In lithium-ion battery manufacturing, the reliable connection between the internal metal foil and the external conductive tabs is crucial in determining the battery's conductivity, cycle life, and safety. The design using aluminum foil for the positive electrode and copper foil for the negative electrode gives rise to three core interconnection scenarios: aluminum-aluminum, copper-copper, and copper-aluminum. Due to differences in material properties, these three types of connections have different focuses in process selection, overcoming challenges, and quality control, collectively forming the "nerve network" of current transmission within the battery cell.
I. Aluminum-Aluminum Interconnection: A Stable Choice for the Positive Electrode
Aluminum-aluminum interconnection is the standard connection scheme for the positive electrode of lithium-ion batteries, combining the positive electrode aluminum foil current collector with aluminum conductive tabs. Aluminum has low density, excellent conductivity, and strong chemical stability in the battery environment. The naturally formed oxide film on its surface resists electrolyte corrosion, making it suitable for high-potential conditions at the positive electrode.
Ultrasonic welding and laser welding are the mainstream processes for this scenario. Ultrasonic welding utilizes high-frequency mechanical vibration to break the oxide film on the aluminum surface, achieving atomic bonding through solid-state plastic deformation. This results in a minimal heat-affected zone, avoiding damage to thin aluminum foil (typically 0.1-0.3 mm thick), making it suitable for multi-layer aluminum foil stacking. The contact resistance at the weld point can be controlled below 0.1 mΩ. Laser welding, with its concentrated energy and controllable precision, achieves a metallurgical bond between the aluminum foil and the electrode tabs, producing dense, defect-free welds suitable for high-speed automated production lines. The core challenge of aluminum-aluminum connections lies in breaking the dense oxide film. The process requires optimization of vibration parameters or laser energy density to ensure a strong connection without issues like incomplete or over-welded connections.
II. Copper-Copper Interconnects: A High-Efficiency Choice for the Negative Electrode
The negative electrode uses a copper foil current collector. Corresponding to copper-copper interconnect solutions, the copper foil is connected to copper tabs. Copper's conductivity is far higher than aluminum, making it an ideal choice for low internal resistance and high current transmission in the negative electrode, especially suitable for the high-power requirements of power batteries and energy storage batteries.
The mainstream processes for copper-copper interconnects are laser welding and resistance spot welding. Laser welding can precisely target copper interfaces, overcoming the challenge of copper's high reflectivity. By optimizing beam oscillation and pulse parameters, it achieves stable fusion between thin copper foil (typically < 20μm thick) and electrode tabs, controlling the heat-affected zone within 0.5mm to avoid damage to the internal separator of the cell. Resistance spot welding, on the other hand, forms a weld nugget through high-current, short-time discharge, suitable for multi-layer copper foil stacking. It requires precise control of electrode pressure and discharge time to prevent copper foil burn-through or a loose weld nugget. The core of copper-copper connections is controlling heat input to prevent copper overheating and oxidation, while ensuring weld strength and conductivity to meet the fatigue resistance requirements of long-term charge-discharge cycles.
III. Copper-Aluminum Interconnection: A Breakthrough Choice for Dissimilar Metals
Copper-aluminum interconnection is a core challenge in cell connection, commonly seen in scenarios involving negative electrode copper foil to aluminum electrode tabs, or positive electrode aluminum foil to copper busbars. Copper and aluminum exhibit significant differences in physicochemical properties: melting point, thermal conductivity, and coefficient of linear expansion differ. At high temperatures, copper readily forms brittle intermetallic compounds such as Cu₂Al and Cu₄Al₃, leading to joint embrittlement, increased resistance, and susceptibility to electrochemical corrosion.
Overcoming this challenge requires a combination of transition layer technology and specialized welding. Ultrasonic welding is the preferred process due to its solid-state bonding characteristics, which suppress brittle phase formation. High-frequency vibration facilitates atomic diffusion at the copper-aluminum interface, resulting in a heat-affected zone of only tens of micrometers, making it suitable for thin-film connections. Laser welding, through optimized parameters (high speed, low power) and the addition of a nickel/tin transition layer, mitigates direct copper-aluminum reactions, reduces brittle phase formation, and improves joint toughness. Furthermore, copper-aluminum composite tabs and nickel-plated transition layer designs are widely used. These metallurgically bonded transition layers reduce interfacial resistance, prevent electrochemical corrosion, and meet the dissimilar metal connection requirements within battery packs.
IV. Quality Control and Industry Value of Type III Interconnects The core quality requirements for all three types of interconnects are low contact resistance, high bonding strength, and a small heat-affected zone. During production, methods such as pull-out testing, contact resistance detection, and metallographic analysis are required to ensure that solder joints are free of cracks and incomplete welds, and that contact resistance is stable at the milliohm level.
The iteration of aluminum-aluminum, copper-copper, and copper-aluminum interconnect technologies directly drives the optimization of lithium battery energy density, safety, and cost. Connecting materials of the same type ensures basic stability, while connecting dissimilar materials breaks through the boundaries of material compatibility, making current transmission within the cell more efficient and reliable. As the new energy industry continues to demand higher battery performance, these three interconnect technologies will continue to develop towards higher precision, automation, and intelligence, becoming the core support for the upgrading of lithium battery manufacturing.
