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

Abstract Thermal management in Li‐ion batteries is critical for their safety, reliability, and performance. Understanding the thermal conductivity of the battery materials is crucial for controlling the temperature and temperature distribution in batteries. This work provides systemic quantitative measurements of the thermal conductivity of three important classes of solid electrolytes (SEs) over the temperature range 150 < T < 350 K. Studies include the oxides Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 , sulfides Li 2 S–P 2 S 5 , Li 6 PS 5 Cl, and Na 3 PS 4 , and halides Li 3 InCl 6 and Li 3 YCl 6 . Thermal conductivities of sulfide and halide SEs are in the range 0.45–0.70 W m −1 K −1 ; thermal conductivities of Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 and Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 are 1.4 and 2.2 W m −1 K −1 , 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.