Methylamine Magnesium Borohydrides as Electrolytes for All-Solid-State Magnesium Batteries

Solid-state magnesium electrolytes may pave the way for novel types of rechargeable, sustainable, and cheap batteries with high volumetric and gravimetric capacities. There are, however, currently no solid-state magnesium electrolytes that fulfill the requirements for solid-state battery applications. Here, we present the synthesis, structure, and properties of six new methylamine magnesium borohydride compounds, α- and β-Mg(BH4)2·6CH3NH2, Mg(BH4)2·3CH3NH2, and α-, α′- and β-Mg(BH4)2·CH3NH2. The β-Mg(BH4)2·CH3NH2 polymorph displays a record high Mg2+ ionic conductivity of σ(Mg2+) = 1.50 × 10–4 S cm–1 at room temperature. The high Mg2+ conductivity of β-Mg(BH4)·CH3NH2 is facilitated by a one-dimensional chain-like structure interconnected by weak dihydrogen bonds and dispersion interactions, forming a migration pathway across the chains. The oxidative stability of Mg(BH4)2·CH3NH2 is ∼1.2 V vs Mg/Mg2+, and the reversible plating and stripping were confirmed by cyclic voltammetry and symmetric cell cycling, revealing high stability toward magnesium electrodes for at least 50 cycles at 60 °C.