Two concepts – one common cell chemistry
SOLSTICE will develop two types of Na-Zn cells sharing the same basic chemistry:
- An all-liquid Na-Zn cell that operates at about 600 °C
- A solid electrolyte Na-Zn cell akin to a classical ZEBRA® cell with a working temperature of ca. 300 °C and using a Na-β“-alumina ceramic as ion conductor
The Na-Zn all-liquid cell concept
The all-liquid cell shares many characteristics with the class of liquid metal batteries (LMBs). Compared to traditional LMBs its cell voltage is with about 1.8 V almost twice as large. This – together with the possibility to use an abundant cathode material – is a big improvement on traditional LMBs that solely rely on the activity of the anode element in the cathode alloy to generate the cell voltage. Due to its similarity with LMBs it shares the advantage of scalability at the cell level. This means that similar to the huge installations‘ characteristic for industrial molten salt electrolysis, Na-Zn cells can potentially reach cross-sections in the square meter range. Such cell sizes entail highly favourable and otherwise unattainable ratios of active to construction material because of the cubic scaling (volume) of the former and the quadratic scaling (surface) of the latter.
The Na-Zn solid-electrolyte cell concept
Sodium metal halide batteries feature a solid, sodium-ion conducting, but electrically insulating ceramic electrolyte consisting of Na-β’’-alumina. The NiCl2 used in today’s ZEBRA® cells is one of the main cost factors and needed for other battery technologies as well. Replacing it by ZnCl2 would save cost and alleviate pressure on limited resources. A voltage of about 1.95 V at standard conditions is likely, but a more complex behaviour including different voltage steps is expected due to the existence of several phases at operating temperature. Technology for ZEBRA® batteries is in very mature state, they have proven to be a very successful product since decades. Retrofitting the existing technology while changing the cathode chemistry from NiCl2 to ZnCl2 is likely possible without serious modifications to the battery concept as such. This will facilitate the further development of the solid-electrolyte Na-Zn system and result in an enormous speed-up in further TRL increases once the initial hurdles are overcome.