High capacity and low potential anode materials like lithium metal are preferred for high energy densities in all solid-state batteries. Due to the highly reducing potential of lithium metal, the electrochemical and thermodynamic stability of a solid ion conductor at the interface plays a crucial role for the performance of all solid-state batteries.

When a solid electrolyte is in contact with lithium metal, three different types of interfaces may occur:¹ (I) A stable interface may form with electrolytes that are thermodynamically stable against Li contact. (II) A reaction occurs and a mixed ionic-electronic conducting (MCI) new interphase forms,² which rapidly growths due to the electronic percolation pathways. (III) A reaction occurs and a solid electrolyte interphase (SEI) forms,³ which only conducts ions. While (I) may be preferred and (II) is exclusively detrimental, due to a proceeding reaction front, the impact of (III) on the battery performance depends on the type of forming SEI.

Here we will shortly introduce the types of possible interphases/interfaces and discuss the expected effects on the overall resistance and performance of an all-solid-state battery. Using time-resolved impedance spectroscopy and time-resolved cyclic voltammetry we introduce a guideline to characterize solid electrolytes for their stability against metal interfaces. The solid electrolyte Li₁₀GeP₂S₁₂ will be shown as an example of an instability,⁴ as has been theoretically predicted.^5,6 SEI formation occurs, affecting the overall cell resistance detrimentally.

Using a novel in situ X-ray photoelectron technique we study the resulting interphase formation when Li₁₀GeP₂S₁₂ is in contact with Li. Using the observed chemical species, in combination with time-resolved electrochemical measurements, we suggest a reaction mechanism for the interphase formation as well as provide an understanding of the growth kinetics.

¹Wenzel S., Leichtweiss T., Krüger D., Sann J., Janek J. Solid State Ion. 2015, 278, 98-105

²Hartmann P., Leichtweiss T., Busche M., Schneider M., Reich M., Sann J., Adelhelm P., Janek J. J. Phys. Chem. C 2013, 117, 21064-21074

³ Wenzel S., Weber D., Leichtweiss T., Busche M., Sann J., Janek J. Solid State Ion. 2016, 286, 24-33

⁴Wenzel S., Randau S., Leichtweiss T., Weber D., Sann J., Zeier W.G., Janek J. submitted

⁵Zhu Y., He X., Mo Y. ACS Appl. Mater. Int. 2015, 7, 23685-23693

⁶Zhu Y., He X., Mo Y. J. Mater. Chem A 2016 DOI: 10.1039/C5TA08574H.