Medium-Temperature Sodium-Iodine Battery System

Summary:

Frank Schäfer a,b, Michael Holzapfel a, Thomas Berger a and Jens Tübke a,b.

a Fraunhofer Institute for Chemical Technology (ICT), Applied Electrochemistry, Joseph-von-Fraunhofer-Straße 7, 76327 Pfinztal, Germany
b Institute for Mechanical Process Engineering and Mechanics, Karlsruhe Institute of Technology KIT, Straße am Forum 8, D-76131, Karlsruhe, Germany
ORCID-ID: 0000-0002-8329-4942

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 [1] against these electrolytes but also an increased sodium-ion conductivity [2], 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 (>2.0 mol/L) [1]. Substitution in the NaSICON composition allows for increased ionic conductivity, enhanced stability against the aqueous cathode and increased total iodine concentration (> 3.0 mol/L). In the case of fissuring of the NaSICON ceramic separator, only solid products are formed. This stops the direct reaction of active materials.

[1] M. Holzapfel, D. Wilde, C. Hupbauer, K. Ahlbrecht, T. Berger, Electrochim. Acta 237 (2017), 12-21.

[2] 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).