Beyond hydrogen storage: The first example of reversible magnesium deposition/stripping onto/from an inorganic salt was seen for a magnesium borohydride electrolyte. High coulombic efficiency of up to 94 % was achieved in dimethoxyethane solvent. This Mg(BH4)2 electrolyte was utilized in a rechargeable magnesium battery.

Unlocking the full potential of rechargeable magnesium batteries has been partially hindered by the reliance on chloride-based complex systems. Despite the high anodic stability of these electrolytes, they are corrosive toward metallic battery components, which reduce their practical electrochemical window. Following on our new design concept involving boron cluster anions, monocarborane CB11H12(-) produced the first halogen-free, simple-type Mg salt that is compatible with Mg metal and displays an oxidative stability surpassing that of ether solvents. Owing to its inertness and non-corrosive nature, the Mg(CB11H12)2/tetraglyme (MMC/G4) electrolyte system permits standardized methods of high-voltage cathode testing that uses a typical coin cell. This achievement is a turning point in the research and development of Mg electrolytes that has deep implications on realizing practical rechargeable Mg batteries.

Summary “...each metal has a certain power, which is different from metal to metal, of setting the electric fluid in motion...” Count Alessandro Volta. Inspired by the first rechargeable magnesium battery prototype at the dawn of the 21st century, several research groups have embarked on a quest to realize its full potential. Despite the technical accomplishments made thus far, challenges, on the material level, hamper the realization of a practical rechargeable magnesium battery. These are marked by the absence of practical cathodes, appropriate electrolytes and extremely sluggish reaction kinetics. Over the past few years, an increased interest in this technology has resulted in new promising materials and innovative approaches aiming to overcome the existing hurdles. Nonetheless, the current challenges call for further dedicated research efforts encompassing fundamental understanding of the core components and how they interact with each other to offering new innovative solutions. In this review, we seek to highlight the most recent developments made and offer our perspectives on how to overcome some of the remaining challenges.

Lithium Ion Battery: Highest growth & major part of the investments Lead acid batteries: By far the most important market (90% market share) 6

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Magnesium batteries have long been pursued as potentially high-energy and safe alternatives to Li-ion batteries; however, fast chargedischarge capability, one of the most desired properties for advanced batteries, remains elusive for this technology. Here, we develop a next generation Mg battery prototype, delivering a specific energy of up to 566 Wh kg 1 and an ultrahigh specific power of up to 30.4 kW kg 1 , which is close to two orders of magnitude higher than state-of-the-art Mg battery. This is achieved by coupling a kinetically fast organic cathode material, operating under bond cleavage-free solidliquid reaction, and an electrolyte capable of providing dendrite-free Mg depositionstripping at a record current density of 20 mA cm 2 .One Sentence Summary: Ultrahigh power, an unprecedented quality of Mg battery, is unveiled and demonstrated using a prototype combining a quinone-based cathode and a second to none Mg(CB 11 H 12 ) 2 electrolyte that enables ultrahigh rate cycling of dendrite-free Mg anode.

In typical chloride-containing electrolytes, storage of MgCl + is dominant in organic cathodes. The negative impact of the MgCl-storage chemistry on the specific energy was elucidated through cell tests with controlled amounts of electrolyte. With the right combination of organic cathodes and chloride-free electrolytes, storage of Mg 2+ in organic electrodes can be realized. The Mg-storage chemistry has also enabled the first Mg battery that operates under lean electrolyte conditions, which has important implications for the practicality of high-energy organic Mg batteries.

Boron clusters are proposed as a new concept for the design of magnesium-battery electrolytes that are magnesium-battery-compatible, highly stable, and noncorrosive. A novel carborane-based electrolyte incorporating an unprecedented magnesium-centered complex anion is reported and shown to perform well as a magnesium-battery electrolyte. This finding opens a new approach towards the design of electrolytes whose likelihood of meeting the challenging design targets for magnesium-battery electrolytes is very high.