Decomposition behavior of unmilled and ball milled lithium alanate (LiAlH 4) including long-term storage and moisture effects

A comprehensive study of the decomposition behavior of as received and mechanically (ball) milled LiAlH 4 has been carried out using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and volumetric hydrogen desorption in a Sieverts-type apparatus. Alfa Aesar LiAlH 4 powder investigated in this work has the average particle size of 9.9 ± 5.2 μm as compared to 50–150 μm for Sigma–Aldrich LiAlH 4 investigated by Ares et al. [9]. High energy ball milling reduced the particle size of the present LiAlH 4 to 2.8 ± 2.3 μm. In general, comparing the results of our microstructural studies with those reported by Ares et al. [9] it is clear that the morphology, microstructure and chemistry of LiAlH 4 can be very dissimilar depending on the supplier from which LiAlH 4 powder was purchased. We do not observe a partial decomposition of LiAlH 4 during milling up to 5 h under high energy impact mode. The observed melting of LiAlH 4 in a DSC test is a very volatile event where the liquid LiAlH 4 starts foaming and flowing out of the alumina crucible. After completion of solidification and desorption at temperatures above melting the powder resembles a lava rock. A thermal sectioning in DSC tests at pre-determined temperatures and subsequent XRD studies show that LiAlH 4 starts decomposing into Li 3AlH 6 immediately after melting. Li 3AlH 6 seems to be already solidified before it starts decomposing in the next stage. All volumetric desorption curves at the 120–300 °C range clearly exhibit a two-stage desorption process, Stage I and II. As received LiAlH 4 is able, in a fully solid state, to desorb at 120 °C under pressure of 0.1 MPa H 2 (atmospheric) as much as 7.1 wt.%H 2 within ∼259,000 s (∼72 h), i.e. ∼93% of the purity-corrected H 2 content from the reactions in Stage I (LiAlH 4(s) → (1/3)Li 3AlH 6(s) + (2/3)Al(s) + H 2) and Stage II ((1/3)Li 3AlH 6(s) → LiH + (1/3)Al + 0.5H 2). The apparent activation energy for Stage I and II for unmilled LiAlH 4 is equal to ∼111 and ∼100 kJ/mol, respectively. For the ball milled LiAlH 4 the apparent activation energy for Stage I and II is slightly lower ∼92.5 and ∼92 kJ/mol, respectively. The water absorption up to 11.7% due to exposure to air for 1 h does not change in any drastic way the hydrogen desorption rate of ball milled LiAlH 4 in Stage I. Flammability tests show that the ball milled LiAlH 4 powder does not self-ignite on contact with air but can only be ignited by scraping the cylinder walls with a metal tool