Manufacturers of electric vehicles (EVs) continue to look for ways to improve battery technologies despite the substantial recent reduction in the cost per kilowatt hour for lithium-ion batteries. Many manufacturers aim to significantly increase energy densities, which will provide greater driving distance between charges, faster charging times, and a reduced risk of overheating. It will also enable the use of nonflammable electrolytes.
Ultrasonic welds provide the most proven and reliable solution for interconnecting the thin copper foils and tabs that are typically used as anode current collectors, as well as the aluminum foils and tabs used in many cathode current collector structures. (Image: Emerson)
Increasing the energy density of EV batteries requires adding substantially more layers of the anode/cathode/electrolyte thin foil materials now being used by the industry. These thin foil layers give EV batteries the ability to charge, store, discharge, and release electrons that produce the electric current that powers the vehicles.
In current EV batteries, graphite-coated copper is the most widely used thin foil material, welded to a single, heavier, nickel, or nickel-coated copper tab. Cathode foils are made from aluminum and are welded to a single, heavier-gauge aluminum tab. These foil-to-tab weld assemblies allow the current to charge and discharge through the external casing of individual battery cells.
By adding layers of these thin foils — while staying within the physical parameters of existing battery designs (for example, 18650/20700 cells or flat prismatic cells) — these more powerful batteries have increased surface area to collect and transport electrons, increasing energy density and battery capacity.
However, increasing energy density by this method can present problems. The ultrasonic metal welding technology typically used for the foil-to-tab bonding process can only produce reliable assemblies using a maximum of 40 to 50 foil layers. Adding foil layers requires higher welding amplitude to bond the layers.
When the number of foil layers (ranging in thickness between five and eight microns) approaches 60 or more, the higher amplitude needed to bond them results in increased particulates and damage to the metal foil, such as wrinkling, cracking, and tearing. Such damage — which increases the risk of short circuits and threatens battery quality by reducing current capacity — can be effectively detected by monitoring weld quality data/limits, performing strength tests, and conducting visual inspections.
Conventional ultrasonic welders could not effectively make the foil-to-tab welds because they used a horizontal oscillation technique that delivered weld energy through a cantilever-type actuator that extended outward before adding clamp force. When more foil layers were added, which required higher amplitude and clamp force, the actuator arm created additional vibration that compromised clamp stability and allowed foil layers to wobble and shift.
The Branson GMX-20DP “Direct Press” Ultrasonic Metal Welder from Emerson. The reliability of the welds produced by this method has been validated by laboratory and customer tests.
Unfortunately, increasing amplitude to compensate made the excessive vibration even worse. Clearly, a new metal welding technology was needed to deliver reliable, foil-to-tab welds on these thicker stacks of metal foils, and engineers have delivered.
The answer: to develop a technology that delivered the needed weld energy more efficiently to the foil stack.
A new form of ultrasonic welding
This new “direct press” ultrasonic metal welder makes it feasible to weld thicker foil layers. It has a vertical actuator that applies more clamp force on the welded parts, holding them with greater stability.
Several factors contributed to the welder’s success: The motion of the vertical actuator could be more carefully controlled, undesirable actuator stress and vibration could be eliminated, and the tooling “grip” on the parts was improved.
The result is a weld process that’s much gentler and provides more consistent, higher quality due to the lower amplitude required. The new method can consistently weld up to 80 or even 100 thin foil layers without damage to the fragile foils.
The reliability of the welds produced by the direct-press ultrasonic method has been validated by laboratory and customer tests. In trials with manufacturers, this new direct-press welding technology has proved its ability to produce high-quality spot welds on 100 layers of 10 micrometer (µm) copper foil to a 0.5 millimeter (mm) copper plate and 86 layers of 10 µm copper foil to a 0.2 mm nickel-coated copper tab.
Using the new direct-press technology, battery manufacturers can produce more reliable, longer-range, more energy-dense batteries, offering the added benefit of high-quality ultrasonic weld power supplies that collect weld data using built-in, weld-quality analytical tools. These batteries, coupled with improved charging systems and nonflammable or even solid electrolytes, should result in electric vehicles with greater driving range and greater operational safety.
The benefits
The direct-press welding technology will provide EV manufacturers with various benefits, including:
- High-quality welds using lower weld amplitudes, typically just 30% to 40% of those required by a conventional ultrasonic welder
- Welds with up to 60% greater pull/peel strength
- Virtually no particulates during the weld process
- Less damage to thin foil layers during welding
- Simplified weld tooling, with minimal knurling required
- Longer tooling life due to improved “grip” on parts and reduced tooling slippage
Additionally, the new technology offers all the other advantages inherent in ultrasonic metal welding, such as solid-state, low-resistance welds that do not corrode and are free of intermetallic compounds.
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