Preparations and de/re-hydrogenation properties of Li_xNa_3-xAlH₆ (x=0.9–1.3) non-stoichiometric compounds

Mixed alkali alanates Li_xNa_3-xAlH₆ have been successfully synthesized by means of grinding mixtures of Li₃AlH₆ and Na₃AlH₆ in specific molar ratios. Non-stoichiometric Li_xNa_3-xAlH₆ compounds with single perovskite-type structures (space group Fm-3m) can be formed only within the composition range of x = 0.9–1.3. Li_1.3Na_1.7AlH₆ exhibits superior hydrogen storage properties over other Li_xNa_3-xAlH₆ compounds. Its onset dehydrogenation temperature (∼423 K) was lowered by more than 40 K from other samples in temperature programmed dehydrogenation (TPD) curves. Also, the dehydrogenation capacity of Li_1.3Na_1.7AlH₆ (3.45 wt.%) is the highest among the compounds. The dehydrogenation enthalpy values of Li_xNa_3-xAlH₆ decreased as x increased from 0.9 to 1.3 according to the results by isothermal pressure-composition (PCI) curves and van't Hoff plots. It shows that the dehydrogenation Li_1.3Na_1.7AlH₆ (49.7 kJ mol H₂⁻¹) was greatly destabilized from that of LiNa₂AlH₆ (68.1 kJ mol H₂⁻¹). Furthermore, the apparent activation energy of dehydrogenation for Li_1.3Na_1.7AlH₆ (138.1 kJ mol⁻¹) was remarkably lowered from that of LiNa₂AlH₆. This illustrates that Li_1.3Na_1.7AlH₆ exhibits enhanced dehydrogenation kinetics from that of LiNa₂AlH₆.