Mg(PF_6)_2-Based Electrolyte Systems: Understanding Electrolyte-Electrode Interactions for the Development of Mg-Ion Batteries

摘要

Mg(PF_6)_2-based electrolytes for Mg-ion batteries have not received the same attention as the analogous LiPF_6-based electrolytes used in most Li-ion cells owing to the perception that the PF_6~- anion decomposes on and passivates Mg electrodes. No synthesis of the Mg(PF_6)_2 salt has been reported, nor have its solutions been studied electrochemically. Here, we report the synthesis of the complex Mg(PF_6)_2(CH_3CN)_6 and its solution-state electrochemistry. Solutions of Mg(PF_6)_2-(CH_3CN)_6 in CH_3CN and CH_3CN/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(PF_6)_2-based electrolytes, with no fluoride (i.e., MgF_2) formed on the Mg surface. Stainless steel electrodes are found to corrode when cycled in the presence of Mg(PF_6)_2 solutions, but Al electrodes are passivated. The electrolytes have been used in a prototype Mg battery with a Mg anode and Chevrel (Mo_3S_4)-phase cathode.