Fast Room-Temperature Mg2+ Conductivity in Mg(BH4)2·1.6NH3–Al2O3 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)2·1.6NH3–Al2O3, reaching a high magnesium conductivity of σ­(Mg2+) = 2.5 × 10–5 S cm–1 at 22 °C assigned to favorable interfaces between amorphous state Mg­(BH4)2·1.6NH3; inert and insulating Al2O3 nanoparticles; and a minor fraction of crystalline material, mainly Mg­(BH4)2·2NH3. 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 ∼1.2 V. The nanocomposite has high mechanical stability and ductility and is a promising Mg2+ electrolyte for future solid-state magnesium batteries.