The effect of preload modulation on laser weld quality of multilayer copper micro-foils and anode disk

In cylindrical lithium-ion batteries, high-quality welding of the stacked copper foil (full-tab structure) and the anode disc is critical. Poor welding can induce porosity and non-uniform formation, which may impair current capability or pose risks of overheating. Laser welding is the preferred choice, owing to its minimal heat input and high-speed capabilities. However, ultrathin interlayer gaps between stacked copper foils and anode disks significantly affect heat conduction during welding, yet it has received limited attention. In this study, the effect of precisely modulating the preload on the laser weld quality of multilayer copper foil stacks and anode disks and the underlying thermal transfer in the micro-gap was investigated. The weld quality was comprehensively evaluated by examining surface and cross-sectional morphology. It was found that within the preload range of 20 N to 60 N, the weld surface defects increased with higher preload, while too little preload led to incomplete penetration. Although the weld depth and width are enhanced with increasing preload, the weld cross-section exhibits lower volatility and fewer porosities at a preload of approximately 40 N. The cross-section fluctuation of the weld was reduced to 34.3 % at 40 N preload, with fewer defects of voids and pores. The weld had a lower resistance value (2.05 mΩ) and higher tensile strength (59.9 N). Theoretical analysis indicates that the ultrathin inter-foil gas gap operates in a slip-flow regime with substantial thermal resistance. The preload governed the thermal resistance of gaps, thereby directly altering thermal transfer efficiency during laser welding. Higher quality welds with greater depth and fewer defects were obtained with an optimum preload of about 40 N, whereas too small preload induced incomplete depth and excessive preloads destabilized the welding process. This research established preload optimization as an effective strategy for enhancing multi-layer copper foil welding reliability, providing novel insights for optimizing fixturing conditions in battery manufacturing processes.