Operando investigation of polysulfide retention approaches in magnesium-sulfur batteries

Abstract

To date the lithium-ion battery (LIB) is the dominant technology for electrical energy storage powering most electronic devices. However, they approach their capacity limits and with increasing safety and ecological concerns other battery systems have been considered for future battery generations. The rechargeable magnesium-sulfur (Mg-S) battery system stands out as a promising candidate because of its potential high-energy density, safety assets and low cost. The cell, however, faces fast cell degradation and low cyclability due to the polysulfide shuttle. Furthermore, challenges lie in finding a suitable electrolyte fulfilling a multitude of requirements.

Furthermore, it is well known that batteries containing sulfur electrodes suffer from fast degradation due to the polysulfide shuttle effect that is mainly observed during charge and there is a strong need to understand the problems related to the polysulfide shuttle. This work aims to investigate different strategies to overcome the polysulfide diffusion to the anode side by operando UV/Vis spectroscopy and microscopy. Approaches to achieve polysulfide retention include the addition of additives during electrode fabrication, different electrode materials and electrolyte solvents. With the aid of reference polysulfide solutions, the absorption wavelengths of specific polysulfide species were identified and correlated to the operando measurements. As the change in polysulfide concentration is the highest at the beginning and thus yielding most information for UV/Vis spectroscopy, most emphasis is put on the first three cycles. Furthermore, pristine and postmortem analysis of the cell component's surfaces is done to compare and evaluate processes that occurred during cycling. With the aid of operando UV/Vis spectroscopy the formation and disassembly of polysulfide species during cycling could be shown and be successfully verified with the reference spectra. Furthermore, incorporation of additives to the cathodes resulted in no significant electrochemical changes but had effects when measured with UV/Vis spectroscopy. Mg-S chemistry is likely more affected by external factors such as type of electrolyte solvent and anode material. The utilization of tetraglyme (TGM) as the electrolyte solvent showed better results than monoglyme (DME). Higher cyclability was demonstrated with Mg-pellets than Mg-foil likely due to the former's greater porosity and thus higher surface area. Recordings via scanning electron microscopy (SEM) showed that the additives were homogenously distributed across the surface. However, postmortem analysis revealed that surface changes are less pronounced on the electrode's surfaces but more on the separator.