Fast Room-Temperature Mg²⁺ Conductivity in Mg(BH₄)₂?1.6NH₃-Al₂O_{3 Nanocomposites}

Design of new functional materials with fast Mg-ion mobility is crucial for the development of competitive solid-state magnesium batteries. Herein, we present new nanocomposites, Mg(BH4)(2)center dot 1.6NH(3)-Al2O3, reaching a high magnesium conductivity of sigma(Mg2+) = 2.5 x 10(-5) S cm(-1) at 22 degrees C assigned to favorable interfaces between amorphous state Mg(BH4)(2)center dot 1.6NH(3); inert and insulating Al2O3 nanoparticles; and a minor fraction of crystalline material, mainly Mg(BH4)(2)center dot 2NH(3). Furthermore, quasi-elastic neutron scattering reveals that the Mg2+-ion mobility in the solid state appears to be correlated to relatively slow motion of NH3 molecules rather than the fast dynamics of BH4-complexes. The nanocomposite is compatible with a metallic Mg anode and shows stable Mg2+ stripping/plating in a symmetric cell and an electrochemical stability of similar to 1.2 V. The nanocomposite has high mechanical stability and ductility and is a promising Mg2+ electrolyte for future solid-state magnesium batteries.