Exploiting grain boundary diffusion to minimize dendrite formation in lithium metal-solid state batteries

Abstract

Maintaining interfacial contact between the Li metal anode and the solid electrolyte is a key challenge in developing Li metal-based solid-state batteries (LMSSB). At moderate discharge rates, relatively slower diffusion within the anode results in roughening and void formation in Li near this interface. The resulting reduction in interfacial contact focuses the Li-ion current during plating to a reduced number of contact points, generating high local current densities that nucleate dendrites. One approach to minimize void formation is to apply high stack pressure, which enhances plastic flow in the anode. Nevertheless, the use of pressure has drawbacks, as it facilitates fracture within the solid electrolyte. Here, an alternative strategy for minimizing void formation is described. Using a multi-scale model, it is shown that targets for capacity and current density in LMSSBs can be achieved by reducing the grain size of Li to exploit fast grain boundary (GB) diffusion. Diffusion rates along a diverse sampling of 55 tilt and twist GBs in Li were predicted using molecular dynamics, and found to be 3 to 6 orders of magnitude faster than in the bulk. Using these atomic-scale data as input, a meso-scale model of Li depletion in the anode during discharge was developed. The model predicts that smaller, columnar grains are desirable, with grain sizes of approximately 1 μm or less needed to meet performance targets. As micron-sized grains are two orders of magnitude smaller than those in common use, strategies for controlling grain size are discussed. In total, the model highlights the importance of the anode's microstructure on the performance of LMSSBs.