Enhanced Performance of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ 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 Li6.4La3Zr1.4Ta0.6O12 (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.