Dopant-vacancy binding effects in Li-doped magnesium hydride

TL;DR

This study combines ab initio calculations and statistical mechanics to analyze how lithium doping affects vacancy behavior and hydrogen diffusion in magnesium hydride, revealing an optimal doping level for enhanced diffusion.

Contribution

It provides new insights into dopant-vacancy interactions and the limited stability of Li-doped MgH2, informing hydrogen storage material design.

Findings

Maximum free vacancy concentration occurs at ~1 mol% Li doping.

Li-doped MgH2 is thermodynamically metastable against phase separation.

Doping enhances hydrogen diffusion only within a limited concentration range.

Abstract

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.