Synchrotron X-ray studies of Al1-yTiy 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 Al1-yTiy 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 Al1-yTiy phase is estimated from quantitative phase analysis of powder X-ray diffraction data to be Al0.85Ti0.15. 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.