Weak segregation and accelerated diffusion of Li at twin boundaries in Cu from DFT: Implications for current collectors in Li-ion batteries

Abstract

Adoption of Li-ion batteries for stationary energy storage requires to prolong their operational lifetimes to several decades calling to inhibit all degradation mechanisms. One of such mechanisms is associated with gradual formation of Cu–Li alloys at copper current collector - a component traditionally assumed inert in a Li-ion cell. Recent studies shows that Li can penetrate inside a Cu upon electrochemical cycling. We demonstrate that this process may be strongly intensified due to the presence of grain boundaries (GB) in copper. Using density functional theory calculations, we discover that Li segregate at copper GBs, which facilitates its GB diffusion according to vacancy-assisted mechanism. The vacancy formation and migration energies at GBs are significantly decreased in comparison to the values in bulk copper causing strong accelerating of Li diffusion. The segregation is caused by atomic-size effect, while the decrease of migration barriers is explained by a more stable coordination of ionized Li at a saddle point. To prevent Li penetration and subsequent formation of Li–Cu alloys, the amount of grain boundaries in thin film copper current collectors should be minimized.