Polysulfide species in various electrolytes of Li-S batteries – a chromatographic investigation

Abstract The HPLC method including derivatisation was used for the determination of polysulfide species concentration during the operation of lithium-sulfur battery cells. A comparison was made between a glyme based electrolyte, which exhibits high polysulfide solubility, and a fluorinated ether based electrolyte, which was proposed to reduce the dissolution and diffusion of polysulfides. A separate analysis was conducted on porous carbon cathodes and separators obtained from cycled battery cells. The determination of the species concentration trends allowed a deeper understanding of the differences in battery cell operating mechanism. We have determined that polysulfide species are formed in similar concentrations in both examined electrolytes, but they remain trapped in the cathode pores in the fluorinated ether based electrolyte lithium-sulfur battery cells. The polysulfide concentration in the separators of the fluorinated ether cells was below the limit of detection, which indicates that the solubilities of polysulfides in the fluorinated electrolyte are below 50 µM. This results in absence of disproportionation or coproportionation reactions between the polysulfide species in solution. This different mechanism influences the change in length and potential of the voltage plateaus of the battery cells during galvanostatic cycling.