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MgTFSI 2 is the only ether-soluble "simple" magnesium salt. The poor electrochemical performance of Mg electrodes in its solutions hinders its practicality as a viable electrolyte for Mg batteries. MgTFSI 2 /DME solutions were demonstrated to dissolve large quantities of MgCl 2 and produce electrolyte solutions with superior performance, though the electrochemical performance, mainly in terms of reversibility, of MgTFSI 2 /MgCl 2 (DME) solutions cannot yet compete with that of organometallic based electrolyte solutions. We believe that the solutions' purity level governs the overall electrochemical performance, especially in solutions where a strong reductant (i.e Grignard reagent) is not present to act as an impurity scavenger. In this work, we alter the performance of the MgTFSI 2 /MgCl 2 (DME) solutions through chemical and electrochemical conditioning and demonstrate the effect on the solutions' electrochemical characteristics. We demonstrate relatively high reversible behavior of Mg deposition/dissolution with crystalline uniformity of the Mg deposits, complemented by a fully reversible intercalation/de-intercalation process of Mg ions into Mo 6 S 8 cathodes. We also investigated LiTFSI/MgCl 2 solutions which exhibited even higher reversibility than MgTFSI 2 /MgCl 2 (DME) solutions, which we attribute to the higher purity level available for the LiTFSI salt. Magnesium is a natural candidate anode material for "next generation" rechargeable batteries due to its high volumetric capacity (3833 mAh/cm 3 ), low reduction potential (−2.3 V) wide abundance, and low price.1 Rechargeable Mg battery research had been developing very slowly since the 1920's but had recently gained a big momentum. Magnesium battery systems will have great difficulties to outperform lithium systems In terms of energy and power density. However, they possess several properties that make them desirable, as they are expected to benefit from a cheaper price and lower hazard levels. One of the core issues developing rechargeable magnesium batteries is the formulation of electrolytic solutions that support reversible magnesium deposition. Other properties such as sufficient ionic conductivity, adequate magnesium ion concentration, and a wide electrochemical window are also mandatory. 15 years ago we synthesized electrolyte solutions that possessed most of the key features listed above. These electrolyte solutions were the product of a Lewis acid/base reaction in which R 2 Mg moieties such as Bu 2 Mg served as the base component, and RAlCl 2 species such as EtAlCl 2 served as the acid component.Hence, we could demonstrate a family of organometallic electrolyte solutions for rechargeable Mg batteries known as di-chloro complex solutions (DCC).2,3 Unfortunately, even the best DCC electrolyte solution does not possess the minimum requirement needed for next generation rechargeable magnesium batteries, e.g. wide electrochemical stability window (>2.2 V), chemical stability and safety. The use of aromatic ligands enabled to develop electrolyte solu...

Rechargeable magnesium batteries have lately received great attention for large-scale energy storage systems due to their high volumetric capacities, low materials cost, and safe characteristic. However, the bivalency of Mg(2+) ions has made it challenging to find cathode materials operating at high voltages with decent (de)intercalation kinetics. In an effort to overcome this challenge, we adopt an unconventional approach of engaging crystal water in the layered structure of Birnessite MnO2 because the crystal water can effectively screen electrostatic interactions between Mg(2+) ions and the host anions. The crucial role of the crystal water was revealed by directly visualizing its presence and dynamic rearrangement using scanning transmission electron microscopy (STEM). Moreover, the importance of lowering desolvation energy penalty at the cathode-electrolyte interface was elucidated by working with water containing nonaqueous electrolytes. In aqueous electrolytes, the decreased interfacial energy penalty by hydration of Mg(2+) allows Birnessite MnO2 to achieve a large reversible capacity (231.1 mAh g(-1)) at high operating voltage (2.8 V vs Mg/Mg(2+)) with excellent cycle life (62.5% retention after 10000 cycles), unveiling the importance of effective charge shielding in the host and facile Mg(2+) ions transfer through the cathode's interface.

Herein the first inorganic magnesium salt solution capable of highly reversible magnesium electrodeposition is presented. Synthesized by acid-base reaction of MgCl2 and Lewis acidic compounds such as AlCl3, this salt class demonstrates upwards of 99% Coulombic efficiency, deposition overpotential of <200 mV, and anodic stability of 3.1 V.

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