Mixed-metal Li 3N-based systems for hydrogen storage: Li 3AlN 2 and Li 3FeN 2

Abstract

The hydrogen storage systems Li 3AlN 2 and Li 3FeN 2 were synthesized mechanochemically by two different routes. In each case an intermediate material formed after milling, which transformed into Li 3MN 2 (M = Al or Fe) upon annealing. The synthesis route had a measurable effect on the hydrogen storage properties of the material: Li 3AlN 2 prepared from hydrogenous starting materials (LiNH 2 and LiAlH 4) performed better than that synthesized from non-hydrogenous materials (Li 3N and AlN). For both Li 3AlN 2 materials, the hydrogen storage capacity and the absorption kinetics improved significantly upon cycling. Ti-doped Li 3AlN 2 synthesized from LiNH 2 and LiAlH 4 showed the best hydrogen storage characteristics of all, with the best kinetics for hydrogen uptake and release, and the highest hydrogen storage capacity of 3.2 wt.%, of which about half was reversible. Meanwhile, Li 3FeN 2 synthesized from Li 3N and Fe displayed similar kinetics to that synthesized from Li 3N and Fe xN (2 ≤ x ≤ 4), but demonstrated lower gravimetric hydrogen storage capacities. Li 3FeN 2 displayed a hydrogen uptake capacity of 2.7 wt.%, of which about 1.5 wt.% was reversible. For both Li 3AlN 2 and Li 3FeN 2, doping with TiCl 3 resulted in enhancement of hydrogen absorption kinetics. This represents the first study of a ternary lithium-transition metal nitride system for hydrogen storage.