First-principles study of the formation and migration of native defects in LiNH2BH3

First-principles calculations based on density functional theory were carried out to investigate the formation and migration of native defects in LiNH2BH3. The structural properties and formation energies of H, Li, N and B related defects in various charge states were examined. Our analysis showed that the dominant atomic H defects are positively and negatively charged H interstitial (I-H(+) and I-H(-)). The Li related defects are dominated by positively charged Li interstitial (I-Li(+)) and negatively charged Li vacancy (V-Li(-)). For N related defects, the energetically favorable defects are positively and negatively charged NH2 interstitial (I-NH2(+) and I-NH2(-)), while B related defect is dominated by neutral BH3 interstitial (I-BH3(0)). Further results indicated that the neutral H-2 interstitial (I-H2) has the lowest formation energy (0.31 eV), suggesting that the major defect in LiNH2BH3 is I-H2. Investigation of migration processes of the defects showed that the migration barriers for V-(B)H(+) (positively charged H vacancy on a B-H site), I-H(+) and I-H(-) are relatively high (0.50-0.68 eV), whereas moderate diffusion barriers are presented for V-(N)H(-) (negatively charged H vacancy on a N-H site) and I-Li(+) (0.29 and 0.32 eV, respectively). The V-Li(-) and I-H2 defects can migrate with low energy barriers of 0.13 and 0.16 eV, respectively. With a low activation energy of 0.47 eV, I-H2 is the major diffusive species in LiNH2BH3. Our calculation results further suggest that the creation of the V-(N)H, I-Li(+) and V-Li defects is the rate-limiting step for their transportation in LiNH2BH3.