Insights into Interfacial Processes in Magnesium-Sulfur Batteries

Magnesium is among the elements considered for post-lithium battery cells due to its high volumetric capacity of 3833 mAh/cm3 and high reduction potential (-2.37 V vs. SHE). Furthermore, Mg offers advantages in case of safety and costs due to the lower tendency to form dendrites and the abundance compared to lithium metal, respectively. The combination with non-toxic and abundant sulfur in an electrochemical cell presents a promising candidate to cope the need of high-energy batteries for future applications like electric vehicles. Theoretically, a cell voltage of 1.77 V and an energy density of 3200 Wh l-1 can be achieved. However, the Mg-S system suffers from fast capacity decay in the first cycles and poor cycle life. Nevertheless, significant research progress is gained since the first report by Kim et al. in 2011 [1], especially in the development of suitable electrolytes. Recently, Zhao-Karger et al. synthesized a chloride-free Mg(BH4)2-derived electrolyte [2] with enhanced properties in terms of electrochemical and thermal stability as well as cycling performance. Still, despite sophisticated studies the mechanisms leading to fast cell degradation are not well understood. To gain further insights, electrochemical impedance spectroscopy (EIS) was applied to Mg/S cells as a function of open cell voltage (OCV), state of charge (SOC) and temperature. Thereby, a limited suitability of the magnesium metal reference electrode for EIS measurements was asserted due to surficial and geometrical aspects. Similar results were found by Tchitchekova et al. [3] and related to a combination of the strong cation-solvent interaction, a generated junction potential in the electrode vicinity and a high impedance of the Mg reference electrode. Latter was also observed in our studies, especially with increasing OCV time and proceeding discharge. To still reliably assign occurring processes in Mg/S cells to a specific electrode, symmetrical Mg/Mg and S/S cells were built and examined by means of the distribution of relaxation times (DRT). The gained results indicate that the magnesium electrode features surfaces layers, which cause severe overpotential and limited cycle life. In addition, the retention of polysulfides in the sulfur electrode is crucial as they not only cause loss of active material but also are incorporated in forming passivating layers on the magnesium anode