Twin boundary and grain boundary engineering to enhance mechanical strength of nanotwinned Cu

Thin Cu foils have been adopted for current collectors in lithium-ion batteries. Nickel is commonly employed to strengthen Cu because of its low lattice mismatch with Cu. This study delves into the effect of different concentrations of Ni ions (0.0017–0.5 M) on the strength of electroplated nanotwinned (NT) Cu. Results showed that the Ni ions refined the NT-Cu films, and the optimal Ni concentration of 0.05 M was found, which maximally strengthened the mechanical strength of NT-Cu by 33.3% (up to 743 MPa). We also achieved a high international annealed copper standard (IACS) of up to 79%. Microstructure analysis reveals that the twin spacing was decreased from 70.2 nm to 21.1 nm, and the grain size was reduced from 2.67 μm to 1.31 μm when adding 0.05 M Ni ions in the electrolyte. Through linear sweep voltammetry (LSV), we found that the presence of Ni ions causes a negative shift in the reduction overpotential of copper, leading to the grain refinement. Confined layer slip model is also proposed to correlate the theoretical values and experimental results.