Molten sodium batteries show remarkable promise as large-scale energy storage systems, but their widespread deployment has been limited by high operating temperatures (～300°C), which impacts their lifetime, cost, and reliability. Lowering the battery operating temperature promises substantial improvements in these areas, but the lower temperature introduces new challenges to the battery chemistry, especially related to the solid state ion-conducting separators used in these batteries. Traditionally, the high temperature operation has been required to not only maintain high Na~+-ion conductivity of solid electrolytes, such as β"-Al_2O_3 or NaSICON, but also to improve the liquid-solid interfacial wetting of molten sodium on the solid electrolyte. We are focused on drastically reducing the operating temperature to near the melting temperature of sodium (97.8°C), which leads to poor interfacial wetting of the molten sodium. Poor wetting and ineffective charge transfer can dominate battery resistances resulting in a cell-limiting interface. Here, we will describe the rational design of engineered coatings to improve wetting of the molten sodium on NaSICON at low temperatures near 100°C, and therefore improve charge transfer across this critical interface. Enhanced mating of the separator-sodium interface by means of engineered coatings is demonstrated to result in lower interfacial resistance and higher battery performance at increased current densities. This strategy promises substantial advances in the operation of molten sodium batteries at low temperatures and opportunities to expand the utility of these batteries to meet emerging grid-scale needs.
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