Whether dealing with square or cylindrical batteries, welding stands as a cornerstone of the manufacturing process. In the intricate dance of lithium-ion battery production, welding techniques are primarily employed in the middle and later stages, where precision and reliability are paramount.
Let’s explore some key welding processes that contribute to the creation of safe and efficient batteries:
1. Safeguarding with Explosion-Proof Valve Welding (Safety Vent Welding)
The explosion-proof valve, often called a pressure relief valve, is a critical safety feature. It’s a thin-walled component located on the battery’s top cover. In the event of excessive internal pressure, this valve is designed to rupture, releasing gas and preventing a catastrophic battery burst.
This valve boasts a clever design, typically involving laser welding two precisely shaped aluminium sheets together. The laser welding process must be meticulously controlled to ensure a reliable seal and consistent rupture pressure. If the weld is too weak or too strong, it can significantly compromise battery safety. Therefore, this process demands extremely precise control over the heat applied.
2. Connecting the Power: Terminal Welding
The battery cover features positive and negative terminals, which serve as pathways for electrical current. Electrons flow from the active materials within the battery through foils, tabs, and terminal extensions, ultimately reaching the vehicle’s electrical components.
Terminal connections are categorized as internal and external. Internal connections involve welding the battery cell tab to the cover’s terminal. External connections involve welding the battery terminal to connecting pieces, forming series or parallel circuits within a battery module.
Typically, the positive terminal is made of aluminum, while the negative terminal is made of copper. A common construction involves riveting followed by full welding. To meet the required tensile strength and conductivity, fibre lasers or composite welding lasers, known for their excellent beam quality and uniform energy distribution, are preferred. These lasers help achieve stable welds in both aluminum-aluminum and copper-copper structures, minimizing spatter and maximizing production yield.
3. Bridging the Gap: Tab Extension Welding
Adapters and flexible connectors are essential components that link the battery cover and the battery cell. These components must withstand high currents, maintain structural integrity, and minimize spatter. Therefore, the welding process must create a sufficiently wide weld while preventing any particles from falling onto the battery cell, which could cause a short circuit. Copper, being a highly reflective material, requires a higher energy density for successful welding.
4. Sealing the Shell: Shell Sealing Welding (Can Sealing Welding)
Power battery shells are typically made of aluminium alloy, with some made of pure aluminium. Stainless steel, particularly 304 stainless steel, is also used due to its excellent laser weldability.
Continuous laser welding of thin-shell lithium-ion batteries significantly boosts efficiency while enhancing appearance and sealing. To achieve even faster welding speeds and a more uniform appearance, many manufacturers are adopting composite welding and annular spot welding, moving away from slower single-fiber welding. In mass production, welding speeds have reached impressive levels while ensuring weld stability.
5. Securing the Seal: Sealing Nail Welding
Sealing nails, or injection hole caps, come in various forms, often round caps. The welding process must ensure a robust seal capable of withstanding high pressure, free from pinholes, cracks, or explosion points.
As the final step in battery core welding, the yield of sealing pin welding is particularly important. Residual electrolytes can lead to defects like explosion points and pinholes. Minimizing heat input is crucial for suppressing these defects. Laser welding significantly enhances stability and consistency, leading to improved yields.
6. Building Modules and Packs: Module and PACK Welding (Busbar Welding)
A battery module is essentially a combination of lithium-ion cells connected in series and parallel, equipped with individual cell monitoring and management devices. The module’s structural design dictates its performance and safety. It must support, secure, and protect the cells while meeting overcurrent requirements, ensuring current uniformity, controlling cell temperature, and providing a means to cut off power in case of severe abnormalities.
Because laser welding copper and aluminium together can create brittle compounds, it is generally used for copper to copper and aluminium to aluminium, in addition to ultrasonic welding. Because copper and aluminium have high laser reflectivity and conduct heat quickly, the connecting piece tends to be thick, requiring higher power lasers.
Conclusion:
Welding is not merely a joining process in battery production; it’s a critical determinant of safety, performance, and longevity. Each welding stage, from the delicate safety vent to the robust module assembly, demands precision and control. The adoption of advanced laser technologies and meticulous process management are essential for achieving high-quality, reliable welds. As battery technology continues to evolve, so too will the welding techniques that underpin its advancement, ensuring safer and more efficient energy storage solutions.
