The origin of conductivity variations in Al-stabilized Li7La3Zr2O12 ceramics

All-solid state batteries have the ability to bring us closer to a zero-emission society. Such battery systems, however, require a powerful solid Li-ion conductor with high stability and exceptionally high Li-ion conductivity. Cubic Li7La3Zr2O12 (LLZO) garnets are attractive candidates combining high ionic conductivity and thermal as well as electrochemical stability. Since the cubic LLZO polymorph is not stable at room temperature, supervalent cations, such as Al3+, are needed to stabilize the highly conductive phase. However, strong variations in conductivity values were reported even for very similar Al contents, which is critical for future battery applications. Therefore, the overall performance of 44 samples with nominally identical composition (Li6.40Al0.20La3Zr2O12), sintered at two different temperatures (1150 °C and 1230 °C, respectively) was analysed by electrochemical impedance spectroscopy. The aim of this work is to elucidate the origin of the strong variations in conductivity reported in literature. This study shows that the effective conductivities of the LLZO samples vary between 2 × 10−5 and 8 × 10−4 S cm−1, which cannot be attributed to obvious differences in sample preparation. Furthermore, the stepwise reduction of the sample volume and repeated measuring of the impedance revealed conductivity variations even within a single sample. Finally, preliminary chemical analysis by laser ablation (LA) - inductively coupled plasma (ICP) - mass spectrometry (MS) identified Al inhomogeneities in LLZO, which may be responsible for conductivity variations of nominally equal samples.