Fast Room-Temperature Mg 2+ Conductivity in Mg(BH 4 ) 2 ·1.6NH 3 -Al 2 O 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(BH 4 ) 2 ·1.6NH 3 -Al 2 O 3 , reaching a high magnesium conductivity of σ(Mg 2+ ) = 2.5 × 10 -5 S cm -1 at 22 °C assigned to favorable interfaces between amorphous state Mg(BH 4 ) 2 ·1.6NH 3 ; inert and insulating Al 2 O 3 nanoparticles; and a minor fraction of crystalline material, mainly Mg(BH 4 ) 2 ·2NH 3 . Furthermore, quasi-elastic neutron scattering reveals that the Mg 2+ -ion mobility in the solid state appears to be correlated to relatively slow motion of NH 3 molecules rather than the fast dynamics of BH 4 - complexes. The nanocomposite is compatible with a metallic Mg anode and shows stable Mg 2+ stripping/plating in a symmetric cell and an electrochemical stability of ∼1.2 V. The nanocomposite has high mechanical stability and ductility and is a promising Mg 2+ electrolyte for future solid-state magnesium batteries.