Magnesium borohydride pyridine derivatives as electrolytes for all-solid-state batteries†

All-solid-state batteries based on abundant elements may pave the way for safer and cheaper energy storage. Magnesium borohydride derivatives with neutral ligands are a new emerging class of solid-state Mg²⁺ ionic conductors, and here we report the discovery of two new pyridine derivatives, Mg(BH₄)₂·xN(CH)₅ (x = 2 or 3). Magnesium tetrahydridoborate tripyridine, Mg(BH₄)₂·3N(CH)₅, crystallizes in the monoclinic space group C2/c and is built from molecular units consisting of trigonal bipyramidal [Mg(N(CH)₅)₃(BH₄)₂] complexes, where the pyridine molecules are packed efficiently as a result of π–π stacking. Magnesium tetrahydridoborate dipyridine, Mg(BH₄)₂·2N(CH)₅, crystallizes in the monoclinic space group C2/c and is built from tetrahedral [Mg(N(CH)₅)₂(BH₄)₂] complexes. The highest ionic conductivity is observed for Mg(BH₄)₂·3N(CH)₅ with σ(Mg²⁺) = 7.2 × 10⁻⁵ S cm⁻¹ at 24 °C, increasing to (Mg²⁺) = 1.4 × 10⁻⁴ S cm⁻¹ at 49 °C. The low activation energy of 0.25 eV shows promise for low temperature conductivity. A low electronic conductivity of σ_e = 2.5 × 10⁻¹⁰ S cm⁻¹ was determined for Mg(BH₄)₂·3N(CH)₅ at 24 °C, providing a high ionic transport number of t_ion = 0.999997.