Synthesis, Structure and Mg2+ Ionic Conductivity of Isopropylamine Magnesium Borohydride

The discovery of new inorganic magnesium electrolytes may act as a foundation for the rational design of novel types of solid-state batteries. Here we investigated a new type of organic-inorganic metal hydride, isopropylamine magnesium borohydride, Mg(BH4)(2)center dot(CH3)(2)CHNH2, with hydrophobic domains in the solid state, which appear to promote fast Mg2+ ionic conductivity. A new synthetic strategy was designed by combination of solvent-based methods and mechanochemistry. The orthorhombic structure of Mg(BH4)(2)center dot(CH3)(2)CHNH2 was solved ab initio by the Rietveld refinement of synchrotron X-ray powder diffraction data and density functional theory (DFT) structural optimization in space group I2(1)2(1)2(1) (unit cell, a = 9.8019(1) angstrom, b = 12.1799(2) angstrom and c = 17.3386(2) angstrom). The DFT calculations reveal that the three-dimensional structure may be stabilized by weak dispersive interactions between apolar moieties and that these may be disordered. Nanoparticles and heat treatment (at T > 56 degrees C) produce a highly conductive composite, sigma(Mg2+) = 2.86 x 10(-7), and 2.85 x 10(-5) S cm(-1) at -10 and 40 degrees C, respectively, with a low activation energy, E-a = 0.65 eV. Nanoparticles stabilize the partially eutectic molten state and prevent recrystallization even at low temperatures and provide a high mechanical stability of the composite.

Automated summary

The discovery of new inorganic magnesium electrolytes may lead to the development of novel solid-state batteries through the investigation of a new type of organic-inorganic metal hydride with hydrophobic domains. The structure of this metal hydride was determined and its properties were optimized through various methods. Nanoparticles and heat treatment were found to enhance the conductivity and stability of the composite material.