Cryo-TEM Captured Insight on the Origin of Li Morphology Disparity in the Same Coin Cell

The morphology of electrochemically deposited Li plays critical role on the cycling stability and safety performance of Li metal batteries. However, what controls the morphology of Li remain largely obscure. Here we use cryogenic transmission electron microscopy and phase field modeling to reveal the correlation between solid electrolyte interphase (SEI) and Li morphology in the system of carbonate-based low concentration electrolyte (LCE) and ether-based localized high concentration electrolyte (LHCE). We discover that for the LCE, the Li morphology is dominated by Li whiskers (Li-Ws) with small fraction of Li particles (Li-Ps). Conversely, for the LHCE, the Li morphology is dominated by Li-Ps with small fraction of Li-Ws. In the same cell, the structural and chemical features of SEI on Li-P and Li-W are different, which consequently leads to differences in ionic conductivity and mechanical properties of SEI. We reveal that SEI layer with high ionic conductivity and strong mechanical confinement favors Li-P formation, while SEI layer with poor ionic conductivity and weak confinement favors Li-W formation. We further delineate that alienation of Li morphology, such as whisker formation in particle dominated system or vice versa, is associated with inhomogeneity of initial SEI layer formed on the Cu. Utilizing dimensionless numbers that incorporates the interplay between reaction-diffusion and stress accumulation-relaxation, a diagram is developed to depict the impact of the chemo-mechanical properties of the SEI on Li morphology. The work provides insights for possible tuning of crucial structural and chemical features of SEI to regulate the morphology of Li deposit.