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 alpha-Al2O3 and YSZ for which the fabrication process is well established. The alpha-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 alpha-Al2O3/YSZ composite directly into a Li-beta structure. Na-beta"-alumina was ion-exchanged successfully in both liquid and vapor phases to form Li-beta"-alumina or LiyNa1-y-beta"-alumina. The liquid phase ion-exchange of Na+ by Li+ in chloride at 700 degrees 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 degrees C) vapor-assist ion-exchange of Na+ by Li+ in the Li-O powder-packed bed. A conductivity of 7 x 10(-5) S/cm at 20 degrees C was obtained by extrapolation of the vapor-assist ion-exchanged sample data. The vapor phase equilibrium of the Li-O phases is quite different from the Na-O 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. (c) 2012 Elsevier B.V. All rights reserved.