Performance and Mechanistic Comparison of Mg-S and Li-S batteries

Kurzfassung

Sulfur is one of the most promising cathode materials due to its high theoretical capacity of 1672 mAh g-1, its low cost and environmental friendliness. In combination with a negative lithium electrode (Li-S) a cell voltage of 2.4 V and an energy density of 2800 Wh l-1 can be theoretically achieved. Yet the use of lithium metal suffers from safety and cost issues due to dendrite formation and limited world occurrence, respectively. In contrast magnesium features significant advantages as it is abundant and shows no dendrite formation during cycling, which ensures superior safety. Furthermore the combination of a negative magnesium electrode and a positive sulfur electrode (Mg-S) offers a higher theoretical energy density of 3200 Wh l-1 at a cell voltage of 1.77 V. Contrary to the Li-S technology the Mg-S system is still in a very early stage of research (first report by Kim et al. in 2011) and suffers from fast capacity decay in the first cycles and poor overall cycle stability. Especially the search for a suitable electrolyte emerged to be difficult as it inter alia has to be non-nucleophilic and capable of reversible Mg deposition/stripping. Very recently Zhao-Karger et al. published a Mg(BH4)2-derived electrolyte with enhanced cycling properties which allows further detailed investigation of Mg-S cells. As both systems rest upon similar electrochemical reactions they display many related properties but also significant differences in the underlying processes. Ensuring the comparability by an identical setup, Li-S and Mg-S cells are investigated by means of galvanostatic cycling and cyclic voltammetryto identify the origin of the fast degradation of magnesium-sulfur batteries.