Enhancing Li1.3Al0.3Ti1.7(PO4)3 Ionic Conductivity via Microstructural and Average Structural Tuning Induced by Sm2O3 Sintering Additives

Currently, further improvement in ionic conductivity remains essential for NASICON-type Li1+xAlxTi2–x(PO4)3 (LATP) solid-state electrolytes to enable their practical use in all solid-state batteries. In this study, Sm2O3 was used as a sintering additive for LATP, and its effects on the microstructure, conductivity, and crystal structure of the LATP were investigated. The substantial reduction in LATP grain size and improved grain connectivity increased the grain boundary conductivity by nearly an order of magnitude, yielding a total conductivity of 0.44 mS/cm at 20 °C for the LATP + 9 wt % Sm2O3 co-sintered sample. The diffraction patterns revealed that the Sm2O3 additive reacted with LATP to form SmPO4 and LiTiOPO4 as secondary phases. Combined Rietveld refinement using X-ray and neutron diffraction data revealed that this reaction could increase the Al3+/Ti4+ ratio in the average structure of LATP, which correlated with the reduced activation energy for bulk conduction. These findings demonstrate that Sm2O3 or other rare-earth element oxide additives provide a simple and effective solution for the microstructural tuning and conductivity enhancement of LATP.