Fluid mechanics of Na-Zn liquid metal batteries

Liquid metal batteries have been introduced as promising option to address the needs for new energy storage technologies. Currently, batteries based on sodium and zinc are under development and a favorable option due to their high theoretical cell potential, readily abundant materials, and cost-advantages. Nevertheless, they face the problem of self-discharge, which makes it inevitable to understand fluid dynamics in the whole cell. Motivated by that, several types of fluid mechanic instabilities in Na-Zn liquid metal batteries are identified and discussed here. On the one hand they can jeopardize secure operation, but on the other hand they can also improve mixing and increase the cell efficiency. In doing so, realistic cell as well as operation parameters are included and dimensionless numbers for identifying critical conditions are presented. The phenomena with highest significance for the discussed batteries are solutal convection, swirling flow, electrocapillary Marangoni convection, and droplet formation. Still, many open research questions remain and we aim at motivating researchers to dig deeper into some of these topics to contribute to an improved cell design and performance.