Operando Investigation of the Polysulfide Dissolution Behavior in Mg-S Batteries

Abstract

Since the first reported magnesium-sulfur (Mg-S) cell by Kim et al. in 2011 interest in this postlithium battery system has steadily increased due to its promising properties in regards of volumetric energy density, safety and costs. In recent years many research groups have attempted to unveil the underlying electrochemical mechanisms by sophisticated structure analysis of the solid product (MgS) as well as spectroscopic investigations of the soluble intermediates (magnesium polysulfides, MgSx). Analogous to lithium-sulfur (Li-S) batteries, the solubility of the latter causes the loss of active material and charge compensation through the polysulfide shuttle. Furthermore, the sulfur species contribute to the formation of an anodic SEI, which - in the case of magnesium metal - exhibits a rather passivating nature. This is supposed to be one of the origins for the fast capacity decay and the short cycle life of magnesium-sulfur battery cells. Benefitting from previous Li-S research to retain sulfur species within the cathode and prevent the polysulfide shuttle, several retention approaches can be considered for the Mg-S system. Among the different strategies, the most promising ones are the sulfur incorporation in micropores, separator coatings with ionomers or carbons, covalently bound sulfur or the use of polar cathode additives as adsorption sites. In this study, the capability of selected attempts on magnesium polysulfide retention was investigated via operando UV/Vis spectroscopy and operando microscopy in optical cells. Besides the analysis of the separator coloration during galvanostatic cycling, the self-discharge during OCV was surveyed. In addition, significant influence of the electrolyte solvent and the polysulfide diffusion length on the reoxidation of the sulfur species was observed.