Decomposition of LiAl(NH2)4 and reaction with LiH for a possible reversible hydrogen storage

This paper deals with the possibility of using compounds in the Li-Al-N-H system as hydrogen storage materials. First, the LiAl(NH2)(4) phase is prepared from LiAIH(4) reacting in liquid ammonia at room temperature. The thermal decomposition of LiAI(NH2)(4) under primary vacuum leads to the release, at 138 degrees C, of two moles of NH3 per formula unit and the formation of LiAl(NH)(2). This study mainly concerns the trapping of ammonia by adding an adequate amount of LiH to LiAI(NH2)(4) in order to desorb hydrogen at low temperature. Indeed, a 1:4 LiAI(NH2)4:LiH mixture prepared by ball-milling is able to desorb more than 5.0 wt % of hydrogen at 130 degrees C. This is remarkable as the desorption occurs at a temperature about 60 degrees C lower than the 1:2 Mg(NH2)(2):LiH mixture reported in the literature. The first attempts of rehydrogenation at 130 degrees C are, however, disappointing, due to the low stability of LiAl(NH)(2). This imide is rapidly decomposed into LiNH2 and AlN, the latter being inert toward hydrogen at low temperature, which explains the lack of reversibility of the hydrogen storage process in this Li-Al-N-H system.