We report the results of a systematic study of the effect of halides on hydrogen release and uptake in lithium amide and lithium imide, respectively. The reaction of lithium amide and lithium imide with lithium or magnesium chloride, bromide and iodide resulted in a series of amide-halide and imide-halide phases, only two of which have been reported previously. On heating with LiH or MgH2, the amide-halides synthesised all released hydrogen more rapidly than lithium amide itself, accompanied by much reduced, or in some cases undetectable, release of ammonia by-product. The imide-halides produced were found to hydrogenate more rapidly than lithium imide, reforming related amide-halide phases. The work was initiated to test the hypothesis that the incorporation of halide anions might improve the lithium ion conductivity of lithium amide and help maintain high lithium ion mobility at all stages of the de/rehydrogenation process, enhancing the bulk hydrogen storage properties of the system. Preliminary ionic conductivity measurements indicated that the most conducting amide- and imide-halide phases were also the quickest to release hydrogen on heating and to hydrogenate. We conclude that ionic conductivity may be an important parameter in optimising the materials properties of this and other hydrogen storage systems.

Non-stoichiometry and bulk cation transport have been identified as key factors in the release and uptake of hydrogen in the Li-N-H system. Amide halide phases have been synthesized that have ionic conductivities several orders of magnitude greater than lithium amide, a faster rate of hydrogen release and elimination of the by-product, ammonia. Here we report the effect of both anion-and cation-doping on the hydrogen desorption properties of lithium amide, focusing in particular on how the presence of chloride anions and magnesium cations affects and controls the structure of the amide and imide compounds at the sub-nanometre level. Reducing the chloride content resulted in new low-chloride rhombohedral phases that contain around half of the chloride present in earlier amide chlorides, but maintained the enhancements seen in hydrogen desorption properties when compared to the halide-free system. These materials may also have potential in a range of other energy applications such as all solid state lithium ion batteries, supercapacitors, and CO 2 capture and storage membranes.

a b s t r a c tAn investigation has been carried out into the lower limits of halide incorporation in lithium amide (LiNH 2 ). It was found that the lithium amide iodide Li 3 (NH 2 ) 2 I was unable to accommodate any variation in stoichiometry. In contrast, some variation in stoichiometry was accommodated in Li 7 (NH 2 ) 6 Br, as shown by a decrease in unit cell volume when the bromide content was reduced. The amide chloride Li 4 (NH 2 ) 3 Cl was found to adopt either a rhombohedral or a cubic structure depending on the reaction conditions. Reduction in chloride content generally resulted in a mixture of phases, but a new rhombohedral phase with the stoichiometry Li 7 (NH 2 ) 6 Cl was observed. In comparison to LiNH 2 , this new lowchloride phase exhibited similar improved hydrogen desorption properties as Li 4 (NH 2 ) 3 Cl but with a much reduced weight penalty through addition of chloride. Attempts to dope lithium amide with fluoride ions have so far proved unsuccessful.Ó 2015 Published by Elsevier B.V.