Diffusion and Deformation Mechanism Maps in Li Metal from Atomistic Simulations

The functioning of Li metal-solid state batteries (LMSSB) requires that interfacial contact between the Li metal anode and the solid electrolyte (SE) be maintained during cycling. A reduction in the Li/SE contact area during stripping will increase the local current density during subsequent plating, fostering dendrite nucleation. This contact area is influenced by the rates of Li transport within the anode towards the interface. Relevant transport mechanisms include diffusion and creep, with faster rates for these processes resulting in improved performance. Given the importance of these transport modes, predicting them as a function of the anode’s microstructure, stress state, and temperature will be helpful in the design of LMSSB. Towards this goal, the present study predicts the rates of diffusion and creep in Li using atomic scale simulations. First, molecular dynamics simulations are used to estimate the rate of Li diffusion along dislocation cores and in grain boundary triple junctions. By combining this data with that from a prior study of grain boundary diffusion, the dominant diffusion mechanisms and overall rates of self-diffusion in Li polycrystals are predicted as a function of grain size, grain shape, dislocation density, and temperature. Secondly, the dominant creep deformation mechanisms are predicted as a function of applied stress, grain size, and temperature using the computed atomistic data as input to constitutive equations from the literature. Grain boundary sliding and coble creep are predicted to be the dominant mechanisms for micron-sized grains. Finally, a 1D model for Li interfacial contact loss is parameterized with atomistic diffusion data for dislocations, triple junctions, grain boundaries, and the bulk. The model predicts that high dislocation densities of ~1012/cm2 enable achieving battery performance targets for Li grain sizes on the order of 10 μm.