Sodium‐based batteries are promising post lithium‐ion technologies because sodium offers a specific capacity of 1166 mAh g⁻¹ and a potential of −2.71 V vs. the standard hydrogen electrode. The solid electrolyte sodium‐beta alumina shows a unique combination of properties because it exhibits high ionic conductivity, as well as mechanical stability and chemical stability against sodium. Pairing a sodium negative electrode and sodium‐beta alumina with Na‐ion type positive electrodes, therefore, results in a promising solid‐state cell concept. This review highlights the opportunities and challenges of using sodium‐beta alumina in batteries operating from medium‐ to low‐temperatures (200 °C–20 °C). Firstly, the recent progress in sodium‐beta alumina fabrication and doping methods are summarized. We discuss strategies for modifying the interfaces between sodium‐beta alumina and both the positive and negative electrodes. Secondly, recent achievements in designing full cells with sodium‐beta alumina are summarized and compared. The review concludes with an outlook on future research directions. Overall, this review shows the promising prospects of using sodium‐beta alumina for the development of solid‐state batteries.

Herein, the proof of concept of a sodium polysulfide battery consisting of two electrode chambers being separated by a solid electrolyte is described. The concept is suited for dissolved polysulfide cathodes and has the advantage that both half reactions can be optimized separately. The formation of solid sulfide discharge products is identified as the major limiting factor for cell cycling. This issue can be alleviated by adding solid P₂S₅. Further improvement can be achieved by replacing diglyme (2G) as the cathode compartment solvent with tetramethylurea (TMU). Using TMU, the cell cycles with Coulombic efficiencies >99% and capacities of 800 mAh g⁻¹ are maintained for at least 30 cycles. Viscosity, density, conductivity, and the electrochemical stability window values of the 2G‐ and TMU‐based electrolytes are compared. The latter shows higher viscosity (2.806 vs 1.603 mPa s), higher density (1.016 vs 0.996 g cc⁻¹), and higher conductivity (4.27 vs 1.45 mS cm⁻¹). The oxidative stability limit of the TMU electrolyte is 3.2 V versus Na⁺/Na, which is sufficient for polysulfide redox reactions. Vis spectroscopy is used to follow the electrode reaction. In case of TMU, the reaction is based on the redox activity of S₃^−• radicals (blue coloration of the catholyte solution).

Sodium-beta alumina is a solid-state electrolyte with outstanding chemical, electrochemical, and mechanical properties. Sodium polyaluminate is successfully employed in established Na–S and Na–NiCl₂ cell systems. It is a promising candidate for all-solid-state sodium batteries. However, humidity affects the performance of this solid electrolyte. In this work, the effect of humidity on disk-shaped samples of Li-stabilized sodium-beta alumina stored in three different environments is quantified. We used impedance analysis and additional characterizations to investigate the consequences of the occurring degradation, namely ion exchange and subsequent buildup of surface layers. Sodium-beta alumina’s ionic conductivity gradually deteriorates up to two orders of magnitude. This is due to layers developed superficially during storage, while its fracture strength of 240 MPa remains unaffected. Changes in microstructure, composition, and cycle life of Na|BASE|Na cells highlight the importance of proper storage conditions: In just one week of improper storage, the critical current density collapsed from the maximum of 9.1 mA cm⁻², one of the highest values reported for sodium-beta alumina, to 1.7 mA cm⁻² at 25 °C. The results validate former observations regarding sodium-beta alumina’s moisture sensitivity and suggest how to handle sodium-beta alumina used in electrochemical cell systems.