Lithium Polysulfide Radical Anions in Ether-Based Solvents

Lithium sulfur batteries have a theoretical specific energy 5 times greater than current lithium ion battery standards, but suffer from the issue of lithium polysulfide dissolution. The reaction mechanisms that underlie the formation of lithium polysulfide reaction intermediates have been studied for over four decades, yet still elude researchers. Polysulfide radical anions formed during the redox processes have become a focal point of fundamental Li-S battery research. The formation of radical species has even been shown to be advantageous to the electrochemical pathways. However, whether polysulfide radical anions can form and be stabilized in common Li-S battery electrolytes that are ether-based is a point of contention in Li-S battery research. The goal of this work was to examine the presence of radical polysulfide species in ether-based solvents. Lithium polysulfide solutions in tetraethylene glycol dimethyl ether and poly(ethylene oxide) are probed using a combination of ultraviolet visible (UV-vis) and electron paramagnetic resonance (EPR) spectroscopy. EPR results confirm the presence of radical species in ether-based electrolytes. Comparison of the UV vis spectra to EPR spectra establishes that the UV vis absorbance signature for radical species in ether-based solvents occurs at a wavelength of 617 nm, which is consistent with what is observed for high electron pair donor solvents such as dimethylformamide and dimethyl sulfoxide.