  For longer than one decade, liquid metal batteries (LMBs) are developed with
the primary aim to provide economic stationary energy storage. Featuring two
liquid metal electrodes separated by a molten salt electrolyte, LMBs operate at
elevated temperature as simple concentration cells. Therefore, efficient mass
transfer is a basic prerequisite for their economic operation. Understanding
these mechanisms cannot be limited at the single layer level. With this
motivation, the effects of solutal- and thermally-driven flow are studied, as
well as the flow coupling between the three liquid layers of the cell. It is
shown that solutal convection appears first and thermal convection much later.
While the presence of solutal flow depends on the mode of operation (charge or
discharge), the occurrence of thermal convection is dictated by the geometry
(thickness of layers). The coupling of the flow phenomena between the layers is
intriguing: while thermal convection is confined to its area of origin, i.e.
the electrolyte, solutal convection is able to drive flow in the positive
electrode and in the electrolyte.
