The Mechanism of Mg2+ Conductivity in Ammine Magnesium Borohydride Promoted by a Neutral Molecule
Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery. Here we present the mechanism of Mg-ion conductivity of ammine magnesium borohydrides, Mg(BH4)2·xNH3 (x = 1, 2, 3, 6). Density functional theory calculations (DFT) reveal that the neutral molecule (NH3) in Mg(BH4)2·xNH3 is exchanged between the lattice and interstitial Mg2+ facilitated by a highly flexible structure, mainly owing to a network of di-hydrogen bonds, N-Hδ+...-δH-B and the versatile coordination of the BH4- ligand. As a result, the Mg-ion conductivity is dramastically improved at moderate temperature, e.g. σ(Mg2+) = 3.3×10–4 S cm–1 at T = 80 °C for Mg(BH4)2·NH3, which is approximately 8 orders of magtitude higher than that of Mg(BH4)2. Our results may inspire a new approach for design and discovery of unprecedented multivalent ion conductors.