Influence of Microwave Sintering and Pellet Pressure on the Phase Formation, Microstructure and Lithium-Ion Conductivity of the Li_6.28Al_0.24La₃Zr₂O₁₂ Solid Electrolyte

The main difficulty in the synthesis of Li₇La₃Zn₂O₁₂ (LLZO) is obtaining a pure cubic phase (Ia-3d) due to the extremely long sintering time (> 36 h) of the conventional method. In this work, an efficient microwave sintering method is used to prepare the Li_6.28Al_0.24La₃Zr₂O₁₂ solid electrolyte with a sintering time fixed at 3 h. The experimental results show that the cubic phase of Li_6.28Al_0.24La₃Zr₂O₁₂ was obtained at 1000 °C to 1200 °C. The transformation of the tetragonal LLZO into a cubic phase above 1000 °C suggests that the additional thermal energy was generated by the coupling of the dielectric loss caused by the microwaves. In addition, the powder samples were pressed into a pellet at 0.68, 1.47 and 2.16 MPa at room temperature. The pellets sintered for 1200 °C and pressed at 2.16 MPa exhibited a good ionic conductivity of 4.58.10⁻⁴ S/cm and the highest density (4.39 g/cm³). We have also found that by increasing the pellet pressure and temperature inside the microwave, the ionic conductivity improves significantly.