Study of thermal material properties for Ta- and Al-substituted Li7La3Zr2O12 (LLZO) solid-state electrolyte in dependency of temperature and grain size

Solid-state electrolytes such as tantalum (Ta)- and aluminum (Al)-substituted Li7La3Zr2O12 (LLZO) are seen as the key component for the next generation of mainstream battery technology. However, this development often lacks specific information on decisive material parameters. Therefore, this work experimentally investigates the thermal material parameters such as the thermal conductivity (kappa) and the specific heat capacity (C-p) of LLZO between room temperature and 1225 K using laser flash analysis (LFA) and differential scanning calorimetry (DSC). The DSC measurements reveal a C-p of 0.55-0.80 J g(-1) K-1. Furthermore, a decomposition of the cubic LLZO phase to pyrochlore La2Zr2O7 is detected between 1500 K and 1750 K, which is substantiated by Raman- and SEM-analysis. The impact of the grain size on kappa is also considered, as the Al-substitution leads to the formation of significantly larger grain sizes compared to a Ta-substitution. The LFA measurements yield a relatively constant kappa between 1.45-1.55 W m(-1) K-1 for both materials and grain sizes, which is the consequence of a phonon mean free path in the range of the interatomic distance of the LLZO crystal. This implies that phonon scattering at grain boundaries is negligible and the main scattering occurs at inhomogeneities in the crystal lattice.