Self-discharge of magnesium–sulfur batteries leads to active material loss and poor shelf life?

Energy & Environmental Science Pub Date: 2021-01-12 DOI: 10.1039/D0EE01578D

Abstract

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. 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 magnesium polysulfide 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.

Graphical abstract: Self-discharge of magnesium–sulfur batteries leads to active material loss and poor shelf life
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