Integrated experimental-theoretical investigation of the Na-Li-Al-H system.

First-principles modeling, experimental, and thermodynamic methodologies were integrated to facilitate a fundamentally guided investigation of quaternary complex hydride compounds within the bialkali Na-Li-Al-H hydrogen storage system. The integrated approach has broad utility for the discovery, understanding, and optimization of solid-state chemical systems. Density functional theory ground-state minimizations, low-temperature powder neutron diffraction, and low-temperature synchrotron X-ray diffraction were coupled to refine the crystallographic structures for various low-temperature distorted Na2LiAlH6 allotropes. Direct method lattice dynamics were used to identify a stable Na2LiAlH6 allotrope for thermodynamic property predictions. The results were interpreted to propose transformation pathways between this allotrope and the less stable cubic allotrope observed at room temperature. The calculated bialkali dissociation pressure relationships were compared with those determined from pressure-composition-isotherm experiments to validate the predicted thermodynamic properties. These predictions enabled computational thermodynamic modeling of Na2LiAlH6 and competing lower order phases within the Na-Li-Al-H system over a wide of temperature and pressure conditions. The predictions were substantiated by experimental observations of varying Na2LiAlH6 dehydrogenation behavior with temperature. The modeling was used to identify the most favorable reaction pathways and equilibrium products for H discharge/recharge in the Na-Li-Al-H system, and to design conditions that maximize the theoretical hydrogen reversibility within the Na-Li-Al-H system.

Mechanochemical synthesis and crystal structure of alpha'-AlD3 and alpha-AlD3.

AlD3 AlD3 was synthesized by ball milling of 3LiAlD4 + AlCl3. Planetary ball milling at room temperature resulted in a mixture of AlD3 (alpha and alpha') and Al in addition to LiCl, whereas cryomilling at 77 K resulted in only AlD3 and LiCl. The AlD3 obtained was a mixture of about 2/3alpha and 1/3alpha'. Alpha' was determined by powder neutron diffraction to take the beta-AlF3 structure with space group Cmcm and a = 6.470(3), b = 11.117(5), and c = 6.562(2) A. It is built up of corner-sharing AlD6 octahedra in an open structure with hexagonal holes of radius 3.9 A. Alpha' slowly decomposes during storage at 40 degrees C. Alpha-AlD3 is also described by a corner-sharing AlD6 network but in a more dense ReO3-type arrangement. Both AlD3 modifications have slightly shorter Al-D distances compared to Na3AlD6, Na2LiAlD6, and K2NaAlH6.

TiCl3-enhanced NaAlH4: impact of excess al and development of the Al1-yTiy phase during cycling.

NaAlH(4) with TiCl(3) and Al were mixed by ball-milling and cycled three times. The hydrogen storage properties were monitored during cycling, and the products were characterized by synchrotron X-ray diffraction. Because of the previously described formation of Al(1)(-)(y)Ti(y) with y approximately 0.15 during cycling that traps Al beyond the amount associated with the formation of NaCl, some Na(3)AlH(6) has no free Al to react with to form NaAlH(4). This was counteracted in the present work by adding a stoichiometric amount of Al that increases the theoretical storage capacity. Due to limitations in metal diffusion small amounts of Na(3)AlH(6) were still detected. When approximately 7 mol % more Al than the stoichiometric amount was added, the observed storage capacity increased significantly, and the Na(3)AlH(6) content was negligible after prolonged rehydrogenation. Cycled NaAlH(4) + 10 mol % TiCl(3) were desorbed to two different levels, and the diffraction patterns were compared. There is no change in unit-cell dimensions during desorption, and there is no sign of changes in the bulk composition of the Al(1)(-)(y)Ti(y) phase during a cycle. Adding pure Ti to a NaH + Al mixture by ball-milling in argon or hydrogen results in formation of TiH(2) that is stable during at least one cycle.

Analytical electron microscopy studies of lithium aluminum hydrides with Ti- and V-based additives.

The microstructure of LiAlD(4) with TiCl(3).1/3(AlCl(3)) and VCl(3) additives has been studied during different steps of the decomposition process using electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope. Energy filtered transmission electron microscopy was used to show elemental distributions in the samples. The spatial distribution of the additives and the main elements within the alanate particles was examined with a resolution of a few nanometers. The analysis of the electron energy loss spectra reveals the chemical state of Al, O, and the additives. Ti and V do not appear to mix chemically with Al to a significant degree. V was found in high concentration in just a few particles, while Ti is more uniformly distributed. All the samples showed evidence of oxidation despite procedures being adopted to avoid exposing the material to air. The additives are oxidized in all the samples, and Al(2)O(3) forms a thin layer at the surface of the particles. This paper gives a comparison between samples at different stages of the decomposition process using different additives.

Synchrotron X-ray studies of Al(1-y)Ti(y) formation and re-hydriding inhibition in Ti-enhanced NaAlH4.

NaAlH4 samples with Ti additives (TiCl3, TiF3, and Ti(OBu)4) have been investigated by synchrotron X-ray diffraction in order to unveil the nature of Ti. No crystalline Ti-containing phases were observed after ball milling of NaAlH4 with the additives, neither as a solid solution in NaAlH4 nor as secondary phases. However, after cycling, a high-angle shoulder of Al is observed in the same position with 10% TiCl3 as that with 2% Ti(OBu)4, but with considerably higher intensity, indicating that the shoulder is caused by Ti. After prolonged reabsorption, there is only a small fraction of free Al phase left to react with Na3AlH6, whereas the shoulder caused by Al(1-y)Ti(y) is dominating. The Ti-containing phase causing the shoulder therefore contains less Ti than Al3Ti, and the aluminum in this phase is too strongly bound to react with Na3AlH6 to form NaAlH4. The composition of the Al(1-y)Ti(y) phase is estimated from quantitative phase analysis of powder X-ray diffraction data to be Al(0.85)Ti(0.15). Formation of this phase may explain the reduction of capacity beyond the theoretical reduction from the dead weight of the additive and the reaction between the additive and NaAlH4.