Native defects in LiNH2: A first-principles study

Abstract

Native defects in lithium amide (LiNH 2 ), a promising candidate for hydrogen storage, are investigated by first-principles calculations based on density functional theory. We examine the structural properties and formation energies of H-, Li-, and N-related defects in all possible states. We find that the dominant H-and Li-related defects are in charged states, i.e., negatively charged H vacancy (V H − ), positively charged H interstitial (H i + ), negatively charged Li vacancy (V Li − ), and positively charged Li interstitial (I Li + ). V Li − and I Li + are present in the highest concentration. The positively charged NH 2 vacancy has the lowest formation energy among N-related defects. Furthermore, migration processes of the dominant defects are investigated. V Li − diffuses most rapidly with the lowest migration energy of 0.20 eV. Both formation and migration energies of Li-related dominant defects are found to be lower than those of H-related dominant defects. With an activation energy of 0.72 eV, V Li − is the major diffusive species in LiNH 2 . Our results further indicate that the formation of H i is the bottleneck for H transport.

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

“…Other groups have recently reported first-principles calculations for native defects in LiNH 2 , using methodologies similar to ours. [26][27][28] The calculated formation energies and migration barriers of individual hydrogen-, lithium-, and nitrogen-related defects reported by Wang et al 28 are in close agreement with our results (to within 0.1 eV for most defects, with a maximum deviation of 0.2 eV in the case of V − H , our value being lower). Comparing to the results of Hazrati et al, 27 the deviations are somewhat larger (up to 0.4 eV), for which we cannot offer an explanation.…”

“…Hazrati et al 27 also proposed that the decomposition process occurs at the surface with the formation of (H + i ,V − H ) Frenkel pairs. Wang et al, 28 on the other hand, did not provide any specific mechanism but suggested that the formation of H + i is the rate-limiting step in hydrogen mass transport.…”

Intrinsic Defects in LiMn2O4: First-Principles Calculations

“…Besides experimental studies on defect-related properties in cathode materials, theoretic calculations, especially first-principles methods based on density functional theory (DFT), have been demonstrated to be a powerful tool for addressing defects in solid. 27 , 28 Some researchers have studied point defects in LiMn 2 O 4 through computational methods. 20 , 21 , 29 − 32 As early as the 1990s, Islam’s group have studied Li- and Mn-related interstitials and vacancies in LiMn 2 O 4 using interatomic potential models.…”

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

“…Some preliminary results and partial conclusions of our work have been reported elsewhere. 25 Other research groups have also recently started investigating native defects, [26][27][28] but our study goes much further in identifying specific mechanisms that can explain the experimental observations. A detailed comparison with the previous papers will be addressed in Sections IV A 3 and V B.…”