MHD driven localized short circuits in liquid metal batteries

Liquid metal batteries (LMBs) are electrochemical devices, which operate as simple concentration cells at elevated temperature. Abundant raw materials and the totally liquid interior promise a very long life time, extreme current densities and a very competitive price of these storage devices. For this reason, LMBs are discussed as an ideal candidate for grid-scale energy storage.
The cathode of the cells (e.g. molten Bi, at the bottom) is typically contained in a metal vessel, while the anode (e.g. molten Li, at the top) is soaked into a FeNi-foam. Both electrodes are separated by a molten salt electrolyte. When discharging the cell, Li is oxidised, crosses the electrolyte layer and alloys into Bi; upon charge, the process is reversed. Due to corrosion or electrochemical reactions of the molten salt layer with the FeNi-foam, the Li-wetting of the foam might decrease during operation. In case of insufficient wetting of this current collector, it might happen that the Li does not penetrate the foam any more, when the cell is charged. Consequently, small Li-droplets will appear below of the foam, and will grow into the electrolyte layer, when charging the battery. As the conductivity of the electrolyte is four orders of magnitude smaller than that of the metals, the current will take the shortest way through the electrolyte, i.e. a large current will flow through the Li-droplets. This current might pinch the droplet locally, possibly deforming, or even cutting it off. Hence, small Li-spheres might be transferred into the electrolyte. This might lead to unwanted self-discharge, if Li reaches the Bi-layer.
In the presentation, the appearance and significance of such localised droplet-transfer and short-circuits will be discussed first. Then, some experimental evidence of similar effects will be presented. Finally, numerical simulations of the transfer of a single droplet will be shown. Moreover, the relevance of the results for practical applications and real cells will be explained.