Optimization of the Tetragonal-to-Cubic Phase Transition of Lithium Lanthanum Zirconate Solid Electrolyte with the Use of Mechanical Activation

We have studied the effect of mechanical activation and subsequent sintering on the formation of solid electrolyte powder with a cubic structure and the composition Li_6.4Al_0.2La₃Zr₂O₁₂ (LLZ) during solid-state synthesis with the use of the low-melting-point crystalline hydrates (ZrO(NO₃)₂∙2H₂O, Al(NO₃)₃∙9H₂O, and La(NO₃)₃∙6H₂O). Mechanical activation of the sinter cake after the first synthesis step at 900°C, in an AGO-2S planetary mill in 4 × 1 min mode at a relative centrifugal force of 20g led to the formation of powder with a particle size d = 0.42 μm, as evaluated from its specific surface area. Subsequent sintering of the mechanically activated LLZ powder at 1000°C for 4 h ensured a complete transformation of tetragonal LLZ into a highly conductive cubic phase, c-LLZ. The room-temperature ionic conductivity of c-LLZ was 2 × 10^–4 S/cm, as determined by electrochemical impedance spectroscopy.