Mechanochemistry under hydrogen gas of Li-N-H and Li-Mg-N-H systems
Fermin Cuevas1, Zhinian Li1,2, Junxian Zhang1 and Michel Latroche1
1
ICMPE/CNRS-UPEC UMR 7182, 2-8 rue Henri Dunant, 94320 Thiais, France
2
GRINM, No.2 Xinjiekou Wai Street, Beijing 100088, China
The safe and efficient storage of hydrogen is widely recognized as a key challenge in the
transition towards a hydrogen-based energy economy. The lithium-nitrogen-hydrogen (Li-NH) system is a promising store media due to its high hydrogen capacity. Li3N absorbs by
solid-gas reaction up to 10.2 wt.% H through consecutive formation of lithium imide Li2NH
and amide LiNH2 [1]. However, reversible hydrogen storage restrains to Li2NH + H2 ↔
LiNH2 + LiH reaction, with an equilibrium pressure of ca. 1.5 atm at 285 °C
(H = -66 kJ/molH2). Interestingly, this system can be destabilized by replacing lithium
hydride by magnesium hydride. Thus, the reaction enthalpy decreases to -44 kJ/molH2 for
Li2Mg(NH)2 + 2 H2  Mg(NH2)2 + 2LiH. Reversibility at atmospheric pressure is estimated
to be 90°C, a temperature close to PEM fuel cell operation conditions [2]. In this work, we
explore the mechanochemistry under hydrogen gas for both Li-N-H and Li-Mg-N-H systems.
Starting reactants are Li3N and 2Li3N+Mg, respectively, and hydrogen gas at 80 atm. In-situ
hydrogen absorption curves were monitored during mechanochemical synthesis [3,4]. The
reaction paths were elucidated by means of ex-situ X-ray (XRD) and neutron diffraction.
Starting from Li3N, hydrogen absorption of 9.8 wt.% H occurs in 2 h following two steps
of equal hydrogen uptake. In the first stage, -Li3N (S.G. P6/mmm) converts to -Li3N (S.G.
P63/mmc) which reacts with one equivalent of hydrogen gas and forms lithium imide
according to the reaction -Li3N + H2  Li2NH + LiH. During the second step, lithium imide
reacts with a second equivalent of hydrogen gas and forms lithium amide following the
reaction scheme Li2NH + H2  LiNH2 + LiH.
Starting from 2Li3N+Mg powder mixture, two reactions steps of 3.9 and 4.7 wt.% were
noticed to complete an overall uptake of 8.6 wt.% H after 4 h. Ex-situ neutron diffraction
studies in deuterated samples were prepared to determine the reaction route. The first step
leads to the formation of LiH and a metastable mixed-cation imide of rationale composition
Li2+x MgN2H2-x with x ~1. The reaction scheme was identified as 2 Li3N + Mg + 2 H2 
Li3MgN2H + 3 LiH. The crystal structure of this metastable phase is closely related to the
high temperature polymorph γ-Li2Mg(NH)2 [5]. As for the second step, the formation of
amorphous magnesium amide Mg(NH2)2 and additional LiH takes place following the
reaction scheme Li3MgN2H + 3H2  a-Mg(NH2)2 + 3LiH. Acquisition of neutron diffraction
patterns on thermal heating evidenced the crystallization of magnesium amide above 160 °C.
This work reveals that mechanochemistry under hydrogen gas is an efficient method for
fast and simple synthesis of hydrides, leading in some cases to the formation of novel
metastable phases with unexplored properties.
[1] P. Chen, Z. Xiong, J. Luo, J. Lin, K. L. Tan, Nature, 420 (2002) 302
[2] W. Luo, J. Alloys Compd., 381 (2004) 284
[3] J. Zhang, F. Cuevas, W. Zaïdi, J.-P. Bonnet, L. Aymard, J.-L. Bobet, M. Latroche, J. Phys. Chem.
C, 115 (2011) 4971
[4] J. Huot, D. B. Ravnsbæk, J. Zhang, F. Cuevas, M. Latroche, T. R. Jensen, Prog. Mater. Sci., 58
(2013) 30
[5] J. Rijssenbeek, Y. Gao, J. Hanson, Q. Huang, C. Jones, B. Toby, J. Alloys Compd., 454 (2008) 233