Macroscale Inhomogeneity of Electrochemical Lithium-Metal Plating Triggered by Gas Phase Evolution

Abstract

Promoting homogenous electrode reactions is essential to suppressing electrode degradation and thus realizing stable energy storage systems. In particular, lithium-metal anodes that undergo drastic morphological change during charging/discharging require strict homogeneity control, more so than conventional intercalation electrodes. Previous studies have mainly focused on the micrometer-scale homogeneity of lithium plating and its control by tailoring the electrolyte chemistry and following the solid-electrolyte interphase formation process. However, the larger-scale, electrode-level homogeneity issue recently emerging for conventional electrodes has received little attention. Here, for the first time, we observe clear electrode-level inhomogeneity during lithium plating that is independent from the micro-scale lithium dendritic growth. Ex-situ and in-situ experiments clearly reveal that this electrode-level inhomogeneity is triggered by gas-generating reactions and the resulting gas-phase (i.e., bubble) evolution that causes localized lithium growth. Experiments using Li|Cu half cells demonstrate that this macroscale inhomogeneity is accompanied by an overpotential increase mainly attributed to limited lithium-ion mass transport through bubbles, as revealed by continuum modeling and simulation. Full-cell experiments using lithium iron phosphate demonstrate that this bubble-inducing macroscale inhomogeneity significantly contributes to decay of the cell capacity and energy density. Our observation of this larger-scale lithium plating inhomogeneity suggests that homogeneity control and its implications for lithium-metal cell performance should be approached from a multi-scale perspective.