Rational design of high concentration electrolytes and MXene-based sulfur host materials toward high-performance magnesium sulfur batteries

Rechargeable magnesium sulfur batteries (MSBs) assembled with earth-abundant and safe Mg anodes less prone to form dendrites on surface and sulfur-containing cathodes offer considerable potential for high energy densities. Nevertheless, suitable sulfur host materials and electrolytes are at present key factors that retard commercial introduction. Here, we explore preparation of Ti3C2@CoO composites as sulfur hosts with high concentration electrolytes consisting of Mg(TFSI)2 and AlCl3 in diglyme as new types of MSBs. Density functional theory (DFT) simulations indicate that CoO adsorbs magnesium polysulfides more strongly than Ti3C2 while Ti3C2 is more conductive to Mg2+ diffusion and transfer than CoO. Coincidentally, use of 1 M electrolyte restrains dissolution of magnesium polysulfides in electrolytes during battery cycling. The experiments indicate that combining Ti3C2@CoO sulfur hosts and 1 M Mg(TFSI)2/AlCl3/diglyme electrolyte provides viable MSBs, maintaining discharge capacities of 540 mAh g−1 after 70 cycles at 100 mA g−1. Mechanistic studies indicate that these MSBs work via a S/MgSx (x = 2–8) redox process although oxidation of polysulfide to sulfur is not efficient in initial cycling. The systems studied here may offer valuable insights and inspiration in the design of appropriate electrolytes and sulfur-host materials for MSBs.