pre-printThe effects of Li doping in MgH2 on H-diffusion process are investigated, using first-principles calculations. We have identified two key effects: (1) The concentration of H vacancy in the þ1 charge state ðVþ1 H Þ can increase by several orders of magnitude upon Li doping, which significantly increases the vacancy mediated H diffusion rate. It is caused by the preferred charge states of substitutional Li in the 1 state ðLi1 MgÞ and of interstitial Li in the þ1 state ðLiþ1 i Þ, which indirectly reduce the formation energy of Vþ1 H by up to 0.39eV depending on the position of Fermi energy. (2) The interaction between Vþ1 H and Li1 Mg is found to be attractive with a binding energy of 0.55 eV, which immobilizes the Vþ1 H next to Li1 Mg at high Li doping concentration. As a result, the competition between these two effects leads to large enhancement of H diffusion at low Li doping concentration due to the increased H-vacancy concentration, but only limited enhancement at high Li concentration due to the immobilization of H vacancies by too many Li
