Mechanochemistry and hydrogen storage properties of 2Li₃N+Mg mixture

The Li–Mg–N–H hydrogen storage system is a promising hydrogen storage material due to its moderate operation temperature, good reversibility, and relatively high capacity. In this work, the Li–Mg–N–H composite was directly synthesized by reactive ball milling (RBM) of Li₃N and Mg powder mixture with a molar ratio of 2:1 under hydrogen pressure of 9 MPa. More than 8.8 wt% hydrogen was absorbed during the RBM process. The phases and structural evolution during the in situ hydrogenation process were analyzed by means of in situ solid–gas absorption and ex situ X-ray diffraction (XRD) measurements. It is determined that the hydrogenation can be divided into two steps, leading to mainly the formation of a lithium magnesium imide phase and a poorly crystallized amide phase, respectively. The H-cycling properties of the as-milled composite were determined by temperature-programmed dehydrogenation (TPD) method in a closed system. The onset dehydrogenation temperature was detected at 125 °C, and it can reversibly desorb 3.1 wt% hydrogen under a hydrogen back pressure of 0.2 MPa. The structural evolution during dehydrogenation was further investigated by in situ XRD measurement. It is found that Mg(NH₂)₂ phase disappears at about 200 °C, and Li₂Mg₂N₃H₃, LiNH₂, and Li₂MgN₂H₂ phases coexist at even 300 °C, revealing that the dehydrogenation process is step-wised and only partial hydrogen can be desorbed.