The composite Mg(BH ₄ ) ₂ ·1.5(CH ₃ ) ₂ CHNH ₂ was found to have an ionic conductivity of 2.7 × 10 ⁻⁴ S cm ⁻¹ at 45 °C, with an activation energy of 1.22 eV. Structural investigation suggests hydrophobic interactions improve the conductivity.

The discovery of new inorganic magnesium electrolytes may act as a foundation for the rational design of novel types of solid-state batteries. Here we investigated a new type of organic-inorganic metal hydride, isopropylamine magnesium borohydride, Mg(BH₄)₂∙(CH₃)₂CHNH₂, with hydrophobic domains in the solid state, which appear to promote fast Mg²⁺ ionic conductivity. A new synthetic strategy was designed by combination of solvent-based methods and mechanochemistry. The orthorhombic structure of Mg(BH₄)₂∙(CH₃)₂CHNH₂ was solved ab initio by the Rietveld refinement of synchrotron X-ray powder diffraction data and density functional theory (DFT) structural optimization in space group I2₁2₁2₁ (unit cell, a = 9.8019(1) Å, b = 12.1799(2) Å and c = 17.3386(2) Å). The DFT calculations reveal that the three-dimensional structure may be stabilized by weak dispersive interactions between apolar moieties and that these may be disordered. Nanoparticles and heat treatment (at T > 56 °C) produce a highly conductive composite, σ(Mg²⁺) = 2.86 × 10⁻⁷, and 2.85 × 10⁻⁵ S cm⁻¹ at −10 and 40 °C, respectively, with a low activation energy, E_a = 0.65 eV. Nanoparticles stabilize the partially eutectic molten state and prevent recrystallization even at low temperatures and provide a high mechanical stability of the composite.

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.

An increased electrification of society calls for a revolution of battery technologies to further improve energy densities, safety and reduce dependencies on critical raw materials. Here we present a new type of fast magnesium electrolytes for all solid-state batteries created as solid solutions of two other fast Mg²⁺ ionic conductors, Mg(BH₄)₂ ∙ NH₃ and Mg(BH₄)₂ ∙ CH₃NH₂. However, the different ligands introduce stacking faults in the structures of the solid solutions, which are eliminated upon heating to T > 40 °C. The stacking faults appear to influence ionic conductivity, as the samples are less conductive after heating. Interestingly, the ionic conductivity does not correlate directly with the relative ligand content, as the highest conductivity is observed for the 1:1 molar composition (σ(Mg²⁺) = 7.3 ∙ 10⁻⁶ S cm⁻¹ at 40 °C), which also has the lowest melting point of 60 °C. Thus, this work demonstrates a new approach to increase cationic conductivity using mixed ligand systems to alter conduction pathways and introduce microstructural strain.

All‐solid‐state batteries based on abundant elements, such as calcium, offer a promising route to safer, cheaper, and more sustainable energy storage. Here, we report a series of fast Ca ²⁺ ‐conducting compounds of methylamine calcium tetrahydridoborates, Ca(BH ₄ ) ₂ · x CH ₃ NH ₂ (0 < x  < 4.9) and related nanocomposites stabilized by inert MgO nanoparticles. Three new crystal structures are identified: a three‐dimensional network of octahedrally coordinated Ca ²⁺ complexes for x = 1, a molecular structure of neutral complexes for x = 4, and a structure of cationic complexes for x  = 6. The thermal stability generally decreases with increasing CH ₃ NH ₂ content, and samples with x > 2 slowly release CH ₃ NH ₂ in “open” atmosphere at room temperature, but are stabilized in “closed” environments, e.g. capillaries. The ionic conductivity increases with CH ₃ NH ₂ content and correlates with increased void space and structural flexibility, reaching σ (Ca ²⁺ ) = 5.0·10 ⁻⁵ S cm ⁻¹ at 60 °C for x = 4. Moreover, the effect of nanocomposite formation provides mechanical stability and a doubling of the ionic conductivity for Ca(BH ₄ ) ₂ ·4CH ₃ NH ₂  − MgO (50 wt%), reaching σ (Ca ²⁺ ) = 1.3·10 ⁻⁴ S cm ⁻¹ at 60 °C. These findings demonstrate how local structure and nanoscale interfacial effects govern calcium transport, offering new design principles for functional calcium solid electrolytes.