Sol – gel Synthesis and Characterization of Sodium beta Alumina Powders

Abstract

Sodium-ion conductivity is a well-known feature of sodium beta alumina ceramics. Sodium beta-alumina electrolyte has been widely employed in large-scale energy-storage systems, such as ZEBRA batteries [1] and high-temperature Na–S [2], due to its strong sodium ion conductivity and outstanding thermal characteristics. The compounds with the formula Na₂OnAl₂O₃, where n is typically between 5 and 11, are commonly recognised as sodium beta-alumina. These compounds have high sodium mobility along the conductive planes due to their layered structure. Based on the changeable ratio of sodium to aluminium, there are two structural variations for beta-alumina: β-Al₂O₃ is a rhombo hedral structure, and β’’-Al₂O₃ is a hexagonal structure with a stoichiometry of Na₂O_(8-11)Al₂O₃ and Na₂O_(5-7)Al₂O₃, respectively[3,4]. The two crystal structures consist of conductive planes stacked in various directions and spinel blocks. The two differ in that β’’-Al₂O₃ has two conducting planes and three spinel blocks, while β -Al₂O₃ has one conductive plane and two spinel blocks. Because of its more conductive planes and higher sodium content in conductive planes, the β’’ -Al₂O₃ phase has 3-5 times stronger ionic conductivity than b-Al₂O₃ [5-7]. Consequently, a high β’’ -Al₂O₃ content is preferred in beta-alumina electrolyte applications in energy-storage devices.
The homogeneous microstructure, densification, β’’ -Al₂O₃ content, and other factors are among the many variables that regulate the electrical properties of beta-alumina electrolyte [1, 8–10]. However, high content β’’ -Al₂O₃ is challenging to synthesize from because of its poor thermodynamic stability. It is usually impossible to avoid unwanted concomitant phases like b-Al₂O₃ and NaAlO₂ (around the boundaries) for the standard solid-state reaction synthesis. Suitable stabilizers such as Li₂O, MgO, or their mixture were often used to raise the β’’ -Al₂O₃ concentration [11–13].

 Zhu et al. observed that the β’’ -phase percentage of beta-alumina sinter stabilized by Li₂CO₃ additive was 88 wt%, in contrast to the 5 wt% of beta-alumina sinter obtained without the addition of stabilizer [7, 14]. This proportion is quite high. By encouraging densification and raising the proportion of β’’ -Al₂O₃, a small quantity of magnesium oxide stabilizer can aid to improve the electrical performance of the beta-alumina electrolyte, according to Chen et al. [15]. It has also been demonstrated that the addition of other stabilizers, such as TiO₂ [16], NiO [17], Y₂O₃ [18], and their composites [18, 19], can raise the β’’ -Al₂O₃ content and enhance the beta-alumina electrolyte's electrical properties.

Moreover, it was found that the β’’-Al₂O₃ content of the beta-alumina electrolyte was significantly impacted by the utilization of alumina sources [20]. Alumina sources such as bayerite, pseudo-boehmite, c-alumina, and a-alumina are commonly used to manufacture beta-alumina electrolyte. Barison et al. [21] conducted a thorough investigation into the impacts of various alumina sources on the β’’ -Al₂O₃ concentration and densification for beta-alumina electrolyte, as seen in Table 1. It was shown that c-alumina was superior for obtaining higher content β’’ -Al₂O₃, even though alpha-alumina assisted in achieving higher densification.

In this work, boehmite was used as an alumina source to prepare highly ionic-conductive sodium beta alumina electrolytes. We looked into how the Na₂O content affected the volume density, microstructure, β’’ -Al₂O₃ content, and the electrical characteristics that resulted.