Aided by Prof. Huggins’s contributions¹, the research and development of sodium beta-alumina solid electrolytes led to the commercialization of the molten sodium-sulfur battery, used today for grid-scale energy storage. Sodium-sulfur and ZEBRA batteries typically operate above 300 °C, and recent efforts have explored lowering the operating temperature to 100-200 °C^2–4. The melting point of sodium is 98 °C, but its alloy with potassium (Na-K) has a eutectic melting point of -13 °C and is a liquid metal at room temperature with a capacity in the liquid region of 580 mAh/g (using K). Na-K has seen recent attention for batteries with organic electrolytes⁵ as well as for flow batteries using K-β′′-alumina ⁶, since polycrystalline sodium beta-alumina is not compatible with Na-K.

A key challenge in using beta-alumina at low temperatures is transport through the interfaces. We discuss transport through the interfaces in a Na-K — K-β′′-alumina — aqueous potassium ferrocyanide (model posolyte) battery. On the Na-K — K-β′′-alumina interface, although Na-K does not wet K-β′′-alumina at room temperature, we find a form of reactive wetting, given certain conditions, that nearly eliminates the interfacial resistance between Na-K and K-β′′-alumina. On the aqueous posolyte — K-β′′-alumina interface, we find that ion exchange of hydrogen/hydronium species and K+ increases the interfacial resistance substantially over time, but this can be greatly decreased through modifying the solution chemistry.⁷ Understanding and engineering the interfaces of beta-alumina may enable novel applications of this amazing fast ionic conductor at low temperatures.

References

1. Whittingham, M. S. & Huggins, R. A. J. Chem. Phys. 54, 414 (1971).
2. Lu, X. et al. Energy Environ. Sci. 6, 299–306 (2013).
3. Lu, X. et al. Nat. Commun. 5, 4578 (2014).
4. Li, G. et al. Nat. Commun. 7, 10683 (2016).
5. Xue, L. et al. Adv. Mater. 28, 9608–9612 (2016).
6. Baclig, A. C. et al. Joule 2, 1287–1296 (2018).
7. McConohy, G. et al. Solid State Ionics 337, 82–90 (2019).