Performance of a Mgs Battery Using Grignard Based Electrolytes

The lithium ion battery has revolutionized energy storage since its introduction to the commercial market in the early 90’s.  Despite its impact, it can be argued that the lithium ion battery is reaching its limit in terms of energy density.  In order to advance energy storage and move beyond lithium ion, new strategies must be considered such as a multivalent ion or a chemical conversion the cathode.  One such system is the magnesium sulfur system which utilizes a chemically safe earth abundant multivalent magnesium metal anode with a high capacity (1675 mA/g) earth abundant sulfur cathode.  One of the promises of using multivalent chemistry with sulfur cathodes is that the formation of polysulfides may be suppressed or eliminated as the multivalent sulfides compounds have low solubility in the solvent systems.  In the current study full cells were constructed using a magnesium metal anode, a coated sulfur cathode, utilizing the well characterized Grignard based electrolytes (EtBuAlCl₂)₂- (dichloro complex, DCC) and second generation Mg(AlCl_4-nPh_n)₂ (all phenyl complex, APC).  Full cells were subsequently analyzed with slow scanning cyclic voltammetry (SSCV), galvanostatic charge/discharge, and post mortem EDX.  SSCV results show electrochemical activity consistent with the formation of polysulfides for both  APC and DCC with slight variations in performance between the two.  Galvanostatic charge/discharge data shows an expected capacity loss between the first and second cycle, however an unexpected recovery of capacity as cycling continued with the capacity reaching 825 mAh/g at the 25^th cycle.  Full cell testing also demonstrated kinetic limitations as well as variations in performance between the two electrolytes consistent with SSCV results.  Postmortem EDX was on each cell component and showed no evidence of polysulfides on the anode surface, and possible trace amounts of sulfur in the separator.  EDX of the cathode showed evidence of a precipitated magnesium sulfur compound previously unreported.  The absence of sulfur on the anode and separator coupled with the precipitated magnesium sulfur compound on the cathode suggests that polysulfides precipitate out in a magnesium system rather than shuttle as they do in the lithium system, presenting a possible solution to the issues of polysulfide formation.