Decomposition behavior of unmilled and ball milled lithium alanate (LiAlH4) including long-term storage and moisture effects

A comprehensive study of the decomposition behavior of as received and mechanically (ball) milled LiAlH4 has been carried out using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and volumetric hydrogen desorption in a Sieverts-type apparatus. Alfa Aesar LiAlH4 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 LiAlH4 investigated by Ares et al. [9]. High energy ball milling reduced the particle size of the present LiAlH4 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 LiAlH4 can be very dissimilar depending on the supplier from which LiAlH4 powder was purchased. We do not observe a partial decomposition of LiAlH4 during milling up to 5 h under high energy impact mode. The observed melting of LiAlH4 in a DSC test is a very volatile event where the liquid LiAlH4 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 LiAlH4 starts decomposing into Li3AlH6 immediately after melting. Li3AlH6 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 LiAlH4 is able, in a fully solid state, to desorb at 120 °C under pressure of 0.1 MPa H2 (atmospheric) as much as 7.1 wt.%H2 within ∼259,000 s (∼72 h), i.e. ∼93% of the purity-corrected H2 content from the reactions in Stage I (LiAlH4(s) → (1/3)Li3AlH6(s) + (2/3)Al(s) + H2) and Stage II ((1/3)Li3AlH6(s) → LiH + (1/3)Al + 0.5H2). The apparent activation energy for Stage I and II for unmilled LiAlH4 is equal to ∼111 and ∼100 kJ/mol, respectively. For the ball milled LiAlH4 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 LiAlH4 in Stage I. Flammability tests show that the ball milled LiAlH4 powder does not self-ignite on contact with air but can only be ignited by scraping the cylinder walls with a metal tool and then the powder burns with an open flame.