Formation of Lightweight Ternary Polyhydrides and Their Hydrogen Storage Mechanism

Abstract

We have studied Na-Li-H compositions by mixing cations of Na and Li under high pressure using ab initio random structure searching and cluster expansion techniques. We have found that high pressure induces the structural stability of Na3LiH12, Na2Li2H12, and NaLi3H12 compounds. Our phonon calculations confirm the dynamical stability of these compounds, whereas our elastic constant calculations confirm their mechanical stability. Convex hull at 25 GPa indicates the minimum of relative enthalpy in Na2Li2H12, which is the new discovery at lower pressure than other alkali-polyhydride substances. The thermodynamic stability with respect to temperature is analyzed using quasi-harmonic oscillation approximation. Furthermore, NaLiH6 exhibits a high gravimetric hydrogen capacity, which makes this material a potential hydrogen storage candidate. We have also reported the formation enthalpy of hydrogen vacancies as well as the hydrogen diffusion in this lightweight ternary polyhydride system for its practical use in energy storage application.