished power battery. Therefore, high-quality materials and high-quality laser welding equipment is to ensure the success of the welding process is the basis. Intelligent technology represented by laser welding path planning, weld identification, defect identification, quality monitoring, etc. is also one of the hot spots for future research. Translated with www.DeepL.com/Translator (free version)
