Solid-state magnesium ion conductors with exceptionally high ionic conductivity at low temperatures, 5 × 10−8 Scm−1 at 30 °C and 6 × 10−5 Scm−1 at 70 °C, are prepared by mechanochemical reaction of magnesium borohydride and ethylenediamine. The coordination complexes are crystalline, support cycling in a potential window of 1.2 V, and allow magnesium plating/stripping. While the electrochemical stability, limited by the ethylenediamine ligand, must be improved to reach competitive energy densities, our results demonstrate that partially chelated Mg2+ complexes represent a promising platform for the development of an all-solid-state magnesium battery.

Eutectic melting in mixtures of alkali and alkali earth metal borohydrides can pave the way for new applications as fast ionic conductors, and facilitate hydrogen release by low temperature chemical reactions and convenient nanoconfinement. Here, we determine the eutectic composition for the lithium potassium borohydride system, 0.725LiBH4-0.275KBH4, with the lowest melting point, Tmelt ∼105 °C, of all known alkali and alkali earth metal borohydride mixtures. Mechanochemistry and manual mixing of LiBH4-KBH4 mixtures facilitate the formation of LiK(BH4)2. However, the melting or heat treatments used in this work do not produce LiK(BH4)2. The bimetallic borohydride dissociates into the monometallic borohydrides at ∼95 °C and partial melting occurs at ∼105 °C. Analysis of the unit cell volumes of LiBH4, KBH4 and LiK(BH4)2 in the temperature range 25 to 90 °C indicates that the formation of the bimetallic borohydride is facilitated by a more dense packing as compared to the reactants. Thus, LiK(BH4)2 is considered metastable and the formation is pressure induced. A phase diagram for the LiBH4-KBH4 system is established, which illustrates the low eutectic melting point and the stability range for the bimetallic borohydride, LiK(BH4)2.

Coordination complexes of magnesium borohydride show promising properties as solid electrolytes for magnesium ion batteries and warrant a thorough microscopic description of factors governing their mobility properties. Here, the dynamics of Mg(BH 4) 2diglyme 0.5 on the atomic level are investigated by means of quasielastic neutron scattering (QENS) supported by DFT calculations, IR and NMR spectroscopy. Employing deuterium labeling we can unambiguously separate all the hydrogen containing electrolyte components, which facilitate Mg 2+ transport, and provide a detailed analytical description of their motions on the picosecond time scale. The planar diglyme chain coordinating the central Mg atom appears to be flexible, while two dynamically different groups of [BH 4 ] − anions undergo reorientations. The latter has important implications for the thermal stability and conductivity of Mg(BH 4) 2-diglyme 0.5 and demonstrates that the presence of excess Mg(BH 4) 2 units in partially chelated Mg complexes may improve the overall performance of related solid-state electrolytes.

The LiBH 4 -NaBH 4 pseudo-binary system has been investigated by X-ray diffraction, temperatureprogrammed photographic analysis, and differential scanning calorimetry, in order to establish the phase diagram. The polymorphic orthorhombic-to-hexagonal phase transition of LiBH 4 was observed at 94 °C in samples containing NaBH 4 , i.e. 15 °C lower than for pure LiBH 4 , which indicates the dissolution of sodium into LiBH 4 . The formation of solid solutions was confirmed by powder X-ray diffraction measurements performed as a function of temperature. A new eutectic composition between Li 0.65 Na 0.35 BH 4 and Li 0.70 Na 0.30 BH 4 , with a melting temperature of 216 °C, is observed. Ab-initio calculations have been performed to establish the relative stabilities of the pure compounds in orthorhombic, hexagonal and cubic structure. The obtained experimental and calculated data were compared with available literature values and they were used for a thermodynamic assessment of the LiBH 4 -NaBH 4 system by the Calphad method. The enthalpy of mixing for solid and liquid solutions has been estimated on the basis of experimental data.The LiBH4-NaBH4 pseudo-binary system has been investigated by X-ray diffraction, temperature programmed photographic analysis and differential scanning calorimetry, in order to establish the phase diagram. Ab-initio calculations have been performed to establish the relative stabilities of the pure compounds in various structures. The obtained experimental and calculated data were compared with available literature values and they were used for a thermodynamic assessment of the LiBH4-NaBH4 system by the Calphad method.

closo-Borates, such as Na 2 B 12 H 12 , are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na 2 B 12 H 12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primaryorder parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling. Raman spectroscopyRaman spectra were collected in a diamond anvil cell on a Renishaw 1000 spectrometer with a 633 nm excitation laser. The pressure was monitored using an internal ruby standard. The spectra were recorded between 500 cm À1 and 2700 cm À1 . DFT calculationsDFT calculations were performed in order to resolve the structure of the -polymorph; to refine the crystal structure of the high-pressure "-polymorph and to determine their ther-research papers Acta Cryst. (2019). B75, 406-413 Romain Moury et al.