Magnesium Polysulfides: Synthesis, Disproportionation, and Impedance Response in Symmetrical Carbon Electrode Cells

Abstract

Magnesium polysulfides have a key role during the redox reaction in magnesium-sulfur batteries. However, kinetics of the Mg polysulfide disproportionation, transport, and solubility in electrolytes are rarely studied. To fill this gap, we here report the synthesis of different chain-length Mg polysulfides and their characterization with 1H NMR, ATR-IR, and XANES spectroscopies. The tendency of polysulfides to disproportionate in solutions is confirmed with UV/Vis spectroscopy. To probe their reactivity and diffusivity, impedance spectroscopy is performed in symmetrical cells consisting of two planar glassy carbon or porous carbon electrodes containing Mg polysulfide catholyte. Several transport-reaction parameters are obtained, such as exchange current densities, chemical diffusion coefficients, transference numbers of polysulfides, and conductivities. Moreover, we show that the solubility of Mg polysulfides in Mg(TFSI)2, MgCl2 ether-based electrolyte is affected at rather moderate concentration of polysulfides, thereby having an impact on the cell performance.