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.