Raman Spectroscopic and X-ray Diffraction Studies of Sulfur Composite Electrodes during Discharge and Charge

The influence of solvents on the discharge behavior of lithium-sulfur cells was investigated by ex situ Raman spectroscopy and X-ray diffraction. Lithium polysulfide species formed in a sulfur cathode during cycling are characterized by Raman experiments for the first time and their structures are examined with regard to three different electrolytes at fully charged and discharged states. Moreover, ex-situ Raman studies give the evidence for the formation of lithium polysulfide on a Li metal anode by shuttle phenomena and the coexistence of soluble lithium polysulfide with elemental sulfur even after full charge. It was found that 1,3-dioxolane (DOX)/1M LiTFSI facilitates the migration of soluble lithium polysulfide toward a lithium anode and initiates a polysulfide shuttle causing a considerable capacity loss in lithium-sulfur cells. Raman results and cycling tests using an air-tight cell demonstrated that tetra(ethylene glycol) dimethyl ether (TEGDME)-based electrolytes hindered the significant overcharge and led to the formation of Li2S2 contributing to high discharge capacity through further electrochemical reduction to Li2S.