Visual observations and charge/discharge behavior of liquid metal cells

Liquid metal batteries (LMBs) consist of a stable density stratification of a molten alkaline metal on the top, a liquid heavy metal on the bottom and a fused salt mixture sandwiched in between. Initially developed as part of energy conversion systems, today they are considered an inexpensive means for stationary large-scale storage of electrical energy. A special feature of LMBs is their very high current density enabled by the fast kinetics at liquid-liquid interfaces and the rapid mass transfer processes in fluids. Scale-up on the cell level will therefore result in large total currents per cell that might however trigger electromagnetic instabilities and/or generate electro-vortex flows.
To allow for an experimental verification of the numerical predictions, a number of low temperature liquid metal cells were tested in order to optimize operating conditions and material combinations. The final aim is the construction of a larger cell able to generate a considerable current.
Na and Li were tested as negative electrode materials versus a positive electrode made of Bi in both cases. Na has a relatively high solubility in its salts and forms a dark metal fog in the electrolyte. While this enables the visualization of an intense flow in the electrolyte, it also leads to an unwanted electronic conduction and accompanying capacity fading. Li solubility in its salts is much weaker and no metal fog formation was observed. Cycling behavior in the tethered drop cell is considerably more stable for Li than for Na. Estimated current densities based on the immersed surface of the drop exceed 2 A/cm2 by far, both for Na and Li. Electrolytes are varied as well since for the metal combinations mentioned above the melting point of the salt mixture determines the cell operating temperature.