Diffusion of lithium in copper current collectors of lithium metal anodes in liquid cells

Conflicting views are found in the literature on the significance of lithium diffusion into copper current collectors of lithium metal anodes. Previously published in situ Neutron Depth Profiling (NDP) measurements in a liquid cell revealed reversible Li diffusion in the current collector during electrochemical cycling, resulting in concentrations of similar to 1 at. %. Herein, a model for these experiments is presented, in which stress in the deposit is induced by Li diffusion accompanying plating and stripping. This in turn produces stress gradients in the current collector, which together with composition gradients drive reversible grain boundary diffusion in Cu during cycling. Quantitative agreement with the NDP results is obtained for a fit diffusivity comparable to previous in situ measurements, but much larger than values from ex situ measurements using Li-Cu bilayers. Creep due to grain boundary diffusion is found to relax stress in the copper foil, resulting in low levels of stress and deformation during cycling. The results clarify the significance of Li diffusion in Cu, and also help elucidate the origin of stress in the lithium anode during cycling, which may induce interfacial instability.