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Abstract Improvements of magnesium ionic conducting materials are important for the development of novel magnesium solid-state batteries. Here we investigate the effect of nanoconfinement of one of the most conductive magnesium electrolytes − the (Mg(BH4)2·NH3) x (Mg(BH4)2·2NH3)1−x composite. The synthesis of nanoconfined Mg(BH4)2·1.47NH3 in a mesoporous silica scaffold (SBA-15, pore size 5.8 nm) is achieved through melt infiltration. Various degrees of pore filling are investigated, ranging from 100 to 300 %. Solid-state 11B nuclear magnetic resonance analysis confirms the successful stabilization of the highly dynamic eutectic molten state of Mg(BH4)2·1.47NH3 through nanoconfinement. The confined sample exhibits notable thermal stability, maintaining integrity up to approximately 100 °C, and the eutectic molten composite does not recrystallize within five months after synthesis, upon storing at room temperature. Notably, among the confined samples, 200 % pore filling displays promising Mg2+ ionic conductivity within the range of 9.1 × 10−6 to 2.7 × 10−4 S cm−1 and a low activation energy of 0.69 eV at temperatures from 32 to 80 °C. Furthermore, the compound was found to have a low electronic conductivity of 7.93 × 10−11 S cm−1 at 70 °C resulting in an ionic transport number close to unity.

Ammine metal borohydrides show large compositional and structural diversity, and have been proposed as candidates for solid-state ammonia and hydrogen storage as well as fast cationic conductors. Here, we report the synthesis method of ammine barium borohydrides, Ba(BH4)2·xNH3 (x = 1, 2). The two new compounds were investigated with time-resolved temperature-varied in situ synchrotron radiation powder X-ray diffraction, thermal analysis, infrared spectroscopy and photographic analysis. The compound Ba(BH4)2·2NH3 crystallizes in an orthorhombic unit cell with space group symmetry Pnc2, and is isostructural to Sr(BH4)2·2NH3, forming octahedral [Ba(NH3)2(BH4)4] complexes, which are connected into a two-dimensional layered structure, where the layers are interconnected by dihydrogen bonds, N–Hδ+⋯−δH–B. A new structure type is observed for Ba(BH4)2·NH3, which crystallizes in an orthorhombic unit cell with space group symmetry P212121, forming a three-dimensional framework structure of [Ba(NH3)(BH4)6] complexes. The structure is built from distorted hexagonal chains, where NH3 groups form dihydrogen bonds to the nearby BH4−-groups within the chain. Ba(BH4)2·2NH3 is unstable at room temperature and releases NH3 in two subsequent endothermic reactions with maxima at 49 and 117 °C, eventually reforming Ba(BH4)2. We demonstrate that the thermal stability and composition of the gas release for the ammine alkaline earth metal borohydrides can be correlated to the charge density of the metal cation, but are also influenced by other effects.

This paper describes the synthesis, crystal structure, and NH3 sorption properties of Mg1-xMnx(NH3)6Cl2 (x = 0–1) mixed metal halide ammines, with reversible NH3 storage capacity in the temperature range 20–350 °C. The stoichiometry (x) dependent NH3 desorption temperatures were monitored using in situ synchrotron radiation powder X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The thermal analyses reveal that the NH3 release temperatures decrease in the mixed metal halide ammines in comparison to pure Mg(NH3)6Cl2, approaching the values of Mn(NH3)6Cl2. Desorption occurs in three steps of four, one and one NH3 moles, with the corresponding activation energies of 54.8 kJ⋅mol-1, 73.2 kJ⋅mol-1 and 91.0 kJ⋅mol-1 in Mg0.5Mn0.5(NH3)6Cl2, which is significantly lower than the NH3 release activation energies of Mg(NH3)6Cl2 (Ea = 60.8 kJ⋅mol-1, 74.8 kJ⋅mol-1 and 91.8 kJ⋅mol-1). This work shows that Mg1-xMnx(NH3)yCl2 (x = 0 to 1, y = 0 to 6) is stable within the investigated temperature range (20–350 °C) and also upon NH3 cycling.