Sodium-Beta Alumina and Its Application in a Novel Sodium-Based Solid-State Cell Concept
Authors: Fertig Micha Philip , Dirksen Cornelius , Skadell Karl , Schulz Matthias , Stelter Michael 
Published: September 2023

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

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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)
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Fertig, M. P., Dirksen, C., Schulz, M. & Stelter, M. Humidity-Induced Degradation of Lithium-Stabilized Sodium-Beta Alumina Solid Electrolytes. Batteries
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[Figure 1. Schematic illustrating the abstract submission´s topics. Pictures adapted from Ref. 2,6]

Figure 1