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.

Secondary magnesium batteries are still in the research stage, after the first prototype of a magnesium-based battery was demonstrated almost two decades ago. Since this breakthrough, despite tremendous efforts by numerous research groups, we are not aware of any system that exhibits better performance in terms of Coulombic efficiency over prolonged cycling. The scientific community is now focusing on the basic phenomena that hinder development of practical magnesium-based rechargeable batteries. Today, we have a better understanding of the structure of electrolyte solutions relevant to rechargeable Mg batteries and its effect on the electrochemical performance. New electrolyte solutions that are not based on organometallic moieties currently surpass the performance of the first generations of complex solutions. There is even an attempt to test alternative anode materials for magnesium-based energy storage systems. In this review, we summarize recent studies conducted in the field, with a focus on the anode/electrolyte solutions side.

Abstract

Mg­(N­(SO2CF3)2)2 (MgTFSI2) solutions with dimethoxyethane (DME) exhibit a peculiar behavior. Over a certain range of salt content, they form two immiscible phases of specific electrolyte concentrations. This behavior is unique, as both immiscible phases comprise the same constituents. Thus, this miscibility gap constitutes an exceptionally intriguing and interesting case for the study of such phenomena. We studied these systems from solutions structure perspective. The study included a wide variety of analytical tools including single-crystal X-ray diffraction, multinuclei NMR, and Raman spectroscopy coupled with density functional theory calculations. We rigorously determined the structure of the MgTFSI2/DME solutions and developed a plausible theory to explain the two-phase formation phenomenon. We also determined the exchange energy of the “caging” DME molecules solvating the central magnesium ion. Additionally, by measuring the ions’ diffusion coefficients, we suggest that the caged Mg2+ and TFSI– move as free ions in the solution. Knowledge of the arrangement of the solvent/cation/anion structures in these solutions enables us to explain their properties. We believe that this study is important in a wide context of solutions chemistry and nonaqueous electrochemistry. Also, MgTFSI2/DME solutions are investigated as promising electrolyte solutions for rechargeable magnesium batteries. This study may serve as an important basis for developing further MgTFSI2/ether based solutions for such an interesting use.

Recently, MgTFSI2/MgCl2 electrolyte solutions in dimethoxyethane (DME) have been shown to function as viable electrolyte solutions for secondary Mg batteries that can facilitate reversible magnesium deposition/dissolution. MgCl2 is a crucial component in these solutions. On its own, however, it is practically insoluble in DME. Therefore, the fact that it is readily dissolved in MgTFSI2/DME solution is remarkable. Addition of MgCl2 greatly improves the electrochemical performance of MgTFSI2/DME electrolyte solutions. Thus, identifying the species formed in MgTFSI2/MgCl2 solutions is intriguing. In this study, we implemented a wide variety of analytical tools, including single crystal X-ray diffraction, multinuclear NMR, and Raman spectroscopy, to elucidate the structure of these solutions. Various solution species were determined, and a suitable reaction scheme is suggested.

The kinetics and thermodynamics of oxygen reduction reactions (ORR) in aprotic Li electrolyte were shown to be highly dependent on the surrounding chemical environment and electrochemical conditions. Numerous reports have demonstrated the importance of high donor number (DN) solvents for enhanced ORR, and attributed this phenomenon to the stabilizing interactions between the reduced oxygen species and the solvent molecules. We focus herein on the often overlooked effect of the Li salt used in the electrolyte solution. We show that the level of dissociation of the salt used plays a significant role in the ORR, even as important as the effect of the solvent DN. We clearly show that the salt used dictates the kinetics and thermodynamic of the ORR, and also enables control of the reduced Li2O2 morphology. By optimizing the salt composition, we have managed to demonstrate a superior ORR behavior in diglyme solutions, even when compared to the high DN DMSO solutions. Our work paves the way for optimization of various solvents with reasonable anodic and cathodic stabilities, which have so far been overlooked due to their relatively low DN.

Rechargeable metal−sulfur batteries (RMSBs) represent one of the most attractive electrochemical systems in terms of energy density and cost. In most of the proposed systems, the anode side is metallic and the cathode side is elemental sulfur impregnated in a porous matrix. Despite the relatively low voltage of these systems, they attract a lot of attention and are considered to be very promising as nextgeneration batteries for the following reasons: (1) utilization of active metal anodes enables a leap in specific energy due to the high capacity of metal anodes in comparison to intercalation compounds, (2) sulfur as a cathode exhibits high theoretical specific capacity (1675 mAh/g), and (3) system components make RMSBs low-cost, less toxic batteries. Nevertheless, the high reactivity of metallic anodes (e.g., Li, Na, Mg, and Al) and the solubility of sulfur species in the electrolyte render these batteries unstable and hinder their practical realization. In this Perspective, we focus on rechargeable sulfur batteries with active metal anodes, present important studies conducted in this field, and summarize the reported methods and techniques that are mandatory for effective and practical studies of RMSB.