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·x(CH2)2CHNH2-Al2O3(50 wt %/18 vol %) (x = 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 Å3. 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 Tion = 0.99999. Additionally, the effects on the ionic conductivity associated with the addition of neutral ligands to metal borohydrides are discussed.