Effect of excess Li on the structural and electrical properties of garnet-type Li₆La₃Ta_1.5Y_0.5O₁₂

Volatility of lithium during preparation of lithium-stuffed garnet-type metal oxide solid Li ion electrolytes is a common problem, which affects phase formation, ionic conductivity, mechanical strength and density. Synthesis of Li-stuffed garnets has been performed generally using the conventional solid-state reactions at elevated temperature in air. The present study describes the effect of excess LiNO₃ (2.5 to 15 wt.%) addition during the ceramic synthesis on the structural and electrical properties of garnet-type Li₆La₃Ta_1.5Y_0.5O₁₂. Powder X-ray diffraction (PXRD) confirmed that cubic phase was formed in all tested cases, and there is no significant variation in lattice parameter with amount of excess LiNO₃ used. However, increasing amounts of excess lithium decreased inter-particle contact and increased grain growth during sintering, producing sharply varied microstructures. PXRD showed no secondary phase and scanning electron microscopy (SEM) analysis showed rather uniform morphology and absence of "glassy" materials at the grain-boundaries. The bulk Li ion conductivity was found to increase with amount of excess lithium, reaching a maximum room temperature conductivity of 1.62 × 10^-4 Scm^-1 for the sample prepared using 10 wt.% excess LiNO₃. Raman microscopy study indicated the presence of Li₂CO₃ in all aged Li₆La₃Ta_1.5Y_0.5O₁₂ samples prepared using excess LiNO₃.