Laser welding technology in new battery manufacturing

Battery manufacturing is increasingly becoming the biggest new challenge in manufacturing in the 21st century. Although the global battery production has reached tens of billions of pieces per year, traditional battery manufacturing technology has been unable to meet the rapidly growing range of battery applications in terms of battery efficiency and cost budget. Most of us are already very aware that batteries are used in hybrid, plug-in hybrid and all-electric vehicles. Although the application of batteries in the automotive industry is mixed, it still cannot stop people's investment enthusiasm for this industry. The American Recovery and Reinvestment Act (ARRA or AR2) mentions that millions of dollars must be injected into companies that invest in batteries in the United States. Borrowing the words of US Secretary of Energy Steven Chu, “These are very effective investments. The future will bring us many times of returns, mainly in the following areas: creating jobs, reducing dependence on foreign oil, and letting us breathe. The air is cleaner and counteracts the effects of climate change."

In addition to applications in the automotive industry, cost-effective, high-performance batteries are also attractive in power and alternative energy applications. Widely installed residential battery storage provides load-level power requirements as well as emergency backup power. This distributed energy storage solution can also increase the productivity of power production sites, offset some of the demand, and invest in and improve some large grid infrastructure. Moreover, electrical energy storage is beneficial to take advantage of unstable alternative energy sources such as solar and Wind Power. In addition, economical and high-performance battery technology will also help reduce the cost of grid expansion.

Today, “high performance and economy” should be used to clearly describe and recognize the heroic role of batteries in helping us solve energy challenges. Despite a lot of concentrated development work so far, the newly developed battery technology is a bit younger than the old and mature technology (such as batteries for flashlights, cameras and computers). Because the energy storage and battery life requirements for battery applications are higher on the market today, the weight and cost of the battery are lower, and the challenges in the manufacturing process are still being solved. Many promising battery solutions exist only in theoretical CAD graphic design, and this design approach has been disconnected from the capabilities and limitations of existing manufacturing technologies. In other words, there needs to be more communication between the designer and the manufacturing staff! We will help them.

Among the many battery technologies used in the above industries, no recognized or obvious winners have emerged. Lithium ions, nickel metal hydrides, zinc air, sodium, sulfur, and many other battery chemistries compete with each other to meet a variety of needs in a variety of applications. And for a given battery variety, there will be many different product shape variations that may result in different manufacturing methods. The new batteries are mostly cylindrical or flat design. The cells are stacked, packaged, or arrayed so that the individual cells are combined into a series and parallel circuit. The connection between a unit and a unit may involve similar or different metallic materials, with two or more different layers. Despite the diversity of designs, there is a common denominator in all of these battery design concepts. The common challenge is to connect thinner and thinner metal materials at a faster rate, which is where lasers are applied.

Batteries usually contain many materials such as zinc, steel, aluminum, copper, titanium, nickel, and the like. These metals may be made into electrodes, wires, or simply outer casings. They may or may not be covered by another metal or battery material. However, no matter what they are composed of, they should be as thin as possible based on weight and cost minimization. Many emerging battery design materials range in thickness from 25 to 250 microns. The two main requirements for soldering these metals include creating a current conduction path and/or the ability to store electrolytes, but for each cell design and application, the complete performance specifications for battery soldering are unique. Conductivity, strength, air tightness, metal fatigue and corrosion resistance are typical welding quality evaluation criteria. Then, after all of these criteria have been identified and implemented, the key determinant is whether the cost is economical.

Battery soldering design plays a key role in the success of the manufacturing process. Fillet and butt welds are often attractive to designers, but lap welding is by far the most likely to succeed. Compared to the other two welding methods, lap welding provides more flexibility, thanks to the fact that it does not require the precise node-to-beam alignment required by other welding methods. The lap welding also provides the possibility of soldering the multilayer battery assembly for the final integration task (see Figure 1), which may involve one or more material types. For example, some metal of the same type may need to be accurately soldered to another type of conductor. There are many types of welding methods that can be used, and they are still rapidly developing.

Series electrical connections and buss (BUSS) connections typically require the connection of non-similar metals. It is in this area that the high speed (100 to 1000 mm/sec) unique to laser welding is unmatched by other welding techniques. High-brightness fiber lasers further drive high-speed welding, enabling low heat input and high solidification rates at the weld. This high cooling rate effectively controls the solidification defects that occur in mixed metal welding. The weld metal combination that is most prone to cracking is copper and aluminum, which happens to be one of the most common metal combinations in lithium-ion batteries. High-speed fiber laser welding exhibits crack-free soldering characteristics in this important metal welding process. Other metal combinations that need to be used or are not common can also be done by laser welding, especially when the solidification speed is high (see Figure 2). Finally, as long as these weld metal combinations are achievable, it is critical to evaluate their performance in expected battery applications (especially strength, toughness, metal fatigue and corrosion resistance).

Some people may think that the combination of this attractive manufacturing process advantage and opportunity must be accompanied by negative factors. The first unfavorable factor is that these unique laser welding applications are relatively new to battery designers. Thus, "laser friendly" soldering designs are not always preferred. In addition, if laser welding technology is considered, those problems that often arise and are not solved will emerge – how is the performance of the weld? The battery industry seems to be more willing to invest quickly in reliable manufacturing capacity. There is also no time, resources or patience in the community to evaluate, develop and implement emerging alternative solutions.

One recognized, frequently used laser welding solution is laser welding based on galvanometer scanning. This "remote welding" technology is not particularly novel in the vast world of laser welding, but improvements in scanning head and laser performance are gaining increasing attention. The ever-increasing power of high-power fiber lasers emits an almost perfect beam that can now be fully utilized within the ultimate speed of the welding process and limits the acceleration problems of other welding motion systems without affecting it. This beam quality also guarantees a larger visual range, longer run times and more angles of incidence, and can be used to weld multiple solder joints simultaneously in many battery solder configurations (see Figure 3).

Other advances in high-speed galvanometer scanning laser welding include the emerging "flying light path" welding technology. In this case, the wide coverage area, high welding speed and very high acceleration that can be achieved can be achieved by precise synchronous scanning axes (A, B) and mutually perpendicular mechanical motion directions (X, Y). EWI developed and demonstrated the feasibility of this technology using a standard Scanlab scan head and a beta version of CNC software and hardware from Aerotech. This high-performance laser welding solution (see Figure 4) is currently being used for challenges in battery soldering and fuel cell soldering process development.

The last laser cell welding challenge is the stability and quality assurance of the process. Based on the high speed and flexibility of laser welding, the success of the manufacturing process depends on the performance of other mechanical components throughout the system to quickly achieve a good weld. This is a very difficult task, especially considering the small size and high speed of the welding, as well as the large number of welds required to complete the battery production. At the same time, considering the number of welds required in the final battery package, the welding quality of the Six Sigma grade is still insufficient and a higher level of quality is required. The solution to these major challenges (process routing and weld quality assurance) is mostly achieved through high-speed image acquisition and analysis. Some of these methods have been tried in some lower speed laser welding applications, but further speed and accuracy are needed, which is a guarantee of the full potential of laser welding in the battery manufacturing industry.

The speed of welding, the performance of mixed metals and the opportunity for quality control are already in place, and are only underdeveloped and widely used. Laser welding may only capture a small piece of this new battery development boom, but it is likely to become the dominant choice for most battery material connection applications in the future.

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