Operando characterisation of Na-Zn molten salt batteries using X-ray radiography: insights into performance degradation and cell failure

The sodium-zinc system for grid-scale energy storage is a compelling solution due to its high cell voltage (1.8 V) and Earth abundance, resulting in minimal active material costs. Since the proposal of the cell concept, the research has been based on the assumption that separating the electrolyte into anolyte and catholyte using a porous medium to limit the molten salt mixing is essential for cell operation. This is deemed crucial in order to confine the produced ZnCl₂ to the vicinity of the Zn pool and to reduce the probability of a direct contact between ZnCl₂ and Na, which would result in self-discharge. However, the constructed cells have not demonstrated consistent performance over extended periods of more than a few weeks. Through in situ operation of the cells at an X-ray beamline, the underlying causes of battery failure have been identified, facilitating a conceptual overhaul of the cell design. It is demonstrated that the separator is not only unnecessary, but is the reason for cell failure. Its elimination enables a significant simplification of the design, while simultaneously ensuring stable cycling.