Synthesis and H- Conductivity of Ba2LiH3-2x O1+X

Abstract

Hydride ions are attractive as charge carriers because these have an ionic radius appropriate for fast ionic conduction and also show strong reducing properties with a standard redox potential of H–/H2 (–2.25 V vs. SHE), which is close to that of Mg/Mg2+ (–2.38 V vs. SHE). Therefore, H– conductors may be applied in energy storage and conversion devices with high energy densities. Recently, we have found La2-x-y Sr x +y LiH1-x+y O3-y (LSLHO) oxyhydride system possessing pure H– conductivity, and have confirmed the capability of the oxyhydride to act as an H– solid electrolyte by demonstrating the battery reaction of an all-solid-state Ti/LSLHO/TiH2 cell [1]. The LSLHO system shows H– conductivity of 10-4 S·cm–1 at 300 ºC. However, a higher H– conductivity will be required to make novel electrochemical devices using H– conduction phenomenon. In this study, we attempted to synthesize a new H– conductive oxyhydride, Ba2LiH3-2x O1+x (BLHO), in order to achieve a higher H– conductivity than that of LSLHO [2]. BLHO was designed on the basis of LSLHO with the K2NiF4-type structure. La/Sr in LSLHO was substituted by Ba to expand the bottleneck size for H– diffusion in the crystal lattice. Furthermore, H– vacancies were introduced into the structure by controlling H/O ratio in the composition of BLHO. Crystal structure of BLHO was determined by X-ray and neutron diffraction. The X-ray diffraction peak of the sample could be indexed as the K2NiF4-type structure. The crystal structure of Ba2LiH3O (BLHO: x = 0) and Sr2LiH3O (LSLHO: x = 0, y = 2) determined by the Rietveld analysis are compared in Figure 1. The anion arrangement of Ba2LiH3O is basically same with Sr2LiH3O. The two apical anion sites in Li-octahedra are preferentially occupied by oxide ions. Oxide and hydride ions randomly occupy the apical site. In contrast, the four axial anion sites of the Li-anion octahedra prefer to be occupied by only hydride ions. As a result, square-planar LiH4 is formed in the Li-octahedra. When the A-site cation in the composition of A 2LiH3O changed from Sr to Ba, the lattice volume expand depending on the difference of ionic radii between Sr2+ (1.58 Å) and Ba2+ (1.75 Å). In particular, influences of the volume expansion mainly appeared to ab plane in the crystal lattice. The details of H– conduction properties for BLHO will be discussed in our presentation.