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• First ever synthesis and isolation of MgCl+ cations containing Mg electrolytes. • Novel MgCl(THF) 5 PhAlCl 3 /THF electrolyte show reversible Mg deposition/dissolution. • Electrochemical "conditioning" increases the cycling efficiency to around 95%. • Investigation of electrolyte solution with Mg prototype full cell configuration. 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 +- PhAlCl 3 , 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) 5 PhAlCl 3 /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 Mo 6 S 8 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 +- PhAlCl 3 −electrolyte may be suitable for practical RMB. [Display omitted] [ABSTRACT FROM AUTHOR]

• We explore here the products of the reaction of the Grignard reagent t -BuMgCl with AlCl 3 (1:1) in THF, and how their presence in solutions affect their electrochemical behavior. • Two complex products were isolated from the precipitated part of the reaction and the counter solution part, which exhibit two distinct single-crystal structures. • Crystallization from precipitated part (compound I) has the molecular structure [MgCl·THF 5 ]+ [AlCl 4 ]– and it is electrochemically inactive in terms of reversible Mg dissolution/deposition. • The molecular structure of the second compound (II) , obtained from solution part is [Mg 2 (μ-Cl) 3 ·THF 6 ]+ [ t -BuAlCl 3 ]- and its solutions in THF exhibit reversible Mg dissolution/deposition behavior. • This study demonstrates the possibility of tuning judiciously the electrochemical response of ethereal solutions of magnesium organohaloaluminate complexes through their structural modifications. The electrochemical response of ethereal solutions containing magnesium organohaloaluminate complexes has drawn great interest in recent decades owing to their relevance to rechargeable magnesium batteries, as demonstrated with solutions containing complexes formed by reacting R 2 Mg and AlCl 2 R moieties in ethers like tetrahydrofuran (THF). However, most of previous reports focused on battery related performances, and less on the structure of the active species. Herein, we focus on (1) identifying electroactive species and (2) correlating the electrochemical properties of their solutions to the preparation modes: either through reactions of their precursors in THF, or by dissolving isolated crystallized products in the ether solvent. Specifically, we explore the products of the reaction of the Grignard reagent t -BuMgCl with AlCl 3 (1:1) in THF, and how their presence in solutions affect their electrochemical behavior. We isolated two complex products, one non-soluble and one soluble in THF, which exhibit two distinct single-crystal structures. The molecular structure of the two complexes obtained from the single-crystal X-ray diffraction (SCXRD) and nuclear magnetic resonance (NMR) measurements was correlated with the electrochemical behavior of their solutions in THF, as reflected by their cyclic voltammetry (CV) response. We found that one product (compound I) has the molecular structure [MgCl·THF 5 ]+ [AlCl 4 ]– and it is electrochemically inactive in terms of reversible Mg dissolution/deposition. The molecular structure of the second compound (II), [Mg 2 (μ-Cl) 3 ·THF 6 ]+ [ t -BuAlCl 3 ]–, has not been reported before, and its solutions in THF exhibit reversible Mg dissolution/deposition behavior. Our study clearly demonstrates the possibility of tuning judiciously the electrochemical response of ethereal solutions of magnesium organohaloaluminate complexes through their structural modifications. [Display omitted] [ABSTRACT FROM AUTHOR]