  We use a combination of ab initio calculations and statistical mechanics to
investigate the substitution of Li+ for Mg2+ in magnesium hydride (MgH2)
accompanied by the formation of hydrogen vacancies with positive charge (with
respect to the original ion at the site). We show here that the binding energy
between dopants and vacancy defects leads to a significant fraction of trapped
vacancies and therefore a dramatic reduction of the number of free vacancies
available for diffusion. The concentration of free vacancies initially
increases with dopant concentration, but reaches a maximum at around 1 mol% Li
doping and slowly decreases with further doping. At the optimal level of
doping, the corresponding concentration of free vacancies is much higher than
the equilibrium concentrations of charged and neutral vacancies in pure MgH2 at
typical hydrogen storage conditions. We also show that Li-doped MgH2 is
thermodynamically metastable with respect to phase separation into pure
magnesium and lithium hydrides at any significant Li concentration, even after
considering the stabilization provided by dopant-vacancy interactions and
configurational entropic effects. Our results suggest that lithium doping may
enhance hydrogen diffusion hydride, but only to a limited extent determined by
an optimal dopant concentration and conditioned to the stability of the doped
phase.
