Effective Liquid Electrolytes for Enabling Room-Temperature Sodium-Sulfur Batteries

Glyme-based electrolytes for sodium-sulfur (Na-S) batteries are proposed for advanced cell configuration. Solutions of NaClO4 or NaCF3SO3 in tetraglyme are investigated in terms of thermal stability, ionic conductivity, Na+-transference number, electrochemical stability, stripping-deposition ability, and chemical stability in Na-cells. Subsequently, versions of the electrolytes doped with fluoroethylene carbonate (FEC) are prepared using 0.5, 1, 2, or 3% additive weight concentrations, and evaluated by adopting the same approach used for the bare solutions. Scanning electron microscopy (SEM) provides morphological details of the passivation layer formed on the Na electrodes, while X-ray photoelectron spectroscopy (XPS) sheds light on its composition. The most relevant achievement of the FEC-added electrolyte is the suppression of the polysulfide shuttle in Na-S cells using a cathode with 70 wt.% of sulfur in the composite. This result appears even more notable considering the low amount of the additive requested for enabling the reversible cell operation. The solutions using 1% of FEC show the best compromise between cell performance and stability. Cyclic voltammetry (CV) displays the potential region related to the FEC electrochemical process responsible for Na-S cell operation. The understanding of the electrolyte features enables additional cycling tests using sulfur cathode with an optimized current collector, increased specific capacity, and coulombic efficiency.