Self Discharge of Magnesium-Sulfur Batteries Leads to Active Material Loss and Poor Shelf Life

Due to its high theoretical energy density and relative abundancy of active materials, the magnesium-sulfur battery has attracted research attention in recent years. A closely related system, the lithium-sulfur battery, can suffer from serious self-discharge behavior. Until now, the self-discharge of Mg-S has been rarely addressed, and even then only indirectly. Herein, we demonstrate for a wide variety of Mg-S electrolytes and conditions that Mg-S batteries also suffer from serious self-discharge. For a common Mg-S electrolyte, we identify a multi-step self-discharge pathway. Covalent S8 diffuses to the metal Mg anode and is converted to ionic Mg polysulfide in a non-faradaic reaction. Mg polysulfides in solution are found to be meta-stable, continuing to react and precipitate as solid MgySx species during both storage and active use. Mg-S electrolytes from the early, middle, and state-of-the-art stages of the Mg-S literature are all found to enable the self-discharge. The self discharge behavior is found to decrease first cycle discharge capacity by at least 32 %, and in some cases up to 96 %, indicating this is a phenomenon of the Mg-S chemistry that deserves focused attention.