In previous articles we’ve highlighted the benefits of aluminium cell housings for 4680 battery cells like improved thermal management and gravimetric energy density and explored the structural performance of 4680 cans made from Speira ION Cell 3-CB and Speira ION Cell 3-CS1,2,3. It has been shown that high strength aluminium grades like Speira ION Cell 3-CS boost the structural performance. On the other hand, higher strength materials enable down-gauging of the can wall while maintaining the same maximum pressure and hence, provide significant lightweighting potential. The impact of strength can be estimated using Barlow’s and Lame’s equation considering Tresca yield criterion:
and is illustrated in Figure 1:
Evidently, strength has a major impact on the feasible wall thickness, where down gauging generates several advantages on cell and pack level: Gravimetric and volumetric energy density increase on cell level as a larger share of the cell’s volume is available for active material within the given dimensional constraints. Consequently, less aluminium is required per kWh of battery capacity saving cost related to aluminium consumption. On top of this the battery cell’s carbon footprint improves as CO2 emissions according to the weight reduction of the cell housing even without improving the carbon footprint of the utilized aluminium.
Weight saving per kWh on cell level reduces the total weight of the battery pack, while lighter packs can reduce the energy consumption of the car. The resulting improved efficiency could either increase range or allow for battery capacity reductions.
Tesla’s 4680 cell cans feature a wall thickness ranging from 0.5mm (Gen1) to 0.3mm (Gen2)4. Considering different strength levels of Ni-plated steel the resulting burst pressure can be estimated and ranges from 45 to 110 bar. Here it should be noted that a structural pack approach might introduce additional mechanical requirements going beyond burst pressure, such as buckling or dent resistance. Such requirements tend to generate geometrical constraints providing a lower limit of wall thickness regardless of the can wall’s strength.
Within Speira’s cell can design we targeted a burst pressure of 100 bar with Speira ION Cell 3-CS resulting in a wall thickness of 0.75mm. Depending on individual cell design and burst pressure defined by the cell manufacturer the can wall thickness could be reduced to roughly 0.35mm using Speira ION Cell 3-CS and to 0.6mm Speira ION Cell 3-CB assuming that a max. pressure of 45 bar would be sufficient. Taking 45 bar as a reference, the weight saving potential of Speira ION Cell 3-CS could be in the range of 40 to 60% compared to a Ni-plated steel can. About 40% weight saving could be feasible compared to moderate strength Speira ION Cell 3-CB.
The foregoing evaluation illustrates that higher strength aluminium grades are desirable in terms of cell performance. On the other hand, strengthening of aluminium requires alloy modifications going beyond the standard for prismatic cell housings AA3003, which excludes solute hardening by elements like Mg. Likewise alloy modifications such as Speira ION Cell 3-CS are required to reduce the share of primary aluminium and to enable high recycled contents as well as significant carbon footprint reductions5 desired by automotive OEMs at industrial scale.
In this regard, the common belief that a Mg content > 0.05wt% deteriorates the weldability due to splashing as well as hot cracking resulting in a significantly reduced yield during battery production hinders the application of higher strength grades or scrap-based materials. However, GB/T-33824-20176 considers alternative alloys such as 3005 – an alloy with Mg contents between 0.2 and 0.6wt% Mg7,8 – highlighting the feasibility of alloy modifications including substantial Mg contents.
Within this study we show that can-cap laser welding is feasible for high strength aluminium cans made from Speira ION Cell 3-CS.
Can-Cap Laser Welding
To evaluate the weldability of Speira ION Cell 3-CS laser welding trials were conducted at Coherent’s Application Center in Hamburg using the FL8000-ARM 50/200 laser system to identify suitable welding parameters within an industrial relevant setup. For all trials Speira ION Cell 3-CS cell cans have been used, while Speira ION Cell 3-CB and 3-CS have been selected as lid materials. Based on Coherent’s experience in cap-can laser welding typical welding parameters in terms of welding speed, power as well as power distribution between ring and center beam were identified. After optimization a weld speed of 10m/min has been achieved with high weld quality in terms of weld appearance, negligible splashing and absence of hot cracking regardless of the material combination.
Figure 2: Welded 4680 cans and a picture of the laser welding process
In addition to visual inspection of the weld seam during and after welding, the welded cans have been evaluated using metallographic investigation as well as helium leakage and burst tests.
Metallographic Investigations
To evaluate the weld porosity as well as the microstructure, metallographic sections have been prepared for both can-lid combinations. Figure 3 displays the metallographic section of the combination Speira ION Cell 3-CS can / Speira ION Cell 3-CB lid in unetched and Barker etched condition. The unetched condition enables the evaluation of weld porosity within the weld seam. Evidently, weld porosity is negligible. Here it should be noted that black spots within the can and the lid correspond to intermetallic phases present in the aluminum alloy. Barker etching reveals the grain structure within the can wall, the weld seam, and the lid. It is evident that the grain structure of the can wall and the lid is much finer due to the thermomechanical processing during rolling and can production. Elongated grains correspond to a strong cold working during cold rolling and wall ironing, which is required to maximize the strength. The weld seam features a much coarser grain structure, which is introduced by melting and solidification during the weld process. In addition, a heat affected zone can be observed adjacent to the weld seam, where the can’s and the lid’s microstructure feature a fine equiaxed grains due to recrystallization.
Figure 4 displays the metallographic sections of the combination Speira ION Cell 3-CS can / Speira ION Cell 3-CS lid. The overall appearance of the weld seam and adjacent areas within the can and the lid are comparable to the combination Speira ION Cell 3-CS can / Speira ION Cell 3-CB lid. The weld quality is good without any signs of hot cracking, splashing or relevant porosity. Again, the area neighboring the weld seam feature fine recrystallized grains displaying the effect of welding induced heat.
Gas Tightness and Burst Pressure
The gas tightness of the welded cell cans has been evaluated by Helium leakage tests of two cans of each combination employing an internal pressure of 2bar (He) and sniffer testing. Although, a current standard for He leakage testing of batteries has not been established, typical leakage rates range from 10-6 to 10-8 mbar.l/s.9 Regardless of the tested combination the maximum helium leakage rate was in the above-mentioned acceptable range.
Considering safety, the burst pressure of the welded cans is of high importance. In addition, failure of the sidewall also known as sidewall rupture must be avoided to prevent thermal propagation during a thermal runaway event of a single cell. Cylindrical cells may contain a current interrupting device or vent features, which should be activated in the pressure range between ~ 10 to 20bar (see Figure 5) 1, while the can has been designed for a burst pressure of about 100bar in case of Speira ION Cell 3-CS.
Burst tests have been performed at 20°C (3x samples) and 100°C (1x sample). As illustrated in Figure 6, the burst pressure ranges from ~38 to 40 bar at 20°C and slightly lower at 100°C regardless of the lid material. In all tests the welded lid was blown of the can without any noticeable damage of the can wall (compare Figure 7).
Metallographic investigations of the blown of lids have shown that the cracking occurs within weld seam and the adjacent heat affected zone, where the material softens during the laser welding process (see Figure 7).
In other words, a breaking point or venting feature has been introduced during the weld process featuring a burst pressure above the traditional safety devices and at ~40% of the cell can’s designed maximum pressure. This feature can be used as a complementary safety precaution to prevent side wall rupture efficiently, as the material selection, cell design and laser welding enforce the expulsion of the lid, jointly.
Implementing this feature in cell design can reduce the risk of thermal propagation significantly, as the lid will be removed in a thermal runaway event of the cell and blow off gas can be guided away from the neighboring cells, efficiently.
References
- Benefits of Aluminium cell housings for cylindrical lithium-ion batteries, Speira Learn
- Benefits of Aluminium Cell Housing for Cylindrical Li-ion Batteries, BatteryDesign.net
- Aluminium 4680 Cell Can Structural Performance, BatteryDesign.net
- Generation 2 4680 Tested // Results & Analysis, The Limiting Factor, YouTube
- Whitepaper Decarbonization of Batteries, Speira LEARN Whitepaper
- GB/T 33824-2017: Aluminum and aluminum alloys plates, sheets and strips for cans and caps of new energy power batteries
- GB/T 3190-2020: Chemical Composition of Wrought Aluminum and Aluminum Alloys
- DIN EN 573-3:2024: Aluminium and aluminium alloys – Chemical composition and form of wrought products
- Leak Detection of Lithium-Ion Batteries and Automotive Components, Agilent
About Speira
Speira is a leading European aluminium rolling and recycling company, comprising a total of eleven recycling and production sites in Germany and Norway plus own research and development. Speira recycles up to 650,000 tons of aluminium per year and produces around one million tons of advanced rolled products. The plants include the Alunorf joint venture, the world’s largest aluminium rolling mill, and Grevenbroich, the world’s largest refining plant. With more than 5,000 employees, Speira is proud to supply some of the best-known industrial companies in the global sectors automotive, packaging, printing, engineering, building and construction. Check out www.speira.com.
About Coherent
Coherent Corp. is a global leader in materials, networking, and lasers for the industrial, communications, electronics, and instrumentation markets. The company is headquartered in Saxonburg, Pennsylvania. It was founded in 1971 to manufacture high-quality materials and optics for industrial lasers. Today, the company operates in more than 20 countries around the world.
Coherent is focused on delivering innovations that fuel market megatrends while pursuing the mission of enabling the world to be safer, healthier, closer, and more efficient. Coherent empowers market innovators to define the future through breakthrough technologies, from materials to systems.