The mechanism of Mg2+ conduction in ammine magnesium borohydride promoted by a neutral molecule

Abstract

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 monoammine magnesium borohydride, Mg(BH4)(2)center dot NH3. Density functional theory calculations (DFT) reveal that the neutral molecule (NH3) in Mg(BH4)(2)center dot NH3 is exchanged between the lattice and interstitial Mg2+ facilitated by a highly flexible structure, mainly owing to a network of di-hydrogen bonds, N-H delta+center dot center dot center dot(-)delta H-B and the versatile coordination of the BH4- ligand. DFT shows that di-hydrogen bonds in inorganic matter and hydrogen bonds in bio-materials have similar bond strengths and bond lengths. As a result of the high structural flexibiliy, the Mg-ion conductivity is dramatically improved at moderate temperature, e.g. sigma(Mg2+) = 3.3 x 10(-4) S cm(-1) at T = 80 degrees C for Mg(BH4)(2)center dot NH3, which is approximately 8 orders of magnitude higher than that of Mg(BH4)(2). Our results may inspire a new approach for the design and discovery of unprecedented multivalent ion conductors.