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 Na2Sn 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.