This progress stimulates and enables the exploration of novel cathode materials. In recent years, this voltage range had expanded from less than 2 V to ca. Hence, the major bottleneck in the development of Mg-ion batteries has been the lack of suitable electrolytes that can support reversible plating of Mg on the anode, yet provide a suitable voltage range for operating the cathode. This is one of the reasons for using highly reductive electrolyte formulations. Thin films of MgO can fully deactivate the Mg anode, as this oxide is neither electronically nor ionically conductive to a sufficient degree. With respect to the anode, highly stable magnesium oxide (MgO) can readily form from traces of moisture or some oxygen-containing solvents/electrolytes. Some of the challenges facing Mg-ion batteries are as follows. Metallic Mg anodes also present numerous other advantages, such as the high natural abundance of Mg and its largely explored reserves, its high volumetric (3833 mAh cm −3) and gravimetric (2205 mAh g −1) capacities, high safety and suitably low electrode potential (−2.4 V vs SHE) 3, 4, 5. Unlike metallic Li, Na or K, metallic Mg foils can be used as anodes due to the predominantly 2 smooth, fast and dendrite-free electrodeposition of Mg, and reduced fire hazards related to this metal. Rechargeable Mg-ion batteries are considered an attractive energy storage system for both mobile and stationary energy storage applications 1. Our results not only point to the important role of nanomaterials in the enhancement of the kinetics of conversion reactions but also suggest that nanostructuring should be used as an integral tool in the exploration of new cathodes for multivalent, i.e., (Mg, Ca, Al)-ion batteries. These materials clearly outperform bulk CuS, which is electrochemically active only at an elevated temperature of 50 ☌. In this context, we present the highly reversible insertion of Mg-ions into nanostructured conversion-type CuS cathode, delivering high capacities of 300 mAh g −1 at room temperature and high cyclic stability over 200 cycles at a current density of 0.1 A g −1 with a high coulombic efficiency of 99.9%. Nanostructuring the cathode materials offers an effective means of mitigating these challenges, due to the reduced diffusion length and higher surface areas. While significant progress has been achieved with magnesium electrolytes in recent years, the further development of Mg-ion batteries, however, is inherently limited by the lack of suitable cathode materials, mainly due to the slow diffusion of high-charge-density Mg-ions in the intercalation-type host structures and kinetic limitations of conversion-type cathodes that often causes poor cyclic stability. Rechargeable magnesium batteries are appealing as safe, low-cost systems with high-energy-density storage that employ predominantly dendrite-free magnesium metal as the anode.
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