Interface Instability of Fe-Stabilized Li₇La₃Zr₂O₁₂ versus Li Metal

The interface stability versus Li represents a major challenge in the development of next-generation all-solid-state batteries (ASSB), which take advantage of the inherently safe ceramic electrolytes. Cubic Li₇La₃Zr₂O₁₂ garnets represent the most promising electrolytes for this technology. The high interfacial impedance versus Li is, however, still a bottleneck toward future devices. Herein, we studied the electrochemical performance of Fe³⁺-stabilized Li₇La₃Zr₂O₁₂ (LLZO:Fe) versus Li metal and found a very high total conductivity of 1.1 mS cm^-1 at room temperature but a very high area specific resistance of ∼1 kΩ cm². After removing the Li metal electrode we observe a black surface coloration at the interface, which clearly indicates interfacial degradation. Raman- and nanosecond laser-induced breakdown spectroscopy reveals, thereafter, the formation of a 130 μm thick tetragonal LLZO interlayer and a significant Li deficiency of about 1-2 formula units toward the interface. This shows that cubic LLZO:Fe is not stable versus Li metal by forming a thick tetragonal LLZO interlayer causing high interfacial impedance.