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·NH₃. Density functional theory calculations (DFT) reveal that the neutral molecule (NH₃) in Mg(BH₄)₂·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^δ+⋯^−δH–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. σ(Mg²⁺) = 3.3 × 10⁻⁴ S cm⁻¹ at T = 80 °C for Mg(BH₄)₂·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.

A new hemiammine borohydride, LiBH₄·1/2NH₃, has been presented, which exhibits fast Li-ion conduction promoted by ammonia.