Stress-Driven Diffusion of Lithium into Anode Copper Current Collectors during Plating and Stripping
Authors: Hebert Kurt 
Published: December 2024

Stresses in lithium metal anodes determine their stability during inevitable volume changes experienced upon cycling. Lithium deformation during cycling in liquid cells can initiate surface morphological instabilities which lead to capacity loss, e. g. whiskers and moss (1-3). Evidence suggests that whiskers grow by solid-state diffusion to the whisker base (3,4), and that whiskers relieve compressive stress in the metal generated by electrodeposition (5,6). While quantitative characterization of these processes is challenging, diffusion accompanying morphology instabilities can be detected when lithium is deposited directly on copper current collectors (7-9). Lv et al. reported operando neutron depth profiling (NDP) measurements of Li diffusion into Cu during plating-stripping cycles (9). NDP demonstrated reversible intercalation of Li in the current collector, and diffusion toward higher concentration during stripping, thus contrary to Fick's Law.
A model is presented for lithium diffusion in a copper current collector during cycling in a liquid cell. Based on our recent analysis of stress measurements during cycling (10), diffusion of plated atoms into the deposit induces lateral expansion of the copper-deposit interface, which is accommodated by stress-driven diffusion of lithium into grain boundaries in the copper foil. During stripping atoms diffuse of the deposit, leading to contraction along the copper interface. Following established practices, stress due to grain boundary diffusion is modeled by tracking evolution of wedges of additional material adjacent to the boundaries (11-13).
The calculations demonstrated growth and contraction of wedges during plating and stripping respectively, accounting for the observed reversible lithium intercalation. When the effective diffusivity is larger than 10-13 m2/s, model calculations successfully captured the linear time-dependent concentration profiles measured by NDP. Creep accompanying grain-boundary diffusion creep resulted in low levels of stress and deformation in the copper foil. The fit diffusivity value was comparable to Li diffusivities previously reported in current collectors and alloy anodes during cycling, but much larger than diffusivities from ex situ experiments. The agreement demonstrated between the model and NDP measurements supports the hypothesis that plating and stripping reactions induce diffusion within the lithium deposit. Stress due to diffusion may play an important role in the morphological instability of the lithium-electrolyte interface during cycling.
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