Efficient magnesium electrolytes are crucial in order to realize rechargeable magnesium batteries, which are an important contender for post-lithium-ion-battery technology.^1,2 Due to the divalent positive charge carried by the Mg²⁺ ion, magnesium mobility is difficult to achieve and so far Mg ion conductivity in inorganic solids has only been reported for few systems and typically, conductivities above 10^-5 Scm^-1 are only reached above 400 °C, see Figure 1.

In this work, we demonstrate that partially chelated ethylenediamine magnesium borohydride complexes show promise as solid-state Mg conductors with unprecedented high conductivity of up to 6x10^-5 Scm^-1already at 70 °C. The compounds are easily prepared by a mechanochemical reaction and allow for reversible Mg plating/stripping at 60 °C, which is recognized to be a major challenge in magnesium ion batteries.

While the electrochemical stability, limited by the ethylenediamine ligand, must be improved to reach competitive energy densities, our results demonstrate that partially chelated Mg²⁺complexes represent a promising platform for the development of an all-solid-state magnesium battery.

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