Current driven flow instabilities in large scale liquid metal batteries and their management

Liquid metal batteries, i.e. batteries in which both electrodes as well as the electrolyte are in the liquid state, usable for grid-scale energy storage have received considerable attention recently . A current and comprehensive account focusing on their applicability in future large scale storage systems is provided by Bradwell, earlier investigations of the technology were oriented on smaller units and thermally regenerative electrochemical systems.

A battery with fully liquid active inventary has a number of advantages: when densities are chosen properly, the battery is self-assembling due to stable stratification. Liquid-liquid interfaces allow for very fast kinetics and thereby rapid charging and discharging. Structureless (liquid) electrodes are insusceptible to aging providing nearly unlimited cyclability. Liquid metal batteries may be built from abundant and cheap feedstock. NaS and ZEBRA batteries share several of the advantages mentioned above, but require large initial investments due to their complicated construction, which is mainly dictated by the fragile ceramic electrolyte. In any case, scalability is a key enabler for cheap grid storage and ease of scale-up is one of the main underlying assumptions of liquid metal battery development. However, large electrode areas and high current densities imply large total current per cell and here electromagnetics together with fluid mechanics - i.e. magnetohydrodynamics - comes into play.

Aluminium reduction cells - which are often mentioned to have sparked the idea for large scale energy storage using liquid metals, see, e.g., - suffer from an interfacial instability which puts a constraint on the minimum electrolyte thickness. While this limitation has also to be considered, our focus is on another kind of instabilty, which limits the upper size of liquid metal batteries and is known in astrophysics under the label Tayler instability (TI). In our context, the TI is a kink-type (i.e. non-axisymmetric) instability that occurs if the current through a column of liquid metal exceeds some critical value in the order of kA, depending on the material properties. If this current threshold is exceeded, the TI would lead to a stirring of the battery inventory destroying the stable density stratification and short-circuiting the electrodes. Due to its potentially dramatic consequences, the TI should definitely be avoided during liquid metal battery operation.

One possibility to circumvent the instability is to use cells with a central bore. Depending on the ratio of bore to cell diameter, the instability can be shifted to higher total currents. Feeding a current through the bore opposing the current in the cell is a means to suppress the TI totally and would therefore be preferred in practical settings.