Effects of Liquid Electrolytes on the Charge–Discharge Performance of Rechargeable Lithium/Sulfur Batteries: Electrochemical and in-Situ X-ray Absorption Spectroscopic Studies

TLDR

A sulfur/carbon composite cathode was prepared for lithium/sulfur batteries. The authors evaluated seven liquid electrolytes—ether, sulfone, and carbonate solvents with common lithium salts—by galvanostatic discharge–charge cycling of coin cells and by in‑situ sulfur K‑edge X‑ray absorption spectroscopy to probe soluble sulfur species. They found that the solvent type, rather than the lithium salt, governs electrochemical performance: low‑viscosity ethereal solvents enable more complete polysulfide reduction, while viscous ethereal solvents leave polysulfides more oxidized, and carbonate solvents react with reduced sulfur species, rendering them unsuitable for elemental sulfur cathodes.

Abstract

A sulfur/carbon composite has been prepared to serve as a cathode for lithium/sulfur batteries. The effects of seven different liquid electrolytes on the electrochemical performance were investigated using galvanostatic discharge–charge tests on coin cells. The electrolytes included ether, sulfone, and carbonate solvents with common lithium salts. It was found that the solvent plays a key role on the electrochemical performance of the lithium/sulfur battery cathode while the lithium salt has no significant effects. Additional characterization, using in situ sulfur K-edge X-ray absorption spectroscopy (XAS), provided insights into the soluble sulfur species in the discharged and charged batteries. We find that the use of low-viscosity ethereal solvents results in a more complete reduction of soluble polysulfides, while soluble polysulfides remained more oxidized in viscous ethereal solvents. Moreover, XAS revealed that reduced sulfur species chemically react with carbonate-based solvents, making this class of solvents inappropriate for elemental sulfur cathodes of lithium batteries.