Electrodeposition of 15 μm Nanotwinned Cu foils with Low Warpage and Excellent Mechanical Properties

Cu foils often significantly warp and deform after removal from titanium substrates, which become a key issue in electrolytic Cu foil preparation. Despite this, the phenomenon has received little extensive investigation. In this study, hydroxyethyl cellulose (HEC) and rare earths (REs) were employed as additives to evaluate their impacts on the warpage and mechanical properties of 15 μm Cu foils. Adding of HEC significantly reduces warpage, creating numerous micropores within the foils that increase the Cu foil interface and reduce residual stress from -110.1 to -40.0 MPa. However, the mechanical properties of the Cu foil are also diminished. Upon further addition of REs on the basis of 10 mg/L HEC, the Cu foil exhibited excellent properties with high strength (651.3 MPa), high elongation (4.1%), and low warpage rate (4.5 mm). The findings reveal that REs alter the residual stress distribution at grain boundaries, reducing warpage. Moreover, the residual stresses at grain boundaries enhance grain and twin boundaries via lattice distortion and dislocation mechanisms, significantly increasing the Cu foil tensile strength and elongation. The high strength and low warpage properties of this Cu foil hold great potential for applications in printed circuit board (PCB) and lithium-ion batteries.