Sub-Ambient Fast Magnesium Ionic Conductivity in Diethylamine Magnesium Tetrahydridoborate and Its Crystal Structure

Magnesium tetrahydridoborate derivatives are promising electrolytes for next-generation solid-state batteries due to their high Mg2+ conductivity and stability towards magnesium metal. Here, we report the synthesis, crystal structure and thermal properties of mono-diethylamine magnesium tetrahydridoborate, Mg(BH4)2∙NH(CH2CH3)2, and the ionic conductivity of the composite Mg(BH4)2∙NH(CH2CH3)2−60wt% Al2O3. Synchrotron radiation powder X-ray diffraction data combined with ab initio structure determination and density functional theory reveal a monoclinic structure (P21/c) comprising one-dimensional zig-zag chains interconnected by dihydrogen bonds and dispersive interactions between organic moieties (–CH2CH3). The compound melts at 83 °C and begins to decompose at 116 °C. Owing to their soft and malleable nature, 60wt% Al2O3 nanoparticles (13 nm) were incorporated to provide mechanical stability, which stabilizes the soft electrolyte while preserving thermal stability. Electrochemical impedance spectroscopy of the composite Mg(BH4)2∙NH(CH2CH3)2−60wt% Al2O3 demonstrates exceptionally high ionic conductivity, reaching σ = 2.58 × 10−6 S cm−1 at −2 °C and crossing 10−4 S cm−1 below 40 °C with an activation energy of 695 ± 11 meV, representing one of the highest conductivities reported for a magnesium hydridoborate derivative. The conductivity decreases significantly when heated above 40 °C, suggesting that slow drying is occurring. This is supported by a decrease in conductivity upon long-term storage. These results highlight the potential of ligand-mediated structural flexibility in enabling highly conductive magnesium solid electrolytes for near room-temperature solid-state inorganic batteries.