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 (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.