Mg(PF₆)₂-Based Electrolyte Systems: Understanding Electrolyte-Electrode Interactions for the Development of Mg-Ion Batteries

Mg(PF₆)₂-based electrolytes for Mg-ion batteries have not received the same attention as the analogous LiPF₆-based electrolytes used in most Li-ion cells owing to the perception that the PF₆-anion decomposes on and passivates Mg electrodes. No synthesis of the Mg(PF₆)₂ salt has been reported, nor have its solutions been studied electrochemically. Here, we report the synthesis of the complex Mg(PF₆)₂(CH₃CN)₆ and its solution-state electrochemistry. Solutions of Mg(PF₆)₂(CH₃CN)₆ in CH₃CN and CH₃CN/THF mixtures exhibit high conductivities (up to 28 mS·cm‾¹) and electrochemical stability up to at least 4 V vs Mg on Al electrodes. Contrary to established perceptions, Mg electrodes are observed to remain electrochemically active when cycled in the presence of these Mg(PF₆)₂-based electrolytes, with no fluoride (i.e., MgF₂) formed on the Mg surface. Stainless steel electrodes are found to corrode when cycled in the presence of Mg(PF₆)₂ solutions, but Al electrodes are passivated. The electrolytes have been used in a prototype Mg battery with a Mg anode and Chevrel (Mo₃S₄)-phase cathode.E.N.K. thanks NSERC for a PGS D as well as the Cambridge Commonwealth, European, and International Trust and Gonville and Caius College for funding. This work was supported by the EPSRC Cambridge NanoDTC, EP/G037221/1