Competing forces in liquid metal electrodes and batteries

Liquid metal batteries have been proposed for low-cost grid scale energy storage. During their operation, solid intermetallic phases often form in the cathode and are known to limit the efficiency of the cell. Fluid flow in the liquid electrodes can enhance mass transfer and reduce or avoid the formation of intermetallics, and fluid flow can be promoted by careful choice of the locations and topology of a battery’s electrical connections, which affect the thermal buoyant forces and electromagnetic forces acting on the electrodes. In this context we study four phenomena that drive flow: Rayleigh-Bénard convection, internally heated convection, electro-vortex flow, and swirl flow, in both experiment and simulation. In experiments, we use ultrasound Doppler velocimetry (UDV) to measure the flow of an electrode made of liquid eutectic PbBI at 160 ◦ C and subject to all four phenomena. In numerical simulations, we isolate the phenomena and simulate each separately using OpenFOAM. Comparing simulated velocities to experiments via a UDV beam model, we find that all four phenomena can enhance mass transfer in LMBs. We explain the flow direction and structure, and give estimates for the magnitude of the mean velocity depending on the cell current. We describe how the phenomena interact and propose dimensionless numbers for estimating their mutual relevance. A brief discussion of electrical connections summarizes the engineering implications of our work.