Engineering Challenges: Joining Dissimilar Metals in Battery Production
Before the battery chemical reaction begins to break down the materials and decrease charge storage ability, the metal connectors will give way because of the fatigue of the process of expansion and contraction as ions move in and out of the plates. ‘It doesn’t matter what’s happening inside the battery if you can’t get in and out of the battery with charge’, comments to Michael Toney at the US National Renewable Energy Laboratory in Colorado.
Why Copper and Aluminum Fight Each Other
Copper’s melting point is about 1085°C; aluminum’s is about 660°C. You could, in general, hold a copper part against an aluminum part in a furnace for a short time without the copper part liquefying. Besides poorly matched thermal conductivity and coefficient-of-thermal-expansion properties, when you apply enough heat to aluminum to melt it fully, the copper is just beginning to lose its strength. You end up with a pool of molten aluminum and still-solid copper, and a horrible, inefficient, lumpy, bumpy weld that’s likely riddled with voids and cracks and a microscopically narrow interface region pretending to be a solid.
Intermetallics are your enemy here on multiple levels. Some brittle intermetallic phases have long names like CuAl₂ and are common in aluminum/copper joints. They’re not great conductors, and they’re not great structural materials. They don’t like to bend; you’ll fracture them. In an automotive battery, given contact with road vibration, thermal cycling, or internal impact, a joint with too much of these guys is well on its way to failure by the time the battery is installed in the car.
For copper and aluminum, the formation of brittle intermetallics accelerates rapidly at temperatures above 120°C. I’ve seen compelling data that a high-resistance, high-intermetallic-content joint can have up to 2.5 times the contact resistance of a pristine joint. In a high-current application like a battery, that’s not a small number; it’s a catastrophe waiting to happen in the form of localized heating and potential thermal runaway.
Solid-State Joining As The Engineering Answer
The best way to prevent intermetallics is to not let the metals melt in the first place. And that’s what ultrasonic metal welding accomplishes. It uses high frequency acoustic vibration at typically 20 to 40 kHz along with precise, controlled pressure to mechanically move the metals across each other. The friction on the surface breaks up any oxides or other contamination, and creates as close to a pure metal-to-metal joint as possible through a principle called “plasticized” bonding instead of by fusing them together as you would in a more traditional welding process. None of the parent metal ever turns into a liquid, and the entire process takes place well below the liquidus temperature of the metals being joined. That means the heat-affected zone is minimized, and intermetallic formation is virtually eliminated.
But make no mistake – this is sophisticated joining technology. The amplitude, or distance the metal is pushed back and forth across itself, is precisely controlled, as is the pressure, the time the pressure is applied, and the total energy delivered. If any of these variables are out of spec, you either don’t generate enough energy to create a bond, or you generate far too much energy and start driving intermetallic formation despite your best efforts. That’s also why statistical process control isn’t nice to have or a luxury – it is the foundational concept for building these processes.
Where The Foil-To-Tab Transition Happens
The copper and aluminum foils present within the battery cell as the anode and cathode current collectors are generally less than 10 microns in thickness. As delicate structures used to carry current throughout the cell in each charge and discharge cycle, these foils are consolidated in the joining process and connected to something sturdy and capable of carrying current out of the cell and into the pack architecture.
This is the transition point of the lithium ion battery tabs – the thicker, stiffer conductive element that bridges the cell’s internal foil stacks and the busbars connecting cells across the module. The significant thickness mismatch between a 10-micron foil and a terminal tab that might be 10 to 20 times thicker creates its own welding challenge. Too much energy and you tear or burn through the foil. Too little and you have inadequate bonding. The engineering solution typically involves multi-layer foil stacking and carefully sequenced energy delivery. Sometimes, this is combined with a foil-side fixture design that controls mechanical constraint during the weld.
Laser Welding and The Wavelength Problem
Laser welding can be used in the production of batteries. However, when it comes to copper, traditional infrared lasers are not suitable. Given that copper reflects about 95% of infrared energy at room temperature, the energy required to reach the melting point is extremely high. After finally reaching this point, the energy will not be absorbed in a stable manner, leading to spattering. The process window is narrow and it is difficult to control the heat input.
On the other hand, blue and green wavelength lasers (operating at approximately 450 nm and 515 nm, respectively) are absorbed with much greater efficiency by copper compared to infrared radiation. As a result, energy can be delivered in a controlled manner at lower peak powers, which reduces spattering and porous areas. Although these systems are more expensive, the advantages are worthwhile for high-speed production lines in which the quality of the bond directly impacts the safety and life cycle of the battery pack.
Quality Control That Keeps Pace With Production
The reality is that there aren’t perfect processes. The best production lines in the world produce some reject material. The trick is to avoid shipping latent defects.
As cell and pack producers ramp up to levels of high automation, they’ll re-learn the lessons the auto industry first had to deal with in the 1950s. Resistance welding isn’t magic – it’s metallurgy. Every weld has a minimum and a maximum energy window that will create an electrically and mechanically sound joint. That window varies with material thickness, but also with the age of the electrodes, the cleanliness of the base material, the concentration of certain elements in the weld seam, and probably a dozen other things that all interact with each other and the process control parameters in ways that aren’t entirely understood.
Electric vehicle batteries aren’t exactly ‘safety critical’ parts, but they’re close enough to bring the same lessons the auto industry learned about getting rid of cottage industries over to cell manufacturing.
Most weld lines right now have process engineers who work for months to get things dialed in, the line starts to ramp up, and the reject rate slowly starts rising. The plant manager realizes that the rate isn’t coming down on its own, panics, and product engineers get yanked into doing a something-that-works job. Cell lines are headed this way too; the battery engineers with a deep understanding of process metallurgy will see far fewer rejects and have far more reliable processes than the battery engineers who only understand the physical layout of their welding machines.
