Liquid metal batteries and their magnetohydrodynamic instabilities

Abstract

Recommended with a Commentary by Brian Skinner, Ohio State UniversityLiquid metals are an old source of fascination in condensed matter physics. For example, understanding their electrical conductivity (even its order of magnitude) was a serious puzzle for many decades, since one cannot use Bloch's theorem in a setting with no periodic structure. (This puzzle was finally cracked in the 1960s -Ref. [1] by Ziman gives a very enjoyable overview.) Recent years have seen a mild revival of interest in liquid metals, with a focus on their mechanical properties [2]. For example, the surface tension of many liquid metals is altered enormously by the formation of an oxide layer on its surface, which presumably grows by the Cabrera-Mott mechanism [3]. (Briefly: an electron first quantumtunnels across the oxide layer from the bulk metal to bind to an oxygen atom on the surface, and then drags a metal ion across the oxide via the Coulomb attraction, so that the oxide thickness is tunneling-limited and grows logarithmically in time.) Consequently, the metal's surface tension can be strongly altered (and even brought to zero) by an applied voltage; Ref.[4] presents some fun experiments in which a spherical liquid metal droplet is reversibly turned into fractal shapes.The two recommended papers illustrate yet another reason why condensed matter physicists might consider reviving their interest in liquid metals. They concern the operation of a certain new type of battery made from liquid metals. The usefulness of these batteries turns out to be limited by a surprising magnetohydrodynamic instability.Before describing these papers, it is worth remarking on the general context that is driving research into new types of battery. The cost of energy generation through renewable sources (and solar panels in particular) has fallen so dramatically in the last decade that this cost is no longer the primary bottleneck for achieving an all-renewable energy grid [5]. A more significant bottleneck now is the cost and efficiency of energy storage. Efficient, large-scale energy storage is needed over a wide range of time scales, from seconds to months, in order