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.

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.

The interface between Mg metal and electrolyte is a key factor affecting Mg battery performance. Switchable interfacial phenomena, involving apparent surface electrochemical inhibition under open-circuit voltage and reactivation upon electrochemical polarization, were investigated with various Mg electrolyte systems, under both electrochemically static and dynamic conditions. Most notably, it was found that such behavior appears to be unique for the Mg system, implying that correct control of the interface is of considerable practical concern in Mg batteries. This new challenge must be addressed in order to achieve high-energy and high-durability rechargeable Mg batteries.

Unlocking the full potential of rechargeable magnesium batteries has been partially hindered by the reliance on chloride-based complex systems.D espite the high anodic stability of these electrolytes,they are corrosive towardmetallic battery components,which reduce their practical electrochemical window. Following on our new design concept involving boron cluster anions,m onocarborane CB 11 H 12 À produced the first halogen-free,s imple-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(CB 11 H 12 ) 2 /tetraglyme (MMC/G4) electrolyte system permits standardized methods of high-voltage cathode testing that uses atypical coin cell. This achievement is aturning point in the researcha nd development of Mg electrolytes that has deep implications on realizing practical rechargeable Mg batteries.Currently,the prospect of attaining energy densities beyond those offered by current lithium-ion batteries is driving interest in rechargeable magnesium batteries.M gm etal offers high volumetric capacity (3833 mAh cm À3 vs. 2036 mAh cm À3 for Li metal) while being non-dendritic and abundant in the earth crust (fifth most abundant element). [1] Since Aurbach et al. demonstrated the first and only rechargeable Mg battery prototype,c hallenges toward realizing Mg batteries still remain. [2] These stem from the absence of practical electrolytes and high capacity/high voltage cathodes. Fori nstance,t he field demands electrolytes capable of operating at high voltages whilst being compatible with Mg metal and all other battery components. [3]

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Despite being considered a promising anode candidate for future battery technologies, the reactivity of Mg metal and its resultant passivation have challenged the development of electrolytes for rechargeable Mg batteries. In this Concept article, we shed light on critical past and current motivations, hurdles, and design strategies of electrolyte development for Mg batteries. Special focus is given to the most recent advancements; in particular, we elaborate on bottom‐up design strategies targeted to overcome the corrosion issue caused by current electrolyte systems. Salts containing the BH motif expanded the portfolio of Mg‐compatible electrolytes and are used as a platform to create a whole new promising family. Here, we explain the approach, challenges, and the path forward for ultimately creating Mg‐compatible, highly stable, and non‐corrosive Mg electrolytes.

The first example of a family of low melting temperature ionic liquids based on the highly stable and inert closo-boron cluster anions is demonstrated. This, combined with excellent physiochemical and electrochemical properties makes this new family of ionic liquids a serious contender towards overcoming key challenges of electrolytes in energy storage systems.