Stability and Disproportionation of Magnesium Polysulfides and the Effects on the Mg-Polysulfide Flow Battery

Abstract

The Mg-S battery suffers from the slow Mg2+ diffusion rate in the solid discharge products (MgS2, MgS). A possible solution to this problem is the Mg-polysulfide flow battery. The formation of the solid discharge products could potentially be avoided with a high voltage cut-off for the discharge process, instead producing soluble magnesium polysulfides (MgSx). Here, the feasibility of the Mg-polysulfide flow battery is investigated, and low active material utilization and lack of reversibility is found. Moreover, the MgSx solutions used in the flow battery are found to be unstable. The UV–vis spectra collected on these MgSx solutions indicate changing solubilized sulfur/polysulfide speciation and/or concentration over time and the formation of sulfur crystals is observed. The instability problem of MgSx is observed in a variety of electrolytes investigated in this work. The solvents and salts used in the electrolyte influence MgSx speciation and stability. In addition, accumulation of electrolyte salt-rich compounds on the Mg anode is observed. This study points out the problem of the competition between magnesium salts and MgSx for dissolution in the electrolyte, an issue which has not gained enough attention in the past but could cause serious problems like poor reversibility and passivation of the Mg anode.