Stability Studies of Fluorinated Alkoxide Electrolytes for Multivalent Batteries

Abstract

Multivalent batteries have the potential to attain high volumetric energy capacities without the safety, cost, or availability barriers which still cast a shadow on lithium-ion battery technology. In recent years, a number of electrolytes have been developed which enable reversible electrochemical processes to take place in Mg-ion batteries (MIBs) and even Ca-ion batteries (CIBs). These include a promising family of electrolytes derived from fluorinated alkoxides, such as the MIB electrolyte Mg[Al(HFIP)4]2 in dimethoxyethane (DME) and its CIB analogue, Ca[Al(HFIP)4]2 in DME. The physical and electrochemical properties of Mg[Al(HFIP)4]2 and Ca[Al(HFIP)4]2 in DME, in their pristine form, have previously been described and are characterized briefly in this work. However, the susceptibility of these electrolytes in a non-pristine state, after having undergone side reactions or decomposition, has never been examined. The degradation mechanisms of Li-ion battery electrolytes have been thoroughly investigated; such degradation severely impacts electrolyte properties and thus battery performance. Now that viable multivalent battery electrolytes are available, it is essential that they be studied from this perspective as well. In this work, stability studies are under- taken on Mg[Al(HFIP)4]2 and Ca[Al(HFIP)4]2 in order to determine the sensitivity of these electrolytes to storage conditions and contaminants, and to begin elucidating degradation mechanisms that could result. The effects of thermal and water-driven decomposition on performance are characterized using a combination of 1H, 19F, and 27Al nuclear magnetic resonance (NMR) spectroscopy and electrodeposition experiments in half-cells. The results show that these electrolytes are highly susceptible to degradation as a consequence of extended thermal exposure and water contamination; these events irreversibly disrupt the heterobimetallic alkoxides which give rise to the excellent electrochemical properties of the pristine electrolytes.