Fast magnesium ion conducting isopropylamine magnesium borohydride enhanced by hydrophobic interactions

Abstract

New materials for the next generation of electrochemical energy storage devices such as batteries are of extreme importance. Here we investigate the structure, ionic conductivity and thermal properties of isopropylamine magnesium borohydride based composites with different compositions, Mg(BH₄)₂·x(CH₃)₂CHNH₂, x = 0.5, 0.9, 1.25, 1.5, 1.75, 2.5, 3.1. Three new compounds are discovered, x = 1, 2, and 3 and the monoclinic structure of Mg(BH₄)₂·2(CH₃)₂CHNH₂ (P2₁/c) is investigated in detail. This structure consists of neutral complexes [Mg(BH₄)₂((CH₃)₂CHNH₂)₂] di-hydrogen bonded to form layers and these layers are connected by hydrophobic interactions via the isopropyl moieties. The orthorhombic unit cell of Mg(BH₄)₂·(CH₃)₂CHNH₂ was also determined, a = 9.78, b = 12.17 and c = 17.24 Å. In general, the samples are thermally stable up to 50 °C where they started to become softer, and at 70 °C isopropylamine release and melting started. The highest Mg²⁺ ionic conductivity was that of Mg(BH₄)₂·1.5(CH₃)₂CHNH₂, σ(Mg²⁺) = 2.7 × 10⁻⁴ S cm⁻¹ at 45 °C, with an activation energy of E_A = 1.22 eV. Furthermore, reversible stripping/plating of Mg was displayed at 45 °C, with an oxidative stability of 1.2 V vs. Mg/Mg²⁺. The addition of MgO nanoparticles (75 wt%) improves the mechanical and thermal stability, and decreases the activation energy, to E_A = 0.56 eV. Thereby the Mg²⁺ conductivity is increased at low temperature. This suggests that the hydrophobic interactions contribute to the high ionic conductivity in the solid state, which opens a new avenue for design and discovery of electrolyte materials.