Magnesium batteries are considered promising solutions for future energy storage beyond the lithium-ion battery era. However, the development of magnesium batteries is hindered by the lack of suitable electrolytes. Here we present solid Mg 2+ electrolytes based on ammine magnesium borohydride composites, Mg(BH 4 ) 2 •xNH 3 , which have conductivities ca. three orders of magnitude higher than the parent compounds (x = 1, 2, 3, and 6). A nanocomposite formed by the Mg(BH 4 ) 2 •xNH 3 composite and MgO nanoparticles exhibits outstanding Mg 2+ conductivity of the order of 10 −5 S cm −1 at room temperature and around 10 −3 S cm −1 at moderate temperature (ca. 70 °C), with an activation energy for Mg 2+ conduction of E a ∼108 kJ/mol (1.12 eV) and high thermal stability (T dec = 120 °C). Characterization using solid-state nuclear magnetic resonance, powder X-ray diffraction, and transmission electron microscopy reveals that the high Mg 2+ conductivity is attributed to amorphization of Mg(BH 4 ) 2 •xNH 3 resulting in a highly dynamic state. This nanocomposite is compatible with a Mg metal anode and allows stable Mg plating/stripping (at least 100 cycles) in a symmetric cell. The results represent a major advancement of solid-state multivalent ion conductors here demonstrated for Mg 2+ .

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.

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

Synthesis, crystal structures, thermal and magnetic properties of the complete series of halide-free rare-earth (RE) metal borohydrides are presented. A new synthesis method provides high yield and high purity products. Fifteen new metal borohydride structures are reported. The trends in crystal structures, thermal behaviour, and magnetic properties for the entire series of RE(BH 4 ) x are compared and discussed. The RE(BH 4 ) x possess a very rich crystal chemistry, dependent on the oxidation state and the ionic size of the rare-earth ion. Due to the lanthanide contraction, there is a significant decrease in the volume of the RE 3+ -ion with increasing atomic number, which correlates linearly with the unit cell volume of the α-and β-RE(BH 4 ) 3 polymorphs and the solvated complexes α-RE(BH 4 ) 3 •S(CH 3 ) 2 . The thermal analysis reveals a onestep decomposition pathway in the temperature range from 247 to 277 °C for all RE(BH 4 ) 3 , except Lu(BH 4 ) 3 , which follows a three-step decomposition pathway. In contrast, the RE(BH 4 ) 2 decompose at higher temperatures in the range 306 to 390 °C, due to lower charge density on the rare-earth ion. The RE(BH 4 ) 3 show increasing stability with increasing Pauling electronegativity, which contradicts other main group and transition metal borohydrides. The majority of the compounds follow Curie-Weiss paramagnetic behaviour down to 3 K with weak antiferromagnetic interactions and magnetic moments in accord with that of isolated 4f ions. Some of the RE(BH 4 ) x display varying degrees of temperature-dependent magnetic moments due to low-lying excited stated induced by crystal field effects. Additionally, a weak antiferromagnetic ordering is observed in Gd(BH 4 ) 3 , indicating superexchange through a borohydride group.

Hydrogen storage properties and polymorphism in KB3H8. The order–disorder polymorphic transition results in disordered B3H8− anions, facilitating cation mobility.

Commencing from metal hydrides, versatile synthesis, purification, and desolvation approaches are presented for a wide range of metal borohydrides and their solvates. An optimized and generalized synthesis method is provided for 11 different metal borohydrides, M(BH) , (M = Li, Na, Mg, Ca, Sr, Ba, Y, Nd, Sm, Gd, Yb), providing controlled access to more than 15 different polymorphs and in excess of 20 metal borohydride solvate complexes. Commercially unavailable metal hydrides (MH, M = Sr, Ba, Y, Nd, Sm, Gd, Yb) are synthesized utilizing high pressure hydrogenation. For synthesis of metal borohydrides, all hydrides are mechanochemically activated prior to reaction with dimethylsulfide borane. A purification process is devised, alongside a complementary desolvation process for solvate complexes, yielding high purity products. An array of polymorphically pure metal borohydrides are synthesized in this manner, supporting the general applicability of this method. Additionally, new metal borohydrides, α-, α'- β-, γ-Yb(BH), α-Nd(BH) and new solvates Sr(BH)·1THF, Sm(BH)·1THF, Yb(BH)· xTHF, x = 1 or 2, Nd(BH)·1MeS, Nd(BH)·1.5THF, Sm(BH)·1.5THF and Yb(BH)· xMeS (" x" = unspecified), are presented here. Synthesis conditions are optimized individually for each metal, providing insight into reactivity and mechanistic concerns. The reaction follows a nucleophilic addition/hydride-transfer mechanism. Therefore, the reaction is most efficient for ionic and polar-covalent metal hydrides. The presented synthetic approaches are widely applicable, as demonstrated by permitting facile access to a large number of materials and by performing a scale-up synthesis of LiBH.

Ammine metal borohydrides show potential for solid-state hydrogen storage and can be tailored toward hydrogen release at low temperatures. Here, we report the synthesis and structural characterization of seven new ammine metal borohydrides, M(BH4)3·nNH3, M = La (n = 6, 4, or 3) or Ce (n = 6, 5, 4, or 3). The two compounds with n = 6 are isostructural and have new orthorhombic structure types (space group P21212) built from cationic complexes, [M(NH3)6(BH4)2]+, and are charge balanced by BH4 –. The structure of Ce­(BH4)3·5NH3 is orthorhombic (space group C2221) and is built from cationic complexes, [Ce­(NH3)5(BH4)2]+, and charge balanced by BH4 –. These are rare examples of borohydride complexes acting both as a ligand and as a counterion in the same compound. The structures of M(BH4)3·4NH3 are monoclinic (space group C2), built from neutral molecular complexes of [M(NH3)4(BH4)3]. The new compositions, M(BH4)3·3NH3 (M = La, Ce), among ammine metal borohydrides, are orthorhombic (space group Pna21), containing molecular complexes of [M(NH3)3(BH4)3]. A revised structural model for A(BH4)3·5NH3 (A = Y, Gd, Dy) is presented, and the previously reported composition A(BH4)3·4NH3 (A = Y, La, Gd, Dy) is proposed in fact to be M(BH4)3·3NH3 along with a new structural model. The temperature-dependent structural properties and decomposition are investigated by in situ synchrotron radiation powder X-ray diffraction in vacuum and argon atmosphere and by thermal analysis combined with mass spectrometry. The compounds with n = 6, 5, and 4 mainly release ammonia at low temperatures, while hydrogen evolution occurs for M(BH4)3·3NH3 (M = La, Ce). Gas-release temperatures and gas composition from these compounds depend on the physical conditions and on the relative stability of M(BH4)3·nNH3 and M(BH4)3.

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