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

Abstract

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 (Li 2NH, Li 2Mg(NH) 2 and Li 2Ca(NH) 2) and lithium amide (LiNH 2). The experimental results show that Li + ions present superionic conductivity in Li 2NH (2.54 × 10 -4 S cm -1) and moderate ionic conductivity in Li 2Ca(NH) 2 (6.40 × 10 -6 S cm -1) at room temperature; while conduction of Li + ions is hardly detectable in Li 2Mg(NH) 2 and LiNH 2 at room temperature. The simulation results indicate that Li + ion diffusion in Li 2NH 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 Li 2NH are well comparable with the experimentally determined values, showing the consistency of experimental and theoretical investigations. The calculation of the defect formation energy in LiNH 2 reveals that Li defects are difficult to create to mediate the Li + ion diffusion, resulting in the poor Li + ion conduction in LiNH 2 at room temperature. © the Owner Societies 2012.