First isolation of solvated MgCl⁺ species as the sole cations in electrolyte solutions for rechargeable Mg batteries

Synthesis of complex magnesium cations in ethereal solutions, is receiving a lot of attention due to their potential utilization in rechargeable magnesium batteries (RMB). The simplest complex cation, namely, solvated MgCl⁺, was hypothesized and reported as the most important cation in nonaqueous magnesium electrolyte solutions chemistry. However, such ions have never been isolated as the only cationic species in ethereal solutions developed for RMB. In this study, we report on successful isolation of the pure electrolyte MgCl(THF)₅⁺- PhAlCl₃, and on the electrochemical behavior of it in ethereal solutions. The structure of this compound was proved by single-crystal X-ray diffraction, Raman, and NMR spectroscopies. The novel MgCl(THF)₅PhAlCl₃/THF electrolyte solutions exhibit reversible Mg deposition/dissolution processes with anodic stability up to 2.7 V. The application of electrochemical cleaning pre-treatment for these solutions enabled to profoundly increase the coulombic efficiency of their Mg deposition/dissolution processes. Examining Mg prototype batteries with Mg foil anodes, composite Mo₆S₈ Chevrel phase (CP) cathodes containing these solutions exhibited a viable battery performance, indicating that upon a further optimization, electrolyte solutions based on the complex MgCl(THF)₅⁺- PhAlCl₃⁻electrolyte may be suitable for practical RMB.