Mg₃(BH₄)₄(NH₂)₂ as Inorganic Solid Electrolyte with High Mg²⁺ Ionic Conductivity

Abstract

Mg3(BH4)4(NH2)2 compound was synthesized through the investigation of the Mg(BH4)2-Mg(NH2)2 phase diagram; its crystal structure was solved in a tetragonal unit cell with the space group I-4. Interestingly, Mg3(BH4)4(NH2)2 has a high thermal stability with a decomposition temperature above 190°C and exhibits a high Mg 2+ ionic conductivity of 4.1x10 -5 S.cm -1 at 100°C with a low activation energy (0.84 eV). The reversible Mg deposition/stripping was demonstrated at 100°C when using Mg3(BH4)4(NH2)2 as solid electrolyte. Thus, Mg3(BH4)4(NH2)2 is a compound that could help to develop rechargeable Mg-ion solid-state batteries.

Hydrides compounds for electrochemical applications

“…Pioneering works on Mg borohydride−amide Mg(BH4)(NH2) showed significant Mg conductivities of 10 −3 mScm −1 at 150°C [57] and oxidative stability above 3 V vs. Mg/Mg 2+ . Similar conductivity but at lower temperature, 100°C, have been later obtained in amorphous/crystalline composites of Mg borohydride−amide, showing the interest of defect structures for Mg-mobility [58], as well as for the crystalline compound Mg3(BH4)4(NH2)2 [59]. A complementary research direction is the association of Mg(BH4)2 to neutral molecules which undergo dynamic exchange with Mg ions and increase the structural flexibility [60].…”

Multivalent Ion Conduction in Inorganic Solids

“…Recently, the search for potential SSEs for MV ion batteries, with ionic conductivities on par with liquid electrolytes and the best performing superionic Li + conductors (≥10 –2 S cm –1 ), has reinvigorated research efforts to find novel electronically insulating solids that are capable of conducting MV ions at RT. To date, a limited number of materials with MV ion conductivity near ambient temperatures have been reported including the spinel MgSc 2 Se 4 , several derivations of Mg­(BH 4 )­(NH 2 ), − MgAl 2 X 8 (X = Cl, Br), and ZnPS 3 . The properties that enable ambient temperature MV conductivity in these materials will be dissected and discussed in detail later.…”

Fast Room-Temperature Mg2+ Conductivity in Mg(BH4)2·1.6NH3–Al2O3 Nanocomposites

“…Temperature-dependent Mg 2+ -ionic conductivities, σ (Mg 2+ ), of (a) samples M1–M6 and (b) sample Mg­(BH 4 ) 2 ·1.6NH 3 –Al 2 O 3 (M4, 67 wt %) (red line) compared to other hydride-based electrolytes. −,,,,− Mg­(BH 4 )­(NH 2 )_1# and Mg­(BH 4 )­(NH 2 )_2# refer to refs and , respectively.…”