Microstructure-controlled Electrodeposition of Mechanically Reliable Double-layered Thin Foils for Secondary Batteries

We investigated an electrodeposition technique for fabricating a high-strength thin metallic foil as a current collector material in secondary batteries. To increase the strength and minimize the increase in electrical resistivity, copper–nickel (Cu–Ni) double-layered foils were considered and the grain structures of each layer were manipulated by controlling the electrodeposition solution and process conditions. Initially, a Cu electrodeposition process was developed to form a bamboo-like grain structure after annealing, which was followed by two Ni processes to produce a foil with a columnar or nano-crystalline grain structure. Subsequently, several foils were annealed at 190 °C for 10 min considering that current collectors experience a thermal load during battery manufacturing. Scanning and transmission electron microscopy-based crystallographic orientation mapping techniques indicated a remarkable change in the grain structure of the Cu foil owing to the grain growth after annealing; conversely, the Ni foil with the nano-crystalline grain structure was insensitive to annealing. By applying these processes to each material, four 10-µm-thick double-layered foils were fabricated. Among these foils, the foil with a nano-crystalline Ni layer exhibited the smallest change in material properties resulting in the highest tensile strength and moderate elongation after annealing. The tensile strength of the best double-layered foil was approximately three-fold higher than that of the single-layered Cu foil after annealing.