Degradation of NASICON-Type Materials in Contact with Lithium Metal: Formation of Mixed Conducting Interphases (MCI) on Solid Electrolytes

We report on the transport properties of lithium ion conducting glass ceramics represented by the general composition Li1+x–y Al x 3+M y 5+M2–x–y 4+(PO4)3 with NASICON-type structure and their stability in contact with lithium metal. In particular, solid electrolyte phases with M = Ge, M = Ti, Ge, and M = Ti, Ta were investigated. AC impedance spectroscopy and DC polarization measurements were applied to determine the conductivity as a function of temperature, and to extract the partial electronic conductivity. The maximum total conductivity at room temperature was found to be about 4 × 10–4 S/cm for the solely Ge containing sample. We demonstrate that the combination of vacuum-based lithium thin film deposition and X-ray photoelectron spectroscopy (XPS) is well suited to study the reactivity of the solid electrolyte membranes in contact with lithium. As a major result, we show that none of the materials investigated is stable in contact with lithium metal, and we discuss the reactive interaction between solid electrolytes and Li metal in terms of the formation of a mixed (ionic/electronic) conducting interphase (MCI) following the well-known SEI concept in liquid electrolytes.