The challenge of developing rechargeable magnesium batteries
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Introduction Mobile devices play an increasingly important role in our everyday lives, from laptop computers and cellular phones to electric vehicles. A major impediment to further develop such devices is the quality of their power sources. These devices should possess high energy densities, operate over a wider temperature range, be user-friendly (low toxicity of components), exhibit prolonged cycle life, and, most importantly, have low manufacturing and handling costs. Lithium-ion batteries can be considered the current leading technology for energy storage and conversion. However, disadvantages of Li-based battery technology are high cost and safety concerns. Mg batteries may provide a safe and cheap alternative to Li-ion battery technology in various applications. Magnesium metal is an ideal anode material1 for rechargeable batteries. It possesses a low reduction potential of –2.37 V (H+/H2 scale, the lowest reduction potential after Li, –3 V), a high volumetric capacity of 3,833 mA h cm–3 (2,046 mA h cm–3 for Li), lower price, and high abundance (eighth most abundant element in the earth’s crust). However, there are many challenges with implementing Mg as a viable anode for rechargeable batteries, with the most challenging being
overcoming the formation of passivating layers. Magnesium is a highly reactive metal, and passivation layers are rapidly formed on the Mg surface when in contact with reducible species such as water, oxygen, and various organics. Unlike the layers formed on Li metal that conduct Li ions, the layers formed on Mg are truly passivating and completely block conduction of Mg ions. The formation of passivating layers on Mg limits the selection of a suitable electrolyte solution; the solvents and salts must be stable with Mg and create a passivation-free environment to allow reversible Mg deposition and stripping. The first suitable electrolyte for reversible Mg deposition/ stripping was reported in the 1920s.2 The electrolyte was based on Grignard reagents (R-MgX, R = C6H5 X = Br,Cl), whose reductive nature made them stable for Mg metal and made it passivation-layer free. Grignard solutions, however, were unsuitable for practical use due to their extremely low conductivity (on the scale of a few µS cm–1) and low anodic stability, which results in a narrow electrochemical window (
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