Cyclopropylamine Magnesium Borohydrides as Solid-State Electrolytes

Solid-state batteries can potentially provide higher energy and power densities than conventional lithium-ion batteries through the utilization of a solid electrolyte instead of a liquid electrolyte. Here we present three cyclopropylamine magnesium borohydride compounds as potential solid-state electrolytes: tricyclopropylamine magnesium borohydride, Mg(BH₄)₂·3(CH₂)₂CHNH₂, dicyclopropylamine magnesium borohydride, Mg(BH₄)₂·2(CH₂)₂CHNH₂, and monocyclopropylamine magnesium borohydride, Mg(BH₄)₂·(CH₂)₂CHNH₂. Additionally, two nanocomposites, Mg(BH₄)₂·<i>x</i>(CH₂)₂CHNH₂-Al₂O₃(50 wt %/18 vol %) (<i>x</i> = 1, 2), were investigated. Four crystal structures were determined, Mg(BH₄)₂·3(CH₂)₂CHNH₂, α-Mg(BH₄)₂·2(CH₂)₂CHNH₂, α'-Mg(BH₄)₂·2(CH₂)₂CHNH₂, β-Mg(BH₄)₂·2(CH₂)₂CHNH₂, and an average effective volume of cyclopropylamine in the crystal structures was determined to be ∼96 ų. The ionic conductivities of the compounds were determined, and the composite Mg(BH₄)₂·(CH₂)₂CHNH₂-Al₂O₃(50 wt %/18 vol %) has the highest value, σ = 1.8 × 10^-5 S cm^-1, at room temperature and an activation energy of 1.18 eV (114 kJ mol^-1). The ionic transference number was determined to <i>T</i>_ion = 0.99999. Additionally, the effects on the ionic conductivity associated with the addition of neutral ligands to metal borohydrides are discussed.