EWI has been working with advanced battery companies on this challenge for several years. As a part of the Symposium on Battery Manufacturing Technology held in September 2010, EWI surveyed the industry about the techniques (see TABLE above) being used to make joints during battery cell and pack assembly. The results of this survey showed that lithium batteries are complex assemblies of multiple layers of several materials (copper, aluminum, nickel, and nickel-plated copper) in a wide range of thicknesses (0.001-0.0625 in.).The manufacturing of the systems requires assembly of a large number of connections, including joints between dissimilar materials. To make these connections, the industry uses a wide range of processes, including soldering, resistance, ultrasonic, and laser welding. The survey results in the TABLE show that no single joining process dominates this application. No one seems to agree on which process to use for each material-geometry combination.
As a result of this survey, EWI and the Ohio State University Center for Automotive Research designed a project to investigate resistance, ultrasonic, and laser welding approaches to making joints between the material combinations involved in assembling lithium ion battery cells and packs. The goal was to develop data to help the industry match a joining process with each joint in the assembly.
Laser welding is attractive because it is a very flexible and precise process. Welds can be sized and shaped to fit into small spaces and to adjust to a wide range of designs. Since laser welding is a non-contact process, an effective welding beam can be delivered into small spaces that are not readily accessible to ultrasonic or resistance welding heads. The welding process is high speed and does not involve a high heat input even though it is a fusion process.
A 600-W IPG YLR-600-SM continuous wave ytterbium fiber laser that delivers a wavelength of 1070 nm was used for this investigation. The beam was delivered to the work piece with a 100 mm focal length lens that theoretically delivers a 9-μm spot. Even though both aluminum and copper are highly reflective (>90%) for infrared wavelengths around 1000 nm, welds could be made using only 55-75% full power (330-450 W).
A two-dimensional motion control system with a scan rate of 200-400 mm/s was used to make the welds. The laser study involved a full factorial designed experiment that incorporated all possible material combinations (aluminum 1100 or 1145, copper 110, nickel 200, electroless plated nickel on copper, electroplated nickel on copper); the presence or absence of argon shielding gas; and sample orientation (which material the laser beam addresses first). The bulk of the study was done with 125-μm-thick materials. All welds were full penetration through both substrates. Cross sections of two tab welds are shown in FIGURE 1.
For simplicity the laser welds were simple single scans across a 25-mm-wide test specimen. Both the mechanical and electrical performance of the welds were evaluated. Mechanical performance was evaluated using both lap shear and t-peel tests. The resistance of the weld was evaluated with a 1 second high current pulse delivered by a resistance welding power supply. Pulses ranged from 400-1000 A.
Laser welding produces strong welds for all material combinations. It was particularly effective for making copper-to-copper and aluminum-to-aluminum welds. It was also the only technique that could weld combinations involving electroplated nickel on copper. In these experiments, the presence or absence of a shielding gas did not make any difference in either the mechanical or electrical performance of the weld. The only difference noted was welds made without shielding gas showed more surface oxidation than those made with shielding gas.
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