The garnet-type solid-state electrolyte, based on the nominal formula Li₇La₃Zr₂O₁₂ (LLZO), is unique in that it is a fast Li-ion conductor, exhibits sufficient mechanical properties, and is also chemically and electrochemically stable against metallic Li. Despite these promising attributes, additional challenges must be overcome before solid-state batteries based on LLZO are viable. Demonstrating low Li–LLZO interfacial resistance (R_Li–LLZO) is a critical milestone along the path to commercialization. While several studies have characterized R_Li–LLZO, nearly all report values significantly higher than conventional LIBs employing liquid electrolytes. Thus, strategies to reduce R_Li–LLZO to values comparable to, or lower than, LIBs are needed [1].

In this study, the impact of surface chemistry on the interfacial resistance between the LLZO solid-state electrolyte and a metallic Li electrode is revealed [1]. Control of surface chemistry allows the interfacial resistance to be reduced to 2 Ω cm², lower than that of liquid electrolytes, without the need for interlayer coatings. A mechanistic understanding of the origins of ultra-low resistance is provided by quantitatively evaluating the linkages between interfacial chemistry, Li wettability, and electrochemical phenomena. A combination of Li contact angle measurements, X-ray photoelectron spectroscopy (XPS), first-principles calculations, and impedance spectroscopy demonstrates that the presence of common LLZO surface contaminants, Li₂CO₃ and LiOH, result in poor wettability by Li and high interfacial resistance. On the basis of this mechanism, a simple procedure for removing these surface layers is demonstrated, which results in a dramatic increase in Li wetting and the elimination of nearly all interfacial resistance. The low interfacial resistance is maintained over one-hundred cycles and suggests a straightforward pathway to achieving high energy and power density solid-state batteries.

[1] A. Sharafi, E. Kazyak, A. L. Davis, S. Yu, T Thompson, D. J. Siegel, N. P. Dasgupta, J. Sakamoto, Chem. Mater. 29, 7961 (2017)