Decomposition pathway of KAlH4 altered by the addition of Al2S3

Altering the decomposition pathway of potassium alanate, KAlH 4 , with aluminium sulfide, Al 2 S 3 , presents a new opportunity to release all of the hydrogen, increase the volumetric hydrogen capacity and avoid complications associated with the formation of KH and molten K. Decomposition of 6KAlH 4 -Al 2 S 3 during heating under dynamic vacuum began at 185 °C, 65 °C lower than for pure KAlH 4 , and released 71% of the theoretical hydrogen content below 300 °C via several unknown compounds. The major hydrogen release event, centred at 276 °C, was associated with two new compounds indexed with monoclinic (a = 10.505, b = 7.492, c = 11.772 Å, β = 122.88°) and hexagonal (a = 10.079, c = 7.429 Å) unit cells, respectively. Unlike the 6NaAlH 4 -Al 2 S 3 system, the 6KAlH 4 -Al 2 S 3 system did not have M 3 AlH 6 (M = alkali metal) as one of the intermediate decomposition products nor were the final products M 2 S and Al observed. Decomposition performed under hydrogen pressure initially followed a similar reaction pathway to that observed during heating under vacuum but resulted in partial melting of the sample between 300 and 350 °C. The measured enthalpy of hydrogen absorption (ΔH abs ) was in the range -44.5 to -51.1 kJ mol -1 H 2 , which is favourable for moderate temperature hydrogen applications. Although, the hydrogen capacity decreases during consecutive H 2 release and uptake cycles, the presence of excess amounts of aluminium allow for further optimisation of hydrogen storage properties

Hydroxylated closo-Dodecaborates M2B12(OH)12 (M = Li, Na, K, and Cs); Structural Analysis, Thermal Properties, and Solid-State Ionic Conductivity

Copyright © 2020 American Chemical Society. Closo-borates and derivatives thereof have shown great potential as electrolyte materials for all-solid-state batteries owing to their exceptional ionic conductivity and high thermal and chemical stability. However, because of the myriad of possible chemical modifications of the large, complex anion, only a fraction of closo-borate derivatives has so far been investigated as electrolyte materials. Here, the crystal structures, thermal properties, and ionic conductivities of M2B12(OH)12 (M = Li, Na, K, and Cs) are investigated with a focus on their possible utilization as new solid-state ion conductors for solid-state batteries. The compounds generally show rich thermal polymorphism, with eight identified polymorphs among the four dehydrated compounds. Both Li2B12(OH)12 and Na2B12(OH)12 undergo a first-order transition, in which the cation sublattices become disordered, resulting in an order of magnitude jump in ionic conductivity for Na2B12(OH)12. K2B12(OH)12 undergoes a second-order polymorphic transition driven by a change in the anion-cation interaction, with no evidence of dynamic disorder. The ionic conductivities of M2B12(OH)12 range from 1.60 × 10-8 to 5.97 × 10-5 S cm-1 at 250 °C for M = Cs and Li, respectively, showing decreasing conductivity with increasing cation size. Compared with the analogous M2B12H12 compounds, such relatively low conductivities are suggested to be a consequence of strong and directional anion-cation interactions, resulting in a more static anion framework