Li+ ionic conductivities and diffusion mechanisms in Li-based imides and lithium amide

In this study, both experimental ionic conductivity measurements and the first-principles simulations are employed to investigate the Li+ ionic diffusion properties in lithium-based imides (Li2NH, Li2Mg(NH)2 and Li2Ca(NH)2) and lithium amide (LiNH2). The experimental results show that Li+ ions present superionic conductivity in Li2NH (2.54 × 10−4 S cm−1) and moderate ionic conductivity in Li2Ca(NH)2 (6.40 × 10−6 S cm−1) at room temperature; while conduction of Li+ ions is hardly detectable in Li2Mg(NH)2 and LiNH2 at room temperature. The simulation results indicate that Li+ ion diffusion in Li2NH may be mediated by Frenkel pair defects or charged vacancies, and the diffusion pathway is more likely via a series of intermediate jumps between octahedral and tetrahedral sites along the [001] direction. The calculated activation energy and pre-exponential factor for Li+ ion conduction in Li2NH are well comparable with the experimentally determined values, showing the consistency of experimental and theoretical investigations. The calculation of the defect formation energy in LiNH2 reveals that Li defects are difficult to create to mediate the Li+ ion diffusion, resulting in the poor Li+ ion conduction in LiNH2 at room temperature.