Good Solid‐State Electrolytes Have Low, Glass‐Like Thermal Conductivity

, respectively. For most of the SEs studied in this work, the thermal conductivity increases with increasing temperature, that is, the thermal conductivity has a glass-like temperature dependence. The measured room-temperature thermal conductivities agree well with the calculated minimum thermal conductivities indicating that the phonon mean-free-paths in these SEs are close to an atomic spacing. The low, glass-like thermal conductivity of the SEs investigated is attributed to the combination of their complex crystal structures and the atomic-scale disorder induced by the materials processing methods that are typically needed to produce high ionic conductivities.