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In this paper, the qualitative and semi-quantitative analysis of polysulfide species dissolved in electrolyte during the discharge-charge process and cycling of Li-S batteries was reported. ICP-OES (Inductively coupled plasma- Optical emission spectrometer) measurement was used to estimate the total sulfur content dissolved in electrolyte. Lithium polysulfide with different order were separated and confirmed by LC-MS (Liquid chromatography coupled with mass spectrometry). Li2S4 and Li2S6 were proved to be the most stable form of lithium polysulfide species. At the end of discharge process, total sulfur content in the form of Li2S4 and Li2S6 remained in electrolyte was about 20% of the active material in initial cathode. At the end of charge process, 45% total sulfur content was preserved in electrolyte mainly in the form of Li2S6. The partial transformation of active material from liquid phase to solid phase resulted in relatively low practical specific capacity than the theoretical. In cycles, active material transferred between liquid and solid phase kept a balance, and the content of total sulfur, Li2S4 and Li2S6 were changed slightly at the end. Consequently, polysulfide dissolved in electrolyte just took limited responsibility for the capacity fading with cycles of Li-S batteries.
Diao et al. (Sun,) studied this question.