Enhanced Performance of Li6.4La3Zr1.4Ta0.6O12 Solid Electrolyte by the Regulation of Grain and Grain Boundary Phases

The application of Li-ion conducting garnet electrolytes is challenged by their large interfacial resistance with the metallic lithium anode and the relative small critical current density at which the lithium dendrites short-circuit the battery. Both of these challenges are closely related to the morphology and the structure of the garnet membranes. Here, we prepared four polycrystalline garnet Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) pellets with different particle sizes (nano/micro) and grain boundary additive (with/without Al₂O₃) to investigate the influence of grain size, the composition of the grain boundary, and the mechanical strength of the pellet on the total Li-ion conduction of the pellet, Li/garnet interfacial transfer, and lithium dendrite growth in all-solid-state Li-metal cells. The results showed that the garnet pellets prepared with nanoparticles and LiAlO₂-related grain boundary phase had decreased total Li-ion conductivity because of the increased resistance of the grain boundary; however, these pellets showed higher mechanical strength and improved capability to suppress lithium dendrite growth at high current densities. By controlling the grain size and optimizing the grain boundary with Al₂O₃ sintering additive, the hot-pressing sintered LLZTO solid electrolytes can reach up to 1.01 × 10^-3 S cm^-1 in Li⁺ conductivity and 0.29 eV in activation energy. LLZTO with nanosized grain and LiAlO₂-modified grain boundary showed the highest critical current density, which is 0.6 mA cm^-2 at room temperature and 1.7 mA cm^-2 at 60 °C. This study offers a useful guideline for preparing a high-performance LLZTO solid electrolyte.