High energy density Na-S/NiCl2 hybrid battery

High temperature (250-350 °C) sodium-beta alumina batteries (NBBs) are attractive energy storage devices for renewable energy integration and other grid related applications. Currently, two technologies are commercially available in NBBs, e.g., sodium-sulfur (Na-S) battery and sodium-metal halide (ZEBRA) batteries. In this study, we investigated the combination of these two chemistries with a mixed cathode. In particular, the cathode consisted of molten NaAlCl4 as a catholyte and a mixture of Ni, NaCl and Na2S as active materials. During cycling, two reversible plateaus were observed in cell voltage profiles, which matched electrochemical reactions for Na-S and Na-NiCl2 redox couples. An irreversible reaction between sulfur species and Ni was identified during initial charge at 280 °C, which caused a decrease in cell capacity. The final products on discharge included Na 2Sn with 1 < n < 3, which differed from that of Na2S3 in traditional Na-S battery. This novel battery demonstrated a 50% increase in energy density over ZEBRA batteries. Despite of the initial drop in cell capacity, the mixed cathode demonstrated relatively stable cycling with more than 95% of cell capacity retained over 60 cycles. Optimization of the cathode may lead to further improvements in battery performance. © 2012 Elsevier B.V. All rights reserved.