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.

Na-β″-alumina is the commercially most successful solid electrolyte due to its application in ZEBRA and NAS^® batteries. In this work, Li-stabilized Na-β″-alumina electrolytes were doped with 3d transition metal oxides, namely TiO₂, Mn₃O₄, and NiO, in order to improve their ionic conductivity and fracture strength. Due to XRD and EDX measurements, it was concluded that Mn- and Ni-ions are incorporated into the crystal lattice of Na-β″-alumina. In contrast, TiO₂ doping results in the formation of secondary phases that enable liquid-assisted sintering at temperatures as low as 1500 °C. All dopants increased the characteristic fracture strength of the electrolytes; 1.5 wt% of NiO doping proved to be most efficient and led to a maximal characteristic fracture strength of 296 MPa. Regarding the ionic conductivity, TiO₂ doping showed the uppermost value of up to 0.30 S cm⁻¹ at 300 °C. In contrast to the other dopants, TiO₂ doping lowered the sintering temperature needed to obtain a dense, stable, and highly conductive Na-β″-alumina electrolyte suitable for applications in Na based batteries.

Solid-state sodium batteries using solid electrolytes have attracted attention as sustainable and powerful electrochemical energy storage systems. Even though the sodium-beta alumina solid electrolyte shows excellent properties, the limited interface contact with both electrodes has limited its wide utilization in room-temperature cell systems. However, interface modifiers can reduce the interfacial resistance, enabling good electrochemical performance. In this paper, we test four different liquid-organic electrolytes (LOE) to modulate the cathode interface. The effect of 1 M NaClO₄ in propylene carbonate on the cell performance and the oxidic solid electrolyte was further investigated. We show that the LOE displacement, not the interphases, is the main reason for the impedance growth using a three-electrode cell, a setup rarely employed in solid-state batteries. The realized semi-solid-state sodium cells with modulated cathode interface achieve an area-specific resistance as low as 63 Ω cm² and a high capacity retention of 99.8% after 100 cycles at 1.0C and 30 °C. Hence, the presented approach is a simple and cost-effective way to enhance the cathode interface in sodium-beta alumina-based cells.

Li-ion batteries often use transition metal oxide (TMO) positive electrodes. Sodium-based analogs are also an excellent option for all-solid-state sodium-based batteries. However, combining TMOs with solid electrolytes is challenging, both being rigid in nature. In this work, we use a polymeric secondary electrolyte to combine a sodium manganese oxide composite positive electrode with a sodium-beta alumina solid electrolyte (BASE) to an all-solid-state sodium battery. The composite electrode exhibits intimate interface contact with the solid electrolyte and well-established intra-electrode conduction pathways because mechanical flexible poly(ethylene oxide) acts as an ionically conducting auxiliary agent. The composite electrode shows an ionic conductivity of 5.5·10⁻⁶ S cm⁻¹ at 80 °C. The careful co-treatment of the Na|BASE interface, resulting in a superior cycling stability of 1000 h at 0.35 mA cm⁻² in a symmetric cell, enhances sodium’s wettability to the BASE surface. The full cell achieves an initial discharge capacity of 80 mAh g⁻¹ at an average voltage of 2.78 V vs Na⁺/Na, corresponding to a specific energy of 155 Wh kg⁻¹ (positive electrode). Hence, we provide a proof-of-concept for all-solid-state cells, which paves the way for numerous combinations of well-conducting polymeric secondary electrolytes with TMO active materials and BASE.

Post-Li-ion technologies are inevitable for solving problems connected to Li-ion cells, namely shortage of raw materials and physicochemical limits in energy density. Substitution of lithium with sodium while simultaneously replacing the liquid electrolyte with a ceramic solid electrolyte is a promising alternative.¹ Sodium-beta alumina is a solid-state electrolyte with outstanding chemical, electrochemical, and mechanical properties.² So far, high-temperature cells have been using it as a solid electrolyte.^3,4 However, we recently showed that the sodium-ion conducting sodium-beta alumina solid electrolyte (BASE) is also a suitable solid electrolyte for solid-state cells operating at middle- to low temperatures. We present methods commonly used to reduce the area-specific capacity at the interfaces, stemming from the poor interaction of the rigid solid electrolyte with its adjacent electrodes. Hence, we tackle the bottleneck of middle- to low-temperature solid-state cell systems utilizing BASE, leading to enhanced cell performance.² To increase the electrochemical performance, adjusting BASE´s properties is necessary. We showed that 3d transition metal doping tunes BASE´s properties effectively. For example, Ti⁴⁺ doping assisted a liquid sintering process, which changed the microstructure. Thus, BASE´s ionic conductivity increased by 50%. Simultaneously, we increased BASE´s fracture strength while reducing energy costs.⁵ Furthermore, we demonstrate the importance of proper storage conditions. We elucidate the effect of humidity on disk-shaped samples of Li-stabilized sodium-beta alumina and quantify the negative consequences of improper storage for cell systems using sodium-beta alumina. Despite detrimental effects on ionic conductivity and the chemical composition, 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 only 1.7 mA cm^-2 at 25 °C. We show how impedance analysis and additional characterizations assist in clarifying occurring degradation mechanisms, namely ion exchange and subsequent buildup of surface layers.⁶ Combining the knowledge from previous works, we provide a proof-of-concept for a novel sodium-based solid-state cell concept utilizing sodium-beta alumina. We use sodium metal as the negative electrode, which is necessary to achieve high specific energy and the requirement to compete with existing cell systems. An environmentally friendly transition metal oxide positive electrode with high specific energy is paired with the negative electrode. A composite positive electrode ensures intra-electrode conduction while enabling a facile charge transfer due to an intimate electrolyte–electrode interface contact, leading to stable cycling over 50 cycles with good energy retention. We think that the proof-of-concept opens the door for dozens of new material combinations and enhances the utilization of sodium-beta alumina in medium- to low-temperature solid-state cell systems. All in all, the presentation points out the excellent performance and the enormous potential of sodium-beta alumina for sodium-based energy storage technologies. References Janek, J. & Zeier, W. G. A solid future for battery development. Nat. Energy 1, 1167; 10.1038/nenergy.2016.141 (2016). Fertig, M. P. et al. From High‐ to Low‐Temperature: The Revival of Sodium‐Beta Alumina for Sodium Solid‐State Batteries. Batteries & Supercaps; 10.1002/batt.202100131 (2022). Sudworth, J. The sodium/nickel chloride (ZEBRA) battery. Journal of Power Sources 100, 149–163; 10.1016/S0378-7753(01)00891-6 (2001). BASF New Business GmbH. Stationary Energy Storage. High-energy, long-duration sodium-sulfur battery (Ludwigshafen am Rhein, 2020). Dirksen, C. L., Skadell, K., Schulz, M., Fertig, M. P. & Stelter, M. Influence of 3d Transition Metal Doping on Lithium Stabilized Na-β″-Alumina Solid Electrolytes. Materials (Basel, Switzerland) 14, 5389; 10.3390/ma14185389 (2021). Fertig, M. P., Dirksen, C., Schulz, M. & Stelter, M. Humidity-Induced Degradation of Lithium-Stabilized Sodium-Beta Alumina Solid Electrolytes. Batteries 8, 103; 10.3390/batteries8090103 (2022). [Figure 1. Schematic illustrating the abstract submission´s topics. Pictures adapted from Ref. ^2,6] Figure 1