Transient Behavior of the Metal Interface in Lithium Metal–Garnet Batteries

The interface between solid electrolytes and Li metal is a primary issue for solid‐state batteries. Introducing a metal interlayer to conformally coat solid electrolytes can improve the interface wettability of Li metal and reduce the interfacial resistance, but the mechanism of the metal interlayer is unknown. In this work, we used magnesium (Mg) as a model to investigate the effect of a metal coating on the interfacial resistance of a solid electrolyte and Li metal anode. The Li–Mg alloy has low overpotential, leading to a lower interfacial resistance. Our motivation is to understand how the metal interlayer behaves at the interface to promote increased Li‐metal wettability of the solid electrolyte surface and reduce interfacial resistance. Surprisingly, we found that the metal coating dissolved in the molten piece of Li and diffused into the bulk Li metal, leading to a small and stable interfacial resistance between the garnet solid electrolyte and the Li metal. We also found that the interfacial resistance did not change with increase in the thickness of the metal coating (5, 10, and 100 nm), due to the transient behavior of the metal interface layer.