Effect of Electrolytes on Room-Temperature Sodium-Sulfur Battery Performance
Room-temperature sodium-sulfur (RT Na-S) batteries are an emergent new technology that are highly attractive due to their low raw materials cost and large theoretical specific energy. However, many fundamental problems still plague RT Na-S batteries that prevent their progression from the research and development phase to the commercial phase. Sulfur and its final discharge product are insulators, and intermediate polysulfide discharge products are soluble in commonly used liquid electrolytes. As a result, RT Na-S cells exhibit large capacity defects, low coulombic efficiencies, and rapid capacity fading. Additionally, the reactive sodium metal anode can form dendrites during cycling, which reduces capacity and shortens cell life. One way to combat these issues is the judicious selection of electrolyte components. In this study, the effects of monoglyme (G1), diglyme (G2), and tetraglyme (G4) glyme ether electrolyte solvents on RT Na-S cell performance are investigated. Galvanostatic cycling of Na/Na symmetric coin cells reveals that the G2 solvent enable stable cycling at low overpotentials over a wide range of current densities. In contrast, the G1-based cells show evidence of dendritic plating, and G4-based cells are not suitable for use at high current densities. Electrochemical impedance spectroscopy during cycling confirms that the G2 solvent facilitates the formation of a strong, stable SEI on the Na electrode surface. Results from galvanostatic cycling of RT Na-S full coin cells demonstrates that G1-based cells deliver the highest initial specific discharge capacities among the three cell types, but G4-based cells are the most reversible. Infinite charging, the indefinite accrual of charge capacity at the high charge voltage plateau, affects all cell types at different cycle numbers and to different extents. This behavior is linked to the strength of the polysulfide shuttle during charge. Optical microscopy experiments show that G2 and G4 facilitate the formation of the S3•- sulfur radical, which reduces capacity. G1 minimizes the radical formation and thus delivers higher initial specific discharge capacity. In order to fully optimize the electrolyte for RT Na-S cells, future work should study glyme solvent blends, additives, and concentrated salts.