Effect of LiCl presence on the hydrogen storage performance of the Mg(NH2)2–2LiH composite

Abstract

The Mg(NH₂)₂–2LiH composite is a promising hydrogen storage material due to its favourable thermodynamics and hydrogen storage reversibility. However, its application is restricted owing to the presence of severe kinetic barriers. In the present work, the Mg(NH₂)₂–2LiH was synthesized by metathesis reaction of LiNH₂ plus MgCl₂ and posterior milling with LiH. LiCl is a co-product that operates as a separation phase favouring the nanostructure of the composite. The synthesized material exhibits good dehydrogenation rate and approximately the theoretical hydrogen storage capacity at 200 °C. However, the formation of Li₄(NH₂)₃Cl and the incomplete rehydrogenation of Li₂Mg₂(NH₂)₃ are progressively favoured during successive hydrogen cycling, deteriorating the storage properties. Two competitive reactions can simultaneously occur involving LiNH₂ as an intermediate phase: the formation of Li₄(NH₂)₃Cl using the co-product LiCl and the complete dehydrogenation of Li₂Mg₂(NH₂)₃ leading to the Li₂Mg(NH)₂ formation. The worst effect of Li₄(NH₂)₃Cl formation was simultaneously demonstrated by kinetics and PCI measurements. The amide-chloride phase was actually the active species for the deterioration of dehydrogenation kinetics and hydrogen storage capacity of the Li–Mg–N–H–Cl system.