Synthesis and decomposition of Li3Na(NH2)4 and investigations of Li–Na–N–H based systems for hydrogen storage

Abstract

Previous studies have shown modified thermodynamics of amide–hydride composites by cation substitution, while this work systematically investigates lithium–sodium–amide, Li–Na–N–H, based systems. Li₃Na(NH₂)₄ has been synthesized by combined ball milling and annealing of 3LiNH₂–NaNH₂ with LiNa₂(NH₂)₃ as a minor by-product. Li_3+xNa_1-x(NH₂)₄ releases NaNH₂ and forms non-stoichiometric Li_3+xNa_1−x(NH₂)₄ before it melts at 234 °C, as observed by in situ powder X-ray diffraction. Above 234 °C, Li_3+xNa_1−x(NH₂)₄ releases a mixture of NH₃, N₂ and H₂ while a bi-metallic lithium sodium imide is not observed during decomposition. Hydrogen storage performances have been investigated for the composites Li₃Na(NH₂)₄–4LiH, LiNH₂–NaH and NaNH₂–LiH. Li₃Na(NH₂)₄–4LiH converts into 4LiNH₂–NaH–3LiH during mechanochemical treatment and releases 4.2 wt% of H₂ in multiple steps between 25 and 340 °C as revealed by Sievert's measurements. All three investigated composites have a lower peak temperature for H₂ release as compared to LiNH₂–LiH, possibly owing to modified kinetics and thermodynamics, due to the formation of Li₃Na(NH₂)₄ and LiNa₂(NH₂)₃.