Fabrication and Characterization of Nanocrystalline and Nanotwinned Cu Foils By Electrodeposition
Authors: Lai Chen-Ching Ching , Chen Chih 
Published: August 2024

Because of the rise of electric vehicles, laptops, and smartphones in recent years, lithium-ion batteries play an important role in powering these electronic devices. As users become more attentive to battery lifespan and performance optimization for prolonged product utility, the selection of battery materials gains paramount significance. Copper, an integral conductor material within electronic devices, specifically functions as the anode current collector in lithium-ion batteries (LIBs). The anode electrode comprises a copper foil substrate coated with active materials, typically encompassing carbon-based materials and silicon-based materials. Although carbon-based materials, such as graphite, currently dominate the market, their relatively lower capacity prompts consideration of silicon-based materials as possessing substantial developmental potential.

Nevertheless, silicon-based materials undergo volumetric expansion during charge and discharge processes, inducing pronounced swelling that may exert compressive forces on the copper foil, thereby elevating the risk of battery rupture. Consequently, the copper foil necessitates a requisite degree of strength to withstand these volumetric variations. Traditionally, copper foil fabrication relies on rolling techniques; however, it is hard to produce rolled Cu with thickness less than 10 mm. Therefore, electroplated copper foils have gradually emerged as the prevailing method in recent years. The purpose of strengthening is achieved by grain refinement, adding other metal impurities, and other strengthening methods. which causes the electrical conductivity to reduce. Nevertheless, nano-twinned copper can simultaneously increase the strength and avoid the problem of electrical conductivity decline. Nano-twinned copper is known to have high mechanical strength, good thermal stability, and resistance to electromigration.

In this study, we employed the rotary electroplating technique to fabricate high-strength nanotwinned copper (NT-Cu) foils. By adjusting electroplating conditions through the incorporation of additives, we investigated the mechanical properties of copper foils. Copper foils electroplated at room temperature exhibited an ultimate tensile strength (UTS) of approximately 830 MPa and a yield strength (YS) of approximately 540 MPa. Compared with the regular copper foil plated under same condition, the UTS is increased by 70%, and the YS is increased by 50%. Moreover, the electroplated NT-Cu foil demonstrated good thermal stability, with the ultimate tensile strength decrease of only 1.54% following annealing for 1 hour at 100°C in a vacuum environment. After an annealing process at 200°C for 1 hour in a vacuum environment, the ultimate tensile strength decreased by approximately 8.74%.

Additionally, electroplating experiments were conducted under both low and high-temperature conditions. The results revealed that copper foils electroplated at a low temperature of 10°C and low current density attained an ultimate tensile strength of 870 MPa, coupled with a yield strength of 560 MPa. In a high-temperature environment of 35°C, under high current density electroplating, the copper foils achieved an ultimate tensile strength of 845 MPa, reaching a yield strength of 540 MPa. These copper foils exhibited a fine-grained and nano-twinned microstructure, characterized by grains with dimensions less than 100 nm.

Figure 1