Hydride-Ion Conduction in AELiH3 (AE = Sr, Ba)

Abstract

Hydride ions (H–) are expected to conduct fast in solids owing to their monovalent state, appropriate ionic radius for conduction, and large polarizability. Thus, H– is a promising class of the charge carrier with high mobility, and may be applicable to the electrochemical reaction in batteries and fuel cells. The K2NiF4–type structure, wherein the perovskite KNiF3 and the rock-salt KF layers are stacked along the c-axis, has been the most researched material as a hydride-ion conductor, e.g., K2NiF4–type oxyhydrides of La2–x–y Sr x +y LiH1–x+y O3–y [1]. In this study, we focused on perovskite-type hydrides, AELiH3 (AE = Ca, Sr, Ba). A simple perovskite structure is expected to have high ionic conductivity, because it ensures a three-dimensional diffusion pathway for the anion species [2-6].First, the migration energy of H– ions in the AELiH3 materials were evaluated using nudged elastic band (NEB) calculations. The migration pathway between adjacent H– sites via vacancies was considered. First-principles calculations were performed using the projector-augmented wave method and PBEsol functional as implemented in the VASP code. The H– diffusivity of sodium-doped SrLiH3 was investigated using density functional theory molecular dynamics. AELiH3 were experimentally synthesized using a mechanochemical method, and their ionic conductivities were evaluated using electrochemical impedance spectroscopy. To further improve the ionic conductivity of these materials, hydride-ion vacancies were introduced by replacing the A-site cation with the monovalent cation of Na+. Neutron powder diffraction (NPD) data were collected at room temperature using a time-of-flight diffractometer (BL09 SPICA) at the J-PARC facility. The distribution of the nuclear scattering length density was analyzed by the maximum entropy method (MEM) using the Z-MEM code. The effect of the alkali-earth element at A-site on the migration energy of H– was evaluated using NEB. The migration energy are estimated to be 24, 35, and 39 kJ mol–1 for CaLiH3, SrLiH3, and BaLiH3, respectively. These low migration energy (< 40 kJ mol–1) suggest that the perovskite-type AELiH3 are suitable substances for hydride-ion conduction. AELiH3 (AE = Sr, Ba) were synthesized via mechanical milling process. The X-ray diffraction patterns of SrLiH3 and BaLiH3 were attributed to a cubic perovskite structure with the space group Pm–3m. For CaLiH3, the starting materials of CaH2 and LiH remained in the product, and the perovskite-type CaLiH3 could not be synthesized. The ionic conductivities of SrLiH3 and BaLiH3 at 100 °C were 4.6 × 10–7 and 6.7 × 10–9 S cm–1, respectively. This clearly denotes the characteristic of hydride ion; a smaller ionic radius of the A-site cation results in a lower migration energy and a higher ionic conductivity. Solid solutions of Sr1–x Na x LiH3–x were obtained in the range of 0 ≤ x ≤ 0.075. The ionic conductivity was enhanced upon Na-doping. The highest conductivity was observed for Sr0.925Na0.075LiH2.925 (x = 0.075) with a bulk conductivity of 5.0 × 10–6 S cm–1 at 25.6 °C. The figure shows the refined crystal structure of Sr0.925Na0.075LiH2.925 obtained by a Rietveld analysis of the NPD data, in addition to the isosurface plot of the nuclear density obtained by MEM. Rietveld analysis confirmed the presence of H defects and a remarkable anisotropy of the thermal oscillations of H. The negative nuclear density at the H sites showed broadening along the {100} direction. These structural features suggest that hydride ions conduct via vacancies between their adjacent sites. The choice of small cations at the A sites and the presence of hydrogen vacancies in perovskite-type hydrides resulted in high hydride-ion conductivity. [1] Kobayashi et.al., Science, 351, 1314–1317(2016). [2] Bai, Q. et.al., ACS Appl. Energy Mater., 1, 1626–1634(2018). [3] Ishihara, T. et.al., J. Am. Chem. Soc., 116, 3801–3803(1994). [4] Li, M. et.al., Nat. Mater., 13, 31–35(2014). [5] Hull, S. et.al., J. Phys.: Condens. Matter, 11, 5257–5272(1999). [6] Yamane, Y. et.al., Solid State Ionics, 179, 605–610(2008). Figure 1