Depending on the operating current density, lithium electrodeposition exhibits three distinct growth modes: root-growing whiskers, surface-growing clusters, and tip-growing dendrites. While dendrites are the most penetrative deposit, they do not occur in practical batteries during normal operation, and the root-growing whiskers and surface-growing clusters fortunately can be blocked by ceramic separators. The detailed mechanisms revealed by this study provide guidelines to increase the safety of rechargeable lithium metal batteries.

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Solid-state electrolytes combined with lithium-metal anodes have the potential to improve the energy density of lithium-ion batteries. However, soft Li metal can still penetrate these stiff electrolytes above a critical current density (CCD). Prevailing methods to determine CCD suffer inconsistencies due to void formations after repeated stripping and plating, leaving significant variations in reported data. Here, we combine one-way linear sweep voltammetry (LSV) with electrochemical impedance spectroscopy (EIS) to uncover the existence of significant polarization in ceramic electrolytes, which can fully relax even without stacking pressure. At high scan rates, LSV experiments showed metal penetration with a diverging transient current, similar to CCD values. However, at a lowered scan rate, the transient current reaches a maximum, suggesting a dynamic electrochemical limiting mechanism. The results and analysis of many consistent samples suggest that polarization of mobile charge carriers preceding the maximum current is critical for accurately understanding dendrite penetration in ceramic electrolytes.

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