Preparations and de/re-hydrogenation properties of LixNa3-xAlH6 (x=0.9–1.3) non-stoichiometric compounds

Mixed alkali alanates Li x Na 3-x AlH 6 have been successfully synthesized by means of grinding mixtures of Li 3 AlH 6 and Na 3 AlH 6 in specific molar ratios. Non-stoichiometric Li x Na 3-x AlH 6 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 1.3 Na 1.7 AlH 6 exhibits superior hydrogen storage properties over other Li x Na 3-x AlH 6 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 1.3 Na 1.7 AlH 6 (3.45 wt.%) is the highest among the compounds. The dehydrogenation enthalpy values of Li x Na 3-x AlH 6 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 1.3 Na 1.7 AlH 6 (49.7 kJ mol H 2 −1 ) was greatly destabilized from that of LiNa 2 AlH 6 (68.1 kJ mol H 2 −1 ). Furthermore, the apparent activation energy of dehydrogenation for Li 1.3 Na 1.7 AlH 6 (138.1 kJ mol −1 ) was remarkably lowered from that of LiNa 2 AlH 6 . This illustrates that Li 1.3 Na 1.7 AlH 6 exhibits enhanced dehydrogenation kinetics from that of LiNa 2 AlH 6 .