In Situ X-ray Absorption Spectroscopic Investigation of the Capacity Degradation Mechanism in Mg/S Batteries

The Mg/S battery is attractive because of its high theoretical energy density and the abundance of Mg and S on the earth. However, its development is hindered by the lack of understanding to the underlying electrochemical reaction mechanism of its charge-discharge processes. Here, using a unique in situ X-ray absorption spectroscopic tool, we systematically study the reaction pathways of the Mg/S cells in Mg(HMDS)₂-AlCl₃ electrolyte. We find that the capacity degradation is mainly due to the formation of irreversible discharge products, such as MgS and Mg₃S₈, through a direct electrochemical deposition or a chemical disproportionation of intermediate polysulfide. In light of the fundamental understanding, we propose to use TiS₂ as a catalyst to activate the irreversible reaction of low-order MgS _x and MgS, which results in an increased discharging capacity up to 900 mAh·g^-1 and a longer cycling life.