First-principles calculations of crystal and electronic structures and thermodynamic stabilities of La–Ni–H, La–Ni–Al–H and La–Ni–Al–Mn–H hydrogen storage compounds

Abstract

First-principles density functional theory (DFT) and lattice dynamical theory (LDT) calculations have been used to investigate the crystal and electronic structures and thermodynamic stabilities of La–Ni–H, La–Ni–Al–H and La–Ni–Al–Mn–H hydrogen storage compounds. We find that all these compounds studied are dynamically stable. For LaNi3.8Al1.2−xMnxH (x = 0.2, 0.4, 0.6) hydrides, Al only substitutes Ni at 3g0 site, H occupies 12n tetrahedral site. The structural optimizations indicate that Mn prefers to substitute Ni at 3g1 site. Mn substitutions for Ni and Al decrease their stabilities. A detailed analysis of bonding interactions reveals that the covalent bonds of H with one Ni or Mn at 3g1 site and two Ni at 2c site are mainly responsible for the stabilities of these compounds. We also present a comprehensive investigation of phonon spectra and vibrational thermodynamics of LaNi5−xAlxH (x = 0, 0.25, 0.5, 0.75 and 1) and LaNi3.8Al1.2−xMnxH (x = 0.2, 0.4 and 0.6). We find that all phonon vibrations have contribution to their vibrational enthalpies; in contrast, the low-frequency phonon vibrations mainly dominate their vibrational entropies. The calculated accuracy of low-frequency phonon vibrations is closely related to crystal symmetry, supercell size and atomic distribution in selected supercell. Generally, calculated enthalpies are more accurate than calculated entropies with respect to their experimental values. We present in the current research a first-principles method to predict the variation of enthalpy with hydrogen content at hydrogenation or dehydrogenation plateau and then to identify the so-called plateau enthalpy of each La–Ni based hydrogen storage alloy. By using this method, we find that the partial substitutions of Ni by Al decrease the so-called plateau enthalpy but impair the hydrogen storage capacity obviously, while Mn and Al substitutions for Ni not only decrease the so-called plateau enthalpy but also extend the plateau length.