Effects of Li doping on H-diffusion in MgH2: A first-principles study

Abstract

The 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 +1 charge state (VH+1) 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 (LiMg−1) and of interstitial Li in the +1 state (Lii+1), which indirectly reduce the formation energy of VH+1 by up to 0.39 eV depending on the position of Fermi energy. (2) The interaction between VH+1 and LiMg−1 is found to be attractive with a binding energy of 0.55 eV, which immobilizes the VH+1 next to LiMg−1 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.