The three most common metal to metal joints in a lithium-ion battery pack are foil to tab, tab to tab, and tab to bus. All three joints pose joining challenges, but of the three, welding multiple layers of foil to a tab is the most challenging. The joint is often made up of dissimilar metals, the metal thickness is mismatched, and one side (the tab) is relatively thick (e.g. 0.2 mm) while the other is made up of multiple, extremely thin, layers. The image below shows a schematic of a large format lithium-ion battery pack cell. The foil to tab weld is needed to gather all the current collector plates (foils) inside the cell and join them to a tab which exits the cell casing and allows the cell’s energy to be transferred to an external source. There are two foil to tab welds in each cell, and hundreds of cells in a typical lithium-ion battery pack. Because of the series and parallel connections, one failure in a foil to tab joint will compromise the output of the entire pack, therefore, a robust joining process is required.
Ultrasonic metal welding (UMW) was evaluated for this particular application. A schematic of the process is shown below. Ultrasonic metal welding is very capable of welding similar and dissimilar combinations of battery related materials such as copper, aluminum, and nickel. Ultrasonic vibrations, typically 20 to 40 thousand Hertz, are used to rub two parts together under pressure. The scrubbing action breaks off oxide and contamination on the surface and breaks down surface asperities creating two ‘smooth’, clean metal surfaces. Once these contact under moderate heat and pressure, a weld is formed.
The process has several advantages. Since it is a solid state process, it can be adapted to dissimilar materials combinations and avoids most concerns about formation of intermetallic compounds. It is ideally suited to welding the highly conductive materials used in batteries including plated copper. It does not require high power and weld cycles are very short, fractions of a second. It also joins multiple layers of thin materials in one operation.
Both resistance spot welding (RSW) and laser beam welding (LBW) were also considered, but lack certain attributes that make UMW a more desirable joining process for the lithium-ion battery application. RSW relies on the resistance of a material to generate heat for joining. However, the aluminum and copper foils typically used in the battery industry have extremely low resistance, in addition, aluminum alloys form a tough surface oxide layer which inhibits RSW and is further compounded by the fact that the oxide layer is present on both sides of each foil layer. UMW does not rely on bulk resistance and inherently scrubs away oxide layers as part of the process. LBW is very sensitive to gaps between material layers in the weld joint. As a general rule of thumb, the gap should be less than 10% of the material thickness. Joining a 12 µm foil would require a 1.2 µm, or less, gap which is very difficult to achieve and requires excessive fixturing. Because UMW is self-clamping, gaps are not an issue.
A typical large format lithium-ion cell uses copper foil as the anode current collector and aluminum as the cathode current collector; therefore, both copper and aluminum have been evaluated with the UMW process. The experimental joints, as shown in the image above, were limited to similar material stacks only, meaning aluminum foils were joined to aluminum tabs and copper foils to copper tabs. The tab thicknesses were held constant at 0.005-inch. Two foil thicknesses, 12 and 25 µm, and two foil stack heights, 20 and 60 layers, were evaluated to prove feasibility and to study the effects on joint properties as the foil thickness and number of foil layers varies.
Cross Section – 20 layers of thin copper
Cross Section – 60 layers of thick aluminum
Analysis of the above cross sections provided a closer look at foil compression, foil damage and the final state of the weld joint. The samples with thinner and fewer layers of foil show an increase in foil movement directly adjacent to the weld zone. In contrast the samples with thicker and more foil layers showed a consolidation of the foils adjacent to the weld zone often resulting in a larger bond region. The consolidation and increase in bond region occurred because the thicker foil stacks bottomed out on the weld tool causing compression in the area adjacent to the weld zone.
Joining multiple layers of thin foils to a tab in a single ultrasonic metal weld operation is feasible. The welds are achievable without fracturing the delicate foil layers. Bonding occurs at the foil to tab interface as well as at each foil to foil interface which results in a strong, highly conductive electro-mechanical joint.
IR videography shows that all joints, with the exception of the copper sample made from 60 layers of 25 µm foil, stayed under 60 ºC during the weld cycle indicating the process will not harm nearby heat sensitive components.
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