A medium-temperature sodium-iodine battery system is presented. The rechargeable molten-sodium system works at approx. 100 °C with high efficiency, and potentially lower cost than existing high-temperature sodium-batteries (which are usually operating at a temperature of around 300 °C). Our battery system uses an aqueous iodine/iodide solution as catholyte and sodium-ion conductive Zr-based NaSICON ceramic material as solid electrolyte. The free halogen, which is formed upon charge, is complexed as highly soluble triiodide. Long-term stability of sodium-ion conductive material in contact with aqueous electrolytes, generally, is a concern. NaSICON-based ceramic material has shown not only an enhanced stability  against these electrolytes but also an increased sodium-ion conductivity , compared to sodium β″-alumina used in sodium-sulfur batteries. The sodium-iodine system has shown to operate in a stable manner with a catholyte allowing for a high total iodine concentration (>3.0 mol/L) . Substitution in the NaSICON composition allows for increased ionic conductivity and enhanced stability against the aqueous cathode. In the case of fissuring of the NaSICON ceramic separator, only solid products are formed. This stops the direct reaction of active materials. M. Holzapfel, D. Wilde, C. Hupbauer, K. Ahlbrecht, T. Berger, Electrochim. Acta 237 (2017), 12-21.  S. Naqash, Q. Ma, F. Tietz, O. Guillon, Solid State Ionics 302 (2017), 83-91.
This work is funded by the German Federal Ministry of Education and Research in the project „MiTemp – Mitteltemperatur-Natriumbatterien mit flüssiger Natriumanode und wässriger Iodkathode“ (03XP0183A).