Medium-temperature molten sodium batteries with aqueous bromine and iodine cathodes.

Medium-temperature sodium-bromine and sodium-iodine battery systems operating at 3.5 V and 3.0 V, respectively, are presented. The rechargeable molten-sodium systems work at approx. 100 °C and use aqueous bromine/bromide or iodine/iodide solutions as catholyte and sodium-ion conductive NaSICON (Na 3 Zr 2 Si 2.3 P 0.7 O 11.85 ) as solid electrolyte. The free halogen, which is formed upon charge, is complexed as highly soluble tribromide and triiodide, respectively. These systems can work in both, static and redox-flow setup, respectively, and aim at stationary energy storage applications. The sodium-halogen batteries were shown to run with high coulombic efficiency over several hundred hours. Long-term stability of the NaSICON ceramic material in contact with aqueous electrolytes, however, is still a concern. The sodium-bromine system suffers from a considerable bromine vapour pressure of the tribromide, whereas the sodium-iodine system was shown to operate in a stable manner with a catholyte allowing for a high total iodine concentration (≥2.0 M). Catholyte concentrations up to 2.75 M total iodine in redox-flow setup and up 3.7 M in static setup are possible without iodine precipitation and correspond to specific energies of 198 Wh kg −1 and 228 Wh kg −1 , respectively, on total active material level. [ABSTRACT FROM AUTHOR]