Synthesis of lithium-beta-alumina by various ion-exchange and conversion processes

Various methods for the synthesis of lithium-beta-alumina solid ion conductors were studied. The starting material was a composite of α-Al2O3 and YSZ for which the fabrication process is well established. The α-Al2O3/YSZ composite was vapor-assist converted into a water-stable Na-beta″-alumina composite structure and then ion-exchanged in either liquid or vapor phases. An attempt was also made to convert the α-Al2O3/YSZ composite directly into a Li-beta structure. Na-β″-alumina was ion-exchanged successfully in both liquid and vapor phases to form Li-β″-alumina or LiyNa1 − y-β″-alumina. The liquid phase ion-exchange of Na+ by Li+ in chloride at 700 °C resulted in mechanically stable Li-beta″-alumina with 60% conversion of Na. The vapor-assist ion-exchange method was more effective, yielding a higher conversion (90%) of Na. Despite the perception that Li ions are mostly intercalated into the spinel blocks preventing the formation of stable Li-beta phases, the stable beta-alumina structure was confirmed by XRD analysis after the high-temperature (1450 °C) vapor-assist ion-exchange of Na+ by Li+ in the LiO powder-packed bed. A conductivity of 7 × 10− 5 S/cm at 20 °C was obtained by extrapolation of the vapor-assist ion-exchanged sample data. The vapor phase equilibrium of the LiO phases is quite different from the NaO phases, and there seems to be an opportunity of enhanced conductivity by optimizing the vapor-assist process. While the vapor-assist ion-exchange of Na-beta-alumina shows a promising route for the synthesis of stable Li-beta-alumina ion conductors, the direct vapor-assist conversion from alpha to beta led to the formation of the LiAl5O8 spinel phase.