Operando UV/vis Spectroscopy Providing Insights into the Sulfur and Polysulfide Dissolution in Magnesium–Sulfur Batteries

Abstract

The magnesium–sulfur battery represents a promising post-lithium system with potentially high energy density and improved safety. However, just as all metal–sulfur systems, it is plagued with the polysulfide shuttle leading to active material loss and surface layer formation on the anode. To gain further insights, the present study aims to shed light on the dissolution characteristics of sulfur and polysulfides in glyme-based electrolytes for magnesium–sulfur batteries. Therefore, operando UV/vis spectroscopy and imaging were applied to survey their concentration in solution and the separator coloration during galvanostatic cycling. The influence of conductive cathode additives (carbon black and titanium nitride) on the sulfur retention and cycling overpotentials were investigated. Thus, valuable insights into the system’s reversibility and the benefit of additional reaction sites are gained. On the basis of these findings, a reduction pathway is proposed with S8, S6 2–, and S4 2– being the present species in the electrolyte, while the dissolution of S8 2– and S3 •– is unfavored. In addition, the evolution of the sulfur species concentration during an extended rest at open-circuit voltage was investigated, which revealed a three-staged self-discharge.

A cost-effective alkaline polysulfide-air redox flow battery enabled by a dual-membrane cell architecture

“…Therefore the 369 nm peak was used to determine the amount of permeated polysulfide ions. However, unlike single-solute solutions, of which the UV-Vis absorbance is generally acknowledged to be proportional to its concentration, the composition of aqueous polysulfide solutions is complex with various polysulfide ions in dynamic equilibria with each other, and the concentration distribution of various polysulfide ions is affected by a number of factors such as concentration, pH and temperature 34 . Nevertheless, in this work, the absorption peak intensity of aqueous polysulfide solution (<10 mM Na 2 S 2 ) was found proportional to its concentration, as demonstrated in Supplementary Fig.…”

Advances in understanding and regulation of sulfur conversion processes in metal–sulfur batteries

“…Great advances have been achieved in developing high-performance LSBs with impressive values reported in the literature, such as cycle life of 2000 cycles, 209,210 high rate capability at 40C, 211,212 or high capacities of over 1200 mA h g À1 . 213,214 However, most of these studies were conducted using coin cell congurations with limited active materials and excessive amounts of electrolytes, which created a huge gap between the achievements in the lab and the expectations from the real world. To approach practical LSBs, several key parameters, namely a low E/S ratio, limited lithium anode and high sulfur loadings, should be emphasized in future studies.…”

“…For Mg–S batteries, the polysulfide structures and concentration evolution in a glyme-based electrolyte were surveyed by in situ UV-vis. 214 A reaction pathway was proposed with S 8 , S 6 2− and S 4 2− being present in the electrolyte as shown in Fig. 13b.…”

“…(b) Schematic illustration of the proposed reaction process of the Mg-S battery based on the operational UV-vis spectroscopy and imaging results. Reproduced with permission: Copyright 2021, American Chemical Society 214. (c) In situ UV-vis spectrum when using the poly (ethylene oxide) electrolyte and using an ultra-high ion-conducting gel polymer electrolyte.…”

The Key Role of Magnesium Polysulfides in the Development of Mg-S Batteries

“…This is a common problem for magnesium batteries which use Mg metal anodes, including Mg-S batteries, and therefore Mg batteries employ unique electrolytes. Additionally, the solubility of MgS x (d) in liquid electrolyte is much lower than that of Li 2 S x . ,, Solution stability of MgS x (d) , anode passivation by sulfur species, cathode reaction kinetics, and reversibility are more substantial concerns for Mg-S systems than Li-S. , The charge/discharge reaction pathways and polysulfide speciation vary between these systems. − Therefore, it is crucial to understand the unique properties of MgS x for the development of highly reversible and stable Mg-S batteries.…”