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 1 H 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 , MgCl 2 ether-based electrolyte is affected at rather moderate concentration of polysulfides, thereby having an impact on the cell performance.

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

“…The socalled solvent-in-salt approach applying a high concentrated electrolyte is reasonable due to lower S 8 and MgS x solubility; 31 however, sulfur dissolution might still occur, leading to electrolyte salt precipitation. 15 Therefore, the most promising approach is the prevention of sulfur reduction at the anode by an artificial SEI as this is not detrimental for the sulfur redox reactions at the cathode and additionally suppresses ongoing SEI growth. To mitigate the active material loss, an additional ion-selective separator coating toward the cathode side is reasonable.…”

“…The same is true when considering lean-electrolyte conditions to have less sulfur present in the liquid phase, because solid-phase kinetics are rather sluggish. The so-called solvent-in-salt approach applying a high concentrated electrolyte is reasonable due to lower S 8 and MgS x solubility; however, sulfur dissolution might still occur, leading to electrolyte salt precipitation . Therefore, the most promising approach is the prevention of sulfur reduction at the anode by an artificial SEI as this is not detrimental for the sulfur redox reactions at the cathode and additionally suppresses ongoing SEI growth.…”

“…DrvaričTalian et al adopted this approach to synthesize distinct MgS x (x = 4, 6, 8) solutions and study their disproportionation and redox kinetics in situ. 15 Recently, an extensive ex situ study by Ford et al highlighted the importance of self-discharge and found it to be severe in all common magnesium electrolytes. 16 However, to the best of our knowledge, there is a lack of systematic operando studies of Mg−S cells to ascertain the cycling and self-discharge properties as ex situ measurements suffer from possible changes of the MgS x species and concentration.…”

“…Drvarič Talian et al. adopted this approach to synthesize distinct MgS x ( x = 4, 6, 8) solutions and study their disproportionation and redox kinetics in situ . Recently, an extensive ex situ study by Ford et al highlighted the importance of self-discharge and found it to be severe in all common magnesium electrolytes .…”

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

“…Meanwhile, the solubility of Li 2 S 8 (expressed in the form of atomic sulfur) in DOL/DME is 6.6 M, while the maximum reported concentration of MgS x (d) solution is 0.25 M (based on atomic sulfur). ,,− ,− Considering the self-discharge and the shuttling effect in sulfur batteries, one could hypothesize that the low solubility of MgS x (d) would be good for the battery performance if reaching the solubility limit prevented further dissolution of S 8 and loss of active material from the cathode. However, as discussed above, the self-discharge problem of Mg-S causes significant capacity decay even under static conditions due to fast sulfur reduction kinetics on the Mg anode surface …”