Redox behavior of Li-S cell with PP14-TFSI ionic liquid electrolyte

Abstract

Li-S batteries, having one of highest theoretical energy densities possible, have been suggested as a way to improve on the current state of the art. However, the dissolution of intermediate polysulfide species in the electrolyte has so far greatly reduced the cycleabilty of these batteries. One proposed solution is to use an ionic liquid electrolyte to decrease the solubility of the polysulfides. This work investigates the polysulfide speciation in Li-S cells with an ionic liquid electrolyte during charge and discharge. Four different room temperature ionic liquids were studied and found to dissolve polysulfides in small concentrations. One of the ionic liquids, PP14-TFSI, was examined further using UV-Vis spectroscopy to identify the polysulfide ions present in the electrolyte at different points of the charge/discharge cycle. It was found that the first step of the reaction during discharge produces S2􀀀 6 with only small amounts of other polysulfides. Most of the S2􀀀 6 was then reduced to S2􀀀 3 and S2􀀀 4 in a second reaction step. These reactions showed good reversibility during charge, however a rapid decline in capacity was observed with continued cycling. This led to the conclusion that the ionic liquid did not by itself prevent the loss of active material associated with the dissolution of polysulfides.