In recent years, due to the implementation of government policies to protect the environment and reduce dependence on petrochemical energy, as well as people's increased concern for the environment, electric vehicles occupy a significant share in the automotive market due to their low-carbon, environmental protection and power-saving features Exponential growth. With the advantages of high power factor, high power factor, long service life, and the safety and reliability of the lithium-ion battery system, lithium-ion batteries have gradually become the most important power source for electric vehicles.
Because the development time of lithium-ion battery technology is shorter than the development time of fuel technology, the current technology has not yet matured. Accidents reported in electric vehicles include spontaneous combustion and collision fires with lithium-ion batteries in electric vehicles. newspapers. There are many factors that affect the safety of the lithium-ion battery system, such as: excessive charging and discharging, short circuit, collision, high temperature, poor connection, etc. Among them, there are many safety related accidents caused by unreliable connections, and loose connections have difficulties such as large uncertainty and difficult diagnosis. Therefore, research in the connection technology of lithium-ion battery systems will improve the connection performance of a lithium-ion battery. systems, Process optimization and strengthening of process management in the field of production are of great importance.
In a lithium-ion battery system, there are many processes related to the connection technology. From the process point of view, basically the connections between three levels: Electrodes-to-Tab at the unit cell level and sealing of the housing, between cells at the module level, and between modules at the battery level, as shown in Fig. 1. Since the lithium-ion battery system is composed of many unit cells, modules, etc., it includes many connection technologies. Common joining methods are: ultrasonic welding, resistance spot welding, laser welding, TIG pulse welding, mechanical joining, etc. This article combines the results of the above jointing technologies in lithium-ion battery systems, and investigates the influencing factors in processing technology various connection technologies.
1 ultrasonic welding
Ultrasonic welding is a solid state welding process. The workpiece does not need to be melted, but the mechanical vibration energy of high frequency ultrasound (typically above 20 kHz) is used to tighten the workpiece and fuse it by friction. Ultrasonic welding can be widely applied in joining various forms of metal foil, not only for welding between workpieces of the same material, but also for welding between workpieces of different materials with different melting points. In addition, due to its high efficiency and speed, low temperature operation and environmental protection, ultrasonic welding is also widely used in battery system bonding. The use of ultrasonic welding in lithium-ion electronic systems is limited mainly due to the small thickness of the welded metal (<3 mm) and the impossibility of welding workpieces from high-strength materials.
The process parameters affecting the quality of ultrasonic welding are similar to the application of ultrasonic welding in other fields, mainly including welding pressure, welding amplitude and welding time. The welding process parameters have a great influence on the welding performance of the lithium-ion battery system, which is mainly reflected in the welding quality, mechanical properties, electrical properties, thermal properties, etc.
Das et al. studied the effect of welding process parameters on the mechanical properties of welding, prepared specimens by ultrasonic welding of 0.3 mm diameter copper pole pieces with 1 mm thick copper leads, and studied the process parameters using a T-pull strength test (Fig. 2). the effect of changes on mechanical properties. It was found that the welding pressure varied from 0.5 bar to 4.0 bar, and the maximum peel strength was achieved at a welding pressure of 1.5 bar. The welding amplitude ranges from 30 µm to 50 µm. As the welding amplitude gradually increases, the peel strength does not change much. When the welding amplitude increases to 45 µm, the peel strength increases rapidly. When the welding time is in the range of 0.15 to 0.55 seconds, the peel strength gradually increases with increasing welding time. Thus, the ultrasonic welding process parameters for welding a 0.3 mm copper pole piece to a 1 mm copper pole piece were finally optimized: welding pressure 1.5 bar, welding amplitude 50 µm, welding time 0.55 s.
Li et al. used copper sheets with a thickness of 0.4 mm and 1.0 mm, respectively, and studied the effect of process parameters on welding quality using ultrasonic welding. It was found that the welding time changed from 0.2 s to 1.0 s. At the beginning of welding, the Vickers hardness increased by 40% compared to the starting material, but as the welding time was further increased, the hardness decreased rapidly, even lower than that of the starting material. material. This is because at the beginning of welding, the mechanical vibration is equivalent to the cold working of the copper sheet, which increases the Vickers hardness of the copper sheet. However, as the welding time increases, the metal material exhibits plasticization behavior when processed in the ultrasonic welding process, and the results of the scanning electron microscope are shown in Figure 3.
Different parameters of the ultrasonic welding process influence each other. In the actual application process, complex adjustments must be taken into account. At the same time, it is also strongly affected by the material. There are two main reasons. One of them is that different materials are sensitive to different welding amplitudes, in addition, even with the same amplitude, the optimal welding process parameters for different materials are also different. For example, when Das et al.  studied the effect of ultrasonic welding process parameters on copper sheet on T-peel strength, they also studied aluminum specimens of the same thickness. The optimized process parameters were as follows: welding pressure of 1.5 bar. , welding. The amplitude is 50 µm and the welding time is 0.35 s.
2 contact spot welding
Resistance spot welding occurs during the welding process, the workpiece is pulled by the electrode, and the current is turned on, and then the resistance of the metal workpiece interface generates local heat to melt the workpiece. Because resistance spot welding has the advantages of relatively mature technology, easy operation, low cost and high work efficiency, it is most widely used in the traditional automotive industry. Similarly, resistance spot welding is also widely used in the bonding of lithium-ion batteries, especially in the manufacture of small cylindrical battery cells, as shown in Figure 4.
Process parameters affecting resistance spot welding include current, pressure, sealing time, etc., among which the peak current and peak current time are the most important. This is because the current is too low or the peak current time is too short, and the workpiece interface is insufficient to generate the heat required for local melting, so a continuous alloy cannot be formed. However, the peak current and peak current time should not be too long, because if the workpiece is exposed to the peak current for a long time, excessive local heat may be generated, causing the material to evaporate. Therefore, in order to produce a good quality lithium-ion battery system, it is necessary to optimize the relevant parameters of the resistance spot welding process.
The use of resistance spot welding to connect lithium-ion battery systems faces many challenges. There are three main points: (1) High conductivity materials commonly used in lithium-ion batteries are not suitable for resistance spot welding, such as copper and aluminum used as electrodes and pole pieces, which are difficult to realize by resistance spot welding due to for high conductivity; (2) resistance spot welding is to melt the workpiece to achieve the purpose of welding. Different materials are difficult to weld due to their different melting points; (3) difficult to apply for welding multi-layer workpieces. , and it is difficult to produce large nuggets, typically 0.9–2.0 mm. However, Das et al. found that when resisting thin workpieces, welding different materials (0.3 mm thick copper-aluminum) had higher productivity than welding the same material (0.3 mm thick copper-aluminum). ). Cu and Al-to-Al) has a larger weld size and exhibits better weld strength. And if copper sheet is used as the top workpiece and aluminum sheet is used as the bottom workpiece, better results will be obtained.
3 laser welding
Laser welding is a non-contact welding process in which the heat generated by a laser beam heats up a workpiece and fuses multiple layers of metal, usually within a few milliseconds. Commonly used for welding electrolyte containers, connectors and busbars. As a non-contact welding process, laser welding has corresponding advantages for joining lithium-ion battery systems. Since laser welding has the smallest heat-affected zone of all welding processes and can be used to join multi-layer sheets, laser welding is considered the most efficient welding process for lithium-ion battery systems. in figure 5. .
There are many factors that affect the laser welding process, mainly the wobble frequency and amplitude associated with the wobble, the power frequency and pulse width associated with the laser, and the travel speed and focus range associated with the equipment. Sheikh et al. used a 150 W quasi-CWIR laser to laser-weld copper contacts to steel cylindrical battery boxes and studied the effect of the respective process parameters on mechanical, electrical, and thermal properties. It was found that the parameters related to the laser, power, frequency and pulse time are positively correlated with the shear strength of the sample, while the pulse time is negatively correlated, so the optimal process parameters are optimized, the power is 40%, the speed is 500 mm/min , pulse time 2 ms, frequency 50 Hz.
However, the relevant researchers also found that laser welding requires high accuracy in the assembly position of the workpiece, and requires welding materials to have high reflectivity, high thermal conductivity and other properties, so laser welding has been applied to join lithium-ion batteries Increased complexity. Through the continuous research of the majority of the workers, the corresponding relevant experience was finally summarized. When welding the tongue to the terminal, the tongue must be thinner than the battery compartment terminal, and the processing parameters must be strictly controlled to ensure sufficient welding energy without penetrating the battery compartment. Brand et al. found that because the weld pool in laser welding is small and the gap tolerance is small, better results can be obtained by tightening the tabs on the battery when welding the tabs.
Tungsten InertGas (tungsten gas) pulsed welding, also known as micro-TIG, is a welding process that uses non-consumable tungsten electrodes to create an arc and work in an inert gas environment. The TIG pulse welding process features easy operation, beautiful weld and high quality. With TIG pulse welding cleaning mechanism, it can clean aluminum alloy resistive oxide film, so it is more suitable for joining aluminum lithium-ion battery pack.
There are many process parameters that affect the quality of TIG pulsed welding, including welding current, welding speed, arc voltage, wire feed speed, weld diameter, and operating factors. For TIG welding of aluminum alloy, Van Zely used a one-dimensional method to study the effect of welding current, welding speed, and wire feed speed on weld quality such as penetration and weld morphology. The results show that the penetration depth is positively correlated with the welding current, as shown in Fig. 6, and negativecorrelates with the welding speed and wire feed speed, and the welding current has the greatest influence on the penetration depth. In combination with surface topography, cladding height and fusion width, the optimized process parameters are a welding current of 125 A, a welding speed of 200 mm/min and a wire feed speed of 1100 mm/min.
At the same time, some researchers noted that due to the high temperature of TIG pulse welding, the aluminum alloy material is prone to deformation and cracking, which is not conducive to controlling the dimensional accuracy of the lithium-ion alloy. block of the battery system and affects the strength of the block. Connection failure may occur during use. Therefore, in the production process, it is necessary to identify possible defects in advance and avoid them.
5 mechanical connection
Existing research data shows that various mechanical methods such as nuts, bolts, screws, or latches can be used to connect lithium-ion battery systems, as shown in Figure 2. 7.
Mechanical joints have the advantage of being strong, easy to dismantle for repairs, and usually operate without heat. In the process of industrial production, people have learned more about the process of mechanical connection, so I will not repeat here. However, the selection process also needs to pay attention to the characteristics of the various mechanical joining processes. studied various bolted connections such as nuts and bolts, drilled thread screws and tapping screws for electrical connections, and tested their electrical resistance. It was found that the electrical resistance of these mechanical joints increased over time. And for bolted and screw connections, the change in the resistance of the connection is also affected by the resistivity, geometry, changes in the parameters of the coating and the process of the connected objects.
Because the Li-ion battery system includes many processes, in addition to the bonding technologies described above, the bonding technologies currently used include friction stir welding, wire bonding, soldering, and bonding. Wait.
With so many connection technologies in the design and manufacture of lithium-ion battery systems, to select the appropriate connection technology, Das et al., in terms of connection strength, resistance, durability, cost, etc., using a decision matrix (Pughmatrix) to analyze various perfect join methods. For example, for modular connection of cylindrical batteries and modular connection of pocket batteries, ultrasonic welding, laser welding and TIG welding are ideal connection technologies.
With the development of electric vehicles, the technology for connecting lithium-ion battery systems has also advanced significantly. At the same time, the production of high-quality lithium-ion battery systems is gradually becoming the most important issue in the field of battery production. In order to meet people's demand for quality lithium-ion battery systems, more in-depth technical research into connection technology is needed. In addition to CT-control and visual control of the state of the connectionCAE analysis can also be introduced to simulate the joining process, optimize process parameters, reduce testing, and improve production efficiency and joint performance. It is believed that with the continuous deepening of research in the field of connection technologies, the process elements of various connection technologies become more and more clear, the production process control becomes more and more perfect, and the quality of lithium-ion battery systems becomes more and more assured, which elevates electric vehicles to a new level.height.
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