Lithium amides have also been studied in some detail for hydrogen-storage applications following the discovery of Chen et al. that mixtures of LiNH2 + 2 LiH can be reversibly cycled according to the following reactions [70]:

LiNH2 + LiH $ Li2NH + H2

6.5 wt.% H2; AH = 45 kJmor1

Li2NH + LiH $ Li3N + H2

5.5 wt.%H2; AH = 161 kJmor1

Experimentally 11.5 wt.% H2 (relative to the hydrogen-free Li3N) are reversibly stored in the mixture, and temperatures of 473 K and 593 K are required for the first and second reaction step, respectively. Pure LiNH2 decomposes by evolving ammonia in contrast to the reaction system LiNH2 + LiH. Some of the Li can be substituted by Mg which leads to lower desorption-temperatures and a lower reaction enthalpy [71]. During the first desorption 2 LiNH2 + MgH2 undergoes a metathesis reaction [72-74] and the resulting reversible reaction is modified to:

Mg(NH2)2+ 2LiH $ Li2Mg(NH)2+ 2H2

8.5

5.6 wt.% H2 AH = 39 kJmor1

The reaction takes place at 473 K and in total 5.6 wt.% H2 can be reversibly exchanged, but hydrogen release takes place in distinct reaction steps. The high reaction temperature suggests the presence of kinetic barriers hampering the hydrogen exchange reactions and research efforts are focused on the understanding of the reaction steps and the mitigation of the kinetic limitations. Neutron powder

diffraction was used to determine the structure of the hydrogen-rich components LiNH2 [70, 75], Mg(NH2)2 [76] as well as of the hydrogen-depleted imide forms Li2NH [77, 78], a-Li2Mg(NH)2, P-Li2Mg(NH)2 [79], and MgNH [80], all of which are observed as intermediate decomposition-products. With the exception of MgNH, the experimentally-observed crystal structures exhibit characteristic simi­larities, i. e. they are related to the antifluorite structure where the N atoms build an approximate face-centred cubic lattice with the Li or Mg ions (and vacancies) residing in tetrahedral interstitials (see Fig. 8.5). Substitution of Li+ with Mg2+ leads to vacancy ordering on the tetrahedral sites, which results in an orthorhombic structure of a-Li2Mg(NH)2 with an approximately doubled unit cell relative to cubic Li2NH [73]. First-principles calculations identified the hydrogen and cation local arrangement as the structural building blocks of the amide/imide structure [81] and their energetics is consistent with the high degree of disorder that is observed in the mixed imide Li2Mg(NH)2, especially when going from the low temperature а-modification to the high temperature P-phase.

The structural phase evolution during hydrogen absorption and desorption of Mg(NH2)2 + 2LiH has been monitored in situ using neutron powder diffraction (Fig. 8.6). Several studies found that the reaction passes through an intermediate

reaction step, however, the composition of the intermediate phase was not solved unambiguously. Weidner et al. suggested an intermediate phase of composition Li2Mg2(NH)3 [82] which was subsequently also identified in systems with LiH excess [83].

Aoki et al. [84] reported an intermediate phase with composition Li4Mg3(NH2)2(NH)4 for the system Mg(NH2)2 + 6LiH which upon further hydrogen release exhibits a continuous transition towards Li2Mg(NH)2 by way of solid- solution-like compounds of the form Li4+xMg3(NH2)2-x(NH)4. Intermediate phases with non-stoichiometric composition have also been observed experimentally during the decomposition of LiNH2 + LiH [85, 86] and possible intermediates have been investigated using first-principles calculations [87, 88]. The defect structure and vacancy ordering for both systems, LiNH2 + 2LiH and Mg(NH2)2 + 2LiH, has a significant influence on energy landscapes of the systems. Further efforts on theory and experiment are needed, however, to fully comprehend this system.

Additions of LiBH4 to the Mg(NH2)2 + 2LiH system have been reported to improve the hydrogen-exchange reaction because of the intermittent formation of Li4(BH4)(NH2)3 [89]. Neutron powder diffraction on annealed samples of 1LiBD4 + 3LiNH2 was used to characterize the structure.

The beneficial effect of the LiBH4 on the desorption properties is thought to stem from a combination of factors, including an altered reaction-pathway from the removal of intermittent LiNH2 from the mixture, improved recrystallization of Mg (NH2)2, and the exothermic heat-effect during the formation of the quaternary compound Li4(BH4)(NH2)3 which has body-centred cubic symmetry [90].

Mixtures of LiNH2/LiBH4 with varying composition have also been investigated as hydrogen storage materials. More than 10 wt.% H2 is released in an exothermic process [91-93] for the stoichiometric composition (LiNH2)0.67(LiBH4)0.33.